砂岩压实三元解析减孔模型及其石油地质意义——以鄂尔多斯盆地十里加汗地区二叠系下石盒子组致密砂岩为例

Ternary analytic porosity-reduction model of sandstone compaction trend and its significance in petroleum geology: A case study of tight sandstones in Permian Lower Shihezi Formation of Shilijiahan area, Ordos Basin, China

以鄂尔多斯盆地十里加汗地区二叠系下石盒子组致密砂岩为研究对象,定量研究砂岩致密化过程中不同埋深、不同时间、不同压实强度对孔隙度的影响。首先深入分析砂岩压实剖面,其次以黏弹塑性应力-应变模型为基础进行理论推导,然后分别利用多元回归和迭代运算方法来确定砂岩弹性模量和等效黏性系数随埋深和时间的变化趋势,最后建立砂岩压实三元解析减孔模型,并通过与镜下薄片观察结果和常用模型进行对比来证明其合理性。新模型能将减孔量分为弹性减孔、黏塑性减孔和胶结减孔3个部分,针对下石盒子组2段岩屑砂岩的模型计算结果与Houseknecht图版上镜下薄片的统计平均值较为接近,基本上能反映砂岩正常演化趋势上压实与胶结作用的相对强弱。此外,该模型能更合理地解释受溶蚀增孔作用影响不大的砂岩的压实作用,而且能够评价不同埋藏和抬升过程的砂岩压实程度及其对储集层致密化的影响。

The tight sandstones in the Permian Lower Shihezi Formation of Shilijiahan area in the Ordos Basin was taken as study object in this research to examine the effects of burial depth,burial time and compaction strength on porosity during densification of reservoir.Firstly,sandstone compaction profiles were analyzed in detail.Secondly,the theoretical study was performed based on visco-elasto-plastic stress-strain model.Thirdly,multiple regression and iterative algorithm were used to ascertain the variation trends of Young's modulus and equivalent viscosity coefficient with burial depth and burial time,respectively.Accordingly,the ternary analytic porosity-reduction model of sandstone compaction trend was established.Eventually,the reasonability of improved model was tested by comparing with thin-section statistics under microscope and the models in common use.The study shows that the new model can divide the porosity reduction into three parts,namely,elastic porosity loss,visco-plastic porosity loss and porosity loss from cementation.And the results calculated by the new model of litharenite in He2Member are close to the average value from the thin-section statistics on Houseknecht chart,which approximately reveals the relative magnitudes of compaction and cementation in the normal evolution trend of sandstone porosity.Furthermore,the model can more exactly depict the compaction trend of sandstone affected little by dissolution than previous compaction models,and evaluate sandstone compaction degree and its contribution to reservoir densification during different burial and uplift processes.