Turbidity Current
A turbidity current is a gravity-driven underwater flow of sediment-laden water that moves along the seafloor or lake floor because the sediment suspension makes it denser than the surrounding clear water, capable of traveling hundreds to thousands of kilometers across the ocean floor and depositing characteristic graded sequences of sand, silt, and clay (turbidites) that form some of the world's most prolific deepwater petroleum reservoirs — including the Paleogene Wilcox turbidites of the Gulf of Mexico, the Upper Cretaceous turbidites of the North Sea (Frigg, Andrew, Heimdal fields), and the Miocene turbidites of offshore Brazil (pre-salt and post-salt Santos and Campos basins) where billions of barrels of oil have been discovered in ancient turbidite sandstones deposited by turbidity currents in deep-water environments hundreds of millions of years ago.
Key Takeaways
- Turbidity currents are triggered by events that destabilize seafloor sediment and introduce large volumes of suspended sediment into the water column — earthquake-triggered slope failures (the most common trigger in tectonically active margins), storm-wave resuspension on the continental shelf, river floods that deliver excess sediment to the shelf edge, volcanic eruptions that deposit ash on the continental slope, and the oversteepening of submarine canyon walls by previous turbidity erosion; once initiated, turbidity currents are self-sustaining as long as the density of the turbid flow exceeds the surrounding water density, meaning that picking up additional sediment from the seafloor as the current flows increases flow density and speed in a positive feedback that can sustain turbidity currents for hundreds of kilometers from the original trigger point.
- The Bouma sequence (or turbidite sequence) describes the characteristic vertical succession of sedimentary structures deposited by a single turbidity current event — from bottom to top: massive or graded sand (Ta, deposited from the most sediment-dense part of the current at maximum flow), parallel laminated sand (Tb, deposited from the upper, lower-velocity part of the current), ripple cross-laminated fine sand (Tc, deposited from the waning current at low velocity), parallel laminated silt and clay (Td, deposited from the slowly decelerating fine sediment suspension), and pelagic/hemipelagic clay (Te, deposited from the residual suspension after the turbidity current passed) — this vertical sequence provides the standard reference for identifying turbidite deposits in cores and outcrops and for predicting reservoir quality from the depositional sequence alone.
- Turbidite reservoir quality is controlled by the depositional facies within the turbidite system — proximal channel fill sands (Ta and Tb Bouma intervals) are typically the best reservoirs, with grain sizes of fine to coarse sand, high porosity (20 to 30% in uncemented reservoirs), and high permeability (100 to 1,000 mD); lobe-fringe and sheet turbidite sands are thinner and finer-grained (silty sand, fine sand), with lower porosity and permeability; and turbidite mudstones (the Te interval and hemipelagic drapes) act as intraformational seals that compartmentalize individual sand bodies within the turbidite system, making the degree of amalgamation (stacking of multiple Bouma sequences without mudstone interbeds) the primary control on vertical reservoir connectivity and production performance.
- Submarine channel systems are the primary conduits that focus turbidity current flow and sediment delivery in deep-water turbidite depositional systems — analogous to river channels on land, submarine channels form sinuous or straight pathways across the continental slope that funnel turbidity currents from the shelf edge to the basin floor, depositing thick channel-fill sand bodies along their axes and thinner overbank levee sands adjacent to the channel margins; the channel fill sandstones are the primary exploration targets in deepwater petroleum systems, and their distribution, connectivity, and internal architecture are the key controls on hydrocarbon resource size and development strategy for deepwater fields.
- Outcrop analogs from ancient turbidite systems exposed on land (the Apennines in Italy, the Ainsa Basin in Spain, the Magallanes Basin in Chile, the Brushy Canyon Formation in Texas) provide the geological training data used to calibrate seismic interpretation and geological model building for subsurface turbidite reservoirs — by measuring architectural element dimensions (channel width, thickness, sinuosity, amalgamation ratio) in outcrops where the 3D geometry is fully exposed, geologists establish the statistical priors for turbidite reservoir architecture that are applied to subsurface reservoir characterization where only well data and seismic provide limited glimpses of the subsurface 3D geometry.
Fast Facts
The turbidity current concept was definitively established by the 1929 Grand Banks earthquake off Nova Scotia, which simultaneously severed multiple transatlantic telegraph cables in a sequence that, when the cable break times were analyzed, revealed a southward-flowing pulse of sediment-laden water moving at 50 to 100 km/h across the ocean floor — the first direct empirical evidence for a turbidity current in action. Philip Kuenen's 1950 flume experiments and subsequent field observations formalized the turbidite facies model. The discovery that deepwater turbidite sandstones could trap oil and gas led to the modern deepwater exploration era beginning in the 1970s, culminating in the discovery of giant deepwater fields including Jubilee (Ghana), Thunder Horse (Gulf of Mexico), Roncador (Brazil), and Tullow's East Africa assets that have collectively added billions of barrels to the world's recoverable hydrocarbon resource base.
What Is a Turbidity Current?
The deep ocean floor is not a static environment. Periodic mass movements of sediment cascade down the continental slope in powerful underwater avalanches that transport hundreds of cubic kilometers of sand and mud thousands of kilometers across the ocean floor. These sediment-laden flows — turbidity currents — are the mechanism by which river-derived sediment bypasses the shallow continental shelf and shelf edge to accumulate in the deep basins of the world's oceans.
