Field Outcrop Research Articles

Provenance characteristics from the Mesozoic to the Cenozoic era and their implications for basin-mountain system evolution in the southwestern Junggar Basin: Insights from field outcrop observations in the Tostai area

The Mesozoic-Cenozoic formations in the southwestern Junggar Basin are a key target for oil and gas exploration. However, complex provenance conditions hinder a comprehensive understanding of sedimentary reservoirs. Based on the petrology and elemental geochemistry analysis of field outcrops in the Tostai area, research is conducted on the tectonic background, sediment transport distances, compositions of parent rocks, and evolutionary characteristics of provenance conditions in the provenance areas. It is generally believed that there have been three notable shifts in basin-mountain relationships during this period. Initially, from the Triassic to the Jurassic, the Northern Tianshan magmatic rock area predominated as the primary provenance, with decreasing basin-mountain proximity and predominantly magmatic rock sediment composition indicating near provenance. Subsequently, heightened Cretaceous tectonic activity in the Tianshan Mountains leads to significant uplift of metamorphic rock regions in the Middle Tianshan Mountains, gradually becoming the provenance for the study area. This increased distance between the provenance and sedimentary areas is marked by a substantial presence of metamorphic rocks in sediments, indicating a far provenance. Finally, the Cenozoic Himalayan movement resulted in the vigorous uplift of the magmatic rock area in the northern Tianshan Mountains, re-establishing it as the primary provenance. This minimized the basin-mountain distance, leading to the deposition of the near-provenance transported alluvial fans, braided rivers, and predominantly magmatic rock sediments.

Intra‐Craton Faults Characteristics, Development and Its Hydrocarbon Accumulation Significance in Mesozoic Strata at Southwestern Ordos Basin, China

ABSTRACTThe Ordos Basin is among China's largest petroliferous basins, with its southwestern part being a key exploration area. Prior studies indicated that the basin's internal structure was relatively simple, featuring minor developmental faults and primarily stratigraphic–lithologic reservoirs. However, recent research has identified numerous strike‐slip faults in the basin, and their relationship to oil and gas accumulations remains unclear. This study, using integrated interpretations of field outcrops, imaging logging and 3D seismic reflection data, clarifies the characteristics, morphology and formation mechanisms of multi‐period faults in the southwestern Ordos basin. Additionally, the study investigates the relationship between these faults and oil and gas accumulations. Results show that Mesozoic fractures in the southwestern basin are primarily NE‐ and NW‐trending. Seismic profiles reveal that these faults exhibit complex geometries, including upright structures in the Middle to Upper Triassic and floral structures in the Cretaceous. Coherence slices show that Lower Jurassic faults have linear structures NE‐ and NW‐trending, while Cretaceous faults exhibit parallel linear structures ENE‐trending. The study of fault displacement and morphology suggests two evolutionary patterns for Mesozoic faults in the basin: layered development and basement‐activated faulting. The widespread ENE‐ and NW‐trending faults represent a specific mode of tectonic stress transfer in stable cratonic areas with minimal basement fault influence. Conversely, some ENE‐trending faults are significantly influenced by basement activation during various geological periods, penetrating deeply into strata and exhibiting distinct segmentation on a planar scale. This differential fault development results in an uneven distribution of Jurassic oil and gas reservoirs. Significant accumulations of Jurassic oil and gas are found in the ENE‐trending tension–torsional strike‐slip sections, whereas many NW‐trending faults may negatively impact oil and gas reservoirs. This study elucidates the characteristics of Mesozoic faults in the southwestern Ordos Basin, offering valuable guidance for oil and gas exploration and development in the region.

