Abyssal

Abyssal refers to the depositional environment and associated sediments of the deepest parts of the ocean basins, generally at water depths greater than 2,000 metres and commonly below 4,000 metres. The abyssal environment is characterized by near-freezing temperatures (1 to 4°C), extreme pressure (40 to 100 MPa), total absence of light, very slow sedimentation rates, and fine-grained sediment types including red clay, calcareous ooze, siliceous ooze, and turbidite deposits. In petroleum geology, abyssal settings are important as the source of turbidite sands deposited by sediment gravity flows that originate from shallower continental shelves and travel down submarine canyons onto the abyssal plain. These turbidite sandstones, buried and lithified over geological time, form some of the world's most productive deepwater oil and gas reservoirs. The definition of abyssal is sometimes used loosely to refer to any very deep marine depositional setting, even shallower than the strict depth threshold, when the context emphasizes organic-rich fine-grained sediments associated with low-oxygen bottom water conditions.

Key Takeaways

The abyssal plain lies below the carbonate compensation depth (CCD), which is the water depth at which the rate of carbonate dissolution equals the rate of carbonate deposition from surface-water organisms (approximately 4,000 to 5,000 metres in the Atlantic and 3,000 to 4,000 metres in the Pacific). Below the CCD, the seafloor receives siliceous ooze (from radiolarians and diatoms that do not dissolve) or red clay (extremely slow settling of windblown dust and cosmic material). Above the CCD, calcareous ooze from foraminifera and coccolithophores accumulates. The CCD position is relevant to petroleum geology because it controls the preservation of organic matter in abyssal sediments: below the CCD, organic matter is also oxidized by the well-oxygenated bottom water, preventing source rock accumulation in most abyssal settings.
Turbidite sandstones deposited in abyssal and deep-slope settings are among the world's largest offshore petroleum reservoirs. The Paleocene-Eocene Forties, Balmoral, and Nelson fields of the North Sea, the Oligocene-Miocene Agbami, Bonga, and Egina fields offshore Nigeria, and the Cretaceous Cascade field offshore Nova Scotia all produce from turbidite sandstone reservoirs deposited in deep-marine settings. The sands were transported as turbidity currents from shallow-water deltaic systems, travelled hundreds of kilometres down submarine canyons, and deposited as sheet or channelized sands on the deep ocean floor. Burial over tens of millions of years and migration of hydrocarbons from organic-rich shales below converted these sands into producing reservoirs.
Submarine fans are the primary depositional architecture for turbidite sands in abyssal settings. A fan system consists of a channel-levee complex in the upper fan (where the turbidity current is confined in a submarine canyon or channel), a mid-fan region where the channel splits into multiple distributary channels (each with flanking levees), and a lower fan where the turbidity current spreads out in unconfined sheet sand deposits. Oil fields are found in all three fan elements: the best reservoir quality is typically in the channel-fill sands (which receive the coarsest grain sizes), while the sheet sands of the lower fan provide large lateral extent at the cost of more variable thickness and quality.
Bottom-simulating reflectors (BSRs) are a seismic anomaly seen in abyssal sediment columns along continental margins where gas hydrates are present. Gas hydrates (ice-like structures of gas molecules caged in a water lattice) are stable under the conditions of high pressure and low temperature that prevail in the abyssal environment. The base of the hydrate stability zone creates a distinctive flat seismic reflector that parallels the seafloor bathymetry (hence "bottom-simulating") and has a reversed polarity, indicating a velocity decrease below the BSR where free gas is trapped beneath the hydrate layer. BSRs are not themselves oil and gas targets but indicate that hydrocarbons are present in the deep water column and may have migrated from a deeper conventional accumulation.
Abyssal sediment packages in the geologic record are recognized in outcrop by their fine-grained background sediments (mudstones, calcareous oozes lithified to chalks or marls), intercalated with coarser turbidite event layers (graded bedding, sole marks, Bouma sequences). In Alberta, there are no Phanerozoic abyssal deposits because the Western Canada Sedimentary Basin was a continental platform environment throughout its geological history. Deep-marine turbidite sequences in Canada are found in the Newfoundland and Nova Scotia offshore (Cretaceous-Paleocene deep-water fans) and in the Canadian Arctic (Sverdrup Basin turbidites). These are the analogues used to calibrate the play models for international deep-water exploration in West Africa and Brazil.

Turbidite Deposition and Abyssal Petroleum Systems

The story of how oil ends up in deep-ocean sands starts on a continental shelf. Large river deltas (Niger, Amazon, Congo, Mississippi) deliver enormous quantities of sand and silt to the shelf edge. During sea level lowstands, when the shoreline retreats to near the shelf edge, the river-supplied sediment piles up at the shelf margin. Submarine landslides and slope failures can mobilize these sediments, sending them down the continental slope as turbidity currents: fast-moving, sediment-laden flows that travel at 20 to 70 kilometres per hour down the continental slope and onto the abyssal plain.

