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One of the most striking geothermal anomalies within the Rhine graben is in the Landau/Pfalz oil field. The temperature at 1,000 m depth is about 100°C in the center of the anomaly. Temperature logs in the oil field are generally disturbed, owing to the former production or injection of fluids. A mathematical model is presented, describing the distribution with depth of the temperature disturbance in a flowing borehole. Based on this, the process of cooling after a period of flowing can be calculated. It is then possible to check whether a borehole is in thermal equilibrium. If favorable conditions prevail (high flow rates and/or small depth of the reservoir) the presented model can be used to estimate the temperature in the reservoir from the surface temperature of he fluid.
The geothermal anomaly is probably associated with deep reaching faults, in which thermal water can rise up from a depth of several thousand meters. The water originates near the crystalline basement and flows into the sedimentary fill, transmitting its heat content to the surrounding rocks.
A numerical model shows that the required water flux to match the observed temperature distribution is very small, and that an anomaly as found in Landau can be created within a time period of about 100,000 years.
The same conditions as in Landau exist in other zones of the Rhine graben, and similar convective systems can account for other temperature anomalies.
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