Homogenization Temperatures Of Fluid Inclusions Research Articles

Implications of lithofacies and diagenetic evolution for reservoir quality: A case study of the Upper Triassic chang 6 tight sandstone, southeastern Ordos Basin, China

The Chang 6 tight sandstone reservoir of the Upper Triassic, developed in a large lacustrine shallow-water delta depositional environment, is the most important target for oil exploration and development in the basin. This study is undertaken with the purpose of investigating the influences of lithofacies, diagenetic fluid activity and diagenetic process on quality of the Chang 6 tight sandstone reservoir situated in the southeastern Ordos Basin, based on core observation and description, petrographic investigations, measurement of electron probe microanalysis (EMPA), homogenization temperature and laser Raman spectroscopy of fluid inclusions, laser microzone sampling carbon and oxygen isotope of carbonate cement, the X-ray diffraction (XRD), back scattered (BSE) image observation. Research result shows that three lithofacies are recognized. Each lithofacies has its initial petrological components that slightly differ from other ones, and this is in turn restricted the diagenetic evolution route and imposed diagenetic process and oil-charging intensity, which led to a different reservoir quality.Three carbonate cementation of microcrystalline calcite, sparry calcite I, sparry calcite II and ankerite cements enriched near the mudstone-sandstone interface, occurred in the Chang 6 reservoir which are unfavorable for the reservoir quality. The Chang 6 reservoir experienced three phases of diagenetic fluid activities. The fluid activity of the meteoric water occurred in the eodiagenetic stage offered a carbon source for the cementation of microcrystalline calcite (δ18OPDB: 13.3‰∼-10.0‰; δ13CPDB: 0.4‰∼2.1‰) and sparry calcite I (δ18OPDB: 19.8‰∼-13.4‰; δ13CPDB: 0.4‰∼0.8‰). The latter two fluids released by the thermal decarboxylation of organic matter during the two oil-charging stages, occurred in the mesodiagenetic stage, providing a carbon source for the formation of sparry calcite II (δ18OPDB: 21.8‰∼-18.7‰; δ13CPDB: 5.1‰∼-0.4‰) and ankerite (δ18OPDB: 21.5‰∼-16.8‰; δ13CPDB: 4.6‰∼-3.9‰). The dominant impact on reservoir quality is mechanical compaction, which destroys more intergranular pore than cementation, especially in the sandstone of lithofacies III (the porosity loss by compaction and cementation is 30.9% and 7.3%, respectively), compared to that of the lithofacies I sandstone (the porosity loss by compaction and cementation is 28.7% and 6.6%%, respectively) and the lithofacies II sandstone (the porosity loss by compaction and cementation is 28.8% and 8.5%, respectively). Laumontite cement formed during the eodiagenetic stage effectively resisted the intensity of mechanical compaction. The authigenic clay minerals in the three lithofacies are dominated by chlorite and illite, and most abundant in the lithofacies III. Primary intergranular pore can be effectively protected when the chlorite cement is less than 4%. The dissolution pores with average of 3.0%, 2.4% and 2.3% in the lithofacies I, lithofacies II and lithofacies III sandstone, respectively, formed during the mesodiagenetic stage improved the reservoir quality to a certain degree. Four models of diagenesis and pore evolution of the Chang 6 reservoir are summarized. The pattern I has the best reservoir quality due its fewer ductile component and more coarser grains, the weaker mechanical compaction and weaker cementation of laumontite and carbonate, as well as a stronger dissolution attributed to the intensified oil-charging during mesodiagenesis.

Reconstruction of the Diagenetic Environments of Tight Sandstone Reservoirs: A Case Study from the Tengger Formation in the Baiyinchagan Sag, Erlian Basin, Northern China

Abstract The Lower Cretaceous Tengger Formation located in the Baiyinchagan Sag of the Erlian Basin comprises mainly deeply buried tight sandstone. The identification of high-quality reservoirs in these thickly stacked and heterogeneous units requires a comprehensive understanding of the diagenetic environmental history of the rocks. This paper reports an integrated study involving thin-section petrography, scanning electron microscopy, X-ray diffraction, fluid-inclusion analysis, and vitrinite reflectance analysis of Tengger Formation sandstones with the aim of characterizing the diagenetic conditions of the reservoir rocks and providing guidance for future petroleum exploration. Observed mineral assemblages, the distribution of authigenic minerals, and the distribution and nature of pores suggest the presence of two types of diagenetic environment, acidic and alkaline, which have varied over time and vertically through the rock column. Acidic conditions are indicated by quartz overgrowths and dissolution of both feldspar and carbonate cement. In contrast, alkaline conditions are indicated by the precipitation of carbonate cement, feldspar overgrowths, quartz dissolution, and occurrences of authigenic illite and chlorite. Changes in pore fluid chemistry controlled the evolution of the diagenetic environment. The early diagenetic environment from 110 Ma to 107 Ma was syndepositional and thus controlled by the chemistry of water in depositional centers, which is interpreted to have been weakly alkaline. Significant burial that occurred at 107 Ma induced pulses of hydrothermal fluids and petroleum into the reservoir rocks, which caused a shift to an acidic diagenetic environment. From 103 Ma to 70 Ma, subsequent episodes of uplift and burial caused periodic alternation between acidic and alkaline diagenetic environments. Three distinct episodes of oil and gas charging interpreted from petrography and the homogenization temperatures of fluid inclusions played a critical role in the enhancement of porosity through time. From 70 Ma to the present, acidic diagenesis gradually weakened because of the consumption of organic acids during the process of interaction between rocks and fluids. This study demonstrates the importance of understanding the diagenetic history of reservoir rocks and provides the basis for improved reservoir characterization and optimized hydrocarbon exploration of the Tengger Formation.

