Incised Valley Fills Research Articles

Challenges and solutions: stratigraphic delineation and sedimentological facies analysis of incised-valley fill reservoirs

Challenges and solutions: stratigraphic delineation and sedimentological facies analysis of incised-valley fill reservoirs

Early–mid Holocene relative sea-level rise in the Yangtze River Delta, China

Regional-scale Holocene sea-level reconstruction is the key to understanding natural climatic variability. The tidal flat, salt marsh, and tidal floodplain in the Yangtze River Delta were very sensitive to morphological changes and sea-level variation during the early Holocene. In this study, the lithology, radiocarbon ages, sediment grain size, benthic foraminifera, and ostracods of three new cores were analyzed. Twenty-four sea-level index points were extracted from incised-valley fills beneath the westernmost part of the Yangtze River Delta and used to construct a detailed relative sea-level curve for 11.03–7.25 ka. The relative sea level were − 38.90 ± 3.48 and 1.59 ± 3.28 m at 11.03 and 7.25 ka, respectively, and the average rate of sea-level rise was 10.71 mm/yr. The relative sea level gradually increased at 11.03–10.10 and 9.29–8.33 ka from 4.80 to 13.30 and 3.19 to 19.59 mm/yr, respectively. The rate of relative sea-level rise gradually decreased from 19.59 mm/yr at 8.33 ka to 13.40 mm/yr at 7.82 ka and further decreased to 5.53 mm/yr at 7.25 ka. The Yangtze River Delta recorded accelerations in the rate of sea-level rise from 8.72 to 8.18 ka of 14.2–19.59 mm/yr, corresponding to the pre-8.2 ka event. A comparison of the sea-level histories of the Yangtze River Delta and other regions indicates tectonics, glacial isostatic adjustment, and coastal levering effects caused by the marine inundation of the continental shelves. These new sea-level data contribute to the understanding of the difference in the sea-level rise rate during the early–mid Holocene.

Bio-sequence stratigraphy of the Neogene: an example from El-Wastani gas field, onshore Nile Delta, Egypt

Abstract. Due to modern hydrocarbon development and exploration activities throughout the onshore Nile Delta of Egypt, a high-resolution biochronologic sequence stratigraphy of the Neogene sequence was conducted to illustrate the gas-bearing reservoirs' depositional sequences. Our study used a multidisciplinary approach comprising biostratigraphy, facies analysis, geophysical logs, and seismic data to shed light on the Neogene stratigraphic framework. The biostratigraphic analysis of planktonic foraminifera and calcareous nannofossils allowed the recognition of six zones and/or subzones and nine zones, respectively. An open-shelf environment was suggested for the Middle–Upper Miocene Sidi Salem Formation, while the Upper Miocene Qawasim and Abu Madi formations were deposited under stressed environmental conditions interpreted as estuary facies. The Lower Pliocene (Zanclean) succession deposited in the middle to outer shelf domains, including the upper-bathyal environments. Seven depositional sequences bounded by six major sequence boundaries were recognized from Serravallian to Zanclean times. These boundaries significantly influenced changes in reservoir properties and architecture of the incised valley fills. The sea-level oscillations are interpreted by correlating the sequence boundaries and flooding surfaces with global eustatic charts.

Accumulation of thick fluvial sediments in the Shinano River incised-valley fills: implications for sequence stratigraphy and alluvial architecture

