Geochemical characteristics of saturate hydrocarbons in crude oils and source rocks of the Qaidam, Tarim, Tupran basins, NW China

Abstract

Based on the systematic analyses of fifteen typical crude oils and ten typical potential source rocks collected from the Qaidam, Tarim and Turpan basins, Northwest China, the geochemical characteristics of the oils and source rocks were investigated and oil-source rock correlations undertaken. The oils and source rocks deposited in saline lacustrine environment from the western Qaidam Basin were characterized by n-alkanes with even carbon-number preference in the C20-C28 range, low pristane/phytane (Pr/Ph) ratios (less than 0.5), and high abundances of C27 steranes, gammacerane and C35 hopanes. The oils and source rocks deposited in marine environment from the Tarim Basin were characterized by n-alkanes with even carbon-number preference in the C14-C18 range, relatively low Pr/Ph ratios (near to 1), high abundance of C28 steranes, and relatively high gammacerane. In contrast, the oils and source rocks deposited in terrigenous bog environment from the Turpan Basin were characterized by relatively high Pr/Ph ratios (oil samples greater than 6) high abundance of C29 steranes, and relatively low gammacerane and C31–35 hopanes. The higher amounts of C37 and C38n-alkanes of source rocks from the western Qaidam Basin and the Tarim Basin suggest an origin of these alkanes from functionalized C37 and C38n-alkadienes and alkenones in prymnesiophytes living in lacustrine and marine environments. Oil-source rock correlations suggest oils in the western Qaidam Basin were derived from the Oligocene Lower Ganchaigou Formation (E3), oils in the Tabei and Tazhong uplifts from the Tarim Basin have a genetic relationship with the Middle-Upper Ordovician source beds. Oils in the Turpan Basin generally fall into two genetic types. Most oils in the Taibei depression from the Turpan Basin were derived from the Lower-Middle Jurassic coal measures, but the fewer oils in this region are a mixed source derived from the Lower-Middle Jurassic coal measure and the Upper Permian source rocks.