Inner architecture of fan delta front deposits: An outcrop example from Xiguayuan formation, Luanping Basin, northeast China

The fan delta front deposit is a notable reservoir type in the basin margin and typically exhibits high heterogeneity in reservoir architecture. With the deepening of exploration and development of oil and gas fields, the study of lower-order depositional architectures is relatively weak, and their implication on remaining oil distribution and enrichment are poorly understood. Field outcrops of the fan delta front deposit exhibit first-hand data of lithofacies, association, and depositional architectures, effectively providing insights for underground reservoir research to guide the development of oil fields, such as high water–cut oil fields in the late development period. Herein, we considered the fan delta front outcrops of the Sangyuan section as an object to reveal the depositional architectures located in the Luanping Basin, northeast China. Using meticulous artificial fieldwork and unmanned aerial vehicle observation, we focused on the 4th-order (single sandbody) and 3rd-order (composite laminaset groups) architectural units. Based on field observations, quantitative statistics, and model matching and guidance, architectural models of the fan delta front in the Sangyuan section were established in response to the high-frequency lake-level. In addition, genetic types of remaining oil controlled by architectural features in Wenchang B oil field during the high water–cut stage were further discussed, and suitable strategies and measures are proposed to produce different types of remaining oil. Results indicate that there are twelve basic types of lithofacies and three types of a single sandbody developed in the Sangyuan section, and evidently, the dimensions and spatial stacking patterns of 4th-order architectural units are closely related to high-frequency lake-level fluctuation, that is, with the rise of lake-level, the thickness and width of a single sandbody gradually decrease, whereas the width–thickness ratio gradually increases. Meanwhile, the spatial stacking patterns of sandbody gradually evolution from the downcut type to the superposition or butted type and finally to the isolated type. A single sandbody is divided into one or more composite laminate groups by the interfaces of 3rd-order architectural elements, which are identified according to the flow regime and sedimentary structures. Different orders of architectural units have different controlling effects over remaining oil distribution and enrichment, and the corresponding technical measures are proposed to enhance the remaining oil recovery. The above mentioned analyses can provide a valuable reference for oil and gas production in the late stage of an oil field with a similar depositional environment.

Early Aptian development and OAE 1a-linked demise of a carbonate platform in the western Tethys: Lower Cretaceous of Sierra Mariola (South Iberian Paleomargin, SE Spain)

A carbonate platform succession of early Aptian age is described from the Sierra Mariola, which is located within the Prebetic Zone of the Betic External Zones (BEZ) of the Southern Iberian Palaeomargin, in SE Spain. The facies and stratigraphic architecture of the studied succession are described and analysed from multiple logged sections to characterize different depositional environments.The cyclic stacking of facies observed suggests a possible orbital forcing of climate transmitted to sedimentation. Superbundles are recognized describing two sequences of Regressive-Transgressive (R-T) evolution. Analysis of carbon-isotope data identifies both the interval before the hyperthermal event of the early Aptian Oceanic Anoxic Event (OAE 1a) and the onset of the latter event, with an abrupt change in sedimentation marking the demise of the carbonate platform. The field outcrop thus preserves a record of the transition from a greenhouse world to a hothouse event.

Sequence stratigraphic analysis of the late Permian Changhsingian platform marginal reef, Western Hubei, South China

This study presents a comprehensive analysis of the late Permian platform marginal reefs in the Sichuan Basin, focusing on reefal lithofacies and sequence stratigraphic patterns. Field outcrop observations and rock sample analyses from the Jiantianba reef were conducted to establish an evolution model of sponge reef development and spatial distribution. Four stages of marginal carbonate platform were documented: open platform, gently sloping reef, steeply sloping reef, and reef bank system. Distinct lithofacies were identified in these stages, reflecting different depositional environments and growth rates. The gently sloping reef was composed of filled skeleton framestone, filled skeleton bafflestone, and micrite organism limestone, indicating limited reef-building capacity. In contrast, the lithofacies of steeply sloping reefs were composed of open skeleton framestone, open skeleton bafflestone, binding skeleton bafflestone, and benthic organism bindstone, indicating stronger reef-building ability. Based on depositional features and carbon isotopic trends, the reef strata were divided into two sequences. Sequence 1 corresponds to the formation of unit 1, and sequence 2 can be further divided into units 2 and 3. In unit 1, reefs developed in a relatively deeper-water setting. It was characterized by rich micrite limestone, forming a gentle margin. Unit 2 witnessed reef development in shallower waters. Early marine cementation and microbial clots were prevalent, contributing to form a steep margin. In the early stage of unit 3, reefs primarily developed in a tidal-controlled environment. Subsequently, reef strata experienced a transition to a wave-influenced environment, leading to the formation of a reef bank system. In general, sequence 1 mainly formed in a heterozoan-dominated factory, and reefs contributed to a relatively gently sloping platform margin. In contrast, sequence 2 formed in a photozoan-dominated factory, and reefs contributed to a relatively steeply sloping platform margin.

