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The Woodford Shale in west-central Oklahoma is an organic and silica rich shale that is a prolific resource play producing gas and liquid hydrocarbons (Gupta et al., 2013). Unconventional shale wells are only producible due to modern hydraulic fracturing techniques. Production surveys from unconventional reservoirs show significant variability between wells and even between fracking stages (Kennedy, 2012). The production potential of a particular shale appears to be related to its brittleness and kerogen content "sweetness". Thus, brittleness analysis becomes important when choosing which shales to produce. A rocks brittleness index can be related directly to elastic properties derived from P- and S-wave velocities, as well as, its specific mineral makeup. This project's main focus is to determine the elastic rock properties that affect or relate to Woodford shale brittleness and how they relate to the rock's specific mineral makeup and kerogen content. Measurements to determine elastic properties, based on ultrasonic laboratory testing, were conducted on available Woodford cores. The estimated elastic moduli were evaluated via cross-plotting and correlation with a variety of rock properties. Elastic properties are of essential relevance to forward seismic modeling in order to study seismic response. Mineral makeup, determined via XRD and XRF analyses done by Kale Janssen (2017), was used to calculate a mineral-based brittleness index for comparison with the elastic moduli. Evaluation of the elastic moduli assisted in determining which elastic properties directly relate to the brittleness of the shales and, in turn, to geomechanical aspects. These properties were correlated with data from previous studies including mineral percentages, total organic content (TOC), and thermal maturity. These correlations were used to determine which elastic properties best predict a rock's brittleness index. The calculated brittleness was used to develop a brittleness index map of the Woodford Formation.
The research project investigated various geochemical aspects of oils, suspected source rocks, and tar sands collected from the Anadarko Basin, Oklahoma. The information has been used, in general, to investigate possible sources for the oils in the basin, to study mechanisms of oil generation and migration, and characterization of depositional environments. The major thrust of the recent work involved characterization of potential source formations in the Basin in addition to the Woodford shale. The formations evaluated included the Morrow, Springer, Viola, Arbuckle, Oil Creek, and Sylvan shales. A good distribution of these samples was obtained from throughout the basin and were evaluated in terms of source potential and thermal maturity based on geochemical characteristics. The data were incorporated into a basin modelling program aimed at predicting the quantities of oil that could, potentially, have been generated from each formation. The study of crude oils was extended from our earlier work to cover a much wider area of the basin to determine the distribution of genetically-related oils, and whether or not they were derived from single or multiple sources, as well as attempting to correlate them with their suspected source formations. Recent studies in our laboratory also demonstrated the presence of high molecular weight components(C4-C0) in oils and waxes from drill pipes of various wells in the region. Results from such a study will have possible ramifications for enhanced oil recovery and reservoir engineering studies.
The Woodford shale is recognized as an abundant source rock across Oklahoma and much of the midcontinent (Lambert, 1990), and up to 8% of the world's hydrocarbon reserves are estimated to have been sourced by the Woodford and its equivalents (Fritz et al, 1991). The Woodford shale is far more complex than other Devonian black shales found in North America due to the presence of alternating bands of chert-like amorphous silica and silica-rich shale. Analysis of chert and its possible role in gas generation and storage in shales has been largely overlooked. The goal of this study is to determine if chert size, amounts, or polycrystallinity can be indicators of thermal maturity within the Woodford shale. Handheld XRF analysis was conducted on the whole rock samples, and a mudrock specific sodium bisulfate fusion was used to separate the non-clay fraction. SEM was performed on the resulting separates to study and observe changes in chert fabric, grain-size, and amount. No correlations were observed to indicate that chert is an indicator of thermal maturity within the Woodford shale. Increase in chert growth and amount was also not detected within the size fractions as thermal maturity increases. Handheld XRF proved to be a good proxy for quick, onsite analysis of silica concentrations, as well as the amount of organic matter within drill core. This could be beneficial as hydraulic fracking produces best results in areas of higher silica content, and the wells with the highest organic matter have the highest potential for petroleum accumulations.
