Cruise Summary For P-1-02-SC: Acoustic Imaging Of Natural Oil And Gas Seeps and Measurement of Dissolved Methane Concentration In Coastal Waters Near Pt. Conception, California
By Thomas D. Lorenson1, Jennifer A. Dougherty1, William Ussler III2, and Charles K. Paull2
1Menlo Park, California
Water Column Methane Analysis
Water-column acoustic anomalies and methane concentrations were documented in coastal waters surrounding Pt. Conception, California, in March 2002. The purpose of this survey, supported by the Minerals Management Service, was to locate active oil and gas seeps in the area as a background for further studies to determine hydrocarbon flux, mainly oil, into the environment.
Objectives in reaching this goal are to (1) document the locations and geochemically fingerprint natural seeps within the offshore southern Santa Maria Basin; (2) geochemically fingerprint coastal tar residues and potential sources, both onshore and offshore, in this region; (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; (5) attempt to predict transport pathways of oil from seep sources to the coastline and; (6) interpret the petroleum system history for the natural seeps.
This survey, addressing objective 1, focused on the area from offshore Surf Beach to the north and Gaviota to the south in water depths ranging from 20 to 500m. In addition, nine stations were sampled outside this area to provide a regional context. Water-column methane concentrations were measured in water samples collected from the R/V Point Sur with Niskin bottles from various depths. A total of 724 water samples from 94 stations were collected.
Several cruises have been planned to establish the geologic setting, sources, and ultimate dispersal of natural oil and gas seeps in the offshore southern Santa Maria Basin, roughly between Pt. Arguello and Pt. Conception.
The first of this series, documented in this report was a 5 day cruise, primarily to measure dissolved methane in seawater. Dissolved methane surveys are helpful in defining the approximate locations of oil and gas seeps, especially when used in conjunction with acoustic imaging of gas in the water column.
Water-column methane concentrations were measured in coastal waters roughly
between Pt. Arguello and Gaviota, California. Concentrations were measured
in water samples collected Nisken bottles aboard the R/V Point Sur (Figure
1) between March 19 and 23, 2002 in order to locate active oil and
gas seeps. Nine stations were sampled outside this area to provide a regional
context. Overall, a total of 724 water samples from 94 stations (Table
1, Figure 2) were measured for
methane concentration (stations 17,28, 43, 47, and 77 were not sampled).
Hull-mounted, echo sounder (3.5 and 12 kHz) data were collected in conjunction
with the methane survey to detect acoustic water column anomalies that
might be associated with gas bubbles and tar-cemented mounds on the seafloor
(Figure 3). Water column properties
were measured for conductivity, temperature and density (CTD), as well
as fluorescence of chlorophyll A as a proxy for phytoplankton concentration.
CTD profiles are given for each station in Appendix
1. Additional cruise data and navigation can be found on the USGS
Offshore oil and gas seeps have been reported to occur in the southern Santa Maria basin, but none has been documented with precise locations and geochemical analyses. In contrast, oil and gas seeps are extensive in the offshore Santa Barbara-Ventura basin, with major seepage occurring south of Coal Oil Point (Hornafius et al., 1999; Quigley et al., 1999).
The offshore Santa Maria basin is the most southern of seven basins in the Central California Province of the Pacific Outer Continental Shelf (OCS) Region. The basin trends northwest, is fault-bounded to the east and west, and is limited on the north by a structural discontinuity and to the south by the Amberjack structural high (Figure 4). Adjacent to and east of this structural high is the Santa Barbara-Ventura basin province. The offshore Santa Maria basin measures about 100 miles long and 25 miles wide, thus covering an area of about 2,500 square miles. This cruise focused on the southern most Santa Maria basin and area to the south and east adjacent to offshore Point Conception where oil and gas seeps have been previously described (Vernon and Slater, 1962, Wilkinson, 1971, Fischer, 1977).
Regional subsidence of the Santa Maria Basin resulted from regional extension during the early Miocene. Uplift and structural inversion of the basin began in the early Pliocene, reactivating normal faults and folding Miocene and Pliocene strata into anticlines, which are the structural traps for much of the petroleum found in this basin. More than 50 exploratory wells have been drilled in the southern and central parts of the offshore section of the basin, and, as of 1995, thirteen oil fields have been discovered (Mayerson, 1997). The explored stratigraphic section ranges from Jurassic to Holocene age and includes as much as 10,000 feet of Neogene strata. The principal hydrocarbon source and reservoir rock in the basin is the middle to late Miocene Monterey Formation.
The FY 2002 field program was conducted using a leased vessel, the 135-ft-long R/V Point Sur, (Figure 1) owned by National Science Foundation (NSF), operated for the Central California Oceanographic Cooperative (CENCAL) by Moss Landing Marine Laboratories. This vessel has a dry laboratory and electronics laboratorr. A van operated by the Monterey Bay Aquarium Research Institute (MBARI) designed for geochemical analyses of water and sediment was loaded on the fantail of the ship during the two-day mobilization at Moss Landing, CA prior to the cruise.
The scientific party for P-1-02-SC included three scientists from the USGS, three from MBARI, two from University of California, Santa Barbara, and one from University of Victoria, Canada. Table 2 lists names, affiliations, and the primary role of each person.
