Geochemical Characteristics And Genesis Of Reservoir Asphalt In Guanwushan Formation In Northwestern Sichuan

For the first time,Petro China Southwest and Gasfield Company has obtained commercial gas reservoir in the Middle Devonian Guanwushan Formation in the northwest of Sichuan basin.And the existence of a large amount of bitumen was found in the field and downhole reservoirs has opened the prelude to the exploration and development of Devonian natural gas in Sichuan basin.The Middle Devonian Guanwushan formation is a new layer of natural gas exploration and development in the Paleozoic marine stratigraphy in Sichuan basin.The genesis and source of the reservoir bitumen and the relationship between bitumen and hydrocarbon accumulation and evolution are unclear.Therefore,this paper carried out a large number of analysis and testing on the source rock of the relevant research area in the northwest of Sichuan basin,the outcrop bitumen of the Hejialiang section in the field,and the shuangtan3 well under the structure of the Shuangyushi-Hewanchang.Specifically,it includes:organic carbon content of source rocks,H/C and O/C,carbon isotope content of kerogen,vitrinite reflectance of kerogen,characteristics of biomarker compound;vitrinite reflectance of bitumen samples,group analysis,testing of carbon isotope,biomarker compound characteristics,and has carried on the detailed analysis,clarified the genetic mechanism and source of bitumen,and established the dynamic relationship between bitumen and oil-gas evolution.It has important guiding significance to the rule of Devonian natural gas accumulation and the next exploration and development in Sichuan basin.The results show that there are three sets of source rocks in the Paleozoic marine strata in the northwest of Sichuan basin,which are the source rocks of the lower Cambrian Qiongzhusi formation,the source rocks of the lower Silurian Longmaxi formation and the source rocks of the lower Permian Maokou formation.Among them,the lower Cambrian Qiongzhusi formation has large sedimentary thickness,high organic matter abundance and good organic matter type.It is a typical type I organic matter with high maturity of organic matter and is generally in the stage of over-maturation.It is the main source of natural gas in the Paleozoic of the source rock stratum;Although the various sources of the lower Silurian Longmaxi Formation source rocks are good,they are in the denuded area in the northwest part of Sichuan,and nearly 2/3 of the southwest part of the study area has been denuded,only in the northeast part of the study area.The residual stratigraphic source rocks in the area have limited distribution and contribute little to the gas accumulation in the Paleozoic in the northwest of Sichuan basin.The lower Permian Maokou formation has a large source rock thickness,high organic matter abundance,good organic matter type,it has a high degree of evolution and also has certain hydrocarbon generation potential and has a certain contribution to hydrocarbon accumulation.The outcrop bitumen is in the mature stage,the bitumen reflectivity is 1.05%?1.22%,the average is 1.13%,the saturated hydrocarbons are 1.59%?3.87%,the average composition is 2.88%;the aromatics are 4.04%?7.95%,the average is 5.92%;the non-hydrocarbon 46.95%?51.65%,average 49.32%;asphaltenes 37.32%?43.59%,average 40.08%.The carbon isotope distribution of bitumen is-33.81‰?-30.69?,with an average of-32.22‰;among the biomarker compounds,the carbon number of the wild outcrop bitumen normal alkane is poorly retained,except for the residual part of the paraffin,most of the normal paraffins and branches.The paraffin has disappeared,the baseline of the gas chromatograph is"saw-tooth-bulk",and the baseline drift is severe.The tricyclodecane in decane is dominant,and the peak of C23 is the highest.Among the pentacyclodecane compounds,C29 is the main peak.The Ts/Tm ratio is low,distributed between 0.31 and 0.46,with an average of 0.38;the gamma wax abundance is higher,the gamma wax/C31 decane ratio is 0.77?0.92,with an average of 0.82;the regular decane is "L" type distribution.The double-exploration 3 downhole sample asphalt is in the stage of over-maturation.The bitumen reflectivity is distributed between 2.13 and 3.17%,with an average of 2.81%.The saturated hydrocarbons in the group are 31.43%-40.93%,with an average of 37.38%.The aromatics are 1.65%?7.54%.3.97%;non-hydrocarbon 27.89%?37.52%,average 32.67%;asphaltene 17.47%?28.04%,average 24.53%;bitumen carbon isotope distribution at-33.730‰?-29.78‰,average-31.69‰;biomarker compounds in normal structure The alkane is relatively intact,and the carbon number is mainly distributed between nC15-nC30,which is characterized by double main peaks.The main peaks are nC18 and nC25,respectively,and nC18>nC25.The low carbon number has certain advantages;the planting is relatively low,and the Pr/Ph is distributed 0.38?0.51,average 0.46;C23 as the main peak in tricyclodecane compounds,C30 hodonane as the main peak in pentacyclodecane,Ts/Tm ratio in 0.64?0.95,average 0.79;lower gamma wax content.The gamma wax/C31 decane ratio is distributed in the range of 0.33?0.52 with a mean value of 0.43;in the regular decane,an irregular "V" type distribution is exhibited.There are differences in the source and genetic mechanism of underground bitumen and field bitumen.The field outcrop bitumen is from the Cambrian source rock and belongs to the biodegradation cause.The underground bitumen is from the mixture of Cambrian and Permian source rocks,which is the cause of thermal cracking.The difference in late tectonic evolution is the key reason for the formation of two different mechanisms of bitumen.After the Jurassic period,the formation of the Longmen Mountains in northwest of Sichuan province increased sharply,and the crude oil from the Cambrian source rocks was destroyed,forming a biodegradable bitumen in the field outcrop.The shuangtan3 well area continues to settle,and a small amount of crude oil from the Permian source rock is mixed in the burial process.In the later stage,the mixed crude oil undergoes thermal cracking under high temperature conditions:the light components form dry gas and at the same time the heavy components precipitates to form the pyrobitumen.