Evaluating Volcanic Reservoirs

36 Oilfield Review

Evaluating Volcanic Reservoirs

Hydrocarbons can be found in volcanic rocksometimes in significant quantities.

Petrophysical methods originally developed for sedimentary accumulations are being

used to evaluate these unusual reservoirs.

M.Y. Farooqui Gujarat State Petroleum Corporation (GSPC)Gandhinagar, Gujarat, India

Huijun HouDhahran, Saudi Arabia

Guoxin LiPetroChina Exploration and Production Company LimitedBeijing, China

Nigel MachinSaudi AramcoDhahran, Saudi Arabia

Tom NevilleCambridge, Massachusetts, USA

Aditi PalJakarta, Indonesia

Chandramani ShrivastvaMumbai, India

Yuhua WangFengping YangChanghai YinJie ZhaoPetroChina Daqing Oilfield CompanyDaqing, China

Xingwang YangTokyo, Japan

Oilfield Review Spring 2009: 21, no. 1. Copyright 2009 Schlumberger.DMR, ECS and FMI are marks of Schlumberger.For help in preparation of this article, thanks to Martin Isaacs, Sugar Land, Texas, USA; Shumao Jin, Brett Rimmer and Michael Yang, Beijing; Charles E. Jones, University of Pittsburgh, Pennsylvania, USA; Andreas Laake, Cairo; and Hetu C. Sheth, Indian Institute of Technology, Mumbai.

In the early days of petroleum exploration, the discovery of hydrocarbons in anything other than sedimentary rock was largely accidental, and such accumulations were considered flukes. Serendipity is still part of exploration, but geologists now know that the presence of oil and gas in such rock is certainly no coincidence. Igneous rockcreated by the solidification of magmahosts petroleum reservoirs in many major hydro carbon provinces, sometimes predominating them.

In general, igneous rocks have been ignored and even avoided by the E&P industry. They have been ignored because of a perceived lack of res-ervoir quality. However, there are many ways in which igneous rocks can develop porosity and permeability.1 Far from inconsequential, igneous activity can influence every aspect of a petroleum system, providing source rock, affecting fluid maturation and creating migration pathways, traps, reservoirs and seals.2

Igneous rocks have been avoided for other reasons. They tend to be extremely hard, although improvements in bit technology are helping drill-ers cope with these tough lithologies.3 Because they typically prevent deep pene tration of seismic

energy, igneous layers are considered an impedi-ment to evaluation of underlying sediments as well. New seismic methods are advancing solutions to this problem, but with their strong refrac-tive qualities, igneous reservoirs remain difficult to characterize.4

Once hydrocarbons are found in igneous reser voirs, assessing hydrocarbon volumes and productivity presents several challenges. Log interpretation in igneous reservoirs often requires adapting techniques designed for other environ-ments. Logging tools and interpretation methods that succeed in sedimentary rock can give mean-ingful answers in igneous rock, but they often require artful application. Furthermore, because mineralogy varies greatly in these formations, methods that work in one volcanic province may fail in another. Usually, a combination of methods is required.

This article describes the complexity of vol-canic reservoirs and presents technologies that have proved successful in characterizing them. The discussion begins with a review of igneous rock types and follows with an examination of the effects of igneous processes on petroleum

1. Srugoa P and Rubinstein P: Processes Controlling Porosity and Permeability in Volcanic Reservoirs from the Austral and Neuqun Basins, Argentina, AAPG Bulletin 91, no. 1 (January 2007): 115129.

2. Schutter SR: Hydrocarbon Occurrence and Exploration in and Around Igneous Rocks, in Petford N and McCaffrey KJW (eds): Hydrocarbons in Crystalline Rocks, Geological Society Special Publication 214. London: Geological Society (2003): 733.

3. Close F, Conroy D, Greig A, Morin A, Flint G and Seale R: Successful Drilling of Basalt in a West of Shetland Deepwater Discovery, paper SPE 96575, presented at the SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, September 69, 2005.

Salleh S and Eckstrom D: Reducing Well Costs by Optimizing Drilling Including Hard/Abrasive Igneous Rock Section Offshore Vietnam, paper SPE 62777, presented at the IADC/SPE Asia Pacific Drilling Technology Conference, Kuala Lumpur, September 1113, 2000.

4. Hill D, Combee L and Bacon J: Over/Under Acquisition and Data Processing: The Next Quantum Leap in Seismic Technology? First Break 24, no. 6 (June 2006): 8195.

White RS, Smallwood JR, Fliedner MM, Boslaugh B, Maresh J and Fruehn J: Imaging and Regional Distribution of Basalt Flows in the Faeroe-Shetland Basin, Geophysical Prospecting 51, no. 3 (May 2003): 215231.

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Oilfield ReviewWinter 09Volcanic Fig. OpenerORWINT09-VOL Fig. Opener

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38 Oilfield Review

systems. Two field examples highlight formation evaluation in volcanic rocks. A case study from a gas-rich reservoir in China presents a technique

that combines conventional logging measure-ments and image logs with neutron-capture spectroscopy and nuclear magnetic resonance.

An example from India demonstrates the impor-tance of incorporating borehole resistivity images in the evaluation of oil-bearing volcanic rock.

About Igneous RocksIgneous rock is formed through the solidification of magmaa mixture of water, dissolved gases and molten to partially molten rock. Igneous rocks vary from one reservoir to another because their constituents have diverse chemistries, origi nating from magma that mixes material from the Earths mantle, crust and surface typically oxides of silicon, iron, magnesium, sodium, calcium and potassium. They also have diverse structures and texturesleading to complex porosities and permeabilitiesdepend-ing on how they were emplaced. Emplacement mechanisms include sudden explosive eruptions, syrupy viscous flows and slow, deep subsurface intrusions. Subsequent weathering and fractur-ing can further complicate rock properties.