A Plea for More Skepticism Toward Fluid Inclusions: Part I. Postentrapment Changes in Fluid Density and Fluid Salinity Are Very Common

Abstract This contribution focuses on fluid inclusions in quartz, which is the most commonly investigated host mineral for fluid inclusions. We demonstrate based on petrographic relationships, cathodoluminescence (CL) images, and microthermometric data that postentrapment migration and/or dismemberment of fluid inclusions in quartz is a very common phenomenon that leads to major changes in the recorded fluid properties. The phenomenon is not restricted to samples that experienced shear stress but is common also in freely grown, euhedral quartz crystals that grew within open-space veins and miarolitic cavities. Generally, both migration and dismemberment lead to substantial increases in fluid density and, thus, in decreased fluid inclusion homogenization temperatures by up to several hundred degrees Celsius. Fluid salinities were commonly less affected, although some high-temperature brine inclusions record salinity increases by up to 30% relative. A good fluid inclusion preservation state is indicated (but not proven) if the inclusions occur on well-defined trails or growth zones, show constant phase proportions, and provide tightly clustered microthermometric data. In contrast, well-developed negative crystal shapes cannot be taken as evidence for a good preservation state. Randomly distributed, regular-shaped fluid inclusions in high-temperature (>300°C) quartz samples are in most cases not of primary origin but rather represent migrated pseudosecondary fluid inclusions. Evidence for fluid inclusion migration and dismemberment includes the occurrence of tiny solid inclusions (mostly mica and rutile) within the host quartz, scattered microthermometric data, and the presence of irregular or c-axis-parallel, dully to nonluminescent quartz patches that depart from healed fracture planes visible in cathodoluminescence images. In summary, pressure-temperature information obtained from fluid inclusions should generally be treated with great caution, unless they meet the above-mentioned criteria for good inclusion preservation.

Fluid inclusions and C–H–O–S–Pb isotope systematics of the Senj Mo–Cu deposit, Alborz magmatic belt, northern Iran: implications for fluid evolution and regional mineralization

AbstractThe Middle Eocene Senj Mo–Cu deposit (1.3 Mt at 1.5 wt % Cu and 0.2 wt % Mo) is located in the central Alborz magmatic belt, northern Iran. Mineralization is characterized by multistage veins, which are hosted in volcanic and volcano-sedimentary strata of the Karaj Formation (c. 41–45 Ma), genetically associated with the Senj Mafic Sill (c. 40 Ma). Four generations of veins are documented based on mineral assemblages and cross-cutting relationships, sequentially: quartz–biotite–chalcopyrite (QBC veins), quartz–molybdenum (QM veins), quartz–pyrite (QP veins) and late calcite (LC veins). Three types of fluid inclusions (FIs) are present in multistage veins: liquid-rich two-phase (L-type), vapour-rich two-phase (V-type) and solid-bearing multi-phase (S-type) inclusions. FIs in the QBC and QM veins are predominantly V-type and S-type, together with minor L-type, whereas the QP and LC vein minerals only contain L-type fluid inclusions. The homogenization temperatures of FIs from QBC to LC veins vary from 320 to 425 °C, 308 to 390 °C, 214 to 288 °C and 145 to 243 °C, and their salinities range from 5.3 to 49.0, 6.2 to 39.7, 5.7 to 13.0, and 2.1 to 7.2 wt % NaCl equiv., respectively. The H–O–C isotope compositions favour a dominantly magmatic origin for the hydrothermal fluids, which were gradually diluted by meteoric waters. The S–Pb isotope data of sulphide minerals indicate that the sulphur and metals were sourced from a deep-seated magmatic reservoir. The Senj Mafic Sill and associated Mo–Cu mineralization was developed in a back-arc extension regime, which faults and crustal fracture systems served as conduits for hydrothermal fluid circulation and Mo-rich veining.

Multistage fracturing history in the Paleocene lacustrine shale oil reservoirs of the Subei Basin, Eastern China

This study examines sealed fractures in core samples from the Paleocene organic-rich lacustrine shales of the Gaoyou Depression, Subei Basin, China, using a multi-pronged approach to elucidate the interplay between fracture propagation and fluid circulation in shaly successions. Based on petrography, geochemistry, and geochronology of fracture-filling minerals, four generations of vein emplacement (denoted Types A-D) were identified. These were attributed to successive episodes of burial diagenesis, tectonism, and uplift occurring at different phases of basin evolution. These vein generations were then inserted into the temporal-spatial framework derived from basin modeling, leading to a reconstruction of the fracture diagenesis history in the Funing shales. Two of the vein generations contain evidence for two phases of oil migration marked by two types of oil inclusions with different fluorescence.It has been shown that the studied lacustrine shales were deposited at ca. 60.2–58.0 Ma. The earliest vein generation, represented by antitaxial beef calcite veins sub-perpendicular to shale bedding, propagated in incompletely consolidated shales within a regional thrust regime related to the Wubao Movement. The vein nucleation during a short-lived burial following initial deposition was also evidenced by the presence of monoliquid inclusions in the veins and calcite LA-ICP-MS U–Pb age (57.7 ± 3.7 Ma). Tectonic compressive stress together with overpressures arising from horizontal and vertical disequilibrium compaction were interpreted as being responsible for this fracturing episode. The second vein generation is bedding-parallel, oil inclusion-rich stretching calcite veins with a crack-seal texture, formed concurrently with kerogen maturation and early oil generation at moderate burial depths. The vein crack-seal fills containing yellow fluorescing oil inclusions provide evidence for episodic oil expulsion in response to cyclic rises and drops in pore pressure during the oil window. Bedding-parallel beef calcite veins characterized by antitaxial growth represent the next vein generation, emplaced during the maximum burial period (ca. 41.9 ± 2.7 Ma). The vein fibers have tracked the horizontal displacement of the wall rocks, likely related to lateral shortening before or at the initiation of Late Eocene basin inversion. The first three vein generations were generated in a relative closed fluid system, with vein fill sourced locally. Host-rock carbonate components and/or earlier diagenetic carbonates were dissolved and then mobilized into these veins. By contrast, the last vein generation with blocky calcite and barite cementation is associated with input of externally derived hydrothermal fluids. The evidence comes from abnormally high fluid-inclusion homogenization temperature (Th) and salinity data, as well as the fact that Ba and sulfate were sourced from the underlying evaporite layers rather than local shales. We thus infer that cross-stratal faulting during the uplift related to the Sanduo Movement resulted in the opening of initial hydraulically closed fluid system. This episode of tectonically-driven hydrothermal circulation from deeper locations in the basin into the Funing shales led to the mixing of oil-bearing pore fluids with exotic basinal hot brines. This also accounts for the presence of abundant blue-green fluorescing oil inclusions and significantly elevated fluid salinity. Overall, this study emphasizes that during basin evolution, differences in tectonic settings and the diagenetic boundary conditions under which veins formed results in different fracturing mechanisms and associated fluid-flow regimes in shale sequences, with consequent effects on vein attributes and mineralization.

Petroleum phase evolution at high temperature: A combined study of oil cracking experiment and deep oil in Dongying Depression, eastern China

Numerous light oil reservoirs have been discovered in the lower part of Mbr 4 of Paleogene Shahejie Fm in the Dongying Depression, Bohai Bay Basin, eastern China. The petroleum phases and reservoir-forming models are crucial to understanding the hydrocarbon accumulation mechanisms in the deep lacustrine layers. The deep formation temperature and pressure, the PVT phase diagrams, as well as the burial-thermal history were simulated and restored based on the results of oil cracking experiments and the analysis of measured data of geological sections in the Dongying Depression. The process and laws of hydrocarbon accumulation were summarized. The cricondentherm envelopes depict a series of variations when the molar percentages of CO2, CH4, C2-5, and C6+ in the fluid composition change. In the process of oil cracking, the cricondenbar increases with the increase in the molar percentage of CH4. With the increase in molar percentage of C2-5 and C6+, the cricondenbar decreases continuously. According to simulations of fluid composition and phase state, the oil reservoirs in this area are primarily oil-rim condensate gas reservoirs and volatile oil reservoirs. Based on burial-thermal history recovered under the constraint of measured vitrinite reflectance (VRo), the homogenization temperature of fluid inclusions suggests two main periods of petroleum charging: in the late deposition of the Mbr 3 of the Paleogene Shahejie Fm and Guantao Fm in the Eocene. In the second charged stage, the oil cracking gas from deep places in the depression was injected into the primary oil and gas reservoirs, causing the changes and differentiation of fluid phases in the reservoirs. The gas invasion and evaporative fractionation caused by late natural gas filling are key factors for the formation of light oil reservoirs.

Dolomite Origins of the Carbonate Successions From the Chihsia Formation, South China: A Case Study From the Pingdingshan Section

The carbonate successions from the Chihsia Formation play an important role for hydrocarbon exploration in the South China. However, the dolomites occurring in the upper section of the Chihsia Formation have been enigmatic, which confined further understanding of the depositional and/or diagenetic fluids and hydrocarbon prediction. In this study, the Pingdingshan section was investigated as a typical outcrop of Chihsian carbonates. On the basis of lithofacies/mineralogic observation of hand specimens and thin-sections, three types of dolomites were classified according to their crystal sizes, that is, micritic dolomite, sucrosic dolomite, and saddle dolomite. Comprehensive, micro-area geochemical proxies, including homogenization temperatures of fluid-inclusions, carbon and oxygen isotopes, and trace and rare-earth elements, were analyzed on different types of dolomites and matrix calcites. The results revealed that the selected samples were representative of temporal seawater and were not contaminated by terrestrial input. The temperatures and salinities of fluid-inclusions, combined with carbon and oxygen isotopic values, revealed that sucrosic dolomites originated from penecontemporaneous micritic dolomites, which had been crystallized and altered into fine- to medium-sized dolomite clusters during the later-stage burying process. In contrast, the saddle dolomites occurring in carbonate matrix or fractures were formed in a hydrothermal environment with a dolomitizing fluid temperature higher than 170°C. This inference is consistent with evidence from rare-earth elements and carbon and oxygen isotopes. Our findings provide new clues for the origin elucidation of dolomitizing fluids in the Chihsia Formation, South China.

