Distributary Channel Deposits Research Articles

Diagenetic alterations and deep high-quality reservoirs within deltaic distributary channel facies: a case study from the Permian Shihezi formation in the Hongde area, southwestern Ordos basin, China

Deep tight gas reservoirs are considered important hydrocarbon exploration targets. High-quality reservoir prediction is critical for successfully exploring and developing deeply buried tight sandstone gas. Previous research has found that the reservoir quality of deeply buried tight sandstones is controlled by diagenesis and sedimentary facies. However, the variation of diagenetic alterations in different facies is still poorly studied on deltaic tight gas sandstone. In this study, core analysis, wireline log data, and 3D seismic were studied for the characterization of diagenetic alterations and sedimentary facies. The tight sandstones were formed in braided river delta deposits. Gravel-bearing coarse-grained sandstone facies and cross-bedded sandstone facies developed in tight sandstones. The Gravel-bearing coarse-grained sandstone is formed in the mid-channel bar of deltaic distributary channels. The major diagenetic processes developed in the tight sandstone include compaction, cementation, and dissolution. Constructive diagenesis can generate secondary pores, mainly including dissolution and kaolinite metasomatism, which can effectively improve reservoir physical properties. Through the diagenesis alterations linked to different lithofacies in cored wells, there are obvious diagenesis variations in different lithofacies. Despite strong compaction, the reservoirs in coarse sandstone facies have developed internal dissolution. After compaction, cementation, and dissolution to increase porosity, the reservoir retains intergranular and secondary pores and forms relatively high-quality reservoirs. The fine sandstone facies with cross-bedding are strongly compacted, with internal ductile particles being compacted and deformed, and the particle orientation is clearly arranged. The reservoir is tight, and the development of dissolution in the reservoir is weak, resulting in poor reservoir quality. In addition, reservoirs located at the interface between sandstone and mudstone are often affected by diagenesis, resulting in the development of calcareous cementation, leading to poor reservoir quality. Therefore, high-quality reservoirs are mostly distributed in coarse sand lithofacies, mainly distributed in the mid-channel bar of distributary channel deposits. Sedimentary facies control the original physical properties of the reservoir with different content and texture; the quality of the original reservoir is subject to diagenetic alteration in different ways. The points in this study could offer insights better to predict deep tight reservoir quality in continental basins.

Sedimentary response and restoration of paleoshoreline of Taiyuan-Shanxi Formations in North China basin

The Permian Taiyuan-Shanxi Formations in the North China Basin are a series of terrestrial-marine strata whose depositional evolution exhibits unique particularities. Current studies involving the paleoenvironment of the Taiyuan and Shanxi Formations are mainly supported by sedimentary facies or geochemical analysis, yet studies on the sedimentary and geochemical responses of the paleoshoreline remain inadequate, which leads to the inconsistency of the concluded paleoshoreline distribution. Based on outcrop data, well data and major and trace element analysis, sedimentary and geochemical features of the paleoshoreline in both formations are identified. Combining sedimentary facies and geochemical analysis, this study aims to restore the distribution of the paleoshoreline in the Taiyuan and Shanxi Formations as well as its migrations.The outcrop and well data show that the paleoshoreline of the Taiyuan and Shanxi Formations features 3 types of sedimentary responses: (1) distributary channel deposits of the delta plain + argillaceous deposits of the delta front; (2) peat mire of the delta plain + argillaceous deposits of the delta front; and (3) distributary channel deposits of the delta plain + tidal flat-lagoon deposits. Based on major and trace element results, Sr/Ba, m = 100 × (MgO/Al2O3), Mn/Fe, f = (Rb/K)/(Zr/Al), CIA, Sr/Cu, V/(V + Ni), and V/Cr are selected as proxies for paleosalinity, paleowater depth, paleoclimate and paleo-redox conditions. The results show that transitional-brackish water was dominant during the transgression period of the Taiyuan Formation with mild changes in paleowater depth, while fresh water primarily covered the North China Basin during regression during the Shanxi Period, which was a time when paleo-water depth exhibited evident fluctuation in the northern region. Generally, the paleoclimate during the Taiyuan-Shanxi Period was warm and moist to hot and humid, and the paleo-redox conditions were anoxic to reductive.Under the influence of transgression during the Taiyuan Period, the boundary of the freshwater zone closely matches the maximum range of marine deposition, indicating a generally steady transgression background; additionally, the paleoshoreline was distributed in the vicinity of Yulin - Shijiazhuang - Baoding. During the regression event of the Shanxi Period, the paleoshoreline migrated southeast to the south of ZK03-2 and southeast of Yu88, extending southward along Sanmenxia - Luoyang - Zhengzhou - Yuzhou, and the freshwater zone extended southward and covered a considerable range of marine deposition, indicating a strong flux of terrestrial freshwater with periodic transgression. The analysis and conclusions in this paper would hopefully provide insights and a reference for the paleoenvironmental evolution and shoreline changes in the Taiyuan - Shanxi Formations of the North China Basin.

