Main factors for large accumulations of natural gas in the marine carbonate strata of the Eastern Sichuan Basin, China

The natural gas accumulation zone, where the marine carbonate rock strata are developed, was formed in the eastern Sichuan Basin under the influence of several main tectonic movements (Caledonian Movement, Indosinian Movement, Yanshanian Movement, and Himalayan Movement). Most natural gas reservoirs exhibit the structural-stratigraphic traps together with multistage accumulation, late-stage adjustment and reformation, et cetera. The natural gas accumulation zone (or so-called gas reservoir groups) is controlled by the following main factors: multi-sourced and multi-formed hydrocarbons for marine source rocks (i.e. Lower Silurian Longmaxi Formation, Lower Permian, Upper Permian Longtan Formation), paleo-uplift, paleoslope, and the hinge belt controlled by the steep dip structures, namely the Lower and Middle Triassic high-quality gypsum. Three sets of high-quality source rocks (i.e. S1l, P1, P2l) account for the abundant hydrocarbon supply for natural gas accumulation in the eastern Sichuan area, especially in the destructed oil reservoir formed earlier. The said destructed oil reservoir not only provides the preservation space for natural gas reservoir that will take place later, but it also provides the hydrocarbon source for thermal cracking of hydrocarbons and thermochemical sulfate reduction (TSR). Although the gas reservoirs in the eastern part of the Sichuan Basin experienced multi-stage adjustment and reformation at later times, the thick and high-quality gypsum as well as the mudstone, as available caprocks, have offered a good preservation condition for the underlying gas reservoirs. The paleohighs (e.g. Luzhou paleohigh and Kaijiang paleohigh), the Permian platform margin slope, and the structurally transformed slope under the function of the steep dip anticline in the eastern Sichuan not only form the high-quality carbonate reservoir, but they also became favorable for oil and gas accumulation. The difference in hydrocarbon generation history of the source rock and multistage accumulation of hydrocarbons in the eastern part of the Sichuan Basin caused the diversity in the location of the gas reservoirs, where the gas reservoirs with low H2S contents are distributed in the eastern Sichuan Basin (e.g. Jiannan gas field), while the gas reservoirs with high H2S content are located in the northeastern Sichuan Basin (e.g. Puguang gas field).