EAGLE FORD SHALE – A HIGHLY COMPARTMENTALIZED RESERVOIR IN A COMPLEX TECTONIC SETTING - INSIGHTS FROM A NOBLE GAS STUDY

Exploitation of unconventional shale gas reservoirs intensified in the last decade and turned the Eagle Ford Shale in Texas into one of the most economic and prolific shale-oil formations in the world. However, uncertainties regarding oil and natural gas sourcing as well as variability in productivity in these reservoirs remain, highlighting our current lack of knowledge and understanding of unconventional systems. Here, we present the complete set of stable noble gases for samples collected in the oil zone in the Eagle Ford Shale. This new dataset, together with previously published data sheds new light on the sources and migration history of hydrocarbons present. The combined datasets portray a region with a complex structural and tectonic history. These datasets reveal the presence of both, a crustal and a mantle noble gas component. Both types of mantle components are present, Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB). These different sources display a spatial distribution pattern, oriented SW-NE, similar to that of most faults in the region, with a pure crustal component in the south followed by a MORB-type signature in the center, and an OIB-type in the west, likely continued north of the studied area. The MORB-type signature suggests the presence of a failed rift system in the region. A complex tectonic area in the oil zone, in which different sample collections from the same wells display both, MORB and OIB-type signatures in addition to displaying very different levels of crustal ⁴He* and ⁴⁰Ar* enrichments is apparent, and suggests the presence of two different hydrocarbon sources in the same wells. These results further suggest the possibility of fracture-connected reservoirs within the Eagle Ford Shale. Deep fault systems must be called upon to account for the OIB-type mantle signature. Gas sample relative ages were inferred on the basis of ⁴⁰Ar volume fractions. These ages are significantly different and display a clear spatial distribution pattern, with decreasing ages from the east toward the west. The different age groups suggest a series of compartmentalized reservoirs, likely separated by faults perpendicular to the rift system, in particular, in the oil zone. In-situ production is largely insufficient to account for most of the crustal ⁴⁰Ar* present and points to an external origin, which may include contributions from deeper Lower Cretaceous and Upper Jurassic source rock systems. Multiple lines of evidence point to exchanges with outside sources and call thus for a more open system for the Eagle Ford Shale.