A turbidity current forms when enough sediment is introduced into the water column at the shelf edge or upper slope to create a mixture denser than the surrounding seawater. This denser mixture flows downslope under gravity, accelerating as it descends the slope, picking up more sediment from the seafloor, and sustaining itself through the positive density feedback of sediment entrainment. The fastest turbidity currents can travel at hurricane speeds (up to 100 km/h), eroding deep submarine canyons and depositing their sediment load hundreds of kilometers from the trigger point when the flow eventually decelerates and spreads out on the relatively flat basin floor.
The deep-water sand bodies deposited by these events — turbidites — are the critical petroleum reservoirs in frontier deep-water exploration. Capped by the impermeable hemipelagic mud deposited between turbidity current events, and sealed by the structural or stratigraphic traps formed by the basin's tectonic history, ancient turbidite sandstones have become among the world's most prolific petroleum reservoirs in frontier deep-water plays where conventional shallow-water exploration has been exhausted.
Turbidity Current Deposits as Petroleum Reservoirs
Turbidite petroleum systems require four elements in coincidence: turbidite sandstone reservoirs with adequate porosity and permeability, a source rock (typically organic-rich deep-water shale deposited between turbidity current events) that has matured to generate oil or gas, a trap (structural closure or stratigraphic pinchout) that prevents the migrating petroleum from escaping, and a seal (the interbedded turbidite mudstones or a regional caprock shale) that maintains the trap's pressure integrity. These elements are often present together in passive continental margins where the same tectonic subsidence that creates accommodation space for thick turbidite successions also creates the burial depth needed for source rock maturation.
Reservoir quality prediction in turbidite systems requires understanding how the depositional process controls grain size, sorting, and porosity — proximal channel-fill turbidites deposited by the high-energy, sediment-dense core of the turbidity current are typically coarser and better sorted than distal sheet turbidites deposited by the lower-energy margins of the flow; diagenetic modification (quartz cementation, carbonate cementation) subsequently modifies the original depositional porosity, and the burial history of the basin determines how much diagenetic porosity reduction has occurred since deposition. The best deepwater turbidite reservoirs are shallow to moderate burial depths (less than 4,000 meters), with sand-rich proximal facies (high amalgamation ratio) and early hydrocarbon emplacement that inhibited diagenetic cementation.
Seismic geomorphology using 3D seismic amplitude extraction and horizon slicing has transformed the exploration and development of turbidite systems by making the plan-view geometry of submarine channels, lobes, and sheet turbidites visible directly from the seismic data — amplitude anomalies related to gas or oil-saturated turbidite sands can be mapped in plan view with resolution of tens to hundreds of meters, allowing explorationists to trace channel geometries, measure lobe dimensions, and identify the most prospective fairways without drilling. This direct seismic imaging of turbidite reservoir geometry, calibrated by well data, is the primary exploration tool for deepwater turbidite systems worldwide.
Turbidity Currents Across International Jurisdictions
Canada (AER / WCSB): The Western Canada Sedimentary Basin contains Cretaceous deep-water turbidite deposits in the Alberta Foothills foreland basin (Nikanassin, Cadomin, and related formations) that were deposited in the foredeep basin formed by Cordilleran thrusting; these ancient turbidites are tight gas reservoirs rather than conventional petroleum reservoirs due to their deep burial and diagenetic modification, but the turbidite facies architecture (channel fills, lobe sheets) controls the natural fracture distribution and sweet spots that are targeted in horizontal development wells. Canada's East Coast offshore (Scotian Shelf, Labrador Shelf) contains Tertiary turbidite sequences deposited on the passive continental margin adjacent to the Grand Banks, with limited exploration to date but potential for turbidite-trapped oil and gas reservoirs analogous to the producing deepwater systems of the conjugate West African margin (the two margins were joined before Mesozoic rifting of the North Atlantic).
United States (API / BSEE): The Gulf of Mexico is one of the world's premier deepwater turbidite petroleum provinces — the Miocene turbidites of the upper continental slope contain the majority of producing GOM deepwater fields, while the Paleogene Wilcox turbidites of the lower slope and abyssal plain represent one of the largest remaining deepwater frontier plays in the world, with discoveries including Tiber, Kaskida, and Anchor accumulating multiple billions of barrels of resource but facing development challenges from extreme water depths (2,000 to 3,000 meters), high temperatures (above 150°C), and reservoir heterogeneity at the scale of individual turbidite sand bodies. BSEE leasing and development regulations govern deepwater OCS exploration, with environmental impact assessments for each exploration and development program addressing the deepwater turbidite reservoir drilling and production activities.
Norway (Sodir / NORSOK): North Sea turbidite fields including Frigg, Andrew, Heimdal, and Grane are Paleocene and Eocene turbidite sandstone reservoirs deposited in the Norwegian-Danish Basin and Viking Graben during the early Tertiary sediment pulse following Mesozoic rifting — these fields have been producing since the 1970s and represent the mature phase of North Sea deepwater turbidite development. Exploration on the Norwegian Barents Sea and mid-Norwegian margin targets Paleogene and Cretaceous turbidite sequences in frontier settings where Sodir licensing rounds have attracted exploration interest from Equinor and international partners. Norwegian petroleum research institutions (NORSAR, NFH) publish on turbidite systems and seismic geomorphology methods that advance the interpretation of NCS turbidite reservoirs.