Humid Paleoclimate Research Articles

Controls of tectonics and paleoclimate on depositional–diagenetic evolution and pore types of Upper Cretaceous successions (Sarvak Formation) in the Abadan plain, Iran

This study examines the influence of tectonics and paleoclimate on the diagenetic evolution of Upper Cretaceous carbonate reservoirs (Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran. Through petrographic analysis and scanning electron microscopy, the sedimentary facies, diagenetic processes, and pore system within the Sarvak Formation were analyzed. Facies analysis identified eight microfacies, which were deposited in shallow to deep settings of a carbonate ramp. Among these, the grain-dominated facies (packstone and grainstone) of shoal complexes and reef-talus settings were identified as the most important reservoir facies. The formation's complex diagenetic history includes marine, meteoric, and burial diagenetic processes, with meteoric diagenesis playing a significant role beneath paleoexposure surfaces. The Sarvak Formation was categorized into eight flow units, seven pore size classes, eleven electrofacies, eight velocity deviation zones, and thirteen Lorenz zones. The study's findings indicate that the most favorable reservoir units are located beneath the Cenomanian–Turonian exposure surface, characterized by significant dissolution of rudist-dominated facies due to meteoric processes and karstification. Tectonic uplift, along with weathering and erosion within a warm and humid (tropical) paleoclimate, significantly impacts the diagenetic evolution, pore types, and petrophysical properties of the Sarvak Formation. This research underscores the complex interplay between tectonics, paleoclimate, and petrophysical properties in the Abadan Plain, enhancing our understanding of carbonate systems in tectonically active and paleoclimatically dynamic regions.

Seasonally dry tropical forests in the late Pleistocene of Mesopotamia, Argentina and their relationship to environmental changes during the Last Interglacial

ABSTRACTThe concept of seasonal forests groups structural types of vegetation that are related to climatic seasonality in the tropics of South America. Consequently, this determines the physiognomy of the vegetation, from semi‐deciduous to strongly deciduous. The strongest link between seasonally dry tropical forests (SDTFs) is their floristic composition, where Leguminosae and Anacardiaceae dominate the woody flora. The fossil records of the Neogene of northwestern Argentina reveal a list of species found in various locations and formations of the Miocene–Lower Pleistocene obtained from studies of pollen, woods (logs), cuticles, impressions of leaves and fruits. The analysis of sediments and woody structures from the Tapebicuá, Toropí/Yupoí and El Palmar Formations (Upper Pleistocene) of the Mesopotamia region allowed us to identify several pollen taxa and silicified wood fragments (mineralized). The woody and shrubby association whose current relatives characterize the SDTF is composed of the 16 fossil species described here belonging to seven families. In our samples, Anadenanthera colubrina and Myracrodruon balansae are the most significant members of the families Leguminosae and Anacardiaceae, which are dominant in the SDTF. The paleobotanical species described in this study confirm the extension of the SDTF to the province of Corrientes, coinciding with various climatic events (dry subtropical, semi‐desert and warm‐humid climate) that would have favored the development of these forests during the Pleistocene in this region. The absolute dates obtained for the Toropí/Yupoí and Tapebicuá Formations confirm their synchronicity and correlation to Marine Isotope Stage 5. The palynological analysis, the presence of Menendoxylon and the sedimentological data allow us to infer the existence of a seasonally dry humid paleoclimate in northeastern Argentina during the Late Pleistocene favorable to the development of the SDTF.

Control of paleoclimate and paleoweathering on chromium contents in a non-ultramafic aquifer hosting high chromium groundwater.

