DEPOSITION, DIAGENESIS AND RESERVOIR QUALITY OF SANDSTONE RESERVOIRS: A CASE STUDY IN THE CUU LONG BASIN, OFFSHORE VIETNAM

Abstract

The sandstone reservoirs are major reservoirs in siliciclastic worldwide. Therefore, a good understanding of the development of internal rock properties is extremely important, especially in terms porosity and permeability (indicate reservoir storage and flow capacity), which are in turn controlled by mineral compositions, rock textures, and diagenetic processes. This project studied the E and F formations in three wells in the Cuu Long Basin, with the aim of better defining controls on porosity and permeability (poroperm), not just in terms of depositional character, but also diagenetic overprints. Core samples were analyzed via thin section observations, scanning electron microscopy (SEM), X-Ray diffraction (XRD), Capillary Pressure (PC) and Helium Porosity-Permeability measurements, together with petrophysical evaluation.  

The E formation was deposited in a fluvial-lacustrine environment that is characterized by claystone/shale interbedded with sandstone, with reduced depositional permeability in the finer-grained intervals. XRD and SEM indicate rock quality in the sandstone reservoirs was influenced by a variety of authigenic minerals, such as carbonate cements, quartz overgrowths, zeolites, and laumontite clays, all of which tend to reduce poroperm. The F formation was deposited in a higher energy setting. This was mostly a braided channel environment indicated by a blocky shape in the wireline across the sandy interval and typically good primary porosity and permeability. As in the F formation, the reservoir quality is strongly controlled by diagenetic evolution. Pore throats in the E and F sandstones are reduced in size by intense compaction and a combination of pore-filling minerals including; calcite cements, authigenic clays, and quartz overgrowths, leading to a negative relationship with poroperm. However, this negative relationship is not as clear in the E formation.