Regional study on potential CO2 geosequestration in the Collie Basin and the Southern Perth Basin of Western Australia

Coal-fired power stations in Collie, Western Australia emit 10 million tonnes of CO₂ per year. This study assesses the potential opportunities of geological storage of CO₂ both within the Collie Basin and the onshore part of the adjacent Southern Perth Basin of Western Australia within 50 km of Collie town site through a desktop evaluation of existing data. The aquifers and coal formations within both basins have been evaluated for their suitability for storage based on geological, geographical and environmental criteria related to storage capacity, injectivity, proximity to sources of CO₂, location of other natural resources and containment security. The study has concluded that there is limited scope for large-scale storage of CO₂ within the Collie Basin. In addition the potential for storage within coals of either basin is not a viable solution. This assessment is based on published criteria for CO₂ storage in sedimentary basins and coal-bearing formations.     

Thermal regime of the northwest Indian rifted margin – Comparison with predictions

We use a simple approach to estimate the present-day thermal regime along the northwestern part of the Western Indian Passive Margin, offshore Pakistan. A compilation of bottom borehole temperatures and geothermal gradients derived from new observations of bottom-simulating reflections (BSRs) allows us to constrain the relationship between the thermal regime and the known tectonic and sedimentary framework along this margin. Effects of basin and crustal structure on the estimation of thermal gradients and heat flow are discussed. A hydrate system is located within the sedimentary deep marine setting and compared to other provinces on other continental margins. We calculate the potential radiogenic contribution to the surface heat flow along a profile across the margin. Measurements across the continental shelf show intermediate thermal gradients of 38–44 °C/km. The onshore Indus Basin shows a lower range of values spanning 18–31 °C/km. The Indus Fan slope and continental rise show an increasing gradient from 37 to 55 °C/km, with higher values associated with the thick depocenter. The gradient drops to 33 °C/km along the Somnath Ridge, which is a syn-rift volcanic construct located in a landward position relative to the latest spreading center around the Cretaceous–Paleogene transition.     

Geochemical overview and undiscovered gas resources generated from Hamitabat petroleum system in the Thrace Basin,Turkey

Since the first drill in 1957, three oil, 19 gas and condensate fields have been discovered in the Thrace Basin. However, any petroleum system with its essential elements and processes has not been assigned yet. This study consists of two parts, (1) geochemical overview of the previous work in order to get a necessary help to construct a petroleum system and (2) calculation of quantitative undiscovered hydrocarbon resources generated from this system. An extensive overview study showed that the primary reservoir and source rocks in the Thrace Basin are the Middle Eocene Hamitabat sandstones and shales, respectively, hence it appears that the most effective petroleum system of the Thrace Basin becomes the Hamitabat (!) petroleum system. Currently, 18.5 billion m³ of in-place gas, 2.0 million m³ (12.7 million bbl) in-place waxy oil as well as minor amount of associated condensate were discovered from this system. This study showed that the regional distribution of the oil and gas fields almost overlapped with the previously constructed pod of active Hamitabat shales implying that short and up-dip vertical migration pathway of hydrocarbons from the source to trapping side was available. Thermal model demonstrated that hydrocarbon generation from the Hamitabat shales commenced in the Early Miocene. The amount of quantitative gas generation based on the mean-original TOC = 0.94 wt%, mean-original HI = 217 HC/g TOC and the volume of the pod of active source rock = 49 km³ is approximately 110 billion m³ of gaseous hydrocarbons that results in a high generation–accumulation efficiency of 17% when 18.5 billion m³ of already discovered hydrocarbons are considered.     

The evolution of rifting on the volcanic margin of the Pelotas Basin and the contextualization of the Paraná–Etendeka LIP in the separation of Gondwana in the South Atlantic

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.     

