Assessment of Controlling Factors in Economic Biogenic Gas Accumulation in Late Pliocene Deep Water Turbidite, Offshore Northwestern Myanmar

Abstract

Generation of biogenic methane is ubiquitously observable at shallow-burial strata where conditions are suitable for the metabolism of methane-producing microorganisms, while distributions of economical amount of biogenic gas accumulation are limited. This shows that the timing and depth of generation of biogenic gas is not only the issue for understanding mechanisms of economic biogenic gas accumulation, but also migration and trap/seal mechanism is need to be considered as closely coupled process. Economically recoverable accumulations of biogenic methane were discovered in deep water turbidite reservoir in the late Pliocene, offshore northwestern Myanmar. The objective of this study is to understand controlling factors and mechanisms of economic biogenic gas accumulation in deep water reservoirs through detailed analysis of literature, laboratory data and petroleum system modeling using PetroMod 1D/2D. Through a process of this study, source rock quality and geochemical environments are important controlling factor for methane generation. Pliocene-Pleistocene age sediment shows predominantly land derived organic matter (type III) and sulfate-deficient pore water condition that micro-organisms are capable of methane generation. The formation of effective stratigraphic trap and seal in early phase provides significant contribution to trap biogenic gas which is generated early during the burial history. Deep marine shale properties and overburden thickness by high sedimentation rates allow sufficient seal capacity. Low relief reservoir geometry in early depositional phase and late growth of structural tilting are considered as the effective accumulation mechanism, because reservoir tilting by structural movement causes increase of gas column height and higher buoyancy pressure can fail the top seal with low seal capacity in early time. Paleo-gas hydrate contributes to the biogenic gas accumulation in deep sea environment. Gas hydrate in early time prevents biogenic gas from escaping to surface. Free gas released from hydrate below P-T stability zone migrates to early formed trap and is conventionally trapped. Finally, paleo-climate change is considered as one of important factor. Low temperature condition during the Plio-Pleistocene glaciation is especially favorable condition for hydrate formation and accumulation of biogenic gas. The recognition of these controlling factors provides insights on future exploration of biogenic gas fields in offshore Myanmar and other deep water environments.

AAPG Datapages/Search and Discovery Article #90336 ©2018 AAPG Asia Pacific Region, The 4th AAPG/EAGE/MGS Myanmar Oil and Gas Conference, Myanmar: A Global Oil and Gas Hotspot: Unleashing the Petroleum Systems Potential, Yangon, Myanmar, November 13-15, 2018