Number 41 09/2018 - iagi.or.id · Wartono Rahardjo University of Gajah Mada, Yogyakarta, Indonesia...

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Berita Sedimentologi

Number 41 – September 2018

Published by

The Indonesian Sedimentologists Forum (FOSI) The Sedimentology Commission - The Indonesian Association of Geologists (IAGI)

Number 41

09/2018

SANDSTONE DIAGENESIS: ESTABLISHING THRESHOLD TEMPERATURE AND DEPTH OF POROSITY DETERIORATION, PENYU BASIN AND TENGGOL ARCH,

OFFSHORE PENINSULAR MALAYSIA

THE EFFECT OF METEORIC PHREATIC DIAGENESIS AND SPRING SAPPING ON THE FORMATION OF SUBMARINE COLLAPSE STRUCTURES IN THE BIAK BASIN,

EASTERN INDONESIA

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Editorial Board

Minarwan Chief Editor Bangkok, Thailand E-mail: [email protected]

Herman Darman Deputy Chief Editor Indogeo Social Enterprise Jakarta, Indonesia E-mail: [email protected]

Rahmat Utomo Bangkok, Thailand E-mail: [email protected]

Ricky Andrian Tampubolon Lemigas, Jakarta

E-mail: [email protected]

Mohamad Amin Ahlun Nazar University Link Coordinator Jakarta, Indonesia E-mail: [email protected]

Rina Rudd Reviewer Husky Energy, Jakarta, Indonesia E-mail: [email protected]

Visitasi Femant Treasurer, Membership & Social Media Coordinator Pertamina Hulu Energi, Jakarta, Indonesia E-mail: [email protected]

Yan Bachtiar Muslih Pertamina University, Jakarta, Indonesia E-mail: [email protected]

Advisory Board

Prof. Yahdi Zaim Quaternary Geology Institute of Technology, Bandung

Prof. R. P. Koesoemadinata Emeritus Professor Institute of Technology, Bandung

Wartono Rahardjo

University of Gajah Mada, Yogyakarta, Indonesia

Mohammad Syaiful

Exploration Think Tank Indonesia

F. Hasan Sidi

Woodside, Perth, Australia

Prof. Dr. Harry Doust Faculty of Earth and Life Sciences, Vrije Universiteit De Boelelaan 1085 1081 HV Amsterdam, The Netherlands E-mails: [email protected]; [email protected]

Dr. J.T. (Han) van Gorsel 6516 Minola St., HOUSTON, TX 77007, USA www.vangorselslist.com E-mail: [email protected]

Dr. T.J.A. Reijers Geo-Training & Travel Gevelakkers 11, 9465TV Anderen, The Netherlands E-mail: [email protected]

Dr. Andy Wight formerly IIAPCO-Maxus-Repsol, latterly consultant for Mitra Energy Ltd, KL E-mail: [email protected]

• Published 3 times a year by the Indonesian Sedimentologists Forum (Forum Sedimentologiwan Indonesia, FOSI), a commission of the

Indonesian Association of Geologists (Ikatan Ahli Geologi Indonesia, IAGI).

• Cover topics related to sedimentary geology, includes their depositional processes, deformation, minerals, basin fill, etc.

Cover Photographs:

Top: Grain to grain contact in Well J-1 at 3320 m

ahbdf (credit: Franz Kessler & John Jong)

Bottom: Thin section photographs in plane

polarised light of cement textures and fabrics

observed in Biak Basin (credit: David Gold)

mailto:[email protected]









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Dear Readers, Berita Sedimentologi No. 41 is finally available for your reading, much later than the publication date that we originally estimated. This unpunctuality has been caused by the constant challenge that we have to overcome prior to publishing many editions of Berita Sedimentologi to date, namely the lack of quality manuscript submitted to us and the lack of manpower for lay out process. As you’re probably aware, Berita Sedimentologi is a non-profit scientific publication managed by a group of volunteers with various other commitment, therefore

priority conflicts have been unavoidable. Nevertheless, we received two high quality research papers to be considered for publication in the current volume and both of them have received high commendation from our international reviewers. These manuscripts are about Sandstone Diagenesis in the Penyu Basin and Tenggol Arch, offshore Peninsular Malaysia and The Effect of meteoric phreatic diagenesis and spring sapping on the formation submarine collapse structure in the Biak Basin. This early September, FOSI will have its 3rd regional seminar that

takes place on the 5th-6th of September in Grha Sabha Pranama, Yogyakarta, Indonesia. This seminar is a jointly-organized event between FOSI, UGM, IAS and SEPM. The seminar is supported by PT Geoservices (Ltd) as the Platinum Sponsor and LEMIGAS as a supporting sponsor. We hope you enjoy reading this publication and as always, if you would like to share your ideas and knowledge through our publication, please contact one of our editors.

Warm regards,

Minarwan

Chief Editor

INSIDE THIS ISSUE

SANDSTONE DIAGENESIS: ESTABLISHING THRESHOLD TEMPERATURE AND DEPTH OF POROSITY DETERIORATION, PENYU BASIN AND TENGGOL ARCH, OFFSHORE PENINSULAR MALAYSIA – F.L. Kessler & J. Jong

5

THE EFFECT OF METEORIC PHREATIC DIAGENESIS AND SPRING SAPPING ON THE FORMATION OF SUBMARINE COLLAPSE STRUCTURES IN THE BIAK BASIN, EASTERN INDONESIA – D.P. Gold

23

Berita Sedimentologi A sedimentological Journal of the Indonesia Sedimentologists Forum (FOSI), a commission of the Indonesian Association of Geologist (IAGI)

From the Editor

Call for paper BS #42 –

to be published in December 2018

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About FOSI

he forum was founded in 1995 as the Indonesian Sedimentologists Forum

(FOSI). This organization is a communication and discussion forum for geologists, especially for those dealing with sedimentology and sedimentary geology in Indonesia. The forum was accepted as the sedimentological commission of the Indonesian Association of Geologists (IAGI) in 1996. About 300 members were registered in 1999, including industrial and academic fellows, as well as students.

FOSI has close international relations with the Society of Sedimentary Geology (SEPM) and the International Association of Sedimentologists (IAS). Fellowship is open to those holding a recognized degree in geology or a cognate subject and non-graduates who have at least two years relevant experience. FOSI has organized 2 international conferences in 1999 and 2001, attended by more than 150 inter-national participants.

Most of FOSI administrative work will be handled by the editorial team. IAGI office in Jakarta will help if necessary.

The official website of FOSI is:

http://www.iagi.or.id/fosi/

Any person who has a background in geoscience and/or is engaged in the practising or teaching of geoscience or its related business may apply for general membership. As the organization has just been restarted, we use LinkedIn (www.linkedin.com) as the main data base platform. We realize that it is not the ideal solution, and we may look for other alternative in the near future. Having said that, for the current situation, LinkedIn is fit for purpose. International members and students are welcome to join the organization.

