Mesozoic Sequence Research Articles

Late Mesozoic–Cenozoic evolution of the Sanjiang Basin in NE China and its tectonic implications for the West Pacific continental margin

This study uses new seismic reflection profiles and exploratory well data to analyze the structural characteristics and evolution of the Sanjiang Basin in eastern Heilongjiang, NE China. The basin sequence was divided into six stratigraphic units, representing discrete Middle-Upper Jurassic, Lower Cretaceous, Upper Cretaceous, Eocene–Oligocene, Miocene and Quaternary phases of development. The Sanjiang Basin initiated as a passive marginal rift flank to the southern Mongol–Okhotsk Ocean during the Middle Jurassic, and primary basin growth was terminated by orogenic deformation related to the Mongol–Okhotsk suture to the north and the accretion of the Nadanhada terrane to the east. Middle-Upper Jurassic strata are controlled by half-graben structures active during this phase of basin growth, and are limited largely to the west Suibin sub-basin. The wider basin experienced renewed rifting and large-scale sag caused by back arc extension along the Eurasian continental margin in the Early Cretaceous, terminating during the Late Cretaceous in a phase of intense compression at the contemporary Andean-type continental margin. This produced a series of thrust-folds through the Lower and Upper Cretaceous successions, and led to the development of a regional-scale erosional unconformity. Minor subsequent Cenozoic extension is attributed to back-arc extension along the West Pacific continental margin, with Eocene–Oligocene, Miocene, and Quaternary deposits unconformably overlying the Mesozoic sequences in half-graben and extensional fault-demarcated depocenters, most prominently in the east Qianjin sub-basin.

New data on correlation of the Middle Mesozoic oceanic and island arc rocks of Eastern Asia

Using a radiolarian method, correlation of stages and substages of the Middle Mesozoic submarine volcanic-siliceous sequences was carried out for the vast territory of Eastern Asia for the first time. The correlation of fragmented outcrops of genetically various siliceous-terrigenous-volcanic rocks showed a broad lateral distribution of sediments from the Norian to Hauterivian stages inclusive within the studied area. This work, based on a combination of paleontological, structural-tectonic, and lithological-petrological methods in contrast to the terrane approach, has demonstrated the possibility of age and facial characteristics of isolated outcrops of the Middle Mesozoic submarine rocks for the huge territory of Eastern Asia.

Dredging up Mawson: implications for the geology of coastal East Antarctica

During the 1911–1914 Australasian Antarctic Expedition samples of bottom sediment were dredged from a wide sweep of coastline extending from the main base at Commonwealth Bay, to the western edge of the Shackleton Ice Shelf. An earlier study showed these sediments to contain palynomorphs recycled from eroding sedimentary sequences. High concentrations of Permian, Jurassic to Early Cretaceous, and Cenozoic microfossils were present in three regions of the continental shelf, namely, offshore from the Shackleton Ice Shelf, from Cape Carr and from close to the Mertz Glacier. The findings of the earlier study are re-evaluated in the light of new information concerning the bathymetry of sampled areas on the continental shelf, the sub-ice topography of ice shelves and the Antarctic interior, and of sedimentary processes controlling the movement of palynomorphs on the sea floor. Data from the vicinity of the Shackleton Ice Shelf raise the possibility of sourcing some recycled material through sub-ice connections with the deep Aurora Subglacial Basin of the interior. From the George V Basin, west of the Mertz Glacier, new echo-sounding data show the dredges collected lie mostly on the edge of a steep trough parallel to the coast. Previous suggestions that Jurassic to Cretaceous sequences there correlate with those of the Otway Basin on the Australian margin are corroborated by recent seismic reflection data showing thick rift and pre-rift sequences offshore from the Adélie and Wilkes coasts. The relationship of these sequences to putative Mesozoic sequences within the inland Wilkes Basin is uncertain. Limited studies suggest that recycled palynomorphs in continental shelf sediments may lie close to the sites of their original deposition.

