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Methods and Applications |
Geologisches Landesamt Nordrhein-Jestfalan, de Greiff Str. 195, 4150 Krefeld, F.R. Germany
In northwestern Germany most natural gas deposits have been sourced by coalification gases derived from the coal-bearing Upper Carboniferous. This is the reason why a new coalification map for the top surface of the Carboniferous (1:500 000) is presented as a supplement and revision of a former map published in 1979 by the same authors. 130 new measurements of vitrinite reflectance (% Rm) from 72 deep boreholes and 60 outcrops have been introduced into the new map. Recent data on the stratigraphy of the top surface of the Carboniferous and its depth below the Permian, Mesozoic and Cainozoic cover, as well as new gas fields (found since 1978) have likewise been incorporated.
The coalification pattern of the uppermost surface of the Carboniferous is demonstrated in a cross section running north-south from the Rhenish massif (south of the Ruhr Basin) to the North Sea. The coalification pattern of the Pre-Permian surface in northwestern Germany is controlled
Rank maxima in the Gifhorn trough at the upper reaches of the River Weser (south of Hameln) are caused by deep subsidence, especially in Jurassic times, and probably also by geothermal maxima. At the mouth of the River Ems the degree of coalification is surprisingly low, although the Upper Carboniferous, comprising early Westphalian (A and B) is covered by 3500–4000 m of younger sediments. It is assumed that because of a basin that is still strongly subsiding, the degree of coalification will not yet have reached the equilibrium which would relate to the present depth and temperature, and that gas generation and accumulation are still taking place here.
The relationship between rank of coal and the occurrence of natural gas deposits generally remain the same as demonstrated in 1979; most gas fields occur where the vitrinite reflectance at the upper surface of the Carboniferous is between 1–2% Rm, — provided, of course, that adequate reservoir rocks of the Upper Carboniferous (Westphalian C, D and Stephanian), Permian (mainly Rotliegendes) and Lower Triassic (Bunter) are present and sealed by evaporites of Permian and Triassic age. A striking exception from the internationally acknowledged ‘death line’ for the occurrence of natural gas fields (commonly assumed at 3.0–3.5% Rm) is represented by the gas field Uchte where the vitrinite reflectance is 4.5% Rm at the surface of the Carboniferous. This anomaly can be explained by a strong but short telemagmatic heating event, with coalification having preceded porosity loss of the reservoir rocks, and/or by fracture porosity gained through tectonic disturbance of the rocks.
This study indicates that the natural gas deposits of the Federal Republic of Germany accumulated in relatively recent geological times, i.e. during the late Mesozoic and/or the Cainozoic, either as a result of an additional subsidence and consequently additional heating of the Upper Carboniferous, or of telemagmatic heating effects.
