DIAGENESIS AND RESERVOIR QUALITY OF TURBIDITE SANDSTONES IN THE BELL CANYON FORMATION, DELAWARE BASIN, TEXAS

Deep-water sandstones of the Permian Bell Canyon Formation in west Texas were studied in outcrop and two subsurface oil fields. Stratigraphic relations in outcrop indicate that the sandstones were deposited in a basin-floor setting by a system of leveed channels having attached lobes and overbank splays that filled topographically low interchannel areas. Individual channel-levee and lobe complexes stack in a compensatory fashion and are separated by laterally continuous, laminated siltstones deposited by the settling of marine organic matter and airborne silt during periods when coarser particles were prevented from entering the basin.

Diagenesis and reservoir quality of the sandstones were examined in cores from Geraldine Ford and East Ford fields. The sandstones are well-sorted, very fine grained arkoses having an average composition of Q₆₇F₂₆R₇. Because the sandstones have a narrow grain-size range and contain no detrital clay, porosity and permeability are controlled by calcite cement, mainly concentrated in layers ranging from 5 to 40 cm thick. Well response and geophysical log correlations suggest that some layers are laterally continuous over a distance of 300 m, causing vertical compartmentalization in the reservoir. Cemented zones are most common near the top and base of sandstones. Areas having high percentages of calcite-cemented sandstone (>20%) occur along the margins of the sandstones, in levee, overbank, and lobe deposits, where the sandstone pinches out into siltstone. The areas having the lowest percentage of calcite-cemented sandstone (<10%) occur where the sandstone is thickest, in the channel facies.

The calcite layers may be associated with pulses of turbidite deposition, although the uniform grain size of the sandstones makes it difficult to differentiate turbidite packages. The source of calcium carbonate is probably dissolution and reprecipitation of detrital carbonate rock fragments and fossils that occur in both the sandstones and siltstones.