Abstract: Fractured Carboniferous proved as prolific exploration target in Northern Germany

HOLLMANN, G., B. KLUG, J. SCHMITZ, E. STAHL and M. WELLENS

The naturally fractured reservoir rocks are Westphalian in age and contained within a thrust-bounded horst block underlying a Zechstein salt plug in the central Lower Saxony Basin. The structuration was generated during the Late Cretaceous to Early Tertiary intraplate inversion. A 3D-seismic survey combined with extensive well data provides a high quality database for a detailed analysis of the fractured reservoir.

The Westphalian strata consist of interbedded mudstones, coal seams and fluvial to deltaic sandstone bodies. Although matrix permeabilities are low a considerable secondary porosity forms the main storage volume. The structural mechanisms of the inversion are described by 3D-balancing which also allows detailed insights on the local stress fields at the block boundaries.

Microscanner and core data suggest two phases of fracture generation. A first paired fracture system is steep, orthogonal and related to a NNW-SSE trending basinal block boundary. It is completely sealed by carbonate and quartz mineralization. A second steeply dipping fracture set has been generated during horst block uplift in the sigma 1 trend of the local stress fields. These fractures as well as reopened older planes have remained open and control the overall effective permeability within the reservoir. The relative timing of fracture generation is supported by fluid inclusion analyses that specify the trapping mechanisms.

Hence, the structure is considered a classical example of a highly productive naturally fractured gas reservoir in porous but low permeability sandstone facies.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria