Seismic Facies Classification

Seismic facies classification refers to the interpretation of facies type from the seismic reflector information. The key elements used to determine seismic facies and depositional setting are bedform internal and external configuration/geometry, lateral continuity, amplitude, frequency, and interval velocity. The classification of seismic facies is an important first step in exploration, prospecting, reservoir characterization, and field development. Classification and interpretation of depositional facies from the chronostratigraphic units can provide initial indication as to whether the area of interest is a viable hydrocarbon system and merits additional research. Furthermore, seismic facies classification can help in the approximation of grain size, sorting, mineralogy, porosity distribution, and permeability of the various deposition units. When combined with open hole logging data, DHI, and advanced processing such as AVO, it is possible to estimate recovery and the potential for an economically viable prospect. In modern seismic interpretation workflows, seismic facies classification is often automated or partially automated using computer algorithms, such as clustering and supervised learning.

Figure 1. Seismic facies interpretation of a prograding shelf in the Santos Basin of Brazil

Workflow

Once a seismic facies unit of interest has been identified, the next step is to attempt to understand the environment of deposition, facies type, and from this infer probable lithofacies. A systemic approach is typically taken to seismic facies classification.

1. Map the major sequence boundaries(note that only 1st and 2nd order boundaries will discernible).
2. Identify chronostratigraphic units, their lapout relationships, and geometries.
3. Consider attributes of the seismic data such as reflection continuity, wavelet frequency, and reflection amplitude.
4. Infer lithology type and extent within facies unit.

Reflection Geometry

Identification of the seismic reflector geometries is the first step towards facies classification and provides information about the depositional processes.

Figure 2. Diagram of various bedform geometries.

Stratified

Simple

• Parallel – Uniform deposition of continuous reflectors.
• Subparallel – Mostly parallel with a small degree of reflector thickening and thinning.
• Divergent – Reflectors disappear internally.

Progradational

• Sigmoidal – S-shapped downlapping character building outward and upward(aggrading) into a basin. Sigmoidal patterns are indicative of fine grained low energy deposition.

• Oblique – Reflectors Toplapping and outward building into a basin.  Oblique patterns are indicative of sedimentation rates and commonly contains clean sand in the upper portion of the bedform.  Oblique bedform types typically require water depths of 500 meters or more.

• Shingled – Thin interval of Toplapping and downlapping reflector geometries outbuilding onto a shelf.  This is similar to the oblique bedform process of deposition but with a shallower water depth. 

Complex

• Mounded – Bi-directional downlapping reflectors with an increasing number of reflectors towards the center on the structure. Adjacent units onlap the mound.  This geometry can be indicative of a carbonate mound or deep water marine fan.   
• Hummocky – Random thickening and thinning of reflectors and can be indicative of shallow marine sands. 
• Deformed – Sliding or slumping of reflectors typically found on a continental slope where once continuous reflectors become discontinuous as they move downward.

Unstratified

Chaotic

• Refers to reflections that that have no continuity with other reflectors in a unit.

Reflection Free

• Having no reflections from internal structures.

Seismic Reflection Characteristics

Figure 3. Diagram showing various attributes used in determining seismic facies.

Identification of the seismic reflection characteristics can aid in understanding lithology type and depositional energy.

Amplitude

Relates to the impedance contrast between beds and provides information about bed spacing, tuning thickness, and to some extent fluid content.

• High amplitude and high lateral continuity indicates low energy quiet water deposition resulting in inter-bedded marine limestones and shales.

• High amplitude and low lateral continuity indicates terrigenous deposition such as inter-bedded channel sands and shales.

• Low amplitude and high lateral continuity can Indicate deep water shales, however, deep water shale often low reflection amplitude contrast making individual reflectors often indiscernible.

Frequency

Relates to the time extent to which a seismic reflection occurred and can provide information about bed thickness and fluid contacts.

Interval velocity

Relates to the wave propagation speed though a given interval and can help to interpret lithology type.

Methods of Classification

ABC method

Published by George Ramsayer in 1979,it involves classifying the lapout relationships and internal configurations according to the coding scheme below.

	Upper Sequence Boundary			Lower Sequence Boundary			Internal Configuraton
Code	Te	Top	C	On	Dwn	C	P	D	c	W	DM	M	Ob	Sig	Rf	Sh
Feature	Erosional Truncation	Toplap	Concordant	Onlap	Downlap	Concordant	Parallel	Divergent	Chaotic	Wavy	Divergent Moundy	Mounded	Oblique Progradational	Sigmoid Progradational	Reflection Free	Shingled

The shorthand nomenclature involves recording of the upper(A) and lower(B) reflector terminations, and the reflection character(C), which is put into an A-B/C template.

These ABC designations are then used to map environments of deposition where like ABC designations are grouped to understand the paleo environments as seen in figure 4. Once an environment of deposition has been identified a lithofacies interpretation can be made through looking at the lithofacies present in analogous environments of the present.

Figure 4. Slides demonstrating the ABC method.

External Links

1. Snedden,J.W., & Sarg, J.F. (2008, January 19) Seismic Stratigraphy-A Primer on Methodology. Search and Discovery Article #40270
2. Futalan, K., Mitchell, A., Amos, K., & Backe, G. (2012, October 2012) Seismic Facies Analysis and Structural Interpretation of the Sandakan Sub-basin, Sulu Sea, Philippines. Search and Discovery Article #30254
3. Ramsayer, G.R. (1979, January 1). Seismic Stratigraphy, a Fundamental Exploration Tool. Offshore Technology Conference. doi:10.4043/3568-MS
4. AAPG Slide Resources - Seismic Facies Analysis