Assessment of the Petroleum Generation Potential of the Neal shale in the Black Warrior Basin, Alabama

Summary With the advent of extended-reach horizontal drilling and multi-stage hy draulic fracturing techniques, organic-rich shale has become an economically attractive source of hydrocarbons th at has transformed the domestic U.S. exploration and production industry. The organic-rich interval of the Flo yd Shale, informally termed the Neal shale, is one such formation that has attracted recent attention. The Neal shale is located in the Black Warrior Basin of Alabama and Mississippi. Formation sediment was transported towards th e southeast along the depositional strike of the Warrior platform, and deposited in epeiric ramp and basinal environmen ts during the Late Mississippian. The rock core and cuttings sampled in this study were taken from th ree wells located in Alabama: the Gilmer 17-12 #1 well, the Ralph W. Holliman #13-16 well, and the Lamb 1-3 No.1 well, with Easy %Ro values of 0.9– 1.0%, 1.5– 1.6%, and 2.2– 2.3%. Petromod ® Easy % Ro values were calibrated to published vitrinite reflectance (R o) data for the Gilmer 17-2 #1 well (Ro = 0.92– 0.98%) and the Ralph W. Holliman #13-16 well (Ro = 1.44– 1.59%). Estimated total organic carbon (TOC) values averaged 2.27 wt%. X-ray diffraction (XRD) and X-ray fluorescence (XRF) spectroscopic analysis of 64 Neal shale samples indicate the formation is cla y-rich and quartz-poor, with the clay content averaging 47.2 wt% and the quartz content averaging 25.2 wt%. Nine samples of the Neal shale, representing 3 different levels of thermal maturity, were analyzed by FIB-FESEM to determine if organic porosity increases with the level of thermal maturation. Based on this analysis, Nea l shale porosity is a mixture of fissure, organophillic, and interparticle porosity. However, no increase in organ ic matter porosity was observed with increasing thermal maturation, possibly due to instrument resolution lim itations preventing observation of kerogen vacuolization at the sub-nanometer scale. Instead, the organophillic porosit y development was found to be linked to bitumen migration/maturation. This suggests that organic porosity d evelopment was significantly affected by the chemical composition of the organic matter, rather than by thermal maturity alone. Kinetic porosity modelin g indicates the formation developed as much as 4.82% kerogen porosity. However, kinetic porosity models like the one used in this study may not be valid because they do not accoun t for the chemical composition of the organic matter. Basin and kinetic modeling indicate that the Neal shale has a large resource potential, with adsorbed gas estimates of 460 Tcf and free gas estimates ranging from 227–4,943 Bcf. Uniaxial strength tests indicate Neal shale samples have an average unconfined axial strength of 3.42 MPa, and samples of the underlying Lewis Limestone have unconfined diametral s trength values averaging 22.22 MPa. This suggests that the underlying Lewis Limestone should serve as an activ e barrier to hydraulic fracturing efforts within the Neal shale. Although the Neal shale has retained a large volume of natur al gas, mapping of the limestone fracture barriers and additional testing of hydraulic fracturing mechanics on clay-rich formations will be necessary before the potential development of the Neal shale as an unconventional petroleum reservo ir can be fully evaluated .