Facies Changes Research Articles

Sedimentology and stratigraphy of the Cenomanian Buda Limestone in central Texas, U.S.A.: Implications on regional and global depositional controls

The Cenomanian Buda Limestone is a distinctive rock formation widely distributed in Texas and its neighboring regions. The extensive character and regional significance of this unit make the Buda Limestone a key stratigraphic datum in the Texas Gulf Coast Basin. Detailed sedimentologic and stratigraphic studies of this formation were mostly focused on the west Texas exposures. In central Texas, on the contrary, very few studies have been performed. Thus, the sedimentary environments and depositional dynamics in this region are still poorly understood. Moreover, the regional correlation between the central and west Texas lithostratigraphic members of this formation remains unknown. Here, based on a detailed sedimentologic and stratigraphic study of central Texas sections (Austin-Kingsbury-San Antonio transect), the Buda Limestone is interpreted as a shallow-subtidal, semi-restricted to nearly open marine (lagoonal) shelf deposit. This unit is subdivided into two members (comprising 4th-order sequences S1 and S2) laterally variable in the study area. Vertical and lateral facies changes in the Buda Limestone are interpreted to be a function of the dynamic interaction of an irregular topography with fluctuating sea levels, under possible eustatic control. The informal (central Texas) “lower Buda” member is laterally correlated with the Lechuguilla Member (lower Buda) and Red Light Member (middle Buda) of western Texas. The informal (central Texas) “upper Buda” member, on the other hand, corresponds to the Love Station Member (upper Buda) from west Texas.

Differentiate allogenic and autogenic controls on the formation of parasequences and depositional sequences of a tide‐ and wave‐dominated carbonate platform (Lower Cretaceous, Kopet‐Dagh Basin, Iran)

The carbonate‐dominated successions of the Tirgan Formation (Latest Barremian‐Early Aptian) in the Kopet‐Dagh Basin (Iran) were studied to decipher the tectonic and eustatic sea‐level effects, as well as the autogenic influence on lateral and vertical facies changes, depositional environment, and sequence stratigraphy of the tide‐wave‐dominated sediments at the northern margin of the Neo‐Tethys Ocean. Fourteen facies were identified in the studied sequences, here grouped into five facies associations according to the depositional setting: tidal flat (A), lagoon (B), shoal (C), shallow open‐marine (D), and deep‐open marine (E). A third‐order depositional sequence, including a transgressive system tract (TST) and a highstand system tract (HST) was also identified, along with several medium‐ and small‐scale, thickening‐ and shallowing‐upward cycles (parasequences). The development of the depositional sequence and parasequences of the studied successions of the Tirgan Formation were controlled by interactions between allogenic (tectonic and eustasy) and autogenic factors (wave and tidal hydrodynamic regimes). The allogenic factors mainly affected the stacking pattern and parasequence development of the TST deposits in the lower part of the Tirgan Formation, characterized mainly by the deep‐open marine association. Autogenic factors display a main influence on the stacking pattern and formation of the HST deposits and related parasequences, characterized by shallow open‐marine‐lagoon–shoal associations developed in the upper part of the Tirgan Formation. The tectono‐sedimentary model for the evolution of the tide‐wave‐dominated carbonate platform represented by the Tirgan Formation may be useful for further studies on sedimentary basin evolution or palaeogeographic reconstructions of the Neo‐Tethys margin.

SediRate‐Fischer plots as a tool to illustrate relative sea‐level and lake‐level changes in subaqueous terrigenous deposits

AbstractFor more extensive and efficient sea‐level and lake‐level reconstructions and correlations in siliciclastic systems, this paper presents a new tool called SediRate‐Fischer plot (SR‐Fischer plot). In SR‐Fischer plots, the linear‐corrected sedimentary rate of each terrigenous sedimentary cycle is plotted from the top of the previous cycle to the top of the current cycle. This process is repeated cycle by cycle, constructing curves of cumulative departure from mean sedimentary rate (CDMR). Based on the assumption that relative sea‐level and lake‐level changes may control sedimentary rates of subaqueous terrigenous sediments within basins, the CDMR could change inversely with relative sea or lake level. Thus, this tool could be interpreted as an indicator of relative sea‐level and lake‐level changes in terrigenous successions. SediRate‐Fischer plots can be obtained from either lithological or proxy cycles formed by sea‐level and lake‐level changes in the case of availability of orbitally tuned data. The SR‐Fischer plots constructed based on different materials are compared with other methods in two cases, as follows: first for the lacustrine Mingshui Formation of the Songliao Basin in China; and second for the deep‐marine Arcillas de Gibraleón Formation of the Guadalquivir Basin in Spain. The SR‐Fischer plots of the Mingshui Formation exhibit low and high CDMR values for shallow‐lake intervals and lakeshore intervals, respectively, and vary inversely with the lake‐level curves derived from changes in sedimentary facies. For the Arcillas de Gibraleón Formation, the CDMR values illustrated by the SR‐Fischer plots exhibit opposite variations with sea‐level curves indicated by dinocyst/pollen ratios. The negative Z‐scores derived from runs tests indicate that these SR‐Fischer plots are not randomly stacked and are minimally influenced by different cycle‐splitting strategies. Thus, the results of comparison with other methods and runs tests demonstrate the validity of this method. Nonetheless, the other allogenic inflows (tectonics and climates) may limit interpretations of SR‐Fischer plots.

A shared earth approach to siliciclastic reservoir characterization: The Zubair Fm of Northern Kuwait

An innovative, integrated characterization workflow is applied to complex siliciclastic reservoirs of the Lower Cretaceous Zubair Formation in the Bahrah Field of Northern Kuwait. A detailed sedimentological and petrophysical assessment, integrated with 3D seismic interpretation, constrained and delineated vertical and lateral facies changes within the field, which is also affected by complex fault and fracture patterns. Siliciclastic geo-bodies have been documented and mapped in 1D, 2D and 3D using seismic stratigraphy, seismic attributes and high-resolution seismic inversion reservoir characterization workflows. The spatial distribution of these geo-bodies appears to show the development of fluvio-deltaic systems in the southern and central sectors of the field, which evolved into marine dominated/tidally influenced deltaic systems towards the northern and eastern sectors. Two major fluvial channels trends have been identified: SW-NE and W-E, with a dominantly NE trending deltaic progradation. Towards the north, elongated (tidal mouth?) sand bars have been interpreted with a NW-SE trend. An integrated sequence stratigraphic framework was consequently developed to better assess the influence of relative sea level changes on sedimentary facies. This framework pointed to the influence of marine incursions and subsequent flooding surfaces, which lead to the development of shale-rich seals and/or baffles that could increase the probability of structural-stratigraphic trapping mechanisms in these depositional environments. This integrated workflow was utilized to update a numerical static reservoir model, which further de-risked future drilling locations and potentially charged reservoir volumes.

Α study on the Gas-bearing Miocene Sediments of MESSARA Basin in Crete (Greece) by Using Seismic Reflection, Geochemical and Petrophysical Data

The focus on exploiting natural gas resources has been increased in the recent years since it was characterized as the transitional fuel to a net-zero era. Consequently, a reevaluation of the gas resources under a new perspective seems to be logical. Within this context, the gas-bearing Miocene sediments of the Messara basin in Crete (Greece) are discussed in this paper. The gas shows have been first reported during the 1990’s, without being thoroughly evaluated. This paper presents the interpretation of eight legacy onshore seismic reflection lines, which led to the construction of the 3D subsurface structural model of the Neogene Messara basin. The seismic data reveals the basin depocenter which accommodates approximately 1500 m of the Miocene sediments. The relationship of the Miocene deposits with the existence of shallow gas is also examined and discussed. New organic geochemical results combined with the published geochemical data are used to discuss the gas generation potential of the Miocene sediments. The porosity and permeability measurements on surface samples are also presented to describe the reservoir characteristics. The integration of the data mentioned above suggests that the Miocene sediments of the Messara basin comprise intervals with adequate organic matter to generate the observed gasses and good reservoir sandstones to accumulate the generated gas. The gas might have been trapped by lateral and vertical facies changes which provide efficient stratigraphic trapping mechanisms.

