Lower Delta Plain Research Articles

Seismic geomorphology and sedimentology of a fluvial-dominated delta: Implications for the Neogene reservoirs, offshore Bohai Bay Basin, China

Fluvial-dominated deltas are common along modern and ancient coasts and act as important hydrocarbon reservoirs. In this paper, an integration of high-resolution three-dimensional seismic, well log, and core data are employed to investigate the seismic geomorphology, depositional facies, reservoir types, and controlling factors of fluvial-dominated delta deposition in the lower segment of the Minghuazheng Formation (N1mL), Bohai Bay Basin, China. Three typical seismic facies and seismic geomorphologic units are identified. Seismic facies 1 displays discrete high-amplitude reflection and a distinct U-shape incision. In plan form, this seismic facies shows a linear, dendritic, and sinuous morphology with high root-mean-square amplitudes. Seismic facies 2 occurs interspersed with seismic facies 1 and shows low-frequency and low-amplitude reflection. Seismic facies 3 shows a continuous high-amplitude reflection and uniform sheet-like morphology covering more than 10 km2 (>3.86 mi2). The N1mL was primarily deposited in the upper delta plain, lower delta plain, and delta front environments and is dominated by three major facies types: (1) distributary channel (seismic facies 1), (2) interdistributary bay (seismic facies 2), and (3) sheet sand (seismic facies 3). Among them, the distributary channel sandstones and sheet sandstones are the major reservoirs in the N1mL. Fluvial processes and lake level cycles were important factors in the development and distribution of reservoirs and traps in fluvial-dominated delta systems. Integration of the seismic geomorphology and a modern geomorphology investigation provide an effective way to predict the sandstone reservoirs and traps in fluvial-dominated delta systems.

Deltaic sedimentary environments in the Niger Delta, Nigeria

This study is focused on the geomorphology of surface sedimentary environments contained in the fluvial, tidal and wave-dominated areas of the Niger Delta. GIS techniques applied to high-quality Landsat and SPOT images were used to identify and map landforms and processes, as well as quantitatively characterise their metrics and spatial distribution. A detailed analysis shows that the extent of the Niger Delta is 70,000 km2, contrary to previous publications that reported 75,000 km2. The delta has also been remapped and classified into mega sedimentary environments: the upper deltaic plain is 69% of the total extent, while the lower deltaic plain and delta front are 25% and 6% respectively. Other subunits were distinguished and mapped within the upper deltaic plain such as fluvial channels, point bars, braid bars, oxbow lakes, and other lakes. Tidal channels, beaches and spits were identified in the lower deltaic plain and delta front. The geometries of these sedimentary bodies (landforms) appear to be relatively scale-invariant, thus meaning that from the measurement of some dimensional parameters (e.g. the length) it is possible to estimate all others (e.g. the width and overall extent). This is an aspect of great relevance in subsurface analyses where it is often difficult to predict the extent of sedimentary bodies. The correlation between channel width and length (r2 = 0.9), is the strongest relationship found between sedimentary body metrics.

Influence of depositional environment on coalbed methane accumulation in the Carboniferous-Permian coal of the Qinshui Basin, northern China

Based on analyses of the lithofacies palaeogeography of the Taiyuan and the Shanxi Formations in the Qinshui Basin, the spatial variations of the coal seam thickness, coal maceral composition, coal quality, and gas content, together with the lithofacies of the surrounding rocks in each palaeogeographic unit were investigated. The results show that the thick coals of the Taiyuan Formation are mainly distributed in delta and barrier island depositional units in the Yangquan area in the northern part of the basin and the Zhangzi area in the southeastern part of the basin. The thick coals of the Shanxi Formation are located within transitional areas between delta plain and delta front depositional units in the central southern part of the basin. The Taiyuan Formation generally includes mudstone in its lower part, thick, continuous coal seams and limestones in its middle part, and thin, discontinuous coal seams and limestone and sand-mud interbeds in its top part. The Shanxi Formation consists of thick, continuous sandstones in its lower part, thick coal seams in its middle part, and thin coal seams, sandstone, and thick mudstone in its upper part. From the perspective of coal-bearing sedimentology and coalbed methane (CBM) geology, the lithology and thickness of the surrounding rocks of coal seams play more significant roles in controlling gas content variation than other factors such as coal thickness, coal macerals, and coal quality. Furthermore, it is found that the key factors influencing the gas content variation are the thicknesses of mudstone and limestone overlying a coal seam. At similar burial depths, the gas content of the Taiyuan coal seams decreases gradually in the lower delta plain, barrier-lagoon, delta front, barrier-tidal flat, and carbonate platform depositional units. The CBM enrichment areas tend to be located in zones of poorly developed limestone and well-developed mudstone. In addition, the gas content of the Shanxi Formation is higher in the coals of the delta front facies compared to those in the lower delta plain. The CBM enrichment areas tend to be associated with the thicker mudstones. Therefore, based on the lithologic distribution and thickness of the rocks overlying the coal seam in each palaeogeographic unit of the Taiyuan and Shanxi Formations, the areas with higher gas content are located in the north-central basin for the Taiyuan coals and in the southern basin for the Shanxi coals. Both of these areas should be favorable for CBM exploration in the Qinshui Basin.

