Sequence Stratigraphy of the Hith/Upper Arab Formations Offshore Abu Dhabi, U.A.E.

Abstract

Abstract The Kimmeridgian Upper Arab zones A, B, and C, are prolific hydrocarbon bearing reservoirs in central and western Offshore Abu Dhabi, (OAD). They were deposited in an arid climate which dominated the Arabian peninsula during Late Jurassic times. The Berriasian to Tithonian Hith Formation which overlies the Arab reservoirs constitutes the cap rock, which just to the east of central OAD gradually pinches out and forms a N-S feather edge. The Hith and Upper Arab zones A, B, and C form 430 to over 600 feet of massive to interbedded anhydrites with varying proportions of limestones and dolomites in central and western OMD. The Arab Formation in OMD is a major regressive unit which was deposited on a broad carbonate platform and prograded eastwards into an open marine shelf environment. The Upper Arab zones A, B, and C consist of small scale shallowing upward cycles of varying thicknesses (6 to 30 fret) which typically consist of shoal grainstones (TST) passing upwards into thin bedded lagoonal burrowed mud/wackestones (MFS). These are overlain by coarse bioclastic grainstones capped by algal laminites and culminate into supratidal anhydrites, which pinchout eastwards. In the west, cycles are thicker and consist of intertidal pack/grindstones overlain by anhydrites. The parasequences or meter scale cycles have layer cake stacking patterns arranged in transgressive, regressive episodes, and are driven by climatic, syndepositional tectonics and their associated low amplitude sea level fluctuations (eustacy). Early evaporative and reflux dolomitization of peloidal and oolitic grainstones has greatly enhanced reservoir characteristics. Meteoric leaching associated with relative drops in sea level resulted in aragonite dissolution and reservoir enhancement Early marine cementation and late post-burial diagensis such as anhydritisation resulted in reservoir deterioration. The objectives of this paper are to develop a sequence stratigraphic framework, emphasizing cyclicity, facies architecture and diagenesis. Core and well log data geared with various inorganic geochemical analyses from four wells are used to constrain the current uncertainties in age dating and integrate the diagenetic signatures into the patterns of relative sea level change which considerably control the formation of those parasequences. This effort will help in better understanding and possible prediction of porosity in such prospective reservoirs. I-INTRODUCTION AND OBJECTIVES Offshore Abu Dhabi (OAD) lies on the eastern part of the Arabian platform, and is located in the southern Arabian Gulf, (Figure- 1). The Late Jurassic in central and western OAD is composed of the Arab Formation which is overlain by the Hith Formation. The Arab Formation (147 to 308 fret thick) in west and central OAD is subdivided into four reservoir which are from top to bottom A, B, C, and D. These Arab zones are prolific hydrocarbon bearing reservoirs in several anticlinal structures extending from central to western OAD. In eastern OAD where open shelf sedimentation persisted, these zones cannot be distinguished. P. 277