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  PROCEEDING SIMPOSIUM NASIONAL IATMI 2001 Yogyakarta, 3-5 Oktober 2001IATMI 2001-06 APPLICATION OF MICROSCOPIC ANALYSIS IN RESERVOIR CHARACTERIZATION: SECONDARY MINERALS AND THEIR INFLUENCED ONCLASTIC RESERVOIR ROCK IN SOME AREAS IN WESTERN INDONESIA Junita Musu & Hadi Prasetyo Sedimentology Lab., Exploration Division, PPPTMGB “LEMIGAS Key words: Petrographic analysis, reservoir characterization, authigenic minerals, reservoir quality, reservoir damage,migration of fines, acid sensitivity, clay swelling. ABSTRACT    An integrated petrographic analysis i.e. thin section, x-ray diffraction (XRD) and scanning electron microscopy (SEM) / energy dispersive x-ray (EDAX) is one of various analyses available, which is relatively cheap and gives quick and accurate resultsin supporting reservoir characterization study. This analysis has documented studied sandstones (mostly taken from North / Central Sumatra, northwest / northeast Java, Barito and Kutai Basins) contained micro porosity and various authigenic minerals such ascements as well as grain alteration or grain replacement which significantly effect on well log characters, reservoir quality, well completion and production. Authigenic phases include authigenic clays i.e. kaolinite, illite, chlorite, and mixed-layer illite /  smectite, and carbonate minerals i.e. calcite, siderite, Fe-dolomite and pyrite. Accordingly, the sandstones evaluated may have three main engineering problems i.e.: migration of fines (kaolinite), acid  sensitivity (calcite, Fe-dolomite, pyrite, chlorite and illite) and clay swelling (mixed-layer illite /smectite). INTRODUCTION Many studies of petrographic analysis have produced adetailed understanding of sandstone diagenesis. The mainobjective of sandstone diagenesis study is not only for  predicting reservoir quality but also for better understandingof certain diagenetic (secondary/authigenic) minerals whichhave greatly effect on the formation evaluation and reservoir damage if improper type of fluid was used during wellcompletion and or production (Pittman, 1982; Schaible et.al,1986; Davies, 1988)The following analytical techniques i.e. thin section petrography, Scanning Electron Microscopy, equipped withEDAX and X-Ray Diffraction are of particular importance for quantifying and determining the development of secondaryminerals within sandstones pore system.The objective of this paper is to illustrate some diagenetic processes resulting in a variety of secondary minerals that issusceptible to formation evaluation and formation damage.The samples that are being used for case studies have beentaken from sandstone reservoirs in some areas containing prolific hydrocarbon in western Indonesia. SANDSTONE DIAGENESIS AND SECONDARYMINERALS Diagenesis is an all-encompassing term for every physicaland chemical change that may affect sediment from themoment it is deposited until the onset of metamorphism. Thisincludes processes of compaction, cementation, dissolution,recrystallisation and replacement. Many workers invoked thatthe main factors controlling the diagenesis are generallytemperature, pressure, detritus composition, pore water,sedimentary facies, tectonic and time. Consequently, variousdistinctive diagenetic processes and their products would beexpected from one area to another.Detailed discussion will be documented by considering somediagenesis studies done by authors and their colleagues onMiocene and Pleistocene sandstone reservoirs in some areasin Western Indonesia ( Figure- 01 ).In a case of Central Sumatra Basin, sixty-four (64) samplesfrom selected Miocene sandstone intervals in CentralSumatra Basin have been prepared for petrographic, XRDand SEM analyses. These sandstones may represent fluvialdeltaic to marginal marine deposits. All sandstones are predominantly composed of monocrystalline quartz withsubordinate feldspars, rock fragments, mica and glauconite.According to quartz, feldspar and rock fragments ratio, mostsandstones can be classified as sublitharenites thoughsubarkoses and litharenites is also contributed.The diagenetic processes affected the sandstones arecompaction, cementation and dissolution of feldspar and theformation of authigenic clays. The most common cement inthe sandstones analyzed is quartz overgrowths ranging from2% to 7%. Other cement such as calcite and siderite / Fe-dolomite can be locally found significant in amount, reachingup to 35% and 