Slope Fan Research Articles

Deposition, deformation, and flexure in a transpressional trough, Queen Charlotte fault, offshore Haida Gwaii (British Columbia, Canada)

Transpressional deformation along the Pacific–North American plate boundary off British Columbia (Canada) generates interactions between tectonic and depositional processes; first-order deformation is creation of a bathymetric trough by flexure of the Pacific plate. Interpretation of a suite of single-channel seismic reflection data in the central and southern trough shows second-order internal structure and depositional systems within the trough. Much of these sediments originated from Quaternary slope fans, although the age of the underlying oceanic crust is 8–13 Ma. These turbidite deposits overlie hemipelagic deposits. Two distinct layers of turbidite deposits in the trough can be explained by sediment deposition systems rather than tectonic events. We infer that the most recent flat-lying deposits, which have been previously interpreted in the northern trough as indicating a lack of compressive deformation, consist, in fact, of along-axis flows based on the general geomorphology of the margin. Second-order deformation consists of small-offset extensional and compressive structures common within 15 km of the foot of the continental slope. A few normal faults are observed tens of kilometers further out on the flexural bulge. Recent faults that cut the seafloor are observed offshore from the rupture plane of the 2012 M7.8 earthquake and match an area of extensional aftershocks. These faults may not be from bending of the entire bulge, which is 30–40 km wide, but from the plate being pulled into the underthrust zone. Maximum depression of the Pacific plate is also offshore from the region of recent rupture, indicating that this rupture is representative of long-term geologic processes.

Spectral decomposition predicts the distribution of steep slope fans in the rift basin of eastern China

Deep reservoirs associated with gravity-flows are garnering considerable attention. Predicting reservoirs deposited by nearshore subaqueous fans is challenging and often underreported in seismic sedimentology analysis. Utilizing post-stack seismic attributes is a quick and straightforward method for quantitatively characterizing these reservoirs. However, reservoir prediction deteriorates when dealing with complex sedimentary volumes and intricate tectonic development. Spectral decomposition (SD) offers an alternative approach to optimize the seismic data. The frequency-dependent S-transform (ST) holds great potential in seismic interpretation. SD based on the ST was employed in the seismic sedimentary characterization of steep slope complex fan reservoirs. Three fourth-order sequence stratigraphic boundaries and three complex fans were ideally shown on seismic frequency decomposition profiles. A 20 Hz seismic sedimentology analysis frequency was determined by comparing three spectral decomposition results following the well-seismic reflection analysis. The internal architectures of fan deltas and the individual outlines of nearshore subaqueous fans were more distinguishable in 20-Hz frequency decomposition data than in full-frequency data. The progradation direction of steep slope fans can be better recognized in frequency decomposition profiles compared to full-frequency seismic data. Three factors influence the seismic sedimentary characterization and prediction of steep slope fans when employing SD. The ability of the ST to preserve phase is crucial for improving the imaging quality of the amplitude attribute. Sedimentary mechanisms control the sedimentary features of steep slope fans, impacting the imaging of seismic attributes. While channelized fan deltas can be better identified, unchannelized nearshore subaqueous fan deposits, which exhibit more heterogeneous sedimentary characteristics, present limitations. The unique volcanic evolution is another factor that impacts the image of the root-mean-square (RMS) attribute. Despite demonstrating excellent local adaptability in signal analysis, the S-transform cannot fully compensate for the combined effects of faults and sedimentary heterogeneity in nearshore subaqueous fans.

Geological characteristics and seismic identification of reservoirs in the Colorado Basin off the Atlantic coast of Argentina

Abstract The Colorado Basin is a hot exploration frontier basin conjugated with the southwest coast of Africa which has undergone four evolutionary stages: Carboniferous-Jurassic pre-rift, Jurassic-Early Cretaceous rift, Late Cretaceous transition and passive marginal drift since Paleocene. Major oil and gas discoveries have been made in the conjugated Orange Basin in recent years, including the Graff field and Venus field discovered in 2022 with recoverable reserves of 550 Mboe and 2.3 Bboe, respectively. This indicates that the basin has good oil and gas exploration potential. Based on the analysis of drilling data and seismic data, it is concluded that the basin has a mature oil-gas generation system and better reservoir conditions. The pre-rift and syn-rift source rocks are confirmed by 5 Wells with oil and gas shows in the shallow water shelf, and all have entered the hydrocarbon generation threshold. Reservoir distribution and trap-sealing conditions have become key factors in oil and gas exploration. this paper describes the reservoir characteristics in the drilling data, and extrapolates the seismic reflection characteristics of the same strata, and predicts the reservoir distribution in the unexplored area of the basin. The reservoir lithology and sedimentary facies are analyzed based on the gamma logging curves. Reservoir identification and reasonable prediction on seismic profile are mainly determined by seismic reflection amplitude and horizon calibration. The basin has three sets of reservoirs, namely, pre-rift Permian underwater fan sandstone, Lower Cretaceous alluvial fan sandstone of syn-rift period and Upper Cretaceous Colorado Formation shallow marine sandstone of transitional period. Among them, Colorado Formation sandstone reservoir has the best physical property conditions, with a maximum porosity of 32% and a thickness of 1000m. The Cretaceous and Paleogene turbidite sandstone of slope fan facies are potential reservoir in the deep waters, which mainly form litho-stratigraphic traps in the continental slope.

Application of Seismic Sedimentology in Identifying Submarine Fan Bodies

Abstract The research area is located in the Yenisai Hatanga Basin of Russia, which is a transitional facies sedimentary rock between sea and land. Oil and gas are mostly concentrated in turbidite sand bodies of the low level domain submarine fans of Lower hutt formation in the Lower Cretaceous, and locally developed in the slope fan sand bodies of the high postion domain. which is good oil and gas reservoir, however, due to limitations of geological research conditions and data, the sedimentary characteristics and planar distribution of the submarine fan sand bodies are not clear in research area. This article conducts research on the identification and plane distribution prediction of submarine fan bodies using seismic sedimentology methods, By combining sequence stratigraphy, seismic profile, and RGB fusion technology, the sand bodies of submarine fans are analyzed. Through the correlation analysis of seismic profiles and seismic facies, corresponding relationship between sedimentary microfacies and typical seismic wave patterns is established. The study shows that the study area mainly develops continental shelf slope facies, submarine fans, and shallow to semi deep marine facies. Sediments formed a turbidite submarine fan composed of sandstone siltstone mudstone in the ancient trough. There are 5 sand groups developed in the Hutt Formation under the main target layer in the study area. Among them, the IV oil group is the main oil-bearing layer. The 0, Ⅰ, and Ⅱ sand groups have limited distribution and poor sand body connectivity. The Ⅲ sand group is mainly developed in the study area. In the north, it is distributed in the middle of the study area on the plane. The IV sand group is widely developed and distributed in the middle of the study area on the plane. Seismic sedimentology can provide technical support for prediction research of reservoirs.

Morphological characteristics and controlling factors of the piedmont fan systems in the Zanskar region, Northwest Himalaya, India

