October 11, 2017
New Publication: Reed, Robert M. (2017) Organic-matter pores: new findings from lower-thermal-maturity mudrocks. GCAGS Journal, v. 6, p. 99–110.
Available on the MSRL Members’ web page or at the Gulf Coast Association of Geological Societies web site.
About This Publication
Based on research presented at the 2016 MSRL meeting, this publication details findings on organic-matter pores in mudrocks with an Ro less than 0.7% from three locations in Texas. The article provides additional information on organic-matter pores that follows up on previous groundbreaking MSRL research on this subject (e.g. Loucks et al., 2009, 2012). MSRL researchers will continue efforts to understand the nature and distribution of this important type of mudrock pore.
New pore analyses of low-thermal-maturity (Ro < 0.7%) organic matter of possible or probable terrestrial origin (Type III kerogen) from three mudrock units in Texas contradict previous pore work on low-thermal-maturity, mostly marine organic matter from the Barnett and Eagle Ford Shales. In early work on mudrocks containing primarily marine organic matter, organic matter at Ro < 0.75% was observed to typically lack pores, whereas some thermally mature organic matter did develop pores. However, this pattern of pore development was not observed by all researchers working on other units, some of whom did see pores in organic matter at lower thermal maturities. To understand if predepositional pores in terrestrial organic matter are responsible for the discrepancy in pattern, this study focuses on organic matter at low thermal maturities, which should not have thermally generated pores.
Two beds of organic-matter-rich mudrock from a terrestrial depositional environment were sampled from the Paleogene Wilcox Group of East Texas. The mudrocks have an Ro of < 0.5%, a siliceous–argillaceous composition, and > 11% total organic carbon (TOC). Based on Rock-Eval pyrolysis results and depositional setting, the kerogen is thought to be dominantly Type III. Some silt-sized organic-matter grains in these samples contain probable predepositional pores. Pore sizes are up to 5 µm in length, and up to 27% of the organic matter has pores. Pores are more common in the larger organic-matter grains.
Samples from one core interval of the basal Cretaceous Eagle Ford Group on the San Marcos Arch of South Texas have abundant vitrinite (Ro ~ 0.45%) based on visual analysis, in contrast with shallower layers, which are vitrinite-poor. Rock-Eval pyrolysis results suggest the presence of Type III kerogen. Although these basal mudrock samples are more argillaceous than the rest of the Eagle Ford, they were deposited in a marine environment. Some of the organic matter in these samples has predepositional pores. Some of these pores are elongate, with sizes > 1 µm.
Pennsylvanian Smithwick Shale samples from the Fort Worth Basin of Texas have thermal maturity ranging from 0.4 to 0.71% Ro. Although much of the unit is marine, observed plant fossils suggest a mixed-organic-matter source. Some Rock-Eval pyrolysis results also suggest Type III kerogen. Much of the organic-matter shows a newly identified organic-matter pore texture. Instead of consisting of a mass of organic matter with spherical holes, the organic-matter is a composite grain made up of smaller, spherical organic-matter bodies. These organic-matter bodies vary slightly in size, but most are 60 to 100 nm in diameter. Pore space is present between the organic-matter bodies. Several possible formation mechanisms exist for these organic-matter bodies, but they strongly resemble “nannobacteria” described by previous researchers.
March 8, 2017
March 1, 2017
VIDEO: Meet The Author: Robert Baumgardner discusses RI 281. (6 minutes)
To purchase this publication as a (PDF) download, please order RI0281D ($15.00).
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