致密砂岩储层成岩相类型及其孔隙度演化定量分析OA

To clarify the differential evolution of porosity in tight sandstone reservoirs,the Chang 6 Member of the Triassic Yanchang Formation in the X255 area,Ordos Basin,was selected as a case study.Cast thin sections,scanning electron microscopy,X-ray diffraction(XRD)analysis of clay minerals,and cathodoluminescence data were integrated to classify the diagenetic facies of the tight sandstone reservoirs and establish the diagenetic evo-lution sequence of each facies.Burial and thermal histories were reconstructed,and the porosity of reservoirs with different diagenetic facies at key stages of geological history was quantitatively calculated to determine their differential evolution patterns.As indicated by the research results,four diagenetic facies are mainly developed in the Chang 6 member reservoirs in the study area:moderate-compaction feldspar-dissolution facies,moderate-compaction chlorite-cementation facies,strong-compaction tight facies,and moderate-compaction carbonate-cementation facies.Their porosity evolution processes differ markedly.In the moderate-compaction feldspar-dissolution facies,com-paction and cementation caused a combined porosity loss of 20.21％during early diagenetic stage A.Dissolution increased porosity by 6.06％during early diagenesis,whereas a further porosity loss of 11.38％occurred during early diagenetic stage B and the mesodiagenetic stage.The final porosity is 11.85％,making this a favorable dia-genetic facies.In the moderate-compaction chlorite-cementation facies,compaction and cementation caused porosity losses of 16.45％and 14.0％,respectively,whereas dissolution increased porosity by 3.15％.Chlorite coatings provided some protection for primary pores,making this the second most favorable diagenetic facies.In the strong-compaction tight facies,compaction and cementation caused porosity losses of 27.61％and 2.70％,respectively,making this an unfavorable diagenetic facies.In the moderate-compaction carbonate-cementation facies,dolomite precipitation occupied approximately 10.0％of the pore space.Subsequent compaction and calcite-dominated cementation rendered the reservoir tight during early diagenesis,making this the least favorable diagenetic facies.