The Woodford Shale is a dark, siliceous mudstone that was deposited in a rift basin during late Devonian to early Mississippian times. Three drill cores containing the Woodford Shale from the Anadarko Basin, Oklahoma, have been geochemically analyzed using a handheld energy-dispersive x-ray fluorescence instrument. Each core was analyzed at 3- to 4- inch intervals, providing high-resolution chemostratigraphy. Analysis of the following elemental concentrations was performed: Mg, Al, S, Si, P, K, Ti, Ca, Mn, Fe, Mo, Cr, Ni, Cu, Zn, Th, Rb, U, Sr, Zr, and V. Major element geochemistry supports that the Woodford is a siliceous mudstone, with little carbonate input. The relationship between iron and sulfur depicts a high degree of pyritization. A portion of the Woodford Shale appears to be iron-limited with respect to pyrite formation. Trace element enrichment factors and ratios (Ni/Co, V/Cr, and V/(V+Ni)) indicate anoxic or euxinic, oscillating with dysoxic to oxic, bottom water conditions during deposition of the Woodford Shale.
A prolific source rock in Oklahoma, the Woodford Shale has sourced petroleum reservoirs for millions of years and produced oil and gas since 1939. Not until the shale boom of the early 2000's was the full potential of the Woodford Shale, as a reservoir, realized. The focus of this research is to characterize three (the main ones) of the four petro-physically distinct zones within the Ardmore Woodford Shale in order to investigate the ideal zone for hydrocarbon extraction. The Woodford Shale is composed of tight, organic-rich, interbedded shale and chert beds deposited along a marine slope. Due to its tight nature, petro-physical characteristics such as porosity, permeability and fluid flow behavior are hypothesized to be related to pore-throat distribution on the nano-meter scale. A total of seven samples from two wells, cover three of the four zones within the Woodford Shale. Pore framework and fluid flow were investigated using mercury injection capillary pressure (MICP), contact angle (wettability), and spontaneous imbibition tests. Pyrolysis tests were conducted to analyze thermal maturity and TOC, while X-ray diffraction (XRD) tests provided the samples mineral composition. The samples exhibit mineral compositions of mostly quartz and clays with high wettability and pore connectivity when interacting with a hydrophobic fluid of n-decane. Porosity and permeability values ranged from 0.5 to 3.1% and 4.4x10-7 to 1.5x10-5 mD respectively, with the majority of pore throats existing within the 5-50 nm range (likely organic matter hosted and/or intraparticle pores). The MICP-derived porosity and permeability results are compared to these obtained from well logs, and a lack of industry standards for measuring tight shale characteristics from core samples makes consistent and repeatable results challenging. An integrated analysis of MICP, imbibition, wettability, geochemistry, well logs, and production data suggests that zone two within the Woodford Shale is the desirable target for hydrocarbon production.
Shales are generally regarded as organic rich source and seal rocks that are unworthy of the amount of research that has been given to their coarser-grained counterparts, even though shales comprise nearly two-thirds of Earth's sedimentary record (Potter et al., 1980). The Woodford Shale is acknowledged as a prolific source rock across much of Oklahoma and the midcontinent (Lambert, 1990). Up to 8% world's original hydrocarbon reserves are estimated to have been sourced by the Woodford and its equivalents (Fritz et al., 1991). Study of the heavy-mineral fraction in sedimentary rocks is important because it can indicate provenance and some of the diagenetic changes that occur in sedimentary rocks. This goal of this study is to describe the heavy-mineral fraction of eight Woodford Shale samples from the Greater Anadarko Basin of Oklahoma, and determine whether or not the constituents that make up the heavy-mineral fraction have any impact on the process of thermal maturity within source rocks. This study utilizes a method designed to efficiently separate the heavy-mineral fraction of shale samples. Scanning electron microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) are used in this study to identify mineralogy, grain size, composition and shape. Mineral distributions in the samples have been determined from point counting. The weight percent of the heavy mineral fraction was calculated for each of the samples. This was then compared to their location within the basin, depth, vitrinite reflectance and total organic carbon (TOC). We found that as the thermal maturity increase, the weight percent of heavy minerals also increases. Pyrite (FeS2) was the most abundant heavy mineral found in the Woodford samples used in this study. From analyzing the different forms of pyrite, it was found that as thermal maturity increases, framboidal pyrite alters to euhedral pyrite.