Provenance of the He 8 Member of the Upper Paleozoic Shihezi Formation, Ordos Basin, China: Insights from Heavy Minerals, Paleocurrents, Detrital Zircon Chronology, and Hf Isotopes

The Ordos Basin is located in the western part of the North China Craton. The Upper Paleozoic Shihezi Formation, particularly the He 8 Member, is one of the main gas-bearing strata. However, the source areas for the north and south sections have not been clearly distinguished, which has constrained oil and gas exploration to some extent. Therefore, understanding the source rock evolution of He 8 Member in both the south and north basins will provide a favorable theoretical basis for oil and gas exploration. The provenance of the He 8 Member of the Shihezi Formation in the Ordos Basin has not been well defined until now. Seven wellbore sandstone samples and three field outcrop sandstone samples from the He 8 Member in the Ordos Basin were analyzed. Based on zircon U–Pb dating and Lu–Hf isotope analyses, zircon assemblages of 520–386 Ma and 350–268 Ma in the southern Ordos Basin might have originated from the North Qinling Orogenic Belt (NQinOB) and the North Qilian Orogenic Belt (NQiOB); the 350–268 Ma age group of zircons from the NQinOB, and a large number of ~320–260 Ma detrital zircons supplied to the southern Ordos Basin by the NQinOB suggest that NQinOB magmatic and/or metamorphic events may have occurred in the NQinOB during the ~320–260 Ma period. From ~320–260 Ma, the NQinOB might have experienced significant tectonic activity that has not been fully revealed thus far. The zircons from 2600–2300 Ma, 2000–1600 Ma, and 450–300 Ma in the northern Ordos Basin might have been derived from the Trans-North China Orogenic Belt (TNCO), the Khondalite Belt, the Yinshan Belt, and the Alxa Belt. The paleocurrent and heavy mineral analyses determined that there are certain differences between the northern Ordos Basin and southern Ordos Basin, with unstable minerals such as barite and pyrite, as well as moderately stable minerals such as garnet, showing an increasing trend from south to north. There are also differences in the dominant paleocurrent directions between the south and north parts of the basin, and the Hf isotope data in the Ordos Basin show two-stage Hf model ages (TDM2) ranging from 918 Ma to 3574 Ma. As a result, the He 8 Member deposits in the southern Ordos Basin and northern Ordos Basin had different sources. The southern Ordos Basin might have derived from the NQinOB, the NQiOB, and the TNCO, and the northern Ordos Basin might have derived from the TNCO, the Khondalite Belt, the Yinshan Belt, and the Alxa Belt.

Multiscale Characterization of Fractures and Analysis of Key Controlling Factors for Fracture Development in Tight Sandstone Reservoirs of the Yanchang Formation, SW Ordos Basin, China

Tight sandstone reservoirs, despite their low porosity and permeability, present considerable exploration potential as unconventional hydrocarbon resources. Natural fractures play a crucial role in hydrocarbon migration, accumulation, and present engineering challenges such as late-stage reformation in these reservoirs. This study examines fractures in the seventh member of the Triassic Yanchang Formation’s tight sandstone within the Ordos Basin using a range of methods, including field outcrops, core samples, imaging and conventional logging, thin sections, and scanning electron microscopy. The study clarifies the characteristics of fracture development and evaluates the relationship between dynamic and static rock mechanics parameters, including the calculation of the brittleness index. Primary factors influencing fracture development were quantitatively assessed through a combination of outcrop, core, and mechanical test data. Findings reveal that high-angle structural fractures are predominant, with some bedding and diagenetic fractures also present. Acoustic, spontaneous potential, and caliper logging, in conjunction with imaging data, enabled the development of a comprehensive probabilistic index for fracture identification, which produced favorable results. The analysis identifies four key factors influencing fracture development: stratum thickness, brittleness index, lithology, and rock mechanical stratigraphy. Among these factors, stratum thickness is negatively correlated with fracture development. Conversely, the brittleness index positively correlates with fracture development and significantly influences fracture length, aperture, and linear density. Fractures are most prevalent in siltstone and fine sandstone, with minimal development in mudstone. Different rock mechanics layer types also impact fracture development. These insights into fracture characteristics and controlling factors are anticipated to enhance exploration efforts and contribute to the study of similar unconventional reservoirs.

