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

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.