Sedimentary Facies Analysis of the Third Eocene Member of Shahejie Formation in the Bonan Sag of Bohai Bay Basin (China): Implications for Facies Heterogeneities in Sandstone Reservoirs

The middle sub-member (Es3z) within the third member (Es3) of the Eocene Shahejie formation is the main source of the generation and accumulation of hydrocarbons in the lacustrine deltas of Bonan depression. Exploration and research work in different blocks is carried out separately. Types of sedimentary facies, and their vertical and lateral evolution in Es3z are not studied in detail. To fill this knowledge gap, we did a detailed analysis of facies and lithological characteristics through integrative studies of cores, well logs and seismic data. Identification of sedimentary structures and lithology of the reservoir zone from cores are calibrated with high-quality well logs and seismic data. Depositional facies in Es3z reservoirs are identified through analysis of sedimentary structures, grain size, log’s trends and seismic sections. Es3z was deposited in the fan delta front setting where five facies associations are found, among them distributary channels consisting of MCS, CSg, PCSs, MS, RCL, WCS, PBSs, RCS and GBS lithofacies, natural levee containing DFs, and furthermore, sheet sand are associated to CBS and SSM lithofacies. GM, GGM and DGM lithofacies are related to inter-distributary deposits, whereas mouth bars consist of PLS, CS and CFS. Depositional history, flow direction of the sediments, and facies distribution are investigated through detailed facies mapping and cross-section profiling to show that the sediments were sourced from southeast to northwest. We found thicker succession of sedimentary profiles towards north and north-west directions. Belt distributary channel deposits, covering a wide range of areas, act as potential reservoirs along with mouth bar deposits, while mudstones in interdistributary channels act as a good source and seal rocks. The methodology adopted has great potential to explore the reservoirs of fan delta front in lacustrine deltas.

Factors controlling reservoir quality of a retreating delta-front in shallow-water lacustrine in the Songliao Basin, Northeast China

Shallow-water lacustrine delta sand bodies with large areas of high-quality reservoirs are widespread in the first member of Yaojia Formation (k2y1) of the Upper Cretaceous in Qijia–Gulong area, central depression of the Songliao Basin. The analysis of elements affecting reservoir quality has always been a focus and difficulty of oil and gas exploration and development. Seismic profile interpretations, well log analysis, lithological associations, core observations, and heavy mineral assemblage determinations are used to interpret sequence stratigraphic framework and sedimentary evolution of K2y1. The latter is interpreted as a transgressive systems tract comprising three fourth-order sequences (SQ1–SQ3). Based on core observations and wireline logs, K2y1 primarily contained a shallow lake, distal bars, sand sheets, interdistributary bay, and subaqueous distributary channel deposits. The sand ratio and scale of the channels in the formations from SQ1 to SQ3 decreased which was reflecting the continuously retrograding sedimentary evolution of deltas to the lakeshore, as observed based on mutual calibration of single well sedimentary facies and root mean square amplitude attributes. Based on petrographic analysis of thin sections, particle sizes, scanning electron microscope (SEM) as well as X-ray diffraction, the effects of diagenesis and sedimentary facies on reservoir quality are investigated. In the process of delta-front retrogradation, the changes in reservoir quality in different sequences are primarily a result of the following factors: (1) the fine sandstone has better reservoir quality and more primary pores than the siltstone, which has higher argillaceous content. Additionally, in the fine sandstone, more primary pores can be preserved in the compaction process with higher reservoir quality. (2) Less authigenic illites have little influence on reservoir quality. When the authigenic illite content is >6%, the flocculent characteristics of authigenic illite crystals easily separate the intergranular pores into tinier micropores, thereby reducing the reservoir physical properties. (3) More than 3% of Pompon-like authigenic chlorites may occupy the pore throats of sandstone reservoirs. (4) The feldspars and rock fragments in the sandstone reservoir adjacent to the source rock easily form numerous secondary dissolution pores and improve the reservoir quality. The present study reveals that large-areas of subaqueous distributary channel sand bodies with stable lateral distribution, high reservoir quality, and significant petrophysical characteristics are the best-qualities of reservoir within the early stage of retreating delta-front system.

Basin margin tectonics and morphology as controls of delta type and architecture: examples from the Mio-Pliocene Yalvaç Basin (SW Turkey)

