Abstract - The objectives of the presented work are (a) to identify the most important geological parameters controlling flow within a range of faulted turbidite reservoir architectures and (b) to quantify links between sedimentological and fault system characteristics, and flow responses in such systems. We have focused on the bed-scale where we find that conventional modelling approaches fail to capture adequately measured correlations between net:gross and sandstone bed amalgamation ratios across the range of typical turbidite architectures. A correct representation of both parameters is crucial in a robust reservoir characterisation, since we find that the amalgamation ratio is by far the most significant parameter controlling both the static and dynamic bed-scale connectivity of unfaulted turbidite sequences. Static and dynamic connectivity are defined as the fraction of the net:gross ratio contained in the largest cluster of mutually connected beds, and as the directional fractional effective permeability of the system assuming impermeable shales.
The connectivity of faulted sequences is much more complex and is dominated by interactions of variables. Of these, the most important are the ratios between fault length and bed length and between fault throw and bed thickness, the original connectivity of the unfaulted sequence and how (and with what cut-off values) shale smears are modelled. A comprehensive modelling suite of idealised systems is used to illustrate these results and to highlight the extremely rare combinations of circumstances in which faulted sequences can have lower connectivities than their unfaulted sedimentological equivalents. Despite this complexity, we find that the dynamic connectivity of a system is controlled by only three geometrical rather than geological variables; a sandstone connectivity term, a system anisotropy term and a system resolution term. If two completely different faulted or unfaulted systems have identical values of these three variables they will have the same flow properties.
We conclude from this study that there is no straightforward answer to the question of what geological factors control flow at a bed-scale in faulted turbidite systems. During the characterisation of particular assets in which likely ranges of potentially significant variables are known or can be estimated, it is therefore recommended that sensitivity studies be performed to assess sandstone connectivity and to explore the relative significance on sandstone connectivity of each variable, thereby permitting the definition of more focused reservoir characterisation programs.
Abstract of talk given to:
Structurally Complex Reservoirs, Geological Society of London, February 2006.