Model of 10,000-m deep petroleum geology and the accumulation and preservation of ultradeep oil and gas in cratonic basins, China

With the gradual depletion of oil and gas resources in shallower strata (<4000 m), exploration toward ultradeep strata (>6000 m) is becoming inevitable. Recent 10,000-m drilling projects in China highlight the petroleum accumulation and enrichment model and reserve potential in ultradeep strata of cratonic basins. This study systematically summarizes 10,000-m deep petroleum geology model, based on recent research progress and exploration discoveries. The lower limit of liquid petroleum preservation depth in basins with low-geothermal gradient (e.g., Tarim Basin with 19.6°C/km) can reach 9000 m, breaking through conventional understanding of a 6000-m depth limit. Different hydrocarbon compounds exhibit distinct thermal stability in that saturated hydrocarbons are more resistant to cracking than aromatic hydrocarbons, whereas diamondoids accumulate during oil cracking and serve as reliable maturity markers. Recent ultradeep drilling confirms this expanded exploration potential. The research reveals preservation mechanisms for paleo oil pools (250 Ma hydrocarbon accumulation age) and gas pools (100 Ma hydrocarbon accumulation age), and thus redefines the ultimate lifespan of oil/gas pools. Experimental simulations have shown that caves and pores are influenced by only lithostatic pressure and can remain even at depths of 75,000 m, carbonate reservoirs therefore have no firmed depth limit. The flow and migration of hydrocarbon fluids in ultradeep strata differ significantly from the buoyancy-driven models in middle to shallow strata. These processes do not fully follow the buoyancy differentiation law and feature multioil-water interfaces in fracture-cavity type reservoirs. Flow and occurrence model of hydrocarbon fluids in heterogeneous reservoirs has thus been established, introducing the concept of large-area stratified cavity-fracture type bead-like pools without unified oil-water contacts. Ultradeep petroleum exploration has revealed oil fields with nearly 1000-m-high oil columns and a series of wells producing up to 1000 metric tons of oil per day (7330 BOPD), challenging traditional theories of petroleum accumulation that focus on high points of traps. These findings provide theoretical support for significant discoveries and strategic changes in ultradeep petroleum exploration and expand petroleum exploration space up to 10,000 m deep.