ABSTRACTThe relationship between fluvial systems and base-level changes, controlled mainly by sea-level fluctuations and tectonic activity, on time scales of 103–104 years, is important for evaluating and improving nonmarine sequence stratigraphy and numerical and physical alluvial architecture models. However, there are not enough quantitative data available from field studies, especially for periods of rapid base-level rise. The Echigo Plain, Japan, is a fluvial–coastal lowland where the tidal range is very small and where a very deep valley was incised by the Shinano River during the latest Pleistocene. The depth of the incised valley near the shoreline, 160 m below mean sea level, is greater than the amount of sea-level fall during the Last Glacial Maximum (LGM) because of tectonic subsidence. We studied an approximately 100-m-long borehole sediment core obtained from the landward part of the plain, where thick fluvial deposits have accumulated since the LGM. Sediment facies, diatom assemblages, sulfur content, carbon to sulfur ratios, and radiocarbon age dates indicate that most of the core sediments were deposited in a fluvial environment during the past 12 kyr. Sediment accumulation rates were 10–15 m/kyr between 12 and 8 cal kyr BP, and relatively low (less than 5 m/kyr) subsequently, mainly because of glacio-eustatic sea-level rise. The timing of the reduction in the accumulation rate is one possible indicator of the position of the maximum-flooding surface in a sequence composed only of fluvial sediments. The presence of many thin layers of very fine to fine-grained sand (20–80 cm thick), interpreted as crevasse-splay deposits, that formed between 11 and 8 cal kyr BP, when the base level was rising rapidly, implies that crevasse-splay deposits are characteristic of aggrading fluvial sediments and the transgressive systems tract in nonmarine sequence stratigraphy. Tectonic subsidence at approximately 2 m/kyr has contributed markedly to the larger accumulation rate of fluvial sediments of the highstand systems tract on the Echigo Plain, compared with rates in other fluvial–coastal lowlands, since 5–6 cal kyr BP, when the eustatic sea-level rise had mostly ceased.

Delta initiation under low sediment discharge: Insights from Holocene incised valley fills north of Tokyo Bay, central Japan

Modern delta formation initiated as a result of the deceleration in the rate of sea-level rise during the middle Holocene. In Asia, many deltas initiated at the mouths of rivers with large sediment discharge at 8 ka, early in the middle Holocene, when the volume of sediment supply exceeded that of the accommodation space created by sea-level rise. However, in the Japanese Islands, even some deltas formed by rivers with quite small sediment discharges initiated at 8 ka. In the area north of Tokyo Bay, a narrow, 70-km-long bay was formed during the last deglacial sea-level rise. The Tone and Watarase rivers flowed into this bay from the west and east, respectively. We carried out facies analysis and radiocarbon dating of two sediment cores obtained in the vicinity of the Watarase River mouth. The delta that formed at the tide-dominated Watarase River mouth was characterized by bay mud sediments with a narrow sand body along the axis. This deltaic sand body, which is 5–15 m thick and coarsens upward, filled a tidal channel that was 1–3 km wide and 30 km long. At the mouth of the sediment-starved Watarase River, the delta initiated at the onset of the sea-level stillstand at 7 ka. In contrast, at the mouth of the sediment-rich Tone River, a delta initiated at 8 ka, when the rate of sea-level rise decreased from 14 to 7 m/kyr. By reviewing delta initiations in the area north of Tokyo Bay and in 17 coastal lowlands of the Japanese Islands, we demonstrate that the sediment discharge threshold governing whether delta initiation occurred at 8 or 7 ka was equivalent to a mean annual water discharge of 100 m3/s.

Leveraging Sequence Stratigraphy to Accelerate Site Remediation: Pliocene Citronelle Formation, Eglin Air Force Base, Florida, USA

AbstractAt Eglin Air Force Base (AFB) in the Florida Panhandle, a groundwater extraction and treatment system was installed to contain and remediate a chlorinated solvent plume. After 2 years of operation, the system was not removing the contaminant mass at the rate predicted or required to meet performance‐based contract terms. As a result, a sequence‐stratigraphic analysis was initiated to develop a strategy to improve performance. Sequence Stratigraphy methods were employed to identify a marine flooding surface (mfs) formed during a relative sea level highstand. The analysis also found that the mfs was locally eroded away, indicating that incised valleys were eroded into the formation during a relative lowstand of sea level. These valleys were backfilled with coarse‐grained fluvial and estuarine strata. The analysis concluded that the groundwater extraction system lacked an extraction well screened within the coarse‐grained valley fill. An additional extraction well was installed, which targeted the incised valley fill and resulted in a significant increase in contaminant mass removal rate without increasing system capacity or operational costs. This case study suggests that efficiency improvements are tenable at many sites where groundwater remediation is occurring within the Surficial Aquifer System of the Gulf Coast (Citronelle Formation) as well as sites in similar geologic settings worldwide.

Coastal Depositional Responses to Relative Sea-Level Rise: Insights from a Superimposed Sandstone–Shale–Coal Reservoir in the Linxing Gas Field, China

The Ximing Sandstone-to-No. 9 Coal succession of the Taiyuan Formation in the Linxing gas field records a complex internal architecture of a transgressive succession developed in the western coast of the late Pennsylvanian North China epeiric sea. Facies and sequence stratigraphic analyses reveal its depositional evolution from fluvial channels through fluvial-dominated and tide-influenced inner estuaries to tide-dominated estuaries and finally to wave-dominated barrier lagoons. The evolution from fluvial- to tide-dominated deposition has been ascribed to the funnel-shaped valley coupled with an increased tidal prism induced by the upstepping and backstepping shoreline. The evolution from tide- to wave-dominated deposition has been ascribed to the wide North China epeiric seaway lacking local coastline irregularities after the incised-valley fill that provided sufficient fetch for the occurrence of large storm waves. Grain-size analysis reveals the relative importance of traction, saltation, dispersed suspension, and flocculated suspension in the development of the transgressive estuarine to lagoonal deposits. This study not only contributes to a proper understanding of coastal depositional response to the relative sea-level rise but also provides a context within which to interpret the symbiotic relationship of the superimposed sandstone–shale–coal reservoirs and predict the distribution of favorable unconventional gas production formation.

Fluvial reservoir architecture, directional heterogeneity and continuity, recognizing incised valley fills, and the case for nodal avulsion on a distributive fluvial system: Kern River field, California

Fluvial reservoir architecture, directional heterogeneity and continuity, recognizing incised valley fills, and the case for nodal avulsion on a distributive fluvial system: Kern River field, California

Late Miocene to Pliocene delta-lacustrine to incised-valley fills sedimentation of the Ransiki Area, Bird’s Head Papua, Indonesia: The sedimentary record of Lengguru Fold and Thrust Belt wedge-top depozone

Late Miocene to Pliocene delta-lacustrine to incised-valley fills sedimentation of the Ransiki Area, Bird’s Head Papua, Indonesia: The sedimentary record of Lengguru Fold and Thrust Belt wedge-top depozone

Sustainability of Regional Groundwater Quality in Response to Managed Aquifer Recharge

AbstractGrowing demands on water supply worldwide have resulted in aquifer overdraft in many regions, especially in alluvial basins under intensive irrigation. This further leads to serious deterioration of groundwater quality. Managed aquifer recharge (MAR) has been shown to mitigate groundwater overdraft, but whether MAR can actually stabilize or reverse the ongoing declines in regional groundwater quality caused by non‐point sources has not been demonstrated. This study was intended to address the question by investigating impacts of different MAR strategies on regional groundwater quality. A geostatistical model was first used to characterize a heterogeneous alluvial aquifer system in a portion of the Tulare Lake Basin. Three‐dimensional numerical models were then employed to simulate groundwater flow and mass transport. Next, MAR strategies were applied in locations with different geological conditions or joint with different irrigation activities, and their performances were evaluated. Results demonstrate the potential of significant, long‐term benefits for regional groundwater quality by applying strategic, high‐intensity recharge operations on geologically favorable subregions. Siting MAR above the incised valley fill (IVF) deposit, a near‐surface paleochannel containing unusually coarse, high‐conductivity hydrofacies, leads to more extensive improvement in the groundwater quality in terms of salinity due to significant vertical flow and lateral outward flow from the IVF. Overall, decades would be required to alleviate groundwater quality concerns in the studied 189 km2 region. The simulations indicate that the deep concentrations remain below the secondary maximum contaminant level as the solute mass migrates downward with the prominent contribution from the attenuation via dispersion and matrix diffusion.

