Log Evaluation of Low Resistivity Fluvial Reservoirs

Evaluation of well logs of low resistivity fluvial reservoirs of Upper Assam, and Krishna Godavari, basins of India, with complex lithological characteristics and distinct depositional settings possess a great challenge to log analyst. It has not been possible to determine the realistic water saturation values since decades against the hydrocarbon zones with poor or reverse resistivity contrasts with water bearing zone because of the complexities of the electrical conductance mechanism varying from Archie to severely non-Archie behaviour, where any shaly sand equation breaks down. Waxman-Smits equation is most simple to apply in any reservoir to model the excess surface conductivity irrespective of its source by least square fitting of multiple salinity core measured conductivity data. Anomalous lower value of initial equivalent conductance of the counter ions i.e. 13-20% of its maximum due to other type of source of surface conductivity like fine silica grains in addition to cation exchange capacity of clay minerals, does not allow the application of empirical Waxman-Smits equation in low resistivity fluvial reservoirs. Accordingly the modified Waxman-Smits equation have been found to be precisely compensating the excess surface conductivity for the computation of accurate water saturation in low resistivity fluvial reservoirs. Another approach of precise computation of water saturation is pseudo Archie method, where reduction of cementation 'm' and saturation 'n' exponents are allowed to find their own levels to take care of excess surface conductivity due to variations of grain sizes. A unified relationship between 'm' and volume of facie has been evolved through linear regression analysis of logs in aquifers. It indicates that composite 'm' is varying as volumetric average, where 'm' for clean and very fine sands are 2.0 and 1.0 respectively indicated on formation factor vs porosity plot. The same relationship has to be extended to the hydrocarbon bearing zones for the computation of realistic water saturation. The main advantage of this approach is that it does not require detail core data. Electrical characterization either using core or log measurements is fundamental to the understanding of mechanism of conduction of electrical currents in reservoir rocks. For example, fluvial reservoirs of Upper Assam and KG basins are showing spread of points respectively parallel to axes on formation factor vs porosity plots due different depositional settings and sorting of sediments. However, spread of points in both cases can only be attributed to the variations of values of cementation factor 'm' from 2.0 in clean sand to 1.0 in fine sand/shales. This forms the basis of applying unified approach of variable 'm' concept in fresh water fluvial reservoirs of different depositional settings. All the published geological and electrical core measurements of shaly sands as well as fresh water fluvial reservoirs have been analysed to develop an unified approach to take care of anomalous reduction of formation resistivity factor with grain size in all type of facies varying from Archie, shaly sands to severely non-Archie reservoirs. Experimental basis of inter relationship evolved through core data has been extended to the log data scale with different in-situ boundary conditions particularly in shale beds. Lithofacies inversion of open-hole logs performed with new approach, has resulted in a realistic log evaluation of low resistivity fluvial reservoirs corroborating with core data in completely cored reservoirs and testing results. This approach takes care of surface conductance effects particularly anomalous lowering of Archie parameters on core and log measurements, in addition to clay effects and this will be very useful to delineate low resistive oil bearing zones which might have been missed earlier to accrete more reserves in future.