Onshore Sector Research Articles

The Cretaceous post-rift series from the Portuguese onshore ranges of the West Iberian Margin and their history of research

The thick post-rift series that covers the Triassic to Lower Cretaceous sedimentary succession of the Lusitanian Basin, in west central Portugal, is crucial to understanding the Mesozoic evolution of the West Iberian Margin, motivating a long history of studies, including those by Daniel Sharpe (1806–56). They record platform carbonates interbedded with marginal marine and alluvial facies. The post-rift infill was deposited above a late Aptian breakup unconformity and is organized into three unconformity bounded sequences: ‘UBS-4A’ (upper Aptian–upper Cenomanian), ‘UBS-4B’ (lower Turonian–lower Campanian) and ‘UBS-5B’ (lower Campanian–Maastrichtian). The first comprises alluvial siliciclastics and shallow-marine, carbonate sequences rich in Tethyan faunas with ammonites and rudists. During the late Cenomanian, the carbonate platform entirely covered the onshore sectors of the Lusitanian Basin. Several diapiric axes controlled the differentiation of inner platform areas with corals and rudists. The infill of ‘UBS-4B’ occurred after the tectonic uplift and emersion of the sectors located to the south of first-order Nazaré Fault Zone. The platform carbonate facies were overlain by regressive micaceous and alluvial coarse siliciclastics. The sequence ‘UBS-5’ overlies a lower Campanian unconformity and has a Campanian–Maastrichtian thick record of alluvial to lagoonal, reddish pelites, rich in vertebrate assemblages.

Dynamics of extreme wind events in the marine and terrestrial sectors of coastal Antarctica

AbstractAntarctic coastal surface winds affect ice‐sheet stability, sea ice, and local ecosystems. The strongest coastal winds are especially important due to the nonlinear relationship between wind speed and wind stress. We investigate the dynamics of extreme coastal winds using a simplified momentum budget calculated across the period 2010–2020 from the ERA5 reanalysis. The pressure‐gradient forcing term in the budget is decomposed into a large‐scale component and one associated with the temperature deficit layer. The role of budget terms across the coastal sector is compared for weak and strong winds. We then calculate composites of the top 100 easterly wind events across six east Antarctic coastal sectors, identifying terms responsible for the evolution of coastal extremes. A simple balance of terms exists offshore, dominated by large‐scale forcing, contrasting with the complex balance in the onshore sector where katabatic forcing is large. Large‐scale forcing explains 57% of offshore coastal wind‐speed variance overall, improving to 81% when budget terms associated with the temperature deficit layer and horizontal advection are included, with significant regional variation. The residual term plays an increasingly active role as wind speed increases. Extremes in all coastal sectors are associated with a synoptic‐scale transient dipole of pressure anomalies driving warm‐air advection. Although katabatic forcing is a very large term in magnitude, it is found to play a passive role, declining as wind speeds increase during extreme conditions. In some regions, an anomalous southerly component develops during extremes, which we attribute to an ageostrophic barrier wind. This research underscores the major role for large‐scale forcing in Antarctica's coastal winds, but also reveals a significant regional locally driven component. The results have implications for improving numerical model simulations of coastal easterlies and for studying their impacts on ocean circulation, sea ice, and ice‐shelf basal melt.

Late Pleistocene to Holocene glacio-eustatic history as recorded in the Pescara paleovalley system (Central Italy, Adriatic basin)