Cr(VI) is a carcinogen with proven mutagenic and genotoxic effects. The effects of the depositional environment (e.g., paleoweathering, paleoclimate, and paleoredox condition) on Crenrichment in non-ultramafic aquifer solids are unclear. In this study, we presented the sedimentary characteristics of a borehole from a typical non-ultramafic aquifer with high Cr groundwater in Jingbian, central Ordos Basin, China. Chromium was enriched in the K1h sandstone aquifer, especially at depths of 400-500m, with the highest value of mass transport coefficient (τAl,Cr) up to 92.13% and τAl,Fe up to 33.5%. The provenance of aquifer Cr was predominantly intermediate and felsic igneous rocks with a mafic rock mixture. This mafic source was inferred from Cr-rich granodiorite and mafic/ultramafic rocks in the Yinshan (Daqingshan-Wulashan) Block, northern Ordos Basin. The Cr-rich aquifer in K1h was developed due to a moderate chemical index of alteration (CIA) (mean, 56.7) under relatively warm and humid paleoclimate, as evidenced by high CIA-temperature (CIA-Temp) (mean, 6.79°C) and paleoclimatic index values (mean, 0.40). Fe-Mn redox cycling in the oxic to suboxic environments contributed to aquifer Cr accumulation. Using path analysis, we identified that paleoclimate created favorable weathering conditions and enrichment of Fe contributed to the formation of high-Cr aquifers. The study reveals the formation of positive Cr anomalies in non-ultramafic aquifers, which is the potential source of groundwater Cr, and highlights the effects of depositional factors on Cr accumulation during aquifer deposition or early diagenesis. It can provide new insights into the natural processes of high-Cr sediments occurring in non-ultramafic aquifers.

Characteristics of Nanoparticles in Late Pliocene Paleo-Mountain Fire Relics in Jinsuo Basin, Yunnan Province and Their Implications for Paleoclimate Evolution

Wildfires significantly affect climate and environmental changes, closely tied to extreme weather responses. Vegetation combustion emits greenhouse gases (CO2, CH4, CO), warming the climate. Climate shifts, in turn, impact vegetation growth, altering combustible material types and quantities, thus affecting wildfire intensity, duration, and frequency. Wildfires profoundly affect ecosystems, influenced by factors like atmospheric oxygen and climate. Their combustion gases impact climate and vegetation growth. Recent advancements in studying ancient wildfires include analyzing nanoparticles as key indicators. This study discovered six types of nanoparticles in ancient wildfire remains, with sizes ranging from 50 nm to 500 nm and diverse compositions including elements such as C, O, Mg, Al, Ti, Fe, S, Ca, and P. These findings indicate that wildfires generate a variety of nanoparticles, offering new insights into ancient fire events. Elemental analysis revealed low magnesium but high calcium and aluminum levels, suggesting a warm, humid paleoclimate during these fires. The presence of high Ti-O ratios and carbon-rich nanoparticles points to ground fires with incomplete combustion. This research underscores the significance of nanoparticles in understanding the history and characteristics of ancient wildfires.

Coupling of Paleosedimentary Environment and Lithofacies: Implications for Shale Oil Enrichment in the Lianggaoshan Formation, Northeastern Sichuan Basin, China.

The sedimentary environment is paramount in dictating depositional processes, ecological settings, and hydrodynamic conditions, all of which are pivotal to the aggregation of organic-rich sediments. A synergistic interplay is observed among the paleosedimentary environment, lithofacies, and enrichment of shale oil. Utilizing an analysis of elemental geochemistry and organic geochemical parameters-such as total organic carbon (TOC) content-this study endeavors to identify the predominant factors that govern the enrichment of organic matter (OM). It reconstructs the evolutionary trajectory of the paleosedimentary environment and introduces a dynamic model for shale oil enrichment within the Jurassic Lianggaoshan Formation of the Sichuan Basin. The results indicate that the Lianggaoshan Formation shale can be categorized into five lithological types, including high TOC felsic-rich shale (A), moderate TOC felsic-rich shale (B), moderate TOC felsic-rich mudstone (C), low TOC felsic-rich shale (D), and low TOC felsic-rich mudstone (E), and high TOC felsic-rich shale has the highest TOC and oil content. The shales of the Lianggaoshan Formation were deposited in an environment characterized by abundant terrestrial input, humid climate, freshwater conditions, reduced oxygen levels, and thriving primary productivity. OM enrichment is mainly controlled by the combination of a humid paleoclimate, high primary productivity, and other factors. The paleoenvironment deposited by lithofacies A coincides with the paleoenvironment conducive to OM enrichment, which makes lithofacies A become the dominant lithofacies for shale oil enrichment. Additionally, the sedimentary environment evolution characteristics and shale oil enrichment model of the Lianggaoshan Formation were established based on geochemical parameters. The paleosedimentary environment of the Lianggaoshan Formation from bottom to top is more and more unfavorable. The No.1 submember of the Lianggaoshan Formation is the most potential shale submember for shale oil exploration due to the paleosedimentary environment that promoted the deposition of lithofacies A and OM enrichment.

