A majority of the world's oil and gas reserves are locked in tight, "unconventional", reservoirs. Without the presence of fractures (natural or hydraulic), these tight reservoirs with matrix permeabilities, usually less than 0.1md and porosities between 3–10PU, do not produce commercially. While hydraulic fracturing is widely used to improve the economic viability of wells, and fields to that matter, the presence of natural fractures plays the same role in improving the flow mechanics. As an industry, there are many tools available which characterize the properties of a created hydraulic fracture from both the predictive and evaluative standpoint. This is important in understanding the impact these properties have in increasing the production from a specific wellbore. Typically, we characterize these hydraulically induced fractures in terms of fracture height, length, width, and conductivity. These fracture characteristics are estimated using a variety of techniques including, pressure transients, production transients, micro-seismic, and various other techniques. However the ability to fully characterize natural fractures in terms other than pure qualitative (number of fractures observed, open or closed) has been limited in application and often only address conductivity. Based on case studies from Cambro-Ordovician tight sandstone reservoirs of the Hassi Messaoud field in Algeria, this paper describes methods that can be used to fully characterize natural fractures in terms commonly used for hydraulic fractures. With the application of combined imagery packages along with analyses of both pressure and production transient data, we now have the means to both understand the long-term impact of natural fracturing by associating quantitative properties that characterize both their locally observed (in-well) and far-field properties. Introduction Large oil and gas accumulations have been discovered in tight sandstone reservoirs of the Algerian Paleozoic sedimentary basins over the past fifty years. The worldwide trend and the drilling plans in Algeria confirm the strategic importance of these unconventional tight reservoirs for the gas production of the years to come. Recent advances in logging technology such as electrical and acoustic imaging used in combination with production data, transient pressure analysis and fracture stimulation techniques have opened new horizons in the characterization and the production of the challenging tight reservoirs of Algeria. Geology Located about 800 km South-Southeast of the capital Algiers (Figure 1) the Hassi-Messaoud field was discovered in 1956. With an original oil in place (OOIP) estimated at 41 billion barrels and a total production to date of 5.4 billion barrels, or 13.2% of the OOIP, this field is one of the biggest hydrocarbon fields in the world producing from Cambrian-age sandstone reservoirs with porosities ranging between 3 and 10 PU and matrix permeabilities typically less than 10md. The field currently produces approximately 400,000 BOP from more than 700 production wells.

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. Introduction The wells studied are located in the Rhourde El Khrouf (RKF) field of the Berkine Algerian basin (Figure-1). Oil was first discovered in this field in the early to mid 90s in the Triassic "Argilo-Gréseux inférieur" - TAGI reservoirs. These sandstones have been interpreted as fluvial deposits (Turner et al, 2001 and Sabaou et al, 2003). The field corresponds to an estimated one billion barrels of oil in place, which makes it one of the most important oil fields in Algeria. Fluvial deposits are usually difficult to map and characterized in detail because of the high frequency of lateral facies changes and difficulty in identifying individual sand packages of channel and overbank deposits with similar geological characteristics. Only an integrated approach combining data from different sources can help reduce the geological uncertainties.

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