An analysis of reservoir characteristics, resources and exploration prospects of the oldest shale gas in the world

Main marine shale formations in southern China, in addition to the Ordovician Wufeng Formation and the Silurian Longmaxi Formation which have become China's shale gas industrial production layers, include marine shale of the Cambrian Niutitang Formation and the Sinian Doushantuo Formation, and other marine shales. So far, the Sinian Doushantuo Formation is the oldest shale gas layer found in China and even in the whole world. Based on previous studies of regional geology and relying on drilling core data obtained in the middle Yangtze region, the authors used multi-scale methods for characterizing the reservoir properties and, combined with the organic matter abundance and evolution characteristics, carried out the comprehensive research. The reservoir and hydrocarbon potential of the world's oldest shale gas were analyzed for the first time. A comparison was made with the commercial shale gas reservoir in the United States and that in Sichuan basin of China. The results show that the Sinian Doushantuo Formation shale in the middle Yangtze area is developed in a relatively stable sedimentary environment, and the mud shale has large thickness and high organic matter content. The total organic carbon (TOC) content of most samples is more than 2%. The brittle mineral content of shale is high, and micro-nano pores and multi-scale fractures are developed. Compared with shale of Longmaxi Formation in Sichuan basin, the thermal evolution degree of organic matter in shale of Doushantuo Formation is moderate, the Raman maturity is between 2.0 and 3.0, the organic matter is in the stage of pyrolysis. The analysis indicates that Doushantuo Formation has the basic geological conditions for shale gas reservoirs and has a good resource and exploration prospect. The research results provide comprehensive basic data for the evaluation, exploration and development of new shale gas fields and new strata in the future.