Preferred Orientation, Microstructures, and Porosity Analysis of Posidonia Shales

Abstract

Synchrotron X-ray diffraction technique was used to characterize composition, microstructure, and texture of four source rock samples of Lower Jurassic Toarcian Posidonia shales retrieved from the Hils Syncline in northern Germany. The samples were obtained from similar depth from boreholes drilled with a lateral distance of roughly 10-20 km from each other. However, the total organic content (TOC) of the samples varies in thermal maturity, with vitrinite reflectance varying over a wide range from 0.68% to 1.45% (Littke et al., 1988). This variation indicates differences in local temperature history due to either a local igneous intrusion or a complex burial history (e.g. Leythaeuser et al., 1980; Rullkötter et al., 1988; Petmecky et al., 1999). The porosity of shales from the four wells is generally low and ranging from 4% to 10%, inferred from the mercury injection (MICP) (Mann, 1987). Mann (1987) reports pore throat sizes between <2.2 nm (estimate) to 60 nm, with most pores being in the order of nanometers. The objective of this work is to investigate whether the thermal history has affected the preferred orientation patterns of Posidonia shales and whether clay minerals may have been diagenetically altered, with consequent changes in preferred orientation. Furthermore, synchrotron X-ray microtomography was used to investigate the three-dimensional (3D) distribution of porosity and constituent phases and to compare the obtained porosity with previous measurements (Mann et al., 1986; Mann, 1987). Microstructures at the nanoscale were studied by transmission electron microscopy (TEM).