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos,Santos and Pelotas basins)

The Campos, Santos and Pelotas basins have been investigated in terms of 2D seismo-stratigraphy and subsidence. The processes controlling accommodation space (e.g. eustacy, subsidence, sediment input) and the evolution of the three basins are discussed. Depositional seismic sequences in the syn-rift Barremian to the drift Holocene basin fill have been identified. In addition, the subsidence/uplift history has been numerically modeled including (i) sediment flux, (ii) sedimentary basin framework, (iii) relation to plate-tectonic reconfigurations, and (iv) mechanism of crustal extension. Although the initial rift development of the three basins is very similar, basin architecture, sedimentary infill and distribution differ considerably during the syn-rift sag to the drift basin stages. After widespread late Aptian–early Albian salt and carbonate deposition, shelf retrogradation dominated in the Campos Basin, whereas shelf progradation occurred in the Santos Basin. In the Tertiary, these basin fill styles were reversed: since the Paleogene, shelf progradation in the Campos Basin contrasts with overall retrogradation in the Santos Basin. In contrast, long-term Cretaceous–Paleogene shelf retrogradation and intense Neogene progradation characterize the Pelotas Basin. Its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation. These temporally and spatially varying successions were controlled by specific long-term subsidence/uplift trends. Onshore and offshore tectonism in the Campos and Santos basins affected the sediment flux history, distribution of the main depocenters and occurrence of hydrocarbon stratigraphic–structural traps. This is highlighted by the exhumation and erosion of the Serra do Mar, Serra da Mantiqueira and Ponta Grossa Arch in the hinterland, as well as salt tectonics in the offshore domain. The Pelotas Basin was less affected by changes in structural regimes until the Eocene, when the Andean orogeny caused uplift of the source areas. Flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic.     

“Uniform geothermal gradient” and heat flow in the Qiongdongnan and Pearl River Mouth Basins of the South China Sea

The Pearl River Mouth Basin (PRMB) and Qiongdongnan Basin (QDNB) are oil and gas bearing basins in the northern margin of the South China Sea (SCS). Geothermal survey is an important tool in petroleum exploration. A large data set comprised of 199 thermal conductivities, 40 radioactive heat productions, 543 measured geothermal gradient values, and 224 heat flow values has been obtained from the two basins. However, the measured geothermal gradient data originated from diverse depth range make spatial comparison a challenging task. Taking into account the variation of conductivity and heat production of rocks, we use a “uniform geothermal gradient” to characterize the geothermal gradient distribution of the PRMB and QDNB. Results show that, in the depth interval of 0–5 km, the “uniform geothermal gradient” in the PRMB varies from 17.8 °C/km to 50.2 °C/km, with an average of 32.1 ± 6.0 °C/km. In comparison, the QDNB has an average “uniform geothermal gradient” of 31.9 ± 5.6 °C/km and a range between 19.7 °C/km and 39.5 °C/km. Heat flows in the PRMB and QDNB are 71.3 ± 13.5 mW/m² and 72.9 ± 14.2 mW/m², respectively. The heat flow and geothermal gradient of the PRMB and QDNB tend to increase from the continental shelf to continental slope owing to the lithosphereic/crustal thinning in the Cenozoic.     

Seismic stratigraphic framework and depositional sequences in the Santos Basin,Brazil

The Santos Basin, situated offshore southern Brazil, is one of nine marginal rift basins in the equatorial South Atlantic. It formed by the collapse of a thermal dome in the late Jurassic and by subsequent rifting and opening of the South Atlantic in the early Cretaceous. Rifting was accompanied by immense volcanic outpouring seen at the surface today throughout the onshore Paraná Basin and thought to underlie the entire Santos Basin, and the adjacent São Paulo Plateau. Vulcanism was followed by subsidence of up to 10 km from Aptian to Recent time, and a coastal hingeline coincides with the Serra do Mar uplift. The basin depocentre, which is 700 km long, is bounded to the north and south by basement and volcanic highs, respectively. A restricted water circulation in the ocean basin, which prevailed up to the Santonian stage, has important repercussions for the hydrocarbon potential of the area. The nine genetically related basins have collective reserves of ≈ 5 billion barrels of oil and associated gas. A stratigraphic framework, based largely on seismic data, has been erected for the Santos Basin. Seven regional unconformities, or ‘R’ reflections, can be traced throughout the basin and form the boundaries for seismic sequences. Isopaching the seismic sequences defines the principal depositional units in the basin and also shows how the basin depocentre shifted with time. Limited well control has enabled the seismic sequences to be correlated with litho-environmental sequences which more fully reflect the geological evolution and provide a working exploration model. Finally, an attempt has been made to recognize and map seismic facies within the seismic sequences and to predict the lithofacies in areas away from well control     

