The effect of clay microstratums and carbonate-anhydrite inclusions on the accuracy of the determining oil saturation coefficient
Key words: textural and non-uniform collector, clay interlay, well logging, conductivity of a layer intersection, saturation coefficient
Authors: E.A. Rakitin (Tyumen Branch of SurgutNIPIneft, RF, Surgut)
The author considers clastic rocks-collectors, textural heterogeneity of which is caused by a combination of lithological and sedimentation factors and manifestation postsedimentary epigenetic changes. Primary sedimentary structures in sandstones associate with the uneven distribution of the thin layers of clay with thickness from several millimeters to 20-30 cm. Secondary texture are formed by postsedimentary epigenetic processes and expressed spotted inclusions of carbonate-anhydrite material in sand and silt rock matrix. An algorithm for determining the saturation coefficient is proposed for collectors with three-textural heterogeneity according to core and well-logging data. For the first time a petrophysical model of collectors with three-textural heterogeneity (sand, clay and carbonate-anhydrite components) is offered. This model describes the relationship between the capacitive and lithological parameters. Determination of the model parameters is based on the results of core studies and photographs of the core saturated by hydrocarbons. On the basis of modern theoretical concepts of heterogeneous environments and analytical calculation electrical conductivity of multicomponent system, an algorithm is proposed and tested for calculating the true values of the coefficient of oil saturation of layer intersection with a three- texture heterogeneity. It is shown that, depending on the ratio of the contents of clay and carbonate-anhydrite layers true value of oil saturation of the formation could be either overstated or understated by well logging data. Oil saturation of sand is determined using adjusted values of the electrical resistivity of clay and carbonate-anhydrite components and petrophysical dependences of electrical parameters on the coefficients of porosity and water saturation for the textural - homogeneous samples. The validity of the oil saturation values is estimated by comparison with its direct determinations on the core.
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