This study describes the sedimentary facies and depositional architectures of Gilbert-type and shoal-water delta deposits developed on opposed margins of the extensional fluvio-lacustrine Yalvaç Basin during the late Cenozoic. The roles of syndepositional tectonism, basin dynamics, and hinterland morphology on the development of different delta types are assessed. This asymmetric trough initially opened as an intramontane molasse basin to the southwest of the Sultandağları massif. Its northern and southern margins are bounded by normal faults, which controlled both tectono-sedimentary evolution of the basin and the surrounding palaeomorphology. The lacustrine deposits consist of thin-bedded limestones, marls, and medium to thick-bedded sandstones and conglomerates. The coarse-grained Gilbert-type delta, up to 150 m thick, was deposited just in the front of the steep northern basin margin. The clinoformal architecture of this delta comprises steeply inclined foreset beds, which are overlain by horizontal alluvial topset units and underlain by subhorizontal bottomset deposits. The Yarıkkaya normal fault controls the northern margin of the basin and plays an active role in both local uplift and subsidence, giving rise to new sediment sources and increasing the accommodation space. Abundant coarse-grained sediment supply into this relatively deep basin across the steep scarp of the Yarıkkaya Fault led to creation of a delta with Gilbert-type architecture. Contemporaneously, on the shallow southern margin of the Yalvaç Basin, several small shoal-water deltas up to 3.5 m thick, were deposited on a smooth basin floor. The gently mound-shaped depositional architecture of these shoal-water delta deposits comprise erosive-based mouth-bar and distributary channel deposits. These observations show that, in this case, the river deposited its bedload shortly after entering the basin, generating small shoal-water deltas that, through stacking over time and some lateral offset, built a shoal-water delta complex.

Reservoir sandstone grain-size distributions: Implications for sequence stratigraphic and reservoir depositional modelling in Otovwe field, onshore Niger Delta Basin, Nigeria

The grain-size distribution and the complexity of the sand-body suggests a high degree of heterogeneity of the depositional environments of clastic reservoirs. To understand the facies and depositional environments distribution of the oil-bearing sandstone reservoir in the Otovwe field, onshore Niger Delta Basin, an integrated sedimentological approach was employed. This involves the integration of well log signatures and log-probability plots of grain-size analyses of ditch-cuttings obtained from 3 wells that showed variability in the rates of oil production. The result suggests that the sediments were deposited in a fluvial environment that contains point bars of a lowstand systems tract; delta front environment that contains mouth bars and distributary channel deposits of a highstand and lowstand systems tracts, respectively; and the nearshore environment with tidal flat sediments of a transgressive systems tract. In some cases, stratigraphic correlation shows variations in facies succession at the same stratigraphic level in individual wells. A systematic variation in grain-size distribution parameters of the various depositional facies, however, appears similar in individual wells. These suggest that the location of the wells within the field experienced different depositional processes, fluvial controlled, and rates of reworking by marine-influenced. Grain-size distribution pattern shows very fine, and fine sands constitute the grain framework of the reservoir sandstone and a minor amount of silt and medium sands with an insignificant amount of sediment size ranging between coarser sand and fine gravel with clay. These suggest that possibly the very fine-grained and fine-grained sands constitute thick beds, silty-sand and shaly-sand as thin beds. Conversely, the clay and gravelly-sand occur as dispersed sediments constituted laminae, and lenticular beds, respectively. The mixture of silt and gravel possibly constituted the sediments that gave rise to the poorly sorted status of the reservoir. Variations in depositional environment and facies, together with grain-size distribution across the reservoir, possibly constituted heterogeneous elements that instituted the variability in the oil production rates of these wells. This study demonstrates the importance of “textural-electrofacies” analytical method in sedimentological studies in a situation where other subsurface data are not readily available for interpretation in anticipation of hydrocarbon reservoir sandstone management and field development in siliciclastic settings. • Very fine and fine sands constitute the framework of the reservoir sandstone. • Dispersed coarser and mud institute a poorly sorted sediment status of the reservoir. • Sediments were transported in a bottom suspension and rolling prior to deposition. • Reservoir facies contains three paleoenvironments: fluvial, delta front and nearshore. • The reservoir experienced more than six sedimentation cycles during development.

Sedimentología e icnología de un delta fluvio-dominado, Formación Lajas (Jurásico Medio), cuenca Neuquina, Argentina

Sedimentologic and ichnologic analysis of the Middle Jurassic Lajas Formation in Bajada de Los Molles area allow recognizing upper offshore-lower shoreface deposits in transition to prodelta, delta front and interdistributary bay succesions. This system is classified as a river-dominated delta due to the presence of distributary channel deposits with huge amounts of particulate organic matter and low diversity and abundance of trace fossils. Hyperpycnites are common in the basal and upper part of the studied section, and form channel-levee and distributary-channel systems, integrated by massive sandstones and load deformation structures, or transitional and recurrent passages of tractive sedimentary structures with abundant particulate organic matter on the foresets. The greatest diversity and abundance of trace fossils are recognized in the upper offshore-lower shoreface deposits where Skolithos and Cruziana ichnofacies occur. Deltaic deposits show trace fossil associations with lower diversity and abundance than the fully marine ones, whereas the hyperpycnite deposits are either unbioturbated or show the lowest diversity and abundance of trace fossils, reflecting the most stressed conditions within the system.

