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Cores from four wells from the West Stuart City trend in LaSalle and Webb Counties, Texas, were analyzed to define depositional and diagenetic facies and to determine factors controlling porosity distribution. In all, 1,187 ft (362 m) of core and 220 thin sections were examined in detail, supplemented by SEM analyses of fractured surfaces and plastic casts of microporosity. Attempts to correlate cores purely on the basis of lithology were unsuccessful owing to the high degree of vertical lithologic variability and the rapidity of lithofacies changes over a relatively short distance (approximately 8,000 ft, 2,400 m). However, when lithofacies were integrated with biofacies (based on gross faunal aspect) and related to submarine hardground surfaces, meaningful correlations ere possible.
The depositional model envisioned for these deposits is that of a broad, discontinuous, shallow bank constructed of coarse skeletal debris (largely caprinid grainstones) with more restricted lagoonal sediments in its lee (miliolid, requienid wackestones, and mudstones). The buttress zone of this bank was periodically subjected to extensive wave action, evidenced by sheets of skeletal grainstones intertonguing with muddy lagoonal facies.
Important diagenetic effects noted include extensive submarine cementation, neomorphism of aragonitic components with virtually no development of moldic porosity, and extensive pressure solution. Cumulative measurements of stylolites with amplitudes greater than 2 cm (0.8 in.), combined with microscopic observations of small-scale stylolites indicate a loss of stratigraphic section of as much as 20%.
A comparison of lithofacies and biofacies logs with permeability and porosity logs compiled from perm-plug data revealed that zones with permeabilities greater than 0.1 md and porosities of at least 6% were associated with rudistid grainstones cemented by
isopachous submarine cement and packstones with a finely crystalline rhombic calcite matrix. This general porosity distribution is modified by basinward tilting so that the most favorable traps lie in updip positions where grainstones intertongue with and are sealed by muddy lagoonal facies, (i.e., in a more landward position behind the bank proper).
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