Hydrocarbon charge of a bacterial gas field by prolonged methanogenesis: an example from the East Java Sea, Indonesia

The Terang-Sirasun Field in the East Java Sea of Indonesia contains 1.0 trillion cubic feet (TCF) of dry gas reserves, which are made up of over 99.5% methane with δ13C of −65‰ and δD of −185‰. The methane was formed exclusively by methanogenic bacteria via the CO2 reduction pathway. The primary source sediments for the methane were identified based on bulk geochemical and absolute biomarker concentrations. Specifically, the C25 acyclic isoprenoid 2,6,10,15,19-pentamethyleicosane (PME), and related isoprenyl glyceryl ethers, which are well known markers for methanogenic archaebacteria, were used as indicators for sediment layers thought to have had the most abundant methanogen activity. Burial history analysis and precise biostratigraphic age control provided the framework for assessing the timing of hydrocarbon fill. Our findings show that methanogenic activity was highest in marine shelfal claystones (mid-outer neritic) ranging in age from 8 Ma (Late Miocene) to present. The gas is reservoired in limestones and sands of the Paciran Member, which are dated from 6.5 to 1.3 Ma (Late Miocene-Pliocene). The top seal for the present accumulation was deposited less than 0.5 Ma ago (Quaternary), although there is strong geological evidence that older seals existed. These older seals were catastrophically removed by submarine slumping resulting in the loss of earlier accumulated gas. Methanogenesis from older source beds continued, and the trap was recharged after deposition of the current top seal. The information obtained from this field study provides evidence that large accumulations of bacterial methane do not necessarily require early entrapment of methane from freshly deposited marine sediments. Traps may be filled with bacterial gas long after the deposition of source beds. provided that the required conditions for active methanogenesis are maintained throughout this period.