HETEROGENEITY OF THE MINERALOGY AND ORGANIC CONTENT IN THE TUSCALOOSA MARINE SHALE

Heterogeneities in the physical and chemical properties of shales play an important role in controlling hydrocarbon distributions and can pose challenges to well drilling and completion strategies. Despite this importance, it remains challenging to quantify these heterogeneities and how they scale in a meaningful way. In this study, we analyze and describe the mineralogical and organic geochemical differences among core and cuttings samples collected from 11 wells in the Tuscaloosa Marine Shale (TMS).

XRD and pyrolysis data were donated by Goodrich Petroleum Company. Data were sorted and analyzed from samples collected in a 60 ft interval near the base of the TMS within the preferred “landing zone” for lateral completions. Among samples from all the wells the mean values for the concentrations of quartz (n = 164), clay (n = 164), calcite (n = 164), and TOC (n = 137) were 22.9%, 47.6%, 17.2%, and 1.6%, respectively. Note that small amounts of feldspars and trace pyrite, dolomite, and siderite were present in addition to these more abundant minerals. The median values for quartz, clay, calcite, and TOC among the individual wells ranged from 16.5% to 36%, 37.3% to 58%, 9.6% to 33.3%, and 1.1% to 2.5%, respectively. The interquartile range (i.e., the range in values that encompasses the middle 50% of the data distribution) for the individual wells proved to be a useful measure of heterogeneity. For example, the interquartile variation for quartz ranged from a low of 3.5% to a high of 13.8% among the different wells, while it ranged from 7.3% to 20.6% for clay. Furthermore, 4 of the wells showed bimodial distributions (two separate peaks on a histogram) of clay concentrations. Three of these wells are located to the south in the central production region of the TMS nearest to the paleo Cretaceous shelf boundary.

This initial quantification of the heterogeneity of key mineralogical and organic parameters in the TMS suggests that sediments were likely sourced from multiple depositional centers with the relative abundances of key minerals influenced by the relative contributions of these sources and the paleo water depth. Additional work to link the kerogen type(s), mineralogical distributions, and depositional settings in the TMS is ongoing.