Differences and main controlling factors of large-scale gas accumulations in typical giant carbonate gas fields: A case study on Anyue gas field in the Sichuan Basin and Jingbian gas field in the Ordos Basin

Anyue gas field in the Sichuan Basin and Jingbian gas field in the Ordos Basin are two giant marine carbonate gas fields with the largest overall scale and the largest single-layer scale respectively discovered in China so far. Based on the analysis of the source-location structure of gas reservoirs, the process of gas accumulation and the space-time configuration of key play elements during reservoir generation, we consider that Anyue and Jingbian gas fields are characterized by a variety of source-location structure types and source-trap configuration of high efficiency. However, Anyue gas field is of “in-situ” accumulation of pyrolysis gas from paleo-oil reservoirs in inherited paleo-uplift, while Jingbian gas field is of adjusted accumulation in the structural transformation zone of a slope. There are three key factors controlling the large-scale enrichment of Anyue and Jingbian gas fields. First, the hydrocarbon source rocks underwent abnormally thermal events, with the duration of thermal events with a heat flow value over 70 mW/m² being about 70 Myr. Different types of kerogen got fully cracked to generate methane-dominated gases in large amount. Second, the multi-layered reservoirs of large scale and high quality are well developed due to the modification of favorable lithofacies by karstification. Third, many types of large-scale traps are well developed including structural, structural-lithologic, stratigraphic-lithologic and lithologic traps, being favorable for large-scale and effective accumulation of gas. The areas with ideal configuration relationship between high-energy beach body and lithologically tight zone within paleo-oil reservoirs or adjacent to hydrocarbon source rocks are favorable for discovering large-scale gas accumulation in the near future.