California Oil, Gas, and Groundwater Program

Publication: Data Releases


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Stephens, M.J., Sowers, T.A., Courser, H.N., Bunch, J.D., and Rosecrans, C.Z.


U.S. Geological Survey data release



The U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board (SWRCB) compiled and digitized mud logs from oil and gas wells in the Oxnard Oil Field area. Scanned copies of the mud logs were downloaded from the California Geologic Energy Management Division (CalGEM) online data and were digitized. Thirteen available mud logs associated with the Pliocene Tar Sand pool code (Vaca Tar Sand) were digitized. Mud-logging technology (mud logs) allows for identification and relative concentration measurement in parts per million by volume (ppmv) of light hydrocarbon gases (methane through pentane) occurring in drilling mud returned to the surface throughout the drilling process as well as estimates of percent oil in mud log cuttings (oil shows). The data provided here are apparent concentrations of hydrocarbon gas (methane, ethane, propane, butane, and pentane) and percent oil shows with depth. Apparent concentrations do not equal actual concentrations or presence of the compound at a given depth. Apparent concentrations reflect a digitized position on a mud log and positive values may not indicate presence of the compound; these values are reported as “mud log RAW.csv” values in this data release (see process step 1 and 2 for further description). Estimates of apparent concentration compared to other depths should be estimated by calculating the deviation with depth from the baseline or minimum value. In the larger work, the reported apparent concentrations represent a relative apparent concentration calculated from the raw digitized data for each mud log and each respective gas component (methane, ethane, propane, butane, and pentane); these value are reported as “mud log PROCESSED.csv” (see process step 3 for further description). A baseline concentration was estimated for each processed mud log and represents the minimum apparent relative gas concentration considered for evaluating the presence of the gas components (see process step 4 for further details). Values above baseline or minimum at a single discrete depth should be regarded with caution. Apparent concentration values should not be compared between sites, as minimum or baseline values result from differences in the digitizing process between sites. These data were analyzed in an accompanying manuscript as part of the SWRCB oil and gas Regional Monitoring Program and the USGS California Oil, Gas, and Groundwater (COGG) program to assess regional groundwater quality overlying and adjacent to the Oxnard Oil Field.