Reconstruction of Paleo-Structure Using 3D Seismic and Well Data - Case Study from a Lower Cretaceous Carbonate Formation Onshore Abu Dhabi

摘要

This paper highlights 3D reconstruction of the paleo-topography of the depositional environment for a Lower Cretaceous carbonate formation onshore Abu Dhabi using 3D seismic and well data. The reconstruction was carried out for two reasons: (1) to understand underlying geologic causes for anomalous lateral variations in pressure, production performance, and logged reservoir properties in the field and (2) to delineate geologic trends away from well control in order to guide further decisions on field development and reservoir management options. Present-day structure of the reservoir top is a high-relief elongated anticline that is open to neighboring giant oil fields. Known hydrocarbon contact is below structural spill-point between the field and its neighbors, however pressure and production data indicate that the field is isolated from its neighbors. No fault was seen on seismic separating the field from its neighbors, thus raising possibility of stratigraphic separation. Further, study on core samples indicated that reservoir quality is controlled by depositional facies and early diagenetic modifications thereof. Thus, reconstruction of paleo-structure was conceived as a means of understanding and delineating geologic drivers for lateral variations in reservoir quality. The reconstruction process relied on an integrated approach: combining information from seismic, well logs, sedimentology, and well test results. Sedimentology studies gave information on expected morphology of depositional environment and controlling factors for reservoir quality; seismic interpretation of structure and stratigraphy at several levels provided basis to understand present-day field architecture and structural evolution through geologic time; reconstruction of three-dimensional structure at time of deposition was achieved by means of restorative velocity models to translate input mapped surfaces to their approximate original morphologies; results validation was achieved by subjecting study outputs to conformance tests with independent data from well logs, pressure tests, production performance, and seismic attributes trends. After reconstruction carried out in this study, the present-day steep anticlinal structure at the target reservoir was translated to a gently dipping ramp with morphology that is consistent with interpreted environment of deposition from cores. Outputs were further validated by conformance of well data and seismic attribute trends with the paleo-structure. Anomalous lateral variations in reservoir properties measured in wells were found to be associated with possible tidal channels that were interpreted to have caused localized diagenetic changes. Thus, findings from the paleo-reconstruction study provided a geologically consistent framework to understand lateral variation in well results, and also provided basis to guide further field development and reservoir management decisions as intended at study inception. Although outputs from the paleo-reconstruction process used in this study were deemed to have given good results, potential pitfalls in applying the method are herein noted.