Delta-front Environments Research Articles

Facies Analysis Using Attribute Seismic With Simply Rock Typing in Field “X” Kutai Basin

Abstract Field X is one of the hydrocarbons producing fields in the Kutai Basin which is one of the largest and deepest tertiary basins producing the second largest hydrocarbon production in Indonesia with 2.47MMBO oil reserves and 28.1 TCF gas. Based on qualitative analysis, the target zone in the four wells is estimated to be within the range of 1050 – 1400 ms. Sensitivity analysis was employed to determine the most sensitive parameters capable of distinguishing lithology within the formation. To ascertain the optimal number of attributes to be used, linear regression was employed, resulting in the selection of 12 attributes with a correlation coefficient of 0.919. Concurrently, log data was utilized for electro facies analysis. Electro facies analysis involved interpreting the shape of the gamma-ray curve, which indicated that the deposition setting in the four wells resembled a delta system. Additionally, rock typing was employed to classify rocks based on wet clay shale parameters, yielding information about the direction and more detailed settings of deposition. The results from the multi-attribute method, rock typing, and electro facies analysis, it was determined that Field X was deposited in a tidal delta to delta front environment, with a deposition direction from North to South.

Palaeoenvironments and palaeoclimate of the uppermost Cuyo Group and lowermost Lotena Group at Quebrada Álvarez, Picún Leufú Sub-basin, Patagonia, Argentina: a preliminary study based on palynology

The palynological (palynostratigraphical/palynofacies) analysis of samples from the Middle Jurassic outcrop at the Quebrada Álvarez section located in the Neuquén Basin, Patagonia, Argentina, allowed us to begin to evaluate the palaeoecological significance of the recovered palynofloras and their importance in the evolution of these ecosystems, during the transition between the uppermost Cuyo Group and the lowermost Lotena Group. The Lajas Formation, characterised by the palynofacies type A, could be interpreted as a tide-modified delta front environment. The high abundance and diversity of the palynomorphs identified in the studied samples, contribute to improve the palynological knowledge of the Challacó Formation (palynofacies type B − F). Considering the palaeoecological requirements of the recognised plant families, relatively humid and warm climate conditions could be inferred during the deposition of this unit. Also, the ‘seasonally dry (winterwet)’ biome was interpreted for the first time in the Neuquén Basin. Based on selected key taxa a late Bathonian–early Callovian age is proposed for the Challacó Formation at the Quebrada Álvarez section. The palynological material recorded in this unit suggests the development of a lacustrine environment with cycles of relative contraction and expansion of the water body due to fluctuations between relatively dry and wet conditions. The dominance of phytoclasts, mainly opaque particles, and freshwater algae (Botryococcus) in the Bosque Petrificado Formation (palynofacies type G − I) could suggest the development of a freshwater to brackish lacustrine environment. Based on the dominance of the same group of palynomorphs (chlorophyte algae) in the Challacó and Bosque Petrificado formations, similar palaeoenvironmental conditions would be inferred for these units. From a lithofacies analysis point of view, the Challacó and Bosque Petrificado formations show isopic facies associations in the study locality.

Upper Miocene ostracods from the Krško Basin, SE Slovenia

The purpose of this study was to identify the ostracod assemblage from a 43 m thick section of the Bizeljsko Formation, which constitutes the middle part of the Upper Miocene Posavje Group. The succession comprises sandstone, siltstone, and marlstone, deposited in a delta front environment. The determined ostracods belong to the families Cyprididae, Cytheridae, Darwinulidae, and Loxoconchidae. In total, 30 species were identified. Additionally, 8 morphotypes were determined at the genus level. The most common genera are Candona, Hemicytheria, and Cyprideis. The assemblage belongs to the Caspiocypris labiata subzone from the upper Pannonian. The ostracod assemblage from the Krško Basin is similar in species composition to Pannonian ostracod assemblages from Croatia and Serbia.

