Research About Geological Conditions Of Shallow Gas (Biogenic Gas) Accumulation In HETAO Basin

Biogenic gas, whose buried depth is shallow, exploration cost is lowliness and has a characteristic of finer benefit comparing to conventional natural gas, is generated by organic matter under reduction condition, with biochemical action. Biogenic gas reservoir are found in Qaidam Basin, song liao Basin, northern area of Jiangsu and Baise area. Having a similarity geologic background about quaternary and tertiary formation comparing to Qaidam Basin ,and the shallow gas having the characteristic of biogenetic gas, therefore, the research about geological condition of shallow gas (biogenic gas ) accumulation in HETAO Basin is carried out in order to find added reserves in new area. The research must be creative of great meanings to HETAO area or even china about biogenic gas exploration field.Applying synthetically Microbiology, geochemistry, geology of natural gas(natural gas genesis theoretics, gas accumulations theoretics), seismic stratigraphy, sedimentology and palaeobiology, using modern geochemical analysis testing technology, microorganism identification and culture techniques and biogenic gas generating simulating testing technique, geological condition of shallow gas (biogenic gas ) accumulation is researched , biogenic gas stock size is calculated and favorable biogenic gas area is selectd in HETAO Basin.It is approved that shallow gas has a characteristic of biogenetic gas. The main Gases constituent are methane, nitrogen, carbon dioxide and hydrogen, these four Gases constituent is above and beyond 97%, the content of ethane, propane, butane and pentane less than 3%, the content of methane carbon isotope ranges from -74.183‰to-77.57‰.The sedimentary facies types of shallow lake facies, semi- deep lake facies and shore lake facies have an intimate relation to biogenic gas. The main gas source rocks its thickness ranging from 123m to 1300m are grey mudstone, sandy mudstone, humus soil and carbon mudstone. The source rocks thickness ratio ranges from 16.4% to 59.8% and the generation gas province area is 2.3 891×10⁴km². Organic carbon ranges from 0.18% to 15.4% and its average is 0.41%. Chloroform bitumen "a" ranges from 0.0021% to 1.424% and its average is 0.011%. The thermal evolution of those organic matter, which mostly contains kerogen Type I and II ,was speculated to have primarily been at the immature mature and biochemistry stage.The palaeoclimate of Pleistocene in Hetao basin is changed during different period: dry and cold-warm and dry-little warm and dry-little cold and damp during eopleistocene,cold and dam-little warm and damp-dry and cold during mid-pleistocene and little warm-dry and cold during epipleistocene.There is a good allocation among the source rock, reservoir and cap rock, the accumulation model is self-producing and self-reserving primary reservoir ,which is optimum combination form in HETAO Basin.There is no obvious correlativity between zymogenic bacteria, cellulose-decomposing microorganisms and buried depth, as well that of organic components.Biochemistry methanogenesis process is of outwardness and its evolution is still processed possibly because of the detection of methanogen. It is reasonable that Organisms methanogenesis function maybe already has been processed, or it will processed.The two main methanogenesis belt on the vertical were proved, one ranges from 200m to 800m,another ranges from 1200m to 1800m. Gas generating center where are lithologic trap, high salinity in formation water and favorable area, lie in south edge of north depressed of HUHE sag, north edge of south slope of HUHE sag and the central and southern parts of north depressed of LINHE sag.It is considered that there are similarity geologic background about quaternary and tertiary formation between Qaidam Basin and HUHE Basin, and having a favourable exploration prospect. The Common gas production is 13.8433×10¹²km³ in HUHE sag, 26.5697×10¹²km³ in LINHE sag and 1.4×10¹²km³ in WUQIAN sag.