Abstract:
The Neal shale, the organic-rich interval of the Floyd Shale located in the Black Warrior Basin of Alabama and Mississippi, has attracted recent attention as a potential economic source of hydrocarbons. X-ray diffraction (XRD) and X-ray fluorescence (XRF) spectroscopic analysis of 54 Neal shale samples indicate the formation is clay-rich and quartz-poor, with the clay content averaging 47.2 wt% and the quartz content averaging 25.2 wt%. Total organic carbon (TOC) values averaged 2.27 wt%, based on the correlation of XRF trace element concentrations to pyrolysis data. 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 thermal maturation. Based on this analysis, Neal shale porosity is a mixture of fissure, organophillic, and interparticle porosity. However, no increase in organic matter porosity was observed with increasing thermal maturation. Instead, the organophillic porosity development was found to be linked to bitumen migration/maturation. This suggests organic porosity development was significantly affected by the organic matter composition, rather than by thermal maturity alone (Figure 8). Kinetic porosity modeling 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 account for the chemical composition of the organic matter (i.e., kerogen vs. bitumen). Basin modeling indicates that the Neal shale has a large resource potential, with an adsorbed gas estimate of 460 Tcf and recoverable 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 the underlying Lewis Limestone samples have an average unconfined diametral strength of 22.22 MPa. This suggests that the underlying Lewis Limestone should serve as an active barrier to hydraulic fracturing efforts within the Neal shale. Although the Neal shale has retained a large volume of natural 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 reservoir can be fully evaluated.
