Offshore Survey, Campeche, MexicoOffshore Survey, Campeche, Mexico


a. To map the presence and distribution of oil and gas seepage to identify areas with a high potential for petroleum reservoirs.

b. Reduce the area to be searched by helping to focus exploration efforts on regions and structures with active seepage.

c. Predict the oil versus gas potential of prospective structures.


a. Regional Exploration Programs to define portions of basin of concession areas with the highest potential for production. Regional studies are often run in conjunction with regional seismic programs and may include grids or other evenly spaced tests.

b. Trend Evaluation Studies can be used to evaluate the potential of regional features and plays. Sample spacing is designed to sample a prospective play without concentrating on individual structures or prospects.

c. Prospect Evaluation studies can be used to rate the relative seepage magnitudes and compositions of individual prospects prior to drilling so that structures with the highest potential are given a higher priority. Close spaced sample grids in conjunction with high resolution geophysical studies are most effective for multiple prospects.

d. Anomaly Detailing including very closely spaced samples and stratigraphic/geochemical drilling programs can be used to detail oil source and maturity of individual high magnitude seeps over specific prospects of interest.

Program Planning

a. Regional Exploration Programs

Regional exploration programs are designed to cover a large study area with evenly spaced core samples to provide a rapid and cost effective method of evaluating the potential of rank, unexplored areas. Typical studies of this type include samples on 2 to 5 km centers. Regional studies are of particular importance in large concession blocks with insufficient subsurface data to provide meaningful guidance on prospect areas or trends. The Green Canyon deepwater oil seeps in the Gulf of Mexico were first discovered by this method.

b. Trend Evaluation Studies

Surface geochemical exploration programs can be used to define the seepage levels and compositions of prospective structural trends. Trend studies generally include collecting cores on local grids of 2 - 4 km centers as guided by seismic data and regional structural interpretations. Surveys of this type can often confirm the existence of seeps along trend away from known reservoirs and predict the oil versus gas potential in these unexplored areas.

c. Prospect Evaluation

Prospect evaluation studies generally include integrated high resolution geophysical evaluation of individual structures of interest and subsequent core sampling to determine seepage levels and compositions. Results should be compared with calibration studies over known fields where seepage levels and reservoir compositions can be compared. Surveys generally include a tight grid of geophysical lines over a structure and close spaced core sampling at 1 to 2 km spacing over faults, gas charged zones and potential migration pathways from the subsurface.

d. Anomaly Detailing

Once a significant anomaly or macro-seep is encountered, close spaced geophysical surveying and coring can be completed to provide precise details on the seepage distribution, composition and the relationship of such seepage to specific subsurface faults. Detail evaluations are often used in deep water and frontier areas where the acquisition of free oil and gas macro-seep samples can provide significant information on the source rock type and maturity which is not available due to limited exploration wells in the area. Sampling patterns include very tight geophysical grids and pinpoint core collection with spacing often less than 1 km. Clustered core sites are often used. Additional subsurface data can be obtained from stratigraphic and geochemical borings into seep areas.

Field Data Acquisition Tools

A wide range of sampling tools have been developed over the years to obtain marine geochemical and geotechnical engineering data. All tools as discussed in the following paragraphs have specific uses and applications, depending on program needs, water depths and available budgets.

a. Sniffer

Sniffer systems pump a continuous stream of sea water from a height of approximately 10 m above the seabed. One or more gas chromatographs are used to continuously analyze stripped gases for methane through butane light hydrocarbons.

- High density sample spacing along lines.
- Compatible with seismic surveys and may be run with ongoing seismic programs or over existing seismic grids.
- Fast, up to 200 km data acquisition possible per day
- Results available in a relatively short period of time after completion of field work. Not necessary to wait for laboratory sample analysis.

b. Piston/Gravity Coring

Gravity and piston coring tools generally include deployment of a weighted steel pipe which is dropped into the seabed from a height of 10 to 20 m. Corer penetration is dependent on seabed soil conditions. Sample lengths of up 20 - 30' are possible in soft bottom areas with corer weights from 400 to 1000 kg.

- Can be deployed in very deep waters.
- Relatively large volume of core is available for hydrocarbon extraction and analysis as well as geologic and engineering studies.
- Multiple samples can be run from each core.
- Large numbers of cores can be acquired per day
- Results can be easily integrated with high resolution geophysical studies and deep seismic data.

c. Vibro-Corer

Vibro Corers are often used in conjunction with piston/gravity coring tools to increase penetration and sample recovery in areas of hard bottom and sand.

