Multiple-scale facies and reservoir quality variations within a dolomite body – Outcrop analog study from the Middle Triassic, SW German Basin

Abstract

This study focuses on a porous dolomite body in the proximal part of an epeiric carbonate ramp (Triassic Muschelkalk, 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 types and is built by a three-fold hierarchy of cycles (3rd-order to 5th-order). 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 late diagenetic modification. Reservoir quality significantly decreases in the non-dolomitized foreshoal section (mean: K = 4.5 mD; φ = 3.6%). Stratigraphically, the most porous dolomite occurs in the upper regressive portion of the 3rd-order 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 on the scale of several 10s of meters. Within correlated 5th-order cycles, considerable facies and poroperm heterogeneity is already observed on the 100s of meters-scale. On a 1000s of meters-scale, 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. Lithology, facies and poroperm data from outcrops and cores were used for 3D-modeling with the software Petrel. The modeled 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 heterogeneity of inner ramp dolomite bodies and can be applied to improve characterization and modeling of similar dolomite reservoirs.