Seismic Facies Analysis

Slide 1

• The second part of stratigraphic interpretation focuses in on the prediction of rock types (lithofacies) within key sequences
• We start we either 2D or 3D seismic data
• We identify and map the major sequence boundaries using reflection terminations
• Then we perform what is referred to as seismic facies analysis
• Our goal is to predict where we have potential reservoirs capped by potential seals
• We may also want to identify regions that have good source rock potential

Slide 2

Some definitions of terms:

• A seismic facies unit – is a mappable, three dimensional seismic unit composed of groups of reflections whose parameters differ from those of adjacent facies units
• Seismic Facies Analysis – is the description and geologic interpretation (environmental setting, lithofacies, etc.) of seismic reflection parameters

Slide 3

This slide lists 4 types of features that we can use in our mapping:

1. Seismic amplitude ….
2. Reflection geometry …
3. Reflection continuity …
4. Wavelet frequency …

Slide 4

This slide shows a 'road map' for stratigraphic analysis

• First we perform seismic sequence analysis to map sequence boundaries using reflection terminations
  • This subdivides the sediment fill into genetically related units – depositional sequences
• Second, we do seismic facies analysis, which has two elements
  • We use a technique to capture geometric information
  • We can also extract seismic attributes, any of a variety of measures of the seismic reflections
  • A commonly used seismic attribute is reflection amplitude, another is reflection continuity
• We combine the geometric information with the seismic attributes to predict the environments of deposition (EODs) that influenced the types of deposits (e.g., a beach where clean sands may have been deposited)

On the right, we have a portion of Line C from exercise 11a

• We talked about the shelf-edge carbonate build-up "reef"
• Below the seismic image is a map view for the youngest Jurassic depositional sequence
• The dark blue is the location of the "reef"
• To the right (landward) is a carbonate shelf
• To the left (basinward) is a carbonate slope

Slide 5

• We need a method to make geometric observations and post them on a map before we can make predictions
• The method we use is called the ABC method
• Although it looks like a formula, it is more a template for recording observations

• The A term is the type of termination pattern at the top of the sequence
  • erosional truncation, toplap, or concordance, i.e., no terminations
• The B term is the type of termination pattern at the base of the sequence
  • onlap, downlap, or concordance, i.e., no terminations
• The C term is the internal reflection pattern, e.g., parallel
• The next few slides show some common internal reflection patterns

• A way to remember the code is to think:
  • A = Above; B = Below; C = Center

Slide 6

• This slide shows a hierarchical method to classify internal reflection patterns
• First, do you think the reflections indicate stratification patterns?
  • If they do, is the stratification simple, progradational, or complex in appearance?
  • If not, the internal reflection pattern may be chaotic – complex structure/stratigraphy or poor seismic imaging
    • or it may be reflection free – which could be due to a nearly homogenous interval (massive sand, salt, etc)
    • or it could be an imaging problem
• The next slides illustrate the terms on the lower half of this diagram

Slide 7

• Simple stratified means the seismic reflections are parallel, sub-parallel or divergent
• The spacing (thickness) between reflections are relatively uniform
• A few words on the divergent pattern
  • In the diagram, there are fewer reflections on the left side than the right
  • The shorter reflections terminate internally, not at the top or base
  • This is probably due to seismic resolution issues
  • The internal layers are thick enough on the right that tops and bases are imaged
  • But moving to the left, units thin and some get too thin for the seismic data to fully resolve
  • Thus the reflections terminate internally as the total sequence thins right to left

Slide 8

• This slide focuses in on progradational internal patterns
• This is commonly associated with deltaic-types of deposits that build out a shelf
• Sigmoid and oblique are two variations
  • Sigmoid exhibits a combination of upbuilding (or aggredation) and outbuilding (or progradation
  • Oblique exhibits outbuilding without upbuilding
  • Oblique is characterized by toplap terminations at the top of the sequence
• Sigmoid and oblique represent outbuilding into relatively deep water; 200 meters of water depth or more
• Shingled is a progradational pattern in a shallower basin or on a relic shelf
  • There is not enough paleotopography for progradation to develop at a scale obvious on typical seismic data