Effects of Fracture Formation Stage on Shale Gas Preservation Conditions and Enrichment in Complex Structural Areas in the Southern Sichuan Basin, China

Fractures have significantly impacted the preservation, enrichment, and productivity of shale gas in the Longmaxi Formation in the complex structural area of the southern Sichuan Basin. Based on outcrop, core, imaging logging, inclusion analysis, acoustic emission experiments, and burial and thermal evolution history data, the formation stage, evolution and preservation conditions of structural fractures were comprehensively studied. Shear fractures are the main features of tectonic origin and are characterized by small widths, wide spacing, and high filling degrees. The structural fractures of the Longmaxi Formation can be divided into three groups in terms of their formation stage. The first-stage fractures were formed in the middle-late period of the Yanshanian tectonic movement (86.2–68.5 Ma), when the palaeotectonic stress was 68.33–71.82 MPa; and the homogenization temperature of the fluid inclusions in the corresponding fracture fillings is 150–178°C. The second-stage fractures were formed at the end of the Yanshanian -early Himalayan tectonic movement (68.5–35.2 Ma), when the palaeotectonic stress was 85.2–100.5 MPa; the homogenization temperature of the corresponding fluid inclusions is 123–148°C. The third-stage tectonic fractures formed during the middle-late period of the Himalayan tectonic movement (35.2–0 Ma), when the palaeotectonic stress was 55.6–63.8 MPa; fillings are scarce in these fractures, but the measured homogenization temperature of the corresponding fluid inclusions is 90–110°C. Shale gas preservation conditions were affected by the fracture formation stage, maximum horizontal crustal stress, fault scale, structural burial depth, formation pressure coefficient, etc. Three types of favourable preservation conditions were identified. The Type I target area is the low slope area in the middle of the syncline, which occurs more than 2.0 km from the fault. The research results have guiding significance for the evaluation of the preservation conditions of deep shale gas in the study area and similar areas.

Solid bitumen reflectance and authigenic clay mineralogy in relation to the Irankuh ore deposition: An innovative approach to basin analysis

This research investigates the relationship of petroleum generation and authigenic clay development in function of the paleo geodynamic setting and ore genesis of the Irankuh Zn–Pb deposit, Iran. Core samples from seven boreholes from the northern flank of the ore field, originating from the Gushfil, Rowmarmar, and Tappehsorkh mines, were studied by macro- and micro-petrography. Three important paleo-indicators of temperature, i.e. homogenization temperature of fluid inclusions (Th), Kübler Index (KI), and solid Bitumen Reflectance (BR) were used to constrain the geodynamic setting of the Irankuh deposit. The paleo-geothermal gradient is determined based on the correlation between KI and the inferred vitrinite reflectance (VR), and the maximum heating time is derived from a simulation study of VR versus Th. Results show that the ore mineralization occurred when Irankuh was located in a fore-arc setting during the Upper Cretaceous or Paleocene. The time of increased heat flux has been inferred to be more than one and likely below 10 million years. Zn–Pb mineralization is concentrated in zones enriched in residue solid hydrocarbon (RSH) which is a pyrobitumen type. Pyrobitumen represents a secondary byproduct of primary-oil solid bitumen reacting with hot hydrothermal mineralizing fluids. These interactions in an oil-water-rock system created secondary porosities that were filled by ores and authigenic clay minerals. Our evidence supported an epigenetic origin for ore formation in Irankuh. This study gives new insights into defining the tectonic setting and timing of ore genesis.

Diagenetic and hydrothermal fluid influence on tight sandstone reservoir quality: Gravity-flow deposits from the Huangliu Formation, Ledong area, Yinggehai Basin, South China Sea

The tight sandstone of gravity-flow deposits in Miocene Mbr 2 of the Huangliu Fm, Ledong area is one of the focuses future exploration in the Yinggehai Basin. Reservoirs have complex sedimentary and diagenesis processes, experienced multi-stage fluid influence. However, the influence of different diagenesis on reservoir quality, and distribution patterns of different diagenetic facies in Mbr 2 of the Huangliu Fm are still short in research. In this paper, a variety of methods, such as thin sections analysis, electron microscopy (SEM), electron probe micro-analyzer (EPMA), cathode luminescence (CL), x-ray diffraction measurement (XRD), in situ carbon and oxygen isotopes composition of cements (SIMS), fluid inclusion homogenization temperature, and high-pressure mercury injection (HPMI), are used to analyze reservoir quality, diagenetic evolution stages, diagenesis, fluid filling processes, etc. Studies have shown that there are obvious differences in diagenetic evolution process, including carbonate cementation, organic acid and hydrothermal dissolution, etc. Which ultimately leads to large differences in physical properties. Although sandstone texture has an impact on reservoir quality, strong compaction and early carbonate cementation were the main reasons for reservoir densification. Early calcite was precipitated in alkaline seawater, and later ferrocalcite was formed by the recrystallization of calcite and polyframboidal pyrites and euhedral pyrites formed under organic acid charged. After hydrothermal fluid charged, dolomite, iron dolomite, cement pyrites and dispersed granular pyrites precipitated. Besides, illite filled the throat. The dissolution of organic acids and late CO2-rich hydrothermal fluids improved the quality of reservoirs. According to the difference of diagenetic process and the quality of the reservoirs, Mbr 2 of the Huangliu Fm is divided into 4 types of diagenetic facies. High-quality reservoirs are mostly developed in H2-Ⅰ, H2-Ⅱ, and H2-Ⅴ sand formation near faults. Early carbonate cements were less developed, which could support primary intergranular pores. Feldspar, rock fragments, and early carbonate cements were strongly dissolved. It includes two diagenetic facies, low cementation - strong dissolution reservoirs and slightly cementation - weaken dissolution reservoirs. On the contrary, tight reservoirs mostly develop in H2-Ⅲ, H2-IV, and H2–V sand formation far away from faults, including late compacted tight reservoirs and early carbonate cementation tight reservoirs. The research results are helpful to analyze the diagenetic evolution of tight sandstone reservoirs, and conducive to the study of similar gravity-flow reservoir quality with the influence of hydrothermal fluids.

