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DESCRmTION AND RESERVOIR CHARACTERIZATION OF A LATE

OFFSHORE EAST JKALIMANTAN, INDONESIAMIOCENE, DELTA-FRONT CORALREEF BUILDUP, SERANG FTELD,

Charles T. Siemers', Sigit Sutiyono*, and Stephen K. Wiman2

ABSTRACT

The Late Miocene sedimentary sequence within Serang Field in the offshoreportion of th e Kutei Basin, East Kalimantan is dominated by fluviaydeltaic and shallow-mar ine siliciclastic deposits, including hydrocarbon-bearing sandstone reservoirs. Alsopresent within this ancient Mahakam River delta depositional sequence ar e numerouscarbonate uni ts indicative of coral reef growth in a delta-front, marine-shelf setting.One such carbonate unit, the 80-6 Limestone, was cored in its entirety an d is the subjectof this IPA Carbonate Core Workshop contribution.

The 88.5 feet of full-diameter material in cores 6-8 of the Serang 3RD2 wellinclude the entir e 67 feet of the 80-6 Limestone unit as well as 11.5 feet of underlyingnon-calcareous, shallow-marine delta-front deposits and lo' feet of overlying calcareousshale. A lower reef buildup 25 ft thick) displays abundant platy corals (withargillaceous/silty carbonate mud matrix) a t the base grading u p to a mix of massive andbranching coral fragments and mud matrix. An upper reef buildup 42 ft thick)begins with a lower, platy-coral-bearing, highly silty an d argillaceous carbonatewackestone nn it t w-c ha ng es ap wa rd to a non-porous, partly-argillaceaus coral rubbledeposit and a highly porous and permeable, 10 ft-thick, reef-core type coral rubbledeposit. The upper 13 feet of the upper reef is a non-porous, reworked mix of avariety of skeletal fragments with an upward-increasing amoun t of si lt hand an d detr italclay. The reef is overlain by slightly calcareous and shelly, silty shale representative ofshallow marine shelf deposition within a delta-front setting.

Based on subsurface correlations within Serang Field and on observations ofsimilar coral-reef buildups in front of the modern Mahakam River delta, it appears th atthe 80-6 Limestone represents a coalescent cluster of reef bui ldups with a lateral extentof at least 2.5 k m and possibly up to several tens of kilometers. Modern coalescentclusters of coral reefs have been observed in water depths of 50-100 m (165-330 ft) infront of a less active (partly abandoned) portion of the Mahakam River delta.Individual buildups up t o 1.5 km across occur within a coalescent cluster that extendsfor greater than 30 km in both depositional strike and dip directions.

Post-depositional modifications within the 80-6 Limestone has resulted insignificant degradation of its reservoir quality., Compactional effects have beenminimal; however, recrystallization of both skeletal fragments (especially coral andmolluscan fragments) and carbonate mud matrix have been extensive. The only partof the reefal buildup with favorable reservoir quality is the 10 ft-thick reef-core facieswith up to 25 porosity. Permeability, however, is developed only to a few tens ofmillidarcies; and, because of the isolated nature of the moldic/vuggy porosity system,permeability does not increase much with a significant increase in porosity.

Because of the probable lateral discontinuity of the porous and permeable reef-core facies within the laterally-extensive delta-front coral-reef buildups, such carbonatereservoirs will continue to be secondary hydrocarbon targets in the Kutei Basin LateMiocene section that is dominated by numerous, favorable sandstone reservoir units.

Consultaot, Sedimentology Division, P.T. Ceoswices, Jakartau d donesia, La. al ikppan, East l G d h ~ ~ ~ h

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IPA, 2006 - Carbonate Rocks and Reservoirs of Indonesia: A Core Workshop, 1992c Contents

Contents

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INTRODUCTION

General Statement and Objectives

This contribution to the IPA Carbonate Core Workshop provides for the display,description and discussion of a coral reef buildup that occurs within a Tertiary sequencedominated by fluvial, deltaic and shallow-marine clastic deposits. Although drilling inSerang and other oil fields in the Kutei Basin has been focused on the development ofsiliciclastic reservoirs, numerous interstratified, carbonate units have been encountered. The67 ft-thick, Late Miocene carbonate unit described herein was cored in its entirety, from topto bottom, in the Serang 3RD2 well in order to evaluate the reservoir potential of suchcarbonate buildups within Serang Field, located in the offshore portion of the Kutei Basin,East Kalimantan. The 88.5 ft of full-diameter core on display for the IPA Core Workshopwill allow participants an opportunity to examine a well-developed reefal accumulation withseveral distinct, and easily recognizable reef subfacies. The objectives of this paper are toprovide a brief description of the-80-6 Limestone cored in the Serang 3RD2 well and topresent a discussion of its origin and reservoir character.

