鄂尔多斯盆地姬塬地区延长组长4+5低渗透储层成因

Formation of low permeable reservoir Chang 4+5 in Jiyuan region,Ordos Basin

碎屑岩储层成岩作用复杂而强烈，对储层物性有着重要影响。应用岩相学研究方法，在对鄂尔多斯盆地姬塬地区延长组长4＋5低渗砂岩储层成岩作用及其对物性影响定量研究基础上，探讨了低渗储层成因机理。成岩作用和孔隙演化研究表明，压实作用虽然造成12％～20％的孔隙度损失，但是压实后剩余孔隙度仍高达15％～23％；早期胶结作用使孔隙度损失很小，压实作用和早期胶结作用并没有使储层致密，不影响油气渗流。溶蚀作用进一步改善了储层，晚成岩阶段A期-B期形成的钠长石、亮晶方解石、白云石、自生高龄石和铁绿泥石等大量胶结物的晚期胶结作用使储层孔隙度仅有4％～6％，储层因此而致密，由此影响油气渗流。晚期裂隙作用对储层孔隙度贡献为6％～8％，改善了储层物性而使其成为有效储层。盆地演化及与之对应的成岩事件研究认为，印支运动前，姬塬地区长4＋5储层处于浅埋藏阶段，经历压实作用和早期胶结作用；早-中侏罗世长7烃源岩进入未成熟-低熟阶段，形成富含有机酸流体进入储层而发生溶蚀作用；早白垩世中-晚期进入深埋藏成岩阶段，晚期胶结物大量形成而使储层致密。生烃增压作用导致的裂隙以及晚白垩世以来的构造运动形成的裂缝对研究区长4＋5有效储层形成具有非常重要意义，同时对低渗油气藏勘探也具有指导意义。

Diagenesis of clastic rock reservoir was complex and intense and it had important influence on reservoir property. On the basis of quantitative research to diagenesis and its influence on physical properties of low permeable sandstone reservoir Chang 4＋5 in Jiyuan region, Ordos Basin, the low permeability reservoir formation mechanism was discussed. Researches to diagenesis and pore evolution show that the residual porosity after compaction was still as high as 15% -23% though 12% -20% of porosity loss caused by compaction. Early cementation made porosity loss only 2% -5%. Compaction and early cementation didn＇t make the reservoir compacted and didn＇t affect oil/gas flow. The porosity of sandstone reservior Chang 4＋5 was only 4% - 6% after formation of late cements such as albite, sparry calcite, dolomite and authigenic clay of kaolinite, illite and chlorite. The reservoir sandstone became condensed which influenced oil/gas infiltration after formation of later cements. Late fractures contributed about 6% -8% to reservoir porosity. The reservoir physical property was improved to become effective reservoir. Studies to basin evolution and diagenetic events indicated that the diagenesis of sandstone reservior Chang 4＋5 in Jiyuan region was at the shallow burial stage which period early compaction and cementation before lndosinian movement. Thermal degree of Chang 7 hydrocarbon source rocks was in immature to low-mature stage. The geofluid riched with organic acid formed from Chang 7 hydrocarbon source rocks flowed into the reservoir which was dissolved to form secondary pore. The diagenesis of sandstone reservior Chang 4＋5 in Jiyuan region was at the deep burial diagenetic stage and large amount of late cement formed and it made the reservoir condensed before later Early Cretaceous. Development degree of fractures, formed because of hydrocarbon generation pressurization and tectonic movement after the early late cretaceous, were very important for formation of effective reservoir Chang4 ＋5 in Jiyuan region. It is also significance for guiding exploration for low permeability reservoir.