Lithological dependency and anisotropy of vitrinite reflectance in high rank sedimentary rocks of the Ibbenbüren area, NW-Germany: Implications for the tectonic and thermal evolution of the Lower Saxony Basin

Abstract Covering a depth interval of 1200 m thirty-one rock samples were collected from a well drilled in the Beust Field of the Ibbenburen anthracite mine, Lower Saxony Basin, NW-Germany. These rock samples consist of high rank coals/anthracites, claystones, siltstones and sandstones of Pennsylvanian age. Analytical methods include the measurement of random vitrinite reflectance VR r , rotational reflectance VR rot as well as maximum and minimum reflectances VR max and VR min , enabling the evaluation of the lithological influence on vitrinite reflectance as well as the indication of tectonic stresses in the Lower Saxony Basin by applying the RIS (reflectance indicating surface) analysis method. This study shows that not only due to the high rank of the studied samples and associated anisotropic character but also due to lithological dependency a partially significant scatter of measured VR r can be observed. The scatter in coals is significantly less than in other sedimentary rocks, e.g., standard deviation in coals can be up to 7× less than in siltstones. The expected increase of standard deviation with increasing rank and accordant anisotropy was not observed for VR r but rather for VR max with the exception of coals displaying a relatively constant and low deviation throughout the depth/maturity interval. Mean VR r values of coals differ from those of other sedimentary rocks at equal depths. VR r measured in claystones is  consistently higher compared to associated coal seams and can be elevated by up to 10%. Silt- and sandstones always display lower VR r with differences of up to 16%. At very high levels of thermal maturity, vitrinite reflectance depends not only on maximum paleotemperature but also on lithology leading to differences in pressure propagation and thus deformation. Additionally, radioactive trace elements in coals and organic matter-rich claystones provide a local secondary heat source. 40 K and 232 Th are preferentially bound to clay particles which are enriched in the studied claystones and depleted in the silt- and sandstones possibly leading to higher vitrinite reflectance observed in the claystones. RIS analysis shows a predominantly negative biaxial distribution indicating the absence of thermal overprinting by a possible intrusion (Bramsche Massif) during the Late Cretaceous which would have induced a uniaxial distribution. Instead, this implies, in addition to vertical stresses due to depositional loading, the presence of a secondary stress field not perpendicular to bedding which is related to the inversion of the Lower Saxony Basin. This information presents, in combination with previous basin modeling results, additional evidence for deep subsidence with subsequent uplift being the cause for the present-day maturity patterns observed in the Lower Saxony Basin.