Depositional environments of the Miocene sediments in northern Song Hong basin

Northern Song Hong Tertiary Sedimentary basin is a classic case study of a pull-apart basin in southeast Asia, whose formation was controlled by the India-Eurasia collision, sinistral and dextral strike-slip motion of the Ailao Shan-Red River Shear Zone and Opening of the East Vietnam Sea during the Cenozoic. Unlike the central and southern parts of the basin, the northern Song Hong Basin experienced a very strong inversion during the Late Miocene. This rapid uplift of the region has led to significantly change in lithofacies and sedimentary environments, which are now still poorly understood. This uncertainty is considered one of the main challingings in prediction of the non-structural traps in the region. The recent results derived from well logging and 2D/3D seismic interpretation allowed us to define the Miocene formation in northern Song Hong basin, which are subdivided into three substrata, namely: the Lower, Middle and Upper stratum, which are characterized by typical characteristics of lithology and depositional environments. The Lower Miocene formation is dominated by deltaic environment at the bottom, transitioning to the overlying shelf environment. Lithology of the section varies from coarse-grained sediment (sandstone) to fine grained material such as shale and mudstone upward; The Middle Miocene stratum demonstrate sandier, coalic materials of the delta plain and delta front environments intercalated with swampy shale. In contrast, the Upper Miocene section is characterized by more fluvial and nearshore elements. It is illustrated by presence of the channel-filled sand bodies and mouth/longshore sand bars. These sand bodies demonstrate good porosity and horizontal permeability, which are considered to be good potential reservoir for both structural and non-structural traps in the Miocene formation.

Lithofacies and Diagenetic Controls on Tight Silty and Sandy Upper Triassic Reservoirs of the Heshui Oil Field (Ordos Basin, North China)

Summary Tight oil, present in reservoirs of low porosity and permeability, can be regarded as a kind of unconventional resource. The tightening process in this kind of reservoir is controlled by the lithology and diagenetic history of the host formation. Upper Triassic Yanchang Formation siltstones and sandstones are the main reservoirs for hydrocarbon accumulation in the Heshui Oil Field (HOF), southwestern Ordos Basin. The reservoirs exhibit low porosity, low permeability, and strong heterogeneity. In recent years, numerous drillcores have been recovered from these units, but the porosity-permeability characteristics and burial history of these silty and sandy reservoirs have not yet been reported in detail. In this study, an integrated analysis of the lithofacies, diagenesis, and reservoir characteristics of the siltstones and sandstones was achieved using a combination of core and thin section, grain size, scanning electron microscopy (SEM), X-ray diffraction (XRD), δ13C and δ18O, mercury intrusion capillary pressure (MICP), and porosity and permeability data. Our primary goals were to quantify the porosity-permeability characteristics of these silty and sandy reservoirs, restore their diagenetic histories, and examine the paragenetic relationship of reservoir tightness to hydrocarbon accumulation. The silty and sandy reservoirs represent braided river delta facies consisting of compositionally and texturally immature sediments. In the burial environment, they underwent complex diagenetic processes that reduced porosity from an initial average of ~38% to the present ~8%. Porosity-destructive processes included compaction (~ –12.5%) and cementation (~ –21%), with increases in porosity related to grain dissolution (~ +2.2%) and tectonic fractures (~ +1.1%). The reservoirs underwent four diagenetic stages: (1) Penesyngenetic and Eogenetic A Stage (Late Triassic-Early Jurassic); (2) Eogenetic B Stage (Late Jurassic); (3) Early Mesogenetic A Stage (Early Cretaceous); and (4) Late Mesogenetic A Stage (Late Cretaceous to recent). Hydrocarbon charging of these reservoirs occurred in three pulses. Existing pore space was partly filled by hydrocarbons during the Eogenetic B Stage. A second hydrocarbon charging event occurred during the Early Mesogenetic A Stage, when residual primary intergranular pores and secondary dissolution pores were filled. A third hydrocarbon charging event occurred during the Late Mesogenetic A Stage, when the reservoirs were tight. Siltstone beds deposited in delta front environments are the main future exploration targets in the Chang 6 to 8 members. The results of this study provide a useful reference framework for future exploration of hydrocarbon resources in the Upper Triassic Yanchang Formation of the HOF, as well as potential insights into the evolution of similarly tight reservoirs in other basins.

