Palaeozoic Basin Research Articles

Late Palaeozoic structural evolution of the Patch Bank Ridge and Utsira High, northern North Sea

AbstractThis study focuses on the Late Palaeozoic development of the area east of Utsira High in the North Sea, where the stratigraphic section below the late Permian Rotliegend Group is undrilled. We use regional 3D seismic data to study structuring, sediment distribution and geomorphology across the Patch Bank Ridge and Utsira High in the North Sea. The results show that the Stord Basin and the bounding Utsira East fault initially developed during the Late Palaeozoic extension, probably during the Devonian, and that the Utsira Shear Zone controlled the location of Late Palaeozoic depocentres. The Patch Bank Ridge is an uplifted part of the Stord Basin where we identify Late Palaeozoic growth strata along the southern and northern flanks, indicating a similar timing of the structural evolution in this area. Two key wells, in the Sele High and Ling Depression, are used to relate a Late Palaeozoic isopach map with regional structuring, surface tilt and basement morphology to the enigmatic parts of the Late Palaeozoic basin system. Our results supplement regional models for the Late Palaeozoic basin development, we suggest that the deeply eroded Devonian half‐grabens preserved on the Utsira High formed parts of an extensive basin system that show stratigraphic expansion towards their bounding faults. The Top Basement surface at these highs offers several distinct geomorphologies that evolved during three periods of exposure, expressed as (i) a tilted and rugose landscape, (ii) distinct drainage networks and (iii) peneplain surfaces. Cover sediments place these landscapes to the (i) Devonian, (ii) Carboniferous/Permian/Triassic and (iii) Late Triassic periods.

Critical analysis between the Ossa-Morena Zone and the Galicia-Trás-os-Montes Zone, Iberian Massif: Geochemical and geochronological comparison and geodynamic implications

Recent studies have suggested the existence of a correlation between the allochthonous units of NW Iberia (Galicia-Trás-os-Montes Zone - GTMZ) with SW Iberia (Ossa-Morena Zone – OMZ) in the European Variscan Belt. Such proposed equivalence is based on lithostratigraphic, tectono-metamorphic, geochronological, and geochemical affinities between these two domains, and implies that both zones had the same Neoproterozoic and early Palaeozoic basins, recording the same magmatic episodes and enduring the same tectono-metamorphic processes related to the Variscan Orogeny. As so, in order to better understand the evolution of these domains during the Ediacaran-Cambrian period, a comprehensive geochemical, isotopic and geochronological analysis of the metasediments that make up the OMZ and the allochthonous units of the GTMZ was carried out. The results show that the metasediments of OMZ and the Lower Allochthonous units of GTMZ present similar geochemical and geochronological features, having been likely deposited near each other, although small, but specific differences between both domains suggest that they do not correspond to the same depocentre. However, no geochemical, isotopic or geochronological correlation can be established with any of these domains and the Upper Allochthonous units of GTMZ. These metasediments display quite exotic features and, as such, were likely deposited in a different narrower basin, located to the W of the basins where OMZ and Lower Allochthonous units of GTMZ were deposited, thus providing precise constraints on the paleogeographic reconstruction of these domains during the Neoproterozoic to Early Ordovician dismembering of Gondwana.

Control of Hercynian Orogeny on basin evolution on the southern margin of the Tethyan region: A review and new insights