Piedmont fans are prominent geomorphic features formed at the transition between mountain slopes and valley floors. This study investigates the morphology of alluvial fans in the Zanskar Basin (ZB) to uncover the key variables influencing their development and morphodynamics. Utilizing advanced GIS and remote sensing techniques, along with field investigations, we conducted a detailed spatial analysis of 103 alluvial fans along the Doda, Tsarap, and Zanskar rivers. This approach allowed for precise mapping and characterization of these fans within complex depositional settings of ZB, particularly where fans merge into bajadas. Our analysis revealed distinct characteristics for the fans, including Fan Area (FA), Fan Slope (SF), Radius (R), Base Length of Fan (BF), Fan Maximum Entrenchment (FME), and Flow Expansion Angle (FEA). A morphometric analysis was then conducted to evaluate the correlation between the fans and their upstream basins. The linear regression analysis demonstrated both positive and negative correlations between these parameters, highlighting the important role of the upstream basins in controlling sediment delivery to fans. The findings suggest that larger basins contribute to the morphological development of fan systems, with larger, less steep fans forming as a result of greater flows and increased sediment supply from basins with denser drainage networks. Lower values of Mountain Front Sinuosity, Valley Floor Width to Valley Height ratio and Basin Elongation suggest that upstream basins in the ZB are significantly influenced by tectonic forces, resulting in linear mountain fronts, V-shaped valleys and elongated upstream basins. The F-99 fan, in particular, has developed a prominent stepped-fan morphology, attributed to differential uplift, vertical incision, and lateral migration of channels across the fan surface. Along the various fronts of the Zanskar, fan morphology is controlled by a complex interplay of long-term tectonic processes, climate, upstream lithology, and basin characteristics. Tectonic forces, particularly the NW-SE-trending ZSZ/STD and ZCT, exert first-order control on fan morphology by influencing sediment-flux and accommodation space. This influence is evident in tectonically modified landforms such as active mountain fronts, fan terraces, elongated basin shapes, wine-glass valleys and triangular facets, all indicating recent uplift and active tectonics in the region. Our results indicate that ZSZ and ZCT exert significant tectonic control over the geometry and evolution of fans, alongside substantial climatic influences.

Tectonic controls on the morphometry of alluvial fans in an arid region, northeast Iran

ABSTRACT The morphology of alluvial fans provides valuable information about changes in tectonic, climate, base level, and drainage basin characteristics. This study aims to evaluate the relationship between active tectonics and morphometric characteristics of alluvial fans in the less studied southern region of the Joghatay mountain, northeast Iran. The study area includes 13 alluvial fans and their corresponding catchment basins in a region with semi-arid climate. Morphometric characteristics of alluvial fans and their corresponding drainage basin including fan area, fan slope, sweep angle, fan toe length, fan radius, basin area, basin slope, fan’s width-to-length ratio, asymmetry factor, and basin shape were measured. Our results revealed a negative relationship between the slope of alluvial fans and their corresponding basin area. The weak correlation between the drainage basin area and the fans area may be attributed to the lithology and active tectonics in the study area. The small sweep angle values of the studied alluvial fans can be attributed to the activity of thrust faults and presence of an uplifting anticline that resulted in formation of elongated alluvial fans in our study area. Analysis of the relationship between the morphometric characteristics of the fans and their corresponding basins indicated a negative correlation, which can be attributed to the sediment transport efficiency of the basins.

Ponding and compartmentalization of a giant Paleogene slope fan by mass transport complexes, offshore Newfoundland, Canada

The Ephesus fan is a giant channel-lobe complex that extends across 460 km2 in the lower structured slope of the West Orphan Basin, offshore Newfoundland, Canada. Based on biostratigraphic dating of offset wells, the fan was likely deposited during the Oligocene. High resolution 3D seismic data reveal that the fan overlies a succession of mass transport deposits that initially created a flatter topographic surface, with later slumping events building elongated ridges. Subsequent high density turbidity currents coursed into the area through a single feeder channel, depositing as lobes onto the flatter area via a network of distributary channels. The lobes were circumferentially ponded and internally compartmentalized by the slump ridges, and the ponding forced the fan to assume its unique, 46 km-wide shape. Distributary channels were initially directed to the south side of the fan, due to the higher relief of the mass transport deposits to the north. After the infill of the accommodation space to the south, subsequent sediment input levelled the remaining rugose topography, and some distributary channels were able to avulse north. Throughout deposition of the fan, distributary channels also continued down-dip from the lobe complex, indicating some sediment bypass continuing down the lower slope. Overlying laminated mudrock displays pervasive polygonal faulting, signaling dewatering processes and the end of deposition by muddy submarine landslides and high density turbidity currents.

