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

摘要： 库车前陆冲断带作为塔里木盆地深层天然气最富集的区带,其天然构造裂缝是气藏高产稳产的关键控制因素。裂缝可通过溶蚀扩容作用将相邻孔隙连接成带。然而,对于裂缝参与溶蚀过程的动态特征、缝-孔-喉结构配置对储集空间微观结构的改造机制,以及选择性溶蚀的矿物响应等问题,目前仍缺乏系统深入研究。选取克深气田2组含裂缝岩心样品,采用高温高压热模拟技术,在设置成藏期有机酸流体介质、温压条件及开放流通体系下,结合扫描电镜分析、离子浓度测定和CT扫描等手段,通过同位观测溶蚀前后样品的矿物形态、孔隙结构和物性变化,揭示了裂缝附近孔隙的溶蚀成岩效应。实验结果显示:随着反应时间延长,有机酸的pH值持续降低,溶液中的Ca²⁺、Mg²⁺、Na⁺、K⁺离子浓度递减,Si⁴⁺离子浓度呈先降后升的趋势;扫描电镜显示样品的粒间孔隙在溶蚀后显著扩大,白云石和长石发生差异性溶蚀;溶蚀实验后,2组样品的孔隙度分别提升30%和50%,渗透率增幅分别达18倍和83倍;CT扫描分析证实,裂缝附近的孔隙空间在溶蚀后有显著扩张,2组样品在单位体积内的孔隙总体积分别增加了5.52倍和2.42倍,喉道体积分别增长了6.65倍和3.07倍,孔喉半径明显增大。相较于孔隙度,沿裂缝的溶蚀作用对渗透率的改善更为显著;溶蚀空间的分布受控于裂缝的充填程度、粒间孔隙数量及连通性。研究表明,裂缝的有效性与储层基质物性是决定裂缝-流体耦合溶蚀效果的关键要素,次生孔隙-裂缝储集系统的发育对超深层优质高孔/高渗储层勘探具有重要指导意义。

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