Integration of seismic inversion and rock physics relationships for determining of shale distribution in one of the reservoirs in the Persian Gulf

Characterization of siliciclastic reservoirs from seismic data is very sensitive to their clay and shale content. Clay can affect P- and S-wave velocities through its type and shape as well as location. Such an elastic behavior of clay infers that reservoir properties can be overlooked if their clay content is not understood and interpreted adequately. Clay can exist in different types with their specific shape and even can be distributed within siliciclastic rocks in various forms- structural, laminar, interstitial and dispersed clay- with different velocity responses. The mixture of the various clay types and forms can make their velocity interpretation for reservoir properties more complicated. Therefore, a proper strategy to separate the effects of clay types and clay forms is necessary for any seismic reservoir characterization on siliciclastic reservoirs with high clay content. Rock physics is a bridge between seismic and reservoir properties. An important goal of this branch of science is to understand the physical properties of the reservoir, so it is important for this kind of study. This study integrates rock physics modeling and simultaneous seismic inversion in order to find different clay distribution (forms) in one of the oilfields in the western part of the Persian Gulf. The well log data (wells A, B, and C) from this field show how the reservoir quality varies within the field with no obvious relationship to their shale content. This independent behavior of shale content and reservoir properties could be an indication that clay distribution may vary and clay type is not the only parameter for clay effects on the reservoir properties. Therefore, Thomas-Stieber rock physics template, first, is used to characterize shale distribution at well location and then the same template is applied on the reservoir properties derived from simultaneous seismic inversion to understand clay distribution in the whole area. Our results confirm that at wells, clay distribution is varying from top to the bottom of the reservoir. We find out that reservoir quality is not changed within the bottom part of the reservoir with high clay content (due to structural clay) while the same clay content reduced reservoir quality in the top and middle parts of the reservoir (due to the dispersed and layered clay). In order to do reservoir characterization, a map of shale content from top Ghar and top lower Asmari is generated. This generated map differentiates proper reservoir interval from the non-reservoir interval. Therefore, by using the proposed method in this study, one can delineate the potential zones of the reservoir for the future plan of drilling and production.