Features of the acid and alkaline diagenetic environment of tight sandstones and the control of the reservoir physical properties: A case study of the Linxing and Shenfu district, eastern Ordos Basin
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摘要: 成岩流体酸碱性对砂岩次生孔隙的形成具有重要影响。从酸性和碱性成岩流体演化角度分析鄂尔多斯盆地上古生界致密砂岩成岩流体酸碱性对物性的控制作用。依据大量铸体薄片观察、扫描电镜、X衍射分析、常规物性测试和高压压汞,详细研究了鄂尔多斯盆地东缘临兴和神府地区上古生界致密砂岩的岩石矿物学特征、物性特征、成岩作用、孔隙成因类型和孔喉结构特征,探讨了致密砂岩储层酸性和碱性成岩流体环境的识别、分带、分布和发育的各类伴生孔隙,并分析了成岩流体环境的演化过程及物性响应特征。在埋藏过程中,工区上组合层系经历了碱性-酸性-碱性的成岩流体演变模式,下组合含煤层系经历了酸性-碱性的成岩流体演变模式。酸性成岩环境以长石溶蚀、石英加大和自生高岭石沉淀为主要特征,发育长石溶蚀孔、黏土矿物晶间孔等孔隙类型,孔喉半径较大,储层质量相对较好;碱性成岩环境以石英溶蚀、自生绿泥石沉淀为主要特征,以石英溶蚀孔、黏土矿物晶间孔为主要孔隙类型,孔喉半径较小,储层质量相对较差。酸碱过渡带致密砂岩储层介于两者之间。但是每类储层的物性和孔喉结构变化较大。成岩流体环境在纵向上呈现分带特征,自上而下发育碱性带、酸碱过渡带、酸性带、酸碱过渡带和碱性带,不同地区之间分带深度边界有差别;在酸碱过渡带中,局部地区发育酸性带、较弱酸性带和较弱碱性带。Abstract: The pH value of plaeofluid exerts great influence on the formation of the secondary pores of sandstones.In the paper, the control of the acid and alkaline diagenetic fluid environment on the reservoir quality was analyzed in the Upper Paleozoic tight sandstones in the Ordos basin.Based on the plenty of survey of casting thin section, scanning electron microscope, X-ray diffraction, routine testing of physical properties, and high pressure mercury injection, the characteristics of petrology and mineralogy, physical properties, diagenesis, genetic types of pores, and pore-throat structure within the Upper Paleozoic tight sandstones were detailedly investigated in the Linxing and Shenfu district, eastern Ordos Basin.The identification, zonage, and distribution of the acid and alkaline diagenetic environment within tight sandstones as well as the evolution of diagenetic fluid environment and its reponse of the physical properties were investigated.Results showed that the upper strata underwent a diagenetic fluid evolution of alkaline-acid-alkaline mode during the burial process, whereas the lower strata experienced a mode of acid-alkaline.Acid diagenetic environment was characterized by feldspar dissolution, quartz overgrowth, and deposition of authigenic kaolinite, in which feldspar dissolution pores, intercrystalline pores of clay minerals developed with relatively large pore radius and better reservoir quality.Alkaline diagenetic environment was featured by quartz dissolution and deposition of authigenic chlorite, in which quartz dissolution pores and intercrystalline pores of clay minerals developed with relatively small pore radius and poor reservoir quality.The case of transitional zone of acid and alkaline zone(TZAA) falls in between acid zone(AZ) and alkaline zone(AlZ).Nevertheless, the physical properties are best in the AZ, followed by TZAA.The distribution of the AZ, AIZ and TAZZ and their responds to the physical properties are significant to the reservoir evaluation.
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图 6 研究区酸性成岩环境发育特征
a.A1,1 797.24 m,下石盒子组,长石颗粒溶蚀,铸体薄片;b.A4,1 781.5 m,下石盒子组,长石颗粒溶蚀,铸体薄片;c.A4,1 993 m,太原组,长石颗粒溶蚀,高岭石晶间微孔,铸体薄片;d.B2,1 672.25 m,下石盒子组,长石颗粒溶蚀,高岭石充填孔隙,铸体薄片;e.B4,2 014.17 m,山西组,长石颗粒溶蚀,高岭石充填孔隙,铸体薄片;f.A1,1 371.4 m,下石盒子组,次生加大石英充填粒间孔隙,边缘为绿/蒙混层,SEM;g.A1,1 373.6 m,下石盒子组,钠长石溶蚀产生粒内孔隙,SEM;h.A1,1 675.7 m,下石盒子组,假六方形高岭石和丝片状伊利石充填粒间孔隙,SEM;i.A4,1 604.43 m,上石盒子组,鳞片状高岭石胶结充填粒间孔隙,SEM;j.A1,1 681.7 m,下石盒子组,钠长石溶蚀产生粒内孔隙,SEM;k.A1,1 713.1 m,下石盒子组,鳞片状高岭石和丝片状伊利石充填粒间孔隙,SEM;l.A3,1 714.5 m,太原组,长石溶蚀向高岭石蚀变,SEM
Figure 6. Characteristics of the acid diagenetic environment in the study area
图 7 研究区碱性成岩环境发育特征
a.A2,1 824 m,山西组,铁方解石,铸体薄片;b.A3,1 714.3 m,太原组,石英溶蚀,铸体薄片;c.B2,2 001.5 m,本溪组,石英边部溶蚀,铸体薄片;d.A1,1 787.32 m,下石盒子组,长石颗粒溶蚀后被铁方解石交代,铸体薄片;e.A4,1 496.5 m,上石盒子组,长石颗粒溶蚀后被铁方解石交代,铸体薄片;f.A3,1 698.9 m,下石盒子组,石英溶蚀,铸体薄片;g.A3,1 230.4 m,上石盒子组,针叶状绿泥石充填粒间缝隙,SEM;h.B3,1 910.92 m,下石盒子组,粒间孔隙内充填针叶状绿泥石,SEM;i.B1,1 751.1 m,山西组,次生加大石英溶蚀,SEM;g.A6,1 769.15 m,山西组,次生加大石英溶蚀,SEM;k.A7,1 807.84 m,太原组,石英颗粒溶蚀,SEM;l.A8,1 390.6 m,上石盒子组,石英颗粒溶蚀,SEM
Figure 7. Characteristics of the alkaline diagenetic environment in the study area
图 9 鄂尔多斯盆地东缘临兴和神府地区上古生界成岩流体酸碱性与储层物性关系(a, b)以及1 710~1 810 m深度段关系(c, d)
Figure 9. Relationship between the pH of diagenetic fluid and physical properties of reservoir from the Upper Paleozoic in the Linxing and Shenfu district(a, b), eastern Ordos Basin, and the relationship between depth of 1 710-1 810 m(c, d)
表 1 鄂尔多斯盆地东缘临兴和神府地区上古生界储层成岩流体环境划分标准
Table 1. Classification standard of diagenetic fluid environment of the Upper Paleozoic reservoir in the Linxing and Shenfu district, eastern Ordos Basin
成岩流体酸碱性划分 长石溶蚀面孔率/% 自生高岭石质量分数/% 方解石质量分数/% 1 酸性 >5 >5 0 2 弱酸性 2~5 2~5 0 3 酸碱过渡 0~2 4 弱碱 2~5 5 碱性 >5 -
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