Salt Tectonics Research Articles

Diagenesis of a light, tight-oil chert reservoir at the Ediacaran/Cambrian boundary, Sultanate of Oman

ABSTRACT The Al Shomou Silicilyte Member (Athel Formation) in the South Oman Salt Basin shares many of the characteristics of a light, tight-oil (LTO) reservoir: it is a prolific source rock mature for light oil, it produces light oil from a very tight matrix and reservoir, and hydraulic fracking technology is required to produce the oil. What is intriguing about the Al Shomou Silicilyte, and different from other LTO reservoirs, is its position related to the Precambrian/Cambrian Boundary (PCB) and the fact that it is a ‘laminated chert’ rather than a shale. In an integrated diagenetic study we applied microstructural analyses (SEM, BSE) combined with state-of-the-art stable isotope and trace element analysis of the silicilyte matrix and fractures. Fluid inclusion microthermometry was applied to record the salinity and minimum trapping temperatures. The microstructural investigations reveal a fine lamination of the silicilyte matrix with a mean lamina thickness of ca. 20 μm consisting of predominantly organic matter-rich and finely crystalline quartz-rich layers, respectively. Authigenic, micron-sized idiomorphic quartz crystals are the main matrix components of the silicilyte. Other diagenetic phases are pyrite, apatite, dolomite, magnesite and barite cements. Porosity values based on neutron density logs and core plug data indicate porosity in the silicilyte ranges from less than 2% to almost to 40%. The majority of the pore space in the silicilyte is related to (primary) inter-crystalline pores, with locally important oversized secondary pores. Pore casts of the silica matrix show that pores are extremely irregular in three dimensions, and are generally interconnected by a complex web or meshwork of fine elongate pore throats. Mercury injection capillary data are in line with the microstructural observations suggesting two populations of pore throats, with an effective average modal diameter of 0.4 μm. The acquired geochemical data support the interpretation that the primary source of the silica is the ambient seawater rather than hydrothermal or biogenic. A maximum temperature of ca. 45°C for the formation of microcrystalline quartz in the silicilyte is good evidence that the lithification and crystallization of quartz occurred in the first 5 Ma after deposition. Several phases of brittle fracturing and mineralization occurred in response to salt tectonics during burial. The sequences of fracture-filling mineral phases (dolomite - layered chalcedony – quartz – apatite - magnesite I+II - barite – halite) indicates a complex fluid evolution after silicilyte lithification. Primary, all-liquid fluid inclusions in the fracture-filling quartz are good evidence of growth beginning at low temperatures, i.e. ≤ 50ºC. Continuous precipitation during increasing temperature and burial is documented by primary two-phase fluid inclusions in quartz cements that show brines at 50°C and first hydrocarbons at ca. 70°C. The absolute timing of each mineral phase can be constrained based on U-Pb geochronometry, and basin modelling. Secondary fluid inclusions in quartz, magnesite and barite indicate reactivation of the fracture system after peak burial temperature during the major cooling event, i.e. uplift, between 450 and 310 Ma. A number of first-order trends in porosity and reservoir-quality distribution are observed which are strongly related to the diagenetic and fluid history of the reservoir: the early in-situ generation of hydrocarbons and overpressure development arrests diagenesis and preserves matrix porosity. Chemical compaction by pressure dissolution in the flank areas could be a valid hypothesis to explain the porosity variations in the silicilitye slabs resulting in lower porosity and poorer connectivity on the flanks of the reservoir. Most of the hydrocarbon storage and production comes from intervals characterized by preserved micropores, not hydrocarbon storage in a fracture system. The absence of oil expulsion results in present-day high oil saturations. The main diagenetic modifications of the silicilyte occurred and were completed relatively early in its history, i.e. before 300 Ma. An instrumental factor for preserving matrix porosity is the difficulty for a given slab to evacuate all the fluids (water and hydrocarbons), or in other words, the very good sealing capacity of the salt embedding the slab.

