Lithofacies and environmental analysis of clastic depositional systems
|Development Geology Reference Manual|
|Series||Methods in Exploration|
|Chapter||Lithofacies and environmental analysis of clastic depositional systems|
|Author||Mark H. Scheihing, Christopher D. Atkinson|
The geological and reservoir properties of sedimentary rocks depend upon an interplay of tectonics, sea level, sediment supply, physical and biological processes of sediment transport and deposition, and climate. At the basin scale, these processes interact to produce the geometric arrangement of different depositional environments or systems tracts through time, known as the stratigraphic architecture of the basin. At smaller scales, these processes control the external geometry and internal “anatomy” of clastic sediment bodies (see Geological heterogeneities). It is at this smaller scale that lithofacies analysis and interpretation of depositional environments become important for reservoir evaluation.
- 1 Data requirements
- 2 Rock description
- 3 Clastic depositional lithofacies and environments
- 4 See also
- 5 References
- 6 External links
Basic data requirements for facies analysis of subsurface rocks are listed in Table 1. Data associated with wells are most often used, but seismic data, particularly three-dimensional data, are becoming increasingly important in defining sandstone body geometries. Conventional core is perhaps the most diagnostic for sedimentological interpretation of vertical sequences (see Core description). However, wireline tools such as dipmeters and formation imaging devices can provide electrical images suitable for sedimentological interpretation with the added ability to determine paleocurrent directions in appropriate cases.
|Slabbed conventional core||Facies, depositional environment|
|Oriented conventional core||Paleocurrent directions|
|Core gamma scan||Shift to wireline logs|
|Sidewall cores, well cuttings, thin sections, sand peels||Mineralogy, lithology|
|Paleontology (micro, macro, trace fossils), palynology||Water depth, depositional environment, time lines|
|Dipmeter log||Paleocurrent directions, lithofacies|
|Formation MicroScanner (FMS) log||Paleocurrent directions, lithofacies|
|Spontaneous potential log||Lithology, curve shape analysis|
|Gamma ray log||Lithology, curve shape analysis|
|Sonic log||Porosity, curve shape analysis|
|Caliper log||Borehole condition, quality control|
|Neutron log||Porosity, lithology (with sonic, density)|
|Density log||Lithology (coal), porosity (with neutron)|
|Repeat formation test (RFT)||Pressure (sand body connectedness)|
Within any given depositional environment, various physical and biological processes act to transport and deposit sediment. These processes result in various distributions of grain size and sedimentary structures that characterize the deposited sediment. Relating these features back to the processes that produced them is the basic method used by geologists to interpret the depositional environment of sedimentary sequences (Figure 1).
One of the first steps in the facies analysis of a clastic reservoir is the description and interpretation of available conventional core. An important result of core description is the subdivision of cores into lithofacies, defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.
Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.
Depositional models are important for predicting the distribution of permeability and porosity within different reservoir types. These models are never exact matches to a reservoir; rather, they serve as guides to aid in the interpretation of any one reservoir. Reservoir properties are generally observed to be correlative with lithofacies types to one degree or another (see Geological heterogeneities). This reflects the fundamental control on permeability and porosity by grain size, sorting, and spatial distribution of different lithofacies types. Even where rocks have experienced later physical and chemical diagenesis, permeability and porosity relationships are controlled, in large part, by the original sedimentary fabric of the rock.
Wireline log calibration and correlation
Interpretations of depositional environment based on individual well data are transformed into a three-dimensional picture of the reservoir by wireline log correlation and, where possible, by three-dimensional seismic data.
Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see Preprocessing of logging data and Core-log transformations and porosity-permeability relationships) and (2) establishing a relationship between lithofacies associations and curve shape. Core gamma scans, obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.
For reservoirs in which no core is available, wireline log shape must be used to interpret sandstone body type and identify depositional environments. If closely spaced cuttings or sidewall cores are available, these can sometimes aid rock to log calibration. Log shapes are deduced from the expected wireline log response of the different environments combined with a knowledge of the paleogeography of the area in which the field is situated. Wireline log shapes are often described as “upward coarsening,” “upward fining,” or “blocky.” However, log shape as determined from a gamma ray or SP log in siliciclastic rocks is related more to argillaceous content than to grain size. Upward coarsening log patterns exhibit an upward decrease in argillaceous content. Upward fining log patterns exhibit the reverse trend. Blocky or cylindrical log patterns exhibit relatively little vertical variation in argillaceous content and are typical of siliciclastic rocks that have low overall argillaceous content. Various publications and reference charts are available to aid in this practice (e.g., Spearing, Cant, and Rider). However, without core control, curve shape analysis is fraught with hazards (e.g., Snedden; also see Quick-look lithology from logs).
Correlation sections that will be used for establishing sandstone body geometry should have a depositionally flat datum (such as a bentonite bed, marine shale bed, or laterally persistent limestone). Sections should be oriented parallel and perpendicular to depositional strike, if known, and represent as straight a line as possible given well density and placement.
