Delta-front Sandstones Research Articles

Sediment supply variation control on Lower Eocene delta sequences (Tremp Basin, Spain)

Sediment supply variations are often overlooked when interpreting depositional sequences, with most studies instead emphasizing changes in accommodation space. Here, we investigated a temporally well-constrained shallow-marine succession in the Tremp Basin to test the control of sediment supply variations on the development of deltaic sequences during the early Eocene. We analyzed the paleoenvironmental record (sedimentary facies and δ13Corg values) of the Morillo Limestone and the Castigaleu Formation (52.2−50.6 Ma). The first progradation of the deltaic system is marked by the abrupt appearance of thick delta-front sandstones and associated with the first negative carbon isotopic excursions (CIEs) in the measured δ13Corg record. Subsequent phases of progradation align with subsequent negative CIEs. Conversely, positive CIEs correspond to finer-grained, more-distal prodelta deposits. A series of hyperthermal events occurred during the deposition, globally identified as negative CIEs on δ13Ccarb reference curves, which we tentatively correlate with our δ13Corg data. We therefore suggest that during deposition of this Lower Eocene shallow-marine succession, the primary trigger behind sequence generation was the high-frequency climate-induced variation in sediment supply, specifically the hyperthermal events, rather than changes in accommodation. This linkage underscores the complex interactions between climate dynamics and sedimentary responses that shape the stratigraphic architecture of shallow-marine settings.

Micro-scale investigation of the pore network of sandstone in the Pannonian Basin to improve geothermal energy development

This study focuses on the Pannonian Basin, specifically in Szentes, Hungary a region of significant geothermal potential, with particular emphasis on the Dunántúl Group; a collective name for the Zagyva and Újfalu formations, which consists of slightly consolidated delta-front sandstone sediments. This research is pivotal in understanding the challenges associated with clogging in geothermal wells, a problem that has led to the premature shutdown of injection wells in the region. Our approach integrates classical petrophysical and mineralogical methods with advanced techniques such as micro-Computed X-Ray Tomography imaging, 3D image analysis, and digital rock simulations. Our findings indicate that the target geothermal rock formations within the Dunántúl Group exhibit high porosity (27–31 %) and variable permeability (60–400 mD), dependent on the location and specific characteristics of the formation. Our micro-CT analyses further identified that the presence of fine-grained materials in smaller pores and generally weak cementation of grains substantially contributes to these challenges.

Modelling diagenetic reactions and secondary porosity generation in sandstones controlled by the advection of low‐molecular‐weight organic acids

AbstractHigher secondary porosity was observed in the centre of a sandstone unit in the Eocene Shahejie Formation fan delta front sandstones from the Bozhong Depression, Bohai Bay Basin. This differs from past studies showing secondary porosity mainly in the marginal parts of sandstones adjacent to shales. This study utilized reactive transport models involving low‐molecular‐weight organic acids (LMWOA) to discuss potential processes resulting in the contrary distribution of secondary porosity. An interface model simulating LMWOA diffusion from adjacent shales to the sandstone resulted in secondary porosity in sandstones adjacent to shales. In contrast, an advection model simulating advective transport of LMWOA parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to the dissolution through LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA‐rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates and ultimately higher secondary porosity. This study indicates that grain sorting during sediment deposition, early carbonate cementation, LMWOA production in adjacent shales, and advection processes collectively control the diagenetic reactions and the distribution of secondary porosity in sandstones.

Sedimentary evolution and distribution of benthic fauna of an Aptian protected bay (Oliete Subbasin, Spain)

