High-resolution Sequence Stratigraphic Architecture of Barremian/Aptian Carbonate Systems in Northern Oman and the United Arab Emirates (Kharaib and Shu’aiba Formations)

Abstract

ABSTRACTA regional sequence stratigraphic model for the Kharaib and Shu’aiba formations (Barremian, Aptian) is proposed based on outcrop and subsurface transects in Oman and the United Arab Emirates. The model shows distinct variations in depositional facies and geometrical patterns in relation to third-order sequences. The sedimentary systems evolved from a low-angle carbonate ramp (Kharaib Formation), to an organic-rich intrashelf basin surrounded by carbonate platforms (Hawar Member and Lower Shu’aiba), to a clay-dominated sedimentation restricted to the intrashelf basin (Upper Shu’aiba). Eustatic sea-level changes and, to a lesser extent, local tectonic controls influenced the development of the systems. This high-resolution sequence stratigraphic model has implications for both exploration and production strategies.This study presents a revised sedimentological facies interpretation based on detailed outcrop observations with geological constraints provided by a regional well-log correlation from Oman and the United Arab Emirates. Time control was provided by biostratigraphy (including new nannofossil data), and carbon isotope stratigraphy. Four third-order depositional sequences bounded by regional exposure surfaces were defined that are composed of at least two higher-order sequences (fourth- and fifth-order).Depositional sequences I and II have a flat ramp setting. The typical ecological succession was orbitolinid and/or miliolid wackestone-packstone deposited in a low-energy, inner ramp environment during early transgression; a mixed-fauna mudstone-wackestone in an open lagoon during maximum flooding and early highstand; and a rudist/miliolid wackestone-packstone-grainstone-framestone of the succeeding late highstand in a high-energy, inner- to mid-ramp environment. The doubling in thickness of the sequences from Oman to Abu Dhabi is attributed to differential subsidence. The Sequence III succession was somewhat similar, but differences were caused by the creation of the intrashelf Bab Basin, and by well-developed microbial boundstones. The basin morphology was primarily due to differential accumulation rates and tectonism was of only minor importance. Rapidly aggrading and backstepping microbial boundstones formed the platform, whereas condensed sedimentation and the accumulation of carbonate source rocks occurred in the basin. Highstand deposits were primarily grainy, high-energy rudist-dominated facies. The sequence ended with a forced regressive wedge along the basin margins. Sequence IV was restricted to the Bab Basin and sedimentation consisted of a clay-rich infill phase. At that time, the Oman and Abu Dhabi platforms were exposed on both sides of the basin. Nannofossils dated this major relative fall in sea level to the early-late Aptian.