Allochthonous salt in the sub-Alpine fold–thrust belt of Haute Provence, France

Abstract In the sub-Alpine chains of Haut Provence, SE France, a very well-exposed Mesozoic sequence showing rapid thickness and facies changes associated with Jurassic and Cretaceous extension on the margin of the Ligurian Tethys has been deformed by ‘Alpine’ compression which occurred from the Late Cretaceous to the Pliocene. Although the geology has been very well known for decades, aspects of the structure remain enigmatic and cannot be explained by either Mesozoic extension or Alpine shortening alone. We infer that some deformation resulted from salt tectonics. A completely overturned, highly condensed Jurassic section near Barles village resembles the elevated roof of a Triassic salt body in a deep-marine basin. This carapace became overturned as a flap in the Middle Jurassic when salt broke out at the seafloor and overran the inverted flap as an allochthonous extrusion, comparable to those in the deepwater Gulf of Mexico or Angola. Later, Alpine compression exploited the weakness of the salt sheet as the Digne Thrust moved over the inverted flap. Although the flap is in the footwall of the thrust, evidence of soft-sediment deformation and other anomalous structures within the flap suggest that it did not originate as an overturned footwall syncline.

The sequence stratigraphy of Mesozoic successions in the Levant margin, southwestern Israel: A model for the evolution of southern Tethys margins

The Levant margin, in the subsurface of the eastern Mediterranean area, formed during the early Mesozoic following rifting and subsequent opening of the southern Tethys Ocean. This work describes the stratigraphic evolution of the shelf edge and slope for this margin in southwestern Israel and in the adjacent Mediterranean Sea. The study is based on the interpretation of 27 wells and 92 seismic reflection lines totaling 2000 km (1243 mi). Depositional sequences and sequence boundaries of the Jurassic and the Cretaceous age inferred from seismic reflection terminations, wireline-log stacking patterns, lithofacies, and biostratigraphic data. Six low-order and 22 high-order depositional cycles were identified. Their stratigraphic architecture reflects shifts of depocenters from the basin to its margin, controlled by eustasy and regional subsidence. Aggrading and backstepping of carbonate platforms in the Levant shelf is associated with relative rises in sea level. Progradation of siliciclastic and carbonate slopes toward the basin is related to relative drops in sea level. The stratigraphic framework of the Levant margin presented here is in accordance with recently published Mesozoic sequence stratigraphy of the Arabian platform, therefore, it may be used as a working model for reconstructing other rifted Tethyan margins in the region. This study further emphasizes the reservoir potential of Jurassic and Cretaceous deep-water lowstand wedges offshore Israel, where extensive exploration efforts are currently occurring.

Contribution of logging tools to understanding helium porewater data across the Mesozoic sequence of the East of the Paris Basin

Helium concentration measurements made on water and rock samples collected at various depths along a 2000 m depth borehole drilled in the eastern part of the Paris basin and reaching the Triassic conglomerates were used to establish a vertical profile of dissolved helium concentration throughout the entire Mesozoic sedimentary pile. Wireline logging tools were used to measure rock density, porosity and U and Th contents every 15 cm in the various formations. Samples from the deepest Muschelkalk and Buntsandstein show a very good agreement between measured and calculated 4He porewater contents. Shallower levels show lower and lower 4He concentrations compared to calculated values when going upwards. The data set obtained by wireline logging measurements was used as inputs for numerical simulations of 1D He production/diffusion throughout the 2000 m profile. Several assumptions regarding the transport properties in the various sedimentary layers were tested and all were found to yield fairly good agreement between modeled and measured He concentrations. There is no need to invoke either a transient regime or a deep crustal He flux in this “quiet” (seismically inactive) part of the Paris basin. Moreover, the modeling results suggest that the Keuper massive halite level associated with the upper Muschelkalk pre-evaporitic series efficiently isolates the overlying layers from any input from deeper formations in the Meuse/Haute Marne area.

