Sequence Stratigraphic Correlation Research Articles

Submarine pyroclastic deposits from 7.3 ka caldera-forming Kikai-Akahoya eruption

Extensive submarine pyroclastic deposits were discovered around the Kikai caldera, Southwest Japan, which originated from the caldera-forming Kikai-Akahoya (K-Ah) eruption that occurred at 7.3 ka. This submarine pyroclastic unit originated from a pyroclastic density current (PDC) that entered the sea and was transformed into a subaqueous density current. Our high-resolution marine seismic reflection surveys reveal that the uppermost unit exhibiting prominent seismic facies thins radially away from the caldera and covers >4500 km2 of the seafloor. The estimated volume of the submarine deposits is >71 km3 based on the isopach map. Seismic sequence stratigraphic analyses and correlation of shard glass compositions from the piston core and remotely operated vehicle samples with those from subaerial outcrops identified the unit as pyroclastic deposits produced by the K-Ah eruption. The geomorphological features of the unit indicate a long runout distance (i.e., 40 km), exponential thinning with increasing distance from the source, and a depression-filling nature of the deposits suggesting that the unit was formed predominantly by a density current that entered the water. The quantitative comparisons of geometric shape and its thinning rate, including that for non-volcanic density currents, indicate that the subaerial PDC entering into the water was transported as a dilute density current in the water. We propose that the formation of these dilute density currents transformed by PDCs probably requires sufficient water depth, as inferred also from previous analog experiments. Adding on-land ignimbrite of 5 km3 and widespread tephra of 249–374 km3, our estimate of submarine pyroclastic deposits of >71 km3 leads to a total bulk volume of the K-Ah eruption of 332–457 km3 (133–183 km3 in DRE), making it probably the largest eruption of the Holocene.

First record of Reticulinella? kaeveri in the Laffan and Ilam formations (Zagros fold and thrust belt, SW Iran): new constraints on the chronostratigraphic calibration of the Arabian Plate sequence stratigraphy

Over the past two decades, sequence stratigraphy has proven to be a valuable tool in addressing challenges arising from the inconsistent application of lithostratigraphic nomenclature across diverse geological provinces. It has been particularly successful in achieving regional correlations within the Cretaceous deposits of the Arabian Plate. The key sequence stratigraphic surface in the currently adopted standard sequence stratigraphic scheme is the K150 SB, marking the transition from a passive to an active margin along the eastern edge of the Arabian plate. Despite its importance, the chronostratigraphic age of this surface is poorly constrained. In the Zagros FTB, the uppermost part of the Sarvak Formation, just below the K150 SB, was recently assigned to the upper part of middle Turonian to upper Turonian, based on the presence of Reticulinella? kaeveri. Our research reveals that the range of R.? kaeveri extends above the K150 SB as this species is also present in the Laffan and base of Ilam formations. The Last Occurrence of this taxon provides a new chronostratigraphic marker of the upper Turonian within the Ilam Formation. This finding proves that more Turonian deposit are present in the study area than was previously recognized. We have studied the deposits enclosing the K150 SB in outcrop sections and wells along a transect from basin to platform margin to inner platform setting. Based on planktonic foraminifera and calcareous nannofossils biostratigraphy of basinal sections and sequence stratigraphic correlation to carbonate platform sections, the age of the uppermost part of the Sarvak Formation, immediately below the K150 SB, is firmly constrained within the interval lower Turonian–lower part of middle Turonian. Sequence stratigraphic correlation from basin to platform interior reveals also that the oldest deposits above the K150 SB are not younger than late Turonian. Our new data represents a significant improvement in the chronostratigraphic calibration of sequence stratigraphy in the Cretaceous of the Zagros fold-and-thrust belt.

EFFICIENT APPROACH TO THE SEQUENCE STRATIGRAPHIC STUDY OF MONOTONOUS HOLOCENE SEDIMENTS FROM THE ARCTIC SHELF

The article faces an approach in which already at sea, based on the results of an express analysis of visually similar cores of Arctic Holocene sediments and their comparison with dated and generally studied in detail cores of region’s sediments, it is possible to determine material that is promising for sequence stratigraphic correlation and paleoreconstructions. Parameters analyzed include lithological, colorimetric (CIE L*, CIE a*, CIE b*), geophysical (magnetic susceptibility – MS), and geochemical (Fe/Rb, Mn/Rb, Ti/Rb). The most informative characteristics are the CIE b* color coordinate, MS, and the Fe/Rb ratio.

