Reservoir architecture in a terminal alluvial plain: An outcrop analogue study (upper triassic, southern Germany) Part II: Cyclicity, controls and models

Abstract

This outcrop analogue study investigates Triassic fluvial sandstones of the Stubensandstein Formation which were deposited on a terminal alluvial plain under semi-arid to sub-humid climatic conditions in the land-locked South German Keuper Basin. The Stubensandstein may serve as an analogue for reservoir units in comparable continental basins. Data came from studies of 13 large sandpits, together with a subsurface database supported by three well-log clusters (comprising up to 20 boreholes) and four single wells. Lithofacies types vary along a palaeogeographical gradient from proximal erosion and non-deposition to fluvial bed-load, to distal suspended-load deposits and finally to playas with lacustrine carbonates. Within a three-fold hierarchy of cyclicity, meso- and macro-scale cycles could be correlated regionally using an approach based on stratigraphic base-level changes. The observed sedimentary patterns, together with palaeosol types, their stable isotope signals and palynological data, indicate that palaeoclimatic fluctuations exerted the principal control on stratigraphic architecture and cyclicity. Progradation of sand wedges into the basin occurred while climate was arid but becoming more humid. As playa lake levels rose, the alluvial sands retrograded. Shorter and longer-term palaeoclimatic fluctuations caused systematic changes in reservior and seal geometries, and in the extent and interconnectedness of sandbodies. Thus, a knowledge of the overall palaeoclimatic trends in comparable continental basins may allow predictions to be made concerning the regional reservoir architecture. Outcrop data were used to generate static reservior models of sandbody architecture in subsurface well log data sets. A stochastic facies and poro-perm simulation was run and calibrated with outcrop data. Different stratigraphic levels could be reproduced, as could the scouring and interbedding of the different fluvial systems.