High-resolution Core Studies of Wolfcamp/Leonard Basinal Facies, Southern Midland Basin, Texas
Robert W. Baumgardner, Jr.¹, H. Scott Hamlin¹, and Harry D. Rowe¹
¹The Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin
Recently, lower Leonard and upper Wolfcamp (operational Wolfcamp A and B) strata have become targets for horizontal drilling and hydraulic fracturing in the southern Midland Basin. High-resolution hand-held XRF, CT scan, and rebound hammer (unconfined compressive rock strength) analyses were collected from core from these intervals. These basinal rocks can be divided into four facies: (1) siliceous mudrock, (2) calcareous mudrock, (3) carbonate-clast conglomerate, and (4) skeletal wackestone/packstone. These facies are interpreted as hemipelagic deposits and sediment density-flow deposits reworked, locally, by bottom currents. Facies thickness ranges from inches to feet. Three scales of cyclicity are observed: (1) cycles of alternating sediment density-flow and hemipelagic deposits defined by 3-inch-spaced XRF elemental data (Ca, Al) (1-5 ft thick), (2) sets of repeating cycles defined by gamma ray log (low gamma = high carbonate, high gamma = high clay) (10s of ft thick), and (3) megacycles of dominantly calcareous or siliceous (defined by facies) cycle sets (10s-100s of ft thick).
The dominant facies, siliceous and calcareous mudrock, have few sedimentary structures, contain relatively high total organic carbon (TOC) (up to 6.3 percent), rare burrows, and common phosphatic nodules and pyrite framboids. Coarser-grained conglomerates and wackestone/packstones have current-related structures, locally, contain low TOC, few phosphatic nodules, and rare pyrite framboids. TOC varies widely by facies over small vertical distances, varies directly with geochemical proxies for marine productivity (Ni, Cu, Zn) and siliciclastic sediment (Al, Si, Ti), and varies inversely with carbonate (Ca, total inorganic carbon). Collectively, these factors indicate that fine-grained sediments accumulated slowly over long periods of time under anoxic conditions, but carbonate-rich, coarse-grained sediments were deposited rapidly on the basin floor and did not include significant marine or terrestrial organic matter.
In the northern Reagan County study area conglomerates up to 9 ft thick are common in the upper Wolfcamp, but absent in the lower Leonard, where thinner, finer-grained wackestone/packstones prevail. The difference in thickness of carbonate-rich beds in these two formations may affect vertical fracture propagation. Measurements of unconfined compressive strength show that most wackestone/packstones are stronger than almost all mudrocks. Even so, mineralized natural fractures are present in all facies. Close vertical proximity of mature organic matter (Type II-III kerogen, calculated Ro = 0.67-1.07, Tmax>438) in mudrock and thin, more brittle beds (brittle-ductile couplets) may explain the appeal of basinal Wolfcamp and Leonard strata as targets for horizontal well completion and fracture stimulation. S2/TOC data from the study cores identify zones of potential hydrocarbon production, which correlate with horizons that have produced 179-460 barrels of oil equivalent/day (30-day initial production) in nearby fields.
AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014