Influence of Deep CO₂-Charged Fluids on the Development of Carbonate Reservoirs in Fault-Controlled, Ultra-Deep burial setting: Insights from Water-Rock Interaction Experiment

Significant breakthroughs have been made recently in petroleum exploration within ultra-deep (burial depth > 6,000 m) carbonates in the Tarim Basin, northwestern China. The discovery of several large-scale oil and gas accumulation (e.g., Shunbei and Fuman) in these deeply buried, highly fractured and vuggy carbonates highlights the crucial role of strike-slip faults in reservoir development. However, the formation mechanisms of these ultra-deep, fault-controlled carbonate reservoirs remain poorly understood. It is therefore essential to conduct experimental simulations investigating the controlling factors and evolutionary trends governing the impact of deep CO₂-rich fluids on carbonate rocks.

For this reason, experiments were performed by using an ultra-deep, multi-tectonic-stage, high-temperature and high-pressure reservoir simulation system. This study focused on two key aspects, the dissolution mechanisms of dolomite in CO₂-saturated solutions, and the evolutionary trends of pore structure in carbonate rocks with different initial pore types during dissolution. Overall, two major findings were obtained. First, within temperature of 40–220 °C and pressure of 10–132 MPa, the saturated dissolution capacity of dolomite in CO₂-rich fluids exhibited an initial increase that was followed by a decrease, with the maximum dissolution occurred approximately at 60–110°C. This provides the theoretical basis for predicting favorable depth intervals where large-scale secondary pores may be formed in dolomite by deep CO₂-rich fluids. Second, influenced by the deep CO₂-rich fluid dissolution, both pore-dominated and fracture-dominated limestones tend to transform into fracture-vug reservoirs. Dissolution preferentially occurred along major fractures, gradually enhancing reservoir space and percolation capacity, ultimately becoming concentrated within these main fracture systems.

These results led to the construction of a genetic model for the development of fault-controlled, fracture-vug carbonate reservoirs. When deep CO₂-rich fluid activity coincides with fault development periods, fluids preferentially migrate into main faults, leading to dissolution-enlarged porosity along fault planes. When fluids migrate to fault intersections, they stagnate and induce dissolution and connectivity to form vugs. The fluids continue to expand along multiple sets of pre-existing faults, stagnating at new fault intersections to create more vugs. Such dissolution cycles are controlled by the episodic regional tectono-fluid activity. Ultimately, early-formed fracture-vug systems may become merged to formwell-connected fracture-vug reservoirs with superior reservoir performance. This model effectively explains the differences in dissolution and modification effects observed in different segments of strike-slip faults and clarifies the underlying mechanisms.