Oil & Gas Geology ›› 2012, Vol. 33 ›› Issue (1): 37-44.doi: 10.11743/ogg20120105

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Reservoir bitumen genesis and its impacts on hydrocarbon migration and accumulation:a case study from Chang 81 of Yangchang Fomation in Huaqing area,the Ordos Basin

Chen Shijia1,2, Yao Jingli3, Lu Jungang1,2, Yang Guoping3, Wen Yanchun1, Zhang Jizhi1, Shi Zhenghao1   

  1. 1. College of Resources and Environment, Southwest Petroleum University, Chengdu, Sichuan 610500, China;
    2. Natural Gas Geology Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500, China;
    3. PetroChina Changqing Oilfield Company, Xi’an, Shaaxi 710016, China
  • Received:2011-03-08 Revised:2011-07-25 Online:2012-02-28 Published:2012-03-22

Abstract:

Bitumen is common in the pores of Chang 81 pay zone of the Triassic Yanchang Formation in Huaqing area of the central part of Ordos Basin,and the oil-water relationship is very complicated.According to the analysis on the geochemical characteristics of reservoir extracts and crude oil,combining with comparative test on the reservoir physical property of cores containing bitumen before and after washing oil and the reservoir fluorescence analysis,authors studies in detail the reservoir bitumen genesis and its impact on reservoir physical property and hydrocarbon migration and accumulation in this area.The results show that the genesis of reservoir bitumen and crude oil are different,and the bitumen has lower maturity with strong biodegradation.Base on the experiment of washing oil,it is found that both the post-washing porosity and permeability of cores containing bitumen increase significantly,and the oil formed in late stage mainly distributes in the residual pore filled with bitumen.It is pointed out that bitumen has an important control on reservoir physical property and hydrocarbon migration and accumulation at late stage.The Chang 81 formation experienced hydrocarbon two accumulation stages,the first stage happened in Middle-Late Jurassic and the second in Early Cretaceous.The early accumulated crude oil with low maturity suffered from strong biodegradation and were turned to bitumen,which filled in the pores of reservoir together with chlorites and ferrocalcites,making the reservoir tighter.The mature oil generated from the source rocks in late stage could only accumulate in the residual pores filled with bitumen or in pores of reservoir with less bitumen.

Key words: Biodegradation, bitumen genesis, reservoir physical property, hydrocarbon migration and accumulation, Huaqing area, Ordos Basin

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