Practical turbidite interpretation: The role of relative confinement in understanding reservoir architectures

Reservoir architecture is key in determining reservoir performance and hydrocarbon productivity but varies greatly in deep-water clastic reservoir systems. However, the interpretation of turbidite architectures can be a daunting exercise for non-specialists or those without experience in deep-water settings. Parameters that can be used to make interpretations are copious, vary in usefulness from one system to another, vary in scale and can take a lifetime to master. In this paper we introduce the simple concept of the relative confinement matrix and explain how it can be used by non-specialist geologists to interpret depositional architectures and hence inform all-important forecasts of performance and productivity. We show how any available parameter can fit into this matrix and be used to derive interpretations in a relatively quick manner. Our central concept is the relationship between the size of a turbidity current and the size of the conduit through which it flows and hence the degree of relative confinement to which a flow is subjected, which can in turn be related to depositional style and resulting reservoir architecture. Predictive interpretations can be made by observing patterns at the core and log scale, relating these to relative confinement and thus to larger, seismic-scale architectures. Variations in relative confinement are expressed through lateral bed continuity, vertical connectivity, amalgamation ratio, net:gross, distribution of hemi-pelagics, distribution and uniformity of facies associations, bed thickness frequency distribution, bioturbation diversity and intensity, distribution of sedimentary structures, mineralogical content, variability & textural maturity, grain size and grain size variability. Notionally the expression of the interaction of flow size and conduit size is described for individual turbidity current deposits, but this can also be translated to the bed-set scale, or larger, in genetically similar units. The careful analysis of the parameters described above allows the prediction of depositional architectures through the understanding of relative confinement for use in exploration and development. The architectural predictions in turn provide the basis for understanding permeability length-scales, kv/kh ratios and hence production performance through these heterogeneous reservoirs.