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Peter A Kukla - One of the best experts on this subject based on the ideXlab platform.

  • deformation of intrasalt competent layers in different modes of salt tectonics
    Solid Earth, 2019
    Co-Authors: Mark G. Rowan, Janos Urai, Carl J Fiduk, Peter A Kukla
    Abstract:

    Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 10 5 . We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured Stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant Stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, Stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured Stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between Stringers, and unlikely in tall passive diapirs because the Stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

  • Regional variations in the structure of the Permian Zechstein 3 intrasalt stringer in the northern Netherlands: 3D seismic interpretation and implications for salt tectonic evolution
    Interpretation, 2014
    Co-Authors: Frank Strozyk, Janos Urai, Heijn Van Gent, Martin De Keijzer, Peter A Kukla
    Abstract:

    The late Permian Zechstein evaporites in the northern Netherlands were exceptionally well imaged in ![Formula][1] of prestack depth migration 3D seismic data. Seismic reflections of a 30–150-m-thick Zechstein 3 anhydrite-carbonate stringer, which was encased in thick layers of rock salt, provided an unparalleled, basin-scale view of the 3D internal structure of a giant salt basin. Seismic data were used to map the regional variation of the intrasalt stringer to analyze its role in deformation styles and salt flow as well as its interaction with the sub- and suprasalt sediments. From our interpretation of the stringer, the salt layers, and the encasing sediments, three regional structural stringer styles can be defined and were analyzed in the context of regional salt kinematics. Our results revealed that the current stringer initially formed a continuous sheet of anhydrite and carbonate, embedded in salt of varying thickness. After the onset of syndepositional gravitational gliding of some of the salt masses and passive salt diapirism triggered by differential loading in the Triassic in other areas, salt flow caused rupture and folding of the stringer on a wide range of scales. The thickness and deformation degree of the individual salt layers controlled the development of regionally distinctive styles of intrasalt structures. Although deformation of the salt and the embedded stringer stopped early on morphologic highs, the basinal areas experienced phases of later activation or reactivation of salt structures and sedimentary basins. This was especially the case during the Late Cretaceous to Early Tertiary plate tectonic reorganization in the Central European plate, causing three-dimensionally complex intrasalt structures observable today. [1]: /embed/mml-math-1.gif

  • the internal structure of salt insights from a regional 3d seismic study of the permian zechstein 3 intra salt stringer in the northern netherlands and its implications for salt tectonics
    EGUGA, 2014
    Co-Authors: Frank Strozyk, Peter A Kukla, Janos Urai, Heijn Van Gent, Martin De Keijzer, Kessler Park
    Abstract:

    In this study we aim to understand effects of internal layering on the structural evolution of the Late Permian Zechstein onshore the northern Netherlands. We study the well-imaged reflections of the 30-150 m thick Zechstein 3 anhydrite-carbonate intra-salt stringer across 6500 km 2 of 3D seismic data. We show that the Z3 stringer can be used to provide an unprecedented, basin-scale view of the 3-D internal structure of the salt. The seismic interpretation is used to evaluate the regional variation in structural style defined by rupture and folding of the stringer, and to assess the relationship with the sub- and suprasalt sediments. Before its disruption the stringer was a continuous sheet with a complex pattern of thickness variation. This thickness variation played an important role in the initiation of down-building of Early Triassic post-salt sediments and associated passive deformation of the salt. The stringer’s further deformation was controlled by regional variation in salt flows, which finally caused a high heterogeneity of its structural styles across the study area. Regional trends of salt flows in the northern study area for example, caused a high disruption and northward drainage of the stringer. In contrast, the southern study area was partially decoupled from this process by fast and deep sediment down-building along major fault zones and shows less disruption of the stringer. The intra-salt structures that formed in both areas are hence very different, while top salt geometries can be rather similar. It is implied therefore that the outer shape of the salt deposits does not reflect its deformation history, but that it is also controlled by differences in the internal structure and cumulative salt flow.

  • Numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics: A case study from the South Oman Salt Basin
    Geological Society London Special Publications, 2012
    Co-Authors: Steffen Abe, Lars Reuning, Janos Urai, Stephan Becker, Peter A Kukla
    Abstract:

    Large rock inclusions are embedded in many salt bodies and these respond to the move- ments of the salt in a variety of ways including displacement, folding and fracturing. One mode of salt tectonics is downbuilding, whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside. We investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ- ated deformation of brittle Stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model, in combination with adaptive remeshing. The model set-up is constrained by observations from the South Oman Salt Basin, where large carbonate bodies encased in salt form substantial hydrocarbon plays. The model shows that, depending on the displacement of the top salt, the Stringers can break very soon after the onset of salt tectonics and can deform in different ways. If extension along the inclusion dom- inates,Stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth.Spacingof the boudin-bounding faults can be as close as 3-4 times the thickness of the stringer. In contrast, salt shortening along the inclusion may lead to folding or thrusting of Stringers.

  • 3d seismic study of complex intra salt deformation an example from the upper permian zechstein 3 stringer western dutch offshore
    Geological Society London Special Publications, 2012
    Co-Authors: Frank Strozyk, Janos Urai, H W Van Gent, Peter A Kukla
    Abstract:

    Mostoftheinformationonsubsurfaceevaporiticstructurescomesfrom3Dseismicdata. However, this data only provide limited information about the internal structure of the evaporites, which is known from salt mines and salt diapir outcrops. Brittle intra-salt layers (carbonate, anhy- drite, clay) ofatleast 10 m thicknessform goodreflectors inevaporites, but the structure and dynam- ics of such 'Stringers' in the salt movement are poorly understood. In this study, we investigate the intra-salt Zechstein 3 (Z3) stringer from 3D seismic data in an area offshore the Netherlands. Observations show complex deformation including boudinage, folding and stacking. Reflections from thin and steep stringer parts are strongly reduced, and we present different structural models andtestsofthese.Wecompareourobservationstostructuralmodelsfromsaltminesandanalogue/ numerical models of intra-salt deformation. A smoothed representation of the upper surface of the stringer fragments follows the shape of Top Salt, but smaller-scale stringer geometries strongly differ from this and imply boudinage. The imaged disharmonic patterns of constrictional folds pro- vide evidence for the complexity of the intra-salt, in agreement with observations in salt mines. This may be explained by interaction of the layered salt rheology, complex three-dimensional salt flow, different phases and styles of basement tectonics and movement of the overburden.

Janos Urai - One of the best experts on this subject based on the ideXlab platform.

  • numerical modeling on deformation of viscous Stringers in deforming salt bodies under a compressive environment a case study from the south oman salt basin
    Arabian Journal of Geosciences, 2021
    Co-Authors: Janos Urai
    Abstract:

    Salt deposits are extremely potent seals for hydrocarbon reservoirs present in the sedimentary basins worldwide. Large rock inclusions (Stringers) encased within various salt bodies may adopt diverse modes of deformation and displacement. Analyzing the movement and segmentation of these elastic and viscous Stringers provides a comprehensive understanding of the intricate internal deformation mechanisms in the associated salt diapirs. The difference in viscosity between the Stringers and the surrounding salt deposit yields a massive impact on the internal complexity of the salt structures. An analytical finite element model was built in combination with an adaptive remeshing process in this study, to achieve a suitable downbuilding simulation. The standard model setup is regulated with observations from South Oman Salt Basin. In this paper, the evaluation of parameter sensitivity concerning the dynamics of viscous Stringers in salt diapirs was carried out, especially the sensitivity study pertaining to varied salt and stringer viscosities, which contribute to the distinct pattern of the internal structure in salt diapirs. The results of the study establish that viscous Stringers exhibit characteristic extension and folding. When the viscosity contrast between the salt and the stringer is 1000:1, strong folding of Stringers dominates the internal structure of the salt diapirs, and when the viscosity contrast between the two is 10:1, only small-scale minor folding of the Stringers takes place. This variation in the viscosity contrast causes disparate internal stress distribution of the Stringers. In addition, the location of the Stringers and the thickness of salt deposit between the Stringers as well as between the Stringers and the base also govern the deformation pattern of the internal structure of the salt domes.