Sedimentary paleoenvironment and its control on organic matter enrichment in the Mesoproterozoic hydrocarbon source rocks in the Ordos Basin, southern margin of the North China Craton

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city, North China Craton, is a potential source rock. Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms. In this study, we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations, mineral composition analysis, organic geochemistry, and elemental geochemistry. We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale. The Cuizhuang Formation was deposited in a late-stage, restricted basin environment during the rift phase, and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic–anoxic water environment, that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities, which promoted the accumulation of organic matter. However, the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions. The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities. Warm conditions promote continental weathering and increase marine productivity, thereby consuming oxygen in the bottom water. Moreover, acidic hydrothermal activity also helps to establish an anoxic environment. Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions, which may explain the development of source rocks in the Cuizhuang Formation.

Sedimentology and geochemistry of the Upper Permian transitional shale, Western Guizhou, China: Implications for the environmental effect of the Emeishan large igneous province and organic matter accumulation

The marine-continental transitional shale of the Upper Permian Longtan Formation serves as a crucial target for both shale gas exploration and development, as well as a significant research object of the dramatic geological events during the Middle-Late Permian period. In this study, X-ray diffraction (XRD), polished section, scanning electron microscopy (SEM), energy spectrum analysis (EDS), total organic carbon content (TOC) and elemental geochemistry were conducted to elucidate the sedimentological and geochemical characteristics of the transitional shale in the Longtan Formation and the underlying Emeishan basalt in Western Guizhou. Additionally, the paleoenvironment conditions, the environmental effect of the Emeishan large igneous province and the organic matter enrichment mechanism of the transitional shale are investigated.The sedimentological and geochemical proxies reveal that influenced by seawater and river, the transitional shale of the Longtan Formation in western Guizhou was deposited in a marginal tidal flat-lagoon environment under a warm and humid paleoclimate, anoxic condition, brackish to saline bottom water, high terrigenous detrital input and high sedimentation rate condition. The mineral and elemental composition characteristics show that the transitional shale was predominantly derived from Emeishan basalt, supplemented by contributions from volcanic ash and hydrothermal activity. This suggests that following the eruption of the Emeishan large igneous province in Middle-Late Permian transition, the paleogeographic pattern of high in the west and low in the east was formed in the Upper Yangtze region. The weathering products of the Emeishan basalt were the main source of the Longtan Formation. Intense volcanic activities in the earliest Wuchiapingian potentially induced cooling of the climate, and the elements carried by hydrothermal input likely enhanced paleoproductivity and increased seawater salinity, and was conducive to the suboxic-anoxic water condition.The organic matter contents in transitional shale are extremely high with the highest organic matter content (>20%) of the shale deposited in mire, followed by the shale deposited in tidal flat (10%), whereas the lagoonal shale, predominant in the Longtan Formation, displays the lowest organic matter content (5%). The organic matter enrichment mechanism of transitional shale is primarily attributed to a “preservation model”. The suboxic-anoxic condition and brackish-saline water were conducive to the organic matter preservation. The primary productivity was low and organic matter mainly came from terrestrial higher plants. The warm and humid climate strengthened terrestrial weathering and erosion, consequently increasing terrigenous organic matter input and sedimentation rate. Elevated sedimentation rate reduced the decomposition time of organic matter, thereby also facilitating organic matter preservation. However, the correlations between TOC content and geochemical proxies in lagoonal shale suggest that the warmer and more humid paleoclimate may have increased terrigenous detrital influx and diluted the organic matter content.

Braided-river architecture of the Triassic Swartberg member, Katberg Formation, South Africa: assessing age, fluvial style, and paleoclimate after the end-Permian extinction

ABSTRACT The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a braided-fluvial body 45 m thick, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2-D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ∼500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist in the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.

