Stratigraphic Model Research Articles

3D stratigraphic modelling of the Bangkok basin using Kriging on borehole data

The Lower Central Plain of Thailand has a deep and highly irregular basement filled with complex layers of alluvial sandy soil and deltaic clay or silt. The Bangkok Metropolitan region with its high level of infrastructure development is located in this plain. With high population density, the problem of land subsidence is critical. This study uses borehole data and the Kriging method to interpolate the data. A detailed 3D stratigraphic model of the basin is presented and several cross sections along two directions. Seventeen layers and the points used for modelling each layer are presented. The model shows eight aquifers in the Bangkok basin, lying between eight layers of clay, and a bottom layer above the basement. The bottom of the lowest aquifer of the Bangkok aquifer system is 610 m deep. The basement extends down to a depth of more than 2000 m in some places. Our 3D model, which extends to the basement, is of interest for mining and site-specific seismic risk analysis. Moreover, our results can be very useful for groundwater and land subsidence studies.

Tectonic and eustatic control on channel formation, erosion, and deposition along a strike-slip margin, San Diego, California, USA

Active margins are characterized by small mountainous streams, which contribute significantly to global sediment transport from land to sea. The response of active margin streams to base-level change due to eustatic sea-level cycles is less well understood than passive margin streams, which form the basis of most sequence stratigraphic models. The southern California margin offers an opportunity to investigate the evolution of small mountainous streams along a strike-slip margin over a sea-level cycle. High-resolution Chirp subbottom data were examined from the Silver Strand littoral cell in San Diego, California, which is located in a region of localized transtension associated with a step in the Newport Inglewood-Rose Canyon fault zone. Chirp data reveal active faulting on multiple fault strands, a subaerial surface likely formed from MIS 4 to MIS 2, fluvial incision associated with MIS 2, and the transgressive surface of erosion formed during sea-level rise from MIS 2 to MIS 1. Interpreted acoustic units observed in the data include the Cretaceous outcropping Rosario Group, Miocene to Pliocene prograding clinoforms, subaerial deposits associated with MIS 4 to MIS 3, channel fill and transgressive lag deposits formed during the most recent sea-level transgression, and Holocene marine sediment. The Chirp data examined in this study build upon findings from other seismic studies, sediment cores, trenching, and outcrops that have been examined on- and offshore San Diego. Combined, these data suggest that the fault-controlled pull-apart basin formed in this region influences sediment transport and deposition throughout the last glacial cycle. During highstand, the fault-controlled wide shelf, subsiding basin, and circulation patterns result in trapping of sediment in San Diego Bay and nearshore, cutting off sediment delivery to the deep-sea. During lowstand, the fault-controlled morphology creates a drainage divide that deflects streams to the south which lengthens and reduces slope of the drainage pathways. These findings are in contrast to observations in other fault controlled littoral cells along the southern California margin where sediment delivery to the deep-sea remains active during highstands, highlighting the variability of stream response to base-level change along active strike-slip dominated margins.

Building a mathematical model for the free groundwater Resources of the plain area in the lattakia basin

أظهرت نتائج النمذجة الستراتيغرافية لمنطقة الدراسة أن التشكيلات الستراتيغرافية تشكل منظومة هيدروجيولوجية مرتبطة هيدروديناميكياً مع بعضها البعض تتمثل بطبقة مائية واحدة ذات جريان جوفي حر. كما بينت نتائج النموذج الرياضي الذي تم بناؤه للمنطقة أن القيم العالية لعامل الرشح تركزت فـي الجــزء الشمالي والأوسط مـن منطقــة البحث، مكان وجود الفوالق ولـم يتجـاوز الفـرق بـين قـيم المدخلات ونتائج المعايرة في معظم مناطق توزع الناقلية الهيدروليكية للطبقة 10%. الموارد المائية في القسم الشمالي الشرقي من منطقة الدراسة ذات نوعية جيدة وكمياتها كبيرة وسهلة الاستثمار حيث بلغ إجمالي الوارد (134607.3) م3/ يوم, تراوحت سماكة الحامل حوالي (50م) في القسم الشمالي وتزداد باتجاه منطقة النطاق الفالقي ضمن مجرى نهر العرب لتصل إلى حوالي (125م) أما بالنسبة لسماكة الحامل في القسم الجنوبي تراوحت بين (75 – 60م) ونظام المياه الجوفية في منطقة الدراسة مستقر ويعود القسم الأكبر من كمية الموارد المائية المتشكلة في المنطقة إلى الوارد الجانبي الجوفي وبلغت نسبته حوالي (90%) من القيمة الإجمالية للموارد.

