Philippe Razin

Stratigraphic organization of carbonate ramps and organic-rich intrashelf basins: Natih Formation (middle Cretaceous) of northern Oman

The middle Cretaceous carbonate deposits in the Middle East are among the most productive oil-bearing stratigraphic intervals in the world, containing numerous giant fields in, for instance, the United Arab Emirates (Mauddud and Mishrif formations), Iran (Sarvak Formation), and Oman (Natih Formation). One of the main reasons for this concentration of hydrocarbons is a geological factor: the coexistence of both reservoir facies and source rocks in the same depositional sequences due to the repeated creation of organic-rich intrashelf basins. This is demonstrated in a high-resolution sequence stratigraphic study of the Natih Formation in Oman, which shows distinct and predictive patterns in the distribution and geometries of reservoir, source rock, and seal facies. The sequence stratigraphic model presented here may serve as a reference for time-equivalent deposits in the Middle East. The sedimentological analysis showed that the Natih Formation was formed by the alternation of two types of depositional systems: (1) a flat-bedded, mixed carbonate-clay ramp, dominated by benthic foraminifera, and (2) a carbonate-dominated ramp bordering an intrashelf basin, with abundant rudists in the mid-ramp environment and organic-rich basinal facies. Three fully developed third-order sequences are distinguished, showing a similar evolution of the depositional system, with a mixed carbonate-clay ramp system at the base, followed by a carbonate-dominated ramp system in the upper part. Variations occur on this pattern, however, depending on the relative influence of eustasy, environmental factors, and tectonism. The late Albiannearly Cenomanian sequence I shows an evolution from a mixed, flat ramp to a carbonate-dominated ramp and organic-rich intrashelf (Begin page 22) basin, and sedimentation is predominantly controlled by eustatic sea level. In the middle Cenomanian sequence II, the evolution from a mixed ramp to a carbonate ramp is also observed, but no intrashelf basin topography was developed in the studied area. This may be due to the high influx of clay that influenced the environment in this sequence, inhibiting the carbonate production, probably in combination with the lack of sufficient creation of accommodation space. The late Cenomaniannearly Turonian transgressive part of sequence III shows a similar evolution to that observed in sequence I, with the development of an organic-rich intrashelf basin. During highstand, however, a tectonically controlled sedimentation pattern is observed, with the development of forced regressive wedges (due to the flexural bulge of the foreland basin). Intrashelf basin formation occurred twice in the transgressive part of the third-order depositional sequences of the Natih Formation. Our study shows that this is mainly the result of differential sedimentation rates, that is, the dynamics of the carbonate sedimentary system itself in response to (rapid) rises in relative sea level, probably of eustatic origin. Tectonism was only a minor factor in the creation of the basin topography, possibly through the creation of small initial relief. The accumulation of the organic matter is not only a result of the creation of a sufficiently deep-water column to guarantee dysaerobic conditions for its preservation. The late Albian and late Cenomaniannearly Turonian were also periods of generally favorable conditions worldwide for high organic matter productivity. The time lines and stratigraphic architecture of the third-order sequences presented here have an application potential at the scale of the Arabian plate. The general sedimentation pattern is predicted by our model, but modifications due to different local conditions are likely to occur.

The Barremian-Aptian Evolution of The Eastern Arabian Carbonate Platform Margin (Northern Oman)

ABSTRACT Carbonate platform margins are sensitive recorders of changes in sea level and climate and can reveal the relative importance of global and regional controls on platform evolution. This paper focuses on the Barremian to Aptian interval (mid Cretaceous), which is known for climatic and environmental changes towards more intensified greenhouse conditions. The study area in the northern Oman mountains offers one of the very few locations where the Cretaceous carbonate margin of the Arabian Plate can be studied along continuous outcrops. Our detailed sedimentological and sequence stratigraphic model of the platform margin demonstrates how major environmental and ecological changes controlled the stratigraphic architecture. The Early Cretaceous platform margin shows high rates of progradation in Berriasian to Hauterivian times followed by lower rates and some aggradation in the Late Hauterivian to Barremian. High-energy bioclastic and oolitic sands were the dominant deposits at the margin. Turbidites were deposited at the slope and in the basin. The Early Aptian platform margin shows a marked change to purely aggradational geometries and a welldeveloped platform barrier that was formed mainly by microbial buildups. The sudden dominance in microbial activity led to cementation and stabilization of the margin and slope and, therefore, a decrease of downslope sediment transport by turbidites. In the Late Aptian, large parts of the Arabian craton were subaerially exposed and a fringing carbonate platform formed. Seven Barremian to Early Albian large-scale depositional sequences reflecting relative sea-level changes are identified on the basis of time lines constrained by physical correlation and biostratigraphy. The reconstruction of the margin geometries suggests that tectonic activity played an important role in the Early Aptian. This was most likely related to global plate reorganization that was accompanied by increased volcanic activity in many parts of the world. Along the northeastern Arabian platform the associated global changes in atmospheric and oceanic circulation are recorded with a change in platform-margin ecology from an ooid-bioclast dominated to a microbial dominated margin. Time-equivalent argillaceous deposits suggest an increase in rainfall and elevated input of nutrients onto the platform. This process contributed to the strongly diminished carbonate production by other organisms and favored microbial activity. The platform margin may thus represent a shallow-marine response to the Early Aptian global changes, commonly associated with an oceanic anoxic event in basinal environments.

Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide-angle, multichannel seismic, and heat flow survey

Continental rifts and passive continental margins show fundamental along-axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault-bounded basin systems change during the late evolution of the new plate boundary, and the role of magmatism may increase as rifting progresses to continental rupture. Along obliquely spreading ridges, first-order mid-ocean ridge geometries originate during the synrift stage, indicating an intimate relationship between magma production and transform fault spacing and location. The Gulf of Aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore. This obliquely spreading rift is considered magma-poor and has several large-offset transforms that originated during late stage rifting and control the first-order axial segmentation of the spreading ridge. Widely spaced geophysical transects of passive margins that produce only isolated 2-D images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models. Using closely spaced new geophysical and geological observations from the Gulf of Aden we show that rift sectors between transforms have a large internal variability over short distances (∼10 km): the ocean-continent transition (OCT) evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed. We suggest that this small-scale variability may be explained (1) by the distribution of volcanism and (2) by the along-strike differences in time-averaged extension rate of the oblique rift system. The volcanism may be associated with (1) the long-offset Alula-Fartak Fracture Zone, which may enhance magma production on its younger side, or (2) channeled flow from the Afar plume material along the newly formed OCT and the spreading ridge. Oblique extension and/or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma-poor margins.

Syn- to post-rift diapirism and minibasins of the Central High Atlas (Morocco): the changing face of a mountain belt

The Atlas Mountains are classically regarded as a failed Mesozoic rift arm subject to Alpine inversion, folding and thrusting. Here, we present new integrated structural and sedimentological studies that have revealed numerous Early–Middle Jurassic diapiric ridges and minibasins, characterized by distinctive halokinetic structures. Diachroneity in halokinesis is observed across the Central High Atlas, waning first in the SW during the Early–Middle Jurassic (Jbel Azourki and Tazoult ridges) and continuing to late Middle Jurassic towards the NE (Imilchil region). The halokinetic structures are readily differentiated from the effects of later Alpine deformation, allowing a new picture of the Central High Atlas to emerge. The most pervasive deformation in the Central High Atlas is associated with Early–Middle Jurassic diapirism, whereas the impact of Alpine inversion is mostly focused at the basin margins. This new understanding helps explain previously problematic aspects of the Atlas Mountains, which we now recognize as an exceptionally well exposed natural laboratory for understanding the interactions between halokinesis, tectonics and sedimentation.

Channelized systems in an inner carbonate platform setting: differentiation between incisions and tidal channels (Natih Formation, Late Cretaceous, Oman)

