Diagenesis of deep sandstone reservoirs and a quantitative model of porosity evolution: Taking the third member of Shahejie Formation in the Wendong Oilfield, Dongpu Sag, as an example

The diagenesis and porosity evolution in the deep 3rd member of Shahejie Formation of the Wendong Oilfield was analyzed using thin-sections, casting thin-sections, X-ray diffractometry, scanning electron micrograph observations, and other data. Sandstone reservoirs are currently at the late diagenetic period. Pores consist of primary pores and the inter-granular dissolved and intra-granular dissolved pores of feldspar, debris and carbonate cements. Physical properties are mainly controlled by carbonate cementation and dissolution, and distribution of abnormally high fluid pressure. The evolution of porosity parameters shows that primary porosity is 36.75%, the porosity loss rate is 40.49% during the process of mechanic compaction, the porosity loss rate is 37.25% during the process of cementation and metasomasis, and the porosity increase rate is 17.88% during the process of dissolution. The proportion of primary porosity is 55.03%, and that of the secondary porosity is 44.97%. The error rate in the quantitative study of porosity is 0.96%, and the main influencing factor of the error rate is sorting coefficient of detrital rock (S0).