Wavelet transform coupled with Fischer plots for sequence stratigraphy: A case study in the Linxing area, Ordos Basin, China

Accurately dividing stratigraphic sequence using traditional data such as core and logging data is challenging due to the insufficient availability of paleontological fossils and rock data, particularly in Carboniferous-Permian strata of the Linxing area of Ordos Basin, dominated by lagoons, barrier islands, and tidal flat sedimentation. This drawback hinders the exploration and exploitation of hydrocarbon reservoirs. To address this issue, this study presents a combination of continuous wavelet transform (CWT) and Fischer plots, enabling the accurate identification of each level of sequence boundary and high-resolution sequence stratigraphic correlation. The optimal scale factors, 120, 60, and 20, are determined by applying different scale factors to gamma ray (GR) curves corresponding to long-term cycle, medium-term cycle, and short-term cycle divisions, respectively. The analysis of sequence characteristics of numerous core segments and the corresponding CWT results (i.e., wavelet coefficient curve and time-frequency energy spectrum) led to the identification of three main types of medium-term cycles (i.e., progradational, regressive, and aggradational) that were then applied to the entire well section of the study area. Moreover, the Fischer plots method was used to identify the long-term cycle based on the determination of medium-term cycles and the division of short-term cycles. This method provided intuitive identification of the relative changes in sea level position. Additionally, the characteristics of sea level changes were consistent with the results of the sea level analysis of rock and logging curves. Therefore, the combination of CWT and Fischer plots can effectively and comprehensively establish an isochronous stratigraphic framework for clastic rock hydrocarbon reservoirs, similar to this study.

Sequence stratigraphic divisions and correlation of the middle sub-member of Eocene Shahejie Formation in the Bonan Sag of Bohai Bay Basin (China): Implication for facies and reservoir heterogeneities

The middle sub-member of the Eocene Shahejie Formation is mainly associated with deltaic environment, and considered as hydrocarbon bearing zone in the southeast of the Bonan Sag, Bohai Bay Basin, China. Previous research and the exploration work in an area were carried out separately, and inconsistency in understanding of stratigraphic division, correlation, and facies was encountered. Furthermore, the facies evolution in the targeted formation is still questionable. To fill these short-comings, sequence stratigraphic correlation and interpretation were conducted on the middle sub-member (Es3z) of the third member (Es3) of Eocene Shahejie Formation in the study area. Integrative facies and paleo-environment analyses are carried out through the correlation study of wireline logs, seismic data and core sedimentology. The sedimentological interpretation reveals that sandstone groups in the Es3z show five different facies associations in both lateral and vertical successions developed in the inner and outer delta front. These depositional facies include deposits in distributary channels, interdistributary channels, mouth bars, natural levees, and sheet sandstones. The Es3z sandstone intervals are characterized by five depositional sequences in terms of the third-order sequence, including Es3z2+3, Es3z4, Es3z5, Es3z6+7, and Es3z8. They are found in all wells consisting of the lowstand, transgressive, and highstand system tracts. These system tracts are separated by the maximum flooding surfaces and correlation boundaries of sandstone groups with unique characteristics of well log motifs, calibrated with the cores by particular lithology. Facies and reservoir heterogeneities are defined within a sequence stratigraphic framework approach for hydrocarbon reservoir prediction. Our results show that sediments were sourced from the southeast toward the northwest. Due to tectonic activity and lake level's fluctuations, reservoir facies are prograding towards the basin side. The steepness of the region impacts the facies and reservoirs evolution together with tectonics and fluctuations in lake level, which turns from the sandy conglomeratic reservoirs into a thick succession of sandstone reservoirs towards the basin side in LST HST and TST system tracts. These reservoir facies are considered as good, and capped by grayish mudstone (inner delta front) to dark gray mudstone (outer delta front) in TST system tracts. The facies architecture of all sandstone groups possesses the hydrocarbons potential in the inner and outer delta front region in which interpreted oil, oil-water layer and water-oil layers show variation in progradation and retrogradation stacking pattern. Among them lower sandstone units of Es3z8 and Es3z6+7 show dominancy in oil rich reservoirs, followed by Es3z2+3, Es3z4 and Es3z5 in LST, HST and TST system tracts. These hydrocarbon units can be correlated laterally with exploration wells for getting new well sites in third order sequence stratigraphic framework. Our findings shed new light on the facies and reservoir heterogeneities, which provide a useful aid for hydrocarbon exploration in the study area.

