Structural and Diagenetic Controls on Reservoir Quality of Tight Formations
Becker I, Busch B, Koehrer B, Hilgers C
The Upper Carboniferous fluvial sandstones and Zechstein Ca2 carbonates represent
important hydrocarbon reservoir units in the Lower Saxony Basin, NW Germany, with
production of more than four decades and a great exploration potential. In those tight, gasproducing
formations, significant exploration and development risks are attributed to their
spatial variability of reservoir quality due to depositional heterogeneities, variations in their
structural characteristics and the presence of temperature anomalies. With a focus on the
Upper Carboniferous sandstones, this study compares three outcrops in the southern part of
the Lower Saxony Basin with analogous reservoirs situated approximately 50 km to the
north. Petrophysical and petrographic data sets are linked to the burial history of the
sandstones to derive control factors on the reservoir quality evolution.
Results indicate similar matrix permeabilities of two outcrops and the two studied reservoirs
(field A and B) but matrix porosities vary. Such differences can be related to differences in
their burial histories. Higher matrix porosities of field B compared to field A are explained by
a lower amount of quartz cementation as the result of stronger basin inversion related uplift
of field B. Further increases in matrix porosities of the outcrop sandstones are related to
carbonate cement dissolution during the late stages of uplift to the surface. However, a third
outcrop reveals significantly reduced reservoir quality which can be associated with intense
additional cementation processes due to high temperature anomalies in the vicinity of a fault.
In contrast, faulting and natural fracturing enhances the reservoir quality of field B by
providing migration pathways for fluids.