The only sedimentologically significant correlation horizons are those that approximate time lines within and between sandstone bodies. This style of correlation requires an understanding of the succession of depositional environments and intervening unconformable surfaces. It often leads to nonparallel and nonhorizontal correlations. For example, in shoreface systems, time lines denoted by shale or silt breaks between shingled shoreface sheets and lenses are inclined in a seaward (depositional dip) direction (Figure 2).
This imbrication does not occur in a strike direction. This style of correlation is especially important for reservoir delineation since the large scale (interwell and field) architecture of the sandstone body exerts a control on the movement of fluids through the volume of the reservoir.
Wireline log correlation is an exercise in pattern recognition combined with the geometry suggested by the interpreted depositional environment(s). The depositional interpretation of a reservoir exerts a major impact on the methods and style of correlation and mapping. For example, correlation in fluvial systems applies very different assumptions about sandstone continuity than does correlation in shoreface or shelf systems.
Clastic depositional lithofacies and environments
Clastic depositional environments range from alpine to abyssal settings (Figure 3 and Table 2). Detailed reviews of these are given by Galloway and Hobday, Walker, Berg, Reading, Beaumont and Foster, and others. The following review is a cursory summary of the origin, lithofacies, geometry, and reservoir properties of major clastic environments and deposits. The reader should be aware that the remarks offered here for each depositional environment are necessarily of a highly generalized and idealized nature. Siliciclastic reservoirs are typically composed of multiple bodies deposited (and eroded) through time under varying tectonic, sea level, and climatic conditions. Corresponding geometry, vertical sequence, wireline log character, and reservoir quality trends for a given reservoir may be, and often are, different from the generalized “single environment” models. In addition, subsequent diagenesis (see Evaluating diagenetically complex reservoirs) may alter the permeability and porosity structure created by depositional (and erosional) processes. However, it has been often observed that in siliciclastic rocks, diagenesis generally follows depositional fabric . Complex structural patterns can reduce reservoir continuity as well.
Major clastic depositional environments
- Alluvial sediments
- Alluvial fans
- Fan deltas
- Braided rivers
- Meandering rivers
- Lacustrine sediments
- Lake deltas
- Freshwater lakes
- Saline lakes
- Eolian sediments
- Interdune deposits
- Sand sheets
- Deltaic sediments
- River-dominated deltas
- Wave-dominated deltas
- Tidally dominated deltas
- Siliciclastic shoreline sediment
- Wave-dominated shorelines
- Microtidal barrier islands
- Mixed wave-tide influenced shorelines
- Mesotidal barrier islands
- Tide-dominated shorelines
- Tidal flats
- Shallow siliciclastic seas
- Tidal sand ridges
- Sand shoals
- Sand sheets
- Deep marine slope and basin
- Slope channel and gully deposits
- Slope canyon deposits
- Intraslope basin deposits
- Sand spillover sheets
- Slope aprons
- Submarine fans
- Glacial sediments
Alluvial fan deposits
An alluvial fan is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.
Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. Gamma ray, SP, and resistivity log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.
Permeability and porosity of alluvial fan deposits vary greatly as a function of depositional process and differential response to diagenesis. In general, streamflow deposits have greater permeability and porosity than debris and mudflow deposits. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, middle and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.
Where alluvial fans prograde into standing bodies of water (that is, oceans or lakes), they are called fan deltas. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by wave and/or tidal processes. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than in more proximal parts of the fan delta because of increased sorting, destruction of compositionally immature grains, and winnowing of fines. Directional permeability trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.
Braided and meandering fluvial deposits
Downdip from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.
Braided rivers and braidplains form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.
Meandering rivers are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (avulsion) and by establishment of a new section in another position on the floodplain.
Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.
The upward-fining character of fluvial channels tends to produce sandstone bodies that have their greatest permeability at the base of the body. However, the common stacking and cross-cutting of channels in both braided and meandering river deposits often produces a complex spatial distribution of permeability within the braided or meander belt. Preferred permeability pathways, and consequently, fluid flow, can be expected to follow the paleochannel direction.
Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits (Figure 3b). Wind transport removes fines and produces rounded and extremely well sorted grains often leading to favorable reservoir quality.
This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits.
Eolian deposits include dune, interdune (marginal to dune complex), and extradune (noneolian) lateral deposits. Dune deposits comprise the major sedimentary bodies in eolian successions. All are characterized by large scale cross-stratification in which foreset dips range up to 35°. Associated deposits may include those of wadi (fluvial), playa (lacustrine), and sabkha (arid tidal flat) origin.
In the subsurface, eolian sandstones generally comprise thickly bedded sequences with few major interstratified shales. The sequences tend to be uniform and lack discernible coarsening- or fining-upward trends and, thus, exhibit blocky to weakly serrated gamma ray, SP, and resistivity log profiles The well-bedded and high angle cross stratified nature of eolian sandstones promotes reliable results from dipmeter logs. Dune and interdune deposits can often be distinguished and paleowind directions inferred using correctly processed dipmeter data.
