Fig 1.
The study area is outlined in black box showing the producing areas of oil fields in the study area highlighted in red.
Fig 2.
Stratigraphy in the study area.
The study area lies near the basin axis. The stratigraphy of the Tulare aquifer is shown in greater detail. Modified from Scheirer and Magoon (2007) [14].
Fig 3.
Structural contour map showing the elevation of the base of the Tulare Formation with respect to sea level.
Pertinent structural and geographic features are labeled. Wells shown on the map are those used to generate the contours. Star shows well 0402938955 in which the contact between the Tulare Formation and overlying alluvium was defined by Milliken (1992) [21]. Contour interval = 150 m. Faults mapped at the surface in eastern Elk Hills [22] are shown in red. Heavy black outlines show oilfield administrative boundaries in the study area. Air photo base from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Table 1.
Fluid production in cubic meters (m3) and United States barrels (bbls) and gas production in cubic meters (m3) and thousand cubic feet (MCF) for the oil fields within the study area in 2018.
The ratio of produced water to oil is shown in the last column. Values from California Geologic Energy Management Division (CalGEM) [12].
Fig 4.
Oil field produced water disposal by injection in the study area.
a) Bubble map showing volumes of produced water disposal in each water disposal well. The size and color of the circle represents the volume of injected water post-1977 (MM bbls = million barrels) [51]. The dates shown on the disposal area labels represent the dates during which disposal by injection into the Tulare Formation occurred within the oil fields. Totals for Buena Vista Oil Field and the North and South Coles Levee Oil Fields combined are shown within the area over which the volumes were totaled (outlined in orange). At the Elk Hills Oil Field, the three main disposal areas (18G, 23/25Z and 27R) are summed separately. b) Wells with surface expressions (fluids breaking through to the surface outside the casing) and c) wells with resistivity anomalies possibly related to the injection of produced water.
Fig 5.
Map showing thickness of the Tulare Formation in the study area.
The formation is thinner over the crests of the anticlines and thicker in the adjacent synclines. Heavy black outlines show oil field administrative boundaries. Wells shown are those used to generate the contours. Contour interval = 100 m. Air photo base from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Fig 6.
Thickness isochore maps of major clay layers in the Tulare Formation.
Heavy black outlines mark oilfield administrative boundaries. a) Net clay isochore map of the Amnicola clay (gross thickness minus sand interbeds). Wells shown are those used to generate the contours. Contour interval = 1.5 m. b) Gross interval isochore map of the Tulare clay. Wells shown are those used to generate the contours. Contour interval = 25 m. Air photo base from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Fig 7.
Groundwater surface elevation maps for the years a) 1980, b) 1990, c) 2000 and d) 2010. The maps were constructed from water surface elevations in groundwater wells outside the oilfields and from density-neutron log data showing the base of the vadose zone at the time of drilling in wells within the oilfields. Because the data coverage is discrete in space and time in the study area, a Gaussian process was used to interpolate the data to create continuous water table maps for selected years and is described in the supplementary material. Contour interval = 10 m. Base map from Esri. "World Hillshade" [basemap]. Scale Not Given. "World Hillshade". July 9, 2015.
Fig 8.
Log-calculated total dissolved solids (TDS) versus depth plots for wells within different elevation ranges for the base of the Tulare Formation.
Log calculated TDS shown by the gray dots. Wells with lab sample analyses are shown as black horizontal lines between the upper and lower perforated interval depths. The vertical position of the line corresponds to the salinity value of the sample. For wells with multiple samples, the highest and lowest salinity values are shown by a gray box and the horizontal line is placed at the average salinity value. Red boxes note log calculated salinity values that are abnormally high and are sometimes associated with produced water disposal wells. The red horizontal line on each graph marks the 10,000 mg/L TDS value. Red vertical dashed lines mark the upper and lower depths at which groundwater salinity exceeds 10,000 mg/L within each depth range. Points with calculated salinities greater than sea water are shown in red. Values in feet in black, values in meters in red.
