Coastal, shallow-marine and deep-marine environments

Four forces distribute the sediment that rivers carry to the sea. Waves rework the sand near the shore and keep it in shallow water. Ordinary waves disturb the bed only down to fair-weather wave base, and storm waves down to the deeper storm wave base (wave base). Tides drive water back and forth, and in narrow passages they speed up currents that can roll sand along. Storms carry sand out of shallow water into deeper water. Gravity flows (slides, debris flows and turbidity currents) dump sediment down the slope to its foot.

We distinguish several belts from the shore to the sea. The beach includes the backshore, which only a storm floods, and the foreshore (the swash zone, where waves run up the beach) between the high-water and low-water levels. Below the beach, down to fair-weather wave base, lies the shoreface, where sand is constantly in motion. Below the shoreface is the offshore transition zone, down to storm wave base, where storm sands alternate with clays. Farther out is the shelf. It descends gently to depths of a few hundred metres, as far as the shelf break. Beyond the shelf break are the continental slope and the slope toe, which leads out to the basin floor.

How strong the tides are depends on the outline of the basin and of the coast. We divide coasts into three classes by tidal range, the difference between high-water and low-water levels. Each class has its own sand bodies:

A delta is a protrusion of the coast at the mouth of a river, built of the river's sediment. The river delivers more sediment to its mouth than waves and tides can carry away, and the excess builds land out towards the basin. On the delta plain, the river divides into distributary channels with levees, and between them lie swampy interdistributary bays with clays and coals. Below water level is the delta front with mouth bars. Deeper and farther out lie the clays of the prodelta.

In a well that cuts through the delta front, the section coarsens upwards: prodelta clays are followed by silts, and these by sands (delta front). This is how the growth of a mouth bar is recorded. The river jet comes out of the channel into standing water, where banks no longer confine it. It spreads out and slows down. Coarse sand settles first, right at the mouth, on the crest of the bar. Fine particles are carried farther out to sea. The bar builds seaward, and over every point of the floor, fine sediments are covered by ever coarser ones. The depth of the water in front of the mouth sets the shape of the bars. In shallow water, the jet drags on the bed and spreads quickly. The bar is wide, and sooner or later it divides the flow in two. The channel bifurcates (splits), and new bars grow at the new mouths. Over deep water, friction with the bed is small, and the light fresh river water spreads over the surface of the denser sea water. The channel continues into the sea between submerged levees, and the bars are drawn out behind it as long narrow bodies, called “fingers”.

The classic example of a river-dominated delta is the modern Mississippi delta with its “finger” bars. An example in Kazakhstan is the delta of the Ural (Zhaiyk) near Atyrau. On a space image of 2004, it is a tree-like, “fingered” delta, of the kind that forms under weak wave action and with a large sediment supply. New distributaries appear in it when the river breaks through its levees. People have strongly modified the delta for flood protection and water diversion. Wave-dominated deltas open onto exposed coasts with strong surf. An example is the delta of the Senegal River in West Africa, where longshore drift of sediment (sand carried along the coast by waves) pushes the river mouth aside along a long sandy spit. Tide-dominated deltas open into bays with large tides, such as the Fly River delta in New Guinea.

When delta sands pile up quickly on the soft prodelta clays, they give rise to growth faults. These are listric normal faults (curved faults that flatten with depth) dipping towards the basin. On the downthrown side of a growth fault, the sands are much thicker, and the beds are bent into rollover anticlines. These are traps (normal faults, extensional structures).

On a coast dominated by waves, the sand near the shore is continuously reworked. On the beach, the swash and backwash of the waves (the rush up the beach and the flow back) leave gently dipping laminae inclined towards the sea. In the upper part of the shoreface, bars and dunes migrate. In the lower part, storms leave hummocky cross-stratification (wave structures). Waves approaching the shore at an angle drive sand along it. This is longshore drift, and it builds spits and bars.

If enough sand arrives, the shoreline advances into the sea, and beach ridges are added to it one after another. This is how a strandplain (a coastal plain built by accumulation) grows. In section, its deposits are a sand wedge that coarsens upwards, from the clays of the offshore transition zone to the sands of the beach (coarsening upward). The depth at which the shoreface ends, plus the height of the beach and ridges, sets the thickness of the wedge. How much sand arrived, and over what time, sets its width.