Application of seismic sedimentology in predicating sedimentary microfacies and coalbed methane gas content

The identification of sedimentary facies and their lithological distribution characteristics is very important for exploration and development of coalbed methane (CBM). However, the thin interlayer that is common in coal-bearing formation, makes it difficult to predict the favorable facies zone of high frequency sequences based on the conventional methods. In this paper, a seismic sedimentology method was founded to reveal the sedimentary microfacies of high frequency sequences in the coal-bearing Shanxi Formation of the Zhongyu area in the central-western of the Qinshui Basin. The first step of this method is the selection of equal-scale stratal slices and extraction of seismic attributes, then several useable seismic attributes were optimized from all seismic attributes based on the relationship between the seismic attributes and net-to-gross ratios of several single-wells, and finally a model was established to depict the spatial distribution characteristics of sedimentary bodies. Results show that the sedimentary environment of SQ8 and SQ9 in Shanxi Formation is lower delta plain and the upper delta plain respectively. Moreover, the distribution characteristics of net-to-gross ratio across the study area were predicted based on the provided seismic sedimentology. Subsequently, the relationship between the net-to-gross ratio and CBM content of several sing-wells was used to predict the CBM gas content in the study area. The results show that the CBM gas content of No. 2 coal seam ranges from 5 to 20 m3/t, and it decreases with the increase of the net-to-gross ratio in SQ8 and SQ9. The relative error for the predicated gas content results ranges from −14.48% to 3.94%, indicating that the method is effective. Finally, based on the distribution of CBM gas content, the study area is divided into four types, in which the muddy sediment control area (Type IV, located in the central and eastern parts of the study area) is the most favorable for CBM preservation. This study provides a methodology to predict the sedimentary microfacies and CBM gas content in the coal-bearing strata, which extends the application of seismic sedimentology in coal and CBM geology.

Widespread nearshore and shallow marine deposition within the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia

Abstract In the Surat Basin of eastern Australia, the Lower Jurassic Precipice Sandstone and Evergreen Formation are a highly prospective reservoir-seal pair for notional future carbon capture and storage. However, the succession remains poorly constrained from a paleodepositional standpoint and this has impacted the capacity to construct predictive reservoir models. Here we integrate sedimentological, ichnological, and palynological data from ten cores located across a large region of the northern and central basin to produce conceptual depositional models for the interval. Our analysis shows that the Lower Jurassic Series consists of fifteen recurring sedimentary facies that are arranged into six facies associations – braidplain, lower delta plain, subaqueous delta, delta-influenced shoreface, tidally-influenced shoreline, and restricted marine shoals. The facies associations occur in the context of large scale fluvio-deltaic systems that occupied the basin. These results are supported by ichnological indications of marine and brackish-water, a coastal suite of palynomorphs, and marine palynomorphs including rare dinocysts, acritarchs, and copepod fragments. The very low abundance of marine palynomorphs are confined to the upper portion of the Evergreen Formation, and in combination with the sedimentology and ichnology suggest that marine influence increased through time as the basin was filled. The elucidation of marine-influenced deposition contravenes all but the most recent facies interpretations of the Precipice Sandstone and Evergreen Formation, and suggests that the paleogeography of the Mesozoic of eastern Australia needs to be reconsidered. Importantly, the nearshore and shallow marine depositional affinity has important implications for the size, orientation, and distribution of geobodies when building geologically-realistic static reservoir models for dynamic flow simulation.