A buried paleovalley system, about 50 m deep and 2 km wide, is documented from the Pescara coastal plain. Based on stratigraphic, sedimentological, paleontological, chronological and geotechnical data, the paleovalley profile and 3D facies architecture of the paleovalley fill (PVF) were reconstructed.The lowermost PVF is a laterally extensive fluvial gravel body, up to 13 m-thick, that represents the lowstand systems tract (LST; pre-11.3 ka cal BP). Above lowstand deposits, the transgressive systems tract (TST), 21 m-thick, shows a deepening-upward trend, from freshwater/inner-estuarine to brackish/outer-estuarine facies associations (11.3–8.0 ka cal BP). The upper part of the succession (highstand systems tract – HST) shows a shallowing-upward tendency from paludal to fluvio-deltaic deposits.Seven millennial-scale parasequences (Ps) were identified within the Pescara Holocene (TST + HST) succession. Transgressive Ps1-3 exhibit a distinctive retrogradational stacking pattern. Highstand Ps4-7 are aggradationally-to-progradationally stacked. During the aggradational phase (P4), the estuary was gradually filled and swamp environments spread onto the valley interfluve. Because of subsequent progradation (Ps5-7), delta plain conditions established. In the research core, TST parasequences show higher accumulation rates (up to 9.4 mm/y) than HST ones (1.3–1.8 mm/y). Thus, the study area evolved from a region of sediment storage (11.3–8.0 ka cal BP) into a sector of prevalent sediment bypass (last 8.0 ky).Major early Holocene flooding events were possibly triggered by Melt-Water Pulses (MWPs) 1B, 1C and 1D. The eustatic rise linked to MWP-1B reasonably caused the P1 flooding event (11.3 ka cal BP). Post-MWPs 1C and 1D sea-level rises likely provoked the complete drowning of the paleovalley system and the subsequent maximum landward migration of the shoreline (about 8.0 ka cal BP).This study provides new evidences, in terms of sedimentary response, of the poorly-documented MWPs 1C and 1D, and the first documentation of MWP-1B eustatic effects in an onshore sector of the Central Adriatic.

Offshore and coastline migration of radio‐tagged Nathusius' pipistrelles

AbstractThe wind energy‐bat conflict is well documented for the onshore sector, with high numbers of casualties, specifically for migratory bat species. Offshore wind turbines might be a threat to bats as well, yet offshore bat migration is poorly documented. Accordingly, potential conflicts between bat conservation and offshore wind energy production are difficult to evaluate. Here, we used automated radio‐telemetry to track 50 km continuous offshore movements of two Nathusius' pipistrelles (Pipistrellus nathusii) within the Motus network. After crossing the marine waterbody, tagged bats traveled over several hundred kilometers along the coastline from Germany towards the Netherlands and Belgium. Our study highlights the possibility for migratory bats to collide with offshore and coastal wind turbines. Therefore, we plead for implementing pre‐ and post‐construction surveys and adequate mitigation schemes at offshore wind turbines in sensitive areas of the North and Baltic Sea if not already practised.

A new concept for the Paleogene lithostratigraphy in the onshore part of the Dolna Kamchiya Basin (eastern Bulgaria) on the basis of 3D modeling

This study is based on primary lithological data from 96 boreholes and lithostratigraphic interpretation of 17 seismic profiles from the onshore part of the Dolna Kamchiya Basin, which belongs to the onshore sector of the Moesian Platform and comprises the south-easternmost part of the Southern platform zone. Nine formal Paleogene lithostratigraphic units were recognized (the Byala, Komarevo, Dvoynitsa, Beloslav, Dikilitash, Aladan, Avren formations, the Dolni Chiflik Member of the Avren Formation, and the Ruslar Formation). For visualization of their spatial relationships and revealing the deep structure of the basin, a 3D lithological model was created. In addition, new data on the lithology, thicknesses and ages of the units were provided.

Hydrocarbon leads and prospects opportunities across a cluster of fields in parts of Onshore Niger Delta Basin

