Prolific hydrocarbon production from chalk reservoirs

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has stimulated detailed analysis of the generation and occlusion of porosity in these rocks. Porosities, which may exceed 45%, reflect conditions of sedimentation and subsequent diagenesis. Sedimentation rate, a function of skeletal productivity and dissolution within the water column, influences cementation at the seafloor. Intraparticle porosity is relatively rare and is found primarily within foraminiferal tests. The more abundant primary interparticle porosity is imprinted during deposition, and reflects calcareous nannofossil preservation. Highest porosities are characteristic of zones containing relatively well-preserved nannofossils. Porosity in chalk may be secondarily enhanced by fracturing.

Initial porosity is reduced by mechanical compaction (dewatering), by solution-transfer of calcite, by growth of authigenic clays, and by precipitation of silica on skeletal elements. These siliceous coatings and locally abundant chert nodules are interpreted as products of remobilization of biogenic opal. Although these observations and interpretations are based on North Sea chalk reservoirs, exploration in frontier areas should reveal chalks with similar features and comparable reservoir potential.

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