Stratigraphic Interpretation Research Articles

Litho- and biostratigraphy of the early Eocenelarger benthic foraminifera-dominated carbonates of the central Tethys domain, Zagros Foreland Basin, SW Iran

The Early Paleogene Tethyan domain saw the development of large platform carbonates rich in large benthic foraminifera (LBF), of which the Taleh Zang Formation in Lurestan Basin, SW Iran, represents an excellent example. The LBF assemblage is dominated by alveolinids, together with a variety of foraminiferal genera such as Nummulites, Rotalia, Sackesaria, Assilina, Operculina, and Glomalveolina. Smaller benthic foraminifera, calcareous algae, gastropods, echinoids, and bivalves constitute minor components of the carbonate. Biostratigraphy analysis of the LBF is conducted on two distinct sections of the Taleh Zang Fm. The sections referred to as TZ-1 and TZ-2 have respective thicknesses of 87 m and 132 m. This analysis has constrained the formation age to the early Ypresian within Shallow Benthic Zones (SBZs) 5–7. Facies analysis supports the major accumulation of these successions in a shallow water environment within the euphotic zone. We recognize 9 distinct facies and microfacies representing four main depositional environments including tidal flat, lagoon, bioclastic shoal, and shallow open marine. These environments are distributed along a low-angle homoclinal ramp and form the foundation of a stratigraphic interpretation characterized by intervals displaying shallowing and deepening trends. Three shallowing-upward intervals have been identified within the Taleh Zang succession. The determined intervals cannot be solely attributed to global sea level fluctuations and support a significant impact of the regional structural context resulting from collision-induced uplift and variable subsidence rates. The identified intervals exhibit local and regional correlation and compatibility on the Arabian Plate, providing robust confirmation of the significant roles played by both tectonics and eustasy at a regional scale.

Alluvial development during climate fluctuations depicted with spectral decomposition of the Middle Triassic to Lower Jurassic post‐rift succession in the Horda Platform (Norway) and the impact on reservoir properties

AbstractFor the first time, this study presents interpretations of alluvial fans in spectral decomposition RGB Blends, analysed in seismic time slices from the Middle Triassic to Lower Jurassic stratigraphical interval of the Horda Platform (northern North Sea). The time slices record a shifting alluvial fan front, fluvial variability, uplift and erosion and reveals depositional elements that may set the common conception of the geological development of this area during the Early–Middle Mesozoic up for discussion. Results show that the Upper Triassic Lunde Formation in the eastern margin of the Horda Platform was characterized by the deposition of coalesced alluvial fans. A variable extent of the fan front through the Upper Triassic is linked to interplaying allogenic factors: uplifted source areas in the aftermath of Early–Middle Triassic rifting determined sediment availability; climate transitioning from arid to semi‐humid, with increasingly fluctuating precipitation, controlled sedimentation, and run‐off; provenance dictated bulk sedimentology and affected prevailing alluvial processes. An overall retreat of the fan system through the Late Triassic coincided with a significant change in landscape characteristics at the transition into the overlying Statfjord Group. Uplift and initial tilting of the Horda Platform caused landscape degradation and the formation of plateaus and incised valleys, contemporaneous with increased humidity and marine transgression, forming estuaries. The shift in depositional style has implications for reservoir properties, creating complexity and heterogeneity in terms of facies distribution and connectivity, which may benefit potential CO2 storage. Upper Triassic alluvial fan development in the Horda Platform (northern North Sea) depicted through spectral decomposition of seismic time slices Enhanced level of seismic stratigraphic interpretation with RGB blending Impact of allogenic factors on alluvial depositional development Implications of alluvial variability on reservoir properties

Using seismic and well data to constrain hydrocarbon trap potential of offshore Lamu Basin, Kenya