The cruise departed Moss Landing, California on March 19, 2002 and returned to Moss Landing on March 23, 2002. Table 3 outlines the overall cruise operations giving start and stop times for each echo sounder line and each CTD station. Figure 5 shows the tracklines and course of the R/V Point Sur in and nearby the study area during the cruise. The transit to Point Arguello took 16-hours, including one practice CTD station (station 51) in the head of Partington Canyon on the Big Sur coastline where high methane concentration have previously been observed.
Originally, 50 CTD stations were planned; however, exceptional weather allowed us the opportunity to sample 94 stations. Figure 2 shows the location of CTD stations, and Table 1 lists the location and depth of each station. Echo sounder records from dual frequencies of 3.5 and 12 kHz to image gas in the water column were recorded and observed in real-time throughout the study area. Acoustic water column anomalies such as the one illustrated in Figure 3 were considered potential seep locations, and the water was sampled when possible. Echo sounder records were also reviewed and cataloged post-cruise to determine possible seep sites for future cruises that are planned to observe the flux rate of oil and gas from seeps. The R/V Point Surs global positioning system (GPS) was linked to an ArcViewTM geographic information system (GIS) database using an algorithm, developed by Gerry Hatcher of the USGS, enabling real-time logging of navigation and station location.
Dual GPS systems by Ashtech and Magnovox (now Leica) were utilized. A SeaBird Electronics 911 Plus profiling CTD with dual conductivity and temperature sensors was used aboard the R/V Point Sur. The profiling cage was also outfitted with a Beckman dissolved oxygen sensor, a Chelsea Aquatracka fluorometer (configured to detect phytoplankton chlorophyll A at the wave length of 685nM), a WetLabs transmissometer, and a Biospherical PAR sensor. The Knudsen 320B/R deep water echo sounder was a hull-mounted high power system featuring a 3.5 and 12 kHz dual frequency configuration option with a high power 5 or 10 kW main transmitter and a 2 kW secondary transmitter with efficient switch mode operation. The Knudsen 320B/R echo sounder used a SCSI link to a personal computer. Equipment details can be seen in Table 4.
Water column samples were collected using 12 five-liter Niskin bottles mounted on a CTD rosette. Upon recovery of the CTD rosette, water samples were processed immediately for shipboard measurement of dissolved methane concentration. Seawater was transferred to 250 mL brown bottles using tygonTM tubing. The tygon tubing conducting bubble-free, flowing seawater from the Niskin bottle was extended to the bottom of each sample bottle. After displacing approximately 2 L of seawater, the bottles were sealed with a screw cap containing a new silicone/teflon septum. In the shipboard laboratory, bottles were placed on their side, a 22-gauge hypodermic needle was inserted for pressure relief, and 10 mL of UHP N2 headspace was added to the bottle. The bottles were stored on their sides and the headspace and water were allowed to thermally equilibrate prior to analysis. Immediately prior to analysis by gas chromatography, approximately 8 mL of headspace gas was displaced into a 10 mL syringe by injecting seawater into the bottle below the seawater/headspace interface. The gas sample was introduced into a Shimadzu mini-2 gas chromatograph through a low volume MgClO4 drying column in series with the injection valve. The gas chromatograph was equipped with a CarbosieveTM 80/100 mesh stainless steel column (1/8 in. diameter X 5 ft.) and a flame ionization detector. UHP nitrogen was the carrier gas, and the oven temperature was held at 145 °C and the detector at 160 °C. A 9.93 ppm (v/v) methane in nitrogen gas standard was used. Laboratory gases were generated in the shipboard laboratory using a Nitrox UHP-40H hydrogen generator, a Whatman 76-803 zero air generator, and a Whatman 76-92 UHP nitrogen generator. This analytical method has a detection limit of ~0.5 nM.
In general, methane concentrations (Table 5) were highest, up to 160 nM, directly adjacent to the coast east of Pt. Conception with an apparent plume of methane extending northwest along the coastline and dissipating about 5 km south of Pt. Arguello. The study area contains three offshore oil platforms, an oil pipeline, with three additional offshore oil platforms just outside the study area. No methane anomalies were associated with these facilities. For reference, minimum and maximum methane concentrations measured at 2m depth in the water column were 3.44 and 71.9 nM, respectively. The maximum methane concentration (160 nM) was measured from a bottom water sample taken over an active oil and gas seep (station 60, depth 35m).
Hull-mounted, echo sounder (3.5 and 12 kHz) data collected in conjunction with the methane survey detected acoustic water column anomalies that might be associated with gas bubbles and mounds on the seafloor. Figure 6 shows that 59 of the acoustic anomalies were associated with methane concentrations of >20nM, and 15 acoustic anomalies occurred in areas where methane concentrations were <20nM. Overlapping occurrences of high methane concentrations and acoustic anomalies are likely areas of active oil and gas seepage.
We thank the captain and the crew of the Point Sur who provided expert, professional and gracious assistance throughout the cruise. The Minerals Management Service provided the funds that made this cruise possible.
Fischer, P.E., 1977, Natural gas and oil seeps, Santa Barbara basin, California; in California offshore gas, oil, and tar seeps; California State Lands Commission Report; 449 p.