This paper is the result of oil and gas exploration engineering.
Objective Two sets of black shales occurred in the Ordovician Ningguo-Hule period in the Lower Yangzi region have some potential for shale oil and gas resources. However, the planar extension and vertical thickness of the shale are still unclear. Accordingly, petrographic paleogeographic studies are of greatest significance for exploring their hydrocarbon resource prospects.
Methods A detailed field geological survey and test analysis were carried out regarding the Ningguo-Huleian sedimentary strata in the Lower Yangtze area. Based on the study of lithology, sedimentary structure and paleontological fossil assemblages, we carry out the research of lithofacies paleogeography.
Results Two types of sedimentary facies are identified: carbonate gentle slope facies and clastic shelf facies. Carbonate gentle slope facies mainly existed in the northern and western parts of the Lower Yangtze region, and can be further divided into shallow water gentle slope and deep water gentle slope according to lithologic and biological characteristics. The shallow water gentle slope is dominated by biological spangled limestone, mud-crystal limestone and argillaceous limestone, while the deep water gentle slope is dominated by nodular limestone, turtle crack biolimestone and shale. The clastic shelf facies mainly developed in the southern and eastern parts of the Lower Yangzi region, and can be further divided into shallow- and deep-water shelves. The main differences between the two subfacies are the increased silica content, biota variation and higher organic matter content in the deep-water shelf.
Conclusions The study reconstructed three stages of the paleogeographic pattern during the Ningguo-Hule Period initially. The Hule Formation of Jixi-Jingde-Ningguo in Southern Anhui province has large shale thickness and high organic matter abundance, which can become a critical prospective zone of Paleozoic shale oil and gas.
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