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Evidence of Naturally Occurring Hydrocarbon within the Aquifers of the Oxnard Plain and Pleasant Valley Groundwater Basins, Ventura County, California

Abstract

The Oxnard Plain and Pleasant Valley groundwater basins serve as important water supply sources for Ventura County. Recent water quality analyses detected traces of biogenic and thermogenic methane from the aquifers of the Oxnard Plain groundwater basin (Rosecrans et al., 2019). Preliminary results from this USGS study suggest these traces to be either naturally occurring from petroleum migration throughout geologic time or from anthropogenic causes (i.e. migration through oil, gas, or water well casing leaks). Shallow hydrocarbon shows were compiled from primary drilling reports, lithological descriptions, mudlogs, core data, and historical documents maintained by the California Geologic Energy Management Division (CalGEM) prior to significant local oil and gas development. This data sheds light on the natural occurrence of hydrocarbon (including methane) within the freshwater aquifers of the Oxnard Plain and Pleasant Valley groundwater basins. Early evidence for the natural occurrence of hydrocarbons in the aquifers of the Oxnard Oil Field include reports from the discovery and subsequent commercial wells drilled between 1936 – 1937. These wells document oil and gas showings from 578’ – 2160’ measured depth (MD), which are above the Pliocene and Miocene tar zones and are geologically separated by transgressive and regressive stratigraphic sequences of the Pliocene – Pleistocene (Swanson and Irvine, 2015). In the Pleasant Valley groundwater basin, exploratory wells drilled near the Conejo Oil Field from the 1920s – 1940s document the presence of hydrocarbon from 45’ – 1486’ MD. A comprehensive review for natural petroleum migration through aquifer systems can help explain hydrocarbon traces in groundwater basins near oil and gas fields. Rosecrans, C.Z., Landon, M.K., and McMahon, P.B., 2019, Groundwater quality results from the Regional Monitoring Program study of the Oxnard Oil Field: Abstract presented at the California State Water Resources Control Board Stakeholder Meeting, Sacramento, California, 25 February. Swanson, B.J., and Irvine, P.J., 2015, Updated Nomenclature for Plio-Pleistocene Formations in the Onshore Ventura Basin, Los Angeles and Santa Barbara 30′ × 60′ Quadrangles: Pacific Section AAPG, SEG and SEPM Joint Technical Conference, Oxnard, California, 3 – 5 May.