鄂尔多斯盆地志丹地区长2储层成岩作用及其对孔隙度演化的影响

Diageneses and Evolution of Porosity of Chang 2 Reservoir in Zhidan Area,Ordos Basin

通过岩心观察、铸体薄片鉴定、粒度分析、扫描电镜、电子探针、X射线衍射分析、核磁共振实验、流体包裹体、真实砂岩微观模型驱替等分析资料,对志丹地区长2储层砂岩成岩作用、成岩演化阶段及孔隙度演化与驱替效率之间的关系进行了研究。结果表明,砂岩经历了压实作用、压溶作用、胶结作用和溶蚀作用的改造,研究区长2油层组现今处于中成岩A期;应用Beard砂岩原始孔隙度恢复公式,计算出长2储层未固结砂岩的原始孔隙度平均39.09%;压实、压溶作用对储层物性影响最大,可以使原始孔隙度减少15.41%,减孔率为39.42%;胶结作用会进一步减少原始孔隙度,使孔隙度损失12.72%,减孔率为32.46%,而绿泥石薄膜的普遍发育则对于原生孔隙具有一定的保护作用;此外,长石和岩屑的溶蚀作用也对储层物性起到明显的改善作用,并且溶蚀作用产生的高岭石充填在孔隙中形成的晶间孔也可以成为储集空间,使孔隙度增加4.2%,增孔率为10.76%;各种成岩作用造成孔隙度变化对于储层的驱替效率影响不同,其中压实作用的增强对降低驱替效率影响最大,而强烈的溶蚀作用会使储层的驱替效率增加,以方解石胶结为主的胶结作用会降低储层驱替效率,而以绿泥石膜胶结为主的胶结作用对储层驱替效率影响相对较小。

Based on the analysis of core observation,cast sections,granularity,scanning electron microscope(SEM),electron probe microanalyser,X-ray diffraction,nuclear magnetic resonance(NMR)experiment,fluid inclusions and waterflooding seepage experiment with real sandstone micromodel,this paper studies the Chang 2reservoir sandstone diageneses,diagenetic evolution stages and the evolution of porosity in Zhidan area,and discusses the influence of the displacement efficiency of the reservoir.The results show that the sandstone has experienced the transformation of compaction and pressure dissolution,cementation,dissolution and metasomatism.This suggests that the Chang 2 oil reservoir set in the research area is now in the middle diagenetic stage A.The average primary porosity of unsoiled sandstone is 39.09% through the original porosity recovery of Beard formula.The compaction and pressure solution affect most destructively the reservoir physical properties,which can reduce 15.41% of pore and the reduction of porosity is 39.42%.The cementation of intergranular authigenic kaolinite and calcite further reduces the original porosity,and the chlorite thin film can protect a part of primary pore,which may decrease 12.72% of pore and the rate of reduction in porosity is 10.76%.In addition,the dissolution of feldspar and rock debris can improve the reservoir physical properties and the aggregate of kaolinite can grow a part of intercrystal porosity,which can increase 4.2% of pore with the rate of growth in porosity reaching 32.46%.All kinds of diageneses cause the change of porosity that impact the displacement efficiency. Compaction is the greatest influence to reduce the displacement efficiency,while strong dissolution increases the displacement efficiency of the reservoir.The reservoir of calcite cementation decreases the displacement efficiency,while the reservoir with chlorite film cementation has relatively little influence on the displacement efficiency.