|Dictionary entry for depositional system|
|A depositional system is a three-dimensional assemblage of lithofacies formed within a particular environmental setting (e.g. alluvial, deltaic, eolian, fluvial, lacustrine, marine, and systems).|
- 1 Overview
- 2 Types of depositional systems
- 3 Applications
- 4 References
- 5 See Also on SEG Wiki
- 6 Additional Resources about Depositional Systems
The concept of a depositional system stemmed from the field of sequence stratigraphy in the late 1970s. During this time, the methodology was focused towards analyzing sedimentary packages not only for their physical properties but also for their associated depositional environments and processes .
A depositional system is a three-dimensional assemblage of sandy or clay-rich facies formed within a particular depositional environment. A depositional system has a definite geometry (e.g. channel geometries are associated with fluvial depositional systems) and has specific sediment transport processes associated with it. Depositional systems are named for their associated depositional environment; common depositional systems are fluvial, marine, eolian, lacustrine, and alluvial. In general, the term depositional system can be applied to a the large scale depositional system of a region as well as the smaller scale systems within that region. A coastal depositional system, for instance, might include deltaic, tidal, and shallow marine systems .
The sediments associated with the various depositional systems are eventually buried and lithified into three-dimensional rock bodies. Core, well-log, and seismic data are commonly used to assess the shape and lithological properties of sedimentary rock bodies and identify the depositional systems associated with their formation . This analysis has important implications for hydrocarbon exploration. Within a basin, the geometric and physical properties of each depositional system can inform and contextualize a hydrocarbon source, migration, reservoir, trap, and seal .
Types of depositional systems
The primary depositional systems are alluvial, deltaic, eolian, fluvial, lacustrine, and marine . These are broad terms that usually apply to large regions of sedimentary successions. These broad classifications can include several depositional styles and an assemblage of different facies. Within a region, it is also acceptable to describe smaller scale depositional systems, such as a tidal flat depositional system. In this case, there is one depositional style acting in this realm and only one or two facies may be associated with this system .
The depositional systems represented in sedimentary rocks are determined from the shape and physical characteristics of the rock bodies. For a eolian depositional system, one would expect to find well-sorted sand in drill cores and low gamma log values. It is possible to image the stoss-and-lee topography associated with lithified dunes with high resolution seismic data (see Seismic geomorphology for more information).
It is important to identify the depositional systems related to a sedimentary package for the purpose of oil and gas exploration. Depositional system analysis can provide porosity and permeability information for sedimentary units. Depositional systems also provide context for hydrocarbon sources, migration, reservoirs, traps, and seals. Depositional systems with clay-rich facies can eventually serve as hydrocarbon sources and seals, while depositional systems with sandy facies are potential reservoirs  . Some depositional system geometries, like systems associated with eolian dunes, incised valleys, and barrier islands, are ideal stratigraphic traps .
Below is a list of clastic depositional systems that are commonly considered for oil and gas exploration; there are many other depositional systems not listed below. For more information, refer to the Additional Resources section at the bottom of this page.
- Anastomosing and braided stream systems  
- Meandering stream system  
- Fluvial plain system 
- Incised valley system 
- Crevasse splay system 
- Distributary channel system 
- River-dominated delta system 
- Wave-dominated delta system 
- Tide-dominated delta system 
- Fan delta system  
- Prodelta system 
- Delta-front mount bar system
Other coastal systems
- Tidal channel system 
- Tidal flats system 
- Barrier island system 
- Shoreface/intertidal sand body system   
- Coastal plain system 
A wide array of data sets and analytical techniques can be used to identify past depositional systems associated with a particular assemblage of lithofacies. Both 2-D and 3-D seismic are used to assess the general depositional style and and geometry of these preserved systems. Attribute analysis, with 3-D seismic data, is particularly useful for imaging unique geometries associated with certain depositional environments. For example, channel-related depositional systems can be clearly imaged in map view with the coherence attribute     Well-log and core data provide additional details, such as grain size distribution, porosity, and permeability     . Together, these three main data types allow interpreters to describe the past depositional systems associated with a particular basin or region  . The following examples are focused primarily on depositional systems associated with sandy facies that are verified or potential hydrocarbon reservoirs.