Slide 9

• Complex internal configurations show significant thickening and thinning
• Mounded shows thinning in at least two directions from a central thick
  • mounds are common in carbonate environments and in association with deep sea fans
• Hummocky shows a complex pattern of thickening/thinning
• Deformed indicates that there has been some post-depositional forces that have partially disrupted the layering
  • The example shown in the lower right is what might exist where some down-slope creep has occurred

Slide 10

• We will code this cartoon section (Line B) to illustrate the ABC method
• Animation shows the steps
• Starting on the left, the first code would be – concordant at the top, onlap at the base, and parallel internally
• This code is good until we start to observe toplap
• What is the code for the second unit? Toplap, Concordant, Oblique
• The third zone? Toplap, Downlap, Oblique
• The fourth zone? Concordant, Downlap, Parallel (or oblique)
  • Not everyone would code every segment the same, which is not a problem
  • Once the codes are posted on a map and interpreted – that is what is important
• The last zone? Concordant, Concordant, Parallel

Slide 11

• We have placed the codes for Line B on the map – we offset the codes to help the illustration
• We also have the codes for Line C (similar string of codes) and Line A (different codes)
• Note where Line B and Line A intersect – see a problem?
  • Line B has Oblique while Line A has Parallel
  • This is not necessarily a conflict
  • Progradation has a dip and a strike component
  • Line B is more of a dip orientation while Line A is more of a strike orientation
  • With these two codes we can conclude the progradation is primarily west to east

Slide 12

• After we post the observations for each line, we have to synthesize the geometric observations
• In this example, there is a zone of oblique progradation to the ESE near the center of the area
• Landward there is an onlaping wedge
• Basinward where is a sheet (C–C/P)

Slide 13

• We are now ready to interpret the EODs – Environments of Deposition
• From left to right we have:
  • A fluvial/non-marine wedge that thins by onlap to the west (tan)
  • A nearshore/marginal marine zone (characterized by toplap) (yellow)
  • The paleo-slope at the end of this depositional period (concordant at top) (green), and
  • The basinal deposits out in deep water (blue)

Slide 14

• Given these interpreted EODs, we can think about the types of depositional bodies we should have
• We use depositional models based on modern analogues, ancient outcrops and/or laboratory experiments
• If we were to drill in the marginal marine (yellow) area of the map on the last slide, we would expect (top-down)
  • Delta front sands
  • Delta front silts
  • Prodelta shales
  • Offshore clays
• We can even anticipate a vertical (and lateral) facies succession as shown on the right
• We can even use geo-statistics to predict, for example, the thickness and lateral extent of channel sands or crevasse splays

Slide 15

• It is time for another exercise
• The data comes from an area in East Texas
• Together we will code Line 1

Slide 16

• Here is line 1 with our interval of interest highlighted
• Some red lines have been added on the peaks (blacks)
• There are two ABC zones – boundary near the yellow vertical line
• What is the code for the first zone – Tp-Dn/Ob (animation)
• What is the code for the second zone – C-Dn/P (animation)

Slide 17

• We have given you the base map with the codes posted for the Woodbine interval - our zone of interest
• Use this map to:
  1. synthesize the observations
  2. predict depositional environments
• Note that some lines have depositional limits for the Woodbine and this sequence is NOT present on the line to the far west

GIVE STUDENTS SOME TIME TO WORK THE EXERCISE

Slide 18

EXERCISE SOLUTION

• This is the synthesis of the observations
  • Brown is C-On/Thin
  • Yellow is Tp-Dn/Ob
  • Green is C-Dn/P

Slide 19

Here are the interpreted depositional environments

• Brown is fluvial and upper delta plain – fluvial sands and shales
• Yellow is lower delta plain – well sorted sands with some silts
• Green is delta slope – shale with minor silt