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Early Miocene Source Rocks and Petroleum System in Kutai Basin, East Kalimantan, Indonesia

Bachtiar, Andang 1
1 Exploration Think Tank Indonesia, Jakarta, Indonesia.

Kutai Basin in East Kalimantan - Indonesia, is one of the major hydrocarbon producing basins in the world with more than 11 BBOE proven reserves, located in the vicinity of the Mahakam delta. The hydrocarbon producers in this basin are mostly of Middle Miocene to Pliocene ages, with petroleum system indicating mostly Middle Miocene deltaic source-rock. Until today, there has not been any prove that older sections can also significantly produce hydrocarbons, neither their source-rocks contribute significantly to the known petroleum system. This study was to position Early Miocene hydrocarbon source-rock in time and space, document their characters, and reveal their genetic relationships as a part of petroleum system analyses for this age level.

The depositional environment of Early Miocene sediments in the study area ranges from deep marine, shallow marine, transitional(deltaic) up to fluvial with general pattern of highstand progradation. The sediment supply and basin subsidence were unique for Kutai Basin; creating anomalies in the general progradational trend, which marked specific tectonic activity during Early Miocene time. Among others are the development of incised valley fill deposits and deep marine turbidites, both of which are associated with lowstand condition in N4, N5, and lower N7 ages. Early Miocene deltas were generally more wave-dominated in the southern part of the basin, while in the central and northern part they were more fluvial and tidal dominated.

Early Miocene source rocks are generally carbonaceous shales and coals with humic-terrestrial character and having gas and mix-oil-gas proneness. Delta front environment proves to be the best craddle of good source rock, while the deep marine shales are considered potential to generate gas only. The best source rock potential is found in the LST section, while the least potential ones relate to TST. The petroleum system involving Early Miocene sediments in Kutai Basin proves to have more gas compare to oil with acummulation located maximum 25 km from kitchen area. The generation of Early Miocene source rocks started at 23 mya, enhanced at 16 mya and reached maximum at 11 mya, while the inversions creating structural traps started at 16 mya. The majority of structural traps involved Early Miocene reservoirs, which may therefore only be charged by late generated hydrocarbons. Stratigraphic traps, however, can be expected to be charged earlier by the system.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009