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Diagenetic effects on reservoir properties in a carbonate debris deposit: Case study in the Berai limestone, Makassar Straits, Indonesia

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After the drilling of four wells that successfully found gas, an unexpected dry well (NW-1) was encountered on what was thought to be the same trend in the “M” field, Paternoster Platform, Southeast Kalimantan – Indonesia. The “M” field is developed
    DIAGENETIC EFFECTS ON RESERVOIR PROPERTIES IN ACARBONATE DEBRIS DEPOSIT: CASE STUDY IN THE BERAILIMESTONE, “M” FIELD, MAKASSAR STRAIT, INDONESIA Chrisna Asmiati Tanos *Corresponding author email:chrisna_asmiati@yahoo.com  Abstract After the drilling of four wells that successfully found gas, an unexpected dry well (NW-1) was encountered onwhat was thought to be the same trend in the “M” field, Paternoster Platform, Southeast Kalimantan – Indonesia. The “M” field is developed in carbonate slope debris reservoirs with gas accumulations within theOligocene - Early Miocene Berai limestone (primary objective). The dry NW-1 well was a surprise as a previous study indicated that this was a favourable location to drill and the wireline from the well indicated asimilar carbonate host.This study has shown that the Berai limestone has a complex depositional and stratigraphic framework and thatit has experienced a multistage diagenetic and tectonic evolution. In order to reduce the risk in any futureexploration drilling, this integrated study was done, using data from all the existing wells in the “M” Field,including the NW-1 well. This study incorporated all cores and developed a new understanding of the controlson the reservoir development based on the integration of petrography, stable isotope, and wireline log data.The dry NW-1 well was cemented earlier in its burial due to pervasive marine cementation, as evidence by theintegration of thin section and isotopic data. In contrast, another well (M-4), containing a siginificant gasaccumulation, experienced a longer diagenetic history a phase of later deeper burial leaching that significantlyenhanced the reservoir quality of the Berai limestone in this well.This study defines a previously unrecognized non-meteoric srcin for porosity development in down-slopeMiocene carbonates. This creates a “greenfields” opportunity for exploration in similar settings locally andelsewhere in Indonesia and the Southeast Asia. Keywords : Carbonate diagenesis, Carbonate debris, Stable isotope 1.Introduction Carbonate reservoirs hold around 60%of the world’s hydrocarbon reserves andaccount for 40% of total production (Chopra et.al., 2005). Diagenetic overprints in carbonatescan completely change the pore structure andmineralogy. In its more extreme degrees,diagenesis can change mineralogy fromaragonite/calcite to dolomite and completelydissolve srcinal grains to form pores while thesrcinal pore space becomes filled with cementto form a completely inverted porositydistribution compared to the originallydeposited sediment (Eberli et. al., 2003).The “M” Field is situated in thePaternoster Platform, Southeast Kalimantan /Borneo in water depths of around 200 ft (Figure1).The study aims to explain why one of the wells (NW-1) in the Berai carbonate is drycompared to other four gas discovery wellswithin the Berai carbonate reservoir that is the“M” Field.This study has shown that the Berailimestone has a complex depositional andstratigraphic framework and that it hasexperienced a multistage diagenetic andtectonic evolution. Therefore, the specificobjective for this study is to look for, if any, arelationship between diagenesis with thereservoir quality. 2.Methodology The study was based on the digitaldataset supplied by Pearl Energy and my ownstudy, logging and sampling of cores. A suite of conventional wireline logs was available for allwells. Stratigraphic units were analyzed petrographically and geochemically using core  material collected by me from the M-4 and the NW-1 well that includes the best recoveryinterval in of the Berai carbonate. A total of 102ft of conventional core from 3 wells, M-3, M-4and NW-1 wells, in the “M” Field wereredescribed. Core plug measurement data,which was only available for M-4 and NW-1wells, was used to know the porosity and permeability value.Detailed thin section petrography provided the backbone of this study and, todate, 44 sections have been examined by me.These were impregnated with a hightemperature blue-dye impregnated epoxy andstained with a standard alizarin-red S and potassium ferricyanide solution for carbonatemineral identification. All thin sections wereobserved in transmitted light opticalmicroscopy. The main cementing phases and porosity types were described, and