T

FOSI Membership

FOSI Group Member as of SEPTEMBER 2018: 986 members

http://www.iagi.or.id/fosi/

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SANDSTONE DIAGENESIS: ESTABLISHING THRESHOLD TEMPERATURE AND DEPTH OF POROSITY DETERIORATION, PENYU BASIN AND TENGGOL ARCH, OFFSHORE PENINSULAR MALAYSIA Franz L. Kessler 1* and John Jong2 1 Goldbach Geoconsultants, O&G and Lithium Exploration, Germany 2 JX Nippon Oil and Gas Exploration (Deepwater Sabah) Limited *Corresponding Authors –[email protected]

ABSTRACT

A review of clastic sandstone reservoirs from the Penyu Basin and Tenggol Arch area has revealed that the deepest, stratigraphically oldest and potentially overpressured reservoirs are affected by diagenetic alteration of reservoir mineral components. There is a marked discrepancy between measured reservoir temperature and calculated reservoir temperature based on vitrinite reflectance data in several investigated wells. Assuming a relatively constant temperature gradient in the basin during the Pliocene to recent time, quartz cementation started at a paleo-depth of ca. 2000m tvdss or 105°C, and porosity was mostly destroyed at a depth of ca. 3000m tvdss and 130°C. This said, there is a strong stratigraphic correlation between pre-Oligocene sediments with high vitrinite reflectivity readings, and a strongly elevated contemporaneous temperature gradient. Therefore, the scope for deep oil and gas drilling maybe reduced in at least some parts of the basin, where oil is found locked in diagenetically altered pore spaces. In addition, geological data also suggest that the Penyu Basin is very complex and may have stronger affiliation with pull-apart rather than with rift basins. Key words: diagenesis, inclusions, Penyu Basin, quartz cementation, Tenggol Arch, vitrinite reflectance

INTRODUCTION The Penyu Basin lies south-west of the Malay Basin and straddles the Malaysia-Indonesia maritime boundary (Figure 1). The stratigraphic summaries of the Penyu and Malay basins are illustrated in Figure 2, and the former is subdivided into lithostratigraphic units such as the Penyu, Pari and the Pilong formations; with age ranging from the Early Oligocene to the Pleistocene, and sometimes described in terms of syn-rift to post-rift sequences separated by erosional unconformities. A seismic type section for the basin is illustrated in Figure 3. In a rich and productive basin, hydrocarbon charge and migration may be inferred from the spatial distribution of discovered accumulations and by assuming the presence of a kitchen down dip of the accumulations. However, rarely is the hydrocarbon migration phenomenon observed or physically demonstrated by well data without further detailed fluid inclusion investigation. In general, the generative source rocks are seldom penetrated by the wells, which are commonly drilled on structural

highs. Therefore, in a less explored basin or frontier basin, more effort is often invested to demonstrate the presence of hydrocarbons, as proof of a key component of a working petroleum system. Quality of reservoir is (perceived as perhaps) one of the most important risk factors in any exploration and appraisal campaign. Oil companies spend significant money in studying reservoir environments, and related statistics for porosity and permeability distributions of various depositional settings. Unfortunately, reservoir diagenesis data are often either forgotten, neglected or the available data are not thoroughly studied by licensed operators. In a previous paper by Maga et al. (2015), the authors highlighted fluid inclusions in quartz as indicators of oil migration timing and reservoir diagenesis, and that oil migration came along with mineral brines. Here, we focus on analysis of reservoir temperatures as determined by logs (present-day basin temperature regime) and vitrinite reflectance data (historical basin temperature regime). Both have important implications for assessing the optimum depth of exploration targets in the Penyu Basin and Tenggol Arch area.

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Figure 1: Structural framework map of the Malay and Penyu basins, offshore Peninsular

Malaysia. The study area covering the Penyu Basin and Tenggol Arch area is outlined in

dashed red line (after IHS Energy, 2010). Blue dots annotate locations of well investigated by

Maga et al. (2015), where quartzes with fluid inclusions are reported.

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EXPLORATION HISTORY AND PETROLEUM SYSTEM

The Penyu basin-filling sediments are of terrestrial and fluvio-lacustrine origin, and punctuated by marine sediments from the occasional periods of ingression (Figures 2 and 3). A number of source rock intervals have been identified (from Late Oligocene to Early Miocene sequences of equivalent Groups K and L in the Malay Basin, and questionable Eocene), and there are at least two pulses of oil migration observed (e.g., Tan, 2009). The main reservoirs in the Penyu Basin are siliciclastic, deposited mainly in fluvial-lacustrine channels and deltas, with varying grain sizes from (rarely) coarse, middle to fine sands. As summarised by Maga et al. (2015), the basin has been explored since 1968 (pre-PSC days) when Conoco was given a concession that once covered the entire basin. To-date, there have been more than 20 exploration wells drilled, mainly on Miocene (post-rift) inversion structures. So far exploration efforts have resulted in one oil discovery at Rhu-1 in 1991 in late “syn-rift” sediments in basement-drape structure (Madon and Azlina Anuar, 1999), and uneconomic discoveries/reasonable shows and in a few other wells (e.g., A-1, J-1; Figures 3 and 4). Recent exploration drilling in the Cherating Half Graben had confirmed the presence of abundant live oil in the mid “syn-rift” section despite the apparent dearth of good reservoir rocks (C-1, J-1; Figure 4). The Rhu-1 well encountered oil in fluvial Oligocene “syn-rift” sandstones. The Rhu oil is light (API 35°), moderately waxy and of high pour point (44 °C). With Oligocene-Miocene levels only marginally mature for oil, charge is inferred to stem from Eocene-Oligocene age isolated pods of lacustrine source with potential contribution from coal and coaly shale layers. The Rhu discovery is evidence of a functioning petroleum basin (Tan, 2009), where in the case of Rhu-1, a clear case of vertical oil migration can be made. Some oils trapped in reservoirs and fluid inclusions are low-gravity, suggesting oil migrated through the conduits at low temperatures, implying limited rock overburden, and likely early migration during the Early Miocene. The Rhu-type light oils probably originated during a secondary, later migration pulse, and the relatively high reservoir temperatures have prevented biodegradation. Hydrocarbon-bearing reservoirs in the Penyu Basin are (with the exception of Rhu) of mostly poor quality and are located in the Oligocene and pre-Oligocene sections. A recent sequence stratigraphic study by Barber (2013) suggests that the depositional system of the Penyu Basin is mainly composed of restricted fluvio-lacustrine facies up to Base Miocene level, and was isolated from the fluviatile sedimentary

pulses of the Malay Basin to the north by the intervening saddle of the Tenggol Arch until Early Miocene time (Figure 3). Post-Early Miocene, the Penyu and the Malay basins practically formed a united sedimentary system of mainly fluvio-terrestrial and shallow marine deposits sourced from the north culminating in a major inversion episode in the study area.

GEOLOGICAL SETTING The Penyu Basin and the more well-known Malay Basin are part of a system of the Cenozoic sedimentary basins developed along a major zone that stretches from the Gulf of Thailand to the Natuna region. Different geological and tectonic models have been proposed to explain the origin of the basins, as discussed recently by Tan (2009). The basins are considered to have originated either in a back-arc setting (Kingston et al., 1983; Mohd Tahir et al., 1994), or as a pull-apart basin developed along a major strike-slip fault (Tapponier et al., 1982), or through thinning of continental crust (White and Wing, 1978). Other tectonic models involve crustal extension over a hot spot (Hutchison, 1989; Khalid Ngah et al., 1996), extensional subsidence along a major left-lateral shear zone (Madon and Watts, 1998; Md Yazid et al., 2014) and as a failed rift arm of a triple junction above a mantle hot spot (Tjia, 1999). Morley and Westaway (2006) proposed a geodynamic model of the Malay Basin, involving lower-crustal flow in response to post-rift sedimentation. Most of these models are regional in nature and were crafted without the benefit of offshore data. However, in recent years we have seen widespread acquisition of Tensor gravity and 3D seismic data, as well as marine gravity data. The data show a thoroughly sheared crust with basement (granite, metamorphic rocks, and basalts) forming elongated slivers along a number of strike-slip faults, particularly at the edges of the Tenggol Arch (Ng, 1987; Figures 1 and 5). Accordingly, we believe that the basins were originated as pull-apart basins on top of a highly sheared continental crust, which started to subside at the end of the Eocene. According to this model, there may not have been any extension in the sense of pure rifting as such, but instead pulses of basement shear that resulted in elongated sag basins (Maga et al., 2015). The Penyu Basin is tectonically complex and has seen at least two phases of major strike-slip tectonism and inversion; the older and very widespread inversion occurred intra-Oligocene, with a weaker and localized one in the Miocene. This may have had the effect that originally channelized reservoir bodies were compartmentalized even further.

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Figure 2: Comparison

of Penyu and Malay

basins stratigraphy

modified after Tan

(2009), originated from

Madon and Azlina

Anuar (1999, Penyu

Basin) and Madon et

al. (1999, Malay

Basin).

