Lateral Accretion Research Articles

Numerical Simulation Research on Horizontal Well to Exploit Remaining Oil of Lateral Accretion Interbed after Polymerflooding

Directing at lateral accretion interbed layers, wedge-shape and pinch-out models are used to analysis polymerflooding development effect using Eclipse numerical simulation soft.. And the distribution location of remaining oil is given after waterflooding and polymerflooding.The recovery is provided including two parts which are lateral accretion interbed and non-lateral accretion interbed. Horizontal well and vertical well are united to exploit remaining oil, in which case, the vertical and horizontal positions of horizontal well are optimized.Numerical simulation results indicate that polymerflooding recovery increasement value is respectively 11.47% and 11.34% about wedge-shape and pinch-out models and polymerflooding works well. The closer to top the horizontal well is , the greater recovery enhancement values is for wedge-shape and pinch-out models. Horizontal hole section starts from the location of seven-eighths of lateral sandbody I left and terminats seven-eighths of lateral sandbody IV, where the recovery is highest, which are respectively 6.69% and 5.61%.

Tidal and fluvial controls on the internal architecture and sedimentary facies of a lobate estuarine tidal bar (The Plassac Tidal Bar in the Gironde Estuary, France)

Abstract Estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers. This study focuses on the Plassac Tidal Bar, which is the unique lobate-shape tidal bar of the Gironde Estuary, located at the very upstream extremity of the bay-head delta, because no detailed sedimentological studies have been made on such geomorphological category of tidal bar. Its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles, ground-truthed by 6 to 9 meter-long cores and time-controlled by historical bathymetric surveys. The base of the tidal bar consists of a tidal erosional surface. This surface has eroded muddy tidalites, corresponding to the maximum flooding surface of the Gironde Estuary. The bar is composed of two vertical sequences of facies, composed of thickening-up cross stratified fine- to medium-grained sand beds at the top and decimeter-thick mud-layers at the base. Those mud layers constitute the master bedding of the bar and are dipping in a channelward direction, perpendicularly to the tidal flows and to the sediment transport directions. One phase of lateral accretion of the Plassac Tidal Bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar. Low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes. During the subsequent flood periods, the muddy master bedding planes are partially eroded and then buried by the fluvially-sourced sandy mini-flood lobes that merge with the tidal bar. The internal architecture of the Plassac Tidal Bar is controlled by a lateral accretion process, its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows. All the other tidal bars of the bay-head delta (around a dozen) exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows. The master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows. Those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits, paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones.

Paleoenvironmental reconstruction of the Lower Mogi Guaçu River Basin (São Paulo State — Brazil), morphopedosedimentary records and fluvial processes

Abstract The Mogi Guacu River rises in Bom Repouso (Minas Gerais State — Brazil) in the Mantiqueira Ridge, and flows into the Rio Pardo river at an elevation of 483 m in Pontal (Sao Paulo State — Brazil), after running a 530 km long course. Especially along the Lower Mogi Guacu Basin, the river morphology is extremely sinuous, characterized by intense processes of channel migration, avulsion, abandonment and reactivation of the channel, producing an extensive alluvial plain composed of a series of associated relief forms and sedimentary facies. Among these forms, point bar deposits by lateral accretion, abandoned meanders, paleo-channels and fluvial terraces are notable features. In this sense, the objective of this work was to investigate whether these features could be linked to environmental changes. To reach this goal, soil properties of a catena of the Jatai Ecological Station (Luiz Antonio — Sao Paulo State) were analyzed in four sectors: Slope, Terrace I, Terrace II and Alluvial Plain. The results from grain-size determination, geochemical and isotopic studies, dating, paleopalynology, coal fragments and micromorphology are presented in this paper. From these analyses, a paleo-environmental evolution divided in three stages is proposed for the area: 130,000 YBP (Upper Pleistocene), when the Mogi Guacu River base level was approximately 6 m above the present one; a drier second phase 10,250 years BP (Lower Holocene), when an organic horizon was formed inside of an abandoned meander (oxbow lake), and a third phase, 2096 YBP (Upper Holocene), of reactivation of a warm and humid climate that promoted the development of a two meters thick Typic Udifluvent in a sector where the Mogi Guacu River no more floods due to the incision of its thalweg, reaching more than six-meters depth in the last 130,000 years BP. Thus, this paper used a fluvial geomorphologic approach and its interplay with climate to understand how the landscape was shaped from Upper Pleistocene to Holocene, however, Neotectonics might have played a relevant role as well, not only in the Mogi Guacu River Basin, but also in the Parana Sedimentary Basin.

