Differences of natural gas accumulation and play fairways in the marginal zone and interior of Sinian platform in Anyue gas field, Sichuan Basin

Anyue gas field in the Sichuan Basin is the largest marine carbonate gas field discovered so far in China, with discovered gas-in-place of 1.03×10¹² m³, of which gas in Dengying Formation accounts for 56.3 %. In recent years, the exploration and development of Dengying Formation mainly focus on its 4^th member (Deng 4 Member) in the marginal zone of platform, and the single well productivity of the platform marginal zone is markedly higher than that of the platform interior. In the study, we discuss the differential gas accumulation based on the geochemical characteristics of natural gas and the correlation between gas reservoirs in the platform marginal zone and the interior, combined with the analysis of accumulation conditions. The results show that the hydrogen isotope value in methane of natural gas in the platform margin zone is obviously larger compared with the platform interior, and the salinity of the paleowater medium of the Cambrian source rocks is correspondingly higher than that of the Sinian source rocks, which reflects the contribution increase of the Sinian source rocks from the platform marginal zone to interior. Second, the reservoirs in the two zones are consistent in multiple aspects like the rock type and reservoir space, but the dissolved pores or cavities of reservoirs in the platform marginal zone is better developed than that in the platform interior, which is mainly controlled by the development extent of mound-shoals and the intensity of epigenetic karstification. Third, there are differences in source-reservoir assemblages between the two zones. The platform marginal zone is adjacent to the source rocks in the rift with its reservoir laterally juxtaposed with the source rocks, resulting in various source-reservoir assemblages such as the side source rock-lateral reservoir assemblage as well as the upper source rocklower reservoir assemblage. However, the platform interior is located far away from the source rocks in the rift, and faults rooted in source rocks are needed to transport the hydrocarbons, resulting in single-type source-reservoir assemblage. Fourth, the difference in the contribution between the Sinian and Cambrian source rocks and the differences in the development position and assemblage type of high-quality reservoirs are the main reasons for the huge differences in productivity between the two zones. Penglai-Jian’ge area in northwestern Sichuan Basin is a favorable play fairway for deep hydrocarbon exploration in the Dengying Formation of platform marginal zone in the future, while the surrounding slope areas of paleo-uplift are favorable for exploration of structural-lithologic gas reservoirs in the Dengying Formation of platform interior.