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Zechstein 2 carbonate diagenesis and porosity study, Lower Saxony Basin, NW Germany

The role of deep-burial dissolution on the creation of secondary porosity in carbonates has been discussed controversially in the recent past. A good study side for this topic is given by the Upper Permian (Zechstein 2) platform carbonates of the Lower Saxony Basin in NW Germany, which is locally characterised by very high amounts of CO₂ and some H₂S (“CO₂ province”). The Zechstein 2 carbonate is used to demonstrate that fluid migration along deep fault zones can create or redistribute burial porosity along fractures in a reservoir characterized by alternating dolomitic and calcitic intervals. The petrophysical properties of carbonates, such as porosity and permeability, can be affected significantly by hydrothermal dolomitization. The studied wells show a dense fracture system, in which hydrothermal saddle dolomite, calcite and fluorite are the major vein cements. In order to evaluate the influence of hydrothermal dolomitization on porosity development, classic carbonate petrography is combined with carbon, oxygen, and sulphur stable isotopes, and rare earth element-and-yttrium (REY) analyses. Gas migration pathways are constrained by Raman spectroscopy and δ¹³C analysis of fluid inclusions trapped in vein cements. Several organic and inorganic sources for the high amounts of CO₂ are conceivable, such as thermochemical sulphate reduction (TSR), mantle degassing, or the thermal decomposition of carbonates.

This study is in cooperation with ExxonMobil Production GmbH, Hannover, Germany (EMPG).

Associated researchers: Bianca Biehl, Lars Reuning