Igneous rocks form under a wide range of condi-tions, and therefore display a variety of properties (left). Molten rock that cools deep beneath the sur-face forms intrusive, or plutonic, rocks. Slow cooling of deep magmas forms large crystals, resulting in coarse-grained rock. These formations typically have low intergranular porosity and insignificant permeability, making them of little interest to the oil industry. The one exception is fractured granites, which can produce hydrocarbons.5 Magmas that approach the surface tend to cool more rapidly. This allows less time for the formation of crystals, which therefore tend to be smaller, resulting in fine-grained crystalline rock.

Extrusive, or volcanic, rocks are created when magma erupts through the Earths sur-face. Magma may extrude in flows of molten lava that, when cooled, form fine- to very fine-grained crystalline volcanic rock. Sometimes, cooling occurs so quickly that crystals cannot form, resulting in volcanic glass, such as obsid-ian. When magmas contain large amounts of water and dissolved gases, buildup of excessive pressure under the ground can cause explosive eruptions of volcanic material. Ejected frag-ments, or pyroclasts, can range in size from fine volcanic ash to bombs tens of centimeters in diameter. Once they have been ejected, indi-vidual fragments accumulate to form pyroclastic rock. Lava flows and pyroclastic deposits may be a few centimeters to a few hundred meters thick, covering thousands of square kilometers. These deposits can have sufficient porosity and permeability to make them viable hydrocar-bon reservoirs.

> Emplacement of igneous rocks. Plutonic rocks, formed by cooling of magma within the Earth, display well-developed crystals with little porosity. Plutons and laccolithsbulging igneous injections into sedimentary layersare examples of plutonic rock. Volcanic rocks, formed when magma extrudes onto the surface and cools rapidly, show very fine crystalline or even glassy textures. Buildup of pressures within the Earth can cause explosive eruptions; these result in the accumulation of fragments of volcanic material in pyroclastic deposits. Rock containing clastic fragments of volcanic origin is termed volcaniclastic. Complex porosities and permeabilities can develop as a result of these different processes.

Traps

Granite wash

Laccolithexposedby erosion

Basement

Volcano

Laccolith

Pluton

Sill

Dike

Country rock

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Eruptioncolumn

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Ash-cloud surge

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Plutonicrock

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Volcaniclastic rocks

Oilfield ReviewWinter 09Volcanic Fig. 1ORWINT09-VOL Fig. 1


PorphyryOne of the most common porphyriticstructures is phenocrysts, 1- to 2-mm [0.04- to 0.08-in.]crystals embedded in a fine-grained, often glassy matrix.Andesite and basalt often have olivine and pyroxenephenocrysts.

PillowLava that erupts under water and quicklydevelops a cool skin around a molten core forms pillowstructures, which are bulbous piles of rock. Pillow lavaoften incorporates seafloor sediments.

PyroclastPyroclasts are sharp, chiseled rockfragments created during a volcanic explosion. Glassshards are often a key component. Sharp shards indicaterapid burial or minimal postdepositional reworking.

FlowFlows form when the fabric of lava aligns inparallel rows or ropy waves.

Structures

VesicularGas expanding in cooling lava createspores called vesicles. Often unconnected, they are thereason very porous volcanic rock, such as pumice, canfloat but has negligible permeability. Vesicles oftenfill with secondary minerals, usually hydrated silicatescalled zeolites. These filled vesicles, called amygdules,reduce intergranular porosity in the same manner asclay in sandstone.

TuffaceousConsolidated pyroclastic material lessthan 2 mm [about 0.08 in.] in diameter is tuff.Unconsolidated tuff is ash. Both can be deposited farfrom their source. A common epiclastic, or weatheredvolcanic, reservoir rock is tuffaceous sand, in whichreworked tuff accounts for less than half the volume ofrock. When tuff makes up more than half the rock,the deposit is called sandy tuff.

BrecciatedMost angular particles exceeding 2 mmin diameter are volcanic breccia. Typically, particlesform from the movement of partially solidified rock, notfrom the ejection of fragments.

GlassyLava that cools rapidly forms volcanic glasssuch as obsidian, tonalite and pitchstone, which differmainly in their alkali feldspar content.

Textures

> Structures and textures in volcanic rocks. Variations in structure and texture give rise to the wide range of porosity and permeability observed in crystalline and pyroclastic rock.

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The different modes of formation of igneous rockscooling of lavas, either under the ground or at the surface, and agglomeration of fragments ejected during explosive eruptionsallow a subdivision of igneous rocks into two groups: crystalline igneous rocks and fragmental igneous, or pyroclastic, rocks.

A simple and common compositional classi-fication of crystalline igneous rocks is based on silica [SiO2] weight percentage. Rocks low in SiO2 (less than 52%) are classed as basic, rocks high in SiO2 (more than 66%) are acidic and those with SiO2 between 52 and 66% are intermediate.6

A parallel classification system groups rocks by weight percent of dark-colored minerals. Rocks rich (more than 70%) in dark minerals, such as olivine and pyroxene, are mafic; those containing few dark minerals (less than 40%), and therefore more light minerals, such as quartz and feldspar, are silicic, sometimes called felsic.7 Mafic rocks, such as basalt, tend to be basic; silicic rocks, such as granite, tend to be acidic.

A different classification encompasses em - place ment mechanism, crystal size and miner-alogy, dividing crystalline volcanic rocks into four main types (above right). The trend from basalt to andesite, dacite and rhyolite forms a continuum of mineralogy.

Pyroclastic rocks, on the other hand, are typically classified by grain size, as are clastic sedimentary rocks. Relative proportions of three grain-size classesblocks and bombs, lapilli and ashare used to classify a pyroclastic rock (right). Pyroclastic and crystalline rock types exhibit differences in texture and structure that lead to differences in porosity and permeability (previous page, bottom).

5. For example, recoverable oil reserves in the fractured granite of the Cuu Long basin offshore Vietnam are estimated at 2 billion bbl [320 million m3] or more. For more: Du Hung N and Van Le H: Petroleum Geology of Cuu Long BasinOffshore Vietnam, Search and Discovery Article #10062, http://www.searchanddiscovery. net/documents/2004/hung/images/hung.pdf (accessed April 6, 2009).

