SOURCES AND DISTRIBUTION OF SALINITY IN HYDROCARBON-ASSOCIATED FLUIDS IN SEDIMENTARY BASINS

Understanding the sources and distribution of saline fluids associated with hydrocarbons in sedimentary basins is critical for oil and gas exploration (e.g. microbial degradation of hydrocarbons and methane production), production (e.g. chemical formulation of hydraulic fracturing (HF) fluids; reservoir integrity and scale issues), and disposal of waste products (e.g. deep injection of produced waters). Fluids injected for HF are relatively dilute, typically composed of local freshwater mixed with HF chemicals or recycled HF fluids. Yet, flowback and produced waters have variable salinities that often quickly (within a few days of production) reach high levels, reflective of basinal brines. Fingerprinting the source of these saline waters and controls on their chemical and isotopic composition is important for determining the fate of injected HF fluids, connectivity of oil/gas reservoirs to adjacent formations, and/or evaluating potential contamination of brine leakage into shallow aquifers. In addition, enhancement of hydrocarbon biodegradation (e.g. for bioconversion of residual oil to methane) requires knowledge of in-situ reservoir conditions. Recent studies of the chemical and isotopic composition of formation waters in midcontinent sedimentary basins illustrate the origin of saline fluids and distribution of salinity, compartmentalization of aquifers, drivers of freshwater-brine mixing, and residence time of fluids.