Along many coasts, sand collects into barrier islands. These are narrow sand ridges, and behind them lies a shallow lagoon. Let us cross such a coast from land to sea. First come the clays and silts of the lagoon. Tongues of sand extend into them from the island side. These are washover fans, which storm waves throw across the crest. Opposite the inlets lie the sand fans of the flood-tidal deltas. Then comes the island itself, with dune and beach sands that pass below sea level into shoreface sands. In front of the inlets, already in the sea, lie the ebb-tidal deltas, and beyond them the shelf clays. The tidal inlets themselves cut through the ridge and connect the lagoon with the sea. In the narrow passage the current is fast, and at the exit it dies away, so sand fans build up on both sides. The inlets also migrate along the island, eroding one bank and building up the other. Where they have passed, channel sands with an erosional base, fining upwards, replace the upper part of the barrier section.

For exploration, what matters most is the shape of the body. A barrier sandstone is a lens elongated along the ancient shoreline and surrounded by clays: lagoon clays on the landward side and shelf clays on the seaward side. It is a classic stratigraphic trap (stratigraphic traps). The tides set the length of the lens. On micro-tidal coasts, inlets are rare and barriers are long. On meso-tidal coasts, frequent inlets break them into short islands. After that, the balance of sand and sea level decides everything. If there is enough sand, the barrier advances into the sea, leaving behind a belt of beach ridges, and the lens spreads out into a sheet. If sea level rises faster, the barrier retreats landward, rolling over the lagoon. Waves cut off its seaward side and throw the sand onto the lagoonal side, and only a thin layer of sand on an erosion surface is left in the section.

Where tides are large and waves weak, tidal flats grow along the shore. Lowest, near the low-water level, is the sand flat. Higher up is the mixed flat, and higher still the mud flat. At the high-water level are the salt marshes, overgrown with salt-tolerant grasses. The current strengthens and then dies away again and again, so sand and mud alternate in flaser, wavy and lenticular bedding (beds and laminae), and cross-sets of opposite directions form a “herringbone” pattern. Meandering tidal channels cut through the flat. Like rivers, they build point bars sideways and leave inclined packages of sand with clay films.

An estuary is a drowned river mouth that the sea has entered as sea level rose. In a tide-dominated estuary, sand collects in ridges elongated along the current. Towards the head of the estuary the sediments become finer, and then coarser again in the river part. In a wave-dominated estuary, a barrier stands at the entrance, behind it is a muddy central basin, and at the head is the delta of the river flowing in. The result is a sand–mud–sand succession.

When relative sea level falls, rivers cut into the exposed shelf and form incised valleys. When sea level rises again, the valleys are flooded and filled with river, estuary and marine sediments (systems tracts). The fill rests on a sharp erosional base that cuts the underlying marine deposits. It can be thick: according to a published compilation, about 30 m on average at a width of about 10 km. The sandstones of incised valleys, surrounded by marine clays, are frequent stratigraphic traps. But inside they are complex: the river sands at the base are overlain by estuarine heterolithic deposits (mixed sand and mud).

On the shelf, mud slowly settles out of the water between storms. Rare events bring sand here. If we trace one horizon from the shore seawards, we can see how it changes. In the offshore transition zone, sandy storm beds with hummocky cross-stratification lie on top of one another and merge into units. Farther out, they are thinner and finer and separated by clays, and below storm wave base they disappear. This is how storms work. The onshore wind raises the water level at the shore. The water flows back to sea in bottom currents that, together with waves, carry sand from shallow water into deeper water. The farther from the shore, the less sand arrives. Where tidal currents are strong, tidal sand ridges and sand waves grow on the shelf. The ridges are tens of kilometres long and elongated along the currents.

