Biogenic gas has become an economic target of exploration and exploitation, due to the high demand for gas. Its geological occurrence is easily interpreted; it is significantly widespread and shallow; gas is good quality with >97% content of CH4, low S and CO2 content. Production tests from this block resulted in a production rate peak of 5 MMscfd at Sagu-1well. This article presents a summary of geophysically,geologically, geochemically aspects in order to assess biogenic gas accumulation in Karangringin Block.
Seismically, existing data clearly exhibits strong amplitude anomalies or a ―bright spot as a Direct Hydrocarbon Indicator. Furthermore, advanced geophysical analysis, AVO, seismic attribute and AI methods, were carried out to confirm gas presence. The result of this analysis has been helpful to distinguish between coal and gas-bearing reservoirs, where coal revealed a similar appearance in the seismic data. Seismic data were also important in delineating lateral gas distribution and exploring prospects and leads in Karangringin Area.
The stable carbon isotopic composition analysis of the gas samples has shown δ13C values between -60.36 and -60.39‰ for methane while C2 of between -32.28 and -35.13‰ suggesting that gases were predominantly originated from bacterial degradation of the organic matter and have only little association with thermogenic origin. The main gas-bearing reservoir is a 7-30 foot thick sand layer over the upper Late Miocene Muara Enim Formation, at a depth of 935-1310 feet below sea level. The Muara Enim Formation was deposited in a litoral environment that reflects the onset of marine regression. The gas is trapped along a NW-SE anticlinal system, related to a thrust fault.
Biogenic gas characteristically occurs at a shallow depth and in high quality, which makes this gas economically attractive for production. Karangringin area, as one of the proven and potential biogenic gas targets, provides a typical setting for integration of geological, geochemical and also geophysical features to assess gas accumulation.
Authors : Sugiri Umar Khiram, Alexis Badai Samudra, Agung Budianto