High temperature wall-rock alteration zoning in the Sanjin deposit, Hishikari gold mine, Japan: Implication for exploration of mature mining districts

The Sanjin deposit is one of three major ore zones hosted by Pleistocene quartz-adularia veins being mined at the Hishikari low-sulfidation epithermal gold mine, which has produced more than 242 t of gold at an extraordinary average grade since 1985. Hydrothermal alteration zoning of the Sanjin deposit was examined with respect to mineralogy, geochemistry, fluid inclusion microthermometry, and oxygen and hydrogen isotopes. Clay minerals are dominated by interstratified chlorite-smectite (C/S) and interstratified illite-smectite (I/S) with <20% smectite. Epidote and prehnite are recorded for the first time at Hishikari in the southeastern part of the Sanjin deposit, typically coexisting with chlorite. Trapping temperatures of fluid inclusions from associated ore zone quartz veins typically range from 195 to 230 °C, with higher temperatures prevalent in the southeastern part of the Sanjin deposit. The calculated fluid δ18O and δ2H values from clay minerals and quartz cannot be explained by a simple water-rock interaction or a simple fluid mixing model, since variable isotopic exchange temperature and endmembers have to be considered. This suggests that both water-rock interaction and mixing of fluids occurred between dynamically variable end members during mineralization. The average estimated formation temperature of chlorites in the Sanjin deposit using chlorite geothermometry is 233 ± 19 °C, in agreement with the highest temperature zone in Hishikari, estimated by homogenization temperature of fluid inclusions of ore veins. In addition, the estimated formation temperatures of chlorite in epidote- and/or prehnite-rich altered rocks are higher (avg. 240 ± 17 °C) than those in epidote- and/or prehnite-poor altered rocks (avg. 216 ± 9 °C). Thus, the chlorite-epidote/prehnite assemblage can be an index of a high temperature alteration zone in the Sanjin deposit. Considering the position of the paleo-water table of the three ore zones, these factors are consistent with the formation of the Sanjin deposit proximal to the upflow zone responsible for gold mineralization at Hishikari. We suggest that our approach could be utilized to understand the thermal structure of epithermal gold system, which may be important to explore for blind veins at mature mining districts.

Geochemical Characteristics and Fluid Properties of the Qixia Formation Dolomites of the Middle Permian in the Shuangyushi Block, NW Sichuan Basin, China

On the basis of petrology, in combination with analysis of the geochemical characteristics of major and trace elements, REE (rare earth elements), carbon isotopes, oxygen isotopes, strontium isotopes and fluid inclusions, the geochemical characteristics and fluid properties of the Qixia Formation dolomites of the Middle Permian Qixiain the Shuangyushi block in the NW Sichuan Basin were systematically studied. The analysis results of the samples show that the Qixia dolomites are mainly composed of medium, coarse, medium-coarse crystalline and dolomites with polymodal crystal size distribution, and cloudy centers and clear rims. They generally show dark purple cathodoluminescence (CL) colors in the centers and orange red CL colors in the rims. The Qixia dolomites are relatively highly ordered (0.79–0.95, averaging 0.90). The CaO and MgO contents show a negative correlation. They are characterized by higher Mg/Ca ratio (0.89–0.92, averaging 0.91), high Fe (94–2,991 ppm, averaging 622 ppm), high Mn (26–185 ppm, averaging 102 ppm), high Na (210–374 ppm, averaging 277 ppm) contents, low Sr (53–218 ppm, averaging 93 ppm) content and lower ∑REE (1.10–5.56 ppm, averaging 2.08 ppm) concentrations. The REE patterns of the dolomites are similar to those of the calcites (C1), and are characterized by light REE (LREE) enrichment, heavy REE (HREE) depletion (LREE/HREE = 5.38–9.58, averaging 7.10), and Eu (0.65–4.19, averaging 1.70) and Ce (0.99–9.52, averaging 2.61) positive anomalies. They have consistent δ13C (1.98–4.27‰ VPDB, averaging 3.01‰ VPDB) and δ18O (−9.54‰–−4.65‰ VPDB, averaging −7.09‰ VPDB) values with the coeval seawater. The salinity index Z is larger than 120, with diagenetic temperature higher than normal temperature. The average value of 87Sr/86Sr is positive (0.707150–0.712273, averaging 0.708669), while some samples are consistent to those of the coeval seawater. Their homogenization temperatures of fluid inclusions (96.3°C–105.0°C, averaging 101.1°C) exceed 100°C, which is equivalent to the buried paleogeothermal temperature, and their salinity (12.3%–14.3%, averaging 13.5%) is 3–5 times that of normal seawater. Based on the regional geological setting and previous research achievements, the dolomitized fluids in the Qixia Formation are probably derived from seawater or marine fluids sealed in pores under shallow-burial environment. The dolomites experienced late alternation by deep high-salinity hydrothermal fluids, and were formed before the Middle-Late Triassic Period.

