Petrophysical and GeophysicalAnalysis of Ordovician Limestone Mounds for the Purpose of Hydrocarbon Reservoir Exploration : Petrofysisk och geofysisk analys av Ordoviciska kalkstensstrukturer för hydrokarbonprospektering

Abstract: Gripen Oil & Gas have extended their prospecting license on Gotland to the north end of the island to search for potential hydrocarbon reservoirs. Earlier prospecting has shown that limestone mound structures from Ordovicium have high potential as hydrocarbon reservoirs due to their antiformal shape and the petrophysical properties of the limestone.This study focuses on the petrophysical properties of the intramound lithofacies of the Ordovician mound structures. Analyses are made to determine the density, porosity and ultrasonic velocity of ten drill core samples (both in dry and water-filled state) from different places on Gotland and these factors are compared to see how they affect one another. Seismic reflection data gathered by OPAB (provided by SGU) is also used to locate potential mound structures in northern Gotland.The results show a clear connection between the density and porosity. Increasing porosities also decreases the ultrasonic velocities of the rocks. The saturated samples show higher velocities for the compressional waves than in the dry samples, but the shear wave velocity is similar in both dry and saturated rocks. Acoustic impedance is used to link the petrophysical analysis to the geophysical data by explaining why one reflector can be seen more clearly than others in a seismic section. Using the seismic sections, five possible mounds are found on northern Gotland. Good correlations are found in the petrophysical analyses, but the porosity of the rock does not seem to be the deciding factor in choosing a reservoir. There is no association between a rock’s measured porosity and its potential extraction volume, clearly shown by the low porosity measured in the Risungs drill cores compared to how much volume of oil that has been extracted. Instead, one should look closer at the seismic sections to find mounds with a high degree of fracturing. None of the mounds seen in the seismic sections show any apparent fracturing, and more seismic surveys should be made before deciding where to drill.