Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (1): 1-25.doi: 10.11743/ogg20220101

• Petroleum Geology • Previous Articles     Next Articles

Diversity in the lithofacies assemblages of marine and lacustrine shale strata and significance for unconventional petroleum exploration in China

Maowen Li1,2(), Xiaoxiao Ma1,2, Zhijun Jin1,2, Zhiming Li1,2, Qigui Jiang1,2, Shiqiang Wu3, Zheng Li4, Zuxin Xu5   

  1. 1.State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Wuxi, Jiangsu 214126, China
    2.Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China
    3.Jianghan Oilfield Company, SINOPEC, Wuhan, Hubei 430223, China
    4.Shengli Oilfield Company, SINOPEC, Dongying, Shandong 257015, China
    5.Exploration Company, SINOPEC, Chengdu, Sichuan 610041, China
  • Received:2021-08-19 Revised:2021-11-25 Online:2022-02-01 Published:2022-01-28

Abstract:

There is a strong correlation between the lithofacies and organofacies of fine-grained organic matter-rich sedimentary rocks,where the carbonate-, quartz-feldspar- and clay mineral-rich lithofacies often show high affinity respectively with type I-S/II-S,I/II and III/IV organic facies. Hence, whole rock mineral X-ray diffraction analysis (XRD) represents a straightforward and effective method to predict the lithofacies, organic matter type, depositional environment, hydrocarbon characteristics, and rock mechanics of geological samples concerned. Based on a thorough review of marine shale data from North America and the Sichuan Basin, a large collection of lacustrine shales from major non-marine sedimentary basins in China were characterized by whole-rock mineral XRD, core observation, thin section analysis and total organic carbon (TOC) measurement. The aims are to classify the lithofacies and organofacies, identify the key difference in lithofacies assemblages, and discuss their implication for unconventional shale oil and gas exploration. The results indicate that lacustrine shales in sedimentary basins in China were deposited in various tectonostratigraphic settings. In general, the fine-grained sediments in freshwater-brackish water lakes in sag stage are dominated by clay-quartz with minor carbonate, whereas the saline and alkaline lacustrine sediments in rift stage consist of carbonate or clay-quartz with carbonate minerals. The sedimentary systems of fine-grained lacustrine deposits are characterized by rapid facies change, complex lithologies, and variable reservior-caprock combinations. Thus, the strong heterogeneity in the lithofacies and their combinations often lead to multiple types of shale oil “sweetspots”; as different lithofacies correspond to varying organofacies, the variation in the thermal behaviors of different organofacies contributes to the change in the states of hydrocarbon occurrence among the various lithofacies combinations. These results confirm that every shale is unique. Therefore, the variation in lithofacies and organofacies of the fine-grained sediments indicates that it is essential to use quantitative and objective parameters in classifying the “sweetspots” and hydrocarbon states of occurrence in order to make the credible assessment of shale oil resources.

Key words: whole rock XRD analysis, organofacies, microfacies analysis, fine-grained sedimentary rocks, Lacustrine shale oil, China

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