Stratigraphy and sedimentary evolution of a modern macro‐tidal incised valley: An analogue for reservoir facies and architecture

AbstractIncised valley fills are complex as they correspond to multiple sea‐level cycles which makes interpretation and correlation of stratigraphic surfaces fraught with uncertainty. Despite numerous studies of the stratigraphy of incised valley fills, few have focused on extensive core coverage linked to high fidelity dating in a macro‐tidal, tide‐dominated setting. For this study nineteen sediment cores were drilled through the Holocene succession of the macro‐tidal Ravenglass Estuary in north‐west England, UK. A facies and stratigraphic model of the Ravenglass incised valley complex was constructed, to understand the lateral and vertical stacking patterns relative to the sea‐level changes. The Ravenglass Estuary formed in five main stages. First, incision by rivers (ca 11 500 to ca 10 500 yrs bp) cutting through the shelf during lowstand, which was a period of fluvial dominance. Secondly, a rapid transgression and landward migration of the shoreline (10 500 to 6000 yrs bp). Wave action was dominant, promoting spit formation. The third stage was a highstand at ca 6000 to ca 5000 yrs bp, creating maximum accommodation and the majority of backfilling. The spits narrowed the inlet and dampened wave action. The fourth stage was caused by a minor fall of sea level (ca 5000 to ca 226 yrs bp), which forced the system to shift basinward. The fifth and final stage (226 yrs bp to present) involved the backfilling of the River Irt, southward migration of the northerly (Drigg) spit and merging of the River Irt with the Rivers Esk and Mite. The final stage was synchronous with the development of the central basin. As an analogue for ancient and deeply buried sandstones, most of the estuarine sedimentation occurred after transgression, of which the coarsest and cleanest sands are found in the tidal inlet, on the foreshore and within in‐channel tidal bars. The best‐connected (up to 1 km) reservoir‐equivalent sands belong to the more stable channels.

Late Ordovician deglaciation of the Murzuq Basin (SW Libya): a core to seismic-scale characterization of the depositional environments and sedimentary architecture

Glacial reservoirs have been the focus of numerous studies in various basins around the world, often with the aim of supporting hydrocarbon exploration and development. These reservoirs present a significant challenge due to their inherently complex internal architectures and significant lateral heterogeneities. This is of significant importance, not only for both exploration and reservoir production optimization, but also as an opportunity for stratigraphic trapping and enhanced prospectivity in mature areas. This study is focused on a major glaciogenic reservoir, of Late Ordovician age, in the Murzuq Basin (SW Libya). It aims to integrate subsurface datasets, such as 3D seismic with well-based sedimentological data (core, conventional wireline logs and wellbore resistivity images). The study proposes a comprehensive facies scheme optimized for the sub-surface but consistent with outcrops, and aims to generate depositional models, characterize the complex internal architecture of the reservoir, establish a genetic stratigraphic framework and evaluate facies in terms of reservoir properties and stratigraphic trap potential. Results, integrating subsurface facies analysis and well-calibrated seismic stratigraphy support previous models, derived from outcrop analysis, suggesting that these complex reservoirs cannot be characterised by purely lithostratigraphic criteria. To aid in the assessment of these formations, a facies scheme is proposed, focused on subsurface data but also considering other published, outcrop-based, schemes. Additionally, an interpretation is presented of the internal architecture of the Upper Ordovician based on a succession of several ice advance-retreat cycles, not all of which are present across the study area. Each cycle corresponds to a seismic package interpreted to comprise the deposits of several key sedimentation phases, resulting in a complex mosaic of genetic packages generated by multiple, often areally-restricted, cut and fill events. Predictive stratigraphic models are proposed for this complex package, based on both seismic stratigraphy and well-based sedimentology. The implications for future prospectivity and the significance for stratigraphic trap potential are also discussed.

