Facies Sequence Research Articles

Early Devonian bioclastic contourites in the High Atlas: a plastered drift recording the convergence between Gondwana and Laurussia (Sub-Meseta Zone, Morocco)

Bottom-current deposits, formed in association with ancient oceanic gateways, preserve records of past global oceanic circulation, plate-tectonic events, and palaeogeographic change. This outcrop study showcases a Pragian to early Emsian carbonate-contourite drift, plastered on the northern upper margin of Gondwana, which recorded the convergence of Laurussia and Gondwana and the (temporal) closure of the nearby American Gateway. The results underscore the value of bed-scale features, including microfacies and primary sedimentary structures, in interpreting bottom-current dynamics. The documented contourite-specific bi-gradational sequences, 40 to 80 cm thick, are largely complete and well-comparable with the standard facies sequence identified in modern oceans. The bioclastic contourites show subtle variations in grain size and mud content in the lower part, distinct sequences of inverse-to-normal grading at different scales in the middle, and a large variety of traction structures in the upper part of the succession. This long-term coarsening-upward trend was caused by a successive increase in maximum bottom-current strength during drift growth. Traction structures such as current ripples and dunes document a westward-directed palaeocirculation of surface or intermediate water during the Pragian-Emsian. They provide evidence of enhanced bottom-current episodes towards the top of the drift, before the contourites were buried by late Emsian shales.

Hydrocarbon accumulation and orderly distribution of whole petroleum system in marine carbonate rocks of Sichuan Basin, SW China

Based on the situation and progress of marine oil/gas exploration in the Sichuan Basin, SW China, the whole petroleum system is divided for marine carbonate rocks of the basin according to the combinations of hydrocarbon accumulation elements, especially the source rock. The hydrocarbon accumulation characteristics of each whole petroleum system are analyzed, the patterns of integrated conventional and unconventional hydrocarbon accumulation are summarized, and the favorable exploration targets are proposed. Under the control of multiple extensional-convergent tectonic cycles, the marine carbonate rocks of the Sichuan Basin contain three sets of regional source rocks and three sets of regional cap rocks, and can be divided into the Cambrian, Silurian and Permian whole petroleum systems. These whole petroleum systems present mainly independent hydrocarbon accumulation, containing natural gas of affinity individually. Locally, large fault zones run through multiple whole petroleum systems, forming a fault-controlled complex whole petroleum system. The hydrocarbon accumulation sequence of continental shelf facies shale gas accumulation, marginal platform facies-controlled gas reservoirs, and intra-platform fault- and facies-controlled gas reservoirs is common in the whole petroleum system, with a stereoscopic accumulation and orderly distribution pattern. High-quality source rock is fundamental to the formation of large gas fields, and natural gas in a whole petroleum system is generally enriched near and within the source rocks. The development and maintenance of large-scale reservoirs are essential for natural gas enrichment, multiple sources, oil and gas transformation, and dynamic adjustment are the characteristics of marine petroleum accumulation, and good preservation conditions are critical to natural gas accumulation. Large-scale marginal-platform reef-bank facies zones, deep shale gas, and large-scale lithological complexes related to source-connected faults are future marine hydrocarbon exploration targets in the Sichuan Basin.

Jurassic sedimentary evolution model and its implication for the sandstone-type uranium mineralization in the Kamusite area in eastern Junggar Basin, NW China