The Chengjiang Biota inhabited a deltaic environment

The Chengjiang Biota is the earliest Phanerozoic soft-bodied fossil assemblage offering the most complete snapshot of Earth’s initial diversification, the Cambrian Explosion. Although palaeobiologic aspects of this biota are well understood, the precise sedimentary environment inhabited by this biota remains debated. Herein, we examine a non-weathered core from the Yu’anshan Formation including the interval preserving the Chengjiang Biota. Our data indicate that the succession was deposited as part of a delta influenced by storm floods (i.e., produced by upstream river floods resulting from ocean storms). Most Chengjiang animals lived in an oxygen and nutrient-rich delta front environment in which unstable salinity and high sedimentation rates were the main stressors. This unexpected finding allows for sophisticated ecological comparisons with other Burgess Shale-type deposits and emphasizes that the long-held view of Burgess Shale-type faunas as snapshots of stable distal shelf and slope communities needs to be revised based on recent sedimentologic advances.

Sedimentary structures associated with hybrid event beds from the Gull Island Formation (Pennsylvanian) of north Clare, Ireland

A previously unreported assemblage of sedimentary structures, including several types of ripple, chevron marks, ridge and furrows, plant-rich laminations, found in association with hybrid event beds, with related muddy debrites and the trace fossil Kouphichnium, is described from a short 10m section through the middle of the Gull Island Formation [GIFm] (Pennsylvanian) of northern County Clare, Ireland. The section exposes a sequence of thickening and coarsening-upwards gravity flow deposits and interbedded mudstone layers. The upper part of the section is dominated by thick, massive and amalgamated sandstone beds; the middle section contains thinner sandstone beds with debritic mudstone caps, while the lowermost section comprises thin beds with a range of sedimentary structures produced by low density, transitional flows influenced by mud content, including mud ripples. 'High-density' turbidity currents operating in a delta front setting are inferred to have transformed to hybrid event beds with dilute but mud-rich suspensions travelling further distally to emplace thin beds, with evidence for clay-induced transitional flow and an assemblage of sedimentary structures not previously described from the GIFm. Small-scale ripple foresets from the upper part of the lower section indicate a southerly component of palaeocurrent flow direction. Chevron marks show size and palaeo-flow direction variation, suggesting temporal changes in current flow. Ridge and furrows have previously been interpreted as being formed under the head of turbidity current; however, the scale of the ridge and furrows described here suggest that interpretation is unlikely and some other unknown mechanism is involved. The presence of Kouphichnium trackways is significant as Carboniferous examples of this ichnofossil are usually recorded from very shallow marine or marginal marine environments. This part of the GIFm in north Clare is interpreted as a shallowing, delta front lobe environment exhibiting an upward transition from marginal or distal lobe to proximal or axial lobe sub-environment. This study highlights the influence of flow transformation in a muddy delta front environment with a high level of sediment supply favouring sediment remobilisation and the generation of a distinctive suite of sedimentary structures.

