Petrophysical evaluation of the Gassum and Frederikshavn formations in the Gassum-1, Voldum-1, Rønde-1, Horsens-1 and Hobro-1 wells. Results of log interpretation, evaluation of reservoir parameters and a description of the interpretation procedure

The present evaluation forms part of an integrated study on the geothermal potential of APMH area of interest at Århus. The present sub-study deals with an analysis of well logs acquired in Gassum-1, Voldum-1, Rønde-1, Horsens-1 and Hobro-1. The objective is characterising the Gassum and Frederikshavn formations with respect to lithology, porosity, permeability, shale volume and thickness of net reservoir sandstone. The intention of the petrophysical study is to provide a basis for deciding on an appropriate exploration strategy within the Århus area. The current sequence stratigraphic subdivision of the Gassum Formation is applied on the well log figures; the subdivision may be slightly adjusted during the progress of sequence stratigraphic part of the study, but a revision will not influence the estimated reservoir parameters.

Log data are available from the five wells, but the log quality varies considerably due to rather old wells and occasionally, also poor borehole conditions. The existing log interpretation has been revised, based on a thorough log quality assessment and supplementary geological information from nearby wells. Cores are cut in the Gassum-1 and Horsens-1 wells, but no drill cores are available from Voldum-1, Rønde-1 and Hobro-1. In addition, the Well Completion Reports provide information about the reservoir rocks including geological and stratigraphic data.

The present log analysis includes a detailed lithological interpretation of the drilled sections, and the lithological interpretation is based on either the GR log or the SP log combined with information from cores and cuttings descriptions. The cores provide valuable information about quality of the reservoir sandstones and the geothermal potential of the sandstones. The logs may be used to induce information from the cored sections into the un-cored intervals and hence, the available cores and measurements of porosity and permeability (CCAL) provide support to the log interpretations.

In the Gassum-1 and Horsens-1 wells, only incomplete log suites are available for porosity evaluation, and a standard petro-physical evaluation cannot be carried out. However, cores are cut in these two wells, and a number of core porosity-permeability data exist, so information about e.g. porosity may be extrapolated to un-cored intervals using the sparse log data. The permeability is estimated from porosity-permeability relations (see below). The GR and SP logs provide information about the shale volume.