Diagenesis of the Zechstein Ca-2 carbonate from the Løgumkloster-1 well, Denmark

Abstract

The carbonate unit Ca-2 found in the Løgumkloster-l well is located on the northern marginal carbonate platform of the North German Zechstein Basin. The ea. 43 m thick sequence includes former oncoidal/algal muds which were deposited in a relatively quiet lagoonal back-barrier environment, and ooidal carbonate sands, deposited in a rather agitated shoal environment. The carbonate sediments of Ca-2 have been subjected to a complex sequence of diagenetic events. However, the present porosity/ permeability, and thus the reservoir quality of the rocks, is primarily linked to four events: two leaching phases, a phase of chemical compaction, and a late anhydritization. The first phase of leaching created mouldic porosity. The last phase of leaching was important as it resulted in a widening of the preexisting pores and fractures and thereby facilitated percolation of sulphate solutions causing the late anhydritization, which considerably reduced the porosity/ permeability of the rock. The present pore geometry of the rock is complex. In the oolitic intervals the intra- and inter-ooidal porosity types are the most widespread. In the oncolitic/algal intervals intra- and inter-oncoidal porosity and vuggy porosity are often combined with intercrystalline porosity. The corresponding Ca-2 rock in the Brøns-l well, situated ca. 20 km northwest of the Løgumkloster- l well, was subjected to the same post-depositional evolution. Comparison with the diagenesis of the upper part of the Ca-la formation in the Aabenraa-1 well and the Ca-1 formation of the Varmes-1 well also shows close similarity in the diagenetic evolution.