Hydrocarbon source rock generative potential of the Sudanese Red Sea basin

To date, the Sudanese Red Sea like the Red Sea as a whole, represents a grossly under-explored basin. Recently, the basin has seen increased exploration interest, with more than 15 wells being drilled, some of which have been petroleum discoveries (gas and condensate with oil shows). A similar exploration trend has been seen in the Saudi, Eriteria and Egyptian sectors of the Red Sea. The Red Sea rift marks the break-up of the Afro-Arabian plate in Eocene–Oligocene time. The tectonostratigraphic evolution can be subdivided into three major phases: Pre-rift, Syn-rift and Post-rift phase. Generally, terrestrials and marginal to deep marine sediments were deposited with both good reservoir quality and potential source rocks being present. The pre-Salt Rudeis/Kareem shales are expected to be good petroleum source rocks for the basin. Moreover, shales of base Zeit (post-Salt) are considered the main source of the known accumulated hydrocarbon (gas and condensate) in the shallow targets with the condensate charge also expected from intra-evaporites shales and pre-salt sources along the salt windows. The measured amount of TOC, HI, and hydrocarbon generation from pyrolysis of kerogen of the examined samples indicate source rock generative potential of these intervals. The TOC contents for base Zeit, and interbeded shales of Dungnab Formations, range from 0.60& to 5.40 wt. %, while the hydrogen index (HI) values are in the range of 50-600mg HC/gTOC suggesting dominantly type III-II kerogen. The high TOC (>1.5) and HI (>300) values of base Zeit and Dungnab Formations are limited to some intervals within these formations. Consequently, the base Zeit source rocks are considered very good for condensate and gas generation. In comparison to their equivalents Burqan and Maqna Group sources (HI = 350–600 mg HC/gTOC) on the Saudi Red Sea sector, Rudeis/Kareem shales could perhaps possess good quality type-II kerogen. The Red Sea’s high pressure and temperature regimes have a significant impact on hydrocarbon maturation, generation and the quality of the preserved accumlations. Presently, the Zeit source varies in maturity across the basin and within the oil window in the Suakin and Bashayer areas and at gas window elsewhere to the east towards offshore. On the other hand, the pre-salt Rudies/Kareem Formations are currently in the late oil to wet gas zone and probably out of gas phase distally offshore. However, the salt tectonics makes for challenging exploration. The salt movement is considered recent as it is only the recent sediments that appear to be controlled by salt movement.

Intra- to post-Messinian deep-water gas piping in the Levant Basin, SE Mediterranean

This study presents evidences for fluid flow manifested as pipe structures in the post-evaporitic overburden characterised from a 3D seismic data from the Levant Basin, offshore central Israel. Interestingly these pipes are genetically and spatially contextualized east and west of the study area, all rooted in the Messinian evaporite substratum. Pipes in the western group accounts for 83% of the pipe population, are crudely cylindrical, oval to elliptical in planform, with diameter and height ranging ∼350–2000 m and ∼320–420 m, respectively. Internal configurations within this group varies from chaotic to concave upward reflections diagnostic of fluid induced collapse. Pipes in the eastern group are seepage pipes appearing conical in shape, with height of ∼350–510 m and diameter of ∼320–420 m. The eastern group pipes are characterized by lack of internal reflectivity and/or disrupted stratal reflections. The western group indicates an episode of fluid flow till the mid-Pliocene, compared to late Pliocene in the eastern group. A conceptual model for the pipes in the western group is proposed to have occurred from subjacent dissolution of the Messinian evaporite under deep-water marine conditions during the Pliocene by vertically focused fluid flow from intra-Messinian realm dissolving the top evaporites and inducing systematic collapse in the overburden, the onset of which may have been triggered by seismicity. Conversely, pipes in the eastern group are proposed to have developed from breaching the top evaporite by pressurized fluids that developed from lateral pressure transfer due to differential loading of the overburden and salt tectonics. These pipe structures may pose a potential risk to deep-water exploration and development. The fluid expulsion phenomena documented in this study has wider implications for the hydrodynamics of many basins where thick evaporite layers are widely developed.

How sandstone porosity and permeability vary with diagenetic minerals in the Scotian Basin, offshore eastern Canada: Implications for reservoir quality

