Homogenization Temperatures Of Fluid Inclusions Research Articles

Source and U-Pb Chronology of Diagenetic Fluids in the Permian Maokou Formation Dolomite Reservoir, Eastern Sichuan Basin, China

The hydrothermal dolomitization, facilitated by basement fault activities, had an important impact on the Permian Maokou Formation dolomite in the Sichuan Basin, which experienced complex diagenesis and presented strong reservoir heterogeneity. The source and age of diagenetic fluids in this succession remain controversial. In this study, various analyses were implemented on samples collected from outcrops and wells near the No. 15 fault in the eastern Sichuan Basin to reconstruct the multi-stage fluid activity and analyze the impact on reservoir development, including petrology, micro-domain isotopes, rare earth elements, homogenization temperature of fluid inclusions, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating. The homogenization temperature of primary brine inclusions in fine-grained matrix dolomite and saddle dolomite is concentrated between 100 and 150 °C, which indicates that the impacts of abnormally high temperatures of other geological bodies. The δ13C and δ18O value and low 87Sr/86Sr value indicate that the diagenetic fluid of fine-grained matrix dolomite is mainly Permian seawater. The U-Pb ages of fine-grained matrix dolomite are ~260 Ma, which coincides with the age of the main magmatism of Emeishan Large Igneous Province (ELIP), and hydrothermal fluid provided a favorable high-temperature environment in the penecontemporaneous stage. While highly radiogenic 87Sr/86Sr compositions suggests those of saddle dolomite, the high-temperature Sr-rich formation fluid. The U-Pb ages of saddle dolomite are 245–250 Ma, which coincides with the age of the 255~251 Ma magmatism of ELIP. This indicates that those should be the diagenetic products of the ELIP hydrothermal fluid in the shallow burial stage. The U-Pb age of coarse-grained calcite is 190–220 Ma, and it should be the diagenetic product of the deep burial stage. Brine inclusions associated with primary methane inclusions were developed in coarse-grained calcite, with a homogenization temperature range of 140.8–199.8 °C, which indicates that the formation fluid activities were related to hydrocarbon charging. The Permian Maokou Formation dolomite was firstly formed in the penecontemporaneous shallow burial stage, and then it was subjected to further hydrothermal dolomitization due to the basement faulting and the abnormally high heat flow during the active period of ELIP. Hydrothermal dolomitization contributed to the formation and maintenance of intercrystalline and dissolution pores, whereas it also formed saddle dolomite to fill the pores, and reduce the pore space. The influence of deep fluid activities on reservoir evolution is further distinguished.

Genesis of the Jianbeigou Gold Deposit on the Southern Margin of the North China Craton: Insights from Fluid Inclusions, H‐O‐S Isotopes, and Pyrite in situ Trace Element Analyses

AbstractThe Jianbeigou gold deposit is a typical lode gold deposit in the Qinling metallogenic belt, located on the southern margin of the North China Craton. Three stages of the hydrothermal process can be distinguished, including the quartz ± pyrite, quartz‐polymetallic sulfide, and quartz‐carbonate ± pyrite stages. From the early to late stages, the homogenization temperatures of primary fluid inclusions are 281–362°C, 227–331°C, and 149–261°C, respectively. The corresponding salinities estimated for these fluids are 3.9–9.9 wt%, 0.4–9.4 wt%, and 0.7–7.2 wt% NaCl equiv. Combined with laser Raman spectroscopy data, the ore‐forming fluid belongs to a H2O‐CO2‐NaCl ± CH4 system with medium–low temperature and salinity. The δ18Ofluid and δD values for the quartz veins are –1.0‰ to 6.0‰ and –105‰ to –84‰, respectively, which indicates that the ore‐forming fluid is of mixed source, mainly derived from magma, with a contribution from meteoric water. Pyrite has been identified into three generations based on mineral paragenetic sequencing, including Py1, Py2, and Py3. The pyrites have δ34S sulfur isotopic compositions from three stages between 3.7‰ and 8.4‰, indicating that sulfur mainly originated from magma. Te, Bi, Sb, and Cu contents in pyrite were all high and showed a strong correlation with Au concentrations. Native gold and the Au‐Ag‐Bi telluride minerals were formed concurrently, and the As concentration was low and decoupled from the Au content. Therefore, Te, Bi, Sb and other low‐melting point chalcophile elements play an important role for gold mineralization in arsenic‐deficient ore‐forming fluid. Combined with the geological setting, evolution of pyrite, and ore‐fluids geochemistry, we propose that the Jianbeigou deposit can be classified as a magmatic–hydrothermal lode gold deposit. Gold mineralization on the southern margin of the North China Craton is related to Early Cretaceous magmatism and formed in an extensional setting.

Hydrothermal silicification along rift border faults in the Rio do Peixe basin, Brazil

This study investigates hydrothermal silicification along rift border faults in the Cretaceous Rio do Peixe Basin, Brazil, through field geology, petrography, mineralogy, and fluid inclusion microthermometry. We distinguish three main structural-diagenetic domains: (1) a non-silicified fault damage zone, (2) a silicified damage zone composed of deformation bands and centimeter-scale quartz veins in the hanging wall and footwall, respectively, and (3) a silicified fault core with meter-scale wide quartz veins and mosaic breccia textures. We recognize three main events of silicification in the fault zone. The first silicification event occurs after the widespread development of deformation bands in the hanging wall damage zone that may reach 270 m in width. During subsequent deformation events, multiple fracturing episodes were accompanied with precipitation of the second quartz generation in hydraulic mosaic breccias and the third quartz cement in vein stockworks. Shallow burial conditions of faulting (<2 km), combined with fluid inclusion homogenization temperatures of the second quartz generation, indicate that silicification occurred in a hydrothermal system characterized by fluid temperatures in the range of 71–391 °C (mean value of 141 °C). We conclude that hydrothermal silicification was imprinted during the late synrift stage, when the rift border faults were already established. This contribution has implications for understanding how structural processes may control the geometrical, petrographic and petrophysical attributes of hydrothermal products.

New constraints from in-situ U-Pb ages and fluid inclusions of calcite cement and structural analysis on multiple stages of strike-slip fault activities in the northern Tarim Basin, NW China

Dating the activity of complex faults and fracture systems is crucial for creating reliable geological models for various tectonics and subsurface engineering applications. This study presents a comprehensive study integrating U–Pb fracture cement dating, trace elements, and fluid inclusion temperature analysis with seismic analysis of faults, fault diagenesis, and burial history studies to better constrain faulting and fracture activities in an intra-cratonic strike-slip fault system in the northern Tarim Basin. Seismic profiles indicate at least three distinct phases of fault activity corresponding to the Middle Ordovician, Permian, and Paleogene periods. Fracture cementation and crosscutting relationships corroborate the identification of three fracturing stages. U–Pb dating of fractured cement has widely detected Middle Ordovician and Early Permian age intervals. Fluid inclusion homogenization temperatures from the fractured cements, ranging across < 50 °C, 70–130 °C, and 150–180 °C, correspond to three episodes of rapid subsidence during the Ordovician, Permian, and Neogene, respectively. These results suggest three phases of fault/fracture reactivation in the Middle Ordovician (prior to 470 Ma), Early Permian (prior to 295 Ma), and Cenozoic. The fault/fracture reactivation in the Ordovician is closely related to the regional tectonic transition from extension to compression, while fault and fracture reactivation in the Early Permian may be related to hydrothermal activity associated with large-scale igneous province and oil emplacement. Fault/fracture activity in the Cenozoic may be related to a reduction in subsidence, gradual reduction of geothermal gradients, and massive oil emplacement. This research underscores the significance of integrating geochemical and subsurface datasets for accurately determining the timing of faulting and fracturing in sedimentary basins.

Geochemistry and accumulation of the ultra-deep Ordovician oils in the Shunbei oilfield, Tarim Basin: Coupling of reservoir secondary processes and filling events

Ultra-deep (7300 m–8900 m) condensed and volatile oil reservoirs have been discovered in the Ordovician reservoirs of the Shunbei oilfield, in the Tarim Basin. This study describes the petroleum accumulation process in the Shunbei oilfield by combining an analysis of its geochemical characteristics with seismic section observations and an analysis of petroleum filling events. Geochemical techniques are used to determine the sources, maturity, secondary processes, accumulation periods, and timing of petroleum filling. Analysis of fluid inclusion homogenization temperatures and burial-thermal histories indicates distinct petroleum filling events. Specifically, the north sub-section of the No. 5 fault zone experienced two petroleum filling periods during the late Caledonian and Indosinian epochs while the No. 1 fault zones and the mid-south sub-sections of the No. 5 fault zones experienced three distinct filling periods, encompassing the late Caledonian, Indosinian, and Himalayan periods. The CO2 concentrations and δ13CCO2 values suggest the presence of secondary microbial gases, primarily thermogenic gases. Gas component analysis reveals a transition from kerogen degradation gas to oil cracking gas from north to south. The detection of intact n-alkanes and compounds of the 25-norhopanes series in oil samples suggests a mixture of early biodegradable oil with late fresh oil in the area. Seismic sections reveal diabase intrusions with strong axial reflections. Local hydrothermal activities during the Permian era rapidly cracked saturated hydrocarbon biomarkers, forming polycyclic aromatic hydrocarbons in the oil. Hydrothermal activity in the south sub-section of the No. 5 fault zone depleted the hydrocarbon-generating capacity of the underlying source rocks. Petroleum charged during the Indosinian period likely originated from potential petroleum reservoirs in the Cambrian system. This explains the comparatively low temperature of the SHB57X reservoir (163.32 °C) and the lack of strong hydrothermal activity in the late stage, but no biomarkers of the second-stage charged oil have been detected. Biomarker detection and gas genesis in the mid-south sub-section of the No. 5 fault zone and in the No. 1 fault zone suggest weak or no hydrothermal activity, with Cambrian source rocks maintaining their hydrocarbon generation capacity. Gas filling in the Himalayan period caused evaporative fractionation in the original reservoirs, resulting in a disproportionate (2-MH+2,3-DMP)/(3-MH+2,4-DMP), producing high K1 values in the light hydrocarbons in the present petroleum reservoirs. Differences in gas charging events in the Himalayan period led to the present distribution of petroleum phases.