Sedimentary Structures Associated With Hybrid Event Beds From the Gull Island Formation (Pennsylvanian) Of North Clare, Ireland

A previously unreported assemblage of sedimentary structures, including several types of ripple, chevron marks, ridge and furrows, plant-rich laminations, found in association with hybrid event beds, with related muddy debrites and the trace fossil Kouphichnium, is described from a short 10m section through the middle of the Gull Island Formation [GIFm] (Pennsylvanian) of northern County Clare, Ireland. The section exposes a sequence of thickening and coarsening-upwards gravity flow deposits and interbedded mudstone layers. The upper part of the section is dominated by thick, massive and amalgamated sandstone beds; the middle section contains thinner sandstone beds with debritic mudstone caps, while the lowermost section comprises thin beds with a range of sedimentary structures produced by low density, transitional flows influenced by mud content, including mud ripples. ‘High-density’ turbidity currents operating in a delta front setting are inferred to have transformed to hybrid event beds with dilute but mud-rich suspensions travelling further distally to emplace thin beds, with evidence for clay-induced transitional flow and an assemblage of sedimentary structures not previously described from the GIFm. Smallscale ripple foresets from the upper part of the lower section indicate a southerly component of palaeocurrent flow direction. Chevron marks show size and palaeo-flow direction variation, suggesting temporal changes in current flow. Ridge and furrows have previously been interpreted as being formed under the head of turbidity current; however, the scale of the ridge and furrows described here suggest that interpretation is unlikely and some other unknown mechanism is involved. The presence of Kouphichnium trackways is significant as Carboniferous examples of this ichnofossil are usually recorded from very shallow marine or marginal marine environments. This part of the GIFm in north Clare is interpreted as a shallowing, delta front lobe environment exhibiting an upward transition from marginal or distal lobe to proximal or axial lobe sub-environment. This study highlights the influence of flow transformation in a muddy delta front environment with a high level of sediment supply favouring sediment remobilisation and the generation of a distinctive suite of sedimentary structures.

Sequence stratigraphic framework of the Upper Eocene–Oligocene succession, northwest Birket Qarun, Fayum-Egypt

The Upper Eocene–Oligocene sequence exposed northwest Birket Qarun; Fayum district covers two rock units, Qasr El-Sagha (Temple and Dir Abu Lifa members) and Gebel Qatrani formations were re-evaluated by sequence stratigraphy. Six depositional sequences were recognized based on facies geometry, and sedimentological aspects. The first sequence (Temple Member) was accumulated under marginal intertidal flat environment indicating highstand deposits. The second (the lower Dir Abu Lifa Member) displays a regressive sedimentary facies starting with progradational delta front environment reflecting lowstand deposits followed upward by a lagoonal environment indicating transgressive/highstand deposits. The third (the top of the Dir Abu Lifa Member) was accumulated under a low sinuosity stream reflecting lowstand deposits. The fourth which constitutes the topmost of Dir Abu Lifa Member initiating near-shore shallow marine points to transgressive sediments, followed upward by fluvial sediments reflecting highstand deposits. The fifth (the Lower of Gebel Qatrani Formation) accumulated as point-bar deposits demonstrating lowstand deposits. The last occupies the main of Gebel Qatrani Formation initiates with restricted riverine marking lowstand deposits, followed upward by several fining upward cycles that were accumulated in fluvial and flood plain conditions with evidence of tidal influence on sedimentation near its top denoting transgressive deposits. Several minor transgressive episodes accompanied by tidal influence on sedimentation are noticed near the top, which may reflect gradual subsidence of the Oligocene alluvial plain. The area is subjected to relative sea-level changes that show a similarity with the global sea level. The base-level fluctuations are mainly controlled by the regional tectonic setting.

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.

Short- and long-term movement of mudflows of the Mississippi River Delta Front and their known and potential impacts on oil and gas infrastructure

