Paleoenvironment and source-rock potential of the Cenomanian-Turonian Eagle Ford Formation in the Sabinas basin, northeast Mexico

Abstract

The Cenomanian-Turonian Eagle Ford Formation is a widespread organic-rich mudrock in Texas and northern Mexico. The economic significance of this unit for oil and gas production in USA has led to numerous investigations documenting its lithological and geochemical variability. Comparatively little is known about facies distribution and source rock potential of this unit in Mexico. Here, we studied a section of the Eagle Ford Formation in the central part of the Sabinas basin. In order to evaluate the lateral continuity of the Eagle Ford Formation, and more specifically to gain understanding of the paleoenvironmental drivers of organic matter deposition and the hydrocarbon source source rock potential of this unit, we combined lithological, gamma-ray spectrometric, petrographic and organic geochemical data.Vertical distribution of main lithologic units and gamma-ray signal of the Eagle Ford Formation of the studied section match those documented in several sites from Texas, with the shale-dominated, bentonite-rich succession of the lower Eagle Ford showing the higher gamma-ray values, and the limestone-dominated succession of the upper Eagle Ford exhibiting moderate-high gamma ray values. Th/K and U/Th ratios suggest that the organic-rich lower Eagle Ford was deposited mainly during a warmer and more humid period, probably related to a sub-humid regional climate phase during the Cenomanian-Turonian global warming event, and under anoxic/dysoxic marine environments. Furthermore, a higher abundance of radiolarians indicates its deposition under nutrient-rich surface water conditions induced by upwelling currents. In contrast, these ratios support a less humid climate and oxic/dysoxic conditions during the accumulation of the upper Eagle Ford. Nevertheless, fluctuations of bottom water oxygen are evidenced by the occurrence of high TOC contents and bioturbated levels. Although TOC values throughout the Eagle Ford Formation point to a poor to very good source rock potential, both equivalent vitrinite reflectance (%VReq) and Tmax values indicate that the organic matter is overmature and has an exhausted hydrocarbon generation capability.