Research and Application of Globally Optimized Sequence Stratigraphic Seismic Interpretation Technology: Taking the Lower Cretaceous Shahezi Formation of Xujiaweizi Fault Depression as an Example

In the study of sequence stratigraphy in continental rift basins, the use of seismic data to track different levels of sequence stratigraphic boundaries laterally is the key to the division of sequence stratigraphic units at all levels and the establishment of an isochronous sequence stratigraphic framework. Traditional seismic interpretation and the establishment of a 3D sequence stratigraphic structure model are a difficult research work. This paper introduces the concept of cost function minimization and performs global stratigraphic scanning on 3D seismic data to interpret horizons and faults in a large grid. Constrained by the results, human-computer interactive intelligent interpretation, by adding iterative interpretation of geological knowledge, established a global stratigraphic model with a relative geological age. The application in the Lower Cretaceous Shahezi Formation of Xujiaweizi fault depression shows that this technology has improved the accuracy and efficiency of sequence stratigraphic interpretation, and the application of this technology has achieved the interpretation of each event horizon under the current seismic data resolution conditions. In this way, a continuous sequence stratigraphic model is established. From this stratigraphic model, any high-frequency sequence-interpreted seismic horizon can be extracted, which provides a basis for the combination of lateral resolution and longitudinal resolution of subsequent reservoir prediction.

Stratigraphic forward modelling of distributive fluvial systems based on the Huesca System, Ebro Basin, northern Spain

AbstractTo better understand the stratigraphic development of sedimentary systems, it is necessary to link the controls on sedimentary processes to the resulting deposits, which in turn allows predictions of stratigraphic architectures at a range of scales. We use a stratigraphic forward model to link the governing parameters to the distribution of deposits within a distributive fluvial system (DFS). The numerical model has been validated against outcrop observations to establish how the depositional processes needed to form the specific sedimentary system have been reproduced. We chose the previously studied Oligocene to Miocene Huesca DFS in northern Spain to investigate and calibrate the model. Additionally, downstream profiles from modern DFS in northern India, and hydrological measurements from the High Island Creek, Minnesota, USA, were used as input parameters for the model in addition to the outcrop data from the Huesca DFS. The resulting model adequately reproduced the real‐world system. Once validated, the analysis of the modelled DFS led to key findings, which expand our understanding of DFS stratigraphic architecture. Reservoir characteristics in radial DFS are dependent on the angle away from the meridian (straight line from the source through the apex to the distal zone of the DFS). The greater the angle is, the coarser the average grain size in the proximal zone is but the finer the average grain size in the medial and distal zones. Lateral variability of net to gross, sandbody thickness and number, and amalgamation ratio is greatest at the transition between the proximal and medial zone and is still significant in the distal part of the DFS. Stratigraphic forward modelling enhanced our understanding of DFS, which leads to reducing risk associated with exploration, production and storage of fluids in subsurface DFS.

