Geology Of Basin Research Articles

New insights on interpretation of velocity anomalies in the offshore Nile Delta basin, Egypt

Several issues associated with existing velocity models and their anomalies affecting the petroleum geology of the offshore Nile Delta basin are described. The paper aims to present velocity and pressure variations and their associated pitfalls in the deeper Nile Delta, where the ambiguities exist in the interpretation of depth structures, especially in between sea bottom and Oligocene sequences. Different conceptualized geological, velocity, and seismic models are generated. We make use of data cubes to generate data views for geological interpretation. The connectivity is established between velocity anomalies and corresponding seismic times, ascertaining that the depth structures have geological implications. The visualization of representative schematic data views of velocity anomalies facilitates the interpretation of new knowledge and the connectivity between seismic and geological models. The velocity attributes significantly influence the seismic data interpretation in the offshore Nile Delta basin and have an impact on the overall cycle of the exploration and production entities.

Quantifying the uncertainties in a fault stability analysis of the Val d’Agri oilfield

Induced seismicity as an effect of injection of fluids in a geological basin is a widely observed phenomenon which is nowadays under public scrutiny. In most cases, the numerical simulation and interpretation of these phenomena requires models with a high degree of realism. In turn, this requires accounting for complex interactions between the fluid and solid components, as well as a detailed description of geometry and a high degree of heterogeneity in the material properties. Also, issues related to the uncertainty of parameters, to an incorrect modeling of sub-scale or multiscale effects and to a limited knowledge of initial conditions are unavoidable and can be detrimental to the reliability of the results. In this work, a statistical analysis including uncertainty quantification and sensitivity analysis on variances has been applied as a post-processing to data coming from a set of numerical simulations of a real world setting (the Val D’Agri oilfield), with the aim of studying the stability of a fault that is known to have experienced a good amount of microseismicity during the modeled period. The uncertainty quantification targeted the effects of fault surface local orientation and pore pressure fluctuations, as well as variability in the friction coefficient of the fault. The analysis of variances focused on the effects of varying the permeability of the fault damage zones (the area enveloping the faults) and the geometrical orientation of the fault as well. The model shows that the zone where the microseismicity has been measured is included in a wider region of moderate instability, which is higher the lower the permeability of the fault damage zone. From the model results the fault seems to be far from a critical state, but the analysis offers, nevertheless, some useful information on the relationship of slip tendency with geometrical and flow quantities in the system, and suggests some improvements in the dynamical model assumptions and settings.

The economic impact of oil and gas development in the Permian Basin: Local and spillover effects

Abstract: The Permian Basin is one of the most prolific oil and natural gas geologic basins in the US. With soaring production in the last decade, the economic impact of oil and gas development in the basin has become a pressing policy question to answer. This paper presents the first basin-wide study that examines both the employment effect and the income effect of the Permian Basin development. We consider not only the local impact but also the spatial spillover effect and the industry-level spillover effect. To correct for the estimation bias due to the potential simultaneity between drilling decisions and economic activities, an instrumental variables (IV) regression model is proposed. We find that both the employment effect and the income effect of shale development in the basin are highly significant. We also show that there are significant spatial spillover effects and spillover effects onto the indirect industries. Relevant policy implications for the long-run economic prosperity in the Permian Basin are discussed.

Multi-objective genetic algorithm optimization of artificial neural network for estimating suspended sediment yield in Mahanadi River basin, India

