Depositional systems: fluvial, deltaic, shoreface, carbonate, deep-water

The five systems to know cold

We focus on the five depositional systems responsible for the vast majority of conventional reservoirs worldwide:

• Fluvial, rivers on continental landscapes; meandering and braided channel belts.
• Deltaic, rivers entering the sea; classic topset-foreset-bottomset architecture.
• Shoreface / coastal, wave- and current-worked sands along paleo-shorelines.
• Carbonate platform, rimmed shelves and ramps whose sediment is grown in place, with reef or shoal margins and inner lagoons.
• Deep-water (turbidite), slope channels and basin-floor fans; the dominant deep-water system.

There are others (aeolian, lacustrine, glacial, estuarine...), but these five hold most of the conventional reservoirs found so far.

Reading the five systems in Figure 4.3.1

Every system in the figure is grown under one ratio, $A/S$: the room that subsidence and sea level make for sediment in a step, against the sediment that arrives (on a carbonate platform, the sediment the platform grows). For the delta, the shoreface and the platform, below 1 the system builds outward, at 1 it builds up, and above 1 the sea gains ground, the same contrast that separates the systems tracts of Section 4.2; for the river $A/S$ sets how far the floodplain rises between avulsions, and for the fan how much sand the shelf lets through. Each model is a kinematic sketch: its rules give the stacking the literature describes rather than computing sediment transport. Plate (a) is the rock, (b) its seismic section and (c) a stratal slice of the same synthetic in map view; the readouts are measured on (a).

• Fluvial. A river leaves channel belts: lens-shaped sand bodies 1 to 2 km wide and 8 to 13 m thick, each with a fining-upward top and a mud-filled abandoned channel at one edge, in floodplain mud with coal where peat survives. When the floodplain rises little between avulsions (low $A/S$), each new belt cuts into the ones below and the sand joins into large bodies; when it rises a lot, every belt is stranded in mud. In (b) a coal seam more than about a metre thick outshines the sands, because coal’s impedance is half the mudstone’s; in (c), where they lie apart, the belts are sinuous ribbons. The risk is connectivity, not sand fraction: in a section the bodies link into one only when they make about half the rock. Amalgamated belts build the Cretaceous Castlegate Sandstone of Utah; isolated belts and coals, the Ness Formation of the Brent Group.
• Delta. A delta builds clinoforms: a nearly flat topset of delta plain, a foreset dipping a degree or two (steep only in a stretched section) and a bottomset of prodelta mud that downlaps a condensed layer. Sand gathers at the rollover as mouth-bar and delta-front sand and follows the shoreline trajectory, so a delta’s reservoir is a sheet whose place and stacking $A/S$ sets. Where a survey resolves the foresets, topsets thinner than a quarter wavelength look absent and the foresets seem to end at the top: toplap, an oblique configuration; thicker topsets show as their own reflections, a sigmoid one (Mitchum, Vail and Sangree 1977). Where the foresets lie closer than a quarter wavelength, as they do at 30 Hz in the figure, the front merges into one sheet and neither can be read. In (c) the front is an arcuate band. Wave-dominated deltas rework the front into cleaner, strike-continuous sand than river-dominated ones; Figure 4.3.2 grows both. Examples: the Cretaceous Ferron Sandstone of Utah; the Niger Delta.
• Shoreface. Each parasequence floods, then progrades: a shoreface that coarsens upward from offshore mud to beach sand, capped by the mud of the next flooding surface. Those flooding muds seal one parasequence from the next, but they lie only a few metres apart, closer than any survey in the figure resolves, so (b) shows the whole stack as one sand body. Stacked in place, all of them lie beneath one well; shingled seaward or stepped landward, a well reaches only the few it crosses. In (c) the shoreface is a strike-parallel band, and single shoreface sands run for tens of kilometres along strike. Examples: the Cretaceous Blackhawk Formation of the Book Cliffs; the Rannoch and Etive formations of the Brent Group.
• Carbonate platform. The sediment is grown in place, by reefs and by the whole shallow platform. When production outruns the new room the margin progrades and sheds debris down a steep fore-slope; when they balance it aggrades; when the sea outruns the lagoon but not the faster-growing reef, the rim keeps up around a deepening lagoon (an empty bucket); faster still, even the rim sinks below the depth where the carbonate factory works and the platform drowns under marl (Kendall and Schlager 1981; Handford and Loucks 1993). Real platforms can grow faster than the sea usually rises, so most drowned platforms also lost their factory to an environmental crisis (Schlager 1981). In (b) the platform’s top and base reflect three to six times as strongly as sand on mud, its interior, in beds thinner than the survey resolves, is nearly blank, and younger basin fill onlaps the slope; in (c) the rim is a band that follows the margin. Texture and, far more than in sandstones, diagenesis set the reservoir quality. Examples: the Late Jurassic Arab-D grainstones of Ghawar; the Permian Capitan reef margin; the Cretaceous Natih platform of Oman.
• Deep water. Turbidity currents spread lobes on the basin floor, each settling where the earlier ones left the lowest ground (compensational stacking), with sand in the axis, thin-bedded fringe at the margins and hemipelagic mud between. In the classic sequence model the shelf decides how much sand gets through (Posamentier and Vail 1988): with little room on the shelf, thick sandy lobes erode through the drapes and amalgamate; with much room, thinner lobes lie sealed apart. Fans also build at highstand where the shelf is narrow (Covault and Graham 2010). In (b) the lobes show as mounded loops only where the survey nearly resolves them, as at 60 Hz; at low frequency the fan merges into one trough over one peak; in (c) they are lobate shapes. Examples: the Paleocene Forties fan of the North Sea; the Oligocene and Miocene channel-lobe complexes of Girassol and Dalia, offshore Angola.