The Spatial and Temporal Evolution of the Sadisdorf Li-Sn-(W-Cu) Magmatic-Hydrothermal Greisen and Vein System, Eastern Erzgebirge, Germany

Abstract The Sadisdorf Li-Sn-(W-Cu) prospect in eastern Germany is characterized by vein- and greisen-style mineralization hosted in and around a small granite stock that intruded into a shallow crustal environment. The nature and origin of this mineral system are evaluated in this contribution by a combination of petrography and fluid inclusion studies, complemented by Raman spectroscopy and whole-rock geochemical analyses. The early magmatic-hydrothermal evolution is characterized by a single-phase low-salinity (7.0 ± 4 wt % NaCl equiv), high-temperature (&amp;gt;340°C), CO2-CH4–bearing aqueous fluid, which caused greisen alteration and mineralization within the apical portions of the microgranite porphyry. The bimodal distribution of brine and vapor fluid inclusions, and the formation of a magmatic-hydrothermal breccia associated with the proximal vein mineralization are interpreted to mark the transition from lithostatic to hydrostatic pressure. The vein- and stockwork-style mineralization (main stage) displays lateral zonation, with quartz-cassiterite-wolframite-molybdenite mineral assemblages grading outward into base-metal sulfide-dominated assemblages with increasing distance from the intrusion. Late fluorite-bearing veinlets represent the waning stage in the evolution of the mineral system. The similarity in the homogenization temperature (250°–418°C) of fluid inclusions in quartz, cassiterite, and sphalerite across the Sadisdorf deposit suggests that cooling was not a significant factor in the mineral zonation. Instead, fluid-rock interaction along the fluid path is considered to have controlled this zonation. In contrast to quartz-, cassiterite- and sphalerite-hosted fluid inclusions, which have a salinity of 0.0 to 10.0 wt % NaCl equiv, the fluid inclusions in late fluorite veins that overprint all previous assemblages have a salinity of 0.0 to 3.0 wt % NaCl equiv and homogenize at temperatures of 120° to 270°C, thus indicating cooling with or without admixture of meteoric fluids during the waning stage of the mineral system. The Sadisdorf deposit shares similar characteristics with other deposits in the Erzgebirge region, including a shallow level of emplacement, similar mineralization/alteration styles, and a hydrothermal evolution that includes early-boiling, fluid-rock interaction, and late cooling. In contrast to most systems in the region, both proximal and distal mineralization are well preserved at Sadisdorf. The recognition of such spatial zoning may be a useful criterion for targeting greisen-related Li and Sn resources.

A numerical model for the quantification of fluid inclusion property variations caused by heterogeneous entrapment and post-entrapment modifications in the H2O-NaCl system

The primary geological conditions for hydrothermal systems can be reconstructed using fluid inclusions, which generally requires that fluid inclusions trapped a single homogeneous phase and experienced no mass and volume changes after entrapment (Roedder's Rules). However, heterogeneous entrapment and post-entrapment modifications have been frequently identified in fluid inclusion studies, making the recovery of primary fluid properties from fluid inclusion data challenging or impossible. In this study, we constructed a specific tool, FlincPro, for the calculations of ideal and nonideal Fluid inclusion Properties in the H2O-NaCl system (ideal here refers to fluid inclusions that adhere to Roedder's Rules). Multiple modules have been developed to quantify the variations of fluid inclusion properties caused by heterogeneous entrapment, volume contraction/expansion, and water loss. The developed software illustrates possibilities in the variation of fluid inclusions in a specific assemblage with the input of a certain starting point.We found that compositions of the vapor endmember fluid are more significantly affected by heterogeneous entrapment than the liquid endmember, as the latter may have several orders of magnitude higher salinity and density. Homogenization pressure and temperature of halite-bearing fluid inclusions trapped on the halite liquidus will increase sharply with the addition of halite, but the homogenization behavior will not be changed. Volume contraction/expansion are effective ways to change the homogenization behaviors, especially for high salinity fluid inclusions. Water loss is not likely to cause the homogenization behavior change from total homogenization by vapor disappearance to total homogenization by halite dissolution for a halite-bearing fluid inclusion without additional volume contraction. Low-salinity and low-density fluid inclusions with total homogenization temperature around the critical temperature of water are least affected by post-entrapment modifications. Therefore, the interpretation of fluid inclusions formation depends on more parameters, e.g., petrographic observations, density and composition variations in fluid inclusion assemblages, and geological information, than only microthermometric data.

Genesis of the Hajibolagh-Zalibolagh Cu-(Ag) intermediate-sulfidation epithermal deposit, Urumieh-Dokhtar magmatic arc, Iran: Evidence from ore geology, fluid inclusions, and stable isotopes

The Hajibolagh-Zalibolagh deposit located northeast of the city of Saveh in the central part of the Urumieh-Dokhtar magmatic arc of Iran, demonstrates vein and breccia vein style copper-(silver) mineralization hosted by Eocene volcano-sedimentary rocks. The main mineralization stage can be divided into four phases, respectively (1) jasperoid + quartz veins, chalcopyrite, and pyrite; (2) quartz veins along with chalcopyrite, bornite, chalcocite, arsenopyrite, and pyrite; (3) quartz-barite veins along with chalcopyrite, bornite, chalcocite and pyrite; and (4) veins of quartz, calcite, hematite, and oligist along with a small number of copper sulfides. The main stages of mineralization are cut by barren post-ore quartz and calcite veinlets. As a result of supergene processes, minerals are oxidized, and malachite, azurite, chrysocolla, chalcocite, covellite, goethite, and hematite are formed, presenting an important exploration guide. The hydrothermal alteration from the center of the vein-veinlets to the outside includes, respectively, silicification, sericitization, intermediate argillic, and propylitic. The average salinity and homogenization temperature (Th) of fluid inclusions is 14.2 wt% NaCl equiv. and 288 °C, respectively. The coexistence of the vapor phase, liquid-rich phase, and halite-bearing fluid inclusions is a sign of boiling in this hydrothermal system, which has contributed to copper saturation. Sulfur isotope values for pyrite, chalcopyrite, chalcocite, and bornite are mainly from –0.16 to 2.98 ‰, indicating a probable magmatic source for sulfur. Boiling, dilution, and mixing are the mechanisms for the ore deposition and facilitated the mineralization. The Hajibolagh-Zalibolagh deposit shows the highest similarity to intermediate-sulfidation epithermal deposits. The best environment for the mineralization and fluid circulation in these epithermal deposits is the intersection of faults with the Eocene volcanic and volcano-sedimentary rocks.

