Key Challenges for Understanding Zechstein Carbonate Reservoirs in NW Europe

Historically, Zechstein carbonate plays in NW Europe have had varying production success. The important influence of reservoir properties is illustrated by a comparison of the Ardmore, Auk and Dalen fields. Ardmore (once known as Argyll, now active as Alma) and Auk lie within the UK Central North Sea, have comparable Zechstein stratigraphy and both are in Quad 30 (ca. 40 km apart). Both fields have strong aquifer drive and Zechstein wells in both have a similar range of cumulative oil production volumes, with pressure depletion over the life of the fields. Whereas on Ardmore the spatial position of a well within the field has an impact on produced volumes, the volumes produced from Auk wells do not have a systematic spatial relationship. The Dalen field in the Drenthe province, onshore Netherlands, is very different from Ardmore and Auk. There is little evidence of aquifer support, only one well shows significant water production; production rates are relatively low and there is no evidence of pressure depletion. Understanding the connectivity of the carbonates is critical in explaining the observed differences.

Zechstein carbonates within the Southern Permian Basin, from the southern North Sea to Poland, have been widely studied, although peer-reviewed literature from different sub-basins includes sequence stratigraphic interpretations that are inconsistent across the whole basin. In the Northern Permian Basin, the central and northern North Sea are relatively under-studied, and there are still divergent opinions on facies assignation in cored wells (e.g. Ardmore and Auk fields designated as shallow-water facies by Trewin et al., 2003, and deep-water facies by Taylor, 1998).

Diagenesis in carbonate-evaporite systems is complex and can variously lead either to permeability reduction or enhancement: effects include lateral and vertical variation between limestone and dolomite, complex history of cementation, evaporite-filled fractures, and enigmatic diagenetic fabrics. Although diagenesis typically follows a particular path, prediction of porosity-permeability properties are difficult.

Another common challenge in most Zechstein carbonate reservoirs is that fracture permeability is significant for production. This can be associated with early water production, a relatively common occurrence for Zechstein carbonate reservoirs. Sometimes fracture permeability is higher or lower than anticipated, with significant impact for well (and field) productivity. Understanding the mechanical stratigraphy, in particular identifying any units which have pronounced fracture properties (either a tendency to retard fractures and act as a mechanical barrier, or units with a tendency to be highly fractured) will be useful in optimising development.

Brecciation is also common in Zechstein carbonate reservoirs, though is not encountered in every field. There are multiple causes of brecciation, including debris flows, karst, or collapse following evaporite removal. The significance of brecciation for fluid flow is dependent upon breccia body shape, degree of cementation, timing, and lateral and vertical extent. Understanding the cause of brecciation and any implications for breccia body permeability and extent can be beneficial.

To tackle these challenges, and improve our understanding of the key factors affecting the performance of Zechstein reservoirs, we have undertaken an extensive multi-disciplinary study involving integration and interpretation of new and legacy data from outcrops and sub-surface. World class Zechstein carbonate outcrops in NE England have been examined in conjunction with cored wells and production data to improve understanding of connectivity within Zechstein carbonates. In particular, the characterisation of fractures has been extensively documented through lidar, photogrammetric and field measurements, and analysed within their structural, stratigraphic and diagenetic context. A revised basin-wide sequence stratigraphic framework allows application of the derived mechanical understanding.