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Offshore Carbonate Facies Characterization and Reservoir Quality of

Miocene Rocks, Central Luconia, Offshore Sarawak, Malaysia

Hammad Tariq JANJUHAH1,2 *

, Abubaker ALANSARI³

1 Department of Geology, American University of Beirut, Riad El-Solh, Beirut, 1107 2020, Lebanon

2 Centre for Seismic Imaging, Department of Geosciences, University Technology PETRONAS, 32610, Sri

Iskandar, Perak, Malaysia

3 Department of Geosciences, University Technology PETRONAS, 32610, Perak, Malaysia

Abstract: Carbonate rocks are important hydrocarbon reservoirs around the globe and in Southeast Asia

particularly, the Central Luconia province. Understanding the internal characteristics, distribution,

geometry and lateral extent of these rocks are essential parts for the exploration and production success.

This study provides a unique and detailed work on carbonate reservoir facies including qualitative and

quantitative analysis of photomicrographs and reservoir quality considering microporosity.

Stratigraphically, these carbonates are known as Cycle IV and V and are represented by eight major type

of facies. The analysed carbonate facies are: coated grain packstone (F-1) (av. Ø = 3%, av. Kh = 0.5 mD)

(av = Average; Ø = total porosity, and Kh = permeability), massive coral lime grainstone (F-2) (av. Ø =

14.7%, av. Kh = 6 mD), oncolite lime grain-dominated packstone (F-3) (av. Ø = 10%, av. Kh = 4 mD),

skeletal lime/dolo packstone (F-4) (av. Ø = 15%, av. Kh = 4.6 mD), coral (platy) lime mud dominated

packstone (F-5) (av. Ø = 4%, av. Kh = 0.5 mD), coral (branching) lime dominated pack-grainstone (F-6)

(av. Ø = 15%, av. Kh = 1 mD), cross-bedded skeletal lime packstone (F-7) (av. Ø = 20%, av. Kh = 2 mD),

and bioturbated carbonate mudstone/chalk (F-8) (av. Ø = 8%, av. Kh = 0.8 mD). Thin sections study

revealed that red algae, foraminifera, corals are the dominant fossil components with a minor admixture

of echinoderms, bivalve, bryozoans, and green algae skeletal fragments. The microporosity value were

quantified using digital image analysis software. All the parameters, e.g., facies characterization,

petrography, porosity-permeability value, and microporosity value were utilized for obtaining a reliable

reservoir quality. Another major achievement of this research is the improvement of the correlation

coefficient (R²) value considering the presence of microporosity than total porosity in carbonate rocks.

The value of correlation coefficient R² has increased from 0.51 to 0.82.

Key words: Carbonate facies, petrography, grain types, porosity-permeability, reservoir quality,

microporosity.

E-mail: [email protected] / [email protected]

1 Introduction

The Central Luconia platforms have been studied intensively, in terms of their geology, stratigraphy and

reservoir aspects (Doust 1981, Epting 1980, Epting 1989, Ali and Abolins 1999, Vahrenkamp 1998, Vahrenkamp

et al. 2004, Madon 1999, Madon, Kim and Wong 2013, Koša 2015). According to Wee and Liew (1988) the

exploration of gas fields in Central Luconia began in 1982, leading to an increase of production rate in the

Sarawak region of Malaysia, where 60 of the 200 mapped platforms were explored and drilled. These carbonate

rocks are economically significant and are believed to hold initial reserves of 65 trillion cubic feet of gas in

place with minor contribution of oil reserves (Abdullah et al. 2012, Khazali, Osman and Abdullah 2013).

According to Epting (1980), carbonate production in Central Luconia was mainly controlled by the growth of

corals and coralline red algae. The same calcified organisms were responsible for the carbonate production in

many contemporary and ancient carbonate platforms (Checconi et al. 2007, Ghosh and Sarkar 2013).

This article is protected by copyright. All rights reserved.

This article has been accepted for publication and undergone full peer review but has not been

through the copyediting, typesetting, pagination and proofreading process, which may lead to

differences between this version and the Version of Record. Please cite this article as doi:

10.1111/1755-6724.13880.

mailto:[email protected]

https://doi.org/10.1111/1755-6724.13880

https://doi.org/10.1111/1755-6724.13880

https://doi.org/10.1111/1755-6724.13880

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The development of shallow marine carbonates in Central Luconia reached its apex between Middle to late

Miocene time (Ting, Chung and Al Jaaidi 2010). The eustatic sea-level, basinal and clastic sediment supply from

the Baram delta in northwest (NW) Borneo has influenced the development of carbonate build-ups (Epting

1989, Zampetti et al. 2004, Menier et al. 2014).

