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Incised Valley Fill Reservoirs, From Outcrop Analogue to Subsurface Interpretations (Upper Carboniferous, Kentucky, USA)

Abstract

Incised valleys are extensive geomorphological features with a high potential of encasing coarse grain sediments in mud-dominated environments, therefore representing an important reservoir potential (e.g. Southern North Sea and onshore Europe). However, their identification and prediction of their size and infill architecture in the subsurface is challenging. Therefore, a better understanding of their mechanisms of formation, as well as the dimensions, geometry, internal heterogeneities and connectivity of their infill, is required. The study of ancient and modern analogues has proven to be a very useful tool to study complex sedimentary bodies. The numerous Quaternary case studies have led to a good understanding of the mechanisms of formation of incised valleys. However, their internal stratigraphic architecture remains less understood. Indeed, their important size dwarfs most the potential outcrops for ancient case studies, and subsurface investigations of modern or recent cases are limited by data resolution and areal coverage. The Pennsylvanian strata extensively exposed along several roadcuts in eastern Kentucky represent an outstanding field laboratory for the study of the stratigraphic architecture of incised valley fills (IVF) developed in coal-bearing fluvio-deltaic successions. One of the numerous outcrops studied in this research displays two IVF's. The first one is a 15m deep erosional surface overlain by valley flank collapse deposits on the sides and fluvial sandstone in the center of the valley, while the rest of the infill consist of tidal flat deposits, in which river influence seems absent, onlapping the underlying deposits. We interpret this as an “abandoned” IVF sequence, meaning that at some point in the valley history, the accommodation-supply ratio increase significantly either due to a very rapid sea-level rise or upstream flow-capture. The second IVF consists of a 17m deep incision infilled by fluvial sandstones transitioning upward from bedload-dominated deposits to suspension-dominated and tidally influenced deposits. It is therefore interpreted as an “active” IVF sequence, meaning that the amount of sediment supplied doesn't change and that change in facies is mainly due to change in slope and sea-level gradually rising. In this paper we present the detailed sedimentologic and stratigraphic analysis of several IVF's, from outcrop and well data, focusing on their reservoir characteristics, geometry and internal heterogeneities.