An exceptional sedimentary record of initial rifting on the East African Plateau in the Miocene: Lessons from depositional cyclicity and palaeoenvironmental proxies

AbstractKnowledge about the initial tectonic and depositional dynamics, as well as the influence of early rifting on climate and environmental evolution remains speculative to a large extent, because sediments are usually deeply buried. Within the East African Rift System, inversion tectonics uplifted a few of these successions to the surface hence presenting rare windows into the pre‐rift depositional history. One such example, an exceptional 700 m long and up to 60 m high fresh road cut provided the opportunity to study in detail initial rift successions of the southern Albertine Rift (Western Uganda). This focusses on the basal and poorly known Middle to Late Miocene in order to unravel the climatic, environmental, hydrological and tectonic evolution of the initial Albertine Rift. A large and robust multi‐proxy dataset was gathered comprising 169 m of stratigraphic thickness, which spans from 14.5 to 4.9 Ma according to a revised lithostratigraphic model. Fieldwork comprised logging of the sedimentary record, spectral gamma ray, magnetic susceptibility and 2D wall mapping with photomosaics. Additionally, the sections were sampled for bulk mineral and clay mineral analysis. The succession exposes a suite of lithofacies and architectural elements detailing the evolution of a fluvio‐lacustrine system. Five depositional environments were identified which show an overall back‐stepping trend from an alluvial plain to a delta plain and finally palustrine/shallow lacustrine conditions. Mesoscale base‐level cycles, preservation potential of architectural elements, and stacking pattern exhibit limited accommodation space. However, it increases over time. This overall trend indicates increasing tectonic subsidence, which can be explained by flexural downwarp within the pre‐rift phase and in the upper part grading into fault‐controlled crustal extension of the syn‐rift phase, which more and more disrupted a large‐scale river system. From the Middle Miocene up to the early Pliocene, this study revealed that palaeoclimate trends become marked by increasing and more fluctuating Th concentrations, loss of feldspar, intercalated lenses of hydroxosulphate minerals, and a shift from smectite‐dominated to kaolinite‐dominated clays. These signals are all interpreted as detrital except for the hydroxosulphates, and they mirror the increasing intensity of chemical weathering and stripping of soils in the catchment. A trend towards increasing humidity is supported by an increase in lacustrine sediment facies and a lake‐level rise. Nevertheless, intercalation of hydroxosulphate, ferricretes and pedogenised horizons prove ongoing seasonality and dry intervals. Finally, based on a revised stratigraphic model a sequence stratigraphic correlation of the outcrop's depositional cycles with basin‐scale cycles is presented. According to these cycles, transition from the pre‐rift to the syn‐rift stage is marked by an unconformity and a tectonic pulse in the latest Miocene. However, the response of fluvial supply, the depositional system as well as climate conditions are less punctuated and characterised by gradual trends and temporal delays. The long pre‐rift phase (ca 10 Myr) and the gradual transition to the syn‐rift phase is in accordance with the active rifting model, which is based on thermal thinning of the lithosphere by asthenospheric upwelling.

Lower Cretaceous (Hauterivian–Aptian) pelagic carbonates in the Danish Basin: new data from the Vinding-1 well, central Jylland, Denmark.

