Neubewertung des rezenten Wärmestroms im Nordostdeutschen Becken

Abstract

(TYPE=abstract)The investigation of heat flow is one of the prerequisites for modeling the thermal structure of sedimentary basins and allows the determination of its geodynamic state as well as the composition and structure of the basement. Although maps of the surface heat flow in the Northeast German Basin (NEGB) are published, our knowledge of this parameter is rather flaw. With this work a well-founded determination and evaluation of heat flow in the NEGB are presented for the first time. Under conductive and stationary conditions heat-flow density (q) is calculated as the product of temperature gradient of a depth interval and its representative thermal conductivity. Therefore, the thermal conductivity of the rocks must be known, and temperature measurements must be available. The surface heat flow (qs) then can be calculated adding to this q the amount of heat generated by the radiogenic elements of the overburden. Deep boreholes, drilled in the course of oil and natural gas exploration, particularly in the years 1962-1990, were used in this study for the determination of q. Due to the targets of drilling and coring the depth intervals examined were predominantly in Permian rocks (Permocarboniferous magmatites and Permian sediments). Laboratory-derived thermal conductivity (TC) was determined in 363 core samples in dry and saturated conditions. The variability of TC within the lithotypes is related to facies, cementation, and degree of diagenesis. Based on the TC of lithotypes, the TC value of formations was estimated taking into account the respective facies conditions in the particular borehole. The TC of lithotypes and formations have higher values than previously supposed and used in modelings of the NEGB. Heat production (A) of rocks was determined on the one hand by direct measurement of the content of U, Th, and K in 52 rock samples and on the other hand indirectly by evaluating borehole measurements. In order to do so, total gamma-ray intensity logs (GR logs) recorded in older gamma units (GE) were converted to modern API units using an empirical equation: GR[APIc]= 4.95433 × GR[GEc] - 27.24. The A value for the entire sedimentary succession of the NEGB calculated from GR logs (using API units) amounts to 1-6 µW/m3 (4 µW/m3 on average). The determination of q is accomplished and described in detail for 13 borehole locations. Possible disturbances were considered, and different correction procedures were applied. The corrections comprise the in-situ temperature correction of TC measured under room conditions, the corrections of temperature disturbances caused by the drilling process, corrections of lateral heat refraction at salt structures, and corrections of palaeoclimatic effects. The determined q values show an uncertainty, which is assumed to be approximately 15% for all locations. The qs values range between 70-90 mW/m2 (average value: 77 mW/m2). It is shown that both, different magmatic successions at the basin basis and the different Mesozoic and Cenozoic deposits (depending on the situation in the basin and the positioning relative to the adjacent salt structures) affect a differentiation of qs. The qs values of this study do not coincide with the distribution pattern of former qs maps. In order to investigate the thermal consequences of different assumptions of structure and composition of the crust and the lithosphere, 2D sections were modelled under steady-state conditions. Variable compositions of the crust and different thicknesses of the lithosphere could explain the determined qs values. It is assumed that the Pritzwalk gravity anomaly of dense rock with less heat production causes a negative thermal anomaly at the surface. Without a better knowledge of the deeper underground of the NEGB it cannot be estimated as to which parts the determined mean q depends on a high heat production of the crust or a high mantle heat flow. It also cannot be ascertained whether the assumption of a thermal stationary regime, which represents a substantial part of the computations, is justified for the NEGB.