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Seismic, Borehole Image Log, Core and Chemostratigraphic Interpretations of the Permo-Carboniferous Unayzah Formation, Central Saudi Arabia: An Integrated Technique for Siliciclastic Reservoir Correlation

Abstract

The siliciclastic of Unayzah formation is stratigraphically and sedimentologically heterogeneous. Reservoir correlation is challenging as these sediments vary in thickness and in vertical and lateral facies distribution. In the absence of good biostratigraphic control, other techniques, such as integration of core/borehole image interpretations and chemostratigraphy are essential for reservoir correlation. Due to this complexity, it was decided to develop an integrated technique for better reservoir correlation. This study assimilate the results from different disciplines on sand fairways, paleo current direction and depositional facies to provide a high resolution correlation for the Unayzah formation encountered in six wells in the study area. Core and cuttings samples were taken every 3ft and 10ft and then subjected to Inductively Coupled Plasma analysis of 50 elements ranging from Na to U. Borehole image data were interpreted over a total of 9786 ft with 1352 ft of core described. Seismic imaging techniques detected porous sand bodies in the study area. Integrated analysis based on borehole image log, core and chemostratigraphic data validated the results of this seismic study and identified trough cross bedded medium to coarse gain sandstone interpreted as fluvial channel with paleo-currents to NNE on top interval, scattered dips with no preferred direction from medium to fine grained lacustrine deposits in the middle interval and coarse grained well sorted eolian deposits with ESE paleo-flows at bottom interval. The resulted three distinctive facies improved the seismically driven sand fairways and also matched hierarchical order of three chemostratigraphic zones (C1/C2/C3) identified by chemostratigraphic scheme. The fully integrated results from this study added geologic understanding of the stratigraphy and sediemnetology, improved resolution and confidence to Unayzah reservoir correlations thereby demonstrating the significant value of this integrated technique. In addition to utilizing this technique to understand reservoir correlation, the technique was also employed to understand paleocurrent directions and to recognize potential ‘sand fairways’. It is thought that such a multidisciplinary technique to reservoir correlation and play fairway mapping could be applied to other sequences throughout the Middle East and beyond, particularly where biostratigraphic control is limited.