Sequence stratigraphy of Oxfordian and Kimmeridgian shelf carbonate reservoirs, offshore Abu Dhabi

Abstract

ABSTRACT Carbonate reservoirs on the eastern flank of the Oxfordian-Kimmeridgian intrashelf basin in offshore Abu Dhabi had received little attention until commercial oil accumulations in structural traps were discovered in the late 1980s and early 1990s. In order to clarify the geometric and chronostratigraphic relationships of the Oxfordian-Kimmeridgian reservoirs, a multidisciplinary study (seismic, lithobiofacies, geochemistry, strontium isotope dating, and well-log data) was used to develop a sequence stratigraphic model. After deposition of the Callovian upper Araej Formation, a differentiated carbonate platform was established in the early Oxfordian in offshore and western onshore Abu Dhabi. Tectonic subsidence coupled with sea-level fluctuations controlled the geometry, deposition, and distribution of the lithofacies. These ranged from organic-rich, limy mudstones in the basinal area, to porous and permeable bioclastic packstones, grainstones, and dolomites in shallow waters on the eastern flank of the intrashelf basin. The upper Kimmeridgian Arab-D Member of the Arab Formation overlies the basinal deposits. Three third-order Depositional Sequences were identified in the offshore area. They are named according to their contained Maximum Flooding Surface; a fourth sequence is an intermediate unnamed Depositional Wedge. Depositional Sequence Jurassic 50 is of Oxfordian age and was deposited during transgressive and highstand periods. The lower Kimmeridgian Depositional Sequence Jurassic 60 is a well-defined lowstand system tract overlain by short-lived transgressive and highstand system tracts. Overlying Depositional Sequence Jurassic 60 is the Depositional Wedge. Finally, Depositional Sequence Jurassic 70 consists of transgressive and highstand system tracts developed on an undifferentiated platform that had localized depressions in the west. The best reservoir developments are in highstand bioclastic packstones and grainstones below the type-1 sequence boundaries that cap Depositional Sequences Jurassic 50 and Jurassic 60. The reservoir units have porosities greater than 20 percent and permeabilities of more than 1,000 milliDarcies. The basinal facies of Depositional Sequence Jurassic 50 have the best source-rock potential in the intrashelf basin.