Several researchers emphasized that understanding the distribution and quantification of microporosity are

especially important when dealing with carbonates (Baechle et al. 2008, Cantrell and Hagerty 1999, Janjuhah et

al. 2017d, Daigle 2014). According to Baechle et al. (2008), Kaczmarek, Fullmer and Hasiuk (2015), Fournier

and Borgomano (2009) the cored measured porosity which is known as total porosity does not entirely

contribute to fluid flow due to the pore geometry which represents dissimilar pore throats that cause a negative

impact on fluid flow. These researchers also stated that carbonate reservoir models mainly depend on different

pore types, such as mouldic and vuggy porosity but the porosity present within the matrix is underestimated and

this uncertainty has a high impact on the reservoir assessment. Also, the subdivision of pores into macropores

and micropores would help to study the abundance and occurrence of microporosity in Central Luconia.

Anselmetti, Luthi and Eberli (1998), Ruzyla and Jezek (1987), Yanguas and Dravis (1985), Milliken and Curtis

(2016), Cantrell and Hagerty (1999) emphasized that the quantification of pore types in blue epoxy resin

impregnated thin section is an effective tool to detect the microporosity zones.

Sedimentological studies (facies, petrography, and reservoir quality) were conducted on Miocene carbonate

rocks (Cycle IV and Cycle V) from the Central Luconia province, offshore Sarawak, Malaysia (Figure. 1). The

main objectives of this study are: (1) to investigate and determine the facies characterization; (2) to examine the

petrographic images qualitatively and quantitatively; (3) to diagnose the reservoir quality of the eight facies, (4)

to distinguish the effect of microporosity on reservoir quality of different facies.

Fig. 1. (a) Geological sectors of the Luconia Province, Offshore Sarawak, East Malaysia. The isolated carbonate platforms scattered mostly

throughout Central Luconia are highlighted in a darker tone, black box indicates the study area Adopted from Janjuhah et al. (2017a).

2 Geological Setting 2.1 Tectonic framework

The study area is located about 170 km offshore north of Bintulu, Sarawak (Figure. 1). The region is

geologically known as Central Luconia province, is distinguished from adjacent tectonic domains by its

relatively shallow occurrence and structural simplicity (Figure. 2). The province is flanked by deep basins on the

west (West Luconia Delta), to the north (North Luconia Basin), and to the east (Baram Delta) borders. A


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prominent lineament, the West Baram Line, separates Central Luconia from the Baram Delta. To the south the

compressed Balingian Province (Figure. 2) (Doust 1981, Haq, Hardenbol and Vail 1987, Mat-Zin and Swarbrick

1997, Hutchison 2004, Cullen 2010). Due to extensional forces, Central Luconia is divided into a number of

local extensional half grabens and grabens, which trend in SSW-NNE, whereas compressional structures mostly

trend in WSW-ENE directions.

Fig. 2. Structural map of Central Luconia Carbonate Platform Offshore Sarawak, Malaysia. Adopted from, Janjuhah et al. (2017a)

2.2 Stratigraphy

The Central Luconia has undergone various episodes of sedimentation. During the early stage of petroleum

exploration in Central Luconia, a number of local stratigraphic units was established to describe the stratigraphy

of this province. The Tertiary formations were defined using paleontological methods because of the absence of

distinctive lithological characteristics. Ho (1978) used a wireline log and well data to subdivide the Late Eocene

to Pleistocene sequence into eight major cycles, separated by major flooding surfaces.

In Central Luconia, most of the sediments are dominated by prograding clinoforms, which served for a

subdivision of entire sequence into eight regressive cycles separated by major transgressions (Epting 1989).

These cycles range from the Eocene to the Present. The Cycle I includes deep-water argillaceous and shallow

marine siliciclastic successions filling an early synrift graben (Figure. 3). This was followed by a late phase of

synrift sedimentation through the Cycles II and III during the opening of the South China Sea. Continuous

subsidence and formation of half- grabens resulted in widespread carbonate deposition during the middle to late

Miocene Cycles IV and V (Figure. 3).


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Fig. 3. A schematic cross section along NNW-SSE direction across the continental shelf of offshore Northwest Sarawak. Adopted from Janjuhah et al.

(2017e) Janjuhah et al. (2017a).

This phenomenon was eventually stopped by the influx of siliciclastic sediments derived from the uplifted

Rajang Fold-thrust Belt during Cycle V and VIII (Madon 1999). In general, the resulting carbonate platforms

become thinner (<500 m) towards the southern part of Central Luconia, and on the northern side, they became

thicker, up to 3000 m (Doust 1981, Hutchison 2004).

3 Material and Methods Facies composition and carbonate reservoir quality were evaluated by detailed core description and laboratory

analysis during which a total of 1610 ft of the core was described representing Cycle IV and V carbonates

(Figure. 4). For the laboratory measurements, a total of 820 samples were collected to prepare thin sections and

to measure the porosity and permeability.


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Fig. 4. Sedimentological log showing the core description of well A, B, C, D and E in Central Luconia, Offshore Sarawak, Malaysia.