Particle fluxes in San Pedro Basin, California: A four-year record of sedimentation and physical forcing

Moored sediment traps were deployed from January 2004 through December 2007 at depths of 550 and 800 m in San Pedro Basin (SPB), CA (33°33.0′N, 118°26.5′W). Additionally, floating sediment traps were deployed at 100 and 200 m for periods of 12-24 h during spring 2005, fall 2007, and spring 2008. Average annual fluxes of mass, particulate organic carbon (POC), ??¹³C_org, particulate organic nitrogen (PON), ??¹⁵N-PON, biogenic silica (bSiO₂), calcium carbonate (CaCO₃), and detrital material (non-biogenic) were coupled with climate records and used to examine sedimentation patterns, vertical flux variability, and organic matter sources to this coastal region. Annual average flux values were determined by binning data by month and averaging the monthly averages. The average annual fluxes to 550 m were 516±42 mg/m² d for mass (sdom of the monthly averages, n=117), 3.18±0.26 mmol C/m² d for POC (n=111), 0.70±0.05 mmol/m² d for CaCO₃ (n=110), 1.31±0.21 mmol/m² d for bSiO₂ (n=115), and 0.35±0.03 mmol/m² d for PON (n=111). Fluxes to 800 and to 550 m were similar, within 10%. Annual average values of ??¹³C_org at 550 m were −21.8±0.2‰ (n=108), and ??¹⁵N averages were 8.9±0.2‰ (n=95). The timing of both high and low flux particle collection was synchronous between the two traps. Given the frequency of trap cup rotation (4-11 days), this argues for particle settling rates ≥83 m/d for both high and low flux periods. The moored traps were deployed over one of the wettest (2004-2005, 74.6 cm rainfall) and driest (2006-2007, 6.6 cm) rain years on record. There was poor correlation (Pearson's correlation coefficient, 95% confidence interval) of detrital mass flux with: C_org/N ratio (r=0.10, p=0.16); ??¹⁵N (r=−0.19, p=0.02); and rainfall (r=0.5, p=0.43), suggesting that runoff does not immediately cause increases in particle fluxes 15 km offshore. ??¹³C_org values suggest that most POC falling to the basin floor is marine derived. Coherence between satellite-derived chlorophyll a records from the trap location (±9 km² resolution) and SST data indicates that productivity and export occurs within a few days of upwelling and both of these parameters are reasonable predictors of POC export, with a time lag of a few days to 2 weeks (with no time lag—SeaWiFS chlorophyll a and POC flux, r=0.25, p=0.0014; chlorophyll a and bSiO₂ flux, r=0.28, p=0.0002).     

Neocomian to early Aptian syn-rift evolution of the normal to oblique-rifted North Gabon Margin (Interior and N'Komi Basins)

The North Gabon coastal rift basins consist of a set of 130–150 long-segment asymmetrically tilted half grabens (Interior Basin) and 000–020 short-segment en échelon half grabens (N'Komi Basin) separated by 040–060 major transverse faults. Tectono-sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation. During Berriasian to early Barremian times, uniform uniaxial 040–060 extension was responsible for the stretching of the brittle upper crust over a 100-km wide domain. During late Barremian–early Aptian times, the main locus of extension stepped westward resulting in severe end-rift uplift and erosion of the failed Interior and N'Komi rift basins. Early Cretaceous coastal rifts in North Gabon display a wide range of styles from oblique rifting (N'Komi Basin), normal rifting (Interior Basin) to transform rifting. The pre-existing Precambrian tectonic fabric exerts a strong control over the mode and over the 100–300 km-scale segmentation of the rifting.     

When did the Falklands rotate?

