Carbonate Reservoirs

Reefs stand up as positive structures and may be draped by mud and form a stratigraphic trap determined by the size of the reef structure. Carbonate reefs, and other carbonate deposits which can be reservoirs, form in a wide range of environments but all require clear water without much clay sedimentation. Reefs are deposited in high energy environments along coastlines exposed to high wave energy.

The Bahamas carbonate platform has welldeveloped reefs on the exposed eastern side but not on the more protected western side. Coral reefs also require warm water (>20◦C) and do not form where cold water is upwelling, e.g. along the coast of West Africa. Reefs build up on the seafloor and may be buried beneath mud during transgressions.

The reef then becomes a perfect stratigraphic trap, often with good permeability both vertically and horizontally

Reefs can form long continuous barriers as in Australia (Great Barrier Reef). In the US much oil was found by following Jurassic and Cretaceous reef trends around the Gulf of Mexico. High energy beach deposits on carbonate banks may consist of well-sorted carbonate sand (grainstones). Ooid sands (ooliths) are formed as beach and shoreface deposits and may have limited vertical thickness, reflecting the wave base.

They may however stack up and form thicker sequences of such rocks. Ooids may also be transported from the shelf into deeper water as turbidites and other slope deposits, but the sorting and reservoir quality is then reduced. Ooids are rather stable mechanically during burial, but at 2–3 km porosity may be strongly reduced by cementation in the intergranular pore space.

The calcite cement may be derived by dissolution along stylolites. Stylolites have a thin layer of clay and other minerals that are not soluble and may present a barrier during oil migration and production. Carbonate muds are very fine grained and do not have high enough primary porosity and permeability to form reservoir rocks, but may gain porosity and permeability by fracturing and become fractured reservoirs.

They can also have mouldic porosity from dissolved aragonite fossils.