3.1 Core description Various Sedimentological features such as depositional texture, nature of skeletal and non-skeletal grains,

lithology, as well as diagenetic features including leaching, stylolites, and visual porosity were manually plotted at

the 1:40 scale onto the core description sheet subdivided into five cores named here wells A-E (Figure. 4).

3.2 Petrography

820 thin sections were prepared for the detailed petrographic study, 80% of the thin sections were prepared at

Sarawak Shell Sdn, Bhd, and the rest 20% was prepared at the thin section lab, University Technology

PETRONAS. These thin sections were impregnated with blue epoxy resin and then studied both under reflected

and transmitted light microscopy as well as under a polarised light microscope Olympus BX 51 with DP-27

attached camera at SEACaRL, University Technology PETRONAS. Half of the thin sections were partially

treated with an acid solution containing red Alizarin S and potassium ferricyanide stains. For the quantitative

analysis of thin sections in terms of grains, matrix, cement and visible porosity, a point counting software J.

Microvision was used based on 700 counts per section.


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3.3 Quantification of microporosity

For the quantification of microporosity, petrographic images (32 images of each thin section) were subsequently

subjected to point counting and digital image analysis (DIA) to determine the amount of visible porosity

(macroporosity). Using the DIA software, a threshold value was set out to estimate the amount of macroporosity

from petrographic images (Figure. 5). False colour images were produced using DIA to obtain the distribution of

macropores (the pores with diameter greater than 10 m in size) in each sample. The microporosity is then

calculated as the difference between the observed porosity in DIA and the measured core plug porosity.

Fig. 5. False color DIA image of mouldic, vuggy and intraparticle porosity developed in foraminifera tests and other bioclasts from Central Luconia.

Total porosity is 19%, the blue color represents macropores which are 8.5% whereas red color representing microporosity which is 10.5%.

3.4 Porosity-permeability analysis

The core measurements under the overburden pressure of 1800-2000 psi were conducted to obtain the total

porosity and permeability values by injecting helium gas, using a porosity-permeability instrument of Vinci

Technologies. The cores were washed before porosity-permeability measurements to clean the fluid which is

present in the pores by toluene fumes in a centrifugal extractor. After the washing/cleaning, these cores were

placed in the oven for 1 day at the temperature of 100 °C. Weight, length, diameter and bulk volume of the cores

were calculated individually and were covered with a transparent tape to avoid grain loss. After cleaning, these

plugs proceeded for the porosity and permeability measurement. Due to the high density of carbonate samples, the

measurements were operated using 1800-2000 psi pressure to get accurate values. The results of this analysis were

also correlated with the petrographic observation.

4 Result and Discussion 4.1 Facies analysis


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Based on the core analysis, the Miocene (Cycle IV and V) carbonates of Central Luconia were subdivided

into eight major facies, namely; coated grain packstone (F-1), massive coral lime grainstone (F-2), oncolite lime

grain-dominated packstone (F-3), skeletal lime/dolo packstone (F-4), coral (platy) lime mud dominated packstone

(F-5), coral (branching) lime dominated pack-grainstone (F-6), cross-bedded skeletal lime packstone (F-7), and

bioturbated carbonate mudstone (Chalk F-8) (Figure. 4; Table-1). A detailed description of each lithofacies is

given below.

Table-1: Facies scheme of Central Luconia based on cores from 8 wells, Offshore Sarawak, Malaysia.

Lithofacies Description

FA- 1

Coated Grain

Packstone

Texture: packstone (floatstone)

Mineralogy: limestone

Components: algae >50%, oncolite algae <40%, corals <30%, separate vugs, skeletal

debris (angular – subangular), forams, echinoderms, gastropods, leaching

Grain size/sorting: fine-medium gravel/moderately - poor

FA- 2

Coral (m)

Lime

Pack-Grainstone

Texture: packstone- grainstone (rudstone)


Components: corals (m) >50% (up to 8cm in diameter), platy coral up to 20%, branching

corals (15%), solitary coral <5%, algae, disconnected vugs, oncolite algae, skeletal

grains (angular-subangular), gastropods, bivalves, echinoid spines

Grain size/sorting: very coarse-granule/moderately – poor

FA- 3

Oncolite Lime

Grain-dominated

Packstone

Texture: packstone (rudstone)


Components: oncolite algae >70% (diameter 2 to 6cm), stylolite, corals >30%, separate

vugs, algae, gastropods, bivalves, echinoid spines, skeletal grains (angular-subangular),

leaching

Grain size/sorting: medium-gravel/ moderately - poor

FA- 4

Skeletal

Lime/dolo

Packstone

Texture: packstone (floatstone-rudstone)

Mineralogy: limestone – dolomitic limestone

Components: skeletal debris >60% (angular-subangular), bivalves, isolated gastropods,

corals (m) <20%, Coral (p) <15%, leaching

Grain size/sorting: fine-coarse grain/ moderately – well sorted

FA- 5

Coral (p)

