Solution Of A Complex Structural Problem By 3D-Seismic

Abstract

van Deemter, Henk,* and van der Kallen, Albert W.H.,* *Shell Internationale Petroleum Maatschappij, assigned to Nederlandse Aardolie Petroleum Maatschappij, assigned to Nederlandse Aardolie Maatschappij, (NAM), Assen, The Netherlands Abstract In 1975 a three-dimensional seismic survey was carried-out near Coevorden (Drenthe Province in the eastern part of the Netherlands some 60 kms-5 mi-south of the Groningen gasfield, see fig. 1) covering a rectangular area of 4, 8 × 3, 25 kms (3 × 2 mi). This area is situated in a region known for its structural complexity. After 3D-processing including 3D-migration (completed in 1976) it appeared that the 3D-data contained much more information than the conventional 2D-seismic sections and allowed a detailed structural interpretation. As a result the subsurface target of a gas appraisal well could be selected in a structurally more favourable position, i.e. around 1250 m. (4100 ft) southsoutheast of the surface location. Subsequent drilling of this well (completed early 1978) confirmed the correctness of the interpretation. Introduction Appraisal drilling for natural gas on a high block in the Coevorden area (fig. 13) was delayed for some years as even a rather dense network (+ 250 × 250 m - 820 × 820 ft) of 2D-seismic data could not provide conclusive evidence for structural closure. In 1975 the Nederlandse Aardolie Maatschappij (NAM) was approached by Shell Internationale Petroleum Maatschappij, The Hague (SIPM) with the question whether NAM would be interested in a 3D pilot project, preferably in an area where it could be of help in a field development plan. NAM accepted SIPM's proposal and selected the rectangular area indicated on fig. 13 for a 3D seismic survey, where the information, besides its value as a pilot study, could help NAM's Production Geological Department in unraveling the structurally complex subsurface. The fieldwork was carried out by a German-based contractor under supervision of both SIPM and NAM. Seismic 3D data processing was done by a major contractor in Dallas (Texas, USA) under supervision of SIPM. The structural interpretation of the processed data was carried by NAM staff. In the following a review will be given of the geological/geophysical problems in the area concerned. The 3D-data will be compared with the original 2D-data to indicate to what extent 3D stacking and migration has contributed to the clarification of the structural picture at the main prospective level, leading to the successful drilling prospective level, leading to the successful drilling of an appraisal well. GEOLOGICAL SETTING IN THE AREA (fig. 2) The primary objective in the area for natural gas-exploration and -development is the Zechstein Z2-carbonate of late Permian age. The directly underlying Z1-sequence of anhydrite, carbonate and Coppershale rests unconformably on a peneplaned surface of the Upper Carboniferous Coalmeasures, the sandy intervals of which form a secondary objective. Natural gas originates from the Coalmeasures, generation and migration having taken place from Upper Cretaceous times to Recent. place from Upper Cretaceous times to Recent. The main seal is formed by the Z2 rock-salt overlying the Z2-anhydrite/carbonate interval. The Z2/Z1 sequence and the Carboniferous formations are dissected by numerous faults, sometimes with considerable throw (up to 500 m - 1640 ft), which predominantly control the traps. Interspersed in the Zechstein salt are slabs of competent rocks (Z3 anhydrites, dolomites and limestones) of largely unknown size and shape. They can be strongly independently tectonized: in some wells duplication and even triplication of the same markerbed has been observed.