Understanding of the shallow shelf system in the Danish Basin during the Early Cretaceous has benefitted significantly from studying the previously overlooked Hauterivian–Aptian section of the Vedsted Formation of the Vinding-1 drill core. The presence of chalks in this section demonstrates that carbonate-rich pelagic sediment accumulated locally in the siliciclastic-dominated Danish Basin and that benthic carbonate production was insignificant. The area was not a carbonate platform in the Early Cretaceous and does not indicate any reworked carbonate supply from platform environments in the vicinity. The scarcity of benthic macrofossils in the cored section is due to the lack of a specialised boreal chalk fauna at that time, and the adjacent nearshore environment apparently did not support any substantial benthic carbonate production. A revised biostratigraphy of the cored section is presented based primarily on calcareous nannofossils, supported by foraminifera, ostracods, and belemnites. Four lithofacies describe the spectrum from marlstone to slightly marly chalk, and the facies succession characterises four depositional units recording two discrete transgressive–regressive cycles. The study provides a depositional record that permits sequence stratigraphic correlation to the Valdemar and Adda Fields in the Central Graben.

Early and middle Jurassic sequence stratigraphic division of M coalfield in Xinjiang

The coal accumulation in the M coalfield occurred in the Early-Middle Jurassic. There are three main coal-bearing strata in the M coalfield, the Early Jurassic Talicike Formation, the Yangxia Formation and the Middle Jurassic Kizilenur Formation. Based on the sequence stratigraphic division and sedimentary environment analysis of 85 Jurassic wells in the Kubai Basin, and the compilation of sequence stratigraphic correlation profiles for 20 well-connected wells, this paper comprehensively studies the drilling, logging, outcrop, core and paleontological data. This paper establishes the isochronous sequence stratigraphic correlation framework of the Jurassic in the Kubai Basin with third-order sequences as units. Finally, this paper studies the sequence stratigraphic characteristics and lithofacies paleogeography of each Jurassic sequence in the M coalfield.

Sedimentary Facies of the Chang 2 Member of Yanchang Formation in Area A of Northern Shaanxi

In order to promote the rolling exploration and development of oil reservoirs in northern Shaanxi, a fine study of sedimentary facies in area A has been carried out. Using core and logging data, on the basis of identifying different levels of sequence interfaces and lake flooding, the Chang 2 Member in Area A was divided into two base level cycles, and a high-resolution sequence stratigraphic correlation was established. On the basis of core description, the analysis of single-well sedimentary facies, continuous-well sedimentary facies and planar sedimentary facies are carried out. It is believed that the Chang 2 member of the study area is a delta plain deposit, dominated by distributary channels, inter-distributary depressions, and natural dike microfacies. There are two to three main rivers in the study area. Both the vertical and the plane show that the length of Chang 22 sand bodies is more developed than the length of Chang 21 sand bodies. Among the three small layers of Chang 22, Chang 221 sand body is relatively well developed, and the channel width and thickness are large, making it the most favorable reservoir.

Giant subaqueous carbonate dunes: a revised interpretation of large-scale oo-bioclastic clinoforms in the middle Jurassic of the Paris Basin and its implications

Recent developments of carbon capture and storage and geothermal subsurface prospects in the Middle Jurassic limestone of the central part of the Paris Basin have rejuvenated interest in the world class outcrops of the Bathonian Oolithe Blanche Formation on the southeastern edge of the Basin. Here, ooid-bioclast grainstones occur as large-scale (10–15 m high) clinoforms in the upper part of the Oolithe Blanche, which remain enigmatic and variously interpreted. High-resolution sequence stratigraphic correlations, constrained by biostratigraphic markers and integrated with regional palaeogeography, indicate that the clinoforms are giant dunes developed during the early part of the regressive phase of a third-order transgressive–regressive sequence (Bt3), in the northern part of the Burgundy Bathonian carbonate ramp. Carbonate sand bodies up to 15 m high with intervening troughs, formed subaqueous giant dune field(s) with calculated wavelengths of about 500–800 m. Using dune height as a proxy for calculating the palaeobathymetry of deposition, it is concluded that the dunes formed under a minimum water depth of about 60 m. A detailed facies analysis of the sediments underlying the dunes, which include beds rich in brachiopods in living position corresponding to the MFS of the third-order transgressive–regressive sequence (Bt3), supports such a water depth. The constitutive facies of the dunes themselves also reflect a significant water depth, despite the presence of ooids that are definitely transported away from their main production zone. This new interpretation of the large-scale clinoforms in the upper Oolithe Blanche and the associated revised palaeobathymetry have implications for the interpretations of Bathonian carbonates across the Paris Basin, which are relevant to ongoing projects of reservoir characterization of subsurface prospects.

