A study in cooperation with the Minerals Management Service and the Energy Division, County of Santa Barbara, California Natural Offshore Oil Seepage and Related Tarball Accumulation on the California Coastline—Santa Barbara Channel and the Southern Santa Maria Basin; Source Identification and Inventory By Thomas D. Lorenson, Frances D. Hostettler, Robert J. Rosenbauer, Kenneth E. Peters, Jennifer A. Dougherty, Keith A. Kvenvolden, Christina E. Gutmacher, Florence L. Wong, and William R. Normark Open-File Report 2009-1225 Also released as MMS report 2009-030 This study was funded in part by the U. S. Department of the Interior, Minerals Management Service (MMS), through an Interagency Agreement No. 18985 with the U.S. Geological Survey, Western Coastal and Marine Geology Team, as part of the MMS Environmental Studies Program. U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt Director U.S. Geological Survey, Reston, Virginia 2009 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Suggested citation: Lorenson, T.D, Hostettler, F.D., Rosenbauer, R.J., Peters, K.E., Kvenvolden, K.A., Dougherty, J.A., Gutmacher, C.E., Wong, F.L., and Normark, W.R., 2009, Natural offshore seepage and related tarball accumulation on the California coastline; Santa Barbara Channel and the southern Santa Maria Basin; source identification and inventory: U.S. Geological Survey Open-File Report 2009-1225 and MMS report 2009-030, 116 p. [http://pubs.usgs.gov/of/2009/1225/]. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Contents Executive summary ..............................................................................................................1 Introduction ..........................................................................................................................4 Chapter 1 Identifing Seeps .................................................................................................10 Chapter 2 Biomarker and Carbon Isotope Analysis ..........................................................44 Chapter 3 Chemometric Modeling, Tarball Sources, and Distribution .............................54 Chapter 4 Beach Monitoring of Tar Samples ....................................................................83 Conclusions and Acknowledgements ..............................................................................104 References ........................................................................................................................107 Appendix 1-1 Maps of beaches with samples locations and families ...............................28 Appendix 2-1 Location, classification, and geochemical data for crude oil, seep oil, and tarball samples. Link to http://pubs.usgs.gov/of/2009/1225/of2009-1225_appendix_2-1/ ...................................... 50 Appendix 2-2 Geochemical parameters used in tarball studies .........................................51 Appendix 3-1 Location, classification, and source/age related geochemical data for crude oil, seep oil, and tarball samples from coastal California, used in the chemometric model with classification of non-model tarballs. Link to http://pubs.usgs.gov/of/2009/1225/of2009-1225_appendix_3-1/ ....................................... 79 Appendix 3-2 Hierarchical cluster diagram of the 388-sample training set for the chemometric model ................................................................................................80 Appendix 4-1 Graphs of tar mass and number as influenced by season and tidal cycle ...............................................................................................................94 iii Natural Offshore Oil Seepage and Related Tarball Accumulation on the California Coastline—Santa Barbara Channel and the Southern Santa Maria Basin; Source Identification and Inventory By Thomas D. Lorenson, Frances D. Hostettler, Robert J. Rosenbauer, Kenneth E. Peters, Jennifer A. Dougherty, Keith A. Kvenvolden, Christina E. Gutmacher, Florence L. Wong, and William R. Normark Executive Summary Oil spillage from natural sources is very common in the waters of southern California. Active oil extraction and shipping is occurring concurrently within the region and it is of great interest to resource managers to be able to distinguish between natural seepage and anthropogenic oil spillage. The major goal of this study was to establish the geologic setting, sources, and ultimate dispersal of natural oil seeps in the offshore southern Santa Maria Basin and Santa Barbara Basins. Our surveys focused on likely areas of hydrocarbon seepage that are known to occur between Point Arguello and Ventura, California. Our approach was to 1) document the locations and geochemically fingerprint natural seep oils or tar; 2) geochemically fingerprint coastal tar residues and potential tar sources in this region, both onshore and offshore; 3) establish chemical correlations between offshore active seeps and coastal residues thus linking seep sources to oil residues; 4) measure the rate of natural seepage of individual seeps and attempt to assess regional natural oil and gas seepage rates; and 5) interpret the petroleum system history for the natural seeps. To document the location of sub-sea oil seeps, we first looked into previous studies within and near our survey area. We measured the concentration of methane gas in the water column in areas of reported seepage and found numerous gas plumes and