The increase in oil and gas production over the years from already discovered fields at relatively shallower intervals, and corresponding decrease in the discovery of new fields has led to observable decline in the available hydrocarbon reserves within the Niger Delta Basin of Nigeria. Therefore, there is need to grow or add to the existing reserves through “searching wider and drilling deeper” across the onshore sector of the Niger Delta Basin, which is the focus of this work. Although, there has been several hydrocarbon prospectivity studies in the Onshore Niger Delta Basin, most of these studies were conducted on individual fields, except where adjacent fields belonged to the same operator. This has provided limited information on stratigraphic intervals, structural features and reservoir zones continuity across several fields. This study is aimed at interpreting recently acquired within the Coastal Swamp Depobelt of the Niger Delta, long-cable regional 3D seismic data over nine producing fields, and data from twenty-five wells, using sequence stratigraphic, structural and reservoir exploration tools, thus providing a regional scale interpretation and information that will unravel new prospective zones across these fields primarily based on amplitude supported structural plays. Results from stratigraphic studies, show the occurrence of regional extensive chrono/sequence stratigraphic surfaces such as maximum flooding surfaces (7.4–12.8 Ma) and sequence boundaries (8.5–13.1 Ma) that depict sediment packages deposited during the Middle-Miocene through Upper Miocene age. Well log sequence stratigraphic and reservoir correlation studies reveal that depositional sequences, bounded by these surfaces, comprise reservoir and non-reservoir packages within their systems tracts. Reservoir architectural studies show sands of lowstand, transgressive, and highstand systems tracts that make up the reservoir units, are relatively thick, laterally continuous and quite extensive across the area. Interpreted seismic sections unravelled the influence of structure on stratigraphy as sediment packages show variable thicknesses across faults. This was linked to syndepositional deformation associated with sediment deposition within the basin. Mapped reservoir tops at intermediate and deeper intervals reveal the dominance of fault-dependent closures. In addition, root mean square amplitude extraction done on these maps, unravelled eight structurally controlled regional hydrocarbon prospects and eight leads with booming amplitude responses that are conformable to structures. Overall, this study has shown that there exist hydrocarbon prospective zones that cut across several fields at deeper depths that are yet to be drilled, which could contribute to the Niger Delta Basin's existing reserve base.

Self-expanded piles: A new approach to unconventional piles development

In onshore and offshore fields of ocean engineering, piles are used as foundation systems for various structures. Piles are classified into different types depending on their materials, geometries, and particularly, installation methods, which have advantages or limitations. Companies and engineers have developed a new group of piles, because of necessity to improve their performance in terms of increasing the bearing capacity, reducing impacts of traditional installation procedures, implementing by low- torque power equipment, and utilizing them in widely different ground conditions, including in a marine environment. In the present study, three different models of a new pile with an expander body are introduced to increase the shaft and pile-toe diameters and its self-expansion in the embedment depth under the titles of the Bubble pile (BP), Self-Expanded pile (SEP) and Wing pile (WP). The main subject of this research is to achieve increased bearing capacity, reduced installation effects, and decreased required installation torque. The frustum-confining vessel of Amirkabir University of Technology (FCV-AUT) was employed for this purpose. Up to 14 axial compressive and tensile load tests were carried out on different model piles on sand collected from Anzali shore located on the northern coast of Caspian Sea in Iran, with relative densities of 45% to 50% within FCV-AUT. Comparing the performance of introduced pile with traditional pile corresponding to the same characteristics, the results indicated a significant increase in the axial bearing capacity and reduced disturbance effect of the pile. Also, a lower installation torque of the SE pile was required compared to the helical pile. The test results also demonstrated that the new pile could resist considerable compressive and uplift loads, and could be a possible alternative to traditional piles in the onshore sector.

3D lithostratigraphic model of the Paleogene of the onshore part of the Moesian Platform (Northeast Bulgaria)

The Paleogene sedimentary rocks in the north-easternmost part of the territory of Bulgaria have been penetrated by numerous boreholes. In terms of regional tectonic zonation, the study area is a part of the onshore sector of the Moesian Platform, which partly includes the South Dobrogea Unit and the easternmost part of the North Bulgarian Dome with its eastern slope. The lithostratigraphy of the Paleogene successions consists of six formal units (the Komarevo, Beloslav, Dikilitash, Aladan, Avren, and Ruslar formations) and one informal unit (glauconitic marker). For compiling an overall conception of the regional aspects (lithology, thickness, spatial distribution, and relationships) of the individual lithostratigraphic units and for illustration of their spatial distribution, a 3D lithostratigraphic model based on reinterpretation of individual borehole sections has been created. The model database was compiled by integration of the original lithological data from 338 borehole sections.

