Clay Mineral Analysis Research Articles

Differential mineral diagenetic evolution of lacustrine shale: Implications for CO2 storage

Understanding the differential diagenetic evolution of different lithofacies is essential for assessing the spatial development of shale reservoirs. These insights are crucial in predicting sealing integrity and storage capacity for sequestered CO2. In this study, we examined seven wells from the Cretaceous Qingshankou Formation in the Songliao Basin, China, with vitrinite reflectance (Ro) values ranging from 0.60 % to 1.62 %. Thin section-based petrographic observations, coupled with QEMSCAN analysis, were used to classify the different lithofacies. X-ray diffraction (XRD) analysis of clay minerals, field emission scanning electron microscope (FE-SEM), and energy-dispersive spectrum (EDS) analyses were employed to analyze the mineral textures, pore types, and diagenetic pathways. The results showed that early diagenetic mineral phases include calcite cement (1st phase), framboidal and microcrystalline pyrite, ferroan and non-ferroan dolomite. Intermediate diagenetic mineral phases were marked by illitization of smectite, chlorite formed by chloritization of smectite and alteration of K-feldspar, and the formation of authigenic albite and quartz, calcite cement (2nd phase) and ankerite. Given the higher potential reaction rate of CO2-fluid‑carbonate systems, we propose that the lithofacies dominated by carbonate minerals are not effective for CO2 storage, even in short-term. In contrast, lithofacies rich in feldspar and clay minerals are likely to be more effective for long-term CO2 storage.

Distinct mechanisms controlling and influencing the supply of clay-sized sediments to the northern shelf of the Sea of Okhotsk since the Last Glacial Maximum

The Sea of Okhotsk is a transitional zone between East Asia and northwestern Pacific Ocean. As a result, its massive thick sediments contain numerous important records of geological past, such as the mechanisms of materials and energy exchanges. However, the scarcity of studies tracing the provenance of clay-sized sediments in the sea has limited the progress of reconstructing the paleoclimate of East Asia. In this study, clay mineral analysis was conducted on 31 surface stations and the LV87–54-1 core. These results were combined with grain-size to provide reliable evidence for distinct mechanisms controlling the influx of clay-sized sediments into the northern shelf of the Sea of Okhotsk since 23 ka. Results of provenance analyses suggested that clay-sized sediments in the northern shelf mainly originated from its northwestern shelf, the Okhotsk–Chukotka volcanic belt, and the Kamchatka Peninsula. Clay-sized sediments of the northern shelf was primarily transported from the northwestern shelf and the Okhotsk–Chukotka volcanic belt after 23 ka, while those analogues from the Kamchatka Peninsula increased after 11.7 ka. Based on variations in clay mineral ratios, sediments from the northwestern shelf were mainly transported by the North Okhotsk Countercurrent, wherein rapid millennial-scale variations were influenced by the high-latitude Arctic Oscillation (from the Last Glacial Maximum to the Last Deglaciation) and the low-latitude East Asian summer monsoon (including the Holocene), respectively. By contrast, the influx of clay-sized sediments from the western Kamchatka Peninsula might be mainly controlled by the intensity of the West Kamchatka Current. From the Last Glacial Maximum to the Last Deglaciation, north and northwest geostrophic winds dominated in winter, and the West Kamchatka Current shifted eastward. Since the Holocene, weakened north and northwest geostrophic winds, but strengthened east winds caused a series of rapid millennial-scale variations in the West Kamchatka Current.

Astrobiological Potential of Rocks Acquired by the Perseverance Rover at a Sedimentary Fan Front in Jezero Crater, Mars

AbstractThe Perseverance rover has collected seven oriented samples of sedimentary rocks, all likely older than the oldest signs of widespread life on Earth, at the exposed base of the western fan in Jezero crater, Mars. The samples include a sulfate‐ and clay‐bearing mudstone and sandstone, a fluvial sandstone from a stratigraphically low position at the fan front, and a carbonate‐bearing sandstone deposited above the sulfate‐bearing strata. All samples contain aqueously precipitated materials and most or all were aqueously deposited. Although the rover instruments have not confidently detected organic matter in the rocks from the fan front, the much more sensitive terrestrial instruments will still be able to search for remnants of prebiotic chemistries and past life, and study Mars's past habitability in the samples returned to Earth. The hydrated, sulfate‐bearing mudstone has the highest potential to preserve organic matter and biosignatures, whereas the carbonate‐bearing sandstones can be used to constrain when and for how long Jezero crater contained liquid water. Returned sample science analyses of sulfate, carbonate, clay, phosphate and igneous minerals as well as trace metals and volatiles that are present in the samples acquired at the fan front would provide transformative insights into past habitable environments on Mars, the evolution of its magnetic field, atmosphere and climate and the past and present cycling of atmospheric and crustal water, sulfur and carbon.