Mesozoic deltaic system along the western margin of the Indian plate: lithofacies and depositional setting of Datta Formation, North Pakistan

Over 1 km thick Mesozoic sedimentary sequence is exposed over a wide area in the Upper Indus basin of north Pakistan along the western margin of the Indian Plate. The Mesozoic sequence is comprised of clastic facies in the lower part, while carbonate facies are dominant in the upper part. About 200 m thick mixed sequence of interbedded sandstone, siltstone, clay, and carbonaceous shale represents the lower Jurassic Datta Formation in the Salt and Trans Indus Ranges in North Pakistan. The Datta Formation constitutes important reservoir horizons in a number of oil fields in the western Himalayan foreland basins where it is encountered at a depth of about 4 km in various wells. The Datta Formation is described from different parts of the range front to understand the internal architecture of various sedimentary facies and their depositional system. The thickness and lithofacies assemblages of the Datta Formation change in different parts of the range front as well as in subsurface of the Upper Indus basin. The Datta Formation represents a coarsening upward deltaic sequence in most parts of the basin. On the basis of lithological variations and sedimentary structures, a number of depositional facies have been recognized which include channel belt facies, floodplain/abandoned channel facies, swamp facies, and lagoonal facies. Further north, in the Kalachitta and Hazara regions, the siliciclastic facies change to more complex assemblages of interbedded bauxite, silcrete, marl, and some limestone. These sediments represent deposition in a delta-plain setting of a fluvial-dominated delta with northwestward flowing channels.

Meso-Cenozoic tectonics of the southern Patagonian foreland: Structural evolution and implications for Au–Ag veins in the eastern Deseado Region (Santa Cruz, Argentina)

Located in the centre of the Argentinean Patagonia between 46° and 49°S, the Deseado Region represents the foreland domain of the Southern Patagonian Andes. Its geology is characterized by thick Mesozoic sequences which, at its eastern sector, present a Mesozoic and Cenozoic geologic evolution which has been strongly determined by the development of three major tectonic phases. The present research is based on field geological mapping, interpretation of seismic and aeromagnetic data, as well as satellite image analysis. This approach has allowed us to identify and characterize the deformation that occurred throughout Jurassic, Cretaceous and Miocene times. We interpret that the most relevant structural features are the result of normal faulting generated as a response to the Jurassic rifting stage. These extensional features have strongly influenced the subsequent geometry and distribution of younger Cretaceous and Cenozoic structures. The Jurassic extensional deformation, which affected major areas of Southern Gondwana, is the product of a major intra-continental rifting stage which was accompanied by synkinematic volcanism. This tectonic regime is characterized by SW-NE directed extension that generated major oblique WNW trending faults accommodating regional dextral-extension. In the study area, this tectonic regime is inferred from the geometries of major fault systems interpreted from available seismic reflection data, as well as from the spatial distribution and orientation of the extensional fracturing associated with the opening of hybrid and dilatational siliceous epithermal Au–Ag veins. Following the Jurassic rifting stage, a more restricted Cretaceous -post-Neocomian-compressional tectonic phase took place. Throughout this period, we interpret the previously formed Jurassic extensional structures to have been reactivated under sinistral transpression. Deformation during this period generated sinistral-reverse WNW belts of deformation, which accommodated reverse faulting, imbricate thrusts, dextral and sinistral R1 and R2 shears and disharmonic folds due to a buttress effect. Under the post-Oligocene Andean regime, W–E directed compression acted on previously-formed N to NNE-oriented normal faults. Compression and shortening uplifted a series of narrow and sub-meridional ranges which run as a 200 km long inversion-related tectonic front along the Patagonian foreland. Between 47°11′ and 48°40′S, one of these NNE ranges divides the entire Deseado Region into two distinctive structural domains. Whilst the western domain presents dominant NNW morphotectonic features, that to the east appears highly dominated by WNW fabrics of Jurassic and Cretaceous age. The structural features of the Eastern domain appear to extend further north of the Deseado Region towards the vicinity of the San Jorge Gulf. This WNW-trending belt hosts pre-Upper Cretaceous rocks and pre-drift basement rocks which include igneous Paleozoic metamorphic rocks and Permian to Triassic sedimentary units. The Deseado region’s epithermal Au–Ag Jurassic vein systems result from the infilling and deposition of low temperature hydrothermal fluids within dilatational and hybrid structures. These spectacular vein systems are compatible with the regional SW-NE extension direction controlled by the Jurassic intra-continental rifting of southern Gondwana. Dilatational and hybrid veins are preferentially hosted by fractures in the Jurassic volcanic rocks, while the veins located within the pre-volcanic basement preferentially infill normal faults. Finally, most of these epithermal vein fields where exhumed during a moderate phase of inversion during Cretaceous times.