High Sulphidation Mineralization and Advanced Argillic Alteration within Concealed Gajah Tidur Porphyry, Grasberg District, Papua

High sulphidation (HS) mineralization and associated advanced argillic alteration have been intersected by three drill holes below the Grasberg porphyry Cu-Au deposit, known as the Gajah Tidur prospect. The prospect is located between 1,500 ̶ 2,750 m level, in Grasberg District, Papua, Indonesia. The holes are of KL98-10-21, KL98-10-22, and GRD39-08 which intersected 3.4 Ma Gajah Tidur monzonite, Grasberg Igneous Complex, the wall rocks of Kembelangan, and New Guinea Limestone Group. This research aims to determine the characteristics of high sulphidation mineralization associated with advanced argillic alteration at Gajah Tidur. Petrographic, XRD, SEM, fluid inclusion, and XRF (geochemical) analyses were applied to identify the mineralogy, geochemistry, and ore fluid properties. The major element plots show a differentiated intrusion. The alteration consists of potassic materials composed of biotite and K-feldspar, overprinted by phyllic and advanced argillic ones typified by alunite, pyrophyllite, and kaolinite. The high sulphidation mineralization characterized by pyrite-covellite, chalcopyrite-chalcocite, enargite, and digenite is also present. Fluid inclusion homogenization temperature of mineralized quartz vein ranges from 393 to 542°C, indicating a magmatic fluid origin predominantly. Two distinct porphyry systems, consisting of the Gajah Tidur Cu-Mo and the Main Grasberg Cu-Au porphyry systems are emplaced at the Gajah Tidur level. Advanced argillic is less intense compared to a pervasive phyllic alteration, overprinting the stockwork and surrounding rocks, emplaced at the upper part of quartz stockwork. It is possibly associated with a late stage of Gajah Tidur porphyry hydrothermal fluids which became cooler and highly acidic. Similar to other porphyry systems such as Oyu Tolgoi in Mongolia, the Gajah Tidur porphyry did not have its hydrothermal fluid ascent to the surface to form lithocap. Instead, the ascending fluids cooled at shallower depths resulting in the formation of advanced argillic alteration.

Стадийность формирования кварц-альбит-алланит-фукситовых прожилков на Au-Pd-месторождении Чудное (Приполярный Урал)

The gold-palladium mineralization of the Chudnoe deposit is represented by mineralized veined zones in fractured and brecciated rhyolites of the Riphean-Vendian age. Native gold and palladium minerals are concentrated mainly in veins of Cr-containing muscovite (fuchsite), in which allanite is present in small amounts; quartz, albite, calcite, potassium feldspar, titanite, apatite, zincochromite and other minerals are also found. Veins of allanite-albite-quartz composition are found in ore zones, in some cases containing abundant fuchsite secretions. It has been established that quartz-albite-allanite-fuchsite and other fuchsite-containing veins are the result of the superposition of quartz-albite vein execution on previously formed gold-bearing fuchsite veins. Quartz-albite vein material fills central parts of the combined veins and cements fragments (xenoliths) of fuchsite, while in some cases part of the gold was redeposited and fixed in the quartz-albite part of the veins. Based on the study of the decomposition structures of solid solutions in native gold, it was previously established that the temperature of gold formation in fuchsite veinlets exceeded 220 °C. The deposition temperature of the main part of the quartz-albite veins was noticeably lower. The homogenization temperature of fluid inclusions in vein minerals (quartz, albite, allanite, calcite) was mainly in the range of 96—168 °C, solutions were enriched with magnesium and calcium chlorides. Under these conditions, allanite, apatite, monazite, xenotime, and molibdosheelite were formed and redeposited, and fuchsite recrystallized.

Coupling of source rock gas expulsion intensity and reservoir quality in time and space-a key to determine gas accumulation in Upper Paleozoic tight sandstones of the Mugua gas field, Ordos Basin, China

