More than half the Earth’s hydrocarbon reserves are contained within pore systems in limestones and dolomites, and therefore an assessment of the amount and type of any porosity in a carbonate sediment is an important part of any thin section description. Porosity can be described as primary, in which case it has been present in the rock since deposition, or secondary when it has developed during diagenesis. Porosity can also be described as fabric-selective if its location is controlled by parti-

cular parts of the depositional or post-depositional fabric of the rock. Porosity which is not fabricselective typically cuts across the fabric of the rock. This division into fabric-selective and non-fabricselective porosity types is the basis of the classification of carbonate porosity proposed by Choquette & Pray (1970), illustrated in 297. As explained on p.8, many porous rocks are impregnated with blue-dye-stained resin before sectioning.