MATURITY ANOMALIES - SOME NUMERICAL MODELS TO CONSTRAIN THE EFFECTS OF HYDROTHERMAL FLUID FLOW ON THE TEMPERATURE HISTORY OF A SEDIMENTARY RIFT BASIN (UPPER RHINEGRABEN, GERMANY)

Measurements of dispersed vitrinite show an unusual vertical maturity trend in a petroleum exploration well in the Upper Rhinegraben. Above and below a 500 m marl layer vitrinite reflectance levels are consistent with modern, conductively dominated thermal gradients. Between about 1000-1500 m depth, however, vitrinite reflectance levels are significantly elevated (about 0.6%) and suggest a reversal in thermal gradient. This anomaly cannot be explained with one-dimensional conductive or conductive/convective heat transfer models. We propose that the maturity anomaly was caused by a transient thermal inversion induced by episodic, lateral flow of hot (130-160°C) groundwater along conductive fractures and bedding planes. To test this hypothesis, a suite of simple, idealized models of groundwater flow, heat transfer, and vitrinite maturation models is presented to determine what transient hydrothermal conditions could have produced the observed thermal anomaly. In these simulations, a single, horizontal aquifer is sandwiched between two less permeable units. The elevation of the aquifer is coincident with the position of the observed thermal maturity anomaly in the Rhinegraben. The total dimensions of the model are 4x10 km. Simulation results indicate that a thermal maturity anomaly could only be produced by a rather restrictive set of hydrothermal conditions. This encompasses a single hydrothermal flow event of 130°C fluid migrating laterally into the aquifer at a rate of 1 m/a for about 10,000 years. The anomaly is spatially confined to near the left edge of the basin. If the flow event lasted longer than 100,000 years, then the maturation anomaly disappears as the lower confining unit approaches steady-state thermal conditions. Spatially confined temperature anomalies resulting in bell-shaped vertical temperature profiles with inverse gradients are not documented in the northern Upper Rhinegraben today. Yet, the observed maturity anomaly gives evidence of past temperature conditions, that caused maturity levels to respond to a transient short-lived, spatially confined heat pulse. It is possible that such an event occurred about 5 million years ago in response to increases in fault permeability associated with far field Alpine tectonism.