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Deep Oil Cracking, TSR, Gas Invasion and Formation Mechanisms of Large Multi-Phase Reservoirs


Multi-phase reservoirs are featured with the co-existence of reservoirs with various fluid phase types. Unraveling their formation mechanisms is crucial for effective fluid prediction and hydrocarbon exploration in deep layers. Oil and gas samples from the large multi-phase reservoirs, recently discovered in the Ordovician carbonates of the Tarim Basin, NW China, were analyzed through the integration of complementary geochemical (e.g. GC, GC×GC-TOFMS, carbon isotope analysis and pyrolysis simulation) and geologic data. The formation mechanisms of the presently analyzed multi-phase reservoirs were attributed to various secondary geochemical alterations including deep oil cracking, TSR (thermochemical sulfate reduction) and gas invasion. Oil and gas in deep Cambrian layers were severely altered by deep oil cracking and TSR due to the high temperature condition (over 170oC, with corresponding depth over 8,500m) and development of evaporite rocks, leading to the generation of secondary products including diamondoids, OSCs (organosulfur compounds) and TSR-altered H2S-rich cracking gases. Such deep Cambrian gases, with diamondoids and OSCs in solution, migrated upward through strike-slip faults (gas source faults) and invaded into the paleo oil reservoirs in the shallower Ordovician carbonates and led to the changes in reservoir fluid characters and phase types. The extent of gas invasion decreased with the increasing distance between paleo oil reservoirs to strike-slip faults and thus resulted in the formation of large multi-phase reservoirs. Paleo oil reservoirs closer to strike-slip faults were impacted more severely by gas invasion to form secondary condensate reservoirs, showing obvious changes of molecular compositions, enrichment of H2S and evident carbon isotopic fractionation in C2-C4 gas components; while away from faults, condensate reservoirs gradually transitioned into weakly altered volatile-oil reservoirs and then to relatively unaltered oil reservoirs.