Abstract: Application of Chalk-Diagenetic Studies to Petroleum-Exploration Problems

Peter A. Scholle

Examination of outcrop and subsurface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, the Gulf Coast, and the U.S. Western Interior indicate that chalks undergo very significant diagenetic changes during their postdepositional history. These changes commonly involve a major reduction in porosity. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis have been used to outline the major factors that control the patterns of chalk alteration.

Depth of burial is the key element; burial is accompanied by loss of porosity because of redistribution of carbonate by pressure solution. The reprecipitated carbonate forms mainly as overgrowths on coccolith plates and as fillings of foraminiferal chambers. Exposure to fresh water alone has little or no effect on porosity, although it may be a factor when combined with overburden pressure. Under normal circumstances, a typical nannofossil chalk-ooze will have 70-percent porosity at the sediment-water interface. At a depth of 1 km, porosity will be reduced to about 35 percent; at 2 km, to about 15 percent; and at 3 km, to essentially 0 percent. Thus progressive lithification of chalks (and their isotopic alteration) occurs 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.

However, in areas such as the Ekofisk field in the North Sea, major quantities of oil are produced from chalks having as much as 40 percent porosity (largely primary) at depths greater than 3 km. Clearly, there is a paradox here. The answer may be that other wells in the Ekofisk area, even ones that have no traces of hydrocarbons, are "anomalously" porous, and that the entire area within this central graben of the North Sea is strongly overpressured. This overpressuring, and to some degree the presence of oil, minimize the differential stresses in the rock and thereby retard pressure solution and cementation. Thus, chalk porosity can be retained to a spectacular degree in overpressured areas. Such zones could be detected 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 have never been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).

AAPG Search and Discovery Article #90977©1975-1976 Distinguished Lectures