March 8, 2017
March 1, 2017
VIDEO: Meet The Author: Robert Baumgardner discusses RI 281. (6 minutes)
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About This Publication
This report, conceived as a follow-up to regional treatment of the Wolfberry depositional systems in Report of Investigations No. 277, focuses on basinal facies of the lower Leonard and Wolfcamp intervals in the southern Midland Basin. The report provides detailed lithologic information about mudrocks and associated facies that were largely ignored before they became prolific producers of hydrocarbons as a result of hydraulic fracturing and horizontal drilling. Readers interested in the regional view presented in RI 277, and those seeking detailed lithologic information about the so-called Wolfcamp A and B zones, should find value in the core-based descriptions of lithofacies in this report, which examines the most continuous basinal cores publicly available from the Lower Permian/Upper Pennsylvanian interval in the Midland Basin. Detailed core description (including thin-section analysis)—combined with total organic carbon (TOC) sampling, handheld XRF scanning, and micro-rebound hammer testing at 1-ft spacing (techniques applied to these cores for the first time)—reveals significant, facies-related differences in TOC content, mineralogy, and rock strength. In addition, programmed pyrolysis (Rock-Eval®) analysis reveals that most kerogen in the lower Leonard/Wolfcamp interval is in the oil-production window and has matured to Type II–III (“oil-gas prone”). Stratigraphers as well as explorationists will find information here to improve their understanding of Permian-age basinal mudrocks in the Midland Basin.
Basinal mudrocks and associated facies assigned to the Wolfcamp and lower Leonard lithostratigraphic units in the Midland Basin are prolific producers of oil and gas. Four facies are recognized in core on the basis of macroscopic examination, XRD analysis, and ED-XRF elemental chemistry: (1) siliceous mudrock, (2) calcareous mudrock, (3) muddy bioclast–lithoclast floatstone, and (4) skeletal wackestone/packstone. Mudrocks are largely hemipelagic deposits of fine-grained sediment delivered by suspension settling. Floatstones were deposited by debris flows that originated on carbonate shelves around the basin. Wackestones/packstones are finer grained sediment density-flow deposits, probably turbidites, reworked by local bottom currents. These sediments were deposited below storm wave base in a basin having limited connection to the open ocean. During deposition, low-oxygen conditions prevailed in bottom waters and sediments, as shown by the presence of agglutinated foraminifera, rarity of burrows, widespread prevalence of small (<6 μm) pyrite framboids, presence of phosphatic nodules, and elevated molybdenum concentrations. Total organic carbon (TOC) content reaches 6.8 percent. TOC is facies-dependent (highest in siliceous mudrock) and varies widely within small vertical distances. Cyclicity is evident in individual meter-thick, upward-fining cycles of floatstone or wackestone/packstone overlain by calcareous and siliceous mudrock. Rock strength increases as calcite content increases, likely as a result of diagenetic cementation. Most kerogen in the lower Leonard–Wolfcamp interval is in the oil-production window and has matured to Type II–III (“oil–gas prone”). On the basis of their TOC and hydrogen content, siliceous mudrocks have the highest potential for hydrocarbon generation.
Keywords: basinal mudrocks, cyclicity, organic matter, Pennsylvanian, Permian, Reagan County, Rock-Eval®, rock strength, stratigraphy