ABSTRACTRivers are an integral part of the hydrological cycle and are the major geological agents which erode the continents and transport water and sediments to the oceans. Thus rivers act an important link between continents and oceans for the mass balance. Estimating the suspended sediment yield is one of the crucial aims in the field of managing, designing and planning of any river system or reservoir. To determine the suspended sediment yield in a river basin demands more labour or it is more costly when automatic sampling devices are used. The number of variables and the inter-relationship existing among them influence the suspended sediment yield and the nature of these inter-relationships is neither linear nor simple. Unfortunately, it is a difficult task to determine the suspended sediment yield when traditional mathematical models are used as complex variables and processes are involved. The major key factors, such as basin geology (rock type), relief, rainfall, temperature, water discharge and catchment area that affect sediment yield, are used as inputs to develop the model for predicting the suspended sediment yield in the Mahanadi River. In this paper, a multi-objective genetic algorithm for artificial neural network (MOGA-ANN)-based approach is used for predicting the suspended sediment yield. The MOGA assists ANN to minimize the two competing objectives, i.e. mean error and variance simultaneously. Thus in this study, a hybrid artificial intelligence-based method, MOGA-ANN model, is developed using the hydro-geological-climatic factors where all parameters associated with the ANN models are optimized simultaneously using MOGAs to estimate the suspended sediment yield in the Mahanadi River basin. The ANN's parameters are optimized globally by the MOGA to accurate estimation. The study has been carried out to develop MOGA-ANN for estimating the suspended sediment load using 20-year data at the Tikarapara gauging station which is the last downstream station in the Mahanadi River. The MOGA-ANN model provided the root mean square error 0.0281, correlation coefficient (r) 0.966 and efficiency factor 0.919 during the testing phase. The results suggested that the hybrid MOGA-ANN model exhibited satisfactory performance with an accuracy of 96.41%.

Regional-scale modelling and predictive uncertainty analysis of cumulative groundwater impacts from coal seam gas and coal mining developments

The geological basins in Australia across which the hydrogeological Great Artesian Basin (GAB) exists, hold significant coal and coal seam gas (CSG) resources. Resource development from deep sedimentary basins often involves the risk of impacting groundwater resources. Predictive analysis of potential impacts on water resources are important for the risk analysis of resource development projects. A regional-scale numerical groundwater model was developed to probabilistically assess potential groundwater impacts due to additional coal resource development from a deep sedimentary basin underlying the GAB. The probabilistic simulation considered the plausible variability of the model parameters and accounted for uncertainties. Predictive uncertainty analysis was undertaken using a rejection sampling method after screening the model runs using predefined objective functions to evaluate the performance of the model runs with respect to historical observations. The predictive simulations were undertaken using 2,618 model runs to obtain maximum head drawdown caused by CSG and coal mining developments. The results showed that the water-table drawdown from an individual coal mining development becomes insignificant (maximum difference in drawdown <0.2 m) beyond 5 km. In general, less water-table drawdown was produced near the CSG development site but small amounts of drawdown spread further from it. Separation of surficial aquifers from deeper coal formations, as generally found in the GAB, limits the propagation of CSG-induced drawdown into the aquifers closer to the surface. While this study was specifically done for the Namoi region, similar outcomes could be expected in the broader GAB and other basins where equivalent hydrogeological conditions exist.

Geology, alteration system, and uranium metallogenic potential of Pasha-Ladoga Basin, Russia

The Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Karelian Craton and Svecofennian Belt in northwest Russia. It is the only basin in Russia that is currently recognized to host unconformity-type uranium mineralization (e.g., Karku deposit). Mineralization is associated with the Priozersk Formation which comprises a basal sequence of immature arkosic sandstones and conglomerates interpreted as high-energy braided stream channel and channel bar facies within an alluvial fan depositional environment. Diagenetic and hydrothermal alteration of the Priozersk sandstones is similar to that observed in unconformity-related uranium deposits of the Athabasca Basin (Canada) and includes three main stages: (1) an early diagenetic assemblage comprising illite-smectite (K-rectorite), calcite, minor chlorite, and kaolinite precipitated at 100–150 °C; (2) a peak diagenetic-hydrothermal assemblage composed of kaolinite-dickite, Fe- and Fe-Mg chlorite, calcite, and sulfides formed at 150–250 °C; (3) late diagenetic-hydrothermal event involving fracture-controlled kaolinite and late hematite. Similar to the Athabasca Basin type uranium mineralization, the Karku deposit exhibits anomalous concentrations of LREE, Nb, Pb, and positive correlations between U and Cu, Zn, Ag, and Pb that is a geochemical signature of unconformity-type uranium mineralization. Based on oxygen and hydrogen stable isotope study, it was found that fluids in equilibrium with early and peak diagenetic clays were consistent with oxidized basinal brines and reduced basement fluids. In contrast to the Athabasca Basin, mineralizing fluids at the Karku area were more reduced and potassic. A new geological model for the Karku deposit is proposed based on petrographic observations and sandstones geochemistry. The proposed model envisages the permeable rocks of paleoregolith and the coarse-grained basal part of the Priozersk Formation as the main pathway for migration of uraniferous fluids responsible for the formation of Karku uranium deposit. From the perspective of the geological setting, the Karku deposit differs from deposits of the Athabasca Basin in that it occurs in a rapidly deposited rift-related basin with associated mafic magmatism, the host sandstone sequence is relatively thin (e.g., Priozersk Formation) and exhibits abrupt lateral facies variations, and the widespread presence of early diagenetic interstitial swelling clays. Based on these sedimentological characteristics, it is considered that the Karku deposit setting precludes optimum conditions to create high tonnage high-grade unconformity-type U deposits as found in the Athabasca basin.

