This paper is the result of oil and gas exploration engineering.
[Objective]The dark shale of the Longtan-Dalong Formation is widely developed in the Upper Permian of the Lower Yangtze region, which is the key formation for shale gas exploration in China. At present, the degree of exploration in this area is low, resulting in unclear understanding of shale gas accumulation conditions.[Methods]The geochemical characteristics, reservoir properties, and preservation conditions of the Longtan Formation and Dalong Formation shales were studied to evaluate shale gas exploration potential and optimize favorable exploration areas.[Results]The organic matter abundance of the Longtan Formation and Dalong Formation is high, TOC content is generally greater than 2.0%, the thermal evolution degree is moderate (between 1.3% and 2.5%), the reservoir space is mainly ink bottle and slit-like mesopores, the brittle mineral content is generally higher than 50%, with good hydrocarbon generation material basis and fracturability. The Longtan Formation shale is thick, with good self-sealing ability, and the preservation conditions are generally better than those of the Dalong Formation. Both of them develop various types of fractures, which are the main channels for shale gas escape. In addition, the late magmatic activity is frequent, which has a strong destructive effect on shale gas, and the preservation conditions are the key to shale gas enrichment.[Conclusions]The evaluation of the Upper Permian shale gas in the Lower Yangtze area should be based on sedimentary environment, hydrocarbon generation capacity, and reservoir conditions, taking the shale gas preservation conditions as the key and adopting the principle of finding weak areas in the context of strong tectonic activity. Finally, five favorable areas were divided in the Dalong Formation and Longtan Formation in the Lower Yangtze area.
Highlights: The Upper Permian shales in the Lower Yangtze region have the material basis for shale gas enrichment; The preservation conditions are the key to shale gas enrichment in the Upper Permian in the Lower Yangtze region.