Research on gas accumulation mechanisms and variations in gas saturation is limited in the newly established Mugua gas field in the Ordos Basin. Understanding these aspects is vital for reduce drilling uncertainty and exploration risk. This study employed a variety of methods, including: (1) the use of organic geochemical examinations to determine the gas expulsion intensity of hydrocarbon source rocks; (2) fluid inclusion analyses to determine the time of gas accumulation; (3) a combination of optical microscopy, scanning electron microscopy observations, pore-simulation and permeability tests, mercury-intrusion capillary pressure experiments, and nuclear magnetic resonance studies to assess the properties of the reservoir quality and identify the tight sandstone rock type; (4) wireline logging and interpretive data analyses to research the gas saturations features; and (5) establish reservoir quality evolution model to systematically study this problem. Integrating fluid inclusion homogenization temperatures with burial and thermal histories indicates that gas accumulation in the Mugua gas fields occurred throughout the Early Jurassic to Late Cretaceous period. Five diagenetic facies have been identified: dissolution, quartz-cemented, clay mineral filling, tightly compacted, and carbonate-cemented facies. The reservoir quality evolution model reveals that the five diagenetic facies experienced densification before the period of gas accumulation. Therefore, for gas accumulation in tight sandstones, it is essential that mature and gas-generating source rocks, provide sufficient expulsion forces to drive gas into the rocks. The mechanism of gas accumulation in the Mugua gas field's tight gas sandstone reservoirs involves the process occurring in sandstones that are either near or interbedded with source rocks, under high pore-fluid pressure generated by gas production. Compared to other Upper Paleozoic gas fields in the Ordos Basin, the gas expulsion intensity in the Mugua gas field is lower, ranging from 6.12–8.39 × 108 m3/km2. Due to the low gas expulsion intensity, the Shihezi 8 Member's tight sandstone reservoirs, situated away from the source rock, typically show low gas saturation. Conversely, the dissolution and quartz-cemented facies within the Benxi, Taiyuan, and Shanxi Formations, intimately interbedded with hydrocarbon source rocks, exhibit good reservoir quality. They offer ideal conditions for gas accumulation, making them prime candidates for exploration and development.

Three-stage hydrocarbon accumulations in the Middle Permian in the Central Sichuan Basin

The Middle Permain Maokou Formation (P2m) is a new region of natural gas exploration in the Sichuan Basin, is characterized by bioclastic limestone with localized dolomitization, and karst fractured-vuggy reservoirs. Currently, on the gas source, hydrocarbon accumulation process and control factors in the Sichuan Basin during the Permian are lacking. To bridge this gap, herein, we identified the filling sequence minerals inside the pores/vugs, along with the oil charge of the Maokou Formation using drill cores, thin sections, oil inclusion analysis, and U-Pb dating of calcite cements. The results showed that the reservoir space of the Maokou Formation was predominated by the residual dissolved pores/vugs, fractures, and dissolved fractures. The pores/vugs underwent four stages of mineral filling by very fine-fine (-crystalline, CC1) calcite → fine-medium calcite (CC2: from 256.4 ± 1.7 to 244.1 ± 6.3 Ma) → fibrous calcite (FC; ∼183.9 ± 8.2 Ma) → coarse-macro calcite (CC3; ∼171.5 ± 5.3 Ma). Combined with the homogenization temperature and salty of fluid inclusion, we considered that three stages of oil charge were present in the Maokou Formation reservoirs. The first stage involved the formation of paleo-oil reservoirs during the Late Permian to Early Triassic, corresponding to the high-maturity aqueous inclusions in CC2, with a homogenization temperature of 106.7°C–137.8°C. At that time, the oil generation from the Lower Cambrian Qiongzhusi Formation rocks peaked, and the generated hydrocarbons migrated upward into the Maokou Formation through the strike-slip faults in the basin center. The second stage involved the formation of paleo-oil reservoirs during the Early Jurassic. The Permian source rocks reached the oil generation window with hydrocarbon expulsion, which was consistent with the oil inclusions in FC. The third stage involved the formation of paleo-gas reservoirs during the Middle Jurassic to Early Cretaceous, corresponding to the high-density methane inclusions and bitumen inclusions occurring in CC3, with the homogenization temperature peaking at 151.9°C–178°C. The natural gas in the Middle Permian of the Central Sichuan Basin is predominantly sourced from the Lower Cambrian Qiongzhusi Formation mudstone and partially from the source rocks of the Middle Permian, indicating a significant source-reservoir conduit of the strike-slip faults in the basin center. The findings provide considerable baseline data to advance further research in the Sichuan Basin.

Diagenetic evolution and origin of cements of the Cretaceous carbonates in the Akal Block, southeast Gulf of Mexico

The Cretaceous succession of deep marine facies (580 m-thick) of the Akal Block, southeast of the Gulf of Mexico, is dominated by lime mudstones that have been extensively affected by dolomitization. Petrographic examination, including cathodoluminescence (CL), and microthermometry reveal three phases of dolomitization that occurred as replacement and pore-/fracture-filling cements. These are (1) an early replacement dolomicrite (D1, <50 μm) with dull orange CL, (2) eu-to subhedral dolomite (D2, 60 μm–150 μm) with distinctive cloudy cores (dull CL) and clean edges (non-luminescent to dull CL), and intercrystalline porosity (6 %), and (3) anhedral vug-/fracture-filling dolomite (D3, 200 μm–500 μm) exhibiting CL zonation. The mean δ18O values of those dolomites show a trend of slight decrease from D1 to D3 (D1 = 0.0 ± 1.3‰ VPDB, D2 = −0.2 ± 1.1‰ VPDB, D3 = −1.2 ± 1.6‰ VPDB), which reflects the influence of progressive burial. The low Sr contents (101 ± 29 ppm) of D1, along with its near-micritic grain size and estimated δ18OD1fluid of parent dolomitizing fluid (−3.6‰–1.9‰ SMOW), support early-stage dolomitization at low temperature of near-surface conditions from solutions of mixed meteoric and seawaters. The higher estimated δ18Ofluid values of D2 and D3 (3.0–13.9‰ SMOW) are expected for fluids in burial settings of high temperatures. The homogenization temperatures of the primary two-phase fluid inclusions of D2 (Th = 99.1 ± 7.9 °C) and D3 (Th = 89.5 ± 11.2 °C) suggest precipitation at deeper burial settings, which is also consistent with their lower Sr contents (80 ± 14 ppm and 98 ± 33 ppm, respectively). The lower mean Th value of D3 than that of D2 likely reflects the influence of tectonic uplift that affected the Akal Block. The shale-normalized (REESN) patterns of the dolomites mimic that of modern seawater and those of D1 and D2, in particular, coincide with that of the lime mudstones (C1), thus suggesting that C1 was their precursor and also D3 originated almost from the same parent fluid that its geochemical composition evolved with progressive burial. The latest fracture-filling calcite cement (C2) postdated D3. The investigation of the products of burial diagenesis, such as fracturing, dissolution, and dolomitization events and their relationship with the tectonic uplift of the Akal Block allows a better understanding of the factors controlling the quality of Cretaceous reservoir rocks.

