Diagenetic Stage Research Articles

Uranium pre-concentration in sandstone-hosted U deposits: A case study from the Hailijin ore field, SW Songliao Basin, NE China

Economic U mineralization in sandstone was commonly attributed to oxygenated fluid infiltration in reduced sandstone, leading to U precipitation in redox front during late diagenetic stage. It was proposed that U can be pre-concentrated before this period based on trace amounts of U in altered products of Fe-Ti oxides, as revealed by electron microprobe analysis (EMPA). However, the obtained trace elements concentrations in altered Fe-Ti oxides (<1 wt%) is generally accompanied with relatively low analytical precision, leading to a skeptical explanation under this situation. In this work, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine trace elements concentrations in hematite from the uraniferous Lower Yaojia Formation (Hailijin ore field, SW Songliao Basin, NE China), aiming to fingerprint this proposed U pre-concentration in details. Some trace elements contents, such as Mo, Ge, Ca, P, Y, Pb, Th, Sn, and W, are positively correlated with U content in hematite. Given that uraniferous oxygenated groundwater with a near-neutral pH is unable to efficiently dissolve detrital phosphate (i.e., apatite and monazite) in the Lower Yaojia Formation, phosphorous must have been released from these P-rich phases before late diagenesis. Such a conclusion is supported by our observations that monazite from the reduced sandstone has a corrosive morphology, and exhibits a corresponding U enrichment in the corrosive rim. The dissolved P and U can be adsorbed by porous titania up to several thousand ppm during early diagenesis, providing a pre-concentrated metal source for remobilization during late diagenesis. It can not only account for a trace-element signature derived from monazite (i.e., Ca, P, Y, Pb, and Th) in hematite geochemistry, but also fingerprint Mo and Ge migrations resulted from oxidation of pyrite and organic matter, respectively. In addition to U, tungsten and Sn would be re-equilibrated with infiltrated oxidized fluid during the recrystallization of porous titania, and partially incorporated into simultaneously formed hematite grains due to high compatibility. The remobilization of pre-concentrated U induced by oxygenated fluid incursion led to U accumulation in a redox front during late diagenesis, forming uraninite nanoaggregates with relatively high concentrations of P, Y and Ti. Our work has verified that trace-element signature in hematite from sandstone is a reliable fingerprint to reveal ore-forming processes, and we propose for the first time that titania and phosphate play a crucial role for U pre-concentration in sandstone-hosted U deposits.

Origin of chamosite and berthierine: Implications for volcanic-ash-derived Nb-Zr-REY-Ga mineralization in the Lopingian sequences from eastern Yunnan, SW China

Clay minerals are generally used as pivotal indicators to reveal diagenetic parameters such as temperature and pH in hydrothermal ore-forming systems. Within the Late Permian Nb-Zr-REY-Ga-mineralized horizons in eastern Yunnan, the elevated enrichment of critical metals has been considered to be closely associated with abundant clay minerals. In this paper, the identification, abundance, morphology, modes of occurrence, and chemical compositions of two phyllosilicate minerals, namely chamosite (chlorite group) and berthierine (serpentine-kaolinite group) were analyzed by XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy equipped with an energy dispersive X-ray spectrometer). The results show that chamosite mainly occurs as massive and book-like aggregates, while berthierine occurs in various forms, including coarse granular, spherulitic, sheaf-like, sharp-angular, and vermicular aggregates. The modes of occurrence and scatter plots of Mg/(Mg + Fe) vs. Al/(Al + Fe + Mg) of chamosite indicate a Fe-Mg-rich siliceous hydrothermal origin. In the Al/Si-Mg/Fe diagram, analysis points for berthierine are distributed in the partially overlapping fields of hydrothermal ore, laterite/bauxite, and marine, indicating that the formation of berthierine was influenced by various geological factors. Based upon its chemistry and modes of occurrence, berthierine mainly originated by direct precipitation from Fe-Mg-rich siliceous hydrothermal fluids, but in some cases, it was derived from the replacement of precursor minerals and the in-situ alteration of volcanic fragments. A scenario of the transformation mechanism of clay minerals in the Nb-Zr-REY-Ga-rich horizons is proposed. Airborne volcanic glasses and primary minerals were initial altered in-situ into vermicular kaolinite and angular berthierine in the early diagenetic stage. Subsequently, some vermicular kaolinite and the oolitic berthierine crystallized from solutions underwent gradual illitization, and some pre-existing minerals (e.g., albite, kaolinite, and illite) were altered into berthierine, while a proportion of chamosite transformed from berthierine through an interstratified berthierine/chamosite (B/C) in the middle to late diagenetic stages. Throughout all stages of diagenesis, the chamosite, berthierine, and their intermediate phases precipitated directly from Fe-Mg-rich siliceous hydrothermal fluids. Moreover, the diagenetic temperature revealed by chamosite is from ∼200 to ∼302℃, indicating that the Nb-Zr-REY-Ga mineralization and chamosite formation occurred under low-temperature thermal alteration conditions.