Eolian sandstones generally comprise excellent reservoir intervals but often possess complex porosity and permeability variations. They are commonly anisotropic with regard to the flow of fluids and exhibit greater horizontal than vertical permeability because of their pronounced lamination.
Deltaic bodies are generally classified into three major categories or end-members on the basis of the dominant sediment transport process that influences their facies constituents and external geometries. These three end-members are as follows:
- River- or fluvially dominated deltas (such as the Mississippi River delta) are those in which wave and tidal energy is low and river transport processes dominate. Sandstone bodies in these systems tend to form well-developed sand bars at the mouths of distributary channels. River-dominated systems periodically abandon their lower course and begin deposition in an adjacent area resulting in the deposition of sandstone bodies over a fairly large area.
- Wave-dominated deltas (such as the Nile and Rhone deltas) are those in which wave energy at the coast exceeds either the fluvial or the tidal energy. Wave reworking causes sand to be formed into shore-parallel bodies that are cuspate at distributary mouths.
- Tidally dominated deltas (such as the Gulf of Papua delta) are those in which the tidal energy exceeds that of either wave or fluvial processes. Sand deposited by the distributaries is reworked by tidal currents into elongate sand ridges that are generally perpendicular to the regional coastline.
Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., Sneider et al.). For more information on deltas, see also Deltaic environments, Delta plain, upper, Delta plain, lower, and Delta plain, subaqueous.
Distributary mouth bars and channel deposits (Figure 3h) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest permeability at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base. Preferred orientation of flow may be expected to follow paleochannel trends.
Distributary mouth bars typically contain a high percentage of interstratified clay that reduces vertical permeability. Hartman and Paynter document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural water drive. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, bypassed oil was accessed by recompleting wells only in the delta fringe interval.
Wave modification acts to winnow delta mouth bar sandstones thus increasing their reservoir quality. In addition, wave processes and longshore currents enhance overall reservoir potential in deltas by redistributing sand as beach and chenier deposits in the interdistributary areas. Tidal reworking can affect reservoir quality at the delta mouth either by acting to winnow fines from the sands or by reducing effective permeability in distributary channels by the introduction of increased amounts of interstratified clay.
Lakes occur in a wide variety of geological settings. They are often very important during the early rifting phase of basin formation on continental crust. Major hydrocarbon-bearing lake deposits are associated with very large and long-lived Tertiary lakes such as those of the western United States, Indonesia, and China. These deposits are characterized by siliciclastic, carbonate, and organic-rich sediments deposited under generally low energy conditions, often by suspension deposition. Other processes include turbidity flows in the lake interior and wave and current reworking along the lake margin.
Lacustrine rocks are generally the source rocks for hydrocarbons found in alluvial fan, fluvial, eolian, and deltaic rocks rather than the reservoirs. However, sandstone bars, beaches, turbidites, and fan deltas associated with lake margins can be reservoirs sourced by open lake deposits. The core and log response characteristics of these deposits are similar to those described from analogous marine environments.
In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate barrier islands and strandplain. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive tidal flats and ebb tidal deltas. A tidal range of 4–6 m (macrotidal) tends to produce estuarine linear tidal sand ridges that are perpendicular to shoreline with associated extensive tidal flats.
Barrier islands (Figure 3f) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.
Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. Gamma ray, SP, and resistivity logs through the barrier core have an upward-coarsening motif (Figure 3f). Logs through the back barrier and lower shoreface are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the barrier inlet may exhibit upward fining.
In general, barrier islands have the best reservoir quality rocks at the top of the sequence. Reservoir quality drops off as one moves either seaward down the foreshore and shoreface into muds of the marine shelf or landward into the [Lagoons|[lagoon]]. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the subtidal area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of bioturbation.
Shallow marine clastic deposits
The marine shelf is an environment affected by storm- and tidal-driven waves and currents and sometimes by oceanic currents. Although shelf sand ridges of either storm or tidal origin formed during transgression are the best known examples (Figure 3g), sand bodies associated with the marine shelf also include reworked delta front and barrier sands, amalgamated storm sheets, and oceanic current deposits.
Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units. In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.
Deep water marine deposits
Reservoir quality sand bodies form on both the continental slope and at the base of the slope. Slope environments include sand bodies formed within submarine canyons and gullies cut into the slope and as spillover sheets. Sands can also accumulate on tectonically formed small basins within the slope itself.
Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure 3i). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.
- Introduction to geological methods
- Reservoir quality
- Carbonate diagenesis
- Geological heterogeneities
- Evaluating stratigraphically complex fields
- Evaluating diagenetically complex reservoirs
- Deltaic environments
- Delta plain, upper
- Delta plain, lower
- Delta plain, subaqueous
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