their abundance was visually estimated; thedissolution events were detected and thediagenetic sequence was reconstructed.Stable carbon (δ13C) and oxygen(δ18O) isotopes were measured on 86 selectedcarbonate samples from the conventional corefrom three wells (M-3, M-4 and NW-1 wells).Stable isotope analyses were carried out onthree sets of samples: carbonate matrix,limestone lithoclasts, and a calcite filled veinsthat had been handpicked using a dentist’s drilland extracted as rock powder from theconventional cores. The isotope analysis wasrun using the facilities of Monash University,Melbourne. The stable carbon and oxygenisotope result use a ‰Vienna Pee DeeBelemnite (VPDB) scale.Coplen, 1988 published the standard conversion equationwhich is δ 18 O (VSMOW) = 1.03091 * δ 18 O(VPDB) + 30.91 which is used to convert thestable oxygen result from‰ VSMOWto‰ VPDB. 3.Results 3.1.Wireline log character The gamma ray log response shows thecarbonate section is present in all 5 wells, but,the character of the gamma ray log clearlydiffers in M-2 well compared to nearby wells tothe southeast and the northwest.The Berai carbonate section within theM-2 well has a different association of lithology which is more an intercalation of carbonate with shale whilst the other 4 wellsshow a blocky gamma ray pattern, implying amuch more consistent, cleaner limestone of about 5 - 30 API (figure 2).3.2.Conventional coresFrom the conventional cores combinedwith petrography observation, five lithofaciesgroup was able to be identified and classified in both M-4 and NW-1 wells. The differentlithofacies association between the 2 wells isthere a more reefal association lithofacieswithin the NW-1 well, which clearly differsfrom the other 2 cored wells (M-3 and M-4).The M-3 and M-4 well consists of more mudand benthic forams with some placktonicforams than the NW-1 well.Petrographic observation also helped toidentify a different diagenetic cementassociation between the M-4 and NW-1 wells.The NW-1 well has a pervasive marine cementas shown in figure 3, which clearly differs fromthe M-4 well.Stable carbon and oxygen isotope resultconfirms the thin section observation, whichindicates the longer diagenetic overprints present in the M-4 well whilst the NW-1 wellshows a shorter time (figure 4).Spectral gamma log within the coreinterval was availabe for only the M-4 well.From the analysis, the gamma log signature hasa poor correlation with the core-derivedlithofacies. Therefore, the lithofacies derivedfrom the core was not able to be applied to allthe non-cored interval for all wells.Likewise, the neutron and density logcharacter illustrates the same poor correlationwhile trying to ditribute the core-derivedlithofacies to the other well that do not havecore. 4.Discussion The evidence preserved of an early nowreplaced widespread aragonite cement type isabundant bladed calcite cement as shown infigure 3. It is abundant in the NW-1 well, lessso in the M-4. Within the M-4 well a longer,and probably deeper set of ongoing burialdiagenetic overprint events took place (asevidence by the isotopic signatures), comparedto the shorter and probably shallower marine-  cement dominated diagenetic span preserved inthe NW-1 well (figure 4).Unlike NW-1, these marine cementtextures are relatively rare in M-4, implying adifferent early burial setting; perhaps indicatingthe sediments in M-4 were deposited in adeeper water setting, less subject to the early pore fluid cross flows that drive early marinecementation.The diagenetic process which droveeconomic levels of porosity in the relatively-tight gas play reservoir in M-4 was deep burialleaching. It began in the lower parts of theshallow burial realm and continued well intothe deep burial realm.The different srcins of the clast biota between these two wells may imply that their clasts were transported from geographicallyseparate platforms. A summary of this newinterpretation of the depositional environmentfor the “M” Field region is shown in figure 5.Table 1 and 2 are a summary of theMiocene carbonate characteristics within theregion (Indonesia and throughout SoutheastAsia) compared with those in the study area.These two tables indicates there is a new playopportunity for future exploration target withina debris flow set carbonate deposits.This deeper water setting for thedeposition of the host reservoir is so far uniquein Miocene carbonate reservoirs in Indonesiaand Southeast Asia and so, as mentionedearlier, defines a new play opportunity inIndonesia and throughout the SE Asian region. 