Organic geochemical analyses (whole oil stable carbon isotopes, sulfur content, gas chromatography, gas chromatography/mass spectroscopy, and gas chromatography/mass spectroscopy/mass spectroscopy) have identified at least eight different genetic groups of oils in the Tarim basin of NW China. These oils derive from two main source rock intervals in Tarim, the Ordovician and the Jurassic. The combination of new organic geochemical data and published subsurface data allows refined interpretations regarding the distribution of hydrocarbons within the basin.

Ordovician sourced oils mostly occur in wells on the Tazhong and southern Tabei uplifts. Biomarker characteristics of most samples from the Tazhong field indicate that their source rocks were probably deposited in a somewhat anoxic, carbonate marine setting, whereas southern Tabei samples are thought to be from somewhat anoxic, marine shales. These findings suggest that Ordovician sourced oils may be derived from a slope facies that occurred between the deep basinal portion of eastern Tarim and platformal deposits of western Tarim, rather than from the basinal graptolitic shales as had been widely speculated. Because the Tazhong area was structurally high during both the Ordovician and today, this model would explain why wells drilled to date in eastern Tarim have all been dry, as any oils generated from slope facies strata would migrate updip to Tazhong or the southern Tabei areas.

Unlike the widespread occurence of Ordovician strata in Tarim, volumetrically significant Jurassic strata are present only in restricted foredeeps in northern Tarim, Minfeng, SW Tarim Depressions. Therefore, any hydrocarbons sourced from Jurassic strata should be confined to restricted areas that have favorable migration pathways.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas