DISCOVERY OF IMMATURE HIGH SULFUR CRUDE OIL AND ITS SIGNIFICANCE IN GYPSUM-SALT BEARING SEDIMENTARY BASIN

Immature high sulfur heavy crude oil is discovered in Bang 1 Well at depth of 610.23-658.5 m in Jianghan gypsum-salt bearing basin.The principal characters of the crude oil are of low content of saturated hydrocarbons,high content of non-hydrocarbons and remarkable even-odd predominance in n-alkyl hydrocarbons.It is particularly rich in the biomark-ers,such as phytane,gamma cerane,nicked porphyrin,and long-chain alkyl thiophene,tetrahydrothiophene,etc.The alkyl thiophene and alkyl tetrahydrothiophene also have an obvious even-odd predominance.The original configuration of biologic materials is ref lectedin strerane distribution.Based on the geological structure and lithologic characters of the occurrence of the crude oil,as well as biomarkers,it is concluded that such a crude oil can be assigned to immature oil generated and accumulated in situ.Its maturity corresponds to the value of R_o0.36%.Certainly,the generation of such an immture heavy crude oil with high content of sulfur could be related to the source rocks rich in sapropelic organic material at shallow depth and low temperature.However,the more decisive factor controlling its generation may be the strong reduction environment of gypsum-salt with high content of sulfur.