Regional Setting and Stratigraphic Framework

Serang Field, located in the offshore portion of the Kutei Basin, East Kalimantan,Indonesia (Figs. 1 and 2), is situated about 25 kin northeast of Attaka Field, a giant oil fieldwhich was discovered in 1970 and has produced in excess of 480 million STB of oil and 700billion SCF of gas Partono, 1992). The geologic setting of the Kutei Basin, one of themajor producing areas in Indonesia, has been discussed in papers by Rose and Hartono(1978) and Samuel and Muchsin (1975). The prolific production of the Kutei Basin isdominantly from clastic reservoirs deposited during progradation of the ancient MahakamRiver delta system, which began in earliest Miocene (latest Oligocene?) time and reached itsmaximum basinward extent in latest Miocene time. Deltaic sedimentation in East Kalimantanis discussed in Allen (1987), Allen et al. (1976), and Allen et al. (1979).

Serang Field was discovered in 1973 by Union Oil Company (now U n o d Indonesia,Ltd.) and Japex (now Inpex). The Serang-1 discovery well is situated on a seismically-defined, faulted structural nose. The discovery well was tested and suspended and noadditional drilling was done in Serang Field until 1990, when the Serang-2 confirmation welltested oil and gas trapped within clastic reservoirs of a separate fault block. Three additionalwells, the Serang-3, Serang-4 (on a separate closure) and Serang-5 were subsequently drilled.The Serm.g-3 was re-entered and side-tracked and is now designated the Serang 3RD2 (3Re-Drill2). A 3-D seismic survey has been acquired over Serang Field and will be processedprior to installation of the Serang Field platform in 1993.

The stratigraphic section penetrated in Attaka and Serang Fields ranges in age fromLate Miocene to Pleistocene. The section in Serang Field is not subdivided into lithostrati-graphic units but is referred to by stratigraphic markers correlated from the Serang-1discovery well. The limestone unit discussed in this paper is designated the 80-6 Limestone.Based on calcweous nannofossil zonation, the 80-6 Limestone is assigned to the CN9 Zoneof Okada and Bukry, 1980 (Fig. 3).

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Materials and Methodology

The Serang-3RD2 well was spudded on 22 May 199 1 and reached a TD of 11,161ft MD (-10,612 ft S S ) on 30 July 1991. The top of the 80-6 Limestone is at 8,445 ft MD-7,960 ft S S ) . Based on a RFT evaluation of the limestone in the Serang-2 well the decision

was made to core the 80-6 Limestone in the Serang-3RD2. Using a LWD (Logging WhileDrilling) approach, Unocal was able to initiate coring about 9.5 feet above the- 80-6Limestone, core the entire 67 ft-thick carbonate sequence and core about 11.5 feet ofunderlying non-carbonate deposits. Thus, both the upper and lower contacts of he carbonatebuildup are well-displayed in the full-diameter core material. Detailed aspects of the LWDapproach utilized are discussed in Sutiyono (1992). Because of the potential for lostcirculation, which is common while drilling limestone units above 6,000 feet in Serang Field,water-based mud was used while cutting Cores 6-8 through the 80-6 Limestone section in theSerang 3RD2 well. Wireline logs run in the well include the following: DLL-MSFL-BHC-SP-GR / LDL-CNL-GR / EPL-PCD and FMS.

Initially, the 88.5 feet of full-diameter core material was handled by Corelab inJakarta where conventional and whole-core analyses were performed and the core wasslabbed into 1/3-2/3 portions. After the core was transported to the P T GeoservicesSedimentology Laboratory, the 2/3 portion was polished (to remove saw marks), HC1-etched(to accentuate rock fabric), mounted in epoxy resin and placed in custom wood boxes fordisplay, photography and long term preservation. The core was described in detail, utilizinga standard binocular microscope and incorporating the detailed petrographic description of33 thin sections which were carefully selected with respect to specific sedimentological andpetrophysical characteristics. Thin sections were vacuum impregnated wth blue-dyed epoxyto emphasize macroporosity and stained for identification of mineral species (e.g., AlizarinRed-S for calcite and potassium femcyanide for iron-rich carbonate minerals). Thin sectionswere particularly useful for the designation of rock fabric (using the designations of Dunham,1962) and delineation of major skeletal and non-skeletal constituents. Basic rock type,carbonate rock fabric (textures and structures) and qualitative determination of constituentgrains for t he core sequence are indicated in the panel diagram illustrated by Figure 5, whichalso provides a suite of key wireline logs and a depth plot of porosity and horizontalpermeability for Cores 7 and 8 of the Serang 3RD2 well.

CORE LITHOLOGY/PETROGRAPHY

No recovery was recorded for Core 6, but Cores 7 and 8 recovered 85.5 feet of full-diameter core material from the 88.5 ft core sequence, including the well-developed 67 ft-thick coral reef carbonate buildup (Figs. 4 and 5). The uppermost portion of Core 7 includes9.5 feet of slightly calcareous marine shale above the carbonate buildup (top at core depth8424.5 ft; approx. log depth 8437 ft). The base of the carbonate occurs at core depth8491.65 ft within Core 8 (approx. log depth 8503 ft), above a thin (0.25 ft), pebblysandstone lag deposit. The lower 11.6 feet of the core is marine shale. The followingsummary description combines binocular microscope description of the slabbed, polished andHCl-etched core surface and petrographic examination of 33 thin sections.