The Scotian Basin is an under-explored offshore basin that has produced gas and minor oil from Jurassic–Cretaceous deltaic sandstones. Reservoir quality is an important exploration risk in the basin. This study evaluates whether there are systematic stratigraphic or geographic variations in diagenetic mineral assemblage control of reservoir porosity and permeability. Three data sets are used: a compilation of all available core-plug porosity and permeability measurements (3271 in total), a subset of 577 corresponding to thin sections with lithofacies assignments, and a subset of 35 thin sections principally from reservoir sandstones, in which modal abundance of diagenetic minerals has been determined by image analysis of scanning electron microscope backscattered electron images. Abundances above a 4% threshold of diagenetic kaolinite, calcite and ankerite correlate inversely with permeability, but chlorite and possibly siderite correlate positively. Diagenetic assemblages show no systematic variation with stratigraphy or geography, both of which may involve variable detrital supply. Permeability shows the well-known variation with grain size and depth of burial, but after correcting for these effects it is greatest in estuarine channel and river-mouth turbidite sandstones. This is because these facies preferentially develop chlorite rims on detrital quartz grains and are also the sites of enhanced flow of late basinal fluids resulting in secondary porosity. Local effects such as facies distribution, architecture of channel sandstones, fluid pathways related to complex salt tectonics, and the effects of overpressure are more important than predictable basin-wide effects in controlling sandstone reservoir quality.

Merganser Field: managing subsurface uncertainty during the development of a salt diapir field in the UK Central North Sea

Abstract The Merganser Field is located in the East Central Graben of the UK Central North Sea, and consists of a gas/condensate column trapped in a structural attic on the flanks of a fully penetrating salt diapir. A large salt overhang obscures the field and structural definition is challenging owing to poor seismic imaging. Exploration drilling established a column of 1327 ft in Paleocene-aged deep-marine deposits of the Forties, Andrew and Maureen Sandstone members, and revealed significant geological complexity. Depositional styles record the relationship between salt tectonics and sedimentation, with variable reservoir distribution influenced by halo-kinetically induced palaeorelief and accommodation space. Re-mobilization of sediments is observed at multiple scales, and includes centimetre-scale de-watering structures, decimetre-scale sand injectites and kilometre-scale olistoliths. Whilst the hydrocarbon properties are consistent, contact depths are variable. Pressure data indicate compartmentalization across the field, which is likely to be caused by either radial faulting or hydrodynamic effects. Owing to the magnitude of subsurface uncertainty, the Merganser discovery could not sustain the investment required for standalone facilities. The development of the neighbouring Scoter Field provided the requisite local infrastructure to progress Merganser into production. The Field Development Plan (FDP) estimated recovery of 100 bscf gas and 3 mmstb condensate, and focused on delivering a low-cost development solution consisting of two horizontal wells and a subsea tie-back that would be robust against the downside, yet maintain flexibility to optimize in an upside outcome. Pilot holes were drilled to establish top reservoir with the subsequent horizontal well trajectories being re-designed to reflect structural geometry. The reservoir sections would maximize connectivity between fault compartments and stratigraphic units, and positioning was optimized with well-site biostratigraphy. Each reservoir section exceeds 4000 ft and maintains at least 1000 ft vertical stand-off from the gas/water contact. The facilities include a 5 km subsea tie back to the Scoter production manifold, with metering at the Merganser manifold for allocation purposes. Gas and condensate are commingled with Scoter, and transported 11 km to Shearwater for processing. The gas is transported onshore through the Shearwater Elgin Area Line and condensate through the Forties Pipeline System to Kinneil. Field performance to date has exceeded the FDP P50 both in terms of daily rate and cumulative production. Early production rates peaked at 100 mmscf/day of gas and 6000 stb/day of condensate and, to end-2014, Merganser has produced 161 bcf of gas and 10 mmstb condensate. This performance is due to a combination of better than expected connectivity, high reservoir k h , lower draw-down afforded by long horizontal wells and compression at the Shearwater platform. Subsurface uncertainties prior to development were considerable and the ‘appraisal through development’ strategy has demonstrated that success is achievable through meticulous planning and scenario analysis.

The pore pressure regime of the northern Gulf of Mexico: Geostatistical estimation and regional controls

