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Geology of the Upper Permian Capitan Shelf Margin - A Study from a Continuous Core in the Northern Delaware Basin
To better understand the geologic evolution, depositional facies, and diagenesis of the Capitan shelf margin (Permian, late Guadalupian), a thorough study was made of a 2400 ft (731 m) portion of a 4800 ft (1463 m) continuous core from the Gulf PDB-04 research well, located in Eddy County, New Mexico, at the northern end of the Delaware Basin.
The PDB-04 core contains the principal components of the prograding Capitan shelf margin, which, in ascending order, are:
1) Basin margin facies (Bell Canyon Formation): alternating beds of fine-grained carbonate and siltstone/sandstone.
2) Slope facies (Capitan Formation): partially to completely dolomitized beds of skeletal wackestone to grainstone and blocks of dolomitized reef boundstone.
3) Reef facies (Capitan Formation): mixture of completely dolomitized sponge-algal boundstones, wackestones, packstones, and grainstones; and a 56 ft thick siliciclastic bed within the reef appearing to be a gully or channel fill.
4) Immediate back-reef facies (Seven Rivers Formation): dolomitized skeletal wackestones and packstones along with peloidal and oolitic grainstones.
5) Pisolite shoal facies (Yates Formation): interbedded siltstones and dolomitized pisolite grainstones.
6) Tidal-flat/lagoon facies (Tansill Formation): dolomitized mudstones and wackestones/packstones that are mixed with thin siltstones.
Dolomitization and dissolution are the principal methods of porosity formation in the core. Dolomitization is pervasive in the shelf and reef facies but variable in slope facies. Preliminary evaluation of subsurface and outcrop data suggests that the limestone lithology of the Capitan reef in the Guadalupe Mountains may reflect a diagenetic window within an otherwise regionally dolomitized shelf margin. Carbon and oxygen isotopic compositions do not discriminate different dolomite types and suggest that the pervasive dolomites had an early, nearsurface origin, probably from waters originating in supratidal or sabkha settings. Dissolution was extensive in the upper portion of the reef facies and less common in the pisolite shoal, immediate back-reef, and slope facies. Vuggy and solution-enlarged fracture porosity in the reef interval may have a telogenetic origin.
Porosity was reduced substantially by marine cements and subsequently by gypsum and anhydrite. Ferroan and nonferroan calcite and dolomite cements are minor diagenetic components in the core. Magnesite replaces carbonate and occurs as a cement in siliciclastic beds of the pisolite shoal and tidal-flat/lagoon facies. A likely origin for the magnesite is movement of dense brines from the overlying Salado evaporites.
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