Saturday, March 6, 2010

Transient Effects Revisited

Today I had a chance to check out the book "Fundamentals of basin and petroleum systems modeling" by Thomas Hantschel and Armin Kauerauf (Springer-Verlag, 2009). It seems that the transient effects may be still fundamentally misunderstood (and underestimated). Their fig. 3.4a on page 109 (shown below) shows a 1D model going through the deposition, hiatus, and erosion stages. With the assumption that the heat flow at the base of the sediment stays constant at 60 mW/m2, the model predicts small  (±5 mW/m2) changes in heat flow in the sediment column. The authors conclude that the transient effect is smaller than that caused by radioactivity within the sediments. You may click on the image to see a version with better resolution.
When evaluating transient effects, it may not be appropriate to assume constant heat flow at base sediments.  You can see from the figure that the forced base boundary is limiting the extent of the transient effects. With a deeper boundary, the heat flow change should be more significant. More importantly, by setting the boundary at base of sediments,  it considers only the process of heating the sediments, but misses the problem that the deposition of the new layer also puts the entire lithosphere out of equilibrium by moving the surface boundary.

The figure below shows this concept. After adding the new sediments, to establish steady state thermal equilibrium again (green curve), temperature, therefore heat flow must change through out the entire lithosphere, not just within the new sediments. Secondly, since the entire lithosphere needs to be heated (not just the sediments) to reach the new equilibrium, it may take much, much longer (lithosphere is typically 10-20 times thicker than the sediments) than heating the sediments alone (see my previous post on this below).

Below is a model with same conditions as the Hantschel and Kauerauf's model, except that it does not assume a constant heat flow at the base of sediments. Rather the temperature at base of the lithosphere at 120 km is fixed at 1330 °C. The transient effects are much stronger compared to the figure at the top.

The following figure shows the predicted heat flow at the base of the sediment column  through time. You see that it is far from constant. From an initial 60 mW/m2, basal heat flow decreases to 48 mW/m2 at the end of the deposition period, and increases gradually during the hiatus. Then it increases to 72 mW/m2 at the end of the erosion period.
This indicates a ±12 mW/m2 change over 10 million years with deposition and erosion rates of 250 meters/my, a bit higher than the average deposition rate. However, the deep water of the Gulf of Mexico has deposition rates several times as high, and the heat flow at the base of sediment today is around 35 mW/m2, while a steady state heat flow would have been about 50 mW/m2.

In recently uplifted parts of North Africa, we see higher heat flows today. Follwing this analysis, it may be concluded that the heat flow prior to the uplift could be 10 mW/m2 lower depending on erosion rates. See this post for details.

The basin modeling literature is littered with papers making assumptions of heat flow at the base of sediments independent of deposition/erosion rates. Where sedimentation rates are high, or vary significantly over time, the application of such thermal models can cause significant errors in estimating the maturity and timing of petroleum generation. To be fair to the authors, this was how I used to do it in the 90s. But I have learned my lessons from those who learned before me.

4 comments:

  1. The blog author objects strongly to Figure 3.4 of the book of Hantschel & Kauerauf (FBPSM). He states that using a fixed heat flow at the basement as a thermal boundary condition is a faulty strategy. Instead of that, he recommends using a fixed temperature of 1330 Celsius at the boundary between lithosphere and asthenosphere. He argues that equilibration of temperature after a disturbance due to deposition or erosion is slow on geological timescales and he demonstrates his view with help of an academic 1D model.

    This is all somewhat problematic because Figure 3.4 is a simple academic example which was created with the intention to illustrate the basic thermal effects of deposition, hiatus and erosion. It is not a real case study and cannot be compared to a realistic scenario. In addition, the blog author erroneously connects a statement about radioactive heat generation, made in reference to figure 3.7, to figure 3.4, which includes no radiogenic heat production at all. Finally, the blog author apparently did not notice that the same approach which he uses is also discussed in FBPSM (see Fig. 3.14).

    In fact this approach has severe disadvantages. It introduces additional uncertainties such as the thickness of the crust, the thermal conductivity in crust and mantle and so on which are usually not known very well. Particularly, his approach is based on a temperature boundary at a fixed non-varying depth of 120 km, which is known to be highly unrealistic, especially in rift basins. Known processes such as stretching of the lithosphere over geological timescales, isostasy, and flexure are ignored. However, crustal models incorporating crustal processes can be used to evaluate the heat flow at the bottom boundary of the basin (FBPSM). The heat flow can afterwards be applied to the basin model, consistent with Figure 3.4, which is an academic example with a constant heat flow. Obviously, this heat flow will not be constant over geologic times in real case studies. A big advantage of this separated approach is the possibility to easily extend the method for lateral heat flow calculations in the sediments. 3D heat flow effects are often non negligible, especially in combination with transient effects.

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  2. I merely tried to show that setting a boundary condition at base sediment will cause the model to underestimate transient effects. I assume 3DModeler does not disagree with that conclusion. If the book authors added a basement layer in their model and set a constant heat flow at a deeper depth (say 50 km), they may see a stronger transient effect.

    Both fig 3.4 and 3.7 use base sediment as boundary which would affect their conclusion about the significance of transient effects.

    I did not suggest not to to vary the depth to asthenosphere in a real basin, or that we should ignore stretching, isostacy and flexure etc. But they are not relevant here.

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  3. Hello. I am trying to find out if you own the copyright for the 2nd image shown above. I see a reference listed that some are from the "Fundamentals of basin and petroleum systems modeling" by Thomas Hantschel and Armin Kauerauf (Springer-Verlag, 2009). Are all of these images from this book or even at minimal based on it? Is just a few? Need source information for the 2nd image. This image is wished to be used within an internal company technical training presentation. Please reply at the earliest convenience. I look forward to your timely response.

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  4. Hi Stephanie, I am sorry I do not monitor the comments on these posts, and just saw your comment today. ZetaWare, Inc (http://www.zetaware.com/ owns the copy right of the images on this post other than the first one. If you simply credit Zhiyong He, or ZetaWare, Inc. or www.zetaware.com, please feel free to use them. This applies to other images posted by Beta Factor. Images on posts by other authors are owned by the respective authors. Thanks very much for visiting and your comments. - Zhiyong He.

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