Subsurface pressures strongly influence the migration and trapping of hydrocarbons and impact the safety and efficiency of drilling operations. The pore pressure field of the northern Gulf of Mexico (GOM) was analyzed at 1000-ft (305-m) depth intervals from 2500 to 17,500 ft (762 to 5334 m) below the sea floor. Two variables were mapped: 12,976 initial hydrocarbon reservoir pressure gradient values and 43,276 observations on drilling fluid (mud) weight. Because of the acute importance of assessing estimate uncertainty, ordinary kriging was employed, providing explicit evaluations of confidence surrounding mapped values. Expected values and confidence intervals for the distribution of both variables were estimated by () grid cells across the GOM for each of the 15 depth intervals. Estimation variances were also used to clip each map to specific extents, within which a uniform minimum threshold of certainty was exceeded.Characteristic of young basins with high sedimentation rates, mean pore pressure exceeded hydrostatic pressure throughout the GOM. Four provinces of internally consistent pressure regimes were defined: three south of Louisiana and one off the Texas coast. They reflect geologic controls on pressure arising from regional patterns of sedimentation and the resultant timing and geometry of salt tectonism. One GOM-wide (shallow) vertical transition in the pressure field was found in the mud weight data, and a second vertical transition (deep) occurred in both variables. Hot spot analysis was also applied to identify specific contiguous areas of abnormally high or low rates of change in pressure gradient and mud weight between depth-adjacent intervals.

Bathymetry of the Levant basin: interaction of salt-tectonics and surficial mass movements

A new high resolution bathymetric map of the Levant Basin between Israel and the Eratosthenes Seamount reveals previously undetected folds, faults and channels. The map facilitates a regional map-view analysis of structures that were previously examined only in cross section. The systematic mapping of morpho-structural elements in the entire basin is followed by a kinematic interpretation that distinguished between two main processes sculpting the seabed from bottom and top: salt tectonics and sediment transport. We show that the contractional domain related to salt tectonics is far more complex than previously thought. Ridges, previously interpreted as contractional folds are, in fact, surficial undulations of the seabed reminiscent of sediment waves. Moreover, other folds previously interpreted as downdip contraction of the westward gliding Plio-Quaternary section are, in some parts of the basin, caused by updip climbing of this section eastwards as a result of the regional pattern of salt flow away from the Nile Cone. In the context of sediment transport, we show that the northern Sinai continental slope is covered by a dense net of turbidite channels, whereas the Levant slope has no channels at all. Particularly interesting is the Levant Turbidite Channel, described and named here for the first time. This feature begins at the southeastern corner of the Mediterranean at water depths of ~ 1100 m, continues along the valley between the Sinai and Levant slopes, and reaches the deepest part of the basin, in water depths of ~ 2500 m, northeast of the Eratosthenes seamount. However, this prominent feature cannot be explained by the current drainage, consisting of two minor rivers that enter the basin at that point, and thus most likely reflects periods of wetter climate and/or lower sea-level, when these rivers were more active and possibly connected to the submarine channel system.

Salt on the move: Multi stage evolution of salt diapirs in the Netherlands North Sea

After more than half a century of research, the dynamics of salt tectonics in the North Sea remains a contentious issue with unresolved questions regarding triggering and driving mechanisms. The North Sea is now covered by high-quality 3D seismic datasets which allow new insights into the interplay between salt tectonics and sedimentation. These insights are of both regional geological, societal and exploration interest as well as more widely applicable as analogues for salt tectonics in frontier regions such as the South Atlantic salt basins. A high quality 3D seismic volume from the Netherlands North Sea is used to show the thick and mobile Zechstein halite sequence of Late Permian age remobilized in several phases into complex salt structures. Early stage structural evolution was dominated by Triassic NNE–SSW and NW–SE trending salt ridges. Salt tectonic reorganization was triggered by regional shortening in the Campanian. Salt was transferred both laterally and vertically, creating salt anticlines and tall asymmetrical salt diapirs. Vertical growth of the tallest salt diapirs continued in pulses in the Plio-Pleistocene likely driven by increased sediment input from southern North Sea delta systems. The most recent deformation affects Late Pleistocene and Holocene sediment and could have impacted on the landscapes of Meso-Lithic human occupied sites. The recent activity of some salt domes should be considered when planning repositories for gas and nuclear waste.

Deformation within an exposed salt wall: Recumbent folding and extrusion of evaporites in the Dead Sea Basin