Similarities in the styles and relative timings of tectonic events in the Outeniqua Basin, South Africa and the North Falkland Basin suggest that basin formation in both regions may have preceded rotation of the Falklands microplate. Contrary to previous models for the break-up of Gondwana, which suggest Jurassic rotation, the data implies Valanginian rotation, contemporaneous with the first recorded motion on the Agulhas Falkland Fracture Zone and South Atlantic rifting. The data also suggests that the formation of the Falkland Plateau Basin may also be a Cretaceous event as opposed to the previously assumed Jurassic age. Such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for Jurassic rotation does not exclude later rotation as a possibility.     

Sequence stratigraphy and importance of syndepositional structural slope-break for architecture of Paleogene syn-rift lacustrine strata,Bohai Bay Basin,E. China

Sequence stratigraphy and syndepositional structural slope-break zones define the architecture of the Paleogene syn-rift, lacustrine succession in eastern China's Bohai Bay Basin. Jiyang, Huanghua and Liaohe subbasins are of particular interest and were our primary research objectives. Interpretation of 3D seismic data, well logs and cores reveals: One first-order sequence, 4 second-order sequences, and ten to thirteen third-order sequences were identified on the basis of the tectonic evolution, lithologic assemblage and unconformities in the subbasins of Bohai Bay Basin. Three types of syndepositional paleo-structure styles are recognized in this basin. They are identified as fault controlled, slope-break zone; flexure controlled, slope-break zone; and gentle slope.The three active structural styles affect the sequence stratigraphy. Distinct third-order sequences, within second-order sequences, have variable systems tract architecture due to structuring effects during tectonic episodes. Second-order sequences 1 and 2 were formed during rifting episodes 1 and 2. The development of the third-order sequences within these 2 second-order sequences was controlled by the active NW and NE oriented fault controlled, slope-break zones. Second-order sequence 3 formed during rifting episode 3, the most intense extensional faulting of the basin. Two types of distinctive lacustrine depositional sequence were formed during rifting episode 3: one was developed in an active fault controlled, slope-break zone, the other in an active flexure controlled, slope-break zone. Second-order sequence 4 was formed during the fourth episode of rifting. Syndepositional, fault- and flexure-controlled slope-break zones developed in the subsidence center (shore to offshore areas) of the basin and controlled the architecture of third-order sequences in a way similar to that in second-order sequence 3. Sequences in the gentle slope and syndepositional, flexure controlled slope-break zones were developed in subaerial region.Distribution of lowstand sandbodies was controlled primarily by active structuring on the slope-break zones, and these sandbodies were deposited downdip of the slope-break zones. Sand bodies within lowstand systems tracts have good reservoir quality, and are usually sealed by the shale sediments of the subsequent transgressive systems tract. They are favorable plays for stratigraphic trap exploration.     

Preliminary research on CBM enrichment models of low-rank coal and its geological controls: A case study in the middle of the southern Junggar Basin,NW China

To date, prospecting work on low-rank coalbed methane (CBM) resources in the middle of the southern Junggar Basin is still in the primary stage, and only a few CBM exploration wells or pilot wells have been deployed in local regions. Systemic understanding of CBM reservoir-forming conditions and geological controlling factors is lacking in the study area, resulting in the mismatch between CBM well deployment and actual geological conditions, as well as poor exploration efficiency. In this paper, the geological controlling effects of the structure, sedimentation, and hydrogeology on CBM enrichment are systematically discussed for the first time, based on the early CBM exploration achievements. The results show that the Xishanyao coal and the Badaowan coal are developed in the upper and lower part of the neutral surface of a fold, respectively. The reservoir-forming conditions of the Badaowan coal are not discussed in this paper due to its poor development. The Xishanyao coal that developed in the axial part of the syncline is most beneficial to CBM enrichment with concentrated extrusion stress and great methane adsorption capacity, while the axial part of the anticline is not favorable for CBM preservation with large tensional stress. The gas content of the Xishanyao thick seams developed in the syncline is higher (average of 4.63–6.34 m³/t) than that in the monocline (average of 2.84–4.56 m³/t). Reverse faulting is more beneficial to CBM enrichment than normal faulting, due to the better sealing capability. The gas content of the Xishanyao coal is obviously influenced by the coal thickness and its roof lithology. The hydrodynamic conditions and total dissolved solids (TDS) values of coalbed water range greatly on regional scale, which leads to a deeper methane weathering zone in the middle-west areas (>1119.62 m) than the eastern Liu-huanggou areas (<501.71 m) and have an important influence on exploration target optimization of CBM exploration wells. Combined with the geological characteristics of the structure, sedimentation and hydrogeology, three CBM enrichment models are proposed in this paper (i.e., broad fold model, northward monocline model and overlying composite model). The reservoir-forming processes and development positions of these CBM enrichment models are discussed systematically to provide a scientific basis for selecting CBM exploration target zones.     

