Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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The Risk Of Benzene Exposure On The Urban Setting From Historic Oil Field Activities


Development of urban uses on top of existing and historic oil fields is common in California and other states. Within the Santa Maria Valley from 1938 to the present, urban development took place over the pre-existing Santa Maria oil field and encroached on the related surface activities. In the late 1980's, some urbanized sites were found to be situated over crude oil impacted soils. A study was undertaken to determine if there was a health risk of benzene exposure to the present urban population associated with historic oil and gas activities. Review of aerial photographs shows that prior to 1987 land use in the valley was primarily oil and gas production or agricultural. The area was developed for oil and gas production in the 1930's and 40's with secondary recovery development initiated in the 1960s. Most of the individual wells had an associated drilling pit or sump which remained in place from the initial drilling until the mid 1970s. Each lease or fee property also included production facilities, commonly referred to as a tank battery, where oil, gas and water were gathered and temporarily stored. These tank batteries were generally connected by a network of underground pipes which transported the oil to refineries, and the gas to gas plants and to market. The water produced with the oil was separated from the oil at the tank batteries and transported to locations for disposal or injection to assist in oil production. Aerial photographs dating from 1938 to 2002 show a steady encroachment of urban mixed uses (i.e., industrial, commercial and residential development) on the historic agricultural and oil field areas. The abandonments of individual wells and well locations were supervised and approved by both the California DOGGR and Santa Barbara County. The closure practices included facility removal, burial of sump or pit residual materials below plow depth, removal or purging of pipelines, well abandonment, and surface restoration prior to the return of the property to the mineral rights owner or to open space. These practices were customary until the early 1990s at which time greater emphasis was placed on site restoration by the mineral producer. As part of this study, thousands of soil samples and numerous soil vapor samples were reviewed that were collected and analyzed for: Total Petroleum Hydrocarbons (TPH); Benzene, Toluene, Ethylbenzene and three isomers of Xylene (BTEX). The sampling locations were selected by environmental professionals to delineate residual crude oil impacted soils based on conventional Phase I site investigation procedures. The procedures employed for selecting soil sampling locations (with regulatory agency oversight) included reviews of documents such as aerial photos, site plans, agency records and individual interviews. The targeted locations included exploration and production tank batteries, individual well locations, associated pits, sumps and field equipment, and pipelines. Geostatistical techniques were used to develop three-dimensional models of the data to determine the extent and concentration of the potential benzene in the soil. The extent of residual crude oil contamination was minimal due to a number of factors such as operator practices and cleanup requirements. The residual crude oil contamination was limited primarily to in exploration and production facilities where there was oil and gas production and processing. When abandonment took place certain regulatory requirements were imposed and mineral lease covenants were met before quit claim. Additionally, there is a physical constraint related to the high viscosity of the locally produced crude oil. The area crude has a low API gravity with high viscosity that does not readily lend itself to transport in the soil. Crude oil is most often characterized by the weight, which is expressed as API gravity in degrees. Crude oils range from as low as 4∞ API, which is much like tar, to as high as 50∞ API, which is much lighter than water. In oil field terminology a crude oil is termed ‘heavy,’ ‘medium,’ or ‘light’ based on the API gravity. The crude oil produced in the Santa Maria area is generally in the ‘heavy’ range. The API gravity in the Santa Maria oil field has consistently been in the range of 15 to 16 ∞ API from as early as 1938 to the present. Benzene is a naturally occurring component in crude oil. Benzene has been measured at concentrations as high as 0.6% for 70 crude oils throughout the world and has been measured at approximately 0.001% (11mg/kg) in the Santa Maria Oil Field. The concentration of benzene in crude oil can be predicted by API gravity. When crude oil is released into or onto soil, the benzene in the crude oil will diminish due to continued evaporation, biochemical degradation, attenuation into soil matrices, and enhanced partitioning due the presence of residual crude oil in the soils. The volume of crude oil-related benzene contained in soils is related to the soil types, connate water, capping mechanisms, depth of burial, and length of time since release into or onto the soils in question. Benzene contamination from crude oil sources in the study area was found to be limited and was not found in measurable quantities in potentially contaminated sites except under narrow circumstances. Of the samples that were gathered in the subject area and tested for benzene, only two percent reported a detectable quantity of benzene. The exception occurs in areas where condensate has been released to soil, resulting in higher benzene detections as well as deeper penetration into the soil. It is important when performing a study of this type to be aware of other potential sources of benzene that are unrelated to production and exploration activities. The benzene, BTEX and ‘other toxic volatile organic compounds’ found in a number of the soil and soil vapor samples were found to be related to the common urban sources such as urban air, paints, paint thinners, gasoline, herbicides, pesticides, and common cleaning solvents.