Sequence stratigraphy of the subsurface cambro-ordovician siliciclastic deposits in the Chotts basin, Southern Tunisia: Petroleum implications

Subsurface study of the Palaeozoic deposits in the Chotts basin, part of the southern Tunisian Atlassic foreland, revealed an incomplete Pre-Hercynian deformed sequence. The remaining truncated Cambro-Ordovician siliciclastic deposits appear to be unconformably overlain by transgressive Late Silurian – Early Devonian shales. Early Ordovician El Atchane sandstones and Hamra quartzite constitute currently the most oil and gas productive levels in the Chotts basin. The main issues encountered in the exploration of these reservoirs are related to their spatial distribution along with their thickness and facies variations. Sequence stratigraphic analysis and correlation of the entire Cambro-Ordovician section using sequence stratigraphic surfaces seems necessary to locate both proven and prospective reservoirs in order to predict their spatial distribution and trend directions in the region. Seismostratigraphic facies analysis of these deposits in subsurface highlights their sequence organizations. The integration of well data revealed in detail the sequence hierarchy of the Cambro-Ordovician deposits throughout the Chotts basin.

Volcanic rocks in Lower Jurassic marine carbonate successions in the southeastern Iberian Range (Spain): biochronostratigraphic dating

The Ligurian Tethys began to open in a roughly NE–SW direction in Pliensbachian times. West of the Ligurian oceanic crust, a carbonate-platform system constituted the eastern Iberian paleomargin. Lower and Middle Jurassic marine sediments deposited in these shallow platforms are well exposed today in the southeastern Iberian Range and host multiple volcanogenic deposits, mainly volcaniclastic in nature. Regional mapping reveals that volcanic outcrops show NW and NE (or NNE) oriented lineaments following tracks of cortical weakness. Volcanic ages were estimated by using biochronostratigraphic methods and sequence-stratigraphic correlations. As a result, ten Lower Jurassic volcanic episodes have been identified. The chronostratigraphic positions of the volcanic deposits span from the early Pliensbachian (Jamesoni Zone or later) to the late Toarcian (Thouarsense Zone). The extension-dominated regime of the eastern Iberian paleomargin also affected the southern Iberian paleomargin (Betic Cordillera) as a result of the eastward prolongation of the central Pangea fragmentation. The break-up of wide platforms and the first volcanic emissions seem to be roughly coeval in the Betic and Iberian domains and synchronous with the onset of rifting and spreading events of the Ligurian Tethys Ocean.

Application of sequence stratigraphy for an enhanced hydrocarbon reservoirs correlation in \u201cOSP\u201d field Niger Delta, Nigeria

The cost of drilling exploration and production wells is high and the price of crude oil in the capital market is fluctuating, therefore, there is the need to mitigate the risk of improper investigation of time/age-dependent reservoir rocks continuity within a field. Sequence stratigraphic analysis was carried out in OSP field with the aim of identifying and correlating hydrocarbon reservoirs that are deposited contemporaneously. Key stratigraphic surfaces (maximum flooding surfaces and sequence boundaries) were interpreted from biostratigraphic, well log and seismic data. Biostratigraphic interpretation subdivide the study area into three floral subzones—P780, P820 and P830. Three maximum flooding surfaces dated 10.4 Ma, 9.5 Ma and 7.4 Ma and four sequence boundaries dated 10.6 Ma, 10.35 Ma, 8.5 Ma and 6.7 Ma were identified. Three depositional sequences (sequences 1–3) were identified from the integration of the biostratigraphic, well log and seismic data. The identified key stratigraphic surfaces and depositional sequences were employed in the delineation and correlation of the hydrocarbon reservoirs which were found to fall within the highstand and lowstand systems tracts. Sequence stratigraphic correlation technique aided the local arrangement and geometries of reservoirs within the field and in locating potential reservoirs.

