GEOCHEMICAL ANALYSIS OF PARASEQUENCES WITHIN THE PRODUCTIVE MIDDLE MEMBER OF THE EAGLE FORD FORMATION AT LOZIER CANYON NEAR DEL RIO, TEXAS

The middle member of the Eagle Ford formation is a heterogeneous, carbonate-shale unit that is the focus of unconventional oil and gas exploration in southern Texas. Exploration results, however, have been mixed because of the apparent heterogeneity of the shale. In this study, we examined the extent of heterogeneities in the Eagle Ford on the “bedding-scale” by evaluating changes in organic and inorganic geochemistry. Samples were collected vertically from each bed in outcrop covering four non-consecutive parasequences. These samples were analyzed using a Rock Eval 6 Analyzer™ to determine total organic carbon (TOC), kerogen type and maturity and using a Niton™ XRF to evaluate inorganic geochemistry. Changes in lithology and depositional environment were evaluated by measuring variations in concentrations of TOC, molybdenum (Mo), uranium (U), and vanadium (V). In Parasequence 1, TOC ranged from 1.4-5%, Mo ranged from 7.2-42.1 ppm, U ranged from 11.4-24.7 ppm and V ranged from 69.8-259.2 ppm. From the basal bed upward, these concentrations increased. In Parasequence 2, TOC ranged from 0.2-5.2%, Mo ranged from 10.2-72.1 ppm, U ranged from 8.6-27.4 ppm, and V ranged from 75.5-241.1 ppm. TOC was at its highest in the middle bed and Mo, U, and V increased upward. In Parasequence 3, TOC ranged from 0.1-2.8%, Mo ranged from 10.8-49.2 ppm, U ranged from 13.1-52.2 ppm, and V ranged from 52.4-249.2 ppm. TOC and U showed no general trend whereas Mo and V increased upward. In Parasequence 4, TOC ranged from 0.5-1.4%, Mo ranged from 23-68.1 ppm, U ranged from 12.7-23.8 ppm, and V ranged from 171.3-279.2 ppm. TOC showed no general trend whereas Mo, U, and V increased slightly upward. These analyses confirm that a substantial degree of heterogeneity is present on the bedding scale. There are no general trends for TOC within the parasequences, but an overall decrease was observed from the base to the top of the entire member. General increases in the concentrations of redox-sensitive trace elements Mo, U, and V toward upper beds in each parasequence indicate that the environment experienced increasingly anoxic conditions during deposition.