Fluid property identification of the Lower Cretaceous reservoirs with complex oil-water contacts in Deseo Basin, Chad

Abstract

Recently, exploration breakthroughs have been made in the Lower Cretaceous sandstone reservoirs in the Doseo Basin, but the identification of reservoir fluid property is difficult due to variable reservoir lithology, complex oil-water contact within and faint responses of the oil zone, which causes the lower accuracy of reservoir fluid property identification with conventional mudlogging and wirelogging techniques. Applying the geochemical logging, fluorescent logging, mud logging and cutting logging technology, in combination with formation test data, this paper distinguishes the crude oil types, analyzes the logging response characteristics of oil zone after water washing, and establishes the interpretation charts and parameter standards for reservoir fluid properties. The crude oil can be divided into two types, namely viscous-heavy and thin-light, based on total hydrocarbon content and component concentration tested by mud logging, features of pyrolysis gas chromatogram and fluorescence spectroscopy. The general characteristics of oil layers experienced water washing include the decrease of total hydrocarbon content and component concentration from mud logging, the decrease of S1 and PS values from geochemical logging, the decrease of hydrocarbon abundance and absence of some light components in pyrolysis gas chromatogram, and the decrease of fluorescence area and intensity from fluorescence logging. According to crude oil types, the cross plots of S1 versus peak-baseline ratio, and the cross plots of rock wettability versus fluorescence area ratio are drawn and used to interpret reservoir fluid property. Meanwhile, the standards of reservoir fluid parameter are established combining with the parameters of PS and the parameters in above charts, and comprehensive multiparameter correlation in both vertical and horizontal ways is also performed to interpret reservoir fluid property. The application in the Doseo Basin achieved great success, improving interpretation ability of fluid property in the reservoir with complex oil-water contact, and also provided technical reference for the efficient exploration and development of similar reservoirs.