Airborne geophysical method images fast paths for managed recharge of California’s groundwater

Given the substantial groundwater level declines in the Central Valley of California, there is an urgent need to supplement the recharge of the groundwater systems by implementing managed aquifer recharge. With approximately 170 km3 (140 million acre-feet) of available groundwater storage space, water deemed to be excess during wet years could be spread on the ground surface at selected locations allowing it to move downward to recharge the underlying aquifer system. Along the eastern edge of the Central Valley there are large paleovalleys that can act as fast paths expediting the downward movement of water. These paleovalleys, incised and then filled with coarse-grained materials—sand, gravel, cobbles—at the end of the last glacial period, are referred to as incised valley fill (IVF) deposits. An IVF deposit has been mapped at one location in the Kings River alluvial fan, with others proposed to exist in the fans of major rivers. If located, these deposits would be optimal sites for managed recharge. In this study, we assessed the use of a helicopter-deployed geophysical method to efficiently locate IVF deposits throughout the Central Valley. We acquired 542 line-kilometers of airborne electromagnetic (AEM) data in the Kings River alluvial fan, with dense line-spacing over the Kings River IVF deposit which had been mapped as ∼2 km wide, extending over 20 km into the Central Valley, from the ground surface to a depth of 30 m. The IVF deposit was unambiguously imaged in the AEM data as an extensive linear feature that was more electrically resistive than the surrounding materials due to the high percentage of coarse-grained sediments. This study provides the evidence to support the rapid adoption of the AEM method to locate IVF deposits along the eastern edge of the Central Valley. These deposits provide valuable natural infrastructure for recharging California’s groundwater.

Sequence paleogeography and coal accumulation model in the fluvio-lacustrine rift basin: The Lower Cretaceous of the Huhehu Sag of Hailar Basin, Inner Mongolia (NE China)

Significant volumes of coals can be preserved in fluvio-lacustrine rift basins but prediction of occurrence and geometry within a sequence-stratigraphic framework is always a challenge. The Huhehu Sag is a continental rift basin in Hailar Basin of NE China, with the Damoguaihe and Yimin formations of Lower Cretaceous as the main coal-bearing strata. In this study, the sedimentary facies, sequence stratigraphy, paleogeography, and coal accumulation characteristics in the Huhehu Sag are analyzed. Data used includes boreholes, geophysical well logs and seismic profiles. The coal-bearing strata are mainly composed of conglomerates, sandstones, siltstones, mudstones, and coals. Alluvial fan, braided fluvial, fan delta, braided fluvial delta and lacustrine depositional systems are identified. Coal seams were mainly formed in lacustrine and delta depositional systems. Six third-order sequences were subdivided according to sequence boundaries represented by regional unconformities, basal surfaces of incised valley fills, facies-reversal surfaces, and abrupt color changes related to the base level changes. Sequence 1 corresponds to the Tongbomiao Formation formed in the early syn-rift phase, Sequence 2 corresponds to the Nantun Formation formed in the syn-rift climax phase, Sequence 3 and 4 span the Damoguaihe Formation, with Sequence 3 formed in the syn-rift climax phase and Sequence 4 formed in the late syn-rift phase, and Sequence 5 and 6 span the Yimin Formation which were formed in the post-rift stage. All six sequences were developed with littoral-sublittoral lake, delta front and delta plain paleogeographic units. Sequence 1 to Sequence 2, Sequence 3 to Sequence 4 and Sequence 5 to Sequence 6 are of progradational patterns, while Sequence 2 to Sequence 3 and Sequence 4 to Sequence 5 display retrogradational features. From Sequence 1 to Sequence 4, the syn-rift led to the thickening of strata as well as the coal seams towards the eastern side of the sag. In Sequence 5 and 6, post-rift thermal sagging caused the wide distribution of the strata with the depositional centers as well as the thick coals being distributed in the central belt of the sag. Coals were mainly developed in the transgressive systems tract (TST) and highstand systems tract (HST) of Sequences 4 and 5. The favorable paleogeographic units for thick coal accumulations were mainly littoral-sublittoral lacustrine and delta. The Huhehu Sag is interpreted as an overfilled basin, we further divided the overfilled basin into three subtype basins by accommodation space, including the low, moderate, and high accommodation overfilled basins. In the low accommodation overfilled basin, the relatively thick coals were developed near the maximum flooding surface, while in the moderate accommodation overfilled basin, the relatively thick coals were developed in the lower-middle part of TST and the middle part of HST. These results could be used to predict the preferred depositional environments and favorable areas for coal and coalbed methane exploration in the fluvio-lacustrine rift basins.