New insights on the late Quaternary geologic evolution of the Ischia Island coastal belt based on high-resolution seismic profiles

The study of the seismo-stratigraphic architecture of Late Quaternary-Holocene volcanic and sedimentary units in the Ischia Island offshore and their correlation with the coastal units of the adjacent onshore sectors allowed for a detailed reconstruction of the seismic stratigraphy of the northern, southern and western sectors of the island. Five (southern Ischia) to nine (western Ischia) seismic units and related seismic facies were identified. The volcanic areas and their detailed seismo-stratigraphic setting fit well in the geological context of a passive margin and confirm that the stratal architecture in the Ischia Island Quaternary marine deposits is controlled by the interaction between glacio-eustatic fluctuations, volcanism, tectonics and sedimentary supply.

VS of the uppermost crust structure of the Campi Flegrei caldera (southern Italy) from ambient noise Rayleigh wave analysis

Shear wave velocities (VS) are defined in the uppermost 1–2km of the Campi Flegrei caldera through the non-linear inversion of the group velocity dispersion curves of fundamental-mode Rayleigh waves extracted from ambient noise cross-correlations between two receivers. Noise recordings, three months long, at 12 seismic stations are cross-correlated between all couples of stations. The experiment provided successful results along 54 paths (inter-stations distance), of which 27 sampled a depth > 1km.VS contour lines are drawn from 0.06km b.s.l. to 1km depth b.s.l. and show difference between the offshore (gulf of Pozzuoli and coastline) and the onshore areas. At 0.06km b.s.l., the gulf of Pozzuoli and the coastline are characterized by VS of 0.3–0.5km/s and of 0.5–0.7km/s, respectively. Such velocities are typical of Neapolitan pyroclastic soils and fractured or altered tuffs. The inland shows VS in the range 0.7–0.9km/s, typical of Neapolitan compact tuffs. Velocities increase with depth and, at 1km depth b.s.l., velocities lower than 1.5km/s are still present in the gulf and along the coastline while velocities higher than 1.9km/s characterize the eastern sector (grossly coincident with the Neapolitan Yellow Tuff caldera rim), the S. Vito plain and the area between Solfatara and SW of Astroni.Such features are much more evident along two cross-sections drawn in the offshore and onshore sectors by integrating our VS models with literature data. Our models join previous noise cross-correlation studies at greater scale at depths of 0.7–0.8km, hence the picture of the Campi Flegrei caldera is shown up to a depth of 15km. VS of about 1.7km/s, corresponding to compression velocities (VP) of about 3km/s (computed by using the VP/VS ratio resulted in the inversion), are found at depths of 1.1km, in the centre of the gulf of Pozzuoli, and at a depth of about 0.7km b.s.l. onshore. An increment of VS velocity (~1.9–2.0km/s) is locally observed onshore which might be attributed to a layer of tuffs and tuffites interbedded with thin lava beds, according to the correlation of VS with stratigraphies in the deep drillings of S. Vito.

Quaternary tectonostratigraphic evolution of the Vlora Basin, south‐western Albania

The Vlora Bay is a roughly north‐west–south‐east‐oriented tectonic depression located at the collision zone between the Apulian Foreland and the Albano‐Hellenic Chain in south‐western Albania. The bay exhibits a triangular‐shaped geometry, which is the result of different tectonic phases that occurred mostly between the end of the Neogene and the Quaternary. The bay consists of two distinct physiographic sections: a shallow water marine offshore area and a recently emerged onshore sector. Along the onshore sector, Pleistocene clastic sediments filling the Pleistocene Vlora Basin crop out due to the recent uplift phases. The sedimentological and stratigraphic organization of the Vlora Basin deposits and the role played by Quaternary tectonic structures in their deposition are the main topics of this paper. A detailed field survey, supported by a geoelectrical survey, reveals a series of unconformity‐bounded stratigraphic units deposited in two north‐west–south‐east‐oriented subbasins. The two subbasins are separated by an intrabasinal, fault‐bounded, pre‐Quaternary morpho‐structural high and are mainly filled by shallow marine to continental deposits. A detailed structural analysis shows different tectonic phases affecting the area during the Pleistocene. Additionally, the geophysical survey facilitates the reconstruction of the deep structure of the basin and provides a better definition of the basin margins. Based on both geological and geophysical data, a tectonostratigraphic evolutionary model is proposed for the Vlora Basin Pleistocene sedimentary infill. This model consists of three phases of sedimentation that were essentially controlled by fault activity and by the progressive uplift of the area.