22% respectively. The dominant authigenicclays are kaolinite (2-23%) and illite (6-11%) with occasionalsmectite (trace - 8%) that is locally identified in severallocations.Study of diagenesis in Northwest Java Basin using integrated petrographic, XRD and SEM examinations were carried outon fifty-four (54) selected sandstones and presented asfollows. The sandstones, based on core description resultsmay indicate as shallow marine deposits.In general, the framework grains are dominantlymonocrystalline quartz (more than 50%), glauconite (5-13%),rock fragments (8-13%) and feldspar (9-12%). Minor grainsinclude mica (2%) and fossil fragments (less than 1%). Manyof feldspar grains have been dissolved creating secondary porosity and kaolinite. Detrital matrix contents are relatively  Application of Microscopic Analysis in reservoir Characterization : Secondary minerals Junita Musu, Hadi Prasetyo and their influenced on clastic reservoir rock in some areas in western Indonesia  IATMI 2001-06 low, approximately less than 10% of bulk rock volume. Allsandstones can be classified as lithic-arkoses and feldspathiclitharenites.The most significant diagenetic cements observed in thesandstones analyzed include (in order of significance) pore-filling calcite, pore-lining chlorite and illite, pore-fillingkaolinite and quartz overgrowths. Pyrite, siderite, dolomite,mixed-layer illite/smectite are only found as minor cements.In some samples, XRD data indicate high amounts of calcite(20-40% by weight percent). This calcite is resulted from principally neomorphism process of many fossil fragments.Chlorite is found in all sandstones (3-14%), coating most of detrital grains. Formation of illite and mixed-layer illite/smectite is a product of early stage diagenesis, in-    situ alteration of glauconite shortly after deposition or a direct precipitation of formation water. Kaolinite formation rangingfrom 1% to 8%, formed as pseudo hexagonal plates and isinterpreted as srcinating from ions released into solution byfeldspar dissolution.In Kutei Basin, sixty-six (66) Miocene sandstone samples are prepared for integrated Petrographic study. On the basis of framework grains composition, chiefly quartz, feldspar androck fragments the sandstones can be mostly classified assublitharenites.The sandstones are dominantly composed of quartz (60-70%)with small amounts of rock fragments (3-6%), feldspar (2-3%) and organic matter (1-7%) and other accessoriesminerals such as mica, glauconite, heavy minerals and fossilfragments.The framework grains are lying within matrix, considered astiny dispersed grains less than 20 microns in size, rangingfrom 4% to 12%.The cementing agents are clays, consisting of grain-coatingchlorite and illite, pore-filling kaolinite, quartz overgrowths, pyrite and carbonates identified as calcite and siderite.Among clays, based on XRD and SEM analyses, kaolinite isthe most significant cement in sandstones compared tochlorite and illite. Pyrite is one of the most common cement precipitated in the sandstones with the proportion rangingfrom 3% to 6%. Siderite and calcite cements occur significantly in several sandstone intervals as pore-fillingcements, while quartz overgrowths are of lesser importance.Fifty-five (55) core chip samples from the Barito Basin, areevaluated using integrated thin-section petrographic, SEMand XRD examinations. According to core descriptionresults, the sandstones envisaged as fluvial deposits.In general, the sandstones are mainly composed of 24%-57%monocrystalline quartz and volcanic rock fragment (13%-37%) with additional feldspar (2%-12%) and small proportion of detrital clay matrix. At the upper part of interval, volcanic rock fragment content is rare. Allsandstones can be classified as feldspathic litharenite,sublitharenite and subarkose. Many of feldspar grains have been dissolved producing both secondary porosity andkaolinite.The main diagenetic cements that have documented from thesandstones analyzed (in order of significance) i.e.: pore bridging / lining smectite, pore filling kaolinite / calcite /zeolite and pore lining chlorite. XRD analysis reveals that thesandstones contain 4%-16% smectite, 2%-19% kaolinite,2%-15% zeolite and 1%-6% chlorite. POTENTIAL RESERVOIR PROBLEMS ANDTENTATIVELY SOLUTION OR MINIMIZINGTHE PROBLEMS Diagenetic minerals, mainly authigenic clays and carbonateminerals are particularly sensitive to reservoir quality.Poupon et.al. (1970), Pittman (1977) and