Integrating multi-scale stratigraphic heterogeneity into a 3D geological model: a case study from an oolitic carbonate ramp system (Middle Jurassic Central High Atlas, Morocco)

Maria Mutti¹, Frederic Amour¹, Max Zitzmann¹, Falko Kraul¹, Sara Tomas¹, Nicolas Christ², Adrian Immenhauser², and Kabiri Lahcen³
¹Universität Potsdam, Germany,
²Ruhr-Universität Bochum, Germany,
³University of Errachidia, Morocco

Three-dimensional geological models based on outcrop analogues are powerful tools to quantify the spatial distribution of geological bodies and provide useful insights into modeling approaches to be used in subsurface reservoir studies. A number of studies have contributed to building single scale-based outcrop models and have provided a geostatistical database for sub-surface reservoir studies at different scales of interest. However, the current challenge for reservoir modeling remains the integration of multi-scale geological heterogeneity and scale-dependent stratal arrangements into a single 3-D model.

Outstanding outcrops of the Middle Jurassic carbonate ramp system (Assoul Fm., Bajocian, Central High Atlas, Morocco) provide an outstanding natural laboratory to build and test 3-D outcrop models at different scales of resolution. Previous studies in this area (Pierre et al., 2010; Christ et al., 2011; Amour et al., 2011) provide a strong framework to analyze in more detail the distribution, morphology, and degrees of heterogeneity observed within shoal complexes. The present study is based on a 1 km by side and 100 m thick study window and describes and models the spatial and hierarchical arrangement of carbonate bodies spanning from (1) high-frequency depositional sequences (1) their depositional domains (EODs) (2) and finally lithofacies mosaic (3). Our multi-scale approach integrates different modeling methodologies, choosen for each hierachical level of interest, and incorporates all information into one single model. Field and petrographic observations (150 thin-sections), have allowed to identify 14 different shallow marine lithofacies types and grouped them into three major depositional domains (EODs), an inner ramp, a proximal middle ramp and a distal middle to outer ramp. The inner ramp setting is composed of an oolithic upper shoal, a peloidal lower shoal and a fore-shoal with rudstone beds. Lithofacies and depositional domains are stacked vertically into medium-scale sequences composed of small-scale sequences.

The bounding surfaces of medium- and small-scale depositional sequences were modeled by georeferencing the identified sequence boundaries with d-GPS mapping in the field and subsequently checked with LiDAR data. On the other hand, the modeling of depositional domains and lithofacies is based on geostatistical data based on 19 measured stratigraphic sections and the physical tracing of lateral facies transitions. The reconstruction of the vertical and lateral facies distribution allowed also specifically to analyze orientation, spatial dimension, shape and distribution of the ooid shoal bodies. Semivariogramm analysis and paleocurrent measurements were carried out to characterize ooid shoal bodies dimensions and orientations.

The data collected suggest that the lithofacies distribution (x1 to x100 m) show a mosaic-like spatial arrangement (a purely stochastic distribution), whereas the depositional domains (x 0.1 to x1 km) shows gradational and linear trend between geobodies. The integration of scale-dependent geological heterogeneity into one single facies model thus requires the use and compilation of different simulation methods due to the various modeling constraints associated with each level of stratigraphic hierarchy. The stacking pattern of medium- and small-scale depositional sequences (1) is built using a surface-based modeling approach. The depositional environments (2) are modeled using Truncated Gaussian Simulation (TGS) because of gradational and linear trend observed between geobodies. Finally, lithofacies distribution (3) is modeled using Sequential Indicator Simulation (SIS) because of its flexibility in honoring spatially independent lithofacies elements (Amour et al., 2011).

The geostatistical data collected on ooid shoal bodies as presented in this study provide an improved understanding of their depositional facies and spatial organization. Three shoal bodies, which show significant differences in facies distribution, reflecting depositional differences and stratigraphic positions, are recognized in the final model and include an Upper Shoal Complex, a Lower Shoal Complex and a Transition Zone.

The Upper Shoal Complex appears as a compact and laterally continuous oolitic-dominated shoal body with a distinct orientation and topography and represents the maximum progradation of the shoals. Furthermore, the Upper Shoal Complex is characterized by a fairly high degree of facies differentiation. The second complex, the Lower Shoal Complex, is characterized by continuous and vertically separated shoal bodies. With a less pronounced orientation of shoal facies, lower topography and intermediate lithofacies differentiation, the Lower Shoal Complex reflects hydrodynamic conditions of lower energy compared to the Upper Shoal Complex. The Transition Zone appears as rare thin patchy sheets of oolithic facies illustrating only minor lateral extension and reduced vertical thickness. The complex is dominated by foreshoal facies types and shows a low facies differentiation indicating the lowest energy conditions of all three shoal complexes.

The case study discussed here emphasizes the importance of making a correct decision in choosing a specific modeling algorithm and highlights how the combination of deterministic and stochastic modeling techniques (TGS and SIS) allows to obtain precise information of the architecture and spatial organization of different hierachical scales within a carbonate ramp system, spanning from depositional sequences, to depositional domains (EODs) to the individual lithofacies. Furthermore, this case study provides a useful outcrop analogue to help better understand scale-dependent geological heterogeneities in other carbonate ramp systems, such as in the Middle East.

References

Amour, F., Mutti, M., Christ, N., Immenhauser, A., Agar, S.M., Benson, G.S., Tomás, S, Alway, R. and Kabiri, L. (2011): Capturing and modeling metre-scale spatial facies heterogeneity in a Jurassic ramp setting (Central High Atlas, Morocco). Sedimentology. DOI: 10.1111/j.1365-3091.2011.01299.

Christ, N., Immenhauser, A., Amour, F., Mutti, M., Tomas, S., Agar, S.M., Alway, R. and Kabiri, L. (2011): Characterization and interpretation of discontinuity surfaces in a Jurassic ramp setting (High Atlas, Morocco). Sedimentology. DOI: 10.1111/j.1365-3091.2011.01251.

Pierre, A., C., Durlet, P., Razin, and E. H., Chellai, 2010, Spatial and temporal distribution of ooids along a Jurassic carbonate ramp: Amellago transect, High Atlas, Morocco: Geological Society of London, Special Publication., v. 329, p. 65-88.

Acknowledgments

This project has been supported by ExxonMobil through the (FC)2 Alliance. We are particularly indebted to Susan Agar and to Gregory Benson for their continuous support. Schlumberger has provided access to the software Petrel (TM of Schlumberger). Thanks to the Agouzil family for their hospitality in the Kasbah Amellago and their invaluable logistic assistance.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012