Quality variations and controlling factors of deepwater submarine-fan reservoirs, Rovuma Basin, East Africa

Abstract

The mechanisms causing quality variations and key control factors of submarine-fan reservoirs in the gas field X of the Rovuma Basin, East Africa are analyzed based on core and well-log data in this paper. Depositional fabric, lithofacies difference and characteristics of genetic units are the fundamental reasons of reservoir quality variations. In the case of weak cementation, porosity and permeability of submarine-fan reservoirs are controlled by grain sorting and clay content, respectively. Reservoir quality variations for 5 main lithofacies are related to variable depositional fabrics and calcite cementation. Among them, massive medium-coarse sandstones with weak cementation have the highest porosity and permeability, and coarser or finer sandstones have poorer reservoir quality. The existence of bottom current can develop laminated sandstones, improving the pore structure and physical properties greatly. Lithofacies vary among different types, locations and stages of genetic units, and they control the distribution patterns of submarine-fan reservoir quality: the physical properties of channel shaft or lobe main body are better than those of the channel or lobe edge. The sandstone sorting and physical properties are gradually improved from near-source to far-source. When multi-stage sand bodies are superimposed, the sand-mud ratio in the later stage is higher than that in the earlier stage, making the physical properties get better in the later stage.