Architecture of the lower terraces and evolution of the Dordogne River at Bergerac (south‐west France) during the last glacial–interglacial cycle

ABSTRACTThe lower terraces of the Dordogne River at Bergerac (south‐west France) were studied in detail using cores, trenches, ground penetrating radar profiles, and 14C, optically stimulated luminescence and infrared stimulated luminescence dating. This study shows that the lowest terrace (Fx) is made up of two major lithostratigraphic units: (i) dominantly horizontal gravel strata interpreted as compound bar deposits in a braided river, which pre‐date 18–17 ka, and (ii) thick lateral accretion gravel units (point bars) formed in an anabranching river during the Late Pleniglacial and Lateglacial. Most of the anabranching channels were abandoned at ca. 11–10 ka following shrinkage of the river bed into a single meandering channel and were partially filled by gyttjas during the Boreal chronozone. Finally, the channels were plugged by overbank fines until about 5 ka. Floods seldom reached the Fx terrace during the late Holocene and accretion became negligible. The main phase of bedrock incision spanned the Holocene, whereas the period of channel adjustment to change in river regime during late Marine Isotope Stage (MIS) 2 only led to a redistribution of the Pleniglacial gravels. The higher terrace (Fw3) formed before the Eemian and was covered by colluvial fans, mainly during MIS 2 and 3. Copyright © 2013 John Wiley & Sons, Ltd.

The storage time, age, and erosion hazard of laterally accreted sediment on the floodplain of a simulated meandering river

AbstractA sediment particle traversing the fluvial system may spend the majority of the total transit time at rest, stored in various sedimentary deposits. Floodplains are among the most important of these deposits, with the potential to store large amounts of sediment for long periods of time. The virtual velocity of a sediment grain depends strongly on the amount of time spent in storage, but little is known about sediment storage times. Measurements of floodplain vegetation age have suggested that storage times are exponentially distributed, a case that arises when all the sediment on a floodplain is equally vulnerable to erosion in a given interval. This assumption has been incorporated into sediment routing models, despite some evidence that younger sediment is more likely to be eroded from floodplains than older sediment. We investigate the relationship between sediment age and erosion, which we term the “erosion hazard,” with a model of a meandering river that constructs its floodplain by lateral accretion. We find that the erosion hazard decreases with sediment age, leading to a storage time distribution that is not exponential. We propose an alternate model that requires that channel motion is approximately diffusive and results in a heavy tailed distribution of storage time. The model applies to timescales over which the direction of channel motion is uncorrelated. We speculate that the lower end of this range of time is set by the meander cutoff timescale and the upper end is set by processes that limit the width of the meander belt.

Investigating fluvial features with electrical resistivity imaging and ground-penetrating radar: The Guadalquivir River terrace (Jaen, Southern Spain)

A geophysical survey has been conducted on the lowest terrace levels and the present day floodplain of the current course of the Guadalquivir River, passing through the province of Jaen (Spain), using two techniques: electrical resistivity imaging (ERI) and ground-penetrating radar (GPR). Three areas have been selected. In one of these sectors (Los Barrios) there is an old quarry where there are excellent outcrops that allow for the calibration of the survey techniques. Facies associations on these outcrops are typical of meandering rivers with sequences of channel fills, lateral accretion of point-bars and floodplain facies.The usefulness of the two methods is analysed and compared as a support for stratigraphic and sedimentological studies. The geometry and lithofacies of subsurface deposits were characterised using ERI and compared with field observations. A total of 5 electrical resistivity imaging profiles were obtained. The changes in electric resistivity highlight granulometric differences in terrace sediments. This technique can thus be used to identify the morphology of these bodies, the lithofacies (silt, sand or gravel) and buried channel pattern.In addition, 16 GPR profiles using 100 and 250MHz antennas were acquired, indicating terrace morphology and the filling of the sedimentary bodies in a more detailed manner than in ERI. The study thus allows for inferring the existence of channel migration, the lateral accretion of point bars and the presence of vertical accretion deposits attributable to the floodplains.

Qiarbahete Sanukitoids and adakitic rocks in NW Tianshan: age and geochemical constraints on their petrogenesis and tectonic settings

The Qiarbahete complex in NW China consists of gabbroic diorite, granodiorite, and late-stage quartz diorite porphyry veins. Zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses show that the gabbroic diorite and granodiorite formed at 368 ± 5.2 Ma and 354 ± 4.1 Ma, respectively, indicating that the complex was emplaced in the Late Devonian–Early Carboniferous. The gabbroic diorites, characteristic of Sanukitoids, exhibit high Mg# (62 average), MgO (6.84% average), Cr (195 ppm average), and Ni (61.4 ppm average) contents. The rocks show moderately fractionated rare earth element (REEs) patterns and weak negative Eu anomalies (δEu: 0.83–0.89), enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs), with low ϵNd(t) values (1.46–1.73). The gabbroic diorites originated from partial melting of a hydrous mantle wedge followed by assimilation of crust during ascent. The granodiorites show a geochemical affinity with adakitic rocks, e.g. SiO2 (64.95–67.87%) > 56%, Al2O3 (15.88–16.56%) > 15%, MgO (1.79–2.31%) < 3%, Sr (315–375 ppm) > 300 ppm, and Yb (1.84–2.06 ppm). They are enriched in light rare earth elements (LREEs) and LILEs and depleted in HFSEs, with weak negative Eu anomalies (δEu: 0.78–0.87). The granodiorites were mainly derived by the partial melting of a subducted oceanic slab, followed by subsequent melt–mantle interaction and crustal rocks contamination. All these indicate that the Qiarbahete complex was emplaced in a continental arc setting attending the southward subduction of the Junggar Ocean during the Late Devonian–early Carboniferous, generating the lateral accretion of continental crust in NW Tianshan.

Point Bar Numerical Simulation and Results Analysis of x-Unit in the Daqing Oilfield D Block

Point bar reservoir is the key development object in the late stage of oilfield development at present. In this paper, numerical simulation is done on the basis of the point bar architecture in the modeling of the point bar. Through dynamic data generating simulated well history and acquiring X-unit splitting factor, history matching is done on this basis. The numerical simulation results reveal that oil saturation has belt distribution characteristics on the pane due to the lateral accretion interlayer affect to the oil-water displacement; injection wells transverse spread is limited by the lateral accretion interbed control; by lateral accretion interlayer updip pinchout control, the remaining oil is mainly located in the middle and upper part of the lateral accretion [1,2]. Because of the above characteristics, taking horizontal wells to develop the top remaining oil of point bar has been very good results. And it has been confirmed by the further results of the numerical simulation.

Numerical Simulation Research on Locations Optimization of Horizontal Well to Exploit Remaining Oil of Lateral Accretion Interbed after Polymerflooding

Conceptual models of lateral accretion interbed (the point bar lateral sandbody model contains three lateral accretion interbed .And the lateral accretion interbeds extends to two-thirds of reservoir thickness)is builded making use of numerical simulation software Eclipse. The effect of injection water on water drive recovery is analysised. The location of remaining oil enrichment is also determined. Horizontal well is placed in remaining oil enrichment and its vertical direction and horizontal hole section are optimization. Horizontal well and vertical well are combined to exploit residual oil. Results indicates that: Injectiong water in the direction of lateral accretion interbed can obtain higher water drive recovery than in the opposite direction. The closer to top the horizontal well is , the greater recovery enhancement values is.Horizontal hole section starts from the location of four-fifths of lateral sandbody I left and terminats nine-tenths of lateral sandbody III, where the recovery is highest. Horizontal well is placed after vertiacal well injecting polymer in which recovery can increase 9.8%. According to numerical simulation optimization results, we have made a physical model with lateral accretion interbeds to do experiments. Numerical simulation results agree with experimental results. Hence horizontal and vertical well uinting together can exploit the remaining oil of lateral accretion interbed effectively .

Formation of a cohesive floodplain in a dynamic experimental meandering river

ABSTRACTField studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers. However, it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering. This is needed because only then does a dynamic equilibrium exist and channels maintain meandering with low width–depth ratios. Our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering. In this study we present two experiments with a self‐forming channel in identical conditions, but to one we added cohesive silt at the upstream boundary. The effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2. The experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels. In contrast, in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel. Overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed: first, overbank vertical‐accretion of silt, e.g. levee, overbank sedimentation or splays; and second, lateral point bar accretion with silt on the scrolls and in the swales. The first style led to a reduction in bank erosion, while the second style reduced excavation of chutes. We conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Palaeoproterozoic terrestrial sedimentation in the Beasley River Quartzite, lower Wyloo Group, Western Australia

Pre-2.2Ga aeolianite deposits are relatively rare in the geological record, due to reworking of aeolianites either by fluvial systems, transgression or non-recognition. Here, we present high resolution sedimentary facies analysis of a section through the 2.2Ga Beasley River Quartzite, lower Wyloo Group, Western Australia. The unambiguous presence of terrestrial (fluvial-aeolian) deposition is documented in the form of fluvial architectural elements (channel, bar, lateral accretion and overbank deposits) and aeolian features (dune, pin-stripe lamination, wind streaks, and adhesion features). These observations contrast strongly with a previous interpretation of marine deposition, which is discounted. Our data is consistent with the dominantly terrestrial depositional mode of the rest of the lower Wyloo Group, including the basal Three Corners Conglomerate Member and the subaerial Cheela Springs Basalt. We conclude that the lower Wyloo succession formed in a terrestrial regime during continental rifting.

Effects of mass-wasting on the stratigraphic architecture of a fjord-valley fill: Correlation of onshore, shear-wave seismic and marine seismic data at Trondheim, Norway

A combined record of onshore, high-resolution, shear-wave seismic and marine, P-wave seismic data shows the near-shore stratigraphic organisation of a fjord-valley fill at Trondheim, central Norway. The overall stratigraphy shows a typical fjord-valley fill of up to 160m in thickness that is composed of glaciomarine and fjord-marine sediments overlain by deltaic deposits which, in the onshore section, are draped by an anthropogenic fill. An interval characterised by low shear-wave velocities below the onshore, anthropogenic fills is shown by sampled drillings to comprise part clay and part organic deposits. The seismic record reveals evidence of recurrent mass-wasting events that include landslide debris, turbidite deposits and channel features. A chronological framework for the stratigraphy is provided by existing radiocarbon dates for local deglaciation and mass-wasting events. The combined onshore–offshore datasets document the Lateglacial to Holocene development of an emerging fjord and delta system affected by major, mass-wasting events. The architectural development of the younger part of the stratigraphy is interpreted as particularly affected by two, known mass-wasting events each involving several million m3 of debris. A thick layer of debris from a 3500calyr event probably resulted in a blockage of the subaqueous channels, diversion of turbidity currents and deposition of lateral accretion packages, whereby local fjord sedimentation temporarily increased. A 2200calyr event resulted in a sudden reorganisation of the distributary system whereby distal delta sedimentation abruptly shifted north- to northeastwards to an area temporarily subjected to marine erosion during fall of relative sea level. The study adds to the understanding of landslides and low-stiffness zones in the area of importance for near-shore geohazard assessment. The study also provides insight into the stratigraphic variability of fjord-valley fills, and highlights the interplay between fjord marine sedimentation, marine abrasion, delta progradation and mass wasting during late stages of fjord filling.

High resolution seismic study of the Holocene infill of the Elkhorn Slough, central California

The seismic analysis of the sedimentary infill of the Elkhorn Slough, central California, reveals a succession of three main seismic units: U1, U2, U3, with their correspondent discontinuities d2, d3. These units are deposited over a paleorelief representing the channel location at least 8k years ago. The location of that paleochannel has not changed with time, but the geometry of the infilling sedimentary packages has done so through the years. Discontinuities d2 and d3 show a relic island or relative high in the center of the Slough that separated the sedimentation into two main small basins at least 3k years ago. There is evidence of erosion in the last two sedimentary units showing that the present erosive pattern began decades ago at minimum. We have correlated radiocarbon data of selected cores with the high resolution chirp profiles and reconstructed the infill for the Elkhorn Slough.In the most recent unit, the occurrence of numerous lateral accretion surfaces on both ends of the main channel is discussed within their environmental setting, tidal currents and the net ebb flux of the area. We have interpreted the presence of gas in the sediments of the slough, with a gas front located at the tops of units 2 and 3, which are discontinuities that reflect an effective seal for the gas. Our data shows no obvious evidence for seepage, although the shallow presence of some of the fronts points out the fragility of the environment in the present erosive conditions.

The influence of chondroitin 4-sulfate on the reconstitution of collagen fibrils in vitro

Collagen fibrils were in vitro reconstituted from the aggregated collagen solution in the presence of a wide range of collagen/chondroitin 4-sulfate (Col/C4S) ratios. As revealed by turbidimetric measurement, the collagen fibril formation is significantly accelerated by C4S. The turbidity values of both the solution after 30min preincubation at 4°C and the gels after 60min preincubation at 37°C become larger with the increase of C4S amount. According to the results obtained from turbidimetric measurement and atomic force microscopy observation of solutions, it is predicted that the preincubation of Col/C4S blends at 4°C is necessary to initiate the Col/C4S binding and then promote the further lateral fusion of collagen aggregates in solution. The interactions between collagen and C4S are also vital in the growth phase of collagen self-assembly. Collagen quantitation data show that the amounts of collagen incorporated into self-assembled cofibrils increase a lot as a result of the presence of C4S. Differential scanning calorimetry measurement shows that the thermal stability of cofibrils keeps increasing with the ascending amount of incorporated C4S. It is suggested that the bound C4S might be captured inside the cofibrils acting as promoter and stabilizer. Atomic force microscopy and scanning electron microscopy observations of self-assembled fibrils indicate that the size increase of the self-assembled cofibrils depends on the lateral accretion of small collagen fibrils, while the self-assembly mode of collagen is not affected.

Climatic, geomorphic, and archaeological implications of a late Quaternary alluvial chronology for the lower Salt River, Arizona, USA

Recent archaeological excavations along the lower Salt River, Arizona resulted in the unexpected discovery of buried late Pleistocene soils and cultural features dating 5800–7100cal YBP (Early Archaic), the latter representing the earliest evidence of human activity in the lower Salt River floodplain thus far identified. Because the lower Salt River floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures, our understanding of the river's geological history is limited. Here we present a late Quaternary alluvial chronology for a segment of the lower Salt River based on 19 accelerator mass spectrometry 14C and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations. Deposits are organized into allostratigraphic units and reveal a buried late Pleistocene terrace inset into middle-to-late Pleistocene terrace deposits. Holocene terrace fill deposits unconformably cap the late Pleistocene terrace tread in the site area, and the lower portion of this fill contains the Early Archaic archaeological features. Channel entrenchment and widening ~900cal YBP eroded much of the older terrace deposits, leaving only a remnant of fill containing the buried latest Pleistocene and middle-to-late Holocene deposits preserved in the site area. Subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments. Our study confirms that the lower Salt River is a complex mosaic of late Quaternary alluvium formed through vertical and lateral accretion, with isolated patches of buried soils preserved through channel avulsion. Although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages, intrinsic geomorphic and local base level controls limit direct correlations of lower Salt River stratigraphy to other large rivers in the North American Southwest.

Geochronology, geochemistry and tectonostratigraphy of Carboniferous strata of the deepest Well Moshen-1 in the Junggar Basin, northwest China: Insights into the continental growth of Central Asia

The Well Moshen-1, the deepest well in the Junggar Basin, is located in the Mosuowan Uplift of the Central Depression. Andesitic magmatic rocks (including the vitric clastic tuff, tuffite and crystal tuff with volcanic–sedimentary rocks) are common in the Carboniferous of the Well Moshen-1. In-situ SHRIMP zircon U–Pb dating and Hf isotope analysis together with geochemical data, seismic profile and magnetic anomaly were carried out on these andesitic tuffs at a depth of 7000m in the Well Moshen-1 in order to reveal the basement character and the evolution of the Junggar Basin and the crustal accretionary process of this region in the Late Paleozoic. By using high-accuracy dating techniques, we obtained the zircon SHRIMP U–Pb age of the andesitic tuff of 331.7±3.8Ma. Geochemical data has also shown that the rocks have typical arc-like geochemical characteristics (low TiO2, Mg#, enrichment in large-ion lithophile elements and depletion in high-field-strength elements, especially Nb, Ta and Ti), suggesting that they are crystallization differentiation products of intra-oceanic arc magmatism. The Carboniferous strata from the Well Moshen-1 are divided into three tectonostratigraphic units with eight periods of volcanic activities, showing a change in the compressional stress of the Mosuowan Uplift from strong to weak and its ending during the Early Permian. The Hf model ages and the formation ages of the zircons from these andesitic tuffs are similar, with 176Hf/177Hf ratios ranging from 0.282906 to 0.283030 and εHf (t) values from 11.7 to 15.2, implying a lateral growth of juvenile crust in the Junggar Basin during the Early Carboniferous. Based on the analysis of regional geologic settings of the Junggar region, the Mosuowan Uplift is considered to be a Carboniferous intra-oceanic arc that resulted from a northward subduction of the South Junggar Ocean, which suggests that the lateral accretion of arcs played an important role in crustal growth in this part of Central Asia.

Lithofacies attributes, depositional system and diagenetic properties of the Permian Gharif Formation from Haushi–Huqf area, Central Oman

Over 70 m thick interbedded sandstone, siltstone and claystone of the upper member of the Gharif Formation are exposed in western Huqf area in Oman Interior Sedimentary Basin. The Gharif Formation, particularly its upper member hosts major hydrocarbon reservoir in the subsurface of the Oman Interior Sedimentary Basin. The upper member of the Gharif Formation is comprised of interbedded thick sandstone, siltstone, carbonaceous clays and intraformational conglomerates. The sandstone lithofacies, on average, constitute 10 m thick multistoreyed sequences, which are composed internally of 2–3 m thick and 100 s of metres across vertically and laterally amalgamated sandstone bodies. Two major types of sandstones (types 1 and 2) are identified on the basis of their lithofacies association and internal architecture. The type 1 sandstone constitutes the lower part of the member and is comprised of pebbly to coarse-grained, planar and trough cross-bedded sandstone, plane bedded sandstone and pebble lags at the base of major sandstone bodies. The cross-beds are, on average, 30 cm thick exhibiting a dominant paleoflow direction towards NW (280–300° N). It is interpreted to be deposited by low sinuosity braided streams. The type 2 sandstone constitutes the upper part of the member and is comprised of medium-grained sandstone, trough to low angle plane bedding associated with lateral accretion surfaces. It is commonly interbedded with carbonaceous clays. Silicified plant fragments are commonly distributed in the upper part of the sandstone. Interbedded clays and siltstones are red, mottled and extensively bioturbated due to root burrows. It is interpreted to be deposited by high sinuosity meandering streams. In the uppermost part of the section, several dark grey to black carbonaceous clay/coal beds with plant matter are interbedded with sandstone and red clay indicating development of swampy conditions during onset of the coastal setting in the uppermost part of the formation. About 30 cm thick bioclastic sandstone deposited by the marine coastal bars mark transition from the Gharif Formation to carbonate dominated Khuff Formation. The sandstone of the Gharif Formation is arkosic in composition. Very small amount of cement and negligible compaction of constituent grains in sandstone indicates shallow burial before uplift.

Geomorphology of carbonate platform‐marginal uppermost slopes: Insights from a Holocene analogue, Little Bahama Bank, Bahamas

AbstractAs the product of a variety of sediment sources and sedimentation (and re‐sedimentation) and erosion processes, the geomorphology and sedimentology of carbonate slopes are highly variable. The purpose of this study is to describe sub‐bottom profiles and side‐scan sonar, multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present‐day platform‐marginal, uppermost slope environments (&lt; 240 m water depth) on the north, open‐ocean facing flank of Little Bahama Bank, Bahamas. Although at time scales of greater than 100 ka this margin is progradational, the data illustrate a complex juxtaposition of erosional and depositional processes and features. Erosion is evidenced by two prominent escarpments (70 m and 120 m) that expose eroded, bedded rocky outcrops. These escarpments are interpreted to represent relict features, related to past sea‐level positions, although they still may be shedding debris. Aside from erosional remnants, sedimentation and active transport is indicated by several features, including active bedforms (especially above the 70 m escarpment, but ripples occur to depths of ca 200 m), several mass transport complexes that overlie and cover the lower escarpment, gravity flow deposits and rare slump features. Similarly, a thick (up to 20 m) onlapping sediment wedge, interpreted to be Holocene in age, suggests lateral accretion of the slope by more than 75 m in this period. Data illustrate that this open‐ocean margin is distinct from windward margins in the Bahamas, which typically include near‐vertical walls of erosion or bypass, flanked downdip by rubble and talus, and leeward margins, which have onlapping muddy wedges, but that lack marked terraces or escarpments. Collectively, the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in Holocene uppermost slope systems, concepts potentially applicable to ancient analogues.

Diverse middle Ordovician palaeoscolecidan worms from the Builth‐Llandrindod Inlier of central Wales

Abstract: Palaeoscolecidan worms are rare, Early Palaeozoic fossils with uncertain affinities within the Ecdysozoa. They are locally abundant in the Cambrian and scattered in the Early Ordovician, but very sparse thereafter. Forty‐four specimens have been collected from the Middle Ordovician of the Builth‐Llandrindod Inlier of Mid Wales and include well‐preserved material assigned to seven new genera, with four additional species in open nomenclature. An additional specimen from the Arenig Pontyfenni Formation of South Wales is also described in open nomenclature. The total demonstrates much greater palaeoscolecid diversity than hitherto suspected for this time. The specimens are preserved as cuticle fragments in shales and siltstones, often of submillimetre size but in many cases with excellent preservation. The level of detail preserved in some is equal to that found in Cambrian phosphatized faunas. The new approach to collecting, and the recognition that this material can yield taxonomically useful information, opens new avenues for palaeoscolecidan research in siliciclastic environments. The new taxa are the following: Radnorscolex bwlchi gen. et sp. nov., Aggerscolex murchisoni gen. et sp. nov., Bullascolex inserere gen. et sp. nov., Wernia eximia gen. et sp. nov., Ulexiscolex ormrodi gen. et sp. nov., Pluoscolex linearis gen. et sp. nov. and Loriciscolex cuspidus gen. et sp. nov. The high diversity, and the taxonomic separation from known groups described primarily from Cambrian carbonates, implies that palaeoscolecidans either diversified significantly during the Ordovician or were taxonomically segregated between carbonate and siliciclastic settings. Palaeobiological findings also include confirmation that some palaeoscolecid basal cuticles were solid and others reticulate, plates (and platelets) could form by lateral accretion, plates were in part primarily phosphatic and in part organic and that in at least some groups, platelet secretion occurred external to plate secretion.

Spatial Distribution of Thin Interbeds in Point Bars of Meadering Channels: Case Study of Block X in Daqing Oilfield

Reservoirs in point bars of meandering channels account for a high proportion in oil and gas reservoirs, with much impact of complicated architecture on remaining oil accumulation and distribution. Thus study on inner architecture of point bars and its impact on remaining oil distribution are important. This paper selects Unit III in Block A in Daqing Oilfield with data in filled well patterns and cores to study inner architecture of subsurface single sand bodies, and analyzes 3D structural features of meandering point bars in Xingbei deltaic plain in Daqing Placanticline. Architecture parameters of lateral accretion shale interbeds are: eastward dip averagely 7°, the upper, middle and lower dip in same accretion face are respectively 10°, 7° and 3°, the width is 15-42m. They are vertically imbricated series and areally corner points of abandoned channels on both sides of meandering rings as track collection intersections, and progressive and curvature reducing arc sets with abandoned channels of meandering rings as outer boundary. This pattern provides geologic basis for successive study on remaining oil.