ABSTRACT: Secondary Porosity in Immature Late Cretaceous and Tertiary Sandstones, Northeast Alaska and Northwest Canada
M. D. Myers, T. N. Smith
Petrographic and scanning electron microscope analysis of Upper Cretaceous to lower Eocene sandstone from outcrops west of the Mackenzie delta and in the central Arctic National Wildlife Refuge (ANWR) reveals secondary porosity. Recognizing this secondary porosity is important for oil and gas exploration because early diagenesis has eliminated most primary porosity in these immature litharenites.
The litharenites are dominated by grains of quartz, cherty argillite, chert, volcanic rock fragments, variable amounts of feldspar, and minor amounts of metamorphic rock fragments. Because of the abundance of ductile grains and deep burial (probable burial to depths in excess of 3000 m), these sandstones have suffered the loss of most primary porosity. Additional reduction of primary porosity has occurred due to the formation of minor amounts of precompaction rim cement (carbonate, chlorite, and illite/smectite) and syncompaction quartz overgrowths.
Dissolution of framework grains and, to a lesser degree, matrix has resulted in secondary porosities of up to 8% in outcrop samples. Framework grains commonly dissolved include volcanic rock fragments, feldspar, chert, cherty argillite, argillite, and quartz. Two processes are responsible for the dissolution. The first process is the direct dissolution of grains. The second process involves two steps in which grains and matrix are initially replaced by carbonate cement followed by dissolution of the cement and creation of secondary porosity. Secondary porosity is reported to exceed 20% in subsurface samples in northwest Canada.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990