Sedimentary architecture and evolution of a Quaternary sand-rich submarine fan in the South China Sea

Investigating the sedimentary architecture and evolution of sand-rich submarine fans is vital for comprehending deep-water sedimentary processes and enhancing the success rate of hydrocarbon resource exploration. Recent drilling activities in the Qiongdongnan Basin, northern South China Sea, have unveiled significant gas hydrate and shallow gas potential. However, exploration in this area faces substantial challenges due to the limited understanding of sandy reservoirs. Leveraging extensive newly acquired extensive 3D seismic data (~9000 km2) and well data, our study reveals five distinct deep-water depositional systems in the Quaternary Ledong Formation, including a submarine fan system, mass transport deposits, deepwater channel-levee systems, slope fans, and hemipelagic sediments. Notably, the targeted sand-rich submarine fan lies within the abyssal plain, situated at a water depth of 1300-1700 m. This fan exhibits a unique tongue-shape configuration and a SW-NE flow direction within the plane and spans an expansive area of ~2800 km2 with maximum length and width reaching 140 km and 35 km, respectively. Vertically, the fan comprises five stages of distributary channel-lobe complexes, progressing from Unit 1 to Unit 5. Their distribution ranges steadily increase from Unit 1 to Unit 3, followed by a rapid decrease from Unit 4 to Unit 5. Our results suggest that the occurrence and evolution of the submarine fan are primarily controlled by sea level fluctuation, confined geomorphology, and sediment supply. Specifically, sea level fluctuation and sediment supply influenced the occurrence of the submarine fan. Concurrently, the confined geomorphology in the abyssal plain provided accumulation space for sediments and shaped the fan into its distinct tongue-like form. In contrast to the deepwater channels within the deepwater channel-levee systems, the distributary turbidite channels within the submarine fan are marked by lower erosion depth with “U” shapes, greater channel width, and higher ratios of width to depth. The comparative analysis identifies turbidite channels as the focal points for offshore gas hydrate and shallow gas exploration in the Qiongdongnan Basin. Furthermore, the temporal evolution of submarine fan offers valuable insights into Quaternary deep-water sedimentary processes and hydrocarbon exploration within shallow strata of marginal ocean basins.

Blast-Fragmentation Prediction Derived From the Fragment Size-Energy Fan Concept

The size-energy fan concept formulation is developed into a prediction model of the fragment size distribution from blast design and rock mass variables. The fragment size is scaled with a characteristic size of the blast and the rock mass discontinuity spacing and orientation. The energy is scaled with the rock strength and the cooperation degree between adjacent holes. A cooperation function is introduced that modifies the energy with the in-row delay, non-dimensionalized with the P-wave velocity and the holes spacing. The cooperation increases as the delay increases up to a certain value, beyond which the cooperation decreases and the fragmentation is coarser. Several prediction models are presented, using the Swebrec-based fan slopes function of the percent passing as starting point, with subsequent improvements involving alternative formulations of that function, that encompass a non-Swebrec underlying distribution of the fragment size. The models include 12 to 14 parameters, controlling the effect on fragmentation of the variables describing the rock mass, the explosive and initiation sequence, and the blast geometry. The parameters are determined from fits to the data base that was used for the xP-frag model, expanded with seventeen additional blasts. All fragmentation data used are mass size distributions determined by sieving and weighing of blasted muckpiles. The different models are introduced sequentially and discussed. The models presented improve the performance of xP-frag, while including a much smaller number of parameters and, unlike xP-frag, keeping the physically sound size-energy fan pattern, effectively extending its nature from a descriptive frame of the fragmentation-energy relations, to a predictive tool.

A Method for Defining Sedimentary Characteristics and Distributions and Its Application in Qinnan Depression, Bohai Bay Basin

A new method incorporating geophysical analysis and geological analysis is proposed to define the sedimentary characteristics and distributions in basins with few drilling wells to promote the exploration of reservoirs. This method is applied to a study, through which its principles, closed-loop workflow and technologies are introduced in detail and the sedimentary characteristics and distributions of the study area are accurately defined. During the application process of the method, a compatible geological model is established, based on which the seismic data are interpreted and the results derived from the interpretation are further verified via seismic forward modeling. The study results exhibit a successive sand-rich deposition from the retrogradational gully-filling gravity flow deposition including near-shore fans, slope fans and basin-floor fans delimited by different slope break belts in transgressive sequences to the progradational delta deposition in a retrogressive sequence including braided river deltas with a long extension distance and fan deltas developed along a steep slope belt. And the potential reservoirs are located at the point-out sites of sand bodies with lower average P-wave velocities than those of muddy sediments. The proposition and application of this method are of great significance for oil and gas exploration.

