Maximizing the Value of 3D Seismic in a Mature Giant Oil Field

Abstract

Zakum field is planned to be extended and developed to the Western area, which is sparsely drilled and has variable reservoir quality. The reservoir target is Thamama 2 (Th 2) that was deposited during early Barremian Early Cretaceous and is divided into six zones that are separated by styolites. The reservoir rocks were deposited in a broad low angle shelf with coarsening upwards sequences of mud-dominated to grain-dominated limestone with reservoir quality degrading westward where the main diagenesis process is calcite cementation, which is retarded due to late hydrocarbon charge. Existing static and dynamic models have sparse, static, and dynamic data points in the West in comparison to closely spaced data points at the crest of the structure. Western area has shown relatively better performance with recent wells in comparison to simulation prediction models. Seismic data with large lateral inter-well resolution was integrated through stochastic inversion leading to higher resolution in the wells to generate several realizations of higher resolution acoustic impedance that show an improved reservoir property distribution in the western part of the field. This is achieved by averaging 25 realizations by incorporating Relative Acoustic Impedance seismic variograms, Depth migration velocity field and data from 249 wells across the field. Three approaches were tested to populate porosity including 1) Regression of porosity against Acoustic Impedance (AI) for all wells, 2) trend and using Sequential Gaussian Simulations (SGS) based on seismic variograms, and 3) co-krigging to average AI with porosity. A trend guided porosity prediction has provided the most reasonable results. Therefore, all available wells (507 in total) were used to populate porosity for each zone and layer respectively. Initially, some mismatches were observed linked to probably changes in zone quality at inter-well scale and saturation rise in transition zone led to relatively higher acoustic impedance in comparison to the relatively high porosity. This issue was mitigated through integrating all available wells. Other reasons might be due to the overburden thickening towards the west exceeding 1000 feet where rocks are much stiffer. The results from this study will help achieving the ultimate goal to optimize the development of the western part of the oil field. The results have shown a superiority to the prior existing model and have better highlighted the reservoir quality variation across the field.

Date of Award	May 2020
Original language	American English