Deepwater

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Deepwater

Also known as turbidites, the deepwater term refers to sediments that were deposited in water depths considered to be “deep,” i.e., those under gravity-flow processes and located somewhere in the upper to middle slope region to the floor of a basin, beneath storm wave base.[1]. In the early years, the studies of the deepwater deposits were more sedimentological, and outcrop focused with the aim to understand the processes and origin of these deposits. But the understanding of deep-water depositional systems has advanced significantly in recent years [2] especially because of the improvement in seismic Dictionary:resolution and the discovery of new technologic approaches that facilitates seismic interpretation and reduction of computing time. This has been led by the interest in deepwater reservoir exploration and production and consequently, the acquisition of data e.g. cores, well logs, 3D and 4-D seismic method

Figure 1. Shallow to deep marine deposits


Definition of deepwater

Geological definition

  • Clastic sediments transported beyond the shelf edge into deep water by sediment-gravity-flow processes
  • Deposited on the continental slope and in the basin
  • They are later buried and become part of a basin-fill

Engineering definition

  • Drilling a well offshore into a basin fill in present-day water depths >500 m (1640 ft) above the mud line (ocean floor)
Figure 2. Architectural elements and their common location on deepwater settings [1]

Elements of deepwater systems

The 'architectural-element' approach provides a systematic means for describing sedimentary systems, organizing observations and measurements for diverse settings. It is becoming common to use this scheme for the description and classification of complex deepwater systems [3] The different types of depositional units and/or architectural elements common to deepwater sedimentary settings include lithofacies assemblages and their geometries, vertical profiles, and other internal and external characteristics that occur repeatedly and are often predictable [3].

In deepwater settings the most common architectural elements are the following:

  • Channels
  • Levees
  • Sheets (sheet sands)
  • Mass transport deposits (MTD)
  • Overbank deposits and splays
Figure 3. Deepwater architectural elements reservoir performance [1]

Architectural elements and Production

Deepwater reservoir systems produce from different architectural elements [1]. The percent of production from these architectural elements varies greatly from basin to basin. Findings from several studies reveal that around 60% of Gulf of Mexico production comes from sheet sands, 25% from channel-fill deposits and 15% come from levees. Lithology, internal arrangement and formation conditions of these elements make them the most prospective (Figure 3). There are exceptions to the rule in areas like the North Sea where the reservoir quality is linked to the age of deposition, producing from different deepwater elements. The conditions in the formation of these are linked to variables that modify their composition, areal extension, and dimension. For example, Jurassic systems are coarse-grained, Cretaceous systems are sand prone and become more mud-rich towards the Oligocene. Also, architecture changes from more widespread amalgamated channels in the Paleocene to more incised and narrower in the Eocene. In addition to this, the concept of petroleum system and migration must be taken into account to evaluate the likelihood of a deepwater element to become a good reservoir.

Figure 4. Scheme of confined and unconfined systems. Modified from [1]

Processes

Deepwater deposits are known as turbidites for being produced mostly by turbiditic processes (turbidity current = sediment and seawater mixture that has higher density than pure seawater). Since sediment gravity flow processes also are present in lakes (shallow water) is preferable the term deepwater systems to refer to marine sediment gravity flow processes, environment and deposits [1] The two major processes involved in the formation of deepwater deposits are known as intrabasinal and extrabasinal (Figure 4):

Intrabasinal processes

They originate in the marine environment; all are mixtures of sediment and seawater

  • Ignitive
  • Episodic
  • Sporadic
  • Mainly triggered by submarine earthquakes
Figure 5. Architectural elements and related system tracts. Note MTD's are developed in FSST and HST considering a carbonate-siliciclastic system [4]

Extrabasinal processes

Originated outside the marine environment

  • Nonignitive flows
  • Quasisteady
  • Longer duration
  • Mainly caused by rivers carrying sediment beyond the shoreline

Sequence Stratigraphy of Deepwater deposits

Sequence stratigraphy is the study of sedimentary rocks within a temporal framework. Vail et al [5]. defined a depositional sequence as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities.” Depositional sequences can be mapped using seismic, wireline logs, and outcrop data.

Figure 6. Deepwater seismic facies offshore Nigeria.[6]

In deepwater deposits sequence stratigraphy, the premise is that marine sedimentary rocks were deposited in a series of cycles that result from the relative fall and rise of sea level. The depositional pattern can vary widely from basin to basin, depending upon variations in sediment supply and basin tectonics [1].