Synergistic application of digital outcrop characterization techniques and deep learning algorithms in geological exploration

In order to meet the needs of geologists for the analysis of data characterizing field outcrops (rock sections or formations exposed on the ground surface), this study developed a field digital outcrop visualization platform based on Cesium (a 3D geospatial visualization technology) digital outcrop characterization technology. The platform was developed based on WebGL (a protocol for rendering interactions on web pages), which overcame the shortcomings of traditional software in terms of visualization, cross-device, cross-platform, and ease of use. Firstly, UAV inclined photography is used for data collection, which transforms a large amount of geological data into an intuitive 3D geological model, while the visualization platform provides rich measurement and mapping tools for the identified features, which more intuitively displays the outcrop information, helps geological explorers to understand the geological conditions in the field more quickly and comprehensively, and improves the analysis efficiency and ease-of-use of outcrop characterization data. Combined with the improved VGG19 (a deep convolutional neural network architecture) algorithm model, it has excellent performance in dealing with the fine texture and complex structure of rocks, which significantly improves the accuracy of lithology identification. The synergistic application of this technology provides geologists with a faster and more comprehensive means to understand the geological conditions in the field. The reliability of combining the Cesium digital outcrop characterization technology with the VGG19 lithology identification algorithm in geological exploration is verified through case studies. The synergistic application of this technology will greatly enhance the efficiency and ease of analysis of outcrop characterization in the field, and provide new perspectives for future research in geosciences.

Discussion on the development practice of low permeability reservoir of beach bar in saltwater lake basin—Take Wu5 block of Wunan Oilfield in Qaidam Basin as an example

Abstract Based on the analysis of field outcrops, drilling cores, casting thin sections, logging interpretation, and development characteristics in the southwestern region of the Qaidam Basin, combined with the sedimentary characteristics, development patterns, and injection production well networks of the Neogene saline lake facies beach and dam in the southwestern region of the Qaidam Basin, his study aims to optimize the displacement system and improve the production rate. This paper counters the difficulty of Severely reservoir heterogeneity and interlayer contradictions low producing degree of water drive utilization and unclear understanding of water drive pattern in the study area; the article adopts Reservoir Engineering Methods, applies water injection development displacement and well network optimization evaluation techniques and bases on the geological characteristics of the reservoir to conduct the first time small well spacing pilot test in Block Wu 5, The results confirm that small well spacing encryption is feasible in this block.Therefore, the deployment and adjustment of small well spacing well network encryption in high structural areas of the area. Through implementation in the past two years, The small well spacing encryption in low-permeability sandbar reservoirs can effectively reduce seepage resistance, increase effective driving capacity, improve recovery rate and recovery rate, improve reservoir development efficiency, and increase economic benefits.

Oolitic Sedimentary Characteristics of the Upper Paleozoic Bauxite Series in the Eastern Ordos Basin and Its Significance for Oil and Gas Reservoirs

In recent years, great breakthroughs have been made in gas explorations of the Upper Paleozoic bauxite series in the Longdong area of the Ordos Basin, challenging the understanding that bauxite is not an effective reservoir. Moreover, studying the reservoir characteristics of bauxite is crucial for oil and gas exploration. Taking the bauxite series in the Longdong area as an example, this study systematically collects data from previous publications and analyzes the petrology, mineralogy, oolitic micro-morphology, chemical composition, and other sedimentary characteristics of the bauxite series in the study area using field outcrops, core observations, rock slices, cast slices, X-ray diffraction analysis, scanning electron microscopy and energy spectra, and so on. In this study, the oolitic microscopic characteristics of the bauxite reservoir and the significance of oil and gas reservoirs are described. The results show that the main minerals in the bauxite reservoir are boehmite and clay minerals composed of 73.5–96.5% boehmite, with an average of 90.82%. The rocks are mainly bauxitic mudstone and bauxite. A large number of oolites are observable in the bauxite series, and corrosion pores and intercrystalline pores about 8–20 μm in size have generally developed. These pores are important storage spaces in the reservoir. The brittleness index of the bauxite series was found to be as high as 99.3%, which is conducive to subsequent mining and fracturing. The main gas source rocks of oolitic bauxite rock and the Paleozoic gas series are the coal measure source rocks of the Upper Paleozoic. The oolitic bauxite reservoirs in the study area generally have obvious gas content, but the continuity of the planar distribution of the bauxite reservoirs is poor, providing a scientific basis for studying bauxite reservoirs and improving the exploratory effects of bauxite gas reservoirs.