The headline of the figure is the share of the section’s reservoir in the bodies one well penetrates. It is measured in the plane of the section, so it is a lower bound on how much of that section’s reservoir the well reaches: bodies that look separate there may join out of the plane.

Deltas, grown rather than drawn

Of the five systems, the delta is the one worth watching happen. A delta is what a river does when it stops being a river: the flow leaves its banks, decelerates, and drops what it was carrying. That single event stacks good reservoir directly against good seal, over and over, across tens of kilometres, which is why deltaic reservoirs hold so much of the world's oil and gas.

Once the sediment arrives, three processes compete for it, and the balance between them sets the shape of the delta. Galloway's classification sorts deltas by that balance of river, waves and tides, read from their plan-view shape, which matters because shape is what you can see on a horizon slice.

• River-dominated. Supply outruns the sea's reworking. Sediment stays near the mouths, lobes build out into open water, and the river switches to a new lobe once the old one has run out of room. The Mississippi is the type example, from its stacked Holocene lobes to the long distributary fingers of its modern birdsfoot.
• Wave-dominated. Waves arriving at an angle drive sediment along the coast, which to first order is a diffusion of the shoreline, so the front's sand is spread along the coast in broad, cleaner sheets. The Nile and the Rhone are the classics.
• Tide-dominated. Tidal currents run in and out through channels at right angles to the coast, so the sand is combed into elongate bars that point out to sea rather than lying along it. The Ganges-Brahmaputra and the Fly are the classics, and their sand bodies are good reservoir that is easy to miss between wells, because they are narrow and run shore-normal.

A delta can also be fed in two different ways: by one trunk river that branches only once it reaches the sea, as the Mississippi does, or by several rivers whose mouths build one composite delta complex. Both build a delta; they do not build the same one.

Figure 4.3.2 grows a delta from these ingredients in a sediment-conserving model of a 140 km stretch of coast and the 90 km of shelf in front of it. You set the sediment supply, the wave energy and the tidal range, choose one river or three, and scrub or play 14,900 years. Each river's plume lays its load down at its mouths; the delta plain stands at most 3 m above sea level; waves spread the delta along the coast and tides spread it out to sea and cut channels across it, reworking only what the rivers delivered; and every 900 to 2,250 years, sooner at higher supply because a lobe fills its room sooner, each river switches its lobe to the least advanced stretch of coast. The bed gains what the rivers deliver to within five parts in a million. Plate (c) places the drivers you set on Galloway's triangle; the readout table reports what the model built from them, measured from the state the plates draw.

Reading Figure 4.3.2

1. The default: a river-dominated delta after 5 kyr. The river has laid down 93.6 km³ and moved the coast 18.1 km seaward on average, 39 km at its furthest point, and has switched lobes three times; the plan view, plate (b), shows the lobes it has left behind. The model builds lobes and the complex they stack into, the way the Mississippi built its Holocene coast, not the long fingers of the modern birdsfoot.
2. The clinoforms. In the dip section, plate (d), the time lines 1,000 years apart step basinward: a flat topset of delta plain, a steeper foreset of delta front and a thin bottomset of prodelta mud. The reservoir is the top of the foreset, and the seal is the prodelta mud that laps over it. The strike section, plate (e), shows the sand bodies themselves: lenses of delta front boxed in by delta plain above and prodelta below.
3. Waves. At the wave preset's supply and 5 kyr, raising the waves from none to 0.92 spreads the delta-front sand from about 510 to 1,420 km² and slows the mean advance from 17.6 to 12.1 km. Net-to-gross rises from 41 to 51 percent, but only because the model gives wave-worked sand a higher sand fraction by rule, standing for the mud that waves winnow out.
4. Tides. The shore-normal anisotropy measures what tides do: how much faster the delta's thickness changes along the coast than out to sea. At 6 kyr it is 0.31 with no tides and 2.62 at a tidal range of 0.9; at the end of the run it climbs from 0.24 with no tides to 1.73 at 0.9 and 1.75 at full range. The bars are delta-front sand, and narrow: a line of wells along the coast can miss every one of them.
5. One river or three. At the same supply the volume is the same, 28.1 km³ after 1.5 kyr, and the readout shows it; three rivers spread it over 1,980 km² of delta front against 500 for one trunk, in a pile 26 m thick at most against 55. But they switch lobes independently and share one in 133 of the run's 298 steps, so by the end theirs is the thicker pile, 125 m against 103.
6. Supply. Doubling the supply from 36 to 72 percent doubles the volume but not the advance: after 10 kyr the coast has moved 34.4 km against 17.7, and by the end of the run the ratio is 1.6. Progradation is the volume divided by the accommodation the delta must fill, and the shelf deepens basinward, so every kilometre further out costs more sediment.

