Facies Architecture of a Net Transgressive Deltaic to Estuarine Succession, the Khafji Sequence, Offshore Northeast Saudi Arabia
The Khafji sequence represents an Aptian to Albian siliciclastic wedge, 800 feet thick, which spans several million years. This sequence records the northeastward advance of a major delta into a carbonate platform. This study utilizes subsurface cores to investigate the evolution of the depositional systems that developed during the retreat of the delta.
Core description has allowed the recognition of fluvial channel, crevasse splay, flood plain, tidal flat, estuarine, shoreface, river mouth bar and shelf facies associations. These are contained in a retrogradational sequence set, 300 feet thick, which comprises three unconformity-bounded sequences. Each sequence contains two or three individually progradational tongues that are arranged in a back-stepping pattern and separated by landward pinching-out marine shales. Each tongue consists largely of fluvial channel sandstones changing distally to the northeast into river mouth bar and shoreface facies associations that are stacked into a retrogradational parasequence set. The basal parasequence, immediately overlying a sequence boundary, is foreshortened and marked by a basal erosional surface. The fluvial sandstones change upwards into crevasse splay deposits, laterally extensive coal beds, tidal flat and bayhead-deltaic facies. These sandstones may also change upwards, across a ravinement surface, directly into backstepping shoreface deposits.
The facies architecture reflects a high sediment supply and a long-term sea-level rise punctuated by brief falls and still-stands. These conditions prompted the formation of the progradational tongues and the development of a mixed tide- and wave-influenced deltaic system. During periods of accelerated sea-level rise, it evolved into a transient wave-dominated estuarine system. The resulting deltaic to estuarine couplets, typified by thick deltaic and thin to absent estuarine components, can be expected in supply-dominated and low-accommodation settings subject to greenhouse sea-level fluctuations. The fluvial channel and river mouth bar facies, deposited therein, can have good reservoir potential.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California