The Junggar Basin in the northern Xinjiang Province, northwestern China, is a large hydrocarbon basin with tremendous reserves of oil, gas and coals. Tens of sandstone-type uranium showings, occurrences and/or anomalies have also been discovered in the Junggar Basin since the 1960s, yet hitherto only one large uranium deposit, the Kamusite deposit, has been identified in the Middle Jurassic Toutunhe Formation sandstones in the eastern part of the basin. Sand bodies with high-permeability are the prerequisite for the formation of the in situ leachable sandstone-hosted uranium deposit, nevertheless the spatial distribution and genetic types of the sand bodies, as well as the deposition model in Jurassic time in the Kamusite area both remain unclear, which largely hindered the understanding of the sedimentary evolution and uranium exploration directions in the region.In this contribution, based on the outcrop observation, core logging and well logging data interpretation, and comprehensive analysis, the sedimentary characteristics, sedimentary facies types, sand body morphology, plane distribution characteristics, as well as the sedimentary evolution history in Jurassic time in the Kamusite area have been illustrated in details. From early to late, the Jurassic sediments in Kamusite can be divided into the Badaowan, Sangonghe, Xishanyao, Toutunhe and Qigu formations, of which the sand bodies favorable for the sandstone-type uranium mineralization have been systematically analyzed, and the depositional model in the Kamusite area have been established. The Early Jurassic Badaowan Formation is a set of coal-bearing coarse-grained clastic sediments of the braided river–braided river delta facies, followed by the lake transgression and fine-grained lacustrine deposition in the Sangonghe Formation. The Middle Jurassic Xishanyao Formation is a set of braided river delta facies coal-bearing clastic sediments, while the uranium orebody-bearing Toutunhe Formation underwent the paleoclimate transition from warm and humid to hot and arid, and developed a set of braided river delta to meandering river facies sequences with an overall positive sedimentary rhythm. The Qigu Formation is dominated by a series of meandering river to lacustrine facies fine-grained sediments and largely denuded by later tectonic activities. Three sets of sand bodies favorable for the sandstone-type uranium mineralization are recognized, including those in the lower Toutunhe Formation, the lower Xishanyao Formation, and the upper Badaowan Formation, from bottom to top. A four-stage sedimentary evolution model from the Badaowan Formation to the Toutunhe Formation has been established thereafter, and general exploration suggestions were proposed in the last of the contribution. More drilling is recommended to the south of the Kalasayi Fault, aiming the lower Toutunhe, lower Xishanyao and upper Badaowan formations. To the north of the Kalasayi Fault, more uranium mineralization could also be expected, and the exploration is suggested to focus on the Jurassic Toutunhe, Xishanyao formations and Cretaceous Tugulu Group.

Subsurface structures and nature of seafloor mounds in the northern South China Sea margin: Implications for Mesozoic hydrocarbon exploration

The Chaoshan Depression in the northern South China Sea (SCS) margin hosts thick (ca. 4–9 km) Mesozoic strata including two sets of hydrocarbon source rocks (the Late Triassic-Early Jurassic and Late Jurassic marine facies sequences). It was predicted to be petroliferous but unproven. Numerous submarine mounds are distributed on its seafloor, but whether they are mud volcanoes or magmatic volcanoes remains controversial due to poor knowledge of the subsurface structures. To reveal the nature of these mounds and the Mesozoic hydrocarbon potential in the Chaoshan Depression, multi-channel seismic (MCS) and ocean bottom seismometer (OBS) data along line CS-L1 across three seafloor mounds are processed and analyzed. The MCS profile reveals the existence of fuzzy reflection zones (FRZs) within the Mesozoic strata beneath these mounds. The overlying sedimentary sequences appear uplifted and deformed. The OBS velocity results exhibit that these FRZs feature significantly lower velocities than their host strata with velocity differences ranging from ∼0.25 km/s to 0.71 km/s, likely resulting from vigorous fluid charging rather than magma intrusion. Thus these FRZs are identified as mud-fluid diapirs. Between these diapirs and the overlying seafloor mounds, numerous high-angle faults exist, serving as vertical fluid feeder conduits. Additionally, several bright spots, acting as indicators of gas reservoirs, are observed near these faults. Moreover, bottom simulating reflectors with reverse polarity are imaged within the shallow strata around the seafloor mounds on an adjacent MCS profile, indicating rich gas trapping and/or gas hydrate accumulation. Therefore, we propose that these seafloor mounds are mud volcanoes with fluids coming from the underlying diapirs. As the diapirs extend downward into the Jurassic and Triassic strata, the hydrocarbon gases feeding the mud volcanoes and diapirs are reasoned to originate from the deep Mesozoic source rocks, implying significant Mesozoic hydrocarbon potential in the Chaoshan Depression of the SCS.

Complex characteristics of suevites at the B. Vanuyta river (Kara astrobleme, Russia)