Integrated organic geochemical/petrographic and well logging analyses to evaluate the hydrocarbon source rock potential of the Middle Jurassic upper Khatatba Formation in Matruh Basin, northwestern Egypt

The present national and international pursuit of converting Egypt into a regional hub of natural gas in the East Mediterranean region resulted in conducting extensive exploration activities in several areas of northern Egypt. Therefore, organic petrographic, geochemical, and geophysical analyses were carried out on the upper Khatatba Formation from the OBA 2-2A well in the Matruh Basin.Restoration of the original sedimentological and geochemical characteristics of the upper Khatatba Formation suggest deposition of this formation in deltaic settings. Few shale and partly calcareous shale units of the Upper Safa and the uppermost Zahra members accumulated in TST in prodelta settings under dysoxic-anoxic conditions. These shale units had an original gas/oil-prone organic facies BC. Other shales and partly calcareous shales of the Upper Safa and Zahra members accumulated in HST in deltafront settings under relatively oxic conditions and had an original lower gas-prone organic facies C.The Upper Safa Member had an original active fair to excellent organic richness (0.76–7.35, avg. 3.38 TOCo(live) wt%) and hydrocarbon source rock potential (S2o: 2.22–17.97, avg. 8.05 mg HC/g org. C). It was characterised mainly by original active mixed gas/oil-prone kerogen type IIIo(live)-IIo(live) (HIo(live): 200.27–297.34, avg. 238.03 mg HC/g TOCo(live)) and subordinately by gas-prone kerogen type IIIo(live) (HIo(live): 181.50 mg HC/g TOCo(live)). The Upper Safa Member had original poor to fair proportions of active oil-prone (0.15–1.47, avg 0.68 TOCo(live, oil) wt%) and higher fair to very good proportions of active gas-prone (0.61–5.88, avg. 2.70 TOCo(live, gas) wt%) kerogens. The Upper Safa Member originally possessed poor to very good oil (S2o(oil): 0.44–3.59, avg. 1.16 mg HC/g org. C) and good to very good gas (S2o(gas): 1.77–14.37, avg. 6.44 mg HC/g org. C) generation potential. The carbonates and partly calcareous shales of the uppermost Safa and the Zahra members possessed poor oil and gas generation potential due to their organic-lean status.The present-day organic geochemical characterization indicates that the Upper Safa Member still shows active fair to excellent organic richness (0.68–6.75, avg. 3.11 TOCpd(live) wt%). However, this member shows a relatively lower present-day good to very good/excellent source rock potential (S2pd: 1.33–10.78, avg. 4.83 mg HC/g rock) and lower active kerogen quality IIIpd(live) (HIpd(live): 115.97–198.21, avg. 151.07 mg HC/g TOCpd(live)). This is related to the current high thermal maturation (gas-window: Tmax: 459–468, avg. 469 °C, thermal alteration index: TAI of 4-) and conversion of the original kerogen IIIo(live)-IIo(live) into oil and gas. The present-day high organic richness, high remaining generative potential, high shale volume of the thermally mature gas-prone kerogen, classify the Upper Safa Member as an active fair to excellent shale gas play in the Matruh Basin. Intrabasinal and interbasinal correlations across northwestern Egypt indicate that the upper Khatatba Formation shows locally and regionally dual source and reservoir rock characteristics due to the lateral facies changes.

Thickness Variation Characteristics of Tidal Rhythmites—An Example From the Pinghu Formation, Xihu Sag, East China Sea Shelf Basin

The Pinghu Formation of the Xihu Sag in the East China Sea shelf basin is influenced by tidal processes, but few studies have focused on its tidal rhythmites. Through detailed observation and description of cores, this article studies the periodicity of the tidal rhythmites of well A-11 by using the grayscale measurement of digital images based on spectral analysis and wavelet transform. According to the statistical data series of millimeter lamination thickness, the sandy lamina thickness, the argillaceous lamina thickness, and the couplet thickness were quantitatively compared and analyzed, to support the interpretation of the main controlling factors of different thickness cycles. The periodicity of sandy laminae, argillaceous laminae, and couplet thickness have distinct differences, which are interpreted to be due to differences in the nature and magnitude of tidal current cycles at the time of deposition. The high-frequency signal represents event deposition, the middle-frequency signal represents tidal current deposition, and the low-frequency signal highlights changes in sedimentary facies. Therefore, the change in the couplet thickness is controlled by event deposition, tidal current deposition, and change of the depositional environment. Our approach to the study of thickness variations in tidal rhythmites supports the reconstruction of the tidal sedimentary environment in the paleostratigraphic sequence.