Pedogenic processes and climatic conditions from Cretaceous (Albian) tropical paleosols of the Itapecuru Formation, Parnaíba Basin, northeast Brazil

Paleosols are an under-utilized tool for paleoenvironmental and paleoclimatic reconstruction in Brazilian intracontinental basins. The Itapecuru Formation, from the Lower Cretaceous of the Parnaíba Basin, consists of a 600-m-thick succession of interbedded very fine-to-fine-grained sandstone, mudstone, and paleosols containing rich fossil assemblages. This paper provides a detailed description of the macro and microscopic pedogenic features of a hydromorphic paleosol profile whithin the Itapecuru Formation. Analyses of clay mineralogy and whole-rock geochemistry in the hydromorphic paleosols at the Prata locality are used to infer pedogenic processes, paleoclimate, paleohumidity and paleovegetation. Estimates of paleoprecipitation and paleotemperature using climofunctions, chemical index of alteration without potassium, and paleosol weathering index proxies (PWIs) show values ranging from 810 to 1042 mm/yr (SE = ±78.1 mm/yr) and 10.9 to 11.8 °C (SE = ±0.23 °C), respectively. Climofunction values (MAP and MAT), paleohumidity, Kӧppen aridity index, and pedogenic clay mineral content suggest a humid paleoclimate during soil-forming and weathering processes. Based on these climate data and previously published paleogeographic and palynological studies, the Cretaceous (Albian) Prata pedotype was formed within the equatorial humid belt with vegetation of wet tropical forests.

Depositional environment variations and organic matter accumulation of the first member of the Qingshankou formation in the southern Songliao Basin, China

The organic-rich shale in the first member of the Qingshankou Formation, which is located in the southern Songliao Basin, is regarded as a high-quality source rock in East China. Geochemistry parameters were utilized to illustrate the formation and preservation conditions of the Qing1 Member in the Changling Sag, southern Songliao Basin. In the present study, from longitudinal continuity, the samples of organic geochemistry and elemental geochemistry were collected and systematically analyzed. This aims at determining the paleosalinity, paleoclimate, paleoredox conditions, and paleoproductivity and reconstructing its depositional paleoenvironment. According to total organic carbon (TOC) content, the Qing1 Member in the Changling Sag can be classified into two intervals, which are the lower part and the upper part, with high TOC content and low TOC content, respectively. It can be proved from the results of geochemical indicators that under warm and humid climatic conditions the relatively lower part was generated in the anoxic environment. Terrigenous input brought nutrients to the water body of the lake, made algae flourish, and had a relatively high paleoproductivity of the lake, which imposed a vigorous impact on the accumulation of organic matter. The upper part is mainly deposited under weakly oxidizing conditions, with gradually enhanced oxidation and reduced productivity. In contrast to the lower shale, the terrigenous inflow is relatively low. TOC content in the Qing1 Member has a positive relevance with the paleoredox conditions, as well as the paleosalinity conditions, indicating that good preservation is favorable for the accumulation of organic matter. A depositional model is proposed for the organic matter accumulation of this shale. The upper part was in a relatively hot and dry paleoclimate, with a low degree of organic matter enrichment, whereas the lower part had a warm and humid paleoclimate, with the input of terrestrial organic matter into the primary productivity of the lake basin and a high degree of organic matter enrichment.

Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin

A set of high-quality lacustrine shales at the bottom of the Chang 7 member of the Yanchang Formation in the Ordos Basin is one of the main source rocks of tight oil and gas and shale oil in the Yanchang Formation. Based on outcrop, core, drilling and seismic data, by the quantitative characterization of outcrops, fine characterization of logging facies and seismic facies, and geochemical tests, the lithofacies types, geophysical response characteristics and organic geochemical characteristics of this high-quality shale are clarified, and the formation paleoenvironment, including redox conditions, paleoclimate, paleosalinity and paleowater depth, is analyzed. The high-quality shale at the bottom of the Chang 7 member is divided into three lithofacies types: black shale, dark massive mudstone and silty mudstone. The organic matter in black shale is mainly interbedded or stratified, the organic matter in dark massive mudstone is dispersed and the organic matter content in silty mudstone is lower. The shale shows high gamma (more than 260 API), a high acoustic time difference (more than 280 μs/m), a high resistivity (more than 330 Ω m) well-logging phase and strong-amplitude parallel–subparallel seismic phase characteristics. Based on the logging and seismic facies characteristics, the plane distribution range of this set of shales is defined. The sedimentary thickness gradually increases from the edge (5–10 m) to the center of the basin, among which the Jiyuan–Huachi–Yijun black shale has the largest thickness (more than 30 m). This set of high-quality shales was mainly formed under a warm and humid paleoclimate, in water depths of 60–120 m, and in an anaerobic reducing and continental freshwater paleoenvironment. The fine identification, distribution range and formation conditions of black shale lithofacies are of practical significance for predicting the distribution of favorable lithofacies of shale oil and gas and the deployment of horizontal wells.

