Search Results (119)

Traps are central to petroleum exploration, where hydrocarbons accumulate during migration. Reservoirs are likewise an essential petroleum system element and serve as the primary medium for hydrocarbon storage. The buried hill is a geological formation highly favorable for reservoir development. However, the factors influencing hydrocarbon accumulation in buried hill reservoirs are highly diverse, especially in areas with complex, active fault systems. Fault systems play a dual role, both in the formation of reservoirs and in the migration of hydrocarbons. Therefore, understanding the impact of complex fault systems helps enhance the exploration success rate of buried hill traps and guide drilling deployment. In the Beibuwan Basin in the South China Sea, buried hill traps are key targets for deep-buried hydrocarbon exploration in this faulted basin. The low level of exploration and research in buried hills globally limits the understanding of hydrocarbon accumulation conditions, thereby hindering large-scale hydrocarbon exploration. By using drilling data, logging data, and seismic data, stress fields and tectonic faults were restored. There are two types of buried hills developed in the Beibuwan Basin, which were formed during the Late Ordovician-Silurian period and Permian-Triassic period, respectively. The tectonic genesis of the Late Ordovician-Silurian period buried hills belongs to magma diapirism activity, while the tectonic genesis of the Permian-Triassic period buried hills belongs to reverse thrust activity. The fault systems formed by two periods of tectonic activity were respectively altered into basement buried hills and limestone buried hills. The negative structural inversion controls the distribution and interior stratigraphic framework of the deformed Carboniferous strata in the limestone buried hill. The faults and derived fractures of the Late Ordovician-Silurian period and Permian-Triassic period promoted the diagenesis and erosion of these buried hills. The faults formed after the Permian-Triassic period are not conducive to calcite cementation, thus facilitating the preservation of the reservoir space formed earlier. The control of hydrocarbon accumulation by the fault system is reflected in two aspects: on the one hand, the early to mid-Eocene extensional faulting activity directly controlled the depositional process of lacustrine source rocks; on the other hand, the Late Eocene-Oligocene, which is closest to the hydrocarbon expulsion period, is the most effective fault activity period for connecting Eocene source rocks and buried hill reservoirs. This study contributes to understanding of the role of complex fault activity in the formation of buried hill traps within hydrocarbon-bearing basins. Full article

The lacustrine to deltaic depositional systems of the Upper Jurassic Qigu Formation in the Yongjin area constitute a significant petroleum reservoir in the central Junggar Basin, China. Based on core observations, petrology analyses, paleoenvironment indicators and modern sedimentary analyses, sequence stratigraphy, lithofacies associations, sedimentary environment, evolution, and models were investigated. The Qigu Formation can be divided into a third-order sequence consisting of a lowstand systems tract (LST) and a transgressive systems tract (TST), which is further subdivided into six fourth-order sequences. Thirteen lithofacies and five lithofacies associations were identified, corresponding to shallow-water delta-front deposits. The paleoenvironment of the Qigu Formation is generally characterized by an arid freshwater environment, with a dysoxic to oxic environment. During the LST depositional period (SQ1–SQ3), the water depth was relatively shallow with abundant sediment supply, resulting in a widespread distribution of channel and mouth bar deposits. During the TST depositional period (SQ4–SQ6), the rapid rise in base level, combined with reduced sediment supply, resulted in swift delta retrogradation and widespread lacustrine sedimentation. Combined with modern sedimentary analysis, the shallow-water delta in the study area primarily comprises a composite system of single main channels and distributary channel-mouth bar complexes. The channel-bar complex eventually forms radially distributed bar assemblages with lateral incision and stacking. The distributary channel could incise a mouth bar deeply or shallowly, typically forming architectural patterns of going over or in the mouth bar. Reservoir test data suggest that the mouth bar sandstones are favorable targets for lithological reservoir exploration in shallow-water deltas. Full article

