Coastal and Marine Geology Program > Seafloor Images Refine Petroleum Exploration Models

Seafloor Images Refine Petroleum Exploration Models

"GLORIA mapping has shown that we need to think again about our conventional models for formation of deep-sea fans. Exploration for hydrocarbon accumulations in ancient fan environments may change dramatically as a consequence of our new understanding of deep-sea fan formation." - Dr. David Twichell, U.S. Geological Survey U.S. Geological Survey (USGS) reconnaissance mapping of the Exclusive Economic Zone (EEZ) is nearly complete after 10 years. Acoustic mapping of the EEZ sea floor using GLORIA side-scan sonar tool includes the margins of the continental United States, Alaska, Hawaii, and Johnston Island. This decade-long program was undertaken in cooperation with the United Kingdom's Institute of Oceanographic Sciences at the Deacon Laboratory (now the Southampton Oceanography Centre). USGS scientists have seen several surprising physiographic features on the Gulf of Mexico seafloor. The Mississippi Canyon and DeSoto Canyon are depositional river-like features with levees and meanders, appearing much as rivers do on land; however, these channels formed under as much as 3,200 meters (10,000 feet) of water. Sediment flowed through these channels for hundreds of kilometers before being deposited. Also, the Florida Escarpment is among the world's largest cliffs at as much as 1,800 meters (6,000 feet) high, and erosional features on the escarpment mimic those seen on land. It is significant that these features were formed in the marine environment and were never subject to subaerial process. (Above) Deep-sea fan. [larger version] (Right) Conceptual diagram of sublobe lithologic, stratigraphic, and backscatter characteristics and their relation to sediment source regions and feeder channels during the late Pleistocene. [larger version] USGS investigations of the Gulf of Mexico seafloor show that patterns of deposition are consistent at several levels of observation. Using GLORIA images, higher-resolution SeaMARC 1A images, and sediment cores, scientists are seeing impressive details of deep-sea fans that have technical interest on two levels. First, patterns of sand deposition in the fans indicate a very slow flow of sediments within restricted paths, as opposed to a flow spread uniformly over large areas in sheets. Second, analyses of cores and SeaMARC images indicate that sand deposition is laterally discontinuous as a result of flow in these channels, rather than laterally continuous as might be expected with sheets of sand. The standard model of deep-sea fan formation is incomplete and, in some important details, incorrect. A - (top) Classic fan model showing a feeder channel and suprafan lobes where continuous sand sheets are inferred to be deposited. B - Observations based on side-scan sonar images and cores showing the association of the sand deposits with channels and the complex, discontinuous nature of the deposits. [larger version] Although scientists have been aware of deep-sea fans for some time, they were not imaged in detail until the USGS produced reconnaissance views using GLORIA technology. Previous theory was based on fragmented information taken from cores or inferred from seismic surveys and from outcrops on land. Until GLORIA and SeaMARC images were available, marine scientists had presumed the seafloor to be flat and fan deposits to be continuous over large areas. For the first time, scientists know the scale of these features and that they are actually discontinuous. Movement of a near-frictionless slurry of sediment and seawater in a narrow, low-relief channel challenges laws of fluid dynamics. The coherent movement of a body of sediment suspended in seawater over a distance of several hundred kilometers and a gradient of about 1 meter per kilometer puzzles USGS scientists. The 'slug' of sediment moves through channels for as much as 600 kilometers before being deposited. Friction between particles in the 'slug' and the underlying channel floor builds slowly enough to allow the material to travel very long distances. The questions raised by this observation may form the basis for a new set of principles of fluid dynamics. Knowledge of the evolution of deep-sea fans is critical to understanding hydrocarbon accumulations. GLORIA imagery can be draped ocer National Oceanic and Atmospheric Administration bathymetry to create three-dimensional images that facilitate analysis. The perspective view shows the Florida Escarpment cut by gullies and a meandering submarine channel formed at water depths of more than 3,000 meters along the escarpment's base. Also shown are mass-wasting deposits (light tones) that bury the channel and originated in the Mississippi Canyon 500 kilometers to the northwest. [larger version] The Mississippi deep-sea fan is believed to be a modern analogue to certain ancient deep-sea fans that have been extensively drilled for oil and gas accumulations. Links between ancient and modern examples are scarce and weak, largely because good opportunities to study modern analogues have not existed before GLORIA mapping began. New interpretations of the formation of deep-sea fans may provide this necessary link. Contact Information Dr. David Twichell 384 Woods Hole Road Woods Hole, MA 02543 Phone: (508) 457-2266 Fax: (508) 457-2310 Email: dtwichell@usgs.gov

Related Research Projects: GLORIA Imagery of the U.S. EEZ USGS Coastal & Marine Geology Program Related Publications: SeaMARC 1A sidescan sonar mosaic, cores and depositional interpretation of the Mississippi Fan: ArcView GIS Data Release - USGS Open File Report 00-352 USGS Coastal & Marine Geology Program Related Links: GLORIA and the EEZ SCAN Program USGS Coastal & Marine Geology Program GLORIA Mapping Program - U.S. EEZ Gulf of Mexico Program USGS Coastal & Marine Geology Program Southampton Oceanography Centre University of Southampton and Natural Environment Research Council

Coastal and Marine Geology Program > Seafloor Images Refine Petroleum Exploration Models