Tracing sources of inorganic suspended particulate matter in the Great Barrier Reef lagoon, Australia

Water clarity on the inshore Great Barrier Reef (GBR) is greatly influenced by terrestrial runoff of suspended particulate matter (SPM). Catchment sediment tracing studies often do not extend into the marine environment, preventing the analysis of preferential marine transport. This study employs novel collection and sediment tracing techniques to examine the transport of the terrigenous ‘mineral’ component of plume SPM within the GBR lagoon for two flood events. Utilising geochemical, radionuclide and clay mineral analysis, we trace terrigenous mineral sediments > 100 km from the river mouth. We show that the SPM geochemistry is highly influenced by particle-size fractionation, desorption, and dilution within the plume, rendering traditional tracing methods unviable. However, the ratios of rare earth elements (REE) to thorium (Th) provide stable tracers of mineral SPM transported across the catchment to marine continuum and allow the identification of discrete catchment sources for each flood event. Plume sediment radionuclides are also stable and consistent with sub-surface erosion sources.

Paleoecology and paleobiogeography of the latest Miocene site of Shuitangba, Zhaotong, China

Apart from northern and central China, the fossil record of the latest Miocene and Early Pliocene of Asia is not well documented and the record of South China during this interval is especially poor. Shuitangba, a site in Yunnan Province, offers a rare window into the paleoenvironment of the latest Miocene in southwestern China. Over 2400 vertebrate and macrobotanical specimens have been recovered from the site. The faunal assemblage is dominated by aquatic avian taxa and many of the mammalian taxa are those that indicate densely vegetated, water-margin habitats. Pollen and carpological remains indicate a temperate to subtropical, broad-leaved/coniferous forest around standing water, with more open areas containing grasses and herbs. Analyses of clay minerals, chemical weathering, and enamel stable isotopes suggest that Shuitangba was warmer and more humid than today, possibly with more pronounced seasonality. Results of community structure analysis indicate that the Shuitangba mammalian community was different from those of other Late Miocene Chinese sites in the high proportions of aquatic-dependent mammalian taxa. While Shuitangba shared mammalian faunal elements with other Late Miocene sites in Yunnan, it was still faunally distinct. Further, Shuitangba was depauperate in its mammalian fauna, which may have been a result of the depositional setting rather than a true reflection of regional faunal diversity.

Distribution of soil minerals along the toposequence of Hyang-Argopuro Volcanic Mountain, Jember, Indonesia

The study was conducted in the Hyang-Argopuro volcanic mountain in Jember, Indonesia, with the aim of assessing the distribution of soil minerals along a toposequence and their relationship to soil genesis. Three soil profiles representing the upper, middle, and lower slopes of the toposequence were analyzed. The results revealed that the predominant sand minerals in the soils are opaque minerals, weatherable minerals, amphibole groups, and ferromagnesian minerals. The presence of magnetite, primarily found in the soil profile on the upper slope, suggests the effect of the well-drained topography on its formation. Clay mineral analysis showed that halloysite dominates in soil profile 1, along with traces of gibbsite and cristobalite in the surface horizon. Soil profile 2 is characterized by a combination of halloysite and illite, while kaolinite and illite dominate in soil profile 3. The presence of illite in these soils aligns with previous studies conducted in volcanic regions. The degree of soil development follows the sequence: Soil Profile 2 > Soil Profile 1 > Soil Profile 3. This corresponds to the soil classification, where soil profile 3 is classified as an Alfisol, soil profile 1 as a Mollisol, and soil profile 3 as an Inceptisol. The Andic properties, such as low bulk density and high pH in NaF, observed in soil profile 1 suggest its development from an Andisol. Overall, the study findings highlight the significant influence of basaltic andesite parent material, mountainous topography, and warm and wet climate on the mineral composition and development in the area.

