Aspects of generation and migration of hydrocarbons from coal-bearing strata of the Hitra formation, Haltenbanken area, offshore Norway

Abstract

The Haltenbanken area, situated 150–200 km offshore mid-Norway is a proven gas/condensate and oil producing province with continuing exploration potential. In this structurally and stratigraphically complex area with multiple source rock possibilities, we have examined the Upper Triassic/Lower Jurassic Hitra formation to determine its possible contribution to the hydrocarbon pools in the area. The 400 m thick Hitra formation consists of interbedded shales, siltstones, sandstones and coal seams. In this study conventional cores of the Hitra formation from depths varying from 2.4. and 4.6 km have been examined with the objective to obtain a better understanding of hydrocarbon generation, migration and expulsion processes in these coals as function of variation in facies and maturity. In addition to basic geochemical measurements (TOC, Rock-Eval and GC analysis of C15+ saturated hydrocarbons), microscopic studies (normal and UV reflected light) were carried out to determine the abundance and initial distribution of hydrogen rich macerals including fluorescent mobile phases such as exsudatinities. Geochemical and microscopal evidence was obtained suggesting that some of the interbedded sandstones show indications of impregnation by migrated hydrocarbons. Some bitumen filled micro fractures were observed extending from the coals towards the adjacent sands. A preliminary interpretation of these data is that these fractures represent conduits carrying hydrocarbons from the coal seams towards the sandstones. The role of the kerogen type variations on the amount of hydrocarbons generated and the mechanism and efficiency of hydrocarbon expulsion from the coals is discussed.