Chalks consist largely of stable low-magnesium calcite. Thus, they undergo diagenetic alteration different from that of more widely studied aragonite and high-magnesium calcite-bearing, shallow-marine carbonate deposits. Examination of outcrop and subsurface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, Gulf Coast, and the U.S. Western Interior indicates that chalks undergo significant diagenetic changes during their postdepositional history. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis outline the major factors that control the patterns of chalk alteration.
The major mechanism of chalk cementation is pressure solution and local reprecipitation. Although small variations in initial grain size, faunal composition, or clay content can lead to significant bed-to-bed variations in cementation, overall patterns of chalk diagenesis appear to be related to two main factors: (1) maximum depth of burial, and (2) pore-water chemistry. With a few notable exceptions, the porosity (and permeability) of chalks decreases as a direct function of burial depth. The exceptions include cases where: (1) oil entered the rock, reducing or terminating carbonate reactions; (2) chalks are overpressured and therefore are not subject to the normal grain-to-grain stresses expected at those depths; and (3) tectonic stresses increase solution and cementation. In areas where fresh water entered the pores before major burial, chalks show a much steeper gradient of porosity loss versus burial depth as compared with regions where marine pore fluids were retained.
Under normal circumstances, a typical nannofossil chalk ooze will have 70% porosity at the sediment-water interface. At a depth of 1 km, porosity will be reduced to about 35%; at 2 km, to about 15%; and at 3 km, to essentially 0. Thus, one observes progressive lithification of chalks (and their isotopic alteration) as one moves downhole or toward areas of greater burial. Petrophysical and isotopic studies can predict maximum depths of burial, paleogeothermal gradients, and proximity to zones of deformation.
In areas such as the Ekofisk field in the North Sea, however, major quantities of oil are produced from chalks having as much as 40% porosity (largely primary) at depths greater than 3 km. This appears to be related largely to the widespread overpressuring of the Central graben in that area. Other such areas of anomalous porosity in thick chalk sections should be detectable by seismic methods. Significant hydrocarbon production from chalks can occur in three major settings: (1) overpressured or oil-saturated zones where these phenomena were initiated early in the subsidence history (e.g., the North Sea); (2) areas where chalks never have been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).