Abstract Mudflows on the Mississippi River Delta Front (MRDF) are recognized hazards to oil and gas infrastructure in the shallow (20–300 m water depth) Gulf of Mexico. Preconditioning of the seafloor for failure results from high sedimentation rates coupled with slope over-steepening, under-consolidation and abundant biogenic gas production. Catastrophic failure of production platforms and pipelines due to seafloor displacement during infrequent large hurricanes such as Camille in 1969 and Ivan in 2004, point to cyclical loading of the seafloor by waves as a primary movement trigger. Due to data limitations, the role of smaller storms and background oceanographic processes in driving seafloor movement has remained largely unconstrained but these are thought to contribute to significant seafloor change. With the aid of new high-resolution multibeam mapping and seismic reflection profiling across sections of the MRDF, several moving features within the deforming delta-front environment are investigated and potential hazards to infrastructure installed and adjacent to the region are discussed. Via repeat mapping surveys of selected areas and records of changing shipwreck locations, we highlight significant seafloor displacement across annual to decadal timescales. For example, individual blocks mapped within mudflow gullies adjacent to Southwest Pass show downslope transport of more than 80 m in a single year, while the SS Virginia , a 153 m-long oil tanker sunk in 1942, has been relocated and found to have moved downslope more than 400 m in 14 years, without a major hurricane (>Category 2) passing through the region.

Facies architecture and stratigraphic sequence of Zubair Formation in Majnoon and Suba oil fields, Southern Iraq

The Zubair Formation was deposited during the Barremain stage within the Early Cretaceous epoch. This study includes the lithofacies analysis and stratigraphic development of the Zubair Formation in four boreholes (Mj-2, Mj-4, Su-2 and Su-7) within the Majnoon and Suba oil fields. This succession is divided into three lithofacies units using well logs such as GR, SP, DT and RT. The upper member is composed mostly of shale layers, the middle member consists of thick layers of sandstone and the lower member consists mainly of layers of shale with less sandy layers. Five major lithofacies are recognized in this succession as follows: quartz arenite well-sorted subrounded lithofacies, quartz arenite poorly sorted sub-angular lithofacies, graywacke poorly sorted lithofacies, sandy claystone and shale lithofacies. Three facies associations (depositional environments) are distinguished in Zubair Formation: delta plain, delta front and backshore. The microfacies analysis and reconstructed paleoenvironments for the Barremain basin in the studied area have contributed to determine two stages of the deposition: the first stage showed clear when the facies was developed from the delta plain to the delta front association facies within the sand-dominated unit of Zubair Formation (lower and middle units). This succession is represented by three depositional cycles of transgression system tracts, which was ended with appeared the shale lithofacies within the well-sorted quartz arenite lithofacies to mark the maximum flooding surface (mfs) in the Suba oil field. While in the Majnoon oil field, these three cycles were represented by alternating delta plain and delta front association with dominated shale lithofacies. The deposition of this part of Zubair Formation was continued during still stand of the sea level to develop at the last time of this stage to delta front environment to mark a start a second stage. The second stage started with the appearance of the backshore association facies as a high-stand system tract (HST) within the upper unit. This pattern of deposition (HST) continued until a change in facies associations from delta front to a backshore in all studied wells. The clear change in the lithofacies occurred from the sand-dominated (delta front association) with high maturity fine-grained sandstone to shale-dominated lithofacies (back-shore) that onsets to still sea level. Therefore, the upper part of the upper unit is represents the HST, and then the Mauddud Formation was formed.

Stacked Parahaentzschelinia ichnofabrics from the Lower Permian of the southern Sydney Basin, southeastern Australia: Palaeoecologic and palaeoenvironmental significance

Trace fossils are sensitive indicators for environmental parameters (e.g., hydrodynamics, oxygen content, water depth, depositional rates, salinity), providing invaluable information to understand past depositional regimes not attainable from other sources of approaches. Here we report a new trace fossil assemblage dominated by a potential bivalve trace Parahaentzschelinia from the Lower Permian Snapper Point Formation, which records deposition in a storm-influenced delta front environment. Dense populations of Parahaentzschelinia occurred in two sections, forming stacked Parahaentzschelinia ichnofabrics and representing multiple burrow adjustments of a single trace maker (tellinid bivalve) subsequent to rapid sedimentation events caused by possible seasonal/annual runoffs. Statistical analysis of burrow lengths and numbers of adjustments suggests up to seven sequences of sediment accumulation events, supplying 3–13 cm thick clastic deposits at each time. A conservative sedimentation rate of 0.24 cm/year was estimated on the basis of two stacked Parahaentzschelinia ichnofabrics and the probable maximum lifespan of the modern tellinid genus. This rate is generally comparable to those derived from modern deltaic environments, lending further support to the interpretation that the ichnofabrics were formed in and characteristic of a deltaic setting. Our research highlights the utility of stacked equilibrium structures as a sensitive indicator of certain depositional regimes, and its applicability for assessing sedimentation rates as well as depositional conditions in the geological past.