Discriminating stacked distributary channel from palaeovalley fill sand bodies in foreland basin settings

Stacked fluvial distributary channel deposits and palaeovalley fills can form major, multi storey sand bodies with similar thicknesses, and with lateral extents often greater than a single exposure. Consequently, they can be difficult to tell apart from one another using outcrop data. This study addresses this problem by quantitatively analysing the architecture of five stacked fluvial distributary channel deposits and two palaeovalley fills from the Pennsylvanian Pikeville and Hyden formations of the central Appalachian Basin, USA. The a priori interpretation of the sand bodies as stacked distributary channels and palaeovalley fills is possible because a robust in-place coal seam correlation framework allows for the recognition of different basin-scale architectures for each type – aspect ratios <1000 and envelopes of fluvial and deltaic strata for stacked distributary channels, and aspect ratios >1000 and a regional basinward facies shift at the bases of palaeovalley fills. Sand body thickness, storey thickness, position and length of storey contacts within the sand body are similar in both types. However, they can be distinguished by different up-system to down-system changes in their respective architectures. Stacked distributary channel sand bodies thin down system, display a decrease in storey thickness, an increase in the mean position of storey heights in the sand body and a decrease in the length of storey contacts. These trends are the result of down-system decrease in channel size, and confinement associated with radially distributive fluvial systems. Palaeovalley fill sand bodies thicken down-system, display an increase in storey thickness, a decrease in the mean position of storey heights, and a decrease in the length of storey contacts. The increase in sand body and storey thickness are the result of down-system increases in original channel size, consistent with trunk axial fluvial systems fed by tributaries that predominate during valley-formation. The down-system increase in amalgamation reflects a down-system decrease in accommodation, from the higher subsidence rate active margin of the basin, and is therefore not necessarily characteristic of palaeovalley fill architectures in all basin settings. This study emphasises the requirement for detailed correlation work and quantitative analysis of external and internal architectures before the interpretation of sand bodies as stacked distributary channels or palaeovalley fills.

Ichnological signatures from wave- and fluvial-dominated deltas: The La Anita Formation, Upper Cretaceous, Austral-Magallanes Basin, Patagonia

Depositional processes operating in deltaic shorelines create a variety of physico-chemical stressful conditions that impact directly on benthic fauna. The ichnological signatures of deltaic successions reflect these physico-chemical stresses, such as rapid sediment influx, elevated water turbidity, periodically reduced oxygenation, salinity fluctuations, and wave- and tidal-energy. In the Campanian La Anita Formation of the Austral-Magallanes Basin, two examples of deltaic successions, each showing different relative dominance of wave and fluvial processes, are described in detail, allowing the recognition of contrasting stresses that acted differentially in a wave-dominated, fluvial-influenced delta and a fluvial-dominated delta. Variations in the burrowing intensity and ichnodiversity allowed us to discriminate the different physico-chemical stresses. In the wave-dominated, fluvial-influenced deltaic deposits, the alternation between fair-weather waves and storm events acted as the main controlling factor on benthic fauna in the facies of the prodelta and distal delta front, displaying facies-crossing and horizontal elements attributable to the Skolithos and Cruziana Ichnofacies, such as Arenicolites isp., Chondrites isp., Cylindrichnus isp., Diplocraterion isp., Gyrochorte isp., Ophiomorpha isp., Palaeophycus isp., Planolites isp., Rhizocorallium isp., Rossellia isp., Schaubcylindrichnus isp. and Skolithos isp. Water turbidity and periodically reduced bottom water oxygenation conditions also played an important but subordinate role. The proximal delta front, and distributary and terminal distributary channel deposits show low burrowing intensities and ichnodiversities with structures attributable to the depauperate Skolithos Ichnofacies with few Diplocraterion isp., Gordia isp., Lockeia isp., Macaronichnus isp., Ophiomorpha isp., Schaubcylindrichnus coronus and Siphonichnus isp., showing stress conditions associated with rapid sediment influx, high energy and reduced salinity conditions related to fluvial discharges as the most important controlling factors. Periodically, fluctuations in salinity conditions due to river discharges and channel avulsion may have also played important roles. The fluvial-dominated deltaic deposits show that the controlling factors were associated with fluvial discharge and river-sediment influx with no clear influence of wave or tidal action. The facies of the distal delta front were controlled by episodic sedimentation, elevated water turbidity, periodically reduced oxygenation and salinity fluctuations, the associated trace fossil suite consists of a monospecific assemblage of Ophiomorpha isp. and sparse Palaeophycus isp. The proximal delta front and the distributary channel deposits are mostly unburrowed, locally displaying a few Arenicolites isp., Lockeia isp., Ophiomorpha isp., Psilonichnus isp. and Teredolites isp. The development of these suites was controlled mainly by heightened sedimentation rates influx triggered by high fluvial discharges, elevated water turbidity due to heightened suspended load and buoyant plumes, periodic fluctuations in salinity conditions caused by river discharges and channel avulsion. Interdistributary areas are characterized by facies preserving evidence that reduced oxygenation was the most important controlling factor in this subenvironment. Fluvial processes generate the most extreme stressful conditions for benthic colonization by tracemakers. Wave processes tend to buffer the stresses produced by river discharge. The effect of waves on fluvial stress is reflected in the higher bioturbation intensities and ichnodiversities that the wave-dominated deltaic deposits of the La Anita Formation delta show in relation to the fluvial-dominated ones.

Coalesced Delta-front Sheet-like Sandstone Bodies from Highly Avulsive Distributary Channels: The Low-accommodation Mesa Rica Sandstone (Dakota Group, New Mexico, U.S.A.)

AbstractLow-accommodation deltaic systems are often challenging to interpret due to their condensed, low-gradient nature, which often results in extensive, sheet-like sandstone bodies. As a result, detailed studies of such systems are scarce, and their internal depositional architecture is still poorly understood. We analyze one such system, the Cenomanian deltaic Mesa Rica Sandstone (Dakota Group), which was deposited in the Western Interior Seaway, in east-central New Mexico, USA. A &gt; 20-km-long escarpment, subparallel to the main delta progradation direction, allows a detailed analysis of facies distribution, depositional architecture, and the spatial extent of stratigraphic surfaces. Results reveal an arrangement of laterally variable shallowing-upward facies successions with three depositional cycles preserved. The first cycle is characterized by deltaic sheet-like sandstone bodies that are consistently overlain by sand-filled amalgamated distributary-channel deposits. The two successive cycles record a progressive reduction of sediment supply into the basin. Vertical and lateral relationships between facies associations and architectural geometries allow the recognition of regional key stratal surfaces, incised-valley fills, and the presence of lagoonal deposits at a sub-regional scale.The Mesa Rica deltaic system represents a river-dominated delta with multiple distributary channels. The sheet-like delta-front sandstone bodies are interpreted as the result of the combined effect of high sandy-sediment supply and low accommodation. The latter acted as an accelerator for autogenic depositional mechanisms such as mouth-bar deposition and abandonment, and for the highly avulsive character of distributary channels. After deposition, minor wave reworking facilitated lateral sand redistribution and favored bioturbation. This study demonstrates that sheet-like delta-front sandstone geometries from low-accommodation systems can be formed without the dominance of wave redistribution processes. This cautions against interpretations of amalgamated shoreline systems based solely on apparent sandstone geometries, without taking into account the preservation potential and postdepositional modification of primary deltaic characteristics.

Retraction notice to “Tight oil accumulation mechanisms of Triassic Yanchang Formation Chang 7 Member, Ordos Basin, China”

Abstract The sedimentary environment, petrology, pore throat structure, migration and accumulation power, and accumulation process are analyzed to study the oil accumulation mechanism and distribution of Triassic Chang 7 Member tight oil in Ordos Basin. Chang 7 tight oil reservoirs are mainly semi-deep lake – deep lake sandy debris flow deposits and delta front underwater distributary channel deposits. The results of field emission scanning electron microscopy, CT scans and other high-resolution microscopy techniques show the physical properties of Chang 7 oil reservoirs are poor, micro- and nano-meter pore throats are developed, inter-granular pores and feldspar dissolved pores are the main pores, and tube- and strip-shaped pores are favorable reservoir space. The hydrocarbon generation and accumulation simulation of tight oil reservoirs shows that the hydrocarbon charging power of Chang 7 high-quality source rocks is strong. The oil saturation of the Chang 7 were more than 50% in the accumulation phase and even reached 70% or more with the continued power increase and oil enrichment. The source and reservoir rocks of Chang 7 tight oil coexist, so the hydrocarbons migrated under high pressure to migrate along relatively high permeable superimposed sands, reticular fractures and other pathways, and formed large-area continuous or quasi-continuous lithologic oil reservoirs.

Linking the High-Resolution Architecture of Modern and Ancient Wave-Dominated Deltas: Processes, Products, and Forcing Factors

