MAJOR AND TRACE ELEMENT CORRELATIONS IN THE UPPER CRETACEOUS EAGLE FORD BLACK SHALE

A comprehensive major and trace element dataset was collected from seven subsurface wells covering ~58 miles along a NE-SW strike within three contiguous counties in south Texas, Live Oak, Karnes and DeWitt. These wells represent the gross Eagle Ford stratigraphy across the Cenomanian-Turonian boundary. In order to geochemically decipher the sources of Eagle Ford sedimentation, we performed the Spearman’s Rank Correlation Coefficient analysis of 292 black shale samples. The resultant positive correlations of 10 major and 37 trace element high precision analytical measurements of these samples are noteworthy. Positive correlation coefficients between 55% to 98% are presented below:

Si with Fe, Al, Ti

Cr with Be

Sc with Al, Si, Ti, Fe, Mg, K

Zr with Sc, Al, Si, Ti, Fe, Mg, Na, K

V with Sc and Mo

Co with Cr, Be

Ni with Ti, Sc, V

Cu with Be, Cr, Sc

Y with Be, Cr, Co, Cu

Nb with Al, Si, Ti, Fe, K, Sc, Zr, Ga, Rb, Y

Mo with Zn, V, Ni

Sn with Be, Cr, Co, Cu, Ga, Y

Cs with Al, Si, Ti, Fe, Mg, K, Sc, Zr, V, Ni, Rb, Nb

Ce with Al, Si, Ti, Fe, Mg, K, Sc, Zr, V, Ni, Rb, Nb, Cs, La

Hf with Al, Si, Ti, Fe, Mg, K, Sc, Zr, Cs, La, Ce, Nd, Gd, Dy, Ho, Yb

Th with Al, Si, Ti, Fe, Mg, K, Sc, Zr, Rb, La, Ce, Nd, Yb, Hf

U with V, Ni, Mo, Cs, La, Ce, Nd, Gd, Yb and Hf

The above correlations are commonly observed in mantle-derived magmatic sources and are uncommon in detrital sedimentary provenances. The observed abundance of multiple bentonite layers in subsurface cores throughout the Eagle Ford Formation further attest to continuous volcanism as the likely source for causing the aforementioned elemental associations. Total Alkali-Silica variations in bentonite from these seven wells document a basalt and basaltic-trachy andesite trend (Darmaoen, Basu and Tinnin, this symposium session). We propose that this scenario of continuous volcanism produced anoxic conditions within the paleo-depositional environment, preserving organic matter that eventually lead to the formation of the prolific Eagle Ford source rock.