--> Genesis of Upper Triassic Xujiahe Formation Tight Sandstone Gas in the Northeast Sichuan Basin

AAPG Southwest Section Annual Convention

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Genesis of Upper Triassic Xujiahe Formation Tight Sandstone Gas in the Northeast Sichuan Basin

Abstract

Xujiahe Formation is one of the important tight sandstone reservoirs in Sichuan Basin. Recent years showed good hydrocarbon exploration potential. However, the origin of gas in the YB- BZ–MLB field of Northeast Sichuan Basin is not entirely clear. The natural gas component is mainly composed of hydrocarbon gas with the average content of methane (CH4) being 93.41% and the natural gas drying coefficient being 0.96-0.99. The carbon isotope of CH4 has little difference ranging from -25.2‰ to 38.2‰. Furthermore, R/Ra falls in between 0.0076 and 0.01608, and 40Ar/36Ar range from 358.48 to 437.2, which all indicate typical dry gas of crustal origin. The calculated mean value of source rocks maturity is 1.90 by using Dai’s δ13C1-Ro regression equation, which is in accordance with the high maturity- over maturity stage of kerogen in Xujiahe formation. The hydrogen isotope of CH4 (δDCH4)is lighter ranging from -154‰ to -174‰ (average value =-160.8‰). Those geochemical data of CH4 show the characteristics of typical coal type gas. The carbon isotopic composition of ethane (δ13C2) is in - 21.5‰--29.7‰ range also tells the coal type gas genesis from Xujiahe source rock in YB and BZ block. The partial or complete inversion of the carbon isotope series may be caused by natural gas mixing in different stages. A relatively small slope of In (C1/C2)-In (C2/C3) in YB and BZ block distinguishing from MLB block prove that natural gas origin from thermal degradation of kerogen. However, the content of δ13C2 in MLB block is less than -28.0‰ and partial or complete reversal of carbon isotope series are probably attributable to marine genetic gas by migrating to the Xujiahe reservoir along the fault and then mixing with coal type gas. It means methane and heavy hydrocarbon gas have different sources. In summary, YB –BZ block is typical coal type gas while MLB block’ methane mainly comes from coal type gas of the coal-bearing source rocks in Xujiahe formation and heavy hydrocarbon gas(C2+) is mainly derived from the crude oil cracking gas in the underlying marine source rock.