Modelling in thrusted areas - sensitivity analysis and application

Compressional areas frequently provide potential exploration targets and therefore an understanding of the effects of overthrusting and structural evolution on rock thermo/physical history and, consequently, on hydrocarbon generation and expulsion processes, can effectively contribute to reducing exploration risks. As many uncertainties in the available information occur in this geological scenario (seismic resolution, structural interpretation, source rock evidence, lack of well measurements, etc.), this paper demonstrates the potential for an integrated approach which improves the quality of the final interpretation, which attempts to predict the petroleum system. We will verify how it may be possible to make use of numerical simulations, both in the structural and geochemical fields, to define the factors which have most impact on the relationship between the timing of hydrocarbon expulsion (primary migration from potential source rocks) and the age of trap formation. The significance of some uncertainties in the available data and the model can also be estimated through `sensitivity analysis' techniques. This allows the investigation of the effects of pressure/temperature linked to successive phases of thrusting, erosion and deposition on potential source rocks, emphasizing the strong tectonic controls not only on the hydrocarbon generation but also on the more crucial expulsion process. In a case history, it will be demonstrated how the use of non standard thermochronology analysis as AFTA (Apatite Fission Track Analysis) and fluid inclusions can better constrain the thermal and burial history of thrust belts, with respect to traditional methods based only on vitrinite reflectance data. This approach is very useful particularly when erosional events, which may cause cooling episodes, heavily control the period in which maximum temperatures are reached. For instance, measured maturity data may not be representative of the present day thermal regime in a basin, especially in the belt's inner part where uplift is more common, and therefore an estimate of the age of erosion and the thickness of the missing sediments has to be determined.