SPE-186979-MS

Development of Carbonate Buildups and Reservoir Architecture of MioceneCarbonate Platforms, Central Luconia, Offshore Sarawak, Malaysia

Hammad Tariq Janjuhah and Ahmed Mohammad Ahmed Salim, Universiti Teknologi PETRONAS; MohammadYamin Ali, PETRONAS Research Sdn. Bhd; Deva Prasad Ghosh, Universiti Teknologi PETRONAS; Meor HakifAmir Hassan, University of Malaya

Copyright 2017, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE/IATMI Asia Pacific Oil & Gas Conference and Exhibition held in Jakarta, Indonesia, 17-19 October 2017.

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AbstractCarbonate platforms in the Central Luconia is the most important province in the Sarawak Basin for gasproduction in the region. These carbonates are economically significant containing 65 trillion cubic feetof gas in place with minor contribution of oil rims. Over 200 carbonate buildups have been seismicallymapped, out of which 120 still remain undrilled. These provide potentially attractive exploration targetsand incentives to discover the remaining hydrocarbons in the region. The Central Luconia Province is akey geological unit for understanding the distribution of hydrocarbon resources in Malaysia. Although thefirst hydrocarbon was discovered more than 60 years ago,nevertheless, little effort has been made until nowto address the proper facies scheme, cyclicity and reservoir quality of these Tertiary carbonates of CentralLuconia.

This paper documents the various facies based on the qualitative and quantitative description of coresof five wells cores in three different carbonate platforms with different cyclicity, carbonate buildupsand reservoir quality. Detailed petrographic and petrophysical data have been validated to establish thestandardized facies scheme. The combination of facies with cyclicity is very important to understand thedepositional setting of the sediments and reservoir behavior. Because the sediments which are depositedduring the transgressive phase can act as a barrier for hydrocarbon to migrate vertically.

Eight facies types have been recognized qualitatively and quantitatively (Table-1), namely F-1 coatedgrain packstone, F-2 coral (massive) lime pack-grainstone, F-3 oncolite lime grain dominated packstone,F-4 skeletal lime/dolo packstone, F-5 coral (platy) lime mud dominated packstone, F-6 coral (branching)lime dominated pack-grainstone, F-7 cross bedded skeletal lime packstone, and F-8 bioturbated carbonatemud stone (chalk). These eight facies reflect different depositional environments ranging from lagoon to offreef settings. Overall, cores from the five wells consist of 80% of limestone, 15% of dolomitic limestone and5% of dolomite and argillaceous limestone. The facies consist of eight dominant components comprising,35%of coral, 30% of red and green algae, 30% of skeletal debris with 5% of other bioclasts. Moldic porosityis dominant (up to 45%), with the remaining are interparticle, intraparticle, vuggy and fracture porosity.

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Six regressive cycles are recognized within the carbonates based on the core and well logs. The 0.1 to 50mthick facies is stacked in vertical sequence within the 50-100m thick transgressive and regressive cycles.The reservoir intervals are divided into seven zones, based on porosity, permeability and major diageneticevents, representing high and tight porosity. The best reservoir intervals are consistently found in most of theregressive cycles, which composed of skeletal lime packstone, coarse coral fragments with touching vugs.The systematic semi-quantitative characterization, portrayed the buildup as coarse limestone with cyclicalgrain size variations creating high-permeability layers.

IntroductionGas field development in Central Luconia, offshore Sarawak started in early 1982 (Wee and Liew, 1988).The Central Luconia carbonates have been studied extensively (Figure. 1), in terms of their geology,stratigraphy and reservoir aspects based on a single platform (Epting, 1980; Doust, 1981; Epting, 1989;Vahrenkamp, 1998; Ali and Abolins, 1999; Madon, 1999).

Figure 1—Geological sectors differentiated in the Luconia Province, offshore Sarawak, East Malaysia.Isolated carbonate platforms scattered mostly throughout Central Luconia are highlighted in a darker tone.