Prototypical basin reconstruction and evolutionary models for the Precambrian of the Qinshui Basin, North China craton

The thick Precambrian strata in the Qinshui Basin (QB) have high potential for hydrocarbon exploration. Tectonic evolution restoration and prototype basin reconstruction are key to hydrocarbon exploration and evaluation for Precambrian strata. We studied the 2D/3D seismic data and Precambrian field outcrops of the QB to elucidate its geological attributes, major regional unconformities, tectonostratigraphic framework, tectonic evolution and prototype basin. The Precambrian strata of the QB are the sedimentary products of the continuous transgression from south to north and are more appropriately attributed to the Xiong'er Aulacogen in the Changchengian and Jixian periods. We identified six regional unconformities: the Archean or Palaeoproterozoic/Xiong'er Group, the Xiong'er Group/Chy, Chc/Chl, Chl/Jxl, the Jxl/Cambrian System, and the Ordovician/Carboniferous. Seven tectonostratigraphic units can be delineated: the Archean–Palaeoproterozoic, Xiong'er Group, Yunmengshan–Cuizhuang, Luoyukou, Jixian, Cambrian–Ordovician, and Carboniferous–Triassic tectonostratigraphic units. The tectonic evolution was divided into seven stages: the formation and evolution of the basement (∼1800 Ma), the synrift stage in the Xiong'er period (1800–1720 Ma), the postrift stage in the Yunmengshan–Luoyukou period (1720–1600 Ma), the passive continental margin basin stage in the Jixian period (1600–1400 Ma), the marine craton stage during the Cambrian–Ordovician (542–440 Ma), the marine–continental transitional–continental craton stage during the Carboniferous–Triassic (316–200 Ma), and the intense deformation stage (200–0 Ma). Three stages of prototype basin models of the Precambrian in the QB have been reconstructed: (1) In the synrift basin model, numerous normal faults have developed, and the section shows a symmetrical composite graben. The topography in the south is low, that in the north is high, and the northeast and northwest areas are palaeo-uplifted. A set of volcanic rocks with coarse-grained clasts was deposited. (2) In the postrift basin model, the fault activity weakened, and the depositional range became larger than that of the synrift stage, and a set of marine clastic–carbonate rocks was deposited showing a gradual coarsening–upwards trend. (3) In the passive continental margin basin model, the fault became almost inactive, the topography was low in the southeast and high in the northwest, and the depositional area was more widespread, resulting in a set of marine carbonates. These results contribute to a better understanding of the break-up process of the Columbia supercontinent and the Precambrian evolution of the North China Craton (NCC) and can be used to predict the distributions of source rocks and hydrocarbons.

Retrogradation of carbonate platforms on a rifted margin: the late Ediacaran record of the northwestern Yangtze Craton (SW China)

The Late Ediacaran Dengying Formation, located in the Sichuan Basin of the northwestern Yangtze Craton, is of significant interest in oil and gas exploration due to its abundant pores and vugs within microbial mound-shoal complexes. However, there is still uncertainty regarding the spatiotemporal distribution and controlling factors of the platform margin. This study comprehensively analyzes the retrogradation pattern of the Dengying Formation platform margin using seismic data, well logs, field outcrops, and petrological characteristics. Our findings reveal that the Dengying Formation strata surrounding the rift basin at the northwestern of the Yangtze Craton can be divided into three main depositional facies: basin facies, slope facies, and platform margin facies. Additionally, based on the integration of lithological, log, and seismic characteristics, the Dengying Formation is subdivided into four third-order sequences, with five sequence boundaries and three seismic facies identified. Supported by sequence stratigraphy and geophysical data, we have reconstructed the tectono-sedimentary evolution of the multiple platform margins on the eastern side of the Deyang-Anyue rift in the Sichuan Basin during the late Ediacaran. Our findings indicate that the platform underwent two phases of retrogradation. The second-stage platform margin underwent retrogradation towards the interior, spanning a distance between 10 and 80 km, based on the initial configuration established by the first-stage platform margin. The main controls for progradation and retrogradation of carbonate platforms are eustatic sea-level changes and tectonic activity. Eustatic sea-level changes can be divided into constructive and destructive phases. Constructive phases are commonly observed in highstand systems tracts, while destructive phases are often associated with transgressive systems tracts and are related to platform retrogradation processes. However, sea-level changes alone cannot fully control the process of platform retrogradation. The thermal subsidence following mantle plume events likely played a significant role in the retrogradation of the platform in the study area. During this period, tectonic processes controlled the geometry of the platform and the deposition of carbonates in the platform margin-slope-basin environment. Additionally, karst-related mound-shoal complexes developed extensively along the platform margin of the Dengying Formation in the northwestern Yangtze Craton. The Lower Cambrian dark shales represent high-quality hydrocarbon source rocks, while the Dengying Formation exhibits an optimal source-reservoir configuration.

Transgression patterns of Muara Enim Formation at Tanjung Agung Village based on stratigraphic data measurements

Abstract Muara Enim Formation is one of the formations which is composed in the South Sumatra Basin in the Late Miocene Epoch. It has a specific lithology which shows how the conditions of the depositional environment were the formation was deposited. The purpose of this research is to determine the pattern of transgression and regression of Muara Enim Formation. Transgression and regression patterns are used to determine how the shoreline moves. It is caused by the supply of sediment material. The finer sediment material is deposited and the smoother the movement of the material towards the land, showing the indication of transgression, and vice versa. This research uses analytical-interpretative as the method, which is using the result of sampling analysis and then interpreting how the pattern of transgression and regression are changed. This method is used to determine the physical properties of the lithology composing the Muara Enim Formation. Based on field observations on the Air Enim River, it is known that the Muara Enim Formation is composed of lithology dominated by fine grain size (claystone) to medium grain size (tuffaceous sandstone) with thin coal in several locations. Based on field outcrop (surface data) it is known that the claystone lithology is thickened to a younger layer (about 2.5 m thickness) with massive sedimentary structure. It is interpreted that the Muara Enim Formation was deposited in a transgression pattern in a depositional phase which was influenced by sediment supply factors and weak depositional currents.

Multi-phase deformation and analogue modelling of the Junggar Basin, NW China

Based on the theory of superimposed deformation and the regional tectonic background, the multi-phase non-coaxial superimposed structures in Junggar Basin were systematically analyzed using seismic interpretation, field outcrop observation, and paleo-stress field recovery methods according to the characteristics of the current tectonic framework. Moreover, the tectonic evolution process of the basin was reconstructed using sandbox analogue modelling technology. The results showed that the study area has experienced five phases of non-coaxial deformation with superimposition: The first phase of deformation (D1) is characterized by NNE-SSW extension during late Carboniferous to early Permian, which formed large graben, half graben and other extensional structure style around the basin. The second phase of deformation (D2) is represented by NE-SW compression during the middle to late Permian, and it comprised numerous contraction structures that developed based on D1. The basic form of the entire basin is alternating uplift and depression. The third phase of deformation (D3) is the NW-SE transpressional strike-slip in the Triassic-Jurassic, which produced numerous strike-slip structural styles in the middle part of the basin. The fourth phase of deformation (D4) is the uniform sedimentation during Cretaceous, and the fifth phase (D5) is the compression along NNE-SSW due to the North Tianshan northward thrust, which produced three rows of fold thrust belts and tear faults in the front of the mountain in the southern margin of the basin. The newly established three-dimensional tectonic evolution model shows that, based on the large number of NW-trending grabens and half grabens in the Carboniferous basement of Junggar Basin, multiple level NE trending uplifts have formed with the joint superposition of the late structural inversion and multiple stress fields. This has resulted in the current tectonic units of alternating uplifts and depressions in different directions in the study area.

