Subsurface characterization of paralic reservoirs
Paralic environments, deltas, estuaries and shoreline-shelf systems respond sensitively to changes in sea-level and sediment supply, commonly by shifts in facies belts across sequence stratigraphic surfaces. As a result, they are often interleaved in highly layered successions. Layering and a wide range of associated sandbody types control the distinctive nature of paralic oil and gas fields. Layering results in stacked reservoirs. Thick extensive sandstones have structurally defined spill points, large aquifers and good reservoir properties. Such sands have high recovery, good reservoir performance and form primary completion targets. By contrast, thin, laterally restricted, isolated sandstones have low recovery factors and poor performance. Each isolated sand may have its own hydrocarbon contacts and a distinct outline that reflects both structural and stratigraphic trapping. Such sands form secondary completion targets. Fault compartmentalization is a recurring risk. Sealing faults increase the number of reservoir compartments and may decrease performance and recovery. They can also generate traps. Sedimentology is pivotal in describing heterogeneity in paralic reservoirs at all scales. Established techniques are likely to be complemented in the future by forward stratigraphic modelling, integration with palynology and more studies of surveillance data to show how sedimentological variability controls fluid movement in real reservoirs.