measured high concentrations of methane in the water column. The result of this work showed that the seeps were widely distributed between Point Conception east to the vicinity of Coal Oil Point, and that they by in large occur within the 3-mile limit of California State waters. Subsequent cruises used sidescan and high resolution seismic to map the seafloor, from just south of Point Arguello, east to near Gaviota, California. The results of the methane survey guided the exploration of the area west of Point Conception east to Gaviota using a combination of seismic instruments. The seafloor was mapped by sidescan sonar, and numerous lines of high -resolution seismic surveys were conducted over areas of interest. Biomarker and stable carbon isotope ratios were used to infer the age, lithology, organic matter input, and depositional environment of the source rocks for 388 samples of produced crude oil, seep oil, and tarballs mainly from coastal California. These samples were used to construct a chemometric fingerprint (multivariate statistics) decision tree to classify 288 additional samples, including tarballs of unknown origin collected from Monterey and San Mateo County beaches after a storm in early 2007. A subset of 9 of 23 active offshore platform oils and one inactive platform oil representing a few oil reservoirs from the western Santa Barbara Channel were used in this analysis, and thus this model is not comprehensive and the findings are not conclusive. The platform oils included in this study are from west to east: Irene, Hildago, Harvest, Hermosa, Heritage, Harmony, Hondo, Holly, Platform A, and Hilda (now removed). 13 The results identify three “tribes” of C-rich oil samples inferred to originate from thermally mature equivalents of the clayey-siliceous, carbonaceous marl, and lower calcareous-siliceous members of the Monterey Formation. Tribe 1 contains four oil families having geochemical traits of clay-rich marine shale source rock deposited under suboxic conditions with substantial higher-plant input. Tribe 2 contains four oil families with intermediate traits, except for abundant 28,30-bisnorhopane, indicating suboxic to anoxic marine marl source rock with hemipelagic input. Tribe 3 contains five oil families with traits of distal marine carbonate source rock deposited under anoxic conditions with pelagic but little or no higher-plant input. Tribes 1 and 2 occur mainly south of Point Conception in paleogeographic settings where deep burial of the Monterey Formation source rock favored generation from all three members or their equivalents. In this area, oil from the clayey-siliceous and carbonaceous marl members (Tribes 1 and 2) may overwhelm that from the lower calcareous-siliceous member (Tribe 3) because the latter is thinner and less oil-prone than the overlying members. Tribe 3 occurs mainly north of Point Conception, where shallow burial caused preferential generation from the underlying lower calcareous-siliceous member or another unit with similar characteristics. It is very desirable to be able to clearly distinguish the naturally occurring seep oils from the anthropogenically derived platform oils. Within the “training set” of oils and tars (388 samples), the biomarker parameters are sometimes sufficient to allow unique discrimination of individual platform oils. More often however, platform samples and seep samples with sources geographically close to each other are too similar to each other, with respect to the biomarker parameters, to definitively differentiate them on that basis alone. In some cases other parameters can be helpful. These other parameters are related to the degree of biogeochemical degradation or weathering that the oils or tars have experienced. These components include the typical oil distribution of n-alkane hydrocarbons and isoprenoids pristane and phytane. All of the platform oils in our sample set contain these components. On the other hand, the seep oils or tars have been exposed to significant biodegradation while in the near subsurface. The majority, but not all of seep oils or tars have been biodegraded up to or beyond the loss of n-alkanes and isoprenoids. Seep oils found in the vicinity of Coal Oil Point or Arroyo Burro are apparently the least weathered and are particularly likely to retain significant n-alkanes and isoprenoids. Therefore the combination of chemometric fingerprinting and the presence or absence of n-alkanes and isoprenoids help to differentiate anthropogenic production oils versus natural seeps oils and tars. The differentiation is not always definitive because of the close chemical similarity of some samples and the variability in the biodegradation progression. This is the case near Coal Oil Point, and near Platform A 2 (Dos Cuadros Field) where seep oils and Platform Holly and Platform A oils are genetically very similar and cannot be definitively distinguished after a period of a few days of weathering. In contrast, oils from the Point Conception platforms can be distinguished on the basis of chemometric fingerprinting alone. In the middle of this spectrum are oils from Platforms Harmony, Heritage, and Hondo, where it is expected that oil weathering would take on the order of two weeks to a month to produce tarballs similar to those seen near Point Conception. In this case there is a much greater degree of weathering needed to proceed from produced oil to the biodegraded tar