Impact of deposition and diagenesis on quality of sandstone reservoirs: A case study in Cuu Long basin, offshore Vietnam

Abstract

Sandstone reservoirs are major reservoirs in siliciclastic rocks worldwide. A good understanding of the development of internal rock properties is, therefore, extremely important, especially in terms of porosity and permeability (which indicate reservoir storage and flow capacity), which are controlled by mineral compositions, rock textures, and diagenetic processes. This paper studied formations E and F in three wells in the Cuu Long basin to better define the impacts of not only depositional characters but also diagenetic overprints on porosity and permeability (poroperm). Core samples were analysed via thin section observations, scanning electron microscopy (SEM), X-ray diffraction (XRD) observations, capillary pressure (Pc) and helium porosity - permeability measurements together with petrophysical evaluation. Formation E was deposited in a fluvial - lacustrine environment that is characterised by claystone/shale interbedded with sandstone, with reduced depositional permeability in finer-grained intervals. XRD and SEM analyses 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, which all tend to reduce poroperm. Whereas, formation F 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 formation F, 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 formation E.