Braided-river architecture of the Triassic Swartberg Member, Katberg Formation, South Africa: assessing age, fluvial style, and paleoclimate after the End-Permian Extinction

The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a 45 m thick braided-fluvial body, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ~500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist within the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.

Organic matter input, paleovegetation and paleoclimate of Upper Cretaceous lignite from Maiganga coalfield, Upper Benue Trough, Nigeria: Insights from biomarkers and stable isotopes

Upper Cretaceous Gombe Formation coals from Maiganga coalfield, Upper Benue Trough, Nigeria were investigated to evaluate the thermal maturity, organic matter input, paleovegetation, paleoclimate, and depositional environment using bulk and molecular geochemical techniques. Vitrinite reflectance (<0.40 %Ro) and biomarker maturity indicators (C31 homohopane 22S/(22S + 22R) < 0.3; 16(H) β/(β+α) phyllocladane <0.5; MPI-1 < 0.5; MPR <2.65) generally signify low thermal maturity. Source-related aliphatic and aromatic hydrocarbon ratios indicate that the coals are derived mainly from terrestrial organic matter but with considerable input from marine algal organic matter. The higher abundance of diterpenoids over triterpenoids and sesquiterpenoids suggests a prevailing contribution of gymnosperms to the Maiganga paleoflora. This is corroborated by the relatively heavier carbon isotopic composition (δ13C = −25.9‰ to −25.0‰) and low angiosperm/gymnosperm ratios. A new proxy, the polycyclic aromatic hydrocarbon ratio (PAHr) which is calculated from PAHr = (phenanthrene + pyrene + fluoranthene + perylene)/(phenanthrene + pyrene + fluoranthene + perylene + retene + cadalene), was proposed to evaluate the abundance and origin of combustion-derived polycyclic aromatic hydrocarbons (PAHs). The mostly high (>0.9) PAHr values for the coals are characteristic of a pyrogenic/combustion origin. Combustion-derived PAHs are abundant in the coals and their ratios indicate a mixed petrogenic/pyrogenic to dominantly pyrogenic origin. However, the high proportion of unburnt organic matter and low abundance of 7-ring PAHs such as coronene indicates limited, high-temperature wildfires. The coronene index values suggest burning temperature in the 700–1000 °C range, which is consistent with the estimated high pO2 of ∼26.5% in the Maastrichtian. n-Alkane proxies (Pwax, Paq, n-C23/n-C29, n-C27/n-C31, etc.) and hydrogen isotope ratios (−117‰ to −112‰) are suggestive of humid paleoclimate with frequent dry periods and lower paleopeat water levels. Moderately high pristane/phytane ratios (2.0–4.8) indicate deposition under sub-oxic to oxic freshwater settings. Additionally, low total sulphur content (0.29–0.90) and dibenzothiophene/phenanthrene ratios (0.05–0.17) imply peat accumulation in lacustrine-swamp to fluvial/deltaic depositional environments with minor post-depositional marine influence.

Stratigraphic architecture and fluvial interpretations of the Upper Cretaceous (Turonian?) Middendorf Formation, Chesterfield County, South Carolina, U.S.A.

