Different aspects of detection of karstified reservoirs – case examples from gravimetry and seismic interpretation.

Here, we present three different case scenarios highlighting different aspects of identifying karstified reservoirs. Karstified carbonates are emerging as a new play model on the Norwegian shelf, ref 7120/1-3 ‘Gohta’ and 7220/11-1 ‘Alta’ discoveries where karstification enhanced the reservoir properties. The first case scenario focuses on the use of microgravimetry in an onshore setting. Open cave passages and other karst features form negative density contrasts expressed in gravity field anomalies. With the aid of 3D forward modelling of surface gravity measurements, we investigated a large karst cave complex, known as the Svarthammarhola cave, in the Caledonian nappe setting of Nordland. Distinct gravity lows detected in the gravity survey were interpreted as previously unknown and inaccessible cave rooms, substantially expanding the overall Svarthammarhola cave volume. Our results also have wider implications on how gravity field data can be used for the understanding of complex subsurface karst features. We present an interpretation approach for microgravimetry, applicable for challenging geological settings with heterogeneous lithologies. The results were published in Norwegian Journal of Geology in November 2018. Our second case scenario addresses the identification of karst elements and karst landforms on seismic data. Such identification is a function of size distribution, seismic resolution and the issue of ruling out isomorphs – similar non-karst landforms with another genesis giving the same seismic expression. Thus, when it comes to identifying karst features on seismic, identification will be made on secondary observations, indicative but not conclusive evidence for karst features. Other plausible explanations for the same seismo-morphological features must be taken into consideration when assessing paleokarst in the subsurface. We present interpretations from the Loppa High of the Barents Sea, where we identified two regions – one with clear karst identification on seismic supported by secondary arguments and one with more equivocal interpretations where alternative models may be valid. The implication of these alternative karst models for exploration studies is significant - especially if expected karst-related high porosity zones are in fact absent. To be submitted. In the 3rd case scenario we compare settings of karst morphology onshore Norway with karstifiable rocks of the stratigraphic record of the Norwegian Continental Shelf. Karst morphologies, such as caves and dolines are common elements of the carbonates onshore Norway. The aim is to get an overview of where to possibly find karstified reservoirs buried on the Norwegian shelf, and what kind of karst one might expect to find in these offshore, submerged settings. In prep.