Understanding deposition and diagenesis of a significant saltwater disposal reservoir using stable isotopes, Grayburg-San Andres intervals, Midland Basin, United States

Abstract

Carbonate sediments record both environmental conditions during deposition along with secondary diagenetic processes. This study investigates how basin restriction, meteoric fluids, and other diagenetic processes contribute to the resulting sediment chemistry and porosity in the Grayburg-San Andres (GRBG-SADR) redisposal reservoir in the Midland Basin of West Texas. Two cores with locations selected in shelfal versus basinal sedimentary environments were described and analyzed for their stable isotope values of carbon and oxygen (δ13Ccarb, δ18Ocarb), chromium-reducible sulfur (δ34SCRS), water-soluble sulfate (δ34SWSS), and of carbonate-associated sulfate (δ34SCAS). Together with mineralogy and lithofacies, the data support a conceptual model for water flow dynamics that suggest an anti-estuarine basin setting for the Midland Basin. Our study shows that the combination of arid climatic conditions, high biologic productivity, and restricted oceanographic setting in the equatorial Midland Basin led to basin-wide δ13C increase of up to +3 ‰. The low δ34Spyr mean value of -29.4 ‰ in deep water deposits indicates open-system conditions for sulfur cycling, implying a sluggish thermohaline circulation did not lead to the development of a stagnant anoxic bottom water layer. Instead, water circulation proceeded and ensured continuous input of oxygenated open marine water. As a result, dissolved sulfate in the basin recorded an open marine δ34S signal of +10 ‰ to +12 ‰. The combination of carbonate mineralogy, post-depositional dolomitization, phase-dependent carbonate dissolution, and meteoric alteration of strata resulted in high porosity (reservoir quality), a significant contributing factor for why the GRBG-SADR Formations are a target for saltwater disposal in the Midland Basin of West Texas.