Palynology of the Freshwater East Formation (Upper Silurian, Pridoli), Pembrokeshire, South Wales, UK

Upper Silurian (Pridoli) cryptospores and trilete spores are described from the type section of the Freshwater East Formation in Pembrokeshire, South Wales. This formation is the oldest stratigraphical division of the Lower Old Red Sandstone succession south of the Ritec Fault in the Anglo-Welsh Basin. It represents an incised valley fill sequence composed predominately of red dryland alluvial sediments inter-bedded with four tidally-influenced green-grey heterothithic units. The sequence is well known for the occurrence of an early land plant Cooksonia flora. The spore assemblages are dominated by cryptospores (67%), particularly laevigate monads and dyads. Trilete spores are less abundant but are diverse in composition and provide the principal means of accurately dating the sequence. Eleven cryptospore taxa and thirty one trilete taxa are identified. Two new species Cymbohilates richardsonii and Velamisporites edwardsae are erected. The Freshwater East microflora contains a distinctive complex of Chelinospora taxa, here named the Chelinospora lavidensis assemblage. It represents a previously unrecognised assemblage within the Pridoli succession of the Anglo Welsh Basin. An accurate age for this assemblage has been determined by correlation with the Upper Silurian succession of the Cantabrian Mountains in northwest Spain, where a continuous spore succession through the Pridoli Series has independent biostratigraphical age control from chitinozoans. The Freshwater East assemblages are correlated with the upper part of the Chelinospora hemiesferica (H) Biozone, which is dated by the chitinozoans Margachitina elegans and Pseudoclathrochitina carmenchui as early mid Pridoli in age. Rare occurrences of small spiny acritarchs, sphaeromorph acritarchs and prasinophyte phycomata in two of the Freshwater East assemblages support the sedimentological data that the grey-green heterolithic beds were deposited in tidally-influenced fluvial and estuarine environments.

Back-barrier evolution and along-strike variations in infilling of the Kosi Bay lake system, South Africa

This study examines both the inlet dynamics, recorded in the back-barrier stratigraphy of Kosi Bay, and the competing effects of waterbody segmentation on along-strike changes in the character and nature of an incised valley fill at Kosi Bay, South Africa. The Kosi Bay system comprises four interconnected lakes (Lake Makhawulani, Lake Mpungwini, Lake Nhlange and Lake Amanzimnyama) subject to varying degrees of marine influence. Formerly, the system's only connection to the ocean was through the Bhanga Nek palaeo-inlet. The location of this system behind a continuous coastal barrier predisposes it to excellent archives of environmental change related to changing sea-level and sediment supply. In this context, two cores were extracted from the system (KB2 and KB4), together with over 150 km of seismic reflection profiling, as well as AMS radiocarbon analyses and legacy data in order to present a new stratigraphic evolution model for this back-barrier system. A total of 5 seismic units were imaged (Units A-E). A major unconformity surface (SB), characterised by broad u-shaped incised valleys, incises the underlying unconsolidated sandy acoustic basement (Unit A). Unit B forms as thickly developed central basin deposits, partially infilling the incised valleys. Two forms of prograding spits are evident -Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space and Unit C2 progrades from the margins of the system into the basin. These are separated by an erosional surface (T-RS) formed by migrating tidal channels as sea-level rose and marine waters entered the system during the Holocene. Unit C2 spits are in turn associated with slump deposits formed on the steepest part of the margins (Unit D) that were dated to 2900 ± 165 cal. BP. The unusual depth of Surface T-RS is attributed to (1) limited sediment supply during sea-level rise, (2) the presence of easily erodible fine sediments in which this surface erodes and (3) a larger tidal prism when the Bhanga Nek inlet was still open and system segmentation by Units C2 and D had not yet occurred. Unit E caps the stratigraphy and is interpreted as lacustrine fines with a thin layer of acoustically transparent material (gyttja) on top. The deposition of Unit E signifies the closure of the system to the ocean and the overall shift from an estuarine/lagoon environment to a lacustrine one. Recalibrated AMS dates place the system closure to between 3160 and 2900 cal. BP, coeval with the formation of the prograding marginal spits and waterbody segmentation (Unit C2). Along-strike variation in the timing of the basin infilling, attributed to changes to the inlet functioning coupled to the segmentation of the waterbody, is characteristic of this area. At 1450 cal. BP, a new tidal inlet formed to the north of Lake Makhawulani, where impounded waters were diverted via a low point in the back-barrier coastal dune cordon. The formation of this inlet suggests a prolonged period of back-barrier flooding until a low point in the barrier was breached during the Holocene sea-level highstand of 1.5 m. Such inlet development and along-strike back flooding of incised valleys is an unusual attribute of coastal waterbodies of South Africa.

