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Geomechanics and Geological Characteristic of Unconventional Plays in the South Sumatra Basin, Indonesia

Abstract

South Sumatra Basin is one of the most prolific back-arc basins in Indonesia. Today, the basin is considered as a mature basin where billions barrels of hydrocarbon have been produced since early 1930. Exploration activity targeting conventional hydrocarbon play has been significantly decreased due to the lack discovery of large economical target. Recently, Indonesian state oil company (PERTAMINA) working together with universities for conducting research to explore possible developing unconventional hydrocarbon plays in Indonesia. This paper is presented the result of regional assessment of unconventional hydrocarbon plays potential mainly shale gas and tight sand using integrated surface and subsurface data. The results of this study summarized that the main source rocks for hydrocarbon in the basin are mainly came from Eocene-Oligocene lacustrine to marine shale of Talang Akar and Lemat Formations. They are excellent source rocks with average TOC (1-3%) characterized by Type 1 and Type 1/III Kerogen. Most of the source rocks was deposited within the half-graben system, which shows three different orientations: NW-SE, N-S and NE-SW. These source rocks are reached at maturity of the oil window (1.0% Ro for Type I) at ~8,000 ft, peak generation (1.0% Ro) at ~10,500ft, and bottom of oil window (1.35%Ro) at~ 13,500 ft. Gas likely derived from high maturity of Talang Akar Fm. XRD data from samples of Talang Akar Shale shows dominant quartz ~(50% -60%) with total clay contents of ~(30-40%) consisting of Kaolinite, Chlorite, Illite and small amount of Smectite (<2%). This data suggested Brittleness Index (BI) of 0.4-0.6, which suggest moderately strong rocks. In-situ stress evaluation indicate strike-slip stress condition at depth where SHmax>Sv>Shmin. Petrophysical evaluation of several wells indicated the occurrence of amalgamated tight sand, which observes mostly in center of the basin. This evidence suggested the possibility to develop tight sand reservoir target. The uncertainty remains in term of performing hydraulic fractures due to the high clay contents. In addition, most of source rock data suggested dominated of Type I, which is oil prone. Therefore, to develop shale gas target, deep drilling operation need to be performed, which is challenging due to high temperature and high-pressure condition in some location. Overall, the study concluded high potential of unconventional resources could be recovered from the South Sumatra Basin in the future.