CALCITE CEMENT DISTRIBUTION IN PERMIAN DEEPWATER SANDSTONES, DELAWARE BASIN, USA

Sandstones of the Upper Permian Bell Canyon Formation in west Texas were deposited in the deep Delaware Basin, which was rimmed by an extensive carbonate shelf-and-reef complex. The sandstones were deposited by turbidity currents in channel-levee systems that terminated in broad lobes; overbank splays filled topographically low interchannel areas.

Diagenesis of the sandstones was examined by standard thin-section petrography on core samples from two subsurface oil fields. The sandstones are well-sorted, very fine grained arkoses having an average composition of Q₆₅F₂₉R₆. Calcite is the most abundant authigenic mineral, having an average volume of 7% and ranging from 0 to 30%. Other cements include pervasive but volumetrically minor chlorite, quartz and feldspar (both K-feldspar and Na-plagioclase) overgrowths, and local anhydrite. Calcite cement is concentrated in zones ranging from 5 to 40 cm thick. Well response and geophysical log correlations reflect lateral continuity of some cemented zones over a distance of 300 m. 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. Isotopic composition of the calcite cements (δ¹³C = -1.8 to -3.0‰ [PDB], δ¹⁸O = -4.6 to -6.3‰ [PDB]) is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris in the sandstones and surrounding siltstones. The calcite is interpreted as forming near maximum burial depth (1.5 km) and temperature (40°C) from modified marine pore waters of ~0‰ (SMOW). Some calcite-cemented intervals, interpreted to be associated with turbidite pulses, occur preferentially near the base of turbidite packages. Repeated scouring by later turbidity flows may have removed carbonate-rich sediment layers within channel deposits, resulting in lower calcite cement concentration in this facies.