- Improved recovery in sand and hard bottom areas.
- Can be deployed from gravity/piston coring deployment systems and quickly interchanged as needed during field program

d. Integrated Geophysical Study in Conjunction With Gravity/Piston Coring Program

High resolution geophysical tools can be used in conjunction with coring methods by identifying seepage zones and targeting coring activities to faults, gas charged zones and other seabed features. Typical tools include sparker or boomer sub-bottom profilers, side scan sonar, pingers and echo-sounders.

- Cores are collected on geologic features with a high potential for seepage.
- Near surface structural information is available for data interpretation and tying seepage zones to deep seismic data
- Preliminary feature maps can be created in the field
- Geochemical studies can share mobilization with site survey studies since vessels and equipment are generally the same.

e. Geochemical Drilling

Geochemical drilling programs include anchoring a geotechnical drilling vessel over a known area and drilling or jetting to obtain samples up to 200 m below the seabed.

- High quality geochemical data from downhole samples below the depth of oxidation.
- Can be combined with geotechnical drilling programs to reduce mobilization costs.
- Continuous downhole profiles increase confidence and reliability of geochemical results.
- Deeper stratigraphic drilling programs can obtain source rock samples for more detailed analyses.

Positioning and Geophysical Tools

a. Differential Global Positioning System (D-GPS)

D-GPS is typically used for accurate navigation and positioning of survey lines and cores. With an accuracy of about +/- 3 m all features are located precisely so that results can be integrated into clients existing exploration data bases.

b. Sub-Bottom Profiler

Sub-bottom profilers such as sparkers or boomers are used to provide high resolution geophysical profiles along existing seismic lines and over structures of interest. Profiles can be interpreted onboard and used to select core locations over areas with faults, gas charged sediments and other preferential migration pathways from depth.
Mapped sub-surface features can be compiled on base maps for comparison with seepage anomalies and deep structure as mapped by conventional seismic interpretations.

c. Side Scan Sonar

Side scan sonar systems records a two dimensional, or map view, of the seabed and of objects and bathymetric features. on the seafloor. Side scan sonar data are used for identify seep related seabed features such as mud volcanoes, pockmarks, authogenic carbonate mounds and gas bubbles in the water column. Since the side scan sonar maps a wide swath of the seafloor it is possible to map features which do not lie directly on the survey line.
Features mapped from side scan sonar data can often be used to identify macro-seep zones as well as the surface expression of outcrop, faults and other structural features.

d. Echo Sounder

Echo sounders are employed during all geophysical studies and coring programs to confirm water depth along geophysical lines. Accurate water depth data is essential so that corers can be deployed and dropped the correct height above the seabed. Bathymetric data is also of general use for planning future exploration programs and can be used to map seabed geologic features.

Core Sample Processing

Sediment cores are processed to provide up to three sets of samples from varying depths in the core. By this method multiple analyses for each sample are available for interpretation, significantly improving the recognition of anomalies.

a. Headspace Gas Samples

Headspace gas samples are processed by adding a 200 cc of sediment to 300 cc of degassed brine solution and sealing the mixture in a 500 cc can. A 100 cc laboratory grade nitrogen headspace is added by displacing 100 cc of brine. Samples are then heated in an oven at 70 degrees C for 12 hours and briskly shaken for 3 minutes to extract hydrocarbon gases from the sediments into the headspace. Samples can then be displaced from the can into an evacuated serum bottle to insure that gases are not altered during the time from sample collection to analysis.

b. Sediment Samples

Sediment samples for extraction of petroleum liquids are double bagged in the field and frozen onboard. Additional samples are recovered as necessary to preserve materials of geologic interest or when macro seepages of petroleum are encountered. Archive samples are also available for client analysis and other geologic and engineering studies.

c. Core Logging

Each core is measured and described onboard to provide a record of the volume and type of sediment recovered. Information recorded includes time, date, water depth, location coordinates, sampler type, core recovery, physical description, samples collected and depths from seabed and other information which may be of geologic or engineering interest. In addition all samples are inventoried on a computerized data base.

Laboratory Analytical Methods - Core Samples

A wide range of analyses can be performed on samples of each core to fully characterize the full range of petroleum hydrocarbons expected in nature from natural gases to heavy oils. Typically 2 to 3 samples from each core are analyzed for both natural gas and oil indicators. Selected samples showing strong indications of liquid petroleum are then extracted and analyzed to further differentiate oil type, maturity and possible sources. These results can then be compared with oil and gas samples from known reservoirs.

Headspace Gas Analysis

a. Methane - Butane Light Hydrocarbons Analysis of Headspace Gases.

