Abstract
In this interim report of research performed as part of the Bureau of Economic Geology's Tight Oil Resource Assessment (TORA) project, I provide documentation and insights gleaned from an ongoing regional stratigraphic analysis of the Bone Spring Formation (BSPG; Leonardian). The BSPG is a major unconventional oil target in the Delaware Basin of southeast New Mexico and west Texas. I will discuss some initial results made from analysis of basin-scale BSPG correlations, cross sections, gross interval isopach maps, and wireline-log facies maps.
Wireline geophysical logs from 18,374 vertical wells comprise the foundation of the data base. The initial BSPG stratigraphic framework was established by building a regional network of cross sections. In establishing these cross sections, eight (8) BSPG mappable units were distinguished based on detailed analysis of wireline logs. Infill correlations made on logs from approximately 3,800 wells have provided a relatively high spatial resolution stratigraphic framework. The resulting stratigraphic architecture, which is comprised of a multitude of “formation tops” that define the eight geological mapping units distinguished in the BSPG, provides a framework for 3D geomodeling. As such, stratigraphic framework informs all of the more ‘downstream’ producibility analyses, e.g., frac parameters, well spacing, etc., that enable stratigraphically- and geographically-specific engineering and economic decisions to be made.
Bone Spring carbonate-dominated units are generally much thicker (up to 3,000 ft; 915 m) immediately adjacent to shelf margin sources and they typically thin into the basin. Carbonate materials were shed off the adjacent shelves by gravity and current-driven forces and formed thick, carbonate-dominated slope deposits during highstands of relative sealevel. The slope deposits are primarily composed of limestone and dolomite facies, represented by chaotic amalgamations of slumps, slide blocks, debris flows, gravity flows, and turbidites. BSPG siliciclastic unit isopach maps tend to be thickest in the basin center and thin toward the basin margins, suggesting that these sediments largely bypassed the carbonate-dominated slopes and infilled the deep basin during lowstands. Recent mapping however, suggests that there may be sandstone-dominated slope fan complexes locally. Confirmation of the latter interpretation will require additional infill correlations in the shelf-margin areas that rim the Delaware Basin.
A BSPG-specific gamma-ray-resistivity wireline facies model has been developed and applied basin wide. Although this methodology is a work in progress, initial mapping of net sandstone and net carbonate wireline facies show promising results. Improved resolution of sediment input fairways and intrabasin sediment distribution patterns are evident in the wireline facies maps.
To my knowledge, there are no published works that provide a quantitative accounting of the stratigraphy and facies distributions of the BSPG at a basin-wide scale. Given the comprehensive scale and scope of this TORA project subtask, when completed, these results may well provide the first, publicly available, stratigraphic characterization of the Bone Spring Formation spanning the entire Delaware Basin.