Research subject. Suevites from the eastern sector of the Kara astrobleme, located in the basin of the Bolshaya Vanuyta river. Aim. To identify the suevitic type and to clarify the facial features of the suevites from the B. Vanuyta River. Materials. Hand-size samples of suevites collecteed in the basins of B. Vanuyta, Anaroga, Kara, and Sopchayu rivers, as well as samples of siltstones collected outside the Kara crater (rivers Halmeryu, Sayakha, Putyu, Anaroga and the creek Lavovyi). The samples were used for preparing polished thin sections and powder specimens for analytical works. Methods. Field observations, optical and electron scanning microscopy, and silicate analysis by the wet chemical method. Results and conclusions. The structural-textural and petrochemical features of the matrix, fragments of impact glasses, and lithoclasts of suevites from the B. Vanuyta river were characterized. The suevite matrix was found to exhibit an intensely sintered character and to be presented by feldspar, quartz, calcite, chlorite, muscovite, glauconite, and titanite. Lithoclasts include siltstones, sandstones, limestones, mudstones, and shales. Vitroclasts are characterized by angular and irregular morphology with different border patterns, and spotted, fluid, and porous textures. The similarity of the chemical composition of siltstone clusters and similar target rocks confirms the genetic relationship and the leading role of this type of target rocks in the formation of the studied suevites. The proximity in the composition of the impact glasses of the studied suevites and the glasses of type I and II suevites indicates the formation of suevite vitroclasts in the river B. Vanuyta by a similar protolith – siltstones and a clay component of the limestones. The matrix lithification character, morphology and nature of the vitroclast outlines indicate that the suevites from the B. Vanuyta river are the rocks of an aerodynamic facies formed under relatively high-temperature conditions, which structurally lie at the lower part of the detrital impactites sequence of the aerodynamic facies. The studied rocks are close to the type I suevites by the composition of the protolith and differ in the conditions of their formation. With the type II suevites, they have similar facies characteristics, but differ in the composition of the target rocks fragments. It is determined that the leading factor in the typification of suevites from the Kara astrobleme is the chemical composition, at the same time, the types can be subdivided more fractionally based on the specifics of the facies environments of their formation.

Archean (3.3 Ga) paleosols and paleoenvironments of Western Australia.

The Pilbara craton of northwestern Australia is known for what were, when reported, the oldest known microfossils and paleosols on Earth. Both interpretations are mired in controversy, and neither remain the oldest known. Both the microfossils and the paleosols have been considered hydrothermal artefacts: carbon films of vents and a large hydrothermal cupola, respectively. This study resampled and analyzed putative paleosols within and below the Strelley Pool Formation (3.3 Ga), at four classic locations: Strelley Pool, Steer Ridge, Trendall Ridge, and Streckfuss, and also at newly discovered outcrops near Marble Bar. The same sequence of sedimentary facies and paleosols was newly recognized unconformably above the locality for microfossils in chert of the Apex Basalt (3.5 Ga) near Marble Bar. The fossiliferous Apex chert was not a hydrothermal vein but a thick (15 m) sedimentary interbed within a sequence of pillow basalts, which form an angular unconformity capped by the same pre-Strelley paleosol and Strelley Pool Formation facies found elsewhere in the Pilbara region. Baritic alluvial paleosols within the Strelley Pool Formation include common microfossil spindles (cf. Eopoikilofusa) distinct from marine microfossil communities with septate filaments (Primaevifilum) of cherts in the Apex and Mt Ada Basalts. Phosphorus and iron depletion in paleosols within and below the Strelley Pool Formation are evidence of soil communities of stable landscapes living under an atmosphere of high CO2 (2473 ± 134 ppmv or 8.8 ± 0.5 times preindustrial atmospheric level of 280 ppm) and low O2 (2181 ± 3018 ppmv or 0.01 ± 0.014 times modern).

Detrital zircon geochronology from the Nueva Segovia Schist, Nicaragua: evidence for the tectonic evolution of the Chortis Block?

Abstract The Chortis Block of Central America is a cratonic-type peri-Gondwanan terrane and is commonly included in Neoproterozoic–Paleozoic palaeogeographical reconstructions. At present, most research has focused on the Mesozoic evolution of the Chortis Block, however, its earlier history remains poorly constrained. As a result, there is considerable debate surrounding the internal complexities of the Chortis Block and its tectonothermal evolution has not been well established by geochronological and geochemical data. New field investigations from the Nueva Segovia Schist (Northern Nicaragua), considered one of the oldest exposed parts of the Chortis Block, reveal it is composed primarily of deformed sequences of greenschist facies marine clastic and chemical sediments in conformable contact with felsic volcanics. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from two samples taken from the Nueva Segovia Schist reveal a youngest age peak of c. 250 Ma with other significant peaks at c. 500 Ma, c. 1.0 Ga and c. 1.2 Ga. Taken together with field observations, these data suggest the Nueva Segovia Schist was likely deposited between c. 250 and 110 Ma proximal to Amazonia during the Late Paleozoic, and they support a Precambrian age for the basement of the East Chortis Terrane. Taken together the data support a Pangaean position of the Chortis Block, adjacent to Amazonia inboard of Oaxaquia.

