Contents Introduction Classification of depositional environments Depositional environments Marginal-marine environments Deltaic system Controls on Delta Environment Sub-environments of delta Variation in delta morphology Processes in model delta Morphological units of delta Classification of Deltas Typical sequence Importance of delta Sedimentary structure in deltas Structural features of deltaic reservoir
Introduction: To discuss deltas we should have great understanding of depositional environment. Depositional Environments: Geomorphic setting in which a particular set of physical, chemical, and biological processes operates to generate a certain kind of sedimentary deposit Physical environment has “static” and dynamic elements Static: basin geometry, sediment composition, water depth, etc. Dynamic: currents (wind, water), precipitation, climate
Chemical elements: pH, Eh, salinity, pCO2, etc. Biological aspects: activities of organisms (burrowing, skeletal particles, etc.) and their remains (e.g., peat)
Classification of DepositionalEnvironments: Nonmarine Environments Colluvial and Alluvial fans Fluvial environments Lacustrine environments Aeolian environments Costal(marginal marine) Environments River mouth environments Regressive river mouths:Deltas Transgressive river mouths:Estuaries Open shoreline(beach) environments Foreshore backshore Marine Environments Shallow marine environments Shoreface Inner and outer shelf Deep marine environments Continental slope, Abyssal plain(basin floor)
Depositional Environments: Objective: Use sedimentary deposits tointerpret depositional environments Physical, biological, chemical parameters of an environment combine to produce a body of sediment characterized by specifictextural, structural and compositional properties Distinctive bodies of sediments or sedimentary rocks are facies
Marginal marine environment: The marginal-marine(transitional) setting lies along the boundary between the continental and the marine depositional realms. It is a narrow zone dominated by Riverine Wave Tidal processes Salinities may range in different parts of the system from fresh water through brackish water to supersaline, depending upon the river discharge and climatic conditions.
Deltaic system The term “delta” the Greek character was used to describe the mouth of Nile by Herodotus nearly 2500 years ago. A modern definition cites delta as “the sub aerial and submerged contiguous sediment mass deposited in a body of water(ocean or lake) primarily by the action of a river”(Moore and Asquith,1971,p.2563).
Deltas seldom form on active, subducting continental margins because there is no stable shallow shelf on which sediments can accumulate. Shape of Delta: The shape of a delta is not always the triangle that suggested the name to Herodotus. Delta shape is influenced bya) Sediment inputb) Wave energyc) Tidal energy.
Controls on delta environment: Magnitude of fluvial discharge Climate, tectonics, s Delta morphology ubsidence, sedimen and sedimentary Delta regime t supply facies topography Magnitude of wave and tidal currentsFactors affecting delta regime, morphology and facies (Elliot 1978a)
Subenvironments of Delta: Deltas are influenced by a complex combination of fluvial and marine processes. Each delta has more than a dozen distinct environments of deposition. These environments can be grouped into three broad divisions:1. The delta plain with the meandering flood plains Swamps and beach complex2. The steeper delta front3. The broadly sloping prodelta, which grades into the open shelf.
Variations in delta morphology: The combinations of factors that control delta morphologies give rise to a wide spectrum of possible delta characteristics. Two main factors are the most important in determining the morphology of deltas:1) Effect of grain size2) Depth of water in delta are going to deposit
(a) a high proportion of suspended load results in a relativelysmall mouth bar deposited from bedload and extensive delta-frontand prodelta deposits
(b) a higher proportion of bedload results in a delta with a higherproportion of mouth bar gravels and sands.
(a) A delta prograding into shallow water will spread out as thesediment is redistributed by shallow-water processes to formextensive mouth-bar and delta-front facies.
(b) In deeper water the mouth bar is restricted to an area close tothe river mouth and much of the sediment is deposited by mass-flow processes in deeper water.