LA–ICP–MS U–Pb geochronology and geological significance of two types of monazite in the Xinjiazui gold deposit, northwestern margin of Yangtze Block, China

The newly discovered Xinjiazui gold deposit situates on the northwestern margin of the Yangtze Block, China. Previously, the absolute age of gold metallogenesis in this area has yet to be well defined due to the lack of suitable dating minerals, significantly hindering the study of ore genesis and metallogenesis of gold deposits. This study presents high precision in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) U–Pb ages of monazite to address this issue. Two types of monazite have been found in this deposit, with Type 1 developed in the sedimentary diagenetic stage and Type 2 formed in the ore-forming stage. Type 1 monazite collected from carbonaceous slate is symbiotic with framboidal pyrite in a skeletal texture. It exhibits a high total amount of rare earth elements (REE) and low Y, Th, and U contents, indicating a low-grade metamorphic origin. In contrast, Type 2 monazite from the auriferous quartz vein is intergrown with hydrothermal mineral assemblages including free gold, Au-bearing pyrite and arsenopyrite, ankerite and sericite, suggesting a hydrothermal origin. The monazite (Type 1) LA–ICP–MS 206Pb/238U age is 460.8 ± 7.0 Ma, representing the sedimentary age of the host rock. A combination of lithologic associations and geochemical characteristics indicates that the Ordovician strata (O2ch) are important surrounding rocks in the Xinjiazui gold deposit. Additionally, the forming ages of monazite symbiotic with free gold in the auriferous quartz–sulfide veins range from 206.4 to 207.4 Ma, representing the gold mineralization age of the Xinjiazui deposit. Field geological observation shows that the vertical vein wall of columnar quartz crystals grows inward in the auriferous quartz–sulfide vein geodes and the euhedral granular sulfides develop in geodes. Therefore, this study proposes that the tectonic system changed from compression to extension in the late orogenic period with the closure of Paleo-Tethys and gold mineralization occurred in the Back-Longmenshan tectonic belt.

Geology and mineral assemblages of the early Cambrian black shales in the South Qinling: Implications for vanadium and barium mineralization

Early Cambrian black shales in the South Qinling (SQL) of China constitute one of the world's most important vanadium- and barium-metallogenic belts. It is controversial on the enrichment mechanics of vanadium (V) and barium (Ba) mineralization. The V and Ba mineralization in the Xichuan area from the northern SQL is mainly hosted by shales and chert of the early Cambrian black shales. We conducted mineralogy and geochemistry studies to reveal the mineralization process. Electron probe X-ray microanalysis (EPMA) studies show V is enriched in jarosite, illite and chlorite in shales. The jarosite develops as concretions or flakes and shows close affinity with apatite, with biogenic structures. EPMA and X-ray maps indicate a V heterogenetic distribution within illite and chlorite. Barite occurs as spots and veins in the dark chert and siliceous shale. K-feldspar in shales occurs as columns and flakes with inhomogeneous Ba concentrations. Calcite is intergrown with quartz and Ba-enriched K-feldspar closely in the shales recording the fluctuation of pH values in the diagenetic stage. An acidity environment favors silicification, and increased pH leads to silica dissolution and calcification along with illitization and chloritization. Slightly enriched heavy rare earth elements, positive Eu anomalies, and fluorite from the shales and chert suggest the hydrothermal fluid has existed. Compared with the representative V and Ba deposits associated with black shales worldwide, these deposits occurred synchronously with extinction or anoxic events as both V and Ba mineralization are redox-dependent and associated with organisms. These events could lead to the initial enrichment of V and Ba. Our study suggests that V and Ba mineralization is mainly associated with pH fluctuation in the diagenetic and biogenic processes.

Uranium Occurrence State and Its Implication for Sandstone-Type Uranium Mineralization within the Hanbazhai Area of the Longchuanjiang Basin, China

The Mangbang Formation in the Hanbazhai area is part of the uranium ore field in the Longchuanjiang Basin, China. Uraniferous sandstones from this formation are examined in this study. The type and mode of occurrence of uranium are investigated in detail using an experiment for the sequential extraction of uranium, as well as an electron probe, scanning electron microscopy, and energy spectrum analyses. The sequential extraction experiment indicates that the proportion of uranium minerals is significantly greater than that of the adsorbed uranium in the samples, with the latter being largely present in framboidal pyrites and clay minerals. The results show that these uranium minerals are mainly composed of coffinite and uranium phosphosilicates, which closely coexist with framboidal pyrites, carbon debris, feldspar minerals, and clay minerals. The discovery of coffinite and uranium phosphosilicates is discussed in context with their symbiotic relationship and geochemical environment. Uraniferous sandstones are considered to have undergone at least two stages of mineralization: the sedimentary–diagenetic stage and the later uranium enrichment by fluid. The geochemical environment of the sedimentary–diagenetic stage is generally a sulfide-reducing environment, and the later fluids are rich in U, Si, P, and Y.

The origin of heterogeneous sucrosic dolomite reservoir of the lower Permian Qixia Formation, NW Sichuan Basin, China

Sucrosic dolomite, an important hydrocarbon reservoir, has long been the focus of carbonate sedimentological and reservoir geological studies. This study investigated a kind of heterogeneous sucrosic dolomite in the Lower Permian Qixia Formation of NW Sichuan Basin, which has recently been the location of giant natural gas discoveries. The heterogeneous sucrosic dolomite is characterized by coexistence of porous euhedral dolomite and tight anhedral dolomite, and it is mainly distributed in the platform-marginal shoal facies with a quasi-layered structure. Further geochemical analysis, including C, O, and Sr isotopes as well as rare earth elements, reveals that the euhedral dolomite and anhedral dolomite have similar geochemical properties to the matrix limestone representing coeval seawater, and they were mainly generated from dolomitization by the closed marine-related fluid (left-leaning REE and δPr &lt; 1) in the shallow burial. The difference in crystal morphology, porosity, and permeability between the euhedral dolomite and anhedral dolomite is mainly related to the compositional and textural heterogeneities of the host rocks. Due to the dissolution of meteoric water (relatively flat REE and low Y/Ho) in the early diagenetic stage caused by high frequent exposures, quasi-layered vugs and caves were formed in the grainstones. In the process of shallow burial dolomitization, the loose-filled carbonate sands formed the porous euhedral dolomite due to sufficient space, while the matrix limestone formed the tight anhedral dolomite due to relatively poor porosity and permeability. Accordingly, the paleogeomorphic highland controlled platform-marginal shoal superimposed by meteoric water dissolution in the early diagenetic stage is the main factor for the formation of Qixia Formation reservoirs, while dolomitization is mainly manifested as the inheritance and adjustment of pre-existing pores in the host rock. Therefore, the exploration direction for dolomite reservoirs in the Qixia Formation in the Sichuan Basin should be shifted to the favorable sedimentary facies-controlled reservoir model, which can also be referential for other cases under similar geological setting.

Reservoir Characteristics and Development Model of Subaqueous Pyroclastic Rocks in a Continental Lacustrine Basin: A Case Study of the Chaganhua Subsag in the Changling Fault Depression, Songliao Basin

Industrial oil and gas eruptions underwater have been found in the pyroclastic rocks of the Huoshiling Formation in the continental lacustrine basin of the Changling fault depression, Songliao Basin. This paper investigates the reservoir space characteristics, physical characteristics, and pore structure differences of subaqueous pyroclastic reservoirs in the Huoshiling Formation, and the causes of physical property differences of different types of reservoirs and their formation and evolution processes are analyzed. (1) The content of volcanic glass in tuff is higher, the reservoir space is dominated by devitrification pores and dissolution pores, and the coarser the grain size, the more favorable the physical properties, with larger pore sizes and higher porosities. The content of clay minerals in sedimentary tuff is high, the pores between clay minerals are the main pores, and the physical properties of sedimentary tuff are poor. The content of soluble components such as feldspar, debris, and laumontite is high in tuffaceous sandstone, which is dominated by dissolution pores. (2) Primary pores are not developed in the pyroclastic reservoirs in the study area, and the reservoirs are relatively dense, with an average porosity of 2.43% and an average permeability of 0.076 mD. The coarse-grained tuff has the highest porosity, followed by tuffaceous sandstone and fine-grained tuff, and the sedimentary tuff has the least favorable physical properties. (3) Devitrification was an important cause of the high-porosity and ultralow permeability of tuff reservoirs. Two oil and gas charges in the middle diagenetic stage led to the organic acid dissolution of rocks. In addition, fractures can provide migration channels for organic acids and deep hydrothermal fluids, leading to late dissolution, and can connect various scattered dissolution pores to improve the effectiveness of the reservoir space. (4) Coarse-grained tuff reservoirs that developed in the proximal facies are favorable targets for hydrocarbon exploration.

