Evaporite Collapse, Karst and Detrital Carbonate Breccias in the Zechstein Reservoir of the Alma Field, Central North Sea: Characterisation, Controls and Implications for Reservoir Quality

Abstract

The Zechstein reservoir of the Alma field (originally Argyll, formerly Ardmore) comprises at least four Zechstein carbonate and evaporite sequences, the latter dissolved during Jurassic exposure, forming a series of collapse breccias that were modified by karst, erosion and faulting. It is essential to identify the different origins of these breccia bodies because these processes produce zones of excess permeability with contrasting stratiform and cross-cutting geometry. In core, these breccia bodies are distinguished by their clast assemblage and fabric, the relationship of clasts and matrix, the presence of sedimentary structures and the nature of their upper and lower boundaries. Predicting the distribution, architecture and reservoir quality of these geobodies is key to managing reservoir development programmes in similar carbonate fields affected by karst, collapse brecciation, reworking and faulting. It requires an understanding of the stratigraphy of the reservoir, particularly that of any internal aquicludes, mapping the palaeogeology of the top reservoir and understanding the onlap history of the exposure surface. The Alma reservoir contains a field-wide impermeable layer, the Sapropelic Dolomite deposited in a basinal setting that controlled the influx of meteoric water during exposure. The lower dolomite breccia, which underlies the Sapropelic Dolomite, represents a stratiform evaporite collapse breccia formed by dissolution in meteoric water that was introduced down-dip beneath the Sapropelic Dolomite. The upper dolomite breccia formed by dissolution of one or more evaporite units by direct infiltration of meteoric water from the top Zechstein surface. During the Jurassic, the top Zechstein surface was modified by karst, apart from the SW part of the Alma field, where the Zechstein was buried by the onlapping impermeable Triassic Smith Bank Formation. Core also shows that there is limited karst development over the sub-crop of the Sapropelic Dolomite. The Zechstein is partly onlapped by Jurassic detrital conglomerates reworked from the brecciated Zechstein and deposited in alluvial fan, shore face and low-energy subtidal settings along the western margin of the field. A well-preserved matrix pore system can be expected within collapse breccias and karst cavities where the Zechstein is overlain by Jurassic detrital sandstone and carbonate breccias. However, in areas onlapped by impermeable sediment, the karst and collapse breccias are likely to contain much poorer reservoir quality.