Tectonic influence on the dimension of incised valley and its sedimentation: the late Pleistocene to Holocene incised valley fills in SE Korean Peninsula

Tectonic influence on the dimension of incised valley and its sedimentation: the late Pleistocene to Holocene incised valley fills in SE Korean Peninsula

Ground motion characteristics and the depth of the S-wave velocity discontinuity in the post-LGM incised-valley fills beneath the Tokyo Bay area, central Japan, based on microtremor surveys

Ground motion characteristics and the depth of the S-wave velocity discontinuity in the post-LGM incised-valley fills beneath the Tokyo Bay area, central Japan, based on microtremor surveys

Allogenic and autogenic controls on facies and stratigraphic architecture of the Lower Jurassic Mashabba Formation, Gebel Al-Maghara, North Sinai, Egypt

The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems.

Transition from a transgressive to a regressive river-mouth sediment body in Tokyo Bay during the early Holocene: Sedimentary facies, geometry, and stacking pattern

Sedimentary facies, geometry, and stacking pattern transitions of river-mouth sediment bodies recording the turnaround from a transgressive estuary to a regressive delta system have not been sufficiently clarified in the Asian sedimentary record. In this study, the temporal and spatial distributions of sedimentary facies of the post-Last Glacial Maximum incised-valley fills (pLGMIVFs), including those recording delta initiation in the Tama River Lowland, on the west coast of Tokyo Bay, are described taking into account 14 sediment cores, 186 radiocarbon dates, and 8745 geotechnical borehole logs. The pLGMIVFs in the Tama River Lowland, which unconformably overlie Plio-Pleistocene deposits, can be divided into braided-river, meandering-river, estuary, and delta systems in an ascending order. The boundaries between the braided-/meandering-river or estuary systems and the estuary/delta systems correspond to a transgressive surface (20 ka) and a maximum flooding surface (8 ka), respectively. In the estuary system, the river-mouth sediment body, accumulated during 10–8 ka, gradually became smaller and coarser-grained landward; in contrast, the delta system's river-mouth sediment body became larger, finer-grained as it prograded seaward during the past 8 ky. With decelerating sea-level rise at 8 ka, the delta initiated as a sediment body, consisting of 5-m-thick, upward-coarsening sand and gravel beds, distributed in an area of 2 km wide and 2 km long in a funnel-shaped river mouth. This initial delta prograded locally in the subaqueous river mouth, with the saltmarsh spreading widely landward at the margins of the delta to the highest sea-level peak at 7 ka. This separation of subaqueous delta progradation and inland saltmarsh expansion occurred because the volume of sediment deposition was almost equal to the increase in accommodation space due to the 7 mm/yr sea-level rise during 8–7 ka.

Controls of facies distribution on reservoir quality in the Messinian incised-valley fill Abu Madi Formation in Salma delta gas field, northeastern onshore Nile Delta, Egypt