Which system lives in which systems tract?

Connecting Section 4.2 and Section 4.3: systems tracts predict the depositional systems you will see within them.

• LST (lowstand): the diagnostic system is the DEEP-WATER TURBIDITE FAN (basin-floor fan + slope channels). Fluvial systems on the exposed shelf cut incised valleys and later fill them with river sands during the late lowstand.
• TST (transgressive): the diagnostic system is the SHOREFACE SAND WEDGE, which retrogrades landward as the shoreline steps back. Basal transgressive lags sit at the contact between the LST below and the TST above. Carbonate ramps may also develop on shallowly drowned shelves.
• HST (highstand): the diagnostic system is the PROGRADING DELTA or the PROGRADING SHOREFACE. Both advance seaward as sediment supply outpaces slowing rise. Clinoforms are the signature geometry. Fluvial systems dominate onshore.
• CARBONATES fit differently, because the platform grows its own sediment and answers to the rate of rise rather than to a river: it is exposed and karstified at lowstand, keeps up or backsteps as the sea rises, progrades by shedding sediment off its margin at highstand, and drowns when the rise outpaces even the reef.

Reservoir quality comparison

• Fluvial sands: typically excellent reservoir (15-25% porosity, 100-1000 mD). Sand body connectivity is the main risk, depends on net-to-gross.
• Deltaic sands: very good reservoir (15-25% porosity, 50-5000 mD). Delta-front foresets are the sweet spot. Distributary mouth bars locally even better.
• Shoreface sands: excellent reservoir (18-30% porosity, 100-10,000 mD). Clean quartz sand from wave-washing. Good lateral continuity.
• Carbonates: highly variable (1-30% porosity). Quality depends on depositional TEXTURE (grainstones > packstones > wackestones > mudstones) AND on diagenesis (dissolution enhances, cementation destroys). World-class reservoirs exist (e.g., Middle East Cretaceous) but so do world-class tight carbonates.
• Deep-water sands: excellent reservoir in lobes (20-30% porosity, 500-10,000 mD). Often the best reservoirs in a basin. Like any sandstone they lose porosity with burial; overpressure and early oil charge can preserve it.

Recognition checklist

When you see a seismic section and want to identify the depositional system:

• Look at the OVERALL GEOMETRY. Is it wedge-shaped (shoreface)? Clinoform-shaped (delta)? Lens-shaped (fluvial/channel)? Mound-shaped (reef/fan)? Sheet-like (marine shelf)?
• Look at the REFLECTOR STACKING PATTERN. Aggrading (carbonate platform)? Prograding (delta, shoreface)? Retrograding (TST shoreface)? Chaotic (mass-transport deposit, slump or reef interior)?
• Look at the MAP-VIEW EXPRESSION on timeslices or horizon amplitude extractions. Sinuous trails (fluvial channel belt)? Arcuate fronts (delta prograding)? Linear shorelines (shoreface)? Mound-like rings (reef rings)?
• Check AMPLITUDE CHARACTER. Sand-rich systems produce stronger amplitudes at top-reservoir boundaries where the sand differs in impedance from the shale above, but coal and carbonate contacts are far brighter than any brine sand, so amplitude alone does not mean sand. A flat spot, a flat reflection from a fluid contact, marks hydrocarbons in some clastic reservoirs.
• Consider TECTONIC/BASIN CONTEXT. Passive margin with shelf and slope? Expect shoreface, delta, deep-water. Rift basin? Expect fluvial, deltaic, and lacustrine. Carbonate shelf? Expect carbonate platforms, shoals, reefs.

Pitfalls

• Relying on one line of evidence. Seismic alone can be ambiguous. Calibrate with wells (petrophysics, biostratigraphy) to confirm.
• Applying clastic reasoning to carbonates. Carbonate sediment is grown in place, by reefs and across the shallow platform, so clastic reasoning (sediment carried in from updip) does not apply. Use carbonate-specific tools.
• Confusing meandering fluvial with tidal estuarine. Both show sinuous channels. Distinguish by context: fluvial in net-downdip flow direction; estuarine near paleo-shoreline with tidal bedding.
• Scale mismatches. A small-scale feature (one channel, 50 m wide) may be invisible at seismic resolution. Interpret the SCALE of the feature against the expected scale of the depositional system.
• Missing thin reservoirs. Shoreface sands as thin as 10 m can be economic reservoirs but may be below seismic resolution. Look for bright amplitude anomalies at their expected depth rather than clear geometric expression.