Compaction and Porosity Prediction in Chert-Rich Sandstones and Conglomerates
COCHRAN, ANN, Esso Resources Canada Limited, Calgary, Alberta, Canada
Chert-rich sandstones and conglomerates are volumetrically important oil and gas reservoir units in western and northern Canada and in Alaska. Unlike quartzofeldspathic sandstones, chert-rich sandstones are highly susceptible to intergranular pressure solution resulting in dramatic loss of intergranular volume (IGV) and porosity with burial. Thin-section petrographic data were used to develop a suite of IGV-decline curves for chert-rich sandstones and conglomerates as the basis for porosity prediction algorithms. The samples studied range in composition from chert-quartz litharenites (Viking Formation, Paddy-Cadotte and Falher members, western Canada basin) to chert-rich litharenites containing appreciable feldspars and volcanic grains (Clearwater and Belly River formations, western C nada basin, and Reindeer Sequence, Mackenzie Delta).
Physical characteristics of chert, such as the degree of microcrystallinity or the presence of impurities and micropores, in part, determine chert's susceptibility to dissolution. Where chert and quartz grains are compacted together, chert is typically penetrated by quartz. Pressure-solution features of chert, such as pocked or dimpled clasts and microstylolitic textures, can be observed in core and cuttings, thin sections, and SEM samples. Early cementation in chert-rich sandstones can limit the effects of compaction. For example, quartz overgrowths in the Viking Formation inhibit later intergranular pressure solution. In chert-rich sandstones and conglomerates, porosity and permeability prediction is complicated by cement timing, cement type and distribution, degree of leaching, and grain textural variety.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)