Abstract:

Tertiary Qianjiang formation is rich in gypsum and salt.The content of chlorine ions in argillites is 0.1—1.5%,the average content of organic carbon and hydrocarbon is 0.61%,996 ppm respectively.The kerogen in Qianjiang formation is of mixed type with a higher genetic potential for petroleum.The normal-paraffins of source rocks from Qianjiang formation mostly fall in the C₂₂-C₃₄,and show a distinct even-odd predominance with EOP values from<1 to 1 approximately.During the deposition of this formation,in part of the fringing areas where the sedimentary layers have relatively nor-mal water salinity,the normal-paraffins also show a odd-even predominance with OEP values from>1 to 1 approximately.These two predominance corres-pond to each other,but the former is the major one.Therefore,the authors consider that the even-odd predominance is the main path of the hydrocar-bon evolution in source rocks of salt lake facies.Among the isoprenoid alkanes in source rocks of Qianjiang formation,phytanes are predominant with pristane-phytane ratios(pr/ph)<1.Because of the well developed gypsum and salt and the lower geothermal gradient(2.3—3.0℃/100m)in the Jianghan basin,the evolution rate is slower and the buried depth is deeper than that of the organic matters in common continental source rocks.In Qianjiang formation,the petroleum generation threshold is about 2000m,mature threshold 2800m,petroleum genera-tion peak 3700m and the lower limit of liquid petroleum generation 4600m.Therefore the thickness of liquid window of hydrocarbon can be up to 2600m.The crude oil in Qianjiang formation can be classified into low-matured oil and matured oil.Its maturity is basically coincident with the maturity of source rocks,which reflects its features of close to oil source,short-distance migration,and which shows oil is generated from and reservoired in Qianjiang formation itself.