burial history

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Exercise 12a: Burial History

Objective Construct a simple burial history diagram and use it to make some predictions about the source and reservoir.

Introduction In this exercise, you are given the stratigraphic column at a single location. From this, you will construct a very simple burial history diagram. Then you will predict:

1. When the source began to generate,

2. What range to expect for reservoir porosity.

There are 8 stratigraphic units sitting on basement. The predicted ages, thicknesses, and water depths for each unit are given in the table below.

UNIT AGE (top) THICKNESS Water Depth (top)

1 0 Ma 150 m 50 m

2 10 Ma 150 m 150 m

3 18 Ma 150 m 250 m

4 29 Ma 150 m 300 m

5 38 Ma 100 m 300 m

Seal 48 Ma 100 m 300 m

Reservoir 60 Ma 100 m 250 m

Source 68 Ma 100 m 300 m

Continued on next page

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Exercise 12a: Burial History, continued

Part 1 Construct a Burial History Diagram

Step Action

1 Figure 1 is the start of a simple burial history diagram. The present-day stratigraphy is based on interpreted seismic horizons, which is displayed on the right side of the diagram. Water depths back through time have been provided. These would come from (1) nearby well data or (2) estimates that are based on seismic facies analysis and interpreted depositional environments. As part of a typical analysis, initial water depth predictions are refined so as to give reasonable burial history diagrams throughout the study area.

Complete the chart showing how each unit has been buried with time. For now we will assume each unit has its present-day thickness back through time, i.e., no compaction.

We will make several simplifying assumptions.

1. Sea level has remain constant back through time

2. There is no compaction effects for the stratigraphic units (thicknesses remain constant)

3. There was no significant erosion

4. Motion was purely vertical, no translation of units laterally (e.g., downdip slumping).


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Part 2 Source Maturity

Thermal History and Source Properties

For divergent (pull-apart) continental margins, we can infer the thermal history from burial history diagrams. In a simplistic fashion, here is what we do.

For this type of margin, the depth to basement is controlled by two factors: (1) thermal subsidence caused by the cooling of extended (thinned) continental crust and (2) the weight (load) of the sediment deposited on top of basement. Again for simplicity, we assume a 1-D loading correct is adequate, i.e., no flexure. Given the sedimentary column, we can estimate the subsidence due to sediment loading. We subtract the loading component from total subsidence to obtain an indication of the subsidence due to the cooling of thinned continental crust - the thermal component.

Figure 2 shows the burial history diagram with (1) a sea level curve (2) sediment compaction effects, and (3) the components of subsidence loading and thermal. Note in Figure 2 that thermal subsidence is close to zero from 68 to 60 million years ago. At 60 Ma the rate of thermal subsidence (slope) is large and subsequently the rate slows exponentially. This is what we would expect if some type of heating event occurred 60 Ma, e.g., a phase of rifting or nearby volcanism.

Figure 3 shows the thermal component of subsidence (thick, solid line) and some theoretical subsidence curves calculated for continental crust that has been thinned 20%, 40%, 60%, 80%, and 100% (rifting to the point of emplacement of oceanic crust). Comparing the thermal component to these curves, we interpret this location is on continental crust that has been thinned about 50%.

This enables us to predict heat flow through time at this location. There is a certain level of background heat flow (e.g., from radiogenic sources). We add the heat flow associated with a 50% rifting event at 60 Ma (bottom of Figure 3). Then we can use basin modeling software to predict the thermal history for all stratigraphic units, e.g., the source and reservoir.


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Step Action

2 Given the burial and thermal history obtained by back-stripping layers (going back through time), we can now model the basin history forward through time and a uniform time step (typically 0.5 or 1 million years). We can predict the effects of burial, temperature, pressure, and time on source and reservoir properties.

Figure 4 shows the burial history along with the tops of te predicted oil and gas windows through time. Based on Figure 4, answer the following questions:

1. When did the base of the source interval begin to generate oil?

2. When did the top of the source interval begin to generate oil?

3. When did the base of the source interval pass from the oil to the gas window?

4. Is the source currently generating oil, gas, or both?

Reminder: These answers are valid only for the single location we have been modeling. Further basinward crustal thinning would have been more than 50%, heat flow would have been higher, and the source would have started generating oil earlier. Further landward the source would have started generating oil later, if at all.


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Part 3 Reservoir Porosity

Reservoir Properties

We can also model the properties of the reservoir unit. We assume that the porosity at the time of deposition was 38%. We then predict how porosity decreased with burial (depth + time) due to mechanical compaction and diagenetic effects.

Step Action

3 Figure 5 shows porosity as a function of time for the top of the reservoir. The solid curve is our most likely case. We can also model optimistic and pessimistic cases (dashed lines) by varying the inputs.

Based on Figure 5, answer the following questions:

1. What is the most-likely present-day porosity at the top of the reservoir?

2. What is the best-case (most optimistic) present-day porosity for the top of the reservoir?

3. What is the worst-case (most pessimistic) present-day porosity for the top of the reservoir?

4. Note the dip in porosity for the worst-case scenario around 30 Ma. What might this be due to?

burial history

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