Characterization of petrography and diagenetic processes influence on porosity and permeability of Oligocene sandstone reservoir rocks, block 15-2 in Cuu Long basin

Abstract

Porosity and permeability are the two most important of reservoir rock characteristics particularly in sandstones. They are controlled by rock fabric, mineralogical composition, diagenetic evolution, etc. To address the unknowns, this investigation examined 98 of core, core plug and side wall core samples taken from oil fields of block 15-2 in Cuu Long basin. The samples were analyzed via thin section, Scanning electron microscope (SEM), X-ray diffraction (XRD) and measured helium porosity and Klinkenberg permeability. The results showed that most of sandstones in sequences BH1.2 and C1 are arkose and lithic arkose and sporadically interbedded by feldspathic greywacke and carbonate cemented sandstones while feldspathic greywacke and zeolite cemented sandstones are commonly present in sequences D1 and E1. The clean sandstones with moderately-well grain sorting have higher porosities and permeable abilities than the very poorly-poorly sorted sandstones. The medium-coarse grained sandstones have a higher porosity and permeability than the very finefine grained sandstones. The diagenetic processes of sandstones are progressively growing with the increasing burial depth as smectite transfers into illite and the amount of illite tends to increase while kaolinite reduces and transfers into dickite. Feldspars and lithic fragments are highly leached and sandstones are heavily restricted by cementation and compaction at deeper. The grain sorting, grain size, compaction and cementation cause to decrease the porosity and permeability of the sandstones. Additionally, clay minerals as illite and chlorite have a negative effect and cause highly decreasing permeability.