Historic paths and future expectations: The macroeconomic impacts of the offshore wind technologies in the UK

Offshore wind power (OSW) plays a key role within the UK strategy for a transition towards a low-carbon economy, offering vast potential for establishing a high-tech manufacturing industry. Previous experiences in the onshore sector (OWP) suggest the UK might fail in fully capturing these macroeconomic benefits. In this work, we investigate the history of UK renewable policies, comparing its national strategy to those of other major OSW-export countries. Through the use of a numerical general equilibrium model, we quantify the macroeconomic impacts under three scenarios: a baseline, which relies on previous estimates and foresee limited local content; a ‘contamination’ scenario, where the UK content reaches the same levels of OWP; and a ‘non-myopic’ scenario, where investors expect governmental support to decrease or disappear, replicating a common path of past renewable policies. We identify the UK as a FDI-oriented country. Our results suggest that increasing the share of locally-sourced capital goods in OSP to OWP-level could generate larger income and employment effects in the UK economy. We find that under forward-looking investors the economic benefits are significantly lower than the case of myopic agents. Our results show an inherent conflict with stated purposes of UK policy for OSW.

Active faulting and continental slope instability in the Gulf of Patti (Tyrrhenian side of NE Sicily, Italy): a field, marine and seismological joint analysis

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called “Aeolian–Tindari–Letojanni” (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.

Buried marine-cut terraces and submerged marine-built terraces: The Carchuna-Calahonda coastal area (southeast Iberian Peninsula)

The Carchuna-Calahonda coastal area is located between the onshore Betic Cordillera and the Alboran Sea. Its onshore sector is formed by detrital sediments that cover a metamorphic basement mostly composed of marbles, contiguous to an offshore shelf setting. New onshore gravity data allow us to characterize the location of flat marine-cut terraces carved into the metamorphic bedrock, which are covered by detrital sediments. In addition, multibeam bathymetry data, 3.5kHz and sparker reflection seismic profiles, reveal offshore flat features linked to marine terraces that are related with the onshore buried marine-cut terraces. Gravity data are newly used to detect marine-cut terraces covered by sediments, enhancing the integration of onshore and offshore data. The marine terraces are distinguished based on the relative sea-level trend (regressive versus transgressive) and on the dominant sedimentary regime (erosional versus depositional). These data help constrain the ages of the marine terraces younger than 150ka, using available Late Quaternary sea-level curves. Although previous geodetic research suggests a rapid sinking of the Carchuna-Calahonda coast, the heights of the marine-cut terraces and depositional terraces are mainly driven by sea-level changes, not tectonics.

Stable isotope stratigraphy of the early Quaternary of borehole Noordwijk, southern North Sea

Between 2.58 Ma and 2.4 Ma, most of the present onshore sector of the Netherlands was an epicontinental sea. Previous studies suggest the occurrence of multiple cold and warm phases in shallow marine successions of the Maassluis Formation in conjunction with the first prominent Northern Hemisphere ice sheet expansions. Nevertheless, correlation with deep marine isotope stages is tentative and relies on relative dating as absolute dating techniques are difficult to apply. Here, we present for the first time an early Quaternary, high-resolution benthic stable isotope record from the onshore sector of the Netherlands, taken from borehole Noordwijk. We tune our record to MIS 100–94 using the characteristic δ18O signature and complementary biostratigraphic data. This new high resolution isotope chronology is providing important insights on stratigraphy and palaeoenvironmental development of the southern North Sea area during the early Quaternary.

Age constraints on the Late Cretaceous alkaline magmatism on the West Iberian Margin

The onshore sector of the West Iberian Margin (WIM) was the locus of several cycles of magmatic activity during the Mesozoic, the most voluminous of which was of alkaline nature and occurred between 70 and 100 Ma. This cycle took place in a post-rift environment, during the 35° counter-clockwise rotation of Iberia and initiation of the alpine compression. It includes the subvolcanic complexes of Sintra, Sines, and Monchique, the volcanic complex of Lisbon and several other minor intrusions, covering an area of approximately 325 km 2. Previous cycles were tholeiitic and transitional in nature, occuring around 200 Ma and 130–135 Ma, respectively. New LA-ICP-MS U-Pb, 40Ar/ 39Ar, K-Ar and Rb-Sr ages on several intrusions distributed along the onshore WIM are presented, which combined with previously published data allows us to constrain the duration of the Late Cretaceous alkaline cycle to circa 22 Ma (94–72 Ma) and define two pulses of magmatic activity. The first one (94–88 Ma) occurred during the opening of the Bay of Biscay and consequent rotation of Iberia and clusters above N38°20′. The second pulse (75–72 Ma) has a wider geographical distribution, from N37° to N39°. This final pulse occurred during the initial stages of the Alpine orogeny in Iberia that led to the formation of the Pyrenees and Betics and to tectonic inversion of the Mesozoic basins. Isotope and trace element geochemistry point to a sublithospheric source for the alkaline magmatism that clearly distinguishes it from the previous cycles which had an important lithospheric mantle component. Also, it allows the discrimination between the two different alkaline pulses in terms of trace element abundance and residual mantle minerology. It is speculated that these differences might be the result of distinct magma ascent rates due to either more or less favourable tectonic settings that avoided or allowed the interaction with metasomatized lithosphere and equilibration with K rich minerals like amphibole and/or phlogopite.

THE PETROLEUM SYSTEM IN THE SANDINO FOREARC BASIN, OFFSHORE WESTERN NICARAGUA

The underexplored Sandino Basin (Nicaragua Basin/Trough) is located within the forearc area of western Nicaragua and NW Costa Rica. Exploration activity since 2004 has focussed on the onshore sector of the basin, and has included the first drilling campaign for over 30 years. Recent 2D basin modelling of the offshore sector together with organic geochemical studies has attempted to reassess the basin's petroleum potential. Geochemical data from the deepest offshore well indicate that Middle Eocene to Lower Oligocene sediments of the Brito Formation, as well as Upper Oligocene to Lower Miocene sediments of the Masachapa Formation, may have source rock potential. A third and perhaps more significant potential source rock interval is associated with the Lower Cretaceous black shales of the Loma Chumico Formation, which has been studied in the adjacent forearc area in NW Costa Rica (Tempisque Basin) and is inferred to be present in the Sandino Basin.The thermal history of the forearc basin is controlled by the low basal heat flow (39 mW/m2). 2D modelling has shown that the Sandino Basin is thermally mature, resulting in the potential for hydrocarbon generation in organic‐rich intervals in the Brito and Masachapa Formations. A petroleum‐generating “kitchen” has tentatively been identified on a NE‐SW seismic section which crosses the basin. Modelling suggests that this kitchen has been active from the Late Eocene until the present day, and that the main phases of petroleum generation in general coincide with phases of maximum subsidence in the Late Eocene, Late Oligocene and Plio‐Pleistocene. Hydrocarbon migration most probably occurred from the deep basin towards the flanks. Significant volumes of petroleum may have been lost prior to the Late Miocene before the formation of a coastal flexure which can be recognised in the NE of the seismic profile.