Gondwana biostratigraphy and geology of West Bengal Basin, and its correlation with adjoining Gondwana basins of India and western Bangladesh

Integrated biostratigraphic and geological studies on the drilled exploratory boreholes of West Bengal Basin have revealed the presence of very thick distinctive Permo-Triassic Gondwana successions in the subsurface, and widely recognized across the basin below the latest Jurassic–Early Cretaceous Rajmahal Traps. Precise geological age of identified Gondwana lithounits of this basin is inferred on the basis of associated palynofloral assemblages along with their correlation with the adjoining Gondwana basins. In addition, two Gondwanic grabens are recognized below the pericratonic set-up whose presence in this basin, till now, are poorly understood. Detailed palynological studies on the Gondwana successions in the key boreholes of this basin enable recognition of 12 established palynological zones ranging from Early Permian (Asselian) to Middle Triassic (Ladinian). Lower Gondwana (Permian) palynofloras of this basin resemble earlier recorded palynofloras from the Talchir, Barakar and Raniganj formations of Indian Gondwana basins, suggesting the occurrence of well-developed above Lower Gondwana lithounits in this basin and the absence of Karharbari and Barren Measures formations which are marked by the non-depositional hiatuses. Upper Gondwana successions (Triassic) of the basin are represented by the Early and Middle Triassic palynofloras only that resemble Panchet and Supra-Panchet (Molangdighi Formation) palynofloral assemblages, and indicate the absence of Late Triassic succession of Upper Gondwana. Newly acquired geological and Bouguer anomaly data, and spatial distribution of Gondwana bearing boreholes across the basin suggest that the boreholes of CHK-A, GB-A, HRP-A, AMD-A, PLS-A and MNG-A fall in a well-defined NNE–SSW aligned graben, defined as the ‘Chandkuri–Palasi–Bogra Gondwanic Graben’ which includes Chandkuri and Palasi Gondwana basins in Indian part and Bogra Gondwana Basin in western Bangladesh where thick Gondwana successions are recognised in the subsurface sections of Singra (Singra-1X, Kuchma-X1, Bogra-X1), Jamalganj (EDH-1, 6) and Barapukuria (GDH-40, 43) coalfields. Gondwana bearing boreholes in the western margins of West Bengal Basin (GLS-A, GLS-B, GLS-C) represent a quite separate Gondwana basin of ‘Galsi’ which is located in the southern part of the N–S trending ‘Purnea–Rajmahal–Galsi Gondwanic Graben’. Latest Jurassic–Early Cretaceous Rajmahal Traps cap the Gondwana successions in the above Gondwana basins of West Bengal Basin along with Bogra and Rajmahal Gondwana basins. Rajmahal Traps and its infra- and intertrappean beds are exlcuded from the Gondwana cycle as these beds are marked by the Early Cretaceous marine dinoflagellate cysts, and the Rajmahal volcanism took place after a very long time gap from Late Triassic to Late Jurassic (ca. 78 my). Rajmahal Traps and its infra- and intertrappean beds indeed represent the post-Gondwana syn-rift sequence in West Bengal, Mahanadi and Rajmahal basins, and closely related with the break-up of Indian Plate from the East Gondwanaland during Tithonian (ca. 150 Ma). The post-trappean Late Cretaceous successions mark the initiation of passive-margin phase in this basin with the deposition of marine Bolpur and Ghatal formations.