Sharp‐based shoreface successions reconsidered in three‐dimensions: A forward stratigraphic modelling perspective

AbstractSea‐level fall is commonly inferred to generate a sharp‐based shoreface succession that displays an abrupt vertical transition from heterolithic, lower shoreface to sandy, upper shoreface deposits across a marine erosion surface. Three‐dimensional, process physics‐based, coupled hydrodynamic‐morphodynamic models are constructed to compare bedding architecture and facies patterns of wave‐dominated delta deposits preserved during normal (static sea level) and forced (falling sea level) regression and then transgression during subsequent sea‐level rise. The models suggest that wave‐dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave‐scoured subaqueous delta top, which is laterally separated from a delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual‐clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion. Significant grain‐size contrasts between these vertically stacked clinoform deposits reflect differences in sediment‐transport directions and sorting under river and wave‐driven littoral currents along the coast, and cannot be related uniquely to sea‐level changes. The marine erosion surface under a sharp‐based shoreface deposit records abrupt facies shift across a kilometres‐wide, wave‐eroded surface and defines a discontinuity in the preserved vertical succession. The continuity of a regressive surface of marine erosion mapped over many tens to hundreds of kilometres across mid‐shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The results of these models suggest caution in inferring sea‐level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures.

Evolution of Paleogene weathering intensity in the Qaidam Basin, northeastern Tibetan Plateau: Insights from clay geochemistry

An integrated multiproxy dataset involving both mineralogical and geochemical investigations of the northern Tibetan Plateau helps in assessing regional weathering processes linked to topographic evolution and environmental change. However, changes in landscape and drainage reorganizations caused by active tectonics and remarkable climate change during the Cenozoic characterize the basins in and around the Tibetan Plateau by frequent facies changes and coarse lithologies, which largely limit the reliability of bulk geochemical indicators for revealing the regional weathering history. Here, we present detailed geochemical investigations of clay-sized sediments collected from the Hongliugou section (54–26 Ma) in the northern Qaidam Basin to reconstruct high-resolution chemical weathering records of the northern Tibetan Plateau. To prevent chlorite bias when calculating the chemical index of alteration (CIA) in the clay-sized sediments, we propose a modified CIA formula (CIAMg+), in which Mg is added as a soluble cation that can be removed during weathering, similar to Na, K and Ca; thus, CIAMg+ = Al2O3/(Al2O3 + Na2O + K2O + CaO* + MgO) × 100. The CIAMg+ and illite chemistry indices are generally the highest values in the early Eocene Climate Optimum (∼52–49.5 Ma) and a long-term decreasing trend is present from ∼ 49.5–26 Ma; the Mg/Al ratios of the clay-sized sediments and hydrochloric acid leachates are the lowest during the EECO and present a long-term increasing trend in the following 49.5–26 Ma, thus collectively indicating a continuous decrease in the regional chemical weathering intensity. The weakening weathering process during the Paleogene is mainly attributed to the decreasing global temperature and decline in precipitation caused by global cooling. Our study suggests that the proposed geochemical index based on clay-sized sediments can serve as an efficient proxy for regional chemical weathering reconstruction, even in thick sedimentary sequences with frequently changing facies and lithology.

Structural geometry and kinematic history of the southern Kicking Horse Rim Cambrian carbonate platform: out-of-sequence thrust faulting in the southern Canadian Rocky Mountains

Abstract The structure of the southern Kicking Horse Rim Cambrian carbonate platform is spectacularly exposed in the Rocky Mountains southwest of Calgary, Alberta, Canada. Episodic uplift along the approximately 100 km long, north-northwest-trending Kicking Horse Rim during the middle and upper Cambrian controlled the position of the Cambrian carbonate platform to basin facies change. Extensive hydrothermal alteration, with local magnesite and/or Pb-Zn mineralization occurred along the west margin of the Rim. Spectacular slump scars, escarpments, megaconglomerates and slide masses locally mark the edge of the southern Kicking Horse Rim platform. Megaconglomerates form a unique and indisputable stratigraphic tie between the basin and platform and show that there is no stratigraphic gap or suture along this boundary. The change from carbonate- to shale-dominated lithologies resulted in a major change in structural style and the boundary between the Eastern and Western Main Ranges structural subprovinces. Serial and regional cross-sections across the area document the southward plunge of Kicking Horse Rim strata and the Eastern Main Ranges structural subprovince and show that this is due to underlying structure, and not stratigraphic thinning of the Cambrian carbonate platform. Prospective horizons for mineralization extend southward in the subsurface beyond their current southern limit of exposure. Kicking Horse Rim and associated Cambrian strata were carried northeastward along the Fatigue Thrust - blind thrust fault system. Because of a prominent hanging wall ramp through the entire Cambrian succession along the east side of the Kicking Horse Rim, the southern Kicking Horse Rim and Cambrian platform formed an asymmetric, east-facing anticline with a long gently-dipping west limb as they were uplifted and carried northeastward. In the northern part of the study area, displacement was along the Fatigue Thrust and the fault climbed through the Paleozoic section. In the south, most displacement on this fault system went into a blind detachment fault near the base of the thick basinal lower Ordovician section and overlying faulted detachment folds. The Simpson Pass Thrust truncated and overrode in its footwall the east-facing anticline and other structures developed in the hanging wall of the Fatigue Thrust. The Simpson Pass Thrust is out-of-sequence relative to the Fatigue Thrust and is one of the few larger displacement, out-of-sequence thrust faults recognized in the southern Canadian Rocky Mountains. Deformation of the Western Main Ranges and thrust faults near the Cambrian facies change occurred prior to the main motion along the Fatigue Thrust - blind thrust system. Motion on this thrust system, and the formation of faulted detachment folds that extend southward into the eastern part of the Porcupine Creek fan structure, occurred prior to motion on the Simpson Pass Thrust. Deformation of all these structures occurred during Jurassic–Early Cretaceous deformation most likely sometime prior to 125 Ma.

