Abstract: Diagenetic Patterns of Chalks

P. A. Scholle

Chalks consist largely of stable low-Mg calcite. Thus, they undergo diagenetic alteration different from that of more widely studied aragonite and high-Mg calcite-bearing shallow-marine carbonate sediments.

The major mechanism of chalk cementation is pressure solution and local reprecipitation. As 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. The porosity and permeability of chalks decrease as a direct function of burial depth, with a few notable exceptions. The exceptions include places where: (1) oil entered the rock, reducing or terminating carbonate reactions; and (2) chalks are overpressured and therefore not subject to the normal grain-to-grain stresses common at those depths. Where fresh water entered the pores before major burial, chalks show a much stee er gradient of porosity loss versus burial depth as compared with regions where marine pore fluids were retained.

This late diagenetic solution-precipitation accounts for reduction of porosity in chalks from about 50 percent near the surface to less than 10 percent at depths of 1,000 m, where fresh water is present, or 2,500 m, where marine fluids are retained. Most of the solution and cementation has taken place on a micron scale and is detectable only by electron microscopy and isotopic analysis. The operation of the pressure-solution mechanism in chalks to this extent implies that this same process may be equally important for porosity reduction in other carbonate sediments.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA