Abstract:

The Kelasu oil and gas area in the Tarim Basin serves as a significant gas source for the “West East Gas Transmission” project in China and stands as the largest ultra-deep oil and gas field developed in the country. The primary reservoir formation targeted in this region is the Cretaceous Bashjiqike Formation, characterized by medium-thick fine to medium sandstone interbedded with thin mudstone layers. Burial depths typically range between 6000 m and 8000 m, and the reservoir matrix’s porosity generally remains below 10%. The pore-throat structure is intricate, with widespread development of structural fractures. The presence of an effective fracture network is essential for achieving high and consistent gas production from this type of reservoir. In the context of deep burial, high temperatures, and pressures, the rapid activity of diagenetic fluids along fractures significantly influences the effectiveness of reservoir fractures. This article conducts microscopic experimental analyses, including CT scanning, main thin section analysis, cathodoluminescence, laser confocal scanning, and scanning electron microscopy on numerous underground rock core structural fractures. The study systematically investigates the diagenetic types of fluids, their configuration relationships, effective fracture openings, and impact distribution range at various scales—from micro-fracture to core fracture, trap, and oil and gas field scales. The impact of diagenetic fluids on the effectiveness of structural fractures within the Cretaceous Bashjiqike Formation of the Kelasu oil and gas field primarily manifests in the cementation, filling, and dissolution of the fracture surfaces themselves and the surrounding reservoir pores. The cementation filling rate of the main fractures, exceeding 60%, typically remains below 5%, with effective openings ranging from 0.2 mm to 2 mm. Diagenetic fluids precipitate or dissolve along microcracks, affecting an approximate range of 4mm to 20 mm around the fractures. Dissolution tends to occur between different minerals in the rocks, particularly quartz and feldspar particles. Vertically, diagenetic fluids follow the fracture network within the upper and middle parts of the anticlinal reservoir, where both cementation and dissolution processes take place. The overall filling rate ranges from 60% to 80%, with the cementation filling rate of fractures in the lower water layer reaching 60% to 90%. Horizontally, diagenetic fluids migrate along the paths of previous sedimentary water systems and structural fractures, with a north-south influence distance spanning 20 km to 40 km, predominantly filled and cemented with calcite. Given the efficient communication of the fracture network in the middle to upper part of the target layer, drilling and completion operations are recommended in these sections to avoid the “basal cementation” layer at the top. In the northern block, acid fracturing techniques are employed to enhance the overall permeability of the reservoir by targeting the large opening fractures filled with calcite material.

Key words: ultra-deep, diagenetic fluids, effectiveness of structural fractures, tight reservoir, Tarim Basin