Eutrophication has intensified in lacustrine systems across the American continent, which has been primarily driven by human activities such as intensive agriculture, wastewater discharge, and land-use change. This phenomenon adversely affects water quality, biodiversity, and ecosystem functioning. However, studies addressing the historical evolution of trophic states in lakes and reservoirs remain limited—particularly in tropical and subtropical regions. In this context, sedimentary records serve as invaluable archives for reconstructing the environmental history of water bodies. Paleolimnological approaches enable the development of robust chronologies to further analyze physical, geochemical, and biological proxies to infer long-term changes in primary productivity and trophic status. This review synthesizes the main methodologies used in paleolimnological research focused on trophic state reconstruction with particular attention to the utility of proxies such as fossil pigments, diatoms, chironomids, and elemental geochemistry. It further underscores the need to broaden spatial research coverage, fostering interdisciplinary integration and the use of emerging tools such as sedimentary DNA among others. High-resolution temporal records are critical for disentangling natural variability from anthropogenically induced changes, providing essential evidence to inform science-based lake management and restoration strategies under anthropogenic and climate pressures. Full article

The lower sub-member of Member 2, Dongying Formation (Paleogene) in the HHK Depression hosts an extensively developed thin-bedded beach-bar system characterized by favorable source rock conditions and reservoir properties, indicating significant hydrocarbon exploration potential. Integrating drilling cores, wireline log interpretations, three-dimensional seismic data, geochemical analyses, and palynological data, this study investigates the sedimentary characteristics, sandbody distribution patterns, controlling factors, and genetic model of this lacustrine beach-bar system. Results reveal the following: (1) widespread thin-bedded beach-bar sandbodies dominated by fine-grained sandstones and siltstones, exhibiting wave ripples and low-angle cross-bedding; (2) two vertical stacking patterns, Type A, thick mudstone intervals intercalated with laterally continuous thin sandstone layers, and Type B, composite sandstones comprising thick sandstone units overlain by thin sandstone beds, both demonstrating significant lateral continuity; (3) three identified microfacies: bar-core, beach-core, and beach-margin facies; (4) key controls on sandbody development: paleoenvironmental evolution establishing the depositional framework, secondary fluctuations modulating depositional processes, strike-slip extensional tectonics governing structural zonation, paleobathymetry variations and paleotopography controlling distribution loci, and provenance clastic influx regulating scale and enrichment (confirmed by detrital zircon U-Pb dating documenting a dual provenance system). Collectively, these findings establish a sedimentary model for a thin-bedded beach-bar system under the strike-slip extensional tectonic framework. Full article

Given the increasing challenges posed by frequent extreme climatic events, understanding the climate–human connection between the climate system and the transitions of ancient civilizations is crucial for addressing future climatic challenges, especially when examining the relationship between the abrupt events of the Holocene and the Neolithic culture development. Compared with the globally recognized “4.2 ka collapse” of ancient cultures, the initial start time and the cultural significance of the 5.5 ka climatic fluctuation are more complex and ambiguous. The Hongshan culture (6.5–5.0 ka) is characterized by a complicated society evident in its grand public architecture and elaborate high-status tombs. However, the driving mechanisms behind cultural changes remain complex and subject to ongoing debate. This paper delves into the role of climatic change in Hongshan cultural shifts, presenting an integrated dataset that combines climatic proxy records with archaeological data from the Hongshan culture period. Based on synthesized aeolian, fluvial-lacustrine, loess, and stalagmite deposits, the study indicates a relatively cold and dry climatic fluctuation occurred during ~6.0–5.5 ka, which is widespread in the Horqin dune field and adjacent areas. Combining spatial analysis with ArcGis 10.8 on archaeological sites, we propose that the climatic fluctuation between ~6.0–5.5 ka likely triggered the migration of the Hongshan settlements and adjustment of survival strategies. Full article