Miocene seismic stratigraphy and geomorphology of Bibiyana gas field, Surma Basin, Bangladesh

An integrated approach using a three-dimensional (3-D) seismic survey over the Chevron-operated Bibiyana field in Bangladesh along with petrophysical well data and cores provides new insights into the stratigraphic development of the Miocene–early Pliocene Ganges Delta. From this, a sequence stratigraphic model was constructed for the Bibiyana field that more accurately depicts the internal architecture of the reservoir units of the field. As of 2019, Bibiyana field was producing approximately 1.3 billion SCF/day of gas and accounted for more than 45% of Bangladesh’s daily gas supply. The 3-D seismic volumes covering the field area were stratigraphically flattened using multihorizon seismic flattening techniques. The well-log data were also flattened to enable the wells to be tied and visualized within the flattened domain. Core data from the field were examined, interpreted, and integrated into the stratigraphic model. Reservoirs at Bibiyana are divided into a series of sequence stratigraphic units, each recognized by a seismically defined sequence boundary at both the base and the top of the units. Two types of major reservoir sandstones were observed: (1) erosional remnants of shallow-marine, tidal sandstones below sequence boundaries and (2) channel sandstones at the base of incised canyons above sequence boundaries. The focus of this investigation was on the architecture of the main reservoir known as the BB60 series (Miocene Bokabil Formation), containing approximately 50% of the field reserves. The BB60 series reservoir is an erosional remnant below a major sequence boundary (SB60) sealed by the thick canyon-fill prodelta shale of the BB55 interval. Channelized sandstones and associated mass transport deposits are present locally at the base of the canyon and have reservoir properties that contrast with the underlying and adjacent shallow-water, tidal-dominated deltaic sandstones of the BB60 reservoir. Multihorizon flattening of the seismic and well data in concert with 3-D visualization reveals the complexly eroded canyon system, incised into the underlying tidal deltaic deposits of the BB60 series. Similar erosional patterns are present in the underlying BH series reservoirs (Miocene Bhuban Formation), and the conceptual model developed for the BB60 reservoir can be applied and aide the understanding of the deeper reservoirs that are less well imaged by the existing seismic data. The recognition that unconformities with overlying canyon-fill shales seal many of the reservoirs at Bibiyana is critical in understanding and modeling the reservoir architecture and performance over time.

The Upper Devonian of the river Sil area: a stratigraphic correlation between León and Asturias, northern Spain

We have studied the Upper Devonian succession of the river Sil area divided into five sections along a SW-NE transect. Thirteen lithofacies types have been identified based on lithology and sedimentological structures. They have been organised into facies associations representing depositional environments from the fluviomarine zone of the nearby part of the basin towards the distal offshore zone. Erosion surfaces and sequence boundaries have been identified and the succession is connected to the Upper Devonian sequence stratigraphic model of the southern Cantabrian Mountains and the Upper Devonian succession of the Coastal Ranges of Asturias in the north. The main portion of the Upper Devonian succession of the Sil area can be correlated with the Nocedo Formation, Gordón Member (sequence A) of early Frasnian age. In the south-westernmost sections of the Sil area parts of the Fueyo and Ermita formations are exposed; they constitute sequence C of early Famennian age. The Upper Devonian Unconformity separates these thick Frasnian-Famennian coarsening upward sequences from the overlying extensive Famennian-Tournaisian condensed succession of the Llombera Beds (sequence D).

Combining single-station microtremor and gravity surveys for deep stratigraphic mapping

Any stratigraphic reconstruction by means of surface geophysical methods is affected by the nonuniqueness of data inversion and by the resolution-depth trade-off. The combination of different geophysical techniques can reduce the number of degrees of freedom of the problem. We have focused on two low-impact single-station geophysical techniques: microtremor and gravity. These have been used by previous authors for stratigraphic mapping only by comparing the results independently. We suggest a procedure to combine microtremor and gravity data into a unique subsoil model and explore to what extent their combined use can overcome their individual weaknesses and constrain the final result. We apply the procedure to the Bolzano sedimentary basin, Northern Italy, to derive a 3D bedrock model of the basin. We use microtremor data to map the ground resonance frequencies and derive an initial 3D bedrock depth model by assuming a [Formula: see text] profile for the sediment fill. Then, we define a density model for rock and sediments and perform 3D gravity forward modeling. We then perturb the [Formula: see text] and density models and find the parameters that best fit the observed gravity anomalies. Data uncertainties are examined to explore the significance of the results. Joint use of the two techniques successfully helps interpret the stratigraphic model: Ground resonance frequencies guarantee the spatial resolution of the bedrock geometry model, whereas gravity data help constrain the frequency to depth conversion.