Microfacies analysis and reservoir discrimination of channelized carbonate platform systems: An example from the Turonian Wata Formation, Gulf of Suez, Egypt

The channelized carbonate reservoirs played important role in the global hydrocarbon production. This study presents a new contribution to the carbonate platform system of the Turonian sediments in the Gulf of Suez. The siliciclastic fluvial channelized system incised the carbonate platform in October Field to deposit excellent reservoir unit. Also, the study interprets the facies change, depositional environment, and petrophysical characteristics of the Turonian Wata sequence, to figure out the fluvial channel reservoir distribution and discrimination using surface outcrops in addition to five cored wells. The petrographic analysis and depositional settings interpretations were conducted to the subsurface and outcrop samples. Then, the conventional core analysis measurements were combined with borehole logs for optimum reservoir discrimination, rock typing, and flow units interpretation. Besides, we studied the reservoir heterogeneity and predict permeability and reservoir rock typing in the un-cored intervals. Our finding revealed that the Wata Formation is subdivided into three main lithological units characterized by 16 microfacies. The lower and upper ones are carbonate dominated, composed of wackestones, packstones, and grainstones which is deposited in platform setting belong to lower/middle shelf and mid/outer shelf to outer ramp setting respectively. The middle unit is classified into lower and upper zones, where the lower clastic zone is composed of vari-sized sandstones, siltstone, and mudstone with some plant remains deposited in a point bar of fluvial channel and flood plains. The upper zone is composed of clastic/carbonate intercalations deposited in intertidal/shelf settings. The integration between the petrography and petrophysics enables discriminating the Wata sandstone reservoir into four static rock types (PSRT 1–4), which are controlling its reservoir quality and heterogeneity. The PSRT1 and PSRT2 have the best reservoir quality in the channelized unit and are recommended to be tracked along with October Field. The HFU2 and HFU3 are the main flow capacity in the Wata reservoir, composed of PSRT1 and PSRT2, and represented by the calcareous and ferruginous quartz arenite microfacies. This study presented a secondary dolomitized microfacies with 85% oil potentiality resulted from the incision of the fluvial channel above the aerially exposed carbonate lower unit. The research addressed the optimized petrophysical properties and reservoir heterogeneity of the channelized Wata sequences. In addition, it highlights a guidelines for further exploration and production of these reservoirs in the basin.

Kilometre-scale coral carpets on mixed carbonate-siliciclastic platforms; a sedimentological study from the Lower Cretaceous of northwestern Africa