Despite the enormous global interest in salt tectonics, which is largely driven by its importance to hydrocarbon exploration, direct field-based studies of salt exposed at the Earth's surface are rare. However, Mount Sedom, located at the western side of the Dead Sea Basin, presents one such opportunity for detailed analysis of salt and the associated sedimentary and structural record of its movement. The Sedom salt wall is a 10 km × 1.5 km N–S trending ridge comprising a range of Late Miocene-Pliocene evaporites and clastics, which have penetrated the overlying Pleistocene clastic sequence. The salt wall displays a moderate-steep west dipping western margin and an overturned (west-dipping) eastern flank. The sedimentary record of passive wall growth includes sedimentary breccia horizons that locally truncate underlying beds and are interpreted to reflect sediments having been shed off the crest of the growing salt wall. Structurally, the overturned eastern flank is marked by upturn within the overburden, extending for >300 m from the salt wall. Deformation within the evaporites is characterised by ductile folding and boudinage, while a 200 m thick clastic unit within the salt wall forms a tight recumbent fold traceable for 5 km along strike and associated with a 500 m wide inverted limb. This overturned gently-dipping limb is marked by NE-directed folding and thrusting, sedimentary injections, and a remarkable attenuation of the underlying salt from ∼380 m to <20 m over just 200 m of strike length. The inverted limb is overlain by an undeformed anhydrite, gypsum and clastics caprock, thought to be the residue from a now dissolved salt sheet that extruded over the top of the fold. Expulsion of salt down the regional slope towards the NE, combined with subsequent dissolution of evaporites, may have resulted in local ‘pinching shut’ of the salt wall, leading to a distinctive hour-glass map pattern. This area also coincides with deposition of a thicker overlying clastic sequence, indicating continued subsidence of this part of the salt wall. Our detailed fieldwork forms the first direct observation and description of large recumbent folds within salt walls, and permits analysis below the limits of seismic resolution. It thereby allows more rigorous testing of salt tectonic models and mechanisms.

Advanced methods for seismic imaging when mapping hydrocarbon traps associated with salt domes

This paper is the first in a series of tutorials on the usage of modern migration methods for seismic imaging in areas of intense salt tectonics. In the paper, various types of hydrocarbon traps associated with salt domes in the Dnieper-Donets depression have been analyzed. Moreover, classification of methods for seismic imaging has been given. With examples from other sedimentary basins, the benefits of pre-stack migration, a consideration of multipathing of seismic energy and the usage of converted waves in imaging of near stock objects have been shown.

Extensional salt tectonics in the partially inverted Cotiella post-rift basin (south-central Pyrenees): structure and evolution

The Cotiella Massif in the south-central Pyrenees hosts upper Cretaceous gravity-driven extensional faults which were developed in the Bay of Biscay–Pyrenean paleorift margin of the Atlantic Ocean. They accommodate up to 6 km of post-rift carbonates above relict upper Triassic salt. Subsequent Pyrenean contractional deformation preserved the main extensional features, but most of the upper Triassic salt was expulsed and then dissolved, leaving little indications of the original salt volume. Nonetheless, several distinctive salt-related features are still recognizable both at outcrop and at basin scale, providing an exposed analogue for salt-floored extensional basins developed on passive margins. Based on field research, we re-interpret the tectonic evolution of the area and suggest that passive diapirs were coeval with gravity-driven extension during the development of the Cotiella basin. The given interpretations are supported with detailed geological maps, original structural data, cross sections and outcrop photographs. The discovery of previously unknown post-rift salt structures in the Cotiella Massif is an extra element to consider in the paleogeographic reconstructions of the upper Cretaceous passive margin of the Bay of Biscay–Pyrenean realm and consequently helps in our understanding of the evolution of current Atlantic-type margins.

Triassic-age salt tectonics of the Central North Sea

The framework of salt tectonics in the Central North Sea was set early in the Triassic. We defined and illustrated five major domains of differing salt tectonic style. The differing structural styles were all interpreted as having evolved under a component of lateral displacement pairing extensional and contractional structures, produced by some combination of decoupled rift extension and gravity sliding. However, the extensional structures are located toward the basin center and the contractional structures near the original updip limits of salt. This suggests a framework driven by gravity sliding of the sediments overlying the Zechstein away from the Central Graben. Possible mechanisms for structural relief away from the Central Graben are the Triassic focus of rifting lying further east at the Norwegian-Danish basin, footwall uplift of a Triassic Central Graben precursor and significant thermal doming occurring much earlier than had previously been thought. The mechanisms are not mutually exclusive and may have acted in concert.