Lime Mud

dominated

Packstone



Components: rich platy corals >70%, solitary coral up to 15%, algae, small fractures,

disconnected small vugs, skeletal debris (angular-subangular), gastropod, forams,

echinoid spines

Grain size/sorting: fine-coarse/poor


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FA- 6

Coral (B)

Lime dominated

Pack-Grainstone

Texture: packstone-grainstone (floatstone)

Mineralogy: limestone-dolomitic limestone

Components: branching coral 50%, 20% red algae, 15% forams, 5% massive coral, 5%

bivalve, 5% other skeletal debris (angular-subangular)

Grain size/sorting: very coarse-granule/poor

FA- 7

Cross Bedded

Skeletal

Lime Packstone


Mineralogy: dolomitic limestone to limestone

Structures: graded bedding

Components: forams 65%, red algae 10%, coral fragments 10%, bivalve 5%,

echinoderms 5%, other skeletal debris 5%

Grain size/sorting: very coarse-pebble/poorly-moderately sorted

FA- 8

Bioturbated

Carbonate Mud

stone (Chalk)

Texture: wackstone - packstone

Mineralogy: dolomitic limestone

Structures: bioturbation

Components: burrowing & bioturbation 60%, forams 10%, coral debris 10%, red algae

5%

Grain size/sorting: very fine-coarse/ moderately-well sorted

4.1.1 Facies-1: coated grain packstone

Facies 1 displays a packstone (floatstone) texture (Figure. 4, 6a; Table-1). It contains a diverse skeletal

assemblage, mainly dominated by calcified algal and coral skeletal fragments and oncolites. Minor constituents

include foraminiferal tests, echinoderm plates, bivalve and gastropod shells. The grains in Facies 1 are poorly to

moderately sorted. Grain size varies from fine to medium gravel. The fossil composition together with a high

degree of disarticulation, fragmentation, and rounding of the grains, as well as the presence of oncolites indicate a

high energy environment. Complete coating of oncolites by algal sheaths suggests a high frequency of rolling of

these structures which further supports the suggestion that the facies formed under high energy conditions

(Alshuaibi, Duane and Mahmoud 2012). The occurrence of packstone is attributed to infiltration of carbonate mud

through the sediment following a change to lower energy conditions (Gawthorpe and Gutteridge 2009).


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Fig. 6. Core images of showing important features of eight different facies identified. A: Facies F-1 (coated grain packstone). B: Facies F-2 (massive

coral lime grainstone). C: Facies F-3 (oncolite lime grain-dominated packstone) D: Facies F-4 (skeletal lime packstone). E: Facies F-5 (coral (p) lime

mud dominated packstone) F: Facies F-6 (coral (b) lime dominated pack-grainstone) G: Facies F-7 (cross-bedded skeletal lime packstone) and G:

Facies F-8 (bioturbated carbonate mudstone (Chalk F-8).

4.1.2 Facies-2: coated (m) lime pack-grainstone

Facies 2 displays a packstone-grainstone (rudstone) texture (Figure. 4, 6b; Table-1). It is contains diverse

skeletal assemblage, mainly dominated by fragments of massive corals and calcified algae as well as by platy and

branching coral debris. Minor constituents include solitary coral skeletons, gastropod, bivalve shells, and echinoid

spines. Grains in Facies 2 are poorly to moderately sorted. Grain size varies from coarse to granule. The diversity

of fossil assemblage indicates fully marine conditions while coarse grain size and asymmetrical oncolitic coating

can reflect a low energy depositional environment (Gawthorpe and Gutteridge 2009). Facies 2 is interpreted as

lagoonal deposits.

4.1.3 Facies-3: oncolites lime grain-dominated packstone

Facies 3 display a packstone (rudstone) texture (Figure. 4, 6c; Table-1). 70% of this facies is represented by

abundant rhodolite/oncolite assemblage dominated by skeletal debris of massive corals, calcified algae, and

foraminifera. Minor constitutes are represented by bivalve shells and echinoids spines. The grains in Facies 3 are

poorly to moderately sorted. Grain size varies from fine to medium gravel. The fossil composition indicates a

shallow marine depositional environment. According to Cook, Shergold and Cook (2005), the oncolite facies are

formed in relatively shallow nearshore waters. While the abundance of algal balls (oncolite/rhodolites) and of

some corals suggests a deposition under medium to occasionally high energy conditions under an influence of sea

currents.

4.1.4 Facies-4: skeletal lime/dolo packstone


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Facies 4 displays a packstone (floatstone-rudstone) texture (Figure. 4, 6d; Table-1). It contains a diverse

skeletal assemblage, mainly dominated by platy coral debris and massive coral fragments. Minor constituents

include calcified algae, bivalve, and rare gastropod shells and foraminiferal tests. The grains in Facies 4 are

moderately to well-sorted. The grain size varies from fine to coarse-grained.

The diverse fossil assemblage again indicates fully marine conditions. The grain size and subangular to

subrounded grain shape of abundant skeletal fragments points to decrease in energy of the environment, probably,

due to rapid sea level rise while gastropod, bivalves, green calcareous algae and small foraminifera are typical for

lower energy conditions of a restricted lagoon (Karami-Movahed, Aleali and Ghazanfari 2016, Shabafrooz et al.

2015).

4.1.5 Facies-5: coral (p) lime mud dominated packstone

Facies 5 displays a dense wackstone-packstone texture (Figure. 4, 6e; Table-1). It is composed of rich platy

coral fragments, with minor remains of solitary corals, gastropods, foraminifera, red calcified algae, and

echinoderms. Grain size varies from very fine to coarse-grained and grains are moderately to well sorted. The

Facies 5 interval is characterised by tight limestone. The presence of abundant platy corals, foraminifera, calcified

algae with a minor contribution of bryozoan, bivalve and some massive coral debris suggests a deeper to open

marine environment low energy environment which is evidence also by the presence of the small benthonic

foraminifera and argillaceous mud. No subaerial exposure features have been recognised in the cores within this

facies.

4.1.6 Facies-6: coral (b) lime dominated pack-grainstone

Facies 6 displays a packstone-grainstone (floatstone) texture (Figure. 4, 6f; Table-1). Here the fossil

assemblage mostly consists of delicate branching coral fragments including some in situ colonies (branches are

0.5-2.0 cm in diameter) in the packstone matrix. Grains are poorly sorted and vary in size from very coarse to

granular. The presence of the corals indicates the normal marine conditions. Miall (2016) ascribed this type of

facies to the colonization stage of the reef development. In turn, Blomeier and Reijmer (1999) noted that such

coral bioclasts in slope sediments are derived from platform margin. Thus, this facies can represent forereef

depositional setting.

4.1.7 Facies-7: cross-bedded skeletal lime packstone

Facies 7 is characterized by packstone (floatstone) texture ((Figure. 4, 6g; Table-1).The fossils here are

mostly represented by foraminifera, with a minor constitution of fragments of red calcified algae, coral, bivalves,

and echinoderms. Grain size in this facies ranges from fine to coarse, and most of the allochems are moderately

sorted. The observed porosity is moderate with a high percentage of isolated vugs. Stylolites and cross-bedding

are common and are particularly restricted to this facies. According to Hamblin (1969), cross-bedding occurs only

in units composed of ooids, pellets and huge skeletal debris. The cross-bedded, skeletal lime packstone facies

represents high variation in grain size. Bioturbation is locally abundant, obliterating the cross bedding. This facies

type is the result of transportation of bioclasts and there accumulation in small submarine banks, usually in the less

protected area of the carbonate platform. Braga et al. (2012), Macintyre et al. (1987) indicated that in a

present-day Brazil shelf, the skeletal sand of a narrow back reef belt runs parallel to the barrier reef, in which sand

is transported from the reef especially during a high storm. However, the skeletal sands, in this case, were not

considered as a back reef setting.

4.1.8 Facies-8: bioturbated carbonate mud (chalk)

Facies 8 displays wackstone-packstone texture (Figure. 4, 6h; Table-1). This lithology is composed mostly of

bioturbated carbonate mud yielding abundant planktonic foraminifera and minor debris of corals and red calcified

algae. The grain size varies from very fine to coarse. The porosity in this facies is mainly associated with

dissolution. Stylolites are present. Due to the abundance of carbonate mud, planktonic and small benthic

foraminifera and ostracodes, this facies is interpreted to be deposited towards the open sea, more specifically as

off-reef deposits (Pomar et al. 2015, Pomar 2001). In off-reef settings, sediments are intensely burrowed; locally

Ophiomorpha, and Thalassinoides are present. The optimum water depth for the carbonate production here is 100

m, below the photic zone.


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4.2 Petrography

4.2.1 Grain types

The qualitative observation of thin sections revealed that a restricted number of components constitute the

bulk rock volume. These are almost exclusively skeletal components. Ooids are absent, and peloids are

uncommon. Predominantly eight skeletal verities are observed including red calcified algae (Figure. 7a),

foraminifera (Figure. 7b), echinoderms (Figure. 7c), bivalves (Figure. 7d), corals (Figure. 7e), green calcified

algae (Halimeda) (Figure. 7f), bryozoans (Figure. 7g), and calcified sponges (Figure. 7h).

Fig. 7. Petrographic images of 8 important components, (a) Red algae: Microstructure is due to micritization and early clacification, (b) Micritization

of test walls saves from late stage dissolution. Only the mud filled in the chambers are dissolved and recrystallized, (c) HMC nature of echinoderm

plates leads to neomorphism, (d) Bivalves, representing multilayer, the brownish color reflecting organic reniments and growing bending layer

representing calcitic layer, (e) Only the corallite crevices/cavities recognizable by corg rich mud, (f) Green algae: Leached in the limestone in which

green algae form a substained part of the total sediments, (g) Bryzones: Forming large and branching masses, showing regular boxlike arrangement of

their zooceia and (h) Sponge: Only the margine of the sponge are selectively micritized. The rest of the sponge was leached and the resulting pores

were filled with cement, which is dominantly present in well A and B, Central Luconia offshore Sarawak, Malaysia Adopted from Janjuhah et al.

