Multi-Scale Stratigraphic Heterogeneity Induced by Allocyclic and Autocyclic Factors in a Low Net-to-Gross Fluvial Succession

Abstract

Although channel bodies in low net-to-gross fluvial successions commonly generate isolated reservoirs, vertical and lateral, multi-scale lithological heterogeneities resulting from combined allocyclic and autocyclic controls can give rise to complex arrangements of channel amalgamation and stacking. Outcrop analyses represent the optimal way to evaluate such heterogeneities without the limitations of resolution and information density associated with geophysical and well-bore data. Laterally continuous exposures (5 km) of the low net-to-gross Esplugafreda and Claret formations (Paleogene, Arén, southern Pyrenees, Spain) are used to develop a reservoir-analog study based on acquisition of quantitative and qualitative sedimentological data. Datasets were obtained at different scales in order to evaluate reservoir heterogeneities at facies, facies-association and architectural-element levels. Constraints on potential eustatic, climatic and subsidence controls in this succession allow evaluation of the architectural response of a fluvial system to allocyclic drivers. Additionally, this succession represents the preserved expression of ephemeral streams developed in an arid environment that undertook repeated and apparently frequent avulsions in response to flash-flood events. It therefore provides an opportunity to model a low net-to-gross channelized fluvial system developed in response to combined allogenic and autogenic controls. Outcropping channel bodies have been mapped using high-resolution orthophotographs, LiDAR DEM's and GPS data; lithofacies, facies associations and architectural elements have been characterized by field analysis. Two facies associations occur in connection with two distinct architectural styles. Sand-prone association (FA1) comprises fine- to medium-grained sandstones, mainly with planar and low-angle cross stratification that have been subject to pedogenic alteration. Conglomeratic facies association (FA2) comprises massive to horizontal or cross-stratified conglomeratic bodies, with minor interbeds of coarse-grained sandstones. Both facies associations occur in isolated ribbons and weakly channelized sheet-like multi-story bodies, however only the FA2 tends to amalgamate laterally and vertically to form channel complexes. Channel complexes with significant lateral continuity are restricted to the uppermost part of the succession, which records a major paleo-climatic change associated with the Paleocene-Eocene Thermal Maximum.

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