.

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Figure 3: Penyu Basin regional seismic section illustrating mapped sequence boundaries (SB) and related depositional facies; yellow = sandy facies, green =

lacustrine shales, purple = meta-sediments. The depositional system of the Penyu Basin comprises mainly restricted lacustrine facies up to Base Miocene, near

Top L event and was isolated from the fluvial sedimentary pulses of the Malay Basin until this time, from Maga et al. (2015) and after Barber (2013).

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Figure 4: Exploration wells and structural elements of the Penyu Basin (well status from IHS Energy). Wells MB-1, J-1 and A-1 as highlighted in

red were investigated for porosity and temperature versus depth relationships (modified after Maga et. al., 2015).

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INVESTIGATION METHODOLOGY – RESERVOIR PETROGRAPHY AND SANDSTONE DIAGENESIS Fluid inclusions in authigenic cements can be measured directly from thin sections. Unfortunately, to the best of our knowledge none of the oil companies that operated in the study area carried out micro-thermometry on any sample. Alternatively, an indirect method is used whereby the depth of occurrence of both quartz cements and fluid inclusions is recorded and compared with borehole temperature and vitrinite reflectance data. Vitrinite reflectance is correlated with temperature using the formula by Barker and Pawlewicz (1995). If there is no significant difference between the measured borehole temperature (which is derived from various sources, in the order of descending quality: DST, MDT, wireline log headers) and vitrinite-derived maximum temperature, it can be

assumed that the heat flow history of the area had been stable over time, and that the present-day borehole temperatures represent the fluid temperatures at the time of cementation. There are only few studies conducted on the depositional histories of sediments in the Penyu Basin, with little published information available on its diagenetic history whereas most studies were focused on the Malay Basin instead (e.g., Nik Ramli, 1986, 1988; Ibrahim and Madon, 1990; Chu, 1992; Madon, 1994). Chu (1992) carried out a comprehensive basin-wide petrographic study of sandstones in the Malay Basin, using 241 samples and established the main porosity reduction mechanism to be mechanical compaction down to a depth of about 1200m tvdss (see also Madon, 1994). Beyond this depth, chemical processes took over, including cementation by quartz or, more rarely, calcite, siderite, pyrite and clay minerals (Figure 6).

Figure 5: Base Tertiary (Top Basement) map over the Penyu Basin and Tenggol Arch showing strike-

slip faulting at the southern edge of the latter. Purple zones represent half-grabens and depocentres and

indicate the land-locked nature of the basin at the time. Inset is a SW-NE seismic section that illustrates

the basement faulting over the Tenggol fault zone (from Maga et al., 2015).

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Uplift and erosion in some parts of the Malay Basin (especially in the south) resulted in secondary porosity by dissolution of framework grains, including feldspar. These are recognised from the presence of oversized pores left by dissolution of framework grains (Nik Ramli, 1986; Ibrahim and Madon, 1990). Hence, more secondary porosity is expected to occur in sandstones with moderate percentage of lithic/feldspathic grains compared to “clean” quartzose sandstones. This is ultimately tied to the provenance and depositional environments of the sandstones (Ibrahim and Madon, 1990). Madon (1994) studied the sandstones in Jerneh Field in northern Malay Basin, and observed increase in quartz overgrowths with depth, from an average of 1% at 1270m tvdss to 7% at 1940m tvdss. Quartz overgrowths begin at about 90 – 100 °C. The amount of quartz cementation appears to be

increasing with age and depth of burial too (Chu, 1992), and also appear more prevalent in “clean” sandstones (Madon, 1994).

DATABASE The study area covers the main Penyu Basin and the Tenggol Arch area (Figure 1), previously operated by PETRONAS Carigali, Conoco, Texaco and more recently Lundin Petroleum and JX Nippon partnership. The database for this study consists of 12 exploration wells previously investigated by Maga et al. (2015; Figure 7), where wireline logs, cuttings evaluation, fluid inclusion stratigraphy (FIS), and thin-section petrographic analyses are available. The latter two studies were provided mostly by Fluid Inclusion Technologies, Inc. (Tulsa) and Corelab (Kuala Lumpur), respectively.

Figure 6: Diagenetic sequence in the Malay Basin, based on Madon (1994).

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Figure 7: Locations of investigated wells and occurrence of oil inclusions as summarised by Maga et al. (2015). Blue areas denote Penyu depocentre,

with depth contour map (in second TWT) of the Oligocene “K” shale level shown (modified after Maga et al., 2015).

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POROSITY, QUARTZ CEMENTATION AND WELL TEMPERATURE RELATIONSHIP Lander and Walderhaug (1999) had established kinetic models of quartz cementation in which quartz cementation starts at an effective threshold temperature of around 70 – 80 °C. Oelkers et al. (2000) suggested that cementation is a continuous process, with cement developing as a function of both temperature and quartz surface area, and with most cement precipitating between 100 - 120°C. Harwood et al. (2013) performed isotope analyses on sandstone from the Jurassic Ness Formation from the North Sea to reveal the growth history of single quartz overgrowths to a resolution of 2 µm. Measured δ18O (cement) range from +28 to +20‰ V-SMOW in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 – 70°C. A comparison of three Penyu wells (MB-1, A-1 and J-1), shows different porosity decline patterns (Figure 8). Most likely, the observed porosity decline is strongly shaped by quartz cementation. In the Penyu Basin, there are two important diagenesis boundaries: (i) Beyond the porosity-reducing mechanism of

mechanical compaction, the onset of destructive diagenesis starts with mineral cementation, in particular quartz cement. This boundary is found in the Malay Basin at around 1200m tvdss (Chu, 1992; Madon, 1994) and in the Penyu Basin between 1800 and 2000m ahbdf (along hole below derrick floor with DFE of 20m, Figure 9). Depending on the temperature gradients in these basins, these depths correspond to temperatures ranging from 105 – 130°C in Penyu Basin (Figure 9), and 90 - 100 °C in the Malay Basin (Madon, 1994). Below this depth, diagenetic processes appear to have affected permeability more than porosity, as the pore throats and smaller pores are occluded by quartz and clay cements. In larger pores, fluids (occasionally oil) may be preserved but these are no longer mobile. A similar pattern of diagenesis is observed in most of the studied wells (Figure 10).

(ii) The depth at which permeability destruction

is deemed complete is at around 2800-3000m tvdss. At this depth, authigenic cements have occluded the pore spaces and only some isolated porosity remains. In clean sandstones, most of the permeability is destroyed by quartz cementation, and clay minerals such as kaolinite, especially in clay- or feldspar-rich intervals (Figure 11). However, as observed in Figure 8, where an increase in porosity values is noticed in J-1 and A-1 wells located in the Cherating Half-Graben at depth around 3250m ahbdf, it is

thought that a deep overpressured formation might have preserved porosity, albeit with the associated reduction in permeability by chemical compaction, which would have likely continued, independently of overpressure, of these deep reservoirs (e.g., Mohammad Jamaal, 2003).