In Central Luconia the dominat carbonate production begans during the Cycle IV and V of Miocene age.The carbonate grown is limited onto the southern part of Central Luconia. As west and east it is bordered btthe geological province of the Balingian, the western Sarawak clastic sheld and the Baram Delta. Core andpetrographic observation in our study reveals that the carbonate production in Central Luconia is dominantlycontrolled by red algae, foraminiferam echinoderms, bivalve, corals, green algae, bryozoans, and sponge(Figure.2). Six regressive cycles are observed in Central Luconia and these cycles dominate Cycles III, IVand V. According to (Epting, 1989); Vahrenkamp (1998); Ali and Abolins (1999); Madon (1999); Madon etal. (2013); Koša (2015); Janjuhah et al. (2017c) carbonate deposition in Central Luconia started during theearly Miocene (Cycle III), but the mega-carbonate deposited during the middle to late Miocene (Cycles IVand V) time. The eight facies scheme is subdivided into four sub-depositional settings namely; protected,reefal, shallow marine and deep off marine (Janjuhah et al., 2017a). The dominant presence of skeletaldebris, sea level fluctuation, clastic input from the Baram Delta and the subsidence of these carbonates arethe major processes involved in the architecture of the carbonate buildups and reservoirs.

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Figure 2—Eight dominant compenets observed in Central Luconia, offshore Sarawak, Malaysia.

Facies Analysis and Petrographic ObservationAs evident from the cores, the dominant appearance of the facies (F-2, F-4, F-6, F-8) and regional sea-levelcurve reflect that during Cycle IV, the rate of carbonate production propogated rapidly, resulting in lateralexpansion of carbonate deposition. Epting (1980), refers the lateral expansion of carbonate deposition as abuild out system. Subsequently in the south east, during the late-early middle Miocene time (Cycle IV) thecentral Luconia experienced a major transgression (Epting, 1980; Koša, 2015). This major transgressionreplaced cycle IV and the development of cycle V began. A major portion of cycle V involved the buildupstage of carbonate growth. The rate of carbonate production is balanced with the rate of sea level rises asfacies F-2, F-4 and F-1 are dominant.

The upper part of cycle V is dominated by high energy facies F-3, indicating that the rate of carbonateproduction could not catch up with the rate of sea level rise and the presence of muddy facies F-5 isalso an indication that the carbonate contains clay/mud due to the slow rate of sediment supply. This isan excellent indication of a build in stage. Each cycle reflectsa different depositional environment, whichis later modified by different diagenetic processes. Petrographic observation reveals that micritization,cementation, mechanical compaction, chemical compaction and dissolution are by far the most dominantdiagenetic processes which have affected the reservoir quality (Figure. 3) (Janjuhah et al., 2017b). Thesubaerial exposure during the regression is evident by the leaching of corals in F-2 and the skeletal debrisin F-4. The metastable carbonate minerals stabilized into calcite and dolomite and high leaching by theinflux of fresh water in the protected and reefal environment resulted in the forming of dominant moldicand intraparticle porosity with an average porosity ranging from 12-30% in the shallow marine condition.These processes affected the carbonate deposition during the buildup and build out stages of cycles VI andV. The shallow marine off reef and deep marine off reef sediments (Facies F-5, F-6, F-7, F-8) reflect adeeper marine environment (Figure. 3).

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Figure 3—Depositional model with respect to facies and sea level fluctuation, Central Luconia, offshore Sarawak, Malaysia.

The cementation, mechanical compaction and chemical compaction diagenetic processes cause by theincreasing overburden pressure or tectonic stress which affected the reservoir quality by compacting theloose grains which were transported from the lagoon. Some of the shallow and deep offreef facies have highporosities, which might reflect that, the sediments which were transported from the lagoon or backreef hadundergone a later stage of dissolution that effected the carbonate sediments to form or enhance porosity.

Table 1—Facies Scheme of Central Luconia based on cores from eightwells, offshore Sarawak, Malaysia, adopted from (Janjuhah et al., 2017a)

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Quantitative Anlysis of Thin sections; Example from Well A and Well BAs discussed earlier based on the full data availability of data set from Well A and Well B. A detailedpetrographic qualitative and quantitative study is conducted. The 868.1 ft of total logged core from Well Aand Well B is composed of averagely 90% of limestone with 10% of dolomitic limestone (Figure. 4, 6). Theoverall absence of shale in both wells are clearly reflected by the clean gamma ray signature (Figure. 5, 7).The Well A and Well B drilled on the same platform and the distance between these two well are almost9km. The core of Well A (Figure. 5) and Well B (Figure. 7) is mostly composed of coarse-grained carbonategrains and are generally depleted of mud. The quantitative distribution of thin sections in terms of grains,matrix, cement, visible porosity, skeletal components and porosity types of whole cored interval is statedin Figure. 4 and 5 for Well A and Figure. 6 and 7 for Well B.

Figure 4—Quantitative distribution of thin sections from Well A; A) lithology, B) grain,matrix, cement and visible porosity, C) texture, D) porosity types and E) is components.