Abstract The Natih Formation (late Albian–early Turonian, Oman) corresponds to a very broad inner carbonate platform extending over more than 800 km between the Arabian Shield to the south and the Tethys continental margin to the north. Two types of channelized systems have developed recurrently on this inner carbonate platform: incisions corresponding to strictly erosive drainage systems which formed at the top of the sub-aerially-exposed platform during significant drops of relative sea level; tidal channels corresponding to partly erosive but mainly constructive/depositional systems which formed during phases of flooding of the inner platform. The comparative analysis of the basal surface and fill of incisions and tidal channels, based on the study of several outcrops in the Oman Mountains allows the recognition of the similarities and the main differences between these two types of channelized systems which both develop in an inner carbonate platform setting. One of the main criteria of differentiation is the stratigraphic context in which incisions and channels develop. Incisions develop at the top of regressive sequences, whereas the channels analysed here developed during phases of flooding or maximum flooding, during which higher energy processes such as tidal currents developed on the platform. The incision surface is clearly defined, with diagenetic effects such as silicification and dolomitization below, and with the systematic fill of subjacent burrows and cracks by sparitic calcite crystals. The basal erosion surface of channels is often multiple and composite, systematically burrowed, and associated with no significant diagenetic effect. Channels are generally less deep than incisions but their width is very similar. Incisions are longer than channels and present a section that is, on the whole, symmetrical and regular, whereas channels locally have one margin that is steeper and more erosive than the other. Finally, the less diagnostic parameter of differentiation is their fill. Indeed, incisions and channels are similarly filled during phases of flooding of the inner platform. Incisions and channels form significant heterogeneities at the reservoir scale. It is therefore necessary to be able to recognize these two types of channelized systems, in order to predict their geometry, extent and fill type, and the eventual occurrence of associated reservoir bodies in the more distal direction (forced regressive wedges/bioclastic shoals).

Mesozoic deep-water carbonate deposits from the southern Tethyan passive margin in Iran (Pichakun nappes, Neyriz area): biostratigraphy, facies sedimentology and sequence stratigraphy

Abstract The objective of this work is to study the Mesozoic turbiditic sediments from the southern Tethys margin in Iran. These sediments are exposed as nappes in the Pichakun Mountains (i.e. the Zagros Mountains in the Neyriz area), which inverted during latest Cretaceous time. Radiolarians are used to both define and date four main lithostratigraphic formations: (1) the Bar Er Formation (undated, probably Late Triassic to Early Jurassic); (2) the Darreh Juve Formation (Aalenian–early Bajocian to middle Callovian–early Oxfordian); (3) the Imamzadeh Formation (middle Callovian–early Oxfordian to Aptian); (4) the Neghareh Khaneh Formation (late Aptian to Turonian–Coniacian). Most of the sediments are deep-sea gravity-flow lobe deposits. Channel deposits occurred during the Bajocian (i.e. the Darreh Juve Fm) and deeply incised channels (canyons?) occurred during the Albian (i.e. the Neghareh Khaneh Fm). Twenty-seven facies, grouped into eight facies associations, are defined. Based on a sequence stratigraphic study (i.e. the stacking pattern), five second-order cycles (10–30 Ma duration), defined between two successive distal facies time-intervals, are characterized: (1) the J2 (Toarcian?–middle Oxfordian, unconformity: Late Toarcian–Aalenian); (2) the J3 (middle Oxfordian–Berriasian, unconformity: middle? Tithonian); (3) the K1.1 (Berriasian–undated top); (4) the K1.2 (undated base–early Aptian, unconformity: late Hauterivian); (5) the K1.3 (early Aptian–at least Turonian–Coniacian, unconformity: Aptian–Albian boundary). The most important tectonic event recorded occurred at the Aptian–Albian boundary (a deposition of olistoliths, from a few metres to 100 m thick, in debris flows; related to Austrian deformations). The Arabian-scale late Toarcian and early Tithonian deformations have been recorded as unconformities. It is expected that another tectonic event occurred during the late Hauterivian. The unconformity of cycle K1.1 could be a late Valanginian eustatic fall of climatic origin.

Diapiric growth within an Early Jurassic rift basin: the Tazoult salt wall (Central High Atlas, Morocco)†

The Central High Atlas (Morocco) constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the Lower Jurassic rift of the Atlas Basin. This paper aims to study the structure and growth evolution of the Tazoult diapiric wall, located in the Central High Atlas, by means of structural and sedimentological fieldwork integrated with remote sensing mapping. The Tazoult salt wall is a 20 km long x 3 km wide NE-SW trending ridge that exposes Upper Triassic red beds and basalts along its core. The succession flanking the salt wall ranges from Hettangian to Bajocian ages displaying spectacular sedimentary wedges in the SE and NW flanks. The Hettangian-early Sinemurian carbonates mainly crop out as blocks embedded in the core rocks. The ~1-km thick Pliensbachian platform carbonates display large subvertical flap structures along the flanks of the Tazoult salt wall with unconformities bounding tapered composite halokinetic sequences. In contrast, the ~2.5-km thick late Pliensbachian-Aalenian mixed deposits form tabular composite halokinetic sequences displaying small-scale hook halokinetic sequences. Passive diapirism resulted in the lateral extrusion of the evaporite-bearing rocks to form an allochthonous salt sheet towards the adjacent SE Amezrai minibasin. The Bajocian platform carbonates partially fossilized the Tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from Alpine shortening. The Pliensbachian carbonate platform evolved as a long flap structure during the early growth of the Tazoult salt wall, well before the onset of the Alpine shortening.