Coral carpets are biostromes that form laterally continuous but low-relief (typically less than 1-metre-high) coral communities with a lack of clear internal zonation. There are few locations that allow analysis of such systems on a regional (tens of kilometre) scale in the rock record. This study describes extensive, well-exposed, coral-rich bodies (coral carpets), deposited on the Moroccan northwest Atlantic shallow-marine margin during the Cretaceous (early Hauterivian). The coral-rich sequence reaches 50 m in thickness, with individual carpets between 10 and 100 cm thick, extending over an area of more than 150 km 2 . Logging and regional correlation, integrated with thin-section analysis, has allowed the evaluation of facies distribution, geometries and temporal evolution. The results provide information on allogenic and/or autogenic driving processes for coral development, and allows the generation of conceptual models for their growth. The Tamanar Fm. corals reflects a shallowing up sequence and the recovery of carbonate productivity following stressed environmental conditions in the late Valanginian. Temporally, coral morphological response relates to high-frequency relative sea level changes that affected water energy and turbidity; thick-branching and massive forms grew in shallow high energy environments whilst platy/flat forms grew in poorly lit and turbid environments. However, local-scale (over hundreds of metres) lateral changes in morphology and facies can be explained by autogenic factors independent of sea level change; varying sedimentation, hydrodynamism and turbidity across the extensive open marine platform. The termination of coral deposition is marked by a hardground surface, indicating slow rates of sedimentation and a transgression, which is followed by an influx of clastic sediments onto the platform, resulting in the deterioration of carbonate platform health, as conditions became intolerable for coral growth. These coral-rich outcrops contribute to the relatively sparse documentation of Hauterivian corals globally and suggest niche palaeoecological and palaeoclimatic relationships that allowed corals to develop despite stressed platform conditions. The low diversity, low-relief coral carpets may provide a proxy for the recovery and demise of corals in response to environmental stress on modern carbonate platforms and future climate change projections. • Kilometre-scale coral-rich outcrops onshore the Moroccan Atlantic Margin. • Non-framework coral carpets show no internal zonation. • Coral morphology and abundance controlled chiefly by water energy and turbidity. • Establishment and demise of corals linked to sea level and climatic change. • Low diversity corals possible proxy for recovery/loss in response to environmental stress.

Depositional elements and evolution of gravity-flow deposits on Lingshan Island (Eastern China): An integrated outcrop-subsurface study

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution.

Recharge and Geochemical Evolution of Groundwater in Fractured Basement Aquifers (NW India): Insights from Environmental Isotopes (δ18O, δ2H, and 3H) and Hydrogeochemical Studies

Considering water as a limiting factor for socio-economic development, especially in arid/semi-arid regions, both scientific communities and policymakers are interested in groundwater recharge-related data. India is fast moving toward a crisis of groundwater due to intense abstraction and contamination. There is a lack of understanding regarding the occurrence, movement, and behaviors of groundwater in a fractured basement terrane. Therefore, integrated environmental isotopes (δ18O, δ2H, and 3H) and hydrogeochemical studies have been used to understand the recharge processes and geochemical evolution of groundwater in the fractured basement terranes of Gujarat, NW India. Our results show that the relative abundance of major cations and anions in the study basin are Ca2+ > Na+ > Mg2+ > K+ and HCO3− > Cl− > SO42− > NO3−, respectively. This suggests that the chemical weathering of silicate minerals influences the groundwater chemistry in the aquifer system. A change in hydrochemical facies from Ca-HCO3 to Na-Mg-Ca-Cl. HCO3 has been identified from the recharge to discharge areas. Along the groundwater flow direction, the presence of chemical constituents with different concentrations demonstrates that the various geochemical mechanisms are responsible for this geochemical evolution. Furthermore, the chemical composition of groundwater also reflects that the groundwater has interacted with distinct rock types (granites/granulites). The stable isotopes (δ18O and δ2H) of groundwater reveal that the local precipitation is the main source of recharge. However, the groundwater recharge is affected by the evaporation process due to different geological conditions irrespective of topographical differences in the study area. The tritium (3H) content of groundwater suggests that the aquifer is mainly recharged by modern rainfall events. Thus, in semi-arid regions, the geology, weathering, and geologic structures have a significant role in bringing chemical changes in groundwater and smoothening the recharge process. The findings of this study will prove vital for the decision-makers or policymakers to take appropriate measures to design water budgets as well as water management plans more sustainably.