The stratigraphy of the Penyu Basin (Figure 2) is characterized by a varying thickness of pre-Oligocene strata, and a mighty Oligocene section covered by a blanket of Neogene in the order of some 1200m thickness. There was mild tectonism during the Miocene, with a locally prominent, Serravallian tectonic inversion along a number of fault lines (Maga et al., 2015). In the Rhu area, the well data indicate a temperature gradient of between 3.3 and 4.0°C/100m (Figure 9). The temperature gradient is fairly uniform throughout the borehole but increases in the bottom hole (Oligocene) section of the wells; most likely as a result of higher formation density and thermal conductivity. This uniformity is broken by, a very important jump in vitrinite readings from 0.7 to 1.0 VRM in the bottom hole section of wells A-1 and J-1 with calculated sediment removal of more than 1000m, indicating an important unconformity (Figure 12). Sandstone samples (cuttings) of the J-1 bottom-hole sequences show strongly welded quartz and feldspar grains (Figure 13). The heat gradient in the bottom hole section (6°C/100 m) is also strongly elevated in respect to the shallower (Oligo-Neogene) sequence with 3°C/100m only. Figure 14 shows a gridded depth map, a compilation of the temperature gradients calculated from the borehole temperature measurements in the Penyu Basin and Tenggol Arch area. The gradients range from 2.7 to 6°C/100m. Recently drilled deep wells (> 4000m, J-1 and A-1) in the Cherating Half-Graben, however, recorded these anomalously high bottom hole section gradients of 5-60C/100m. The cause of these high gradients is yet unknown but maybe the consequence of the wells being located in a “hotspot” area. In respect to oil migration and reservoir diagenesis, it is possible or even likely, that pulses of hot water brines flushed carrier beds together with oil or alternated with the latter. Sections of the bottom hole sequence in wells J-1 and A-1 presented strong oil shows with reasonable porosity preservation (Figure 11). However, there was evidence of low permeability: oil appeared to be locked in diagenetically secluded pore space and could not escape from the reservoir fabric. This may also explain the overpressures observed in hydrocarbon-bearing intervals. The process is shown schematically in Figure 15. Based on the temperature/geothermal gradient data, and our understanding of quartz cementation and its relationship with temperature gradient, a spatial distribution of porosity destruction by quartz cements is shown in Figure 16. Ultimately, these

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temperature gradients and porosity threshold maps are indicative of the exploration risk that oil companies are facing when exploring deep reservoir targets.

Discovery of oil-bearing but tight reservoir rock is regarded as technical success, but economic success depends to a large extent on reservoir permeability, as well as reservoir continuity.

Figure 8: Porosity comparison at depth (in meters along hole below derrick floor) between three

Penyu Basin exploration wells; MB-1, J-1 and A-1. See Figure 5 for well locations.

.

Figure 9: In the study area, quartz overgrowth starts at a depth > 1800 - 2000 m ahbdf, depending

on the respective temperature gradient rather than on stratigraphic unit (from Maga et al. 2015).

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Figure 10: Common patterns

of diagenesis affecting

reservoir rock at increasing

depth, temperature, and

pressure. Mobilized quartz

builds up layers of cement,

with the outcome that

permeability in sandstone

reservoirs is destroyed

gradually (from Maga et al.,

2015).

Figure 11: Miscellaneous cuttings from well J-1, show a strong recrystallization, leading

to the occasional growth of authigenic quartz crystals. There is little or no reservoir

potential present in these samples (from Maga et al., 2015).

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Figure 12: Well J-1 temperature, VRM and depth relationship. Borehole temperatures and vitrinite

reflectivity data, converted to °C using the Barker and Pawlewicz formula (1995) are well aligned on the

bottom hole section of the well (> 2500m tvdss) with a much higher temperature gradient than the upper

hole section, which belongs arguably to a much older (? Mesozoic) stratum. This observation points to a

significant stratigraphic gap of some 1000 - 2000 m missing sediment.

Figure 13: Grain-to-

grain contact in Well J-

1 at 3320m (ahbdf).

The cuttings show

strongly welded quartz

and feldspar grains,

with hardly any

cement being visible.

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Figure 14: Penyu Basin and Tenggol Arch - temperature gradients (°C/100m) calculated from

borehole measurements, DST and wireline temperatures (from Maga et al., 2015).

Figure 15: A schematic summary of oil migration and reservoir diagenesis. a) Pre-diagenetic stage < 80

°C - moderately saline water fills pore spaces between clastic grains. b) Oil migration and salty brine ca.

100 °C - saline water with oil enters pore spaces. Mixing of salty brine with porefill water, and oil forms

bubbles (green) in the centre of pore spaces. c) Destructive diagenesis ca. 120 °C - quartz and kaolinite

(yellow) grow along the margins of the clastic grains (= syntaxial overgrowth). Oil is trapped, there is no

permeability left, and porosity is strongly reduced. Oil bubbles were pressurised and shrank.

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DISCUSSION Temperature and vitrinite reflectivity comparisons indicate, that the Penyu Basin may be far more complex than previously thought. In particular, we notice the effect of repeated strike-slip movements, having affected the basin possibly during pre-Oligocene, and confirmed during Oligocene and, somewhat less so, in Neogene time. Therefore, the Penyu Basin may constitute a pull-apart basin, rather than a rift basin. Furthermore, a comparison between the wells: MB-1, J-1 and A-1 and Rhu-1 points to the presence of an older remnant, a pre-Oligocene sediment sequence, located in the centre of the basin, and displaying high vitrinite reflectivity values. This sequence is poorly dated but may contain Eocene and perhaps even Cretaceous strata. Most likely, this part of the basin has seen removal of pre-Oligocene sediments of more than 1000m. This may point to pre-Oligocene uplift, followed by Early Oligocene drowning. This invokes comparisons with parallel observations of the Sarawak margin evolution on north-western Borneo (Kessler and Jong 2016). Further evidence of strike-slip tectonism in the Malay Basin is given by Md Yazid et al. (2014), the authors described the basin was developed partly as

a result of tectonic collisions and strike-slip shear of the Southeast Asia continental slabs, as the Indian Plate collided into Eurasia, and subsequent extrusion of lithospheric blocks towards Indochina: “The Sunda Block epicontinental earliest rift margins were manifested by the Palaeogene W-E rift valleys, which formed during NW-SE sinistral shear of the region. Later Eocene NW-SE dextral shear of (2nd order) Indochina Block against East Malaya Block rifted open a 3rd order Malay Basin. Developed within it is a series of 4th order N-S en-echelon ridges and grabens. The grabens and some ridges, sequentially, host W-E trending 5th order folds of later compressional episodes. The Malay Basin Ridge and Graben Model explains the multi-phased structural deformation which started with the, a) Pre-Rift Palaeo/Mesozoic crystalline/ metamorphic Basement, b) Syn-rift phase during Paleogene, c) Fast Subsidence from Late Oligocene to Middle Miocene, d) Compressional inversion of first Sunda fold during Late Miocene, and e) Basin Sag during Plio-Pleistocene with mild compressional episodes. The subsequent Mio-Pliocene folding history of Malay Basin is connected to the collision of Sunda Block against subducting Indian-Australian Plate”.

Figure 16: Penyu Basin and Tenggol Arch - porosity is largely annihilated by quartz

cement/overgrowth, below indicated depth levels (m tvdss), @ 130 °C (modified after Maga et al

2015).

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Adding to the above model, strike-slip tectonism is also observed in the Tenggol Arch area, which separates the southern edge of the Malay Basin from the Penyu Basin, and also in the small Naga Hitam pull-apart graben offshore Pahang. Further west, however, we see areas of the Peninsula Malaysia rising at least since the Oligocene, and shedding sediment into both the Malay and Penyu basins. The boundary between Oligocene pull-apart basins (Malay, Penyu, Naga Hitam) and Oligocene uplift (Peninsula Malaysia), however, appears difficult to pinpoint.

CONCLUSIONS Economic viability of deep oil and gas reservoirs depends on the presence of porosity and permeability. The latter is of particular importance, as it controls the production rates of oil and gas. This case study demonstrates that reservoir porosity and permeability prognosis cannot rely on sedimentary facies patterns and compaction criteria alone, and that an understanding of diagenesis is vital. The results of this study could be used to assist in the assessment of the optimum depth for exploration targets in the study area, which suggest that at depths of greater than 3000m tvdss sandstones are likely to have poor reservoir properties. Note also that some vitrinite reflectance data appear to be inconclusive and should be reviewed. However, it is worth mentioning that the current observations from the rather limited extent of the Penyu Basin may not necessarily hold for the bigger and well-developed Malay Basin, where a number of mechanisms for porosity preservation in deeper reservoirs have been documented. These include grain/rim coating in detrital quartz-rich environment, emplacement of pore spaces by early hydrocarbon migration, secondary porosity development as described in this paper and, preservation of pore spaces by overpressures. The latter overpressure mechanism is believed to be the main common process for porosity preservation of deeper reservoirs in the Malay Basin, and likely the same scenario for the deeper section of J-1 and A-1 wells. Interestingly, and as noted in this study, research by Mohammad Jamaal et al. (2003), suggested the possible role of chemical compaction, which would be facilitated by the high geothermal (gradient> 5°C/100m) in the Malay Basin. Chemical compaction appears to commence around 100 - 110 °C and is independent of overpressure with the associated reduction in permeability that may need to be taken into account for assessing hydrocarbon prospectivity of deeper parts of the basin.