ABSTRACT The Upper Cretaceous (Turonian?) Middendorf Formation is a sand-rich stratigraphic unit of fluvial origin that forms a large aquifer in the U.S. Atlantic Coastal Plain. In Chesterfield County (South Carolina), which is the site of the type locality, the formation ranges in thickness from 66.5 to &amp;gt; 119.7 meters. The base of the formation is an unconformity above Paleozoic metasiltstone, and the upper contact is an unconformity above which lies sand of the Quaternary Pinehurst Formation. Outcrops display the following five facies assemblages: 1) sandstone to conglomeratic sandstone (fluvial bar and channel deposits), 2) beds of alternating laminae of sandstone and mudstone (fluvial overbank or floodplain deposits), 3) ≥ 1 m-thick beds of clay (swamp deposits, floodplain deposits, and/or sediment that accumulated in abandoned fluvial channels), 4) 0.2–0.5 m-thick planar to slightly undulatory beds of framework-supported sandstone with a mud matrix (traction-dominated current deposits at the top of fluvial bars, upper-flow-regime bedform deposits in subsidiary fluvial channels, or coarse-grained overbank deposits), and 5) sandstone to conglomeratic sandstone cemented by iron (interpreted as fluvial bar and channel deposits, with the iron cement being a diagenetic “groundwater ferricrete” that formed via the circulation of shallow groundwater and the oxidation of iron-bearing minerals). Kaolinite in various forms is pervasive throughout the formation and is interpreted as an early diagenetic phenomenon that formed by prolonged postdepositional weathering and flushing by meteoric water under a warm and humid paleoclimate. The fluvial system that formed the Middendorf Formation prograded into the area from the west or northwest from uplifted margins of Mesozoic rift basins and/or the Appalachian Mountains. This progradation was a response to a base-level fall and the sediment accumulated during base-level lowstand and subsequent early transgression. In Chesterfield County, the Middendorf Formation can be subdivided into three fining-upward sequences. Each sequence consists predominantly of medium to coarse sand with a greater abundance of gravel in the lower part of the sequence and a greater abundance of clay and silt beds in the upper part. Each sequence is interpreted as either a response to autogenic processes or a response to allogenic sea-level changes, specifically a higher-order (higher-frequency) progression from relative lowstand conditions to early transgression whereby coarse sand and gravel (e.g., fluvial bar and channel deposits) were preserved during initial lowstand conditions and a greater proportion of mud and finer-grained sand (floodplain deposits) were preserved during subsequent early transgression. The Middendorf Formation is correlative with several other kaolinite-rich fluvial sandstones in North America including the Raritan Formation in New Jersey, the Tuscaloosa Formation of the eastern Gulf of Mexico (Alabama, Mississippi, Louisiana), the Woodbine Formation of the central Gulf of Mexico (Texas), and the Frontier Formation of Wyoming. The accumulation and preservation of these formations occurred in response to a Turonian eustatic sea-level fall and subsequent transgression, and the early diagenetic kaolinite in these formations is attributed to similar warm and humid paleoclimate conditions.

Cycles of fine-grained sedimentation and their influences on organic matter distribution in the second member of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin, East China

According to the theory of sequence stratigraphy based on continental transgressive-regressive (T-R) cycles, a 500 m continuous core taken from the second member of Kongdian Formation (Kong 2 Member) of Paleogene in Well G108-8 in the Cangdong Sag, Bohai Bay Basin, was tested and analyzed to clarify the high-frequency cycles of deep-water fine-grained sedimentary rocks in lacustrine basins. A logging vectorgraph in red pattern was plotted, and then a sequence stratigraphic framework with five-order high-frequency cycles was formed for the fine-grained sedimentary rocks in the Kong 2 Member. The high-frequency cycles of fine-grained sedimentary rocks were characterized by using different methods and at different scales. It is found that the fifth-order T cycles record a high content of terrigenous clastic minerals, a low paleosalinity, a relatively humid paleoclimate and a high density of laminae, while the fifth-order R cycles display a high content of carbonate minerals, a high paleosalinity, a dry paleoclimate and a low density of laminae. The changes in high-frequency cycles controlled the abundance and type of organic matter. The T cycles exhibit relatively high TOC and abundant endogenous organic matters in water in addition to terrigenous organic matters, implying a high primary productivity of lake for the generation and enrichment of shale oil.

New palaeoclimatic constraints from paleosols on the Middle-Late Jurassic landscape, Western Colorado, U.S.A

