Control of heterogeneity in overmature shales by sedimentation and diagenesis: evidence from the Wufeng–Longmaxi Formations, southern Sichuan Basin

The heterogeneity of overmature shales is a research hotspot, but there is a lack of deep understanding of its origins. Taking the Wufeng–Longmaxi shales as an example, this study investigates the controlling factors of reservoir and petrological heterogeneity in systems. The transgressive systems tracts (TST) are the sweet-spot intervals of shale gas. Their sedimentary processes were controlled by volcanism and glaciation. Both processes directly/indirectly injected nutrients into the ocean, causing the high primary productivity. The produced organic matter (OM) and siliceous skeletons were preserved well in anoxic water. The siliceous skeletons were transformed into authigenic quartz through diagenesis, which filled interparticle pores to resist compaction and maintain pore connectivity. OM migrated and accumulated in the interconnected pore network and formed abundant OM pores. The early highstand systems tract (EHST) was deposited during the regression, and productivity was weakened, resulting in reduced siliceous skeletons. Moreover, dysoxic-oxic water was not conducive to OM preservation. Extrabasinal quartz and feldspar significantly increased, becoming the main brittle particles resisting compaction. These factors made the number of full-scale pores was far smaller than that of TST. During the late highstand systems tract (LHST), further regression occurred, and extrabasinal clay rapidly increased. Oxic water facilitated oxidation of OM, making it more difficult to accumulate. Clay aggregates undergo continuous compaction during diagenesis to form a dense structure, resulting in the lowest porosity. The results suggest that the good compaction resistance of authigenic quartz may be the key to the hydrocarbon potential of deeply buried overmature shales.