ACKNOWLEDGEMENTS This paper is written based on the extended findings of an earlier publication by Maga et al. (2015). We

thank past and present authors who contributed to the ideas presented in this paper, and to whom we are indebted. However, any mistakes or shortcomings remain the responsibility of the authors. Fruitful discussions with Dr. Mazlan Madon, our colleagues in PETRONAS Carigali, Lundin Petroleum and JX Nippon are gratefully acknowledged. Our gratitude is extended to our reviewers for offering constructive comments, which help to improve the quality of this paper.

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THE EFFECT OF METEORIC PHREATIC DIAGENESIS AND SPRING SAPPING ON THE FORMATION OF SUBMARINE COLLAPSE STRUCTURES IN THE BIAK BASIN, EASTERN

INDONESIA David P. Gold* SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, United Kingdom *Corresponding author: [email protected]

ABSTRACT

The islands of Biak and Supiori, situated in the Bird’s Head region of New Guinea, comprise predominantly Neogene age carbonate units that extend offshore into the adjacent Biak Basin. Unusual geomorphologic features including pockmarks, headless canyons and semi-circular collapse structures identified in multibeam bathymetric imagery occur on the southern margin of the Biak Basin. These features have a bathymetric expression distinct from strike-slip faults of the Biak Fault Zone which bound the eastern margin of the basin. The Biak Fault Zone comprises several seismically active, segmented and parallel fault strands. Seismicity along the Biak Fault Zone is responsible for the shedding of mass transport deposits into the basin, however these are absent from the geomorphologic features along the southern margin of the basin. Instead, these features appear isolated and unrelated to activity of the Biak Fault Zone and are interpreted to have formed as a result of ‘spring sapping’ by submarine aquifers. Rapid uplift during the Pliocene caused exposure and karstification of carbonates from onshore Biak which extend into the offshore Biak Basin, providing conduits for a freshwater lens to develop within older Miocene strata. Diagenetic cement textures and fabrics indicate that many Miocene carbonates were subjected to meteoric diagenesis within freshwater aquifers that overprinted burial cements. This is supported by stable isotope analyses of diagenetic cements which record negative δ18O values. Keywords: Spring sapping, meteoric diagenesis, carbonates, aquifer

INTRODUCTION The islands of Biak and Supiori are situated in the Indonesian province of Papua on the Pacific island of New Guinea. These islands form part of a small archipelago of islands north of Cenderawasih Bay, a large embayment to the west of New Guinea (Figure 1). Biak is the largest island within this archipelago, with the island of Supiori located to the northwest. The islands of Biak and Supiori are separated by the Sorendidori Fault (Figure 1), an oblique normal fault that downthrows younger Neogene sediments of the island of Biak to the SE from Early to Middle Miocene carbonates of the island of Supiori to the NW (Gold et al., 2017). Neogene sediments from Biak and Supiori are predominantly carbonates that extend SW into the offshore Biak Basin, which is situated south of Biak and Supiori, and north of

Yapen Island (Figure 1). The Biak Basin is bounded on its eastern margin by the Biak Fault Zone, a series of parallel, NW-SE trending strike-slip faults that form the linear west coast of Supiori and Biak (Gold et al., 2017). These faults also form clearly expressed lineaments on the seafloor that are readily observed in multibeam bathymetric data (Gold et al., 2017; Figure 1). Several unusual geomorphologic collapse features are observed along the southern margin of the offshore Biak Basin (Figure 1). This study aims to test whether these features are fault-controlled or diagenetic in origin by examining the bathymetric expression of structural features of the basin using multibeam imagery and the burial history of analogous outcrop samples collected from formations that extend offshore. This paper



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contributes to the understanding of geomorphologic and diagenetic responses to regional tectonic events in a frontier basin of Eastern Indonesia through application of laboratory techniques to identify the sedimentary processes that control geomorphologic features.

NEOGENE GEOLOGICAL HISTORY During the Early to Middle Miocene, carbonate platforms flourished across much of the Bird’s Head and are recorded in outcrop and the Salawati, Bintuni, and Biak Basins. These carbonates form part of the ‘New Guinea Limestone Group’ which includes several contemporaneous carbonate formations found across much of western New Guinea (Visser and Hermes 1962; Pieters et al., 1983; Brash et al., 1991; Gold et al., 2017). From the Middle to Late Miocene, a reduction of carbonate accumulation rates due to environmental deterioration which were outpaced by the rate of relative sea-level rise led to the drowning of the New Guinea Limestone Group platform beneath deep-water strata (Gold et al., 2017). Rapid uplift of New Guinea, validated by fission track ages of metamorphic units, is recorded from 10 Ma, and in many areas since 5 Ma (Hill and Gleadow, 1989). This culminated in the formation of the regional ‘intra-Pliocene unconformity’, dated within the Salawati basin to have occurred at

approximately 4 Ma (Decker et al., 2009). This unconformity is related to rapid uplift of the Misool-Onin-Kumawa ridge, an arcuate anticline sub-parallel to what is now the Seram Trough (Pairault et al., 2003). The collision of the Banda Arc with the Australian margin in the Timor area caused large scale surface deformation across the Bird’s Head and Banda Arc from slab-mantle decoupling (Spakman and Hall, 2010). The rapid isostatic uplift resulting from this decoupling caused the formation of this unconformity and the rapid exhumation of the Neogene sediments. The Biak Fault Zone is interpreted to be a young feature as it incises Pliocene strata (Gold et al., 2017). Recent sedimentation within the Biak Basin is controlled by activity along the Biak Fault Zone (Bertoni and Garcia 2012; Memmo et al., 2013).

INVESTIGATION OF OFFSHORE FEATURES ‘Pock marks’, headless canyons and semi-circular collapse features observed in bathymetric multibeam data occur several kilometres offshore south-west of the island of Biak (Figure 1). In multibeam data, a narrow ‘Y-shaped’ headless canyon-oriented NW-SE is approximately 10 km in length and 700 m wide (Figure 1). Approximately 20 km east of this structure, a semi-circular collapse feature is approximately 7 km in diameter (Figure 1).

Figure 1: ASTER digital elevation and bathymetric multibeam data provided by TGS of the Biak Basin and islands of Biak and Supiori displaying key structural and bathymetric features (MTDs - Mass transport deposits).