Introduction to special section: Salt tectonics and interpretation

Advances in the understanding of salt tectonics generally result from interpretations of modern seismic data, field studies of exposed salt basins, experimental and numerical modeling, and cross-section restoration. In this special section of Interpretation , a journal emphasizing the mutual

A 3D model of the Wathlingen salt dome in the Northwest German Basin from joint modeling of gravity, gravity gradient, and curvature

In the past few decades, numerous attempts have been made on modeling of salt tectonics and deciphering the geometry of salt domes, which is a key challenge in petroleum exploration. We have derived a 3D density model of the Wathlingen salt dome, situated in the southern part of the Northwest German Basin from joint modeling of reprocessed torsion balance measurements. Gravity, gravity gradients [Formula: see text], curvature derived from horizontal gravity gradients [Formula: see text], and horizontal directive tendency are jointly modeled to decipher the geometric structure of the salt dome. The model was constrained by geologic and borehole information. We found that the Wathlingen salt dome is a mushroom-structured salt body, which is 14-km long, 4–8-km wide extending up to [Formula: see text] depth. The top mushroom structure of the salt is horizontally spread up to [Formula: see text]. It would not have been possible to derive the complex 3D structure from modeling of gravity data alone.

Restoration of basins driven by extension and salt tectonics: Example from the Cotiella Basin in the central Pyrenees

The structure of the Cotiella Basin consists of four well differentiated salt-floored sub-basins which contain more than 6 km of halokinetic growth-strata and have been interpreted to be formed by gravity-driven extensional faults and diapirism during the middle Coniacian – early Santonian. The subsequent Pyrenean orogeny (late Santonian to Miocene) inverted the extensional basins and squeezed the diapirs leaving little trace of the original salt volume, although both the extensional and the salt-related features of the Cotiella Basin were preserved. This paper provides new insights into the evolution of the Cotiella Basin by establishing a correlation between sub-basins by means of structural restorations. The workflow followed and the assumptions made during the restoration of the inversion and especially of the extensional structures are outlined in detail.

Five Tectonic Settings in Five National Parks and Forests: A Field Camp Experience

In summer 2014, Southern Utah University’s field camp visited five national parks and forests to study five different tectonic settings in five weeks. These included: thick-skinned contraction of the Laramide Orogeny at Capitol Reef National Park; normal faulting due to salt tectonics of the Paradox Formation at Arches National Park; thin-skinned folding and thrusting of the Sevier Orogeny at Fish Lake National Forest and Kolob Canyon of Zion National Park; foreland sedimentary transitions in the Book’s Cliffs areas of Utah; thrusting and conjugate fracture development due to the gravitational collapse of the Marysvale volcanic field at Bryce Canyon National Park; and a metamorphic core complex at Great Basin National Park and Hendry’s Creek in the Northern Snake Range of Nevada. This article provides a brief summary and photographic tour of the students’ experiences studying each tectonic environment, and it highlights wonderful geological features exposed in our national parks.

The Naval diapir (southern Pyrenees): Geometry of a salt wall associated with thrusting at an oblique ramp

The southern Pyrenees provide several examples of diapiric structures linked to migration of Triassic evaporites associated with the movement of thrust sheets. Among them, the Naval diapir, the westernmost diapiric body in the Sierras Marginales unit, is key to understanding the mechanisms controlling salt tectonics in this sector of the Pyrenees. In this paper, we use geological mapping, field data, cross-sections, seismic reflection profiles and detailed gravity data to characterize the geometry of the Naval diapir, a body of Middle and Upper Triassic evaporites and shales, and its relationship with the sedimentary cover and compressional structures. The information obtained from these methods was used to construct 2.5D and 3D models. These models indicate the significant role of compressional oblique structures and crestal weakening in the origin of the N-S salt-wall geometry, during the translation–rotation of the western end of the Sierras Marginales thrust sheet in the Late Eocene–Oligocene times. The uplift of low-density materials outlived compressional activity in the area and continued during the Oligocene–Early Miocene.