The sand bodies of the shelf are possible stratigraphic traps. They are thin storm beds that are continuous over a large area, and isolated sand ridges surrounded by clays. But on the shelf, what matters most to the petroleum engineer is the clays. When sea level rises quickly, the shoreline retreats and the supply of sand to the shelf stops. Over a huge area, a thin layer of sand-poor mud slowly accumulates. This is a condensed section (a thin interval that formed slowly), and in it lies the maximum flooding surface. This surface corresponds to the time of the greatest advance of the sea onto the land (sequence stratigraphy). Such clays are regional seals and reliable markers for correlation. On the GR log they often show up as a maximum of radioactivity (Gamma ray log and natural gamma ray spectrometry).

The deep-water section is made up of two kinds of interval. Long intervals are times when fine clays and calcareous mud slowly settle on the floor. Short events are times when a gravity flow brings a layer of sand in hours or days (gravity flows and turbidites). Slides and slumps on the slope turn into debris flows. These take up water and turn into turbidity currents, which travel along the floor for tens or hundreds of kilometres. Their deposits are turbidites, graded sand beds with an erosional base. Each bed is one event, which is why it is called an event bed.

Flows travel through submarine canyons cut into the edge of the shelf, especially opposite the mouths of large rivers or where the shelf is narrow. They also travel through channels on the slope. At the sides of the channels, flows that spill over the banks build levees of thin sand–mud layers. At the foot of the slope, the gradient drops and the channels lose their walls. The flows spread out and deposit sand in lobes. Together, channels, levees and lobes make up a submarine fan. If sediment arrives through many small channels along the whole slope, the lobes merge into a sand apron at its foot. On the slope and at its foot there are also mass-transport deposits: slumped and crumpled units that can be both a seal and a baffle.

• Sand-rich systems: a short path from land and coarse sediment. They have channels and lobes of amalgamated sands, and a high net-to-gross ratio.
• Mud-rich systems: large rivers with fine sediment. They have long sinuous channels with high levees and thinner lobes. The sand is mainly in the channels and in the thin beds of the levees.
• Logs: channel fills give a cylinder or a bell, with the beds fining and thinning upwards. Lobes give a cylinder or a funnel. Levees and lobe margins give a serrated curve (GR curve shapes). GR smooths beds thinner than its vertical resolution (the thinnest bed the tool can show), so the thinly bedded sands of levees are easily mistaken for clays (vertical resolution of logging).

The sizes of lobes in very different systems are remarkably close. According to a comparison of six modern and ancient systems, lobes fall into two groups. On a flat basin floor, they average 14 km wide, 35 km long and 12 m thick. In depressions where the floor relief restricts spreading, they average 5 km wide, 8 km long and 30 m thick. The volumes of the lobes of both groups are of one order, about 1–2 km³. The means for individual systems are from 0.4 to 3.3 km³. A large system builds not larger lobes but more lobes in the same time. If you multiply the mean width, length and thickness of a group together, you get a larger volume than the real one, because a lobe thins towards its edges.

A slope channel is heterogeneous inside. In its axis, the sand beds are thickest. Each new flow erodes the mud “cap” of the previous one, so sand lies on sand, and the beds amalgamate (merge). At the margin, the beds are thinner, the mud caps are preserved, and there is less sand. We estimate the position of a well in a channel from two numbers: the sand fraction and the amalgamation ratio.

For a first estimate of body sizes, we use published compilations of measurements in outcrops, on seismic data and in modern environments. The mean values below show the order of magnitude. The scatter within each type is from a few times to an order of magnitude and more.

• The orientation of bodies sets the well pattern. Coastal sands are elongated along the ancient shoreline. The bars and channels of river-dominated deltas and the tidal ridges are elongated across it. Slope channels run down the slope. We orient rows of injection wells and horizontal wells along the long axis of the bodies (orientation of rows).
• Correlation relies on the clays of flooding surfaces. They are of one age and continuous. The same coastal or delta sandstone in different wells, however, may be of different ages, because the belts shifted together with the shoreline (Well correlation).
• Heterogeneity differs by body. Coastal and delta bars have permeability that increases upwards, and inclined clay films. Distributary, tidal and submarine channels have erosional bases with clay pebbles and clay films at the base. Lobes have thin clay interbeds that are continuous at the distal margins (the far edges).
• Stratigraphic traps: barrier lenses, coastal sands pinching out updip, incised valleys, shelf sand ridges, channels and lobes enveloped by clays (stratigraphic traps).