The Hercynian Orogeny was caused by the collision of Gondwana and Laurussia in the Mid‐/Late Carboniferous–Early Permian. This orogeny strongly influenced the pattern and evolution of basins and hydrocarbon accumulation on the southern margin of the Tethyan region (i.e., North Africa and the Middle East). Research on the Carboniferous–Early Permian Hercynian Orogeny is of great value in both plate tectonic reconstructions and hydrocarbon exploration. For its influence on basin evolution, some previous studies have considered the modification of the early Palaeozoic basins, but there has been less work on the evolution of the post‐Hercynian Mesozoic basins. We review the available data and re‐analyse the spatiotemporal impacts of this key tectonic event on the evolution of the basins in this region. The Carboniferous–Early Permian Hercynian Orogeny was an important turning point in basin evolution in North Africa and the Middle East. It interrupted the stability of the Gondwana platform and heralded a huge change in the pattern of basins from the uniform pre‐Hercynian Palaeozoic basins to the post‐Hercynian Mesozoic basins characterized by strong segmentation. The Carboniferous–Early Permian Hercynian Orogeny not only modified the pre‐Hercynian Palaeozoic basins but also controlled the distribution and evolution of the post‐Hercynian Mesozoic basins. Large‐scale uplifts were generated among the basins through modification of the inherited palaeohighs and five large NE‐trending arches were newly formed. These uplifts divided the early unified basin into multiple separate basins. Five large‐scale, almost equidistant, NE‐trending arches were generated on the northern margin of Gondwana (the Allala, Sirte, Levant, Al‐Batin and Oman arches), forming the present‐day Palaeozoic Basin pattern with an alternating distribution of uplifts and depressions. The northern part of the large, NE‐trending Hercynian arches collapsed as a result of the extensional stress caused by the opening of the Neotethys Ocean in the Late Permian–Triassic and formed Mesozoic basin depocenters (the Oued Mya–Pelagian basins, the Sirte Basin, the Eastern Mediterranean Basin and the Central Arabian Basin). These depocenters are important oil and gas enrichment zones. The NE‐trending Hercynian arches, the Mesozoic basin depocenters, the opening of the Neotethys Ocean and the segmentation of the basins by the Alpine Orogeny all show relatively consistent east–west‐directed segmentation on the northern margin of Gondwana. This shows that they may have a unified dynamic genetic background. The zone of structural weakness formed by the collision of multiple terranes during the Pan‐African Orogeny may have controlled this segmentation. The inheritance between reformation of the pre‐Hercynian Palaeozoic basins and influence on the post‐Hercynian Mesozoic basins by Carboniferous–Early Permian Hercynian Orogeny, is the result of the tectonics repetitively focused in pre‐existing weaker domains, under the lithospheric mantle modification in the polyphase assembly and break‐up of the supercontinent.

Migration of the Palaeozoic magmatic front from Zagros to Alborz mountains with progressive closure of the Palaeo-Tethys Ocean; Insights from Zagros detrital zircon U[sbnd]Pb age and Hf isotopic composition

The northwest-southeast-trending Zagros Fold-and Thrust Belt in south Iran resulted from the late Oligocene to the early Miocene collision of the Arabian and Central Iran blocks. The Zagros Palaeozoic basin holds the key to reveal the tectonic setting of the earlier stages in the development of the Tethyan realm. We aim to document the Palaeozoic tectonic environment of the Zagros Basin and provide critical constraints on the progressive closure of the Palaeo-Tethys. This work presents a combined detrital zircon UPb age and Hf isotopic composition from the Zagros and Alborz basins in an attempt to identify and date major geologic events along the southern margin of the Palaeo-Tethys and reconstruct the regional tectonic history of the northern margin of Gondwana-Land during the Ediacaran-early Palaeozoic interval. Detrital zircons UPb age and Hf-isotope ratio from Zagros second-cycle Devonian and third-cycle Permian sandstones and Alborz Palaeozoic strata indicate main magmatic flare-ups in the Arabian-Nubian Shield including the Tonian island arc magmatism, the Ediacaran post-orocline-tightening magmatism, the late Ediacaran-early Cambrian Protogonos continental arc magmatism. In addition, detrital zircons recorded the Ordovician continuation of the arc activity in the Protogonos during the early Palaeozoic. Therefore, repetitive recycling of the Neoproterozoic crystalline basement and related sedimentary cover of the Arabian-Nubian Shield, i.e., the Saharides, is a plausible source of the Devonian and Permian strata in Zagros. Our results show a close geological tie between Alborz and other Gondwana-Land-derived units in terms of Palaeozoic detritus sources which is in concordance with the location of the suture along the northern margin of the Alborz Mountains. Separation of the Alborz from the Sanandaj-Sirjan zone is accomplished by inserting from the east, the Central Iranian Blocks (Yazd, Posht-e Badam, Tabas, Lut) by strike-slip stacking and anti-clockwise rotation. This created the extensional zone of Northwest Iran.

Přirozená radioaktivita sedimentárních hornin v severovýchodní části moravskoslezského paleozoika

The paper provides detailed information on the natural radioactivity of sedimentary rocks in the northeastern part of the Moravo-Silesian Palaeozoic Basin in the Bohemian Massif. The studied area is dominated by the Lower Carboniferous flysch sediments belonging to the Horní Benešov, Moravice, and Hradec-Kyjovice formations (from botttom to the top). These three flysch formations consist of siliciclastic sediments, mainly graywackes, siltstones, silty shales, and clay shales, while conglomerates are less abundant. The overlying sediments of the Ostrava Formation (Namurian) occur on the eastern margin of the studied area. The Ostrava Formation has a character of a paralic coal-bearing molasse with alternating beds of sandstone (or arkose), siltstone and claystone. The sediments often have an elevated content of organic matter, concentrating into coal seams. The rocks of the Ponikev Formation (Devonian) occur in a narrow belt in the western part of the studied area. The Ponikev Formation consists mainly of silicites. Potassium, uranium and thorium contents were measured using a laboratory gamma-ray spectrometer in 1612 rock samples from 672 localities on five map sheets (1 : 50 000 scale). The calculated values of mass activity of 226Ra equivalent (am) demonstrate that the natural radioactivity of sedimentary rocks of all three flysch formations is lower or similar to that of the average Earth crust (about 180 Bq . kg–1). The radioactivity of flysch sediments increases with decreasing grain size from psephites (average am is 122 Bq . kg–1) to psammites (144 Bq . kg–1) and to aleurites + pelites (188 Bq . kg–1). Siltstones and claystones with a low organic matter content (226 Bq . kg–1), coaly siltstones and coaly claystones (240 Bq . kg–1), coaly sandstones and arkose sandstones (232 Bq . kg–1), and coal (202 Bq . kg–1) of the Ostrava Formation show slightly higher average am values. The relatively high radioactivity of these sediments is related to the elevated content of uranium (7.9 ppm on average).

Age constraints of the Guttulapollenites hannonicus–Cladaitina veteadensis Biozone in Argentina reveal the first record of Early Triassic (Olenekian) palynofloras in Western Gondwana

The upper Palaeozoic basins of central-western Argentina include continuous fossiliferous successions spanning the Carboniferous-Permian interval. The palynostratigraphic biozones comprise Late Mississippian, Pennsylvanian and Cisuralian assemblages. Recently, new palynofloras of the La Veteada Formation were referred to the Lopingian (late Permian). However, they are characterized by spores and tetrads of Lundbladispora spp. and Densoisporites spp., pollen grains of Lunatisporites pellucidus, L. noviaulensis and Protohaploxypinus samoilovichii, and the alga Syndesmorion stellatum, that distinguish the post-Permian recovery floras worldwide. A new uranium-lead chemical abrasion-isotope dilution-thermal ionization mass spectrometry (U-Pb CA-ID-TIMS) age confirms the Olenekian age of this stratigraphic unit and allows the identification of the first Early Triassic palynofloras in this region of western Gondwana. Comparison and correlation with similar assemblages from the southern and northern hemispheres supports the Early Triassic turnover with an increase of lycopsid cavate spores associated with some diagnostic species of taeniate and non-taeniate bisaccate pollen.

REVIEW OF THE LOWER-PALAEOZOIC RESERVOIR PROSPECTIVITY OF W. IRAQ WITH REFERENCE TO JORDAN

The Palaeozoic basin of Jordan and Iraq occupies the NW part of the Arabian Plate. The Lower Palaeozoic succession is the most under-explored succession in Iraq, particularly in western Iraq where it is considered the most promising target for future exploration. The western Iraqi subsurface and Jordan geology are characterized by similar major chrono-stratigraphic rock units. In western Iraq, the Akkas-1 well is the only gas and condensate discovery in sandstone reservoirs of Early Silurian and Late Ordovician age. Lower Silurian sandstones of the Akkas Formation are an oil reservoir characterized by a mean porosity of 6.5% and permeability of 0.2mD. Underlying Upper Ordovician sandstones of the upper part of the Khabour Formation are also reservoir, but for sweet gas and condensate. The latter are comparable to both the Risha and Wadi Sirhan discoveries in Jordan. Several deeper potential sandstone reservoirs (of older Ordovician and Cambrian ages) are also present across the region. Equivalent Ordovician age Jordanian reservoirs are primarily developed in homogeneous, highly mature quartz arenites of storm-dominated clastic-shelf stratigraphies. Within the Risha Field and the Wadi Sirhan area, are also end-Ordovician (Hirnantian) glaciogenic and periglacial sandstones with more variable reservoir quality characteristics compared to the underlying quartz arenites. These often comprise highly immature arkoses (as diamictites) that provide a mineralogically much less stable assemblage in which there is a possibility of developing secondary porosity. Sedimentological and XRD mineralogical studies suggest that diagenesis is an important factor in developing of reservoir quality. Nevertheless, reservoir quality is a major exploration risk where burial was significant that led to quartz overgrowth and clay cementation. It is notable that some sandstone reservoirs have chlorite-clay coatings, which prevented formation of the quartz overgrowth and preserved good reservoir quality. The widespread occurrence of associated source and seal rocks together with the proven and potential reservoirs, suggests that Lower Palaeozoic prospects below the deserts of western Iraq are promising targets for future hydrocarbon exploration.