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The potential for these structures to be fault-controlled or diagenetic in origin was examined. Active faulting on the Biak margin The ‘Y-shaped’ canyon and semi-circular collapse structure are situated west of NW-SE striking segments of the Biak Fault Zone which forms a transtensional flower structure to the east of the Biak Basin (Figure 1). The orientation of the ‘Y-shaped canyon’ and semi-circular collapse structures is also NW-SE and are parallel to the strike of the Biak Fault Zone (Figure 1). Recent earthquake CMT focal mechanisms in the Biak and Supiori region plotted between 1st January 1976 and 1st January 2018 show that the Biak Fault Zone is currently seismically active (Figure 2). Focal mechanisms indicate that presently principal displacement along the Biak Fault Zone has a dextral strike-slip component along a NW-SE striking plane parallel to the orientation of the faults

identified in multibeam bathymetry (Figure 2). Lobate mass transport deposits (MTDs) are common along strands of the Biak Fault Zone, indicating the shedding of material during fault movement (Figure 2), however they are absent from the collapse structures on the southern margin of the Biak Basin. The ‘Y-shaped’ canyon displays no evidence for seismicity within the last 40 years, nor is it associated with any MTDs (Figure 2). The semi-circular collapse structure is associated with an earthquake that occurred on 24th November 1990 at a depth of 15 km, however this may be related to the slumping of overlying material into the collapse feature. The surface expression of the ‘Y-shaped’ canyon and semi-circular collapse feature is markedly different to that of the Biak Fault Zone (Figures 1 & 2). Segments of pure strike-slip often appear as straight or wavy faults of modest topographic expression (Le Pichon et al., 2001). This is clearly shown along the

Figure 2: Recent earthquake CMT focal mechanisms in the Biak and Supiori region plotted from the International Seismological Centre catalogue using MIRONE software in between 1st January 1976 to 1st January 2018. Focal mechanisms are plotted over ASTER DEM and multibeam bathymetric imagery. Mass transport deposits (MTDs) are common along strands of the Biak Fault Zone (Biak F.Z.) which exhibit a predominantly dextral strike-slip component along computed fault planes that are parallel to structures observed in multibeam bathymetry. MTDs are absent from the collapse structures on the southern margin of the Biak Basin. The ‘Y-shaped’ canyon displays no evidence for seismicity within the last 40 years, nor is it associated with any MTDs. The circular collapse structure is associated with an earthquake that occurred on 24th November 1990 at a depth of 15 km displaying oblique slip with either a dextral N-S component, possibly relating to the Biak F.Z., or a sinistral E-W component. However, this earthquake may also have been related to the collapse of overlying material into an undercut cavity.


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principal strands of the Biak Fault Zone which are laterally continuous, with evidence for displacement and high relief fault scarps between minor faults parallel to the main strands of the transtensional flower structure (Figure 1). In contrast, the ‘Y-shaped’ canyon is observed in low relief except for a deep narrow incision along its central course. Neither the ‘Y-shaped’ canyon nor the semi-circular collapse feature display evidence for displacement or lateral continuity. It is, therefore, interpreted that these structures are isolated and are not fault-controlled by activity of the Biak Fault Zone. Submarine spring sapping The erosional undercutting of a slope results in mass wasting of overlying material (Orange and Breen, 1992) and is known by a variety of terms including ‘seepage erosion’ (Hutchinson, 1968), ‘artesian sapping’ (Milton, 1973) and ‘spring sapping’ (Johnson, 1939; Small, 1965; Bates and Jackson, 1980; Robb, 1990). In this article the term submarine spring sapping is favoured. There are many examples of where spring sapping has resulted in the formation of submarine canyons worldwide (e.g. Johnson, 1939, Robb, 1984; Paull and Neumann, 1987; Paull et al, 1990; Robb, 1990; Orange and Breen, 1992; Orange et al., 1994; Dugan and Flemings, 2000; 2002; Green et al., 2007; Flemings et al., 2008; Bratton, 2010). Lateral migration of meteoric water within marine basins is well documented (Wu and Chafetz, 2002; Bratton, 2010). Bratton (2010) defined three spatial scales of submarine groundwater discharge: 1) nearshore – 0-10 m offshore, 2) embayment – 10 m – 10 km offshore, 3) shelf – width of the entire continental shelf. Fresh water has been reported in a well 100 km off shore Florida in the Gulf of Mexico, 10 km offshore of Saudi Arabia and offshore Bahrain in the Persian Gulf, and beneath the continental shelves of the North Atlantic (Kohout, 1966; Fetter, 1980; Chafetz et al., 1988; Chafetz and Rush, 1995; Edmonds, 2001; Wu and Chafetz, 2002; Person et al., 2003; Fleury et al., 2007). Meteoric diagenesis of ancient carbonates through lateral flow of fresh water in palaeoaquifers is also well-documented (Grover and Read, 1983; Dorobek, 1987; Niemann and Read, 1987; Wu and Chafetz, 2002; Moore and Wade, 2013). Submarine spring sapping results in headward erosion and slope undercutting that leads to repeated slope failure and the formation of gullies and/or canyons (Orange and Breen, 1992). Orange and Breen (1992) attribute the cause of spring sapping to be seepage induced slope failure controlled by critical pore pressure gradients whereby flow through a porous medium exacts a force on grains greater than the frictional or cohesive force holding the grains in place and material is

transported away leading to slumping of overlying material. It has been suggested that this process may be the most significant mechanism for causing slope failure leading to the development of headless canyons (Johnson, 1939; Sangrey, 1977). Due to the isolated nature of the collapse features, and their tendency to form headless canyons, the potential for these structures to be created through a process of submarine spring sapping was examined. Carbonate units that are exposed on the islands of Biak and Supiori are interpreted to extend into the offshore Biak Basin (e.g. Gold et al., 2014). Therefore, samples were collected from Biak and Supiori to determine whether evidence for meteoric diagenesis is observed onshore in units interpreted to be present in the subsurface of the Biak Basin

MATERIAL AND METHODS OF ONSHORE

ANALOGUE ANALYSES Fieldwork was conducted in 2011 and 2013 on the islands of Biak and Supiori. Carbonates were described in the field and sampled for analysis at Royal Holloway, University of London. In total 47 samples were selected for petrographic analyses using thin section petrography to determine their post-depositional burial history. Cement types observed during petrographic analysis were divided into those that form in meteoric waters, the marine realm, and shallow and deep burial environments based on features described by Tucker and Wright (1990) and Scholle and Ulmer-Scholle (2003), and depicted in Figure 3. Nine samples (Biak 1-5, Supiori 1-4) deemed representative of the varying diagenetic cement types identified through thin section petrography were later selected for bulk-rock stable isotope (δ18O and δ13C) analyses to determine the presence of meteoric cements. Samples were milled to extract powdered calcite specifically from areas in which cements were abundant, avoiding bioclastic grains to ensure bulk isotope values indicative of diagenetic cement. Carbon dioxide was extracted from samples by reacting the milled powder with phosphoric acid using the procedure described by McCrea (1950). Three standards were used to fix the calibration curve, NBS19, LSVEC, and RHBNC. RHBNC is the Royal Holloway standard taken from Iceland spar which forms at low temperatures. One standard was used for NBS19 and LSVEC, and three samples of RHBNC were used as a control to monitor the run. Analytical precision, based on the RHBNC standard, was less than 0.05‰ for both oxygen and carbon ratios (Table 1). Consistency of results was achieved by comparing laboratory standards against NBS19 using the calibration curve. The stable isotope data are recorded in relation to the heavier isotope (δ18O and δ13C), and Peedee Belemnite (VPDB) standard.


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PETROGRAPHIC ANALYSIS The diagenetic features of the samples including porosity forming episodes, cross cutting relationships, and overgrowth of cements was examined (Figure 4). Different carbonate cement textures and fabrics form within different diagenetic environments relating to the chemistry of the waters they are bathed in, saturation with respect to carbonate, and levels of oxygen upon burial (Figure 2). Description Samples Biak 1 of the Pleistocene age Mokmer Formation and Supiori 1 of the Early Miocene age Wainukendi Formation are classified as grainstones which contain inclusion-rich fibrous fringes and botryoidal cements, with intervening primary interparticle porosity (Figure 4). The remainder of the samples which are Pliocene age or older (Biak 2-5, Supiori 2-4) contain abundant isopachous or uneven bladed calcite cements fringing grains, pore-filling inclusion-free equant calcite cements and a packstone fabric undergoing aggrading neomorphism of originally aragonitic micrite to

calcite micro- or pseudospar (Figure 4). This fabric is later cross cut by the development of secondary mouldic porosity (Figure 4). Interpretation Samples from the relatively young Pleistocene age sediments of the Mokmer Formation are interpreted to have been deposited in a reefal environment on the reef crest and reef front. These environments are home to photosynthetic organisms and high hydrodynamic energies that act to remove CO2 away from the site of deposition. This increases alkalinity and encourages precipitation of early marine cements such as fibrous fringes that often do not fully occlude interparticle pore space (Figures 3 & 4). Sediments influenced by meteoric phreatic diagenesis are characterized by pervasive calcitization of aragonite, extensive dissolution with well-developed mouldic porosity, and the occurrence of isopachous bladed and pore-filling equant calcite cements (Quinn, 1991; Figures 3 & 4). Meteoric diagenesis is often responsible for aggressive dissolution and porosity enhancement due to undersaturation of meteoric waters with respect to

Figure 4: Thin section photographs in plane polarised light of cement textures and fabrics observed during petrographic analysis of samples. Different cement textures and fabrics are observed to be characteristic of the marine, shallow burial and meteoric phreatic diagenetic realms.


