Geochemistry of the Upper Permian Ca-2 Deposits of the Løgumkloster-1 Well, South Denmark

Abstract

Rock samples of the Upper Permian Ca-2 unit of the Løgumkloster-l well have been subjected to chemical analysis. The Ca-2 interval was undoubtedly originally an aragonite which have undergone several episodes of diagenesis including a dolomitization and a heavy anhydrite mineralization. Two original facies types are recognized, an oolitic shoal facies, and a lagoonal carbonate facies. The samples have been characterized by neutron activation, atomic absorption, Xray diffraction, palaeomagnetism, optical and electron microscopical examination. Samples of the drilling mud has been analyzed as well, and it is found that contamination of the samples included in the present study from the drilling mud is unlikely to have happened. Imprints of both the original facies and the diagenetic events are found in the present work. The Sr and Ba abundances reflect the original facies-bound distribution, which is also reflected in the normalized REE distribution patterns. A low-Sr anhydrite component is clearly distinguishable in several parameter cross plots. Iron, Sc, REE and several other elements are found to be situated in the EDTA-insoluble residue, which has been identified primarily as a muscovite or clay mineral phase. Pyrite is present, although of small relative volume it is thought to be responsible for the occurrence of As, Sb, and possibly Se. A small fraction of the Cr, As, and Sb may originate from the heavy part of the crude oil, the introduction of which was the last process of alteration the rock experienced. A geomagnetically reversed period is encountered in the middle of the Ca-2 interval, and is tentatively identified as one of the Tatarian reversed events.