The giant Suban gas field in southern Sumatra contains estimated reserves of 5 Tcf [140 billion m3] in fractured granites. For more: Koning T: Oil and Gas Production from Basement Reservoirs: Examples from Indonesia, USA and Venezuela, in Petford N and McCaffrey KJW (eds): Hydrocarbons in Crystalline Rocks, Geological Society Special Publication 214. London: Geological Society (2003): 8392.

Landes KK, Amoruso JJ, Charlesworth LJ Jr, Heany F and Lesperance PJ: Petroleum Resources in Basement Rocks, Bulletin of the AAPG 44, no. 10 (October 1960): 16821691.

6. An acidic rock contains proportionately more nonmetallic oxides than a basic rock and forms an acid when dissolved in water. A basic rock contains proportionately more metallic oxides than an acidic rock and forms a base when dissolved in water.

7. The term mafic is derived from the words magnesium and ferric, whereas felsic is a combination of feldspar and silica.

Hyndman DW: Petrology of Igneous and Metamorphic Rocks, 2nd ed. New York City: McGraw-Hill Higher Education, 1985.

> Classifying igneous rocks by mineral composition. Fine-grained and coarse-grained rocks of similar composition have different names. For example, a magma containing quartz, potassium feldspar, sodium-rich plagioclase and biotite may cool slowly and form coarse-grained granite. If the same magma is extruded, it will form fine-grained rhyolite. Olivine-rich magmas do not commonly extrude, but crystallize at depth, and so form only coarse-grained rocks.


Quartz

Coarse Grained

Fine Grained RhyoliteDaciteAndesiteBasalt

GraniteGranodioriteDioriteGabbroPeridotite

BiotiteAmphibole

Pyroxene

Olivine

Sodium-richplagioclasefeldspars

Calcium-richplagioclasefeldspars

Min

eral

com

posi

tion,

vol

ume

perc

ent

80

60

40

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0

Increasing temperature of crystallization 700C [1,300F]1,200C [2,200F]

Potassiumfeldspar

100

Increasing silica content 75%45%

Increasing potassium, sodium and aluminum content

Increasing calcium, magnesium and iron content

> Classifying pyroclastic rocks by grain size. Pyroclastic rocks are identified based on grain size, in a similar fashion to clastic sedimentary rocks.

Clast orCrystal

Size, mmSedimentary

ClastsPyroclasticFragments

Crystalline Rocks:Igneous, Metamorphic

or Sedimentary

Boulders

Cobbles

Blocksand bombs

Pebbles Lapilli

Granules

Very coarse sand

Coarse sand

Medium sand

Fine sand

Very fine sand

Silt

Clay

G r a

v e

lS

a n

dM

u d

Coarse ashgrains

Fine ashgrains

Cryptocrystalline

256

64

16

4

2

1

0.5

0.25

0.125

0.032

0.004

Very coarsegrained

Very coarsecrystalline

Fine grained

Fine crystalline

Medium grained

Medium crystalline

Coarse grained

Coarse crystalline

Very fine grained

Very fine crystalline


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Volumes of VolcanicsPetrologists have calculated that the shallow part of the Earths crust contains a volume of volca-nic rockformed by the ejection of lava at the surfaceof 3.4 to 9 x 109 km3, an order of magni-tude greater than the volume of sedimentary rock. This estimate includes extrusions at seafloor rift zones, where oceanic plates are pulling apart and new crust is created by volcanic activity.

The presence of volcanic rocks in hydro-carbon provinces is common because volcanic activity has taken place in or near many sedi-mentary basins at one time or another. Volcanism can also affect distant basinslarge volcanoes can push pyroclastic flows up to 1,000 km [about 600 mi] from their origin and wind can carry ash thousands of kilometers (left). Consequently, blankets of ash and tuffs, or consolidated ash, may be found far from their source.

Hydrocarbon-producing igneous rocks occur the world over (below). The earliest docu-mented oil discovery in volcanic rock may be the Hara oil field of Japan, which began pro-ducing in 1900.8 The field produced oil from three tuffaceous layers. Other early produc-tion was recorded in Texas, in 1915, along a trend of seafloor volcanoes that erupted dur-ing deposition of the Austin Chalk.9 The buried volcanic formations produced 54 million bbl [8.6 million m3] of oil from 90 fields in more than 200 igneous bodies.

> Image of the Chaitn volcano, southern Chile, from the NASA Terra satellite. The volcano, thought to be dormant before its May 2, 2008, eruption, sent a plume of ash and steam 10.7 to 16.8 km [35,000 to 55,000 ft] into the atmosphere. This image, acquired three days after the eruption, shows the plume extending eastward more than 1,000 km across Argentina and into the Atlantic Ocean. The volcanic plume (white) is distinguishable from the clouds (turquoise). The land surface is dusted with tan-gray ash. [From Chiles Chaiten Volcano Erupts, http://earthobservatory.nasa.gov/IOTD/view.php?id=8725 (accessed April 6, 2009)].


km

miles0

0 100

100

CHILE Ash cover

ARGENTINA

ChaitnPlume

A T L A N T I C O C E A N

> Distribution of hydrocarbon-bearing igneous rocks. Gold dots represent locations of hydrocarbon seeps, shows and reservoirs in igneous rocks. (Adapted from Schutter, reference 36).

Hydrocarbons associated withigneous rocks or igneous activity


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Volcanic reservoirs may contain significant accumulations. As of 1996, cumulative production from the volcanic tuff and associated layers of the Jatibarang field, West Java, was 1.2 billion bbl [190 million m3] of oil and 2.7 Tcf [76 billion m3] of gas. Speculated reserves are 4 billion bbl [635 million m3] of oil and 3 Tcf [85 billion m3] of gas.10 Reservoir analysis yields porosity values of 16 to 25% and permeability up to 10 darcies. In this reservoir, the volcanic rocks are also source rocks.11

Petroleum SystemsVolcanism can affect all aspects of a petroleum system, producing distinctive source rocks, accelerating fluid maturation, facilitating fluid migration, and creating traps, reservoirs and seals.