  • deformation of intrasalt competent layers in different modes of salt tectonics
    Solid Earth, 2019
    Co-Authors: Mark G. Rowan, Janos Urai, Carl J Fiduk, Peter A Kukla
    Abstract:

    Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 10 5 . We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured Stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant Stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, Stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured Stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between Stringers, and unlikely in tall passive diapirs because the Stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

  • Regional variations in the structure of the Permian Zechstein 3 intrasalt stringer in the northern Netherlands: 3D seismic interpretation and implications for salt tectonic evolution
    Interpretation, 2014
    Co-Authors: Frank Strozyk, Janos Urai, Heijn Van Gent, Martin De Keijzer, Peter A Kukla
    Abstract:

    The late Permian Zechstein evaporites in the northern Netherlands were exceptionally well imaged in ![Formula][1] of prestack depth migration 3D seismic data. Seismic reflections of a 30–150-m-thick Zechstein 3 anhydrite-carbonate stringer, which was encased in thick layers of rock salt, provided an unparalleled, basin-scale view of the 3D internal structure of a giant salt basin. Seismic data were used to map the regional variation of the intrasalt stringer to analyze its role in deformation styles and salt flow as well as its interaction with the sub- and suprasalt sediments. From our interpretation of the stringer, the salt layers, and the encasing sediments, three regional structural stringer styles can be defined and were analyzed in the context of regional salt kinematics. Our results revealed that the current stringer initially formed a continuous sheet of anhydrite and carbonate, embedded in salt of varying thickness. After the onset of syndepositional gravitational gliding of some of the salt masses and passive salt diapirism triggered by differential loading in the Triassic in other areas, salt flow caused rupture and folding of the stringer on a wide range of scales. The thickness and deformation degree of the individual salt layers controlled the development of regionally distinctive styles of intrasalt structures. Although deformation of the salt and the embedded stringer stopped early on morphologic highs, the basinal areas experienced phases of later activation or reactivation of salt structures and sedimentary basins. This was especially the case during the Late Cretaceous to Early Tertiary plate tectonic reorganization in the Central European plate, causing three-dimensionally complex intrasalt structures observable today. [1]: /embed/mml-math-1.gif

  • the internal structure of salt insights from a regional 3d seismic study of the permian zechstein 3 intra salt stringer in the northern netherlands and its implications for salt tectonics
    EGUGA, 2014
    Co-Authors: Frank Strozyk, Peter A Kukla, Janos Urai, Heijn Van Gent, Martin De Keijzer, Kessler Park
    Abstract:

    In this study we aim to understand effects of internal layering on the structural evolution of the Late Permian Zechstein onshore the northern Netherlands. We study the well-imaged reflections of the 30-150 m thick Zechstein 3 anhydrite-carbonate intra-salt stringer across 6500 km 2 of 3D seismic data. We show that the Z3 stringer can be used to provide an unprecedented, basin-scale view of the 3-D internal structure of the salt. The seismic interpretation is used to evaluate the regional variation in structural style defined by rupture and folding of the stringer, and to assess the relationship with the sub- and suprasalt sediments. Before its disruption the stringer was a continuous sheet with a complex pattern of thickness variation. This thickness variation played an important role in the initiation of down-building of Early Triassic post-salt sediments and associated passive deformation of the salt. The stringer’s further deformation was controlled by regional variation in salt flows, which finally caused a high heterogeneity of its structural styles across the study area. Regional trends of salt flows in the northern study area for example, caused a high disruption and northward drainage of the stringer. In contrast, the southern study area was partially decoupled from this process by fast and deep sediment down-building along major fault zones and shows less disruption of the stringer. The intra-salt structures that formed in both areas are hence very different, while top salt geometries can be rather similar. It is implied therefore that the outer shape of the salt deposits does not reflect its deformation history, but that it is also controlled by differences in the internal structure and cumulative salt flow.

  • Numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics: A case study from the South Oman Salt Basin
    Geological Society London Special Publications, 2012
    Co-Authors: Steffen Abe, Lars Reuning, Janos Urai, Stephan Becker, Peter A Kukla
    Abstract:

    Large rock inclusions are embedded in many salt bodies and these respond to the move- ments of the salt in a variety of ways including displacement, folding and fracturing. One mode of salt tectonics is downbuilding, whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside. We investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ- ated deformation of brittle Stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model, in combination with adaptive remeshing. The model set-up is constrained by observations from the South Oman Salt Basin, where large carbonate bodies encased in salt form substantial hydrocarbon plays. The model shows that, depending on the displacement of the top salt, the Stringers can break very soon after the onset of salt tectonics and can deform in different ways. If extension along the inclusion dom- inates,Stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth.Spacingof the boudin-bounding faults can be as close as 3-4 times the thickness of the stringer. In contrast, salt shortening along the inclusion may lead to folding or thrusting of Stringers.

Zuwena Rawahi - One of the best experts on this subject based on the ideXlab platform.

  • halite cementation and carbonate diagenesis of intra salt reservoirs from the late neoproterozoic to early cambrian ara group south oman salt basin
    Sedimentology, 2009
    Co-Authors: Joerg Schoenherr, Lars Reuning, Peter A Kukla, Michael G Siemann, Janos Urai, Ralf Littke, Zuwena Rawahi
    Abstract:

    Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra-salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self-charging hydrocarbon system within partly highly overpressured carbonate bodies (‘Stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post-dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate Stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re-precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack-seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.

  • polyphase thermal evolution in the infra cambrian ara group south oman salt basin as deduced by maturity of solid reservoir bitumen
    Organic Geochemistry, 2007
    Co-Authors: J Schoenherr, Peter A Kukla, Janos Urai, Ralf Littke, Zuwena Rawahi
    Abstract:

    Petrographic and organic geochemical analyses on solid reservoir bitumen from the Late Precambrian to Early Cambrian Ara Group indicate polyphase thermal evolution in the South Oman Salt Basin (SOSB). The Ara Group constitutes a complex petroleum system in the deep subsurface (3–5 km) of interior Oman. Salt diapirs encase isolated, partly highly overpressured carbonate bodies (intra-salt ‘‘Stringers’’) which represent a predominantly self-charging hydrocarbon system. A large number of stringer cores show that reservoir quality is decreased by the presence of solid reservoir bitumen. By using different microscopy techniques, this solid reservoir bitumen has been identified within pores, pore throats and microfractures of the carbonates. Solid bitumen reflectance measurements (BRr) were converted to vitrinite reflectance (VRr), using a new improved calibration. Vitrinite reflectance geothermometry shows that maximum paleo-temperatures have a highly heterogeneous distribution. Paleo-temperatures of 380 � C were obtained from coke-like solid reservoir bitumen, which most likely formed from thermal cracking of oil. In our model, this process is related to hydrothermal fluids, deriving from deeper pre-salt strata of the SOSB, which entered the intra-salt carbonate Stringers during times of basement tectonic activity via hydrofractures. This caused the formation of high maturities (‘‘pyrobitumens’’) in close vicinity to the fractures and precipitation of ‘‘migrabitumens’’ with increasing distance to the HT source, which is supported by the geochemical composition of the solid reservoir bitumens. The influx of these fluids into the carbonate Stringers is considered to represent a major contribution to their strong overpressures. � 2007 Elsevier Ltd. All rights reserved.

Lars Reuning - One of the best experts on this subject based on the ideXlab platform.

  • Numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics: A case study from the South Oman Salt Basin
    Geological Society London Special Publications, 2012
    Co-Authors: Steffen Abe, Lars Reuning, Janos Urai, Stephan Becker, Peter A Kukla
    Abstract:

    Large rock inclusions are embedded in many salt bodies and these respond to the move- ments of the salt in a variety of ways including displacement, folding and fracturing. One mode of salt tectonics is downbuilding, whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside. We investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ- ated deformation of brittle Stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model, in combination with adaptive remeshing. The model set-up is constrained by observations from the South Oman Salt Basin, where large carbonate bodies encased in salt form substantial hydrocarbon plays. The model shows that, depending on the displacement of the top salt, the Stringers can break very soon after the onset of salt tectonics and can deform in different ways. If extension along the inclusion dom- inates,Stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth.Spacingof the boudin-bounding faults can be as close as 3-4 times the thickness of the stringer. In contrast, salt shortening along the inclusion may lead to folding or thrusting of Stringers.

  • halite cementation and carbonate diagenesis of intra salt reservoirs from the late neoproterozoic to early cambrian ara group south oman salt basin
    Sedimentology, 2009
    Co-Authors: Joerg Schoenherr, Lars Reuning, Peter A Kukla, Michael G Siemann, Janos Urai, Ralf Littke, Zuwena Rawahi
    Abstract:

    Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra-salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self-charging hydrocarbon system within partly highly overpressured carbonate bodies (‘Stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post-dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate Stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re-precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack-seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.

Ralf Littke - One of the best experts on this subject based on the ideXlab platform.

  • halite cementation and carbonate diagenesis of intra salt reservoirs from the late neoproterozoic to early cambrian ara group south oman salt basin
    Sedimentology, 2009
    Co-Authors: Joerg Schoenherr, Lars Reuning, Peter A Kukla, Michael G Siemann, Janos Urai, Ralf Littke, Zuwena Rawahi
    Abstract:

    Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra-salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self-charging hydrocarbon system within partly highly overpressured carbonate bodies (‘Stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post-dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate Stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re-precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack-seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.

  • polyphase thermal evolution in the infra cambrian ara group south oman salt basin as deduced by maturity of solid reservoir bitumen
    Organic Geochemistry, 2007
    Co-Authors: J Schoenherr, Peter A Kukla, Janos Urai, Ralf Littke, Zuwena Rawahi
    Abstract:

    Petrographic and organic geochemical analyses on solid reservoir bitumen from the Late Precambrian to Early Cambrian Ara Group indicate polyphase thermal evolution in the South Oman Salt Basin (SOSB). The Ara Group constitutes a complex petroleum system in the deep subsurface (3–5 km) of interior Oman. Salt diapirs encase isolated, partly highly overpressured carbonate bodies (intra-salt ‘‘Stringers’’) which represent a predominantly self-charging hydrocarbon system. A large number of stringer cores show that reservoir quality is decreased by the presence of solid reservoir bitumen. By using different microscopy techniques, this solid reservoir bitumen has been identified within pores, pore throats and microfractures of the carbonates. Solid bitumen reflectance measurements (BRr) were converted to vitrinite reflectance (VRr), using a new improved calibration. Vitrinite reflectance geothermometry shows that maximum paleo-temperatures have a highly heterogeneous distribution. Paleo-temperatures of 380 � C were obtained from coke-like solid reservoir bitumen, which most likely formed from thermal cracking of oil. In our model, this process is related to hydrothermal fluids, deriving from deeper pre-salt strata of the SOSB, which entered the intra-salt carbonate Stringers during times of basement tectonic activity via hydrofractures. This caused the formation of high maturities (‘‘pyrobitumens’’) in close vicinity to the fractures and precipitation of ‘‘migrabitumens’’ with increasing distance to the HT source, which is supported by the geochemical composition of the solid reservoir bitumens. The influx of these fluids into the carbonate Stringers is considered to represent a major contribution to their strong overpressures. � 2007 Elsevier Ltd. All rights reserved.