Study on pore structure characterization of strong diagenesis sandstones and the logging response characteristics in Tazhong area, Tarim Basin

The original framework of pore structure is determined by sedimentation; however, reservoirs with different rock components undergo different diagenetic processes with different intensities, so they have different pore structure characteristics. The logging response characteristics of strong diagenetic sandstone show a series of geophysical responses, such as acoustic, nuclear, and electrical, of the final pore structure style formed by diagenetic transformation after primary sedimentary structures. In low permeability reservoirs, the response characteristics are more reflected as the signal coupling of different geological factors instead of a linear correlation with porosity or permeability like in conventional reservoirs. The formation studied, the Silurian system in the Tazhong area, is mainly characterized by a large area and low abundance of lithological reservoirs with complex geological characteristics such as variable lithology, large physical property differences, and strong heterogeneity. Due to the early formation of the paleo-uplift in the Tazhong area, the Indosinian and Yanshan orogeny had negligible effects and formed a stable tectonic background. Furthermore, the evolution of pore structure is reflected in the transformation from primary pores to secondary or the remaining pores by cementation and dissolution in different diagenetic stages of different sand bodies in tidal flat faces. Therefore, the analysis and characterization of the pore structure system are particularly important. Using data based on 314 MICP samples, continuous changes of the pore structure, and the logging response in single layer and correlated layers, we conducted observations and analytics in the study. Based on the continuous MICP data, the relationship between the changes in pore structure, the geological principles, and the logging response characteristics is analysed. Our results show that the pore system heterogeneity is mainly caused by dissolution and cementation and differs in distribution. The main controlling factors of pore structure changes are transformed from sedimentation controls mainly to sedimentation–diagenesis or mainly diagenesis effects from the bottom-up case. Six types of pore structures are characterized, and we analysed the coupling effect of sedimentation and diagenesis factors on the logging response in the inconsistency features of curves. The classification of pore structure can help to analyse the law of pore structure change in sand body, and quantitatively characterize it from the perspective of control factors and logging response characteristics, which lays a foundation for the subsequent pore structure prediction.

Petrography, Mineralogy and Diagenetic history of the Sarmord Formation (Valanginian-Aptian) in selected areas, Northeastern Iraq

This research studies the petrography, mineralogy and diagenetic history of the Sarmord Formation at Sulaimani and Erbil Governorates in Northeastern Iraq. The Sarmord Formation consists mainly of alternated yellowish to bluish gray bedded marly limestone with yellowish gray soft marl beds. Petrographic investigations of the marly limestones were based on 270 thin sections and show that the skeletal grains include bioclasts, foraminiferas, radiolarians, calcispheres, echinoderms, ostracods and cephalopods. The groundmass is composed of micrite with light brown to gray colors, rich in organic materials and iron oxides. The X-ray diffraction analysis of the marls revealed that the abundant clay minerals in the Sarmord Formation are kaolinite and smectite -illite mixed layers, in addition to chlorite traces, which may indicate an increasing influx of detrital minerals from the continental weathering because of the humid Early Cretaceous climate. Non-clay minerals include calcite and quartz with subordinate feldspars. The main diagenetic processes affecting the Sarmord Formation carbonates are micritization, dissolution, cementation, compaction, dolomitization, dedolomitization and authigenic minerals (pyrite and silica). These belong to three diagenetic stages; early (shallow burial), middle and late (deep burial).

Origin of breccia dolomites in the middle Devonian Guanwushan Formation, NW Sichuan Basin, China: Implications for ultra-deep carbonate reservoirs

Deep to ultra-deep carbonate rocks have become a target for hydrocarbon exploration. The formation mechanisms of these deep to ultra-deep reservoirs are very complicated, and no consensus has yet emerged. This paper reports an investigation into the Middle Devonian Guanwushan Formation in the NW Sichuan Basin, China, which is a typical ultra-deep dolomite reservoir. Drilling-core and thin-section observations identified breccia matrix dolomite (MD) and interbreccia-filling dolomite (FD), of which the FD is commonly associated with sphalerite and barite. Geochemical analyses, including C and O isotopes, trace and rare earth element (REE) contents, and fluid inclusions, were conducted to investigate the origins of the different types of dolomite. Results indicate that the MDs generally retain the signature of syn-depositional seawater (relatively positive δ18O, low Fe and Mn, left-leaning REE patterns, and negative Ce anomalies), with involvement of meteoric water (relatively high Al, Ti and ΣREE). In contrast, the FDs were the diagenetic product of deep hydrothermal fluids (depleted δ18O, high Fe and Mn, and enrichment in MREEs), and the homogenization temperature of fluid inclusions is much higher than the normal burial geothermal temperature. The breccia dolomite reservoirs of the Guanwushan Formation were deposited as reef-core micro-facies and formed during the depositional to early diagenetic stages, and were the result of subaerial seawater dolomitization and meteoric water karstification. However, hydrothermal fluids subsequently modified the reservoir by cementation and filling. The Guanwushan Formation is a typical case of an ultra-deep carbonate reservoir that formed during the depositional to early diagenetic stages but was filled at the burial stage. Therefore, the favorable reservoir zones associated with deposition and early diagenesis should be the targets of future exploration. The findings of this study are expected to be of general significance for deep and ultra-deep oil and gas exploration in other regions and countries.

