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Relationship Between Reservoir Quality, Facies and Depositional Environment: Working Towards a Predictive Model for the Deepwater Wilcox


Successful development of the Wilcox deepwater reservoirs is predicated on understanding the reservoir quality distribution. Wilcox reservoir quality is not simply a function of mechanical compaction with depth. Reservoir intervals at similar depths below mud-line in the same well have very different reservoir quality. In addition, the range in reservoir quality for the most abundant reservoir facies, the massive sandstones or what are interpreted to be Bouma Ta turbidite divisions, was much greater than had been observed in other deepwater reservoirs. This range in reservoir quality caused a large amount of uncertainty in predicting and modeling of permeability distribution. In response to this important business challenge, a major focus has been on significantly improving the understanding of the linkages between reservoir quality, depositional architecture and lithofacies distribution for the Wilcox and in developing new workflows and models for improved reservoir description, classification and quality prediction. The first step to solving this problem was a detailed classification of all the reservoir facies and interpretation of the reservoir architecture. From this analysis, it was interpreted that the best reservoir quality was associated with channelized and proximal sheet reservoirs. It was observed that a significant amount of massive “sandstones” had large amounts of silt, which is unusual for Ta sandstones. These silty massive sandstones were more common in the interpreted sheet deposits and most common in the distal sheet setting. Preliminary analysis suggests that massive sandstones with high fine-grained content are not Ta turbidite deposits. Instead, these sandy siltstones with poorer reservoir quality represent a transitional facies between turbidites and debrites. These transitional facies need to be treated separately from the cleaner turbidites in forward modeling of reservoir quality. It can now be demonstrated that reservoir quality is strongly controlled by reservoir architecture and in particular proximal to distal changes in depositional sand quality and not solely related to mechanical compaction. This has resulted in more accurate modeling and prediction of Wilcox reservoir quality. This fundamentally new view of the controls on reservoir quality has the potential to significantly impact the forward modeling of permeability distribution and impact current Wilcox developments and future exploration wells.