Low-latitude ?upper Barremian–lower Aptian palynoflora and paleovegetation of the Biyadh Formation (Arabian Plate, eastern margin of northern Gondwana): evidence for a possible cold snap

A palynological analysis of core and cuttings samples from successions assigned to the Biyadh Formation of Saudi Arabia in the Arabian Gulf (eastern margin of northern Gondwana) revealed diverse and well-preserved palynological assemblages including bryophyte and fern spores, gymnosperm and flowering plant pollen, dinocysts, and acritarchs. Palynological data corroborates earlier geological and stratigraphic models, indicating a ?late Barremianearly Aptian age. The fluctuations of dinocyst abundance and diversity proved extremely useful to detect fine-scale sea-level changes in carbonate platforms with episodic terrigenous inputs. In agreement with current understanding of dinocyst paleoenvironmental distribution trends, Cribroperidinium and Aptea? vannophora are restricted to inner neritic/?fluvial-influenced settings, while Spiniferites and Trichodinium have been confirmed as more open marine water (middle/outer neritic) indicators. The composition of the terrestrial assemblages is dominated by fluvially-transported spores, mainly released by ferns and probable tree ferns. The overall floral composition is considered to reflect a seasonal, humid, climate punctuated by episodic droughts. Recurrent increases in abundance of wind-pollinated cheirolepid and Araucariaceae pollen, observed in assemblages otherwise characterized by low dinocyst content, are interpreted to represent sea-level falls and basinward expansion/migration of conifer-dominated coastal vegetation. The abundance of the dinocysts Cepadinium variabilis and Pseudoceratium nohrhansenii sp. nov., previously recorded at higher latitudes, is interpreted to reflect the invasion of cooler/colder water masses, confirming a significant ?latest Barremian/early Aptian global cold episode, possibly related to the OAE1a event. The following new dinocyst taxa are described: Pseudoceratium nohrhansenii sp. nov. and Protobatioladium alhajrii sp. nov.

Geometrical Breakdown Approach to interpretation of depositional sequences

AbstractSeismic and sequence stratigraphic analyses are important methodologies for interpreting coastal and shallow-marine deposits. Though both methods are based on objective criteria, terminology for reflection/stratal stacking is widely linked to eustatic cycles, which does not adequately incorporate factors such as differential subsidence, sediment supply, and autogenic effects. To reduce reliance on model-driven interpretations, we developed a Geometrical Breakdown Approach (GBA) that facilitates interpretation of horizon-bound reflection packages by systematically identifying upward-downward and landward-seaward trajectories of clinoform inflection points and stratal terminations, respectively. This approach enables a rigorous characterization of stratal surfaces and depositional units. The results are captured in three-letter acronyms that provide an efficient way of recognizing repetitive stacking patterns through discriminating reflection packages objectively to the maximum level of resolution provided by the data. Comparison of GBA with selected sequence stratigraphic models that include three and four systems tracts and the accommodation succession approach shows that the GBA allows a greater level of detail to be extracted, identifying key surfaces with more precision and utilizing more effectively the fine-scale resolution provided by the input seismic data. We tested this approach using a synthetic analogue model and field data from the New Jersey margin. The results demonstrate that the geometric criteria constitute a reliable tool for identifying systems tracts and provide an objective and straightforward method for practitioners at all levels of experience.

