Salt kinematics

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Salt kinematics is the study of how salt structures are made and deformed through various kinds of forces. Salt can cause massive structures within the Earth, and can be crucial to finding hydrocarbons. Imaging salt in the subsurface can be quite a challenge. Salt often is associated with weak reflections, and makes it hard to interpret between salt and other sediments within the Earth. Various seismic processing techniques and methods have been made to better image subsurface salt. New techniques are still being invented to interpret salt in seismic today to better image salt in seismic data.

Background Geological Salt Kinematic Information:

Salt can be a tricky process to image in seismic. To better interpret salt bodies in seismic, geologic information needs to be integrated in order to accurately image salt. Salt and sediment interactions can cause reservoir traps and hydrocarbon traps which are very beneficial to oil and energy companies[1]. Salt bodies are generally formed in tabular sheets composed of either halite or gypsum. These tabular sheets are formed by tectonics to form them into salt bodies. Regional extension deformation is a common tectonic force that cause the movement of salt into diapirs[2]. These diapirs in deep water can be up to 6 kilometers in thickness[1]. A geologic feature associated with salt bodies are called sutures. When two salt sheets combine into one it is called an allosuture. When sutures form between two lobes of the same salt sheet it is called an autosuture[3]. There are 3 different phases when it comes to salt tectonics which are reactive, active, and passive (Figure 1). Reactive is when regional extension occurs and the buoyant salt rises to fill in the voided space. Active occurs when salt structures rise and break the overlying strata due to thinning from the regional extension. Passive occurs when the salt body is exposed on the surface[3]. Knowing where the salt is being supplied from can make for a better seismic interpretation. Also knowing the geological structures that are below the salt can help give a better insight to the evolution of the salt body[4].

Figure 1 - (Photo presented by Experimental studies of the controls of the geometry and evolution of salt diapirs (Karam and Mitra)

Seismic Imaging of Salt Kinematics

Imaging salt can be a very tricky process due to salt having around the same acoustic impedance as other sediments, so it be can be hard to distinguish between salt layer reflections and sediment layer reflections. Only using seismic to image salt can lead to inaccurate interpretation unless geological information is integrated into the interpretation[4]. There is multiple methods to imaging salt such as Pre Stack Depth Migration (PSDM), Two Way Time Structure Maps (TWT), Kirkchoff migration, Wave Equation Migration, and RTM Imaging.

Pre Stack Depth Migration (PSDM)

Sequence boundaries of salt can be determined by three different categories including biostratigraphic data, well logs, and seismic reflections. Before starting to model the salt body the interpreter should check the pre-processing parameters and the geology of the region[4]. Mapping the salts formation tops and base can help interpret the flow of hydrocarbons due to the movement of salt. Pre-stack depth migration provides better imaging of salt complexes and subsalt geology. The geology underneath the salt is crucial to give insight to the evolution of the salt body. PSDM shows strong differences between the salt and other sediments velocities. This in return provides better velocity models for processing seismic. Salt penetrating wells can be drilled to improve the velocity model, however if the salt body drilled is small then that velocity should not be used due to it not representing the entire salt body. Deep water wells can additionally be correlated with the pre stacked data in order to provide a better resolution of the salt imaging. A workflow for mapping deepwater salt with PSDM  starts with removing the salt velocity to create a sediment velocity model. The salt velocities are found from interpolated well logs that have been drilled through the salt structure. The sediment velocity is then combined with the picked formation top to create a salt body. The seismic data between the formation top and base is generally disorderly (Figure 2). This information can help interpolate the base of the salt with the new salt velocity model. The depth gathers can periodically checked in order to maintain consistent velocities. If the velocities are flat then the velocity picks are correct. If the velocity picks are too fast then the gathers will curve up. If the velocities are too slow then the velocities will curve down[1]. This well help make the velocities between the salt top and base reliable to use. [[Anisotropic migration]] of the final salt velocities will make the subsalt geology even more accurate[4].

Figure 2 - (Seismic Data of Salt Strucutre contributed by Simon Stewart in 2008)

Two Way Time Structure Maps (TWT)

Two Way Time Structure Maps are used to define the salt structures by picking horizons along the diapir at different depths. The variance volume attribute can help make the salt bodies more apparent due to the attribute outlining fault surfaces and steeply dipping angles. This is computationally done by “comparing trace by trace variability in acoustic impedance”[2]. Going through different thickness maps varying in depth from the top of the salt to the base of the salt identifies the different parts of the salt body The top picked horizons are the youngest, and looking through the older surfaces at greater depths determines how the diapir evolutionized into the shape it is today (Figure 3)[5]. This gives clear indication of the formation of the top and base of the salt structure. A cross sectional iew of the two way time map can show the dip angles of the salt diapir[2].

Figure 3 - (Two Way Time thickness maps credited to Harding and Huuse 2015 and Huuse)

Kirkchoff Migration and Wave Equation Migration

Kirkchoff Migration has a better subsalt resolution due to it being more versatile with the velocity model’s accuracy.

The Wave Equation Migration is affected more by the inaccuracy of the velocity model which results in worse subsalt resolution imaging.

Gravity surveys can be used to also determine information about the salt body which might be beneficial in knowing when interpreting the salt movement. If the residual gravity values are low then the salt body formation is thick with a low density causing it to be buoyant[4].

Figure 4 - (Photo presented by CGG: Dual Flood RTM by Li, Dy, and Agnihotri)

Reverse Time Migration (RTM Imaging)

RTM is an algorithm that is favored to use when mapping complicated salt structures[6]. The algorithm consists of a two wave equation. RTM uses image partition gathers in order to obtain a better image of the salt. When using RTM the first step is to build the model of the Salt Body [6]. Dip angles are then recorded from the Vector Image Partition (Figure 4). The image is then clarified by having several traces added up over several shots. RTM is the most advanced technique for imaging salt, however there are many leading researches to improve how salt is mapped[6]

External Links

  • [1] - Diapir
  • [2] - Amplitude
  • [3] - Acoustic Impedence
  • [4] - Seismic Processing

References

  1. 1.0 1.1 1.2 Borton, Laura, & Abu, Chodhury (20087). Regional Geology of deepwater salt architecture: New plays in the GOM. World Oil May 2007 issue, pgs 93-96.
  2. 2.0 2.1 2.2 Harding and Huuse 2015, R., & Huuse, M. (2015). Salt on the move: Multi stage evolution of salt diapirs in the Netherlands North Sea. Marine and Petroleum Geology,61, 39-55. doi:10.1016/j.marpetgeo.2014.12.003
  3. 3.0 3.1 Dooley, T. P., Hudec, M. R., & Jackson, M. P. (2012). The structure and evolution of sutures in allochthonous salt. AAPG Bulletin,96(6), 1045-1070. doi:10.1306/09231111036
  4. 4.0 4.1 4.2 4.3 4.4 Zhang, Q., Chang, I., & Li, L. (2009). Salt interpretation for depth imaging ‐ where geology is working in the geophysical world. SEG Technical Program Expanded Abstracts 2009. doi:10.1190/1.3255627
  5. Jackson, M. (1994). Structural Dynamics of Salt Systems. Annual Review of Earth and Planetary Sciences,22(1), 93-117. doi:10.1146/annurev.earth.22.1.93
  6. 6.0 6.1 6.2 Obriain, M., Smith, D., Montoya, C., Burgess, B., Koza, S., Zdraveva, O., . . . King, R. (2014). Improved Subsalt Imaging and Salt Interpretation Using Reverse Time Migration Scenario Testing and Image Partitioning. Proceedings 76th EAGE Conference and Exhibition 2014. doi:10.3997/2214-4609.20141342