GEOCHEMICAL IDENTIFICATION OF FAVORABLE BASINS FOR BIOGENIC GAS EXPLORATION IN INDONESIA

摘要

Biogenic gas is commonly formed at shallow depths and low temperatures (less than 75°C) by anaerobic bacterial decomposition of sedimentary organic matter and thus is unrelated to the processes that form oil. Therefore, it is important to distinguish between bacterial and thermogenic gas, as bacterial gas provides no clues to the presence of an effective petroleum system. Biogenic gas is generally very dry (> 95% methane) and is isotopically light (< -55‰). Natural gases occurring in cool, shallow reservoirs are commonly presumed to be a product of biologic processes that have been estimated to contribute over 20% of the global gas resources.In Indonesia, biogenic gas has been documented as a large component of the gas produced from the East Java Basin. Other Indonesia basins have been identified as potential favorable sites for biogenic gas generation which may contain large methane resources. These basins are characterized by young sediments (Plio-Pleistocene) with high sedimentation rates (>50 m/million-years), low temperature during sedimentation and compaction (0-75°C), and total organic carbon content >0.5 wt%).A geochemical and basin modeling study was conducted to document the origin of shallow gases from several basins representing a variety of tectonic settings. These include: thick deltaic depocenters of the Batanghari River in Central Sumatra, and Sesayap River in Tarakan Basin (northeast Kalimantan) and a foreland tectonic basin (Tomori block) in central Sulawesi. The results of this study show that biogenic gas is present in all three areas supporting the earlier hypotheses. However, thermogenic gases were also identified in the Central Sumatra and Tarakan deltaic basins. The presence of these thermogenic gases indicate thatthe origin of some of the biogenic gas may be the result of bacterial degration of migrated thermogenic gas.