Fig 9.
Map showing depth to log-calculated salinity of 10,000 mg/L total dissolved solids (Base 10K) in the Tulare aquifer.
Wells shown are those used to generate the contours. Empty area over Elk Hills Oil Field contains no groundwater with less than 10,000 mg/L TDS. Black stars show location of wells in Table 2. Air photo base from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Table 2.
Completion depths for sampled wells with salinity values near 10,000 mg/L TDS compared to mapped depth to water with 10,000 mg/L TDS at the wells’ locations in the study area as a check on map accuracy.
Fig 10.
Map showing the thickness of the Tulare aquifer interval containing water with Total Dissolved Solids (TDS) less than 10,000 mg/L.
Empty area in Elk Hills Oil Field contains no groundwater with less than 10,000 mg/L TDS. Wells shown are those used to generate the contours. Heavy black outlines mark oilfield administrative boundaries. Air photo base from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Fig 11.
Northwest cross section A-B across the Elk Hills Oil Field.
Location is shown in inset map at bottom of figure. Well API numbers used to identify oil and gas wells in the United States are shown at the top of each well and year drilled and logged is shown below the log. Gray shaded area marks the vadose zone. Blue shaded area denotes groundwater with less than 10,000 mg/L TDS (Base 10K). Well use abbreviations: WD = water disposal, EOR = enhanced oil recovery. Left track of each log is the spontaneous potential (SP) curve shaded yellow for sand layers. Right track contains the deep and shallow resistivity curves. Deep resistivity above three ohm-m is shaded yellow. Given the temperature gradient in the study area, sands with deep resistivity values above three ohm-m usually contain water with less than 10,000 mg/L TDS. Air photo base on inset map from https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/.
Fig 12.
A comparison of two wells located 610 m (2000 feet) south of the initial disposal operations that commenced in 1981 in the Elk Hills Oil Field 18G WD area (location shown in Fig 13B).
Depths between the wells do not align exactly due to differences in the elevation of the rig floor from which the depths were measured. Well API 0402967266 was drilled and logged in 1982 near the beginning of WD injection. Well API 0402967266 was drilled and logged 183 m (600 ft) east in 1995—thirteen years later. a) The TDS versus depth graphs for each well with abnormally high calculated salinity values highlighted in red for the 1995 well. b) The log response (SP in the left track and shallow and deep resistivity in the right track) of the wells between 91 and 400 m (300 and 1,300 ft). Sand intervals are highlighted in yellow on the SP curve and resistivities above three ohms are highlighted in yellow on the deep resistivity curve. Low resistivity anomalies in the 1995 well are highlighted in pink.
Fig 13.
Post-1980 wells drilled in the 18G WD area on the south flank of Elk Hills Oil Field.
The Tulare Formation dips to the south. Wells drilled during this time period but not logged through the Tulare Formation are not shown. Northern limit of Tulare clay (outcrop) shown as black dot-dash line (from California Resources Corporation [42]). a) wells drilled in the 1980’s with initial 18G WD area in red circle b) additional wells drilled in the 1990’s. Wells highlighted in pink show resistivity anomalies indicating the presence of saline produced water in the Tulare Formation. X’s mark the locations where injected water has broken through to the surface (surface expressions). Water supply wells (blue triangles) were sampled multiple times during the 1990’s [41] but showed no effects of disposal water migration. No samples are available for the post-1990’s period. c) wells drilled from 2000 to 2003. Well 0403021374 contains perched brackish water in a sand directly above the Amnicola clay and lies updip from the 18G disposal wells. However, this brackish perched water is also present in wells dating back to the 1940’s and does not appear to be related to water disposal in the 18G area. d) wells drilled post-2003. Well 0403054207 was drilled in 2014 and resistivity anomalies indicate that disposal water had migrated beyond the boundary of the Elk Hills Oil Field into the Buena Vista Oil Field at that time.