Rigless well intervention and trees on wire from a DPII vessel: a case study

Since the 2006 bidding rounds, exploration activity in both the Timor-Leste exclusive offshore area (TLEA) and joint petroleum development area (JPDA) has progressed steadily. Within the JPDA, exploration remains largely focussed in the Jurassic-age Plover and Elang Sandstone formations in the Flamingo Trough-Sahul Syncline region. Over 700 MMbbls of liquids and 4Tcf of gas have been discovered in this western region of the JPDA, including the 2008 Kitan oil field discovery by Eni, which is currently scheduled for production in 2011. Recent seismic survey activity within the JPDA by PSC holders Petronas and Oilex has resulted in the combined acquisition of 2,800 square kilometres of new 3D data. These surveys, together with on-going re-evaluation of existing well data by these companies, has helped further refine the knowledge of petroleum systems within the JPDA, in preparation for drilling campaigns scheduled for late 2009 and early 2010. Within the TLEA, multi-client seismic surveys undertaken in 2005, together with on-shore academic research, indicates that the prospective Mesozoic sequence of the Northern Bonaparte Basin underlies the Timor Trough, greatly enhancing the petroleum prospectivity of this region. Further detailed 2D and 3D seismic surveys in the TLEA have been recently completed by PSC holders Reliance Exploration and Eni. The first wells to be drilled in this deeper water frontier region are scheduled for 2010.

Tectonics of the western part of the Polish Outer Carpathians

The western part of the Polish Outer Carpathians is built up from the thrust, imbricated Upper Jurassic-Neogene flysch deposits. The following Outer Carpathian nappes have been distinguished: Magura Nappe, Fore-Magura group of nappes, Silesian, Subsilesian and Skole nappes. Interpretation of seismic and magnetotelluric survey from the region South of Wadowice, allows observation of relationship between basement and flysch nappes in the Outer Carpathians. It also allows identification of dislocation cutting both flysch nappes and their basement. All the Outer Carpathian nappes are thrust over the southern part of the North European Platform. The platform basement is composed of older Precambrian metamorphic rocks belonging to the Bruno-Vistulicum terrane. Sedimentary cover consists of Paleozoic, Mesozoic and Neogene sequences. The characteristic features of this boundary are horsts and troughs of general direction NW-SE, turning W-E. Faults cutting only the consolidated basement and the Paleozoic cover were formed during the Hercynian Orogeny in the Carboniferous and the Early Permian. Most of the older normal faults were covered by allochtonous flysch nappes forming thus the blind faults. During the last stage of the geodynamic development the Carpathians thrust sheets moved towards their present position. Displacement of the Carpathians northwards is related to development of dextral strike-slip faults of N—S direction. The orientation of this strike-slip fault zones zone more or less coincides with the surface position of the major faults perpendicular to the strike of the Outer Carpathian thrustsheets. The huge fault cuts formations from the Paleozoic basement through the flysch allochton between the boreholes in Sucha Beskidzka area. The displacement of nappes of the Carpathian overthrust and diapiric extrusion of plastic formations of the lower flysch units occurred along this fault.