Current status and ecological roles of Zostera marina after recovery from large-scale reclamation in the Nakdong River estuary,Korea

Large Zostera marina meadows (covering 13.6 km²) existed in the Nakdong River estuary on the south coast of Korea until the mid-1980s, but these Z. marina beds nearly disappeared due to reclamation of adjacent mud flats for the construction of a port and industrial complex during the late 1980s. Partial recovery of Z. marina meadows occurred recently, and Z. marina coverage of about 0.3 km² was observed in this estuary. In this study, shoot morphology, density, biomass, productivity, and tissue nutrient content were measured to evaluate the current status of the Z. marina meadows by comparing these data to those for persistent seagrass meadows in similar geographical areas. Additionally, we examined the ecological roles of Z. marina in this estuary after recovery from the large-scale disturbance. Shoot density (151 shoots m⁻²) and total biomass (141 g DW m⁻²) in the estuary were similar to those reported from other Z. marina meadows in Korea. Annual leaf production (1726 g DW m⁻² y⁻¹) was higher than generally observed for Z. marina in other geographical areas. These results imply that the existing Z. marina meadows in this estuary have adjusted to local environmental conditions that changed after large-scale reclamation. Estimated annual whole plant carbon (C) and nitrogen (N) incorporations based on shoot production and tissue C and N content were 810.0 g C m⁻² y⁻¹ and 59.7 g N m⁻² y⁻¹, respectively. These values were equivalent to 2.4 × 10⁵ kg C y⁻¹ and 1.8 × 10⁴ kg N y⁻¹ for all Z. marina beds in the Nakdong River estuary. This high C and N incorporation into Z. marina tissues suggests that existing Z. marina meadows play important roles in C and N cycles in this estuary. Although the currently existing Z. marina beds in this estuary are persisting and play an important ecological role, anthropogenic factors that cause seagrass declines still affect the estuary. Thus, effective management and monitoring of Z. marina beds and environmental factors are critical to protecting and conserving this invaluable component of the Nakdong River estuary.     

Stratigraphic architecture of an Early–Middle Jurassic tidally influenced deltaic system (Plover Formation), Browse Basin,Australian North West Shelf

Understanding the internal stratigraphic architecture of sand-dominated deltas is critical to assessing the extent and distribution of petroleum reservoirs. The stratigraphic architecture and evolution of a major Early–Middle Jurassic fluvio-deltaic system (Plover Formation) on the Australian North West Shelf has been established through integrated analysis of core, borehole image logs and wireline logs for the Calliance field in the Browse Basin. Six facies associations identified in cored intervals are interpreted as tidally influenced channel- and tidal channel-fill complexes (FA1–FA2), crevasse-splay deposits and interchannel marshes (FA3), heterolithic mouthbars and sandflats (FA4), sandy mouthbars (FA5) and offshore transition to offshore (FA6). Therefore, the overall depositional system in the study area is proposed to be a tidally influenced deltaic system, in which FA6 represents prodelta deposits, FA5 and FA4 constitute distributary mouthbar deposits of the delta front and tidally influenced channel and interchannel deposits represent the lower delta plain (FA2, FA1, FA3). Analysis of image lithology and fabric are used to extend interpretation to uncored intervals and to identify intrusive and extrusive igneous units and associated volcaniclastic facies within the formation. Five third-order stratigraphic sequences (S1–S5) record progradational (S1, S2 and S4) and retrogradational (S3 and S5) phases of delta evolution. Paleocurrent indicators derived from borehole image logs indicate common southerly directed sediment dispersal in S2 and S3 and increasingly complex with westerly directions in S4 and S5. Two rift-related depositional phases are recognised separated by a phase of uplift between S3 and S4. The stratigraphic succession of S2 is consistent with the depositional pattern expected in a synrift setting and deposition of the over-thickened sandy succession (FA5 in S4), which is also the major reservoir interval, was most likely controlled by syn-tectonic faulting.     