Comparison of optical and geochemical thermal maturity parameters over a stratigraphically controlled natural transect

Thermal maturity is a fundamental parameter in understanding organic matter evolution during the burial of sedimentary rocks. To this end, numerous optical and geochemical thermal maturity parameters have been proposed, most commonly applied are reflectance or programmed pyrolysis Tmax. Reflectance of vitrinite macerals, or increasingly commonly of solid bitumen macerals, is the classical and well-standardized technique in thermal maturity studies. However, this technique can be influenced by operator bias in acquisition and interpretation. The low cost and wide availability of programmed pyrolysis data has led to the adoption of Tmax as a primary thermal maturity proxy, particularly in industrial studies. However, programmed pyrolysis was originally conceived as a screening tool and significant inconsistencies of Tmax with thermal maturation have been reported.Detailed comparison of maturity proxies is often hampered by the availability of suitable sets of samples from a maturity gradient along uniform facies. Since many of these proxies are influenced by formation specific parameters, the most valuable comparisons must be made using stratigraphically and lithologically controlled sample sets, which are relatively rare. This work examines a natural thermal maturity transect, integrating multiple methodologies to give a detailed discussion of the applicability of optical and geochemically based thermal maturity indicators in the immature to early generation range. This study utilizes a stratigraphically constrained sample set of Upper Cretaceous strata in western Canada taken from 10 wells ranging in thermal maturity from approximately 0.2 to 0.8 %VRo and is based on application of three independent methodological approaches: reflectance measurements, Tmax, and hopanoid and steroid based molecular maturity ratios.Data and correlation equations between major thermal maturity indicators demonstrates solid bitumen reflectance at pre-oil window maturity levels lies along a continuum. Reporting values within somewhat inconsistent classification schemes of bituminite and solid bitumen maceral, as suggested in some recent studies, diminishes the applicability and usefulness of this parameter. Comparison of microscopy and pyrolysis-based maturity proxies with the diagenetic evolution of hopanoids and steroids revealed a robust covariance. Individual molecular maturity parameters are unsuitable to cover the full range of thermal maturity but the wide array of proxies applicable in narrower maturity ranges allows for an optimized selection of those parameters within a given maturity interval. Pyrolysis-derived Tmax and optical determination of OM properties are effective covering a wide range of conditions ranging from the eogenetic to catagenetic stages. Fluorescence analysis is most robust in the eogenetic to diagenetic stages, whereas reflectance measurements provide higher confidence levels at elevated diagenetic or catagenetic stages. Given the limitations of individual thermal maturity parameters, thermal maturity investigations following an integrative, multi-disciplinary approach will limit the impact of uncertainty from any one parameter, improving the odds of achieving a more reliable maturity assessment.

In situ characterization of pyrite from multiple microenvironments in middle Jurassic strata, northeastern Ordos Basin, China: Evaluation of synsedimentary/diagenetic fluid evolution