Correlações Estratigráficas Conflitantes no Limite Permo-Triássico no Sul da Bacia do Paraná: O Contato Entre Duas Seqüências e Implicações na Configuração Espacial do Aqüífero Guarani

The stratigraphic register of the Guarani aquifer system in the Parana basin involves deposits of the Triassic to the Cretaceous. The register in the south region differs from the other areas of the basin, causing confusion in the stratigraphic conception of the Pirambóia Formation. This formation is correlated with paleozoics deposits whose register is only found in the south region of the basin. The correlation that intends for these units has implications in the space configuration and distribution of the aquifer. The space relationships of this record are reinterpreted. The Rio do Rasto Formation, of late Permian age record alluvial plain system, in a progressive semiarid environment, including dune fields; in the southern part of the basin these dune field are extensive and is represented by thick record, covered again by extensive fluvial deposits of the Sanga do Cabral Fm at the PT boundary. The unconformity above records a generalized uplift, associated to the Gondwanides orogeny and to climatic changes, accompanied by progressive supply and sin-sedimentary deformation. Above the unconformity, aeolian and fluvial deposits of the Mesozoic sequence present division organized in 3 blocks: West, Central and East. Three depositional cycles were identified. The first, unconformable over Permian rocks, occurs only in the Central Block, with fluvial and lacustrine deposits filling small rifts (Santa Maria and Caturrita fms). The second cycle extends over the whole basin with fluvial deposits and humid aeolian (formations Guará and Pirambóia); the fluvial and aeolian Guará Fm lies unconformable over Sanga do Cabral Fm in the West Block. In the East Block the aeolian and fluvial unit identified as Pirambóia Fm correlative; it occurs unconformable over the Rio do Rasto Fm, omitting the Sanga do Cabral Fm and the rocks of the first cycle. The third cycle system tract records super arid dune fields of the Botucatu Formation. The Late Permian aeolian facies of the Sanga do Cabral Fm, previously called by some authors as the Pirambóia Fm, doesn't present interfingers and doesn't constitute a hydrostratigraphic unit connected to the Guarani System.

Arcabouço geológico e hidrofácies do Sistema Aquífero Guarani, no município de Ribeirão Preto (SP)

Subsurface geology and hydrogeological studies using geophysical well logs allowed to define the geologic framework and to characterize the hydrofacies of Guarani Aquifer System (GAS) in the city of Ribeirão Preto. The GAS is composed by the sediments of Pirambóia and Botucatu formations and rests upon an impermeable pelitic substrate, the Corumbatai Formation, which is considered an aquiclude, and is mostly covered by basic volcanic rocks of Serra Geral Formation. The typical geophysical patterns of Pirambóia and Botucatu formations are well correlated and their complex facies variations are compatible to the stratigraphy of the GAS. The thickness variations of the Pirambóia Formation were not defined because only one of the studied wells penetrated the entire Mesozoic sequence. In the case of Botucatu Formation the paleodune relief preserved by the volcanic lavas and the depressions created by deep aeolian erosion are responsible for the thickness variations. This situation emphasizes the inadequacy of using the Botucatu and Serra Geral formations contact as a datum for structural analysis. The shape of the structural contour of the unconformity at the top of Pirambóia Formation, that is considered to be a good local marker, confirmed the existence of a structural low in the central-southwestern portion of the city. The Ribeirão Preto depression plunges southwest dropping over 100m across 15 Km. Three hydrofacies - A (lower), B (medium) and C (upper) - were identified in the Pirambóia Aquifer. The upper and lower hydrofacies show log signature typical of reduced permeability, whereas the medium one shows log signature of high permeability. The Botucatu Aquifer has a single hydrofacies (D) with log signature indicative of elevated permeability.

