Water, water, everywhere, Nor any drop to drink!—The Rime of the Ancient Mariner, Samuel Taylor Coleridge
This study shows that Wolfcamp-produced waters in the Permian Delaware Basin are predominantly in situ Wolfcamp shale formation water with δ18O ∼6.5 ± 0.5‰ (standard mean ocean water) and a salinity as low as 20,000 ppm, consistent with illite-water equilibrium at peak burial conditions.
Produced waters in the Delaware Basin have highly radiogenic 87/86Sr ratios of ∼0.7085–0.7095 believed to be sourced from evaporative brines in the Salado salts and overlying shallow Ochoan evaporites. Despite Wolfcamp-produced waters in the Midland Basin routinely having total dissolved solids of up to ∼250,000 ppm, which is double that in the Delaware Basin, chloride-bromide systematics of produced waters show that only minimal halite dissolution was involved in both basins.
High-salinity produced waters in the Bone Spring Formation and the upper Wolfcamp formation from the Delaware Basin (∼50,000–125,000 ppm) are mixtures of Wolfcamp formation water and Ochoan evaporative brines that have mixed with local meteoric water. These brines infiltrated deep into the Delaware Basin during uplift of the western edge of the Delaware Basin via permeable Guadalupian and Leonardian sandstone and siltstones.
Due to the high illite content in the Wolfcamp shale, the shale-siltstone interface likely behaved as a clay membrane. Salinity differences of up to approximately 100,000 ppm across this interface created potential gradients in ion and water activity (aw), producing an osmotic pressure gradient.
Ion diffusion into the shales results in the flow of water out of the shales (high aw) into high-salinity siltstones (low aw). The coupled osmosis–diffusion model predicts high absolute osmotic pressures of up to ∼1680 psi and cocurrent flow of oil and water out of the shale. However, the flow of water out of the shale into adjacent siltstone faces an opposing osmotic pressure. This may explain the high fluid pressures encountered in the Wolfcamp shale and why oil production in the Delaware Basin produces so much water.