Munson (1988)explained that mode of occurrences of authigenic pore claysignificantly controlled permeability rather than porosityvalues ( Figure-02 ). Introduction of drilling, stimulation andrecovery fluids can have a profound effect on these diageneticminerals and resulting either enhanced reservoir or reducedreservoir quality.For typical sandstones analyzed in this paper, it appears thatdiagenetic modification on sandstones results in variableamounts, mode of occurrence and types of authigenic claysand carbonate minerals that lined and or filled the pores. Dueto the presence of both authigenic minerals (with in order of significance summarized in Table-01 ) the sandstones mayhave three main engineering problems i.e.: migration of fines(kaolinite), acid sensitivity (calcite, Fe-dolomite, pyrite,chlorite and illite) and clay swelling (mixed-layer illite/smectite and smectite).All of the sandstones presented in this paper containsignificant amounts of kaolinite. Kaolinite occurs ashexagonal plates that are generally loosely bound to porewalls ( Figure-03) . Kaolinite clay crystals are easilydislodged and migrated under conditions of high fluidturbulence and if contact with fresh water based fluids or high pH fluids (higher than 10). The migration of fines problem related to high fluid turbulence results from highwell bore/formation pressure differentials during perforatingor production may be minimized using low under balanceddifferential pressures (no exceed than 2000 psi). The use of KCL, CaCL2 and NaCL as completion fluid candidates andthe pH of drilling mud below 10 are recommended ininhibiting the migration of fines due to contact with freshwater based fluids and with high pH fluids respectively.The sandstone reservoirs containing calcite supplementedwith iron carbonate (siderite, Figure-04 ; and Fe-dolomite),iron clay (chlorite, Figure-05 ; and illite, Figure-06 ) and pyrite ( Figure-07 ) are susceptible to damage caused byincorrect acidization, in case of iron-hydroxide precipitation.A routine use of mud clean-up acid (15% HCL or 12% HCL plus 3%HF) should probably be avoided in the reservoirs.The use of 15% HCL will liberate iron from the commonsiderite, Fe-dolomite, chlorite, illite and pyrite and thisresults in reprecipitation of ferric hydroxide occurring as a brown gelatinous mass which will occlude both pore spacesand throats. Hydrofluoric acid should not be used in theformation unless care has been taken to remove some of thecalcium prior acidization with HCL. Weak HCL (less than  Application of Microscopic Analysis in reservoir Characterization : Secondary minerals Junita Musu, Hadi Prasetyo and their influenced on clastic reservoir rock in some areas in western Indonesia  IATMI 2001-06 7%), together with appropriate iron and calcium chelatingagents, will prevent formation damage results from incorrectacidization.The sandstones from the Central Sumatra and the NorthwestJava Basins comprise small amounts of mixed-layer illite/smectite and smectite, except for Pleistocene sandstonein Northeast Java and Barito Basins containing smectite andzeolite in significant quantities ( Figure-09 ). They occur as athin, pore lining material and as structural shale fragmentsthat replaced feldspar. If the reservoirs containing mixed-layer illite/smectite requires fracing or flushing, it will berecommended to use of a 3% KCL solution (or oil-basedsolutions) to avoid any potential swelling problems caused bythe presence of smectite clay. SECONDARY MINERALS INFLUENCE ON LOGRESPONSE The combined thin section petrographic, SEM and XRDanalysis exhibit mineralogical compositional variation in thevertical sequence of the studied sandstones. These verticalchanges in mineralogy have certain influence on log response. 1) Gamma Ray Log  The radioactive elements that have been detected by thesetools are mainly K-40 in both K-feldspar and illitic clay. Theanalyses show that the studied sandstones containing of significant proportion of K-feldspar and illitic clay.Therefore, the gamma-ray reading will be increased.Consequently, determination of shale volume for the studiedsandstones directly from GR. log is difficult or will be over-estimated. 