KUHN, MARTIN, Wintershall AG, Barnstorf, Germany

ABSTRACT: Synsedimentary Tectonics, Distribution, and Reservoir Quality of Rotliegende Sandstones: Emsland Province, Northwest Germany

The exploration for gas in the Rotliegende of the Emsland province, northwest Germany, has had very limited success so far. This fact is contrasted by less than 30 km distance to the giant Groningen gas field. Risks concerning Rotliegende hydrocarbon reservoirs in this area are (a) severe deterioration of permeability by widespread growth of illite in pore space and (b) absence of upper Rotliegende strata in certain areas. The Rotliegende of the Emsland was deposited in an embayment of the main west-northwest-east-southeast Rotliegende basin. Intensive growth of illite in pore space is confined to the northern part of the investigated area. A southern part, about 50 km southward, with only limited well control in between, displays an erratic spatial distribution of clay- and sandstone . A total of 450-m conventional cores of 22 wells in the Rotliegende were evaluated as to their sedimentary facies and petrography. A plateau basalt represents the lower Rotliegende, with a maximum thickness of 135 m. It was covered by a red-laminated claystone of up to 65 m thickness. After deposition of this claystone, a half-graben structure was formed, filled up with clean mature medium-grained sandstones up to 150 m thick, and deposited in a desert environment. Subsidence in the southern part ended after deposition of the so-called Haren sandstone, which is of good reservoir quality. The northern part continued to subside, and sandstones of a regressive lakeshore environment alternate with transgressive claystones. Erosion of the southern area is documented (a) in the distinct amoun of detrital basalt fragments in the sands shedded to the north and (b) the absence of upper Rotliegende strata. We assume the composition of detritus, together with depth, to be the main factors creating illite growth in this northern area.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.