Lobe Sub-Structure in Deep-Sea Fan Systems — The Distinction Between Accommodation-Limited and Compensational-Stacking Patterns

Abstract

Sandy lobes are a key architectural element in many deep-water successions. Recent attention has focussed on whether depositional hierarchies similar to those proposed for up-slope channel systems can be applied to lobes. Nesting of lobe elements within lobe bodies can account for the diverse bed stacking trends in some lobe successions and lobe sub-structure can have a significant impact on the volumes of connected sand. To date, much of the focus on lobe development has been on progradation and compensation; accommodation is generally assumed to be unimportant despite the value of slope equilibrium concepts in predicting channel behaviour up-dip. As lobe elements are relatively small yet build larger composite bodies extending basinwards, this means the former can locally blanket earlier elements in what are accommodation-limited proximal settings. Two end-member styles of lobe stacking might thus be envisaged – compensational stacking driven by local re-orientation of the flow field in fringe settings where existing lobe hierarchy schemes might apply, and accommodation-limited settings where accommodation is rapidly filled and the lobe surface evolves to bypass. In the former setting, lobe elements are separated by local abandonments or offset fringes, in the later, by erosional surfaces. The two types of lobe sub-structure are well expressed in the Pennsylvanian Ross Sandstone Formation, western Ireland where they have been cored and characterised in a behind-outcrop drilling programme. Compensationally-stacked lobe elements comprise trendless sandy stacks of either turbidites or hybrid event beds (HEBs) in bundles up to 8 m thick separated by dcm-thick thin-bedded and fine grained inter-lobe elements. These are interpreted as offset stacked axis and fringe deposits respectively. Accommodation-limited lobes are generally 2-5 m thick and characterised by prominent upward-sandying trends. They are dominated by HEBs and are capped by erosional surfaces with megaflutes that locally extend beneath channels cut into the lobe tops. Proximal avulsions released energetic flows into accommodation-limited sectors of the fan that were close to the equilibrium profile leading to flow expansion, deposition, rapid aggradation to bypass, and auto-incision as the fan surface was healed. The distinction between the two styles of lobe sub-structure is important for reservoir quality prediction, larger-scale architecture, and drainage of reservoir fluids.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016