Detailed facies and sequence stratigraphic analysis of an Aptian shallow-marine succession illustrates the evolution of sedimentary environments and their benthic faunal associations in a marginal bay developed in northeastern Iberia (Maestrazgo Basin, Oliete Subbasin). In this bay, marine circulation was hampered by wide stationary shoals located in the low-subsidence transitional areas between subbasins. The Aptian sedimentary record of this bay (Josa and Oliete formations) is composed of four transgressive–regressive (T–R) sequences bounded by major unconformities generated during relative sea-level falls. The Josa Fm (formally defined herein) encompasses an asymmetrical T–R cycle (sequence J). It comprises transgressive prodelta clays with episodic orbitolinid blooms, grading upwards to regressive freshwater-influenced littoral and delta-front sandstones and sandy limestones which yield diverse bivalve communities dominated by trigoniids. The Oliete Fm encompasses three T–R cycles (sequences O1–O3), mostly deposited in a protected lagoon. The transgressive hemicycle of each sequence is dominated by marls containing low-diversity faunal associations composed of ostracods and mm-sized gastropods and bivalves, indicating stressed (probably dysoxic) conditions in the deepest parts of the lagoon, related to a lack of water circulation. The regressive hemicycle of sequences O1 and O2 is characterised by low-energy intertidal to shallow subtidal deposits including sponge spiculites, mud-dominated accumulations of articulated bivalves and gastropods with low-diversity microfaunal associations composed mainly of Choffatella and few other textulariids, and marginal oyster-serpulid patches. The uppermost part of sequence O2 is characterised by the development of siliciclastic tide-influenced coastal plains. Sequence O3 consists of massive intertidal to shallow subtidal glauconite accumulations related to the transgressive reworking of siliciclastic deposits. Foraminiferal associations in sequence O3 show an increase in diversity (including miliolids, lageniids and diverse textulariids), which was probably fostered by a more stable connection with open seas, caused by tectonic activity in the bounding shoal areas. The analysis of these four sequences represents a comprehensive and detailed characterisation of shallow-to-marginal marine benthic communities integrated in a sequence stratigraphic framework.

Reservoir Characteristics and Controlling Factors of Large-Scale Mono-Block Gas Field Developed in Delta-Front Sandstone—A Case Study from Zhongqiu 1 Gas Field in the Tarim Basin

Taking the Zhongqiu 1 Gas Field in the Tarim Basin as an example, the heterogeneity of large-scale mono-block gas fields and their primary controlling factors have been analyzed. Based on drilling core data, well log data, scanning electron microscopy, thin-section analysis, and mercury injection experiments, combining sedimentological interpretation, research on the reservoir characteristics and variability was carried out. The results showed that: (1) The lithologic characteristics showed obvious variations among wells in the Zhongqiu 1 gas field. Specifically, the main lithology developed in the Zhongqiu 1 well is feldspar lithic sandstone, while the remaining wells predominantly consist of lithic feldspar sandstone. These differences in rock composition maturity reveal that a higher proportion of stable mineral components leads to poorer reservoir properties; (2) the main factors controlling oil and gas productivity include the variations in petrology, mineralogy, and diagenetic process characteristics. The high content of unstable mineral components and constructive diagenesis could increase reservoir porosity together. (3) Sedimentary facies of the Bashijiqike Formation in the Zhongqiu 1 Gas Field played a dominant role in the reservoir distribution. The division of sedimentary facies zones reflects variations in material composition and grain size, serving as the main material basis for reservoirs. Differences in mineral composition reflect the sedimentary environment of the reservoir. Additionally, mineral composition indicates the relationship between diagenetic processes and reservoir evolution. The high feldspar content in well ZQ1 corresponded to relatively favorable reservoir properties. The dominant feldspar type was plagioclase, suggesting that early-stage chemical weathering had undergone significant alteration. The above conclusions provided a microscopic perspective to explain the differences in oil and gas production capacity of large delta-front gas fields, serving as a geological basis for the exploration and exploitation of similar fields.

Movable fluid evaluation of tight sandstone reservoirs in lacustrine delta front setting: Occurrence characteristics, multiple control factors, and prediction model

