Petrographical characteristics and post-depositional alteration affecting porosity and permeability of Oligocene sandstones, block 15-1/05, Cuu Long basin

Abstract

Petrographical characteristics and post-depositional alteration studies of sandstones are the two important factors to reservoir rocks, which affect oil and gas storage and permeability of reservoir rocks. This study revealed petrographical characteristics, post-depositional alteration, and their influence on the porosity and permeability of Oligocene sandstones, including C, D, and E and F sequences, block 15-1/05, Cuu Long Basin. The results show that most of the sandstones were arkose, lithic arkose, and sporadically interbedded by feldspathic greywacke. The post-depositional alteration was progressively increasing following the burial depth from early diagenesis of sequence C, to intermediate diagenesis of sequence D and advanced diagenesis of sequence E and F. The post-depositional alterations significantly influenced on the porosity of the Oligocene sandstone were the cementation and mechanical compaction. They reduced the porosity and permeability of the sandstone. Additionally, authigenic clay minerals have a negative effect on permeability in which sandstones were rich illite and illite-smectite clay minerals, and the permeability tended to decrease stronger than others. Our results showed that the potential reservoir rocks of Oligocene sandstones, block 15-1/05 were sequence E and F sandstones that are in well grain sorting, well grain roundness shape, and contained a small number of cement, particularly the absence of illite and illite-smectite.