The sulfur isotopic composition and trace element content recorded in sedimentary pyrite have been widely used as a proxy to establish the geochemistry of pyrite-forming fluids. However, a continuous and complete evolution process of these fluids from the sedimentary period to diagenetic period remains not well constrained. The middle Jurassic strata (the Yan'an Formation containing coal seams and its overlying Zhiluo Formation hosting U deposits in the sandstone) in the northeastern Ordos Basin contain four kinds of microenvironments favorable for pyrite formation, namely coal, carbonaceous debris in barren gray sandstone, barren gray sandstone without carbonaceous debris and U mineralized gray sandstone from bottom to top, providing a good opportunity to decode pyrite formation in response to synsedimentary/diagenetic fluid evolution. In these microenvironments, pyrite named as PC, PD, PB and PM respectively, displays a variety of micro morphologies (framboidal, euhedral, infilling, irregular and cement) formed during sedimentary and diagenetic stages. PC has δ34S values ranging from −36.1 to +26.5 ‰ and low contents of total trace elements (from 6.7 to 58 ppm of Co + Ni + As + Se + Mo). PD and PB present δ34S signatures from −33.6 to +14.4 ‰ and from +9.7 to +22.2 ‰, respectively, with moderate trace element contents (from 21 to 111 ppm and from 23 to 80 ppm, respectively). PM resulting from the oxidation of PD and PB shows negative δ34S values (from −16.9 to −11.1 ‰ for framboidal pyrite and from −56.7 to −34.8 ‰ for euhedral pyrite + pyrite cement) and high trace element contents varying from 350 to 931 ppm. Detailed sulfur isotope and trace element microanalyses indicate that S involved in the formation of pyrite is derived from the fractionation process of organic sulfur in the coal, while Fe and trace elements originate from the surface-derived meteoric water. A paragenetic sequence from early PC, to PD and PB, and finally PM implies an evolution trend of δ34S values increasing first and then decreasing (bacterial sulfate reduction for framboids and Ostwald ripening for euhedral pyrite and pyrite cement of PM) and gradual enrichment of trace elements from synsedimentary to diagenetic fluids. The quantitative results can contribute to a better understanding of synsedimentary/diagenetic fluid evolution both locally and regionally for sedimentary strata in the low-temperature system.

Hydrothermal silicification in Ediacaran Dengying Formation fourth member deep dolomite reservoir, Central Sichuan Basin, China: Implications for reservoir quality

The Ediacaran Dengying Formation fourth member in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of hydrothermal silicification on deep dolomite reservoir quality. By integrating petrology, geochemistry and physical property measurement, this study aims to reveal the genesis of different quartz phase and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the quality of deep dolomite reservoir in Dengying fourth member. Three phases of quartz cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: (1) first‐stage quartz filled in primary reservoir spaces, (2) second‐stage quartz mainly filled in secondary reservoir spaces, and (3) third‐stage quartz filled in secondary reservoir spaces with Mississippi Valley‐Type minerals. By analysing petrographic and temporal relationships between these fabrics, it is suggested that the first‐stage quartz precipitated during the syndiagenetic stage, the second‐stage quartz precipitated during middle diagenetic stage, and the third‐stage quartz precipitated during middle diagenetic stage as well. Silicification during syndiagenetic stage is characterized by replacement of cryptocrystalline quartz and cementation of first‐stage quartz within matrix dolomite (MD). After syndiagenetic silicification, the mineral composition of the MD changed from magnesium calcium carbonate to silica, and the later constructive diagenetic alterations were almost ineffective. The distribution of syndiagenetic silicification during syndiagenetic stage gradually decreases from the platform‐interior to the platform‐margin. The second‐stage and third‐stage quartz cements, derived from two‐stage hydrothermal silicification in the middle stage, are both destructive for reservoir preservation. This study, combined with coeval vapour–liquid inclusions Th data and thermal‐burial models, constrains the time of the second‐stage quartz to ca. 403–408 Ma, suggesting an hydrothermal silicification related to the Caledonian movement. The time of the third‐stage quartz cement is constrained to ca. 259.4 Ma, associated with Late Permian eruption of Emeishan flood basalts.

Evolution of the Pristane/Phytane Ratio with Maturity of Lacustrine Source Rocks: Implications of a Thermal Simulation Experiment

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.

Diagenesis and Pore Formation Evolution of Continental Shale in the Da’anzhai Lower Jurassic Section in the Sichuan Basin

As an unconventional oil and gas reservoir, the diagenesis and evolution of continental shale controls the formation and occurrence of inorganic and organic pores. In order to quantitatively characterize the pore characteristics of a continental shale reservoir and their influence on the evolution of the diagenesis stage, the characteristics of organic and inorganic pore types of continental shale in the Da’anzhai section of the lower Jurassic Ziliujing Formation were identified by means of X-ray diffraction mineral composition analysis and argon ion polishing scanning electron microscope measurements and observations, and the influence control of the diagenesis stage on the pore development of the continental shale reservoir and its control were clarified. The results show the following: ① The organic matter pores in continental shale are developed in large quantities, including organic matter pores in the mineral asphalt matrix and organic matter pores in the kerogen; the pore types of inorganic minerals are very rich, the main pore types are linear pores between clay minerals, intergranular (intergranular) pores, and intragranular corrosion pores, and microcracks are also developed. ② When affected by compaction diagenesis, the inorganic pores of continental shale decrease with an increase in the burial depth and diagenesis degree. ③ The burial depth of continental shale is 2000–3000 m in the middle of diagenetic stage A, and a large number of organic matter pores and dissolved inorganic pores develop at this depth, meaning that the total porosity of shale increases significantly. The burial depth of continental shale is 3000–4000 m at diagenetic stage B, where kaolinite and other clay minerals are dehydrated and converted into illite, the brittleness of shale is increased, and the interior of the shale is subject to external stress, causing microcracks to form. In the late diagenetic stage, when the buried depth of the continental shale is more than 4000 m, the organic matter is subject to secondary cracking and hydrocarbon generation, the organic pores of shale increase in number again, and the inorganic pores decrease in number due to compaction. In conclusion, we found that the burial depth is the main control factor for the development of pores and microfractures in continental shale reservoirs; diagenesis caused by burial depth is the main factor affecting the development of pores and microfractures in continental shale reservoirs; and the shale burial depth in this case is more than 3500 m, which is in the middle of diagenetic stage B. Inorganic porosity in shale is reduced, and the number of microfractures is increased. When the shale is buried more than 4000 m deep in the late diagenetic stage, the thermal evolution of organic matter in shale is high, and methane gas is generated in large quantities, which is conducive to the formation and development of organic matter pores in continental shale.

