FRETWELL, P. NICHOLAS, Research Student, Department of Earth Sciences, University of Cambridge, Cambridge, UK

ABSTRACT: The Evolution of Microporosity in Jurassic Limestones: A Comparison Between the Cotton Valley Limestone of Eastern Texas and the Lincolnshire Limestone of Eastern England

The upper Jurassic Cotton Valley Limestone of East Texas and the mid-Jurassic Lincolnshire Limestone of eastern England, show the development of microporosity in the micritic fractions (principally ooids and peloids) of carbonate grainstones.

In Texas, microporosity occurs between euhedral microrhombs (1 to 5 micrometers length) of low-magnesium calcite. Micropore-casts show pyramidal-shaped micorpores up to 3 micrometers in length interconnected by narrow (0.2 micrometers) tubular and sheet-like micropore-throats. The microtexture is more chaotic in Lincolnshire, and is developed between subhedral to anhedral micrograins (0.5 to 2 micrometers length) of low-magnesium calcite. The relationship between micropores and micropore-throats is less distinct due to the effects of dissolution.

Microtextural variation is more pronounced in the Cotton Valley Limestone. Distinctly less porous horizons occur where microporosity is reduced or totally occluded. This occurs by continued microrhomb growth, until a non-porous crystal mosaic results. Grain size analysis shows microrhombs are smaller in the less porous textures. This can be explained by a finer-grained and less porous precursor (depositional) microporous texture. Microrhomb growth may occur over a longer time period in a highly microporous precursor, before complete occlusion occurs.

Post-compactional non-ferroan poikilotopic calcite, reservoir bitumen, and saddle dolomite cements post-date microrhombic textures in Texas. In Lincolnshire microrhombic calcite is leached, and is infilled by poorly-ferroan, post-compactional calcite syntaxial on echinoderm fragments.

Microrhombic calcite and associated microporosity may be developed during early burial. Its formation may span a considerable time gap, depending on the variation in quantity of depositional microporosity.

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