Geology of recently recognized lithologies inside the Red Bed Series in the Chwarta-Mawat area, Kurdistan Region, NE-Iraq

The sequence of the coastal siliciclastic facies of the Paleocene-Eocene Zagros Foreland Basin is well exposed in the Chwarta-Mawat area, north of Sulaimani Governorate, northeastern Iraq. According to previous studies, this sequence is designated as Red Beds Series (RBS), consisting of more than 2000 m of fine and coarse red and grey clastics. The present study found recently recognized lithologies (RRLs) about 100 thick inside the series and encompassed green marlstone with subsidiary detrital limestone which has significant tectonic and paleogeographic results. The petrography, biostratigraphy, and boundary conditions studies are applied to uncover their origin. The new lithologies are deposited in the subsiding basin in the Imbricate Zone during the Middle Eocene. During this age, the Tertiary Foreland Basin was separated by a paleohigh into two basins, (northern and southern basins). In the northern basin, the RRLs have deposited as deep facies of the Naopurdan Formation while in the southern basin the Pila Spi Formation was deposited as lagoonal facies. The former formation was deposited as reefal facies in the shelf area of the northern basin to the northeast Chawrta and Mawat Towns. The RRLs, units one and two of the series (in the Thrust Zone) are correlated respectively with Pila Spi, Sinjar and Gercus formations in the High Folded Zone. These correlations are shown on the stratigraphic columns and paleogeographic models with the aid of nannofossil and benthonic foraminiferal studies. This study is the most important step for solving the tectonic and paleogeographic setting of northeastern Iraq and for a better understanding of the Zagros Collisional belt.

Sedimentation and erosion at Majuli Island, Assam, India: analysis of paleo-depositional environment

ABSTRACT The World’s one of largest inhabited river-made island Majuli, developed in response to depositional processes of large sediment load carried by the Bramhaputra River and Subhansari River from the Himalayan hinterland. But, for decades it has been going through a significant rate of erosion, which is severely threatening the existence of the island and its rich cultural heritage. These erosional processes result in exposure to island depositional records and provide an opportunity to study these sedimentary deposits. This paleo depositional record is very well analysed by adopting the methodology of facies identification and their relationship diagram. A total of 18 exposed outcrop sections are studied with detailed sedimentological documentation, facies identification and symbols assigned to facies. Then calculations and tabulations are carried out for Facies Relationship Diagram (FRD) along with the construction of the sequence model, which provides a lateral and vertical lithofacies association developed in the entire island sequentially. On the basis of validation of obtained data through field survey and analysis of more vertical profiles, we study the correlation between existing bank erosion and dominated sedimentary facies at Majuli Island. This analysis results in two facies sequences: one mud-dominated and the other sand-dominated facies. The Massive Mud and Massive Sand beds are represented as the main facies between these two sequences. Therefore, mud-dominated sequence represents the Flood Plain deposit, whereas the sand-dominated sequence represents Bar and Channel deposits. This study also reveals that sedimentary sequences developed by the Bramhaputra River are mud dominated, while that of the Subhansiri River is both mud and sand sequences. This reflects the differential source to sink transport of sediments, as the Bramhaputra River carries sediment load for a longer distance than the Subhanshiri River. This study also reveals that areas of sand-dominated facies show higher erosion than mud-dominated facies. Therefore, the sedimentary record of the Subhansiri River is more vulnerable to erosion.

Age and causes of the Yellow River dissecting the Zoige Basin in the eastern Tibetan Plateau, China

Through extensive field investigation in the Zoige Basin of the eastern Tibetan Plateau, a typical sedimentary profile was found on the front scarp of the second terrace of Maqu reach of the Yellow River at the outlet of the Zoige Basin. This profile contains the stratigraphic sequence of paleolake facies and paleo-riverbed floodplain facies. Field observation and the analysis of particle size characteristics and optically stimulated luminescence dating showed the following results: (1) The profile includes a pale blue-grey paleolake deposit, which formed in a deep-water anoxic environment. This deposit was overlain by a mottled pebble layer with pale yellow-orange lenticular sand, which represents typical paleo-riverbed floodplain deposits, indicating a strong hydrodynamic process. The typical unconformity contact relationship between the two deposits shows that the profile recorded the sedimentary information of the transformation of the Zoige paleolake environment into the Yellow River paleo-river environment, which provides powerful sedimentary evidence for the Yellow River dissecting the Zoige paleolake. (2) The optically stimulated luminescence dating results showed the Zoige paleolake was dissected by the Yellow River at 37 ka BP, resulting in the discharge of lake water. At 35 ka BP, the lake water gradually became shallow and disappeared, and the Yellow River subsequently connected the water system of the Zoige Basin. (3) The East Kunlun fault induced strong tectonic movement in the late Pleistocene, and the Tibetan Plateau had a warm and humid climate at 37 ka BP. Under these conditions, the headward erosion of the Yellow River was aggravated in the faulted wide valley grassland between the Anyemaqen mountains and Xiqing mountains, and the Zoige paleolake was dissected by the Yellow River from west to east in the Maqu bottleneck reach. The results of this study have important scientific significance for in-depth understanding of the evolution of river-lake water systems and the formation of the Yellow River source water system in the eastern Tibetan Plateau.

