COOPER, MARK R., STEPHEN S. FLINT, ROBERT H. HUNTER, JAMES D. MARSHALL; and ANDREW J. C. HOGG, BP Research, Sunbury-on-Thames, U.K.

ABSTRACT: Quartz Cementation and Its Effect on Porosity Evolution and Permeability within the Lower Permian Penrith Sandstone (Rotluegend Equivalent), NW England

The precipitation of quartz as syntaxial overgrowths reduces porosity but more significantly, pore-interconnectivity. This study examines the role of quartz cementation during porosity evolution within a transverse aeolian dune from the Penrith Sandstone. Three-dimensional controls on cement distribution and permeability have been examined at both the dune and lamina scale. Forty, 2.5 cm diameter cores cut parallel to lamination, were collected at regular intervals vertically and horizontally over two quarry faces parallel to depositional dip and strike and measured for porosity and permeability. Modal and textural analysis of thin-sections cut perpendicular to lamination indicate that mechanical compaction and minor pressure solution have significantly reduced primary depositional po osity; the mean minus cement porosity is approx. 26% bulk volume (B.V.). The extent to which porosity is effected is dependent upon depositional fabric; grainflow laminae generally preserve higher minus cement porosities (>30% B.V.) than wind-ripple laminae (<25% B.V.). Precipitation of externally-sourced silica, although preventing further compaction, reduces porosity to a mean residual of approx. 8% B.V. Cement distribution is controlled by minus cement porosity and detrital mineralogy. Cementation reduces the mean permeability of wind-ripple lamination to approx. 150 mD; however, permeability increases sharply to approx. 500mD where coarse grained well sorted grainflow laminae intertongue at the top of the preserved dune. At the lamina scale, 3-D geometrical quantification of de rital framework, cement distribution, minus cement and residual porosity networks has been undertaken utilising precision serial grinding, at 20-100 micrometer intervals, coupled with scanning electron microscope backscatter and cathodoluminescence imaging and 3-D reconstruction techniques.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.