南华北盆地太康隆起太原组砂岩储层致密成因及天然气充注特征

Tightening genesis and gas charging characteristics of the Taiyuan Formation sandstone reservoir in the Taikang uplift, southern North China Basin

南华北盆地晚古生代时与鄂尔多斯盆地同属于华北克拉通盆地的一部分,发育广泛的石炭-二叠系煤系地层。其中,太原组不仅是重要的烃源岩层,而且具备良好的致密砂岩气等非常规天然气成藏条件。然而,关于砂岩储层的研究目前尚处于早期阶段,砂岩储层的特征、致密成因和天然气充注特征等一系列关键性问题仍然缺乏详细的研究。本文综合利用铸体薄片、扫描电镜、物性分析、X射线衍射、流体包裹体等实验测试方法研究了南华北盆地太康隆起太原组致密砂岩储层的岩石学特征、成岩特征及成岩作用下孔隙度演化历史,确定了储层致密时间和成因,同时结合地层埋藏史-热演化史,恢复了致密砂岩储层中天然气的充注过程,最终确定了砂岩储层的致密-气体充注之间的耦合关系。研究结果显示:太原组致密砂岩储层主要经历了压实、胶结和溶解等多种成岩作用,当前处于中成岩阶段B期,孔隙类型以次生溶蚀孔隙、黏土矿物晶间微孔和微裂缝为主。通过定量分析不同类型成岩作用对孔隙度的影响,确定压实作用是导致储层致密最主要的因素,造成储层原生孔隙度从38.6%降低了31.01%,黏土矿物、碳酸盐矿物及硅质胶结作用仅使孔隙度损失7.27%,长石、岩屑等颗粒溶蚀作用增孔能力有限(仅为3.28%);孔隙度演化恢复结果表明太原组砂岩储层在中成岩A期的晚期(晚三叠世)开始进入致密阶段。通过对流体包裹体的类型和成分分析,确定太原组砂岩经历过单期次的气体充注过程。含烃盐水包裹体均一温度与地层埋藏史-热史对比结果表明气体充注对应地质时间为229~213 Ma,对应地质历史时期为晚三叠世,与储层致密化时间基本一致,表明太原组砂岩储层属于“边致密边成藏”型储层。

In the Late Paleozoic, both the southern North China Basin and the Ordos Basin formed part of the North China Craton basin, and Carboniferous-Permian coal measure strata were widely developed. The Taiyuan Formation is not only an important source rock, but also has good conditions for the accumulation of unconventional natural gas such as tight sandstone gas. However, research into the sandstone reservoir is still at an early stage, and a series of key issues, such as sandstone reservoir characteristics, tight genesis and gas charging characteristics, still lack detailed research. By comprehensive utilization of a series of experimental methods including casting thin sections, scanning electron microscopy(SEM), X-ray diffraction, petrophysical property analysis and fluid inclusion observations, this work studies the petrology characteristics, diagenetic features and porosity evolution history under diagenesis of the tight sandstone reservoir of the Taiyuan Formation in the Taikang uplift, southern North China Basin. The tightening time and genesis of the sandstone reservoir are also determined, and the charging process of natural gas in the tight sandstone reservoir is deduced in combination with the burial history and thermal evolution history of the formation, and the coupling relationship between the sandstone reservoir tightness and the gas charging process is finally clarified. The results show that the tight sandstone reservoir of the Taiyuan Formation has undergone a variety of diagenesis, mainly including compaction, cementation and dissolution, and currently entered phase B of the middle diagenetic stage. The pore types are dominated by secondary dissolution pores, intergranular micropores associated with clay minerals and microfractures. Through quantitative analysis of the influence of different diagenesis on porosity, it is concluded that compaction is the most important factor leading to the density of the reservoir which results in the decrease of the original reservoir porosity(38.60%) by 31.01%, and only 7.27% loss of porosity due to the clay minerals, carbonate minerals and siliceous cementation. The feldspar and debris particles have limited ability(only 3.28%) to increase porosity by dissolution. The porosity evolution results show that the Taiyuan Formation sandstone reservoir entered the densification stage in the late stage of middle diagenetic A(Late Triassic). By analyzing the type and composition of fluid inclusions, it is confirmed that the Taiyuan Formation sandstone experienced a single gas charging process. The comparison of homogenization temperature of hydrocarbon-bearing saline inclusions with burial history and thermal history shows that the gas charging time was 229~213 Ma(corresponding to the Late Triassic), which is basically consistent with the time of reservoir densification, indicating that the densification process and the gas accumulation process may have occurred simultaneously.