Geochemical Significance of Discovery in Cambrian Reservoirs at Well ZS1 of the Tarim Basin, Northwest China

Gas condensate and natural gas samples from wells ZS1 and ZS1C in the Cambrian strata of the Tarim Basin, NW China are analyzed geochemically to assess thermal maturity, extent of oil cracking and secondary alteration. Hydrocarbons are derived from the Cambrian source rocks with gas condensate and solution gas reservoired in the Middle Cambrian Awatage Formation and dry gas in the Lower Cambrian Wusonggeer and Xiaoerbulake Formations, where caverns from dolomitization and local fractures provide spaces for petroleum accumulation. Gas and gas condensate in the Middle Cambrian are mainly generated from kerogen thermal degradation at high maturity stage (condensate–wet gas window) rather than from in-reservoir thermal cracking of preexisted oils. Evaporative fractionation exerts certain impacts on gas condensates as evidenced by enrichment of light molecular weight alkanes, depletion of diamantanes in one condensate, and low aromaticity. Condensate in well ZS1C is a thermochemical sulfate reduction (TSR) alteration residual. The dry gas in the Lower Cambrian reservoirs is mainly derived from thermal degradation of kerogen with minor contribution from secondary cracking of preexisted oil. Variable H₂S content in the Lower Cambrian reservoirs is primarily migrated from deep buried strata as in situ TSR alteration is on its initial stage. The Cambrian in the Tarim Basin is an unexplored zone. Condensate without extensive oil cracking in the Middle Cambrian and TSR altered condensate in the Lower Cambrian discovered at ZS1 structure suggest a likelihood of down-dip oil. Geologic framework of the Tarim Basin also favors the preservation of liquid hydrocarbons in ultradeep strata as it reaches its maximum burial depth in last 5 millions of anna (Ma). A short heating time requires much a higher temperature than the current reservoir temperature for completion of oil thermal cracking.