The tight sandstone, deposited in a lacustrine delta front setting, constitutes substantial new petroleum reserves. Tight sandstone reservoirs' fluid mobility directly affects oil production and recovery efficiency. Consequently, it is crucial to quantitatively and synthetically characterize fluid mobility. This study presented a detailed investigation of the movable fluid of the Yanchang tight sandstone in the Ordos Basin. Multiple experiments were conducted to investigate movable fluid, including casting and cathodoluminescence thin section, nuclear magnetic resonance, X-ray diffraction, and high-pressure mercury intrusion. According to pore-throat structure investigation with petrographical observations, delta front sandstone was divided into four types of lithofacies, pore-throat structure, and three pore-throat spaces (nano-scale, submicron, and micron), which correspond to various occurrence characteristics of the movable fluid. The result shows that the fluid mobility of the micron pore-throat space is higher than the sub-micron scale, and the nano-scale is the lowest. The fluid mobility is driven by petrophysical property, pore-throat structure, diagenetic mineral, lithofacies, and sandstone strata pattern. With the deterioration of the pore-throat structure and petrophysical property, fluid mobility gradually decreases. Movable fluid content significantly correlates with quartz and feldspar content, showing that residual intergranular and feldspar dissolved pores can significantly improve fluid mobility. On the contrary, it is negatively correlated with clay and carbonate cement content. The carbonate cementation and clay mineral filling increase the proportion of nano-scale pore-throat, leading to the deterioration of the reservoir quality and pore-throat structure. Detailed researches on the spatial distribution of fluid mobility show it related to sandstone bed thickness, mudstone proximity, and mudstone/formation ratio. Typically, medium-grained sandstones forming thick beds in mudstone-poor sequences are commonly low in carbonate and clay mineral cement and highest in movable fluid content. In contrast, fine-grained sandstones occurring as thin beds or located near the mudstone-sandstone interface in mudstone-rich sequences generally exhibit high carbonate and clay cement. Hence, its movable fluid content is lowest. The frequent material exchange between sandstone and adjacent mudstone prompts the enhancement of the cementation of clay and carbonate minerals, which is a devastating factor for fluid mobility. The proposed model for predicting fluid mobility can be used for similar delta front sandstones of other lacustrine basins. These findings provide new insights into predicting the best reservoir sandstones with high fluid mobility and effectively exploiting tight sandstone reservoirs.

The influence of bioturbation on reservoir quality: Insights from the Columbus Basin, offshore Trinidad

Nine facies are recognised in two mid-Pliocene cores from the Columbus Basin, offshore east coast Trinidad. Facies successions are consistent with deposition within a deltaic-shoreface continuum. Lower shoreface deposits are heavily bioturbated by Scolicia, Asterosoma, Thalassinoides, Teichichnus, Palaeophycus and other elements of the Cruziana Ichnofacies whereas middle shoreface sandstone beds contain sand-filled burrows of the Skolithos Ichnofacies. Deltaic deposits record coarsening-upward successions with evidence of freshwater influx such as synaeresis cracks, fluid muds, and dispersed organic detritus. Prodelta and distal delta front deposits comprise heterolithic beds and contain sporadically distributed and diminutive structures belonging to the Phycosiphon Ichnofacies. Hummocky cross-stratification is prominent in storm-dominated proximal delta front sandstone deposits that are sparsely bioturbated by Ophiomorpha, Skolithos and other elements of the Rosselia Ichnofacies. Cryptic bioturbation is common at the top of these beds. Thin section examination, core plug analysis, and numerical modelling of the facies were undertaken to better understand fluid flow in bioturbated strata of the Columbus Basin. Whereas lithology and texture remain the primary control on reservoir quality, examples of biogenically enhanced permeability are present in the P3 and P7 cores. The highest permeability values are associated with highly bioturbated middle shoreface deposits that comprise clean, sand-filled burrows. Favourable permeability measurements are also attributed to cryptically bioturbated delta front sandstone beds. The latter display higher permeabilities when compared to unbioturbated and sparsely sandstone beds. Biogenic activity contributed to reduced permeabilities in lower shoreface and distal delta front deposits owing to extensive sediment packing and sediment mixing behaviours. Relationships between permeability and bioturbation intensity indicate that effective permeability in bioturbated intervals of the P3 and P7 cores are best characterised by the arithmetic mean of permeabilities and that simple statistical calculations can aid in the analysis of highly bioturbated facies (BI = 5–6).