Comparative Study on Shale Characteristics of Different Sedimentary Microfacies of Late Permian Longtan Formation in Southwestern Guizhou, China

Organic-rich marine-continental transitional shales are widely distributed in Guizhou Province, China. Samples from the Late Permian Longtan Formation were investigated using organic petrography analysis, X-ray diffraction (XRD) analysis, field emission-scanning electron microscopy observations, mercury intrusion capillary pressure and gas adsorption experiments to better understand the organic geochemical characteristic, mineralogical composition, full-size pore structure characteristics and fractal characteristic of shale reservoir. The relationships among the total organic carbon (TOC) content, mineral composition, pore structure parameter, methane adsorption capacity and fractal dimension of shale samples are discussed, along with the differences between different sedimentary microfacies. Results show that shale samples are characterized by high TOC contents, low permeability, complex mineral composition and pore structure. Shales of YV-1, deposited in delta and lower delta plains, have an extreme high clay content (71.33% average) and the clay minerals mainly consist of the I/S and kaolinite, while shales of XD-1, deposited in the lagoon-tidal flat, have a relatively low clay content (37% average) and the I/S occupies an absolute advantage. The pore volume, specific surface area and average pore diameter of YV-1 (0.02881 cm3/g, 20.806 m2/g and 11.07 nm, respectively) are larger than that of XD-1 (0.02110 cm3/g, 20.101 m2/g, and 8.40 nm, respectively). The mesopore of YV-1 is the predominant contributor to the pore structure, while the micropore of XD-1 also occupies a certain proportion in addition to the mesopore. Organic matter (OM)-hosted pores are largely developed in XD-1 samples, while clay mineral-hosted pores dominate the pore system of YV-1. Shale samples with higher TOC content and clay content generally have larger specific surface area and pore volume, which provides more adsorption space and enhances pore structure heterogeneity. Samples of XD-1 have high TOC contents, suitable mineral composition and complex pore structure, suggesting that shales deposited in a lagoon-tidal flat environment may have a greater potential for shale gas development.

Using neural networks and the Markov Chain approach for facies analysis and prediction from well logs in the Precipice Sandstone and Evergreen Formation, Surat Basin, Australia

Abstract Facies analysis is crucial for reservoir evaluation because the distribution of facies has significant impact on reservoir properties. Artificial Neural Networks (ANN) are a powerful way to use facies interpretations from core to determine equivalent facies from wireline logs in uncored wells. However, ANN do not incorporate information that relates to facies successions. This has limited the ability to effectively use facies information derived from logs alone in reservoir modelling, especially at the regional scale where data is often sparse, clustered, or incomplete. In this study, based on observations of 8 cored wells with a total thickness of ∼2000 m, 20 core facies were defined that range from 0.22 m to 11.56 m thick. Facies were based on grain size, sedimentary structures, and ichnological characteristics; Each facies corresponds to a distinct depositional sub-environment within the broader context of a large nearshore to shallow marine system. It was essential that these facies were incorporated into reservoir models to accurately map the distribution of reservoir and seal geobodies for CO2 storage assessment in the Surat Basin, Australia. However, core data are few and far between in the Surat Basin. To use core-defined facies in the absence of core, six wireline log parameters – gamma ray, density, sonic, neutron, photoelectric factor, and deep resistivity were plotted in multidimensional space and examined using Linear Discriminator Analysis. Combined with model recognition and Fisher Canonical Discriminance, the 20 core facies were simplified into 10 representative wireline log facies (WLF) with unique petrophysical parameters. We then used the Markov Chains Approach (MCA) to determine the significance of vertical facies transitions, which supported the interpretation that facies group into 5 distinct associations: (1) channel-levee complex; (2) lower delta plain; (3) subaqueous delta; (4) shoreface and; (5) tidal flats and channels. Based on the facies analysis and statistical classification, Multilayer Perceptron Classifier, a type of neural network method was applied using a training set of three cored wells that had all 6 wireline log data and using the facies successions determined from the MCA. Results show that the accuracy of WLF prediction ranges from 66% to 99% (ca. 83%). The accuracy of facies recognition decreased step wise with a decreasing number of logs as input data, such that when only gamma ray, density, deep resistivity, and sonic were used to train neural networks the accuracy dropped to between 45 and 98% (ca. 67%), depending upon the facies. This was considered the lowest acceptable threshold of accuracy for facies determination for input into reservoir models for carbon capture and storage. The results of this study show that sedimentary facies can be accurately predicted for uncored intervals in the Precipice Sandstone and Evergreen Formation to improve facies mapping and static reservoir modelling. Additionally, wireline log facies are helpful for interpreting Lower Jurassic stratigraphy, depositional setting, and basin evolution in the Mesozoic of Eastern Australia.