Arcabouço geológico e hidrofácies do Sistema Aquífero Guarani, no município de Ribeirão Preto (SP)

Subsurface geology and hydrogeological studies using geophysical well logs allowed to define the geologic framework and to characterize the hydrofacies of Guarani Aquifer System (GAS) in the city of Ribeirao Preto. The GAS is composed by the sediments of Piramboia and Botucatu formations and rests upon an impermeable pelitic substrate, the Corumbatai Formation, which is considered an aquiclude, and is mostly covered by basic volcanic rocks of Serra Geral Formation. The typical geophysical patterns of Piramboia and Botucatu formations are well correlated and their complex facies variations are compatible to the stratigraphy of the GAS. The thickness variations of the Piramboia Formation were not defined because only one of the studied wells penetrated the entire Mesozoic sequence. In the case of Botucatu Formation the paleodune relief preserved by the volcanic lavas and the depressions created by deep aeolian erosion are responsible for the thickness variations. This situation emphasizes the inadequacy of using the Botucatu and Serra Geral formations contact as a datum for structural analysis. The shape of the structural contour of the unconformity at the top of Piramboia Formation, that is considered to be a good local marker, confirmed the existence of a structural low in the central-southwestern portion of the city. The Ribeirao Preto depression plunges southwest dropping over 100m across 15 Km. Three hydrofacies – A (lower), B (medium) and C (upper) – were identified in the Piramboia Aquifer. The upper and lower hydrofacies show log signature typical of reduced permeability, whereas the medium one shows log signature of high permeability. The Botucatu Aquifer has a single hydrofacies (D) with log signature indicative of elevated permeability.

Geology and evaluation of hydrocarbon prospects of Tethyan sediments in Spiti Valley, Spiti and Zanskar, Himanchal Pradesh

The Tethyan sediments in the Spiti Basin stretch from Pir Panjal in the south to the Zanskar Range in the north. The Tethyan sediments lie within the Higher Himalayan Physiographic Zone. Since such huge thicknesses of sediments are deposited in this area, it is anticipated by the geologists that favourable conditions might have prevailed for the generation and accumulation of hydrocarbon in the Tethyan sediments. It has been continuous endeavour of ONGC to explore and enhance hydrocarbon reserves from all Indian sedimentary basins including the category IV and frontier basins. Seismic data acquisition and geological modelling have been carried out for Lesser Himalayas and even parametric, structural, or wild cat wells have been drilled and one such well is under drilling at Sundernagar. The Tethyan part being inaccessible has not been subjected to detailed geological modelling. Mesozoic sequences from Kibber Gate Tashegang and Lidang Domal traverses have been subjected to sedimentological, Palaeontological, Palynological and source rock investigations to reconstruct microfacies, biochronostratigraphy, depositional environment, and organic matter maturation. The petrographic study of the Mesozoic Tethyan sediments exposed along the selected traverses shows occurrence of Kioto Limestone, Spiti Shale, Giumal Sandstone, Tashegang Limestone and Chikkim Limestone formations. The Kioto Limestone is highly sparitised and has poor porosity. The Giumal Sandstone consists of glauconitic sands which are well indurated and have very poor intergranular porosity. The porosity is further reduced by calcite cementation. Similarly, the Tashegang Limestone is also highly sparitised and has poor porosity. Spiti shale is dark grey, black, carbonaceous, occasionally oxidised in nature. Brachiopod fauna supportive of Lower Carboniferous age has been recorded from Lipak Formation and Ordovician Silurian fauna from Takche Formation. Cephalopods supportive of Oxfordian Callovian age have been recovered from Spiti Shale. Lilang Group has yielded rare Cephalopods of Triassic Jurassic age. On the basis of FAD of Riguadella filamentosa and Egmontodinium torynum Late Bathonian to Late Tithonian age has been suggested to Spiti Formation. On the basis of FAD of E. cinctum and Batioladinium micropodum, Late Tithonian to Early Valanginian age has been suggested to Tashegang Formation. On the basis of FAD of B. micropodum the base of Giumal Formation is dated as Early Valanginian. The absolute pollen frequency (APF) value of the palynofloral assemblage from Spiti Formation suggests inner neritic to marginal marine environment of deposition, while palynoflora from Tashegang Formation indicate marginal marine to lagoonal environment and Giumal Formation was laid under marginal marine conditions. The Organic matter recorded from the studied samples shows Humic-Wood (H-W) to Humic-Sapropelic-Wood (HS-W) to Humic-SapropelicCharcoal (HS-C) facies.

