Deepwater Reservoir Elements: Sheet Sands and Sandstones
Sheet sands and sandstones are considered to be some of the best high-rate, high-ultimate-recovery (HRHU) reservoirs in deep water (Chapter 2). This is because of their tendency toward the simplest reservoir geometries: good lateral continuity, potentially good vertical connectivity, high aspect ratio, narrow range in grain size (and thus greater porosity and permeability), and few erosional features. Because of the initial successes with these reservoirs in the northern deep Gulf of Mexico, industry has studied sheet sands and sandstones in great detail to better understand them and hence to find more of them. One problem, however, is that reservoirs initially interpreted as sheet sands were determined later to be amalgamated channel sands.
Sheet sands are deposited from decelerating flows at the termini of channels. Sheet sands and sandstones reflect the sediments that have bypassed through updip channels (confined flow) and are deposited in a primarily unconfined setting. They are characterized by high-aspect-ratio reservoir sand bodies (>500:1), which differ markedly in aspect from the updip channels that feed them (which have aspect ratios of 30:1 to 300:1). Unlike other deepwater reservoir elements, sheet sands commonly have an areal extent that exceeds the area of the trap. Sheet sands and sandstones are most prevalent in mixed-mudsand to mud-dominated systems (Richards and Bowman, 1998). Sheet sands and sandstones are less common in sand-rich to gravel-rich systems (Chapter 1 of this book).
Sheet sands and sandstones are characterized by their tabular external form and their excellent continuity