Supratidal (sabkha) to intertidal (microbial mat), and lowermost intertidal to shallowsubtidal (peloid-skeletal tidal flat) environments were studied along the Abu Dhabi coastline in the vicinity of Al-Qanatir (Al-Rufayq) Island. A transect from land to sea displays the following classic examples of supratidal to shallow-subtidal facies belts:(1) Inner, upper sabkha (upper supratidal), buckled polygonal halite crust displaying teepee structures; (2) Stranded beach ridges, forming low-relief topographic highs paralleling the coastline that are mainly composed of cerithid gastropods; (3) Outer, upper sabkha (upper supratidal), buckled polygonal halite crust displaying teepee structures; (4) Middle sabkha (middle supratidal), whitish anhydrite polygons on surface; (5) Lower sabkha (lower supratidal), soft, shiny surface due to sparkling gypsum crystals (gypsum mush); (6) Upper intertidal, thin, leathery, crinkled or crenulated microbial mat; (7) Middle intertidal, blistered microbial mat and pinnacle or domed microbial mat; (8) Lower intertidal, thick, smooth polygonal microbial mat and tufted or cinder-like microbial mat; (9) Lowermost intertidal to shallow-subtidal, peloid-skeletal tidal flat (lagoonal and shallow tidal-channel/tidal-creek deposits) displaying cerithid and littorinid gastropod grazing-traces, Skolithos-type burrows, and eroded wave ripples.The stenohaline bryozoan species Disporella sp., which has not been recorded previously from the United Arab Emirates, was found within a thin channel lag deposit of the outer, upper sabkha environment. Significant amounts of dolomite were found within a subsurface crinkly-laminated microbial mat (middle sabkha environment). The fine-crystalline dolomite displays subhedral to euhedral dolomite rhombs embedded in an organic matrix. The formation of dolomite is interpreted to be related to sulphate reducing microbial organisms which form the widespread microbial mat along the Abu Dhabi coastline.Radiocarbon dating of 15 samples (10 hardground samples, 3 microbial mat samples, and 2 samples from anhydrite-dominated layers) show an age range from ca 3500 uncalibrated 14 C yr BP (outer, upper sabkha environment: subsurface hardground, seaward of stranded beach ridges) to ca 900 uncalibrated 14 C yr BP (intertidal environment: subsurface microbial mat); thereby supporting the seaward progradation of the facies belts since the last Holocene sea-level highstand (formation of cerithid gastropod stranded beach ridges). An anhydrite layer within aeolian deposits (inner, upper sabkha environment: landward of stranded beach ridges) showed a radiocarbon age of the host sediment of ca 12,900 uncalibrated 14 C yr BP, corresponding to Pleistocene dune deposits, pre-dating the Holocene flooding event. The distribution of radiocarbon ages indicates a complex stratigraphic history in which chronostratigraphic time lines clearly cross-cut depositional lithofacies and diagenetic boundaries. This is significant in that depositional lithofacies and diagenetic facies are commonly used i...

A new sequence stratigraphic framework is proposed for the Lower Cretaceous Kharaib Formation (Barremian and Lowermost Aptian) of the United Arab Emirates. This framework is based on the integration of core and well-log data from Abu Dhabi oil fields with outcrop data from Wadi Rahabah, Ras Al-Khaimah (U.A.E.). The Kharaib Formation is part of the late transgressive sequence set of a second-order supersequence, built by two third-order composite sequences. Fourteen fourth-order parasequence sets build into two third-order composite sequences and show predominantly aggradational and progradational stacking patterns, typical of greenhouse cycles. On the basis of faunal content, texture, sedimentary structures, and lithologic composition, eleven reservoir lithofacies and eight non-reservoir "dense" lithofacies are identified from core. These same lithofacies are also identified in time-equivalent rock exposures studied in Wadi Rahabah. The analyzed lithofacies range from open platform, lower ramp to restricted platform subtidal to intertidal environments. Intensively bioturbated wackestone and packstone, and interbedded organic- and siliciclastic-rich limestone characterize the three so-called dense zones (Lower, Middle, and Upper Dense Zone). Locally, mud-cracks, blackened grains, and rootlets are observed. The two reservoir zones (Lower and Upper Kharaib Reservoir Unit) correspond to the late transgressive and, dominantly, highstand systems tracts characterized by parasequence sets that show shallowing-upward trends from open platform, burrowed skeletal wackestone to skeletal, peloidal packstone and algal, coated-grain grainstone/rudstone, and rudist, algal floatstone/rudstone. Well-developed Thalassinoides firmgrounds (Glossifungites surfaces) indicate temporary cessation in sedimentation and cap several parasequence sets and parasequences. Stylolitic intervals within the reservoir units predominantly correspond to major facies changes related to third-, fourth-, and fifth-order sequence boundaries, parasequence set boundaries, and parasequence boundaries. In outcrop, low-angle clinoforms that cannot be seen in core data are observed within the highstand systems tract of the upper third-order composite sequence (Upper Kharaib Reservoir Unit). Integration of subsurface and outcrop data leads to more insightful and realistic geological models of subsurface stratigraphy. Introduction Large hydrocarbon accumulations have been discovered and produced from platform carbonates of the Upper Thamama Kharaib Formation in Abu Dhabi1,2,3. The Kharaib Formation (Barremian and Lowermost Aptian) contains two reservoir units (Lower and Upper Kharaib Reservoir Unit) separated and encased by three zones of very low porosity and permeability, subsequently referred to as dense zones (Lower, Middle, and Upper Dense Zone). Thickness of the Upper Kharaib Reservoir Unit is between 150 and 170 feet, and thickness of the Lower Kharaib Reservoir Unit is about 80 feet. Highly permeable beds (several Darcy) that mainly consist of coated-grain grainstone and rudist floatstone/rudstone facies are recognized in Field-A (Fig. 1). Generally, reservoir quality (porosity and permeability) decreases from crest to flank across the structure. This is due to compaction and cementation caused by the interaction with formation water. Diagenetic patterns of Field-B, Field-C, and Field-D (Fig. 1) are significantly different to those observed in Field-A. Whereas Field-A displays very good petrophysical properties, only locally affected by minor early- and late-diagenetic events, the reservoir quality of Field-B, Field-C and Field-D is impacted negatively by early-diagenetic porosity-destructive calcite cementation. This early-diagenetic calcite cement prevented significant compaction but occludes much of the primary interparticle porosity of the grainstone facies.