Conectividade entre a Grande Bacia Artesiana da Austrália, bacias subjacentes, e a cobertura cenozoica

A defined hydrogeological basin is convenient for understanding and classifying groundwater systems. However, groundwater flow results generally from pressure and density gradients and may pass through boundaries defined by hydrostratigraphy. Earlier conceptual models of Australia’s Great Artesian Basin (GAB) ascribed all artesian water in the region to the GAB. Although this may have facilitated regulation and control of groundwater usage, it created an unwieldy and unpredictable boundary definition for the GAB. As the understanding of hydrodynamics in the GAB steadily grows, so does the appreciation of interconnectedness of this Jurassic-Cretaceous system with underlying sedimentary basins and overlying cover. In 2012, the definition of the GAB as a hydrogeological basin was modified to align with the Eromanga, Surat and Carpentaria geological basins to simplify this issue for the GAB Water Resource Assessment (GABWRA). This revised definition has since been broadly adopted and remains in use; however, this definition has necessitated a more consistent regional understanding of connectivity with deeper basins that have been of increasing interest and importance for the exploitation of petroleum and gas reserves. Extraction commonly produces groundwater as a by-product, modifying pressure in the host reservoir and potentially in adjoining aquifers, as well as causing possible contamination through fluid transfer. Twenty-seven sedimentary basins are currently known to underlie the GAB and this number is steadily growing through regional seismic studies and exploration.

Teleseismic Scattered‐Wave Imaging Using a Large‐N Array in the Albuquerque Basin, New Mexico

AbstractThe advent of low‐cost continuously recording cable‐free autonomous seismographs, commonly referred to as nodes, enables dense spatiotemporal sampling of seismic wavefields. We create virtual source reflection profiles using P waves from five teleseismic events recorded by the Sevilleta node array experiment in the southern Albuquerque basin. The basin geology records a structurally complex history, including multiple Phanerozoic orogenies, Rio Grande rift extension, and ongoing uplift from a midcrustal magma body. The Sevilleta experiment densified the long term, regionally sparse seismograph network with 801 single channel vertical‐component 10 Hz geophone nodes deployed at ∼300 m spacing for 14 days in February 2015. Results show sediment‐basement reflections at &amp;lt;5 km depth and numerous sub‐basin structures. Comparisons to legacy crustal‐scale reflection images from the Consortium for Continental Reflection Profiling show agreement with structural geometries in the rift basin and upper crust. Comparisons of the teleseismic virtual reflection profiles to synthetic tests using 2D finite‐difference elastic wave propagation show strong P‐to‐Rayleigh scattering from steep basin edges. These high‐amplitude conversions dominate the record sections near the western rift margin and originate at the Loma Pelada fault, which acts as the primary contact between rift‐bounding basement‐cored fault blocks and rift basin sediments. At near offsets, these signals may interfere with interpretation of upper crustal structure, but their relatively slow moveout compared to teleseismic P‐wave multiples provides clear temporal separation from sediment‐basement reflections across most of the array. The high‐signal‐to‐noise ratio of these converted Rayleigh‐wave signals suggests that they may be useful for constraining short‐period (∼1 Hz) dispersion with strong sensitivity in the uppermost ∼1 km of the rift basin sediments.

Open Uruguay Round: strategy to overcome the challenges of frontier exploration

Abstract Uruguayan geological basins imply a high exploratory risk and therefore present a frontier status. However, Latin America and West Africa are the regions in which exploratory activity has seen a remarkable increment, with major discoveries and significant investment in the last decade, and plenty of E&amp;P opportunities nowadays. Within this context, The Uruguayan Government has recently approved a new bidding system for onshore and offshore areas, Open Uruguay Round, including very competitive features and contractual terms, after evaluating other countries’ petroleum fiscal regimes.

Overpressure Generation Mechanisms and Its Distribution in the Paleocene Shahejie Formation in the Linnan Sag, Huimin Depression, Eastern China