The Genesis of Pyrite in the Fule Pb-Zn Deposit, Northeast Yunnan Province, China: Evidence from Mineral Chemistry and In Situ Sulfur Isotope

The Fule deposit is a typical Cd-, Ge- and Ga-enriched Pb-Zn deposit located in the southeast of the Sichuan–Yunnan–Guizhou Pb-Zn polymetallic ore province in China. Zoned, euhedral cubic and pentagonal dodecahedral and anhedral pyrites were observed, and they are thought to comprise two generations. First generation pyrite (Py1) is homogeneous and entirely confined to a crystal core, whereas second generation pyrite (Py2) forms bright and irregular rims around the former. Second generation pyrite also occurs as a cubic and pentagonal dodecahedral crystal in/near the ore body or as an anhedral crystal generally closed to the surrounding rock. The content of S, Fe, Co, and Ni in Py1 are from 52.49 to 53.40%, 41.91 to 44.85%, 0.19 to 0.50% and 0.76 to 1.55%, respectively. The values of Co/Ni, Cu/Ni and Zn/Ni are from 0.22 to 0.42, 0.02 to 0.08 and 0.43 to 1.49, respectively, showing that the Py1 was formed in the sedimentary diagenetic stage. However, the contents of S, Fe, Co, and Ni in Py2 are in the range from 51.67 to 54.60%, 45.01 to 46.52%, 0.03 to 0.07% and 0.01 to 0.16%, respectively. The Co/Ni, Cu/Ni and Zn/Ni values of Py2 are from 0.40 to 12.33, 0.14 to 13.70 and 0.04 to 74.75, respectively, which is characterized by hydrothermal pyrite (mineralization stage). The different δ34S values of the Py1 (−34.9 to −32.3‰) and the Py2 (9.7 to 20.5‰) indicate that there are at least two different sources of sulfur in the Fule deposit. The sulfur in Py1 was derived from the bacterial sulfate reduction (BSR), whereas the sulfur in the ore-forming fluids (Py2) was derived from the thermochemical sulfate reduction (TSR). The main reasons for the different morphologies of pyrite in the regular spatial distribution in the Fule deposit are temperature and sulfur fugacity.

The diagenesis and evolution of conglomeratic reservoirs in the Wutonggou Formation in the Yingyeer area

The conglomeratic reservoirs of the Permian Wutonggou Formation in the Yingyeer area of the Turpan–Hami Basin are key petroliferous horizons, with enormous potential for hydrocarbon exploration. Existing studies on the conglomeratic reservoirs in this area mainly focus on the sedimentary environment and sand body distribution, while few of them concern the diagenesis and evolution of the conglomeratic reservoirs, leading to deficient knowledge about relevant mechanisms. By comprehensively utilizing the data from ordinary and casting thin section identification, the x-ray diffraction analysis of whole rock and clay, scanning electron microscopy, and other analyses and assays, this study analyzed and investigated the petrological characteristics, diagenetic types, and characteristics, and diagenetic evolutionary stages and mechanisms of the conglomeratic reservoirs of the Permian Wutonggou Formation in the Yingyeer area. The study results are as follows. Under the influence of sedimentary provenances, tectonic evolution, and paleogeomorphology, the conglomeratic reservoirs of the Permian Wutonggou formation have low clastic grain maturity and low structural maturity, suggesting proximal provenances. The reservoirs are subjected to various diagenesis, including carbonate cementation, which fills pores and throats; compaction as a major factor leading to poor reservoir quality, and dissolution, which significantly improves the reservoir properties. The conglomeratic reservoirs in the Wutonggou formation are generally in substage A of the middle diagenetic stage. The diagenetic evolution of the reservoirs generally underwent early mechanical compaction, calcite precipitation, secondary quartz overgrowth, the injection of organic fluids, the dissolution of feldspars and lithic grains, the transformation of clay minerals, and calcite cementation in sequence. Hydrocarbon charging mainly occurred during the late Middle Jurassic and was greatly influenced by early compaction and dissolution.