Karstified layers and caves formed by superposed epigenic dissolution along subaerial unconformities in carbonate rocks – Impact on reservoir-scale permeability

Superposed events of rock exposure, fracturing, and the hydrological dynamics, are key elements that influence epigenic karstification in carbonate reservoirs. However, they are not fully identified in seismic or well data. One way forward is a multiscale characterization of the porous system to understand karst development, from the pore scale to large caves, confined to high-permeability layers that can reach the basin scale. The present study investigates superposed meteoric dissolution events in carbonate rocks by combining the analyses of depositional setting, diagenesis, stratigraphic architecture, tectonics, and hydrological zones. We performed analyses of 130 plugs collected in 11 outcrops using petrographic, petrophysical, mineralogical, and geochemical analyses. The study area is the Cretaceous Jandaíra Formation in the Potiguar Basin, Brazil, where we identified a regional subaerial unconformity-SU (sequence boundary-1). This surface is characterized by a change in facies and depositional system between dolomitic peritidal deposits (depositional sequence 1) and tight-cemented calcite subtidal deposits (depositional sequence 2). A concentration of caves occurs below the SU. We conclude that subaerial unconformities (vadose surfaces) can develop better reservoir qualities with superposed freshwater leaching and karstification. They control the lateral reservoir zonation, whereas faults and fractures provide vertical pore distribution within the vadose/phreatic hydrological zones. Water table fluctuations can develop multilevel caves and conduits along the epiphreatic zones, increasing the reservoir thickness, which reaches 50–100 m in the Potiguar Basin. We recognized two previous subaerial unconformities at the basin scale using well and seismic data. These findings may have important implications for assessing reservoir location and quality in large areas.

Controlling factors of acoustic properties in continental carbonates: Implications for high-resolution seismic imaging

Continental carbonates are characterized by multi-scale heterogeneous porous networks, making the geological interpretation of seismic imaging difficult. We investigate two sedimentary sections exhibiting a similar facies succession, combining geological characterizations and multi-scale acoustic measurements. Based on outcrop investigation and petrographic description, we define nine sedimentary facies displaying contrasted early diagenetic evolutions. According to the vertical facies variations, we develop a depositional model corresponding to a low gradient valley fed by freshwaters, subdivided into three main domains (alluvial plain, palustrine and paludal). To understand the acoustic properties of the studied sedimentary rocks while remaining representative of their multi-scale heterogeneity, we acquire acoustic measurements at two different scales: i) at log-scale, directly on the outcrop surfaces using a frequency of 250 kHz; and ii) at plug-scale as usually done in laboratory using a frequency of 500 kHz. Based on these multi-scale geophysical acquisitions, we link in-situ P-wave velocities with the different sedimentary facies while characterizing centimeter-scale Representative Elementary Volumes (REVs). Conversely, based on laboratory measurements and thin-section petrography, we define relationships between P-wave velocity, porosity, facies, and diagenesis, corresponding to millimeter-scale REVs. Using both in-situ P-wave velocity measurements and plug densities, we construct 1-D synthetic seismograms showing meter-scale seismic reflectors equivalent to crosswell seismic frequency ranges. This approach shows the following: i) high-amplitude seismic reflectors fit with facies changes associated to diagenetic contrasts (e.g. cemented versus uncemented carbonates); ii) reflection free-zones match with a succession of facies changes affected by diagenetic homogenization (e.g. intensely to pervasively recrystallized and cemented carbonates). Our work highlights the importance of relating an extensive geological description of carbonates (facies, depositional model, diagenesis) together with multi-scale acoustic measurements and synthetic seismic modelling to predict the high-resolution heterogeneities of subsurface reservoirs.

Extracting Time-Series of Wet-Snow Facies in Greenland Using Sentinel-1 SAR Data on Google Earth Engine

Accurate information on the dynamic change of wet-snow facies (WSF) in Greenland is important in investigating the mass balance of Greenland ice sheets (GrIS) and assessing global sea level rise. In this study, we proposed a rapid WSF extraction algorithm, referred to as difference method, for extracting WSF at a continental scale and with high spatial resolution using the Sentinel-1 synthetic Aperture radar (SAR) data available on Google Earth Engine (GEE). We used this method to extract the WSF in Greenland from May to October in recent 6 years. Accuracy of the extracted results was evaluated using air temperature data from Greenland Automatic Weather Station (AWS). Results suggested that the difference method has good precision in extracting WSF from Sentinel-1 SAR data, and can process a large amount of data simply, quickly, and effectively.

