Reservoir Potential of Fluvial Sheet Sandstone: Ten Boer Claystone, Southern Permian Basin

Marinus E. Donselaar¹, Irina Overeem¹, Johannes H. Reichwein¹, and Clemens A. Visser^2
1Applied Earth Sciences, Delft University of Technology, Delft, Netherlands
²Nederlandse Aardolie Maatschappij B.V., Assen, Netherlands

Gas production is declining in the small fields surrounding the giant Dutch Rotliegend Slochteren Gas Field. To postpone end-of-field-life and abandonment of infrastructure the potential as secondary reservoir of thin sheet sandstone beds in the overlying Ten Boer Claystone Member (TBCM) was investigated. First perforation tests in thin Ten Boer sheet sandstone proved very successful and produced 60 Mm³ gas at only 50Keuro extra cost. The success boosted research in the reservoir architecture of the sheet sandstone. Challenges are to assess the size, shape, lateral connectivity and spatial distribution from wide-spaced well data and outcrop analogues.

The TBCM is a 40-110 m thick shale-prone succession with thin sheet sandstone beds deposited as unconfined terminal lobes at the end of fluvial channels on a mud flat bordering the Silverpit Desert Lake. The succession was subdivided in cycles on the basis of combined core and well-log analysis. Each cycle comprises a shaley base, a sandy-silty middle part and a shaley top, interpreted as the result of wet-dry-wet climate cyclicity. Net sand maps based on the log-correlated succession in a 300 by 160 km area showed the development in space and time of nested sandstone sheets at the end of widely spaced south-north fluvial pathways.

An outcrop analogue study in the distal part of the Huesca Fluvial Fan (Miocene, Spain) yielded size and shape data sets of terminal lobe sandstone sheets and associated fluvial ribbon sandstone. The data were used in a numerical forward modeling study to test reservoir architecture scenarios of unconfined flow paths at the fluvial channel termination.

The study yielded a facies model of fluvial sandstone ribbons acting as permeability conduits in sand-on-sand contact with sheet sandstone. The results will be applied to refine reservoir architecture models in a distal fluvial setting.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas