车西洼陷陡坡带沙三下亚段近岸水下扇储层成岩演化及其对储层物性影响

利用岩芯观察、铸体薄片、扫描电镜、阴极发光及岩石物性测试等多种资料,本文对济阳坳陷车西洼陷陡坡带沙三下亚段近岸水下扇储层进行了探讨。结果表明,车西洼陷沙三下段近岸水下扇储层岩石类型宏观上主要表现为泥质杂基支撑砾岩、碎屑支撑砾岩、砾质砂岩等沉积,微观上以岩屑砂岩和长石质岩屑砂岩为主,灰泥质杂基含量最高可达40%。车西洼陷沙三下段近岸水下扇储层为典型的特低孔、特低渗储层类型,其平均孔隙度和渗透率分别为3.42%、2.02×10-3μm2;但在3 500~4 000 m深度段发育异常高孔带,孔隙度可达10.2%,储集空间多为溶蚀孔和微裂缝。研究区储层经历了复杂的成岩演化过程,目前处于中成岩B期,其成岩演化序列为:压实作用/碳酸盐胶结→长石溶蚀/碳酸盐胶结物溶蚀→石英溶蚀/碳酸盐胶结/灰泥重结晶→石英加大/碳酸盐胶结溶蚀→碳酸盐胶结。不同亚相所经历的成岩演化有所差异,其中中扇辫状水道中远端及中扇前缘经历了多期溶蚀作用,储层物性相对较好,平均孔隙度分别可达4.5%,7.25%,为研究区近岸水下扇砂砾岩储层的“甜点区”。

Diagenetic Evolution of Esx3: Nearshore Subaqueous Fan Reservoir and Its Influence on Property in the Steep Slope Zone of Western Chezhen Sub-sag

Based on the core, casting thin section, SEM, cathodeluminescence and petrophysical properties analysis data, the characteristics of the Es₃x nearshore subaqueous fan reservoir in the steep slope zone of Western Chezhen Sub-sag are discussed. The results show that lithology types are mainly the mud matrix-supported conglomerate, clast-supported conglomerate, pebbly sandstone and so on at macro level, and the microscopic lithology types are showed as lithic sandstone and feldspathic lithic sandstone and their plaster matrix content can be up to 40% for the nearshore subaqueous fan reservoir of Es₃X in the steep slope zone of the Western Chezhen sub-sag. The nearshore subaqueous fan reservoir is characterized with extremely low porosity and permeability, and their porosity and permeability are respectively 3.42%, 2.02×10-3 μm2, but there is an abnormal high porosity zone between 3 500 m and 4 000 m in the verticle section, where the porosity can be 10.2% and the reservoir space are mainly dissolved pores and micro-fractures. The reservoir had experienced complex diagenesis in the study area and it is in the B sub-stage of middle diagenesis. The reservoir diagenesis evolution sequence in the study area is as follows: compaction/carbonate cementation→ dissolution of feldspars and carbonate cements→dissolution of quartz/carbonate cementation/plaster recrystallization→quartz overgrowth/carbonate cementation dissolution→carbonate cementation. The different sub-facies of the nearshore subaqueous fan had experienced different diagenesis evolution, because the middle- distal zones of the braided channel of the middle fan and middle fan front had experienced multi-period dissolution, the relative reservoir quality is very good and their average porosity respectively are 4.5% and 7.25%, which can be regarded as the "sweet spot" of the nearshore subaqueous fan glutinite reservoir in the steep slope zone of the Western Chezhen sub-sag.