Spatial Changes in Depositional Processes in a Progradational Mixed-Influence Deltaic Succession, Jurassic Lower Lle Formation, Halten Terrace, Offshore Norway

Abstract

Recognition of depositional processes in clastic coastal-deltaic deposits forms the basis for predicting their key reservoir properties, including dimensions, geometries, orientations and the three-dimensional distribution of petrophysical properties and reservoir heterogeneities. The application of this well-established approach can be problematic in mixed-energy coastal-deltaic systems, in which process interpretation is weakly constrained. We have addressed this problem through a detailed sedimentary facies analysis of exceptionally well-preserved and widely distributed cores, calibrated with their wireline log signatures, which enable confident process and environmental interpretations in all preserved parts of the depositional system. This study focusses on the lower, progradational part of the Lower to Middle Jurassic Ile Formation in the southern part of the Halten Terrace on the Norwegian shelf. The 100 m thick Ile formation forms the top of a 300 m thick megasequence, comprising progradational to aggradational deltaic sediments, which are overlain by a succession of retrogradational coastal deposits. The progradational to aggradational succession contains three types of regressive parasequences: (1) mixed-wave-tide influenced parasequences consist of bioturbated mudstones, which coarsen upward from micro-hummocky cross-stratified sandstones and mudstones, through intervals with increasing bidirectional current-ripple cross-lamination and into cross-bedded sandstones with mud-draped foresets and toesets; (2) mixed-wave-fluvial influenced parasequences consist of mudstones that coarsen upwards through micro-hummocky cross-stratified fine-grained sandstone and mudstone into poorly sorted coarse- to very coarse-grained sandstone; and (3) wave-dominated parasequences consist of bioturbated mudstones which pass upwards into mudstones interbedded with hummocky cross-stratified sandstones that amalgamate upwards. Parasequences observed in core have a thickness varying between 3 and 28 m, with a decrease in thickness upwards. The number and thickness of parasequences are variable laterally. The mixed-wave-fluvial influenced and mixed-wave-tide influenced parasequences are found in close association with each other, and represent active and abandoned parts of the delta, respectively. Wave-dominated parasequences are interpreted as linear coastlines lateral to the deltas.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015