Source RockAlthough most hydrocarbons found in volcanic rocks come from sedimentary source rock, some volcanic rocks are also source rocks. Vegetation entrained in ash flows may contain enough water to protect it from the heat of emplacement. Subaerial volcanism may create lakes and swamps with kerogenrich sediments, and the volcanically warmed water in these basins encourages nutrient growth, further enhancing the production of organic material.

MaturationBy adding heat, igneous bodies can accelerate hydrocarbon maturation. Large intrusive bodies, such as thick dikes and sills, cool slowly and may affect great volumes of surrounding rock, causing overmaturation.12 Volcanic flows cool relatively quickly, so they usually have less impact on maturation. The impact of igneous activity on fluid maturation can be assessed by petroleum systems modeling.13

In addition to direct heat, the circulation of hydrothermal fluids in the heated zone also may affect maturation. For example, scientists working in the Guaymas basin of the Gulf of California have reported that hydrothermal fluids heated to 400C [752F] are responsible

for alteration of organic matter and the creation of petroleum.14 The process is rapid, taking hundreds to thousands of years rather than the millions of years typically needed to generate oil.15

MigrationThere are several ways for hydrocarbons that originated elsewhere to become trapped in volcanic rocks:Hydrocarbons can pass vertically or later

ally from sedimentary rocks into structurally higher volcanic rocks.

Compaction of sedimentary rocks can forcehydrocarbons downward into volcanic rocks.

Hydrothermalfluidsarecapableofdissolvinghydrocarbons and depositing them in igneous rocks.

Ifthevaporpressureinvolcanicrocksbecomeslow enough during cooling, hydrocarbons may be drawn into the pore spaces.

TrapsIgneous intrusions into surrounding sedimentary layers, called country rock, often result in closed structures within the intruded formations. The Omaha Dome field in the Illinois basin, USA, was formed by this type of trap. The trapping structure is a Christmas tree laccolith produced by an ultramafic intrusion (above).16 The field was discovered in 1940 and produced about 6.5 million bbl [1 million m3] of oil from sandstones that are in contact with the intrusion.

ReservoirsIgneous rocks share another characteristic with sedimentary reservoir rocks; they can have primary porosity and sometimes develop secondary porosity. But unlike sedimentary rocks, igneous rocks lose their porosity quite slowly with compaction. Primary porosity may be intergranular or vesiculara type of porosity resulting from the presence of vesicles, or gas

> Traps caused by laccolith intrusion. The trap of the Omaha Dome field in Illinois was caused by a Christmas tree laccolith (left ) of mica-peridotite intruding into limestones and sandstones. Traps (green) can also be caused by punched laccoliths (right ), which lift overlying layers along bounding faults.


Christmas Tree Laccolith Punched Laccolith

8. Mining in Japan, Past and Present. The Bureau of Mines, Department of Agriculture and Commerce of Japan, 1909.

9. Ewing TE and Caran SC: Late Cretaceous Volcanism in South and Central TexasStratigraphic, Structural, and Seismic Models, Transactions, Gulf Coast Association of Geological Societies 32 (1982): 137145.

10. Kartanegara AL, Baik RN and Ibrahim MA: Volcanics Oil Bearing in Indonesia, AAPG Bulletin 80, no. 13 (1996): A73.

11. Bishop MG: Petroleum Systems of the Northwest Java Province, Java and Offshore Southeast Sumatra, Indonesia, USGS Open-File Report 9950R (2000), http://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50R/ ardj_occr.html (accessed April 7, 2009).

12. Schutter, reference 2.13. Yurewicz DA, Bohacs KM, Kendall J, Klimentidis RE,

Kronmueller K, Meurer ME, Ryan TC and Yeakel JD: Controls on Gas and Water Distribution, Mesaverde Basin-Centered Gas Play, Piceance Basin, Colorado, in Cumella SP, Shanley KW and Camp WK (eds): Understanding, Exploring and Developing Tight-Gas Sands: 2005 Vail Hedberg Conference, AAPG Hedberg Series, no. 3 (2008): 105136.

14. Simoneit BRT: Organic Matter Alteration and Fluid Migration in Hydrothermal Systems, in Parnell J (ed): Geofluids: Origin, Migration and Evolution of Fluids in Sedimentary Basins, Geological Society Special Publication 78. London: Geological Society (1994): 261274.

15. Kvenvolden KA and Simoneit BRT: Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean, AAPG Bulletin 74, no. 3 (March 1990): 223237.

16. English RM and Grogan RM: Omaha Pool and Mica-Peridotite Intrusives, Gallatin County, Illinois, in Howell JV (ed): Structure of Typical American Oil Fields, Special Publication 14, vol. 3. Tulsa: American Association of Petroleum Geologists (1948): 189212.

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bubbles, in igneous rock. Porosities in vesicular basalts and andesites may reach 50%.17 Secondary porosity is important for many volcanic reservoirs and is sometimes the only porosity present. It may result from hydrothermal alteration, fracturing and late-stage metamorphismmetamorphism during the late stages of igneous activity that alters the minerals formed earlier. Sills and lac-coliths may become reservoirs, especially when they intrude into source rocks. They may fracture upon cooling, providing porosity, permeability and migration pathways.

SealsIgneous rocks can provide seals. After alteration to clay, extrusive layers may act as tight seals. Impermeable intruded rocks, such as laccoliths that form traps, also may seal hydro-carbons in formations beneath them.