Delta‐to‐fan source‐to‐sink coupling as a fundamental control on the delivery of coarse clastics to deepwater: Insights from stratigraphic forward modelling

AbstractWe propose that a more readily studied, secondary source‐to‐sink (S2S) systems can be formed on direct‐fed margins, in which shelf‐edge deltas are ‘sources’ and deep‐water fans are terminal depositional ‘sinks’, with channels working as delivery ‘conduits’ in between. DionisosFlow stratigraphic‐forward model, coupled to seismic and borehole data from middle Miocene Pearl River margin, are used to explore physical and conceptual linkages of delta‐to‐fan S2S systems, with a focus on the predictability of when and how coarse clastics are delivered from the deltas down to the submarine fans. Middle Miocene Pearl River delta‐to‐fan S2S coupling was stratigraphically enacted in three main ways: (a) deltas that lack downdip fans: high sea level or low sediment supply caused coarse clastics to be stored mainly on inner to outer shelf areas; (b) deltas that are linked downdip to fans: coarse clastics were funneled to submarine fans through slope channels, via direct delta‐to‐fan S2S linkages created by delta overreach at shelf break or channels extending back to shelf‐margin prodeltas; (c) fans that lack updip, coeval deltas: coarse shelf clastics were carried laterally by longshore or other shelf currents, but eventually captured by canyon heads, and then delivered directly to the basin floor. Moreover, our DionisosFlow stratigraphic‐forward models suggest that an oscillation in sea‐level behaviour from slowly falling to rapidly falling would result in a within‐system tract surface occurring within the falling‐stage systems tract. This surface is identified as a significant lower‐order unconformity in its proximal reaches and as a correlative conformity distally. Within‐system tract surfaces are identified by a change in shelf‐edge trajectory regimes from flat to slight falling to moderately falling and in architecture from mixed progradation and degradation to dominant degradation. They are coeval with the onset of the deposition of submarine fans linked updip to deltas or lacking updip deltas, highlighting that sandy deposits can be compartmentalized even within a single systems tract.

Inferring the Shallow Layered Structure at the Chang’E‐4 Landing Site: A Novel Interpretation Approach Using Lunar Penetrating Radar

AbstractThe current paper investigates the shallow layers of the lunar regolith at the Chang’E‐4 landing site. Four layers between 0 and 10 m were identified using lunar penetrating radar. Based on these outputs, a revised stratigraphic model is suggested for the post‐Imbrian ejecta at the Von Kármán crater. The layers were previously unseen due to the smooth boundaries between them. The revised model was inferred using an advanced hyperbola‐fitting scheme. Applying conventional hyperbola‐fitting to non‐homogeneous media results in errors and inaccuracies that are often wrongly assumed to be negligible. We propose a novel hyperbola‐fitting scheme that is, not constrained to homogeneous media and can be applied subject to any arbitrary one‐dimensional permittivity distribution. Via this approach, we can estimate the permittivity profile of an investigated area and detect layered structures that were previously transparent to electromagnetic waves due to the gradational dielectric properties at their interfaces.

Sedapp v2021: a nonlinear diffusion-based forward stratigraphic model for shallow marine environments

Abstract. The formation of stratigraphy in shallow marine environments has long been an important topic within the geologic community. Although many advances have been made in the field of forward stratigraphic modeling (FSM), there are still some areas that can be improved in the existing models. In this work, the authors present our recent development and application of Sedapp, which is a new nonlinear open-source R code for FSM. This code uses an integrated depth–distance related function as the expression of the transport coefficient to underpin the FSM with more alongshore details. In addition to conventional parameters, a negative-feedback sediment supply rate and a differentiated deposition–erosion ratio were also introduced. All parameters were implemented in a nonlinear manner. Sedapp is a 2DH tool that is also capable of running 1DH scenarios. Two simplified case studies were conducted. The results showed that Sedapp not only assists in geologic interpretation but is also an efficient tool for internal architecture predictions.