Structural analysis of seismic data, combined with well data, provides information for hydrocarbon exploration in a prospective basin. These techniques have been employed in the offshore Lamu Basin to characterize structural and stratigraphic setting of carbonate reefs and assess structural deformation and its effect on trapping mechanisms in the offshore basin. Structural interpretation was based on seismic and well data. A combination of microfossil, nannofossil and palynomorph data provided broad age-control for the stratigraphic interpretations. Seismic amplitude brightening attribute was used for tracing structural continuity. Faulted anticlinal traps were formed in the Upper Cretaceous and Tertiary periods. There are inversion features with recent extensional faulting. Favourable structural traps are in the near coastal region. The carbonate reefs in the offshore basin are interpreted from the Early Paleocene to Miocene periods. These reefs formed in the near coastal region, are likely to be quality traps/reservoirs for hydrocarbon accumulation. Five main structures are identified from the depth seismic maps for the Early Paleocene, Late Paleocene and Early Miocene. The reef front may have subsided early, the overlying shales offering the sealing mechanisms. The Davie-Walu transformation was likely to affect the trapping mechanism in the central and southern regions of offshore Lamu Basin. The study provides better understanding of the carbonate reefs’ hydrocarbon potential and trapping mechanisms in the offshore basin.

ClinoformNet-1.0: stratigraphic forward modeling and deep learning for seismic clinoform delineation

Abstract. Deep learning has been widely used for various kinds of data-mining tasks but not much for seismic stratigraphic interpretation due to the lack of labeled training datasets. We present a workflow to automatically generate numerous synthetic training datasets and take the seismic clinoform delineation as an example to demonstrate the effectiveness of using the synthetic datasets for training. In this workflow, we first perform stochastic stratigraphic forward modeling to generate numerous stratigraphic models of clinoform layers and corresponding porosity properties by randomly but properly choosing initial topographies, sea level curves, and thermal subsidence curves. We then convert the simulated stratigraphic models into impedance models by using the velocity–porosity relationship. We further simulate synthetic seismic data by convolving reflectivity models (converted from impedance models) with Ricker wavelets (with various peak frequencies) and adding real noise extracted from field seismic data. In this way, we automatically generate a total of 3000 diverse synthetic seismic datasets and the corresponding stratigraphic labels such as relative geologic time models and facies of clinoforms, which are all made publicly available. We use these synthetic datasets to train a modified encoder–decoder deep neural network for clinoform delineation in seismic data. Within the network, we apply a preconditioning process of structure-oriented smoothing to the feature maps of the decoder neural layers, which is helpful to avoid generating holes or outliers in the final output of clinoform delineation. Multiple 2D and 3D synthetic and field examples demonstrate that the network, trained with only synthetic datasets, works well to delineate clinoforms in seismic data with high accuracy and efficiency. Our workflow can be easily extended for other seismic stratigraphic interpretation tasks such as sequence boundary identification, synchronous horizon extraction, and shoreline trajectory identification.

Chapter 12. Well sequence stratigraphy interpretation methodology

Abstract This chapter describes the methodology employed to recognize depositional sequences in well data, involving the integration of wireline logs, biostratigraphy, lithostratigraphy (and lithological interpretations), seismic and sedimentology (particularly facies analysis), with an emphasis on a pragmatic approach to sequence recognition in North Sea Jurassic–lowermost Cretaceous well successions. Wireline log profiles characterizing individual depositional sequences or parts of sequences are illustrated by reference to key North Sea well sections.

Integrating seismic attributes and rock physics for delineating Pliocene reservoir in Disouq field, Nile Delta, Egypt

The Nile Delta is known as the most prolific gas province in North Africa. Gas exploration and production were the main targets over decades in the Nile Delta. The integration of petrophysical analysis, seismic interpretation, and seismic attributes could indicate the presence of gas in Disouq field, which is located in Nile Delta, Egypt. Well log data showed that the reservoir is classified into three zones (A, B, and C). These zones are characterized by low gamma ray (around 35%), with relatively high porosity (around 29%) and low water saturation (36–38%). Structural and stratigraphic interpretation was conducted in order to investigate the extension and geometry of the reservoir, in addition to the faults affecting the study area. The reservoir is represented by a channel of Pliocene age, characterized by the low impedance sand saturated by gas. Near and far offset data show the gas effect. There is an increase of the negative amplitude from near to far offset. The amplitude map and structural contours match well and show good conformance. Seismic attributes including surface attributes and “Red Green Blue/colour blending” help in delineating the extent and geometry of the reservoir, in addition to the possible prospect. Finally, the volume of original gas in place was estimated what showed that the study area is of great economic interest.