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.

The evolution of hypoxia off the Changjiang Estuary in the last 3000 years: Evidence from benthic foraminifera and elemental geochemistry

Estuarine hypoxia has exerted great influence on the ecological system and fishery production in the past decades, consequently attracting intensive concern. In this study, the upper 18.0 m of Core YD0902, retrieved from the hypoxic area off the Changjiang Estuary, was dated by AMS 14C method and analyzed with lithology, grain size, benthic foraminifera, and geochemical elements to decipher the paleoenvironmental change and hypoxic history in the last 3000 years. The core location was in the neritic to prodelta transitional environment during 3000–2160 cal yr BP and the delta-front environment after 2160 cal yr BP. No hypoxia occurred during 3000–2600 cal yr BP, potentially related to the contemporary occurrence of a rapid cold climate event in the northern hemisphere. Hypoxia has likely existed persistently since 2600 cal yr BP. The period 2600–600 cal yr BP characterizes persistently severe hypoxia mainly linked with the warm climate, strong East Asian Summer Monsoon (EASM) and rainfall, intensified Taiwan Warm Current intrusion and upwelling, and high primary production. Lessened hypoxia followed in the period 600–260 cal yr BP because of the cold climate, weak EASM, and minor precipitation. The recent period (post 260 cal yr BP) was not disclosed by the core due to lost sediments in the top 4.5 m, and could only be surmised to have severe hypoxia because of climate factors similar to the period 2600–600 cal yr BP. Our findings shed new light on long-term hypoxic evolution and controlling factors in mega-river estuaries.

Subaqueous sandy mass-transport deposits in lacustrine facies of the Upper Triassic Yanchang Formation, Ordos Basin, Central China

Abstract Subaqueous sandy mass-transport deposits (SMTDs), which are a type of sedimentary deposit formed in deep water environments, have attracted increased attention in recent years. Based on the analysis of outcrops, cores and thin sections using X-ray diffraction and scanning electron microscopy, numerous sandy mass-transport deposits or sandy debrites have been identified in the delta-front and deep lake environments recorded by the Chang 6-7 Member of the Yanchang Formation in the Ordos Basin; thus, a mass-transport model has been established in this paper. These sandy mass-transport deposits are characterized by two distinct features: first, sandstones are characterized by a low matrix content and are defined as massive clean sands; second, massive sandstones are characterized by the widespread development of a rimed chlorite cement with a two-layer structure, which is identical to that present in distributary channel sands of the delta-front environment of the Yanchang Formation. The occurrence of and components of clay minerals indicates that the isopachous rimed cements in the inner layer are related to the original sedimentary environment. Rimed chlorite cements are formed from isopachous clay films that originally adhered to the surfaces of the detrital particles and represent syn-depositional products. Further studies suggest that during the process of retransporting delta-front sediments and forming the sandy debris flows, this material, which was adhered to the surfaces of clastic particles as isopachous clay coatings, together with the small amount of clay-water matrix present in the flow, acted as an “adhesive agent” between particles during the subsequent remobilization of the deposits and the formation of sandy debris flows. The adhesion strength and local matrix support strength created by the clay-water matrix (gels), together with the frictional strength of the sediments, provided support strength for the subaqueous mass transport of the Yanchang Formation. The adhesion formed by the isopachous clay rim or the clay-water matrix is especially dominant and is considered to be the root cause holding the sandy debris flows together during their mass transport. This research may have global implications for better understanding analogous subaqueous mass-transport processes and the distribution of sandstone reservoirs in other locations.