As a strategic replacement area for hydrocarbon exploration in the Tarim Basin, the Kuqa Depression has been the subject of relatively limited research on the sedimentary characteristics of the Triassic strata within its western Wushi Sag, which constrains exploration deployment in this region. This study focuses on the Wushi Sag, systematically analyzing the sedimentary facies types, the evolution of sedimentary systems, and the distribution patterns of the Triassic Kelamayi and Huangshanjie formations. This analysis integrates field outcrops, drilling cores, wireline logs, and 2D seismic data, employing methodologies grounded in foreland basin theory and clastic sedimentary petrology. The paleo-geomorphology preceding sedimentation was reconstructed through balanced section restoration to investigate the controlling influence of foreland tectonic movements on the distribution of sedimentary systems. By interpreting key seismic profiles and analyzing vertical facies successions, the study classifies and evaluates the petroleum accumulation elements and favorable source–reservoir-seal assemblages, culminating in the prediction of prospective exploration areas. The research shows that: (1) The Triassic in the Wushi Sag mainly develops fan-delta, braided-river-delta, and lacustrine–shallow lacustrine sedimentary systems, with strong planar distribution regularity. The exposed strata in the northern part are predominantly fan-delta and lacustrine systems, while the southern part is dominated by braided-river-delta and lacustrine systems. (2) The spatial distribution of sedimentary systems was demonstrably influenced by tectonic activity. Paleogeomorphological reconstructions indicate that fan-delta and braided-river-delta sedimentary bodies preferentially developed within zones encompassing fault-superposition belts, fault-transfer zones, and paleovalleys. Furthermore, Triassic foreland tectonic movements during its deposition significantly altered basin configuration, thereby driving lacustrine expansion. (3) The Wushi Sag exhibits favorable hydrocarbon accumulation configurations, featuring two principal source–reservoir assemblages: self-sourced structural-lithologic gas reservoirs with vertical migration pathways, and lower-source-upper-reservoir structural-lithologic gas reservoirs with lateral migration. This demonstrates substantial petroleum exploration potential. The results provide insights for identifying favorable exploration targets within the Triassic sequences of the Wushi Sag and western Kuqa Depression. Full article

The Lower–Middle Jurassic of the Kuqa Depression consists of terrestrial clastic deposits containing coal seams and thick lacustrine mudstones, and is of great significance for oil and gas exploration. Based on the comprehensive analysis of core, well-logging, outcrop, and seismic data, the sequence stratigraphy, depositional systems, and the controlling factors of the basin filling in the depression are systematically documented. Four primary depositional systems, including braided river delta, meandering river delta, lacustrine, and swamp deposits, are identified within the Ahe, Yangxia, and Kezilenuer Formations of the Lower–Middle Jurassic. The basin fills can be classified into two second-order and nine third-order sequences (SQ1–SQ9) confined by regional or local unconformities and their correlative conformities. This study shows that the sedimentary evolution has undergone the following three stages: Stage I (SQ1–SQ2) primarily developed braided river, braided river delta, and shallow lacustrine deposits; Stage II (SQ3–SQ5) primarily developed meandering river, meandering river delta, and extensive deep and semi-deep lacustrine deposits; Stage III (SQ6–SQ9) primarily developed swamp (SQ6–SQ7), meandering river delta, and shore–shallow lacustrine deposits (SQ8–SQ9). The uplift of the Tianshan Orogenic Belt in the Early Jurassic (Stage I) may have facilitated the development of braided fluvial–deltaic deposits. The subsequential expansion of the sedimentary area and the weakened sediment supply can be attributed to the planation of the source area and widespread basin subsidence, with the transition of the depositional environments from braided river delta deposits to meandering river delta and swamp deposits. The regional expansion or rise of the lake during Stage II was likely triggered by the hot and humid climate conditions, possibly associated with the Early Jurassic Toarcian Oceanic Anoxic Event. The thick swamp deposits formed during Stage III may be controlled by the interplay of rational accommodation, warm and humid climatic conditions, and limited sediment supply. Milankovitch cycles identified in Stage III further reveal that coal accumulation was primarily modulated by long-period eccentricity forcing. Full article

The Aptian–Albian interval represents a significant cooling phase within the Cretaceous “hothouse” climate, marked by dynamic climatic fluctuations. High-resolution continental records are essential for reconstructing terrestrial climate and ecosystem evolution during this period. This study examines a lacustrine-dominated succession of the Shahezi Formation (Lishu Rift Depression, Songliao Basin, NE Asia) to access paleo-weathering intensity and paleoclimate variability between the Middle Aptian and Early Albian (c. 118.2–112.3 Ma). Multiple geochemical proxies, including the Chemical Index of Alteration (CIA), were applied within a sequence stratigraphic framework covering four stages of lake evolution. Our results indicate that a hot and humid subtropical climate predominated in the Lishu paleo-lake, punctuated by transient cooling and drying events. Periods of lake expansion corresponded to episodes of intense chemical weathering, while two distinct intervals of aridity and cooling coincided with phases of a reduced lake level and fan delta progradation. To address the impact of potassium enrichment on CIA values, we introduced a rectangular coordinate system on A(Al₂O₃)-CN(CaO* + Na₂O)-K(K₂O) ternary diagrams, enabling more accurate weathering trends and CIA corrections (CIA_corr). Uncertainties in CIA correction were evaluated by integrating geochemical and petrographic evidence from deposits affected by hydrothermal fluids and external potassium addition. Importantly, our results show that metasomatic potassium addition cannot be reliably inferred solely from deviations in A-CN-K diagrams or the presence of authigenic illite and altered plagioclase. Calculations of “excess K₂O” and CIA_corr values should only be made when supported by robust geochemical and petrographic evidence for external potassium enrichment. This work advances lacustrine paleoclimate reconstruction methodology and highlights the need for careful interpretation of weathering proxies in complex sedimentary systems. Full article

The Dixi area of the Junggar Basin has favorable petroleum geological conditions, with the Cretaceous system representing one of the principal hydrocarbon-bearing strata. However, the genetic origin and mixing characteristics of natural gas across different tectonic zones remain insufficiently understood. In this study, a total of 65 natural gas samples were analyzed using molecular composition and stable carbon isotopic data to determine gas origins and quantify the contributions of different source rocks. A novel multivariate mathematical analysis method was developed and applied to convert compositional and isotopic data into quantitative parameters, enabling the accurate estimation of end-member mixing ratios in natural gas. This methodological innovation addresses the challenge of interpreting multi-source gas systems under complex geological conditions. The results show that the Cretaceous natural gas in the Dixi area is derived from three main sources, comprising both oil-type gas from Permian lacustrine source rocks and coal-type gas from Carboniferous coal-measure source rocks. The calculated mixing proportions exhibit significant spatial variation: in the northern Dixi area, coal-type gas dominates (67.8–84.3%), while the southern zone presents a broader mixture (25.6–68.4% coal-type gas). In the Dongdaohaizi Depression, oil-type gas is predominant, accounting for 89.4–97.7%. This study not only clarifies the genetic classification and mixing dynamics of natural gas in the Dixi area but also provides a quantitative framework for evaluating accumulation processes and source contributions in multi-source gas reservoirs. The proposed method offers valuable guidance for assessing resources and optimizing exploration strategies in the Junggar Basin and other similar basins. Full article

The lacustrine mudstones and shales of the Triassic Yanchang Formation in the Ordos Basin serve as critical hydrocarbon source rocks. However, previous studies predominantly focus on individual lithologies, with comparative investigations into the sedimentary environments of dark mudstones and black shales remaining relatively limited. The study systematically compares sedimentary environment parameters (e.g., paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity characteristics) between mudstones and shales, and how these distinct environmental factors governed the differential enrichment mechanisms of organic matter within the depositional aquatic system has been elucidated. Geochemical proxies (e.g., CIA, Sr/Cu, Rb/Sr, Sr/Ba, V/Ni, U/Th, V/Cr, Rb/Zr, P/Ti, Cu/Ti) reveal marked contrasts: In comparison with the Chang 7 and Chang 8 dark mudstones, the Chang 7 black shales exhibit (1) warmer–humid paleoclimatic regimes, (2) higher paleosalinity, (3) intensely anoxic conditions, (4) deeper paleowater depth, and (5) elevated paleoproductivity. These environmental divergences directly govern the significant total organic carbon content disparity between black shales and dark mudstones. Organic enrichment in the Chang 7 dark mudstones and black shales is primarily controlled by paleoproductivity and paleoredox conditions, with secondary influences from paleoclimate and paleowater depth. Based on the above studies, this research established a differential organic matter enrichment model. This research is of significant importance for guiding oil and gas exploration and development in the Ordos Basin. Full article