Hydrothermal alteration and the remobilization of rare earth elements during reprecipitation of nano-scale apatite in phosphorites

Sedimentary phosphorites enriched in rare earth elements (i.e., REE) are an important reservoir of these critical metals and may have the potential to resolve the global REE supply risk in the future, which is widely distributed around world. However, the occurrence and utilizability of REE in phosphorites remain contentious. In this study, multiple-scale analysis techniques were used to investigate the mineralogical and geochemical characteristics of typical REE-enriched phosphorites from Zhijin, SW China. The mineralization was divided into two stages: early fluorapatite (Fap1) that formed during the sedimentary–diagenetic stage, and late fluorapatite (Fap2) that formed during the hydrothermal alteration stage. The in situ REE geochemical data (i.e., quantitative data and mapping analysis) and scanning electron microscopy images show that Fap1 is the main REE host phase, because it is abundant and high REE concentration. Data obtained by both laser-ablation–inductively coupled plasma–mass spectrometry and high-resolution transmission electron microscopy reveal that most REE are resident in the lattice of nano-scale fluorapatite, which could be remobilized by hydrothermal fluids during the process of dissolution of nano-scale Fap1 and reprecipitation of micro-scale Fap2. In addition, the formation of synchysite-(Y) in the altered phosphorites involved reprecipitation of the mobilized REE. We suggest that F− and SO42− in fluids play a major role in the mobilization of REE in fluorapatite and that CO32− sourced from dolomite alteration contributes to the precipitation of REE. Our results indicate that REE in fluorapatite can be remobilized and further enriched by hydrothermal alteration to REE fluorocarbonates (the current predominant economic source of REE minerals). Overall, our results reveal that REE occur mainly in the lattices of fluorapatite and synchysite-(Y) in phosphorites, and provide new insights into the mineralization mechanism and industrial availability of REE-enriched phosphorites.

Diagenetic Evolution and Its Impact on Reservoir Quality of Tight Sandstones: A Case Study of the Triassic Chang-7 Member, Ordos Basin, Northwest China

The Upper Triassic Chang-7 Member of the Ordos Basin contains typical tight sandstone reservoirs. Reservoir quality is affected by diagenesis, which is a critical factor in tight oil exploration. In this study, the Chang-7 Member tight sandstones were studied by a variety of experimental methods, including thin sections, scanning electron microscopy (SEM), and X-ray diffraction (XRD), to determine the reservoir characteristics and diagenesis and discuss their influences on the reservoir quality. The Chang-7 Member sandstones are mainly lithic arkose and feldspathic litharenite with an average porosity and permeability of 7.17% and 0.13 mD, respectively. The pore type is mainly primary intergranular pores, and the secondary pores are feldspar dissolved pores, which are relatively developed, with pore radii of 2–20 µm. Diagenesis of Chang-7 Member tight reservoirs mainly includes compaction, quartz cementation, carbonate cementation, clay mineral cementation, and dissolution. The diagenetic stage develops into Mesodiagenesis A. The average porosity loss from compaction and cementation of sandstone in the Chang-7 Member is 15.93% and 18.67%, respectively. With the increase in burial depth, the porosity and permeability of the reservoir gradually decrease. In mesodiagenesis, the authigenic illite and carbonate cementation compacts the reservoir. The acid fluid carried by the two stages of oil and gas filling during diagenesis dissolved feldspar and carbonate cement, which plays a certain role in transforming the tight reservoir.

A review of possible mechanisms for mercury migration in diagenesis: Clay to pyrite

Though sedimentary mercury in marine sediments has been regarded as a geological indicator of volcanic activity based on volcanism activity is the dominant natural source of Hg to the oceans, the influence of diagenesis on mercury cannot be dismissed. Marine mudstone has been selected to explore the migration of mercury from the syn-sedimentary stage to the diagenetic stage in this review. Marine mudstones undergo a series of significant transformation processes, including the illitization of smectite and the formation of framboidal pyrite aggregates during the diagenetic stage. This process makes the adsorption capacity of minerals change significantly that clay minerals are weakened, while pyrites are enhanced. In this reason, it is inferred that the sedimentary mercury may re-migrate from clay minerals to pyrite. This at least means that the indication of volcanic activity by mercury enrichment in marine mudstone need to be re-evaluated.