Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geological Conditions and Accumulation Mechanism of Shallow Biogenic Gas Reservoirs in Andaman Basin
(1) Petrochina Hangzhou Research Institute of Geology, Hangzhou, China.
Many shallow biogenic gas reservoirs have recently been discovered in Miocene- Pleistocene strata in Andaman offshore area. And most of them occurred within or adjacent to the depocenter filled with approximately 5000 m thickness of Neocene sediments. The large biogenic gas field Zawtika 1A in Block M09 was discovered in 2007, with proven gas reserves greater than 2 TCF. And its reservoir is the Miocene-Pliocene Delta Front Sandstones, which were buried generally 750-1580 m in depth. Besides, the shallower biogenic gas reservoirs exist in Pleistocene strata, with burial depth less than 500m.
The seismic data of biogenic gas sections in Andaman basin displays reflection characteristics such as blanking, enhanced reflection patterns, bright spot, flat spot, reverse polarity, shadows in instantaneous frequency, and increase in amplitude with the offset. So it’s easy to be identified.
There are eight beneficial geological factors of Andaman basin for controlling biogas generation and accumulation: 1.Favorable biogenetic gas generation conditions including rapid deposition, abundant immature organic matter, reducing environment, low geothermal gradient etc. 2.Massive unconsolidated clastic reservoirs,which were characterized by wide distribution, shallow burry, vertical superimposition, approximately 24-40% porosity and 500-3000 md permeability. 3.Effective seal and preservation conditions. 4.Increase of temperature and decrease of pressure caused by faults, which are favorable for melting of hydrate and releasing of gas. 5.Effective secondary migration pathways, including faults and permeable sandstones. 6.Synsedimentary traps, which are favorable for accumulating biogas. 7.The biogas reservoirs were compensated by the biogas generated uninterruptedly. 8.Additional thermogenic gas supplement from deep mature source rocks.
We establish a possible accumulation model of this large biogenetic gas reservoirs based on destabilization of the paleo-hydrate and secondary migration. There are six stages of geological processes worked together in the following sequential order: deposition of organic rich sediments → rapid deposition and methanogenesis at reducing environment → formation of gas hydrate → increase of temperature and decrease of pressure (fault activity) leading to melting of hydrate and releasing of gas → secondary migration and subsequent entrapment in porous and permeable clastic reservoirs → dynamic balance between continuing escape and supplement.