Control of structure and fluid on ultra-deep fault-controlled carbonate fracture-vug reservoirs in the Tarim Basin, NW China

Abstract

This study comprehensively uses various methods such as production dynamic analysis, fluid inclusion thermometry and carbon-oxygen isotopic compositions testing, based on outcrop, core, well-logging, 3D seismic, geochemistry experiment and production test data, to systematically explore the control mechanisms of structure and fluid on the scale, quality, effectiveness and connectivity of ultra-deep fault-controlled carbonate fractured-vuggy reservoirs in the Tarim Basin. The results show that reservoir scale is influenced by strike-slip fault scale, structural position, and mechanical stratigraphy. Larger faults tend to correspond to larger reservoir scales. The reservoir scale of contractional overlaps is larger than that of extensional overlaps, while pure strike-slip segments are small. The reservoir scale is enhanced at fault intersection, bend, and tip segments. Vertically, the heterogeneity of reservoir development is controlled by mechanical stratigraphy, with strata of higher brittleness indices being more conducive to the development of fractured-vuggy reservoirs. Multiple phases of strike-slip fault activity and fluid alterations contribute to fractured-vuggy reservoir effectiveness evolution and heterogeneity. Meteoric water activity during the Late Caledonian to Early Hercynian period was the primary phase of fractured-vuggy reservoir formation. Hydrothermal activity in the Late Hercynian period further intensified the heterogeneity of effective reservoir space distribution. The study also reveals that fractured-vuggy reservoir connectivity is influenced by strike-slip fault structural position and present in-situ stress field. The reservoir connectivity of extensional overlaps is larger than that of pure strike-slip segments, while contractional overlaps show worse reservoir connectivity. Additionally, fractured-vuggy reservoirs controlled by strike-slip faults that are nearly parallel to the present in-situ stress direction exhibit excellent connectivity. Overall, high-quality reservoirs are distributed at the fault intersection of extensional overlaps, the central zones of contractional overlaps, pinnate fault zones at intersection, bend, and tip segments of pure strike-slip segments. Vertically, they are concentrated in mechanical stratigraphy with high brittleness indices.