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Figure 5—Sedimentological log showing qualitative and quantitativedescription of Well A in Central Luconia, offshore Sarawak, Malaysia.

Figure 6—Quantitative distribution of thin sections from Well B; A) lithology, B) grain,matrix, cement and visible porosity, C) texture, D) porosity types and E) is components.

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Figure 7—Sedimentological log showing qualitative and quantitative description of Well B in CentralLuconia, offshore Sarawak, Malaysia. Tens of feet-scale, high-frequency cycles are the vital genetic units

that shape the reservoirs of the Central Luconia carbonates. These genetic units are the sedimentaryrecord of cycles in the making of accommodation space (Homewood and Eberli, 2000; Lerat et al., 2000).

In Well A, Packstone comprises 50% of the cored rock thickness and grainstone, floatstone and rudstoneconstitutes the remaining 40% (Figure. 5, 4B), whereas in Well B, floatstone covering the most of the coreinterval, packstone is the second dominant texture with 27% followed by rudstone 14%, grainstone 9% andboundstone 5% respectively (Figure. 7, 6B). The plug porosity values of these two wells are ranges from 5to 30% and permeability is from 0.04 to 10 mD in all the observed facies (Figure. 5, 7).

The quantitative observation of thin section revealed that in Well A carbonate grain covers an area of 35%followed by matrix 30%, cement 25%, and visible porosity is 10% (Figure. 5, 4C), in other hand Well Bcomposed of 29% of grains with 33% of matrix, 31% of cement and 7% of visible porosity (Figure. 7, 6C).

Depositional Sequence in Carbonate BuildupsBased on the comparison of carbonate buildups in five wells of Central Luconia, it was possible to interpretthe successions within a regional genetic framework in terms of depositional model (Figure. 3, 8). Thesewells are located in different sectors in Central Luconia.

Well A and Well B intersect a deeper carbonate platform located in the NE of Central Luconia. WhereasWell C, Well D and Well E are located in the SW side of Central Luconia and intersect shallower platform.The distance between these two wells are 52 km (Figure. 8). By considering well locations and the depthof the carbonates, these wells are divided into two platforms. Well A and Well B are in platform 1, whereas

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Well C, Well D and Well E are in platform 2. The correlation of these wells was also validated/correlatedwith the biostratigraphic dataset from one legacy core in Central Luconia. Following the Epting (1989)terminology, these carbonates consist of the following depositional units.

1. Transgressive Cap Phase Cycle V (Build In Phase)2. Main Buildup Phase Cycle V (Buildup Phase)3. Main-Buildout Phase Cycle IV4. Transgressive Basal Phase Cycle IV/III

Figure 8—Correlation of five wells from two different platforms, Central Luconia, offshore Sarawak, Malaysia.

Transgressive Basal Phase Cycle III/IV (Basal Unit)The transgressive basal phase is characterized by dark gray colors, with abundant coral debris and containingsilty carbonate (mud-dominated) with large foraminifera. Within each unit, the transition from one facies(Facies-7) to another (Facies-8) is usually gradual, whereas the unit boundaries are defined more clearly by

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depositional breaks or changes. The distribution and reoccurrence pattern of the facies within and amongstunits suggest that cyclicity changes of probably local extension occurred during the regression phase of thecarbonate deposits.

Main Buildout Phase Cycle IVThe Cycle IV in platform-2 was characterized by sedimentation in a protected back environment, probablyduring a prolong period of relatively stable sea-level (Figures. 3, 8, 9). This buildout phase is composedmainly of massive coralline algae, foraminifera and coral dolomitic lime packstone (Figure. 3). Thedominant facies in this phase are Facies-2, Facies-4, Facies-6 and Facies-8. The top part of the unit is sharpand is tentatively interpreted to reflect a sea-level low stand (Figure. 9). The calcitic dolomites are welldeveloped in the lower part of the section (Figure. 8).

Figure 9—Buildups relative to sea level fluctuation in five wells, Central Luconia, offshore Sarawak, Malaysia.

Epting (1980) also highlighted that in the development of a build-out system, the lagoon becomesenlargedas a result of the migration of reef flat and forereef towards the sea. Based on that, the protectedpart of the carbonate complexes reflects different sedimentary environments. In Miocene buildups, all the

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situations mentioned above confirm that the platform-2 buildups grow during the phase of a carbonatebuildout(Epting, 1980; Epting, 1989) (Figure. 10). Once the growth started on uplifted highs during lateearly Miocene time (Cycle IV), the rate of carbonate production was more than the subsidence, whichresulted in a later expansion of the carbonate production (Epting, 1980; Epting, 1989).

