A JOINT GAUSSIAN PROCESS MODEL OF GEOCHEMISTRY, GEOPHYSICS, AND TEMPERATURE FOR GROUNDWATER TDS IN THE SAN ARDO OIL FIELD, CALIFORNIA, USA

Declines in fresh groundwater availability have expanded interest in brackish resources; however, the distribution of brackish groundwater is poorly understood. Water resources in sedimentary basins across the United States often overlie oil and gas development. Mapping of groundwater total dissolved solids (TDS) using data from oil well geophysical logs has become an important technique for identifying fresh and brackish groundwater in sedimentary basins.

Existing geophysical log analysis methods use porosity and temperature to relate formation resistivity to TDS. Typically, natural geothermal gradients are used to estimate temperature at the location of collected resistivity. However, in thermally enhanced oil fields steam is injected into the subsurface to mobilize high viscosity oil, creating variable temperature distributions. Furthermore, TDS derived from resistivity also depends on the fractions of dominant ions in the water. Typically, dissolved chloride and bicarbonate fractions must be determined. It is also necessary to model TDS across many geologic units with heterogenous porosity distributions. Collectively, each quantity needed to estimate TDS (resistivity, porosity, temperature, bicarbonate fraction) varies in space and time, and available data points are rarely collocated.

Here, we present a new method of mapping groundwater TDS that continuously models each quantity together with a joint Gaussian process. The model yields volume maps of all five quantities. We apply this method to the steamed San Ardo Oil Field in Monterey County, California. In some areas of the aquifer system overlying the oil zone, the temperature is ~75 °C, roughly twice the natural background value. Groundwater TDS is typically <1,500 mg/L in the aquifer and increases with depth to ~9,000 mg/L in the oil-producing zone. Weaker lateral TDS trends may be controlled by recharge patterns associated with the Salinas River. Our model reveals with high certainty that groundwater has freshened in one localized part of the oil-producing zone in San Ardo and suggests with less certainty that more widespread freshening has also occurred. The lowering of TDS was possibly from decades of low-TDS steam injection and the associated fluid production and disposal operations.