砂岩储层孔隙保存的定量预测研究

STUDY ON QUANTITATIVE PREDICTION OF POROSITY PRESERVATION IN SANDSTONE RESERVOIRS

对北方地区若干盆地的砂岩孔隙发育特征的研究表明，地温场、地质年代和盆地沉降方式对砂岩孔隙的演化和保存有制约作用。地温梯度每增加１℃，砂岩孔隙度平均减小约７％；在地温梯度２—４℃／１００ｍ范围内，有效储层的保存深度差异可达２５００—３０００ｍ。地质年代每增加１Ｍａ，砂岩孔隙度降低约０．０１８％—０．００９％。地层超压可最大保存５％—７％的孔隙度。盆地沉降方式不同引起的孔隙保存量的差异为２％—５％，相应的有效孔隙保存深度的差值约１０００ｍ。

The porosity preservation and evolution of sandstone are mainly controlled by geotemperature field,geologic age and subsidence styles,in which geotemperature field is the most important parameter.It is found that average porosity of sandstone decreases by 70%± as average geotemperature gradient increases by 1℃/100m,in which ddecreasing rate of sandstone porosity in high geotemperature gradient area(e.g.4.0℃/100m),with increasing burial depth,is about 3 4 times that in low geotemperature gradient area (e.g.2℃1/100m).As a result,the same porosities may correspond to different burial depths ranging from 2500 to 3000m under geotemperature gradients between 2-4℃/100m.It is also presented that decreasing rate of sandstone porosities with increasing geologic age is about 0.018%-0.009% per million years,which shows that the effects to porosities resulting from geologic age and geotemperature.are supplimentary to one another. Furtherly,the relation between geotemerature geologic age and porosity is established and may be applied to quantiative prediction of underground sandstone porosity.Basin subsidence styles also have an evident even key influence on porosity preservation.It is indicated that sandstone porosity may be preserved by 2%-5% due to subsidence style change,in which burial depth difference of effective reservoirs may reach 100m.An integrated study of the above three parameters amy well explain why high porosity but deep burial reservoirs occur in basins in the western China,characterized by low geotemperature field and low porosity but shallow burial reservoir in basins in the eastern China,with high geotemperature field.In addition,the dynamic relation between porosity and the above three parameters may be more accurately described by time temperature index (TTI) presented by Lopatin.Quantitative relations between thermal matutrity porosity,thermal maturity and burial depth,thermal maturity and effective sandstone ratio for the three types.of sandstones have been established in this paper,which may exactly predict porosity,burial depth and ratio of effective reservoirs for exploration,and have been proved to be an effective method in reservoir evaluation.