Postrift deltaic sedimentation in western Kachchh Basin: Insights from ichnology and sedimentology

The postrift stage in the pericratonic Kachchh Rift Basin of western India is represented by a thick sequence of a diachronous megadelta system. The deposition of this deltaic sequence was initiated during the mid-Kimmeridgian and ended in the Albian. The sedimentation was interrupted by two transgressive events demarcating three subcycles within the megadelta system. The present study addresses the older mid-Kimmeridgian to Tithonian “Jhuran Delta”. An attempt is made here to delineate the nature of the Jhuran Delta using the ichnological and sedimentological parameters. The Jhuran Delta, which consists of 14 lithofacies, is grouped into seven lithofacies associations corresponding to the prodelta, subaqueous gravity flow, delta front, distributary channel, distributary mouth bar, and shoreface depositional environments along with a transgressive unit overlapping the lower delta of the megadelta system. Three distinct stages of deltaic evolution are delineated along with the fourth stage of transgression and delta destruction. The prodelta environment of Stage I is characterized by moderate ichnodiversity represented by nine recurring ichnotaxa, suggesting alternating brackish and fully saline water conditions, which indicate river-dominated deltaic sedimentation. Stage II shows a marked increase in the influence of river (fresh water) water, as indicated by a submarine mass failure that includes debris flow, chaotic and contorted structures, sediment gravity-failure and water-escape structures and suggesting a higher rate of sedimentation. Ichnologically, this phase indicates low ichnodiversity with sporadic occurrences of Ophiomorpha and Palaeophycus. Stage III indicates the mixed dominance of river and wave processes. Sediments deposited in the delta front environment show moderate ichnodiversity with nine ichnotaxa. The distributary mouth bar environment comprises 6 recurring ichnotaxa including the ichnoassemblages of both river and marine wave environments. Integrated ichnological data suggest deposition in a mixed environment. The Jhuran Delta stage ended with a marine transgressive event and reworking of the deltaic sands, which were overlain by a thick shoreface deposit. Thus, the Upper Jurassic Jhuran Delta represents a river-dominated, wave-influenced delta.

Geological characteristics and main controlling factors of Permian lacustrine tight oil in the eastern part of the Junggar Basin

The Junggar basin is rich in oil and gas resources, and recently, unconventional oil and gas reservoirs have become important targets. Significant amounts of oil and gas are located in Permian formations in the Shazhang–Dajing area, however, fewer studies have focused on unconventional oil and gas reservoirs, particularly on the factors controlling the distribution of unconventional reservoirs. In this study, a sequence stratigraphic framework of the study area was developed based on cores, well logs and seismic data. Source rock samples were analyzed, including tests of kerogen, vitrinite reflectance, chloroform bitumen, organic carbon and group composition. The physical properties and pore characteristics of reservoir samples were also analyzed using scanning electron microscope and high resolution electron microscope. The Permian Pingdiquan Formation comprises three thirdorder sequences in which individual sequence can be divided into a low-stand system tract (LST), a transgression system tract (TST), an early high-stand system tract (E-HST), and a late high-stand system tract (L-HST). Several source rock lithologies have high total organic carbon (TOC), and are matured, having excellent hydrocarbon generation potential. Clastic and dolomitic rocks are the main reservoirs, and nanometer-scale pores are the main reservoir space in the tight reservoirs. The distribution of tight oil is controlled by the sequence stratigraphy, palaeogeomorphic units, and sedimentary facies. The fine sediments, which were generally deposited near the initial flooding surface (FFS) and the maximum flooding surface (MFS), are favorable zones for tight oil. The distributions of the hydrocarbon source rocks are controlled by the depositional environment, depending on the palaeogeomorphic units; the deep depression in the Huoshaoshan area and the moderately-deep depression in the Shishugou area are two distinct hydrocarbon generating centers. The distribution of tight oil is clearly controlled by the sedimentary facies; the tight oil is mainly located in the fan delta front and lacustrine depositional environments. The distribution of tight oil in this area is summarized, and favorable areas of tight oil are proposed.