Research Article| February 21, 2019 Linking the High-Resolution Architecture of Modern and Ancient Wave-Dominated Deltas: Processes, Products, and Forcing Factors R. Bruce Ainsworth; R. Bruce Ainsworth 1Australian School of Petroleum, University of Adelaide, Adelaide SA 5005, Australia Search for other works by this author on: GSW Google Scholar Boyan K. Vakarelov; Boyan K. Vakarelov 2SEDBASE OOD, 21B Moskovska Street, Sofia 1000, Bulgaria Search for other works by this author on: GSW Google Scholar Christian H. Eide; Christian H. Eide 3Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway Search for other works by this author on: GSW Google Scholar John A. Howell; John A. Howell 4School of Geosciences, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, U.K. Search for other works by this author on: GSW Google Scholar Julien Bourget Julien Bourget 5Centre for Energy Geoscience, School of Earth Sciences, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information R. Bruce Ainsworth 1Australian School of Petroleum, University of Adelaide, Adelaide SA 5005, Australia Boyan K. Vakarelov 2SEDBASE OOD, 21B Moskovska Street, Sofia 1000, Bulgaria Christian H. Eide 3Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway John A. Howell 4School of Geosciences, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, U.K. Julien Bourget 5Centre for Energy Geoscience, School of Earth Sciences, University of Western Australia, Crawley, WA 6009, Australia Publisher: SEPM Society for Sedimentary Geology First Online: 21 Feb 2019 Online Issn: 1938-3681 Print Issn: 1527-1404 Copyright © 2019, SEPM (Society for Sedimentary Geology) Journal of Sedimentary Research (2019) 89 (2): 168–185. https://doi.org/10.2110/jsr.2019.7 Article history First Online: 21 Feb 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. Bruce Ainsworth, Boyan K. Vakarelov, Christian H. Eide, John A. Howell, Julien Bourget; Linking the High-Resolution Architecture of Modern and Ancient Wave-Dominated Deltas: Processes, Products, and Forcing Factors. Journal of Sedimentary Research 2019;; 89 (2): 168–185. doi: https://doi.org/10.2110/jsr.2019.7 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract Wave-dominated deltas are often fed by single trunk distributary channels which can remain the primary source of sediment supply to the delta for periods of thousands of years. Consequently, the sedimentary architecture of the delta can record subtle changes in sediment supply and wave intensity over significant periods of time. The geomorphological expression of these variations are beach-ridge elements and disconformity-bounded, beach-ridge element sets. There are two types of beach-ridge element sets observed on modern deltas, those associated with mouth-bar progradation (mouth-bar element sets), and those associated with delta-lobe flank accretion (lobe element sets). When the ratio of the rate of sediment supply by the fluvial system (F) is relatively high with respect to the rate of sediment removal at the mouth-bar location by waves (W) (i.e., the F/W ratio is high), the mouth-bar element sets are deposited. When the F/W ratio is low, sediment is preferentially transported to the lobe flanks and the lobe element sets are deposited. The mouth-bar and lobe element sets are bounded by the same unconformity and disconformity surfaces and are together termed element-set pairs. Analogous cyclical patterns of deposition have also been recognized in plan view and vertical sections from studies of ancient wave-dominated deltas from outcrop and subsurface data (seismic, well logs, and cores).Dating of beach-ridge elements on deltas deposited in the last 6000 years (Holocene) indicate a rate of formation of individual ridges on the order of decades to one hundred years. The beach-ridge element sets and beach-ridge element-set pairs are typically formed in periods of hundreds of years. Groups of beach-ridge element sets, beach-ridge element-set pairs, and associated genetically related distributary channel deposits form individual delta lobes. The delta lobes are generated by fluvial avulsion episodes, which are autogenic events intrinsic to the fluvial deposystems, and which typically occur on the order of multiple hundreds to thousands of years. Individual beach-ridge-element formation has previously been attributed to autogenic events. We propose that centennial-scale climate cycles may provide a mechanism for generating and controlling the intra-lobe changes in F/W ratio that generate the beach-ridge element-set and beach-ridge element-set-pair morphology of wave-dominated deltas. It follows that observations of such morphologies in the ancient could potentially be used as a proxy for subtle centennial-scale climatic forcing of wave-dominated deltas through deep geological time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Controls on reservoir quality in fan-deltaic conglomerates: Insight from the Lower Triassic Baikouquan Formation, Junggar Basin, China

Abstract Fan delta sandy conglomerates form important oil and gas reservoirs. Recently, a new giant oilfield was discovered in the Lower Triassic Baikouquan Formation (T1b) in the Mahu Sag of the Junggar Basin, northwestern China. However, the fan delta sandy conglomerate reservoirs are strongly heterogeneous. Most have poor permeability and host little hydrocarbons, while a subset at the delta front exhibit high porosity and permeability, forming high-quality reservoirs. To explain this disparity, the sedimentary facies, diagenesis, and reservoir quality of the sandy conglomerate strata, and their interactions, were studied. Detailed logging of 253m cores indicates that the T1b was deposited in a lacustrine fan delta complex, comprising ten lithofacies from coarse matrix-supported pebbly conglomerate (Gm) to fine massive mudstone (Fm) in four Facies Associations. Interpretation of optical microscope, backscattered and scanning electron microscope imaging show that these ten lithofacies underwent differential diagenesis, especially dissolution of minerals such as orthoclase. Dissolution was particularly prevalent in the subaqueous distributary channel deposits of sandstones (St), hyperpycnal-flow massive sandstones (Sm) and grain-flow deposits of sandy gravelly conglomerates (Gcs) at the delta-front, forming abundant secondary pores. These pores enhanced the reservoir performance and led to high porosity and permeability within these three lithofacies. For these reservoirs, continuous winnowing at the delta front led to relatively high primary porosities, and subsequent mineral dissolution during burial diagenesis, closely related to acidic Mn-rich hydrocarbon-bearing fluids, significantly improved the porosities further. Migration of hydrocarbon-bearing fluids was facilitated by faults cutting through the reservoir and underlying source rocks. This led to the development of belts of high-quality reservoir at the delta front near the fault conduits. This model for reservoir development and charge suggests that further reservoirs can be expected within the T1b of the Mahu Sag.

Influence of sedimentary facies on reservoir quality and distribution of diagenetic features in the Funing Formation, Wanglongzhuang Oilfield, Subei Basin, Eastern China

The Paleogene clastic sediments of the Funing Formation, which is characterized by slump-fan, delta-front, and beach-bar deposits, are important targets for oil and gas exploration in the Wanglongzhuang Oilfield, Subei Basin, eastern China. Three categories of reservoir quality are distinguished on the basis of petrographic and petrophysical experiments such as nuclear magnetic resonance (NMR) and mercury injection (MI). A paragenetic sequence for diagenetic evolution of each sedimentary facies has been reconstructed to determine the mechanisms for the variations in reservoir quality of the formation and the influence of the sedimentary facies on the distribution of diagenetic features in each of its members. The maturity of the organic material, textural relationships, and the distribution pattern of diagenetic minerals suggest that the study area underwent mainly mesodiagenesis. The mesodiagenesis A and mesodiagenesis B stages induced different diagenetic features in the target reservoir; the beach-bar sand of the E1f2 unit and the delta-front sand of the E1f4 unit underwent mostly only cementation; the delta-front deposits of the E1f3 unit, however, suffered more from mechanical and chemical compaction than from cementation. The loss of porosity in the fluxoturbidites of E1f4 due to compaction was just as large as the loss by cementation. The gravity-flow channel, sand-bar, and distributary-channel deposits have a higher porosity and permeability than the sand-beach and estuarine-bar deposits.

Predictive distribution of high-quality tight reservoirs of coarse clastic rocks by linking diagenesis to sedimentary facies: Evidence from the upper Sha 4 Member in the northern Bonan Sag, Bohai Bay Basin, eastern China

Coarse clastic rocks of the upper Sha 4 Member in the northern Bonan Sag of the Bohai Bay Basin in eastern China are important hydrocarbon reservoirs. The deposits are tight reservoirs owing to the low porosity (less than 10%) and low permeability (less than 1 mD). Because of the strong heterogeneity, although the reserve in the northern Bonan Sag is remarkable, only 4.9% of the reserves are recovered. We have studied these tight reservoirs by linking diagenesis to sedimentary facies to help predict the distribution of high-quality tight reservoirs. Petrographic analysis is undertaken based on cores, thin sections, X-ray diffraction and scanning electron microscope, helpful to understand the impacts on tight reservoirs of sedimentary factors and diagenesis factors. Sedimentary microfacies, lithologic characteristics, reservoir property, diagenesis, and diagenetic minerals are studied. Coarse clastic rocks are deposited mainly in nearshore subaqueous fans and fan deltas. The multistage sandstones are the valid reservoirs of coarse clastic rocks and dominated by feldspathic litharenite, lithic arkose, and arkose. The reservoir property is poor principally owing to the strong compaction and cementation. Pores are composed of secondary pores and primary pores. The secondary pore, generated in the dissolution of detrital minerals and/or cements, is the major type of pores and important to porosity improvement. By linking diagenesis to sedimentary facies, it can be concluded that high-quality tight reservoirs of coarse clastic rocks of the upper Sha 4 Member in the northern Bonan Sag of Bohai Bay Basin in eastern China are associated with medium to coarse-grain sandstones, found in the middle part of underwater distributary channel deposits in fan deltas and in the middle part of underwater channel deposits in nearshore subaqueous fans, with abundant secondary porosity but low cement contents, vertically at depths ranging from 3500 to 4100 m.

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.

The imprint of Late Holocene tectonic reactivation on a megafan landscape in the northern Amazonian wetlands

The modern Amazonian ecosystem outcomes from the complex interplay of different factors performed over the geological history, with tectonics being long speculated as perhaps a fundamental one. Nevertheless, areas where tectonic activity can be fully characterized are still scarce in view of the large dimension of this region. In this work, we investigate the signature of neotectonics in one megafan paleolandform that typifies a large sector of the Negro-Branco basin in northern Amazonia. The approach joined regional morphostructural descriptions of the Viruá megafan surface and the acquisition of topographic, sedimentological, and chronological data focusing on the central sector of the megafan. The results revealed an abundance of rivers that form dendritic, subdendritic, and trellis patterns. These rivers also have numerous straight segments, orthogonal junctions, and orthogonal shifts in courses. Structural lineaments, defined by straight channels and also straight lake margins, are aligned along the NW-SE and NE-SW directions that are coincidental with the main regional structural pattern in Amazonia. This study also led to recognize two large areas of lower topography in the south-central part of the megafan that consist of rectangular depressions parallel to the morphostructural lineaments. A sedimentological survey indicated that cores extracted external to the largest depression have only distributary channel and overbank sand sheet megafan deposits. Optically stimulated luminescence (OSL) ages ranged from 17.5±2.0 to 46.9±3.4ky and radiocarbon ages ranged from 5.9–5.7 to 20.1–19.6calky BP. In contrast, cores extracted within the depression consisted of fluvial deposits younger than 2.1–1.9calky BP that increased in thickness toward the central part of the depression. We propose that the studied megafan was affected by tectonic reactivation until at least a couple thousand years ago. Tectonics would have produced subsiding areas more prone to flooding than adjacent terrains, which constituted sites for renewed deposition of fluvial sediments reworked from the megafan surface following its abandonment. A comparison of our data with those from other Amazonian areas with similar records of late Holocene tectonics suggests a landscape imprinted by faulting, probably of strike-slip motion. This finding increases the record of neotectonic activity in the Amazonian wetlands and may be useful in studies aiming at discussing the origin and extension of late Holocene deformation in the South American intraplate. In addition, we present a megafan with an unusual development in a cratonic region under the combined effect of climate and tectonics.