(2018a).

Fig. 8. Photomicrographs of representing different porosity types, (a) Intraparticle and moldic porosity, (b) Fracture porosity, (c) Vuggy porosity and


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(d) Interparticle porosity with partial pore space filling by dolomite cement and the rate of dolomitization increase with time in well A and B,

Offshore Sarawak, Malaysia Adopted from Janjuhah et al. (2018b)

4.2.2 Pore types

The pore system in Central Luconia is dominated by isolated vugs (Lucia 1995, Lucia 2007). It is attributed to

leaching of skeletal allochems leading to a partial or complete destruction of primary rock fabric. Such a selective

dissolution was caused by the primary composition of skeletons which consists of either aragonite (corals, green

algae, some foraminifera, bivalve, and sponges) or a high-magnesium calcite (red algae, echinoderms, some

foraminifera, bivalve, and sponges); both these carbonate mineralogies were highly labile and dissolved soon after

burial (Zhuravlev and Wood 2009). The initial observation of the thin section indicates that the mouldic pores are

dominated. The process created poorly connected enlarge mouldic porosity. Pore types include mouldic,

intraparticle, interparticle, fractured and vuggy porosity (Figure. 8), are observed based on Choquette and Pray

(1970). Most of the observed pores are almost exclusively secondary in nature.

4.2.3 Quantitative analysis of thin sections

A detailed petrographic quantitative study revealed that the considered core lithology is composed of 85% of

limestone, with 10% of dolomitic limestone and 5% dolostone (Figure. 9a). The core of these wells are mostly

composed of coarse-grained carbonate grains that are generally depleted of mud. In terms of texture, packstone

account for 50% of the cored rock, and grainstone, floatstone and rudstones constitute other 40% (Figure. 9b).

This sectioning allowed us to quantify the grain, matrix, and cement content as well as visible porosity and

revealed that carbonate grains cover an area of 33%, followed by 31% of matrix, 29% of cement and 7% of visible

porosity (Figure. 9c). The grains and visible porosity is further classified based on the quantitative observation of

8 dominant components and different pore types. The grains are dominated by eight components namely, by red

algae (30%), corals (30%), foraminifera (25%) and green algae (10%) while echinoderm, bryozoans, and bivalves

account to the remaining 5% (Figure. 9e). Besides, five different pore types are observed in descending order,

mouldic porosity is the dominant porosity, covering an area of almost 50%, vuggy porosity (20%) intraparticle

porosity (15%), interparticle porosity (10%), and fracture porosity (<5%) (Figure. 8, 9d).


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Fig. 9. Quantitative distribution of skeletal grains in thin sections from Central Luconia; A) lithology, B) grain, matrix, cement and visible porosity, C)

texture, D) porosity types and E) fossil components Adopted from, Janjuhah et al. (2017b)

4.3 Petrophysical properties and reservoir quality based on deposition, diagenesis, and microporosity

4.3.1 Facies and petrography

The porosity and permeability measurement and petrographic analysis of the eight facies revealed that the visible

porosity varies from poor to fair in facies 1, 5 and 8 while it is locally good to very good in facies 2, 3, 4, 6 and 7.

Considering total porosity, the hydrocarbon reservoir potential is suggested to be generally thought-out to be fair

for the middle to upper Miocene (Cycles IV and V) carbonates of the Central Luconia, Offshore Sarawak,

Malaysia. The porosity-permeability values of numerous reservoir facies also expresses that the facies-2 (av. Ø =

14.7%, av. Kh = 6 mD), facies 3 (av. Ø = 10%, av. Kh = 4 mD), facies 4 (av. Ø = 15%, av. Kh = 4.6 mD), facies 6

(av. Ø = 15%, av. Kh = 1 mD) and facies 7 (av. Ø = 20%, av. Kh = 2 mD) exhibits better reservoir qualities

(Figure. 10). On the contrary, the facies 1 (av. Ø = 6%, av. Kh = 1 mD), facies 5 (av. Ø = 4%, av. Kh = 0.5 mD)

and facies 8 (av. Ø = 8%, av. Kh = 0.8 mD) are thought to be dense limestone characterizing by a poorer reservoir

quality. In hand-specimen/petrographic observation revealed that large mouldic pores commonly occur within

coral fragments, while smaller mouldic pores appear to be formed from leaching of finely dispersed coral,

foraminiferal and algal debris. It is also observed that most of the porosity present in the corals are completely

cemented by calcite cement (Figure. 7e), representing tight reservoir intervals. Janjuhah, Salim and Ghosh

(2017b) also documented that calcite cement filled the void spaces in Central Luconia carbonate rocks. As has


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been shown, mouldic pores are the dominant pore types. Diagenesis also play an important role in controlling the

reservoir quality (Janjuhah et al. 2017c, Janjuhah et al. 2017b). Janjuhah et al. (2017b), Janjuhah et al. (2017c),

Janjuhah et al. (2017a) stated that mouldic pores which are present in Central Luconia are isolated in nature. The

presence of low permeability value in different facies is strongly related to the non-touching pore system or poorly

interconnection due to the small size of matrix intercrystalline pores as well as the presence of organic rich mud

and pressure solution seams. These phenomenon may constitute a potential barrier to fluid flow (Janjuhah et al.

