Deep petroleum accumulation models revisited: Case studies from the Tarim basin

The progressive expansion of oil and gas exploration to the deep and ultra-deep strata suggests a great potential in deep parts of petroliferous basins for oil and gas exploration. A number of scientific issues have also emerged including hydrocarbon generation? reservoir development and hydrocarbon accumulation in the deep and ultra-deep strata. On the basis of extensive literature review? this paper elaborates upon the unique environments for deep oil and gas accumulations and factors controlling the deep source, reservoir and cap rock formation? and focuses on four fundamental issues that need to be paid more attention in studying the formation and evolution of deep and ultra-deep oil and gas reservoirs: research on oil and gas generation? reservoir storage space formation? oil and gas phase behavior and hydrocarbon migration in deep strata needs to be carried out on the premise of the basic laws of material balance and energy conservation; the deep strata has experienced the entire basin evolution process from shallow to deep; it is thus necessary to study oil and gas accumulation from the perspective of dynamic evolution; it is necessary to explore new experimental methods and enhance our understanding on the stability of liquid hydrocarbons in deep basin and their maximum preservation depth (temperature) ; more attention needs to paid to the integration of multidisciplinc and intersecting multiple technologies to unravel deep and complex geological issues. Two possible reservoir forming models for deep and ultra-deep reservoirs are proposed: middle-shallow depth emplacement and deep-burial preservation type; prolonged shallow burial and rapid late-stage deep burial (condensate) gas accumulation type. Taking the Shunbei area in the central carbonate platform of the Tarim basin and the Bozi-Dabei area in the Kuqa depression as two typical examples? oil and gas accumulation in the two areas were studied using reservoir geochemical analysis? a suite of fluid inclusion analysis techniques? in-situ calcite U-Pb geochronology and basin modelling. The results show that the Ordovician deep oil and gas reservoirs in the Shunbei area are characterized by "an early middle-shallow accumulation? a late-stage deep burial and sustained preservation". Relatively stable structural settings after oil and gas being entrapped is the key factor for oil and gas reservoirs to be preserved. The formation of the Cretaceous ultra-deep condensate gas reservoirs in the Bozi area mainly occurred in deep- to ultra-deep settings. A "prolonged shallow burial and late-stage rapid deep burial" is crucial to the formation of this type of deep condensate gas reservoirs. Both examples attest the two reservoir forming models proposed for deep oil and gas reservoirs. The findings may have significant implications for enhancing our understanding of deep and ultra-deep oil and gas accumulations and may provide new insights for deep and ultra-deep oil and gas exploration elsewhere. © 2023 Geological Society of China. All rights reserved.