Abstract: In 2024, according to the exploration idea of "fracture-cavity gas reservoirs formed after fracture reconstruction of carbonate rock", three exploration wells were deployed in the Ordovician subsalt area of Ordos Basin to test gas and obtain high-yield industrial gas flow, revealing good exploration potential for the subsalt fracture-cavity gas reservoirs. However, the high-yield and enrichment regularity and comprehensive prediction model of fracture-cavity gas reservoirs are still unclear, making it difficult to determine favorable zones and achieve drilling targets. Therefore, based on cores, thin sections, seismic data, logging data, and production performances, this paper investigates the basic geological conditions and high-yield and enrichment mechanism of the Ordovician subsalt fracture-cavity gas reservoir in Ordos Basin. The results show as follows. (1)The subsalt fracture-cavity gas reservoirs have dual source hydrocarbon supply conditions of the Upper Paleozoic coal measures and Lower Paleozoic marine source rocks, of which the latter are the main source rocks, with the maximum hydrocarbon generation intensity of 1.2×10⁸m³/km² and sufficient hydrocarbon supply capacity. (2)The reservoir spaces of the subsalt fracture-cavity units are mainly composed of fractures and dissolution pores developed along the fracture zone, mixed with a small amount of matrix intergranular pores. The average porosity can reach more than 10 %, and the average permeability can reach up to 10 mD, demonstrating good reservoir performance. (3)The subsalt faults and associated fracture systems can not only improve the physical properties of dolomite reservoirs and form fracture-cavities with good reservoir performance, but also communicate source rocks with reservoirs, thus providing effective channels for oil and gas to accumulate in fracture-cavity reservoirs. (4)The thick layer of gypsum-salt rock developed in the 6th submember of Member 5 of Majiagou Formation serves as the regional cap rock for the fracture-cavity gas reservoir. The gypsum-salt rock developed in the 10th submember of Member 5 of Majiagou Formation, as well as the Member 3 of Majiagou Formation, serves as the cap rock overlying on the gas reservoir. Moreover, the tight carbonate rocks around the fracture-cavities form lateral sealing, and those good preservation conditions are conducive to natural gas enrichment and formation of fracture-cavity gas reservoirs. Through comprehensive evaluation, it has been preliminarily determined that the favorable exploration area for the Ordovician subsalt fracture-cavity gas reservoirs is about 2.5×10⁴km², and the estimated natural gas reserves can reach 5000×10⁸m³, indicating great exploration potential. It is an important direction and real target for exploration of the Ordovician subsalt natural gas.

Key words: Ordos Basin, Ordovician, subsalt fracture-cavity gas reservoir, dual source hydrocarbon supply, strike-slip fault, fracture system

摘要： 2024年,按照"断裂改造碳酸盐岩形成缝洞体气藏"的勘探思路,针对鄂尔多斯盆地奥陶系盐下勘探层系部署实施了3口探井,试气获得高产工业气流,揭示了盐下缝洞体气藏具有良好的勘探潜力。然而,目前对于缝洞体气藏的高产富集规律及综合预测模式尚不明确,难以落实有利区带与钻探目标。基于岩心、岩石薄片、地震、测井和生产动态等资料,对鄂尔多斯盆地奥陶系盐下缝洞体气藏的基本地质条件及高产富集机理进行了分析。研究结果表明:①鄂尔多斯盆地奥陶系盐下缝洞体气藏具备上古生界煤系与下古生界海相烃源岩双源供烃条件,其中,下古生界海相烃源岩为主要烃源岩,最大生烃强度为12×10⁸m³/km²,具有充足的供烃能力;②盐下缝洞体的储集空间以裂缝及沿裂缝带发育的溶蚀孔洞为主,含少量基质晶间孔,平均孔隙度可达10%以上,平均渗透率最高可达10 mD,具有良好的储集性能;③盐下断裂及伴生裂缝系统不仅能够改善白云岩储层的物性,形成储集性能良好的缝洞体,还可以沟通源岩与储层,为油气向缝洞体储层聚集提供有效的运移通道;④马家沟组五段6亚段发育的厚层膏盐岩为缝洞体气藏的区域盖层,马家沟组五段10亚段和马家沟组三段等层系中发育的膏盐岩为气藏的直接盖层,缝洞体周围的致密碳酸盐岩构成侧向遮挡,这些好的保存条件有利于天然气富集并形成缝洞体气藏。通过综合评价,初步落实鄂尔多斯盆地奥陶系盐下缝洞体气藏的有利勘探面积约为2.5×10⁴km²,预计天然气储量规模可达5000×10⁸m³,勘探潜力大,是奥陶系盐下天然气勘探的重要方向与现实目标。

关键词: 鄂尔多斯盆地, 奥陶系, 盐下缝洞体气藏, 双源供烃, 走滑断裂, 裂缝系统