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.

Tiñuformationen i Oaxaca, Mexiko är den enda fossilförande enheten från undre paleozoikum mellan den Laurentiska plattformen i nordvästra Mexiko och de Gondwanska lagerföljderna i Sydamerika. Det har nyligen visats att Tiñusektionen är en koncentrerad passiv kantlagerföljd med Gondwanakaraktär. Formationen delas upp i två delar; Yudachica från översta kambrium, som vilar okonformt på mellanproterozoisk grund, och Río Salinas från undre ordovicium (tremadoc). Formationen har tidigare studerats med avseende på dess avsättningsmiljöer och det fossila innehållet av trilobiter och konodonter vilka båda ger utmärkt biostratigrafisk kontroll över formationen. Ungefär 1000 exemplar av Linguliforma brachiopoder från tolv kalkstenslager från Tiñuformationen har studerats. Detaljerade undersökningar om taxonomi och stratigrafisk distribution av taxa har utförts. Faunan omfattar nio acrotretida taxa och en ny siphonotretid art och ett fåtal lingulida fragment. De acrotretida och det siphonotretida taxa har beskrivits grundligt och klassificerats till släkte. / The Tiñu Formation of Oaxaca State (Mexico) is the only fossiliferous lower Palaeozoic unit between the Laurentian platform in northwest Mexico and the Gondwanan successions in Andean South America. It has recently been shown that the Tiñu section is a condensed passive margin succession with Gondwanan character. The formation is divided into two members, namely, the uppermost Cambrian Yudachica Member, which rests nonconformably on a middle Proterozoic basement, and the Lower Ordovician (Tremadoc) Río Salinas Member. The formation has been studied with respect to its depositional environments and its fossil content of trilobites and conodonts, both providing excellent biostratigraphical control for the formation. About 1000 specimens of Linguliform brachiopods of twelve limestone horizons of the Tiñu Formation have been studied. Detailed investigations on taxonomy and stratigraphic distribution of the taxa have been made. The fauna comprises nine acrotretid taxa, a new siphonotretid species and a few linguloid fragments. The acrotretid and siphonotretid taxa have been thoroughly describes and classified to genus.