Oil & Gas Geology ›› 2019, Vol. 40 ›› Issue (1): 133-141.doi: 10.11743/ogg20190113

• Petroleum Geology • Previous Articles     Next Articles

Origin of reservoir bitumen and its implications for adjustment and reformation of hydrocarbon-accumulation in Baiyuan Sag, Pearl River Mouth Basin

Li Meijun1,2, Zhang Zhongtao3, Chen Cong3, Yang Chengyu2, Lu Xiaolin2, Fu Jian2, Dai Jinhui2, Tang Youjun1   

  1. 1. Key Laboratory of Exploration Technologies for Oil and Gas Resources(Yangtze University), Ministry of Education, College of Resources and Environment, Wuhan, Hubei 430100, China;
    2. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum(Beijing), Beijing 102249, China;
    3. CNOOC East Nanhai Corp. Ltd., Shenzhen, Guangdong 518000, China
  • Received:2018-07-15 Revised:2018-08-18 Online:2019-02-28 Published:2018-12-08

Abstract:

Reservoir bitumen has been discovered in multiple Paleogene series of strata in the Baiyun Sag,Pearl River Mouth Basin.The abundance of reservoir bitumen gradually increases from the shallower Zhuhai and Zhujiang formations to deeply buried Enping and Wenchang Formations.The reservoir bitumen only sporadically occurs in coarse-grained sandstones with better permeability and porosity, while occurs in siltstones and mudstones in the form of veins.Almost all reservoir bitumen samples do not show apparent anisotropy with bitumen reflectance lower than 2.0%,suggesting that the reservoir bitumen in the Baiyun Sag doesn't mainly result from high-temperature cracking.All bitumen extracts lack 25-norhopane series according to the analysis of molecular geochemical composition.No gas chromatogram base-line "hump" was discovered in all saturated hydrocarbon of reservoir bitumen.The reconstructed burial history of representative individual wells shows that the reservoir strata have not experienced large-scale uplifting and denudation.Therefore,the reservoir bitumen in the Baiyun Sag is not of biodegradation origin.Based on the analyses of the geological setting and characteristics of oil and gas reservoirs,the origin of reservoir bitumen could be attributed to later "gas washing" for deeply buried hydrocarbon-accumulation generated by source rocks of Enping and Wenchang Formations during the early thermal maturity stage.The time differences of thermal evolution for source rocks and reservoirs of various stratigraphic horizons leads to the adjustment and altering of oil and gas reservoirs in the Baiyun Sag,subsequently resulting in the formation of light and volatile oil reservoirs.

Key words: bitumen reflectance, cracking, gas washing, reservoir bitumen, Baiyun Sag, Peral River Mouth Basin

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