Sequence Stratigraphy and Geochemistry of Oil Shale Deposits in the Upper Cretaceous Qingshankou Formation of the Songliao Basin, NE China: Implications for the Geological Optimization of In Situ Oil Shale Conversion Processing

The Songliao Basin contains some of the largest volumes of oil shales in China; however, these energy sources are located in areas covered by arable land, meaning that the best way of exploiting them is likely to be environmentally friendly in situ conversion processing (ICP). Whether the oil shales of the Songliao Basin in the Qingshankou Formation are suitable for ICP remain controversial. In this paper, through sequence stratigraphic correlations, three main thick oil shale layers (N1, N2, and N3) of the Sequence1 (Sq1) unit in the first member of Qingshankou Formation (K2qn1) are confirmed as consistently present throughout the Southeastern Uplift region of the basin. The spectral trend attributes reflect that the lake reached a maximum flood surface of the K2qn1 in N2 oil shale layer, and the total organic carbon (TOC) and Fischer assay (FA) oil yield are significantly increasing. The N2 and N3 oil shale layers were deposited in a high lake level environment associated with ingressions of ocean water. The oil shale in these layers with the characteristics of high TOC (maximum of 23.9 wt %; average of 7.2 wt %), abundance of aquatic organic matter (OM) (maximum hydrogen index (HI) of 1080.2 mg/g; average of 889.9 mg/g) and carbonate contents (maximum of 29.5%; average of 15.4%). The N2 and N3 oil shale layers have higher brittleness index (BI) values (generally 40–50%), larger cumulative thicknesses (maximum of 13.3 m; average of 12.0 m), and much higher source potential index (SPI) values (0.92 and 0.88 tHC/m2, respectively) than the N1 oil shale layer within Sq1 transgressive system tracts (TST), indicating that the N2 and N3 layers are prospective targets for ICP. In addition, oil shales buried to depths of <1000 m have strong hydrocarbon generation capacities that make them suitable for ICP.

Sequence stratigraphy of the Upper Jurassic mixed siliciclastic-carbonate deposits in the North German Basin (Lower Saxony, Hildesheimer Wald)

The Wendhausen-6 core, drilled in the Hildesheimer Wald, 30 km SE of Hannover, provides an insight into the Oxfordian deposits of the North German Basin consisting of the Ornatenton, Heersum, and Korallenoolith formations. Eight facies are recognized based on the different fossil combinations, sedimentary structures, and the carbonate content. A facies model of a mixture of a deltaic system and a carbonate ramp is presented based on the interpretation of these facies, which helps the understanding of the facies and sequence stratigraphic evolution in the mixed system in the North German Basin and could be applied to other shallow-water mixed depositional systems. Sequences of three hierarchies are delimited. Small-scale sequences are defined by the changes in components, grain size, and the vertical facies stacking patterns. Medium-scale sequences are bounded by distinctive exposure surfaces. These surfaces are characterized either by the enrichment of iron minerals or by irregular base of the lowstand channel-fill sandstones overlying on them. Large-scale sequences are derived from the facies proportion statistics of each medium-scale sequence. The similar trend between the regional sea-level curve and the facies statistic curve implies that sea-level fluctuations have a great control on the large-scale sequence evolution during the Oxfordian age. Sequence stratigraphic correlation was achieved at the medium-scale sequence level, and four surfaces of Oxfordian age were correlated across the North German Basin.

Hydrocarbon leads and prospects opportunities across a cluster of fields in parts of Onshore Niger Delta Basin

