Hydrocarbon-generating potential, depositional environments, and organisms of the Middle Permian Tarlong Formation in the Turpan-Hami Basin, northwestern China

The Middle Permian Tarlong Formation (P(2)t) source rocks in the Turpan-Hami Basin were studied based on multiple geochemical analyses on outcrop samples from the basin edges to reveal the hydrocarbon-generating potential, depositional environments, and organic composition. Due to long-term exposure to the surface, some outcrop source rock samples suffered weathering and displayed abnormally low S-1 + S-2, hydrogen indices, and H/C atomic ratios, and abnormally high T-max and R-o values. Hopane and sterane biomarkers in these samples were almost unaffected by weathering and likely reflect real and unaltered values. After eliminating the effects of weathering, the P(2)t source rocks in the Turpan-Hami Basin were found to exhibit high hydrocarbon-generating potential and are dominated by good to excellent source rocks and by type II kerogens. The P(2)t source rocks in the Turpan-Hami Basin display low to medium pristane/phytane (Pr/Ph, < 2.0), gammacerane/alpha beta C-30 hopane, and C-27 diasteranes/C-27 steranes ratios; low C-19 tricyclic terpane/C-23 tricyclic terpane (< 0.5) and C-24 tetra-cyclic terpane/C-26 tricyclic terpane (< 2.0) ratios; medium 4-methyl-steranes/Sigma C-29 sterane ratios; and high sterane/hopane ratios. These source rocks were deposited in a "balance-filled-lake" with minor or no terrestrial organic matter, significant algal input, especially freshwater to brackish water microalgae and acritarch organic matter input, and weakly alka-line, anoxic to suboxic brackish water.