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calcite and the development of secondary mouldic porosity (Tucker and Wright, 1990). These characteristics are observed in samples from Pliocene and older sediments, with the exception of sample Supiori 1 of the Wainukendi Formation. It is therefore interpreted that onshore samples from Biak and Supiori were subject to pervasive overprinting by meteoric cements which is likely to extend into the offshore. This supports Hendardjo and Netherwood’s (1986) observations from the nearby Salawati Basin where most offshore samples were subject to meteoric phreatic diagenesis after burial. Through petrographic analyses a paragenetic sequence of cement phases precipitated with increasing time and burial was determined based on cross-cutting and over-printing relationships (Figure 5). Over printing relationships suggest that they underwent diagenesis in a meteoric phreatic environment late on during their paragenetic history (Figure 5).

RESULTS OF STABLE ISOTOPE ANALYSIS The results of the bulk-rock stable isotope analyses are given in Table 1. The results show that calcite comprised >94% of the powdered carbonate material in all but one sample. Sample Biak 3 of the Korem Formation contained only ca. 70% calcite. The δ18O values of the calcite cements range from -5.36 to -1.48‰ VPDB (Table 1) and the δ13C values range from -7.61 to +2.74‰ VPDB (Table 1). Carbon-

oxygen cross plots of the analysed samples are shown in Figure 6. The results of the stable isotope analysis indicate that the majority of the cements show a trend from precipitation in normal shallow marine waters to precipitation in the meteoric phreatic diagenetic realm, supporting observations made through petrographic analysis (Figure 6). Meteoric cements have negative δ18O values as fresh water is more enriched with the lighter 16O isotope. However, during late diagenesis pore fluids also often exhibit negative δ18O values, and less negative δ13C, (Figure 6) due to higher temperatures of precipitation on burial and fractionation (Dickson and Coleman, 1980; Tucker and Wright, 1990). Samples Biak 1 and Supiori 1 of the Mokmer and Wainukendi Formations, respectively, plot close together in the low positive end of δ18O and δ13C values (Figure 5). Both these samples exhibit very obvious early marine diagenetic features such as inclusion-rich fibrous fringes and botryoidal cements (Figure 4), and plot with carbon and oxygen isotope values expected for normal marine carbonate cements (Figure 6). Samples from the Wardo (Biak 2-3) and Wafordori (Biak 4-5) Formations exhibit highly negative δ18O values between -3.79 and -5.36‰ VPDB, typical of values expected of meteoric phreatic cements (Figure 6). The oldest Early Miocene samples from the Wainukendi Formation (Supiori 2-3) fall within the mixing zone between normal marine and meteoric phreatic cements (Figure 5). However, sample Supiori 4 of the Wainukendi Formation exhibits δ18O

Figure 5: Paragenetic scheme of cement phases forming with increasing time and burial. Evidence for diagenesis in the marine, shallow burial, meteoric phreatic and deep burial diagenetic realms is interpreted. Based on overprinting relationships, the meteoric phreatic diagenetic realm is encountered late on in the paragenetic sequence.


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and δ13C values approaching those expected for deep burial cements (Figure 5). The results of the bulk-rock stable isotope analyses allowed palaeothermometry calculations to determine temperatures at which the cements precipitated. A geothermal gradient of 3°C/100m was calculated from bottom-hole temperatures in the similar Salawati (Redmond and Koesoemadinata, 1976), Bintuni (Chevallier and Bordenave, 1986), and North Irian (McAdoo and Haebig, 1999) regional basins was used to convert temperature to depth. The method for calculating palaeodepths and precipitation temperatures was taken from work on the cement stratigraphy of Miocene carbonates from Sabah, Malaysia (Ali, 1995). It was assumed that the parameters of Ali’s (1995) method would closely match that of the Biak Basin since samples used in both experiments were of similar age, latitude,

geothermal and hydrothermal gradients, and were likely to have similar starting sea-water temperatures and isotopic values. To equate the calculated δ18OVPDB values obtained by mass spectroscopy to burial depth, it is necessary to know the isotopic composition of the ambient pore fluids, the geothermal gradient for the time of each cement stage, and the degree of openness of the system (Ali, 1995). As the isotopic composition of the pore fluids is unknown, it is impossible to relate the δ18OVPDB values precisely to a burial depth. However, an estimate of palaeo-precipitation temperature can be given using Equation 1. This equation follows a standard palaeotemperature calculation given by Epstein et al. (1953), later refined by Irwin et al. (1977) and Anderson and Arthur (1983), and used by Ali (1995) on Miocene carbonates from Sabah.

T = 16.0 - 4.14 (δc - δw) + 0.13 (δc - δw)2 [Eq.1]

Table-1 : Oxygen and carbon isotope data including standards and precision information. The stable

isotope data are recorded in relation to the heavier isotope (δ18O and δ13C), and Peedee Belemnite

(VPDB) standard. Isotopic composition of sea water taken from Shackleton and Kennett (1975).

Temperature calculated using method described by Anderson and Arthur (1983). Calculated burial

depth using a geothermal gradient of 3°C/100m.

Samples Formation Age δ13C(‰) δ18O(‰) Est % carb.

δ18O sea water

Temp (°C)

Depth (m)

Standards

RHBNC 3.31 -10.37 95.49

RHBNC 3.23 -10.35 102.36

RHBNC 3.22 -10.42 96.00

Average 3.25 -10.38 97.95

External Precision 0.04 0.03

NBS-19 1.94 -2.20 100.00

Known 1.95 -2.20

LSVEC -46.50 -26.70 112.37

Known -46.50 -26.70

Outcrop

Biak 1 Mokmer Pleistocene 2.66 -1.48 94.34 -1.20 17.2 572.3

Biak 2 Wardo Pliocene -7.21 -5.03 97.52 -1.20 33.8 1126.0

Biak 3 Wardo Pliocene -6.95 -3.79 71.92 -1.20 27.6 920.5

Biak 4 Wafordori Middle - Early Miocene -7.61 -4.81 98.23 -1.20 32.6 1087.7

Biak 5 Wafordori Middle - Early Miocene -2.44 -5.36 98.96 -1.20 35.5 1182.5

Supiori 1 Wainukendi Early Miocene 2.74 -1.71 97.73 -1.20 18.1 604.8





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Where: T = precipitation temperature (°C) δc = oxygen isotopic composition of CO2 produced from calcite at 25°C δw = oxygen isotopic composition of CO2 in equilibrium with formation water, given as -1.2‰ for seawater composition prior to the establishment of polar ice caps (Shackleton and Kennett, 1975; Ali, 1995).

D = T/Gg [Eq.2] Where: D = depth (m) T = precipitation temperature (°C) calculated using Equation 1 Gg = geothermal gradient, here given as 3°C/100m (0.03)

Using the 3°C/100m geothermal gradient, maximum burial depth can be calculated using Equation 2. From the recorded values of δ18O, it is calculated that meteoric cements from sample Biak 5 of the Wafordori Formation attained the greatest burial depth and temperature values of ca. 1.2 km and ca. 35.5°C, respectively (Table 1).

