Origin of deep oil accumulations in carbonate reservoirs within the north Tarim Basin: Insights from molecular and isotopic compositions

Abstract

Large amounts of light oil have been produced from carbonate reservoirs at depths below 7 km in the recently discovered Shunbei and Yuecan oil fields within the north Tarim Basin. Eighteen samples were investigated for molecular and isotopic compositions to help understand the origin of these deep oil accumulations. Several ratios based on C6 and C7 hydrocarbons suggest that they were not affected by evaporative fractionation. Linear relationships between the carbon number and log of the mole fraction of n-alkanes support no fractionation alteration. The correlation between concentrations of methyldiamantanes and stigmastane indicates that these oils were not severely cracked and probably at the beginning or an early stage of intense oil cracking. C29 sterane isomerization ratios 20S/(20S + 20R) and ββ/(ββ + αα) have reached their equilibrium points while a good linear relationship between the concentrations of terpanes and steranes suggests variations in maturity. Calculated vitrinite reflectance of source rocks based on phenanthrene and diamondoid hydrocarbons shows different ranges, which may reflect multiple oil charges to these deep oil accumulations. A narrow range of δ13C values of whole oil and oil fractions suggest a genetic correlation of these oils. Triangular plots based on C6 and C7 hydrocarbons and diamantanes also indicate a very similar source. In addition, several ratios based on n-alkanes, isoprenoids, tri-, tetra-, and pentacyclic saturated hydrocarbons support a common source. However, oil SB7 seems to differ from the others in terms of source rock depositional environment, indicated by its much higher pristane/phytane ratio and lower dibenzothiophene/phenanthrene ratio as well as a much lower dibenzothiophenes concentration.