Organofacies characteristics and key controlling factor of the Cretaceous Albian-Turonian marine source rocks in the southern MSGBC Basin

doi:10.3969/j.issn.1672-7703.2023.02.010

Abstract

Abstract: The organic geochemical characteristics of crude oil and source rocks in the Cretaceous Albian, Cenomanian, and Turonian from wells Z-1 and X-1 in the southern continental margin of MSGBC Basin are systematically analyzed. In Well Z-1, the average values of TOC,I_H, and R_o of the Albian source rock are 1.9%, 283 mg/g, and 0.88%, those of the Cenomanian source rock are 2.1%, 456 mg/g, and 0.81%,and those of the Turonian source rock are 3.4%, 538 mg/g, and 0.66%, respecitvely. In Well X-1, the average values of TOC, I_H, and R_o of the Albian source rock are 1.3%, 246 mg/g, and 0.67%, respectively. Based on the organic geochemical and sedimentary characteristics of oil and source rock samples, six organofacies are classified, i.e., predelta dysoxic humic organofacies, delta front dysoxic humic organofacies, shallow marine dysoxic sapropelic-humic organofacies, bathyal anoxic sapropelic organofacies, bathyal dysoxic sapropelic organofacies, and pelagic dysoxic sapropelic-humic organofacies. By analyzing the organofacies characteristics, source rock deposition patterns during the Albian and Cenomanian-Turonian are constructed, and influence factors are discussed, including organic matter source, marine paleo-productivity, redox condition, and depositional process, among which the oceanic anoxic event was the key controlling factor for the deposition of high-quality source rocks. From Albian to Turonian, the organic matter source changed from a mixture of terrestrial higher plants and marine algae to marine algae, showing an increasing abundance of marine algae, increasing marine paleo-productivity, and enhancing bacteria modification on organic matter. From Albian to Turonian, the depositional environment changed tremendously, and the reductive water environment was enhanced. In the transition period during Cenomanian-Turonian, the water oxygen content decreased rapidly, while the salinity increased rapidly, which might be related to the Oceanic Anoxic Event. The study results enable to determine the organofacies characteristics and the key controlling factor of the Cretaceous marine source rocks in MSGBC Basin, which have great significance to identify source rock series, delineate the distribution of source rocks, and guide the petroleum exploration in the basin.

CLC Number:

TE112.113

Zhan Xin, Zhao Hongyan, Yang Songling, Kong Lingwu, Li Dan, Pang Lin’an, Du Wei, Zhang Penghao, Liu Zheng. Organofacies characteristics and key controlling factor of the Cretaceous Albian-Turonian marine source rocks in the southern MSGBC Basin[J]. China Petroleum Exploration, 2023, 28(2): 102-118.

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