Taphonomy ofIsisfordia duncanispecimens from the Lower Cretaceous (upper Albian) portion of the Winton Formation, Isisford, central-west Queensland

Taphonomic analysis of fossil material can benefit from including the results of actualistic decay experiments. This is crucial in determining the autochthony or allochthony of fossils of juvenile and adult Isisfordia duncani, a basal eusuchian from the Lower Cretaceous (upper Albian) distal-fluvial-deltaic lower Winton Formation near Isisford. The taphonomic characteristics of the I. duncani fossils were documented using a combination of traditional taphonomic analysis alongside already published actualistic decay data from juvenile Crocodylus porosus carcasses. We found that the I. duncani holotype, paratypes and referred specimens show little signs of weathering and no signs of abrasion. Disarticulated skeletal elements are often found in close proximity to the rest of the otherwise articulated skeleton. The isolated and disarticulated skeletal elements identified, commonly cranial, maxillary and mandibular elements, are typical of lag deposits. The holotype QM F36211 and paratype QM F34642 were classified as autochthonous, and the remaining I. duncani paratypes and referred specimens are parautochthonous. We propose that I. duncani inhabited upper and lower delta plains near the Eromanga Sea in life. Their carcasses were buried in sediment-laden floodwaters in delta plain overbank and distributary channel deposits. Future studies should refer to I. duncani as a brackish water tolerant species.

Tracing the sources, fate, and recycling of fine sediments across a river-delta interface

Deltaic floodplains are thought to be long-term depositional environments, however there remains a limited understanding regarding timescales of depositional and erosional events, sediment delivery pathways and sediment storage. This study uses sediment concentration and sediment fingerprinting to examine the contribution of surface and subsurface sources to suspended sediment transiting the Lower Roanoke River, North Carolina, United States. The Lower Roanoke is disconnected from its high-gradient uplands in the Piedmont and Appalachian Mountains by a series of dams, which effectively restricts suspended sediment delivery from the headwaters. Accordingly, sediments from the Lower Roanoke River basin are the primary source of suspended sediment downstream of the dams. The fingerprinting method utilized fallout radionuclide tracers (210Pbxs and 137Cs) to examine the spatial variation of sediment-source contributions to suspended-sediment samples (n=79). Three end-member sources were sampled: 1. surface sources (floodplains and topsoils; n=60), 2. subsurface sources (channel bed and banks; n=66), and 3. deltaic sources (delta front and prodelta; n=11).The results demonstrate that with decreasing river slope and increasing influence of estuarine-driven flow dynamics, the relative contribution of surface sediments to the suspended-sediment load increases from 20% (±2%) in the upper reach, to 67% (±1%) in the Roanoke bayhead delta (BHD). At the river mouth, the surface-sediment contribution decreases, and the delta front and prodelta sediments contribute 74% (±1%) to the suspended load. These results indicate, that during the delta transgression, erosion of the lower delta provides an additional source of sediment to the upper delta. At the same time, the lower deltaic plain, considered a sediment sink and long-term sediment-storage site, becomes erosional. The lower river and distributary network of the delta plain, which were thought to only disperse sediments in a seaward direction, may also have an important landward-directed sediment-dispersal component that provides nourishment and fortification to the upper BHD, at the cost of the eroding lower delta. Recognition of these contrasting sediment pathways in the Roanoke River highlights that these complex bidirectional processes may exist in other eroding deltas. Understanding these bidirectional processes will be necessary for the ongoing management of deltaic environments under increasing anthropogenic stress such as land use change and accelerating sea-level rise.