Exploration in Volcanic ProvincesHydrocarbon exploration in and around igneous rocks may involve a variety of geological, geo-physical and geochemical techniques. Traditional

surface mapping of elevated structures has revealed volcanic deposits. For example, in Japan, rhyolitic volcanic rocks containing large hydrocarbon accumulations have been discov-ered by mapping structural highs.18 Another tra-ditional method, the recognition of hydrocarbon seeps at the surface, is used to find deeper reser-voirs. Oil and gas sometimes rise to the surface along contacts between igneous and sedimentary rocks. Seeps in the Golden Lane area of eastern Mexico have been associated with steeply dip-ping igneous rocks that have penetrated thick oil-rich carbonate layers.19

Advanced techniques are also used. Satellite imagery has been applied to evaluate the basalt-covered Columbia basin in Washington and Oregon, USA.20 Geochemical analysis of ground-water in the same region has detected significant levels of methane over a large area, indicating potentially commercial quantities of natural gas in Columbia River basalts.21

Depending on the properties of the volcanic rocks, gravity and magnetic techniques may be useful. These were among the earliest geophysi-cal approaches applied, and they contributed to the successful exploitation of the 1915 Texas vol-canic play mentioned previously. Mafic igneous rocksricher in dense and magnetic minerals than felsic igneous rocksoffer better contrast with regional sediments, so they may show up distinctly on gravity and magnetic surveys. Aeromagnetic surveys have been effective in identifying prospects in mafic flood basalts in the Otway basin, southeastern Australia.22

Magnetotelluric (MT) methods have also been used, usually in conjunction with other techniques, to investigate high-resistivity vol canic rocks as potential reservoirs (for more on MT, see Electromagnetic Sounding for Hydrocarbons, page 4). For example, MT surveys in the Yurihara oil and gas field in Japan are aiding exploration of areas surrounding producing reservoirs.23 On some MT lines, resistive uplifted volcanic layers have been identified as possible prospects. Integration of MT surveys with surface seismic information was valuable in characterizing the internal struc-ture of an oil- and gas-producing basalt layer.

Seismic methods, while extremely useful for detecting sedimentary structures, have had mixed success in volcanic provinces. Massive basalts with-out internal layering have high effective seismic quality, meaning they are not highly absorptive, so seismic waves pass through them with little attenuation. Seismic surveys are relatively suc-cessful in delineating the tops and bottoms of such layers. However, layered basalts, especially those with interspersed weathered surfaces, tend to scatter seismic energy and may yield poor data.24 To improve the quality of seismic data in volcanic provinces, survey planners use satellite sensing to determine lithology and topography, and are incorporating the results in assessments of survey logistics, acquisition parameters and processing requirements (above left).25

In areas with highly attenuating volcanic lay-ers, borehole seismic surveys have shown some promise in improving seismic image resolution. Such was the case with an offset vertical seismic profile (VSP) acquired in a 4,750-m [15,600-ft] exploratory well in the Neuqun basin, Argentina.26 At the well location, the surface was covered by approximately 150 m [490 ft] of basalt that strongly attenuated surface seismic energy. The VSP produced an image with higher resolution than the surface seismic results and illuminated other igneous bodies in the subsurface.

> Remote sensing in volcanic provinces. Satellite data from visible, near-infrared, infrared and thermal bands help geophysicists assess topography and ground surface character before planning seismic survey acquisition. In this example from Argentina, satellite data (bottom) from several spectral bands are combined and color-coded to distinguish different surface characteristics. Recently erupted basalt flows are highlighted as dark red in both satellite images. Acquisition crews use the information to determine whether the terrain is accessible to vibrator trucks and other equipment (top). The photograph of the survey vehicles shows the Payun volcano seen from the south.


Nonbasalt rocks

Fresh basalt

Weathered basalt

Basalt with sparse vegetation

Nonvolcanic sediments

Vegetation

Fresh basalt

Weathered basalt

PayunPayun

20mi

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26. Rodrguez Arias L, Galaguza M and Sanchez A: Look Ahead VSP, Inversion, and Imaging from ZVSP and OVSP in a Surface Basalt Environment: Neuquen Basin, Argentina, paper SPE 107944, presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Buenos Aires, April 1518, 2007.

27. Li G, Wang YH, Yang FP, Zhao J, Meisenhelder J, Neville TJ, Farag S, Yang XW, Zhu YQ, Luthi S, Hou HJ, Zhang SP, Wu C, Wu JH and Conefrey M: Computing Gas in Place in a Complex Volcanic Reservoir in China, paper SPE 103790, presented at the SPE International Oil and Gas Conference and Exhibition in China, Beijing, December 57, 2006.

28. Barson D, Christensen R, Decoster E, Grau J, Herron M, Herron S, Guru UK, Jordn M, Maher TM, Rylander E and White J: Spectroscopy: The Key to Rapid, Reliable Petrophysical Answers, Oilfield Review 17, no. 2 (Summer 2005): 1433.

Once a hydrocarbon-bearing volcanic deposit is discovered, evaluating the reservoir can be a challenge. Methods for assessing porosity, permeability and saturation in sedimentary rocks must be modified to work in volcanic provinces. Case studies from China and India demonstrate such techniques.

Gas-Bearing Volcanic Formations in ChinaThe giant Daqing field, discovered in 1959, is the largest oil field in China and one of the largest in the world. The field has produced more than 10 billion bbl [1.6 billion m3] from sedimentary layers 700 to 1,200 m [2,300 to 3,900 ft] deep. Stratigraphic wellsdrilled to understand the basin-scale relationships between the reservoirs and the surrounding strataencountered gas in volcanic layers at depths between 3,000 and 6,000 m [10,000 and 20,000 ft]. Because of the difficult environment and challenging reservoir rocks, these reserves were not immediately tar-geted for development.

In 2004, PetroChina initiated a nine-well appraisal program and entered into a joint proj-ect with Schlumberger to better understand these deep volcanic reservoirs. The study area covered 930 km2 [360 mi2] and incorporated 3D seismic data along with wireline logs, borehole images and core analyses from 15 wells. To sup-port development decisions, analysts constructed a workflow to evaluate these complex reservoirs and estimate the amount of gas in place.27

The initial step in the workflow involved building a structural model from seismic data. The top of the Yingcheng volcanic group is a sig-nificant seismic reflector, and interpretation of this horizon supplied the major structural control for the model. In addition to the top of the group, seismic interpreters distinguished three main volcanic sequences, with interbedded and bound-ing sedimentary sequences (above right). Within the structural model, each sequence was divided

into smaller cells that were later populated with physical properties.

The reservoir consists mainly of interlayered crystalline rhyolites and rhyolitic pyroclastics, but a full spectrum of volcanics was encountered, rang-ing from basaltic to rhyolitic in composition and from crystalline igneous to pyroclastic in texture.

