Rock Physics Properties and Seismic Signatures of Hot Shales on the Norwegian Shelf
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Hot shales are made up of mainly clay, organic matter, silt and quartz, which cause hot shales to have a complex composition and texture. This makes it more advanced to do rock physics studies and AVO modeling. Complex relations and models are needed to do studies and to predict their seismic response. As the complexity increases, several input parameters are needed, which may not always be available. Seismic inversions of one selected random line from a 3-D seismic survey and eight wells with well logs such as compressional- and shear-wave velocity, density, gamma ray and resistivity were available for this thesis. The focus was on Spekk and Melke Formations. Rock physics studies and AVO modeling of hot shales show that organic rich shales are strongly affected by organic matter. Factors such as anisotropy, compaction, porosity, fractures, fluids and maturity will also affect the properties of Spekk significantly. A crossplot of acoustic impedance versus Vp/Vs ratio, which includes templates that shows the trend of shales and brine saturated sands, was made to see where hot shales plot compared to these formations. Spekk did mainly plot at Vp/Vs ratios similar to brine saturated sands.An AVO class III is located in a syncline between two wells in the Yttergryta area. AVO analysis was done at top Spekk and at top Draupne to study the response of hot shales. The results showed that hot shales may occur as AVO class III, and not necessarily as AVO class IV. Spekk in well 6507/8-6 was capped by a shaly claystone and had a different AVO response than hot shales capped by marls and carbonates.Mature hot shales could cause an amplitude anomaly similar to AVO class III, but because of minimal tectonic uplift and a burial depth of Spekk that has not yet reached the maturity window, the formation is most likely not mature. Spekk found at a deeper depth than the syncline, do not show any indications of mature Spekk, which may imply that the amplitude anomaly is caused by hydrocarbon filled sands. Based on my results, it was not possible to separate hot shales from hydrocarbon saturated sands. This is due to uncertainties in how properties of hot shales are affected by geological variability and uncertainties associated with data quality and model assumptions.