Solid bitumen as an indicator of petroleum migration, thermal maturity, and contact metamorphism: A case study in the Barrandian Basin (Silurian - Devonian), Czech Republic

Silurian and Devonian marine shales and limestones of the Barrandian Basin host abundant black solid, non-fluorescing bitumens that fill tectonic fractures and veins, and occlude fossil moulds and diagenetic concretions. Solid bitumen, interpreted as thermally degraded petroleum, entered the rocks during several successive episodes of fracture-bound petroleum migration that occurred during deeper burial of the strata. Regional distribution of bitumen reflectance values that range between ∼0.9–2.3% Rr, correlate with variations of its FT-IR and Raman spectroscopic characteristics and aromatic hydrocarbon composition, and collectively evidence the maturity trend increasing across the basin from the southwest to the northeast. The reflectance of chitinozoans and graptolites (∼0.8–1.9% Rr) in the country rocks and homogenization temperatures of hydrocarbon fluid inclusions document palaeotemperatures ranging between ∼90–150 °C, characteristic of the oil window zone grading into the gas/condensate zone. Although in a basin-wide perspective the averaged values of solid bitumen and zooclast optical reflectance converge and indicate the same northeastern-increasing regional diagenetic trend, solid bitumen reflectance values vary considerably at individual localities and even within some bitumen samples. The wide scatter of optical reflectance values and the heterogeneity of optical properties, which were attributed to the presence of multiple source rocks in the basin, the variable lithology of bitumen host rocks, or other variables, hamper the use of solid bitumen as a simple alternative to zooclast/vitrinite reflectance palaeothermometers in a given basin. On the other hand, the highly anisotropic domain and the mesophase “coking” textures of the solid bitumen that were recognized in the NE part of the basin provide unique evidence on an anomalous, hitherto unrecognized, geologically short-lasting thermal event that affected the Palaeozoic rocks. A line of indirect evidence suggests that the coking of the bitumen was caused by a cryptic intrusion, possibly a concealed branch of the Central Bohemian Pluton, which intruded into the strata during the Variscan orogeny. More rarely occurring semi-solid, vividly yellow fluorescing waxy bitumen, that postdates solid bitumen in some fractures and voids, does not reveal a regional thermal maturation trend. It precipitated from relict waxy oils that migrated through the strata during a post-Neogene uplift of the Barrandian region.

Evaluation of sweet spots for a tight sandstone reservoir: A quantitative study of diagenesis in the fourth member of the Oligocene Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Reservoir sweet spot evaluation is very important for tight sandstone gas exploration and development. Due to the characteristics of large burial depth, strong diagenesis heterogeneity, and prominent importance of diagenetic facies, sweet spot evaluation is the focus and difficulty of unconventional oil and gas exploration and development. By combining with core observation, conventional core analysis, clay x-ray diffraction analysis, bulk rock x-ray diffraction analysis, powder grain size analysis, thin section quantitative statistics, fluid inclusion homogenization temperature, scanning electron microscopy, scanning electron microscopy mineral quantitative evaluation, well logging data, and pre-stack seismic data, integrated simulation approach of seismic diagenetic facies and diagenetic numerical simulation was used to simulate porosity evolution and spatial distribution of tight sandstone of second submember (Smbr 42) of the fourth member (Mbr 4) of the Oligocene Huagang Formation in the Xihu Depression in the East China Sea Shelf Basin, and to evaluate reservoir sweet spot distribution. Based on mineral texture relationship and diagenetic sequence of Smbr 42 sandstone compaction, clay mineral transformation, calcite cementation, quartz cementation, and dissolution, five diagenetic facies were identified. Based on various seismic elastic parameters and core-derived diagenetic facies, the distribution of seismic diagenetic facies was interpreted by seismic diagenetic facies prediction method. Seismic diagenetic facies distribution can provide a “hard-constraint” three-dimensional diagenesis heterogeneity model for diagenetic history reconstruction, and seismic diagenetic facies distribution provides constraints on grain size and quartz grain content distribution in diagenetic numerical model. Based on the detailed consideration of input parameters of burial history, thermal history and diagenetic history, integrated diagenetic numerical simulation was used to reproduce porosity evolution and spatial distribution of Smbr 42 sandstone, and was applied to reservoir sweet spot evaluation of tight sandstone. Smbr 42-1 sublayer developed type III reservoir sweet spot and non-sweet spot. Smbr 42–2 to Smbr 42-6 sublayers mainly developed type II1, type II2, type III, and minor type I reservoir sweet spot. Simulation results were consistent with the locations of multiple sets of drilled gas layers and proven gas reservoirs.