The Atlas-East Variscan -Elbe shear system and its role in the formation of the pull-apart Late Palaeozoic basins

The Variscan orogeny occurred as a result of the Late Devonian to Late Carboniferous collision and accretion of Gondwana-derived microcontinents and continental masses with those of Laurussia. The irregular boundaries of the colliding continents caused isochronous transpressional and transtensional tectonics, accompanied by a complex pattern of intracontinental shear zones at the scale of the southern European Variscides. These shear zones and their configuration controlled the subsequent evolution of Permian to Middle Triassic paleogeography. The geographic distribution, from Morocco to the Eastern Alps, of the Late Carboniferous–Permian up to Triassic basins, most of which are considered as pull-apart basins, was related with the development of the Late Palaeozoic intracontinental shear network. Our analysis of the stratigraphic, tectonic and volcanic features of the Late Carboniferous/Permian continental basins across the Laurussia/Gondwana boundary reveals the role of the East Variscan Shear Zone during this time as a precursory lineament for the development of the Permian to Triassic rifting of Pangaea and the following opening of oceanic basins (e.g., the Neothetyan Ocean).

In-situ stress partition and its implication on coalbed methane occurrence in the basin–mountain transition zone: a case study of the Pingdingshan coalfield, China

The basin–mountain transition zone presents complex geologic structures and non-uniformly distributed in-situ stress. Studying the spatial distribution laws of in-situ stress and their influences on coalbed methane (CBM) occurrence in coal seams plays a significant role in CBM extraction and prevention of coalmine disasters. Based on the actual measured in-situ stress data, CBM content and gas pressure data in the Pingdingshan coalfield, located in the basin–mountain transition zone in the south of the late Palaeozoic basins in the North China block, this research investigated the distribution characteristics of geologic structures and partition of in-situ stress as well as the effects of in-situ stresses on CBM occurrence in the research area using evolution theories of geologic structure and a statistical analysis method. The research results show that geologic structure and in-situ stress distribution in the research area have obvious partition characteristics. The research area is divided into three tectonic zonations. In-situ stress distribution is controlled by tectonic types and tectonic stress field evolution of different tectonic zonations, which are divided into high tectonic stress zonation, tectonic stress zonation and vertical stress zonation from east to west. Also, the research results reveal the characteristics of each stress zonation and the relationship between CBM occurrence and in-situ stress in this research area.

Artificial neural networks as a tool for pattern recognition and electrofacies analysis in Polish palaeozoic shale gas formations

Unconventional oil and gas reservoirs from the lower Palaeozoic basin at the western slope of the East European Craton were taken into account in this study. The aim was to supply and improve standard well logs interpretation based on machine learning methods, especially ANNs. ANNs were used on standard well logging data, e.g. P-wave velocity, density, resistivity, neutron porosity, radioactivity and photoelectric factor. During the calculations, information about lithology or stratigraphy was not taken into account. We apply different methods of classification: cluster analysis, support vector machine and artificial neural network—Kohonen algorithm. We compare the results and analyse obtained electrofacies. Machine learning method–support vector machine SVM was used for classification. For the same data set, SVM algorithm application results were compared to the results of the Kohonen algorithm. The results were very similar. We obtained very good agreement of results. Kohonen algorithm (ANN) was used for pattern recognition and identification of electrofacies. Kohonen algorithm was also used for geological interpretation of well logs data. As a result of Kohonen algorithm application, groups corresponding to the gas-bearing intervals were found. Analysis showed diversification between gas-bearing formations and surrounding beds. It is also shown that internal diversification in gas-saturated beds is present. It is concluded that ANN appeared to be a useful and quick tool for preliminary classification of members and gas-saturated identification.

