致密砂岩孔隙演化及其对含油饱和度的影响——以鄂尔多斯盆地甘泉地区三叠系延长组长6—长8段储层为例

Pore evolution in tight sandstone and its impact on oil saturation:a case study of Chang 6 to Chang 8 reservoirs in Triassic Yanchang Formation,Ganquan area,Ordos Basin

致密砂岩储层的微观非均质性强,含油饱和度变化大。为了探究致密砂岩储层成岩过程中孔喉大小分布的变化及其对含油饱和度的影响,以鄂尔多斯盆地甘泉地区三叠系延长组长6—长8段致密砂岩储层为例,利用铸体薄片、扫描电镜、高压压汞等测试手段,定量计算了成岩作用对孔隙度的影响。在此基础上,以孔喉参数的统计模型为约束条件,建立了主要成藏期孔喉大小分布模型,利用积分方法计算出了主要成藏期的可动流体饱和度。甘泉地区长6—长8段致密砂岩储层在早成岩期—中成岩期受到强烈的压实作用,平均压实减孔率为81.85%,胶结作用进一步使孔隙度平均降低11.00%左右,溶蚀作用虽然增加了孔隙空间,但增幅较小,平均为4.38%;主要成藏期开始(128 Ma)和结束(111 Ma)对应的平均古孔隙度分别为13.82%和8.68%,大于最小流动孔喉半径(0.1μm)的孔喉体积占比低,可动流体饱和度为35.05%~93.27%。主要成藏期孔喉半径及可动流体饱和度偏低是含油饱和度低的原因之一。受储层自生黏土矿物对岩石润湿性的影响,现今含油饱和度值并未急剧降低。成藏时期孔喉大小分布模型计算油层可动流体饱和度为研究类似地区油气储层孔喉大小演化及其与含油饱和度的关系提供了一种可行的分析方法。

Tight sandstone reservoirs exhibit strong microscopic heterogeneity and significant variations in oil satura-tion.To investigate the variations in pore and throat size distribution during the diagenesis of tight sandstone reservoirs as well as their impact on oil saturation,the study takes the Chang 6 to Chang 8 tight sandstone reservoirs of the Triassic Yanchang Formation in the Ganquan area of the Ordos Basin as a case study.The influence of diagenesis on porosity was quantitatively calculated using methods such as cast thin sections,scanning electron microscopy(SEM),and high-pressure mercury injection.On the basis of the test results,a pore and throat size distribution model during the main hydrocarbon accumulation period was established,constrained by statistical models of pore and throat parameters.The movable fluid saturation during the main hydrocarbon accumulation period was then calculated using integration methods.The results showed that the Chang 6 to Chang 8 tight sandstone reservoirs experienced strong compaction during the early and middle diagenetic stages,with an average porosity reduction of 81.85%due to compaction.Cementation further reduced porosity by about 11.00%on average.Although disso-lution increased pore space,the increase was relatively smaller,with an average value of 4.38%.The average paleoporosity at the beginning(128 Ma)and the end(111 Ma)of the main hydrocarbon accumulation period was 13.82%and 8.68%,respectively.The volume proportion of pore and throat radii greater than the minimum flow throat radius(0.1μm)was low,and the movable fluid saturation ranged from 35.05%to 93.27%.The low pore and throat radii and movable fluid saturation during the main hydrocarbon accumulation period was one of the reasons for low oil saturation.However,due to the influence of authigenic clay minerals on reservoir rock wettability,the current oil saturation has not decreased sharply.The pore and throat size distribution model during hydrocarbon accumulation provides a feasible method for analyzing the evolution of pore and throat size as well as its relation-ship with oil saturation in similar reservoirs.