Light hydrocarbon analyses of headspace gas can be used to identify active gas seepage from depth and as tool to help predict the oil versus gas potential of the study area. Measured gases include methane, ethane, propane, iso-butane and normal butane by Flame Ionization Detector (FID) gas chromatography.

b. C5+ Gasoline Range Hydrocarbons Analysis of Headspace Gases.

C5+ gasoline range hydrocarbon analyses by FID gas chromatography can be used to quantify the gasoline range liquid hydrocarbon content of the sediments. Resultant chromatograms are subdivided by molecular weight ranges of Pentane - Benzene, Benzene - Toluene, Toluene - Xylene and Xylene+ hydrocarbons for comparison with signatures from oil samples from known reservoirs.

c. Stable Carbon Isotope Measurements

Stable carbon isotope measurements of selected headspace gas methane samples can be used to help differentiate biogenic versus petrogenic influences on the measured gases.

Sediment Analysis

a. Synchronous Fluorescence Analysis of Sediment Extracts.

Synchronous fluorescence scanning of sediment extracts identifies the aromatic hydrocarbon content of migrated petroleum liquids. Fluorescence signatures can also be used to identify heavy versus light oil types for comparison with known oils from the study area.

b. Total Petroleum Hydrocarbon Screening

Total petroleum hydrocarbon screening methods provide a rough estimate of the total amount of oil in the sample. TPH screening can be used as a very rapid onboard screening tool to identify samples for more detailed extraction and analysis.

c. CS2 Extraction and C15+ Hydrocarbon Analysis

CS2 extraction and chromatographic analyses of sediment extracts provides an excellent method of characterizing the gasoline to diesel range signature of selected samples. This technique allows the comparison of extract signatures with petroleum signatures from nearby reservoirs.

d. Capillary Gas Chromatography.

Capillary gas chromatography analyses are run on selected samples showing significant liquid petroleum levels as defined by C5+ and fluorescence tests. Results provide a more detailed differentiation of the C15+ component of migrated oils for comparison with known oils.

e. Other Analyses.

Biomarkers and other whole oil and source rock tests can be run if the sufficient quantities of oil rich sediments are recovered from core samples.

Interpretation of Results


a. Seep Magnitudes

To map the horizontal distribution of seepage areas with respect to known production, prospective trends or structures and low potential areas.

b. Seep Compositions

To make a reliable prediction of the oil versus gas potential of the areas of interest and statistical support of such predictions.

c. Petroleum Characterization

Provide detailed characterization of the petroleum constituents and the possible sources and contrast possible source type or maturity changes between anomalous areas.

d. Integration with Geologic Model

Interactive integration of geochemical results with regional geologic or prospect data to help improve understanding of the basin of interest and to help identify additional prospective trends or structures.

Typical interpretive products include:

a. Numerical Data Tables

Numerical listings of all data sets including plotted fluorescence profiles and CS2 extraction chromatograms.

b. Summary Statistics

Statistical tables including the maximum, minimum, mean and standard deviation of each geochemical magnitude and compositional component.

c. Histograms of Magnitude and Compositional Components

Histograms are generated for each magnitude component to help differentiate natural background levels from anomalous seepage zones. Compositional histograms are used to help differentiate the oil versus gas potential of the study area and define compositional populations which may suggest variable sources in the study area.

d. Interpretive Dot Maps

Interpretive dot maps are used to allow both magnitude and compositional data to be posted on a single product. Individual sites are color coded to reflect seepage composition while the dot size reflects magnitude of seeping gases. By this method the low magnitude and background sites are posted as very small dots. Interpretive dot maps are of particular use in areas with widely space data point where insufficient sample coverage is available for contouring.

e. Summary Interpretation

Summary interpretive products are used to combine the various products into a single map which clearly show the magnitude and compositional signature of all data sets acquired during the survey. Products include projecting compositional histograms, fluorograms and chromatographs onto a summary anomaly map. Summary maps may also include regional geologic or structural data.

f. Seismic Feature Maps

High resolution geophysical interpretations are compiled onto seabed features and shallow structural interpretations to overlay geochemical survey maps. Seabed features maps include seep mounds, mud volcanoes, pockmarks and other information mapped from side scan sonar data. Structural interpretations include a map of faults, fractures, channels, gas charged zones and other subsurface features mapped from the high resolution geophysical data.

Marine Sniffer Database - Gulf of Mexico

Deep Water Marine Geochemical Coring. Data Example: Green Canyon, Gulf of Mexico

Integrated Marine Geochemical Exploration Programs. Data Example: Northern Chukchi Sea, Alaska

Offshore Sniffer Survey: High Island Area, Gulf of Mexico

Geochemical Discoveries Offshore Venezuela and Trinidad

Calibration Survey, Central Bonaparte Basin, Australia