Sedimentary Facies Computation and Stratigraphic Analyses Using Well Logs, Borehole Images and Cores in Triassic Fluvial Sandstones of the Algerian Sahara

Abstract

Abstract The Triassic reservoirs of the eastern Sahara province represent one of the main oil and gas accumulations in Algeria. This clastic succession corresponds to fluvial, estuarine and shallow marine deposits characterized by common lateral and vertical facies changes that are responsible for uncertainties in the modeling of the reservoir heterogeneities. A realistic identification of the depositional environment is critical to the delineation and prediction of the best quality reservoir facies so that optimized exploitation of the reservoir can be achieved. This paper describes a methodology that was used to generate a depositional model in the Lower to Upper Triassic reservoirs of the Rhourde El-Khrouf field based on subsurface data from six wells including well logs, borehole images, cores and the regional knowledge of the basin. Structural and sedimentary dip analyses were performed both on borehole images and oriented full-bore core photos that provided at least twice as many dips than borehole images. Facies logs were derived from the integration of core calibrated electrical borehole image analysis results with open-hole log data using neural network techniques. Depositional environments were then determined from correlations based on logs stacking patterns, facies associations and dip data. These results show that the fluvial reservoirs of the Rhourde El-Krouf field are characterized by large variations from laterally extensive bodies with good interconnectedness and high net-to-gross ratios, to multi-storey ribbon bodies with poor interconnectedness and low net-to-gross ratios. This integrated approach using high resolution image logs and full-bore core photos provided a much more robust reservoir model than would be obtained from traditional approach based solely on open-hole logs.