Hydrocarbon Seep Detection
Hydrocarbon seep detection is a method for identifying, confirming, and analyzing hydrocarbon presence and potential in ocean basins. While hydrocarbon seeps have been known and studied for many decades, seep studies are becoming more common with better remote sensing technologies. Seep studies are performed utilizing hydrographic tools and geochemical analysis.
What is a Seep?
Seeps are locations along the seafloor where hydrocarbons (predominately methane gas but also other hydrocarbon fluids) have breached the seafloor surface. Hydrocarbons are generated deep in the subsurface from kerogen rich source rocks (ex. coals and shales). After maturation, hydrocarbons migrate into a reservoir (ex. porous sand) that is capped by a seal (ex. shale formation). In cases where seals are poor and have been affected by networks of faults, fractures, and fissures, hydrocarbons can migrate up through the seal to the seafloor. Hydrocarbons that breach the seafloor can create gas filled pockets (bubbles) that float to the surface as well as other geomorphological features such as mud volcanoes. These gas pockets and geomorphological features can be identified from remote sensing data such multibeam echosounders (MBES) and 3D seismic data.
Hydrographic Remote Sensing Tools
Seeps can be detected through multiple methods including sonar, 2D and 3D seismic data, sub-bottom profilers, and AUV. The use of MBES have become more efficient and cost effective in seep detection with the incorporation of backscatter (BS) and water column (WC) data being collected during MBES operation. A brief overview of each of these techniques is provided below.
Multibeam Echosounders (MBES)
MBES systems started being adapted in the 1990s and replaced previously used single beam echosounders (SBES). MBES systems operate in similar fashion to SBES by transmitting acoustic waves from a transducer located beneath the ship and calculating the travel time for the sound to return. The water velocity is previously recorded using a sound velocity profile (SVP) and correlated with arrival times to determine seafloor depths. Modern day MBES systems range from 10kHz to 30kHz signals allowing for high resolution imaging of the seafloor. They record more data than previous SBES by recording sounding collected across ship track in what is known as a “swath” up to four to six times the water depth. This allows for faster data acquisition and more complete coverage of the ocean bottom surface.
BS is recorded during MBES bathymetry acquisition in modern systems. It is similar to side scan sonar which is based on transmitting acoustic waves and measuring the relative intensity of the return. By examining the relative amplitudes of returning acoustic waves, information regarding the surface texture and composition can be determined. For example, hard or coarse reflectors such as carbonate limestones, basalts, and some clastic sediments can produce stronger returns than softer sediments such as pelagic sediments often found on the seafloor. This informs the interpreter about lithological variations on the seafloor in conjunction with depth estimates.
Water Column (WC)
WC data is another product obtained from MBES systems that gives insight into aspects of marine environments between the ocean surface and the seafloor. WC data can identify gas bubbles, biology (ex. schools of fish), and physical processes occurring in the ocean. WC data combines both the multibeam and backscatter data to give information about amplitudes of returning signals with depth. When the acoustic wave from the transducer interacts with things such as bubbles in the water a disturbance is created in the wave propagation creating backscatter. This data can be analyzed to determine how strong, how variable, and where the signal originated from.
In most hydrocarbon exploration phases, sparse 2D seismic is gathered or made available prior to collecting 3D seismic. By using MBES, BS, and WC data along with subsequent geochemical analysis, seep detection and analyzation can help identify hydrocarbon types and presence before collected 3D seismic data. This can help delineate and rank potential plays, leads, and prospects in order to more accurately locate and plan 3D surveys.
Seeps contain multiple geological and biological signatures that can be identified with remote sensing geophysical equipment. Geological indicators of seeps include mud volcanoes, pockmarks, localized depressions, subsurface faults, gas hydrate deposits, and shallow gas accumulations. The features can also have detectable features associated with them that can be detected as distinct geophysical features such as gas bubbles and oil droplets. Biological features include dense accumulations of chemosynthetic communities comprised of clams, mussels, and tubeworms that feed on hydrocarbon seeps.
MBES data is first used to identify geomorphological features on the ocean bottom known to have seeps such as mud volcanoes or faults. BS data is then merged with the MBES data and overlain to distinguish hard and soft reflectors such as a carbonate buildup or chemosynthetic community formed around a hydrocarbon seep. WC data is then used to identify potential gas bubbles extending out of the seep locations. By combing all three data types targets can be selected for coring in order to confirm hydrocarbon presence, type, and thermal history.
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