The basal 1 1.6 feet of laminated to ripple-bedded and slightly burrowed, silty marineshale (lith. unit 13; Fig. 7F) is overlain abruptly by a very thin (0.25 ft) pebbly, clayey andshelly, medium- to very coarse-grained sandstonebed (lith. unit 12; Fig. 7F). Thin sectionsdisplay a very poorly-sorted mix of quartz grains, detrital mud and reworked skeletal

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fragments, including pyritized larger foraminifera, molluscs and even coral fragments (Fig.11F) Minor (trace to 1 ) greenish glauconite and brownish coalified plant/wood fragments(2-4%) also were noted. Thin section visible (macro) porosity, including both primary (inter-granular) and secondary (grain-dissolution) porosity, is present but rather insignificant owingto the limited distribution of the thin unit.

The pebbly-sand lag deposit is overlain immediately by a 3.65 ft-thick, highly siltyand argillaceous transitional unit (lith. unit 11) containing abundant platy corals andHulimedu green algal fragments. The distinct, 0.5 cm thick to 10 cm+ wide, platy whitecoral fragments (Figs. 7E-F, 11E) and small (0.5-1.0 mm thick x 1 cm long) Halimcdaflakes (Fig. 11D) are oriented parallel to stratification within a detrital (non-calcareous) silty,microporous mud matrix . Lithologic unit 10 (core depth 8476.5-8488.0 ft; 11.5 ft thick)grades upward from a highly argillaceous, silty and dolomitic wackestone with abundant platycorals and Huliineda (Figs. 7D and 11C) to a carbonate mud-rich skeletal wackestone/-packstone with abundant branching corals. Lithologic unit 9 (core depth 8472.5-8476.5 ft;4.0 ft) is a poorly-organized (non-stratified; burrowed?), slightly argillaceous and silty coralgrainstone/packstone/wackestone(?) unit containing abundant branching coral fragmentswithin a microporous, carbonate mud-dominated matrix (Figs. 7C, 1 1A-B). The highlyrecrystallized coral fragments are 0.5 to 1.0 cm thick and 5 to greater than 10 cm long witha muddy, rubble-like accumulation that is overlain by a large (15-30 cm across), highlyrecrystallized massive coral fragment at the base of lithologic unit 8. The silty/sandy andargillaceous coral grainstone deposit of lithologic unit 8 (core depth 8467.0-8472.5 ft; 5 5ft), with a mix of pebble-size branching, platy and massive coral fragments with a sandy andargillaceous carbonate mud matrix (Figs. 1OE-F), seems to mark the top of a lower reef 'buildup.

The basal part of an upper reef' buildup is the very sandy (10-20%) and argillaceous(30-50%) lithologic unit 7 (core depth 8461.0-8467.0 ft; 6.0 ft) which contains well-orientedplaty, and some branching, corals and common Hulinieda fragments within a microporous,carbonate mud-dominated matrix (Figs, 7B, IOC-0). Lithologic units 6 and 5 (core depthfrom 8447.5-8461.0 ft; 13.5 ft) transcends the gap between Cores 7 and 8 and are composedof a poorly-organized (non-stratified) coral grainstone/packstone rudite (i.e., coral rubble)accumulation of mostly highly-recrystallized, branching and massive corals (Figs. 6F, 7A,1OA-B). Irregular coral fragments commonly are 3-5 cm across with many ranging up togreater than 8 cm. They are mixed with minor fragments of coralline algae, Hulimeduechinoderms and larger foraminifera, in a slightly silty and argillaceous, microporouscarbonate mud matrix .

The 10 ft-thick lithologic unit 4 (core depth 8437.5-8447.5 ft; approx. log depth 8450-8460 ft) comprises the most porous and permeable zone within the carbonate buildup. Itis a, coral boundstone to grainstone/packstone rudite (i.e., coral rubble) accumulation oflarge, mostly massive type coral fragments and abundant intraclast matrix of finer skeletalfragments in a carbonate mud (Figs 6D-E). Extensive recrystallization and leaching of coralsand other skeletal fragments within this central-reef type deposit has resulted in excellentmoldic/vuggy porosity (Figs. 6D-E, 9A-E) and moderate permeability development (i.e.,measured porosity of greater than 25% and permeability of 10's of millidarcies). Theoverlying 10 ft-thick lithologic unit 3 (core depth 8427.5-8437.5 ft) is a transitional, highlymixed (reworked) skeletal (coral/coralline algal) grainstone/packstone/wackestonewith bothbranching and massive coral fragments and abundant coralline algal fragments (Figs. 6C, 8E-F) along with a slightly silty/argillaceous, microporous carbonate mud matrix (30-40% of therock). The upper 3 feet of the upper reef' (lith. unit 2; core depth 8424.5-8427.5 ft) is asilty/sandy and highly argillaceous skeIetaI packstone/grainstone with abundant Iarge bivalve

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fragments mixed with coralline algal fragments and coral fragments (Figs. 6B, 8C-D). Thereef is overlain in Core 7 by an upper 9.5 feet of slightly shelly and calcareous, silty shalecontaining common coral, bivalve and larger. foraminifera fragments and minor Hulimeduflakes and tests of small benthonic and planktonic foraminifera (Figs. 6 A , 8A-B).

