Three-dimensional facies modeling of carbonate sand bodies: Outcrop analog study in an epicontinental basin (Triassic, southwest Germany)

Abstract

This article is the first part of an integrated study to characterize the anatomy and geometries of carbonate shoal reservoir bodies in epicontinental settings. It is based on outcrop analog data from Triassic layer-cake carbonates in the south German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The database of this study consists of 56 measured sections supplemented by six shallow cores and wireline logs, covering an area of 25 by 36 km (15 by 22 mi). The potential reservoir bodies consist generally of midramp shoal and shoal-fringe facies types, which are composed of skeletal and oolitic carbonate packstones and grainstones with significant amounts of porosity. The upper Muschelkalk is composed of a lower transgressive and an upper regressive interval; within these, shoal bodies show not only similarities but also major differences in character, geometry, and distribution. These reflect the conditions of an epeiric system, which reacts strongly to small changes in accommodation. The accommodation seems to be mainly triggered by the interaction of hierarchically organized large-, medium-, and small-scale relative sea level changes and a subtle paleorelief. At the level of medium-scale cycles, distinct two-dimensional cyclicity styles can be differentiated, which record the lateral facies distribution along the depositional gradient. Different styles of medium-scale cycles include different types of shoal bodies: (1) transgressive crinoidal shoal style: shoal bodies (mean width: 21 km [13 mi], mean length: 37 km [23 mi], mean thickness: 4.2 m [13.7 ft]) with low facies differentiation, deposited on top of subtle paleohighs; (2) skeletal sheets style: thin reservoir sheets (mean width: 5.1 km [3.1 mi], mean length: 11.1 km [6.8 mi], mean thickness: 0.3 m [0.9 ft]) of reservoir facies on top of the paleohighs; (3) regressive oolitic shoal style: shoal bodies (mean width: 14.6 km [9.0 mi], mean length: 20.8 km [12.9 mi], mean thickness: 0.69 m [2.2 ft]) with high facies differentiation on the flanks of paleohighs; and (4) low-accommodation style: patchy and mosaic distribution of shoal bodies (mean width: 11.2 km [6.9 mi], mean length: 26 km [16 mi], mean thickness: 0.69 m [2.2 ft]). All data were loaded into three-dimensional (3-D) modeling software to distribute 14 facies types and model the 3-D stratigraphic architecture. The resulting facies distribution implies that volume and dimensions of the shoal bodies are mainly controlled by the combination of stratigraphic cycles and a subtle paleorelief, which is indicated by overall thickness changes of succession. High-energy shoal facies types occur only in and around areas with a reduced overall thickness, whereas areas with a thicker development are dominated by low-energy, muddy facies types. All observations combined point to the presence of a subtle paleorelief, which could be induced by slight differential subsidence of inherited paleotectonic basement blocks. On the kilometer to regional scale, previous studies suggest simple layer-cake stratal patterns. However, in the full 3-D view, the apparent layer-cake stratigraphy turned out to be a pseudolayer cake, with very gentle (0.01–0.001 dip) clinoform geometries. These very subtle offlapping, pinch-out geometries may have been overlooked in epeiric reservoir systems elsewhere.