Processes in a model delta: Reduced to its simplest elements, a delta forms due to “unique hydrodynamic interaction” between river water and seawater. There is sharp contrast in water density due to salinity. As a result river water forms a plane jet that spreads out and forms a layer over the seawater. Current velocity diminishes radially from the jet mouth, depositing sediments whose settling velocities allow grain size to diminish radially from the jet mouth.
Developmentof the deltathrough time byprogradation (A,B, D) anddistributaryswitching (C)(From Davis,1983)
Levees: Ridges on either side of the distributary channels are termed “levees”. The sand carried in the stream is deposited along the sides of the jet in the subaqueous levees, where friction and mixing slow the flow. Distributary channels: It is channel that branches off and flow away from a main channel or stream. Common feature of delta. Distributary channel sands are abundantly:
I. Cross-bedded, with plenty of ripple cross- lamination.II. Scour-and-fill structuresIII. Discontinuous clay lenses.
Distributary mouth bars: Further offshore, where friction and spreading begins to slow the jet, sediments is dropped in the distributary mouth bars. The distributary mouth bar sands are even more complexly cross stratified because of the complex current system that pass over them. Wood, debris and other organic matter carried down the river during floods end up in the distributary mouth bars. Between the distributaries on the delta plain are wide, shallow “inter-distributary bays” and “marshes” like the flood plains of the meandering river. Much of inter-distributary sequence is built of sand sheets from “crevasse splays” deposited.
Morphological units of deltas: Three main morphological units appear. Delta platform/plain: The delta platform is the sub-horizontal surface nearest the jet mouth. It is basically composed of sand and traversed by the distributary channel and its flanking levees. Delta slope/front: The delta platform grades away from the source into delta slope on which finer sands and silts come to rest. Commonly burrowed, sand coarsing upward, and have good porosity and permeability.
Pro-delta: Delta slope in turn passes down into the delta slope on which finer silts and clays settle out of suspension.pro-delta deposits rest unconformable on marine shelf deposits, which may include nonclastic components such as algal reefs.Classically these three elements termed as 1- bottomset, 2-foreset, 3-topset respectively.
Classification of delta: Deltas can be classified in several ways (Nemec, 1990), however classification on the basis of delta-front regime (Galloway, 1975) appears to be favored by most geologists. Deltas are classified thus as:1) Fluvial-dominated delta2) Tide-dominated delta3) Wave-dominated delta
The forms of modern deltas: (a) the Nile delta, the „original‟ delta, (b) theMississippi delta, a river-dominated delta, (c) the Rhone delta, a wave-dominated delta, (d) the Ganges delta, a tide-dominated delta.
Fluvial-dominated delta: A fluvial or river dominated delta has a large volume of sediment and tends to be “lobate” when there is a moderate sediment supply and “elongate” when the sediment supply is large. If the sediment supply cannot keep up with the erosive powers of tides, than the delta tends to be very small. It occur where the tidal range is very low and the tidal current action is very weak.
Example:Mississippi delta: It is created when very large amounts of sediment are carried into relatively quiet water. Partly because dredging has kept the major distributary channels (locally called “passes”) fixed in position for many decades, the Mississippi‟s distributaries have built long fingers of sediment out into the sea. The resulting shape has been termed a “birdfoot” delta. Because of the dominance of stream sedimentation that forms the fingerlike distributaries, birdfoot deltas like the Mississippi‟s are also referred to as stream-dominated deltas.
Stream-dominateddeltaAster satellite photo of the Mississippi River delta taken in 2001.
Tide-dominated delta: A tide dominated delta has many linear channels parallel to the tidal flow and perpendicular to the shore. It occur in regions where wave action is limited and tidal ranges are generally in excess of 4 m, generating strong tidal currents -- have a major effect on mixing of river water and seawater and on sediment redistribution. These deltas form along a coast that is dominated by strong tides, and the sediment is reshaped into tidal bars that are aligned parallel to a tidal current.
Example:The Ganges-BrahmaputraDelta inBangladesh is agood example ofa tide-dominateddelta.