Hydrocarbon habitat of the offshore Bohai Basin, China

The Bohai Basin is a major continental petroliferous basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total petroleum resources in the basin. Although the basin has been explored for nearly 40 years, the offshore part of the basin is just reaching a moderately mature stage of exploration, and its petroleum geological features are gradually becoming known. The subduction of the Pacific Plate to the Eurasia Plate resulted in the convective upwelling of the mantle in the Bohai area and triggered the development of NNE and NE striking extensional faults along the Mesozoic structures in the Late Paleocene, causing the formation of the rift basin. During the Oligocene, the depocenters and subsidence centers of the greater Bohai Basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin. Since the Oligocene, especially during the Neogene, due to the indentation of the India Block to the Eurasia Plate, the right-lateral strike-slip movement of the Tan-Lu fault was relatively intense in the offshore part of the basin. These characteristics led to the differences between the offshore and onshore sectors in sedimentation, structural features, and petroleum geology. During the deposition of the Dongying Formation, the offshore sector became the depocenter and subsidence center of the basin, resulting in the deposition of the lower Dongying Formation mudstone source rocks; while these mudstones are immature in the onshore part of the basin. In the offshore Bohai, most major discoveries have been made in the drape anticlines on the rises in the Upper Tertiary reservoirs; while the Lower Tertiary reservoirs within the half-grabens only serve as secondary reservoirs. As the Neogene depocenter of the basin, the offshore part accommodated some fine-grained shallow lacustrine clastics of the lower Minghuazhen Formation, serving as important regional seals for the Neogene reservoirs. A set of shallow faults and a central faulted anticlinal belt in the shallow Upper Tertiary sequences were formed during the Neogene in the offshore sector under the influence of right-lateral strike-slip faulting of the Tan-Lu fault. These shallow faults networked with deep extensional faults, serving as hydrocarbon conduits. The central faulted anticline belt, along the Tan-Lu fault system, also provides significant traps.

Hydrocarbon habitat of the offshore Bohai Basin, China

The Bohai Basin is a major continental petroliferous basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total petroleum resources in the basin. Although the basin has been explored for nearly 40 years, the offshore part of the basin is just reaching a moderately mature stage of exploration, and its petroleum geological features are gradually becoming known. The subduction of the Pacific Plate to the Eurasia Plate resulted in the convective upwelling of the mantle in the Bohai area and triggered the development of NNE and NE striking extensional faults along the Mesozoic structures in the Late Paleocene, causing the formation of the rift basin. During the Oligocene, the depocenters and subsidence centers of the greater Bohai Basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin. Since the Oligocene, especially during the Neogene, due to the indentation of the India Block to the Eurasia Plate, the right-lateral strike-slip movement of the Tan-Lu fault was relatively intense in the offshore part of the basin. These characteristics led to the differences between the offshore and onshore sectors in sedimentation, structural features, and petroleum geology. During the deposition of the Dongying Formation, the offshore sector became the depocenter and subsidence center of the basin, resulting in the deposition of the lower Dongying Formation mudstone source rocks; while these mudstones are immature in the onshore part of the basin. In the offshore Bohai, most major discoveries have been made in the drape anticlines on the rises in the Upper Tertiary reservoirs; while the Lower Tertiary reservoirs within the half-grabens only serve as secondary reservoirs. As the Neogene depocenter of the basin, the offshore part accommodated some fine-grained shallow lacustrine clastics of the lower Minghuazhen Formation, serving as important regional seals for the Neogene reservoirs. A set of shallow faults and a central faulted anticlinal belt in the shallow Upper Tertiary sequences were formed during the Neogene in the offshore sector under the influence of right-lateral strike-slip faulting of the Tan-Lu fault. These shallow faults networked with deep extensional faults, serving as hydrocarbon conduits. The central faulted anticline belt, along the Tan-Lu fault system, also provides significant traps.

Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (İzmit) earthquake on 17 August 1999

Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avcilar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the Cinarcik Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the Cinarcik Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through B uy uk cekmece and K u c uk cekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthquake in the NAFZ may have extended through Avcilar. Indeed, Avcilar and Izmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avcilar and Izmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avcilar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.