The Khuff Formation can be described as a second-order transgressive-regressive sequence.The transgressive sequence set comprises Khuff third-order composite sequences KS7 through KS5 and the third-order lowstand/transgressive systems tract of KS4. The regressive (highstand) sequence set comprises the highstand systems tract of KS4 as well as Khuff third-order composite sequences KS3 through KS1. Major Khuff gas reservoirs are associated with the second-order highstand sequence set.

The Mussafah Channel is a man-made canal cut perpendicular to the coastline, located to the southwest of the city of Abu Dhabi, United Arab Emirates, and is ideal for studying coastal depositional processes in an arid environment. The channel walls reveal a few meters of Pleistocene reworked dune deposits, unconformably overlain by Holocene carbonates and sabkha evaporites. The Holocene succession consists of intertidal to shallow subtidal sediments that vary significantly along depositional strike direction. Bladed gypsum crystals, gypsum rosettes, and nodular to highly contorted, discontinuous bands of classic sabkha anhydrite are present along the channel walls. Sedimentology, petrography, SEM, X-ray diffraction, and radiocarbon age-dating analyses of the sabkha sequence show the following profile from base to top: (1) non-bedded carbonate-rich sand: reworked aeolianite with an approximate (ca.) radiocarbon age in years (yrs) before present (BP) ca. 26,800 14C yrs BP; (2) cross-bedded to non-bedded carbonate-rich sand: aeolianite/reworked aeolianite (ca. 24,000–23,500 14C yrs BP); (3) crinkly-laminated stromatolitic bindstone: intertidal, low-energy microbial mat (ca. 6,600–6,200 14C yrs BP); (4) lower, discontinuous and in places reworked hardground: cemented channel-lag deposits (ca. 6,400 14C yrs BP); (5) peloid-skeletal packstone with rootlets or microbial-laminated peloid-skeletal packstone, laterally grading into fine- to coarse-grained, cross-bedded, cerithid-rich, bioclastic packstone, grainstone, and rudstone: lowermost intertidal to shallow subtidal, low-energy, mud-rich rooted and microbial-laminated lagoonal deposits and moderate- to high-energy, intertidal to shallow subtidal tidal-channel, tidal-delta, and tidal-bar deposits (ca. 6,200–5,200 14C yrs BP); (6) upper discontinuous and shingled hardground: cemented beach rock (ca. 5,700 14C yrs BP); (7) cross-bedded, bioclastic rudstone/grainstone, grading laterally into intervals displaying bladed gypsum crystals and nodular to enterolithic anhydrite: intertidal to shallow subtidal, high-energy longshore beach bar and beach spit deposits; overprinted by sabkha gypsum and anhydrite (ca. 5,000 14C yrs BP). Significant amounts of dolomite were found within the rooted and microbial-laminated mud-rich lagoonal carbonates, some of the tidal-channel/lagoonal deposits, the buried crinkly-laminated microbial mats, and within some of the Pleistocene carbonate-rich sands. The dolomite is very fine-crystalline and displays spherical morphologies as well as subhedral to euhedral dolomite rhombohedra. The formation of dolomite is interpreted to be related to dolomite-mediating microbial organisms which form the widespread microbial mat along the Abu Dhabi coastline. Microbial organisms are also present within the rooted and microbial-laminated lagoonal carbonates and, most probably, within all the other studied carbonates and the Pleistocene carbonate-rich sands. Biopolymers of microbial origin, referred to as Extracellular Polymeric Substances (EPS), are interpreted to play a key role in primary dolomite formation. The sabkha sequence at Mussafah Channel formed during the post-glacial Flandrian transgression, resulting in the reworking of the Pleistocene aeolian dunes and the deposition of intertidal to shallow subtidal carbonates. Recent find of whale bones within tidal-channel deposits overlying the microbial mat further document the initial Holocene transgression. During a subsequent slight sea-level fall (regression), these carbonates were overprinted by gypsum and anhydrite. The observed lateral and vertical facies variations reflect primary reservoir quality variations, an important aspect to be considered for geological facies and reservoir quality modeling.

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