Evolution of the sedimentary environment on the continental shelf of southeastern Hainan Island under the influences of sea level and climate during the Holocene

The South China Sea, which connects the Indian Ocean and the Pacific Ocean, is an important part of the Indo-Pacific Warm Pool. This study examined the provenance and evolution of the sedimentary environment on the southeastern Hainan Island continental shelf in the northwestern South China Sea through AMS 14C dating, grain size analysis, and clay mineral analysis of sediments from Core QZ4. The clay mineral analysis indicated that the terrigenous detritus originated from Hainan Island, while the grain size data identified three endmembers: EM1 and EM2, corresponding to the fine-grained material transported by the oceanic current system, and EM3, corresponding to the coarse-grained material input by rivers from proximal sources. The sedimentary environment in the study area underwent three stages: foreshore (11.2–9.2 ka BP), nearshore (9.2–6.2 ka BP), and shallow shelf (6.2 ka BP to present). From 11.2 to 6.2 ka BP, the sedimentary environment was mainly controlled by sea level changes, although extreme cold events (such as the 9.4 ka BP and the 8.2 ka BP event) had a significant effect on the southeastern continental shelf of Hainan Island. From 6.2 ka BP to the present, the sedimentary record has mainly been influenced by climate change, and an increase in riverine contribution and the enhancement of chemical weathering may have been caused by rising typhoon intensity or frequency and possibly by an increase in human activities. These findings will be valuable for conducting high-resolution research on typhoons by analyzing sediments on the eastern Hainan Island shelf and neighboring areas.

Identification of three stages of paleochannels and main source analysis beginning in the middle Pleistocene in the western Bohai sea in north China

Significant global sea transgressions–regressions have occurred under the influence of glacial–interglacial climatic change during the Quaternary period. Various types of paleochannels developed on the continental shelf of the western Bohai Sea as a result of sea level decline. Currently, the source of the paleochannels and the sedimentary environment and evolutionary pattern of the paleochannels are disputed. This study interprets high-resolution single-channel seismic data using a chronological framework based on accelerator mass spectrometry 14C (AMS14C) dating and optically stimulated luminescence (OSL) dating of core DZQ01. Furthermore, this research correlates this data with the cores of TJC-1 and H6 in the Western Bohai Sea and analyzes sedimentological characteristics based on grain size and clay mineral analyses. The results indicate that (1) three stages of paleochannels, namely Seismic Unit 6 (SU6), SU4, and SU2 are identified from bottom to top during Marine Isotope Stage 8 (MIS 8), MIS 6, and MIS 4, respectively; (2) from 300 to 243 cal. ka B.P., the SU6 paleochannel has four pathways, with the Yellow River serving as a primary source; from 191 to 130 cal ka B.P., the SU4 paleochannel has three pathways, and the main sources are the Yellow River, the Liao River, and the Luan River; from 74 to 14 cal. ka B.P., the SU2 paleochannels have two large tributaries, with the principal sources being the Yellow River and the Luan River. We, therefore, it can be inferred that the uplift of the Tibetan Plateau and glacial–interglacial climatic change constitute the formation mechanism for the paleochannels.

A 60–50 ka African Humid Period modulated by stadial Heinrich events HE6 and HE5a in northwestern Africa

A Late Pleistocene lacustrine sedimentary succession has recently been discovered and investigated in Northwest Africa (Morocco, Anti-Atlas, Agadir-Tissint Feija). By integrating new radiometric 230Th/U and OSL ages and new clay mineral analyses with previous sedimentological evidence, we significantly refine the origin and the timing of this succession and its significance for African paleoenvironments and paleoclimate. The sedimentary succession reflects the evolution of a paleolake, referred to as paleolake Tissint, that indicates the abrupt establishment of a wet period at ∼60 ka and a gradual return to drier conditions at ∼50 ka. Establishment of this paleolake coincides with a peak of precession-forced high summer insolation centered around ∼60 ka suggesting that orbital parameters triggered humid conditions in northwest Africa during this time interval. Comparing this observation with others from the literature, it appears that continental-scale wet conditions prevailed across North, Central and East Africa at that time and add evidence in support of a 60–50 ka African Humid Period (AHP). Given that the Holocene African Humid Period was associated with “Green Sahara” conditions, wet conditions between 60 and 50 ka likely sustained coeval greening, at least partially, of the Sahara region. In northwest Africa, the 60–50 ka AHP is affected by Heinrich Event HE6 which via adjustment of the Atlantic Meridional Ocean Circulation (AMOC) seems to have delayed the onset of this AHP. In a similar way, the younger Heinrich Event 5a resulted in short-term regional aridification about 55 ka ago that briefly interrupted the generally wetter period. Finally, as the AHP between ∼60 ka and ∼ 50 ka coincides with a major Human dispersal out of Africa; these particular conditions may have offered a climatic window of opportunity for Homo sapiens to open migration routes across Africa towards the Levant.