A screening assessment of the impact of sedimentological heterogeneity on CO2 migration and stratigraphic-baffling potential: Johansen and Cook formations, Northern Lights project, offshore Norway

• Johansen-Cook storage unit consists predominantly of wave-dominated deltaic sandstones. • Novel method used to screen impact of sedimentological heterogeneity on CO 2 migration and stratigraphic-baffling potential. • Screening method combines experimental design, sketch-based reservoir modelling and single-phase flow diagnostics. • Multiple types of sedimentological heterogeneity have significant effects. We use a method combining experimental design, sketch-based reservoir modelling, and single-phase flow diagnostics to rapidly screen the impact of sedimentological heterogeneities that constitute baffles and barriers to CO 2 migration in the Johansen and Cook formations at the Northern Lights CO 2 storage site. The types and spatial organisation of sedimentological heterogeneities in the wave-dominated deltaic sandstones of the Johansen-Cook storage unit are constrained using core data from the 31/5-7 (Eos) well, previous interpretations of seismic data and regional well-log correlations, and outcrop and subsurface analogues. Delta planform geometry, clinoform dip, and facies-association interfingering extent along clinoforms control: (1) the distribution and connectivity of high-permeability medial and proximal delta-front sandstones, (2) effective horizontal and vertical permeability characteristics of the storage unit, and (3) pore volumes injected at breakthrough time (which approximates the efficiency of stratigraphic baffling). In addition, the lateral continuity of carbonate-cemented concretionary layers along transgressive surfaces impacts effective vertical permeability, and bioturbation intensity impacts effective horizontal and vertical permeability. The combined effects of these and other heterogeneities are also influential. Our results suggest that the baffling effect on CO 2 migration and retention of sedimentological heterogeneity is an important precursor for later capillary, dissolution and mineral trapping.

High-resolution sequence stratigraphy and facies assemblage in flood-dominated and mixed deltaic systems: Insights from early Eocene Figols Group, South-central Pyrenean basin

The upper part of the Early Eocene Figols Group cropping out near the town of Tremp (province of Lerida, Spain), is a wonderfully exposed sedimentary succession mainly constituted by mouth bar deposits and delta-front sandstone lobes, forming a fluvio-deltaic system dominated by catastrophic fluvial floods. These deltaic sandstones are vertically associated with carbonate and hybrid (mixed carbonate and siliciclastic) sediments that are physically related to a carbonate ramp platform which crops out in the southernmost part of the study area. The vertical and lateral juxtaposition of these depositional systems allows investigating how the stratigraphic controls (as climate and relative sea level) affect the resulting facies assemblages. The studied succession has been subdivided following a pragmatic physical stratigraphic approach into three 3rd order large-scale depositional sequences (FG2, FG3, and FG4), six 4th order small-scale depositional sequences and 21 EDSs (elementary depositional sequences). The latter are considered the stratigraphic building block of the succession, made of 10s of meter scale forestepping sandstones followed by, backstepping sandstones, bioclastic and mudstone sediments. During the lowstand system tract (LST), the depositional sequences FG2 and FG3 are characterized by the occurrence of flood-dominated deltaic systems (mouth bars and lobes). These deltaic deposits mainly developed in the northern sector characterized by higher subsidence. The overlain transgressive system tract (TST) is dominated by hybrid sediments and carbonates that initially accumulated in the southern zone and then progressively spread out over the entire area. The highstand system tract (HST) is mainly constituted by shelfal fine-grained sediments reflecting the reactivation of the fluvial input and deltaic clastic deposition in a more landward position. This sequential model resulted to be applicable at all hierarchical scales and in particular to the EDS scale. In FG4, the fluvio-deltaic systems became coarser-grained and channelized, the carbonate production on the shelf decreased and lagoonal fines were accumulated so as to separate vertically the fluvio-deltaic bodies. Syn-sedimentary tectonic activity capable to produce local variations of the available accommodation space is inferred by the geometry and facies distribution shown in the high-resolution correlation framework, particularly if considered at the physical scale of EDSs.

