Autocyclic Generation of Upper Kharaib Reservoir Unit Parasequences Recognised by Numerical Forward Models

Abstract

Parasequence thickness and frequency are traditionally interpreted to be controlled by allocyclic processes such as oscillations in eustatic sea-level. However, the use of numerical forward models is challenging these concepts. Outcrop data from Ras Al-Khaimah (UAE) were incorporated into numerical forward models and used to replicate parasequences from the Upper Kharaib Reservoir Unit. Results indicate that clinoform geometries within the parasequences can form by autocyclic, rather than allocyclic, processes. Stratigraphic, sedimentological and palaeoenvironmental interpretations made from outcrops of Upper Kharaib carbonate clinoform parasequences at Wadi Rahabah, Ras Al-Khaimah, were used to build a numerical stratigraphic forward model. Numerical stratigraphic forward models produce fully quantitative three-dimensional deterministic models that replicate and predict the spatial distribution of stratal geometries, stacking patterns, sedimentary thickness and facies formed under a set of predefined input parameters and boundary conditions. A CarboCAT numerical model of carbonate deposystems that uses cellular automata to determine the distribution and lithofacies of heterogeneous carbonate strata in three dimensions (Burgess, 2013) was used to replicate the Upper Kharaib parasequence geometries. Results of the numerical forward model show that carbonate clinoform parasequences from the Upper Kharaib Reservoir Unit can be generated by an autocyclic Ginsburg-type mechanism of sediment transport and shoreline progradation (e.g. Burgess and Wright, 2003; Burgess, 2006). Ginsburg (1971) proposed that progradational, regressive, cycles in carbonates can be formed independently of external eustatic sea-level or tectonically driven forces by autocyclic processes including wind-driven or tidal sediment transport. As sediment is transported towards the shore by autocyclic processes, seaward progradation of the sediment wedge occurs forming a regressive cycle. Alternating transgressive-regressive autocycles are deposited as production rates are less than or exceed the rate of subsidence, respectively. CarboCAT models indicate that the degree of clinoform progradation within the Upper Kharaib is controlled by the rate of relative sea-level rise, driven by localised subsidence, and sediment transport. Thick aggradational parasequences are produced where the rate of relative sea-level rise is the primary control on the deposition of the clinoforms. Parasequence frequency increases where the rate of sediment transport exerts a stronger control than the rate of relative sea-level rise on the deposition of the clinoforms. Observations from numerical forward models have implications for the distribution of reservoir intervals within the Upper Kharaib. Parasequences formed by autocyclic process produce heterogeneous reservoirs with complex facies mosaics. Lateral heterogeneity and variable thicknesses within these reservoirs is more difficult to correlate and trace across fields than simple stacked, layer cake, parasequences created by sea-level oscillations.