Geological setting and Ediacaran–Palaeozoic evolution of the western slope of the East European Craton and adjacent regions

A set of geological maps and geological cross-sections was prepared to document the geological setting of sedimentary basins developed on the western slope of the EEC and adjacent areas to the west. On the basis of these data and literature on the subject, the evolution of the sedimentary basins in the study area was reviewed, with special emphasis on the Ediacaran–Lower Palaeozoic basin. The basin originated during late Ediacaran rifting, related to the latest stages of breakup of the Precambrian super-continent Rodinia/Pannotia, associated with large-scale igneous activity. The rifting ultimately led to the formation of the Tornquist Ocean and subsequently, during the latest Ediacaran to Middle Ordovician, the SW margin of the newly formed Baltica became a passive continental margin. The upper Cambrian depocentre in the Bilgoraj-Narol Zone and the Łysogory Block tentatively is interpreted as a small, narrow foredeep, related to the docking of the Malopolska Block to the western margin of Baltica. From the Late Ordovician through the Silurian, a gradual change to a collisional tectonic setting is observed across the entire SW margin of Baltica, as well as in the zones adjacent to it from the west, which together became the site of development of the extensive Caledonian foredeep basin, related to the convergence and collision of Avalonia and Baltica. The oblique character of the collision resulted in a prominent diachronism in the development of the foredeep basin. This refers to the initiation of basin subsidence, the starved basin phase, the main phase of rapid subsidence and supply of detritus from the west, and the termination of basin development. The Early Mississippian (Bretonian) phase of uplift and erosion and, to a lesser degree, also the Late Pennsylvanian one significantly affected the structure of the western EEC. During the Mississippian, extensive magmatic activity took place at the SW margin of East European Craton, in the region referred to here as the Baltic-Lublin Igneous Province.

Interpretation of a Permian conjugate basin margin preserved on the outer Northwest Shelf of Australia

The Northwest Shelf (NWS) of Australia is characterized as a series of northeast-southwest trending Mesozoic offshore depocenters which both juxtapose and partially overprint a series of onshore, northwest-southeast trending Palaeozoic basins. An integrated interpretation of well bore data, regional seismic data and plate tectonic models suggests that the Palaeozoic section is also present below the Mesozoic depocenter. Referred to as the East Gondwana Interior Rift, the primary rift axis is oriented in a (present day) NE-SW direction, with orthogonal marginal rift basins such as the onshore Canning and Southern Carnarvon basins. While precise age dating for the formation and stratigraphy of the axial rift is speculative, our integrated interpretation suggests that a significant portion of the pre-existing rift was modified by a Mid-Permian extensional event, forming the Northern Carnarvon basin. Interpretation of recently acquired 3D reflection seismic data suggests that the conjugate basin margin from this Permian rifting event is preserved, and is visible below the Mesozoic section. A series of back-stepping, Late Permian carbonate ramps and banks is interpreted to form on a thermally subsiding rift flank. Our interpretation of these carbonate banks is based primarily on seismic geometries, and is supported by area well control and regional paleogeographic models. This interpretation suggests that a deep marine intra-continental basin bisected the NWS in the Late Permian. Shallow marine conditions then persisted across the conjugate margin through the Triassic and into the Jurassic. After Late Jurassic rifting associated with Gondwanan break-up, the region subsided into deep water.

Геологическая модель Предъенисейского верхнепротерозой-палеозойского осадочного бассейна на юго-востоке Западно-Сибирской провинции

Геологическая модель Предъенисейского верхнепротерозой-палеозойского осадочного бассейна на юго-востоке Западно-Сибирской провинции

How far did the Cadomian ʽterranesʼ travel from Gondwana during early Palaeozoic? A critical reappraisal based on detrital zircon geochronology

ABSTRACTIn this paper, laser ablation ICP-MS U–Pb detrital zircon ages are used to discuss provenance and early Palaeozoic palaeogeography of continental fragments that originated in the Cadomian–Avalonian active margin of Gondwana at the end of Precambrian, were subsequently extended during late Cambrian to Early Ordovician opening of the Rheic Ocean, and finally were incorporated into and reworked within the European Variscan belt. The U–Pb detrital zircon age spectra in the analysed samples, taken across a late Neproterozoic (Ediacaran) to Early/Middle Devonian metasedimentary succession of the southeastern Teplá–Barrandian unit, Bohemian Massif, are almost identical and exhibit a bimodal age distribution with significant peaks at about 2.1–1.9 Ga and 650–550 Ma. We interpret the source area as an active margin comprising a cratonic (Eburnean) hinterland rimmed by Cadomian volcanic arcs and we suggest that this source was available at all times during deposition. The new detrital zircon ages also corroborate the West African provenance of the Teplá–Barrandian and correlative Saxothuringian and Moldanubian units, questioned in some palaeogeographic reconstructions. Finally, at variance with the still popular concept of the Cadomian basement units as far-travelled terranes, we propose that early Palaeozoic basins, developed upon the Cadomian active margin, were always part of a wide Gondwana shelf and drifted northwards together before involvement in the Variscan collisional belt.

