June 14th, 2022 Meeting Abstract
“Stratigraphic Controls on Groundwater Salinity in the Tulare Formation, Elk Hills Area, Kern County, California“
Presented by: Tony Reid
Elk Hills, a large anticlinal dome located in the southwestern San Joaquin basin, contains a section of the Plio-Pleistocene Tulare Formation 1,000 to 2,500 feet thick. The Tulare crops out at the irregular topographic surface of the hills, and an unknown amount of the section is presumed eroded following later uplift.
The Elk Hills Tulare section contains five informal members: from bottom up, Lower Tulare, Amnicola Claystone, Upper Tulare Interval B, Tulare Clay, and Upper Tulare Interval A. The Lower Tulare lies conformably on the shallow marine San Joaquin Formation, and contains delta front sand and fluvial channel sands and gravels, interbedded with floodplain mud, silt and sand. The Amnicola claystone is a lateral continuous interval of primarily claystone deposited in a quiet lake setting. The Upper Tulare Interval B member contains fluvial channel and floodplain deposits, similar to the Lower Tulare. The Tulare Clay is another quiet lake deposit and in outcrops contains clay and limestone secreted by the algae Clara. The Upper Tulare Interval A member is present in outcrop at the lower flanks of the Elk Hills anticline, but extends in the subsurface into the basins located north and south of Elk Hills. Interval A is similar to Interval B, but the channel sand content in much higher. In summary, the Elk Hills Tulare section represents the transition from shallow marine conditions of the San Joaquin Formation to stream channel, floodplain and lake settings on the basin floor.
Although Elk Hills rises as much as 1,200 feet above the modern basin floor, the top of groundwater is 200 to 300 feet above sea level. As a result, much of the Tulare lacks groundwater and, at the crest of the structure, the entire Tulare section is dry. Below the air-water contact, salinity increases upward from the base of the Tulare. Salinity at the base of the Tulare is in excess of 20,000 mg/l TDS. At 200 to 300 stratigraphic feet above the base of the Tulare, salinity is 10,000 mg/l TDS and marks the base of USDW. Salinity continues to decreases upward through the remainder of the Lower Tulare.
The upward decreasing salinity trend likely represents the original connate waters which became less saline as the basin transitioned from marine to fluvial influences. With uplift of the Elk Hills dome, groundwater drained structurally high areas, moving parallel to stratigraphic layering. Poor vertical permeability prevented mixing of groundwater of different salinities.
Stratigraphic architecture of the Tulare Formation is complex, and no one area characterizes the Tulare as a whole. Layering, whether stacked channels or interbedded floodplain and braided streams deposits, have corresponding different vertical profiles in salinity and groundwater movement. Whether oil migration or groundwater movement, one needs to define each area’s unique stratigraphy to understand fluid movement.
I am a native Californian and grew up in the San Fernando Valley. I attended nearby CSU Northridge, earning BS and MS degrees in geology. After graduation, I relocated to Bakersfield and worked for Getty (later Texaco) in production and exploration roles in the San Joaquin and Salinas basins. After I joined Bechtel, I began a long association with the Elk Hills Field, learning about the complex Stevens sandstone and Monterey porcelanite reservoirs. Following Occidental’s purchased of the Elk Hills Field from the DOE, I transferred to Houston and worked exploration projects in Nigeria, Libya, Indonesia, Philippines, Papua New Guinea, and Australia. Another transfer took me to Muscat, Oman, where I worked as Chief Geologist for Oxy’s carbonate reservoirs.
Returning to Bakersfield, I worked exploration projects in the Ventura and San Joaquin basins. Back at Elk Hills, I was the Chief Geologist when we learned that aquifer exemptions were required for many of Oxy (and later California Resources Corporation) fields. In a new position as aquifer exemption coordinator, I applied lessons from petroleum geology to defining underground sources of drinking water.
I am currently a consulting geologist and reside in Bakersfield with my wife Susan. I am a California Professional Geologist, and Editor for the Pacific Section AAPG.