Caspian transgressive-regressive cycles across the Lower Volga region during the Quaternary reconstructed from the borehole at Ulan-Khol (Kalmykia, Russia)

During the Quaternary, the Lower Volga region was typically a shallow shelf of the Caspian Sea. This region regularly and distinctively responded to sea level fluctuations by changing both sediments and biota. This region is therefore a key to understanding the history of the transgressive-regressive cycles of the Caspian Sea during the Quaternary. There is a wealth of information about the Quaternary deposits of the Lower Volga region; however, their diversity of facies, erosional processes, and fragmented sections create difficulties in stratigraphic interpretation and the development of paleogeographic models. The studied 500-m section of borehole 8 Ulan-Khol is among the few in the Caspian region that allows for the sequential reconstruction of the Quaternary history of the region in one section. These borehole data offer a unique opportunity to reconstruct transgressive-regressive cycles in Pleistocene basins located in the most southeastern part of Europe (the Caspian region). Based on biostratigraphic, sedimentological, and paleomagnetic methods, we were able to define main Early, Middle, and Late Pleistocene units, which were correlated with regional stratigraphical units. Each unit corresponds to regional paleogeographic events and sedimentation modes characterized either by basinal (brackish water) or continental conditions when lagoons and fluvial landscapes dominated. The borehole interpretation integrated with regional data demonstrates a complex history of Caspian basin development during Apsheronian time (MIS 63–20) with the presence of a strong regression in its final stage, accompanied by a previously unknown advance of the Paleo-Volga valley to the latitude of Astrakhan city and further south. This regression and the formation of the Paleo-Volga valley continued during Tyurkyanian time (MIS 19). By the end of the Tyurkyanian, the regression was followed by a transgressive stage, which culminated in Bakunian time (MIS 18–12). The Khazarian stage (MIS 11–5) of Caspian Sea development was characterized by a constant change from alluvial-marine to marine and estuarine environments. The borehole data support our earlier conclusion that the Singilian deposits separate the lower (MIS 11–7) and upper (MIS 5) Khazarian units. New data support the existence of only one transgression of the Caspian Sea during Khvalynian time (MIS 3–2); after its maximum stage the sea level gradually decreased with the formation of coastlines at several terrace levels. Due to its high deposit thickness and rich paleontological, stratigraphic and lithological information, Ulan-Khol borehole 8 can be considered a key site for the Northern Caspian area.

A facies- and sequence stratigraphy-based reservoir model for a carbonate reef complex: South Swan Hills oil pool, Alberta

We present a reservoir model for the South Swan Hills oilfield, Alberta, Canada, developed in a Devonian atoll reef, based on facies analysis and sequence stratigraphy. The analysis illustrates the complex reservoir geometries in such a reef, caused by facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale sequence boundaries. The model is built on lithofacies analysis from high-resolution core descriptions that recorded lithologies, bioclast type and characteristics, and sedimentary structures. Core descriptions were correlated to well logs, enabling lithofacies to be mapped across the field. Lithofacies distributions permit definition of three distinct reef environments: a reef interior, a reef margin and a terminal shoal. Vertical variability is characterized using sequence stratigraphic interpretations. Two 3rd order and four 4th order sequence boundaries were identified and correlated across the reef. The analysis illustrates the complex reef geometries linked to facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale unconformities. These and the intervening deposits represent the response to fluctuating sea level.The geological analysis was transformed into a 3-D geocellular model using a sequential indicator simulator. The simulator models lithofacies and, in turn, porosity and permeability distributions based on strong relationships between lithofacies and petrophysical properties for the three representative regions; the interior, margin and terminal shoal. The interior deposits formed forms the bulk of non-reservoir material in the centre of the reef build-up. The margin surrounded surrounds the interior and formed forms a continuous 1–2 km band of reservoir around it. Reservoir thickness is 20–30 m and primarily composed of reef flat deposits. The terminal shoal forms a reservoir body that caps the reef complex, but is disconnected from the bulk of the underlying reef by a thick layer of impermeable clay associated with a sequence boundary. The high-resolution realization displays the geometry and continuity of the reservoir and exposes demonstrates the compartmentalized and variable producibility of the atoll.