New insights into sedimentological and geochemical conditions of marine Pleistocene sediments in the Tangier region (Morocco): First implications for paleoenvironmental and paleoclimatic reconstructions

The Pleistocene period is characterized by important periodic climatic variations, marked by alternating arid and humid phases. This work aims to reconstruct the prevailing Pleistocene paleoenvironment and paleoclimate in the Tangier area based on a multidisciplinary study of a marine formation dating from 125 ka. Field investigations and sedimentological, micropaleontological, mineralogical, and geochemical analyses were carried out on samples from the Pleistocene Achakkar formation. The granulometric study, combined with a morphoscopic, exoscopic analysis of quartz grains and evidence for diverse benthic foraminifera, suggests a low-energy marine depositional environment, most likely foreshore to shoreface. Moreover, analysis of clay minerals and major elements suggests mineralogical evolution, indicating an arid climate. The sedimentary successions of the Pleistocene Achakkar littoral are progressively more and more coarse; emplacement was subject to several interactive factors, such as climatic and eustatic responsible for reconstituting the geological history of the Tangier Peninsula.

Lowering of the kinetic barrier of clay-mineral reactions along a seismogenic fault: Example from the Nankai subduction zone, Japan

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S–I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is ∼15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20–30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura–Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20–30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite–illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones.

Spatial and temporal distributions of clay minerals in the central Andaman Sea: Provenances and sedimentary processes over the last 42 kyr

The uplift of the Tibetan Plateau has led to the formation of the world’s largest sediment “source–sink” system in the northeastern Indian Ocean, and seafloor sediments record rich environmental and climatic information during the accumulation process, making it an ideal area to reconstruct past sedimentary processes. To reveal fine sediment provenance and climatic implications, we present a sedimentary reconstruction during the past 42 kyr based on clay mineral analyses of sediments from core ADM-159 retrieved from the central Andaman Sea. The clay minerals mainly consist of illite (I, 27%–47%) and kaolinite (K, 8%–37%) with minor smectite (S, 11%–37%) and chlorite (C, 11%–37%). Provenance analyses show that the illite-dominated clay minerals are mainly derived from terrestrial sources of detrital material input from the Irrawaddy River, indicating a primary supply of fine sediment from Myanmar terrestrial material. Considering the stable sediment provenance, S/K, S/(I+C), and the illite chemical index were chosen to reconstruct the transport pattern of the fine fraction sediment over the last 42 kyr. S/K and S/(I+C) showed relatively consistent trends over time with relatively high values at 32–25 and 15–6 ka BP, indicating a strong supply and transport capacity of fine particulate matter. In contrast, relatively low values of S/K and S/(I+C) occurred at 25–15 ka BP, indicating a relatively weak deposition process of fine sediments in the study area. The trends in S/K and S/(I+C) are roughly consistent with the variations in solar radiation. Together with the illite chemical index, these trends indicate that solar activity controlled the weathering and sedimentation of fine-grained sediments. We also found significant periodicities in our clay mineral records at 25.05 and 0.473–0.477 kyr, suggesting that the parameters of the Earth’s orbit and solar radiation were potential driving mechanisms of the changes in regional climate. Moreover, monsoon precipitation triggered by the movement of the intertropical convergence zone was another important factor controlling the supply and transport of fine particulate matter in the study area, especially the weak precipitation during the last deglacial period and the relatively strong precipitation during the Holocene, so we could draw a conclusion that precipitation during these periods had significant effects on the weathering, erosion, and transport of fine particulate terrestrial materials over the last 42 kyr.

