The Oil Charging Pathways Of Sinian-Lower Paleozoic Ancient Oil Reservoirs Of In The Gaoshiti-Moxi Regions (Sichuan Basin)

Comprehensive geochemical analysis has been performed on rock,bitumen and gas samples from the Gaoshiti-Moxi regions of Sichuan Basin.These samples include reservoir bitumen from Lower Cambrian Longwangmiao and Sinian Dengying formations,source rocks from Cambrian Qiongzhusi,Sininan Dengying and Doushantuo formations and gas samples from Longwangmiao Formation.Based on Gas-Chromatograph(GC)and Gas Chromatograph-Mass Spectrometry(GC-MS)analysis of these samples,this study conducted bitumen-bitumen,bitumen-gas and bitumen-source correlations.Then the family of reservoir bitumen was classified and source rocks for ancient oils reservoir were characterized.Particularly,charging and migration pathway of ancient oil reservoir in Gao-Mo regions was preliminarily studied.The results show that the reservoir bitumen samples in Gao-Mo region are in high thermal maturation and possess similar geochemical characteristics.The gas chromatograms of saturated fraction of the reservoir bitumen exhibit smooth baselines with normal alkanes ranging from C14 to C33.The m/z 191 mass chromatograms show a relatively high abundance of C23 tricyclic terpane,and m/z 217 show a “L” shape distribution pattern of C27-C28-C29 regular steranes.All data points were scattered together in the ternary diagram of relative abundance of C27-C28-C29 regular steranes,indicating their genetic affinity.The data of the samples are concentration distribution in ternary diagram of the stable carbon isotopic values of reservoir bitumens and their fractions are generally higher than-30‰.The results of bitumen-bitumen and bitumen-source correlation indicate that the reservoir bitumen from three different formations(Cambrian Longwanmiao,the second and forth member of Sinian Dingying Formation)belong to the same population and have been principal sourced from source rocks of Qiongzhusi Formation.Dengying and Doushantuo Formation source rocks may be minor contributor for the reservoir bitumen.The bitumen-gas correlation indicates that the natural gas of Longwangmiao Formation were converted from reservoir bitumen of Longwangmiao Formation.In addition,The tracing indicators of molecular parameters such as 4-/1-MDBT(4-menthyldibenzothiophene/1-methydibenzothiophene)and 4,6-/(1,4+1,6-DMDBT)[4,6-dimenthyldibenzothiophene/the sum of 1,4-dimenthyldibenzothiophene and 1,6-dimenthyldibenzothiophene] show that the general migration direction of ancient oil reservoir was from west to east for Sinian-Lower Paleozoic ancient reservoir in the Gaoshiti-Moxi regions.Therefore,it can be predicted that the source kitchen should be located to the west of the Gao-Mo regions,probably on the Deyang-anyue sag.This study have practical implication in characterizing the ancient oil reservoir and predicting the location of natural gas pools.