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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 42 (1992), Pages 161-172

Effects of Fibrous Illite on Permeability Measurements from Preserved Cores Obtained in Lower Wilcox Group Gas Sandstones, Lake Creek Field, Montgomery County, Texas

Jeffry D. Grigsby (1), Jose M. Vidal (2), Don L. Luffel (2), Joe Hawkins (2), John M. Mendenhall (1)


The strategic location of fibrous illite in the pore network reduces reservoir permeability in lower Wilcox Group gas sandstones in Lake Creek field. Although quartz cement is the dominant authigenic mineral in these sandstones, it is the fibrous illite, which ranges from 3% to 8% of the whole rock volume and extends into and bridges the main permeability pathways, that largely influences reservoir quality.

Detailed petrographic study of preserved core indicates that fibrous illite coalesces and collapses upon extraction and drying during conventional core plug analysis. This results in dry core permeabilities (k) two to seven times greater than preserved core brine permeabilities (kw). For example, a plug with a porosity of 13.9% and k equal to 1.99 md had a measured kw of 0.29 md. Because the illite texture is maintained in preserved core, this procedure may more closely approximate actual reservoir permeability at connate water saturation in lower Wilcox Group gas sandstones.

A log interpretation model, calibrated to preserved core permeabilities, predicted the true matrix permeability thickness (kh) of the lower Wilcox G4 gas reservoir in the LCU No. 48 well to be 22 md-ft. Well test analyses, after adjustment for stimulation, indicate the kh of the G4 reservoir to be 7 to 9 md- ft. Although the kh predicted by the model is higher than the kh obtained from the well test, the model prediction falls within the range of uncertainty associated with log and core analyses. This correlation between predicted and measured reservoir permeabilities in the G reservoir emphasizes the importance of accurate permeability measurements at connate water saturations for lower Wilcox Group gas sandstones at Lake Creek field.

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