The Messinian incised-valley fill sediments “IVFs” of the Abu Madi Formation comprise the most prospective hydrocarbon-bearing reservoirs in the Nile Delta onshore and offshore. An integrated seismic-stratigraphic, sedimentological and petrophysical approach is applied to identify the factors controlling reservoir heterogeneity within the Abu Madi IVFs. Two separate channelized systems are interpreted and reflect the variation in flow dynamics and depositional energy. Morphologically, the channel system at the base constitutes the deeply-incised (average relief ∼ 80 ms “ms”) flat-filled channels corresponding to the braided fluvial pattern, whereas the channels at the top consists of shallower (average relief ∼ 20 ms) concave-up and shingled-filled channel facies of the meandering fluvial pattern. Coarse-grained quartzose sandstones are predominant in the channel sand bars, channel lags and channel-base drapes of the braided channel system. Higher in the section, the meandering fluvial facies hosts higher proportion of fine-grained sediments accumulated in the abandoned channels and their floodplains.The relative proportion of clays and pseudomatrix materials exercise the first-order control on the pore-scale heterogeneity and reservoir quality. Four reservoir rock types (RRT) were interpreted having different lithological and compositional characteristics and thereby petrophysical flow properties. Capillary pressure, irreducible water saturation, permeability anisotropy and pore-system heterogeneity increase from RRTI to RRTIV. RRTI intervals consist the quartzose sandstones preferentially accumulated at the base of the fining-upward cycles in the braided fluvial sediments. This study highlights the impact of sedimentological and compositional controls on the pore-scale heterogeneity, and therefore will enhance the predictive models for exploration in the incised-valley fill reservoirs.

Integrating facies analysis with dipmeter data to characterize point bars of the Lower Cretaceous McMurray Formation, Christina River, AB, Canada

The Lower Cretaceous McMurray Formation contains crude bitumen reserves of the Athabasca Oilsands that formed by channel amalgamation and lateral migration of fluvio-tidal point bars. These point bars are widely accepted as incised valley fills during episodic base-level rises in tidally influenced estuary (Mathison, 2004; Hein et al., 2013). Historically, well logs such as gamma-ray and spontaneous potential were used in combination with core data to interpret facies and to conduct stratigraphic correlations of the McMurray Formation. Microresistivity image logs, dipmeter logs, and seismic data have greatly improved the identification of facies and recognition of point-bar geometry. In comparison to microresistivity image logs and seismic data, dipmeter data is more regionally accessible; the tool indicates the orientations of fractures, bedding planes and contacts and it is commonly used to delineate boundaries between different point bars in the McMurray Formation. This work provides examples of dipmeter tadpole analyses and discusses how dipmeter-log interpretations can be supported with core-based facies analyses.This study integrates dipmeter tadpole patterns, sedimentological and ichnological characteristics from 47 sedimentary cores. Vintage dipmeter tadpole patterns were observed and digitized from 52 wells to establish a detailed facies characterization with a particular emphasis on McMurray point-bar complexes and sheltered shoreface/delta deposits. In particular, dipmeter data are useful in indicating paleocurrent directions and lateral accretion orientation of point bars, which helps to reconstruct paleodepositional history and stratigraphic relationships. Point bars in the study area are characterized by these sedimentological and dipmeter characteristics: 1) a lower point-bar section characterized by trough cross-bedded sandstone (locally inclined stratified master bedding) with local mud clasts and disorganized, low-to high-angle dipmeter readings; 2) a middle section represented by inclined heterolithic stratified lateral accretionary deposits that exhibit consistent low-angle dips (5°–18°), and display subtle shallow-to-steep-to-shallow dipmeter pattern; and 3) an upper section of the point bar or in some cases abandoned channel strata comprising laminated and lenticular mudstone or bioturbated heterolithic intertidal strata, each showing low-angle (<4°) dipmeter measurements.The dipmeter characteristics of estuarine point bars are integrated with core observations to delineate distinct point-bar stacking patterns and stratigraphic units. In particular, dipmeter tadpole patterns help to refine facies interpretations, such as cross-stratified sandstone facies. Similar point bar scales and stacking patterns are observed in four channel sets. Point bars show either NW-SW or NE-SE lateral accretion surfaces, suggesting northward paleocurrent flow direction.The analyzed datasets support a transgressive fill incised valley fill model that is consistent with earlier McMurray studies in different areas. The four channel sets descend from different stratigraphic levels, but share similarities in scale, orientation and sedimentology/ichnology. The similarity attests to persistent transgression over McMurray time punctuated with regular sea-level fall and rise until the McMurray Formation strata were replaced by Clearwater Formation sedimentation.