PETROPHYSICS AND RESERVOIR PERFORM ANCE

Conventional porosity and horizontal permeability data for the carbonate sequence ofCores 7 and 8 are displayed by the depth-plot in Figure 5 and the cross-plot of Figure 12.Most of the measurements represent the carbonate mud-rich reef facies (i.e., lith. units 2-3and 5-11) with very low porosity (1.1-9.5%; avg.=3.7%; n=47) and very low permeability(mainly less than 0.1 md, with a few values to nearly 1 md). The reef-core type sedimentaryfacies of lithologic unit 4 displays significantly higher porosity (mainly 15.8-28.4%;avg.=21.5%; n=lO) and permeabilities of 20-85 md. It is very interesting to note,however, that increasing porosity values within the rnoldic/vuggy macroporosity system donot tend to result in significantly increased permeability (Fig. 12).

Although the resistivity through the porous portion of the 80-6 Limestone is relativelylow (4-6 ohm-m), oil shows were observed during initial examination of the core, and lowwater saturations calculated from the logs using formation water salinity of 15,000 ppmindicated the presence of hydrocarbons. Also wireline logs indicated good porosity,displayed by a density-neutron cross-over through the upper part of the carbonate section(Fig. 5 . Based on these observations the decision was made to run a drill stem test toevaluate the interval 8430-8476 ft, which includes the porous and permeable reef-core facies(lith. unit 4). After loading 40 barrels of diesel cushion, the well was shot underbalancedat about 1200 psi using Tubing Conveyed Perforation (TCP). The well died after flowingat the rate of 945 barrels of fluid per day. The string fluid content was reversed out and atotal of 42 barrels of formation water were recovered. Water sample analysis yielded asalinity of 10,000 ppm, which was much fresher than expected. The oil shows within thecore, in combination with the water production, indicate that the 80-6 Limestone has highresidual hydrocarbon saturation, probably due to flushing by fresh water.

SEDIM ENTO LOG Y, DIAGENESIS AND RESERVOIR QU ALITY

Sedimentologically he 67 ft-thick carbonate unit (80-6 Limestone) represented byCores 7 and 8 of the Serang 3RD2 well is interpreted to represent a conlplex, multi-geneticcoral reef that developed on a relatively-shallow to moderately-deep (100-300 ft) marine shelfsetting in front of a prograding delta. The thin, pebbly and clayey sandstone bed at the baseof the carbonate buildup probably represents a lag-type, reworkedhon-depositionalsurface.The coral-rich carbonate buildup started on that relatively firm substrate. A lower reef'buildup (i.e., the 25 ft-thick sequence of lithologic units 8-1 1) displays an abundance of platycorals (with argillaceous/silty carbonate mud) in the lower part, grading upward to a mix ofmassive and branching coral fragments and carbonate mud matrix. An upper reef buildup(i.e., the 42 ft-thick sequence of lithologic units 2-7) begins with a lower, platy coral-bearing, highly silty and argillaceous carbonate wackestone unit that changes upward to anon-porous, partly-argillaceous coral rubble deposit and a highly porous and permeable, 10ft-thick, central-reef-core type coral rubble deposit. The upper 13 feet of the upper reefis a non-porous, reworked mix of a variety of skeletal fragments and an upward-increasing

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amount of silt/sand and detrital clay. The reef is overlain by slightly calcareous and shelly,silty shale representative of shallow marine shelf deposition, which may have been influencedby delta-front contributions.

On a regional basis, it is interesting the note that the 80-6 Limestone is present in allfive Serang wells drilled to date (Fig. 13) The carbonate buildup ranges from 67 f t (20.5m) thick in the Serang 3RD2 well to less than 20 ft (6 in) thick in the Serang-1 and Serang3ST wells, which indicates that the carbonate buildup had an extent of at least 2.5 km in anortheast-southwest direction and 1.5 km in a northwest-southeast direction (Fig. 13). Suchcarbonate buildups also have been recognized on the modern Mahakam River delta and wereimaged on sparker and side scan sonar surveys in 1985 and 1986 (Rachwad, 1987). Withinapparent coalescent clusters of reef-like carbonate features, individual buildups were encoun-tered to be up to 1.5 km in aerial extent, up to 40 m (130 ft) in thickness and growingoptimally in water depths ranging from 50 to 100 m (165-330 ft). The top of such modernbuildups commonly are at water depths of 40-50 m (130-165 ft). The growth of individualcarbonate buildups appear to be restricted by pulses of sand and silt from the land mass tothe west, and to a lesser extent, by southward flowing longshore currents carrying siliciclasticsediment in suspension. The maximum extent of such coalescent clusters of reef-like buildupin front of the modem Mahakam delta apparently extend for up to greater than 30 km in both .depositional dip and strike directions and appear to be present in front of less active (partlyabandoned) portions of the delta.