Depositional styles of deltaic systems from the inner shelf to shelf edge and their controlling processes: A case study from the Upper Oligocene to Lower Miocene in the Pearl River Mouth Basin

Late Oligocene to Early Miocene deltaic systems (the Zhuhai Formation) comprise the most important oil and gas reservoirs in the Pearl River Mouth Basin in the northern South China Sea and their changes in depositional architecture reflect the interplay of basin kinetic energy, sea level change and sediment supply. Based on the analysis of well log, seismic and core data, depositional styles of deltaic systems, sequence stratigraphy and their controlling factors are systematically documented. Three kinds of deltaic systems (braided river, meandering river and incised river deltas) are distinguished from the inner shelf, middle–outer shelf to shelf edge within a dynamic sequence framework. The braided river and braided river deltaic deposits along the inner shelf are mainly composed of vertically-stacked, thick-bedded channel sandstones and broad-lobate shaped delta front sandy deposits. The meandering river deltaic systems distributed in the middle–outer shelf are characterized by relatively thick delta front sandstones, interdistributary bay fills and prodelta mudstones with turbidite sheets in places. Incised river deltaic systems were generally developed at the shelf margin, with clinoform thickness ranging from 200 to 1200 m, and were commonly associated with large-scale slump and slope fan deposits. The temporal–spatial distribution of deltaic systems within the sequence framework shows a distinct pattern: the braided river delta system mainly developed in the early stage of highstand systems tracts; meandering river delta systems occurred in the middle–late stage of highstand systems tracts; and incised river delta systems generally formed in the falling stage systems tract. Depositional architectures of deltaic systems reveal that the basin kinetic condition varied from the inner shelf to shelf edge. Inner shelf braided river deltaic systems were dominated by fluvial processes, whereas middle–outer shelf meandering river delta and shelf edge incised river delta systems were reworked by waves and alongshore currents. The migration of deltaic systems across the entire wide shelf within the transgression–regression cycle is regarded as the result of high-amplitude sea level fluctuations under coolhouse or icehouse climatic condition. High progradation rates of shelf edge deltaic systems reflect enormous sediment supply resulting from the continuous uplift of the drainage area during the early post-rift period and the intense erosion triggered by glacial climate.

Lacustrine to fluvial depositional systems: The depositional evolution of an intracontinental depression and controlling factors, Lower Cretaceous, northern Tarim Basin, northwest China

The Lower Cretaceous (Baxigai and Bashijiqike Formations) of the northern Tarim basin contains over 1000 m of siliciclastic fluvial-deltaic deposits and is of great significance for oil and gas exploration. Based on integrated cores, outcrops, well-logs, and seismic data, the sequence stratigraphy, depositional architecture and sedimentary infilling process from a shallow lake basin to a braided fluvial-dominated basin, as well as their controlling factors, were documented. Four major depositional facies, including lacustrine and braided-river delta in Baxigai Formation, braided river, and sheetflood, sand-bed river in Bashijiqike Formation, were identified based on the analysis of lithofacies and facies associations. Four third-order sequences（SQ1 in Baxigai Formation, SQ2-4 in Bashijiqike Formation）bounded by regional unconformities are recognized in the deposits. SQ1 consisting of deltaic deposits can be classified into Lowstand (LST) + Transgression (TST) and Highstand (HST) systems tracts based on recognition of regressive-transgressive cycle. The third-order sequences in Bashijiqike Formation consisting of fluvial deposits can be divided into Low and High Accommodation Systems Tracts （LAST and HAST） according to the degree of channel amalgamation. Sheetflood, sand-bed river deposits developed in LAST, and is characterized by highly amalgamated channels; the HAST is dominated by braided-river and floodplain deposits, with a higher floodplain ratio and more isolated channels. The tectonic uplifts at the early deposition of Bashijiqike Formation caused the overcompensation of sediments, and thus the depositional system shifted from deltaic to fluvial. The unconformity at the base of the Bashijiqike Formation was attributed to tectonic uplifts occurred in the study area. The cycle of arid to semi-arid paleoclimate led to the periodic changes from sheetflood, sand-bed river in LAST to braided-river in HAST of Bashijiqike Formation. The delta front sandstones comprise the potential favorable reservoirs in the study area.

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 > 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.