An integrated high-resolution image log, sequence stratigraphy and palynofacies analysis to reconstruct the Albian – Cenomanian basin depositional setting and cyclicity: Insights from the southern Tethys

Borehole image logs are used to overcome the lack of core samples and other conventional open-hole logs in many subsurface reservoirs around the world, significantly reducing the likelihood of petroleum resource exploration. These image logs can provide high-resolution data to visualize sedimentary structures, facies types, lithofacies, geomechanical characteristics, and depositional trends. Sedimentologists recognize the Bahariya Formation of the North-Western Desert of Egypt as the typical coastal -shallow marine depositional model that emphasizes a typical heterogeneous sandstone reservoir with many obstacles in production profile. One of the most significant issues related to the heterogeneous Bahariya reservoir that confronts hydrocarbon producers is the loss of productive target zones due to spatial facies changes. This research aims to overcome the exploration and development challenges of the complex and heterogeneous sandstone reservoir of the Albian – Cenomanian Bahariya Formation that lies in the Abu Gharadig Basin, Southern Tyths, Egypt. In this work, we used integrated image logging in conjunction with palynofacies, core samples, and sequence stratigraphy to provide high resolution information that can aid in better resource exploitation. Furthermore, we utilized the integrated work in the definition of the high-resolution para-sequences, sequences boundaries, lithofacies characteristics and paleocurrent analysis of each facies association. The interpretation of coastal shallow marine facies from four BHI-logs was established and calibrated with two core sets in order to define the best hydrocarbon reservoir intervals and focus on improving the development plan of the study area. Our findings show that the Bahariya Formation is characterized by eleven facies associations which have been interpreted based on the integrating of the twenty-seven electrofacies extracted from the FMI images, wireline logs, and core photographs. The identified facies associations were classified into five depositional sequences and their boundaries were determined using dipmeter patterns and extracted lithofacies from BHI. The LST and MFS deposits in both the fourth and fifth depositional cycles, respectively, whereas the HST deposits are found only in the third depositional cycle. According to our findings, the development of the shoreface sandy facies of cycles three and five represents the maximum rate of sea-level rise, as well as the development of the offshore sand bar system, which represents high-quality sandstone reservoirs. This case study demonstrates how image logs can be used to better understand the spatial distribution and geometries of sandstone bodies in the Bahariya Formation. Furthermore, it provides a reference imaging fingerprint of high-resolution coastal shallow marine, high-frequency sequences analysis, sedimentary structures, and extracted electrofacies analogues that can be used in the basin and elsewhere.

A closer look at sedimentation processes in two dam reservoirs

Dam reservoirs are complex depositional systems, which provide barriers to natural sediment transport along river courses. In addition, reservoir sedimentation causes serious problems in their operational management. Given the wide variability of reservoir bottom shapes, total volumes of stored water, river discharges, sediment loads and sediment textures, universal depositional models are difficult to achieve for dam reservoirs. This article focuses on the need to improve current understandings of sedimentation processes, and thorough spatial particle size distribution in dam reservoirs. To achieve this aim, sonar and ground penetrating radar surveys were combined with high-resolution stratigraphic data from 44 short sediment cores in two valley-type reservoirs, Seč and Křižanovice, part of a reservoir cascade in the Chrudimka River, which is a left-hand tributary of the Labe River (Czech Republic). Sonar survey was used to construct bathymetric maps and bottom reflection intensity images. The results showed topography inherited from pre-reservoir times and a good degree of correspondence between bottom reflection intensity and grain size. The sedimentation patterns exhibit distinct proximal-to-distal and transversal changes in sediment facies and grain size. Pre-reservoir and present morphologies have substantially shaped sediment accumulation. Spatial grain size sorting is a function of travel distance within reservoirs, which is conditioned by their bended and narrow geometry, as well as by reservoir management. Two stages of reservoir sedimentation are recognised. The initial stage was characterised by the filling of local depressions, which were overlain by the second stage, typified by continuous and horizontally deposited sediments. Relatively low sediment accumulation rates (from 0.2 to 1.06 cm/year) are the consequence of reservoir characteristics (capacity and associated accommodation space) and factors at a basin scale (low discharge, geology and land-use). Slow reduction of reservoir capacity was observed. The calculated trap efficiency is very high in the Seč Reservoir and much lower in the Křižanovice Reservoir. The main factor controlling trap efficiency is reservoir capacity and shape combined with low inflow. Ultimately, the trap efficiency change over time because of varying reservoir capacity due to seasonal water level fluctuation and varying hydrologic conditions. Due to the slow silting of the reservoir, the trap efficiency has decreased only slightly. Aside from general trends, some specific features exist for every reservoir based on local geometry and reservoir management strategy. This implies that an innovative approach combining map, sonar and geophysical surveys with core data is required to develop better facies models and to optimise sediment management strategies.

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.