Middle Eocene-Early Miocene larger foraminifera from Dhofar (Oman) and Socotra Island (Yemen)

Here, the larger foraminifera found in Middle Eocene-Early Miocene rocks from Dhofar (Oman) and Socotra Island (Yemen) are studied in detail. The architectural analysis leads to the description of five new genera and nine new species: five agglutinated foraminifera, Pseudolituonella robineti n. sp., Socotraella ashawqi n. gen. n. sp., Pseudoaccordiella ayaki n. gen. n. sp., Barattolites andhuri n. sp., and Rogerella aydimi n. gen. n. sp.; and four porcellaneous foraminifera, Idalina grelaudae n. sp., Idalina pignattii n. sp., Macetadiscus incolumnatus n. gen. n. sp., and Omanodiscus tenuissimus n. gen. n. sp. The larger foraminifera identified in a composite section located in western Dhofar, in the Shuwaymiyah section located in eastern Dhofar, and in the Wadi Ayak section located on Socotra Island have facilitated the identification of the following larger foraminifera zones: SBZ 14–SBZ 15 (middle Lutetian), SBZ 16 (late Lutetian), SBZ 17 (Bartonian), SBZ 18 (latest Bartonian-earliest Priabonian), SBZ 19–SBZ 20 (Priabonian), SB 21–SB 22A (Rupelian), SB 22B–SB 23 (Chattian), and SB 24 (Aquitanian). All these data permit to assess the age of the following lithostratigraphic units: Dammam Fm.—Andhur Mb. lower Lutetian?-middle Lutetian age (SBZ 13?–SBZ 14 partim), Qara Mb. middle Lutetian (SBZ 14–SBZ 15), and Uyun Mb. upper Lutetian (SBZ 16); Aydim Fm.—Heiron Mb. Bartonian (SBZ 17), Moosak Mb. upper Bartonian-Priabonian (SBZ 18–SBZ 20), Tagut Mb. Priabonian (SBZ 19–SBZ 20), and Haluf Mb. Priabonian (SBZ 19–SBZ 20) to lower Rupelian (SBZ 21) on Socotra Island; Ashawq Fm. Rupelian (SB 21–SB 22A); and Mughsayl Fm. Chattian-Aquitanian (SB 23–SB 24).

The mid-Cretaceous Natih Formation in Oman: A model for carbonate platforms and organic-rich intrashelf basins

The Natih Formation in Oman is an exceptionally well–exposed and studied example of a large (>1000 km [621 mi]) epeiric, shallow marine, tropical carbonate platform system with adjacent organic-rich intrashelf basins. It serves as an outcrop analog for many of the mid-Cretaceous giant oil reservoirs as well as for unconventional reservoirs in the Mesozoic of the Arabian plate. A key feature of the Natih Formation is its strict hierarchical stratigraphic organization at multiple orders of depositional sequences (third to fifth), which was first demonstrated in outcrop and subsequently adopted in the seismic and log data sets of the nearby subsurface. It has provided a high-resolution time framework for the robust sequence stratigraphic model that has been developed for an area of 6000 km2 (2317 mi2). Deposited in a calm tectonic setting, the Natih Formation provides an excellent example to demonstrate the principles of carbonate systems’ response to eustatic sea level change and some fundamental differences with siliciclastic systems. Key aspects that are addressed include the influence of the rate of sea level rise on the transition from ramp to platform, the creation of organic-rich intrashelf basins through differential accumulation rates, changing facies models within third-order sequences, and the importance of different platform top channel types. The economic relevance of this case study is the improved prediction of reservoir, seal, and source rock facies distribution and the associated early diagenetic overprint, insights that have been applied in the reservoir modeling of nearby oil fields as well as in exploration-scale studies.

Stratigraphic Architecture of the Latest Jurassic - Early Cretaceous Carbonate Platform System of Abu Dhabi, United Arab Emirates (UAE)

The integration of subsurface and outcrop data has allowed to build a 200 km NW-SE correlation transect across 7 oil fields in Abu Dhabi. It reveals new possible concepts on the stratigraphic organization of the Late Jurassic-Early Cretaceous systems in the U.A.E.