ABSTRACT The Wanakah Formation and Tidwell Member of the Morrison Formation record intervals of paedogenesis in the Paradox Basin and Central Colorado trough, western Colorado during the Middle-Late Jurassic. Detailed field description of paleosols and paedogenic carbonates at different stratigraphic horizons from three localities document four main pedotypes: vertisols, gleysols, oxisols, and protosols. Generally, gleysols reflect reducing conditions, protosols and oxisols reflect oxidizing conditions, and vertisols reflect fluctuations in oxidizing/reducing conditions (seasonality). Major elemental geochemical ratios in samples from these paleosols suggest variable redox conditions and a sub-humid to humid paleoclimate with seasonal precipitations during paleosol development. Estimated mean annual temperature based on elemental analyses of paleosol B in paleosols from Ribbon Trail and Escalante Canyon in western Colorado suggest mesic – thermic paleoclimate. Mean annual precipitation indicates sub-humid to humid regional palaeoclimatic conditions marked by seasonal precipitation. Clay mineralogy of these paleosols, determined by X-ray diffraction, shows the abundance of illite most likely formed from smectite alteration due to episodic wetting and drying. δ13C and δ18O isotope analyses of carbonate veins in these paleosols suggest that they formed recently from meteoric water and therefore are not considered for palaeoclimatic reconstructions. Mean annual temperature and mean annual precipitation estimates in the Wanakah Formation and Tidwell Member of the Morrison Formation provide new information on the local and regional palaeoclimatic and paleoenvironmental conditions during the Middle Jurassic in western Colorado as well as data for comparison with the Upper Morrison paleoclimate from northern and southern localities in the western United States.

Geological Characteristics and Paleoenvironmental Evolution of Fine-Grained Sediments in the Third Member of the Xujiahe Formation in the Western Sichuan Depression, SW China

This study investigated, in detail, the characteristics of the Late Triassic fine-grained sediments in the third member of the Xujiahe Formation (Xu-3 Member), in the Western Sichuan Depression, and the paleoenvironmental evolution during their deposition through petrological interpretation, mineralogical composition characterization, and element geochemical analysis. According to the mineralogical composition, the Xu-3 Member can be divided into two petrological types, namely clayey fine-grained felsic sedimentary rocks and lime fine-grained felsic sedimentary rocks. The main mineral components are siliceous, clay, and carbonate minerals. Through the cluster analysis of major elements, all samples could be divided into two types with different major elemental characteristics. Trace elements exhibited distinct Sr depletion, relative enrichment of large ion lithophile elements, and high field strength elements. Two REE enrichment patterns were observed, which could be attributed to differences in the provenance area and tectonic background. The paleoclimate of the sedimentary area was warm and humid, but it was hotter and drier in the southern and central parts of the depression. The change trend of paleo-productivity was consistent with the paleoclimate. The waters in the sedimentary environment were mainly brackish water to saline water, with fresh water in the southern part of the depression. The paleo-redox conditions of the waters were mainly sub-oxidation to sub-reduction, but the southern part of the depression was more oxidative. The provenance area experienced a moderate degree of chemical weathering under a warm and humid paleoclimate, same as the depositional area. However, the depositional environments differed between the northern and south-central parts of the depression.

High-quality sandstone reservoir evaluation and prediction in a half-graben rift sag under the influence of igneous intrusions, Songliao Basin, Northeast China

Tight sandstone reservoirs of continental lake basins are one of the most important types of unconventional hydrocarbon resources. The first member of the Yingcheng Formation (k1y1) in Baojia Sag, Songliao basin, is an important tight sandstone reservoir and also a region rich in hydrocarbon. However, volcanic rock intrusions increase the stratum thickness and change the characteristics of the reservoirs, thus making the distribution of high-quality reservoirs more difficult to predict. Therefore, according to the identification of sequence boundaries, the five 4th-order sequences of k1y1 in Baojia sag are divided, into SQ1–SQ5, which are mainly dominated by lacustrine and fan-delta depositional systems. The stratigraphic characteristics in the study area were affected by two stages of volcanic activity. In the first stage, a large area of tuff was developed in SQ5. In the second stage, sheets of intrusive rocks were formed in SQ1–SQ4. Through the identification of cast thin sections and Scanning Electron Microscopy (SEM) data, the types of reservoir micro-pores mainly develop primary pores, microfractures, intergranular, and intragranular dissolved pores. Given the complex strata under the interference of volcanic rocks, it is considered that sandstone with a 10–40% argillaceous content is a high-quality reservoir. The results showed that under the background of episodic tectonic activities, humid paleoclimate, and sufficient paleoprovenance, large-area fan-deltas of SQ2 and SQ4 are developed in both steep slope and gentle slope zones. Compared with SQ1 and SQ3, small-scale fan-deltas are formed under the background of a stable structure and arid paleoclimate. The reservoir quality gradually improved from SQ1 to SQ4, this is mainly due to the progressive reduction of reservoir argillaceous content, the gradual reduction of compaction, and the gradual enhancement of dissolution, resulting in the expansion of pore throats to improve pore connectivity. Geostatistical inversion based on argillaceous content was proven to be more effective than wave impedance inversion as it distinguishes lithology and predicts high-quality reservoirs disturbed by volcanic rocks. Comprehensive analyses show that the sandstone of SQ3 and SQ4 with a 10–40% argillaceous content is the highest quality of reservoirs, which interact with lacustrine source rocks and easily form “self-generated and self-stored” and “the lower-generation and upper-storage” patterns of hydrocarbon reservoirs. The prediction method and distribution law of high-quality reservoirs under the influence of volcanoes can provide guidance for hydrocarbon exploration and development.

