青海柴达木盆地南八仙油气田油源与成藏机理

Source of oils and mechanism of reservoir-formation of the Nanbaxian oil and gas field, Qaidam Basin, Qinghai Province, China

柴达木盆地南八仙油气田不同油气藏间原油性质特征变化明显,油层在纵向井段上分布范围逾3000余m。通过地球化学实验并结合构造等地质条件分析,认为该区各种深、浅层油气藏中的原油具有煤成烃的基本地球化学特征,并且为同一来源,油源为临近地区较深部位中侏罗统煤系地层;该区深、浅层原油物性间的差异也反映在地球化学指标上,其成因或成藏机理是:深部早期油藏在断裂等地质作用下发生了明显的蒸发分馏作用,其分馏出的产物在浅部再次聚集成藏。

The oil properties of different reservoirs show remarkable variations from the Nanbaxian oil and gas field of Qaidam Basin, with the oil measures reaching more than 3 000 m. There are black oils in deep reservoirs (<IMG SRC='IMAGE/08130154.JPG' HEIGHT=11 WIDTH=10>) and condensate oils in shallow reservoirs (N1- <IMG SRC='IMAGE/08130155.JPG' HEIGHT=11 WIDTH=10>). Based on the study of organic geochemical experiment of oil and the analysis of petroleum geologic background, the following conclusions have been made. (1) These oils are of basic geochemical characters of hydrocarbons generated from coal measures, which are related with source inputs of high plants in mild oxidation mild reduction sediment environment and come mainly from Middle Jurassic coal measures near the Nanbaxian oil and gas field indicated by data from oil and source rock correlation. (2) The oils from deep reservoirs are of the same geochemical feature in biomarker distribution and carbon isotope components as those from shallow reservoirs, they both from common source rock. (3) The variation of chemical property, composition and some geochemical indexes also exists in the different oils from deep and shallow reservoirs, for example, the contents of some aromatic compounds (such as benzene, toluene and dimethylbenzene) in the oils from deep reservoirs are unconventionally high compared to the oils (condensate) from shallow reservoirs. These phenomena are caused by evaporative fractionation, in that case the oils from deep reservoirs are residuals, and the condensates from shallow reservoirs are products. (4) In geological history, special structure geological events happened in this area, which induced the evaporative fractionation, as a result, the early deep reservoirs were transformed and the oils from them migrated up to the shallow strata of lower stratum pressure due to fracturation, then these oils accumulated to form the shallow reservoirs.<P>