Abstract
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 low- resolution 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.
