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ABSTRACT: Sedimentology, Diagenesis, and Origin of Gas in the Upper Miocene Parigi and Pre-Parigi Carbonate Buildups, Northwest Java Sea, Indonesia

BUKHARI, TAHIR, and FIRMAN YAMAN, Atlantic Richfield Indonesia Inc., Jakarta, Indonesia

The upper Miocene Parigi and pre-Parigi carbonates contain large gas accumulations at shallow depths. They consist of foraminiferal-skeletal wackestone to grainstone, and sparse boundstone. Grains are predominantly amphisteginid and lepidocyclinid foraminifera and fragments of echinoderms, mollusks with sparse rotalids, miliolids, coralline and codiacian algae, and planktonic foraminifera. They were deposited at buildups in moderate to high-energy environments on a shallow-marine carbonate platform during the late Miocene before the deposition of the Cisubuh claystones. These buildups generally trend north-south and are abundant along an east-west belt. They are sparse and lower in relief north of the belt where they tend to form isolated biostromes. Most buildups are structurally con rolled and develop over both compactional and inversional topographic highs.

Diagenesis plays a major role in reservoir development of Parigi and pre-Parigi carbonates. Both intervals have been subjected to marine and fresh-water phreatic diagenetic conditions. Granular glauconite is commonly observed and contributes as must as 20% of the grains in some areas. Dolomitization is common to abundant generating intercrystalline porosity. It is interpreted to be formed during early diagenesis, possibly associated with mixed fresh-marine-water conditions. Later state diagenetic processes occurring at depth include neomorphic recrystallization of micrite, nonfabric selective dissolution of calcite to produce vugs and solution channels, and minor pressure solutions. High porosity (35-40%) and permeability (1-2 darcys) have been observed. Common porosity types includin fine intercrystalline, mouldic and vugular are uniformly distributed throughout the buildup.

The reservoired gas is primarily methane with minor fractions of ethane and carbon dioxide. The isotopic data analyzed to date indicate the gas origin to be a mixture of mostly thermogenic and some biogenic gas. More analyses are currently underway. The gas was primarily sourced in the underlying Oligocene Talang Akar formation, and faults appear to be the main conduits of gas migration from Talang Akar to the shallower reservoirs. Direct hydrocarbon indicators from seismic have been useful in high-grading some prospects.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)