Mud-coated intraclasts: A criterion for recognizing sandy mass-transport deposits – Deep-lacustrine massive sandstone of the Upper Triassic Yanchang Formation, Ordos Basin, Central China

Numerous examples of sandy mass-transport deposits (SMTDs) in deep-marine and deep-lacustrine environments have been documented by researchers worldwide in recent years. However, it is an enormous challenge that how to distinguish the subaqueous mass-transport processes from the depositional records. After the rigorous examination of several outcrops in the Ordos lacustrine basin in central China, we propose that the “mud-coated intraclasts” in the deep-water thick massive sandstone of the Yanchang Formation are a viable criterion to recognize deposition from sandy debris flows in delta front environments. These so-called “mud-coated intraclasts” are a newly discovered special depositional phenomenon in the massive clean sandstones in China’s lacustrine basins. A mud-coated intraclast generally consists of two components: an inner core and a mud coating. The inner core is generally composed of muddy blobs or sandy lumps. The mud coatings are generally composed of thin shales or mud-rich fine-grained sediments. The inner core, which is generally coated by nearly concentric ring-shaped mud coatings, is embedded in massive deep-water sandstones. Because these features have never been documented in the geological literature, the present authors called them “mud-coated intraclasts” in the present study. The depositional features suggest that the sediments that contain these mud-coated intraclasts were characterized by en masse emplacement (i.e., a Bingham plastic behavior) and were supported by the strength of the plastic medium at all times (i.e., no flow transformations occurred during their transport and deposition). Accordingly, mud-coated intraclasts are one of the most significant criteria that indicate sandy debrites as the origin of the deep-water thick massive sandstones in the Yanchang Formation. This diagnostic feature may be useful to identify analogous SMTDs in other locations.

Process regime variability across growth faults in the Paleogene Lower Wilcox Guadalupe Delta, South Texas Gulf Coast

The Wilcox Group in Texas is a 3000m thick unit of clastic sediments deposited along the Gulf of Mexico coast during early Paleogene. This study integrates core facies analysis with subsurface well-log correlation to document the sedimentology and stratigraphy of the Lower Wilcox Guadalupe Delta. Core descriptions indicate a transition from wave- and tidally-influenced to wave-dominated deposition. Upward-coarsening facies successions contain current ripples, organic matter, low trace fossil abundance and low diversity, which suggest deposition in a fluvial prodelta to delta front environment. Heterolithic stratification with lenticular, wavy and flaser bedding indicate tidal influence. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies and structureless sandstones with Ophiomorpha record deposition in wave-influenced deltas. Tidal channels truncate delta front deposits and display gradational upward-fining facies successions with basal lags and sandy tabular cross-beds passing into heterolithic tidal flats and biologically homogenized mudstones.Growth faults within the lower Wilcox control expanded thickness of sedimentary units (up to 4 times) on the downdip sides of faults. Increased local accommodation due to fault subsidence favors a stronger wave regime on the outer shelf due to unrestricted fetch and water depth. As the shoreline advances during deltaic progradation, successively more sediment is deposited in the downthrown depocenters and reworked along shore by wave processes, resulting in a thick sedimentary unit characterized by repeated stacking of shoreface sequences. Thick and laterally continuous clean sandstone successions in the downthrown compartments represent attractive hydrocarbon reservoirs. As a consequence of the wave dominance and increased accommodation, thick (tens of meters) sandstone-bodies with increased homogeneity and vertical permeability within the stacked shoreface successions are created.