CHARACTERIZATION OF A SHALLOW MARINE SANDSTONE RESERVOIR IN A SYN-RIFT SETTING: THE BENTHEIM SANDSTONE FORMATION (VALANGINIAN) OF THE RÜHLERMOOR FIELD, LOWER SAXONY BASIN, NW GERMANY

Abstract

ABSTRACT The Bentheim Sandstone Formation is of Valanginian age and consists of several sheet-like bodies (each up to tens of meters thick) located in the western part of the Lower Saxony Basin of NW Germany. This study compares the reservoir characteristics of the Bentheim Sandstone Formation at outcrop in the Bad Bentheim area and in the subsurface where it forms an oil reservoir in the Riihlermoor field. The formation is interpreted to have been deposited along a tidal, embayed coastline at the margin of a small, partially enclosed sea. Rapid thickness variations of the total unit suggest it was deposited in a syn-rift setting of horst and graben. Toward the south and east, the sandstone interfingers with basinal shales. Outcrop facies analysis has identified three main facies associations: (1) a tide-influenced fluvial deltaic association; (2) a wave-influenced fluvial deltaic association, and (3) a tide-dominated deltaic association. All three facies associations are sand-dominated. Progradation of the deltaic system in all three cases resulted in the formation of sandier-upward facies successions composed of mudstone or heterolithic facies in the lower part and cleaner, more massive sandstone in the upper part. Comparison with contemporaneously deposited reservoir sandstones has been assisted by the collection of outcrop gamma ray and permeability data. A stratigraphic methodology derived from study of outcrop can be applied to the subsurface. In both outcrop and subsurface, the Bentheim Sandstone Formation can be subdivided into three genetic sequences (Bentheim 1, 2 and 3) which are retrogradationally stacked, back-stepping toward the basin margin in a north-westerly direction. Genetic sequences are used to define progradational-retrogradational phases of deposition within which stacked, sandier-upward successions form a stratigraphic subunit correlatable over several kilometers. These subunits are recognized in the subsurface and are interpreted to possess a sigmoidal geometry. Sedimentation was strongly influenced by synsedimentary tectonism. This tectonic influence focused deltaic depositional systems into a series of W-E and NW-SE oriented grabens. In both the outcrop and subsurface areas this influence is strong and has a marked control on the distribution of facies and, consequently, on reservoir thickness and quality.