Geochemistry of the middle-upper Miocene Bathan Formation, Al-Rehaili area, Jeddah, Saudi Arabia: provenance, tectonic setting, and paleoweathering implications

The present work investigates geochemical and mineralogical signatures preserved in the fluvio-lacustrine sediments of the middle-upper Miocene Bathan Formation at Al-Rehaili area, northern Jeddah, Saudi Arabia to constrain the composition and tectonic setting of the source area and to appraise the influence of weathering. Sixteen sediment samples were collected from the lacustrine delta plain and delta front sandstones (SS I), lacustrine offshore mudrocks, and the younger fluvial channel sandstones (SS II). The sandstones are compositionally immature first cycle. The sandstones (SS I) and the mudrocks of the first depositional stage were derived from an intermediate igneous source rock, whereas the sandstones (SS II) of the second depositional stage were derived mainly from a felsic igneous source rock with subordinate contribution from an intermediate igneous source rock. They indicate an active continental margin to oceanic arc tectonic setting. The values of the Chemical Index of Alteration (CIA) and the Plagioclase Index of Alteration (PIA) as well as the Al2O3/Na2O, Rb/Sr, and Al2O3/(CaO+Na2O) ratios indicate that the source rocks experienced low to moderate degree of weathering consistent with arid to semi-arid climate that prevailed during the Miocene. Subaerially exposed lacustrine offshore mudrocks displayed excessive depletion in the concentration of mobile elements comparing to the bedrock. The low values of the paleoweathering indices are further explained by the enhanced uplift of the Red Sea Escarpment after the cessation of the first stage of the Red Sea floor spreading at 14–15 m.y. ago.

Hierarchical Architecture of Sequences and Bounding Surfaces In A Depositional-Dip Transect of the Fluvio-Deltaic Ferron Sandstone (Turonian), Southeastern Utah, U.S.A.

Abstract: Depositional-dip-oriented transects through coastal-plain to nearshore marine domains are essential elements of generic sequence stratigraphic models for continental-margin successions. Many questions remain about the validity of these models, however, because well-constrained outcrop examples are sparse. Presented herein is the first detailed study of a 30-km-long, almost continuously exposed, depositional-dip transect through nonmarine and marine facies of the fluvio-deltaic Ferron Sandstone. This transect is connected to the southern end of a previously documented, 67-km-long, depositional-strike-oriented section. Excellent exposure reveals relationships among facies, stacking patterns, and bounding surfaces across three orders of sequences and various shoreline trajectories. Deposits of sequences are dominated by falling-stage systems tracts, suggesting a regime dominated by long, gradual relative falls in sea level punctuated by shorter relative rises. Widespread fluvial downcutting is generally absent. Distributary-channel sandstones pass down dip into broadly contemporaneous shoreface and delta-front sandstones. A composite sequence boundary divides the study interval into two composite sequence sets. Sequence sets I–III comprise eastward-dipping, offlapping clinoform sets defining a descending, regressive shoreline trajectory (falling-stage deltas). Sequence sets IV–VI overlie a thin transgressive deposit and consist of progradational to aggradational clinoform sets (highstand deltas). Key bounding surfaces are similar at all scales of the hierarchy: they are diachronous, cryptic, and pass down depositional dip into conformable surfaces where they are almost impossible to recognize as key surfaces. Differentiation between units and surfaces that develop at different hierarchical levels is possible only by integrating detailed outcrop observations with regional stratigraphic architecture.

Enhanced bioturbation on the down‐drift flank of a Turonian asymmetrical delta: Implications for seaway circulation, river nutrients and facies models

AbstractExtant models predict delta front environments down‐drift of river mouths as unfavourable for organisms because of the physico‐chemical stresses caused by sediment and fresh water influx. This study, however, finds evidence for near‐optimal living conditions down‐drift of contemporaneous mouth bars and distributary channels, as well as at the tops of abandoned lobes, in part of the asymmetrical ‘Notom Delta’ complex of the Ferron Sandstone (Turonian, south‐eastern Utah, USA). Presented herein is a sedimentological and ichnological model using thirty‐two detailed measured sections along a 16 km transect through two continuously exposed, ca 10 m thick allomembers containing delta front, mouth bar and distributary channel facies. Azimuths from sedimentary structures show south‐eastward deflection of near‐shore palaeocurrents relative to the inferred north–south shoreline, as well as minor reversal of flow. Two end‐member trace fossil suites are recognized in delta front sandstones: (i) a stressed suite of low abundance, low diversity, diminutive traces reflecting mobile deposit feeding, resting and locomotion behaviours; and (ii) a comparatively unstressed, high abundance, moderate diversity suite with a regular, heterogeneous distribution of deep, vertical or U‐shaped suspension‐feeding burrows which, in places, thoroughly homogenize the sandstones. The down‐drift side of the delta was colonized by suspension feeders during seasonal reversal of the seaway gyre when mud plumes were swept northward or when river‐derived nutrients were sufficiently concentrated relative to fresh water and sediment input. During normal seaway circulation, very high sedimentation rates and mud‐laden, wave‐dampened waters down‐drift of the river mouths heightened the preservation potential of the pervasively bioturbated facies. Up‐drift of the river mouths, these bioturbated facies were either not preserved or not developed until the lobe was abandoned. This alternative model for delta planform asymmetry contributes to the refinement of facies models for deltaic systems and provides a framework for predicting the distribution of bioturbation‐enhanced porosity and permeability in lobe‐deflected deltas.

