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The Exercises

The exercises below involve the analysis of parasequence using well logs and cores. The parasequences are seperated from each other using major surfaces that include TS (transgressive surfaces), mfs (Maximum Flooding Surface), and SB (sequence boundary). This followed by the examination of the stacking patterns of the parasequences. Finally Fisher diagrams are drawn to help predict the extent of the bounding and interior surfaces of parasequences

An analysis of parasequences should the lead to the interpretation of the depositional settings of the lithologies that are enveloped by the surfaces listed above. Finally the parasequence sets can be matched to global sea level curves in order to identify potential source rocks, reservoirs and seals. A final product is the definition of "zones" or "compartments" whose facies are asssociated with different lithologies which have different characteristics with different vertical and lateral distributions.


Exercise 1 - Introduction to parasequence identification

Click on thumb nails to expand images.

The objective of this exercise is to learn how identify parasequences using previously described lithofacies within a core and well logs

The interpretation process is divided here into 2 steps:

1. Using the well with described lithofacies, identify all parasequences (cycles). As given in the definition of cycles, each shoaling upward cycle is bounded by a maximum flooding surface. Therefore, the lower surface of the cycle is the base of the deeper lithofacies layer which overlies the top of a shallowing upward cycle. The upper boundary is the top of a shallower lithofacies layer which is overlain by a deeper lithofacies layer. Mark each cycle with a curved arrow to indicates the grain size variation and so its shoaling upward character. An arrow that moves to the left indicates that the grain size is coarser and so the water is becoming shallower.

2. Using the sonic and gamma ray logs, establish the patterns of deepening and shallowing and determine whether the cycles you identify from the cores can be tied to variations in the log character. This step is important since it enables the subsequent regional correlation to be based on specific log markers. For example, there are a few gamma ray spikes that indicate sudden deepening. You will see later that one of these spikes is identified as the most significant maximum flooding surface for the whole sequence. Overall the third order sequence character may be deduced by identifying the deepening and shoaling trend (from one mfs to the next). Additionally, some cycle boundaries also have clear log character change.

Saudi Aramco provided the data for this high frequency sequence stratigraphic analysis of the first two wells and the later14 wells from an axial line of dip (NE-SW) at the crest of the Berri Field that intersects the coast of the Arabia Gulf about 100 km north of Dhahran, Saudi Arabia. This section is approximately 55 km long and 30 km wide, with each well havng gamma ray, porosity, permeability, sonic, and density log information tied to a core description and lithofacies analysis based on the original Hanifa Formation interpretation of the Berri field by the Saudi Aramco-Mobil team in 1991 and McGuire et al. (1993).
Parasequences within cored wells should be established on the basis of subdividing stratigraphic surfaces and lithology. This first exercise consists of two separate wells and a lithological key (right figure). Using the lithological key and well log character identify the parasequences or cycles within the well sections.


Well # 1 - Shallow shelf region:

Lithofacies.Lithofacies + Well Logs. Solution.

Well # 2 - Deeper region:

Lithofacies.Lithofacies + Well Logs. Solution.

If you are confused in this exercise you should work you way through the earlier sections of:

1) The Introduction and

2) Parasequence correlation


Exercise 2 - Correlate four well logs on the basis of well logs and cores used together

Exercise 2
The objective of this exercise is to introduce the concept of well log correlation. The exercise utilizes the logs of four wells (right side is north, left side is south). The interval which is bounded by transgressive surfaces represents a complete sequence (TS to TS). All wells are datumed to the transgressive surface (TS) datum at the base of the section. The exercise is subdivided into two steps:
1. Using the well logs only:
a) For each well, identify the trends in the paleobathymetry and grain size of the lithology and the logs (shoaling or deepening) and identify parasequences (cycles) which are bounded by mfs.
b) See if you can find clear electric log events (mainly gamma log spikes) that can be identified in all or most wells. Use these markers for correlating all four wells.
c) Determine the overall profile of the section (north to south).
2. By combining the log interpretation with lithofacies cycle identification, the interpretation process can be much more definite.
a) For each well, identify all cycles.
b) For the overall lithofacies character (shallowing, deepening, thickness changes, etc.), and in combination with the interpretation of step 1, identify the third-order sequence major surfaces including the maximum flooding surface (mfs) and the sequence boundary (SB).