Identifying rock types within the sequences and correlating them between wells were diffi-cult tasks. Lithology classification for most types

of rocks relies on mineralogy, which cannot be determined easily for the very fine-grained or glassy textures common in volcanic rocks. This led scientists studying volcanic rocks to focus on chemical composition as the key factor in classi-fication schemes. With elemental concentrations from an ECS elemental capture spectroscopy tool, interpreters used these chemistry-based classification schemes to provide a continu-ous lithology description.28 However, chemical

> Structure of the Yingcheng volcanic group beneath the Daqing field. Interpretation of seismic data determined the top of the volcanic group, and integration of seismic and log data allowed delineation of the upper volcanic, lower volcanic and predominantly basaltic sequences.

2

km0

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ConglomerateShaleUpper volcanicSedimentaryLower volcanicBasalt

XS8 XS401 XS4 XS602 XS6 XS601X,200

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R U S S I A

Daqing

S. KOREA

N. KOREA


17. Chen Z, Yan H, Li J, Zhang G, Zhang Z and Liu B: Relationship Between Tertiary Volcanic Rocks and Hydrocarbons in the Liaohe Basin, Peoples Republic of China, AAPG Bulletin 83, no. 6 (June 1999): 10041014.

18. Komatsu N, Fujita Y and Sato O: Cenozoic Volcanic Rocks as Potential Hydrocarbon Reservoirs, presented at the 11th World Petroleum Congress, London, August 28September 2, 1983.

19. Link WK: Significance of Oil and Gas Seeps in World Oil Exploration, Bulletin of the AAPG 36, no. 8 (August 1952): 15051540.

20. Fritts SG and Fisk LH: Structural Evolution of South MarginRelation to Hydrocarbon Generation, Oil & Gas Journal 83, no. 34 (August 26, 1985): 8486.

Fritts SG and Fisk LH: Tectonic Model for Formation of Columbia Basin: Implications for Oil, Gas Potential of North Central Oregon, Oil & Gas Journal 83, no. 35 (September 2, 1985): 8589.

21. Johnson VG, Graham DL and Reidel SP: Methane in Columbia River Basalt Aquifers: Isotopic and Geohydrologic Evidence for a Deep Coal-Bed Gas

Source in the Columbia Basin, Washington, AAPG Bulletin 77, no. 7 (July 1993): 11921207.

22. Gunn P: Aeromagnetics Locates Prospective Areas and Prospects, The Leading Edge 17, no. 1 (January 1998): 6769.

23. Mitsuhata Y, Matsuo K and Minegishi M: Magnetotelluric Survey for Exploration of a Volcanic- Rock Reservoir in the Yurihara Oil and Gas Field, Japan, Geophysical Prospecting 47, no. 2 (March 1999): 195218.

24. Rohrman M: Prospectivity of Volcanic Basins: Trap Delineation and Acreage De-Risking, AAPG Bulletin 91, no. 6 (June 2007): 915939.

25. Laake A: Remote Sensing Application for Vibroseis Data Quality Estimation in the Neuquen Basin, Argentina, paper presented at the IAPG VI Congreso de Exploracin y Desarrollo de Hidrocarburos, Mar del Plata, Argentina, November 1519, 2005.

Coulson S, Grbak O, Cutts A, Sweeney D, Hinsch R, Schachinger M, Laake A, Monk DJ and Towart J: Satellite Sensing: Risk Mapping for Seismic Surveys, Oilfield Review 20, no. 4 (Winter 2008/2009): 4051.

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44 Oilfield Review

composition is not the whole story; for example, if a particular rock has a rhyolitic composition, chemistry alone cannot distinguish between a crystalline rhyolite and a pyroclastic rhyolite tuff. Textural information from borehole images obtained by the FMI fullbore formation microim-ager provided the basis for distinguishing these rock types and tying together log data from all the wells. Magnetic resonance T2 distributions provided additional information to complete the lithology classification.

By combining all available information, geologists were able to identify 11 igneous rock types in each well and then correlate them across the field using seismic data and con-ceptual geological models from other volcanic environments (left).

Evaluating the petrophysical properties of each rock type was particularly challenging.29 Compared with the clastic and carbonate rocks that form conventional hydrocarbon reservoirs, these volcanic rocks exhibit the most problem-atic features of both; the complex mineralogy, including the presence of conductive minerals such as clays and zeolites, parallels that of the most challenging clastic rocks, and their tex-ture and pore structure mimic those of the most complex carbonate rocks. This combination of features presents difficulties for the evaluation of porosity, permeability and fluid saturations.

A robust scheme for lithology-independent evaluation of porosity in low-porosity, gas-bearing formations is the DMR densitymagnetic reso-nance interpretation method, which combines bulk density and magnetic resonance porosity measurements.30 A relationship between matrix density and elemental concentrations derived from core analysis was applied to the ECS results to produce a continuous log of matrix density. The matrix density provided input to the DMR process for calculating high-quality estimates of porosity and indications of gas saturation in each well. To extrapolate porosity information to areas away from the wells, interpreters developed probability distributions of porosity for each rock type and used them to populate the model.

Estimating gas saturation was a challenge because the complex rock texture prevented development of a suitable Archie-type saturation equation, so a capillary pressurebased approach was used to estimate saturation. Pseudocapillary-pressure curves were derived from well-log magnetic resonance T2 distributions and cali-brated to mercury-injection capillary-pressure

> Correlation of igneous rock types with seismic data. Rock types were identified using FMI images, NMR T2 distributions and ECS elemental concentrations. Rock types were classified into seven crystalline lithologies (greens, pinks and purples) and four pyroclastic lithologies (orange and yellows). A sample correlation (bottom) shows an FMI image acquired through an interval of predominantly pyroclastic layers. A seismic section (top) through the central well is used to extend rock types across the field. The rock types observed in the central well are displayed at the well location using the color codes for volcaniclastic and crystalline lithologies. Rock types extrapolated away from the central well are displayed as semitransparent colors on the seismic section.


