The Formation And Evolution Of Solid Bitumen In The Anyue Gas Field Of The Sichuan Basin

Based on organic petrology,Rock-Eval pyrolysis,elemental analysis,infrared spectrum,carbon isotopic compositions and gas chromatography-mass spectrometry analysis of molecular compositions,the geochemical characteristics of reservoir bitumens in the Dengying Formation from Anyue gas field of the Sichuan basin have been investigated to understand their origin and evolution.The microscopic observation illustrated that the reservoirs are mainly composed of dolostone dissolution pores accompanied by pyrite.Primary-insitu bitumen and secondary migrated bitumen are two typical solid bitumen types in the studied sample suite.Contact relationship between bitumen and mineral particles reflects that bitumens are derived from high temperature cracking.The bitumen content in Moxi area is higher than that in Gaoshiti area.Overall maturity of solid bitumen is very high with average equivalent vitrinite reflectance（Ro）of 2.47%.The elemental analysis shows that bitumen is characterized by carbon-rich,hydrogen-poor and oxygen-poor nature in the Dengying Formation.Infrared spectrum analysis show no detectable peaks.These bitumens are seriously dehydrogenated with very high carbonization degree,therefore,the hydrocarbon generation potential is very low.Rock―Eval pyrolysis analysis results show that total organic carbon content is almost equal to the residual carbon content,suggesting complete thermal cracking.Meanwhile,very high Tmax of about 600°C also suggests almost no gas generation potential.All data suggest that these solid bitumens were derived from oil cracking residues without any further hydrocarbon generation potential.Solvent extracts of solid bitumens show full suite of n-alkane distribution with carbon number in the range of C₁₃-C₃₁1 and no obvious biodegradation.Terpanes are dominated by pentyclic terpenes with relatively high proportion of gammacerane.Small amount of 25-norhopanes has been detected,which may be related to high thermal evolution.The steranes were dominated by regular steranes with relatively low proportion of diasteranes.The pregnane is more enriched than homopregnane and the C₂₇ steranes were dominated in the sterane domain.Our result suggest that source rocks of bitumen were formed under highly reducing depositional environment and the organic matter were mainly originated from aquatic algae.The carbon isotope analysis of the reservoir bitumen in the Dengying formation shows that the average value of carbon isotope ¹³C is-35‰,which is roughly the same as that of the carbon isotopes of the bitumen in the Qiongzhusi Formation source rock,which shows that the solid bitumen in the Dengying Formation is likely originated from the Lower Cambrian source rocks.According to the burial history and thermal history of the study area,the reservoir bitumen belongs to oil thermal cracking origin.