The production of gas and condensate from liquid-rich shale reservoirs, particularly within heterogeneous lacustrine systems, remains a critical challenge in unconventional hydrocarbon exploration due to intricate multiphase hydrocarbon partitioning, including gases (C₁–C₂), volatile liquids (C₃–C₇), and heavier liquids (C₇₊). This study investigates a 120-meter-thick interval dominated by lacustrine deposits from the Lower Cretaceous Shahezi Formation (K₁sh) in the Songliao Basin. This interval, characterized by high clay mineral content and silicate–pyrite laminations, was examined to identify the factors controlling hybrid shale gas condensate systems. We proposed the Hybrid Shale Condensate Index (HSCI), defined as the molar ratios of (C₁–C₇)/C₇₊, to categorize fluid phases and address shortcomings in traditional GOR/API ratios. Over 1000 samples were treated by geochemical pyrolysis logging, X-ray fluorescence (XRF) spectrum element logging, SEM-based automated mineralogy, and in situ gas desorption, revealing four primary controls: (1) Thermal maturity thresholds. Mature to highly mature shales exhibit peak condensate production and the highest total gas content (TGC), with maximum gaseous and liquid hydrocarbons at T_max = 490 °C. (2) Lithofacies assemblage. Argillaceous shales rich in mixed carbonate and clay minerals exhibit an intergranular porosity of 4.8 ± 1.2% and store 83 ± 7% of gas in intercrystalline pore spaces. (3) Paleoenvironmental settings. Conditions such as humid climate, saline water geochemistry, anoxic bottom waters, and significant input of volcanic materials promoted organic carbon accumulation (TOC reaching up to 5.2 wt%) and the preservation of organic-rich lamination. (4) Laminae and fracture systems. Silicate laminae account for 78% of total pore space, and pyrite laminations form interconnected pore networks conducive to gas storage. These findings delineate the “sweet spots” for unconventional hydrocarbon reservoirs, thereby enhancing exploration for gas condensate in lacustrine shale systems. Full article

The occurrence of gypsum in clastic rocks of continental saline lake basins reflects complex depositional and diagenetic processes. However, its genesis remains relatively understudied. Based on core descriptions and thin-section analyses, this study investigates the occurrence types and genetic mechanisms of gypsum in the Bottom Sandstone Member of the northern Tabei Uplift. Five types of gypsum occurrences are identified: layered gypsum, gypsum clasts, spotted gypsum, gypsum nodules, and a mixed deposition of clastic rocks and gypsum. The mixed deposition of clastic rocks and gypsum includes gypsiferous mudstone, muddy gypsum, gypsiferous mudstone containing muddy clasts, and sandy gypsum. Layered gypsum, spotted gypsum, gypsiferous mudstone, and muddy gypsum mainly result from in situ chemical precipitation during periods of high evaporation and reduced runoff. In contrast, gypsum clasts, gypsiferous mudstone containing muddy clasts, and sandy gypsum reflect processes of transportation and reworking induced by flood events. Seasonal variations in hydrodynamic conditions play a critical role in the formation and distribution of gypsum. During dry periods, surface runoff weakens or ceases, and the salinity of lake water or pore water in clastic deposits increases due to intense evaporation, promoting gypsum precipitation. During flood periods, increased runoff can erode previously formed gypsum, which is subsequently transported and deposited as gypsum clasts. The morphology of gypsum varies with its transport distance. These findings enhance our understanding of clastic–evaporite mixed systems in arid continental lacustrine settings and provide insights into sedimentary processes influenced by seasonal climatic fluctuations. Full article

