Shoal Research Articles

The controlling effect of chair-shaped slope break landforms on sedimentation in continental faulted basins: A case study in the Xilinhaolai area, Baiyinchagan depression, Erlian Basin, China

To reveal the geomorphic characteristics of the gentle slope zone in a continental faulted basin and its control on sedimentary systems, we have performed the geomorphic units division and comprehensive analysis of seismic facies, well-log facies, and core facies in the Xilinhaolai area in the western area of the Baiyinchagan depression, North China. The results find that the gentle slope in the Xilinhaolai area developed a chair-shaped slope break, which includes four geomorphic segments: steep slope of the depression margin (SSDM), shallow-water gentle slope (SWGS), deepwater steep slope (DWSS), and depression center. The chair-shaped slope break controlled the distribution of the sand body by controlling the water depth and hydrodynamic type of sedimentation. The SSDM was a favorable place for the deposition of beach bars. It has become a good oil reservoir not only because of its shallow burial and good physical properties but also because of the location configured with stepped faults. On the SWGS, the active river current and lake waves caused the deposition of large-scale braided river deltas and shoal bars, which are already the main oil and gas areas discovered in the Xilinhaolai area. On the DWSS, a large-scale slump turbidite fan developed and became a very favorable oil and gas reservoir near the oil and gas source rock. The slumped turbidite fans on the DWSS and the shoal bar on the SSDM are potentially favorable reservoirs for future oil exploration.

A Modeling Approach for Beach-Bar Sand Reservoirs Based on Depositional Mode and Sandbody Volume

Beach-bar sand in lacustrine facies represents one of the most significant reservoirs. Depending on the depositional characteristics, it can be further divided into two different sedimentary microfacies, beach sand and the bar sand. Favorable reservoirs are often developed in bar sand. The lower section of the upper part of the 4th member of the Shahejie Formation in the Gao89-1 block is a typical nearshore shallow water beach-bar deposit. Oil distribution is influenced by lithofacies and physical properties. In order to better characterize the heterogeneity within beach-bar sandbodies, a modeling method based on the depositional mode and sandbody volume is proposed. Firstly, a sandbody model is established. On this basis, an algorithm for distinguishing between beach and bar sand based on vertical thickness is proposed. The model is post processed based on the sandbody volume to remove unreasonable sandbodies. The method allows for a more realistic three-dimensional geological model of the beach-bar sands in the study area than the classical two-point geostatistical, object-based, and multi-point simulation method. A facies-controlled modeling approach is used to establish a petrophysical property model on this foundation; the result shows that the property models better reflect the characteristics of the petrophysical distribution in the Gao89-1 block.

Shaking table test and numerical research on seismic response of a CFST arch bridge for different soil grounds

Severe damages or failures of arch bridges on different soil grounds have been reported during previous strong earthquakes. The influence of different soil grounds on the seismic response of arch bridges is unclear. In this study, three large-scale shaking table tests were conducted to investigate the seismic response of a model bridge abutment of a concrete-filled steel tubular (CFST) arch bridge, which was set inside a model soil box at a reduced scale of 1/90. Grounds with three types of soil were simulated: silty clay, clay-gravel layered, and bare gravel soil. The accelerations and displacements of the model soils and the sliding displacement and settlement of the model bridge abutment were evaluated through the tests. The test results showed that the soil types significantly affected the seismic response of the soil and bridge abutment system, and the spectral characteristics of input waves significantly affected the acceleration amplitude ratio (AAR) of the bridge abutment; however, the changing trend of the AAR was usually related to the corresponding soil types. The soil grounds exhibited a remarkable filtering effect along the soil heights under seismic excitations, and the filtrations of the frequency components of the input waves for different soil grounds shows significant difference. Furthermore, the dynamic shear stress–strain curves showed that the silty clay soil remained in the nonlinear behavior development stage under strong seismic excitations, while the bare gravel soil remained in the linear behavior development stage. The energy dissipation capacity of the bare gravel soil ground could not be utilized. The test results in this study can be used for seismic risk assessment of different soil grounds, to guide the optimal seismic design of arch bridges for different soil grounds.

Impacts of Fracture Roughness and Near-Wellbore Tortuosity on Proppant Transport within Hydraulic Fractures

For unconventional reservoir hydraulic fracturing design, a greater fracture length is a prime factor to optimize. However, the core observation results from the Hydraulic Fracturing Test Site (HFTS) show that the propped fractures are far less or shorter than expected, which suggests that the roughness and tortuosity of hydraulic fractures are crucial to sand transport. In this study, a transport model of sands is first built based on experimental measurements on the height and transport velocity of the sand bank in fractures with predetermined width and roughness. The fracture roughness is quantified by using the surface height integral. Then, three-dimensional simulations are conducted with this modified model to further investigate the impact of tortuous fractures on sand transport, from which a regression model is established to estimate the propped length of hydraulic fractures at a certain pumping condition. The experiment results show that the height of the sand bank in rough fractures is 20–50% higher than that in smooth fractures. The height of the sand bank decreases with the reduction in slurry velocity and increases with the increase in sand diameter. Sand sizes do little effect on the transport velocity of the sand bank, but the increase in slurry velocity and sand volume fraction can dramatically enhance the migration velocity of the sand bank. The appearance of tortuous fractures decreases the horizontal velocity of suspended particles and results in a higher sand bank compared with that in straight fractures. When the sand bank reaches equilibrium at the tortuous position, it is easy to produce vortices. So, there is a significant height of sand bank change at the tortuous position. Moreover, sand plugging can occur at the entrance of the fractures, making it difficult for the sand to transport deep into fractures. This study explains why the propped length of fractures in HFTS is short and provides a regression model that can be easily embedded in the fracturing simulation to quickly calculate dimensions of the propped fractures network to predict the length and height of propped fractures during fracturing.

NATURAL FEATURES AND HYPOTHESES OF ORIGIN NEASHORE BARS IN THE COASTAL ZONE OF NON-TIDAL SEAS

Problems of the research. The article considers the origin, morphology and dynamics of unique forms of coastal-marine relief in the coastal zone of non-tidal seas, under the influence of the hydrogen factor, an exceptionally strong potential of mechanical energy, sufficiently large reserves of sand deposits (fractions of 0.05–1.00 mm), active influence of wave impulses and wave currents in the zone of energy dissipation, active nanoexchange between the shore and the underwater slope. It is shown that this issue has a long history of scientific research in different countries due to its important practical significance. Long years we researching the alongshore sand drifting between C. Bol. Fontain in North to Jebriyan Bay in South as a experimental natural object of the North-Western part of the Black Sea. Theoretical elaborations and methods. Among the natural features of the named landforms, the researchers distinguish 8 leading ones, which are manifested only in the environment of the coastal zone. Therefore, the purpose of the article is to consider the genetic hypotheses of the formation of underwater sand bars in the coastal zone of the seas over the past 100 years, and on this basis – to outline, evaluate, analyze several hypotheses about the formation of bars, show their positives and disadvantages, name the genetic features for the physical geographical. coastal environments As part of the eight most important and most common features, we have identified the following: distribution of wind wave sizes and wave current velocities; the value of the steepness of the underwater slope covered with sediments; value of sediment sizes and hydraulic value of coastal zone sediments; some processes of wave transformation and dispersion of wave energy; certain individual processes of sediment movement of different hydraulic magnitudes in a cell of a sandy coastal zone; steepness of wind waves during storms in the spray source; active action of sand drifts by movement along the coastline and under the influence of the regime of transverse sediments; the nature of underwater swells is different, because one of them moves from the underwater slope to the shore, another – from the shore towards the sea, and the third is dynamically stable. Main results and conclusions. As can be seen, the main hypotheses and the corresponding features of the formation of underwater swells act on their own, separately from the general process of development of the coastal zone of the seas. Such a principle does not meet the elementary requirements for the definition and evaluation of complex geographic natural systems. This is especially true for the coastal zone of the seas, where all the genetic components of the continental, oceanic, and coastal-marine types interact closely in a complex. They act unanimously. It is from such positions, according to this principle, that underwater sand bars and other coastal-marine landforms should be investigated. In other elements of the exogenous part of the geographic envelope, they are not found anywhere and determine the uniqueness of the coastal zone of the seas. The authors of these variants build their hypotheses on separate parts of natural coastal phenomena. Until now, researchers have avoided complex, systematic approaches to this topic. Therefore, the article provides recommendations for improving the theory of the genesis and development of underwater swells within different sites of alongshore sand drifting.

Characteristics of water alternating CO2 injection in low-permeability beach-bar sand reservoirs

Water flooding can be ineffective in highly heterogeneous low-permeability beach-bar sand reservoirs. The introduction of CO2 flooding helps boost the oil production of the reservoirs but only in an early stage. During the late stage of flooding, gas channeling would occur. Water alternating gas (CO2) (WAG) process can be used to delay gas channeling and improve the effect of CO2 injection, though its adaptability to beach-bar sand reservoirs remains unclear. In order to clarify CO2 injection characteristics in these reservoirs, experiments were carried out in high-temperature high-pressure NMR on-line displacement experiment apparatus to simulate different flooding modes on synthetic cores that can reflect the vertical heterogeneity of beach-bar reservoirs. Different CO2 injection modes were implemented on these cores and the displacement characteristics and residual oil distribution features during both WAG injection and continuous CO2 injection were analyzed quantitatively and qualitatively. The results show that the scheme of WAG injection after continuous CO2 injection can obtain better oil displacement efficiency than that of the scheme of continuous CO2 injection after WAG injection, but there is no significant difference in respect of oil displacement efficiency of WAG flooding between the mode of bar-injection – beach-production (injection into bar sand – production from beach sand) and the mode of beach-injection – beach-production (injection into and production from beach sand), with the former mode having a higher oil recovery rate. The wider pore-size distribution range of microscopic residual oil after WAG injection shows great potential of enhancing oil recovery from subsequent continuous gas injection. When WAG injection is implemented prior to continuous CO2 injection, the displacement effect of the latter is more significant. This research may provide a theoretical basis for CO2 EOR in this type of reservoirs.

Seismic diagenetic facies prediction of tight sandstone in the offshore sparse well area: An example from the Xihu Depression of the East China Sea Basin

The purpose of this study is to forecast the spatial distribution of diagenetic facies (DF) of tight sandstone in the offshore sparse wells area. Analytical test data, rock physics experiment data and prestack seismic data were used to study the spatial distribution of seismic diagenetic facies (SEISDF) of tight sandstone of the second (H4b) submember of the fourth (H4) member of the Huagang (E3h) Formation in the N gas-field, Xihu Depression, East China Sea Basin. Based on diagenesis and diagenetic minerals, core-based DF of the H4b sandstone were divided into chlorite-coated facies (CHF), calcite-cemented facies (CAF), dissolution facies (DIF), tightly compacted facies (TIF), and quartz-cemented facies (QUF). Various seismic elastic parameters were obtained by prestack elastic parameter inversion method, and sensitive seismic elastic parameters were selected according to the seismic identification principle of DF. In order to predict the spatial distribution of DF, four DF and sensitive seismic elastic parameters were integrated to establish a SEISDF prediction model based on supervised learning, and obtain a diagenetic facies comprehensive index (DFCI) that can distinguish four DF. Then, the 90° phase conversion technique of seismic lithology was used to realize the consistency of the DFCI between isochronous stratigraphic interfaces with core-based DF. The stratal slicing technique of seismic geomorphology was used to make stratal slices of the DFCI between isochronous stratigraphic interfaces. Representative stratal slices were singled out, and the spatial distribution of the SEISDF was interpreted. This study shows that the H4b1 sublayer primarily develops the TIF, while the H4b2 and H4b3 sublayers chiefly develop the QUF, CHF, and DIF. Compared with the H4b2 and H4b3 sublayers, the areas of the QUF and DIF in the H4b4, H4b5, and H4b6 sublayers slightly increase. The QUF are mainly distributed in the subaqueous distributary channel and a small number of sheet sand and channel bar depositional facies in the form of continuous sheets. The CHF are primarily developed in lenticular shape in the channel bar and a small amount of sheet sand and subaqueous distributary channel depositional facies. The most favorable DIF are chiefly developed in the subaqueous distributary channel, channel bar, and sheet sand depositional facies near the fault, which are distributed in strips. This study proposes a very valuable method for the SEISDF prediction of tight sandstone, which can provide a reference for the study of DF of similar strata in other sedimentary basins.

Developing and monitoring an innovative NNBF to nourish a bay bar: An example from the southeast shore of Lake Ontario

This project tests a cost-effective, innovative dredged sediment placement technique targeted to nourish an eroded barrier bar. The dredged sediment was placed in the nearshore in a Cobble Bell, a form designed to allow for efficient dispersal and migration of sediment by natural processes in this location (such as predominant waves and longshore transport) in a desired direction along the bar. We describe the regional and site assessment undertaken, and explain the way design parameters including technical feasibility, assumptions about wave climate, and constructability were accounted for. A monitoring protocol was designed and implemented to measure the vertical and horizontal accretion on the barrier bar. This included survey transects using RTK equipment and surface analysis of beach volume and morphology change via drone images and photogrammetry. Field measurements of the bar indicated that within the first few days, the piled material eroded rapidly, and over the next few months, migrated downshore, widening the previously breached area. This suggests that the cobble bell nature-based feature has the potential to both accelerate and slow down natural processes in ways that enhance the performance and attractiveness of the beach while minimizing costs. In addition to the monitoring results, we describe a monitoring protocol that is both simple and effective which can be adapted by local entities interested in the management of their coastal landscapes.

RELATIONSHIP BETWEEN OVIPOSITION OF Podocnemis unifilis AND WATER LEVEL IN PERUVIAN AMAZONIAN FLOODPLAINS

In the Amazon floodplains, hydrological variability plays a crucial role by shaping the habitat, offering shelter and feeding or breeding areas. During the dry season, some fresh water turtles such as the yellow-spotted turtle (Podocnemis unifilis Troschel, 1848) use the sand banks at point bar areas as breeding sites, which must be available in time for the start of oviposition. In this study, we used altimetry-derived water level data from radar satellites combined with in-situ hydrological measurements to evaluate water level variations and relate them to oviposition starting dates (2003-2012) from Pacaya-Samiria National Reserve, Peru. Synchronous hydrological behavior was found between the major river stems (Ucayali and Marañon) and the inner rivers (Pacaya and Samiria), thus enabling the utilization of water level in the main river stem to correlate with oviposition in the inner rivers. Mean oviposition date was July 23nd with selection of a narrow water level range (92 masl ± 0.9 SD). Consistent shifts towards early/late oviposition were found for years with early/late dry season onset. Extreme biological shifts in terms of oviposition date onset, were identified for specific years (2004, 2010 and 2011) mainly for the extreme dry year of 2010.

Drivers of functional diversity in the hyporheic zone of a large river

The effects of regional (hydrogeology and geomorphology) and local (sediment and hydrology) characteristics on hyporheic assemblages were studied along a 40-km reach of a large gravel-bed river. Hyporheic water and fauna were sampled at the upstream and downstream positions of 15 large gravel bars. The resulting 30 stations varied in their sediment grain size, stability and direction of river-aquifer exchanges. The study concludes that at the 40-km (sector) scale, the longitudinal distribution of hyporheic fauna was controlled by 1) the hydrogeology of the valley (i.e. gaining vs loosing sectors) that modifies abundance and taxonomic richness of stygobites 2) current channel morphometry of the river (i.e. shape and location of meanders), and 3) historical changes (i.e. river incision) which modify abundance and richness of assemblages. At the local scale, we found that surface grain size and stability of the sediment evaluated by visual observation were poor predictors of hyporheos composition. In contrast, the local hydrology (i.e. downwellings, upwellings, low vertical exchanges) explained a large part of the abundance, taxonomic richness and composition of the hyporheic assemblages. Stations with low vertical exchanges were found poorly colonized, while the upwelling zones were rich in stygobites and downwelling areas harbor abundant and species-rich temporary hyporheos. It was also observed that functional diversity was controlled by the same parameters, with high relative abundances of stygobites in upwelling zones and POM feeders in downwelling zones. The heterogeneity of hydrological patterns, with alternation of upwellings and downwellings may represent the optimal spatial structure for hyporheic biodiversity conservation and resilience in rivers.

Three-Dimensional Hole Size (3DHS) Approach for Water Flow Turbulence Analysis over Emerging Sand Bars: Flume-Scale Experiments

The many hydrodynamic implications associated with the geomorphological evolution of braided rivers are still not profoundly examined in both experimental and numerical analyses, due to the generation of three-dimensional turbulence structures around sediment bars. In this experimental research, the 3D velocity fields were measured through an acoustic Doppler velocimeter during flume-scale laboratory experimental runs over an emerging sand bar model, to reproduce the hydrodynamic conditions of real braided rivers, and the 3D Turbulent Kinetic Energy (TKE) components were analyzed and discussed here in detail. Given the three-dimensionality of the examined water flow in the proximity of the experimental bar, the statistical analysis of the octagonal bursting events was applied to analyze and discuss the different flume-scale 3D turbulence structures. The main novelty of this study is the proposal of the 3D Hole Size (3DHS) analysis, used for separating the extreme events observed in the experimental runs from the low-intensity events.

Subaerial and upper-shoreface morphodynamics of a highly-dynamic enclosed beach in NW Baja California

Estimating sediment budget is a challenge that requires understanding the morphological relationship between the subaerial beach and the upper and lower shoreface. Based on a unique dataset of monthly measured topographic and bathymetric measurements, this study describes the morphodynamics of the subaerial and subtidal sections of a highly-dynamic beach over a 5-year period (2015–2020). Typically, the beach presents a longshore bar trough (LBT) state at the end of the winter, driven by net offshore sediment transport. The nearshore morphology evolves into more reflective states (rhythmic bar beach, RBB; transverse bar rip, TBR; and low tide terrace, LTT) driven by onshore sediment transport when the incident wave-energy decreases over the spring and summer. Seasonal variations of the incident wave-energy resulted in large subaerial-subtidal exchanges of sediment (200–300 m3 m−1), but these exchanges were an order of magnitude smaller in yearly scales. Subaerial and upper-shoreface morphological changes varied alongshore. Larger variations existed in the south, influenced by persistent rip channels. Instead, alongshore processes resulting from changes in wave direction may control the morphodynamics in the center and north. The subtidal section in north and center gained significant amount of sand (300 m3 m−1) during high-energy conditions, which contributed to a net annual gain of 60 m3 m−1. The origin of that sediment is currently unknown, it could have bypassed from an adjacent beach, shifted alongshore or inputted from the lower shoreface deeper than the depth of closure. This research shows the presence of very large monthly-to-seasonal total volume fluctuations relative to net annual or multi-annual changes, and highlights that comparatively small additions to the annual sediment budget could potentially contribute to the long-term stability of the beach.

‘The wait for the Flood is also the wait for Fish!’: Water Commons among the Mishing on Majuli Island (Assam), India

Abstract Flood is often perceived as a destructive force in state and other hegemonic discourses on Majuli Island, India. However, for the indigenous Mishing who live near river banks or on chapories (sand bars), flood is not perceived as an abnormal external force, but as deeply intertwined with their daily living, culture and economy. More importantly, for the Mishing, flood represents the continuous annual creation of water commons. This paper examines the water commons formed by the annual deluge and describes the fishing culture of the Mishing in these commons. Contrary to privatised fishing grounds and private fisheries, where public access is limited, water commons are open to everyone and all villagers have equal access to this “common pool of resources.” Further, the paper examines the different aspects of Mishing subsistence fishing culture on the commons: community fishing; gender; fishing as subsistence and livelihood. This article is based on an ethnographic study of a Mishing village on Majuli Island. Participant observation and interviews were the main tools of data collection for this research. The paper concludes that Mishing subsistence fishing is economically and socio-culturally significant for the community, and Mishing fishing culture on the commons highlights their everyday life with the river on Majuli Island. Keywords: Chapori, commons, flood, Majuli Island, Mishing

Survival of the Strong and Dense: Field Evidence for Rapid, Transport‐Dependent Bed Material Abrasion of Heterogeneous Source Lithology

AbstractBed material abrasion is a major control on the partitioning of basin‐scale sediment fluxes between coarse and fine material. While abrasion is traditionally treated as an exponential function of transport distance and a lithology‐specific abrasion coefficient, experimental studies have demonstrated greater complexity in the abrasion process: the rate of abrasion varies with clast angularity, transport rate, and grain size. Yet, few studies have attempted to assess the importance of these complexities in a field setting. Here, we develop a new method for rapidly quantifying baseline abrasion rate in the field via Schmidt Hammer Rock Strength. We use this method, along with measurements of gravel bar lithology, to quantify abrasion in the Suiattle River, a basin in the North Cascades of Washington State in which sediment supply to the channel is dominated by recurrent debris flows from a tributary draining Glacier Peak stratovolcano. Rapid downstream strengthening of river bar sediment and a preferential loss of weak, low‐density vesicular volcanic clasts relative to non‐vesicular ones suggest that abrasion is extremely effective in this system. The standard exponential model for downstream abrasion, using single‐lithology abrasion rates fails to reproduce observed downstream patterns in lithology and clast strength. Incorporating heterogeneity in source material strength as well as transport rate‐dependent abrasion largely resolves this failure. Further work is needed to develop a comprehensive quantitative framework for the dependence of bed material abrasion on grain size and transport rate.

Harmonic Extrapolation of Seismic Reflectivity Spectrum for Resolution Enhancement: An Insight from Inas Field, Offshore Malay Basin

For decades, how to improve the resolution limit of seismic data has been a concern for seismologists. For this reason, several geoscientists have proposed various methods of improving the bandwidth of the data. Discovering an easy and scientifically reliable means of improving seismic data resolution would undoubtedly help geophysicists interpret more complex details of the subsurface geology. In this study, we transformed the bandlimited time-domain seismic data to the frequency domain using the Fourier analysis method, and a basis pursuit atomic algorithm was applied to decompose the real and imaginary parts of the spectrum into summations of cosines and sines. The resultant reflectivity spectrum (in the frequency domain) was deconvolved by a pre-estimated wavelet spectrum to obtain the true earth’s reflectivity data spectrum and was subsequently extrapolated to beyond the original band limit. The result shows an extended bandwidth from 68 Hz to 161 Hz and 80 Hz to 170 Hz for both synthetic trace model and the main seismic data, respectively. Consequently, this improved vertical resolution of sub-seismic geologic features, such as crevasse splay, levee, barrier bar complex, lagoon inlet channels, alluvial fans, and fluvial channels, and shows subtle facies variations in Inas field.

Monitoring 23-year of shoreline changes of the Zengwun Estuary in Southern Taiwan using time-series Landsat data and edge detection techniques

More than 80% of sandy beaches in Taiwan have been experiencing a severe recession, although the sediment discharge of rivers in Taiwan is significantly higher than the world average producing almost 2% of global fluvial sediment discharge. This contradiction is primarily due to the widespread constructions of reservoirs and intensive anthropogenic activities in coastal regions. In addition, coasts are particularly vulnerable to hazards due to climate change, such as sea-level rise, as they are located at the transition zone of terrestrial and marine environments. Along with the fact that Taiwan is an island and is one of the most climate-vulnerable regions globally, coastal management and sustainability are nationally critical topics, especially considering the ongoing reformation and legislation of Taiwan's coastal conservation laws.As stated in the Sustainable Development Goals (SDGs) goal 14, accurate and continuous shoreline positions information is essential for coastal conservation. However, by reviewing previous global studies and projects commissioned by the Taiwanese government aiming at monitoring shoreline changes, they usually exhibit several limitations, such as limited band selections or conservative band ratio-derived water indices, relying on either manual digitization or simple thresholding methods, focusing on either artificial or smoothly shaped coasts, and using images acquired at considerably different tidal height levels.Therefore, in the present study, a subpixel shoreline extraction approach based on a sustainable cross-generation dataset and a robust edge detection algorithm is proposed. This approach is exemplified by the Zengwun River Estuary located in southwestern Taiwan—Taiwan's most critical coastal preservation region. By quantitatively analyzing the resultant time-series shoreline positions from 1999 to 2021, several hotspots of shoreline recession have been identified: an extreme erosional rate up to −69.4 m year−1 is revealed in the northern sand bank; while the offshore sand bar demonstrates an overall landward retreat rate of −35.4 ± 1.24 m year−1.

Microfacies and Reservoir Connectivity of Shore Sandbar, Southern Indus Basin, Pakistan

Shore sand bar reservoirs have attracted much attention as one of the target intervals with the greatest potential for petroleum exploration and development in marine sedimentary basins. Nevertheless, due to lack of effective research methods, it is difficult to analyze the rapid lateral change and heterogeneity in a sand bar reservoir, which has a major impact on the efficient petroleum development that seriously restricts the rolling evaluation and efficient development of sand-bar reservoirs. In this study, based on integrated analysis of cores, thin sections, logging, 3D seismic data, production test and dynamic data, through a combination analysis of drilling and seismic interpretation data—the petromineral composition, microfacies and reservoir connectivity of the shore sand bar in the southern Indus basin are investigated, which is used for the fine description of the sandbar reservoir. The results show that the shore sand bar is located in a relatively high-energy shore sedimentary environment, which is conducive to forming a favorable lithologic reservoir. Four sedimentary microfacies types are identified, including center bar, bar edge, inter bar and local mudstone interbeds. The sandbar microfacies are changed rapidly, and different microfacies types overlap each other, especially the inter bar and local mudstone interbeds that overlap and intersect in the center bar and bar edge, which significantly reduces the internal reservoir connectivity as well as intensifying the heterogeneity of the sandbar reservoir. The sandbar reservoir is not connected transversely, the physical properties are changing rapidly, and the sandbar reservoir is cut into several relatively independent oil reservoirs.

Lower limits and controlling factors of the petrophysical properties of coal-measure sandstone oil reservoirs: A case study of the Jurassic Yan’an Formation in the Huanjiang area, Ordos Basin, China

Coal-measure sandstone oil reservoirs of the Jurassic Yan’an Formation in the Ordos Basin, China, have strong heterogeneity. Reservoir quality is the core factor that determines the large-scale accumulation of hydrocarbons. Taking the Yan’an Formation in the Huanjiang area, Ordos Basin, as an example, we have calculated the lower limits of petrophysical properties based on empirical statistics, porosity and permeability intersections, mercury injections and irreducible water analysis, oil tests, oil-bearing property analysis, and distribution functions. In addition, we have investigated the main controlling factors of effective reservoirs in the Yan’an Formation. The results demonstrate that the lower limits of petrophysical properties of the effective reservoir in the Yan’an Formation display a gradually decreasing trend as the depth increases. Furthermore, we have obtained the functions of the lower limits of the petrophysical properties and the depth. Sedimentation and diagenesis have a significant control effect on the quality of reservoirs in the Yan’an Formation. High-quality reservoirs have mainly developed in the sedimentary microfacies of point sand bar and distributary channel sand bodies. The strong compaction and cementation in the diagenesis process are the internal reasons for the poor petrophysical properties of the Yan’an Formation reservoirs. Overall, the effect of compaction on the petrophysical properties of the reservoir is stronger than that of cementation, whereas the secondary dissolution pores significantly improve the petrophysical properties of the reservoir.

Controls of sandstone architecture on hydrocarbon accumulation in a shallow-water delta from the Jurassic Shaximiao Formation of the western Sichuan Basin in China

The thin but widely distributed sandstones in the shallow-water deltas have the potential to hold large deposits of natural gas. The fluvial sand bodies formed from river channels with different sedimentary facies and sandstone architectures have remarkable heterogeneities. The results of this investigation suggest that the sandstone architectures of river channels have a significant impact on the hydrocarbon accumulation and production of sandstone reservoirs. Information regarding the sandstone origin and architecture of shallow-water deltas, especially of the fingered bar type, is still very limited. This investigation focused on the channel sandstones in the shallow-water delta of the Jurassic Shaximiao Formation on the eastern slope of the western Sichuan Basin in China. Based on data from cores and logs of sandstones, seven types of lithofacies and four sedimentary facies were identified. Sand-rich sedimentary facies, including distributary channels, mouth bars, and overbanks, have merged to form four types of compound sandbodies corresponding to the fourth-order architectural unit in the architectural classification. Four types of compound sand bodies were classified, including CS1 (variable distributary channel sand body), CS2 (diverting channel and residual overbank sand body), CS3 (diverting channel and residual mouth bar sand body), and CS4 (terminal channel and residual mouth bar sand body). Relatively good physical properties occur in the sandstone architectures of types CS3 and CS4, with an average porosity of 11.06% and an average permeability of 0.97 mD. Sandstone architectures from the Jurassic Shaximiao Formation of the study area were controlled by water depth and climate, tectonic subsidence, distance from provenance, erosion effect, and channel sinuosity and discharge. With strong lateral erosion, a shallow-water delta would have more sandstone architectures of types CS3 and CS4 in the high sinuosity of channels far from the provenance. Furthermore, sandstone architectures of types CS3 and CS4 are prone to form in connected channel sandbodies with relatively high contents of natural gas of approximately 4.48 × 104 m3/d. The results of this investigation are beneficial to improve the understanding of the characteristics of sandstone distributions in shallow-water deltas, especially in fingered bar types, and to guide oil and gas explorations in shallow-water delta systems.

Evaluation of hydrocarbon resources of the Devonian terrigenous rock basin of Perm Region

The geological structure of the Devonian terrigenous complex within the Bashkirian arch located in the well-explored southern part of Perm Krai has been considered in the paper. Due to the limited extent and complex structure of the bulk of reservoir rock in the prospective horizons, these deposits are explored unevenly, usually alongside with the overlying oil and gas bearing complexes. On the basis of the performed analysis of systematic data, the authors justify quantitative parameters for evaluation of non-localized resources after the method of comparative geological analogies in the Devonian terrigenous deposits. The estimated quantity of hydrocarbon resources in the territory under study unequivocally defines it as prospective for exploration and discovery of reserves in the Devonian oil and gas bearing complex. Therefore, the authors have identified areas for priority prospect evaluation survey taking into account the extent of the most promising facies zones of deltaic channels and beach bars.