5.Conclusion The diagenetic overprint in the Beraicarbonate section occurred across a shorter timeframe within the NW-1 well than within the M-4 well. The early occlusion of porosity in NW-1explains this di-fference, while the longer anddeeper burial diagenetic overprint in the M-4well explains why gas is present in this well butnot the NW-1 well. The NW-1 well issignificantly different in terms of the lithofaciesand diagenetic overprint phases especially itsnarrower burial diagenetic band (as seen in itsisotopic character). Its shallower depositionalsetting (compared to M4) explains its low porosity and permeability character. Porosityand permeability were lost in early diagenesisin this well due to pervasive marinecementation, porosity was never regained inthis well during its deeper burial.In contrast the M4 well was depositedas rudstones in a muddier deeper setting than NW-1 and probably was sourced from amuddier deeper platform compared to NW-1. Itnever experienced the pervasive early marinecementation seen in NW-1. It still retainedsufficient remnant permeability on entering thedeeper burial environment to allow later dissolution and the enhancement of reservoir quality in its Oligocene - Miocene Berailimestone section.This new model of a non-meteoricsrcin for porosity development in down-slopeMiocene carbonates creates a “greenfields”opportunity for exploration in similar settingslocally and elsewhere in Indonesia. By knowingthat there are several other producing fields inSoutheast Asia, where the better reservoir intervals are a response to deep burial leaching,strengthens the likelihood of undiscoveredhydrocarbons across the region. What is notknown from the other regions of such burial-diagenetic related platform production is thatthe same set of deep burial processes can createreservoir quality in appropriate traps in deeper water carbonate debris aprons located somedistance away from crestal positions (thetypical seismically defined targets) in theMiocene carbonate platform of SE Asia. 6.Recommendation 1)Future core and core plug work shouldutilise stable isotope analyses as astandard tool in order to better define acomplete diagenetic history within thecarbonate section. 2) The integration of core, stable isotope, petrographic, and wireline logs offers a better method of reservoir understanding and prediction. Even so, it would be even better toinclude the seismic data and so generate a morecomprehensive regional result. 7.Acknowledgement The writer would like to thank her supervisor, Dr. John K. Warren, for all thesupports and commentsin the preparation of the manuscript.  She would also like to thank PearlEnergy for the use of their data. Many thanks toAngus Ferguson and Julie Kupecz of PearlEnergy for all the support and assistance. 8.References Coplen, T. B., 1988, Normalization of oxygenand hydrogen isotope data: ChemicalGeology - Isotope Geoscience section,v. 72, p. 293-297.Chopra, S., N. Chemingui, and R. D. Miller,2005, An introduction to this specialsection carbonates: The Leading Edge, v. 24, p.488-489.Eberli, G. P., F. S. Anselmetti, C. Betzler, J. H.Van Konijnenburg, and D. Bernoulli,2004, Chapter 10: Carbonate platformto basin transitions on seismic data andin outcrops: Great Bahama Bank andthe Maiella Platform margin, Italy, inG. P. Eberli, J. L. Masaferro, and J. F.Sarg, eds., Seismic imaging of carbonate reservoirs and systems,American Association of PetroleumGeologists Memoir 81, p. 207– 250. Fournier, F., and J. Borgomano, 2007,Geological significance of seismicreflections and imaging of thereservoir architecture in theMalampaya gas field (Philippines):American Association of PetroleumGeologists Bulletin, v. 91, p. 235-258.  Nelson, C. S., and A. M. Smith, 1996,Stable oxygen and carbon isotopecompositional fields for skeletal anddiagenetic components in New ZealandCenozoic non tropical carbonatesediments and limestones: a synthesisand review: New Zealand Journal of Geology and Geophysics, v. 39, p. 93-107.Park, R. K., A. Matter, and P. C. Tonkin, 1995,Porosity evolution in the Batu Rajacarbonates of the Sunda Basin -Windows of opportunity: Proceedingsof the Indonesian PetroleumAssociation - Twenty Fourth AnnualConvention, October 1995, p. 211-235.Pireno, G. E., C. Cook, D. Yuliong, and S.Lestari, 2009, Berai carbonate debrisflow as reservoir in the Ruby Field,Sebuku Block, Makassar Straits: A newexploration play in Indonesia: Pro-ceedings of the Indonesian PetroleumAssociation - 33 rd Annual Convention,October 2009, 19 pp.Pireno, G. E., and D. N. Darussalam, 2010,Petroleum system overview of theSebuku Block and the surroundingarea: Potential as a new oil and gas province in South Makassar Basin,Makassar Straits: Proceedings of theIndonesian Petroleum Association - 34 th Annual Convention, October 2010, 16 pp.   Figure 1. Location of the study area; Inset shows the relative position of the “M” field within thissimplified regional geological map .  
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