Abstract
Regulatory decisions on oil and gas industry produced water injection have been based on the principle that some faults act as fluid flow barriers and provide a natural boundary for related fluid migration. Often, little evidence is shown to demonstrate whether a fault plane or damage zone behaves as a flow barrier or conduit. Furthermore, it is usually unknown if the entire fault plane behaves uniformly, or transitions from sealing to leaking along-strike and/or vertically. Fluid flow near faults is likely controlled by the damage zone architecture, variable amounts of clay-rich gouge, and juxtaposition of stratigraphic layers. While much work has been done to assess the sealing nature of faults in the context of oil exploration and production, little has been done to characterize how water in oil fields behaves near faults. Here, we use groundwater total dissolved solids (TDS) (mg/L) trends near faults to gain a better understanding of groundwater-geologic structure interactions in the Poso Creek oil field, Kern County, California. We constructed a groundwater TDS model using 51 produced water geochemistry samples and 141 borehole geophysical logs. The model is used to predict TDS at discrete locations in 3-D space, then uses a Gaussian process to interpolate TDS over a volume. The TDS model is parameterized via mathematical optimization using Google's TensorFlow. Root-mean-square error for fitted log TDS is 0.23. The volume model of TDS was then compared to structural contour maps emphasizing faulting that were constructed from mapping the base of the Macoma Claystone, a regionally extensive unit. We found that some faults, in conjunction with clay layers, play a role in controlling TDS trends. This was shown by abrupt TDS gradients located across faults with higher displacement, likely due to recharge dominantly perpendicular to strike, combined with effective fault seal. However, some TDS gradients across mapped faults vary along-strike, disappearing toward the fault tips where fault displacement decreases. The nature of TDS gradients is interpreted to represent changes in sealing ability along fault strike.