characteristic of tarball stranded on the beach. Tar deposition on beaches was monitored as part of cooperative with the County of Santa Barbara Energy Division and the U.S. Geological Survey during 2001-2003. We found tar deposition varies on a seasonal basis. In general, tarballs accumulate at a faster rate or remain longer on all beaches during the summer and fall months. The reasons for this are unclear based on our limited observations, however we speculate that factors such as prevailing winds and currents combined with more quiescent wave conditions favors the accumulation and preservation of tarballs on the beach during the summer and fall months. In contrast, winter storms, with much greater wave action remove beach sand and other materials, and stormy seas tend to break up oil that might weather into tarballs. Natural seepage is affected by the spring/neap tidal cycle; however, the link to tar deposition is unclear. Longer periods of monitoring are needed to address the variability in the data and provide a more robust statistical analysis. 3 Introduction This study has developed a living geochemical chemometric (fingerprint) model tuned for oils and tars sourced from the California Monterey Formation. The model allows for inquiry of new unknown tars or oils to build upon our library of coastal tar fingerprints as a database for future investigations. Our study area includes the entire coastline of California (Fig. I-1) however we are currently concentrating our efforts in southern California. We have also examined the possible origins of tars and provides qualitative rates of deposition measured during a three-year period on Santa Barbara county beaches from 2001-2003. The California coastline contains long stretches of sandy beaches, rocky inlets, high cliffs hanging precipitously over crashing waves, and many other scenic wonders. This beautiful natural resource is, however, continually exposed to contamination from both natural and anthropogenic sources. In particular, the coastline is impacted by petroleum hydrocarbons that occur as tarballs washed up all along the shorelines and as onshore seepages from rocky outcrops and cliff faces. Natural sources for these petroleum hydrocarbons include prolific, frequently chronic, onshore and offshore shallow oil seeps, especially prominent along the southern California coast (State Lands Commission Staff Report, 1977). Anthropogenic sources include possible accidental oil spills from commercial vessel traffic, from offshore drilling rigs, and from ships involved in the processing and transport of oil along the coastal shipping lanes. Differentiating between natural and anthropogenic petroleum sources and determining specific sources of coastal contamination is essential to evaluate threats to the ecosystems and to limit contaminant impact. Although crude oils and source rocks in the California borderland oil fields have been extensively characterized (Curiale and others, 1985), published geochemical work on the substantial (approximately 20,000 tonnes/year discharged into the ocean, as estimated by a U.S. Academy of Sciences report, NAS, 2002) hydrocarbon beach tar accumulations along the California coast is limited. Reed and Kaplan (1977) used stable isotopic ratios of sulfur, nitrogen, and carbon to distinguish seep oils, beach tars, and crude oils from the southern California Borderland. Another early study utilized stable isotopic ratios of carbon and sulfur and total sulfur content of asphaltene fractions to correlate beach tars deposited near Los Angeles with their probable sources, to distinguish natural seep oils from imported tanker crude oils and local production wells, and to evaluate seasonal distribution patterns and transport (Hartman and Hammond, 1981). Hartman and Hammond (1981) determined that more than 50 percent of asphalt found in Santa Monica Bay is from the Coal Oil Point (COP) seeps, which are ~150 km to the west. They proposed that asphalt transport from COP to Santa Monica is dependent on seasonal ocean currents and gyres. A significant decrease in asphalt deposition along Santa Monica Bay beaches was observed during winter months. Hartman and Hammond (1981) proposed that during the winter, COP asphalt is transported westward in the Santa Barbara Channel (SBC) and subsequently northward by the Davidson Current that emerges near the Channel Islands. A more recent study used various molecular parameters of tar residues on beaches within the Monterey Bay National Marine Sanctuary to try to ascertain sources (Kvenvolden and others, 2000). Reports on coastal tar and oil seeps considers the 4 geologic framework and some potential tarball correlations related to this study (Kvenvolden and Hostettler, 2003; Hostettler and others, 2004). The Santa Barbara County Energy Division, in conjunction with the United States Geological Survey (USGS), conducted a two year long “fingerprinting” and monitoring study of stranded asphalt on Santa Barbara County beaches, oil samples from natural oil seeps and offshore oil platforms (examples given in figs. I-2, 3, 4, and 5) (Lorenson and others, 2004). The samples were analyzed for biomarkers (persistent hydrocarbons) and various isotopic compositions, and then incorporated into an asphalt “fingerprint” database. The fingerprints of COP beach asphalt, COP natural seep oil, and Platform Holly crude oil are very similar and require sophisticated chemical analysis