Factors of soil diversity in the Batumi delta (Georgia)

Abstract. The aim of this study was to determine certain basic properties of soils in the Batumi delta (southwestern Georgia) to determine the relationships of studied properties and to identify differences with regards to these properties between different sampling sites in the delta that were selected based on the delta morphology. In this context, a total of 125 soil samples were collected from five different sampling sites, and the clay, silt and sand content of the samples were determined along with their mean weight diameter (MWD) values, aggregate stability (AS) values, amount of water retained under −33 (FC) and −1500 kPa (WP) pressure and organic matter (OM) content. Correlation analysis indicated that clay content and OM were positively correlated with MWD, and OM was positively correlated with AS. However, the sand content was found to be negatively correlated with MWD. In addition, clay, silt and OM content were positive correlated with FC and WP. Variance analysis results determined statistically significant differences between the sampling sites with respect to all of the evaluated properties. The active delta section of the study area was characterized by high sand content, while the lower delta plain was characterized by high OM and AS values, and the upper delta plain was characterized by high MWD values, high FC and WP moisture content levels and high clay and silt content. In conclusion, it was demonstrated that the examined properties were significantly affected by the different morphological positions and usages of these different areas. These results may help with the management of agricultural lands in the Batumi delta, which has never been studied before.

Doomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta

The Ganges-Brahmaputra-Meghna (Bengal) Delta in Bangladesh has been described as a delta in peril of catastrophic coastal flooding because sediment deposition on delta plain surfaces is insufficient to offset rates of subsidence and sea level rise. Widespread armoring of the delta by coastal embankments meant to protect crops from flooding has limited natural floodplain deposition, and in the tidally dominated delta, dikes lead to rapid compaction and lowered land surface levels. This renders the deltaic floodplains susceptible to flooding by sea level rise and storm surges capable of breaching poorly maintained embankments. However, natural physical processes are spatially variable across the delta front and therefore the impact of dikes on sediment dispersal and morphology should reflect these variations. We present the first ever reported sedimentation rates from the densely populated and human-controlled floodplains of the central lower Bengal Delta. We combine direct sedimentation measurements and short-lived radionuclides to show that transport processes and lateral sedimentation are highly variable across the delta. Overall aggradation rates average 2.3 ± 9 cm y–1, which is more than double the estimated average rate of local sea level rise; 83% of sampled sites contained sediment tagged with detectable 7Be, indicating flood-pulse sourced sediments are widely delivered to the delta plain, including embanked areas. A numerical model is then used to demonstrate lateral accretion patterns arising from 50 years of sedimentation delivered through smaller order channels. Dominant modes of transport are reflected in the sediment routing and aggradation across the lower delta plain, though embankments are major controls on sediment dynamics throughout the delta. This challenges the assumption that the Bengal Delta is doomed to drown; rather it signifies that effective preparation for climate change requires consideration of how infrastructure and spatially variable physical dynamics influence sediment dispersal on seasonal and decadal time scales.

Coal facies characteristics and its control on methane content in South Yanchuan Block, Southeast Ordos Basin, China

ABSTRACTThe variability in depositional environments in ancient peat bog and coal facies characteristics was investigated with 29 coal samples from four coalbed methane wells in South Yanchuan Block, Southeast Ordos Basin, China. The control of methane content is described as coal facies parameters and the corresponding measurements of methane content. The results indicated that the depositional environment of ancient peat bog in South Yanchuan Block is primarily a lower delta plain, and a low tissue preservation index (TPI) and high gelification index (GI) are typical features of reservoir’s coal facies, reflecting a deep water depositional environment in ancient peat bog. Compared with the coal facies characteristics of the western area, the coal reservoir in the eastern area displays low GI and high ash content as a result of ancient peat bog’s proximity to sea and source area. Various lateral methane contents are influenced by burial depth, whereas the characteristics of coal facies have a strong control on methane content within the single well in vertical direction. The methane content increases with increase in GI and vitrinite to inertinite ratio (V:I), while there are indistinct relationships between TPI and transportation index with methane content. With high GI and longer distance to sea and source area, the methane content in the western area is higher than that of the eastern area. High desmocollinite content also has a negative influence on methane content.

