ARE SHOAL RESERVOIRS DISCRETE BODIES? A COQUINA SHOAL OUTCROP ANALOGUE FROM THE MID TRIASSIC UPPER MUSCHELKALK, SW GERMANY

A. Petrovic^1,2,* and T. Aigner¹

¹ Department of Geosciences, Centre of Applied Geosciences, University of Tübingen, Hölderlinstr.10, Tübingen, Germany.

² present address: Faculty of Geosciences, University of Bremen, Klagenfurterstraße, Bremen, Germany.

^* corresponding author, email: alexander.petrovic@uni-bremen.de

Carbonate ramp reservoirs may show a much higher degree of complexity than the schematic and often layer-cake type depositional models which are widely used in exploration and reservoir characterisation, especially in three dimensions. This complexity was investigated in an outcrop study of the Ladinian (Middle Triassic) Quaderkalk Formation (Upper Muschelkalk) from SW Germany. The formation was deposited on an epicontinental, gently inclined carbonate ramp in the regressive part of a third-order depositional sequence and represents a sub-seismic-scale coquina-dominated shoal reservoir analogue. In the study area, coquina carbonates make up a complex system of four stratigraphically separated shoal complexes. The present paper investigates the 1-D to 3-D facies distribution, reservoir architecture and sequence stratigraphic evolution of the largest shoal complex of the Quaderkalk Formation, the Oberer Hauptquader. The object of this outcrop analogue study was to improve the understanding of internal reservoir heterogeneities (such as connectivity and continuity) in similar carbonate shoal complexes in the subsurface. To that end, detailed analyses of 71 quarry outcrop and core samples were carried out, together with the study of more than 400 thin sections, the 2-D analysis of three quarry wall panels and regional-scale 2-D correlations, and facies mapping.

This investigation documents significant sedimentological heterogeneities at different scales within the studied shoal reservoir analogue. Instead of a continuous facies belt, a number of separate potential reservoirs form a shoal complex “mosaic”. Shoal development is controlled by sequence stratigraphic architecture, mainly by cycles which were responsible for shoal expansion during times of regression (forming potential reservoir-volume) and shoal drowning during times of transgression (forming seals and potential stratigraphic traps). Within the individual shoal complexes, thickness and lithofacies types show gradational lateral changes. Correspondingly, lithofacies associations within the studied shoal analogue are not defined by sharp boundaries. Thus the present “shoals” do not in fact represent discrete “bodies”, as often depicted in reservoir models and as commonly used in reservoir simulation. In addition, shoal-internal sedimentological heterogeneities (potential flow baffles in the subsurface) are strongly influenced by a combination of small-scale cyclicity and event-type deposition. This study therefore demonstrates the need for sedimentological studies of reservoir heterogeneities at various scales in order to decrease uncertainties and risks in exploration and production.

Keywords: carbonate ramp, SW Germany, Muschelkalk, Middle Triassic, reservoir architecture, coquina, outcrop analogue, sequence stratigraphy, reservoir body, facies boundary.

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