Characterization of reservoir quality in tight sandstones from the Benxi Formation, eastern Ordos Basin, China

The Benxi Formation in the eastern Ordos Basin harbors abundant natural gas resources and shows promising exploration and development potential. However, the reservoir characteristics are complex, and the primary controlling factors are unclear, presenting significant challenges for reservoir characterization. In response to these challenges, we conducted a systematic study on the characteristics of tight sandstone reservoirs in the Benxi Formation by integrating thin section analysis, scanning electron microscopy, high-pressure mercury injection, and conventional petrophysical analysis alongside well log data analysis. By applying empirical calculation formulas of a porosity evolution quantitative model, we elucidated the primary controlling factors of reservoir heterogeneity. Our research identified that the reservoirs in the eastern Ordos Basin, Benxi Formation, are predominantly composed of quartz sandstone and lithic quartz sandstone, with pore-filling cementation as the dominant cement type and the main storage spaces being intergranular pores and dissolved pores. The homogenization temperatures of fluid inclusions in authigenic quartz range from 92.8 to 185.7°C, indicating that the target layer is in the mesodiagenesis phase B. The main reasons for the differences in reservoir quality in the Benxi Formation are attributed to both sedimentation and diagenesis. In terms of sedimentation, two distinct sedimentary microfacies control the distribution of reservoir quality differences based on variations in quartz content and soluble components. Regarding diagenesis, the Benxi Formation underwent compaction, cementation, and dissolution, with compaction being the fundamental cause of widespread reservoir compaction.

Hydrocarbon accumulation history in Lower Cretaceous in northern slope of Bongor Basin in Chad, Central Africa

Based on the analysis of the fluid inclusion homogenization temperature and apatite fission track on the northern slope zone of the Bongor Basin in Chad, this paper studied the time and stages of hydrocarbon accumulation in the study area. The results show that: (1) The brine inclusions of the samples from the Kubla and Prosopis formations in the Lower Cretaceous coexisting with the hydrocarbon generally present two sets of peak ranges of homogenization temperature, with the peak ranges of low temperature and high temperature being 75–105 °C and 115–135 °C, respectively; (2) The samples from the Kubla and Prosopis formations have experienced five tectonic evolution stages, i.e., rapid subsidence in the Early Cretaceous, tectonic inversion in the Late Cretaceous, small subsidence in the Paleogene, uplift at the turn of the Paleogene and Neogene, and subsidence since the Miocene, in which the denudation thickness of the Late Cretaceous and after the turn of the Paleogene and Neogene are ~1.8 km and ~0.5 km, respectively. The cumulative denudation thickness of the two periods is about 2.3 km; (3) Using the brine inclusion homogenization temperature coexisting with the hydrocarbon as the capture temperature of the hydrocarbon, and combining with the apatite fission track thermal history modeling, the result shows that the Kubla and Prosopis formations in the Lower Cretaceous on the northern slope of the Bongor Basin have the same hydrocarbon accumulation time and stages, both of which have undergone two stages of hydrocarbon charging at 80–95 Ma and 65–80 Ma. The first stage of charging corresponds to the initial migration of hydrocarbon at the end of the Early Cretaceous rapid sedimentation, while the second stage of charging is in the stage of intense tectonic inversion in the Late Cretaceous.

Distribution and Genesis of the Deep Buried, Fractured and Vuggy Dolostone Reservoir in the Lower Ordovician Succession, North Tarim Basin, Northwestern China

Recently, a series of prolific fracture-vug reservoirs have been discovered in the lower Ordovician dolostone successions of the northern Tarim Basin. However, the genesis of these reservoirs remains unclear. In this study, observations on drilling cores and thin sections identify the pore space characterized by dissolved fractures, fissures, and vugs. Petrology, cathodoluminescence, and homogenization temperatures of fluid inclusions aid in establishing the diagenetic paragenetic sequence. Dissolving enlargement occurred after chemical compaction of overlying limestone and before the Permian volcanic activity. Breccia pores containing unique fillings of terrestrial materials (quartz sand and allogenic kaolinite) and calcite cements with negative δ18OPDB values (−18.4‰) along with 87Sr/86Sr ratios (up to 0.71026) indicate that the dissolving fluid originated from meteoric freshwater at the surface. The δ18OSMOW values of the calcite precipitating fluid (−2.1‰ to −8.7‰) further suggest freshwater as the source of the dissolving fluid, buffered by the Ordovician wall rocks or formation water. As the distance from the unconformity surface increases, both the homogenization temperature and δ18OPDB values of the breccia pore-filling calcite in the southern study area gradually elevate and deplete, respectively, indicating a rise in temperature during the infiltration of meteoric freshwater with increasing subsurface temperatures. The abnormal reflection bodies identified as reservoirs in seismic profiles along deep-seated strike-slip faults delineate these faults as the channel for the infiltration of meteoric freshwater. The penetrating strata of these faults and the high 87Sr/86Sr values of breccia pore-filling calcite suggest that karstification occurred during the Devonian period. Accordingly, we establish a deep karst model in which the Devonian meteoric freshwater penetrated along the strike-slip faults and dissolved the Ordovician dolostones, resulting in the development of deep buried karstic fault reservoirs in the southern region of the northern Tarim Basin.