Geomorphology of the Pinios River delta, Central Greece

ABSTRACTThis paper presents the results of geomorphological investigations carried out on the Pinios River delta, which is a Late Holocene arcuate type delta, located in the southern Thermaikos Gulf (Central Greece). Digital elevation model (DEM) analysis and the study of maps of the last two centuries, accompanied by field survey and aerial photo interpretation have led to the production of a geomorphological map at 1:15,000 scale which outlines the features of the deltaic plain and coastal zone. The evolution and the associated morphology of the delta are the result of the complex interplay of fluvial sedimentation, wave activity and prevailing longshore currents. The dominant landforms of the delta are the numerous abandoned meandering channels, as the river has changed its course several times, and a series of sub-parallel linear sandy beach ridges (cheniers) occupy the lower delta plain. The shoreline of the delta is generally retreating due to marine processes, especially where former river mouths occur whereas the presently active mouth of the river and its immediate surrounds are prograding.

Paleoenvironmental Reconstruction of A Late Cretaceous, Muddy, River-Dominated Polar Deltaic System: Schrader Bluff–Prince Creek Formation Transition, Shivugak Bluffs, North Slope of Alaska, U.S.A.

Abstract Upper Cretaceous (Santonian to late Campanian) strata from the upper 196 m at Shivugak Bluffs along the Colville River of northern Alaska preserve a paleopolar, fluvial–deltaic succession deposited along a prograding paleo–Arctic Ocean coastline. The uppermost lower Schrader Bluff Formation is best classified as a river-dominated delta based on muddy prodelta and delta-front deposits that contain abundant hyperpycnites, subaqueous terminal distributary channels, interdistributary bays, and mouth bars. This deltaic succession is overlain by distal distributaries and a braided fluvial system of the Prince Creek Formation. Channel measurements and paleo-discharge calculations from Prince Creek Formation distributaries indicate that rivers were the appropriate size and had the appropriate discharge to regularly produce hyperpycnites. Delta front and interdistributary bays transitioned into lakes and floodbasins on the lower delta plain, alongside distributary channels during delta lobe reorganization and abandonment. These floodbasins were subsequently transgressed and are overlain by proximal shelf and prodelta muds containing Phycosiphon. Overlying strata containing delta front, interdistributary bay, terminal distributary channel, distributary mouth bar, and distributary channel deposits signaled the reactivation of the delta lobe. Ultimately, these deltaic environments were incised into and became overlain by braided channel deposits during a marked drop in relative sea level and progradation of the fluvial system. Combining ichnologic and sedimentologic observations was critical to identify eleven distinct deltaic and continental paleoenvironments. This study documents the architecture and evolution of an ancient, high-latitude, muddy, river-dominated deltaic system and highlights the autogenic and allogenic processes preserved in the system.

Formation Conditions and Sedimentary Model of Over‐Flooding Lake Deltas within Continental Lake Basins: An Example from the Paleogene in the Jiyang Subbasin, Bohai Bay Basin

AbstractA large quantity of drilling core, paleontology, geochemistry and geophysics data revealed several features of the Jiyang subbasin during the deposition of the Ek1‐Es4x members: (1) the paleotopography of the gentle slope belt had an extremely low gradient; (2) the paleoclimate frequently alternated between dry and wet periods in a generally arid setting; (3) there was strong weathering around the periphery of the basin; (4) the lake was very shallow; (5) the lake level frequently rose and fell; and (6) the sedimentary environment of the gentle slope belt was an over‐flooding lake. All of these factors provided favorable geological conditions for the development of an over‐flooding lake delta. The lithologies of the continental over‐flooding lake delta deposits are complex and diverse. The compositional maturity is moderate to low, and the grain size distribution curves and sedimentary structures indicate the presence of both gravity and traction currents. The sedimentary microfacies associations consist of a combination of ordered superposition of flood channels, distributary channels and sheet sands. The delta exhibits a weak foreset seismic reflection. The over‐flooding lake delta deposits are laterally extensive. The sandstone content is high, and the individual sandstone beds are thin. The flood channel and distributary channel deposits exhibit evidence of bifurcation and lateral migration. The distribution of the sandbodies and the oxidation color of the mudstones provide evidence of cyclic deposition. The paleoclimate was the dominant factor controlling the development of the over‐flooding lake delta. Due to the frequently alternating wet and dry paleoclimates, the over‐flooding lake delta is characterized by the development of a broad upper plain and a lower delta plain. The upper delta plain is characterized by flood channel deposits, whereas the lower delta plain is represented by distributary channel deposits. The transition zone is characterized by the interaction of flood channels and distributary channels. Due to fault activity, the sandbodies of the over‐flooding lake delta were juxtaposed against hydrocarbon source rocks, which was favorable for the development of lithologic reservoirs or structural‐lithologic reservoirs. The lower delta plain deposits comprise the most favorable reservoirs.