Linking the Surface and Subsurface in River Deltas—Part 2: Relating Subsurface Geometry to Groundwater Flow Behavior

AbstractUnderstanding subsurface structure and groundwater flow in deltaic aquifers is essential for evaluating the vulnerability of groundwater resources in delta systems. Deltaic aquifers contain coarse‐grained paleochannels that preserve a record of former surface river channels as well as fine‐grained floodplain deposits. The distribution of these deposits and how they are interconnected control groundwater flow and contaminant transport. In this work, we link depositional environments of deltaic aquifers to stratigraphic (static) and flow and transport (dynamic) connectivity metrics. Numerical models of deltaic stratigraphy were generated using a reduced‐complexity numerical model (DeltaRCM) with different input sand fractions (ISF) and rates of sea‐level rise (SLR). The groundwater flow and advective transport behavior of these deltas were simulated using MODFLOW and MODPATH. By comparing the static and dynamic metrics calculated from these numerical models, we show that groundwater behavior can be predicted by particular aspects of the subsurface architecture, and that horizontal and vertical connectivity display different characteristics. We also evaluate relationships between connectivity metrics and two environmental controls on delta evolution: ISF and SLR rate. The results show that geologic setting strongly influences both static and dynamic connectivity in different directions. These results provide insights into quantitatively differentiated subsurface hydraulic behavior between deltas formed under different external forcing (ISF and SLR rate) and they are a potential link in using information from delta surface networks and depositional history to predict vulnerability to aquifer contamination.

A sequential record of the Llandovery δ13Ccarb excursions paired with time-specific facies: Anticosti Island, eastern Canada

The Silurian climate was volatile and punctuated by multiple short-lived climate events, each associated with biotic crisis and perturbations to the global carbon cycle. In total, seven positive carbon isotopic excursions are globally recognized in the Silurian with the large excursions of the Wenlock-Ludlow interval in the Baltic succession garnering the most research; these excursions are recorded in association with cyclic facies changes and the enigmatic resurgence of microbial carbonates. Climate models, hypothesizing cycles of alternating humid and arid climate states, have been developed to explain these paired lithologic-isotopic fluctuations; however, little work has been done to test these models on lower Silurian strata. To test for the presence of δ13Ccarb excursions and the stratigraphic motif of proposed climate events, we developed a high-resolution integrated stratigraphic framework for the upper Hirnantian to mid-Telychian (Upper Ordovician to Llandovery) carbonate-dominated succession of Anticosti Island, by examining ~450 m of strata from a recent drill core, supplemented by ~120 m of outcrop. Four facies assemblages and three time-specific facies are identified and organized into three orders of superimposed transgressive-regressive cycles. High-resolution isotopic curves reveal the presence of four global positive carbon isotope excursions all sequentially recorded in the study interval; the HICE (peak of +5‰), Early Aeronian (peak of +2‰), Late Aeronian (peak of +6‰), and Valgu (peak of +3.5‰) excursions. Common cyclic stratigraphic trends are recognized with these δ13Ccarb excursions, where each excursion is associated with a shift from shale-rich to shale-poor facies, an intermediate-order transgression, and a minor δ18O excursion. These repetitive lithologic-isotopic patterns in the Llandovery succession of Anticosti Island are linked to cyclic fluctuations between low-latitude humid and arid climate states. In addition to these patterns, microbial-rich carbonates are associated with the Late Aeronian and Valgu δ13Ccarb excursions on Anticosti Island; this suggests a genetic link between the elevated δ13C values and the proliferation of calcified cyanobacteria and associated microfossils. This integrated stratigraphic model linking carbon excursions with climate and carbonate factory shifts provides an excellent framework for future studies, using a wide variety of geochemical systems, to better understand the processes that drove environmental and biodiversity changes through the Llandovery.

