DIAGENESIS OF UPPER TRIASSIC RESERVOIR DETRITAL ROCKS IN WEST AND NORTHWEST OF SICHUAN BASIN
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摘要: 川西—川西北地区上三叠统包括须家河组和香溪群;主要沉积相类型有河流三角洲、海湾、湖泊和冲积扇.上三叠统储集岩主要由长石岩屑砂岩、岩屑长石砂岩、岩屑砂岩、长石石英砂岩、岩屑石英砂岩及少量长石砂岩组成.岩石碎屑颗粒含量81%~95%,碎屑中有较多长石和岩屑,具低的成分成熟度和高的结构成熟度.原始孔隙度的45.0%~77.5%是由于压实作用而丧失的.胶结作用对储层的破坏性仅次于压实作用,包括自生石英、长石、粘土矿物和碳酸盐矿物的沉淀.石英加大的深度可能大于1 800~4 080 m.受有机质影响,碳酸盐胶结物具较低的δ13C值(PDB),为-4.85‰~2.91‰;由于淡水淋滤和深埋藏,有较低的δ18O值(PDB),为-16.97‰~-7.86‰.溶蚀作用是对储集岩最重要的建设性作用,以长石溶蚀最为常见.粘土矿物以伊利石为主,无蒙皂石.镜质体反射率为0.81%~2.48%,推测古地温大部分在85~160℃.储集岩成岩作用已进入到晚成岩作用阶段的中期.裂缝在很大程度上改善了储集性能.由于压实作用和胶结作用影响大,溶蚀作用影响小,储集岩以低孔、低渗为特征.Abstract: The upper Triassic includes the Xujiahe Formation and the Xiangxi Group in the west and north-west of Sichuan Basin.The main types of sedimentary facies are fluvial-dominated deltas,embayment,lake and alluvial fans.The upper Triassic reservoir rocks consist of feldspathic litharenite,lithic arkose,litharenite,subarkose and sublitharenite.Clastic grains,which contain abundant feldspar and rock fragments,make up 81% to 95% of rocks by volume.Sandstones are characterized by low composition maturity and high texture maturity.Primary porosity lost 45.0% to 77.5% due to compaction effect.Cementation,including precipitation of authigenic quartz,feldspar,clay minerals and carbonate minerals,is the second process contributing to porosity reduction.Influenced by organic materials,the δ13C values for carbonate cements,which range from-4.85‰ to 2.91‰(PDB),are low.Because of leaching with meteoric water and deep burial depth,the δ18O values for carbonate cements,which range from-16.97‰ to-7.86‰(PDB),are also low.Dissolution effect,which is mainly the leaching of feldspar,is the most important process for reservoir quality improvement.Clay minerals are mainly illite,and smectite is absent.Vitrinite reflectance ranges from 0.81% to 2.48%,indicating 85-160 ℃ for most ancient geotemperatures.Diagenesis proceeds to middle-late stage.Fractures have improved reservoir characteristics to a great extent.Because the influence of compaction and cementation is strong,and the influence of dissolution is weak,reservoir rocks are characterized with low porosity and permeability.
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