REGIONAL VARIATIONS IN MATURITY TRENDS AROUND DEEP GEOTHERMAL WELL GROß-BUCHHOLZ GT-1, HANNOVER, LOWER SAXONY BASIN

Geothermal well Groß-Buchholz GT-1 (GBGT-1), located at the city limits of Hannover, SE Lower Saxony Basin (LSB), penetrated lower Cretaceous to lower Triassic sediments down to a depth of 3900 m. The LSB is an active hydrocarbon-producing basin in a mature state of exploration. Well GBGT-1 affords the opportunity to investigate the major source rocks for almost all hydrocarbon accumulations of the LSB (Wealden Fm and Posidonia Shale) at the site of hydrocarbon generation and to understand the conditions leading to their maturation.

To reconstruct the subsidence and heating history at GBGT-1 we determined the thermal maturity on more than 100 cuttings and core samples, using petrographic studies on organic particles (vitrinite reflectance) and bulk-rock geochemical analyses. The resulting maturity trends indicate a slightly increased maturity at the recent surface (0,4 0,6 %Ro). By reference to numerical 1D simulation, this can be explained for the vitrinite-trend as a consequence of about 1800 m of overlying Cretaceous sediments and a marginally increased heat-flow (~65 mW/m2), whereas a lesser overburden (c. 1000 m) and a strongly increased heat flow (~95 mW/m2) in the late Cretaceous yielded the best fit for the bulk-rock geochemical trend.

For regional comparison, numerical simulations were carried out on data from two more wells to the north-east (Großburgwedel) and east (Hämelerwald) of GBGT-1. Although the three wells lie within a distance of only 20 km from each other, they experienced strikingly different heating and subsidence histories: 700 m of Cretaceous overburden and an increased late Triassic heat flow were calculated for Hämelerwald and only 200 m of overburden and a heat flow peak in the early Cretaceous were determined for Großburgwedel.

The resulting variations in the eroded sedimentary overburden can be explained with a variable magnitude of structural inversion and/or salt movement processes over short distances across the “Hannover Scholle” and the adjacent "Hildesheim Scholle". Regional aspects of deformation, including balanced sections across the study area, will be shown in a separate presentation (Jähne and Kley, this volume).