Growth of complex volcanic ash aggregates in the Tierra Blanca Joven eruption of Ilopango Caldera, El Salvador

Aggregation processes control both the residence time and dispersal of volcanic ash during eruptions yet remain incompletely understood. The products of aggregation vary from simple ash clusters to large, complexly layered accretionary lapilli. Here we detail the micro-stratigraphy of a single population of accretionary lapilli that grew during the ∼431 CE Tierra Blanca Joven eruption from Ilopango Caldera, El Salvador. The accretionary lapilli were sampled 10 km from the caldera source within a sequence of ash-rich pyroclastic density current deposits and intercalated fall material, known as unit D, which is traceable >40 km from Ilopango. Scanning electron microscopy and image analysis reveal common facies that form distinct layers within the accretionary lapilli. Each facies is distinguished by quantitative and qualitative variations in particle size distribution, porosity, and particle fabric. We infer that these textures resulted from aggregation conditions that differed in terms of liquid water availability, particle concentration and grain size distributions. In our proposed model, a characteristic sequence of facies accreted from core to rim in the accretionary lapilli during passage through ash clouds generated by vent-derived plumes and pyroclastic density currents. The accretionary lapilli are mostly composed of smaller aggregates (ash clusters, ash pellets) and grew predominantly by accretion of already-formed aggregates, rather than by grain-by-grain accretion of individual particles. This finding is consistent with observations of rapid aggregate growth in volcanic plumes, suggesting a common evolutionary pathway for accretionary lapilli formation across diverse eruptions.

High-resolution taphonomy and sequence stratigraphy of internally complex, bakevelliid-dominated coquinas from the Aptian Romualdo Formation, Araripe Basin, NE Brazil

The Aptian Romualdo Formation (Araripe Basin, NE Brazil) was deposited in a restricted epeiric sea, during the opening of the South Atlantic Ocean, with a coeval record to that in the Brazilian Pre-Salt succession. The upper part of the Romualdo Formation encompasses the Highstand Systems Tract of a third-order stratigraphic sequence, and contains shell beds interbedded with shale, siltstone, and fine-grained sandstone. Based on distinct taphonomic features (shell sorting, fragmentation, abrasion, rounding, and orientation), sedimentological criteria (shell abundance, type of matrix, sedimentary structures, and presence/absence of quartz grains), and stratigraphic attributes (bed thickness, geometry, contacts), the paleoenvironmental conditions and the bed-by-bed accumulation history of the coquinas is disentangled. Eight carbonate microfacies were recognized, including ostracod carbonate mudstone, bivalve-gastropod wackestone, gastropod-bivalve packstone, bivalve-gastropod grainstone, bivalve-gastropod floatstone, bivalve-gastropod rudstone, bakevelliid floatstone, and bakevelliid rudstone, which are organized in six, cm-thick shell concentrations. Mudstone and wackestone facies mark the settling of carbonate muds without significant bottom currents and wave action, probably below storm wave base, whereas packstone, floatstone, and rudstone, with fragmented, rounded, and oriented shells, were deposited in agitated waters, between the fair-weather and the storm wave bases. Ripple cross-laminated grainstone is interpreted as high-energy facies deposited above fair-weather wave base. The shell beds are internally complex and consist of cm-scale alternations of microfacies differing in packing, sorting, and composition. They tend to fine upwards within dm-thick shell beds, generating shallowing-upward facies sequences. The concentrations, the uppermost one with in situ bakevelliid shells in the top, record high-frequency base level oscillations influenced by eustasy and climate changes. These bioclastic accumulations originated by the superposition of sedimentologic and primary biologic processes (e.g., post-deposition meiofaunal bioturbation). The cm-thick, low-diversity shell beds are excellent examples of multiple-event carbonate deposits generated in a siliciclastic-dominated, restricted epeiric sea, revealing high-frequency cycles in proximal to distal restricted marine settings.

Predicting sequence stratigraphic architecture and its implication for hydrocarbon reservoir potential of the uppermost Silurian through Lower Devonian Winduck Interval, central Darling Basin of western New South Wales, SE Australia

This paper provides the results of lithostratigraphy and depositional environment of the uppermost Silurian through Lower Devonian Winduck Interval section from three areas widely distributed in the central Bancannia Trough from Bancannia South 1 well, the southern Pondie Range Sub-basin from Pondie Range 1 well and the northwestern part of the Blantyre Sub-basin from two wells, the Booligal Creek 1 and 2. These wells were studied using wireline log, core and cutting data to predict sequence stratigraphic architecture and its implication for hydrocarbon sandstone reservoir potential. This study necessitated the use of detailed facies prediction relationships and geometry of lithostratigraphic concepts for the purpose of establishing a sequence stratigraphic architecture framework of the studied areas. Two units were established, lower and upper Winduck Interval, within the Winduck Interval identified by lithological data examination and wireline log characteristics of the sediments from the four wells in the study area. Results of the lithology type analysis showed that the Winduck Interval is interbedded richly in sandstone and siltstone complex deposits. This paper presents a clear picture of the sequence stratigraphic model, as well as that of the three third-order depositional sequences (DS1, DS2 and DS3, respectively) that are identified from changes in systems tracts patterns (composed of lowstand, transgressive and highstand), vertical and lateral facies sequence thickness changes and the lithostratigraphic units distribution in the Winduck Interval section. Potential reservoir prospectivity targets are identified in lowstand system tracts and highstand system tracts deposits consisting of sandstone facies and thin silts, while the transgressive system tracts deposits consist of shales and some interbedded shaly siltstones, many having excellent potential source and seal properties. The contribution of sequence stratigraphy as an approach to understanding lithostratigraphy of the Winduck Interval has important significance in regard to future hydrocarbon reservoirs exploration across the central Darling Basin.

Textural and Paleoenvironmental Characterization of the Campano-Maastrichtian Patti Sandstone, Southern Bida Basin, Northcentral Nigeria

Field and granulometric investigation of Patti Sandstone were carried out with a view towards establishing the lithological sequence, geological processes, textural characteristics and paleoenvironment of deposition of the sediments. Field study entailed basic field procedures and sedimentological descriptions which encompass physical factors such as textural parameter, composition, colour and structures; biological inference in terms of fossil content and chemical character in terms of chemical composition and post depositional diagenesis effect. Granulometric laboratory procedure includes weighing and sieving of samples with set of sieves; calculated cumulative percentages, software input of data to generate statistical parameters such as mean, standard deviation (sorting), skewness, kurtosis and ternary diagrams. Field observations show that the lower part of the outcrop section has alternation of claystone and shale; the mid-section shows coarsening-up sequence of shale, clay and sandstone facies of a retrogradational system, while the upper part is composed of intercalation of claystone and sandstone in repetitive pattern, various thicknesses and colours. The textural characteristics of the sandstone are generally coarse-fine grains, reddish to greyish in colour, rounded to subrounded and moderately to well sorted. Structurally, the outcrops are planar cross bedded, wavy laminated and concretional. Granulometric result categorized the sandstones into fine, medium and coarse grained particles; well sorted to moderately well sorted, characterized by lateral retrogradational sequence from the shoreline. The skewness varies from fine skewed, very fine skewed and symmetrical. Kurtosis values suggest multi-sourced particles deposited in marine and fluviatile paleoenvironments.

Reservoir facies classification based on random forest and geostatistics methods in an offshore oilfield

Machine learning methods are increasingly employed in various seismic and petrophysical methods for parameter estimation, interpretation, prediction, and classification. Reservoir facies classification assists the interpretation of seismic data as an important step in petroleum exploration and production monitoring. In this study, we estimate a reservoir facies model by integrating random forest (RF) algorithms and geostatistics modeling. The Surmeh Formation with the Jurassic age is known as one of the most important hydrocarbon reservoirs in the Middle East. The upper part of the Surmeh Formation is equivalent to the Arab Formation, which includes sequences of evaporitic carbonate facies in the study area. Well log data including DT, GR, RHOB, and PHI are used in the RF method for reservoir facies classification. Cross-validation verifies the high accuracy of our classification, with an average accuracy of 95%. The predicted reservoir facies consistently describe the carbonate and evaporitic facies with the geological information of this formation. The decision tree diagrams of the RF algorithm give valuable information on decision limitations and how to select features for efficient computation. We use the classification results for facies modeling. The comparison between facies models and drilling core data shows that the APE value of the sequential indicator simulation model is less than that of the indicator kriging model.