2) Density and Resistivity Logs The presence of iron bearing and conductive minerals (suchas siderite, pyrite and chlorite) will be influenced on bothdensity and resistivity logs ( Figure-10 ). The density valuewill be increased, consequently, calculated porosity from loganalysis is under-estimated. On the other hand, the resistivitylog will be decreased, therefore, determined water-saturationwill be too large. CONCLUSIONS An integrated thin-section petrographic, SEM and XRDanalysis provides useful tool for supporting reservoir characterization study.Study of sandstones diagenesis in some areas in WesternIndonesia with references sandstone samples from the North /Central Sumatra, Northwest / Northeast Java and KuteiBasins indicates various distinctive diagenetic processes andtheir products. The products of cementation process, chieflyauthigenic clays, carbonate minerals and also pyrite playmain role not only to the reduction of porosity and permeability of sandstone reservoirs but also to give effect onthe log response, reservoir sensitivity and reservoir damage.According to authigenic minerals present the sandstonesmay have three main engineering problems as follows: 1)    Migration of fines  due to the presence of kaolinite. Thefines migration problem appears to have seriouslyaffected the sandstones in all basins studied. 2)    Acid sensitivity  due to the commonly presence of calcite,Fe-dolomite, pyrite, chlorite and illite.The result of an integrated thin section petrographic,XRD and SEM analysis illustrate that acid sensitivity problem of: The sandstones in North Sumatra basin will be affected by a combination of chlorite and Fe-dolomite; the sandstones in Central Sumatra Basin iscaused by a combination of illite, calcite and siderite;the sandstones in Northwest / northeast Java Basin isdue to the presence of illite, chlorite, illite/smectite,heavy minerals and calcite, and the sandstones in KuteiBasin is greatly affected by calcite, siderite and pyrite. 3)   Clay swelling   caused by the presence of mixed-layer illite/smectite. This clay-swelling problem, though it isof lesser importance, would be present in the sandstonesin local area in the Central Sumatra, and Northwest JavaBasins, and especially in Barito and Northeast Java. ACKNOWLEDGEMENTS We would like to thank our colleagues for supporting thisstudy. SELECTED REFERENCES Almon, W.R. & Davies, D.K., 1981. Formation damage andthe crystal chemistry of clays, Clays and the ResourceGeologist, Mineralogical Association of Canada, ShortCourse, F.J.Longstaffe ed., Calgary, p.81-103.Crowe, C.W., 1985. Evaluation of agents for preventing precipitation of Ferric hydroxide from Spent Treating Acids,  JPT (April),  p.691-695.Krueger, R.F., 1986. An Overview of Formation damage andwell productivity in oil-field operations,  JPT (February),  p.131-152.Davies, D.K., 1988. Sandstone Reservoirs, with emphasis onClays, Stimulation and Formation evaluation, a three dayscourse.Pittman, E.D., 1982.   Problems Related to Clay Minerals inReservoir Sandstones. In Oil Field Development Techniques,  AAPG Memoir No.28,  p. 237 - 244 . Prasetyo, H., and Wicaksono, B., 1996.   The RelationshipBetween Diagenetic Minerals and Formation Damage of Clastic Reservoirs in Some Areas in Indonesia. One daySeminar and Workshop on Formation Damage Paper, LEMIGAS, Jakarta.Prasetyo, H., 1997. Applied Microscopic Analysis for Petroleum Industry.  Makalah Ceramah Ilmiah dan Diskusi, Jurusan Geologi, Fakultas Teknologi Mineral, UPN“Veteran” Yogyakarta.  Application of Microscopic Analysis in reservoir Characterization : Secondary minerals Junita Musu, Hadi Prasetyo and their influenced on clastic reservoir rock in some areas in western Indonesia  IATMI 2001-06 Schaible, D.F., Akpan, B., and Ayoub, J.A., 1986.Identification, Evaluation and Treatment of FormationDamage, Offshore Louisiana. SPE Paper No.14820. Poupon, A., et.al., 1970. Log Analysis of Sand-ShaleSequences – A Systematic Approach,  JPT,  p.867-881.PPPTMGB “LEMIGAS” & THE BRITISH GEOLOGICALSURVEY, 1993. The North Sumatra Basin: Hydrocarbon potential of the Pertamina UEP-I area,   Vol.1 & 2, unpublished report, Lemigas. SAMPLE LOCATIONSKaoliniteIlliteIllite /SmectiteChloriteCalciteFe -SideritePyriteZeolites SmectiteDolomiteN.Sumatra Basin XXXXXXX C.Sumatra Basin XXXXXXXXXX N.E Java Basin XXXXXX N.W Java Basin XXXXXXXXX Kutei Basin XXXXXXXXXXX Barito Basin XXXXXXXXN o t e :XX = significantX = less-significant C L A Y SC A R B O N A T E SO T H E R S SECONDARY MINERAL TYPES IN THE STUDIED AREAS Table-1 Secondary Mineral Types in Studied Area Figure-1 Studied Area Location Map Related To Indonesia Basins
2001 TEE Paper

2001 TEE Paper

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