PETROGRAPHY AND ORIGIN OF DEPOSITS AT THE BENTHEIM BITUMEN MINE, NORTH-WESTERN GERMANY

The fracture-bound deposit at the Sieringhoek (Bentheim) bitumen mine in NW Germany has yielded a substantial quantity of bitumen ore. Empla cement of the bitumen into the fracture system occurred under high pre ssure, causing brecciation and impregnation of the host Gildehaus Sand stone. Subsequently the bitumen was brecciated and cemented by calcite and calcite-entrained sand. Bitumen injection followed fracturing int o a hydrocarbon reservoir at depth, generated from a Jurassic/Cretaceo us source rock. Diagenesis of the host rock was largely completed by t he time of bitumen injection, reflecting the long time gap during whic h source rock maturation occurred. Impurities in the bitumen include l arge quantities of rock fragments, abraded during injection, and cryst als of metal oxides and sulphides.