The Northwest German sedimentary basin has a complex geological history. Its deeper underground has amazingly complicated lines of structure, with high blocks separated by deep sedimentary troughs. It can now be proved that the 68 oil and gas fields discovered in the Northwest German basin are closely related genetically to the distribution of such subsurface troughs within the basin. These subsurface troughs were developed from the Liassic onward, but the first framework had probably already originated in the Zechstein (Upper Permian). Their characteristic feature is the variation in thickness of sediments, which in the basin deeps may increase two- and threefold above the more normal thicknesses. The Liassic, for instance, which has a normal thickness of 200–300 meters, may increase in parts of the troughs up to 1,200 meters.
A further characteristic is the principal part played by bituminous rocks. Oil shales are known from the Rhaetic (Upper Triassic) to the Barremian (Lower Cretaceous), but primary bituminous remains can also be found widely spread in marls, dolomites, and limestones. These bituminous series are not only restricted to the purely marine facies of the basin sediments but they are also fairly well developed in the saliferous facies of the Portland (Upper Jurassic) and in the brackish-limnic Wealden.
About 200 salt domes have been proved within the limits of the Northwest German basin. The time of their origin varies; it was proved, however, that the first uprising of salt must have already taken place in the upper Dogger. The migration of oil and gas from the source rocks in the deepest parts of the troughs into higher reservoir rocks had already occurred at the time of formation of the pronounced upper Dogger traps. It must further be expected that from the beginning of sedimentation, especially at the boundaries of the troughs, stratigraphic traps were also already existent and that, therefore, formation of oil fields had already started during the Jurassic. There is evidencc for such pre-Neocomian oil deposits, which were later completely or partly destroyed. The formation of the present oil deposits is, however, much younger; it took place, in most cases, in the Upper Cretaceous or in the early Tertiary, and in some places even later.
A repetition of processes leading to the formation of the Northwest German oil fields must be assumed; that is, we see a multiple origin for the oil series from the Permian to the Lower Cretaceous. We are concerned with multiple migration from the troughs and multiple accumulation in the different types of oil traps—salt domes, anticlines, and stratigraphic traps.
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Habitat of Oil
The history of oil exploration in a large basin is very much like the history of research in most fields of investigation. In the history of research into the subject of oil occurrence, however, the rate of increase of knowledge has fluctuated greatly. Sourced from the 1955 AAPG Annual Meeting, this publication contains many of the papers presented at that meeting, which discuss the habitat of most of the oil found in the world prior to 1955.