The Umiak field discovery, Northwest Territories, Canada

This paper examines the discovery process in terms of changing exploration paradigms and describes a field discovered because of this change in mindset. Prior to the Umiak discovery all wells in the Mackenzie Delta had been drilled on structural highs. Reevaluation of two 30-yr-old dry holes along with existing two-dimensional (2-D) seismic data resulted in discovery of the Umiak gas field. This reappraisal led to recognition that a stratigraphic trap might exist between these two wells drilled in the early 1970s. The Kilagmiotak M-16 well contained 290 m (951 ft) of porous sandstone in the Eocene Taglu Formation, whereas the Umiak J-37 well, up dip and 11 km (6.8 mi) to the west, had no sandstone in the same interval. Examination of 2-D seismic lines found evidence of an updip sandstone pinchout beneath an angular unconformity on a tilted fault block. Strata in the tilted fault block below the unconformity contain strong amplitudes and flat spots. Interpretation of a subsequent three-dimensional (3-D) survey supported the play. A partnership of Alberta Energy Company (operator, now Encana), Anadarko, and Gulf Canada (now ConocoPhillips Canada) drilled the Umiak N-16 discovery well during 2004. Gas and some oil was found in gently dipping Eocene Taglu shoreface and delta front sandstones, and within gently folded foreset beds of the Eocene Richards Formation above a mid-Eocene unconformity. The Umiak N-05 appraisal well drilled a year later confirmed the discovery. Together these two wells delineate the fourth largest onshore gas accumulation on the Mackenzie Delta.

Lowstand Turbidites and Delta Systems of the Itararé Group in the Vidal Ramos region (SC), southern Brazil

Turbidites have captioned the attention of sedimentologists during the last decades due their importance as hydrocarbon reservoirs. However, their relationship to delta systems still deserves further studies. This paper presents examples from a late deglacial to early post-glacial deltaic and turbidite strata exposed in the surroundings of Vidal Ramos (Santa Catarina State, Brazil), southern portion of the Parana Basin. There, the uppermost part of the Mafra Formation and the Rio do Sul Formation onlap the Proterozoic basement and comprises an up to 360 m thick package. It includes (base to top) black shales, mass transport deposits (MTD) and sandy turbidites (Mafra Formation) as well as thin bedded turbidites (tbt), including one interval of black shales and sandy turbidites, overlain by proximal delta front sandstones (Rio do Sul Formation). The analysis of the succession shows two more than 150 m thick coarsening-upwards deltaic successions composed of turbidite sand sheets at their base (prodelta), followed by partially collapsed thin bedded turbidites (delta slope wedge) and delta front sandstones. Both turbidite sand-sheets abruptly overlay black shale intervals related to maximum flooding surfaces and therefore record correlative conformities. A detailed stratigraphic section elaborated from the correlation of four logs (1/100) suggests that distal delta front sands includes both thin bedded turbidites and wave reworked sands whereas the proximal delta front was dominated by long-lived underflows (hyperpycnal flows). The succession suggests that the most expressive turbidite beds (base of the delta systems) have resulted from relative sea-level falls (early lowstand) whereas the thin-bedded turbidites were related to the development of the late lowstand wedge. Black shales represent the transgressive systems tract and HST were not deposited or preserved in the area. High sediment supply associated with lowstand tracts could explain the occasional (Vidal Ramos) to common occurrence of slope failures (slumps and diamictites) involving thin bedded turbidites and delta front sandstones. This situation is quite logical in terms of deglacial periods, and resulting high sediment supply, within a long-term icehouse context, with prevalence of lowstand to transgressive settings.