Characteristics of Cambrian tectonic-lithofacies paleogeography in China and the controls on hydrocarbons

In early Cambrian, the earth was in a state of active super-continental disintegration, ocean basin expansion, and seawater transgression. It was a critical period for hydrocarbons source rocks development on a global scale. In China for example, hydrocarbons reservoirs, including conventional and unconventional, have been discovered in the Cambrian rocks. By using regional tectonic analysis, stratigraphic correlation, and lithofacies comparison, we compiled a tectonic-lithofacies paleogeographic map and determined the lithofacies paleogeographic characteristics of the main plates of the early Cambrian period in China. Our results show that although the North China Plate was deposited in shallow-water shore facies, it was later uplifted and exposed to long-term denudation; and thus, lacked the depositional conditions necessary to form hydrocarbon source rocks. In contrast, both the South China and the Tarim Plates contain the complete platform-slope-basin facies sequence, which is the required depositional conditions to promote the formation of hydrocarbon source rocks. Furthermore, our results demonstrate that, during the period of Meishucun and Qiongzhusi formations, South China Plate was composed of deep-water sediments. Correspondingly, during the sedimentation of the Yuertusi Formation, several areas such as the Kalpin, Tabei, and Taxinan in the Tarim Basin, formed a gentle-sloping continental shelf sedimentary sequence of phosphorous siliceous rocks, muddy shales, marlstones, and doloarenite. In addition, we demonstrate that (1) abundant nutrients provided by the hydrothermal and coastal upwelling events, combined with the “periodic anoxic” bottom water conditions, resulted in the formation of high-quality hydrocarbon source rocks in the Yuertus Formation in the Tarim Basin; and (2) the intracratonic sag and continental shelf edge slope settings controlled the deposition of hydrocarbon source rocks in the Sichuan Basin.

Modeling of subsurface sedimentary facies using Self-Attention Generative Adversarial Networks (SAGANs)

Understanding subsurface distribution features is crucial for reliable sedimentary facies modeling. Obtaining the distribution feature of complex subsurface sedimentary facies is challenging, especially non-stationary sedimentary facies which are location-specific, well-ordered, facies sequence (e.g., deltas). The application of machine learning algorithms has potential to assist with this imaging problem, particularly using the Generative Adversarial Networks (GANs) method. Recently, GANs have proven to be outstanding for unsupervised learning on complex distributions of training images. However, in most GAN-based model convolution processes, the information in a local area is computationally invalid for reproducing the global features of training images. To remedy this, we introduce an advanced Self-Attention Generative Adversarial Network (SAGAN) for subsurface geological facies modeling. Compared with the basic GANs, SAGANs introduce a self-attention mechanism to attain details from a long distance in the image, reproducing global features of training images. SAGAN case studies involve stationary channels and non-stationary delta facies. We use probability maps, variograms, connectivity functions, and visualization results to evaluate and compare our simulation realizations. For channel cases, SAGANs’ realizations can reproduce different distributions of channels and point bars in different river systems. For the delta case, the SAGAN method shows a better ability to reproduce delta non-stationary characteristics than the MPS and basic GAN methods. All results are of high quality and diversity, reproduced the known geological sedimentary patterns, and compared with the basic GANs, SAGANs can better reproduce the global features of non-stationary training images. It is demonstrated that our first proposed SAGANs for geological facies modeling represent a powerful method for reproducing depositional facies distribution pattern.

Pollen and Spores Recovery in Tunga Buzu Carbonaceous Shale Type Section Member: Significance in Sequence Stratigraphy, Age Dating and Paleoenvironment Deduction of the Early Miocene Gwandu Formation, Sokoto Basin, Northwestern Nigeria