Possible geological interpretation of the volcanic seismic facies based on volcanostratigraphy elements: A case analysis of the Yingcheng Formation in the Changling Fault Depression, Songliao Basin, NE China

Volcanic rocks in faulted depressions are one of the most interested areas for hydrocarbon exploration. In a frontier basin exploration, integrated interpretation of the seismic facies of volcanic rocks is a key success. The key information of boundary system and deposited unit in the volcanostratigraphy did not attract sufficient attention during the process of seismic stratigraphic interpretation of volcanic rocks. In this paper, the volcanic rocks of the Yingcheng Formation in the Changling Fault Depression of the Songliao Basin are taken as an example. According to the maximum average slope, the volcanic seismic facies were divided into three types: mounded seismic facies unit (MSFU), tabular seismic facies unit (TSFU), and mounded-tabular seismic facies unit (M-TSFU). MSFU consists of lava dome and simple lava flows, which has fewer reservoir layers, medium to poor reservoir quality with limited extension. The TSFU comprises more complex component of simple lava flow and base surges, and contains more layers, with good-medium reservoir quality and a large reservoir extension. The M-TSFU has numerous reservoir layers with good reservoir quality and a significant reservoir extension, as well as a higher proportion of braided lava flows and pyroclastic flows. Going from the most favorable reservoir targets to the least favorable, the following units were deposited in the Changling Fault Depression: pyroclastic flow, braided lava flow, base surge, simple lava flow, lava dome, and reworked volcanic clastic flow. The study suggested that mounded-tabular facies units are potential favorable exploration targets in the Songliao Basin. This paper provides a technical route for seismic facies interpretation of volcanic rocks and the analysis of favorable volcanic exploration targets in faulted volcanic basins.

Vendian of the eastern flank of the Avdyrdak anticline of the Alatau anticlinorium (Southern Urals)

Object of study. Deposits of the Asha series of the Vendian of the eastern flank of the Avdyrdak anticline, divided by a meridional fault into two monoclinal blocks, in which researchers give different stratigraphic interpretations of the sections. Results. A study was made of the deposits of the Asha series of the eastern flank of the Avdyrdak anticline along the highway (80K-031) Ufa–Beloretsk, where these deposits are most fully exposed by road cuttings and quarries. Detailed descriptions and sketches of fragments of the sediment section are given, the lithological and petrographic composition of the rocks, their ratio and thickness in the sections of the western and eastern tectonic blocks are specified. Conclusions. It has been established that according to the lithologic-petrographic composition of the rocks and the structure of the sections of the Asha series of the Vendian of the eastern flank of the Avdyrdak anticline along the highway (80K-031) Ufa-Beloretsk, the western and eastern tectonic blocks of the structure are composed of deposits of the Basa, Kukkarauk and Zigan formations. The stratigraphic similarity of the sections of the western and eastern tectonic blocks of the eastern flank of the Avdyrdak anticline indicates the thrust character of tectonic movements.

Untrackable distal ejecta on planetary surfaces

Impact ejecta are important references to establish regional and global stratigraphy of planetary bodies. Canonical views advocate radial distributions of distal ejecta with respect to the source crater, and their trajectories are significantly deflected on fast-rotating bodies. The Hokusai crater on Mercury formed a peculiar ray that features a hyperbola shape, and the sharp swerve of orientation was interpreted as a sign of a faster planetary rotation in the near past. Here, we show that this ray was not caused by a hypothesized larger Coriolis force, but due to abruptly-steepened ejection angles. Heterogeneous shock impedances of pre-impact impactor and/or target, such as topographic undulations, affect local propagation paths of shock and rarefaction waves, causing sudden changes of ejection angles. Distal ejecta with non-radial distributions are an inherent product of planetary impacts, and their unobvious provenances could mislead stratigraphic interpretations and hamper age estimations based on spatial densities of impact craters.