The Linnan Sag is one of the main oil-producing units in the Huimin Depression, Eastern China, and the pore pressure gradients obtained from drill stem tests (DSTs) range from 9.0 to 16.0 MPa/km. Uncertainty about the origin and distribution of abnormally high pressures in the Linnan Sag has led to different interpretations of hydrocarbon accumulation and resource assessments, and it interferes with safe drilling. In the Linnan Sag, mudstone compaction curves are substantially affected by several non-compaction factors, and the normal trend of the compaction curve is difficult to determine. The determination of the origin and distribution of overpressure in the Linnan Sag is a challenge. In this study, the factors that may affect mudstone compaction—such as the shale volume, higher calcareous, and organic matter content—were carefully examined and processed. The pressures in the mudstones were estimated by the corrected mudstone compaction curves, which were compiled from acoustic, density, and neutron logs, and calibrated using DST and mud weight data. The log response–vertical effective stress and acoustic velocity-density crossplots were used to identify the mechanisms that generate overpressure. The comprehensive compaction curve shows that the mudstones in the overpressured layer exhibit clear disequilibrium compaction characteristics. The logging response crossplots demonstrate that those overpressured points were consistent with the loading curve. The findings suggest that, the fundamental mechanism resulting in overpressures is the disequilibrium compaction of thick Paleocene mudstones. Hydrocarbon generation and vertical transfer of overpressure may be the main unloading mechanisms, which corresponds to the overpressure points that deviate from the loading curves. Since organic matter cracking may occur in formations at depths greater than 4000 m (Ro &gt; 1.0%), the contribution of hydrocarbon generation to overpressuring is expected to be limited. The transfer of overpressure through opening faults is therefore considered to be the main cause of higher overpressure in local sandstones. The overpressures in the mudstones are characterized by a gradual decrease from the center to the margin in the Linnan Sag. The pressure in the isolated sand bodies are generally similar to that in the surrounding mudstones, whereas it can be lower or higher when the overpressure in the sand bodies are vertically transferred by faults to other pressure systems. The results of this analysis provide an indication of the magnitude, mechanism, and distribution of overpressure in the Linnan Sag. This insight can be used to guide further exploration of the Linnan Sag and similar geological basins.

Evaluation of Building Collapse Risk and Drift Demands by Nonlinear Structural Analyses Using Conventional Hazard Analysis versus Direct Simulation with CyberShake Seismograms

AbstractLimited data on strong earthquakes and their effect on structures pose challenges of making reliable risk assessments of tall buildings. For instance, although the collapse safety of tall buildings is likely controlled by large‐magnitude earthquakes with long durations and high low‐frequency content, there are few available recorded ground motions to evaluate these issues. The influence of geologic basins on amplifying ground‐motion effects raises additional questions. Absent recorded motions from past large magnitude earthquakes, physics‐based ground‐motion simulations provide a viable alternative. This article examines collapse risk and drift demands of a 20‐story archetype tall building using ground motions at four sites in the Los Angeles (LA) basin. Seismic demands of the building are calculated form nonlinear structural analyses using large datasets (∼500,000 ground motions per site) of unscaled, site‐specific simulated seismograms. Seismic hazard and building performance from direct analysis of Southern California Earthquake Center CyberShake motions are contrasted with values obtained based on conventional approaches that rely on recorded motions coupled with probabilistic seismic hazard assessments. At the LA downtown site, the two approaches yield similar estimates of mean annual frequency of collapse (λc), whereas nonlinear drift demands estimated with direct analysis are slightly larger primarily because of differences in hazard curves. Conversely, at the deep basin site, the CyberShake‐based analysis yields around seven times larger λc than the conventional approach, and both hazard and spectral shapes of the motions drive the differences. Deaggregation of collapse risk is used to identify the relative contributions of causal earthquakes, linking building responses with specific seismograms and contrasting collapse risk with hazard. A strong discriminative power of average spectral acceleration and significant duration for predicting collapse is observed.

Ionic composition, geological signature and environmental impacts of coalbed methane produced water in China

ABSTRACT Ionic composition data of coalbed methane (CBM) produced water from five CBM blocks in China were used to investigate the variability in and the geological controls on water quality across geologic basins and the resulting environmental impacts. The assessment tools included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO3 ternary diagram. Techniques such as principal component analysis (PCA) and R-mode hierarchical cluster analysis were also applied to the ionic database to reveal the relationships between the ions. The results from this study indicate that three major ions (Na, Cl, and HCO3) and two primary processes (sulfate reduction and mixing of the coal seam water with the fracturing fluid) affect the ionic compositions of CBM produced water. The burial depth of the coal seam, the fresh water influence and the fracturing fluid contamination jointly control the water quality variability of produced water. The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of CBM produced water, such as coal seam water in the partially equilibrated region and exotic water (including the fracturing fluid or shallow cold groundwater) in the immature water region. Most of the CBM produced water in China has high concentrations of Na, HCO3, Cl and TDS compared to the standards for drinking water and irrigation water. The produced waters from deep formations (>1000 m), or those produced during the early drainage periods, are always saline, “Na-Cl” type waters. Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.