The increase in oil and gas production over the years from already discovered fields at relatively shallower intervals, and corresponding decrease in the discovery of new fields has led to observable decline in the available hydrocarbon reserves within the Niger Delta Basin of Nigeria. Therefore, there is need to grow or add to the existing reserves through “searching wider and drilling deeper” across the onshore sector of the Niger Delta Basin, which is the focus of this work. Although, there has been several hydrocarbon prospectivity studies in the Onshore Niger Delta Basin, most of these studies were conducted on individual fields, except where adjacent fields belonged to the same operator. This has provided limited information on stratigraphic intervals, structural features and reservoir zones continuity across several fields. This study is aimed at interpreting recently acquired within the Coastal Swamp Depobelt of the Niger Delta, long-cable regional 3D seismic data over nine producing fields, and data from twenty-five wells, using sequence stratigraphic, structural and reservoir exploration tools, thus providing a regional scale interpretation and information that will unravel new prospective zones across these fields primarily based on amplitude supported structural plays. Results from stratigraphic studies, show the occurrence of regional extensive chrono/sequence stratigraphic surfaces such as maximum flooding surfaces (7.4–12.8 Ma) and sequence boundaries (8.5–13.1 Ma) that depict sediment packages deposited during the Middle-Miocene through Upper Miocene age. Well log sequence stratigraphic and reservoir correlation studies reveal that depositional sequences, bounded by these surfaces, comprise reservoir and non-reservoir packages within their systems tracts. Reservoir architectural studies show sands of lowstand, transgressive, and highstand systems tracts that make up the reservoir units, are relatively thick, laterally continuous and quite extensive across the area. Interpreted seismic sections unravelled the influence of structure on stratigraphy as sediment packages show variable thicknesses across faults. This was linked to syndepositional deformation associated with sediment deposition within the basin. Mapped reservoir tops at intermediate and deeper intervals reveal the dominance of fault-dependent closures. In addition, root mean square amplitude extraction done on these maps, unravelled eight structurally controlled regional hydrocarbon prospects and eight leads with booming amplitude responses that are conformable to structures. Overall, this study has shown that there exist hydrocarbon prospective zones that cut across several fields at deeper depths that are yet to be drilled, which could contribute to the Niger Delta Basin's existing reserve base.

Incorporation of Sequence Stratigraphy in Gas Reservoir Correlation: A Case Study

The application of sequence stratigraphy to resolve the miscorrelation between different genetic units in reservoir characterization in a gas field of Surma Basin is dealt with. Interpretation of available seismic and wireline logs (gamma ray, resistivity, density and neutron porosity) give the sequence stratigraphic correlation of reservoir sands. The reservoirs geometry, its extent, seal architecture and trapping styles have been revealed better with this correlation. There is juxtaposition of two reservoir sands, namely A1 and A2. A1 is located at older highstand sand, whereas A2 is in the younger lowstand sand. Lithostratigraphically they might be same but sequences stratigraphy reveals that they are different and deposited at different times. Moreover, the concept reveals that lowstand sand has better reservoir quality than any highstand and transgressive sand.
 Asiat. Soc. Bangladesh, Sci. 45(2): 209-216, December 2019

Demarcation and correlation of stratigraphic sequences using wavelet and Hilbert-Huang transforms: A case study from Niger Delta Basin

Demarcation and correlation of stratigraphic sequences are particularly significant in hydrocarbon exploration, in which signatures of well logs are commonly used in conventional interpretation methods. In this study, an intelligent and effective way of sequence stratigraphic analysis is proposed through combining the continuous wavelet transform (CWT), the discrete wavelet transform (DWT), and the Hilbert-Huang transform (HHT) together. By using MATLAB software, these mathematical methods (CWT, DWT, and HHT) are applied on GR logs to interpret sequence surfaces and cycles in the Agbada Formation, Niger Delta Basin. Within the context of base-level and A/S variation, the studied interval is interpreted as a long-term (3rd-order) base-level stratigraphic cycle and three middle-term base-level (4th-order) stratigraphic cycles regarding individual well and cross-section. The comparative results indicate that the CWT and DWT, compared with the HHT, have better resolution in identifying base-level cycles and their periodicity. By contrast, the HHT is more suitable to interpret specific positions of sequence surfaces. Generally, together with conventional interpretation methods, the exploratory methods of this study provide a relatively objective and acceptable way in high-resolution (4th-order or lower rank) sequence stratigraphic analysis at scales of hundreds of meters. Besides, a source to sink depositional model is presented as an example to illustrate multiple well log (GR) interpretations. The idea of this study aims to be extended to more studies to reduce subjective uncertainties in model-driven interpretations.