Figure 10—Platform-2 Comparison with Epting (1989) model of carbonatebuildups growth in Central Luconia, offshore Sarawak, Malaysia.

Main Buildup Phase of Cycle VFaces-1, Facies-2, and Faces-4 which are protected, occasionally reefoid depositional environments inplatform-1 (Figure. 3, 8). The abundance of small benthic foraminifera, clean and massive corallinealgae, Amphistegina spp. together with corals, miliolids and echinoid fragments suggests a shallow marineenvironment to a protected environment of deposition. The texture can be of any type varying frompackstone to floatstone with a net predominance of packstone. According to Epting (1980); Epting (1989)in a typical buildup setting, the rock forming organisms kept pace with the rising sea level. Reef debrisis the major source of carbonate sedimentation in the buildup setting which leads the carbonate complexactivity to grow upwards. Two shallowing-upward cycles can be recognized, at intervals 10490 – 10300ftand 10156 – 10022 ft (Figure. 8).

The sequence shows interbeds of Facies-2 and Facies-4 passing upward into Facies-1. The Facies-2intervals consist of deposits rich in corals and encrusting algae. In a shallow marine environment, atransgressive interval is commonly recognized by fossil accumulations produced locally as the transgressionensued (Kidwell, 1989; Cattaneo and Steel, 2003), and ravinement surfaces commonly act as a substratumfor benthic communities and corals (Kidwell, 1989). Both cycles led to the subaerial exposure of largeparts of the buildups, which caused leaching and minor dolomitization. Reservoir properties are better inthe upper part of the cycles and they decrease downward due to the diminishing effect of leaching andincreasing effect of calcitization.

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Transgressive Cap Phase Cycle V (Buildln).This is a heterogeneous unit, comprising various rock types and reflecting the depositional environmentranges from shallow open marine to deeper marine. The interval is characterized by Facies-1, Facies3,Facies-4 and Facies-5. Predominantly, the presence of platy corals in Facies-5, small benthic forams inFacies-4 together with miliolids suggests a deeper open marine to shallow marine environment. The shallowopen marine deposits are characterized by in abundance of algal balls (rhodolites), which are present inFacies-3, and some corals which suggest deposition under medium- to occasionally high-energy conditions,possibly with some currents.

Compared to the underlying unit, the environment has deepened and overall carbonate growth has shiftedfrom a reefal buildup to a rhodolite-dominated carbonate shoal (Facies-3). The unit is interpreted to havebeen deposited during the second third-order, sea-level cycle (Figure. 9). The growth of carbonate buildupsis terminated due to the rapid encroachment of Baram Delta sediments, which has been observed in seismicand wireline logs (Figure. 8).

Seismic evidence suggests, according to the Epting (1980); Epting (1989) model, that carbonate growthin platform-1 occurred during the time of the main build up and build in phases (Figure. 11).

Figure 11—Platform-1 Comparison with Epting (1989) model of carbonatebuildups growth in Central Luconia, offshore Sarawak, Malaysia.

SummaryIn the southern part of Central Luconia, the growth of carbonate was terminated earlier by clastic influx fromBahram delta at the end of Cycle IV. However, towards the northern part, the growth of buildups was activefor a considerable period of time known as Cycle V, but was later replaced by Cycle VI clastic sediments.These carbonates were highly affected by different diagenetic processes which altered the reservoir quality.However, these diagenetic processes vary considerably on a regional scale. Overall the architecture of thecarbonate buildups is determined by four dominant processes as described by Epting (1989). The first

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process is the production rate of carbonate in Central Luconia which is followed by active subsidence,fluctuation of sea-level and the influx of clastic sediments from the Baram Delta. The carbonate buildupmodels of buildup and buildin reflect the accuracy of Epting (1989) model However, the buildout phase hassome uncertainties with respect to our own idea. Epting (1989) model for buildout phase is based on singleplatform. However, there are different types of platforms present in Central Luconia as described by Koša(2015). We need to consider more models when dealing with the buildout phase.

AcknowledgmentThe senior author would like to thank Petroliam Nasional Berhad (PETRONAS) for providing the data, andto University Technology PETRONAS and to Prof. Dr. Deva Prasad Ghosh for their support and economicassistance throughout the research under the grant YUTP 0153AA-A14.

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