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SSP 1998 Project Summary: |
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Jean-Christophe Desplat, EPCC
Douglas Smith, EPCC
Hydrocarbons (oil and gas) are produced when sedimentary rocks rich in organic material (source rocks) are heated to high temperatures (60-150 degrees Centigrade) for long periods of time (millions to tens of millions of years). This process of conversion from primary organic material to hydrocarbons is known as maturation. The accurate assessment, and prediction, of the maturity and maturation history of hydrocarbon source rocks is of great importance to the oil industry when exploring for hydrocarbons in sedimentary basins like the North Sea.
The maturation history of a source rock depends upon its heating history. Temperature increases with depth of burial and vertical heat conduction is the main control on the temperature structure of sediments and the amount of heat supplied to source rocks. It is assumed that this is the only control on temperature variations in standard, one dimensional models of source rock maturation.
Rather than having a simple layered structure, as is assumed in one dimensional basin models, it is clear that rifted sedimentary basins have a complex structure and basement topography that varies in three dimensions. Faults can produce sudden steps in basement topography of a kilometre or more that juxtapose high conductivity basement rock and low conductivity sediments or even water. Most hydrocarbon deposits are found on basement highs and that is where most exploration wells are drilled. Present-day temperature and maturity data from these wells are then used to constrain the thermal history of the downthrown areas, where hydrocarbons are first generated before moving (migrating) to the highs where they are concentrated and trapped in structural culminations.
In a one dimensional simulation no correction can be made for lateral heat conduction or for the variation in thermal structure that is likely to result because of basement topography. The results of an SSP project last year ( 3D Modelling of Heat Flow in Sedimentary Basins by Dirk Carstens) have shown that heat flow is concentrated within basement highs due to their higher conductivity. This means that sediments overlying these highs will have been more intensely heated than those in adjacent lows. Using maturity data from wells on the highs may therefore lead to an overestimate of the maturity of source rocks in the main downthrown "kitchen" areas.
The principal objective of this study will be for the student to develop a three dimensional model for heat transport. They will then use this model to test the impact of a range of basement topographies and structures on the temperature structure of overlying sediments using geologically appropriate rock properties. The impact of having a water-filled rather than sediment-filled basin adjacent to the basement high will also be investigated. Based on the experience of Dirk Carstens, the objectives detailed above should be achievable within the 8 week period available. The project could be extended in a number of ways. This could be by looking at more detailed case studies, and perhaps comparing model predictions to real values, or by looking at the evolution through time of a single basement fault block.
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