2017b), it is worth saying that the micritization is the major diagenetic process which effect the reservoir quality

because it destroys the internal structure of the grains and reduced the porosity. The porosity in carbonate rocks

are usually reduced by the micritization which decreases the pore throat radius or grain size by filling them with

micrite (Taghavi, Mørk and Emadi 2006). The same phenomenon is also mentioned by Shakeri and Parham

(2014), Janjuhah et al. (2017c), who stated that micritization and compaction severely reduced porosity in

carbonate rocks.

Fig. 10. Total porosity versus permeability crossplot labelled with the eight different corresponding facies with average coefficient of determination

R² = 0.49, Central Luconia.

4.3.2 Reservoir quality enhancement based on microporosity

The macroporosity includes all pore types (mouldic, intraparticle, interparticle, vuggy) greater than 10 m.

However, the total porosity which is measured from the core plug and the macroporosity which is quantified

from thin sections revealed that this is the microporosity that comprises a significant percentage in Miocene

carbonates. (Figure. 11) illustrated clearly a contribution of microporosity by comparing of total porosity vs.

permeability cross plot with macroporosity vs. permeability cross plot (Figure. 10, 11). The relationship

macroporosity versus permeability cross plot represents a good fit with an increase of R² (coefficient of

determination) values as compared with total porosity verse permeability cross plot. The R² increased from 0.51

(total porosity vs. permeability) (Figure. 10) to 0.81 (macroporosity vs permeability) (Figure. 11).


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Fig. 11. Macroporosity versus permeability cross plot with better coefficient of determination R² = 0.81, Central Luconia.

Furthermore, to obtain the contribution of the microporosity with respect to the different facies encountered

within wells, a separate cross plot for each facies was build. Among all identified facies, the measured porosity is

higher than the macroporosity but when the macroporosity is cross plotted with permeability, it shows a better fit

with an increase of the R² values compared to the total porosity versus permeability curve. The Facies-1 coated

grain packstone represented a mean total porosity of 10% with R² = 0.51 (Figure. 12a), considering the

macroporosity as a function of permeability, the R² increased to 0.67 (Figure. 12b). Facies-2 (Coral (m)

lime-dominated pack-grainstone) total porosity versus permeability cross plot resulted a coefficient of

determination R² = 0.51 (Figure. 12c). Considering macroporosity than the total porosity vs permeability - an

increase in the value of R² which is 0.70 (Figure. 12d). The cross plot of total porosity vs permeability for Facies-3

(Oncolite lime-grain-dominated packstone) gave a correlation coefficient of R² = 0.76 (Figure. 12e). The

regression coefficient R² increased up to 0.90 when the permeability value of Facies-3 is plotted with respect to

macroporosity (Figure. 12f). Facies-4 (skeletal lime dominated dolo-packstone) revealed a total porosity up to

20%, the average valued of R² in total porosity vs permeability cross plot is 0.59, (Figure. 12g), whereas by

considering macroporosity the R² increased up to 0.72 (Figure. 12h). Same phenomena observed in other facies as

well. As Facies-5 (Coral (p) lime mud-dominated packstone) where R² increases from 0.66 to 0.81 (Figure. 12i-j),

Facies -6 (Coral (b) lime dominated packstone) from 0.61 to 0.78 (Figure. 12k-l), Facies-7 cross-bedded skeletal

lime packstone from 0.50 to 0.69, and Facies-8 (Bioturbated carbonate mudstone (Chalk) the coefficient of

determination R² increased from 0.44 to 0.67 (Figure. 12 m-p).


16


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Fig. 12. Total porosity, macroporosity and permeability cross plot of separate facies to observe the contribution of microporosity in total porosity for

(A) the cross plot of total porosity versus permeability for Facies-1 coated grain packstone (B) considering macroporosity vs permeability for

Facies-1 indicating better correlation by increasing the R² value (C) Facies-2 (Coral (m) lime-dominated pack-grainstone) porosity versus

permeability cross plot (D) Considering macroporosity than total porosity vs permeability (E) Cross plot for total porosity vs permeability for

Facies-3 (Oncolite lime-grain-dominated packstone) (F) Macroporosity vs permeability cross plot for Facies-3(G) the cross plot for total porosity vs.

permeability for Facies-4 (skeletal lime-dominated dolo-packstone) (H) macroporosity vs permeability cross plot of facies-4. (I) Total porosity Vs

permeability cross plot of Facies-5 (Coral (p) lime mud-dominated packstone) (J) Macroporosity vs permeability cross plot of Facies-5 (K) Facies - 6

(Coral (B) lime dominated pack-grainstone) total porosity vs permeability cross plot (L) Macroporosity Vs permeability cross plot of Facies-6 (M)

Facies-7 (Cross-bedded skeletal lime packstone) total porosity vs permeability cross plot (N) Macroporosity vs permeability cross plot of Facies-7 (O)

Facies-8 (Bioturbated carbonate mudstone) Total porosity vs permeability cross plot (P) Macroporosity vs permeability crossplot of Facies-8

respectively.