DISCUSSION Synthesis of petrographic and stable isotopic data Carbonate cements that have undergone meteoric phreatic diagenesis specifically related to subaerial exposure are reported to display variable δ13C values with relatively constant δ18O values (Allan and Matthews, 1982). However, relatively constant δ13C and variable δ18O values are an indicator of meteoric

Figure 6: A) Carbon-oxygen cross plots for Neogene carbonate samples analysed for stable isotope geochemistry. Samples Biak 1 and Supiori 1 which display obvious marine cements lay within the carbon and oxygen isotopic values expected for the precipitation of marine cements. There is a trend towards freshwater cements occurring during burial, supporting the interpretation of a submarine freshwater aquifer beneath the burial diagenetic environment. B) Location map of samples collected from the islands of Biak and Supiori


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diagenesis at relatively low water-rock ratios (Wu and Chafetz, 2002). Samples from Biak and Supiori display increasingly negative δ18O and δ13C values which indicate considerable water-rock interactions suggesting meteoric waters had a progressively greater influence on the isotopic composition of samples during burial (e.g. Wu and Chafetz, 2002). Observations of overprinting of cements indicate that meteoric diagenesis occurred late on in the paragenesis of the carbonate samples (Figure 5). This is supported by the temperatures calculated for

the precipitation of meteoric cements during stable isotope analysis which suggest they were precipitated at depths ca. 1 km (Table 1). During the Early to Middle Miocene, carbonates originally formed in the marine diagenetic environment were progressively buried, passing through underlying diagenetic environments precipitating deeper burial cements (Figure 5). Samples dated from the Early Miocene through to the Pliocene are interpreted to have attained burial depths of approximately 1.2 km (Table 1). This

Figure 7: Schematic model showing development of the freshwater lens during the Neogene. A) During the Early Miocene, carbonate platforms grow within marine diagenetic realms, burying older to deeper burial diagenetic environments, B) As relative sea-level rises, Middle Miocene carbonate strata backstep across former Early Miocene platform, burying it within the shallow burial diagenetic realm, C) As relative sea-level continues to rise, the Early Miocene platform is progressively buried to deeper diagenetic environments, D) Uplift forming the intra-Pliocene unconformity exposes Pliocene sediments. Karstification forms conduits for freshwater lens to develop and penetrate older strata. Strata previously buried within deep diagenetic environments are uplifted into meteoric realm causing overprinting of meteoric diagenesis over burial cements. The oldest sediments are not uplifted far enough to reach fresh water lens and retain deepest burial diagenetic signature.


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suggests relatively rapid uplift of Neogene strata since the Pliocene. There may have been gradual burial of carbonate strata in the Bird’s Head region up to the Pliocene until the formation of the regional ‘Intra-Pliocene’ unconformity at 4 Ma (Decker et al., 2009) which is responsible for the rapid exhumation of Neogene sediments. Meteoric phreatic diagenesis within carbonate rocks is usually attributed to periods of low relative sea-level, especially within shallow water facies rocks (Meyers and Lohmann, 1985; Quinn, 1991; Frank and Lohmann, 1995; Melim, 1996; Moore and Wade, 2013). Karstic joints that are developed in subaerially exposed carbonates of the hinterland act as a conduit for freshwater aquifers to extend offshore. In the Biak and Supiori region, relative sea-level lowstand is attributed to the tectonic uplift, exhumation subaerial exposure and karstification of

the youngest Neogene sediments during the formation of the ‘Intra-Pliocene’ unconformity (Figure 7). Karstic joints acted as conduits for freshwater to develop a subterranean lens bathing older strata in meteoric waters. It is interpreted that this freshwater lens is the cause of meteoric phreatic diagenesis within samples analysed by this study. The process of exhumation uplifted strata previously buried within deep diagenetic environments up into the region influenced by the freshwater lens, causing overprinting of meteoric cements on interpreted burial cements (Figure 7). Some of the oldest Early Miocene samples from the Wainukendi Formation exhibit carbon and oxygen isotopic values close to those expected for deep burial cements (Figure 6). It is interpreted that these samples have not been uplifted through the freshwater lens, and are exposed updip of the lens

Figure 8: A) Present day topographic and bathymetric profile along transect A-B across southern Biak and into the offshore Biak Basin. The freshwater lens extends southwest from the island beneath the Biak basin, comparable to the Floridian Aquifer. B) Transect A-B displayed in map view across the southern margin of the Biak Basin


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on the island of Biak. Inversely, the youngest Pleistocene sediments have not been buried to such an extent to have reached the freshwater lens, and remain unaffected by meteoric diagenesis (Figure 8). Effect of meteoric diagenesis in the Biak Basin ‘Pock mark’ and collapse features, such as those observed west of Biak and Supiori (Figure 1), are often associated with gas seepage (e.g. Hovland and Judd, 1998; Yun et al., 1999). However, here they are interpreted to be caused by submarine ‘spring sapping’. The process of submarine spring sapping is interpreted to be driven by the freshwater lens responsible for the meteoric phreatic overprinting of samples collected onshore Biak and Supiori extending a considerable distance offshore (Figure 8). The Biak and Supiori aquifer extending offshore into the Biak Basin represents the shelf scale of Bratton (2010). At the shelf scale the freshwater aquifer extends as far as the shallowest overlying confining unit, typically comprising fine-grained sediments (Bratton, 2010). The confining unit in Biak and Supiori are interpreted to be Late Miocene to Pliocene deep-water sediments (e.g. Gold et al., 2017). The thickness of the meteoric lens at the shelf scale is typically several hundred metres and has a width of approximately 80km (Bratton, 2010). In the offshore Biak Basin, the width of the lens extends approximately 55 km offshore and is interpreted to be approximately 250 m thick (Figure 8). The interpreted ca. 1 km depth of the Biak freshwater lens is comparable to that of the Floridian Aquifer and extends almost as far offshore (Figure 8). It is unknown whether the ‘Y-shaped’ canyon or semi-circular collapse feature observed in multibeam bathymetry (Figure 8) formed during subaerial exposure or subaqueously. However, it is likely that both are relatively recent structures, no older than the Pliocene. The lateral, rather than vertical, displacement of carbonate units by the Biak Fault Zone permitted the freshwater lens to extend beyond the transtensional flower structure via well-developed karstic joints acting as conduits to the southern margin of the Biak Basin.

CONCLUSIONS Petrographic and stable isotope analysis of Neogene carbonates from the Bird’s Head region of New Guinea enables the reconstruction of their subsequent burial history and potential as hydrocarbon reservoirs. The following conclusions can be drawn from these reconstructions. Calculation of burial temperatures and depths reached by samples suggest that they attained a maximum temperature of ca. 35.5°C and depth of ca. 1.2 km up until the Pliocene. Sediments were

rapidly exhumed during the creation of the ‘intra-Pliocene unconformity’ formed when rapid isostatic uplift as slab-mantle decoupling close to Timor affected the wider Banda Arc and Bird’s Head region. This uplift resulted in a period of low relative sea-level in the Biak and Supiori region with the development of a freshwater aquifer formed as a response. Most samples show evidence of meteoric phreatic diagenesis through petrographic recognition of meteoric cements and presence of light oxygen isotopes. Precipitation of these cements is interpreted to have occurred late on in the paragenetic history of the samples as they passed through a freshwater lens during uplift. Subsequent ‘spring sapping’ by this freshwater lens is responsible for various collapse structures observed in multibeam bathymetry of the Biak Basin.

ACKNOWLEDGMENTS

I would like to thank TGS for providing multibeam bathymetric data, Ramadhan Adhitama and Ferry Yulien of Institut Teknologi Bandung for assistance during field work and collection of the samples, the Southeast Asia Research Group consortia and staff members at Royal Holloway, University of London for enabling this study to take place, Dr. Dave Lowry and Dr. Nathalaie Grassineau for help in stable isotope analysis, and Dr. Jim Hendry for continued support and assistance in the production of the isotope cross plot. Finally, I would like to thank Robert Hall for continued mentoring, guidance, helpful discussion and facilitating the progression of our understanding of a geologically complex and exciting region of the world through his work with the Southeast Asia Research Group.

FUNDING

This research was funded by industrial companies as part of a consortium supporting the Southeast Asia Research Group at Royal Holloway, University of London.

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