Post-depositional modifications within the carbonate buildup have been significant.Gompactional effects appear to have been minimal with only local stylolite development.However, recrystallization of both skeletal fragments and carbonate mud matrix has beenextensive. The abundant coral fragments, which originally were composed of unstablearagonite, have undergone recrystallization to stable, low-magnesium calcite with resultantdestruction of most coral skeletal microstructure. Most molluscan fragments and the greenalgae, Halimeda also have undergone the same form of recrystallization. Other skeletalelements such as coralline algae and benthonic foraminifera have undergone less recrystalliza-tion, owing to their original calcite composition. Where non-calcareous, detrital clay isrelatively abundant within the carbonate deposits, recrystallization of coral material has notproduced significant amounts of macroporosity; however, where the carbonate deposit isrelatively free of such clay, moldic/vuggy macroporosity and associated permeability developto significant levels (e.g., 25 and tens of millidarcies in the reef-core facies of lithologicunit 4). The presence or absence of detrital clay appears to be a major control on thedevelopment of secondary porosity within these reefal carbonate buildups. The lack of suchclay within the reef-core facies probably is due to high current and wave energy associatedwith such deposits.

~ Although porosity and permeability are well-developed within the 10 ft-thick high-energy, reef-core sedimentary facies, the reservoir quality of carbonate buildups such as the80-6 Limestone is difficult to evaluate. The carbonate unit appears to be fairly extensiveacross Serang Field, and similar modern, coalescent buildups appear to be laterally extensiveover tens of kilometers; however, the porous and permeability reef-core type deposits maybe much less extensive and probably are not interconnected throughout the entire buildup.Obviously, such carbonate reservoirs will continue to be secondary hydrocarbon targets ina Tertiary section dominated by numerous, favorable sandstone reservoir units.

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F'IGURE 1. Map showing location of Kalimantan and the offshore portion of the Kutei Basin,East Kalimantan see Figure 2 for location of Attaka and Serang Fields).


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4J

rN

ATTAKA FII

1:

r.

V S E R A N C FIELD1

1

FIGURE 2. Location of Attaka and Serang Fields within the offshoreportion of the KuteiBasin, East Kalimantan (see Figure 1).


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GENERALIZED STRATI GRAPHIC CO LUM N

SERANG FIELD IIPOCH

CALC.

NANNO

CN 14

and

CN 13

ZN 12

CN 11

to

CN 10

CN 9

_. or

Older

' M54-7 MARKER

RT MARKER

3 Cored interval

80-6 limestone

Serang-3 RD2

S98-9 MARKER

DEPOSITIONAL ENVIRONMENT

Delta front

Distributary mouth bars

and tidal bars

to

Prodelta

Coral patch-reef buildups

Fluvial to tidal delta p lain

Distributary channels

Point bars

or

Channel bars

Delta front

Distributary mouth bars

and tidal bars

to

Prodelta

Coral patch-reef buildups

FIGURE 3. Generalized stratigraphic column for Serang Field, off-shore portion of KuteiBasin, East Kalimantan. The calcareous nanoplankton zonation is from Okada and BukryI + o n \


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S u b s e aD e p t h

. .

. .> .

. . . .

-. .

. . .. .

. . . .. .:. .

. -s79-

579-4

. .

. *

558-1

. . .

. . .

. .

. .

S80-I. . .

.-_-.-...

. .

. s8l .

S82-

S82-

S82-

Log D e p t h. . .

FIGURE 4. Portion of the Serang 3RD2 wireline log showing the location of Cores 6-8 throughthe 80-6 Limestone.


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I E

MF

B

I I IU


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i. .

8

I

0(0tm

0t-t0

0

mtm

00

O

00vO


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FIGURE 6. Selected core p hotographsfrom lithologic uni t s 1-5 of t he 80-6 Limestonesequence, Serang3RD2 well.

A. 8416.5 ft. Portion of core from shelly shale bed near the top of lithoiogic unit1. Thelarger, light-colored skeletal fragments include mainly corals and bivalves.Also present arecommon larger foraminifera and minor Halimeda flakes,small benthonic and planktonicforamin ifera. Matrix is 'composed of detrital clay and10-15% quartz silt, along with tracesofgreenish glauconite and fine coalified plant fragments.

B. 8425.5 t. Portion of ore from sandy and argillaceous skeletal packs tone/grains tone withinlithologic unit2, composed of mix of detrital tlay , abundant quartz sil th er y fine sand(15-35 )and wide variety of mostly fine-grained skeletal fragmen ts. Note vague stratification andbioturbate fabric. Porosity and permeability are mostIy less than5 and 0.2 md.