Subglacial Geology and Geomorphology of the Pensacola‐Pole Basin, East Antarctica

AbstractThe East Antarctic Ice Sheet (EAIS) is underlain by a series of low‐lying subglacial sedimentary basins. The extent, geology, and basal topography of these sedimentary basins are important boundary conditions governing the dynamics of the overlying ice sheet. This is particularly pertinent for basins close to the grounding line wherein the EAIS is grounded below sea level and therefore potentially vulnerable to rapid retreat. Here we analyze newly acquired airborne geophysical data over the Pensacola‐Pole Basin (PPB), a previously unexplored sector of the EAIS. Using a combination of gravity and magnetic and ice‐penetrating radar data, we present the first detailed subglacial sedimentary basin model for the PPB. Radar data reveal that the PPB is defined by a topographic depression situated ~500 m below sea level. Gravity and magnetic depth‐to‐source modeling indicate that the southern part of the basin is underlain by a sedimentary succession 2–3 km thick. This is interpreted as an equivalent of the Beacon Supergroup and associated Ferrar dolerites that are exposed along the margin of East Antarctica. However, we find that similar rocks appear to be largely absent from the northern part of the basin, close to the present‐day grounding line. In addition, the eastern margin of the basin is characterized by a major geological boundary and a system of overdeepened subglacial troughs. We suggest that these characteristics of the basin may reflect the behavior of past ice sheets and/or exert an influence on the present‐day dynamics of the overlying EAIS.

Variations on Petroleum Geology among Major Basins and Their Formative Mechanisms, Central and Southern South China Sea

AbstractThe South China Sea (SCS), one of the largest marginal seas in West Pacific, has experienced two marginal sea tectonic cycles, the pro‐SCS and Neo‐SCS, forming a tectonic trend of “compression in the south, extension in the north, subduction in the east and strike in the west”, with various kinds of sedimentary basins developed. The Central and Southern South China Sea (CSSCS) mainly has Zengmu, Brunei‐Sabah, Wanan, Zhongjiannan, Nanwei, Beikang, Reed, Palawan and Nansha Trough basins (Fig. 1). Since the exploration in the early 20th century, hundreds of oil and gas fields have been discovered in the CSSCS, making it one of giant oil and gas provinces in the world. However, the oil and gas potential of the CSSCS varied a lot, even among adjacent basins. Oil and gas resources in the southern Zengmu and Brunei‐Sabah basins are huge in scale, with recoverable reserves of nearly 5.3 billion tons of oil equivalent, which is ten times of that the Wanan basin in the west. The oil and gas discoveries in the Beikang basin on the Nansha block are only one three‐hundredth of Zengmu basin. No commercial discoveries have been made in the Reed and Nanwei basins. Although several studies have focused on the petroleum geology of separate basins, no systematical comparison has been made among various basins to reveal their differences and gain an overall perspective, largely due to limited datasets. The present study aims to investigate these aspects, using 90,000‐km 2D seismic profiles, 34 well logs, three cores, 36 outcrops, as well as paleontology, gravity and magnetic data.

Geologie des Neuwieder Beckens und seiner Randgebiete (Exkursion C am 25. April 2019)

Geologie des Neuwieder Beckens und seiner Randgebiete (Exkursion C am 25. April 2019)

Application of Radar Imagery to Tectonic, Structural and Geological Studies in Afikpo Basin, Southeastern Benue Trough, Nigeria

Radar imagery was used to identify geological features and the production of geological maps of the Afikpo basin. The essence of this study was to delineate significant geological and geomorphological features that have not been imaged by the traditional ground field mapping. The application of Radar imagery technique will be relevant to the discovery of subsurface structures that will aid the accumulation or concentration of certain economic minerals or natural resources. It will enhance the identification of significant geological information such as lineaments, geologic structures, drainages, etc, that will serve as a guide to the actual ground field work investigation. Results from radar imageries revealed drainage pattern, major geomorphological units, mega lithostratigraphic units, lineaments and structures. The geology of Afikpo basin consist of Cretaceous, Tertiary and Quaternary rocks are recognizable; the highly deformed Cretaceous rocks are characterized by regional lineaments. Some of the rivers are aligned along the regional lineaments. The NW-SE and NE-SW lineaments truncate one another, suggesting different ages for the lineament groups. A few sinistral tensional movements occur in the eastern sector of the basin. The major folds in the Afikpo basin have NE-SW and NW-SE trends, and occur as anticlines and synclines. The basement structuring and basin framework suggest a tectonic setting in the southeastern Benue Trough resulting from the Early Cretaceous opening of the South Atlantic Ocean and interpolate movement in Africa. From the evidence currently available we may conclude that the post Santonian Afikpo basin falls within the transtenstional basin categories associated with transform faults. The regional tectonics interpreted from the radar imageries are likely hydrocarbon related structural features.

