3D computer-modelling of fluvial deposits: outcrops studies of the Lower Troncoso Member (Neuquén Basin, Argentina) and implications for the characterisation of subsurface reservoirs.

VEIGA, G.D.; HOWELL, J.A.

S. M. de Tucumán

Congreso; 9th International Conference on Fluvial Sedimentology.; 2009

Computer-based modelling of fluvial deposits has become a crucial tool for understanding and predicting the variability of hydrocarbon reservoirs in the subsurface. The precise definition of their key parameters requires quantitative geometric data that can be only obtained through the direct observation of the rocks. Therefore, outcrop analysis of potential targets or the study of suitable analogues is important as a means of constructing quantitative models that can be directly applied to subsurface exploration. In order to evaluate the relative importance of some of these variables, a detailed study of the distal fluvial deposits of the Lower Troncoso Member of the Huitrín Formation in the central Neuquén Basin (west-central Argentina) was performed. The aim of the study was, while building robust models that could be applied to the subsurface in neighbouring areas, evaluate how key elements that can be only assessed through the 3D observation of the sedimentary bodies influence the outcome of dynamic flow simulations. Models were built considering the distribution of potential reservoir units (three types of channels and sandstone lobes) and its contrast with low permeability units (floodplains) at a complete unit scale (~80 m thick) in a 56 km2 area. Reservoir units were modelled stochastically and populated with petrophysical properties from analogue units in the North Sea. Alternative models were built changing some of the modelling parameters (channel-belt width, sinuosity, detailed channel-belt vertical distribution) in order to compare the results of flow simulations and evaluate their influence in the final results. Of all the parameters analysed the main differences raised in the analysis of the vertical distribution of sandstone bodies. The stochastic distribution of channel bodies considering only ten vertical logs gave a more uniform channel distribution and more optimistic results, in contrast with the observed architecture where channels are more constrained to a restricted vertical zone. Results derived from changes in the channel-belt width proved more significant than changes in the channel-belt sinuosity (this one calculated empirically through palaeocurrent distribution within a channel body). Finally, slight changes in the petrophysical characteristics of floodplain deposits changed the results of the flow simulations significantly, suggesting that a detailed study of fine-grained facies is also of great importance in the characterisation of distal fluvial successions.