关键词: 库车前陆冲断带, 白垩系, 深层砂岩, 裂缝, 溶蚀作用, 实验模拟

Abstract: The Kuqa foreland thrust belt possesses the most abundant deep natural gas in Tarim Basin, where natural structural fractures serve as the key control factors for high and stable production of gas reservoirs. Fractures can connect adjacent pores into continuous zones through dissolution and capacity expansion. However, critical knowledge gaps still exist in the dynamic characteristics of fractures participating in dissolution processes, modification mechanism of fracture-pore-throat structural configuration on the microstructure of reservoir space, and mineral response to selective dissolution. In this study, two sets of fractured core samples were selected from Keshen gas field, and further processed using high-temperature and high-pressure thermal simulation technology. Under the conditions of organic acid fluid, temperatures and pressures, coupled with the open circulation system during hydrocarbon accumulation period, combined with scanning electron microscopy (SEM)analysis, ion concentration measurement and CT scanning, a collocated observation was performed on the mineral morphology, pore structure and physical properties of the samples before and after dissolution, thus revealing the dissolution and diagenesis effect of pores near fractures. The experimental results show that the pH value of organic acid solution continuously decreases, the ion concentrations of Ca²⁺, Mg²⁺, Na⁺ and K⁺ in the solution progressively decline, and the ion concentrations of Si⁴⁺ decrease first and then increase with the extension of reaction time. SEM observations reveal the significant enlargement of intergranular pores and differential dissolution of dolomite and feldspar after dissolution. For the two sets of post-experimental samples, the porosity is increased by 30 % and 50 %, and the permeability is increased by 18 and 83 times, respectively. CT scanning has confirmed that the pore space near fractures is expanded significantly after dissolution. The total pore volume per unit volume of the two sets of core samples is increased by 5.52 and 2.42 times, and the throat volume is increased by 6.65 and 3.07 times, respectively. The pore throat radius is obviously increased. Compared with porosity, the improvement of permeability by dissolution along fractures is relatively more pronounced. The spatial distribution of dissolution is governed by fracture filling condition, number of intergranular pores, and connectivity. Studies demonstrate that fracture effectiveness and reservoir matrix properties are the key decisive factors for the fracture-fluid coupling dissolution effect. The development of secondary porosity-fracture reservoir systems has important guiding significance for the exploration of high-quality, high porosity and high permeability reservoirs in ultra-deep formations.

Key words: Kuqa foreland thrust belt, Cretaceous, deep sandstone, fracture, dissolution, experimental simulation