Lithofacies and Clay Mineral Analysis of the Upper Cenomanian Ms'ad Formation, Rutbah Area, Western Iraq

The Cenomanian succession in Rutbah City, western Iraq, is composed of siliciclastic carbonate with multicolored paleosoil layers that refer to unconformity surfaces within the Ms'ad Formation. The current study will examine the clay mineralogy and facies to determine the source of these clay minerals and depositional settings. Fifteen carbonate facies have been identified as three distinct facies associations (FA1–FA3). These FAs are supratidal to intertidal (FA1), semi-restricted lagoon (FA2), and shallow open subtidal ramp with patches of a rudist-bivalve reef (FA3). The reported Multispirina Iranica-Nezzat simplex-Chrysalidina gradata assemblage Zone within the Ms'ad Formation indicated the Cenomanian age. Clay mineral analysis displays three distinctive clay mineral assemblages; kaolinite-dominated with illite (assemblage A), R0-dominated (smectite-rich illite/smectite mixed layers) with kaolinite and illite (assemblage B), and illite-dominated (illite and illite-rich illite/smectite mixed layers) with kaolinite and R0 (assemblage C). Most of these clays are detrital, generated by weathering source rocks (igneous, metamorphic, and sedimentary) under various paleoclimatic circumstances. The Kaolinite-dominated assemblage A represents intense chemical weathering from hot, humid circumstances. Smectite-dominated R0 assemblages were created by weathering alkaline rocks in flat pedogenic soils with seasonal humidity. The source of clay minerals in the studied region was subject to a dry climate towards the end of the Cenomanian period, which encouraged physical weathering and the production of illite-rich assemblages.

Holocene sediment source analysis and paleoclimatic significance of core KZK01 from the eastern part of the Beibu Gulf

Identifying the sources of sediments is of great significance in reconstructing Holocene paleoclimate evolution in the Beibu Gulf and in understanding the characteristics of regional responses to changes in global climate. The Holocene paleoclimatic evolutionary history of the Beibu Gulf was investigated by chronological, geochemical, and mineralogical means using the sediments of Core KZK01 from the eastern part of the Beibu Gulf. The rare earth element (REE) distribution curves, (Gd/Yb)N and (La/Yb)N discriminant diagram, and (Gd/Lu)N and ∑LREE/∑HREE discriminant diagram indicated that the detrital materials in the eastern part of the Beibu Gulf primarily originated from Hainan Island and its proximal sources, with considerable contributions from Taiwan and Pearl River materials. Source analysis of clay minerals showed that Luzon Island was the main source of smectite, followed by Hainan Island. Rivers in Taiwan were the main sources of illite in the study area, followed by the Red River. The Red River was the main contributor of chlorite, followed by the Pearl River. Kaolinite mainly originated from Hainan Island and Guangxi. Coastal currents, surface currents, and warm currents were the main drivers of material transport. Paleoclimatic variations since the Holocene in the Beibu Gulf were divided into three stages: 12–9 cal kyr BP, 9–1.3 cal kyr BP, and 1.3 cal kyr BP to the present. During different stages of climatic evolution, drought was often accompanied by cold and humidity coexisted with warmth, and cold-dry-warm-humid alternation is characterized by significant phases. The illite crystallinity clearly recorded the extreme cold events, such as Bond Events (except Bond6) and the Younger Dryas, and the change trend was essentially consistent with the regional climate record, reflecting the control of global climate change on the process of land–sea interaction in the tropical region. Furthermore, it highlights the great potential of illite crystallinity as a proxy indicator for reconstructing the surface chemical weathering processes of the region.

Infrared spectra of mixtures of heated and unheated clay: Solving an interpretational conundrum

AbstractFourier transform infrared (FTIR) spectroscopy is frequently used for archaeological studies related to fire, allowing, among other things, researchers to distinguish between unheated and heated clay minerals. However, heat signatures are not always clear‐cut in infrared spectra of bulk sediments, as spectra occasionally appear with ambiguous absorbance bands attributed to hydroxyl (OH) in clay minerals. This paper presents an experimental study addressing this interpretational problem by considering the effect of mixtures of heated and unheated clay, a phenomenon expected in archaeological sites. After creating experimental mixtures and testing them using bulk FTIR spectroscopy, our results indicate that even a relatively small amount of unheated clay—only ca. 5%–10% mixed into a fully heated deposit—will result in ambiguous infrared spectra that are difficult to interpret. For comparison, ambiguous bulk FTIR spectra from two archaeological contexts—an ashy fill within a pit installation and a hearth—were studied with FTIR microspectroscopy, which demonstrated the presence of unheated clay within a largely heated deposit. Micromorphological observations explain the mixed nature of the investigated archaeological contexts, in this case, primarily via bioturbation. Our results thus emphasize the importance of microcontextual analysis of clay minerals. Furthermore, these results indicate that heated deposits are likely missed altogether in some archaeological contexts where only bulk FTIR analyses have been conducted.