C 8434.5 ft. Portion of core from lower part of lithologic unit3 which is a complex mixtureof a variety of sand to fine-pebble size skeletal fragments (mainly corals and coralline algae)within a carbonate mud-dominated matrix(30-40 of rock). Th e brownish skeletal fragmentsare small branching corals. A few pyritized benthon ic foraminifera also are visible as sm all darkspecks. Most of the rock is light-colored carbona te mud matrix . Porosity and permeabilitymeasured at8434.4 t was 2.5 and 0.01 md, which is typical of lithologic unit3 with averageof 5.1 porosity ( ~ = 1 0 )nd permeabilityof mostly less than0.05 md.

D. 8443.0 t. Exampleof coral-moldic porosity with theco ral reef-core facies of lithologic unit4. Such porosity is common within this facies with average of21.5% (n=10). Permeabilityranges mainly from around20-85 md.

E. 8446.5 ft. Portion of core from lower portion of reef-core facies (lithologic unit4)displaying coral rubble composedof sma ll massive and branching corals. Coral-m oldic porosityis well-developed throughout such material and ranges up to greater than25 , with 10's ofmillidarciesof permeability. The lack of non-calcareous detrital clay seems to have been a majorcontrol on the preservation of permeability within this reef facies.

F. 8452.5 ft. Portion of core from the lowerpart of lithologic unit5 which is a slightly siltyand argillaceous skeletal grainstone/Packstone Rudite composed ofpoorly organized mix ofmostly coral fragments (mainly massive and branching types) and some coralline algal fragments(both encrusting and bran ching types) alongwith carbonate mud-dominated matrix. Porosity/-permeability measurement at core depth8452.2 t was 3.9% and 0.01 md, which is typical forthis unit with average of3.5 porosity (n =5 ) and less than0.20 md permeability.


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FIGURE 7. Selected core photographs from lithologic units 6-13 of the 80-6 Limestonesequence, Serang 3RD2 well.

A. 8459.5 ft. Core slab from portion of lithologic unit 6 showing part of in-situ, large massivecoral which has been extensively recrystallized. Such coral material commonly is recrystallizedand infilled with porosity-occluding, sparry calcite. Partly-open fractures shown here mayprovide some permeability enhancement; however, such features are not common within the 80-6Limestone core from the Serang 3RD2 well.

B, 8466.0 ft. Portion of lithologic unit 7 displaying abundant, irregular, platy corals (upper, half) and finger-like branching coral fragments (lower half) within a very sandy and argillaceous,

carbonate mud matrix. Also present to common within this unit is Halimeda. Porosity andpermeability within t s lithology is poor (avg.=4.0%; n=6; and less than 0.5 md). Thislithology is interpreted to represent the lower, deep-water part of the upper reef within the80-6 Limestone sequence.

C. 8474.5 ft. Portion of lithologic unit 9 displaying abundant, finger-like branching coralfragments within a slightly silty and argillaceous carbonate mud matrix. Porosity/permeabilitywithin this unit averages only 3.2 porosity and less than 0.1 md permeability, owing to theabundance of matrix and extensive recrystallization of coral material.

D, 8482.5 ft. Portion of ithologic unit 10, characterized by abundant platy corals and Halimedawithin microporous carbonate mud matrix. The platy corals are well-displayed subparallel tobedding; some appear to have been slightly pyritized (dark color). Halimeda flakes are thenumerous small, brown specks within the gray carbonate mud matrix. This lithology has beeninterpreted to represent the lower, deep-water portion of the lower reef within the 80-6Limestone of the Serang 3RD2 well. Porosity/permeability is low with only average of 3.7porosity (n= 11 and less than 0.5 md permeability.

E, 8491.5 t. Lower portion of lithologic unit 11 showing numerous, whitish platy corals withina silty, argillaceous and slightly calcareous matrix. This lithology represents the deep-waterinitiation of reef development for the 80-6 Limestone in the Serang Field area.

F. 8492.0 ft. Narrow, extensively-sampled portion of core representing the coarse-clastic Iag-like deposit of lithologic unit 12 and portions of the underlying and overlying units. The pebble,clayey and shelly sandstone contains abundant pyritized benthonic foraminifera and coralfragments, in addition to fine-sand to fine-pebble size quartz and detrital clay matrix. This layermay represent the somewhat irm substrate upon which coral growth initiated. The underlyinglithologic unit (13) is a non-calcareous, pro-deldmarine shelf type sequence.


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FIGURE 8. Selected thin section photomicrographs for lithologic units 1-3 of the 80-6Limestone sequence, Serang 3RD2 well. Units include the uppermost portion of the upper reefand overlying, calcareous marine shale. All depths are core depths.

A. 8416.4 ft, Lith. Unit 1. General, very low-magnification view of fossiliferous, silty shale(mudstone) from a calcareous bed about 7 feet above the upper reef . Skeletal materialdisplayed here include mainly benthonic foraminifera and bivalve fragments. An echinoidfragment is present in the lower left and a small coral fragment is visible at central right edge.Plane-polarized light and stained with Alizarin Red-S and potassium ferricyanide. Visible

\ porosity (blue) includes possible minor matrix microporosity and unnatural (sample damage), fracture porosity.

