Abstract: Zechstein 2 Carbonate Reservoir Prediction and Development in the West-Emsland Area, Upper Permian, Northwest Germany - An Integrated Approach
Werner Dauben, Christian Strohmenger
The Zechstein 2 Carbonate (Ca2) comprises a complete 3rd-order sequence, which is subdivided into seven parasequences recognized by subfacies successions in cores and combined gamma-ray and sonic-log responses. Besides higher-order sea-level changes and synsedimentary structuring, the paleotopography built by the underlying Werra-Anhydrite (A1) controlled the deposition of the Ca2.
The A1-platform was extensively karstified during a third-order sea-level lowstand at the end of A1 time and during the early transgressive phase of the Ca2-basin and -slope deposition. The Ca2 maximum flooding corresponds to the inundation of the A1-platform and the filling of the A1-karst cavities.
Four shallowing-upward parasequences represent the Ca2-highstand systems tract on the platform. They are defined by flooding surfaces in cores and by an abrupt increase in sonic velocity. High energy, shallow subtidal facies at the base of each parasequence pass gradually upward into tidal flat facies and, at the top of the 5th (and the 6th) Ca2-parasequence(s), even into supratidal sabkha facies. This dominant intra-Ca2 lowstand separates two major shallowing-upward cycles. The lower one is typically dominated by intertidal, mud-rich facies, whereas the upper one usually contains thick packages of originally porous, shallow subtidal shoal and bar facies. The ideal example of this subfacies succession occurs at the Ca2-platform margin. A lowstand wedge, which prograded over thick uppe and middle slope facies at the end of Ca2 time, expands the reservoir basinward. Correlating these potential pay zones in cores and well logs finally led to a 3-D model of the reservoir architecture.
Thickness relationships among the A1, the Ca2, and the overlying Basal Anhydrite (A2) and the Staßfurt Salt (Na2), emphasized by abrupt thickness changes of these lithologic units in a predominant prograding geometry, help to predict top-Ca2 facies belts. The dramatic thickness changes occur at the A1- and the A2-platform margins, limiting the most prospective reservoir zone.
3-D seismic interpretations delineate these margins and support the reservoir characterization by seismic attribute analysis.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France