Numerical Simulation of Deep Biogenic Gas Play Northeastern Bay of Bengal (Offshore Northwest Myanmar)

Chung, Younghoon ¹; Yang, Su-Yeong ²; Kim, Jae Woo ¹
(1)Myanmar E&P, Daewoo International Corporation, Yangon, Myanmar. (2) Energy Division, Daewoo International Corporation, Seoul, Korea, Republic of.

Commercial quantities of methane gas were discovered in the Late Pliocene reservoir, offshore northwest Myanmar in 2003-2005. The gas is stratigraphically trapped in deepwater turbidite sandstones, and occurs between 2,900 and 3,300 meters sub-sea. Gas accumulations are of biogenic origin, which was proven by its dryness (>99% methane) and enriched light isotope character (δ13C ranging from 60 to 70 ‰). Little is known about the source rock potential in the area due to limited offset well data. A basin-modeling study using PetroMod 1D/2D packages was employed to test Pliocene and Miocene source rock potential, and to analyze gas generation, migration, and trapping history of the study area. Three seismic sections and 8 wells were selected to construct 2D models. The method of Clayton 1992, which gives a nomogram for the conversion of source rock volume to volumetric yields of biogenic gas, was applied for biogenic gas generation modeling. The model output was calibrated with vitrinite reflectance, and bottom hole temperature data.

The model indicates that the stratigraphic section in the study area is thermally immature. There were three biogenic gas generation peaks at Late Miocene, Early Pliocene, and Late Pliocene. Most of biogenic gas was generated from Middle Miocene and Early Pliocene shale. The model suggests the expelled biogenic gas migrated vertically, with a large amount of the generated gas being lost due to a lack of effective seals encountered. Accumulation of biogenic gas in commercial quantities requires relatively unusual geologic conditions, including formation of traps and seals in early stage. Simulation results indicate that the existence of paleo-hydrates during Miocene to Pliocene period played an important role in gas accumulation, with the hydrates acting as seals in the initial gas generation stage. The presence of paleo-hydrates is supported by the occurrence of low-saline formation water. The paleo-hydrate was later released by change in P-T condition due to sediment burial. The later stage of sealing is thought to be provided by thick shale overburden as well as homogeneous shale interbeds and authigenic carbonate cement beds within reservoir sandstones.

The numerical simulation shows that the Pliocene sediments in this region are far above thermogenic hydrocarbon generation windows. This study will provide insights on the biogenic gas potential of the Pliocene Bengal fan sediments in the northeastern Bay of Bengal.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.