鄂尔多斯盆地西南部长7和长6-3致密砂岩成岩作用与成岩相

Diagenesis and diagenetic facies of the Chang 7 and Chang 6-3 tight sandstone reservoirs in southwestern Ordos Basin,China

研究鄂尔多斯盆地西南部长7和长6-3致密砂岩成岩作用,探讨有利成岩相,为油气勘探开发提供理论依据。通过扫描电镜、流体包裹体、铸体薄片和X射线衍射分析等手段,结合埋藏史研究,结果表明:长7和长6-3砂岩储集空间以石英、长石等溶孔和残余粒间孔为主;为低孔-超低渗大孔微喉型储层;影响储层物性最主要的成岩作用是压实作用和胶结作用,其次是溶蚀作用和云母的蚀变膨胀;主要成岩序列为:压实作用-云母蚀变、绿泥石膜沉淀、石英次生加大-方解石和白云石胶结-石英溶蚀-长石溶蚀、石英和伊利石胶结-高岭石胶结-含铁碳酸盐、黄铁矿胶结。优质储层展布受沉积相和成岩相展布控制。最有利的成岩相是粒间孔-绿泥石膜胶结相;其次是溶蚀孔-长石石英溶蚀相。这2个成岩相叠合的相带是最有利的成岩储集相带。由于长7和长6-3段砂岩紧邻烃源岩,受有机酸的影响,溶蚀孔-长石石英溶蚀相最发育(溶蚀孔隙占全部孔隙的72.4%)。微裂隙-强压实相是相对不利的成岩储集相带。由于研究区云母和塑性岩屑含量较高,而机械压实使塑性岩屑变形甚至假杂基化,导致储层变得非常致密。微孔-碳酸盐胶结相和微孔-伊利石胶结相物性最差,是最不利的成岩储集相带。

Rock thin section observation, scanning electronic m icroscopy （SEM ）, X -ray diffractionanalysis and other analysis technologies are used, com bined w ith the analysis of burial history,diagenesis and diagenetic facies of Chang 7 and Chang 6-3 tight reservoir beds, so as to providetheoretical basis for petroleum exploration and developm ent of Chang 7 and Chang 6-3 tight sandstonein southw estern O rdos Basin. It show s that the m ain reservoir spaces of sandstones are quartz andfeldspar dissolution pores and residual intergranular pores. T he sandstones of Chang 7 and Chang 6-3 are characterized by low porosity, ultralow permeability, macropore, and micro-throat. It reveals thatthe main diagenesis affecting the reservoir physical properties is compaction and cementation, followedby dissolution and mica swelling and alteration. The main diagenetic sequences are compaction-micaalteration, chlorite cementation, quartz overgrowth-calcite and dolomite cementation-quartzdissolution-feldspar dissolution, quartz and illite cementation-kaolinite cementation-iron-bearingcarbonate and pyrite cementation. Distribution of high quality reservoir is controlled by sedimentaryfacies and diagenetic facies. The most favorable diagenetic facies are intergranular pore-chloritecementation facies, following by dissolved pore-feldspar and quartz dissolution facies. Chang 7 andChang 6-3 sandstones are adjacent to source rocks and affected by organic acids, dissolved porefeldsparand quartz dissolution facies are well developed （dissolution pores account for 72. 4 % of thetotal pores）. The superimposed or transformed area of these two facies is the most favorablediagenetic reservoir facies, while the microcrack-strong compaction facies is relatively unfavorablediagenetic reservoir facies. The contents of mica and plastic fragments are very high in the study area.Some mica and plastic fragments even underwent plastic deformation and filled into intergranularvolume and became false matrices. Compaction plays a more important role in destroying the primaryporosity, starting at the beginning of deposition and continuing to the present. Micropore-carbonatecementation facies and micropore-illite cementation facies are the most unfavorable diagenetic facieswhich result in the worst quality reservoir.