About the Accumulation Ratio (A/S)

This Accumulation Ratio calculator provides a fundamental analysis in sequence stratigraphy by quantifying the relationship between accommodation space (A) and sediment supply (S). This ratio is a powerful predictive tool used by geologists to understand and forecast the stacking patterns of sedimentary layers, which form the basis of reservoir characterization, basin analysis, and hydrocarbon exploration.

What This Calculator Does

The calculator determines the A/S ratio by synthesizing four critical geological inputs: changes in base level (like sea level), tectonic movement, sediment compaction, and the rate of sediment supply. The resulting ratio indicates whether the depositional system is likely to build outwards (progradation), build upwards (aggradation), retreat landwards (retrogradation), or be eroded due to a fall in base level (forced regression).

When to Use It

The A/S ratio is essential in various geological contexts:

• Petroleum Exploration: To predict the geometry and location of potential reservoir rocks (e.g., shoreline sands) and seals within a sedimentary basin.
• Basin Analysis: To reconstruct the geological history of a basin, understanding the interplay between tectonics, sea-level changes, and sediment input over millions of years.
• Reservoir Characterization: To model the internal architecture of a discovered oil or gas field, helping to optimize production strategies.
• Academic Research: To test hypotheses about sedimentary processes and controls on the stratigraphic record.

Inputs Explained

• Change in Base Level: This refers to the vertical change in the surface to which sediments accumulate, most commonly the eustatic (global) sea level. A rise is positive, while a fall is negative.
• Tectonic Subsidence / Uplift: This is the vertical movement of the Earth’s crust. Subsidence (sinking) creates space and is entered as a positive value. Uplift (rising) reduces space and is a negative value.
• Sediment Compaction: As new sediments are deposited, the layers beneath them compact under the weight, losing volume and creating a small amount of additional accommodation space. This is always a positive value.
• Sediment Supply: This represents the potential thickness of sediment delivered to the area. It must be a positive value, as it is the “material” used to fill the available space.

Results Explained

The calculated A/S ratio and the sign of the accommodation value (A) lead to one of four interpretations of the depositional system’s behavior:

Formula and Method

The calculation is performed in two steps. First, the total accommodation space (A) is determined:

Accommodation (A) = (Change in Base Level) + (Tectonic Subsidence) - (Sediment Compaction)

Next, the accommodation is compared to the sediment supply (S) to find the ratio:

A/S Ratio = Accommodation (A) / Sediment Supply (S)

Step-by-Step Example

Consider a coastal basin with the following conditions over a specific time interval:

• Base level (sea level) rises by 20 meters.
• The basin experiences tectonic subsidence of 5 meters.
• Compaction of underlying muds accounts for 2 meters.
• The sediment supply from rivers is enough to deposit 15 meters of sediment.

1. Calculate Accommodation (A):
   A = 20 m (Base Level Rise) + 5 m (Subsidence) - 2 m (Compaction) = 23 m
2. Calculate A/S Ratio:
   A/S = 23 m / 15 m = 1.53
3. Interpret the Result:
   Since the A/S ratio is greater than 1, the system is retrogradational. The shoreline would move landward as the creation of space (23m) outpaces the supply of sediment (15m).

Tips and Common Errors

• Consistent Units: Ensure all inputs (base level, subsidence, compaction, supply) are in the same unit (e.g., meters or feet) before calculating.
• Sign Convention: Remember that base level fall and tectonic uplift are negative values, as they reduce accommodation space. Subsidence and base level rise are positive.
• Progradation vs. Forced Regression: A common mistake is to assume any A/S ratio less than 1 means progradation. This is only true if accommodation (A) is positive. If A is negative (due to a significant fall in base level), the system is a forced regression, which has very different geological implications.
• Zero Supply: The calculator requires a sediment supply greater than zero, as division by zero is undefined. In reality, a zero-supply scenario would lead to a drowned basin (if A > 0) or an erosional surface (if A < 0).

Frequently Asked Questions (FAQs)

What is the difference between transgression and retrogradation?

Transgression is the landward movement of the shoreline. Retrogradation is the landward stacking pattern of sedimentary deposits that results from transgression. They are related concepts, where one is the process (transgression) and the other is the resulting geological product (retrogradation).

What is the difference between regression and progradation?

Similarly, regression is the basinward (seaward) movement of the shoreline. Progradation is the basinward stacking pattern of deposits that results from regression. Progradation is the product of the regression process.

Can the A/S ratio be negative?

Yes. If the total accommodation (A) is negative, the A/S ratio will be negative. This happens when the combined effect of base level fall and/or tectonic uplift is greater than subsidence and compaction. A negative A/S ratio always signifies a forced regression and potential erosion.

What does an A/S ratio of exactly 1 mean?

An A/S ratio of 1 represents a perfect balance between the creation of space and the sediment filling that space. This leads to aggradation, where sedimentary layers stack vertically on top of each other with little to no horizontal movement of the shoreline.

How does sediment compaction influence the calculation?

Compaction reduces the volume of previously deposited sediment, effectively creating a small amount of new space at the surface for more sediment to accumulate. It is therefore subtracted in the formula (since it adds to the available space, it’s treated like subsidence). In many large-scale models, its effect is minor compared to eustasy and tectonics, but it can be significant in areas with thick, compressible sediments like muds and peats.

Is this model applicable in non-marine environments?

Yes, the concept is applicable. In non-marine settings like lakes or alluvial plains, “base level” refers to the level to which rivers grade or the lake surface level. Changes in lake level or tectonic movement of the basin floor can be used in place of sea-level change and subsidence.

What are the main limitations of the A/S ratio model?

The A/S model is a one-dimensional (vertical) simplification. It doesn’t account for lateral variations in sediment supply, wave/tidal energy, or basin floor topography, which all influence the final stratigraphy. It is a conceptual tool best used for first-order predictions.

Why is a Forced Regression significant for hydrocarbon exploration?

Forced regressions can expose previously deposited coastal and marine sediments to subaerial erosion, creating a significant geological boundary called a sequence boundary. These boundaries can act as seals or migration pathways for hydrocarbons. Additionally, sands deposited at the new, lower shoreline position (lowstand shorelines) can form excellent, isolated reservoirs.

References

1. Catuneanu, O. (2006). Principles of Sequence Stratigraphy. Elsevier. ISBN: 978-0-444-51568-1.
2. Posamentier, H. W., & Vail, P. R. (1988). Eustatic controls on clastic deposition I—Conceptual framework. In Sea-Level Changes: An Integrated Approach (SEPM Special Publication No. 42, pp. 109-124). https://doi.org/10.2110/pec.88.01.0109
3. Schlager, W. (1993). Accommodation and supply—a dual control on stratigraphic sequences. Sedimentary Geology, 86(1-2), 111-136. https://doi.org/10.1016/0037-0738(93)90127-2
4. Muto, T., & Steel, R. J. (2000). The accommodation concept in sequence stratigraphy: some new perspectives. In Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories (GCSSEPM Foundation 20th Annual Research Conference, pp. 1-14).