Mesoproterozoic delta systems of the Açuruá Formation, Chapada Diamantina, Brazil

Facies analysis, distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct, independent river-dominated and tide-dominated braid delta systems in the Mesoproterozoic Açuruá Formation. The river-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 30m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front, distributary channels, and floodplain; (c) grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse; (d) lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin; (e) paleocurrent readings indicate delta front sandstone bodies shifting laterally towards NE to SE directions. The tide-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 25m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front and tide-influenced distributary channels; (c) grain size of sandstone from delta front and tide-influenced distributary channels varies respectively from very fine to coarse and fine to granular; (d) abundance of tide-influenced sedimentary structures suggest tidal dominated delta development; (e) consistent bidirectional paleocurrents with northwards ebb-tide direction and no lobe shifting. The studied braid delta systems belong to a highstand system tract and as such, they exhibit classic parasequence sets with progradational stacking pattern. The facies association variability and overall characteristics of these Mesoproteozoic delta systems are compatible with delta systems of all ages. However, contrary to typically Phanerozoic deltas – which are characterized by progradational clinoforms – the coalescence of the studied braid delta facies associations promoted fairly sheet-like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea.

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Sedimentary facies and stratigraphic architecture in coarse-grained deltas: Anatomy of the Cenozoic Camaná Formation, southern Peru (16°25′S to 17°15′S)

In the external forearc of southern Peru (Arequipa region), the sedimentary facies and the stratigraphic architecture of the Cenozoic Camaná Formation are presented in the context of tectono-eustatic controls. The Camaná Formation is defined as ∼500 m thick coarse-grained deltaic complex that accumulated in a fault-bounded elongated depression extending from the Coastal Cordillera in the east to the offshore Mollendo Basin in the west and likely up to the Peruvian Trench. Based on the analysis of facies associations, we propose a refined stratigraphic scheme of the Camaná Basin fill. The Camaná Formation was formerly divided into the Camaná “A” and Camaná “B” units (CamA and CamB, respectively). We reinterpret the stratigraphic position and the timing of the CamA to CamB boundary, and define three sub-units for CamA, i.e. sub-units A1, A2, and A3. Each depositional unit shows individual stacking patterns, which are linked with particular shoreline trajectories through time.Strata of A1 form the basal succession of the Camaná Formation and consist of distributary channels and mouth bars, unconformably overlain by beds of A2. A2 consists of delta front deposits arranged in voluminous clinothems that reflect a progradational downstepping complex. A3 consists of delta front sandstones to prodelta siltstones arranged in retrogradational onlapping geometry. A pebbly intercalation in proximal onlapping A3 deposits is interpreted to reflect pulses of uplift in the hinterland. The overlying CamB unit is characterized by a thick alternation of fluvio-deltaic conglomerates and sand bars. The ages of the individual units of the Camaná Formation are not yet well defined. Based on the available information and stratigraphic correlations we tentatively assign A1 to the Late Oligocene, A2 to the Early Miocene, A3 to the late Early Miocene to early Middle Miocene, and CamB to the Late Miocene to? early Pliocene.The sub-units A1 and A2 represent a regressive systems tract, where the shoreline was forced to migrate seaward. This scenario differs from the Early Miocene eustatic sea-level rise suggesting that significant tectonic uplift along the Coastal Cordillera controlled the high sediment influx during A2 deposition. The sub-unit A3 represents a transgressive systems tract, triggering landward migration of the shoreline. This scenario is well in line with the global sea-level chart suggesting that A3 has been deposited during a phase of eustatic sea-level rise with minor tectonic activity. The fluvial deposits of CamB reflect an increased sediment flux due to uplift of the hinterland. The observed stratigraphic patterns support predominant tectonic control on sedimentation in the Camaná Basin and the established stratigraphic framework provides an essential baseline for future correlations of the Cenozoic sedimentation in the forearc area of the Central Andes.