600 650 700 750 800 850 900

Lower lava flow

Water laid

Pyroclastic flow

Pyroclastic fall

Surge flow

Upper lava flow

Middle lava flow

Pyroclastic flow

Pyroclastic fall

Extrusive

Lava flow

Common depth point number

FMI Image50 0

FaciesPorosity

Lavaflow

Tuff

Pyroclasticflow

Surgeflow

Pyroclasticfall

%

Outer dome-building volcanic

Middle dome-building volcanic

Inner dome-building volcanic

Intrusive

32. Kumar R: Fundamentals of Historical Geology and Stratigraphy of India. New Delhi: New Age International Publishers Limited, 2001.

33. Negi AS, Sahu SK, Thomas PD, Raju DSAN, Chand R and Ram J: Fusing Geologic Knowledge and Seismic in Searching for Subtle Hydrocarbon Traps in Indias Cambay Basin, The Leading Edge 25, no. 7 (July 2006): 872880.

34. Pal A, Machin N, Sinha S and Shrivastva C: Application of Borehole Images for the Evaluation of Volcanic Reservoirs: A Case Study from the Deccan Volcanics, Cambay Basin, India, presented at the AAPG Annual Convention and Exhibition, Long Beach, California, USA, April 14, 2007.

29. Li GX, Wang YH, Zhao J, Yang FP, Yin CH, Neville TJ, Farag S, Yang XW and Zhu YQ: Petrophysical Characterization of a Complex Volcanic Reservoir, Transactions of the SPWLA 48th Annual Logging Symposium, Austin, Texas, June 36, 2007, paper E.

30. Freedman R, Cao Minh C, Gubelin G, Freeman JJ, McGinness T, Terry B and Rawlence D: Combining NMR and Density Logs for Petrophysical Analysis in Gas-Bearing Formations, Transactions of the SPWLA 39th Annual Logging Symposium, Keystone, Colorado, USA, May 2629, 1998, paper II.

31. Short NM Sr and Blair RW Jr (eds): Geomorphology from Space. NASA (1986), http://disc.gsfc.nasa.gov/geomorphology/ (accessed March 3, 2009).

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measurements performed on cores. Saturation values computed in this way showed a strong dependence on pore network geometry. For example, the core measurements showed the air-fall tuffsvolumetrically the most signifi-cant reservoir rock typeto be microporous, or having pore throats less than 0.5 m in radius. Saturation profiles across these formations exhib-ited long transition zones extending hundreds of meters and covering most of the reservoir. The saturation results, validated with gas indications from the DMR method, downhole fluid analysis measurements and production data, were consis-tent with the assumption that the reservoir was a single-pressure system with one free-water level. The capillary pressurebased approach was subsequently used to populate the model with saturation values.

Gas in place for the reservoir was calculated by summing the gas contained in each model cell. However, reservoir rock quality in this field is extremely heterogeneous. In addition, well control was limited, and the seismic data were imperfect in guiding the distribution of petrophysical properties. To cope with these diffi-culties, engineers employed a stochastic method to populate cells with porosity and gas satura-tion. Nearly 60 realizations were performed to evaluate the potential quantities of gas in place for the study area, providing an understanding of the range of uncertainty associated with field volumetrics. The results of the overall study sup-ported the decision to develop the field.

Oil in Indias Deccan TrapsThe Deccan Traps were formed by Late Cretaceous extrusion of flood basalts that today cover more than 500,000 km2 [190,000 mi2] of central western India. They are called traps, from the German word treppen for step, because they give rise to topography characterized by stepped terraces of resistant basalt layers (above right).31 The episode of volcanism was synchronous with the rifting of the Indian continent from southern Africa. Although the genesis and the mechanism of emplacement of these basalts are still debated, the general consensus is that they erupted under water.32 More than 40 such basalt layers have been identified, many of them interbedded with fluvial and estuarine limestones, shales and sandstones. In some places, total thickness of the traps exceeds 3,000 m.

During the last 40 years, Cambay basin, one of the oldest hydrocarbon plays of western India, has produced hydrocarbons from sediments overlying the Deccan basalts.33 Until recently,

the top of the volcanic deposits was considered economic basement, below which commercial hydrocarbon reservoirs were not expected to be found. However, in the past few years, oil has been discovered in these deeper volcanic rocks.

In 2003, Gujarat State Petroleum Corporation (GSPC) initiated a six-well campaign in Block CB-ONN-2000/1. The first three wells exhibited oil shows in the volcanic layers. In 2004, the fourth well, PK-2, proved to be a signifi-cant oil discovery, testing at 64 m3/d [400 bbl/d]. For planning the next well, a simplistic reservoir model was constructed that assumed the hydro-carbon-bearing topmost basalt layer penetrated

> The Deccan Traps of India. The Deccan Traps are a sequence of approximately 40 basalt layers covering portions of central western India. Differences between the basalts, which are competent, and interlayered sands, shales and limestones, which are more easily eroded, give rise to the rough terrain (right ). This photograph was taken at the Mahabaleshwar escarpment in the Western Ghats. The Cambay basin (left ) is a downdropped graben with oil-bearing sediments overlying the basalts. Basalt outcrops are shown in orange. (Photograph courtesy of Dr. Hetu C. Sheth, Department of Earth Sciences, Indian Institute of Technology, Mumbai.)

km

miles0

0 500

500

P A K

I S T A

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NEPAL

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BANGLADESH

SRI LANKA

I N D I A

Deccan Traps

Cambay basin

Mahabaleshwar


by Well PK-2 was laterally extensive. Based on this model, Well PK-6 was drilled in 2005 just 600 m [1,970 ft] to the southwest of PK-2, but unfortunately it did not flow any hydrocarbon. This unexpected result encouraged GSPC to update the reservoir model through further data analysis, specifically considering the rock facies and fractures and their interplay with faults within the volcanic layers.34

As a first step, geologists developed a textural classification of the volcanic layers. Three main faciesvesicular basalt, nonvesicular basalt and volcaniclastic unitswere identified using borehole image logs, petrography from Well PK-1

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and hand specimens of basalt (above). Next, the facies were correlated from well to wellan exercise that was far from straightforward. Lava flows can commingle, and after solidification other changes can occur, such as hydrother-mal alteration, weathering, cementation and structural deformation. These changes can be identified in outcrop, but tracking them in the subsurface is not easy. Based on image facies and log signatures, three main basalt layers, A, B and C, could be correlated between key wells PK-2 and PK-6 (above right).

