Origin and Genesis of Solid Bitumen from the Sinian Dengying Reservoir in the Deyang–Anyue Rift within the Sichuan Basin (SW China): Evidence from Biomarkers and Microscopic Features

[Image: see text] Deep-carbonate gas reservoirs are a significant field for conventional gas exploration in the Sichuan Basin, and industrial-grade gas reservoirs have recently been discovered in the second member of the Dengying Formation in the Deyang–Anyue rift. Drilling results have indicated that the large amount of solid bitumen that is commonly found in pores and fractures in the reservoir of the second member of the Dengying Formation. However, systematic studies have not yet been conducted on the solid bitumen of the second member of the Dengying Formation and its genesis and parent source remain unclear. This study is based on the systematic analysis of solid bitumen samples collected from new exploratory wells, with the aid of optical microscopy, scanning electron microscopy, and gas chromatography–mass spectrometry to clarify the genesis, thermal maturity, and parent source deposition environment of the solid bitumen. Electron microscopy and biomarker results indicate that bitumen is in a high-maturity stage and is a product of the thermal cracking of crude oil. Energy dispersive spectrometer results and the continuous distribution of n-alkanes indicate that the bitumen is not subjected to strong thermochemical sulfate reduction and biodegradation. Isoprenoids and terpenes suggest that the palaeoreserves associated with the bitumen of the second member of the Dengying Formation originate in reductive marine carbonate sediments. The combination of regional stratigraphic contact relationships, lithology, and biomarker characteristics implies that the Maidiping Formation muddy dolomite and the Dengying Formation algal dolomite are the potential parent sources of solid bitumen in the second member of the Dengying Formation.