From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, October 7, 2011
Bone Spring Formation Deep-Water Successions of the Delaware Basin, Texas and New Mexico:
Facies, Rock-Body Geometries, Depositional Model, and Developing Investigations of Mudrock
Link to streaming video: available 10.07.2011 at 8:55am
Dr. Seay Nance
Bureau of Economic Geology, UT Austin
The Bone Spring Formation (Leonardian/Guadalupian?), up to 3,000 ft in thickness, records deep-water accumulation of carbonates and siliciclastics in an intracratonic basin bounded by regionally extensive platforms during a period of regional aridity. Siliciclastics are represented by amalgamated sandstone to siltstone turbidite complexes and hemipelagic drapes. Carbonates are represented by amalgamated, dip-elongate rock bodies of coarse-grained debris-flow breccia-conglomerates and associated bioclastic packstone/grainstone to mudstones on slopes, as well as finer-grained carbonate turbidites in more basinward locations. Carbonate sources were platforms subject to marine-current sweeping and shelf-margin collapse. The writer's preferred depositional model places Bone Spring siliciclastic deposition during lowstand to early-transgressive phases when platforms were covered by sandy deserts and carbonate deposition in sea-level, late-transgressive to highstand phases.
The Bone Spring comprises six distinctive intervals over a large area; three are siliciclastic dominated and three are carbonate dominated. Carbonates in the north part of the basin are pervasively dolomitized, whereas those in the west part of the basin contain abundant limestone. These six intervals are well defined on geophysical well logs. Log subdivision is less apparent in more updip areas, where radioactive and electrically conductive siliciclastics pinch out, and in downdip areas, where less radioactive, more resistive, debris-flow carbonates pinch out. Bone Spring hydrocarbon production has been prolific in New Mexico, where one or two of the aforementioned intervals dominate production in a given field. The lowermost—Third Sand—interval has dominated Texas production.
Recent interest in the resource potential of mudrocks has focused attention on the finer-grained facies of the Bone Spring. Log-based facies interpretations and correlations in thin-bedded mudrock successions in basinward settings are particularly problematic wherein rock-based log calibrations are imperative. Present work focuses on characterization of subfacies, which include siliciclastic- to carbonate-dominated varieties that range from massive to well laminated. Soft-sediment deformation is common, and identification of enigmatic sedimentary structures is critical to environmental interpretation. Early results from chemical and isotopic analyses reveal coordinated, systematic variances between constituents. These variances signal changes in (1) siliciclastic-dominated (elevated quartz, clay, and titanium) to marine-carbonate-dominated (elevated % carbonate, +2–4‰ shifts in carbonate δ13C) sediment sources and (2) greater accumulation of terrigenous-sourced organic carbon (elevated TOC, avg. organic δ13C = -27.4‰). These variances suggest that most organic-carbon accumulation occurred from Northwest Shelf sources during sea-level lowstands.