Delineation of Gas Sand Reservoir Prospect in the “A” Field, Deep Water Area of Kutai Basin

Kutai basin hydrocarbon’s potential is not only in the delta area but extends to the deepwater area of East Kalimantan. The result of the integration method based on petrophysical, inversion, and seismic multiattribute aim to delineate the gas reservoir potential zone in the research area. The reservoir rocks consist of graywacke to sublitharenite of Upper Miocene sandstone with an average shale volume content 19%, porosity 23%, and water saturation 55%. The reservoir gas is reflected in the Vp/Vs ratio range of 1.58 to 1.8, the Poisson ratio value of 0.17 to 0.3 GPa, and the incompressibility value of 8 to 22 Gpa*g/cc. The distribution potential reservoir zone is indicated by the low acoustic impedance anomaly value of 4000 to 7500 m/s*gr/cm3, high RMS attribute anomaly value of 2000 to 10000, and distribution of low shale volume, high porosity, and low incompressibility will be estimated by using multi-attribute linear regression and probabilistic neural network methods that concentrated on the upper slope fan in the southern area.

The Effects of Porosity on The Anisotropy Parameters of The Slope Fan Facies Sand Reservoirs in The Deepwater Kutai Basin, East Kalimantan, Indonesia

For good interpretation and modelling, knowledge about the effects of anisotropy and its relationship with reservoir properties is needed. Related to this issue, this study evaluated the effects of porosity on the anisotropy parameters for sand reservoirs deposited in the upper slope fan facies. The study objects are obtained from core plug samples of sand reservoirs in the deep-water Kutai Basin. Core plug samples were collected from 13602.5 ft to 13704.5 MD depth. The ε, γ, δ, and η anisotropy parameters data were obtained by ultrasonic measurements, and porosity data were obtained by laboratory measurement with Coreval 700 apparatus and Boyle’s law. The analysis results show that the relationship of the anisotropy parameters with porosity appears when high-porosity sandstone and low-porosity sandstone are separated. The plots of anisotropy ε, γ, and δ, show trends for greywacke, with increasing anisotropy value the porosity increases. The effect of porosity on the high porosity (29%-37%) sandstone shows a steeper change than on the lower porosity (12%-13%) sandstone. The analysis also shows that the higher composition of lithic mineral grain reduces the effect of anisotropy on porosity.

Using machine learning to predict processes and morphometric features of watershed

The research aims to classify alluvial fans’ morphometric properties using the SOM algorithm. It also determines the relationship between morphometric characteristics and erosion rate and lithology using the GMDH algorithm. For this purpose, alluvial fans of 4 watersheds in Iran are extracted semi-automatically using GIS and digital elevation model (DEM) analysis. The relationships between 25 morphometric features of these watersheds, the amount of erosion, and formation material are investigated using the self-organizing map (SOM) method. Principal component analysis (PCA), Greedy, Best first, Genetic search, Random search as feature selection algorithms are used to select the most important parameters affecting erosion and formation material. The group method of data handling (GMDH) algorithm is employed to predict erosion and formation material based on morphometries. The results indicated that the semi-automatic method in GIS could detect alluvial fans. The SOM algorithm determined that the morphometric factors affecting the formation material were fan length, minimum height of fan, and minimum fan slope. The main factors affecting erosion were fan area (Af) and minimum fan height (Hmin-f). The feature selection algorithm identified (Hmin-f), maximum fan height (Hmax-f), minimum fan slope, and fan length (Lf) to be the morphometries most important for determining formation material, and basin area, fan area, (Hmax-f) and compactness coefficient (Cirb) were the most important characteristics for determining erosion rates. The GMDH algorithm predicted the fan formation materials and rates of erosion with high accuracy (R2 = 0.94, R2 = 0.87).

