A New Approach for the Realistic Evaluation of Very Thin Reservoirs of Krishna Godavari Basin, East Coast, India

In thin bedded reservoirs high resolution borehole images are generally used to determine distribution of high quality, productive sands. Deep water Tertiary reservoirs of Krishna Godavari basin are generally very complex and heterogeneous, ranging from massive thick sands to highly laminated very thin sand/shale sequences with average bed thicknesses less than the vertical resolution of microresistivity-imaging tool. The image tool responds to the beds and features smaller than the vertical resolution but can not accurately determine laminar bed thickness which can be used as an integral part of lowresolution technique. In the absence of a well defined set of thin bed boundaries from image tools, an integrated of approach of forward modeling and inversion is also impractical for the accurate thin bed evaluation. A new approach of laminated shaly sand analysis is developed where laminar sand/shale estimation is based on generation of binary lithology on shale volume curve derived from micro-electrical resistivity. Formation beds as thin as the twice of the sampling interval can be generated through this technique in such a way that cumulative net sand fraction is restricted with integrated sand volume. Laminar shale volume from image tool calibrated to the LamCount core data has been considered to be the ground truth in the analysis. The volume of dispersed shale and the total and effective porosities of the laminar sand fraction are determined using a Thomas-Stieber volumetric approach. Water saturation is estimated from laminar sand-fraction resistivity derived from electrical anisotropy. Application of the methodology leads to an improved accuracy of reserve estimates and productivity predictions. Entire procedure is illustrated with an example of completely cored reservoir section. Results indicate that low resistive laminated reservoirs with very high shale fractions can be highly productive with reservoir quality equivalent or some times even better than massive thick beds. Introduction The earliest proposed solution, to the thin-bed problem (Poupon et al., 1954), is based on parallel resistor model where formation conductivity represented as volumetric average of sand and shale fractions. The sandstone conductivity arrived after correcting the average value for shale fraction is then used to calculate sandstone–water saturation. Since then, more than fifty derivatives, mostly empirical, of Archie’s original equation (1942) exist to address the complex problem of shaly sand water saturation (Worthington, 1985). These techniques were developed to address the effects of dispersed clay in sandstones, rather than macroscopically inter bedded sandstones and shales. Many are derived from local or regional data to address specific formation characteristics of porosity, electrical properties, and shale distribution effects. Investigations demonstrated that an empirical bulk shale approach using shale corrected effective porosity and shale resistivity cannot accurately deal with the wide range of combinations of both laminar and dispersed shale effects on the formation properties of porosity and saturation (Thomas and Stieber, 1975; Givens, 1987; Yuan and Diederix, 1987; de Wall, 1989). A truly general porosity and saturation petrophysical model must include the concepts of shale distribution for a quantitative approach to the differing effects of laminar and dispersed shale. Patchett-Herrick (1982) proposed such a saturation model based on a combination of the Poupon, et al., 1954, laminar shale equation and the Waxman and Smits, 1968, dispersed clay equation. Both classes of “shaly sand” saturation equations are in wide use. A review of the assumptions that underlie any of the equations in both classes shows that there are fatal flaws in all (Herrick and Kennedy 2009). Maute (1992) speculated about this variability of nature with regard to shale distribution and the inability of any empirical or