Unlocking the Mystery of Zawtika Biogenic Gas Accumulations Offshore Gulf of Moattama Myanmar

Abstract

Abstract For the past decade, biogenic gas production from the offshore Gulf of Moattama has significantly contributed to both the domestic and international gas market in this region. And yet, despite success in exploring for biogenic gas, our understanding of how these generative and trapping systems work, to some extent, remains a mystery. The primary goal of our study has been to unlock the mysteries of biogenic gas generation through a review of available literature and a detailed analysis of the geology and hydrocarbon occurrence in PTTEP's Zawtika Field. Our study began with a detailed literature review in order to develop an understanding of the fundamental mechanisms of biogenic gas generation. We realised at this point that while biogenic gas generation is a common phenomenon; trapping of such gas in commercial quantities is unusual. Through a process of intensive lab analysis of hydrocarbons and sediments, the geochemical properties of hydrocarbon gas and potential biogenic source rocks were determined. 1D burial history modeling applying biogenic gas kinetics was conducted to determine the key parameters contributing to the Zawtika Field success case. Success case analysis outputs integrated with latest literature findings has resulted in formulation of the recipe for biogenic gas generation and accumulation. Depositional setting appears very important in the generation and trapping of significant volumes of biogenic gas. The series of progradational packages deposited within the Ayeyarwady Delta are instrumental in establishing the source-reservoir system hosting such biogenic gas deposits. These sedimentary fluctuations dictate change in lithological character resulting in coarsening upward sediment cycles. Extremely high deposition rates are calculated for sediments deposited through Pliocene to recent times, commonly greater than 1,000m to 2,000m/million years. Such high deposition rates are instrumental in providing the sediment overburden that allows sealing of such biogenic systems. The Zawtika field is situated west of the termination of the Sagaing and Mergui fault systems creating NE-SW trending splays at the end of the Moattama Basin. Within this specific depositional setting, post-oxic depositional environment conditions are required. This environment of deposition is initially oxic where high sulphate content in water in the depositional system inhibits biogenic methane generation. Post burial, reduction of sulphate to few ppm levels provides a suitable environment for biogenic methane generation. Rate of removal of sulphate vs. depositional rate will have a significant impact on depth at which biogenic gas generation occurs and therefore possibility of trapping. Low sediment water interface temperature (SWIT) and geothermal gradient/heat flow regime are required to place the biogenic gas generation window as deep as possible. Such is required so that generation occurs for as long as possible providing time for trap formation and sealing. Subsurface geothermal gradient or heat flow less than 3.5°C/100m and 50mW/m2 respectively is likely necessary. Bio-gas generation window depth range of between 500m to 2,000m is considered typical with generation window lengths around 1 to 1.5 million years. Continuous delivery of humic, land derived organic matter into the depositional setting is essential. However, provision of high amounts of high quality organic matter (High Hydrogen Index (HI)) is not required for the generation process. It appears that methane generating micro-organisms are capable of metabolising organic matter with HI as low as 50, though, HI greater than 100 to 150 appears advantageous. Typical amounts of organic matter in such sediments range from as low as 0.5 to 1%wt. The results of our study have clearly delineated the geological and geochemical parameters that have primary influence on the successful generation and trapping of biogenic gas in the offshore Gulf of Moattama. From this we have been able to formulate a specific biogenic gas risking index that allows us to more effectively explore for biogenic gas in this and similar geological settings. With this knowledge and methodology in mind, we consider that PTTEP is positioned as a leader in ongoing exploration for biogenic gas in S.E. Asian and other biogenic gas prone basins.