A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia

A three-dimensional high resolution sequence stratigraphic model of an Ypresian nummulitic carbonate ramp and organic-rich basin is presented based on outcrops in Central Tunisia. The sedimentation pattern is influenced by the interplay of different orders of variations in eustatic sea-level (third to fifth order), the pre-existing palaeotopography, and probably some synsedimentary tectonism (differential subsidence). Time-equivalent rocks deposited in a comparable structural and depositional setting along the northern margin of the African plate are hydrocarbon bearing (Tunisia and Libya). This example may thus serve as an outcrop analogue for this petroleum system, providing valuable information on the sub-seismic-scale distribution pattern, geometries and heterogeneities of both the reservoir and source rock facies.

The studied outcrops cover an area of 10 by 20 km where present-day valleys provide three-dimensional access to the rocks. In addition, the transition from the inner/mid-ramp, with nummulitic reservoir facies, to the carbonate source rocks in the basin is exposed in continuous outcrops. This transition takes place in about 3 km, a distance generally beyond the resolution of well spacing. Based on the physical tracing of beds, and the recognition of three orders of depositional sequences (third to fifth) a high resolution time framework is constructed. The accumulation of large nummulites (best reservoir facies) is stratigraphically controlled, and occurs in the transgressive phases of the landward-stepping fourth order cycles (overall transgression). Carbonate production was at that time so high that aggrading geometries are observed during these transgressive pulses. Our observations show that size, morphology and reproduction strategy of foraminiferal assemblages and, particularly, nummulites and Discocyclina, are related to changes in water depth and, consequently, in accommodation space. A regional east–west cross-section shows significant thickness variations of the Ypresian succession that were probably controlled by synsedimentary differential subsidence. The detailed, sub-seismic-scale, geometrical information on stratal patterns and lateral facies change are quantified, and used in a 3D numerical stochastic modelling (HERESIM) of this petroleum system.