New Understanding of the Early Cambrian Uplift–Depression Framework and the Large-Scale Source–Reservoir Distribution along the Margin of the Awati Sag in Tarim Basin, NW China

The uplift–depression framework controls the source–reservoir assemblage. However, the exploration breakthrough is restricted by an insufficient understanding of the uplift–depression differentiation framework in the Early Cambrian Keping–Wensu area. In this paper, based on field outcrops evaluations, thin section analysis, logging data, drilling data, and 3D seismic data, Wensu low paleo-uplift was discovered in the northern Tarim Basin, and the planar distribution was demonstrated in detail, generally shown as a SW–NE trending nose structure, extending roughly 114 km in length to the southwest, about 35 km in width to the northeast, and with the overall characteristic of being high in the west and low in the east. During the Early Cambrian, the Tabei paleo-uplift evolved into the Wensu low paleo-uplift and largely died out by the Middle Cambrian, with the development of ramps and rimmed carbonate platforms. The tectonic-sedimentary evolution of the uplift–depression framework controlled the development of a set of main source rocks and two sets of large-scale effective reservoir rocks in the Lower Cambrian, constituting two sets of effective hydrocarbon accumulation in the upper and lower stratigraphic parts of the basin. Among them, the upper assemblage holds more potential for hydrocarbon exploration, and is expected to be a next strategic target area for hydrocarbon exploration of Cambrian subsalt in the Keping–Wensu area.

Estimation of the dolomite content of carbonate rock outcrops based on spectral knowledge and machine learning

Accurately estimating the dolomite content in carbonate rocks is crucial for optimizing oil and gas exploration and production strategies. Hyperspectral techniques for estimating dolomite content have advantages in terms of efficiency, cost-effectiveness, and non-destructiveness compared with traditional laboratory methods. Despite the abundance of hyperspectral data, feature selection and extraction remain challenging. In this study, hyperspectral data collected from surface outcrop in the field using the analytical spectral device (ASD) were applied to construct model for estimating dolomite content. Firstly, the data were preprocessed via outlier analysis and continuum transformation. Next, a hybrid approach integrating spectral knowledge with machine learning was proposed and applied to facilitate efficient and precise feature selection of the hyperspectral data; in this approach, preliminary screening based on spectral knowledge is followed by further hyperspectral data feature selection using a random forest algorithm. The selected features were then combined using a support vector regression algorithm to obtain the estimation model. Finally, the accuracy of the model was evaluated using the hyperspectral data from field outcrop samples. To further verify the effectiveness of this method, various combinations of eight input variables and four machine learning algorithms were compared. Among all combinations, our model achieved the highest accuracy with a test R2 value of 0.91 and a root-mean-square error of only 0.122. The proposed method is practical and efficient and provides precise quantitative data for field geologists to identify the mineral distribution in outcrops. Thus, our method provides robust support for understanding reservoir characteristics and has significant practical value in geological surveys and mineral exploration.

Regolith mapping using airborne gamma-ray spectrometry in central Brazil

In deeply weathered terrains, with high geological diversity and scarce outcrops, the lateritic regolith features support mineral research. Predictive regolith mapping uses spatial modeling and airborne gamma-ray spectrometric data to locate weathered residual products, such as the ferruginous lateritic duricrusts found in tropical regions. Although the regolith materials exhibit qualitative similarities in their gamma-ray spectrometric responses in RGB (KThU) images, the Boolean and fuzzy approaches highlighted differences that allows for mapping the regolith weathering residual products on a wide geodiversity protoliths. Two inference systems were used, one identified the ferruginous lateritic duricrusts and oxisols derived from the felsic/pelitc and mafic rocks types (TP test procedures), and the other identified solely those derived exclusively from mafic rocks (TPM test procedures). Cutoff values for the conjunction eTh/K ∩ eU/K ∩ eTh ∩ K (TP-4) using Boolean logic, provided the most accurate regolith map (accuracy = 95%) among the various tests carried out and indicate that in 28.6% of the total study area, there are lateritic duricrust and oxisols relative to rocks, saprolites, mottled horizons and other soils type. The eTh cutoff value > 4.4 ppm was used to exclude mafic rocks and associated soils, and the K value < 1.3% to exclude rocks with eTh/K and eU/K values similar to the lateritic duricrusts and oxisols. For mapping the lateritic duricrusts derived exclusively from the mafic rocks the conjunction of eTh ∩ eU/K ∩ eTh/K ∩ K ∩ eU (TPM-3) fuzzified by the large membership function was the most efficient. The eU variable with values < 1.21 ppm helps improve the effectiveness of this procedure. The generated predictive map indicated that in about ∼12% (657.9 km2) of the total study area, there are lateritic duricrusts and oxisols derived from mafic rocks. Therefore, a successful method of mapping vast areas from complicated lateritic regolith mosaic derived from a large geodiversity is to apply specific plugins for data fuzzification, supported by observations from field outcrops.

