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Methods and Applications |
G.B. Petroleum Consultants Ltd., 18 Grosvenor Street, London, W1X 9FD
Zechstein carbonates form important gas reservoirs in Germany and the Netherlands and in at least one commercial field in the U.K. southern North Sea but elsewhere they are largely unexploited. The carbonates occur in four separate units and all are known to contain potential reservoir rocks. Distribution of porosity is controlled either by a combination of depositional facies and diagenesis or by the occurrence of collapse breccias. Intragranular porosity resulting from early leaching predominates in shallow-marine grainstones and intercrystalline porosity caused by late leaching occurs in deepermarine carbonate mudstones. Using a knowledge of facies distribution, it is possible to predict where these potential reservoirs might occur and as facies distribution can be related to the overall thickness of each carbonate unit, it is also possible to make tentative predictions of porosity distribution from isopach maps. Collapse breccias, however, are not related to any particular facies and potential reservoirs resulting from this process are most likely to be found around structural highs that became exposed at various times since, the Zechstein.
