Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral‐template properties of exopolymeric substances play a crucial role.

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The Dohat Faishakh sabkha in Qatar is one of the rare modern environments where it is possible to study the formation of dolomite, a mineral whose origin has been long debated. In previous studies, dolomite formation in this area was considered to be the result of a penecontemporaneous replacement of aragonite, occurring in the presence of Mg-rich evaporated pore-waters. However, a re-investigation of the sabkha revealed that dolomite is not forming exclusively under the evaporitic conditions that characterize the supratidal zone, but also in microbial mats that colonize the lower intertidal zone, indicating that evaporated pore-waters are not a strict requirement for the mineralization process. Moreover, in the supratidal zone, portions of the sediment that are rich in dolomite are also relatively richer in organic material, which derives from partially degraded microbial mats buried in the sediments. Extracellular polymeric substances (EPS) that constitute microbial mats are recognized as an important component for the formation of Mg-rich carbonates. The presence of living and decaying microbial mats comprising EPS, rather than a replacement process, may be the key factor for dolomite formation in the Dohat Faishakh sabkha.

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...

Reservoir rock typing is a process by which geological facies are characterized by their dynamic behavior. The dynamic behavior of the facies is assessed by studying the rock texture, the diagenetic processes which overprinted the initial fabric, and the interaction between the rock itself and the fluids. Porosity, permeability and pore size distributions characterize the rock texture while capillary pressure, relative permeability and wettability describe the rock-fluid interaction. Reservoir rock typing is a synergetic process between geology and petrophysics/SCAL. It is therefore a process by which various petrophysical parameters and dynamic measurements obtained from SCAL are integrated in a consistent manner with geological facies (lithofacies) to estimate their flow (dynamic) behavior. The relationships between lithofacies and reservoir rock types (RRTs) is complex because of the inter-play between facies, diagenetic processes and the rock-fluid interaction (wettability changes) in the reservoir. Similar lithofacies, deposited under the same depositional environments, may exhibit different petrophysical properties due to diagenesis. Therefore, lithofacies deposited under similar geological conditions may experience different diagenetic processes resulting in different petrophysical groups with distinct porosity-permeability relationship, capillary pressure profile and water saturation (Sw) for a given height above the Free Water Level (FWL). On the contrary, lithofacies deposited in different depositional environments, might exhibit similar petrophysical properties and dynamic behavior. The authors emphasize on the need to have a good understanding of the original facies, depositional environments, subsequent diagenetic processes and rock-fluid interaction (via SCAL) to be able to unravel the relationships between lithofacies, petrophysical groups and rock types. A workflow for carbonate rock typing addressing some of the industry pitfalls and the differences between lithofacies, petrophysical groups and rock types are presented in this paper. Introduction -NomenclatureBefore proceeding into the rock type description and its link with geology and SCAL, it is important to provide a few basic definitions of the common technical terminologies found in the literature such as lithofacies, facies associations, petrophysical groups, rock types and flow units. In this paper we define lithofacies or lithofacies types as a depositional facies, or lithotype, based on sedimentary texture (Dunham 1962; Embry and Klovan 1971), grain types (skeletal grains, peloids, ooids, etc.), and, optionally, sedimentary structures (cross-bedding, bioturbation, lamination, etc.). Typical lithofacies types are skeletal wackestone, skeletal-peloid packstone or cross-bedded ooid grainstone. Facies associations are groups or bins of lithofacies from the same depositional environment/facies tracks with common φ-k relationships/trends. Petrophysical groups are units of rocks (can consist of multiple lithofacies) with similar petrophysical ...