Deep water gas reservoirs in the Western Black Sea consist of highly laminated heterogeneous and complex sand/shale sequences. A petrophysical volumetric model was created by combining the triaxial induction resistivity data with the Thomas-Stieber (1975) sand-shale volumetric model to better evaluate low-resistivity pay zones in highly laminated shaly sand sequences. Although the results from this method are volumetrically almost correct, they are quite insufficient to deal with the distinctions between finely laminated sand, silt and shale.
In order to calculate volumetric petrophysical parameters, such as porosity and water saturation, which are linearly or non-linearly derived from the sand/shale fraction of the rock volume, accurate measurement of laminar shale volume in shaly sand sequences is an essential first step. Even though GIIP does not change much, the volumetric method presents a particular challenge in terms of continuity of gas bearing layers when there is water located in both above and below the gas-bearing sands. High-resolution wireline image tools with a vertical resolution of 0.5 cm can capture bed thicknesses and boundaries of these sand layers, enabling the determination of the net to gross sand ratio. However, imaging technologies by themselves are unable to determine precise porosity and water saturation. By statistically evaluating extensive volumes of core data (RCA and SCAL) and image logs, a new high-resolution methodology offers a simple and innovative method to compute porosity and saturation.
A partially cored reservoir section example is used to demonstrate the entire technique. This approach does not require standard log deconvolution. The uncertainty has been understood after comparing the results between the volumetric model and high-resolution model. By using the methods described in this paper, it is possible to position the perforation interval more precisely and reduce uncertainty for volumetric petrophysical calculations in complex highly laminated clastic reservoirs. The results show that low resistive laminated clastic reservoirs can be extremely productive, showing reservoir quality comparable to that of productive thick sands.