The development of fluvial systems over long time scales is a complex interplay of tectonic, climatic, and lithological factors. The initiation and location of fluvial channels in the landscape is less well understood. Recently exposed surfaces provide opportunities to determine factors controlling fluvial channel initiation. During the Würm Last Glacial Maximum (c. 20 ka), the Ain valley in eastern France transformed into a large proglacial lake. Following deglaciation, new drainage channels initiated on the drained lake floor. Extensive morphological and sedimentological mapping and lithogenetic interpretation of the valley fill enable to determine the forcing factors of fluvial channel initiation. The location of the postglacial channels is determined by the initial topography of the lake floor and lithological variability of the sediments. Tributary channels of the Ain preferentially initiated in depressions of gently sloping former delta bottomsets, which prograded from different directions. In addition, the location of channels is determined by the presence of low-permeability, glacio-lacustrine deposits, that favored overland flow and erosion, compared to the highly permeable terrace deposits on the former lake floor. The differences in erodibility of the fine-grained and coarse-grained deposits resulted in relief inversion. Full article

The carbon isotopic behavior of hopane compounds during thermal maturation remains ambiguous due to limitations in current detection techniques. In this study, a low-maturity lacustrine shale sample was pyrolyzed in a hydrous semi-open pyrolysis system. The hopane compounds from the artificially matured samples (R_o = 0.72–1.28%) have been separated and enriched for the test of their carbon isotopes (δ¹³C). The results show that thermal maturity can somewhat affect the carbon isotopes of monomeric hopane compounds, with a maximum difference value over 21‰. However, thermal maturity has different effects on the δ¹³C values for different monomeric hopane compounds. For example, the carbon isotopic values of 22S-homohopane at different thermal stages can vary up to 21‰, while only 3‰ for C₂₉βα. In addition, the carbon isotopes of different monomeric hopane compounds show distinct evolution trends. For C₂₉αβ and C₂₉ Ts, their carbon isotopes first become slightly heavier and then become lighter, reaching the lightest value at 350 °C. When the pyrolysis temperature continues to increase, the δ¹³C values become heavier and finally become lighter. However, the δ¹³C values of Ts, Tm, 22S-homohopane, and 22R-homohopane show a completely reversed trend. They initially become slightly lighter and then become heavier, reaching the maximum value at 350 °C. When the pyrolysis temperature continues to increase, the δ¹³C values become lighter and finally become heavier. Meanwhile, the carbon isotopes of C₂₉βα, C₃₀αβ, C₃₀βα, and non-hopane gammacerane almost remain constant at different thermal stages. When the carbon isotopes of hopane compounds are used in the studies of oil–source correlation, it is prudent to consider the effects of thermal maturity on these values. Full article

Taking the Lucaogou Formation in the Junggar Basin as the research object, this study draws on core mineral data, thin-section observations, and geochemical test results to systematically investigate the enrichment mechanism and migration characteristics of shale oil. The findings show that the Lucaogou Formation is primarily composed of Type I and Type II kerogen, with high hydrocarbon-generation potential; its organic matter mainly originates from lacustrine algae, rich in low-carbon alkanes and tricyclic terpanes, and is well-preserved under reducing conditions. The upper and lower “sweet spots” of the Lucaogou Formation each form an independent source–reservoir–seal system. Shale oil in the upper sweet spot is characterized by low density, low viscosity, high wax content, and a relatively high pour point. Reservoir space is dominated by intergranular pores, dissolution pores, and intercrystalline pores, which are well-developed and exhibit relatively high permeability. By contrast, shale oil in the lower sweet spot is marked by high density, high viscosity, low wax content, and a relatively low pour point. Its reservoir space is dominated by dissolution pores and intercrystalline pores, which are unevenly developed and result in poorer permeability. Overall, shale oil enrichment in the Lucaogou Formation is jointly controlled by organic matter source, diagenesis, and sedimentary environment. This study further clarifies the controlling factors for shale oil enrichment in the Lucaogou Formation and provides a scientific basis for the exploration and development of unconventional oil and gas resources. Full article