Deformation Characteristics and Sealing Capacity Evaluation of Dolomite-Bearing Anhydrite and Dolomitic Anhydrite Cap Rocks—A Case Study of the Middle Cambrian in the Eastern Tazhong Area

The Middle Cambrian salt–anhydrite succession in the Tarim Basin has been regarded as an effective regional cap-rock. However, numerous Ordovician hydrocarbon reservoirs have been discovered above the anhydrite, and recent drilling has identified industrial oil and gas flows beneath anhydrite-bearing intervals. These findings call into question the sealing effectiveness of anhydrite rocks in deep subsalt settings. In this study, X-ray diffraction (XRD), petrographic analysis, scanning electron microscopy (SEM), and triaxial compression tests were conducted to investigate the mineral composition, deformation behavior, and failure mechanisms of anhydrite rocks. The results indicate that: (1) dolomite-bearing anhydrite undergoes plastic deformation at depths greater than 4400~4600 m (~70 MPa confining pressure), whereas dolomitic anhydrite enters the plastic deformation regime below 5200~5400 m (~80 MPa confining pressure); (2) the deformation evolution of the cap rocks can be divided into four stages. Stages I–III are dominated by brittle deformation, with plasticity progressively increasing with confining pressure, whereas Stage IV is characterized by pervasive plastic deformation and strong sealing capacity, representing an effective cap rock during the critical period of hydrocarbon accumulation; (3) Middle Cambrian reservoirs in the eastern Tazhong area were destroyed by reverse faults that cut through brittle Middle Cambrian cap rocks. In contrast, Lower Cambrian gas reservoirs were charged during the Himalayan period, when the cap rocks remained intact, and exhibited strong sealing capacity. This study demonstrates the temporal variability in the sealing effectiveness of Middle Cambrian anhydrite cap rocks in the eastern Tazhong area and provides a methodological basis for deep and ultra-deep subsalt hydrocarbon exploration.