Alluvial fan and fan delta facies architecture recording initial marine flooding in the Mio‐Pliocene syn‐rift sequence of the Fish Creek‐Vallecito Basin, southern California

AbstractThe timing and character of the initial marine flooding of extensional basins has implications for their tectonic history. Yet, the recognition of such flooding is difficult along rift basin margins due to the dominance of coarse‐grained systems and the lack of marine fauna. This study conducts detailed facies and stratigraphic analysis of a Mio‐Pliocene alluvial fan and fan‐delta succession in the Fish Creek Vallecito Basin in southern California. Our goal is to characterize the marine flooding surface, determine the paleogeographic position of the shoreline and estimate the magnitude of relative base‐level rise that occurred during the marine incursion associated with the opening of the Gulf of California. Our results show that the flooding of the Elephant Trees alluvial fans is often marked by an abrupt lithologic and facies change from meter‐scale boulder‐rich subaerial debrites (proximal alluvial fan facies association) to centimetre‐scale granule‐rich subaqueous debrites, ripple‐laminated sandstones and mudstones (prodelta facies association). By delineating the zone of transition between the subaerial and subaqueous facies, we place the initial flooding paleo‐shoreline 4 km up the alluvial fan's paleo‐depositional slope. Considering alluvial fan slope gradients between 1° and 5°, this 4 km transgression would require an estimated 70–350 m of water depth during the initial marine incursion. Interfingering of fan‐delta deposits with subaqueous marine and planktonic‐rich evaporites suggests that the basin was below sea‐level after, and perhaps even before, the marine flooding. Subaerial subsea‐level basins exist in Death Valley and the Salton Trough today within similar extensional and transtentional tectonic regimes. This subaerial subsea‐level interpretation might explain the high magnitude and abrupt relative base‐level rise recorded by the facies transitions in the Fish Creek Vallecito Basin. These results suggest that the Fish Creek Vallecito Basin underwent significant extension during its early and nonmarine depositional phase, allowing it to reach subsea‐level elevations. The tectonic history of the Fish Creek Vallecito Basin maybe similar to other extensional basins where rapid subsidence allows the accumulation of nonmarine strata below sea‐level prior to the marine flooding, then restricted and deep marine strata immediately after flooding.

Late Oligocene to early Miocene delta and linked slope fan systems: Depositional architecture and sediment dispersal, the Pearl River Mouth Basin

AbstractThe Upper Oligocene to Lower Miocene (Zhuhai Formation) in the Pearl River Mouth Basin of the northern South China Sea contains extensive shelf‐edge delta and linked slope fan deposits, and records the sediment dispersal and constraint processes. According to the integrated analysis of well‐log, seismic and core data, depositional architecture and delivery mechanism of sandy deposits from the shelf margin to the lower slope are documented systematically. Four major depositional facies are identified: shelf‐edge incised channels and shelf‐edge deltas; amalgamated slump‐channel complexes; slope gullies; and slope fan deposits. Incised channels at shelf‐edge delta front (2.0 to 4.4 km wide) comprise V‐shaped single channels and W‐shaped or irregular‐shaped channels which eroded shelf‐edge sandy deltaic deposits and retransferred them to the lower slope. Further slumping along channel walls may be triggered by the continuously channel downcutting process; and then these incised channels amalgamated and collapsed downslope to generate large‐scale coalesced slump‐channel complexes (15 to 35 km wide), which are regarded as the main delivery system to transport sandy deltaic deposits to the deep‐water basin. There are a series of V‐shaped or W‐shaped slope gullies (tens to hundreds of metres wide) developed along the lower slope, which further delivered sediments to slope fan systems. Slope fan systems are composed predominantly of basal debris flow massive and sandy turbidite channel and frontal splay deposits comprising one of the most important oil and gas reservoirs in the study area. Channelization and large‐scale slumping along the shelf‐edge delta front are regarded as the most important dispersal mechanisms delivering sandy sediments across the shelf margin. Falling sea level was suggested to be one of the important factors causing the incision and large‐scale shelf margin slumps, whereas the unstable high (800 to 1200 m) and steep (5° to 8°) shelf‐edge delta and slope clinoforms provide a pre‐condition for large‐scale shelf‐edge collapse.

