Outcrop Analogue for a Mixed Silicicoclastic-Carbonate Ramp Reservoir: A Multi-Scale Facies Modeling Approach

Amour, Frédéric ¹; Mutti, Maria ¹; Christ, Nicolas ²; Immenhauser, Adrian ²; Benson, Gregory ³; Tomás, Sara ¹; Agar, Susan ³; Kabiri, Lahcen ⁴
(1) Institute of Earth and Environmental Science, Potsdam-Golm, Germany. (2) Institute for Geology, Mineralogy and Geophysics, Bochum, Germany. (3) ExxonMobil Upstream Research Company, Houston, TX. (4) University of Errachidia, Errachidia, Morocco.

The understanding of fluid flow in carbonates is key to the improvement of reservoir characterization models. Due to the relatively long distances between wells in subsurface reservoirs, outcrop analogues are used to provide insights on the geological complexity at inter-well spacings. The present work models geological heterogeneities of the same outcrop at three scales of organization, i) the depositional sequence, ii) the depositional environment and iii) the lithofacies type.

The study area, located in Amellago (Morocco), is 1 km on each side and 100 m thick. The formation consists of prograding shoals deposited on a low-angle carbonate ramp. The outcrops allowed the acquisition of 19 sections with spacings that range down to 40 m, georeferenced by using d-GPS and LiDAR methods. Based on field data and microfacies analysis, 11 lithofacies grouped into four depositional environments (EODs), were identified. A marly open ramp (EOD 1) deposited from the middle to outer ramp, changing laterally to a semi-restricted ramp (EOD 2) composed of cyanobacteria, oncoids and gastropods within the middle ramp. The inner ramp records shoals bodies in a high energy ramp (EOD 3). “Reef” buildups (EOD 4) are composed of oysters.

The multi-scale approach allows the investigation of spatial variability within shoal belts that extend tens of kilometers, specifically variabilities of morphology, dimensions, heterogeneity and connectivity of geobodies at the three scales of organization. At the kilometre scale, the depositional sequence model indicates a thickening trend of the sequences basinward. For EODs, the model shows that shoal geobodies (23,46% in vol.) average 333 m long, 192 m wide and 4.1 m thick, and their orientation is 125°N. Three types of shoal morphology have been observed, i) planar and continuous, ii) domal with thin connecting bodies and iii) isolated and domal. At the lithofacies scale, the model shows that the shoals are composed of three lithofacies with unique morphological characteristics, a peloidal oolithic packstone-grainstone (1) 330x145x4 m, an oolithic grainstone (2) 281x163x2 m and a cyanobacteria rudstone (3) 210x191x1 m. The degree of lithofacies heterogeneity within the shoals is variable. These changes in morphology and heterogeneity are likely controlled by the position of the fair-weather wave base (FWWB) in comparison to the sea floor. Around the FWWB, the interplay of everyday waves and storm waves seems to increase heterogeneity.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.