Evaluating the Links between Turbidite Facies and Gross System Architecture: Upscaling Insights from the Turbidite Sheet-system of Peira Cava, Southeast France

Amy, Lawrence, Ben Kneller, and Bill McCaffrey, School of Earth Sciences, University of Leeds, Leeds, LS2 9JT

Abstract

Understanding how the characteristics of individual turbidites measured along 1D sections may relate to reservoir-scale system geometry remains a significant upscaling problem. Yet the ability to make this link with an acceptable degree of accuracy can be fundamentally important when evaluating turbidite reservoirs. Insights into the key relationships are perhaps best established within well-exposed outcrops in which bed-to-bed correlations can be established over reservoir-scale distances.

The Peïra Cava outlier of the Tertiary Annot Sandstone preserves a 1.2 km thick section of dominantly sheetform-confined and ponded turbidites (together with subordinate mass-transport deposits). A 600 m stratigraphic interval, which is exposed over a 10 km by 5 km area, has been selected for study. Within this interval reliable bed-to-bed correlations have been established for 70% of beds greater than 0.5m thick, and over 6 km of section have been measured. The combined data set allows the 3D geometry of the system to be constrained at the reservoir scale, and linked into sedimentary characteristics of individual beds.

Certain stratigraphic intervals exhibit dramatic and occasionally abrupt down-stream and cross-stream changes in bed-thickness and sand/shale ratio, whereas others vary modestly and progressively. A number of different architectural types can be distinguished on the basis of the degree to which these properties vary spatially. We test the significance of individual turbidite characteristics (including sand thickness, mud-cap thickness, sand/shale ratio, grain-size, and the presence or absence of erosional structures, tractional structures, and chaotic units) for their value as predictors of downstream and cross-stream bed geometry. Statistical analysis of bed thickness-decay versus bed sedimentary characteristics shows that a link can be established between sedimentary facies and bed geometry. The results are compared with models currently used to predict the spatial distributions of sediment properties, such as fan-based concepts of proximality, flow-efficiency concepts, and the influence of topographic control.