Using Noble Gas Geochemistry to Characterize Sources and Migration of Fluids in the Eagle Ford Shale

Abstract

The Eagle Ford Shale in south Texas has become one of the most prolific shale plays in the United States in recent years. While production data suggests that oil and natural gas can be produced across a vast area of the field, the source of H2S and hydrocarbons, and the extent to which fluids have migrated into and out of the Eagle Ford, have yet to be determined. This study uses noble gas isotopes, gas composition, and stable isotopes to evaluate the source gases, to characterize the fluids-in-place, and to characterize the extent of fluid migration from the Eagle Ford Shale. The inert nature and distinct isotopic compositions make noble gases ideal tracers of crustal fluid processes. In most shales, the noble gas isotopic composition reflects a binary mixture of: 1) air-saturated water (ASW), containing 20Ne, 36Ar, and 84Kr derived from solubility equilibrium with the atmosphere during groundwater recharge and 2) radiogenic noble gases such as 4He*, 21Ne*, and 40Ar* sourced from the decay of U, Th, and K. Once noble gases incorporate into crustal fluids, they fractionate only by well-constrained physical mechanisms (e.g., diffusion, phase-partitioning). For example, although the decay of U and Th, produces a fixed ratio of 4He/21Ne (2.2×107) and the initial 4He/21Ne of minerals in shale are fixed, 4He will be preferentially released with respect to 21Ne at hydrocarbon-producing temperatures. Over time, the isotopic ratios vary as fluids equilibrate with the shale matrix. Variations occur as a function of temperature, porosity, and the volume of fluid flow. Thus, the isotopic values can be used to reconstruct the history of fluid flow in specific formations. Our data from the Eagle Ford show that mantle-derived gases (elevated 3He/4He= 0.15–0.25Ra and 20Ne/22Ne=10.2–11.1) and radiogenic gases (4He, 21Ne, 40Ar) dominate the overall gas composition. We anticipate that volcanism during Cretaceous/Cenozoic rifting activity caused the observed mantle-gas contributions. Interestingly, higher mantle contributions appear to correlate with elevated H2S in the production wells from this study suggesting thermal sulfate reduction induced by magmatic activity. Additionally, ASW and radiogenic noble gases can be used to model the relative volume of residual fluids-in-place for this Eagle Ford play. Initial data suggests that there has been minimal fractionation of noble gases implying minimal loss of the initial hydrocarbon fluids.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015