B. 8419.5 ft, Lith. Unit 1. High-magnification view of silty and slightly carbonaceous shalecomposed mainly of non-calcareous, detrital clay and with quartz silt (white specks) andcarbonaceous matter (brownish black, coalified plant fragments). Also note small, pyrite-filledbenthonic foraminifera at lower right. This shallow marine (pro-delta?) .shale blankets andeffectively seals the 80-6 Limestone reservoir.

C. 8427.4ft, Lith. Unit 2. Very-low magnification view of argillaceous, coral grainstone/pack-stone illustrating recrystallized coral fragments (top and bottom) separated by a layer of non-calcareous detrital clay. Recrystallized coral skeletal walls have been stained reddish withAlizarin Red-S solution to demonstrate their low-iron calcite composition. Corallite voids havebeen filled by a more iron-rich calcite, as indicated by the bluish potassium ferricyanide stain.Porosity (blue) within the non-calcareous layer probably is unnatural owing to sample damage.Measured porosity/permeability 4.1%/0.267 md.

D. 8426.3 ft, Lith. Unit 2. Low-magnification view of sandy and slightly argillaceous, skeletal(bivalve) grainstone from the reworked top of the upper reef . Most skeletal fragmentsillustrated in this non-stained cross-polarized light view are.elongate bivalves. A small, pyrite-filled benthonic foraminifera is present at central right. Quartz silt and fine sand grains arevarious shades of gray. No porosity is visible. Measured porosity/permeability 5.0 /0.021md.

E. 8435.4 ft, Lith. Unit 3. Very low-magnification view of skeletal packstone from thereworked top of the upper reef which is dominated by recrystallized coral fragments. Notebenthonic foraminifera (central upper portion) which have been partly replaced by pyrite (black)and coralline algal fragment (lower right) with minor intraskeletal porosity (blue). The veryfinely-crystalline, brownish-appearing, carbonate mud (microspar) matrix fills completely theinterkkeletal area. Non-stained and plane-polarized light. Measured porosity/permeability2.0 /0.005md.

F. 8429.3 ft, Lith. Unit 3. High-magnification view of skeletal packstone/wackestoneillustrating the finely crystalline nature of the carbonate mud (microspar) matrix which is stainedreddish with Alizarin Red-S solution in this plane-polarized light view. Also note therecrystallized skeletal material (reddish crystals) and coralline algal fragments (upper and lowerleft) with intraskeletal microporosity . Measured porosity/permeability 5.3 /0.004 md.


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FIGURE 10. Selected thin section photomicrographs for lithologic units 6-8 of the 80-6Limestone sequence, Serang 3RD2 well. These units represent the lower low-porosity and low-permeability portion of the upper reef and the upper reworked portion of the lower reef .Sample depth are core depths. .

A. 8458.3 ft, Lith. Unit 6 . Very low-magnification view of slightly-silty, highly-recrystallizedcoral grainstone/boundstone ?) with massive-type corals. Note the recrystallized nature of coralsand extensive corallite-filling carbonate mud matrix withim the upper fragment. Quartz si l t h e -sand is shown by the white specks in this plane-polarized light view of a stained (Alizarin Red-S)sample. Note the lack of macroporosity . Measured porosity/permeability 2.9 %/O.O03 md.

B. 8458.3 ft, Lith. Unit 6 . Very low-magnification view of highly-recrystallized coral material.N o original skeletal structure is visible, except for a thin layer of encrusting coralline algae inthe central lower portion of view. A minor amount of quartz silt/sand (white grains) is visible.Note the lack of visible porosity in this plane-polarized view of a stained (Alizarin Red-S)sample. Measured porosity/permeability 2,9%/0.003 md.

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C 8462.4 ft, Lith. Unit 7. Very low-magnification view of fossiliferous, argillaceous andslightly carbonaceous sandstone illustrating numerous large fragments of platy corals, whichdisplay flat bases and irregular tops. The corals have been recrystallized from aragonite to low-iron calcite (reddish Alizarin Red-S stain). Intercoral material is clayey, very fme-grained,quartzose sandstone which represents a major break in the 80-6 Limestone, separating it into

lower- and upper-reef portions. Note the lack of visible porosity in this plane-polarized light*view. Measured porosity/permeability 4.3 %/0.019 md.

D. 8462.4 ft, Lith. Unit 7. Low-magnification view of fossiliferous, argillaceous and slightlycarbonaceous sandstone. Note the brownish-appearing non-calcareous mud and carbonatemud/cement within the quartzose (white grains), very fme-grained sandstone. A portion of alarge, platy coral is visible at top edge. Also note echinoid spine at left center and elongate,brownish-black coalified plant fragments oriented parallel to stratification throughout thesandstone matrix . Note lack of visible porosity within this plane-polarized light view of anunstained portion of the sample. Measured porosity/permeability 4.3 %/0.019 md.