Microfacies types and distribution of epicontinental shale: A case study of the Wufeng–Longmaxi shale in southern Sichuan Basin, China

For black shales, laminae and bedding are hard to identify, grain size is difficult to measure, and trace fossils do not exist. Taking the Ordovician Wufeng – Silurian Longmaxi shale in southern Sichuan Basin, China, as an example, the types, characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections, argon-ion polishing field emission-scanning electron microscopy (FE-SEM), and kerogen microscopy. The epicontinental sea develops delta, tidal flat and shelf facies, with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies, the calcareous and clayey flats under intertidal flat facies, the calcareous and clayey shelfs under shallow shelf facies, the deep slope, deep plain and deep depression under deep shelf facies, and the overflow under gravity flow facies. Basinward, silty lamina decreases and clayey lamina increases, the grain size changes from coarse silt to fine mud, the silica content increases from about 20% to above 55%, the carbonate and clay minerals content decreases from above 40% to around 10%, and the kerogen type changes from type II2 to type II1 and type I. Provenance and topography dominate the types and distribution of shale microfacies. The underwater distributary channel, interdistributary bay, clayey flat, clayey shelf, and overflow microfacies are developed in areas with sufficient sediment supply. The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply. The deep shelf shale area is divided into deep slope, deep plain, and deep depression microfacies as a result of three breaks. The formation of epicontinental shale with different microfacies is closely related to the tectonic setting, paleoclimate, and sea level rise. The relatively active tectonic setting increases the supply of terrigenous clasts, forming muddy water fine-grained sediment. The warm and humid paleoclimate is conducive to the enrichment of organic matter. The rapid sea level rise is helpful to the widespread black shale.

Organic matter enrichment mechanism of Youganwo Formation oil shale in the Maoming Basin

The Youganwo Formation oil shale located in the Maoming Basin represents a large commercially valuable lacustrine oil shale resource and a potential bio-shale gas reservoir in South China. With the aim of deepening the understanding of factors that influence organic matter enrichment, this research conducted a geochemical investigation to reconstruct the depositional paleoenvironment of bioproductivity, preservation and dilution. Youganwo Formation oil shale is mainly deposited in semi-deep to deep-lake environments with relatively warm and humid paleoclimate in the subtropical-temperate zone. The total organic carbon (TOC) content (1.46–11.85%), S2 values (4.79–115.80 mg HC/mg rock) and HI (328–1040 mg HC/mg TOC) indicate that the oil shale has a good oil source rock potential. TOC content, (S1 + S2) values and vitrinite reflectance values show that its marginally mature organic matter (OM) belongs to kerogen type I-III with good oil-generating potential.A 3rd order sequence was identified in the Yougnwo formation. Subsequently, the multiple factors including bioproductivity, preservation and dilution that control the OM enrichment of oil shale within system tracts were discussed. Moderate-quality oil shales (Oy-1) were developed in the transgressive systems tract (TST) in an oxidizing condition with abundant detrital input. High-quality oil shales (Oy-2) were deposited during the high-stand systems tract (HST) with increased accommodation space, improved preservation conditions, warm and humid climate, higher water bioproductivity and minimum detrital matter input. During the regressive systems tract (RST, Oy-3), higher detrital matter input and fresher water led to lower TOC values. Among these multiple factors, dilution condition was the major one that influences OM abundance and variation on the basis of sufficient organic matter input. Thus, OM enrichment models of Oy-1, Oy-2 and Oy-3 sub-members were established. The OM enrichment and quality in oil shale were controlled by the combined effect of bioproductivity, preservation, and dilution.