Wave-dominated delta: A wave dominated delta is smoothly arcuate; the wave action reworks the sediments and make such deltas much sandier than other types of deltas. It occur where wave energy is high; out-flowing freshwater behaves as a countercurrent, slowing down oncoming wave crests and causing waves to break in deeper water than normal. this leads to vigorous mixing, rapid deceleration of the freshwater flow, and sediment deposition; wave action reworks the deposited sediments to form sand bars and beaches, creating a straight shoreline with only a small protuberance at the distributary mouth.
Example:The Nile DeltaIt is a wave-dominateddelta that containsbarrier islands along itsocean-ward side
mixed-process deltas: The examples discussed above illustrate some differences in characteristics of modern deltas that are shaped by processes that are predominantly fluvial, tidal or wave related. Many deltas have characteristics that are transitional between these end members types. Example: The Copper River delta in the Gulf of Alaska provide an example of a delta that is strongly influenced by tides but also experiences high wave power(Galloway, 1976).
General delta patterns: A well-developed delta provides the whole gamut of clastic sediment types from carbonaceous mudstones to conglomerates. The proportions of the sediments types are controlled chiefly by the interaction of fluvial agency supplying the material and the marine agency receiving it. This interaction leads to four general deltas patterns:I. High-destructive deltasII. High-constructive deltas of birdfoot typeIII. High-constructive deltas of lobate typeIV. Fan deltas
Typical sequence: The classical stratigraphic profile of deltaic deposits shows a coarsening upward sequence from the delta slope muds and silts to the distributary mouth bar sands. This is opposite to the fining upward sequence found in most meandering fluvial system.
Why deltas are so important??? Ancient deltaic deposits are extremely important economically. Due to variety of environments in the deltas, it makes more important for a reservoir geologists. Many oil and gas-fields are in sedimentary deposits associated with deltas. Located near the boundaries between marine deposits, which include source sediments, and non-marine deposits which represent the supply zone for reservoir rocks, deltas are ideally situated for the maximum interplay between source and reservoir facies.
They hosts most of the world‟s coal, and many major petroleum provinces. The deltaic process is a way of deposition lobes of sand (potential reservoir) into envelopes of organic-rich marine muds (potential source beds). Deltaic environments deposit many potential stratigraphic traps, including mouth bars, barrier bars, and channels. Rapid deposition often leads to over-pressuring. This may generate diapiric traps and roll-over anticlines.
Sedimentary structures and fossils: Numerous types of sedimentary structures such as:1. Cross bedding2. Ripple marks3. Bioturbation structures4. Slump structure and5. Mud diapirs occur in deltaic deposits.A “Mud diapir” is a dome or fold in sediments that is formed by the plastic deformation of mud underlying sand or other sediments. Diapirs called mudlumps, frequently emerge in distributary mouth bar deposits,
Structural features in deltaic reservoir: Structural features result from deformation of sediments and rocks includes faults, folds, tilting(dip), and fractures. Structural features can broadly divided into two classes based on the timing of the deformation: Syndepositional deformation features: Syndepositional deformational processes, which includes:a. Slumpingb. Mud diapirismc. Growth faulting are common in lower delta plain environments and operate during delta formation.
Post-depositional deformation: Post-depositional deformational features includes:a. Foldingb. Tilting, faulting, fracturing. This is due to tectonic forces and consequent movement of earth‟s crust. Significances: Structural features can modify the sandstone body geometry. These features are important in the migration, accumulation and trapping of petroleum.
Migration of hydrocarbons from the source rock enhanced by: Faulting and Fracturing. These accumulation or trapping of oil is caused by permeability barriers which prevent further migration of the petroleum. When the permeability barriers is a structural feature, the reservoir is considered structural traps Fracture and faults are important on an inter-well scale where they control the movement of both injected and naturally occurring reservoir fluids and may significantly affect the production of hydrocarbons.
Examples in Pakistan: Samber Formation is source rock of deltaic environment. Goru Formation is a reservoir rock of deltaic environment. Its upper part acts as a seal rock.