There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.

There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.

Fault-controlled dolostone bodies have been described as potential hydrocarbonbearing reservoirs. Numerous case studies have described the shape and size of these often non fabric selective dolostone bodies within the vicinity of crustal-scale lineaments, usually from Palaeozoic or Mesozoic carbonate platforms, which have undergone one or more phases of burial and exhumation. There has been little attention paid, however, to fault-strike variability in dolostone distribution or the preferential localization of these bodies on particular faults. This study focuses on dolostone bodies adjacent to the Hammam Faraun Fault (HFF), Gulf of Suez. This crustalscale normal fault was activated in the Late Oligocene, coincident with the onset of extension within the Suez Rift. Dolomitization in the prerift Eocene Thebes Formation occurred in the immediate footwall of the HFF forming two massive, non facies selective dolostone bodies, ca. 500 m wide. Facies-controlled tongues of dolostone on the margins of the massive dolostone bodies extend for up to 100 m. The geochemical signature of the dolostone bodies is consistent with replacement by Miocene seawater, contemporaneous with the rift climax and localization of strain along the HFF. A conceptual model of dolomitization from seawater that circulated within the HFF during the rift climax is presented. Seawater was either directly drawn down the HFF or circulated from the hanging wall basin via a permeable aquifer towards the HFF. The lateral extent of the massive dolostone bodies was controlled by pre-existing HFF-parallel fracture corridors on the outer margins of the damage zone of the fault. The behaviour of these fracture corridors alternated between acting as barriers to fluid flow before rupture and acting as flow conduits during or after rupture. Multiple phases of dolomitization and recrystallization during the ca.10 Ma period in which dolomitization occurred led to mottled petrographical textures and wide-ranging isotopic signatures. The localization of dolomitization on the HFF is interpreted to reflect its proximity to a rift accommodation zone which facilitated vertical fluid flow due to perturbed and enhanced stresses during fault interaction. It is possible that the presence of jogs along the strike of the fault further focused fluid flux. As such, it is suggested that the massive dolostones described in --

Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.

Petrographic and petrophysical data from different limestone lithofacies (skeletal packstones, matrix-supported conglomerates and foraminiferal grainstones) and their dolomitized equivalents within a slope carbonate succession (Eocene Thebes Formation) of Hammam Faraun Fault Block (Suez Rift, Egypt) have been analyzed in order to link fracture distribution with mechanical and textural properties of these rocks. Two phases of dolomitization resulted in facies-selective stratabound dolostones extending up to two and a half kilometers from the Hammam Faraun Fault, and massive dolostones in the vicinity of the fault (100 metres). Stratabound dolostones are characterized by up to 8 times lower porosity and 6 times higher frequency of fractures compared to the host limestones. Precursor lithofacies type has no significant effect on fracture frequency in the stratabound dolostones. At a distance of 100 metres from the fault, massive dolostones are present which have 0.5 times porosity of precursor limestones, and lithofacies type exerts a stronger control on fracture frequency than the presence of dolomitization (undolomitized vs. dolomitized). Massive dolomitization corresponds to increased fracture intensity in conglomerates and grainstones but decreased fracture intensity in packstones. This corresponds to a decrease of grain/crystal size in conglomerates and grainstones and its increase in packstones after massive dolomitization. Since fractures may contribute significantly to the flow properties of a carbonate rock, the work presented herein has significant applicability to hydrocarbon exploration and production from limestone and dolostone reservoirs, particularly where matrix porosities are low.

Determination of the distribution and mechanism for carbonate-dominated mass transport sediments is often compromised by the scale and access to exposures. Consequently, many studies lack the resolution to capture the heterogeneity and dimensions of mass transport deposits. This study documents the size, shape, and stratal assemblage of remobilized carbonates in the Eocene Thebes Formation in the Hammam Faraun Fault Block (HFFB) of western Sinai, revealing the complexities of carbonate mass transport deposits at sub-seismic scale. Present day pseudo three-dimensional exposure of the Thebes Formation in a large fault block, formed during the opening of the Gulf of Suez, allowed for lateral and down-dip measurement of slope and basinal facies in the field and from photos. Remobilized facies were digitized in the photos and evaluated using image analysis software, a technique with a wide range of applications to outcrop studies of sedimentary architecture. Debris flow deposits in the lower section of the Thebes Formation comprises clasts with differing fossil assemblages. A relative sea level rise at the start of upper Thebes Formation deposition resulted in basinal sediments comprising periodic incursions of high-density turbidite grainstones encased within a background of planktonic foraminiferal wackestones. Foraminiferal assemblages of remobilized facies imply multiple sources on the carbonate platform, demonstrating the effect of short-lived tectonism on slope instability and deposition of mass transport deposits. The results of the study confirm that tectonism associated with the Syrian Arc Fold Belt, which altered the style of basin sedimentation between Egypt and Syria, persisted into the Eocene at least as far south as Wadi Araba in the western desert and Hammam Faraun in Sinai. In addition, the shape, size, and extent of the two dominant remobilized facies, debris flows and grainstone turbidites are influenced by their mud-rich versus grainy compositions.

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The Lower Cretaceous Minagish Formation forms one of the most prolific oil reservoirs in onshore Kuwait, with key reservoir units located in thick oolitic grainstones forming the lower half of the Middle Minagish Member. Within the central and eastern part of Kuwait, the upper part of the Middle Minagish consists of inner ramp skeletal packstones, that are variably cemented, for which the reservoir potential is poor. Following exploration campaigns carried out by KOC in the unexplored south-western area of Kuwait, unusual oil staining has been observed in the uppermost part of the Middle Minagish Member highlighting a potential new hydrocarbon play. As part of a multiwell study investigating the Middle and lowermost Upper Minagish Members cored in south-western Kuwait, this work focuses on understanding the occurrence of hydrocarbon stained deposits within the uppermost Middle Minagish and assesses their extent. The study aims to characterise the sedimentological make-up and analyse the field-scale depositional organisation to assist in the prediction of reservoir architecture. A facies analysis has been carried out and has resulted in the interpretation of the depositional environments. The key surfaces characterised in core along with openhole log data have helped in the interpretation of a sequence stratigraphic framework across both the reservoir and non-reservoir units. The facies analysis and vertical facies evolution across the cored Minagish succession suggest deposition in an intertidal to marginal/proximal mid-ramp setting, with the development of oolitic geobodies both in a marginal shoal corridor and sand bars formed in a more landward position on the inner ramp. The more proximal sand bars are typically recorded in the south-west Kuwait area and are not present further towards the east (e.g. Minagish and Umm Gudair Fields; Davies et al., 2000; Davies et al., 2001). The vertical depositional organisation indicates that the Middle Minagish is part of a large-scale regressive trend, characterised by the upward evolution from marginal inner ramp/proximal mid-ramp oolitic grainstones to skeletal packstones terminating with deposition of internal oolitic/carbonate sand bar to locally intertidal deposits. Hydrocarbon staining is observed where internal oolitic/carbonate sand bar/intertidal deposits are capped by key surfaces, locally associated with dissolved cavities suggesting exposure. These surfaces are interpreted as a sequence boundary above which a distinct change in deposition from clean carbonates to clay/organic matter-prone deposits (Upper Minagish) occur. The well correlation highlights a condensation of the sand bar/intertidal deposits in the south-western part of the study area, which is a result of a progressive decrease in accommodation at the end of the regression that is amplified in the landward position of the system. The work has been carried out prior to the establishment of the reservoir architecture and has helped in the prediction of rock type distribution.