Sequence stratigraphic characteristics and hydrocarbon prospect of Mesozoic Formation in Suibin Depression, Sanjiang Basin

Abstract According to the detailed analysis of drilling data, together with the interpretation of seismic section, the Mesozoic of Suibin Depression is divided into four second-order sequences and eight third-order sequences. The third-order sequences at the bottom of second-order sequences develop low-stand systems tracts (LSTs), and the other sequences do not. The analysis result of the balanced section and Milankovitch cycle indicates that the Mesozoic sequences above third-order in the Suibin Depression are mainly controlled by structure, and that the Milankovitch cycle only controls high frequency sequences (systems tracts). Based on the above analyses, the second-order sequence stratigraphy model of the Mesozoic of Suibin Depression is established. The model is of typical tripartition, the early basin fill (LST), the middle basin expansion (TST) and the late basin abortion (HST). The transgressive systems tracts (TSTs) overlying the LSTs in the second-order sequences may be good seal and source rock, so the LSTs have the potential for oil and gas.

Late Paleozoic and Mesozoic deformations in the southwestern Primorye region

Two types of tectonic deformations indicating different geodynamic settings are defined in the southwestern Primorye region. Near-latitudinal compression forces were responsible for the oldest, Late Paleozoic deformations. The Permian stratified complexes host a near-meridional system of folds and zones of dynamothermal metamorphism, cleavage, and foliation oriented orthogonally relative to the compression. Late Proterozoic (?) mafic-ultramafic rocks are characterized by similar deformations. In the Late Permian, the deformations were accompanied by granitoid magmatism controlled by fold and cleavage structures. The younger, Mesozoic deformations produced by near-meridional compression are represented by NE-trending sinistral strike-slip faults and their structural parageneses: an ENE-trending system of folds and downdip-thrusts both superimposed on Paleozoic protostructures and manifested in Mesozoic and Cenozoic sequences. It is inferred that, at the Paleozoic-Mesozoic boundary, near-latitudinal compression was replaced by near-meridional compression, probably, in response to the corresponding change in direction of the lateral displacement of the interacting Asian continent and (or) Pacific Plate.

Paleozoic composite petroleum system of North Africa: hydrocarbon distribution and main controlling factors

North Africa, which is one of the main oil and gas producing regions in the world, is best known for its sub-salt Paleozoic-Triassic reservoirs and Paleozoic source rocks. Hydrocarbon abundance varies greatly from one structural domain to another areally and from one stratigraphic interval to another vertically. Analyses of the essential elements and geological processes of the Paleozoic petroleum system indicate that the distribution of the Lower Silurian shale source rocks, the development of a thick Mesozoic overburden, the presence of the Upper Triassic-Lower Jurassic evaporite seal are the most important factors governing the distribution of the Paleozoic-sourced hydrocarbons in North Africa. The Mesozoic sequence plays a critical role for hydrocarbons to accumulate by enabling the maturation of the Paleozoic source rocks during the Mesozoic-Paleogene times and preserving the accumulated hydrocarbons. Basins and surrounding uplifts, particularly the latter, with a thick Mesozoic sequence and a regional evaporite seal generally have abundant hydrocarbons. Basins where only a thin Mesozoic overburden was developed tend to have a very poor to moderate hydrocarbon prospectivity.

