A Quantitative Thermodynamic Model for Diabase Intrusions in the Amazonas Basin - Brazil, and their Effects on Maturation of Organic Matter

In the Amazon basin extensive volcanism occurred during the Mesozoic with wide-spread intrusion of diabase sills into the Palezoic sedimentary infill. In some areas there exists a close relationship between hydrocarbon accumulations and distance from Mesozoic dikes and sills: Vitrinite reflectance, spore coloration and abundance of gas increase upwards towards the sills, contrary to what would be expected from sedimentary burial by subsidence alone. At greater distances away from the sill the geothermal effect due to burial becomes dominant and an increase in vitrinite reflectances away from the sill is observed. To investigate this relationship we developed both an analytic and numerical model of a finite width heat source (dike or sill) which takes also into account the geothermal heat flux due to sediment burial. This allows us to calculate the complete temperature history of the intrusive and the surrounding sedimentary rock. The model demonstrates that the temperature history is primarily sensitive to the thickness of the intrusive, the latent heat of fusion and the thermal conductivities and diffiuivities of the intruaive and the host rocks. The maturation of organic matter is evaluated using a vitrinite reflectance model proposed by Lerche (1988), for which we determined the relevant parameters through an inverse fitting procedure to actual well data. The resulting relationship can be used to evaluate hydrocarbon maturation in areas where sill intrusions are a major thermal factor.