MORB and Solar-OIB like Noble Gas Signatures in the Eagle Ford Shale, Southwest Texas Implications for the Earths Geological Evolution and Framework

Heterogeneity of oil and gas production in the Eagle Ford Shale, southwest Texas is significant, and an understanding of how fault-fracture networks and deep crustal structures may have affected the Eagle Ford Shale from deposition through maturation is missing. This study presents the complete set of stable noble gases for gas samples collected in the Eagle Ford Shale. This new dataset, together with that of Byrne et al. (2018) and noble gas information from Harrington et al. (2015) sheds light on the tectonic and magmatic history of the region. They reveal the presence of both crustal and mantle components. Two mantle end-members are present, a weak one, corresponding to a MORB signature, suggesting the presence of an old SW-NE oriented rift, and a more prevalent one, corresponding to a primordial, solar-like and OIB signature. The noble gas pattern displays an almost pure crustal component in the south, a MORB-type signature in the center, and a solar-like/OIB signature to the west, likely continuing north. The MORB-identified rift follows closely the southern Laurentian lithospheric margin and is parallel to all major known faults in the area. It is consistent with the palinspastic continentally reconstructed Ouachita Rift (~530 Myrs), if two small clockwise rotations are applied in the area. These rotations can be accommodated through the presence of small transform faults. The latter are consistent with calculated relative 40Ar ages in these gas samples, pointing to the presence of discrete blocks within the Eagle Ford Shale. The primordial, solar-like component is consistent with an origin in the subcontinental lithospheric mantle, the shallow refractory reservoir beneath the Laurentian craton, an hypothesis consistent with the presence of a heterogeneous mantle. Both, intensive Late Cretaceous volcanic activity in the area as well as reactivation of all major fault systems in the Miocene are consistent with the presence of a recent, prevalent solar-like signature in the Eagle Ford Shale. This solar-like component displays characteristics similar to that identified in the Michigan Basin, also part of the Laurentian craton, supporting the hypothesis by Castro et al. (2009) that primordial noble gas signatures do not necessarily fingerprint the presence of a deep mantle plume.