Origin of calcite cements in the Permian Lucaogou Formation tight reservoirs, Jimsar sag, Junggar basin, NW China: Constraints from geochemistry

Understanding the diagenetic evolution of carbonate cementation in hydrocarbon reservoirs is critical to better reconstruct the history of fluid flow during petroleum accumulation. The extensive occurrence of carbonate cements within the lower Permian Lucaogou Formation tight reservoirs is regarded as a major diagenetic factor that causes strong hydrocarbon reservoir heterogeneities. Petrographic and geochemical analyses, including optical microscopy fitted with cathodoluminescence, scanning electron microscopy, carbon and oxygen stable isotopes, in situ rare earth element, as well as fluid inclusion microthermometry, were carried out to decipher the origin of calcite cementation and understand its control on reservoirs quality. Petrography shows that the precipitating cements mostly consist of pore-filling blocky/mosaic calcite, with a dull orange luminescence color. Chondrite-normalized rare earth element (REE) patterns and C–O isotopic compositions of these calcite cements exhibit strongly negative Eu and Tb anomalies, enrichment in light REE, positive carbon (δ13CPDB = 0.58‰–7.45‰) and negative oxygen (δ18OPDB = −11.38‰ to −8.61‰) isotopic values, similar to the dolostone and micritized peloidal grains, suggesting that the calcite-precipitating fluids were derived from dissolution of lacustrine depositional carbonates. The relative timing of calcite precipitation in reservoirs was constrained based on the thermal-burial history of the study region and precipitation temperatures obtained from fluid inclusions. The results show that this cementation episode mainly occurred during the Late Triassic to Early Jurassic, which was synchronous with the tectonic uplift event. Moreover, homogenization temperatures of fluid inclusions and geochemical signatures of the calcite cement are similar to those of the fracture-fillings, suggesting that these two calcite phases may be genetically related and form coevally within one fluid flow system. It can be interpreted as a response to the compressional tectonic deformation given by the collision of the Qiangtang Block with the Eurasia Plate. This tectonic deformation event is regarded as the driver for calcite precipitation during the early burial stage, resulting in a considerable reduction of reservoir quality. Calcite cementation is pervasive and may play a main role in halting hydrocarbon migration through some of the permeable pathways in reservoirs.

Genesis of tungsten mineralization in the Yangla copper-polymetallic deposit, northwest Yunnan, China: Evidence from in situ U-Pb dating, trace elements, Sr isotopes, fluid inclusions, and H-O isotopes of scheelite

Scheelite is one of the main ore minerals in tungsten deposits, and its geochemical features can be used to examine and constrain tungsten mineralization. The Yangla deposit is one of the most important Cu-polymetallic skarn-porphyry deposits in the “Sanjiang” polymetallic metallogenic domain, Southwest China, hosting 150 Mt of Cu ores with economic concentrations of Pb, Zn, and Sb. The newly discovered tungsten mineralization in the Yangla deposit remains poorly understood. Hence, in order to ascertain the timing of tungsten mineralization, investigate its genetic relationship with magmatism and ore genesis, and elucidate the source, nature, and evolution of the ore-forming fluids, we performed a combined study of geochronology, in-situ trace elements, fluid inclusions, H-O isotopes, and in-situ Sr isotopic compositions of scheelite. In-situ U-Pb dating of scheelite showed that the timing of tungsten mineralization was 30 Ma. This mineralization age is significantly later than the published Cu mineralization age (∼230 Ma), skarn (∼231 Ma), and granitoid emplacement age (∼230 Ma) in the Yangla ore district. In-situ trace element analyses revealed that scheelite is depleted in Mo, Mn, and Ta and slight enriched in Sr, Y, REEs, and Au. It is also characterized by HREE-depleted chondrite-normalized patterns with strongly positive Eu anomalies. Our data reveal that 3Ca2+ = (2REE)3++□Ca (site vacancy) is the main substitution mechanism that controls REEs in scheelite. The low concentrations of Mo, negative Ce anomalies, and positive Eu anomalies of scheelite indicate a reduced condition. Preliminary REEs features indicate that the genesis of scheelite may be related to vein-type W (Au) deposits. The homogenization temperatures for fluid inclusions present a strongly decreasing trend (328 to 173 ℃), indicate that cooling is the predominant mechanism for the formation of scheelite. The δD and δ18Ofluid values from fluid inclusions of scheelite vary broadly from −161.70 ‰ to −133.30 ‰ and from 6.32 ‰ to 9.80 ‰, respectively, indicating a mixture of magmatic and meteoric water, with possible involvement of magma degassing and fluid–rock interaction. The initial 87Sr/86Sr ratios of scheelites (0.71931– 0.72117) are significantly higher than those of the granodiorite (0.71149 – 0.71990) and basalt (0.70562 – 0.70995) in the Yangla ore district, indicating that the Oligocene magmatism and Devonian marble with radiogenic Sr isotopic compositions contributed to tungsten mineralization. The fluid-rock interaction between the ore-forming fluids derived from the concealed granitoids and the Devonian wall rocks may play an important role in tungsten mineralization and provide significant Ca for scheelite precipitation. Thus, the tungsten mineralization should be associated with Oligocene magmatism, ruling out the genetic relationship with Late Triassic magmatism and being significantly different from the skarn-porphyry Cu-Pb-Zn mineralization at Yangla.