鄂尔多斯盆地晚三叠世湖盆中部延长组长7储层致密化因素分析

鄂尔多斯盆地晚三叠世湖盆中部长7油层组砂岩孔隙度平均为9%，渗透率为0.1×10^-3 μm²，属致密储层范畴。储集体主要为半深湖—深湖相重力流环境下砂质碎屑流砂岩及浊积砂岩，具有粒度细、填隙物含量高、孔喉形态弯曲复杂，分布集中以及次生长石溶孔发育、原生粒间孔较少等特征。运用铸体薄片、X-衍射、包裹体及电子探针等微观测试分析资料，对储层物性影响因素进行分析。分析认为，沉积、成岩及烃类充注是影响储层物性的三大因素，三者相互作用，相互影响。半深湖—深湖相砂岩特有的沉积特征导致储层压实作用强烈，使孔隙减少22%，是储层物性变差的关键因素；长7烃源岩生、排烃过程中生成的有机酸溶蚀长石等易溶物质，产生次生溶孔，对致密储层物性改造有益；烃类大量生成后的偏碱性环境利于晚期含铁碳酸盐等胶结作用进行，是生排烃高峰期后储层变致密的决定性因素。

Compaction factors of tight reservoirs in the seventh member of Yanchang Formation in the central Ordos Basin during the Late Triassic

The Chang7 reservoir (the seventh member of Yanchang Formation) in the Ordos Basin, with an average porosity of 9% and permeability of 0.1× 10-3 μm2, is a tight reservoir. Reservoirs are mainly sandy debris flows and turbidite sandstones in a semi-deep and deep lacustrine gravity flow environment, featured by fine granularity, high filler content, complicated pore and throat shape, enriched secondary dissolved feldspar pores, and poor primary intergranular porosity. The influencing factors for reservoir physical properties were analyzed according to micro test data such as cast thin section, X-ray diffraction, fluid inclusion and electron probe microscopy. Sedimentation, diagenesis and hydrocarbon charging are the three major factors affecting reservoir physical properties. The sedimentary characteristics of semi-deep and deep lacustrine sandstones were key factors that caused intense compaction and cementation in reservoirs. As a result, sandstone porosity may decrease by 22%. On the other hand, feldspars were dissolved by organic acid liquids produced by the reaction of hydrocarbon emplacement, which in turn improved physical property. Alkaline reservoir fluid after hydrocarbon charging was beneficial to the cementation of ferrous calcites, which was critical to reservoir tightening.