Biostratigraphy of the Numidian Formation and its underlying Tellian unit, based on calcareous nannofossils in Northern Tunisia: Implication on their stratigraphic vs. tectonic relationships

A detailed biostratigraphic study, based on calcareous nannofossils, was performed to re-evaluate the stratigraphic interpretation of the Numidian Formation and its structural relationship with the underlying El Haria, Boudabbous and Souar formations Tellian units (i.e., stratigraphic vs. tectonic). The new biostratigraphic results indicate that the Numidian Formation is not restricted to a Miocene age as considered in some recent works but spans from Oligocene (Rupelian and Chattian, NP23 to NP25) to early Miocene (Aquitanian and Burdigalian, NN1 to NN3). The Cap Serrat succession can be retained as the type section for the Tunisian Numidian Formation, where both the Chattian-Aquitanian (Oligocene/Miocene boundary) and the Aquitanian-Burdigalian transitions have been characterized.Based on the new calcareous nannofossil findings, several duplications of the sedimentary succession were identified. They include the thick shales and marls of El Haria (Maastrichtian-Paleocene) and Souar (Eocene) formations, thrusted by the Numidian Formation. The updated regional structural cross-sections, confirm that the Numidian Formation is currently juxtaposed by southward major thrust contact to the underlying Kasseb structural unit in the studied area. The latter is intensively affected by poly-phased tectonics, resulting in the thrust events associated with the Alpine phase inversion. This has to be taken into consideration in oil and gas exploration in the area of the fold-thrust belt system, in both onshore and offshore Northern Tunisia.

Analogue modelling of a Tabberabberan fold-thrust belt in the Cobar region and implications for the origin of Cobar-type mineral deposits

The Cobar Supergroup is a package of Siluro-Devonian sedimentary, volcanic and volcaniclastic rocks in central New South Wales that underwent significant deformation during the Middle Devonian Tabberabberan Orogeny. The Cobar Supergroup, particularly within the Cobar Basin, constitutes one of the most mineral-rich rock packages in New South Wales, being endowed with Au, Cu, Pb, Zn and Ag, mainly in what are commonly referred to as Cobar-type deposits. Inconsistencies in the traditional stratigraphic interpretation of the area have led to a re-evaluation of its structural history, in particular the role of the Rookery Fault, which bounds the eastern side of the basin. Evidence suggests that that structure may dip to the east, rather than the west, as is traditionally assumed, leading to the possibility that Tabberabberan deformation in the area took the form of a west-verging fold-thrust belt. Analogue modelling is employed to examine the consequences of that scenario. Such a mechanism explains the known distribution of stratigraphic units of the Cobar Supergroup and its basement, as well as observed structures, in particular a high strain zone immediately west of the Rookery Fault. The modelling indicates that deeper parts of the crust were thrust up along the Rookery Fault, which, along with substantial burial, led to perturbation of the geotherm and heating of the footwall. This accounts for the higher metamorphic grade immediately west of the Rookery Fault. Furthermore, heating and overpressuring of basement rocks in the footwall led to mobilisation of metal-bearing fluids and transport of them along the high strain zone and subsequent deposition of those metals within favourable structural traps, with variable interaction with basinal fluids, to form Cobar-type deposits. A fold-thrust mechanism with ongoing erosion of the thrust wedge can also explain the distribution of Middle to Upper Devonian rock sequences in the region. KEY POINTS A west-verging fold-thrust model is proposed to explain Tabberabberan deformation of the Cobar Supergroup and its basement. Analogue modelling is employed to demonstrate the fold-thrust model. A fold-thrust model explains the distribution of Ordovician to Upper Devonian stratigraphic units in the Cobar region, as well as associated structures, metamorphic grade distribution and the origin of syn-deformational Au, Cu, Pb, Zn, Ag mineral deposits.

Formation and reorganization time scales of aeolian landscapes

AbstractLandscapes created through sediment transport are shaped by the interaction of flow and form. In landscapes where wind is the agent of geomorphic work, this is clear at the small scale; equilibrium dune morphology is linked to the wind climate and the supply of sediment. At larger scales, this linkage becomes ambiguous because the form of giant dunes and dune fields integrates long histories of varied wind and sand supply. Without a framework to assess aeolian landscape evolution at this scale, the time taken to form and reorganize dune fields has been largely unexplored quantitatively. We show that these time scales can be understood by linking modern wind and topographic data sets for one of the most expansive and morphologically diverse unvegetated dune fields, the Rub’ al Khali (southern Arabian Peninsula). By linking sediment flux to the surface area and slope of dunes, and growth to the divergence in that flux, we fully couple form and flow at the dune field scale. Our results show quantitatively how dune field formation and reorganization are outpaced by climate change and the implications for stratigraphic interpretation.

