Distribution And Hydrocarbon Generation Intensity For The Permian Source Rocks In Southern Sichuan Basin

The Maokou Formation in the Lower Permian and the Jialingjiang Formation are the main producing-gas layers of Petro China's exploration area in south Sichuan Basin.In recent years,the Maokou Formation and the Jialingjiang Formation both have industrial gas outputting in the SINOPEC's exploration area of the eastern study area,presenting some potential.Natural gas in many gas fields discovered of the Maokou Formation and the Jialingjiang Formation is derived from the Permian source rocks,but the main layers and hydrocarbon generation potential of which,so far,however,have not been studied systematically,restricting the exploration progress of natural gas.Based on outcrop geology,investigating and sampling of cores and cuttings,and applying petrology and technique of organic geochemistry for oil/gas of the research area,together with logging data processing and interpretation,the distribution and development of the source rocks,geochemical characteristics,the gas generation intension of the source rocks are studied in this paper.The following conclusions are drawn:(1)There are two sets of source rocks in the Permian of south Sichuan Basin,i.e.,the Lower Permian marine carbonate rocks and the Upper Permian transitional mudstone rocks.(2)The Longtan Formation are coal-measure source rocks.The Longtan Formation dark mudstone source rocks and coal are 20m?100m and 4m?16m thick in the research area,respectively.Mudstone in the Longtan Formation are characterized by high organic matter abundance.The TOC values are 0.10%?28.84%,with an average value of 5.61%.The source rocks can reach an excellent level in quality.The type of kerogen is mainly type-?and the maturity level is in stage of high maturity and stage of post mature,mainly generate dry gas.The gas generation intensity of the Longtan Formation source rocks is(15?50)×10~8m~3/km~2 that possess the high hydrocarbon generation potential,laying a robust foundation for the generation of a large gas pool.(3)The Maokou source rocks are mainly distributed in the 1st member and the lower section of the second member of the strata,and the Qixia ones are mainly developed in the 1st member of the formation.The Maokou Formation source rocks are better than the Qixia ones:the thickness and the TOC values of the former are60m?110m and 0.07%?4.86%with an average value of 1.08%,respectively,and source rocks can reach a medium level in quality,while those of the latter are 20?45m and0.21–0.79%with an average value of 0.46%,respectively,and source rocks in a poor quality.The organic matter type of the Maokou Formation and the Qixia Formation are both mainly type-?,few type-?,and thermal evolution degree is high maturity to post mature,mainly generate dry gas.The gas generation intensity of the Qixia Formation source rocks is(0.2?2.5)×10~8m~3/km~2,is very low,with the poor hydrocarbon generation potential,while that of the Maokou Formation is much greater with a range of(10?22)×10~8m~3/km~2 that possess the medium hydrocarbon generation potential,laying a robust foundation for the generation of a medium gas pool.