Paleo-geochemical Studies of Eagle Ford black shales of south Texas.

Abstract:

We report the analyses of zircons, high-resolution global ⁸⁷Sr/⁸⁶Sr seawater curve, porosity, total organic carbon (TOC), maturity and mineralogy for a 300-ft long full-sized (3.5-inch diameter) subsurface core samples of carbonate-rich (marl) Eagle Ford black shale of south Texas. Zircons from Eagle Ford cores yield three ages of 93.7±1.9 Ma, 93.2±1.66 Ma and 94.13±1.25 Ma, consistent with the Cenomanian-Turonian (C-T) age of deposition in three contiguous cores, spatially separated by 140 miles. Our zircon ages from volcanic ash study shows that the Eagle Ford was deposited during 10 Ma, at a rate of 28 ft/Ma covering the Oceanic Anoxic Event 2 (OAE2) at the C-T boundary. In addition, ⁸⁷Sr/⁸⁶Sr sea water curve from 28 closely spaced carbonates in black shale cores displays a less radiogenic input of ⁸⁷Sr/⁸⁶Sr during, and leading up to, C-T event. On the other hand, the carbonate weight percent shows a negative correlation with the porosity measured for 33 continuous samples, and porosity also shows a positive correlation with the TOC and vitrinite reflectance (VRe, by pyrolysis (Geomark)) of 28 samples being analyzed, especially around the C-T boundary. The VRe decreases right at the boundary and remains almost constant till the end of deposition of the Eagle Ford shales. The OAE2 can be directly related to the lowering of radiogenic input of ⁸⁷Sr/⁸⁶Sr and TOC, resulting in an increase in the porosity of the shales along with the carbonate content. Recent studies of black shales of Wufeng, Longmaxi formations and Lower Cambrian black shales, China, have shown the porosity to be positively correlated to TOC and carbonate content while being negatively correlated to clay content. Our study however shows a different relation between these parameters.

Volcanogenic clays, such as montmorillonite, vermiculite, dickite and halloysite 10Å, ranging from 2 to 12% in modal abundance were determined from X-ray diffraction (XRD) analyses of 100 Eagle Ford bulk marl samples from different depths and contiguous cores, indicating continuous volcanism throughout the Eagle Ford deposition.

This continuous volcanism during the EF deposition of volcanic silicic sediments and carbonates was part of the global continental arc flare-ups in the Cretaceous, responsible for greenhouse conditions and subsequent anoxia during marl deposition.