Feld samples were collected from an outcrop at Tunga Buzu with the intention of comparing field relationship between the claystone member and the underlying carbonaceous shale Type Section for palynological study. The study was carried out in order to determine the geologic age relationship of the facies sequence, sequence stratigraphy and paleoenvironment of deposition of the sediments. Lithological description of the beds follows notation of colour, facies composition, texture, fossil content, structure and post depositional diagenetic effects. Laboratory preparation of palynological slides involved decarbonisation, sample digestion using hydrofluoric acid, sieving with 10µm mesh, bleaching of organic debris with nitric acid, maceral separation with zinc bromide, rinsing of separated maceral with ethanol and final mounting of the organomaceral on the slides for microscopic analysis.The carbonaceous shale is light grey in colour, fissile, carbonaceous, slightly bioturbated and ferruginized. Pollen, spores, dinoflagellates and algae were recovered from samples N1-N4. The samples N1-N3 are claystone samples characterised by relatively low assemblage assemblage of miospores with downhole increase in dinoflagellate cysts. The carbonaceous shale (sample N4) is relatively rich in palynomorphs with corresponding high palynomorph population and diversity compared with the claystone facies. Both the claystone and carbonaceous shale contain marker fossils such as Striamonocolpites sp., Retibrevitricolporites obodoensis, Verrutricolporites sp., Praedapollis sp., Belskipollis elegans and Peregnipollis nigericus; indicative of Early Miocene age. The litho-sequence of claystone and carbonaceous shale indicated alternation of continental and marginal marine deposits characterised by paucity and preponderance of peridinacean dinocysts. This suggests eustatic change in sea level of continental Lowstand Systems Tract (LST) and marginal marine Transgressive Systems Tract (TST) deposits respectively. The Tunga Buzu Carbonaceous Shale Type Section constitutes a member of the Early Miocene Gwandu Formation of the Sokoto Basin, Nigeria.

Recognizing key sedimentary facies and their distribution in mixed turbidite–contourite depositional systems: The case of the Pacific margin of the Antarctic Peninsula

AbstractInterplay of deep‐water sedimentary processes is responsible for building a myriad of features and deposits across mixed turbidite–contourite systems, from <5 cm beds to >200 km long sedimentary drifts. Investigations of the spatial and temporal variability of their sedimentary facies and facies associations is crucial to reveal the dynamics between along‐slope bottom currents and down‐slope turbidity currents, as well as their impact on drift construction and channel erosion. This study focuses on extensive modern mixed (turbidite–contourite) systems, developed across the continental rise of the Pacific margin of the Antarctic Peninsula. Nine sediment cores were sampled and analysed, through grain size and geochemical methods, to study the sedimentary facies at high‐resolution (ca 1 to 20 cm). Three main facies associations have been identified across distinct morphological features (i.e. mounded drifts and trunk channels), comprising intercalations of hemipelagites, bottom current reworked sands (which include fine to coarse‐grained contourites) and gravitational facies (turbidites and mass‐transport deposits). These facies associations reflect fluctuations of the background sedimentation, oscillations of the bottom‐current velocity and of the frequency of gravity‐driven currents. The sedimentary record features cyclic alternations during the Late Quaternary (>99 kyr), suggesting that variations between along‐slope bottom currents and down‐slope turbidity currents are strongly linked to glacial–interglacial cycles during Marine Isotope Stages 1 to 6. Sedimentary records affected by bottom currents on polar margins, such as those of the Antarctic Peninsula, are essential to decipher the facies and facies sequences of bottom‐current deposits, as the low degree of bioturbation throughout most of the sediments allows us to observe the original sedimentary structures, which are poorly preserved in similar deposits from other continental margins.

Facies and Sequence Stratigraphic Study of Bema Field, Niger Delta, Nigeria

Facies architecture and sequence stratigraphic framework of a potential hydrocarbon Prospective Area of Bema Field, Central Swamp Depobelt, Niger Delta was investigated. This study utilized Well Logs, Side Well Samples, Biostratigraphic Data and 3-D Seismic. The study revealed four facies unites, namely, Sandstone facies (SF), Shaly-Sandsone facies (SSF), Mudsone facies (MF), and Heterolithic facies (HF). The Sequence Stratigraphic analysis of the field revealed Nine (9) stratigraphic surfaces, comprising of (Five Maximum Flooding Surfaces (MFSs) with ages ranging from 20.7 to 15.0 Ma, and Four Sequence Boundaries (SBs) with ages between 20.4 and 15.5 Ma respectively). Five Vail’s third order depositional sequences (SQ1, SQ2, SQ3, SQ4, and SQ5) and four (4) Galloway’s genetic sequences (GQ1, GQ2, GQ3 and GQ4) were recognized within the study area based on Stratigraphic Marker tops and system tracts. Hydrocarbon play assessment revealed that the HST and TST are key reservoir rocks in the study while the shale of the TST therefore forms the seal for the potential traps in the study area.