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Oil From Granitoid Rocks. Reservoir Characterization of The Fractured Basement of The Neuquén Basin, Octogono Field, Argentina

Abstract

Located in Argentina, the paleozoic basement in the Octógono field, Huechulafquen formation, consists of granitoid rocks underlying Los Molles shale which is seal and one of the source rocks altogether with Vaca Muerta formation. The field was discovered in 1918 and produced from shallow reservoirs. Development activity in the basement is a recent upside. The main tectonic pillar was formed in the Early Jurassic. After the rifting, sedimentary units of Cuyo group deposited above the Basement in hemigrabens lately elevated during the tectonic inversion in the Upper Jurassic. The structure was already formed when hydrocarbon migration started. All permeability and storage had been originated from fracturing and alteration. Fracturing is the result of tectonic processes that raised the basement more than 1000 m above the surrounding rocks. These happened before the initial stages of the basin and several reactivations took place. Six alteration zones corresponding to the differential weathering can be correlated based on 3D seismic and well logs. Resistivity is affected by iron content. Also, due to the low matrix porosity, small variations in the mineralogy strongly affects indirect storage estimations. Resistive imagery, NMR and acoustic logs allow getting an estimation of total fluid as well as storage and permeability indicators. A fracture intensity index can be obtained from picking of open discontinuities. NMR allows the characterization of total pore space in addition to the interpretation of fluids by the analysis of diffusion maps and comparison of T2 spectra. A permeability indicator is based in the reduction of velocity and amplitude of the Stoneley wave in presence of mobile fluids, a function of mobility that can be seen as a coupling into a formation wave known as slow wave. The reservoir is described by partition coefficient and classifies as type BA (35 % of storage in macrofractures). Recovery factor is estimated in 25% considering expansion of a 300 m thick gas cap. The oil leg is 450 m thick, down to a depth chaotic seismic reflectors, typical of unaltered basement. Faults at seismic resolution and each of the alteration zones are mapped in detail. The integration of the processed information into a 3D model allows identifying regions where storage capacity is higher. Mapping of fracture intensity helps to orientate the development to sectors where flow capacity is higher, thus optimizing EUR.