4.3.2 Facies distribution

The eight identified facies have been grouped into two classes known as good reservoir facies and poor reservoir

facies (Figure. 13). Facies-2, Facies-3, Facies-4, Facies-6, and Facies-7 are ascribed to good reservoir facies,

whereas Facies-1, Facies-5, and Facies-8 possess a poor reservoir quality. Figure. 13 provides a correlation of two

grouped reservoir facies in five wells within two carbonate platforms (1 and 2). The platform-1 is a deeper

platform towards the NE direction of Central Luconia. Where the poor reservoir quality is overlay the good

reservoir quality and this is due to the deepening of platforms towards the open sea (Figure. 13). This phenomenon

is also supported by the dominant presence platy corals and argillaceous material in Facies-5 in Well-B. While in

the platform-2, the good reservoir facies are dominant towards the landward but as the platform extent towards the

open sea, the thinker good reservoir quality are interbedded with thinner poorer reservoir facies (Well D and Well

E) (Figure. 13).

Fig. 13. Correlation of two different carbonate platforms based on the petrophysical properties of observed

facies from Central Luconia, offshore Sarawak, Malaysia.

5 Conclusion The carbonates in Central Luconia, Offshore Sarawak, Malaysia, consist mainly of limestone with minor

constituent of dolomitic limestone and dolomite. A detailed core description revealed that these carbonates

consist of eight facies: coated grain packstone (F-1), massive coral lime grainstone (F-2), oncolite lime

grain-dominated packstone (F-3), skeletal lime/dolo packstone (F-4), coral (p) lime mud dominated packstone

(F-5), coral (b) lime dominated pack-grainstone (F-6), cross-bedded skeletal lime packstone (F-7) and

bioturbated carbonate mudstone (Chalk F-8). Foraminifera, red algae, and corals are the main fossils

components. The porosity-permeability measurement shows a good porosity but low permeability value at a

given total porosity, the presence of high isolated mouldic pores that are observed under the micropores is the

main reason to have a high porosity with low permeability. In carbonate rocks, the porosity and permeability

value is directly related to the diagenesis, which alter the rock properties. It is also documented that the impact

of microporosity cannot be simply neglected, especially when dealing with carbonate reservoirs. The

microporosity has an obvious impact on the quality of reservoir. While assessing reservoir properties or

reservoir performance, taking macroporosity into consideration can improve the reservoir quality prediction in

\Central Luconia, Offshore Sarawak, Malaysia.


18

Acknowledgements

The senior author would like to thank Petroliam Nasional Berhad (PETRONAS) for providing the data and to

Prof. Dr. Deva Prasad Ghosh (Head: Centre for Seismic Imaging, Department of Geoscience, University

Technology PETRONAS, Tronoh, Perak, Malaysia) for his support and economic assistance throughout the

research under the grant YUTP 0153AA-A14.

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About the first author Hammad Tariq Janjuhah, PhD., joins the Department of Geology as an assistant professor after gaining a doctorate at the

University Technology PETRONAS, Malaysia in March 2018. He obtained his master degree in Petroleum Geology, from the


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University of Malaya, Malaysia in February 2014. His PhD focused on addressing the problems that are lacking in the guide

towards hydrocarbon reserves exploration that industry is faces today, efforts on this front focus on understand the carbonate

reservoirs in terms of sedimentology, the presence of microporosity and their relationship with the petrophysical properties.

During his doctoral studies, Hammad was also involved in various industrial projects, including Sarawak Shell Berhad and

PETRONAS research Berhad for four years. His research interest and work extends from the core scale to the laboratory scale,

focusing on the understanding of carbonate reservoirs by providing the proper/appropriate facies scheme to understand the

depositional setting, the diagenetic history that modifies the reservoir quality by enhancing or destroying the pore spaces for

hydrocarbon accumulation, and cyclicity to identify the nature of the cycle where the reservoir is present. The comprehensive

analysis of microporosity in these reservoirs provides a new quantitative method for evaluating hydrocarbon reserves and

predicting the reservoir performance. In the geology department of the American University Bruit, Hammad is involved in

teaching and research in the field of Petroleum Geology. Email: [email protected]; phone: 00961-81970143.


mailto:[email protected]

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