Significant hiatuses in the terrestrial Late Variscan Central and Western Bohemian basins (Late Pennsylvanian–Early Cisuralian) and their possible tectonic and climatic links

Abstract Significant changes in the stratigraphy of the Central and Western Bohemian Upper Palaeozoic basins occur during or shortly after hiatuses. The different extent and changes in the depocentres of the Radnice and Nýřany members (Moscovian) in the Plzeň Basin clearly indicate changes in the structure of this basin taking place during a break in sedimentation between these two units (311.9–308.3 Ma). Thick weathered rocks that occur in boreholes in the Mšeno–Roudnice Basin indicate another sedimentation break (305.9–304.1 Ma) between the Nýřany Member and the Týnec Formation (Kasimovian). Another possible hitherto undiscovered hiatus occurred between the Týnec and Slaný formations (Kasimovian–Gzhelian, about 304–303 Ma). The most significant changes in the configuration of the basins occurred between the Slaný and Líně formations (Gzhelian–Asselian, 301.6–300.6 Ma). This is indicated by deeply cut river valleys at the top of the Slaný Formation, by high thickness of weathered deposits occurring immediately beneath the Líně Formation, and mainly by the shift of depocentres from the southern to the northern part of the Central Bohemian basins. The hiatuses between the Radnice and Nýřany members and between the Slaný and Líně formations are accompanied by significant changes in the depocentres, and they are therefore interpreted primarily as tectonic events related to the extensional collapse of the Variscan orogenic belt. By contrast, the hiatuses beneath and above the Týnec Formation are interpreted as being the products of lower sedimentation rates during drier climates, which is consistent with the characteristics of the sediments, correlation with other Central European basins, and with climate models for this period. Due to the characteristics of the Líně Formation, in which the effects of climate aridization are clearly seen, the authors presume that tectonic as well as climatic changes occurred at the hiatus between the Slaný and Líně formations.

Geochemistry and Nd isotope signature of the Collserola Range Palaeozoic succesion (NE Iberia): Gondwana heritage and pre-Mesozoic geodynamic evolution

AbstractThe Collserola Range includes a representative stratigraphic succession of the Palaeozoic of the central part of the Catalan Coastal Ranges, ranging from Cambro-Ordovician to Carboniferous times. In this paper we present an up-to-date review of the stratigraphy and structure of the Palaeozoic of the Collserola Range, and provide geochemical and Sm–Nd isotope data to constrain the pre-Mesozoic crustal evolution of this sector of the Variscan Belt. Geochemical compositions indicate that the Palaeozoic siliciclastic rocks of the Collserola Range were fed by a relatively mature heterogeneous source of sediment, comprising quartz-rich sediments to intermediate igneous rocks. The siliciclastic rocks of the Collserola Range show great geochemical affinity with the turbidites of passive margins. The Sm–Nd signature of the siliciclastic rocks is compatible with those of the Palaeozoic and late Proterozoic fine-grained siliciclastic rocks of the neighbouring terrains of SW Europe. There is a small decrease of the εNdT with decreasing age of sedimentation from the Cambro-Ordovician to the Carboniferous, suggesting an increase of the amount of more ‘juvenile’ material. The presence of small volumes of alkaline basaltic rocks provides evidence for the input of juvenile material in the early Palaeozoic basin and suggests that an extensional tectonic regime prevailed during the Cambro-Ordovician sedimentation. From a geodynamic point of view, the overall analysis of the data implies that the Palaeozoic rocks of the Catalan Coastal Ranges were part of the Northern Gondwana passive margin before the closure of the Rheic Ocean and the subsequent Variscan Orogeny.