3D thermal modelling versus down-hole temperature extrapolation in the Sydney-Gunnedah-Bowen Basin and the implications for targeting potential geothermal anomalies

Geothermal exploration programs require accurate subsurface temperature information and currently this information primarily comes from temperature maps created from shallow extrapolated down-hole temperature measurements. These extrapolations, often taken from measurements made in non-equilibrated boreholes, do not account for variations in geological structure or thermal conductivity. Here we explore temperature maps at 5km for extrapolated equilibrated and non-equilibrated bores and for modelled basin temperatures. Modelled temperatures are derived from finite element models using 3D basin geology and defined thermal properties. When 3D thermal modelling is compared to extrapolated temperatures at both shallow (500m) and deep (5km) intervals it can be shown that shallow surface temperature variations (from extrapolated measurements) are often not seen at depth. Depending on whether equilibrated or non-equilibrated temperature measurements are extrapolated, the positions of the potential target anomalies shift. In general both equilibrated and nonequilibrated extrapolated measurements either overestimate or underestimate temperatures at 5km. These extrapolated down-hole temperature measurements may even create false target anomalies, which significantly impacts on exploration.

Post‐seafloor spreading magmatism and associated magmatic hydrothermal systems in the Xisha uplift region, northwestern South China Sea

AbstractSubmarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multibeam bathymetric data to examine the morphology and internal architecture of post‐seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flow through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post‐seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/metamorphic dehydration reactions in sediments produced local overpressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results show that post‐seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma, whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post‐seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea.

Differentiating the effects of logging, river engineering, and hydropower dams on flooding in the Skokomish River, Washington, USA

A decades-long, progressive loss of channel capacity in the Skokomish River, a 622 km2 basin draining the southeast Olympic Mountains of Washington State, has caused increasing flooding with severe consequences to endangered salmon runs, infrastructure, and private property. To differentiate among multiple potential drivers of the capacity loss, we analyze the geomorphic evidence for the potential effects of: flow regulation by two dams constituting the Cushman Hydroelectric Project, which began regulating flow in the river's North Fork in 1925 and diverting water out of the basin in 1930; sediment production from mid-twentieth century logging in the river's South Fork basin; and twentieth century river engineering in the mainstem. Bankfull channel capacity in the mainstem has steadily declined since about 1940 from 370 m3 s−1 to <100 m3 s−1 due partly to the narrowing of the Skokomish River, which in 2015 was only 45% as wide as it was in 1938. The capacity loss is also due to sediment filling the channel, with nearly 2 m of aggradation measured at a stream gauge since 1965. Comparison of channel cross sections surveyed in 1994, 2007, and 2016 show that about 20,000 m3 yr−1 (34,000 Mg yr−1) of sediment is accumulating in the Skokomish River. The nature, timing, and spatial pattern of this channel narrowing and shallowing are consistent with the response expected from the Cushman Project, which exports water out of basin and thus substantially reduces downstream flows, but, because the dams were built below a natural lake, does not reduce the sediment supply. While sediment yield from the South Fork is high, accounting for about three-fourths of the total sediment supplied to the Skokomish River, it is dominated by the progressive widening of the channel and recruitment by lateral fluvial erosion of glacial sediments in alluvial terraces; landslides associated with logging in the South Fork basin produced a small amount of sediment relative both to the sediment produced by channel widening in the upper South Fork and to the rate of aggradation in the mainstem Skokomish River. The naturally-high sediment load from the South Fork and the flow reduction in the North Fork result in the unusual effect of flooding having increased downstream of the dams despite substantial reductions to peak flows. This case study illustrates how a watershed-scale analysis of multiple land uses and flow management and their interaction with the basin's geology and geomorphology can make use of geomorphic evidence to differentiate among the possible drivers of channel change and associated flooding.