Effect of authigenic clay minerals and carbonate cements on quality of tight sandstone reservoirs: Insight from Triassic tight sandstones in the Huaqing area, Ordos Basin, Northern China

• Pore and throat structure can be characterized separately by XCT scanning. • Chlorite and illite produced most of the intercrystalline pores in tight sandstone . • Chlorite coatings are effective in preserving the large primary pore-throat. • The densification was completed by cementation instead of compaction. • Tight sandstone with high chlorite is the best quality reservoir in the Ordos Basin. As an important part of tight sandstone, authigenic clay minerals and carbonate cements have a strong influence on the densify of tight sandstone, but the details of these effects on pores and throats need be further clarified. The Ordos Basin is the second-largest sedimentary basin in China with the highest annual oil output and is characterized by tight sandstone. In this paper, the thin section, HPMI, XCT scanning and fluid inclusion homogenization temperature were combined to study the densification process of the tight sandstone in the Ordos Basin. The results show that early chlorite coating can effectively reduce further damage to the pore-throat space. But the pore-lining chlorite and illite led to reduce of the radius of intercrystalline micropores to <1 μm. After mechanical compaction, authigenic illite and the late ferrocalcite and ferrodolomite cements continued to grow and eventually complete the densification. Authigenic clay minerals and carbonate cements played contrasting role on evolving of pores and throats: tight sandstone with high chlorite showed obvious “right peak” PSD and TSD in XCT scanning with the largest number of throats, formed the best-quality reservoir; tight sandstone with high carbonate cement suggesting flat “right peak” TSD and increased the average length of throats; tight sandstone with high illite showed flat and “left shift” PSD and obvious “bimodal” TSD with the fewest number of throats, and formed the worst-quality reservoir.

Geology, mineralogy, geochemistry and conditions of forming the Tukan gold-quartz deposit (Khudolaz trough, South Urals)

Research subject. Detailed mineralogical and geochemical studies of rocks of the Tukan gold-quartz deposit (quartz veins, dolerites, gabbro-diorites) were carried out. The deposit is one of numerous small gold-quartz occurrences in the Khudolaz trough of the West Magnitogorsk zone.Methods. The morphology and composition of native gold was studied using an Axios kop 40 A optical microscope and Tescan Vega 3 and JEOL-6390LV scanning electron microscopes. The chemical composition of rocks was determined by X-ray fluorescence (Carl Zeiss VRA-30) and atomic emission (Shimadzu ICPE-9000) analyzes. Fluid inclusions were analyzed using Linkam TMS-600 thermostage equipped with Link-System 32 DV-NC software and an Olympus BX51 optical microscope. The gas composition of fluid inclusions was examined by Raman spectroscopy (Horiba LabRam HR800 Evolution).Results. Gold-bearing quartz veins and dolerites intersect ing the gabbrodiorite intrusion of the Khudolaz complex are associated with submeridional strike-slip faults. Native gold has a various morphology and is confined to cracks in quartz veins and altered dolerites. The composition of gold is stable, with the average fineness of 871 ± 8.3‰. An analysis of fluid inclusions in a gold-bearing quartz vein showed that gold was formed at temperatures of no less than 186–230°С from a fluid with a salinity of 4–8 wt % NaCl-eq. The presence of gases СО2, N2 and CH4 was found in fluid inclusions. Dolerites and gabbrodiorites hosting gold-bearing veins contain sulfides (pyrite, pyrrhotite, chalcopyrite, pentlandite, violarite), sulfoarsenides (cobaltite and its analogs), as well as mine rals of Ag (hessite) and Pb (kuranakhite and phases similar in composition to minerals of the dugganite group and burckhardtite). Conclusions. The sustained composition of native gold and a narrow range of salinity and homogenization temperatures of fluid inclusions indicate the formation of gold during a single stage of mineralization. The main source of gold could have been the dolerite dikes of the Ulugurtau complex. The presence of CO2, N2 and CH4 gases in fluid inclusions from goldbearing quartz indicates that not only magmatic fluid, but also fluid from the host rocks took part in mineral formation. The low value of the ratio X(CO2)/X(CH4) ≤ 0.7 indicates a moderately reduced ore-forming fluid. Low Au concentrations (0.1– 0.4 ppm) in quartz veins, dolerites, and gabbro-diorites, including those containing visible gold, indicate a weak commercial potential of the deposit

Oil Charging Power Simulation and Tight Oil Formation of Fuyu Oil Layer in Sanzhao Area, Songliao Basin

A thermodynamic and hydrocarbon composition model based on VTFlinc software was conducted based on the homogenization temperature of fluid inclusions and gas-to-liquid ratio, and the fluid inclusions’ trapped pressure was calculated. This software does not require the determination of individual inclusions, the composition of the simulation procedure is simple, and the simulation result has certain reliability. The VTFlinc software was used to calculate the trapped pressure of oil inclusions in the Fuyu tight reservoir of the Sanzhao area in the Songliao Basin. The results show the existence of two phases of oil charge. The first phase occurred at the end of the Nenjiang Formation at 79–74 Ma with an oil entrapped pressure of 16.922–19.068 MPa, and pressure coefficient of 1.18–1.26. During the second period in the Sifangtai Mingshui group at 69–64 Ma, the oil entrapped pressure was 20.216–28.830 MPa, and the pressure coefficient was 1.21–1.33. During the two periods, the pressure of the trapped oil inclusions is abnormal, and the pressure coefficient is above 1.2. Abnormal high pressure is the main driving force of hydrocarbon source rock and oil in the Fuyu Formation, as well as an important driving force for the lateral migration of oil in the tight reservoir. The reservoir formation model of the Fuyu tight oil reservoir in the Sanzhao area is a direct high-pressure charging and multi-channel migration accumulation model. The Q-1 source rock and Fuyu oil layer has a direct contact relationship between the source rock and the reservoir.

