Why I use THR

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"Geotrack offers advise (at no cost) and a range of services on the most appropriate service for a specific exploration problem. However, the service I use most is THR (Thermal History Reconstruction) which represents the integration of AFTA, VR and basin modelling to give a constrained thermal or burial history for a sampled section. I use THR because without it I am forever running a vast suite of basin models, based on a series of assumptions (e.g., stretching factors, tectonic environment, porosity as a function of time and depth) which I am nervously aware are very poorly constrained. These endless models can give hugely varying predictions on hydrocarbon prospectivity. With THR, I can corner, by direct measurement, some of the most important factors which influence the generation and migration of hydrocarbons. Principally, these critical parameters are the TIME of maximum temperature and the MAGNITUDE of maximum temperature experienced by the section.

Face it, if my team can reduce the number of viable basin models by first measuring some of these important factors, then not only do our models have greater application and use, but also, we will be spending less time developing (tweaking) alternative models which are inappropriate. Reduce the number of options, reduce personnel time and reduce risk - THR is now used routinely in our company at the front end of an exploration program.

Also, (although I didn't appreciate this at the outset), I realised after about 5-6 THR jobs that we had been dedicating considerable time and resources to modelling parameters that were not directly influencing the bottom line - i.e., the time and magnitude of generation and expulsion. For example, I can remember one guy spending weeks looking at the possible heatflow history for this particular well location in South America. He reconstructed the tectonic setting and determined periods of rifting, drifting, foreland setting, etc. On this basis and following an extensive literature survey, he assigned heatflow values through time corresponding to each of these tectonic settings. Another few weeks were dedicated to resolving the thermal conductivity of the section (so he could get to temperature versus time) which meant another literature survey on porosity variations through time for specific lithologies. You can well imagine, at this point, we had some very detailed models and beautifully coloured output (really impressed management!), but no real handle on which of a dozen models was most appropriate. Sometime later we ran THR on this well (about 5 AFTA samples and 12VR samples). What we found was that for this Mesozoic and Cenozoic section, cooling from maximum paleotemperature commenced at ~5 Ma and that ~2500 metres of section had been removed in relation to this event at this well site. Knowing this, we were then able to model with considerably more confidence the generation and migration history at this location and the region. About 10 of our 12 models immediately became obsolete. Also, regrettably, we realised how much time we had wasted in trying to understand the Mesozoic and Early Tertiary heatflow history (and related investigations into sediment conductivities and porosity). Although arguably interesting, it just didn't matter to the hydrocarbon story, because all the action was in the last 5 Ma. I don't think management was ever aware, but on that particular project, we could have saved probably 4 months of team time, if we had used THR at the start of the project.

So finally, I like to refer to this THR approach as CONSTRAINED basin modelling. Everybody does basin modelling these days (off-the-shelf software has made this activity very common, and commonly abused ...), but not necessarily CONSTRAINED basin modelling where the factors most critical to the bottom line are measured first."

Note from Geotrack: Central to this approach is the application of AFTA (Apatite Fission Track Analysis) and VR (vitrinite reflectance). AFTA provides the timing information, and both AFTA and VR constrain the magnitude of paleotemperature. In the related series of slides, we give a step-by-step example, using the Anglesea-1 well from the Otway Basin in southeast Australia, of how AFTA and VR results are applied to give a constrained thermal history for the well section. Note that not only is the time of multiple episodes identified, but the value of the paleogeothermal gradient at the time of the episode is calculated, together with the amount of eroded section, thus reducing the number of viable models.