Influence of fine particle content in debris flows on alluvial fan morphology

Alluvial fans are large-scale depositional structures commonly found at the base of mountain ranges. They are relatively soil-rich compared to the rocky terrains, or catchment areas, from which their material originates. When frequented by debris flows (massive, muddy, rocky flows) they contribute significantly to local hazards as they carry focused, collisional, fast-moving materials across alluvial fans, unpredictable in size, speed, and direction. We research how fine particle content in debris flows correlates with directional changes, i.e., debris flow avulsions. Toward this, we analyzed field data from two neighboring alluvial fans in the White Mountains (California, USA) that exhibit dramatically different topographies despite their proximity and associated similar long-term climates. Informed by these measurements, we performed long-term and incremental alluvial fan experiments built by debris flows with systematically-varied fine particle content. We found that (1) decreasing fine particle content increases the variability of fan slopes and associated channelization dynamics, and (2) for all mixtures longer-term continuous alluvial fan experiments form more complex surface channelizations than repeated flows for the same total time, indicating the importance of both particle sizes and timescales on alluvial fan surface morphology.

Computational morphology of debris and alluvial fans on irregular terrain using the visibility polygon

A computational method is proposed to simulate the morphology of debris and alluvial fans formed on irregular terrain, where they may abut or bend around steeper valley sides or obstacles. Fan elevation and slope are assumed to depend only on the distance from the apex, measured along straight or winding paths. When there are no obstacles, the resulting fan morphologies are simply surfaces of revolution, with a straight (for constant slope fans) or curved generatrix (for concave fans). When obstacles are present, however, fan surfaces of great variety can be produced. To capture the resulting morphology, we exploit a powerful tool of computational geometry: the visibility polygon algorithm. Starting from one or more initial fan apexes, the proposed algorithm iteratively generates fan sectors with new apexes along their margins, until the fan complex has attained its maximal extent. To validate the method, we first apply it to several cases with analytical solutions, such as single fans on faceted topography, and multiple coalescing fans with weld lines. We then apply the method to real valleys, and check that it can reproduce observed fan morphologies even in complex cases involving diffluences and confluences.

Depositional architecture and aggradation rates of sand-rich, supercritical alluvial fans: Control by autogenic processes or high-frequency climatic oscillations?

Alluvial fans are important paleoclimatic archives, that may record high-frequency climatic oscillations. However, climate signals may be overprinted or even be destroyed by autogenic processes caused by channel avulsion and lobe switching. Here we present new data from two different Late Pleistocene (MIS 3–2) alluvial fan systems in northern Germany and compare these systems to experimental alluvial fans and other field examples. The selected fan systems formed under similar climatic and tectonic conditions, but differ in size, type, and drainage area allowing to estimate the role of climate and autogenic controls on flow processes, facies architecture, and fan-stacking patterns. Luminescence dating is used to determine the timing of fan onset and aggradation. Fan onset occurred in response to climate change at the end of MIS 3 when temperatures decreased and periglacial climate conditions were established in northern central Europe. A related increase in sediment supply and strongly variable precipitation patterns probably promoted fan formation. The major period of fan aggradation was approximately between 33 and 18 ka, followed by fan inactivity, abandonment, and incision during the Lateglacial. The highest aggradation rates occurred during the early stage of fan building, when up to 35 m thick sediment accumulated within a few thousand years. Sand-rich, sheetflood-dominated fans are related to larger, low-gradient fan catchments. Steep depositional fan slopes (5°–17°) and short-lived high-energy floods promoted supercritical flow conditions. Well sorted, sediment-laden, rapidly waning flows favored the deposition and preservation of supercritical bedforms and allowed for the aggradation of stable antidunes. Steep, dip-slope catchments enhanced stream gradients and promoted the transport of coarser sediments. These fans have lower gradient slopes (2–6°) and are dominated by channelized flows, alternating with periods of unconfined sheetfloods. Meter-scale coarsening upward successions, characterized by sandy sheetflood deposits at the base, overlain by multilateral or smaller single-story gravelly channel fills may be related to high-frequency climatic fluctuations or seasonal fluctuations in water and sediment supply. These coarsening-upward successions are commonly bounded by a paleo-active layer, from which ice-wedge casts penetrate downwards. The comparison to experimental fans and other field examples implies that the recurrent pattern of multistory, multilateral and single-story channel bodies with a lateral offset to vertical stacking pattern most probably was controlled by autogenic switch in an avulsion-dominated system. The change in deposition from alluvial-dominated processes to aeolian sedimentation with minor alluvial influences during the Lateglacial records alternation of dry and ephemeral wetter phases that are related to rapid climatic variations. The main phase of aeolian sand-sheet deposition probably correlates with Heinrich event H1 between approximately 18–16 ka and reflects sedimentation in response to aridification and high mean wind speeds.

Geophysical analysis to delineate a Class-I AVO prospect in the offshore east coast of India: A case study

Characterization of deep-water reservoirs poses enormous challenges to interpreters because of its inherent complexity. Therefore, defining reservoirs' extent and predicting their properties in absence of drilled well or away from drilled wells has always been difficult. In deeper targets, the sensitivity of seismic amplitude to fluid change gets considerably reduced, therefore detection of fluid and their contact remains challenging. In such a case, the application of extended elastic impedance can be helpful in the delineation of prospective reservoirs and detection of fluid contact. This is demonstrated with a case study of a Miocene slope fan prospect in the deepwater Mahanadi basin on the east coast of India. The prospect is typical Class-I in terms of amplitude variation with offset where the presence of fluid is hard to identify in conventional full-stack data. An appropriate extended elastic impedance attribute for fluid has been generated and interpreted to define the reservoir boundary and detect fluid contact. The amplitude of the fluid attribute map is corrected for tuning effect as there are variations in reservoir thickness across the area. Additionally, in order to detect fluid contact, a common contour binning technique has been applied which helps to locate the depth contour where possible fluid contact could be present. The case study has added significant values in de-risking a Class-I prospect without any anomalous amplitude increase with offset.

An integrated source rock potential, sequence stratigraphy, and petroleum geology of (Agbada-Akata) sediment succession, Niger delta: application of well logs aided by 3D seismic and basin modeling

We investigated the source rock potential, sequence stratigraphy, and characterized hydrocarbon reservoirs at Otumara field, Niger delta, using integrated 3D seismic, wireline log analysis, and basin modeling. The burial history and thermal maturity were modeled, the reservoirs were delineated, and the petrophysical parameters were also estimated from the wireline logs. The Passey “ΔLog R” method for estimating the preliminary evaluations of the total organic carbon (TOC) from integrating sonic, neutron, and density with resistivity has been used. The results indicate that the primary source rock of hydrocarbons is the Upper Akata Formation, despite a higher TOC percentage in the Agbada Formation. Based on sequence stratigraphy analysis, TA4, TB1, TB2, and TB3 second-order supercycles were obtained in the studied well TD46. The results also revealed that the field has two large net pays with high-quality reservoir facies: a deltaic slope fan at the upper shoreface and a river mouth sandbar at the lower shoreface. Furthermore, the reservoirs were faulted by a series of growing faults that faulted the basin slope. The reservoir facies are characterized by an average of 18% porosity, 1200 mD permeability, 16% volume of shale, and high hydrocarbon saturation of about 85%. Finally, the petroleum system elements have been defined for improved hydrocarbon exploration. In the absence of complete or partial core samples, this case study emphasizes the importance of using wireline logs to estimate organic richness and investigate sequence stratigraphy in clastic sediments.