Oil & Gas Geology ›› 2009, Vol. 30 ›› Issue (5): 519-529.doi: 10.11743/ogg20090501

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Carbon isotope features of alkane gases in the coal measures of the Xujiahe Formation in the Sichuan Basin and their significance to gas-source correlation

Dai Jinxing1, Ni Yunyan1, Zou Caineng1, Tao Shizhen1, Hu Guoyi1, Hu Anping2, Yang Chun1, Tao Xiaowan1   

  1. 1. PetroChina Exploration and Development Research Institute, Beijing 100083, China;
    2. Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
  • Received:2009-05-26 Online:2009-10-25 Published:2012-01-16

Abstract:

The coal measures in the Upper Triassic Xujiahe Formation in Sichuan Basin are coal-bearing continental strata.Its first,third,and fifth members are dominated by dark mudstone and coal and are gas source rocks.The kerogen of the mudstone are mainly of type Ⅱand Ⅲ with an average TOC of 1.96%.They are mature in the central Sichuan Basin and are highly mature in the western Sichuan Basin.Since the organic matter of source rocks are dominated by huminite,the source rocks are gas prone and the gas generated only contains minor light oil or condensate.The second,fourth and sixth members consist mainly of sandstone.There are three sets of source-reservoir-caprock combinations,resulting in many indigenous coal-derived gas fields.The gas reserves discovered in the Xujiahe Formation is only next to that in the Lower Triassic Feixianguan Formation.The Guangan gas field,the second largest gas field in the basin,produce gas from the Xujiahe Formation.The carbon isotope of the coal-derived gas have the following features:1) most of them are positive carbon isotopes(i.e.,δ13C113C213C313C4),2) its δ13C2 value is between-20.7‰--28.3‰,the heaviest among that of the 9 gas pays in the whole basin,3) there are lighter δ13C1 in central Sichuan Basin with the lowest value of-43.0‰.A few gas reservoirs sourced from oil-prone kerogens had been discovered in eastern and southern Sichuan Basin,where the Xujiahe Formation thins out.These reservoirs have light carbon isotopes(typically lighter than-30‰,and in extreme cases as light as-36.3‰) and can be easily distinguished from coal-derived gases.

Key words: carbon isotope, gas-source correlation, alkane gas, Xujiahe Formation, Sichuan Basin

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