Regional constraints of the Sørkapp Basin: A Carboniferous relic or a Cretaceous depression?

The Sørkapp Basin (NW Barents Shelf) contains a comprehensive sedimentary succession that provides insight into regional tectonics and depositional development of the shelf from the Devonian to the Cretaceous. With its location east of the mid-Atlantic spreading ridge and south of Svalbard, the Basin serves as an important link between the offshore and onshore realms.This study subdivides this sparsely studied basin into six main seismic units (three Paleozoic and three Mesozoic). A metamorphic basement together with assumed Devonian sedimentary deposits form the foundation for a chiefly Carboniferous basin. The Basin forms a syncline with infill showing limited fault-influence. Overlying the early infill are Late Carboniferous deposits which show less lateral variation in thickness but also active growth on the few faults showing significant displacement. The overlying platform deposits of the latest Carboniferous and Permian show a change in depositional geometry, with onlapping deposits towards the east probably resulting from uplift of the Stappen High and regional flooding. Subsequent, particularly Late, Triassic sedimentation shows a more distinctly progradational pattern with a dominantly southeastern source for sediments. During this shallow shelf-filling stage, the Sørkapp Basin is sheltered by the Gardarbanken High, blocking the Early Triassic clinoform development. The High was transgressed in the Middle Triassic and the platform-edge progressively approached the present Svalbard coastline.The youngest Mesozoic unit forms a separate saucer-shaped depocenter west of the Sørkapp Basin, where deposits are truncated by the seafloor in a mid-basin position and across the Gardarbanken High. The depositional pattern for this succession correlates with the outcrop pattern of the Adventdalen Group implying a post Middle Jurassic to Cretaceous age. The Sørkapp Basin has been referred to as a Cretaceous feature based in this depocenter. However, the foundations are much older and the Cretaceous depression is located west of the deeper basin. Accordingly, we propose the informal term Sørkapp Depression for the Cretaceous basin.     

Determination of residence time and mixing processes of the Ubatuba,Brazil, inner shelf waters using natural Ra isotopes

Coastal waters contain elevated dissolved activities of short-lived radium isotopes, ²²³Ra and ²²⁴Ra, having half-lives of 11.4 and 3.66 days, respectively. The input of these isotopes near the coast must be balanced by decay and mixing into the open ocean, where excess activities are zero. Since the decay rate is known, in the ideal case the mixing rate may be determined from the offshore distribution of these isotopes. This study found that samples collected in June 2000 followed the expected exponential decrease with distance offshore. We assign a dispersion coefficient of 28–39 m² s⁻¹ for this study. During January 2002 and November 2003, there was not a consistent decrease of activity with distance offshore. This is likely due to the ruggedness of the coastline, where many bays and small islands interrupt simple mixing patterns. To estimate exchange rates during 2002 and 2003, we used a model based on the decrease in the ²²⁴Ra/²²³Ra activity ratio (AR) with time for samples isolated from fresh inputs of Ra. This model yielded residence times of 1–2 weeks for samples collected within 20 km of the coast. We used this residence time to calculate the flux of ²²⁸Ra (half-life = 5.7 years) to the study area necessary to maintain the enrichment relative to ocean water. This enrichment is a factor of ten greater than the flux of ²²⁸Ra expected from submarine groundwater discharge (SGD) occurring within 50 m of shore.     

Seismic imaging of Paleogene sediments of Kachchh Shelf (western Indian margin) and their correlation with sea-level fluctuations

The sedimentary history of Kachchh offshore (central western Indian margin), especially since the eruption of the Deccan Traps (∼65 Ma), has remained scantily studied despite an area with promising resource potential. Of late, new marine surveys combined with industrial drilling along the Kachchh shelf are beginning to elucidate the depositional history of this region. Here, we attempt to synthesize interpretation of new offshore seismic data, along with borehole information and long-term sea-level variations to provide a coherent sedimentological and lithostratigraphic framework over the past ∼65 Ma in this area.     

