Oil and gas reservoirs in chalks of the Gulf Coast, Denver basin, and North Sea show similar porosity relations. Most of the storage capacity in the three areas comes from the preservation of primary porosity. Normally, the high initial porosity (60 to 75%) of chalks is progressively lost during burial owing to mechanical and chemical compaction effects. Thus, in many areas of the Gulf Coast and the Western Interior, paleoburial depths of about 1,000 to 1,500 m (3,300 to 5,000 ft) form an economic lower limit for exploration because primary porosity has been drastically reduced at greater depths.

Three factors can strongly influence this relation of porosity and burial depth. First, fracturing can greatly improve the effective permeabilities of chalk reservoirs. Fracturing related to gentle flexuring, salt-dome tectonics, or fault zones has a major influence on the reservoir characteristics of North Sea and Gulf Coast fields and may be involved in Western Interior fields as well. Second, abnormally high pore-fluid pressures (geopressures) reduce or completely halt mechanical and chemical compaction and thus aid in the preservation of primary porosity. In the North Sea and offshore Louisiana, geopressuring has allowed preservation of as much as 40% porosity at depths of greater than 3,000 m (10,000 ft). Finally, early formation of biogenic methane (from bacterial decomposition f organic matter contained within the chalks) or early introduction of migrated hydrocarbons to the point of virtual oil or gas saturation (as in some North Sea chalks) may also be instrumental in porosity preservation during burial.

The porosity relations in chalks, although fairly complex, are far simpler than those typically seen in shallow-water limestones. Thus, based on relatively sparse data, reservoir properties and petroleum potential of chalks can be reliably predicted throughout large areas.

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