Thin-bedded turbidites and channel sands : the connectivity bridge

Abstract

Turbidite deposits are a common facies in many deepwater systems, and within these facies, thin-bedded turbidites (TBTs) and very thin-bedded turbidites (VTBTs) are most abundant, but their potential for exploration and production has been largely underdeveloped. This study seeks to develop a quantitative approach for characterisation of TBTs and VTBTs, using their sedimentary attributes from ancient and subsurface systems, to assess the impact of multi-scale sedimentary heterogeneity on the connectivity between TBT/VTBT intervals and other turbidite facies within selected deepwater systems. It further seeks to provide valuable insights into the hidden reservoir potential of TBT/VTBT facies, with a view to assessing their capability to enhance or reduce the impact of net-to-gross-associated uncertainty on connectivity between these facies and associated primary reservoir facies (channel sands). Analysis of 1,107 ft (337.4 m) of cores from the North Brae Field enabled identification of the sedimentary attributes of TBT/VTBT facies and their combination into attribute indices. These indices are facies Net-to-Gross Index (NGI), Sand Connectivity Index (SCI), Facies Ratio Index (FRI), and Sediment Textural Index (STI). From these indices, NGI and SCI form the central focus of this study. The interpreted sedimentary logs together with the insights gained from an additional 16,275 ft (4, 961 m) of cores logged from Pierce, Starling, and Fram Fields, enhanced the appreciation of multi-scale lateral and vertical variabilities of their facies attributes. The detailed study undertaken on an 892 ft (272 m)-long interval in the Zumaia section, serves to provide the input data for the high-resolution modelling and flow simulation studies conducted to better understand the importance of bed-scale sedimentary heterogeneities on fluid flow in a typical basin-plain setting. In addition, the resultant multi-resolution models reflect significant variations in the distribution of key parameters that affect flow performance within the types of connectivity bridge (defined in this thesis as the connectivity between primary and secondary reservoir facies) considered. The results of this study demonstrate that attribute indices can be applied to characterise different deepwater architectural elements, and also serve as quantitative input parameters for conditioning reservoir models and ground-truthing flow simulation. This application can be extended to all turbidite types. Furthermore, the results indicate the importance of siting injector wells strategically to improve oil sweep, and minimise the risk of high water cut, whilst optimising oil recovery. The findings establish the effect of bed-scale sedimentary heterogeneity, particularly bed stacking pattern, net-to-gross distribution, and lateral variation in bed thickness, on connectivity, which in turn has profound implications on the producibility of deepwater turbidite reservoirs, and can therefore have a significant effect on reserve estimation and overall project value. On the basis of the Continuity-Connectivity Scheme, developed from isolating individual continuity-connectivity relationships in the various scenarios considered, TBT-associated deepwater turbidite systems can be classified into six broad categories, namely: (1) low continuity and high connectivity systems (single-storey channel); (2) low-to-intermediate continuity and intermediate connectivity systems (laterally-stacked channel-levee); (3) intermediate continuity and low connectivity systems (distal basin plain); (4) intermediate-to-high continuity and intermediate-to-high connectivity systems (vertically-stacked channel-levee); (5) high continuity and intermediate connectivity systems (proximal basin plain); and (6) high continuity and high connectivity systems (channel-splay lobe). Following the results of this study, deepwater turbidite reservoirs that are characterised by an effective oil displacement, efficient areal sweep, and lowest risk to early water breakthrough offer the best potential, occasioned by a favourable connectivity threshold, for economic recovery of bypassed pay in hydrocarbon-producing turbidite fields.