California Oil, Gas, and Groundwater Program

Oxnard Oil Field

Summary of findings from Rosecrans and others, 2021

No evidence of water1 from oil bearing formations mixing into overlying aquifers

Oil-related gases, however, were present in low concentrations in the same aquifer. There is no known risk of these gases in drinking water and no regulatory thresholds for them.

The thermogenic gases were detected near areas with many oil wells, where large volumes of water and steam have been injected, in relatively shallow oil-bearing formations, and in aquifer zones located above producing oil sands.

There is also oil and hydrocarbon gases from oil naturally occurring within the aquifer.

We could not conclusively differentiate between the natural occurrence and movement of oil-related (thermogenic) gases, and those mobilized by oil-field operations.

Gases potentially indicate that there are pathways between areas of oil-field operations and the aquifer used for groundwater supplies; they may also be a result of hydrocarbons occurring in the aquifer.

An upward pressure gradient from oil-producing zones to the overlying aquifer across a broad area was present; this could drive upward movement of fluids if there are pathways connecting these zones. The upward pressure gradient was determined from extensive groundwater level measurements by local water management agencies and idle well fluid level information from operators.

The risk of water from oil-producing zones moving along the same pathways as the gases is at present unknown.

Denser sampling of active oil wells (geochemical end members), groundwater wells inside the field, ongoing paired oil- and water well fluid level measurements, and focused sampling at different depths could help improve differentiation and pathway identification.

1 When used in a scientific setting, the term “fluid” includes both gas and waters. In public settings, the term usually means liquids of some type. To avoid confusion, we are using “water” to refer to liquids and “gas” as something distinct. In the field, waters and gases are present in mixtures.