Abstract
California Senate Bill 4 (SB4) was passed in 2013 prohibiting the injection of wastewater from oil and gas activities into aquifers containing fresh water. The enactment of California SB4 has renewed interest in determining groundwater quality, measured in parts per million total dissolved solids (ppm TDS), in and near oil fields in the southern San Joaquin Valley. Current efforts to map the distribution of underground sources of drinking water, or waters with less than 10,000 ppm TDS, rely on measured groundwater chemistry or processed borehole geophysical logs, but these mapping efforts can be limited by lack of spatial coverage of the available produced-water chemistry measurements. Previous work reveals a positive linear relationship between groundwater TDS and oxygen isotope composition. This, along with the well-known relationship between the oxygen isotope composition of groundwater and precipitated calcite, may allow for the determination of groundwater TDS using the oxygen isotope composition of calcite cement. Here, we test if oxygen isotope composition of calcite cements in local sandstones could be used to estimate groundwater salinity in the San Joaquin Valley. Samples from oil-and-gas drill cores were selected from a variety of San Joaquin Valley oil fields based on the criteria that they came from an interval with known TDS measurements. Samples were made into polished thin sections and the calcite cement was observed for cathodoluminescence and analyzed for δ18OVPBD (the relative ratio of 18O to 16O normalized to the Vienna Pee Dee Belemnite standard) via secondary ion mass spectrometry (SIMS) at a 10-µm scale and bulk isotope ratio mass spectrometry. The measured δ18OPDB in the cements and the bottom hole temperature from each well were used to calculate δ18OVSMOW (the relative ratio of 18O to 16O normalized to the Vienna Standard Mean Ocean Water standard) of the porewater. These calculated δ18OVSMOW porewater compositions were then compared with the known TDS measurements to verify the relationship between porewater δ18OVSMOW and salinity. Results showed that natural oxygen isotopic variability within a single generation of cement ranges from 2 – 3 ‰. Also, SIMS results show a positive linear relationship between observed cement oxygen isotope composition and measured TDS, indicating oxygen isotope composition present in calcite cements could be a good predictor of TDS in the San Joaquin Valley. Bulk isotope ratio mass spectrometry (IRMS) δ18O data was also collected to verify whether bulk analysis could be used in-lieu of SIMS due to bulk IRMS producing δ18O data with less extensive sample preparation and lower cost. However, bulk data show a positive bias when compared with SIMS data. More work should be done to understand why bulk and SIMS data do not agree and if heating the sample prior to analysis as an effort to drive off any additional phases in the sample other than the calcite cement could improve results.