Using brackish groundwaters as indicators for potential impacts from deeper oil and gas activities Mark Engle, Francisco Reyes U.S. Geological Survey Dept. of Geologial Sciences. Univ. Texas at El Paso El Paso, Texas

Acknowledgements Funding: USGS Energy Resources Program Assistance with sampling: Madalyn Blondes (USGS), Matt Varonka (USGS), Tanya Gallegos (USGS), Josue Magana (NMSU) Assistance with analysis: Stephanie Ray (UTEP), Belinda Gonzalez (UTEP), Jasper Konter (Univ. Hawaii), Tiffani Schell (USGS) Thanks to the oil and gas operators whom allowed to sample their water and oil wells.

Potential exposure pathways to drinking water sources Water Volume vs. Quality Impacts Mostly operation engineering problems Modified from U.S. Environmental Protection Agency, 2015

Potential for upward water and hydrocarbon migration Interception with fractures or old wells Fluid movement may be slow (decades or longer) Distances of several km Difficult to assess potential Reagan et al., 2015

Brackish groundwater as a proxy – Dockum Aquifer, Permian Basin, Texas Approach: Examining brackish groundwater-brine interface Pressure Data Drill Stem and Initial Pad Data Chemical Data Na/Cl, Ca/SO4 Isotopic Data δ18O, δ2H, 87Sr/86Sr ? Reyes, 2014 Engle and Blondes, 2014; Engle et al., in review

The Permian Basin A A’ A A’ Study Area Major US oil producer (2012 Texas RRC data) 312 MMbbl oil 12% of US production 1,200 BCF gas A A’ Source: Texas BEG A Triassic Dockum Group Upper Aquifer System A’ Evaporite Confining System (ECS) Deep Basin Aquifer System (DBBAS) Source: UT Permian Basin

The Triassic Dockum Aquifer Upper: Mud rich with thin interbedded s.s. Lower: Sandstone rich, conglomerates, mudstones, & interbedded mud- & sandstone Mineralogy: Qtz, calcite, alb, illite, muscovite, kaolinite Limited uses High salinity hazards Natural occurring radioactivity Used for irrigation, municipal water supply, and hydrocarbon operations. Label graphs I’ I I’ I Modified from Texas Water Development Board (2003)

2 models for pressuring in the basin Delaware Basin model: Over-pressuring Palo Duro Basin model: Under-pressuring Drill stem test data (Luo et al., 1994) Depths of 3-5 km, gradient up to 20 kPa/m Clay compaction disequil., dewatering, and hydrocarbon generation Modeling work (Senger et al., 1987) Cenozoic uplift and tilting and erosion produced substantial under-pressuring West East Dockum aquifer Modified from Matchus and Jones, 1984

Reservoir Pressure Gradient Data Salt Data for 3 counties Data from below aquitard Source rocks are over-pressured Upper-most brine bearing reservoirs under-pressured since 1940s Oil Reservoirs Data from IHS Energy (2015) and Friedrich and Gene (2015)

Dockum Chemistry Overview – Durov Diagram Na-rich waters Higher salinity Generally deeper Ca-rich waters Lower salinity Found on basin margins

Water types vs. depth and location Ca-rich waters tend to be shallower than Na-rich waters Ca-rich waters tend to be more common in the basin margins

TDS ranges of the produced waters TDS increases with depth until the Devonian Lower TDS in the lowermost reservoirs and in the shales

Origin of the produced waters Leonardian to Penn reservoirs Evap’d Seawater Matches data from fluid inclusions Guadalupian and Devonian/Silurian reservoirs Meteoric

Origin of the strontium in produced waters Leonardian to Penn. Reservoir data overlap with halite + polyhalite in the evaporites

Dockum - Salinity Sources Dockum waters suggest evaporite dissolution source Brine salinity from paleoseawater and evap dissolution w/ SO4 reduction Possibly some connection Direction unclear SO4 reduction

Dockum – Sr sources Na-rich Dockum waters approach the composition of halite + sylvite + polyhalite Some overlap with brines (?) Doesn’t fit with Dockum evap dissolution model

Dockum - Water Sources Dockum entirely meteoric Ca-rich vs Na-rich Disconnect between shallow and deep brine No evidence for upward brine migration into the Dockum

Conclusions Pressure system is a hybrid of Delaware and Palo Duro models Over-pressured source rocks Downward pressure gradients through the evaporites Na-rich and Ca-rich Dockum waters have different origin Neither suggest influx from deep basinal brines Upward flow of oil and gas related fluids from source rocks likely to be stopped in below the evaporites

Caveats and Final Thoughts Patterns in vertical groundwater flow may be delayed to current conditions Access to brackish groundwater samples is relatively rare Pressure data for source rocks can also be difficult to obtain But, it may have potential as a screening tool to identify relative risk from different basins

Conventional vs. Continuous Hydrocarbon Accumulations Modified from Schenk and Pollastro, 2002

Continuous hydrocarbon accumulations Examples Shale Gas Tight Gas Tight Oil Coalbed Methane Characteristics Low permeability reservoir Pervasive across a large area, non-discrete Not affected by hydrodynamic processes No down-dip water contact The Marcellus Shale, Virginia Photo: Cathy Enomoto, USGS