How is stratigraphic modeling of frontier basins dependent on data: A case study of the Shelburne sub-basin, offshore SE Canada

Frontier basins are difficult to model with forward stratigraphic modeling software, due to the limited amount of geological information available from sparse wells and limited seismic coverage. Five different simulations with varying amounts of geological information are compared to a geologically constrained reference case model using all available data for the Shelburne sub-basin, in order to understand the relative importance of various geologically constrained parameters to modeling frontier basins. The modeling has been performed with DionisosFlow™, a 4D multilithology diffusion-based deterministic forward stratigraphic modeling software. All five less constrained models resemble the major features of the reference case model, with a similar distribution of sand on the shelf and localized progradation of sand across a delta into deep-water. In detail, there are significant deviations in sedimentation patterns. Not all geologically constrained parameters are equally important to modeling; the most influential parameters are paleobathymetry and sediment thickness defined by seismic surfaces. In this case study, the model could not be calibrated unless three sediment input points were chosen, as demonstrated independently from detrital petrology studies. Predefining certain modeling parameters generally reduces computational time. Unconstrained models offer a first overview of the probable architecture of the basin, and are a useful tool which can reduce the overall computation time of the modeling process and give insight into sedimentation patterns.

Lower Visean sea-level changes in the northern Precaspian Basin

This study presents the sequence stratigraphic interpretation of subsurface data in a Lower Carboniferous lagoon setting in the northern boundary zone of the Precaspian Basin. The proposed stratigraphic and facies distribution model of the lagoon fill enables the reconstruction of the relative sea-level history and refines the understanding of the geological evolution and eustatic changes in the Caspi region during the Lower Carboniferous. Semi-circular carbonate mounds were built in the Devonian period and Tournaisian stage enclosing a lagoon with an opening towards the Caspi Sea in a southeast direction. A sea-level fall at the end of the Tournaisian stage terminated the growth of the carbonate buildups; during the Lower Visean (Lower Carboniferous) time, the lagoon was filled with mixed siliciclastic and carbonate sediments. The lagoon fill is interpreted as a third-order composite sequence composed of three fourth-order sequences representing sea-level changes with an increased magnitude and frequency relative to the quiet conditions that prevailed during the previous carbonate buildup in Famennian-Tournaisian. The highstand systems tracts of the fourth-order sequences are likely Waulsortian-type carbonate banks, while the falling-stage and lowstand systems tracts are mostly clastic progradational wedges. The transgressive systems tracts include retrograding shale and grain shoal facies. A prominent falling-stage systems tract was detected in the third fourth-order sequence in the lagoon fill representing a significant sea-level fall in lower Visean time. The falling-stage systems tract is composed of three downstepping-prograding lobes indicating a continuously falling sea level punctuated by minor pauses. Seismic imaging revealed a series of distinct prograding wedges inside the falling stage lobes that are either accretionary bedset or high-frequency nested sequences. The observed fall in sea level during the Lower Visean may be associated with, and it may provide stratigraphic evidence for the beginning of Paleozoic glaciation on Gondwana.

Mimicking the process of manual sequence stratigraphy well correlation

Stratigraphic correlation of well logs is based on interactive interpreter-based pattern recognition. A skilled interpreter identifies similar patterns (such as upward fining and coarsening) in user-defined well sections and links them using either a conscious or subconscious stratigraphic model. This manual stratigraphic correlation of numerous wells in mature fields can be time-consuming and error-prone. To expedite the process of stratigraphic correlation, we have performed semiautomatic stratigraphic correlation of wireline logs from multiple wells using improved dynamic time warping (IDTW). IDTW uses semblance, which compares the shape of the well logs, to replace the Euclidean distance in pairwise error computation. The resulting error matrix is compatible with the lateral nonstationary variation of well logs in the same formation. The workflow begins with interpreting stratigraphic well tops on user-defined well sections that is similar to the current process of stratigraphy analysis. The interpreted wells are then treated as reference wells to aid in interpreting well tops for other wells. Necessary manual interventions are incorporated during the semiautomatic stratigraphic correlation process. We applied our method to two experimental fields: a sand-rich reservoir and a mud-rich reservoir. The applications illustrate that our method performs well in aggradational strata and successfully predicts discontinuities with manual interventions.