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps-Ligurian basin junction during the Cenozoic

We focus on the northern Ligurian margin, at the geological junction of the subalpine domain and the Ligurian oceanic basin, in order (1) to identify the location of the southern limit of the Alpine compressive domain during the Cenozoic, and (2) to study the influence of a compressive environment on the tectonic and sedimentary evolution of a passive margin.Based on published onshore and offshore data, we first propose a chronology of the main extensional and compressional regional tectonic events.High-resolution seismic data image the margin structure down to ∼3 km below seafloor. These data support that past rifting processes control the present-day margin structure, and that 2800-4000 m of synrift sediment was deposited on this segment of the margin in two steps. First, sub-parallel reflectors indicate sediment deposition within a subsident basin showing a low amount of extension. Then, a fan-shaped sequence indicates block tilting and a higher amount of extension. We do not show any influence of the Miocene Alpine compression on the present-day margin structure at our scale of investigation, despite the southern subalpine relief formed in the close hinterland at that time. The southern front of the Miocene Alps was thus located upslope from the continental margin.Finally, a comparison with the Gulf of Lions margin suggests that the tectonic influence of the Alpine compression on the rifting processes is restrited to an increase of the subsidence related to flexure ahead of the Alpine front, explaining abnormally high synrift thicknesses in the study area. The Alpine environment, however, has probably controlled the sedimentary evolution of the margin since the rifting. Indeed, sediment supply and distribution would be mainly controlled by the permanent building of relief in the hinterland and by the steep basin morphology, rather than by sea-level fluctuations, even during the Messinian sea-level low-stand.     

Vodyanitskii mud volcano,Sorokin trough,Black Sea: Geological characterization and quantification of gas bubble streams

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to ∼650 mmol L⁻¹ at ∼150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at ∼60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (δD-CH₄ = −170.8‰ (SMOW), δ¹³C-CH₄ = −61.0‰ (V-PDB), δ¹³C-C₂H₆ = −44.0‰ (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 × 10⁶ mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.     

Speculative petroleum systems of the southern Pelotas Basin,offshore Uruguay

The Pelotas Basin of Brazil and Uruguay represents a frontier basin with under-explored hydrocarbon potential. Although oil and gas accumulations have yet to be identified, only 21 exploratory wells have been drilled in an area of more than 330,000 km², 20 of which are located in the Brazilian portion of the basin. A detailed study of the petroleum system of offshore Uruguay has strong potential to contribute to a better characterization of the capacity of the basin to generate and accumulate hydrocarbons. Three stages have previously been recognized during the evolution of Pelotas basin: (1) a prerift phase which preserved Paleozoic and Mesozoic units of the Paraná Basin; (2) an Early Cretaceous volcano-sedimentary synrift phase; and (3) a Cretaceous to Cenozoic postrift phase deposited during the passive margin stage. In this study, we use sequence stratigraphy methodology to interpret 2D multichannel seismic sections of the southern segment of the Pelotas Basin in the Uruguayan Atlantic margin. This analysis allows us to identify depositional sequences, systems tracts and the distribution of the main elements of the potential petroleum systems. Following our analysis, we propose six speculative petroleum systems (SPS) in the Pelotas Basin. The first SPS is related to the prerift phase and is represented by a Lower Permian restricted marine source rock and reservoirs related to Permian to Upper Jurassic aeolian and fluvial sandstones. The second SPS corresponds to the synrift phase and is constituted by a Barremian lacustrine source rock with reservoirs of alluvial/fluvial sandstones of the same age. The other four proposed SPS are associated with the postrift phase, represented by marine source rocks related to Aptian-Albian, Cenomanian-Turonian and Paleocene transgressions, all of which are identified in the region and interpreted in seismic lines from Uruguay. These postrift SPS have predominantly siliciclastic reservoirs represented by Early Cretaceous aeolian sandstones and Cretaceous to Cenozoic deltaic sandstones and turbidites.