In outcrop studies, volcanic rocks can be cor-related using geochemical analysis of major and minor elemental composition. In the subsurface, similar data can be acquired using the ECS tool. Crossplots of elemental silicon versus calcium,

aluminum, iron and titanium for Basalts A, B and C showed that Basalt A, the top unit, is composi-tionally different in the key wells, while Basalts B and C are compositionally similar (next page). This suggests that the top basalt layer is discon-tinuous laterally between the two wells, contrary to the assumption in the original model.

Following the facies analysis, the next phase of the study involved characterizing natural frac-tures, which are abundant within the volcanic layers. In the discovery Well PK-2, the top basalt that flowed hydrocarbon is thick, comprising a nonvesicular basalt layer overlying a vesicular basalt section with a number of fractures that appear conductive on borehole images.35 The presence of open fractures and vesicles creates a good-quality reservoir with a dual-porosity system, and the fracture network enhances per-meability. In contrast, in Well PK-6, the top basalt layer, which is thinner, essentially nonvesicular and less fractured, is not a good reservoir.

In addition to facies type and the presence of fractures, the geometrical relationship between

> Textural classification of Deccan basalt facies. Images from the FMI borehole resistivity imaging tool helped geologists identify three main rock types. Vesicular basalts (left ) exhibited vesicles in image (top), in hand specimen sample (bottom) and also in sidewall cores from a neighboring well. Nonvesicular basalts (center ) showed no such gas bubbles in borehole images or in sidewall cores. Images of volcaniclastic basalts (right ) showed fine-scale layering of angular particles. (Basalt photograph courtesy of Charles E. Jones, University of Pittsburgh, Pennsylvania.)

Vesicular Basalt Nonvesicular Basalt Volcaniclastic Rock

3 cm


> Initial well-to-well facies correlation. Texture-based facies classification allowed correlation of three basalt layers between Well PK-2 and Well PK-6. Basalt A (blue) is the producing zone in Well PK-2, but not in PK-6. Basalts B and C are nonproductive.


VesicularbasaltBrecciated zone innonweathered basalt

Volcaniclastics

Nonvesicularbasalt

1,800

1,825

1,850

1,875

1,925

1,775

1,900

1,800

1,825

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Dept

h, m

1,775

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Top

TopA

Well PK-2 Well PK-6

fractures and faults also seems to play a crucial role in localizing hydrocarbon accumulations. In Well PK-2, the open fractures occur at high angles to a seismic-scale fault, while fractures in Well PK-6 are aligned approximately parallel to the fault. Interpreters developed a conceptual model in which the seismic-scale fault facilitates fluid communication, allowing the open fractures that intersect it to conduct hydrocarbons to produc-ing wells. Fractures aligned with the fault are less likely to intersect it, and therefore are unlikely to conduct hydrocarbons. This concept was vali-dated in a new well, PK-2A1, which contained conductive fractures oriented perpendicular to seismic-scale faults and also produced oil.

Future Volcanic ActivityEvaluation of hydrocarbons in volcanic rock pres-ents many challenges, but creative application of techniques designed for sedimentary reservoirs is helping oil and gas companies characterize and exploit these complex accumulations. The

35. In the absence of acoustic or testing data, conductive frac-tures on borehole images are considered open to flow.

36. Schutter SR: Occurrences of Hydrocarbons in and Around Igneous Rocks, in Petford N and McCaffrey KJW (eds): Hydrocarbons in Crystalline Rocks, Geological Society Special Publication 214. London: Geological Society (2003): 3568.

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combination of borehole resistivity images with neutron-capture spectroscopy and magnetic resonance logs is becoming the new standard data suite for evaluation of volcanic reservoirs. With increased understanding of the capacity of volcanic rocks to contain oil and gas, other companies may consider reassessing volcanic formations they have bypassed, with a view to reevaluating their potential.

Unlike their sedimentary counterparts, vol-canic rock reservoirs have not been studied systematically. In addition to the few examples described in this article, hydrocarbons occur in or around igneous rocks in more than 100 coun-tries.36 In many instances, only oil shows and seeps have been documented, but further explo-ration may uncover significant reserves.

The presence of volcanic rocks in a basin may not ever become a basis for exploration, but the possibility of such basins sustaining a viable petroleum system should be included within an array of options. While some operators might stop drilling after encountering basement, those with a better understanding of the potential of volcanic rocks may treat them like any other pro-spective reservoir rock. LS

> Comparison of basalts in two wells. Elemental concentrations (right) from the ECS tool are expressed as ratios of calcium, iron, aluminum and titanium to silicon (Ca/Si, Fe/Si, Al/Si and Ti/Si). Ratios are plotted for Basalts A (blue oval), B (green oval) and C (red oval). In each of the ratio plots, the red and green ovals have approximately the same relationship to each other, but not to the blue ovals. For example, in the Ca/Si plot for Well PK-2 (top), the red and green ovals are next to each other, and the blue oval is inside the red oval. However, in the Ca/Si plot for Well PK-6, the red and green ovals are still next to each other, but the blue oval is inside the green oval. This arrangement indicates that Basalts B and C correlate from one well to the other, but Basalt A does not.

0.15

0.10

0.05

00.10 0.15 0.20 0.25 0.30 0.35

Ca/Si Fe/Si0.20

0.15

0.10

0.05

00.05 0.15 0.25 0.35

Ti/Si

0.100 0.20 0.30

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Al/SiBasalt C

Basalt B

Basalt A

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Ca/Si

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0.14Al/Si

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00 0.10 0.20 0.30

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1,770

1,780

1,790

1,800

1,810

1,820

1,830

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1,850

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Basalt A

Basalt C

ImageLogs Elemental Concentrations, kg/kg

Well PK-2Depth, m Gamma Ray Lithology

Well PK-6Depth, m Gamma Ray Lithology


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Evaluating Volcanic Reservoirs

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