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1st Research Conference - Gulf Coast Section SEPM

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Preservation of Porosity in the Deep<br />

Woodbine/Tuscaloosa Trend, Louisiana<br />

Thomson, Alan, Shell Oil Company, New Orleans, LA (p. 32-34)<br />

Cores of the Woodbine/Tuscaloosa Formation from False RIver Field, Pointe<br />

Coupee Parish, Louisiana, contain sandstones from 20,000 ft with anomalously<br />

high porosities and permeabilities. Porosities greater than 25% and<br />

permeabilities of hundreds of millidarcies are common. Scanning electron<br />

microscopy shows that individual grains of these olive-green, semi-friable<br />

sandstones are coated with chlorite. The chlorite occurs as 7-10µ-wide<br />

hexagonal plates which arrange themselves edgewise one crystal thick on grain<br />

surfaces. Sandstones with more or less continuous chlorite coatings around<br />

quartz grains display little framework compaction and minor development of<br />

secondary quartz overgrowths; however, interbedded sandstones with little or<br />

no chlorite are often completely cemented by secondary quartz. Intermediate<br />

between these extremes are sandstones with incomplete or poorly developed<br />

chlorite coatings; these display outgrowths of secondary quartz rather<br />

overgrowths of an envelope nature.<br />

Petrographic and SEM data indicate an early diagenetic origin for the chlorite<br />

which apparently ceased to form once detrital grains were coated with a single<br />

layer of crystals. This layer was sufficient to mask nucleation sites for silica<br />

overgrowths, and in addition may have prevented compaction by pressure<br />

solution, thereby allowing the sandstones to be buried to great depths without<br />

appreciably reducing porosity.<br />

The chlorite was probably derived from ultrabasic volcanic detritus which is<br />

present in the sandstones to varying degrees. The source of this detritus can be<br />

traced to the peridotite belt of southern Arkansas.<br />

DEPTH [FT] s (Millidarcies) POROSITY (%) Oil Sat. (% Pore) Total Water (% Pore)<br />

19,783 0.1 2.2 0.0 59.2<br />

19.901 0.1 7.6 0.0 76.3<br />

19,902 0.2 14.9 0.0 48.3<br />

19,903 0.1 9.9 0.0 70.7<br />

19,908 0.1 13.7 0.0 59.8<br />

19,909 28.0 21.4 0.5 60.7<br />

19,910 26.0 20.1 0.5 53.7<br />

19,911 36.0 25.9 1.9 72.5<br />

19,927 10.0 22.5 1.8 72.5<br />

19,928 1249.0 26.0 1.9 74.6<br />

19,929 680.0 29.0 1.4 81.0

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