to discriminate between the samples. All beach asphalt analyzed was determined to be natural oil from the Monterey Formation, the main petroleum source and reservoir rock in the area (Lorenson and others, 2004). Although the county study was inconclusive in identifying sources, information was gathered on the distribution of beach asphalt (tarballs), its variation, and possible sources. Lorenson and others, 2004 observed higher asphalt accumulations during the summer and fall months and proposed variations in tides, currents, winds, and surf zone energy to be the cause. Of the southern beaches, COP accumulated the most asphalt mass. Of all the beaches surveyed, the largest number of tarballs was observed at COP, but not the most asphalt in terms of mass. COP tarballs on average were smaller than those collected at beaches in the northern part of the county. The northern beaches generally had much larger tarballs than the southern beaches, resulting in more asphalt mass. Differences in observed tar balls sizes between northern and southern beaches were attributed to different sources and confirmed by geochemical analyses. Tarballs on northern and southern beaches were chemically determined to have different sources (Lorenson and others, 2004). Lorenson and others, 2004 proposed that the difference in sizes was due to the different sources. These works all conclude that much of the tar accumulation originates from the Miocene Monterey Formation. Source rock in the Monterey Formation shares several 13 chemical characteristics with local tars, including 1) unusually “heavy” δ C (around – 23‰); 2) aliphatic biomarker parameters 28,30-bisnorhopane indicating an anoxic marine depositional environment (Curiale and others, 1985), high C35 αβ-hopane 22S and 22R epimers compared to C34, and the presence of gammacerane (Peters and Moldowan, 1993); 3) a characteristic value (>3) for the biomarker parameter called “the triplet” (Kvenvolden and others, 1995), defined in appendix 1; 4) a small but consistent presence of oleanane; 5) sterane parameters indicating low maturity as opposed to fully mature hopane parameters; 6) very low diasteranes relative to regular steranes, indicating a clastic-poor marine source rock; 7) abundant aromatized steranes, especially monoaromatics relative to triaromatics, indicating low thermal maturity (Curiale and others, 1985); and 8) sulfur-rich PAH, such as dibenzothiophenes. Although the above chemical components are common to all the tarballs, their relative proportions vary. A fingerprinting technique utilizing ratios of these constituents, and other biomarker parameters from both the aliphatic and aromatic hydrocarbon suites, allows discrimination among different tar samples. Tars can be correlated with each other and with distant sources. 5 The chemical composition of the tarballs is linked to its geochemical history. Despite the large number of offshore shallow hydrocarbon seeps, and the constant impingement of tar onto the shoreline, little is known about the mechanics of hydrocarbon formation in shallow seeps, specific sources of tarballs, or their transport from the marine environment onto the shore. At present there is no irrefutable data linking tar on beaches to specific offshore natural seeps (Leifer and others, 2002, Del Sontro, and others, 2007). Because many of the tarballs from offshore seeps are transported significant distances from their sources by ocean currents, geochemical assignment of their origin provides insight into the circulation patterns of the coastal currents. The circulation patterns within the Santa Barbara Channel are well studied (Hickey, 1998; Harms and Winant, 1998; Winant and others, 1999, 2003). Persistent cyclonic circulation, upwelling conditions, and wind-relaxing drive the currents in a seasonally dependent pattern. The net result of drifter studies is a combination of in-channel deposition, both on the mainland coast and on the Channel Islands, with flow predominantly toward the south and east in the spring and summer (California Current) and to the west and north in the late fall and winter (Davidson Current and the Southern California Countercurrent). Mapping depositional sites of tarballs that also drift with these ocean currents, complement these drifter studies, as well as provide information on the fate of these petrogenic contaminants in the coastal environment. Our results demonstrate that tar accumulations on California beaches can be related to natural sources and that there is extensive offshore seepage as documented in this report. Offshore seepage results in producing tarballs, some of which find their way to nearby beaches. Seepage is also responsible for creating unique seafloor oases for sessile organisms that would otherwise not survive on the sand-covered seafloor. We have also have shown that natural seeps can, in many occurrences, be distinguished from produced oil with gas chromatography - mass spectrometry, chemometric fingerprinting, and statistical analyses provided that the produced oil is not biodegraded. In some cases even produced oils, biodegraded near the sea floor or on the sea surface are sufficiently different from natural seepage and can be distinguished. Further, this conclusion can be applied to California Monterey Formation-sourced oils common to the study area. 6 Figure I-1. Map showing locations of beaches, sampled oil platforms and natural seep samples. 7