Effective reservoir development model of tight sandstone gas in Shanxi Formation of Yan'an Gas Field, Ordos Basin, China

Through a large number of rock thin section microscopic observations, scanning electron microscopy analysis, and fluid inclusion homogenization temperature measurement, the lithological composition of tight reservoirs in the Shanxi Formation of the Yan'an Gas Field in the southeast of the Ordos Basin was studied, lithofacies types were classified, and the diagenetic evolution of different types of rocks was combined with the burial history, thermal history, and hydrocarbon charging process to analyze the time matching relationship between the key oil and gas charging period and reservoir densification. Studies have shown that pure quartz sandstone and quartz-rich low-plasticity lithic sandstone mainly develop mechanical compaction, secondary dissolution, and kaolinite precipitation. Before the two essential hydrocarbons are charged, the porosity is 15.8%–31.5%, which is typical of medium and high permeability reservoir rocks. The porosity of the high tuffaceous heterogeneous quartz sandstone, plastic-rich granular lithic sandstone, and carbonate tight cemented sandstone ranges from 4.6% to 10.8%, indicating that the reservoir is ultra-low porosity-low porosity reservoirs before the first key hydrocarbon charge. It is difficult to charge the hydrocarbons in the later stage. Therefore, the diagenesis of pure quartz sandstone and quartz-rich low-plastic granular lithic sandstone that maintained high porosity and permeability during early oil and gas charging has been inhibited, and the physical properties of the reservoir are relatively good. They are the primary migration channels and accumulation spaces for late-natural gas. This constitutes a sweet spot in tight sandstone gas reservoirs. The research findings are critical for understanding the coupling relationship between reservoir densification and accumulation, clarifying the formation mechanism of effective reservoir rocks, and predicting the distribution of sweet spots.

Diagenesis and hydrocarbon charging period in the Lower Cretaceous Bayingebi Formation, Hari sag, Yin`E Basin, Northern China

The Lower Cretaceous Bayingebi Formation at the Hari sag, Northern China is a significant bearing oil-gas tight reservoir in the Yin`E Basin. In order to understand petrophysical properties of the reservoirs, the diagenesis and hydrocarbon accumulation phases were investigated using casting thin sections, scanning electron microscopy (SEM), X-ray diffraction and fluid inclusion analysis. Additionally, the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge are investigated. The results showed that the pore types include primary pore, intergranular pore and dissolution intragranular pore. The diagenesis of sandstone reservoirs in the process of burial and diagenesis mainly includes compaction, fracture, cementation, metasomatism and dissolution. The comprehensive analysis of various diagenetic stage indexes shows that Bayingebi Formation is in the middle diagenetic stage A-B. The homogenization temperature and salinity of fluid inclusions are low temperature and low salinity (120 °C ∼ 130 °C, 0% ∼ 9%), medium temperature and high salinity (140 °C ∼ 150 °C, 10% ∼ 17%) and high temperature and medium salinity (160 °C ∼ 170 °C, 9% ∼ 11%) respectively. According to the homogenization temperature of the inclusions and the history of burial evolution, the Bayingebi Formation has mainly three phases of hydrocarbon accumulation (100 ∼ 97Ma, 93 ∼ 87Ma, 84 ∼ 81Ma).

Geology, U-Pb geochronology and stable isotope geochemistry of the Heihaibei gold deposit in the southern part of the Eastern Kunlun Orogenic Belt, China: A granitic intrusion-related gold deposit?

The Heihaibei gold deposit is a newly discovered gold deposit in the southern part of the Eastern Kunlun Orogenic Belt. Its most distinctive features are that the gold mineralization is hosted in monzogranite, and that the presence of pre-ore (possibly syn-ore) monzogranite and post-ore gabbro allows to constrain the mineralization’s formation age. Zircons from the monzogranites yield U-Pb ages of 454 ± 3 Ma, while zircons separated from the gabbro dikes cutting the monzogranites and gold mineralized body yield U-Pb ages of 439 ± 3 Ma, which is interpreted to be the minimum age of the Au mineralizing event. Combined with the regional geological background, we proposed that the Heihaibei Au mineralization occurred during the subduction stage of the Early Paleozoic Proto-Tethys ocean.The ore assemblage is dominated by pyrite, arsenopyrite and native gold. The hydrothermal alteration that has led to the peculiar enrichment of Au is not systematically distributed and displays no clear concentric zoning pattern. The main mineralization formed during three stages: the K-feldspar-quartz-pyrite (Py1)-arsenopyrite-sericite-epidote stage (I), the quartz-pyrite (Py2)-native gold-chlorite stage (Ⅱ), and the quartz-carbonate stage (III). The main gold mineralization occurred during stage Ⅱ. Fluid inclusion homogenization temperature and salinities decrease from stage I (Th., 268–412 °C; W., 6.87–16.63 wt% NaCl equiv.) to stage Ⅱ (Th., 183–288 °C; W., 3.69–14.84 wt% NaCl equiv.). The δ18O and δD values (δ18OH2O = 4.9 to 9.7‰; δDV-SMOW = -84.1‰ to −81.1‰) of quartz samples from stage I and stage Ⅱ are comparable to a magmatic-hydrothermal ore-forming fluid that possibly underwent fluid-rock interaction with the Nachitai Group metamorphic rocks during the early ore-forming stage. The relatively uniform δ34S values (δ34SV-CDT = 7.7 to 8.5‰) are slightly elevated compared to magmatic δ34S values, but could be derived from a magma if a significant crustal melt component is present. Moreover, the δ34S values are within the S isotopic composition range of a granitic reservoir, suggesting that they are probably inherited from the Heihaibei monzogranites. The Pb and Hf isotope compositions imply a close genetic association between the gold mineralization and granitic magmatism, which are both the products of the mixing of crustal and mantle sources. The trace element compositions of pyrite provide additional evidence that the gold mineralization in the Heihaibei deposit was related to the magmatism. Compared with the typical characteristics of orogenic gold and intrusion-related gold systems (IRGS) deposits, the Heihaibei gold deposit may instead be classified as a granitic intrusion-related gold deposit.

