Deposition, diagenesis and its reservoir quality of sandstone reservoirs: A case study in the Cuu Long Basin

Summary The sandstone reservoirs are major reservoirs in siliciclastic basin worldwide. Therefore, a good understanding of the development of internal rock properties are extremely important, especially factors such as porosity and permeability, in turn controlled by mineral composition, rock texture 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, not just in terms of depositional character, but also diagenetic overprints. The core samples were analyzed via thin section observation, 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. The XRD and SEM indicate rock quality in the sandstone reservoirs were influenced by a variety of authigenic minerals, such as carbonate cements, quartz overgrowths, zeolite and laumontite clays, all of which tend to reduce porosity and permeability. 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. In the E formation, XRD and SEM studies show that, as in the F formation, the porosity and permeability are strongly controlled by diagenetic evolution. Pore throats in the E and F sandstones are either minimized in size by intense compaction and a combination of pore-filling minerals including; calcite cements, authigenic clays and quartz overgrowths, leading to negative relationships with porosity and permeability. However, this negative relationship is not as clear in the interval with calcite cementation in E formation.