Sedimentary lithofacies characteristics and sweet-spot interval characterization of the Sinian Doushantuo Formation in Upper Yangtze Platform, South China

ABSTRACT The Sinian Doushantuo Formation is the oldest shale gas reservoir discovered in the world, which contains good shale gas shows as the Eyangye-1 and Zidi-1 wells in the Yichang Region of western Hubei province, China. The shales in the Doushantuo Formation feature considerable thickness (135 m), high gas content (4.83 m3/t) and high fragile mineral content (up to 75%). Due to the influences of the sedimentary environment, the shale reservoirs here have high dolomitic content (54%) and abruptly changes in vertical lithofacies. Moreover, the characteristics of the shale sweet-spot differ significantly from that in the Wufeng Formation and Longmaxi Formation in southeast Sichuan Basin. A high-resolution sequence stratigraphic correlation was performed on the shales of the Doushantuo Formation at the Eyangye-1 well, Zidi-1 well, Zidi-2 well and some outcrop profiles in Yichang area for the identification of their sedimentary microfacies characteristics. A comprehensive comparative analysis was made by incorporating the elemental geochemistry, wireline and mud logging data of the well to further identify the sweet-spot interval therein. With the analysis of Eyangye-1 well, the sweet-spot interval of the Doushantuo Formation is 3360−3408 m, of which sedimentary microfacies is deepwater reduced environment with many pyrite laminaes, and the TOC is high to 3.42%, the Ro is 3.3%, the organic and matrix pore are well developed and the aperture more than 50 nm, the porosity is 2.7%, the desorption gas content average is 2.16%, and the main mineral is dolomitic (54%) and siliceous quartz (21%). Additionally, these parameters combined with previous studies will shed light on evaluating and characterizing the layers therein, also provide referential geological data for the following exploration and development activities of this shale system.

Sea-level changes vs. organic productivity as controls on Early and Middle Devonian bioevents: Facies- and gamma-ray based sequence-stratigraphic correlation of the Prague Basin, Czech Republic

Abstract The Devonian marine stratigraphic record is characterized by a number of bioevents – overturns in pelagic and benthic faunal assemblages, which are associated with distinct changes in lithology. The coincidence of lithologic and biotic changes can be explained by the causal link between biotic evolution, carbonate production and relative sea-level changes. To gain insight into the sea-level history of Early and Middle Devonian bioevents (the Lochkovian/Pragian Event, Basal Zlichovian E., Daleje E., and Chotec E.) we carried out a sequence-stratigraphic analysis of carbonate-dominated successions in the Prague Basin (peri-Gondwana), a classic area of Devonian bioevents. The study is based on a basin-wide correlation of facies and field gamma-ray spectrometry (GRS) logs from 18 sections (Lochkovian to Eifelian), supported by element geochemistry and published biostratigraphic and carbon isotope data. Devonian carbonate deposition in the Prague Basin alternated between two end-member modes: an oligotrophic, homoclinal ramp (Praha and Daleje-Třebotov Formations) and a mesotrophic, distally steepened ramp (Lochkov, Zlichov, and Chotec Formations). They show contrasting facies, particularly the absence/presence of gravity-flow deposits, allochem composition, U/Th ratios, and geochemical composition (productivity proxies such as P/Al, Si/Al, Zn/Al, TOC and stable carbon isotopes). The mesotrophic systems reflect an increased availability of nutrients on the shelf during the late Lochkovian, early Emsian (Zlichovian), and Eifelian periods when sea surface temperature, pCO2, and silicate weathering rates were higher. The oligotrophic systems deposited during the Pragian–to-earliest Emsian and late Emsian (Dalejan) periods reflect reversed palaeoclimatic trends. We identified three depositional sequences (DS), DS1 (base of Pragian to early Emsian); DS2 (early Emsian to mid Emsian); and DS3 (mid Emsian to mid Eifelian). These sequences were integrated into a peri-Gondwana relative sea-level curve, which was then compared with the Euramerican sea-level curve of Johnson et al. (1985). The bioevents coincided with several sequence stratigraphic surfaces, representing variable limbs of the relative sea-level curve. On the other hand, their conspicuous coincidence with the switching intervals between the colder oligotrophic and warmer mesotrophic modes suggests that organic production linked to global climate was the primary control on biotic overturns, while sea-level fluctuations may have only amplified its effects.