Sedimentary record off the Yangtze River estuary and its response to typhoons and human activities over the past 70 years

The muddy area off the Yangtze River estuary serves as an archive that can be investigated to unravel the impacts of environmental changes and human activities. However, in the context of increasing human activity and global climate change, it is unclear how the sedimentary records in this area respond to these changes. Radionuclide activity, magnetic susceptibility, grain size, clay minerals and chemical elements of samples in a sediment core (T1) collected off the Yangtze River estuary were analyzed in this study to identify the sediment provenance and elucidate the impacts of typhoon events and human activities on the sedimentary records over the past 70 years. Based on the analyses of clay minerals, Cr/Th vs. Sc/Al, element ratios, and correlations between grain size and magnetic susceptibility, sediments at 44–90 cm were mainly derived from the Yangtze River, while sediments at 0–44 cm were influenced by the Yangtze River, old Yellow River, and relict sand. Since AD 1950, the mineralogical–geochemical compositions of sediments in core T1 have significantly altered, which was attributed to typhoon events and human activities. Before 2003, typhoon activity influenced the frequency of sand layer deposition, which was related to El Niño-Southern Oscillation (ENSO) variations. However, following the operation of the Three Gorges Dam in 2003, the relationship between typhoon event records and ENSO was disrupted. Human activities have affected the sedimentary records of typhoon events off the Yangtze River estuary. This study emphasizes that sediments at the edge of the muddy area off the Yangtze River estuary can serve as good records of typhoon events, but the influence of human activities should be considered when studying the relationship between typhoon activity and climate change, especially during the past several decades.

Intra‐Seasonal Variability in Sediment Provenance and Transport Processes in the Brahmaputra Basin

AbstractSediment composition in modern fluvial settings is commonly assessed regarding spatial but rarely temporal variability, potentially leading to a bias of unknown extent. Here, we present the grain‐size distribution, bulk chemical and mineralogical composition of a time‐series set of 36 suspended sediment samples from the Brahmaputra river, as well as clay and heavy mineral analysis of selected samples. Sampling covers the June–November 2021 period, which included two major flooding events. We show that the two flooding events are characterized by contrasting grain size, with the first event characterized by a grain‐size minimum and the second by a grain‐size maximum. Although grain sizes of the first flood and the period after the second are similar, their compositions differ significantly, highlighted by a factor‐two decrease of biotite largely compensated by an increase in quartz. By contrast, the content of garnet, clinopyroxene, sillimanite, and rutile increased compared to epidote and amphibole during the second flood event. By relating the results to spatio‐temporal rainfall and discharge patterns and basin morphology, we conclude that the first flooding primarily mobilized hydraulically pre‐sorted sediments from the exposed sandbars of the floodplains, while those sandbars are already submerged during the second flooding in a single‐channel system, resulting in higher sediment contributions from highland tributaries draining igneous and high‐grade metamorphic rocks. Such temporal variations pose constraints on the interpretation of compositional differences between individual samples regarding sediment provenance and dispersal and should be considered in studies of modern drainage basins as well as ancient sediment routing systems.

Reservoir characteristics of lacustrine mixed siliciclastic-carbonate sedimentary rocks: A case study of the Paleogene Funing second member in the Subei Basin, China

Unique lacustrine mixed siliciclastic-carbonate sedimentary rocks accumulated during the deposition of the second member of the Paleogene Funing Formation in the Jinhu and Gaoyou Sags, Subei Basin, which has significant scientific and petroleum exploration value. The reservoir characteristics of the deposits were summarized by means of core observation, petrographic analyses, scanning electron microscope analysis, porosity and permeability tests, carbonate content measurement, and X-ray diffraction clay mineral analyses. These special deposits consisting of siliciclastics and allochems were mainly found in a shallow lake at the front and side of the deltas and also were found in the gravity flow deposits of a deep lake. The results indicate that the lithology and dissolution of early calcite blocks of cement together influence the reservoir performance. From a sedimentological perspective, two types of lacustrine deposits are present in the study area. The first one preserves massive or graded bedding composed of allochems (ooids and intraclasts) and siliciclastics (mainly quartz) of various grain sizes as well as marl; this type of reservoir is poorly sorted. The second one preserves wavy cross-lamination composed of ooids and quartz of similar size; this type of reservoir is well sorted. The diagenesis of the reservoirs in the study area mainly includes compaction, cementation, and dissolution. Reservoirs with massive or graded bedding were impacted by compaction, with current porosity of less than 5%. Reservoirs with wavy cross-lamination could form high-quality reservoirs with porosity of up to 22%, in which the secondary dissolution pores represent the main type of porosity. The early calcite cementation may have inhibited compaction prior to the formation of solution vugs and the formation of high-quality reservoirs.