This exercise requires the identification of the major stratigraphic surfaces that can be correlated from well to well.

The surfaces should be identified on the basis of the log character and the stacking patterns of the wells.



Make the correlation of the four wells on the basis of parasequences.








Exercise 3 - The final and complete regional sequence stratigraphic interpretation

Using the results of exercise 2, make an interpretation of all fourteen regional wells. The well spacing is approximately 4 km. The objective of this exercise is to identify the architecture of the sequence and its major system tract components (TST, HST, SWM or LST) and to relate their occurrence to the position of the sea level. The reason for doing this is to better understand the processes that lead to the deposition of these different system tracts.

The exercise is subdivided into two steps:

  1. Interpret the well logs in order to identify the parasequences and the surfaces that subdivide them (See step 1 of exercise 2).
  2. Using the character of the superimposed lithofacies, the interpretation as stated in step 2 of exercise 2. Additionally, major surfaces should be identified or confirmed by utilizing cycles geometries and their relationships to the major surfaces. For example, a maximum flooding surface may be identified as the surface on which overlying cycles downlap and aggrade then prograde. A sequence boundary (SB) may be recognized if there are terminating (truncating) cycles in the shallow regions (north), etc.
The final interpretation should provide a fairly accurate representation of the reservoir architecture and can be used for recommending hydrocarbon extraction based on juxtaposition of source, reservoir, and seal.

Unlike some of the earlier exercises this one does not use seismic but instead allows you to make an interpretation of regional sedimentary geometries from 14 (fourteen) well logs:

  1. Make regional well log interpretation and identify major surfaces
  2. Superimpose lithofacies and improve interpretation
  3. Construct Fisher diagram based on shallow sector of the updip wells.
  4. Improve and enhance lithofacies interpretation.
  5. Explain results in terms of
      i. Walther's Law of Correlation of Facies
      ii. Starved Basin
      iii. Ramp to Rim configuration
  6. Produce 2D models and define best reservoir, source and seal.

Interpretation based on well logs.



Interpretation based on lithofacies character + well logs.



Final Solutoin.



Exercise 4 - Improve correlation - the use of Fischer Diagrams

The Fischer plot is a graph with two axes. The horizontal axis represents time and the vertical axis represents thickness. Having established the length of time involved to accumulate the thickness of sediment within the section a line representing the rate of subsidence of this section is drawn across the graph from upper left to lower right, with the upper left end starting at the top of the section at the time of the initiation of deposition of the section and the lower right end terminating at the time of the termination of deposition and at the base of the section.

Then working from the base of the section up, plot the thickness of the first parasequence as a vertical line, with its base resting on the diagonal subsidence line at the time of the start of deposition of that parasequence. Now draw a further line of subsidence from the top of the vertical line representing the first parasequence to the vertical axis on the lower right. Next plot the thickness of the second parasequence as a vertical line, with its base resting on the new diagonal subsidence line at the time of the start of its deposition. Now plot another line of subsidence from the top of this parasequence to the vertical axis on the right. Repeat this process for each of the parasequences until the vertical lines for each has been plotted. When this is finished you will note that the subsidence is represented by a series of inclined parallel lines and the horizontal spacing of the vertical lines representing the parasequences is equally spaced. This is because each parasequence is assumed to have the same time duration. This oversimplification explains some of the character of this plot.

Now draw a curving line that connects the tops of the each of the vertical lines representing the thicknesses of each parasequence. This curve represents sea level position through time. Because this solution to the exercise is a smooth interpolation, this curve may it lie just below the crests of some of the parasequences. However, in this particular case it should not be inferred that the crests of these parasequences were exposed. Some exposure has been seen in the local updip stratigraphy but this is ambiguous. In the case in point the peaks or tops of the parasequences are mfs, and so might not be expected to show exposure!!! Not the spacing of the inclined subsidence lines varies. This reflects variation in the thickness of each parasequence. An increase in thickness of the parasequences means that accommodation space was increasing and may be the result of an eustatic rise or increased subsidence. If the parasequences thin then this means that there is a decrease in accommodation space and sea level is falling or subsidence is decreasing.

Fischer Diagram cycles mapping.





Fischer Diagram Solution.

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Last Revised: October 13, 2005