Geochemical criteria for reservoir quality variations in chalk from the North Sea

The research project i EFP-86) systematically investigates the influence of chalk geochemistry on petrophysical parameters determining hydrocarbon reservoir quality, i.e. porosity and permeability. Two wells of the North Sea Tyra gas Held were chosen for the present investigation: the central well TWB-8 and eastern marginal well E-lx. Geophysical logging data with interpretations exist for both wells. Drill core sections of Upper Maastrichtian and Danian chalk were selected for the geochemical investigations. Chemical dara on chalk samples were gathered by using both conventional (X-ray fluorescence) and special instrumental analytical techniques (instrumental neutron activation). The geochemical data are compared with the well-logging results. Geophysical logging suggests that there is reduced porosity in the Danian reservoir units LDP and UDT in both the central and marginal wells. The chalk drill core samples from the section with reduced porosity also show a lower Ca content. At the same time, a high Si content is observed in these samples and a number of trace elements in chalk show a similar distribution with depth. Silicon diagenesis is therefore regarded as being responsible for reservoir quality variations in Tyra rocks. A linear dependence is observed between chalk porosity and silicon content of chalk, i.e. reservoir porosity may be estimated from the Si content of chalk. Chalk permeability may also be determined by Si content but the linear dependency is less significant. Geochemically, depth distributions of elements Al, Fe and Sc show the same trends as that for Si. Therefore, diagenetic changes in chalk also include clay minerals. Other features of the Tyra gas reservoir are displayed through the chemical data. The gns zone in TWB-8 is characterized by low contents of Na and CI, i.e. lower water saturation is indicated. Low concentrations of rare earths in all chalk samples show a shale-normalized pattern that is characteristic of marine sediments laid down under oxic conditions. Some changes that occur with depth in the Ce anomaly probably indicates a slight change in the depositional environment. A most pronounced feature of the Tyra chalk is the depth distribution of manganese: the content continuously decreases with depth, i.e. from Danian (about 2000 ppm) to Maastrichtian strata (less than 200 ppm). In this respect, no other chemical element in chalk correlates with Mn. At present, there is no indication as to which mineral or mineral phase one is likely to find the element.