Pore structure characteristics of different lithofacies of the Longmaxi shale, Western Hunan‐Hubei Region, China: Implications for reservoir quality prediction

Lithofacies and micropore characteristics of shale reservoir directly affect the exploration and development of natural gas. To determine the development potential of shales of the Longmaxi Formation in the Western Hunan‐Hubei Region, the lithofacies types and micropore structure characteristics of the Longmaxi Formation shales have been systematically studied in this paper, based on ordinary thin sections, argon ion polishing–scanning electron microscopy, physical property testing, x‐ray diffraction whole‐rock and clay mineral analysis, isothermal nitrogen, and carbon dioxide adsorptions. According to the principle of ‘laminar structure + total organic carbon (TOC) + mineral composition of felsic, clay, and carbonate minerals’, eight types of shale lithofacies were identified. The effective porosity of the Longmaxi shale ranges from 1.8% to 14.4%, with a peak range from 3% to 6%, while the permeability ranges from 0.001 to 1.27 mD, with an average of 0.106 mD. Combined with the isothermal adsorption experiments, the proportion of mesopores is the largest in the Longmaxi shale, followed by macropores and micropores. Moreover, the pore diameters of mesopores are concentrated between 3 and 10 nm, while those of micropores are concentrated between 0.4 and 0.9 nm. The pore volume of shale is most developed when the content of brittle felsic minerals is 50% ~ 75% and the content of clay minerals is 25% ~ 50%, and the TOC content is positively correlated with the pore volume and specific surface area. There are significant differences in the pore development characteristics of different lithofacies. The shale reservoir space analysis indicates that micropores are controlled by TOC content and clay minerals, while the macropores are controlled by depositional intergranular pores, TOC content, and felsic content, as well as diagenetic pyrite mould pores, microfractures pores, and intercrystalline pores. On the other hand, mesopores are related to a mix of depositional and diagenetic factors with dominant contribution of intergranular pores, intercrystalline pores, organic pores, and intergranular shrinkage fractures pores. It is found that the pore development of massive organic‐rich siliceous shale lithofacies has the best reservoir quality, including well‐connected intergranular pores, intercrystalline pores, organic pores, and shrinkage fractures. Therefore, the massive organic‐rich siliceous shale lithofacies is the dominant fine‐grained reservoir lithofacies in the studied area. Based on understating the pore characteristics of different lithofacies, this study can provide a scientific basis for the prediction of favourable sweet spots of shale gas reservoir in the Longmaxi Formation, western Hunan‐Hubei Region.

The sources and transport pathways of sediment in the northern Ninety-east Ridge of the India Ocean over the last 35000 years

The Ninety-east Ridge (NER) is located in the southern Bay of Bengal in the northeast Indian Ocean and is composed of pelagic and hemipelagic sediments. In addition to contributions from marine biomass, the ridge also contains terrestrially sourced sedimentary material. However, considerable disagreement remains regarding the origin of these terrestrial materials and transport pathways. This paper discusses the collection of seafloor surface sediments and three sediment cores recovered from the northern region of the NER, as well as the analysis of clay minerals, Sr-Nd isotopes, and sediment grain size. The ages of the three core sediments are constrained by AMS 14C dating to better establish the source and transport pathways of the terrestrial materials within NER sediments over the past 35000 years. The research results show that the Qinghai-Tibet Plateau is the predominate source of terrigenous sedimentary material in the NER. In the plateau, the crustal materials were weathered and stripped and then transported to the Andaman Sea via the Irrawaddy River. From there, the material was transported westward by monsoon-driven circulation to the northernmost part of the NER before being transported to the south for final deposition. This transport mode has changed little over the past 35000 years. However, during the rapidly changing climate of the Younger Dryas (12.9~11.5 ka BP), there were some variations in the input amount, grain size, and Sr-Nd isotope value of the source material. The above conclusions are significant for re-evaluating the source of terrigenous sediments, the temporal and spatial changes in transport modes, and the sensitivity of the NER to climatic shifts.