Sedimentological and Geochemical Characterization of the Early Cambrian Eastern Yunnan, Southwestern China

The accurate reconstruction of the early Cambrian paleoclimate and paleoceanographic conditions on the Yangtze Plate is crucial for understanding the ancient environment during the Cambrian Explosion. It is also a key factor in understanding the ecological habits of organisms during the Cambrian Explosion. The study utilized field outcrops, thin section analysis, and major and trace elements to investigate the sedimentary environment, provenance, paleoweathering, and paleoclimate of the Lower Cambrian Hongjingshao (HJS) Formation (Cambrian Stage 3, ~515 Ma) in the Yangtze Basin, eastern Yunnan, SW China. The HJS sandstones are composed of 10 lithofacies, including massive and weakly bedded gravel supported by coarse sandstone (Gm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), ripple cross-laminated sandstone (Sr), horizontal bedded sandstone (Sh), scour-fill sandstone (Se), massive sandstone (Sm), fine to medium sandstone with thin bed muddy siltstone (Fl), muddy siltstone (Fsc), and mudstone (Fm). On the basis of these lithofacies, channel fill and over-bank deposits in delta and shallow shelf depositional environments are suggested for HJS Formation. The major elements-based provenance discriminant function and mineral composition indicate that felsic rocks from the recycled orogen and continental block are the main sediment source terrane for the HJS sandstones of the study area. CIA, PIA, and CIW values range from 71.29 to 93.72, indicating an intermediate to intense chemical weathering and semiarid to humid climate conditions in Cambrian Stage 3. The research findings have clarified the paleoclimate and paleoceanographic environment of the Early Cambrian in the Yangtze region, which is of significant importance for understanding the early biological and ecological marine environment in the study area.

Sedimentary characteristics of mixed source fine-grained gravity-flow and its significance for shale oil exploration in a lacustrine depression basin: A case study of the Chang 73 Sub-member of the Triassic Yanchang Formation in Ordos Basin, NW China

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang73 Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 73 Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.

Differential sedimentary evolution of typical aulacogens of Meso-Neoproterozoic in North China craton

Many countries and regions in the world have obtained industrial oil flow in the Meso-Neoproterozoic sedimentary strata and formed commercial exploitation in recent years. The development horizon of high-quality source rocks in the Proterozoic in North China can be compared with the international, indicating that the Meso-Neoproterozoic in North China has great exploration potential. The sedimentary characteristics of typical aulacogen in multiple cratons in the Meso-Neoproterozoic North China Craton are compared and studied by using field outcrop data, drilling data and analysis and test data, aiming to provide sedimentary support for the prediction of oil and gas distribution and evaluation of exploration field in the Meso-Neoproterozoic in this area. The results show that there are four sedimentary systems in the study area, including Marine clastic rock sedimentary system, Marine carbonate sedimentary system, Marine-continental transitional facies sedimentary system and glacial sedimentary system. They are divided into seven sedimentary facies types: barrier coastal facies, non-barrier coast facies, shallow shelf facies, carbonate platform facies, reef facies, fan delta facies and glacial facies, and further divided into 15 subfacies and 21 microfacies. On this basis, the Meso-Neoproterozoic sedimentary filling sequences of two typical aulacogens, Yanliao and Xiong 'er, in the study area are clarified, showing that the formation time of each sedimentary filling sequence stage of different aulacogens is different, and the rock characteristics, lithology combination, lithologic structure, contact relationship, vertical sequence and sedimentary facies assemblage of the same sedimentary filling sequence stage are obviously different. The filling characteristics of the two aulacogens completely record the geological events related to the breakup of the Colombian supercontinent.