Allogenic Versus Autogenic Controls on Sedimentation in the Fluvial-to-Marine Transition Zone: A Subsurface Study From the Dhruma Formation, Rub’ Alkhali, Saudi Arabia

Abstract

Allogenic Versus Autogenic Controls on Sedimentation in the Fluvial-to-Marine Transition Zone: A Subsurface Study from the Dhruma Formation, Rub’ Alkhali, Saudi Arabia

Bassam Alshammari, Nigel P. Mountney, Luca Colombera

Fluvial Research Group and Shallow-Marine Research Group, School of Earth and Environment, University of Leeds, LS29JT, UK

Discerning the roles of autogenic and allogenic controls on the accumulation and preservation of sedimentary successions requires characterization at multiple scales. This is especially true in fluvial-to-marine transition zone (FMTZ) settings for which the preserved stratigraphic record is complicated by tempo-spatial interactions of fluvial, wave and tidal processes that give rise to a broad range of sub-environments. Approaches to lateral correlation for this type of succession in the subsurface is problematic. Typically, the distribution of architectural elements representative of various paralic sub-environments cannot readily be determined from subsurface data. One solution is to construct depositional models that are constrained by subsurface well data but for which the architectures of geobodies are informed by information stored in a database of global analogs.

This study presents a workflow for the construction of depositional models to account for stratigraphic complexity inherent in a FMTZ succession. The approach integrates techniques in lithofacies analysis, ichnology and sequence stratigraphy based on analysis of subsurface data from 15 wells that penetrate the middle Jurassic, Lower Dhruma Sand member of Dhruma Formation, Rub’ Alkhali Basin, Saudi Arabia. The data set includes facies description of ~1650 ft. (~500 m) of core, 77 core thin sections, 5 image logs and a series of 2D seismic lines. Analog information is derived from >300 analogs held in relational databases developed in-house. A specific objective of this study is to predict the nature of juxtaposition of geobodies of various type and to investigate their lateral continuity, thereby demonstrating the presence of a range of depositional sub-environments and establishing a sequence stratigraphic framework.

Lithofacies cycles revealed in core record staked upward-fining sandstone successions, heterolithic and bioturbated sandstone, cross-bedded sandstone and siltstone, shale, and thin beds of coal. Associations of these facies record tidally influenced fluvial (up-dip settings), coastal plain, mouth bar, and shoreface (down-dip) sub-environments. Facies transitions observed vertically in core record the presence of progradational and retrogradational parasequence sets indicative of multiple transgressive-regressive cycles. Database-informed analog examples show how the lateral extent and juxtaposition of sub-environments might have changed in response to relative sea-level change.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019