Stratigraphy of the Albian–Cenomanian (Cretaceous) phosphorite interval in central Poland: a reappraisal

Several closely-spaced phosphorite beds stand out at the Albian–Cenomanian transition in the mid-Cretaceous transgressive succession at the northeastern margin of the Holy Cross Mountains, central Poland. They form a distinctive condensed interval of considerable stratigraphical, palaeontological, and economic value. Here, we correlate the classical section at Annopol with a recently investigated section at Chałupki. We propose a new stratigraphic interpretation of the phosphorite interval, based on lithological correlations, Rare Earth Elements and Yttrium (REE+Y) signatures of phosphorites, age-diagnostic macrofossils, and sequence stratigraphic patterns. This interval has long been considered as exclusively Albian in age. However, new macrofossil data allow us to assign the higher phosphorite levels at Annopol and Chałupki, which were the primary target for the phosphate mining, to the lower Cenomanian. In terms of sequence stratigraphy, the phosphorite interval encompasses the depositional sequence DS Al 8 and the Lowstand System Tract of the successive DS Al/Ce 1 sequence. The proposed correlation suggests that lowstand reworking during the Albian–Cenomanian boundary interval played an important role in concentrating the phosphatic clasts and nodules to exploitable stratiform accumulations. Our conclusions are pertinent to regional studies, assessments of natural resources (in view of the recent interest in REE content of the phosphorites), and dating of the fossil assemblages preserved in the phosphorite interval. On a broader scale, they add to our understanding of the formation of stratiform phosphorite deposits.

The oldest Andean synorogenic fossiliferous deposits of the Miocene Cerro Azul Formation, central Argentina

Cerro Patagua is a locality with fossiliferous outcrops of the Cerro Azul Formation in La Pampa Province (central Argentina). First fossil remains were known in 1999 and assigned to the Chasicoan Stage/Age (Late Miocene, Tortonian) after the presence of the Octodontoidea rodent Chasichimys bonaerense. Later references to vertebrate remains were scarce and mostly limited to some mentioned taxa in broader faunal contexts. Now, we present a detailed taxonomic study of the entire mammal assemblage from Cerro Patagua, which evidences that most taxa are typical of the Chasicoan Stage/Age. A peculiar enterolith-like structure, recovered from loess levels, is also described, and several taphonomic aspects observed on fossils are related to their level of provenance. In addition, we offer stratigraphical interpretations of the Cerro Azul Formation and a geomorphological analysis of the Cerro Patagua area, and provide a detrital zircon dating of 12.3 ± 1.8 Ma. This result reflects the time interval 14.1–10.5 Ma, a temporal range that includes the Serravalian Stage (late Middle Miocene) and could indicate that the oldest synorogenic deposits could have begun to accumulate at the end of this stage. Even considering the minimum age of this interval (Tortonian Stage), this dating is the oldest obtained for the Cerro Azul Formation, compared with previous published data from Arroyo Chasicó locality (9.43–9.07 Ma). This assignment could explain that taxa from Cerro Patagua show affinities with Late Miocene mammal assemblages from central Argentina, as well as with older faunas from Patagonia and Bolivia.

Productivity prediction in the Wolfcamp A and B using weighted voting ensemble machine learning method

This study investigates the applicability of ensemble machine learning to predict cumulative gas production for the first 36 months of shale gas reservoirs in the Wolfcamp A and B, Delaware basin. The machine learning application was conducted from three notable perspectives: (1) Well log data generation based on stratigraphy interpretation, manual well correlations, and pre-process of category type data to utilize it for machine learning training. (2) General applicability of machine learning using well data without production rate information. (3) Ensemble methods (ESM) with voting weight composed of four individual machine learning methods based on bagging and boosting: random forest (RF), gradient boosting (GB), extreme gradient boosting (XGB), and light gradient boosting machine (LGBM) providing reliable shale gas productivity prediction performance and consistent results from different training and test data compositions. Coefficient of determination (R2) and mean square error (MSE) are notably improved in comparison between the primary level data and proposed data package with well log data (7% increase and 19% decrease, respectively). Furthermore, the ESM with voting weight produces the best predictive performance on average for the ten different training and test data pools among the six cases (RF, GB, XGB, LGBM, and ESM with even/voting weight). In the ESM with voting weight, the average weights of the RF, GB, XGB, and LGBM are 0.259, 0.243, 0.281, and 0.216, respectively. Although the RF is the best predictive model among the four individual prediction results, XGB produces the highest weight. The voting weight system of the ESM lets the four individual machine learning methods complement one another, balancing the final ESM model. Besides, the final ESM model brought around 12% uncertainty reduction MSE-wise compared to the RF model. Therefore, enhanced universality and reliability are obtained for the given learning data, permitting the successful productivity prediction of shale gas reservoirs.