Sedimentary characterization of a braided delta using well logs: The Upper Triassic Xujiahe Formation in Central Sichuan Basin, China

The image logs played a key role in interpreting the lithology and sedimentary structures of uncored units. An integrated analysis of the borehole images and open-hole logs, which are calibrated with core observations, is conducted to reveal the detailed sedimentary characteristics of the Upper Triassic Xujiahe Formation in the Central Sichuan Basin. To establish borehole image interpretation guidelines, the interpretations of sedimentary textures and lithology from cores and outcrops are calibrated with borehole images and conventional logs. Lithologies in the Xujiahe Formation include conglomerates, fine to medium-grained sandstones, siltstones, mudstones and coals. Sedimentary structures in terms of bedding, scour surface and stacking style of beds are interpreted from the image and open-hole logs. Depositional microfacies are defined on the basis of grain size and sediment texture, sedimentary structures, thickness, basal and upper contacts derived from core observations, openhole logs and image logs. The Xujiahe Formation sandstones in Central Sichuan Basin are deposited in a braided delta front, of which the depositional microfacies consist of underwater distributary channel, river mouth bar, distal bar or sand sheet and underwater interdistributary bay. The depositional microfacies is predicted in a single well using a combination of openhole conventional logs, borehole images, and cores. The distributary channels, which show box-shaped or bell-shaped GR log character, are generally infilled with fine to medium-grained sandstone with massive, tabular, wedge-shaped or trough-cross-laminated bedding. Repeated fining-upwards cycles, as well as the stacked nature of individual bedsets and the presence of scour surfaces represent multistory, amalgamated, channel-fill deposits. The underwater distributary channel deposits can also be overlain by interdistributary bay mudstones, known as channel abandonment. Mouth-bar deposits, which are characterized by coarsening- up successions, infill the channels as they migrate laterally, and sometimes could be overlain by a channel deposit. The methodology has potential application to other braided delta front reservoirs with appropriate calibration and scaling.

An assessment of the Upper Succession and the related secondary reservoirs in the Welton Field, onshore UK

AbstractThe Welton oil field has produced nearly 20 MMBO (million barrels of oil) since discovery in 1981. Now in post-plateau decline, there is increasing reliance on a series of secondary reservoirs. Production has been from a suite of stacked reservoirs deposited by large-scale prograding delta-plain systems of early Westphalian age. Whilst the bulk of production has been from the Basal Succession, a considerable upside is considered to exist in the less well-studied Upper Succession that comprises predominantly distributary channel and crevasse splay deposits which have produced in excess of 3 MMBO. These accumulations occur within the Deep Soft Rock, Deep Hard Rock and Tupton reservoirs.This paper focuses on a sedimentological analysis of cored intervals, integrated with petrophysical logs and detailed production data to enable further recommendations to identify areas of undrained pay, along with identifying additional reservoir management activities that could optimize future offtake from the field. These reservoirs consist predominantly of very fine-grained sandstone, with permeability values rarely attaining 100 mD and average porosity values of 10–12%.Recommendations include executing tracer communication tests and building a detailed field model, as well as a pilot water-injection scheme to increase production from some of Welton's secondary reservoirs.Supplementary material: A full set of detailed sedimentological logs for each of the cored wells in this study is available at https://doi.org/10.6084/m9.figshare.c.3593984

RETRACTED: Tight oil accumulation mechanisms of Triassic Yanchang Formation Chang 7 Member, Ordos Basin, China

Abstract The sedimentary environment, petrology, pore throat structure, migration and accumulation power, and accumulation process are analyzed to study the oil accumulation mechanism and distribution of Triassic Chang 7 Member tight oil in Ordos Basin. Chang 7 tight oil reservoirs are mainly semi-deep lake – deep lake sandy debris flow deposits and delta front underwater distributary channel deposits. The results of field emission scanning electron microscopy, CT scans and other high-resolution microscopy techniques show the physical properties of Chang 7 oil reservoirs are poor, micro- and nano-meter pore throats are developed, inter-granular pores and feldspar dissolved pores are the main pores, and tube- and strip-shaped pores are favorable reservoir space. The hydrocarbon generation and accumulation simulation of tight oil reservoirs shows that the hydrocarbon charging power of Chang 7 high-quality source rocks is strong. The oil saturation of the Chang 7 were more than 50% in the accumulation phase and even reached 70% or more with the continued power increase and oil enrichment. The source and reservoir rocks of Chang 7 tight oil coexist, so the hydrocarbons migrated under high pressure to migrate along relatively high permeable superimposed sands, reticular fractures and other pathways, and formed large-area continuous or quasi-continuous lithologic oil reservoirs.