For exploration, sequence stratigraphy establishes the chronostratigraphic framework for the basin, thus determining the relationships between shallow marine and continental sediments (highstand- HST) and subsequent deep marine deposits (lowstand- LST). Sequence stratigraphy can also help predict the distribution of producing elements (Figure 5)

Seismic expression of Deepwater systems

Three-dimensional seismic data afford an unparalleled view of the deep-water depositional environment, in some instances with vertical resolution down to 2–3 m. Seismic time slices, horizon-datum time slices, and interval attributes provide images of deepwater depositional systems in map view that can then be analyzed from a geomorphologic perspective. Geomorphologic analyses lead to the identification of depositional elements, which, when integrated with seismic profiles, can yield significant stratigraphic insight. Finally, calibration by correlation with borehole data, including logs, conventional core, and biostratigraphic samples, can provide the interpreter with an improved understanding of the geology of deep-water systems [7]. Seismic data allows for the study of deepwater deposits succession and evolution vertically and aerially if tools like seismic attributes and horizon slicing are performed. Even detailed sequence stratigraphy studies can be performed with the use of well log data. Three among the myriad examples currently existing on the seismic expression of deepwater system studies are presented to show some of the advantages in the use of seismic data for this type of study.

In Figure 6, a table with a collection of seismic expressions of deepwater seismic facies identified by Hansen, L [6] on his study offshore Nigeria is presented.

Figure 7. The images on the right show the interpretation performed with well-logs where 8 aggradational and progradational cycles were identified. Sequences 1 to 4 correspond to progradational sequences deposited in outer shelf, slope and basin floor environments during the Paleocene to the Eocene. High frequency fourth order sequences were deposited later from the Oligocene to the Miocene in progradational wedges of deepwater deposits. The lateral continuity of these high frequency cycles is limited due to constant erosion by channels coming from the shelf carrying sediments down to the basin floor. This lateral variation for the interpreted sequences corresponds to changes in the compensated depositional style. Sequences 7 and 8 are thicker at northern locations and thinner to the south possibly because of the influence of paleo-topography during sedimentation. [4]

An example of an interpretation of sequences in seismic data is presented in Figure 7. Where Tellez et al,[4] with the use of well logs and application of seismic attributes to the seismic volume defined the sequence stratigraphic framework for Cenozoic strata at the Rankin platform sub-basin, north Carnarvon basin, Australia.

Figure 8. Plane evolution relation of the O73 channel system [8]

In the work of [8] the evolution of a deepwater channel complex in the M oilfield in West Africa has been studied and interpretations are shown in time and vertical slices.


See also

External links

  1. http://www.sepmstrata.org/page.aspx?pageid=39
  2. http://www.searchanddiscovery.com/documents/2011/30214mutti/ndx_mutti.pdf
  3. http://www.sepmstrata.org/page.aspx?pageid=41

References

Contribution from The University of Oklahoma

This page is currently being authored by a student at the University of Oklahoma. This page will be completed by December 1 2019.

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Slatt, R., Weimer, P., 2004, Petroleum systems of deepwater settings. Society of Exploration Geophysicists
  2. Posamentier, H., and Kolla, V., 2003, Seismic geomorphology and stratigraphy of depositional elements in deep-water settings. Journal of sedimentary research 73.3 : 367-388. https://doi.org/10.1306/111302730367
  3. 3.0 3.1 Sprague, A. R., Patterson, P., Hill, R., Jones, C., Campion, K., Van Wagoner, J., Sullivan, M., Larue, D., Feldman, H., Demko, T., Wellner, R., Geslin, J., 2002, The Physical stratigraphy of fluvial strata: A hierarchical approach to the analysis of genetically related stratigraphic elements for improved reservoir prediction. AAPG annual meeting
  4. 4.0 4.1 4.2 Tellez, J.J., Slatt, R., 2017, Seismic Geomorphology and Characterization of Deep Water Architectural Elements and its Applications in 3D Modeling: A Case Study in North Carnarvon Basin Australia. Search and Discovery Article #42097 http://www.searchanddiscovery.com/documents/2017/42097tellez/ndx_tellez.pdf
  5. Vail, P., 1987, Seismic stratigraphy interpretation using sequence stratigraphy, Part 1, in A.W. Bally, ed., Atlas of Seismic Stratigraphy: AAPG Studies in geology 27, p. 1-10
  6. 6.0 6.1 Hansen, L., Janocko, M., Kane, I., Kneller, B., 2017, Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria. Marine Geology 383. 10.1016/j.margeo.2016.11.011
  7. Posamentier, H., and Vail, P., 1988, Eustatic controls on clastic deposition II-sequence and systems tract models: SEPM Spec Pub. 42, p. 125- 154
  8. 8.0 8.1 Zhang, W., Zhong, T., Liu, Z., Liu, Y., Zhao, L., Xu, R., 2017, Architecture mode, sedimentary evolution and controlling factors of deepwater turbidity channels: A case study of the M Oilfield in West Africa Petroleum Science. Volume 14, Issue 3, pp 493–506| https://link.springer.com/article/10.1007/s12182-017-0181-2.