LA DORSAL ASUNCIÓN - RÍO GRANDE: UN ALTOFONDO REGIONAL ENTRE LAS CUENCAS PARANÁ (BRASIL, PARAGUAY Y URUGUAY) Y CHACOPARANAENSE (ARGENTINA)

The Dorsal Asuncion-Rio Grande (DARG) is a first magnitude tectonic megastructure that constitutes a regional depositional uplift responsible for the partitioning of the post-Permian-Triassic sedimentation of the Parana (Brazil, Paraguay and Uruguay) and Llanura Chacoparanense (Argentina) basins. The northern end of the DARG, in Paraguay, is represented by the Asuncion High materialized by an outcrop of the crystalline basement (Tebicuary Craton) plunging toward the SSE and fringed by Paleozoic and Mesozoic sedimentary sequences. Contrarily, the southern extreme of the DARG is constituted by the Rio Grande High, located in the Rio Grande do Sul State, Brazil; and it is equally characterized by an outcrop of crystalline basement fringed by Paleozoic and Mesozoic sequences, but plunging toward the NNW. The intermediate tract, in the territories of the Argentina and Uruguay, is less exposed and required a regional correlation by means of subsurface information. The DARG separates two huge tectonosedimentary environments that correspond to the adjacent Parana Basin, toward the cast, better exposed and the different sedimentary basins covered by the Llanura Chacoparanense, toward the west. On the basis of geographical disposition, kinematics and chronology of its evolution, the DARG's origin can be correlated with the convergent tectonics of the Late Paleozoic that structured the Sierras Australes of Buenos Aires (Argentina). Finally, the evaluation of the influence of the DARG contributes to the understanding of the tectonosedimentary mechanisms that exhibit the sequence constituting the infill of the basins of the Llanura Chacoparanense and Parana.

Prediction of the phase state of hydrocarbon accumulations in the Mesozoic deposits of northwestern Siberia

The phase state of hydrocarbon accumulations was predicted using recent drilling data for three oil and gas complexes of the northern regions of the West Siberian petroliferous basin: Lower and Middle Jurassic, Upper Jurassic, and Berriasian-lower Valanginian (Achimov member); these results were compiled in simplified maps presented in the paper. In order to develop a plausible model and distinguish the zones of hydrocarbon accumulation of various phase states, we analyzed and systematized the existing concepts on the distribution of oil, oil-gas condensate, and gas condensate pools in the Mesozoic sequences of this region, types of organic matter in rocks, and stages of catagenetic alterations. Based on the data of the Tyumen SG-6 superdeep well, the oil-generating capacity of source deposits was assessed. Three zones differing in the phase state of hydrocarbons, oil, gas condensate-oil (transitional), and gas condensate, were distinguished in the map of the distribution of hydrocarbon accumulations in the Early-Middle Jurassic petroliferous complex of northwestern Siberia. Only two zones of hydrocarbon accumulations, oil and transitional, were detected in the Late Jurassic complex of the Nadym-Taz and Ust’-Yenisei regions. The same zones but within different boundaries were distinguished in the Achimov sequence. In contrast to the Jurassic beds, the zone of occurrence of oil accumulations in the Achimov member occupies a larger area and extends further north. In terms of density, the oils of the Achimov deposits were subdivided into three types: ultralight, light, and medium. The zones of their occurrence are shown in the map.

Paleotectonic evolution of the Zürcher Weinland (northern Switzerland), based on 2D and 3D seismic data

A new interpretation of the Permo-Carboniferous (PC) troughs in the Zurcher Weinland region in northern Switzerland is proposed on the basis of 2D and 3D reflection seismic surveys. The presence of two WSW-ENE oriented grabens separated by a crystalline basement horst block, confirmed by the Benken borehole, is well established, though the presence of PC deposits in its surroundings remains uncertain. The interpretation of the 3D seismic data shows a transtensive tectonic regime for this period confirming the results of earlier interpretations of 2D-lines. There is no evidence for a compressive Saalian phase (around 265 Ma) as postulated for the PC trough west of Weiach. Several observed faults show signs of reactivation during a late Triassic inversion phase. Detailed mapping of the Mesozoic sequence reveals that further and minor reactivations along the border of the PC trough south of Benken may have influenced deposition of the younger sediments and the local structuration of the base-Tertiary unconformity. The findings of the local investigations are interpreted in the context of large-scale paleotectonic models.