FACIES AND POROPERM CHARACTERISTICS OF A CARBONATE SHOAL (MUSCHELKALK, SOUTH GERMAN BASIN): A RESERVOIR ANALOGUE INVESTIGATION
M. Ruf1* and T. Aigner1
1 University of Tübingen, Department of Geosciences, Sigwartstr. 10, D-72076 Tübingen, Germany.
*Corresponding author, e-mail: michael.ruf@uni-tuebingen.de
Outcrop analogue investigations contribute to the understanding of the architecture and behaviour of subsurface hydrocarbon reservoirs. This study was designed to provide input for static reservoir modelling of shoal bodies on carbonate ramps.
We investigated an oolitic and shelly carbonate shoal of Late Muschelkalk (late Ladinian) age, about 30 km long and 15 km wide, exposed in a series of quarries in southern Germany. The dimensions of this shoal body correspond to both modern analogues in the Persian Gulf and to oilfields in the Middle East.
Fifteen outcrop locations were chosen to cover the entire shoal. A regional correlation based on facies, sequence analysis and outcrop gamma-ray logs was used as a framework for mapping the distribution of shoal (i. e. reservoir) facies and associated reservoir properties.
Three major depositional environments were recognized: foreshoal, shoal and backshoal. In each environment, specific vertical facies successions composed of thick shallowing-upward units and thin deepening-upward units forming metre-scale cycles were deposited. These small-scale cycles are stacked and form large-scale shallowing and deepening trends. Shoal facies developed preferentially in the regressive parts of small-scale cycles and were found to be laterally continuous over distances of at least several kilometres.
Porous intervals within the shoal facies are concentrated along the tops of the small-scale shallowing-upward cycles. The porous intervals in successive small-scale cycles increase in thickness and in absolute values of porosity upwards through the large-scale shallowing trend. Separate vug pore types are characterized by relatively low permeabilities (0 to about 10 mD) in spite of the high porosity (up to 20 %). The combined occurrence of separate vug and interparticle porosity leads to higher permeabilities (tens of mD).
Isolines of porosity, permeability and net/gross follow the contours of the carbonate shoal complex. The highest values of porosity appear more or less at the centre of the shoal, while the highest values of permeability are found at the shoal margins. The distribution of maximum porosity and maximum permeability zones are thus controlled by the cyclic seaward-stepping and landward-stepping of the shoal body.
The systematic variations in extent, stacking pattern and related petrophysical characteristics of the carbonate shoal bodies may be used for modelling shoal reservoirs in producing oilfields.