Organic geochemistry of oils and condensates in the Kekeya Field, Southwest Depression of the Tarim Basin (China)

This study shows that the oils and condensates in the Kekeya Field in the Tarim Basin, NW China, belong to a single family, most likely derived from marine shale source rocks deposited under oxic–suboxic conditions with mixed terrigenous and algal–bacterial organic inputs. The maturity data clearly indicate that the paraffinic condensates were not formed by thermal cracking of oil during late catagenesis (Ro>1.2%). Both the oils and condensates were generated within the normal oil window, whereas addition of gaseous hydrocarbons from a separate source resulted in migration fractionation and hence spuriously high heptane indices. Age specific biomarkers show that the oils and condensates were not generated from the Mesozoic–Cenozoic strata, but from the Carboniferous-Permian sections. 1D basin modeling results are consistent with this interpretation, suggesting that the Mesozoic–Eocene strata are currently immature with respect to hydrocarbon generation and expulsion. Deep-seated faults may have provided routes for upward fluid migration at the time of active deformation during several pulses of the Himalayan orogeny. The favored targets for further petroleum exploration in front of the Kunlun Mountains include the deep structures within the Carboniferous–Permian strata for indigenous petroleum accumulations and anticlines and stratigraphic traps within the Mesozoic–Cenozoic sections that are cut by deep-seated thrust faults where secondary petroleum accumulations most likely occurred.