Sequence stratigraphy and paleogeography of the Middle Jurassic coal measures in the Yuqia coalfield, northern Qaidam Basin, northwestern China

Detailed stratigraphic and paleogeographic analyses of data from 72 boreholes for the Middle Jurassic intermontane fluvial-lacustrine coal-bearing sequences were conducted in the Yuqia coalfield of the northern Qaidam Basin, northwestern China. Three third-order sequences lasting in total ca. 10.6 m.y., and an internal lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST), and falling-stage systems tract (FSST) have been identified. A series of sequence-specific paleogeographic maps have been constructed based on the contours of lithological parameters. The paleogeographic units include alluvial fan-braided (meandering) fluvial plain, upper delta plain, lower delta plain, subaqueous delta, shore-shallow lake, and deep lake. The preferred sites of coal accumulation are interdelta bays, upper delta plains, lower delta plains, and fluvial back swamps. The sequence stratigraphic and sedimentological analysis of the Middle Jurassic coal-bearing measures of the Yuqia coalfield provides a basis for a comprehensive coal accumulation model that involves a six-period evolution from the LST, early TST, late TST, early HST, late HST to FSST. The major coal seams were accumulated in the early and late TST of the sequences S1 and S2. These results are of practical significance for coal resources exploration and enhance geological effects of prospecting engineering in the northern Qaidam Basin.

Facies architecture of asymmetrical branching distributary channels: Cretaceous Ferron Sandstone, Utah, USA

AbstractDistributary channel systems are an important component of deltaic systems, but details of their branching pattern, stream‐order, internal variability and relation with adjacent levée, bay and bayhead delta are rather poorly documented in ancient examples. Photomosaic and measured sections collected along a gooseneck‐shaped canyon in southern Utah allow direct mapping of the branching pattern of an ancient distributary system. The main channel belt is ca 250 m wide and narrows to ca 200 m downstream of the branching point. A subordinate channel belt, ca 80 m wide, branches off of the main channel, forming a distinctly asymmetrical branching pattern. Water discharge in the main channel is estimated to be 85 to 170 m3 sec−1. Comparison with palaeodischarge estimates of trunk rivers mapped in previous studies suggests that the branching documented in this study probably is a fourth‐order split. The distributary channels are characterized by a U‐shaped geometry filled with medium‐grained, cross‐bedded sandstone, and are dominated by lateral accretion, suggesting limited lateral migration and moderate sinuosity. Tidally influenced facies and limited trace fossils indicate direct marine influence. The distributary channels erode into adjacent levée and underlying heterolithic bay‐fill deposits, and the marine influence suggests that they were deposited on a lower delta plain, rather than on a non‐marine floodplain. The subordinate channel fed a bayhead delta, suggesting that it was formed by a partial avulsion, rather than bifurcation around a mouth bar, as is more characteristic of terminal distributary channels. Channel‐floor drapes, bar‐accretion drapes and abandoned channel fills within the sandstone channel belts represent the most important heterogeneity from the perspective of reservoir characterization.

The Late Aalenian Polyplocussandstein Formation in SE Lower Saxony, Germany: Meandering distributary and crevasse splay sedimentation on a lower deltaic plain

The Late Aalenian Polyplocussandstein Formation in SE Lower Saxony, Germany: Meandering distributary and crevasse splay sedimentation on a lower deltaic plain

Factors Controlling the Textural Characteristics of the Lower Delta Plain Sediments, Wadi Al-Hamd,NE Red Sea, Saudi Arabia

Factors Controlling the Textural Characteristics of the Lower Delta Plain Sediments, Wadi Al-Hamd,NE Red Sea, Saudi Arabia

Factors Controlling the Textural Characteristics of the Lower Delta Plain Sediments, Wadi Al-Hamd,NE Red Sea, Saudi Arabia

Factors Controlling the Textural Characteristics of the Lower Delta Plain Sediments, Wadi Al-Hamd,NE Red Sea, Saudi Arabia