As the turbidity current decelerates on the flat abyssal plain, it deposits the coarser particles (sand, silt) first, then the finer clay last, creating a characteristic graded bed: coarse at the bottom, fine at the top. This Bouma sequence (named after Arnold Bouma, who described it in 1962) is the hallmark of turbidite deposition and is recognizable in both outcrops and well cores. Successive turbidity currents stack these graded beds into thick sequences of interbedded sandstone and mudstone that, when buried and structurally trapped or stratigraphically pinched out against the slope, form the reservoir for deep-water oil and gas accumulations.

Fast Facts

The word abyssal comes from the Greek abyss, meaning bottomless. The abyssal zone of the ocean (2,000 to 6,000 metres) and the hadal zone (below 6,000 metres, in oceanic trenches) together cover approximately 75 percent of the ocean floor by area. The deepest abyssal plain is in the Challenger Deep of the Mariana Trench, at approximately 11,000 metres depth. In petroleum exploration, the commercial frontier for deepwater drilling has progressively extended from the 200-metre shelf edge limit in the 1970s to more than 3,000 metres today. The world's deepest producing oil well is in the Gulf of Mexico in approximately 2,900 metres of water. The largest undiscovered petroleum resources remaining on Earth are believed to be in deepwater passive margins of the South Atlantic, East Africa, and the Arctic, in abyssal and upper abyssal-slope settings where turbidite reservoir systems have been identified on seismic but not yet drilled.

Abyssal Settings and Source Rocks

Organic-rich source rocks do not generally accumulate in fully abyssal settings because the well-oxygenated, cold bottom water oxidizes organic matter before it can be buried and preserved. The best marine source rocks form in restricted, semi-enclosed basins where bottom water circulation is sluggish or absent (anoxic conditions): settings like the Cretaceous Western Interior Seaway, the Jurassic Tethys Sea, and Miocene-Pliocene restricted basins along passive margins. These settings, which are technically not fully abyssal, accumulate the organic-rich black shales and marls that generate oil and gas on burial.

In deep-water petroleum systems, the source rocks are typically in slope or restricted-basin settings above the true abyssal plain, while the turbidite reservoir sands are deposited on the abyssal plain below. The oil migrates updip from the source rock into the overlying turbidite sands in the submarine fan systems.

Abyssal is also used as a prefix in abyssal plain, abyssal fan, abyssal zone, and abyssal environment. Related terms include turbidite (a sedimentary deposit formed from a turbidity current; the primary reservoir rock type in abyssal and deep-slope petroleum accumulations; characterized by graded bedding and Bouma sequences), deepwater (a general term for water depths greater than 300 to 500 metres in the petroleum industry; deepwater exploration targets turbidite and mass transport deposit reservoirs in abyssal and upper slope settings), submarine fan (the fan-shaped depositional system formed by repeated turbidity currents in deep-water settings; the primary reservoir architecture for deep-marine oil and gas accumulations), carbonate compensation depth (CCD, the water depth at which carbonate dissolution rate equals carbonate supply rate from surface waters; below the CCD, abyssal sediments are carbonate-free siliceous ooze or red clay), and Bouma sequence (the characteristic vertical succession of sedimentary structures in a turbidite bed: massive sandstone at the base, grading upward through ripple-cross-laminated silt to laminated fine sand to mudstone cap; the diagnostic fingerprint of turbidite deposition in abyssal settings).

How Abyssal Fan Mapping Guided a 400 Million Barrel Discovery Offshore West Africa

An exploration team was evaluating a deepwater block offshore Equatorial Guinea in approximately 1,600 metres of water. The block sat in the Cretaceous-age deep-water Congo Fan system, where turbidite sands had been deposited over an area of several thousand square kilometres during sea level lowstands of the late Cretaceous and Paleocene. Legacy 2-D seismic data showed several bright amplitude anomalies on structural highs, but the geometry was unclear at the 2-D line spacing available.

A high-resolution 3-D survey was acquired and processed. The team used seismic amplitude analysis and horizon slicing at the Campanian-age target horizon to map the paleogeography of the fan system. Time slices at the target level showed a well-defined channel-levee complex trending northwest-southeast, with a mid-fan distributary zone that spread into sheet sand deposits on the abyssal plain to the southeast. The structural and stratigraphic trap was identified where the channel sands pinched out against a fold closure on the northwestern flank of the fan.

An exploration well drilled into the structure encountered 42 metres of net pay in the turbidite channel sand, with porosity of 24 percent and permeability of 450 millidarcys. A flow test produced 5,200 barrels of oil per day from a 3-metre perforated interval. The field was subsequently appraised with three additional wells and mapped at approximately 420 million barrels of recoverable oil. Development planning used the 3-D seismic fan architecture mapping to design an 8-well subsea tieback system targeting the highest-connectivity channel-sand areas. The field is now in production, contributing approximately 50,000 barrels per day to the block operator's portfolio. The abyssal fan mapping framework was the analytical tool that identified the trap geometry and predicted the reservoir distribution before a single appraisal well was drilled.