Geological, geochemical and fluid inclusion investigations on the Duna Pb-Ba-(Ag) deposit, Central Alborz, North Central Iran

The Duna Pb-Ba-(Ag) mine is located ca. 155 km north of Tehran in the central Alborz structural zone, northern Iran. The ore mineralization occurs as stratabound, epigenetic, E-W and WSWENE trending veins and veinlets in fracture-controlled as well as massive and open-space filling textures within a Permian dolomitic limestone host rock. Field observations as well as mineralogical and petrographic studies show dolomitization, silicification and possibly haematisation in the host rock. In the mineralization zone, galena and barite are the main minerals, followed by pyrite, quartz, chalcopyrite, tetrahedrite, sphalerite, calcite and supergene minerals such as covellite, malachite, azurite, cerussite, anglesite, and Fe-oxides. The chemical analyses of the highgrade ore samples show an average grade of 18.66 wt. % for Pb, 19.99 wt. % for Ba, and 120 ppm for Ag together with substantial quantities of Zn (0.15 wt. %), As (690 ppm), Cu (0.86 wt. %), Sb (0.25 wt. %), and Sr (0.56 wt. %). The amount of silver in some samples from the tunnel and discordant layers is up to 7030 ppm. The positive Eu/Eu⃰ ratio and the weak negative Ce/Ce⃰ anomaly in the ore samples were most likely inherited from magmatic water. The presence of minerals such as pyrite and chalcopyrite together with the co-precipitation of sphalerite and chalcopyrite suggest a high-temperature for mineralizing fluids. The homogenization temperatures of fluid inclusions from barite in concordant layers span between 135 and 165 ºC with salinities between 18.54 and 23.65 wt. % NaCl equivalent, while the homogenization temperatures of fluid inclusions from barite of discordant layers span between 113 and 285 ºC with salinities between 7.34 and 23.65 wt. % NaCl equivalent. The structural, geological, geochemical, and mineralogical studies together with the paragenesis of the ore minerals and fluid inclusion data allow consideration of the Duna Pb-Ba-(Ag) mine as a two stage mineralization scenario; 1st stage /older/MVT-type (Early Cimmerian tectonic phase), and the second stage/younger/Irish-type (Laramide orogenic movements). The structural data, high temperature of the fluid inclusions, positive Eu/Eu⃰ ratio and high silver content, especially in the discordant layers, indicate the involvement of a magmatic water mixed with meteoric and connate fluids comparable to the Irishtype mineralization in the second stage, which formed along brecciated zones of the thrust faults. The second stage of mineralization was most likely influenced by the Akapol granitoid intrusive mass, which overprinted the 1st stage/older/MVT-type.

Petroleum Charging History of the Paleogene Sandstone Reservoirs in the Huangtong Sag of the Fushan Depression, South China Sea

The Paleogene sandstone reservoir in the Huangtong Sag of the Fushan Depression is one of the most commercially prolific oil accumulations in the Beibuwan Basin, South China Sea. In this study, twenty-seven crude oil samples from four oilfields in the sag were geochemically analyzed and classified into three oil groups. Oils in the Meitai and Hongguang oilfields show similar geochemical characteristics, with relatively abundant C30 4α-methyl-24-ethylcholestanes and low contents of oleanane and C19+20 tricyclic terpanes (TT), and therefore may be derived from the same source kitchen. Oils from the Yong’an oilfield have a greater proportion of oleanane and C19+20 TT compounds. Oils from the Chaoyang oilfield have intermediate contents of these biomarkers and proportionate values of other related parameters. The unimodal distribution pattern of homogenization temperatures of fluid inclusions found in all the Paleogene reservoirs indicates a single episode of charging. Combining this understanding with the reconstruction of stratigraphy-burial and geothermal histories by 1-D basin modeling, the petroleum charging time was determined to be between 8–2 Ma (the end of the middle Miocene to the early Pliocene). Hydrocarbon migration orientation and charging pathways were traced using molecular indicators (4-/1-methyldibenzothiophene and 1-/4-methodibenzofuran). In the Chaoyang oilfield, the values of these geochemical parameters decreased with decreasing burial depth, indicating vertical oil migration along faults, which are plentiful in the sag. Sandstone bodies with deeper burial depths may therefore be the most promising exploration targets in the Chaoyang area. The Yong’an oils generally migrate from the northwest toward the southeast, suggesting that the source kitchen for the Yong’an oilfield is in the northwest. By similar inference, the hydrocarbon source kitchen for the Hongguang-Meitai area is likely to be on the north side of the Hongguang-Meitai area. By tracing these hydrocarbon charging pathways, the northwest of the Yong’an area and the northern part of the Hongguang-Meitai area are identified as the most promising areas for exploration.