STROHMENGER, CHRISTIAN, and GUNTER JAGER, BEB Erdgas und Erdol GmbH, Hannover, Germany; JOHN C. MITCHELL and KAREN M. LOVE, Exxon Production Research Co., Houston, TX; MARCUS ANTONINI, REINHARD GAST, ERIK VAN DE POL, MICHAEL STEFFAN, and KONRAD ROCKENBAUCH, BEB Erdgas und Erdol GmbH, Hannover, Germany

ABSTRACT: An Integrated Approach to Zechstein Ca2 Carbonate Reservoir Facies Prediction in the South Oldenburg Area, Upper Permian, Northwest Germany

Integrating seismic, well log, and core data from the Zechstein Ca2 makes it possible to predict reservoir facies distribution, increasing likelihood of drilling successful wells.

Well logs and cores show that Ca2 thickness and facies distribution are related to thickness of underlying Werra Anhydrite (A1) and overlying Basal Anhydrite (A2). Thin (30-80 m), dolomitized, porous Ca2 platform facies occur where the A1 sulfate platform is thick (about 300 m). Thick upper and middle slope facies (100-250 m) lie basinward of an abrupt decrease of A1 thickness from 300 to 40 m, but platformward of an abrupt thinning of the A2 from 80 to 2 m. Slope facies are porous where not calcitized. Platform-influenced carbonates overlying the slope facies lie basinward of the A1 thickness change. Nonporous lower slope facies (40-100 m) occur farther basinward, overlying thin A1 (about 40 m). Syndepositional structuring causes anomalies in Ca2 thickness and facies distribution.

Intra-Ca2 mineralogical variation makes mapping intra-Basal Zechstein with thicknesses from three-dimensional (3-D) surveys difficult. However, the total thickness of the Basal Zechstein can confidently be mapped from 3-D seismic. Abrupt changes in Basal Zechstein interval thickness allows the location of A1 and A2 anhydrite "cliffs," providing control on Ca2 facies limits. Integrating total Basal Zechstein thickness maps made from seismic interpretations with maps made from well log interpretations yields maps that honor all data.

Potential reservoir zones are low-impedance layers in seismic models and appear as anomalies on seismic attribute maps. Lithologic variation hinders direct interpretation of anomalies as porosity. Improved prediction of calcitization trends will enhance recognition of porous zones.

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