A Study On The Origin And Kinetic Model Of Natural Gas Generation In The Oolitic Reservoir Of Feixianguan Formation In Northeast Sichuan Basin

Due to a series of large or oversize scale oolitic gas pools were found in Feixianguan Formation in NE Sichuan Basin, the gas exploration have a good prospect. The questions about origin and accumulation of the natural gas in oolitic reservoir have been one of the focus in the field of petroleum geology.Systemic geochemical analyses such as organic petrology, geochemistry, catalytic hydropyrolysis and GC-IRMS show that, the solid bitumen in reservoir of Feixianguan Formation in northeastern Sichuan basin are residuals(pyrobitumen) generated from oil cracking. Changxing Formation is the primary hydrocarbon source rock for the solid bitumen, while Feixianguan and lower Silurian are the subordination, according to the carbon isotopic compositions of the insoluble constituent and the biomarkers such as sterane and terpane in saturated hydrocarbons of the solvent extracts. There is an obviously genetic relationship between the solid bitumen in Well Po2 of Feixianguan Formation and the kerogen(derived from the hydrocarbon source rock) in Well Guan5 of Feixianguan Formation according to distribution and molecular carbon isotopic compositions of the saturated hydrocarbon of catalytic hydropyrolysis products. A evidence that the source rock in Feixianguan Formation has contributions to the gas pools of Feixianguan Formation can be acquired.The differences or similarities of gas generation mechanism between crude oil and its asphaltenes were discussed based on that pyrolysis was carried out in anhydrous closed system (gold tubes). The yields and carbon isotopic composition of gaseous hydrocarbons and the yields of pyrobitumen were determined. The yields of gaseous hydrocarbon have analogy change trend with the yields of pyrobitumen. The yield of methane generated from asphaltene cracking is 50% of that of oil cracking when oil and alsphsltene crack completely, and the quality rate of total gas to pyrobitumen generated respectively from oil and its alsphsltenes is 6: 4 and 3: 7 here. In the process of pyrolysis, the relation ofδ¹³C₁<δ¹³C₂ <δ¹³C₃ exist as a whole, and theδ¹³C value of methane, ethane and propane generated from oil is more than that of asphaltene correspondingly.The pyrolysates including gas, liquid and solid of oil cracking were quantitatively analyzed. Based on the pyrolysis data and kinetic calculation, the yield correlativity among gas, liquid and solid products was regressed with high correlative coefficients to establish a prediction model suitable for the resource estimation of oil cracking gas. The verification formula for this model was also established on the principle of mass conservation. The affecting factors and the application preconditions of this prediction model were discussed. This model would enlighten and provide some new ideas for the resource assessment of natural gas in the high and over mature marine carbonate source rock areas in China.The kinetics parameters were extrapolated to geological conditions to establish a kinetics and geological model of gas generation of oil cracking in paleo-oil reservoir, according to the history of subsidence, paleo-temperature and tectonics of Feixianguan Formation. And then the course of generation, migration and accumulation of oil cracking gas in oolitic reservoir was recurred.