Impacts of Tectonic Subsidence on Basin Depth and Delta Lobe Building

AbstractChannel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan‐shaped deposit. These studies often assume a steady subsidence and uniform basin depth. In nature, however, lobe building is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active areas. Herein, we explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe‐switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel‐scale avulsions occur relatively frequently because of in‐channel sediment aggradation, dispersing sediment to regional lows of the delta. Avulsion settings for the Selenga Delta preserve discrete stratal packages that could contain predominately deep channels. Exploring the interplay between tectonic subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models.

Integration of Sequence Stratigraphic Analysis and 3D Geostatistical Modeling of Pliocene–Pleistocene Delta, F3 Block, Netherlands

AbstractThis research is focused on the analysis of the sequence stratigraphic units of F3 Block, within a wave‐dominated delta of Plio–Pleistocene age. Three wells of F3 block and a 3D seismic data, are utilized in this research. The conventional techniques of 3D seismic interpretation were utilized to mark the 11 surfaces on the seismic section. Integration of seismic sequence stratigraphic interpretation, using well logs, and subsequent 3D geostatistical modeling, using seismic data, aided to evaluate the shallow hydrocarbon traps. The resulting models were obtained using System Tract and Facies models, which were generated by using sequential stimulation method and their variograms made by spherical method, moreover, these models are validated via histograms. The CDF curve generated from upscaling of well logs using geometric method, shows a good relation with less percentage of errors (1 to 2 for Facies and 3 to 4 for System Tract models) between upscaled and raw data that complements the resulted models. These approaches help us to delineate the best possible reservoir, lateral extent of system tracts (LST and/or HST) in the respective surface, and distribution of sand and shale in the delta. The clinoform break points alteration observed on seismic sections, also validates the sequence stratigraphic interpretation. The GR log‐based Facies model and sequence stratigraphy‐based System Tract model of SU‐04‐2 showed the reservoir characteristics, presence of sand bodies and majorly LST, respectively, mainly adjacent to the main fault of the studied area. Moreover, on the seismic section, SU‐04‐2 exhibits the presence of gas pockets at the same location that also complements the generated Facies and System Tract models. The generated models can be utilized for any similar kind of study and for the further research in the F3 block reservoir characterization.

Fluvial and Marine Depositional Environments Determined From Groundwater Chemistry and Calculated Clay‐Cation Compositions

AbstractCation mole precents (XCa, XMg, and XNa) on clay minerals in a siliciclastic coastal aquifer calculated from groundwater compositions, can aid in distinguishing fluvial from marine sediments and locate the fluvial/marine transition zone. The clay‐cation compositions of two clay‐rich siliciclastic coastal aquifers calculated from the composition of the groundwater were compared to the location of fluvial, fluvial‐marine transition and marine environments identified by traditional core, well log and paleontological analyses. The traditionally defined fluvial depositional environment is distinguished by clays with low XNa < 8% (for 92% of the samples) and high XCa from 55% to 98% with the remainder XMg. In comparison, the traditionally defined marine depositional environment is distinguished by clays with higher XNa > 8.0% (for 86% of the samples) and lower XCa ranging from 5% to 39% with the remainder XMg. The traditionally defined fluvial/marine transition zone has values overlapping the fluvial and marine end members, with 59% with XNa < 8% (fluvial) and 41% with XNa > 8% (marine), and XCa ranging from 45% to 98% with the remainder XMg. Calculations support that the excess Na (XNa > 8%) on marine clays can be retained for 10s of millions of years despite flushing with meteoric water, because of the high clay content in marine sediments. The calculated cation composition of clays deposited from river water are XNa < 8%. In contrast XNa 8%–54% is possible for seawater equilibrated clays supporting that the values found in the different depositional zones are possible. Calculated cation compositions might supplement traditional stratigraphic interpretations in distinguishing fluvial/marine depositional environments.