California Oil, Gas, and Groundwater Program

Occurrence of water and thermogenic gas from oil-bearing formations in groundwater near the Orcutt Oil Field, California, USA

Summary of Anders and others (2022)

<em>Figure 1. Study area map (figure 1a in paper).</em>
Figure 1. Study area map (figure 1a in paper).
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Figure 2. Water well in vineyard near Santa Maria, California, and the Orcutt Oil Field.

This figure highlights the locations where new groundwater samples were collected (blue dots) to obtain information on chemical constituents beyond that available in historical samples. Most of the groundwater is of good quality.

Three wells are highlighted on the map with red circles. These samples showed evidence of mixing between groundwater and water or thermogenic gas from oil-bearing zones.

Map
Figure 3. Groundwater sampling locations (Figure 1a in paper).

Multiple lines of evidence were used to examine sources of fluids mixed into the three groundwater samples. This slide shows information from oil well records upgradient and near the wells where small amounts of oil field fluids were found.

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Figure 4. Groundwater sampling locations (Figure 9 in paper).

Fluid migration risk factors associated with oil field operations were also explored.

The most intensive development has occurred in the northern part of the field. Water (orange dots) and steam (green dots) have been injected to enhance oil production. These injection zones sometimes overlap areas with old oil wells. Because old wells were not generally constructed to modern standards, the wells could provide potential pathways for the fluid to move into groundwater. The combination of injection and potential pathways makes this area a zone where there is a higher risk of fluid migration.

Map
Figure 5. General location of oil field activity as denoted by oil well function in 2021 (Figure 2 in paper).
  • Oil-related gases were found in 3 of 15 groundwater samples and present in low concentrations in the same aquifer. There are no known risk of these gases in drinking water, and regulatory thresholds have not been developed for these gases.
  • We could not conclusively differentiate between occurrences and movements of oil-related (thermogenic) gases mobilized by oil-field operations and natural occurrences and movements of thermogenic gases.
  • Further insights on potential risks to groundwater could come from:
    • Additional analysis of existing borehole resistivity and temperature data
    • Oil-well integrity analyses
    • Additional groundwater-quality and water-level monitoring that includes sampling in/near active oil wells and focusing on sampling at different depths
  • Patterns in the Orcutt Oil Field are similar to those found in the Oxnard Oil Field.
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Figure 6. Pumpjack in Orcutt Oil Field.