Structure of the Paleozoic basement in the Senegalo-Mauritanian basin (West Africa)

Abstract The interpretation of 2D seismic surveys from the Senegalo-Mauritanian basement and the reappraisal of rock-samples from oil exploration drillings provide new insights into the structure of the Appalachian-Mauritanian belts and the overlying Carboniferous and Permian basins. Two different units have been evidenced below the Mesozoic sedimentary cover : a lower unit (Unit 1 : basement) and an upper unit (Unit 2 : Late Palaeozoic basins). Unit 1 shows two distinct tectonic areas : the northern area with two different belts evidenced on both sides of the Senegalese block and the southern area exhibiting various tilted blocks making part of the Palaeozoic (Ordovician to Devonian) Bové basin. In the northern area the Western Thrust Belt is thrust over the Senegalese block while to the east, the so-called Mauritanian belt is thrusted over the West African craton. In the Mauritanian belt area, the youngest deformed sediments belonging to the outcropping Bove basin are Famennian in age. The Mauritanian inner belt underwent two tectono-metamorphic events (circa 330 and 270 Ma). Unit 2 which is unconformably capping both the Western Thrust Belt and the southern tilted blocks of the Paleozoic Bové basin (Casamance basin) is locally affected by eastward and westward-verging thrusts. Unit 2 is dated to the Early Permian by palynomorphs, and could have formed between the early (340–320 Ma) and late (270 Ma) Variscan tectonic events. The subsurface Late Palaeozoic basins from Senegal and Mauritania are interpreted as intra Variscan belt basins, similar to those suspected by seismic investigations off shore Guinea and Mauritania.

The nature of the Palaeozoic oceanic basin at the southwestern margin of Gondwana and implications for the origin of the Chilenia terrane (Pichilemu region, central Chile)

The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane.

2013 PESA industry review: exploration

Exploration activity in Australia in 2013 occurred across a broad spectrum of conventional and unconventional plays. Competition for acreage was buoyant with large tracts of key onshore basins either licensed or under application. Offshore, there were new awards on the western Australian margin and in the Bight Basin off SA. Offshore 3D seismic acquisition was reduced from anomalously high levels in 2012. Onshore 2D seismic acquisition was at historic highs and onshore 3D was the most ever recorded. Overall drilling levels were maintained despite a decline offshore. Of 13 offshore wells drilled, six were discoveries. Sixty-nine exploration wells (excluding CSG wells) were drilled onshore. Fifty addressed conventional, and 19 were unconventional shale or basin-centered gas targets. Sixty of the 69 wells were drilled in the Cooper/Eromanga Basin where conventional oil and gas exploration yielded 11 oil and six gas discoveries. Drilling and fraccing campaigns in the Nappamerri Trough unconventional gas plays provided early encouraging results. 213 exploration and appraisal CSG wells were drilled in the CSG basins of Queensland and NSW. In Queensland a record total of 1,317 CSG wells were drilled in fiscal year 2012/2013. Shale gas exploration activity was increasingly focused on the Palaeozoic and Proterozoic Basins of Western, Central and Northern Australia with major oil and gas companies involved in joint ventures preparing for drilling in 2014. The results of these programmes will have an important bearing on the future direction of exploration in these plays.

Geothermal energy in sedimentary basins in the UK

Deep onshore Mesozoic basins have favourable geothermal aquifers at depth comprising basal Permo-Triassic sandstones. The principal basins are the Wessex and Worcester (southern England), Cheshire (northwest England), Eastern England, Larne and Lough Neagh (Northern Ireland). Measured temperatures are up to 80 °C and could reach 100 °C in the deepest parts of some of the basins. Porosity and permeability data from depth are limited, but values high enough to allow adequate yields have been measured in many of the basins. Productive sandstones vary from a few tens of metres to hundreds of metres thick resulting in productive transmissivities. The estimated heat in place (Inferred Geothermal Resource) has been calculated as 201 × 1018 to 328 × 1018 J. New heat demand maps illustrate that many of the centres of high heat use are coincident with Upper Palaeozoic basins. Within the Carboniferous and Devonian there are thick sequences of deeply buried arenaceous deposits. Some productive local aquifers occur at shallow depth, but most depend on fissure flow that is anticipated to diminish rapidly with depth. The exception may be the Carboniferous Limestone where warm springs and a pronounced thermal anomaly in Eastern England demonstrate groundwater flow at depth, possibly along pathways of many kilometers.