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Multiple-Scale Facies and Reservoir Quality Variations within a Dolomite Body

Koehrer, Bastian S.1; Aigner, Tom 1
1 Institute of Geosciences, University of Tuebingen, Tuebingen, Germany.

This study focuses on a porous dolomite body marking the proximal part of an epeiric carbonate ramp (Triassic Muschelkalk in the South German Basin), analogous to hydrocarbon-bearing dolomite reservoirs in the Middle East (e.g. Khuff and Arab Formations).

The dolomite body is made up of shoal, lagoonal and peritidal facies and is composed of a three-fold hierarchy of cycles, possibly representing 3rd-order, 4th-order and 5th-order cyclicity. The best reservoir quality occurs in finely-crystalline lagoonal dolo-mudstones (mean: K = 37.8 mD; φ = 20.2 %) with micro-intercrystalline and vuggy porosity. Coarsely-crystalline, shoal-related dolo-grainstones show variable reservoir potential (mean: K = 30.1 mD; φ = 4.4) due to diagenetic modification. Reservoir quality significantly decreases in the non-dolomitized foreshoal section (mean: K = 4.5 mD; φ = 3.6%). Highest poroperm values are reached by a combination of interparticle, separate vug and touching vug pore space. Stratigraphically, the most porous dolomite occurs in the upper regressive portion of the large-scale cycle. Reservoir quality progressively decreases towards the limy bottom of the section.

Outcrop mapping showed high lateral continuity for the distribution of facies types and poroperm values over several 10’s of meters. Within correlated small-scale cycles, considerable facies and poroperm heterogeneity is already observed on the 100’s of meters scale. On a regional scale (1000’s of meters), the facies and lithology distribution is controlled by the geometry of a gently inclined carbonate ramp. During regression, a progradational pattern is recognized, marked by a basinward migration of backshoal facies and a down-dip decrease of dolomite body thickness.

Based on correlations on multiple scales, outcrop and core data was used for 3D-geological modeling with Petrel. The modelled variations in the poroperm distribution are consistent with the degree of heterogeneity expected for a combination of laterally persistent lithology and highly variable facies.

Results of this study may help to better understand the reservoir compartmentalization of inner ramp dolomite bodies and can be applied to improve characterization, prediction and modeling of similar dolomite reservoirs.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009