Geochemical Study Of Solid Bitumen From Reef And Oolitic Beach Gas Reservoirs In Northeastern Sichuan Basin

The solid bitumen in hydrocarbon source rocks has a distinctive style by its low content, broad distribution, variable optic character and complex origin. Due to high thermal maturity, the bitumen occurred widely in the gas reservoirs of Feixianguan Formation of lower Triassic and Changxing Formation of upper Permian in the northeastern Sichuan basin, has become the highly condensed, insoluble solid bitumen. They are considered as residues generated by the thermal cracking of liquid hydrocarbons migrated into reservoirs based on the solid bitumen's occurrence. It is significant for understanding gas generation, migration, accumulation and concentration to study nature, origin and evolution mechanism of these solid bitumens.The solid bitumen samples from the Changxing Formation and the Feixianguan Formation reef and oolitic beach gas reservoirs are studied in this paper. Firstly, their form, optic structure and elemental composition were determined by using the microscope and SEM-EDS (scanning electron microscope/energy dispersive X-ray spectroscopy). Based on the results, the solid bitumens were extracted and degraded by the selective chemical degradation. The extracts and degradation products were determined by GC(gas chromatography),GC-MS(gas chromatography mass spectrometry),GC-IRMS(gas chromatography isotope ratio mass spectrometry) respectively. The investigation indicates that: (1) The solid bitumens from the Changxing Formation of upper Permian and the Feixianguan Formation of lower Triassic in the northeastern Sichuan basin can be divided into two different types according to their configuration and structure, i.e.isotropic autochthonic solid bitumen and anisotropic immigrated solid bitumen. (2) The main elements of solid bitumens are carbon, sulfur and oxygen. Carbon is the highest element of content. The solid bitumens are divided into two categories according to S/C (atomic ratio): high-sulfur and low-sulfur solid bitumen. (3) The precursor of the solid bitumen may be mainly derived from bacteria and algae input and formed in a marine facies environment. Biomarker parameters and bitumen reflectance indicate that the solid bitumens have experienced a relatively high thermal maturity, over the main stage of oil generation.