E. 8468.2 ft, Lith. Unit 8. Very low-magnification view of very sandy and argillaceous skeletal(coral/Halimeda) grainstone representing the upper, reworked portion of the lower reef .Elongate Halimeda flakes with moderate intraskeletal porosity (bluish) are well displayed orientedparallel to stratification. Also note the common platy coral fragments (especially lower left) andscatlered benthonic foraminifera within the very sandy (white grains), argillaceous and partlycalcite-cemented 'Matrix . Also note the lack of visible porosity within this plane-polarized lightview of a stained sample. Measured porosity/permeability 2.6 %/0.018 md.

F. 8471.2 ft, Lith. Unit 8. Low-magnification view of a portion of a recrystallized fragmentof a massive-type coral. The original skeletal structure (stained reddish with Alizarin Red-Ssolution) has been recrystallized from aragonite to calcite. Corallite areas (stained purplish withpotassium ferricyanide) have been filled with iron-bearing calcite. Note the total lack of visibleporosity within this plane-polarized light view. Measured porosity/permeability 1.3 %/0.014 md.


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FIGURE 11. Selected thin section photomicrographs for lithologic units 9-12 of the 80-6Limestone sequence, Serang 3RD2 well. Units represent the lower reef and underlying,marine shelf, reworkedhon-depositional lag deposit on which the reef developed. Sampledepth are core depths.

A. 8474.1 ft, Lith. Unit 9. Very low-magnification view of argillaceous, coral packstone/-wackestone illustrating relatively large, branching coral fragments surrounded by a silty, argilla-ceous matrix; also note the small, elongate, brownish-black fragments of coalified plant material.The reddish-colored (Alizarin Red-S stained) coral fragments have been highly recrystallizedwith corallite areas filled with both finely-crystalline dolomite and sparry calcite. Note the lackof visible porosity within this plane-polarized light view. Measured porosity/permeability5.7%/0.065 md.

B. 8475.2 ft, Lith. Unit 9. Very low-magnification view of argillaceous/sandy and stylolitizedcoral grainstone/packstone illustrating a cluster of branching coral fragments which have beenrecrystallized to low-iron calcite (reddish, Alizarin Red-S stained) and filled with high-ironcalcite (purplish, potassium ferricyanide stained). Sandy (white grains) and argillaceous,carbonate mud matrix is visible between coral fragments. Also note the irregular coral fragmentcontacts which have been slightly stylolitized (marked by dark insoluble residue), and the lackof visible porosity in this plane-polarized light view. Measured porosity/permeability2.9 /0.017 md.

C. 8481.4 ft, Lith. Unit 10. Very low-magnification view of slightly silty, skeletal (cor-al/Halirneda) wackestone illustrating fragments of branching corals (lower left and right center)and elongate Halimeda flakes within a dense, carbonate mud matrix. Fragments, recrystallizedto low-iron calcite have been stained reddish with Alizarin Red-S solution. Note the lack ofvisible porosity within this plane-polarized light view. Measured porosity/permeability2.2%/0.023 md.

D. 8489.2 ft, Lith. Unit 11. Very low-magnification view of a slightly silty and argillaceous,skeletal (Halimeda) wackestone/mudstone showing numerous, elongate Halimeda flakes withina slightly silty (white specks), dense carbonate mud matrix. Note the lack of visible porositywithin this plane-polarized light view of a stained (Alizarin Red-S) sample. Measuredporosity/permeability 2.8 %/0.042 md.

E. 8491.3 ft, Lith. Unit 11. Very low-magnification view of a fossiliferous, silty shale(mudstone) composed mainly of detrital quartz silt (white specks) and non-calcareous clay withcommon brownish-black, coalified plant fragments and scattered, large platy corals. The platycoral.fragment illustrated here displays the typical flat bottom and digitate top; it has beenrecrystallized to calcite (reddish, Alizarin Red-S stain) and partly filled with iron-rich calcite(purplish, potassium femcyanide stain). Apparent visible porosity (blue) probably is due tosample damage (porosity/permeability not measured).

F. 8491.8 ft, Lith. Unit 12. Very low-magnification view of a fossiliferous and calcareous, verypoorly-sorted sandstone, which represents a marine shelf, reworkedlnon-depositional ag-typedeposit/surface, upon which the 80-6 Limestone reefal growth started. A wide variety ofelements are illustrated here; including fine- to medium-grained quartz sand (white), pyritizedbenthonic foraminifera (blackish grains), coral fragments (center), brownish-black codified plantfragments, brownish argillaceous matrix, and minor intergranular porosity (blue). Calcite isstained reddish (Alizarin Red-S) and iron-calcite stained purplish (potassium ferricyanide) in thisplane-polarized lightview.


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;

1 100- 1c- . PERMEABILITY md)

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10 10,

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FIGURE 12. Cross-plot of conventional core analysis helium porosity and horizon&permeability for samples from the coral reef buildup (80-6Limestone) represented by Cores 7-8of the Serang 3RD2 sell.


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L I M E S T O N E

3 O 1

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D I S T R I B U T I O N M A P

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FIGURE 13. Fence diagram illustrating the distribution and wirelinelog character of the 80-6Limestone in the five wellsof Serang Field, Kutei Basin, East Kalimantan.

1serang field

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