Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (6): 1119-1128.doi: 10.11743/ogg20200601

• Petroleum Geology • Previous Articles     Next Articles

Geochemical comparison of mudstone and shale—A case study of the 7th member of Yanchang Formation in Ansai area, Ordos Basin

Bojiang Fan1(), Qiliang Mei2, Xiaojun Wang2, Yue Meng2, Qijiang Huang2   

  1. 1. School of Petroleum Engineering and Environmental Engineering, Yan'an University, Yan'an, Shaanxi 716000, China
    2. Changqing Oilfield Branch of PetroChina, Xi'an, Shaanxi 710018, China
  • Received:2019-06-12 Online:2020-12-28 Published:2020-12-09

Abstract:

High-quality shale and mudstone source rocks are developed in the 7th member of Yanchang Formation (Chang 7 member) in Ansai area within the Ordos Basin.A study of geochemical differences of the two rocks was considered necessary for evaluating their quality and identifying their contributions to oil and gas reservoirs.A core description of the Chang 7 member was then carried out based on 25 key exploration wells and a geochemical comparison of the rocks was also performed based on 81 mudstone and shale samples gathered from the member in the wells.The results show that both shale and mudstone samples are good source rocks with their thermal maturation and organic matter abundance values within favorable ranges, except that the shale samples are dominated by Types Ⅰ and Ⅱ1 organic matters whereas the mudstone samples are dominated by Types Ⅱ1 and Ⅱ2 organic matters.Compared with mudstone in terms of geochemical features, shale has significantly higher abundance of organic matter with more shale samples having TOC values greater than 4% and is more oil-prone as it contains higher content of sapropel.Shale also has higher contents of free and pyrolysis hydrocarbons and greater hydrocarbon-generating potential than mudstone.However, it should be noted that the occurrence of more pores and micro-fractures in shale may have a greater impact on the content measurements of free hydrocarbon.Compared with shale, in which pores and micro-fractures are easily developed and serve as pathways for hydrocarbon expulsion and migration, mudstone demonstrates stronger retention capacity for hydrocarbons.In addition, there is no significant difference between shale and mudstone in terms of biomarker parameters.

Key words: mudstone, shale, geochemical difference, Yanchang Formation, Ansai area, Ordos Basin

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