High thermal maturity in the Lower Saxony Basin: intrusion or deep burial?

Abstract

The Lower Saxony Basin (LSB) is situated at the southern rim of the Northern Permian Basin, the most important hydrocarbon-bearing province in Europe. In its southern part, areas of high thermal maturation (up to 6% R r) occur which were previously explained as being caused by magmatic intrusions of Turonian to Santonian age that affected Carboniferous to Lower Cretaceous rocks. In this area, 2D numerical basin modelling was carried out in order to reconstruct the temperature histories along two parallel, N–S-trending seismic sections. Along these sections, new data about maturity distribution and temperature history were obtained by vitrinite reflectance measurements and apatite fission track analysis. For the areas with highest maturity values, low coalification gradients were established which agree well with earlier published data on wells further towards the west. This new information does not support previous hypotheses on high heat flows from magmatic intrusions, even if lateral heat transport is taken into account. Instead, comparisons with a recently published map of the subsurface structure of the LSB indicate that maturity distribution may be closely related to major tectonic structures. Deep burial of individual structures of the LSB, which are bordered by deep E–W- to ESE–WNW-trending faults is further suggested by (a) rapidly increasing thicknesses of preserved Cretaceous strata towards the south and (b) structural restoration of one of the seismic sections. To achieve a fit between measured and calculated palaeo-temperature and vitrinite reflectance data, deep burial of Carboniferous source rocks during Cretaceous time has to be assumed. Due to an inversion phase in the early and middle Upper Cretaceous, major amounts of sediment were removed and further uplift occurred in Tertiary times. In contrast, the northern part of the LSB experienced maximum burial and temperatures during Tertiary times.