Relationship between petroleum generation, migration and sandstone diagenesis, Middle Jurassic, Gifhorn Trough, N Germany

Abstract

The Gifhorn Trough in North Germany contains an interbedded succession of Lower to Middle Jurassic source and reservoir rocks. Regional variations in the burial history made it possible to investigate different stages in the maturation history of source rocks and the diagenetic history of reservoir rocks. This study focuses in particular on the temporal and spatial relation between secondary petroleum migration and diagenetic events in the reservoir sandstones. Reconstruction of the burial and geothermal history indicate the initiation of petroleum generation at the beginning of the Upper Cretaceous. Lower Toarcian shales have been identified as the common source rock of all oils, based on biomarker and stable isotope studies. Differences in the composition and maturity of reservoired oils are interpreted to be related to separated oil kitchens of different maximum source rock maturity, rather than in-reservoir alteration after migration and accumulation. Major diagenetic events in the shallow marine, arenitic reservoir sandstones are linked to various processes operating during burial: (a) Fe-calcite cement formation — isotope analyses indicate that this is facies-related from carbonate redistributed at shallow burial depth (800–1200 m); (b) Fe-dolomite — dolomitization of early formed Fe-calcite is discussed based on pore water and oxygen isotopic composition; (c) late kaolinite — two models are discussed to explain the observed simultaneous formation of kaolinite and the creation of minor amounts of secondary porosity; (d) late quartz overgrowths — diffusive silica transport from quartz grain dissolution in shales into adjacent sandstones is proposed as a formation mechanism. Geochemical analyses of carbonate-cemented intervals suggest that early generated condensate was trapped during carbonate precipitation, prior to the main phase of petroleum migration. These early formed carbonate-cemented horizons probably compartmentalized the reservoir, controlled petroleum migration routes and promoted the formation of diagenetic traps.