Examples of depositional system analysis
Lake Maracaibo, Venezuela 
This study focused on using well-log and 3-D seismic data to describe the geomorphology and lithofacies of Miocene units in Venezuela. The study uses a workflow termed "seismic sedimentology", which makes use of particular seismic attributes and well-log data. After this analysis, these Miocene units include braided stream, meandering stream, lacustrine delta, coastal plain, lagoon, and fan delta systems. The sandier facies associated with some of these depositional systems are potential hydrocarbon reservoirs. See figure showing stratal slice with fluvial channels and other depositional systems.
Gold River North Field 
A high-resolution depositional system analysis was carried out on the upper Cretaceous units in the Gold River North field in South Texas. The large scale depositional system for the region is characterized as a shelf-edge deltaic system. This study identifies the sediment transport processes associated with these Cretaceous units. Several smaller scale depositional systems are found within the overall shelf-edge deltaic framework, including fluvial plain, fluvial channels, crevasse splay, and deltaic plain, deltaic-front mouth bars, prodelta, and incised valley systems. The incised valley and distributary channel systems are possible stratigraphic traps, while the deltaic system sandstones are known gas reservoirs. Five interpreted stratal slices are shown in the bottom figure with various depositional systems outlined.
Niger Delta Basin 
The hydrocarbon potential of the Niger Delta Basin is investigated with seismic attributes and well-log data. These coastal system sand bodies in this region were investigated with the Iso-frequency, Chaos, and Root Mean Square Amplitude attributes. Using these tools, this study describes five depositional systems: fluvial, delta, distributary channel, tidal channels, and shoreface systems. The shaley units associated with the shoreface and deltaic systems constitute the hydrocarbon sources and seals for this region, while the sandy units associated with the distributary channel system serve as reservoirs.
- Galloway, W.E., 1998. Clastic depositional systems and sequences: Applications to reservoir prediction, delineation, and characterization. The Leading Edge 17, 173–180., doi:10.1190/1.1437934.
- Klein, G. de V., 1985. Focusing on SEG Continuing Education: Sandstone depositional systems: a primer for geophysicists. The Leading Edge 4, 96–97., doi:10.1190/1.1439123.
- Li, X., Zeng, H., Zha, M., 2011. Mapping depositional systems using seismic sedimentology: a case study of Gold River North field, Webb County, South Texas, in: Beijing 2009 International Geophysical Conference and Exposition, Beijing, China, 24–27 April 2009. pp. 286–286., doi:10.1190/1.3603811.
- Zeng, H., Ambrose, W.A., Villalta, E., 2001. Seismic sedimentology and regional depositional systems in Mioceno Norte, Lake Maracaibo, Venezuela. The Leading Edge 20, 1260–1269., doi:10.1190/1.1487259.
- Onayemi*, J., Oladele, S., 2014. Analysis of facies and depositional systems of ‘Ray’ Field, onshore Niger Delta Basin, Nigeria, in: SEG Technical Program Expanded Abstracts 2014. pp. 2724–2728., doi:10.1190/segam2014-0019.1.
- Langhi, L., 2013. 3D seismic stratigraphy and seismic attributes analysis: a powerful approach to maximise the characterisation of Palaeozoic depositional systems (Australian Northwest Shelf), in: ASEG Extended Abstracts 2004: 17th Geophysical Conference. pp. 1–3., doi:10.1071/ASEG2004ab084.
See Also on SEG Wiki
- Introduction to 3-D seismic exploration
- Seismic attributes
- Seismic geomorphology
- Seismic stratigraphy
- Sequence stratigraphy
- Stratigraphic interpretation
- Structural interpretation