Shiyou shiyan dizhi (Mar 2024)
Lithofacies characteristics and reservoir capacity of fine-grained sedimentary rocks of second member of Qianfoya Formation in Yuanba area, Sichuan Basin
Abstract
The shale oil and gas exploration potential of the Jurassic Qianfoya Formation in the Sichuan Basin is promising. However, there is still a lack of understanding regarding the basic sedimentary characteristics and development patterns of fine-grained sedimentary rocks. Therefore, utilizing the principles of fine-grained sedimentology, this study thoroughly examines the geological features of the fine-grained sedimentary rock reservoir in the second member of the Qianfoya Formation in the Yuanba area. This investigation includes the analysis of material composition, sedimentary structures, reservoir space types, and classification of lithofacies types. Subsequently, the physical characteristics and reservoir space types of the main lithofacies reservoirs are compared and analyzed. Finally, the study predicts the favorable intervals for development. The results reveal that the fine-grained sedimentary rocks in the second member of the Qianfoya Formation in the Yuanba area consist of two rock types: clay rock and siltstone, with a total of 15 rock facies types. Organic-rich massive clay rock, organic-rich massive silty clay rock, organic-rich laminated silty clay rock, organic-rich banded silty clay rock, organic-rich massive silty clay rock, and organic-rich laminated silty clay rock are the six major lithofacies types of fine-grained sedimentary rock. Additionally, various types of reservoir spaces exist, such as interlayer fractures among clay minerals, shale bedding fractures, intercrystalline pores among clay minerals, intercrys-talline pores among iron pyrite, dissolved pores, and organic matter pores. Among these, intercrystalline pores of clay minerals are the most widely distributed. The average porosity and permeability of the six major lithofacies types were found to be 2% and 0.023×10-3 μm2, respectively. The lithofacies of organic-rich laminated silty clay rock, organic-rich laminated silty clay rock, and organic-rich massive silty clay rock exhibit rich reservoir space types, wide distribution, large quantity, higher porosity and permeability compared to other lithofacies, as well as higher TOC values, making them the most favorable reservoir lithofacies types. The dominant stratum of the second member of the Qianfoya Formation in the study area is situated in substratum 1, the bottom of substratum 2, and the middle and lower parts of substratum 3.
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