ABSTRACT

Cores of the Woodbine/Tuscaloosa formation from False River field, Pointe Coupee Parish, Louisiana, contain sandstones from 20,000 ft with anomalously high porosities and permeabilities. Porosities greater than 25 percent and permeabilities of hundreds of millidarcies are common. Scanning electron microscopy shows that individual grains of these olive-green, semi-friable sandstones are coated with chlorite. The chlorite occurs as 7-10 micron-wide hexagonal plates which arrange themselves edgewise one crystal thick on grain surfaces. Sandstones with more or less continuous chlorite coatings around quartz grains display little framework compaction and minor development of secondary quartz overgrowths; however, interbedded sandstones with little or no chlorite are often completely cemented by secondary quartz. Intermediate between these extremes are sandstones with incomplete or poorly developed chlorite coatings; these display outgrowths of secondary quartz rather than overgrowths of an envelope nature.

Petrographic and SEM data indicate an early diagenetic origin for the chlorite, which apparently ceased to form once detrital grains were coated with a single layer of crystals. This layer was sufficient to mask nucleation sites for silica overgrowths, and in addition may have prevented compaction by pressure solution, thereby allowing the sandstones to be buried to great depths without appreciably reducing porosity.

The chlorite was probably derived from ultrabasic volcanic detritus which is present in the sandstones to varying degrees. The source of this detritus can be traced to the peridotite belt of southern Arkansas.