Hydrocarbon migration and accumulation in a thrust fault-controlled deep reservoir: Insight from the THM coupling numerical modeling

Faults critically control hydrocarbon migration and accumulation, especially in deep to ultra-deep environments where reservoir quality is generally poor. However, current understanding of fault-controlled hydrocarbon accumulation remains largely qualitative, relying on geological interpretation and conceptual models. A quantitative reconstruction of episodic hydrocarbon expulsion, migration, and accumulation during fault activity under in-situ temperature, pressure, and stress conditions remains lacking, thereby constraining a mechanistic understanding of fault-controlled petroleum systems. In western China, thrust-fault-controlled hydrocarbon reservoirs are widely developed in superimposed basins. This study establishes a geological conceptual model based on typical deep reservoirs, incorporating multiple reservoir–seal assemblages and fault systems. Numerical simulations of hydrocarbon migration and accumulation under fully coupled thermo‑hydro‑mechanical (THM) conditions were conducted using COMSOL Multiphysics. The research quantitatively evaluates the effects of fault geometry, reservoir–seal configurations, and fluid properties on accumulation dynamics. The high-resolution simulations of the fully coupled THM processes reveal that during active faulting periods, hydrocarbons preferentially migrate vertically along the high-permeability damage zone on the fault zone, and are blocked by the seal rock, showing a top-down charging into the reservoirs. During transitional periods, diminished vertical conductivity leads to hydrocarbon accumulation preferentially in proximal, bottom reservoirs. Hydrocarbon enrichment is jointly controlled by fault type (reverse faults being more favorable than normal faults), fault activity sequence, and the relationship between strata and fault tendency. Notably, a “seal-before-break” fault activity pattern can lead to instantaneous release of overpressure-driven hydrocarbons, facilitating highly efficient hydrocarbon accumulation. This study provides a quantitative reconstruction method for fault‑controlled hydrocarbon migration and accumulation under realistic subsurface conditions. It advances the mechanistic understanding of fault‑controlled petroleum systems and offers theoretical support for exploring deep fault‑related reservoirs.