Salt tectonics in the SW Alps (Italy–France): From rifting to the inversion of the European continental margin in a context of oblique convergence

The SW Alps result from the inversion of the European continental margin during the oblique convergence between Europe and Adria since the Cretaceous. The orogenic deformation is controlled by two factors: the inherited sedimentary and structural record and the geodynamic interaction between the two plates. In this paper we present a stratigraphic and structural analysis of the external SW Alps (Ventimiglia–Menton area) in order to define the sedimentary and deformational geometries of the chain and to reconstruct the evolutionary history. The field-data highlight the preeminent role played by inherited salt-structures, which derive from the depositional history experienced by the European margin since the Mesozoic onwards. From Late Triassic to Jurassic, evaporites and carbonates deposited as a response to the Thetyan rifting. The following emplacement of the Cretaceous flysch and of the Eocene foreland basin succession was strongly influenced by the extensionally-triggered salt diapirism and by the interactions with deformations connected to the Pyrenees dynamics. The resulting geologic discontinuities (i.e. diapir-induced highs and basins, inherited normal and trasform faults) strongly influenced the successive Oligo-Miocene evolution of the belt in the study area. Observed changes in the thrusts and folds kinematics are considered as the results of rotation during their approaching to inherited highs. Furthermore, the overturning of thrusts and folds in the front of the diapiric flanks are associated with the progressively salt squeezing into the anticlines cores promoted by ongoing Alpine compression.Finally, the kinematic data from the study area show radical differences in the tectonic transport direction with respect to the rest of the SW Alps (NW- to W-ward in the Ventimiglia–Menton area, S- to SW-ward in Provence and Ligurian Alps). This difference is interpreted to be caused by the relative motions of crustal blocks dominated by transpressive tectonics in the frame of the Oligo-Miocene western Mediterranean geodynamics.

Gas hydrate destabilization and methane release events in the Krishna–Godavari Basin, Bay of Bengal

Methane release events have been linked to global warming, alteration of the carbon cycle and influence on biota. However, unequivocal evidence of paleomethane release events are limited. We report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core (MD161-8) from the Krishna-Godavari (K–G) Basin, Bay of Bengal. The most negative δ13C spikes are recorded during the marine isotope stages MIS-4 and at the transition of MIS-5 to 4. Occurrence of highly 13C depleted (average δ13C = −48 ± 2.4‰ VPDB) authigenic high magnesian calcite are also reported within this time window from the core MD161-8. In the present work an unequivocal explanation for the observed 13C depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical/electron microscopy or C–O stable isotope ratio analyses due to possible influence of diagenetic alteration. We attribute the observed episodic methane expulsion events, as inferred from the negative δ13C excursions and earlier reports on the occurrence chemosynthetic bivalves and Mo concentration anomaly to the destabilization of the base of gas hydrate stability zone (BGHSZ). Sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here. Shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage. Shale and salt tectonics in the passive continental margins being a globally observed phenomenon, its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time. Additional evidence of methane emission from site MD161-15 further supports the link between shale tectonics and methane emission.

Depositional facies and platform architecture of microbialite-dominated carbonate reservoirs, Ediacaran–Cambrian Ara Group, Sultante of Om

Intrasalt carbonates of the Ediacaran–Cambrian Ara Group constitute a significant reservoir element of the intrasalt “stringer” play in Oman, in which dolomitic carbonates are encased in salt at depths of 3 to 7 km (1.9 to 4.3 mi). These reservoir carbonates have significant microbial influences. Although Ara Group reservoirs are mostly latest Precambrian, the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded, in lacustrine settings where calcified invertebrates are not a significant source of carbonate, or after periods of mass extinction before faunal recovery. A broad range of carbonate facies provides the context in which to understand the origin of the microbialite-dominated reservoirs developed across both ramp and rimmed shelf profiles. Major facies associations include carbonate-evaporite transition zone, deep ramp and slope, subtidal microbialites, clastic-textured carbonates, and restricted peritidal carbonates. Microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties, and that help in predicting the location of reservoir fairways. Microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities (>50%), shallow subtidal pustular laminites with cm-scale variability of lamina morphology, deeper subtidal crinkly laminites that show mm-scale variability of lamina morphology, and intertidal tufted laminates that show mm- to cm-scale tufted textures. Other reservoir facies are more conventional grainy carbonates including ripple cross-stratified grainstone–packstone, hummocky cross-stratified grainstone–packstone, flat pebble conglomerate, ooid and intraclast grainstone–packstone, and Cloudina grainstone–packstone. These facies are almost invariably dolomitized and all have moderate to excellent reservoir quality. These facies comprise carbonate platforms, broken up during salt tectonics, that range up to 160 m (525 ft) in thickness and extended laterally, prior to halokinesis, for tens to over 50 km (31 mi). The distribution of reservoir facies follows sequence stratigraphic predictions, with microbialites occurring in every accommodation profile. Late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build-ups, whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies. Pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods. Crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate.