Hydrothermal Dissolution Research Articles

Hydrothermal Dissolution of Reactive Silica Materials in Sodium Hydroxide Lyes

AbstractReactive silica materials like vitreous silica or cristobalite were dissolved at temperatures between 100 and 220 °C for up to 7 days in NaOH lyes. The liquid and solid reaction products were analyzed chemically. Sodium water glasses with silica concentrations of up to 27 wt % and molar SiO2/Na2O ratios of up to 3.7 were obtained. The experimental data were evaluated by an approximation algorithm. The initial size reduction rates ranged between 0.1 and 7 µm h−1, depending on the temperature and the choice of starting materials. Correlations between thermodynamics and kinetics were discussed.

Differential fault-fluid alterations and reservoir properties in ultra-deep carbonates in the Tarim Basin, NW China

The development of high-quality carbonate hydrocarbon reservoirs in the ultra-deep (7000–10000 m) Lower Paleozoic strata of the Tarim Basin was substantially related to tectonic evolution, strike-slip faults, and diagenetic fluids. Detailed studies of the influences of tectonic evolution and strike-slip faults on the properties of diagenetic fluids and the development mechanisms of ultradeep carbonate reservoirs under the control of both faults and fluids are required to identify high quality reservoirs. In this study, end-member geochemical indicators of meteoric water, hydrothermal fluid, and formation fluid were constructed based on typical diagenetic mineral and geochemical data obtained from representative wells. Meteoric karst associated with strong tectonic uplift was the main diagenetic event along strike-slip faults in the Tahe area. The Shunbei area was generally affected by buried formation water, and locally by weak meteoric water or hydrothermal fluids. The Tazhong, Shunnan, and Gucheng areas were predominantly affected by strong hydrothermal fluids along strike-slip faults associated with strong volcanic activity, while some wells, such as TZ12, reveal the influence of thermochemical sulfate reduction (TSR). Differences in fault fluid types and properties resulted in differential development of ultra-deep carbonate reservoirs among the different areas. In the Tahe area, meteoric water alteration along faults formed karst fracture-cavity reservoirs. In the Shunbei area, fault-cavity carbonate reservoirs were formed by strong strike-slip faults, whereas fluid alteration was weak. Hydrothermal dissolution reservoir in the Tazhong area, hydrothermal silicification reservoir in the Shunnan area, and hydrothermal dolomitization reservoir in the Gucheng area are developed, respectively.

Hydrothermal alteration mechanisms of an Archaean metamorphic buried hill and the models for reservoir zonation, Bozhong depression, Bohai Bay Basin, China

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.

Igneous intrusion contact metamorphic system and its reservoir characteristics: A case study of Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin, China

Taking the Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin as an example, this study comprehensively utilizes seismic, mud logging, well logging, physical property analysis and core thin section data to investigate the metamorphic reservoir formed by contact metamorphism after igneous rock intrusion. (1) A geological model of the igneous intrusion contact metamorphic system is proposed, which can be divided into five structural layers vertically: the intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and floor. (2) The intrusion is characterized by xenoliths indicating intrusive facies at the top, regular changes in rock texture and mineral crystallization from the center to the edge on a microscopic scale, and low-angle oblique penetrations of the intrusion through sedimentary strata on a macroscopic scale. The metamorphic aureole has characteristics such as sedimentary rocks as the host rock, typical palimpsest textures developed, various low-temperature thermal metamorphic minerals developed, and medium-low grade thermal metamorphic rocks as the lithology. (3) The reservoir in contact metamorphic aureole has two types of reservoir spaces: matrix pores and fractures. The matrix pores are secondary “intergranular pores” distributed around metamorphic minerals after thermal metamorphic transformation in metasandstones. The fractures are mainly structural fractures and intrusive compressive fractures in metamudstones. The reservoirs generally have three spatial distribution characteristics: layered, porphyritic and hydrocarbon impregnation along fracture. (4) The distribution of reservoirs in the metamorphic aureole is mainly controlled by the intensity of thermal baking. Furthermore, the distribution of favorable reservoirs is controlled by the coupling of favorable lithofacies and thermal contact metamorphism, intrusive compression and hydrothermal dissolution. The proposal and application of the geological model of the intrusion contact metamorphic system are expected to promote the discovery of exploration targets of contact metamorphic rock in Nanpu sag, and provide a reference for the study and exploration of deep contact metamorphic rock reservoirs in the Bohai Bay Basin.

Rapid dissolution without elemental fractionation by laser driven hydrothermal processing

Traditional dissolution of geologic samples often requires a significant time investment. Here, we present an alternative method for the dissolution of geologic materials using laser-driven hydrothermal processing (LDHP). LDHP uses laser energy directed onto a submerged sample, which increases the temperature and pressure at the liquid–sample interface and drives the hydrothermal dissolution coupled with photomechanical spallation, an ablative process. This uses focused 527 nm laser energy at 40 W average power, 1 kHz pulse repetition rate, and 115 ns pulse duration. When LDHP is performed on basalt geostandards (BCR-2 and BHVO-2) using the conditions outlined, we show that LDHP does not produce significant elemental fractionation and, thus, can be considered an alternative processing method to traditional mechanical crushing and acid digestion. Additionally, it is possible using LDHP to utilize the spatially confined beam to target and selectively isolate individual phases in a rock, potentially alleviating the need for mechanical separation of inclusions that are difficult to physically isolate. Furthermore, using this outlined method of LDHP, we demonstrate full dissolution of 120 mg of obsidian in 85 minu, meaning that LDHP is a potentially very useful method when sample processing is time sensitive.

Discovery of Pseudomorph Scapolite and Diagenetic Indication from the Permian Volcaniclastic Rocks in Western Sichuan (SW China)

Volcaniclastic rocks are important unconventional oil and gas reservoirs from which it is difficult to determine the protolith due to strong metasomatic alteration. Intensive alteration has occurred in much of western China, but few robustly documented examples are known from which to assess the alteration processes. Further recognition from the petrological and mineralogical record is essential to quantify the diagenetic environment, the degree of alteration, and its impacts. Permian volcanic rocks are widely developed in the western Sichuan Basin (southwestern China), with a thickness of more than 200 m. The thickness of volcaniclastic rocks in the Permian Emeishan Basalt Formation is up to 140 m, with a 5600~6000 m burial depth. In this study, we demonstrate an approach to recognizing hydrothermal alteration by the occurrence of scapolite megacryst mineral pseudomorphs (SMMPs) in Permian volcaniclastic rocks in the Sichuan Basin (southwestern China). The results show that SMMPs in the Permian volcaniclastic rocks in the western Sichuan Basin mainly occur in the lower part of the Permian basalts as intragranular minerals and rock inclusions. Scapolite is transformed into quartz and albite, and only the pseudomorph is preserved, indicating secondary hydrothermal fluid metasomatic alteration. Scapolite is formed after microcrystalline titanite and is the product of the high-temperature pneumatolytic metasomatism of plagioclase from the mafic protolith during the post-magmatic stage. The mixing of meteoric water and barium-rich hydrothermal fluid leads to the precipitation of barite; additionally, the pores are filled with barite and halite after the alteration of scapolite. The silicification and hydrothermal dissolution of scapolite and the albitization of sodium-rich matrix minerals increase the pore volume, which is conducive to the later recharge by hydrothermal fluids. The discovery of SMMPs can serve as an indicator of the high-temperature pneumatolytic metasomatism and mixing of meteoric water and deep hydrothermal fluid.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-paleogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Analysis of the filling patterns and reservoir development models of the Ordovician paleokarst reservoirs in the tahe oilfield

The characteristics of Ordovician paleokarst reservoirs in the Tahe Oilfield are complex. Many prediction studies on the distribution of karst fractures and caves have been carried out, but the filling types and distribution characteristics of caves have been less studied and are thus less understood. First, the development characteristics of the early paleohydrological landforms in the early Hercynian are considered, the paleokarst landforms are restored, the characteristics of the paleohydrological network are clarified, and the lateral distribution of the surface and groundwater systems is illustrated. Additionally, based on a qualitative analysis of core thin section photographs, conventional logging curves, full-bore formation microresistivity scanning, imaging, logging and other characteristics, the existing filling types are divided into four classes: vertical flow filling, river transport filling, cave collapse and hydrothermal transformation. Moreover, reservoir facies division standards for the different filling types are established. Different filling types are constrained in the context of paleogeomorphology and paleohydrology to further study the filling effects and the corresponding controlling factors. As research progresses, five common joint control modes are proposed: joint control of sinkholes and underground rivers, joint control of ancient landforms and underground rivers, joint control of multilayer caves and ancient landforms, control due to collapse and fracture, and joint control of early collapse and late hydrothermal transformation. Based on the model, the relevant mechanisms and the concentrated areas of development are analyzed in depth. The distribution map of the plane filling type is obtained by combining the development model with the identification of the filling type. The relationships between oil and gas production and the reservoir filling degree and between oil and gas migration channels are analyzed by the classification and statistics of different filling types laterally, and the relationships between oil and gas production and the reservoir filling degree and between oil and gas migration channels are analyzed. Hydrothermal action has the strongest ability to transform reservoirs, mainly due to hydrothermal dissolution and fault zones transporting oil and gas; the yield of caves filled with sinkholes is relatively low, which is related to the good reservoir space retained by oil and gas filling and the migration channels provided by fault zones. The yield of the underground river filling type is lower than that of the water-falling cave filling type, mainly because the underground river filling type is located in the karst slope and lower part of the basin. The water energy in the karst slope is relatively strong, and the interior of the cave is filled with medium-fine sandstone. In the lower part of the karst zone, the water energy becomes weak; both of these conditions lead to further filling of the cave reservoir space and a lack of migration channels. The collapse filling type has the strongest impact on the reservoir because this discussion is mainly about late collapse, which causes the cave to collapse, and the internal filling is severe; even if a fault zone acts as a migration channel, oil and gas production is relatively low.

Petrological, Geochemical and Chronological Characteristics of Dolomites in the Permian Maokou Formation and Constraints to the Reservoir Genesis, Central Sichuan Basin, China

The Middle Permian Maokou Formation in the Sichuan Basin has huge resources and is an important target for natural gas exploration. In recent years, significant exploration breakthroughs have been made in the dolomite field of member Mao-2 in central Sichuan, and the gas production of several wells has exceeded 1 × 106 m3/d, indicating promising prospects for exploration. However, the origin of the dolomite reservoir in member Mao-2 remains ambiguous, which restricts the accurate prediction of favorable reservoirs. This study focuses on drilling in the Hechuan area as its research object, by using a detailed description of the cores from member Mao-2 of seven wells; samples were selected for tests of the degree of dolomite cation ordering, stable carbon and oxygen isotopic compositions, strontium isotopic composition, rare earth elements, LA-ICP-MS element mapping and U-Pb dating. It is clarified that: (1) The crystalline dolomite of member Mao-2 in the Hechuan area is the main reservoir rock, and the heterogeneous vugs and fractures are the main reservoir space. The dolomite in member Mao-2 has been characterized by a low degree of cation ordering value (avg. 0.59), with values of δ13C (avg. 3.87‰), δ18O (avg. −7.15‰) and 87Sr/86Sr (avg. 0.707474) having similar geochemical characteristics to Middle Permian seawater; the REEs normalized distribution patterns have similar characteristics to limestone; and the U-Pb age (261.0~262.0 Ma) corresponds to the age in the Capitanian stage of the Permian Guadalupian Series. (2) Petrological studies show that member Mao-2 has vertical karstification zonation characteristics; syngenetic karstification controls the formation of a large-scale fracture-cave system in the phreatic zone; the dolomitization of sediment in the fracture-cave system occurred during the penecontemporaneous period with locally restricted seawater. (3) The main controlling factors of the reservoir were syngenetic karstification, early dolomitization and hydrothermal dissolution related to Emei taphrogenesis. The research results are of great significance for dolomite reservoir prediction; the highlands of paleogeomorphology with syndepositional faults are favorable areas for dolomite reservoirs.

Hydrothermal alteration and corresponding reservoir significance of the Permian Emeishan basaltic lavas, west Sichuan, China

Reservoir spaces, such as vesicles, ‘secondary’ amygdales, dissolution caverns and geodes, are widely developed in the Emeishan basaltic lavas in the Zhoudaping section, Leshan, west Sichuan, China. The dissolution characteristics, cementation sequences, hydrothermal activity stages, as well as fluid types, and their effects on the reservoir capacity were investigated for each stage. Macroscopically, the dissolution features present as irregular dissolution zones, which are characterised by a light red colour. Microscopically, in the dissolved zone, the cementation-filling minerals are associated with complex fill sequences, such as quartz/laumontite/chlorite–chlorite/saponite–epidote/celadonite–cryptocrystalline chlorite–laumontite/calcite/quartz. The U–Pb geochronology shows that the age of chlorite fill in amygdales is 235.3 ± 19.6 Ma; the coarse-crystalline quartz inside dissolution caverns/geodes, 124.47 ± 5.63 to 123.84 ± 5.63 Ma; and the siliceous mineral-filled amygdales, 118.34 ± 3.70 to 114.08 ± 3.76 Ma, which correspond to the early Late Triassic and the mid–late Early Cretaceous, respectively. Combined with geochemical characteristics of post-dissolution fill, the amygdales are affected by two stages of hydrothermal activity: chlorite filling of the amygdales corresponds to post-magma hydrothermal fluids during the early Late Triassic, and the siliceous mineral-fill in amygdales corresponds to deep-sourced hydrothermal fluids during the mid–late Early Cretaceous. The geodes/dissolution caverns result from a single stage of hydrothermal activity related to the mid–late Early Cretaceous deep-source low-temperature hydrothermal fluid. The Late Triassic post-magma hydrothermal fluids are generally destructive to pores, and tectonic-related dissolution of deep-sourced hydrothermal fluids has a positive effect on the formation of reservoir spaces, greatly enhancing fluid storage and flow capacities of the volcanic lavas. We recommend the multi-stage hydrothermal dissolution during Late Triassic–Early Cretaceous and faults, fractures and columnar joints be the focus of hydrocarbon exploration. KEY POINTS The reservoir spaces developed in the Zhoudaping section, such as amygdales, dissolution caverns and geodes, were controlled by different stages and types of hydrothermal alteration. Amygdales are the product of two hydrothermal events, which correspond to post-magma hydrothermal fluids during the early Late Triassic and the deep-sourced hydrothermal fluids of the mid–late Early Cretaceous. Geodes/dissolution caverns are affected by deep-source low-temperature hydrothermal fluids in the mid–late Early Cretaceous. The multi-stage hydrothermal dissolution during the Late Triassic–Early Cretaceous and faults, fractures and columnar joints should be the focus of hydrocarbon exploration.

Characteristics and Formation Mechanism of Multiscale Storage Spaces in Ancient Deep Tight Reservoirs: Examples from the Cambrian Yurtus Formation in the Northern Tarim Basin, China

AbstractUnder the background of complicated diagenetic fluids, high-temperature pressure in superimposed basin, the pore-forming and pore-preserving effects of deep tight reservoirs are complex, and the formation mechanism of high-quality reservoirs has always been the core issue. With the discovery of oil and gas in ultradeep drilling in Tarim Basin, the most unconventional tight reservoir of Lower Cambrian Yuertus Formation in Tabei area began to receive attention. Based on the research of typical field outcrops in Tabei area, the lithofacies, reservoir space differences, and reservoir formation mechanism are systematically analyzed. (1) The stratum has undergone complex multistage diagenetic fluid transformation, and the rock types are diverse with great differences; siliceous rock and dolomite are the main rock types, often in thin-medium layered distribution. (2) After long-term deep burial transformation, siliceous rock and dolomite can still retain a large number of effective storage spaces; hydrothermal dissolution pores, organic acid dissolution pores, atmospheric freshwater dissolution pores, and intercrystalline pores are the main pore types, which provide main storage spaces. Multiscale pores are developed in siliceous rock and dolomite, with good connectivity and good pore structure. (3) The large-scale storage spaces mainly come from the effective maintenance of primary pores, organic matter, and hydrothermal dissolution. The siliceous minerals and dolomite have stable properties and strong resistance to compaction and can effectively maintain the early pores. (4) The large-scale reservoir space was formed in the early diagenetic stage; the pressure-solution and cementation are the two most important diagenetic processes for reducing storage spaces; however, under the pore-forming and pore-preserving effects of early silicification and dolomitization, various types of dissolution, and oil charging, the tight lithology can still maintain effective storage spaces. The related research has important theoretical and practical significance for studying the formation mechanism of tight reservoirs in deep ancient strata and predicting high-quality reservoirs.

Contribution of Remote Sensing and GIS to the Inventory and Analysis of Factors influencing on the Development of Karst Features in the Middle Atlas, Morocco

The development of karst features is a complex and interactive process of many factors with varying influence over time. The following main factors seem to have influenced the occurrence and development of karst features in the Middle Atlas in Morocco: Precipitations after extreme weather events depending on the precipitation pattern modified by orographic effects and exposition of the terrain towards the main rain-bearing clouds are determining surface water input and infiltration as well as the outline of morphologic watersheds and drainage basins. The lithologic conditions such as the outcrops of lower Jurassic limestones, the geomorphologic disposition, the volcanic activity within limestones combined with hydrothermal dissolution processes, and the structural conditions influencing water permeability in fault and fracture zones and, thus, the dissolution intensity, are further factors. Ongoing aseismic vertical and horizontal neotectonic movements due to plate tectonic activities and shallow earthquake activities are causing the development and reactivation of fault zones and as consequence modifying groundwater flow conditions in the karst hydrologic systems. Seismic activity can lead as well to the development of sinkholes and the collapse of caves. The development of a systematic Karst-Geo Information System (GIS) is recommended that should be updated regularly.

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.

Rock Magnetic Signatures of Hydrothermal Mineralization in the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field

AbstractThe Ocean Drilling Program Leg 158 drill holes from the Trans‐Atlantic Geotraverse hydrothermal field are investigated to understand the rock magnetic signatures of hydrothermal mineralization. A composite columnar section has been constructed through hole correlation to understand the stratigraphic variation of magnetomineralogy within the stockwork. Isothermal remanent magnetization components unmixing, first‐order reversal curve diagrams, low‐temperature magnetic signatures, and electron microscopic analyses disclose magnetic minerals of disparate occurrences related to predominating hydrothermal mineralization reactions in three broad zones: For basaltic basements, serpentinization of olivine phenocrysts during preliminary hydrothermal alteration produces magnetite, in addition to primary titanomagnetite; Chloritized and silicified zone samples contain relict titanomagnetite and exsolved magnetite that survived hydrothermal dissolution; Anhydrite and sulfide zone samples are dominated by magnetite and hematite, likely from oxidation of polymetallic sulfides due to exposure in oxidative seafloor environments during drilling. Our findings suggest that seafloor oxidation potentially modifies the magnetic properties of polymetallic sulfides in hydrothermal deposits, which applies to magnetic tomography of sophisticated subseafloor vent structures and prospecting seafloor massive sulfides (SMS) deposits therein. Meanwhile, we alert future deep‐sea mining that drilling may promote physicochemical alteration of SMS deposits, causing environmental risks. The established magnetic signatures ultimately contribute to understanding the in situ geological preservation of SMS deposits and optimizing exploitation procedures in the future.

Mineralogical and Geochemical Evidence of Dissolution-Reprecipitation Controlled Hydrothermal Rare Earth Element Mineralization in the Amba Dongar Carbonatite Complex, Gujarat, Western India

Abstract The Amba Dongar carbonatite complex in western India comprises an inner ring of carbonatite breccia surrounded by a sövite ring dike. The various carbonatite units in the body include calcite carbonatite, alvikite, dolomite carbonatite, and ankerite carbonatite. The carbonate phases (calcite and ankerite) occur as phenocrysts, groundmass phases, fresh primary grains, and partially altered grains and/or pseudomorphs when hydrothermally overprinted. Rare earth element (REE) enrichment in the groundmass/altered calcite grains compared to the magmatic ones is ascribed to the presence of micron-sized REE phases. Fluorapatite and pyrochlore constitute important accessory phases that are altered to variable extents. Higher concentrations of Sr, Si, and REEs in fluorapatite are suggestive of a magmatic origin. Fresh pyrochlore preserves its magmatic composition, characterized by low A-site vacancy and high F in the Y-site, which on alteration becomes poorer in Na, Ca, and F and displays an increase in vacancy. The C-O isotope compositions of the carbonates also corroborate the extensive low-temperature hydrothermal alteration of the carbonatites. The REE mineralization is the result of interaction of the carbonatite with a sulfur-bearing, F-rich hydrothermal fluid that exsolved from late-stage carbonatitic magmas. The hydrothermal fluids caused dissolution of the primary carbonates and simultaneous precipitation of REEs and other high field strength element (HFSE)-bearing minerals. Complex spatial associations of the magmatic minerals with the REE fluorocarbonates, [synchysite-(Ce), parisite-(Ce), bastnäsite-(Ce)] and florencite-(Ce) point to the formation of these REE phases as a consequence of postmagmatic hydrothermal dissolution of the REEs from fluorapatite, pyrochlore, and carbonates. Ubiquitous association of fluorite and barite with REE minerals indicates transport of REEs as sulfate complexes in F-rich fluids. Precipitation of REE fluorocarbonates/florencite resulted from fluid-carbonate interaction, concomitant increase in pH, and decrease in temperature. Additionally, REE precipitation was aided and abetted by the removal of sulfur from the fluid by the precipitation of barite, which destabilized the REE sulfate complexes.

Petrology, mineralogy and geochemistry of Ordovician rocks in the southwest of Tarim Basin, implications for genetic mechanism and evolution model of the hydrothermal reformed-paleokarst carbonate reservoir

Paleokarst carbonate reservoir plays an important role in the exploration of oil and gas from shallow to ultra-deep strata. Multistage karstification is superimposed to form the present ultra-deep karst-hydrothermal reformed carbonate reservoirs in the Yubei area of the Tarim Basin. Based on petrology identification, stable carbon and oxygen isotope analysis, inductively coupled plasma mass spectrometry (ICP-MS), and Laser Ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we investigated paleokarst periods, chemical property of hydrothermal fluids, and the evolution model and formation mechanism of paleokarst-hydrothermal reformed Ordovician carbonate reservoirs. The results show that the Ordovician strata experienced five periods of karstification from the middle Caledonian period to the late Himalayan period. The Caledonian quasi-contemporaneous karst and the Hercynian epigenetic karstification provided the most fundamental pore – fracture framework for carbonate reservoirs, and the later multistage deep burial karstification re-transformed the established reservoir space by hydrothermal dissolution under thermochemical sulfate reduction (TSR) reaction. It is proposed that the thermal convection between the hydrothermal fluids from the upper mantle and carbonate rocks formed two kinds of hydrothermal fluids in Ordovician strata. The first kind of hydrothermal fluid is performed with a SiO2-enriched fluid under the acidic environment based on TSR, forming siliceous cements and hydrothermal dissolution pores in the Lower Ordovician strata. The second kind of hydrothermal fluids appeared as an alkaline CO32—enriched fluid with the reversible reaction between HCO3− and CaCO3, forming hydrothermal calcite and dolomite near the migration channel in Upper Ordovician carbonate rocks. The periodic changes of in situ trace element concentrations of multistage quartz fillings indicated that paleokarst reservoirs are controlled by thermal convection under episodic tectonic activity. As a summary, we established the migration pattern of hydrothermal fluids in the thin section scale and the evolution model of the hydrothermal reformed paleokarst reservoirs in basin scale.

Characteristics and Formation Mechanism of the Lower Paleozoic Dolomite Reservoirs in the Dongying Depression, Bohai Bay Basin

The Lower Paleozoic carbonate strata experience multi-stage tectonic activity and post-depositional volcanic activity in the Dongying Depression, Bohai Bay basin. These tectonic and magmatic activities have caused the reservoir to undergo severe diagenesis, resulting in strong reservoir heterogeneity. This study aims to identify the characteristics of dolomite, various reservoir spaces’ characteristics, the origin of different types of dolomite, and the porosity evolution. According to crystal size and morphology, dolomites can be divided into three kinds of matrix dolomites and four kinds of dolomite cements. The petrology and geochemistry of the dolomite suggests that matrix dolomite is formed from seawater. The medium-to-coarse-crystalline dolomite cement (D3) has a higher 87Sr/86Sr ratio (0.7119 to 0.7129) and a higher homogenization temperature (>125 °C), suggesting that the fluid for the precipitation of D3 is a mixed fluid formed by hydrothermal fluid eroding the 87Sr-rich feldspar sandstone. The strikingly negative δ18O values (−23.7 to −25.7‰ VPDB) of saddle dolomite (D4) indicate that D4 precipitated from hydrothermal fluids and the Mg2+ source may be due to dissolution of the host dolomite that formed in the evaporation environment. The reservoir spaces of the target strata in the study area mainly include fractures, dissolution vugs, intercrystalline pores, and moldic pores. Dissolution is the basis for forming high-quality dolomite reservoirs. The faults and fractures provided favorable conditions for dissolution. Hydrothermal fluid and organic acid were the main dissolution fluids for the dolomite reservoir, which were beneficial to the development of secondary pores. In the study area, organic acid dissolution was shown to contribute more than hydrothermal dissolution in the study area.

Impact of fluorine-bearing hydrothermal fluid on deep burial carbonate reservoirs: A case study from the Tazhong area of Tarim Basin, northwest China

Deep burial carbonate reservoirs (>4000 m) have great potential of reserves for oil and gas exploration, although the matrix reservoirs generally become extremely heterogeneous and tight due to various diagenesis. Hydrothermal dissolution is supposed to be important for formation of high-quality deep burial carbonate reservoirs. However, few studies have focused on influence of minor fluorine-bearing fluid on carbonate reservoir quality. This study integrates petrography, geochemistry and fluid inclusion data to understand impact of fluorine-bearing fluid on deep burial carbonate reservoirs. Mineralogy and mineral distribution suggest that fluorite and concomitant calcite are found in the Ordovician and Cambrian strata of the whole Tazhong area, especially in the vicinity of faults. Calcite associated with fluorite is probably derived from progressive mixture between formation water and fluorine-bearing hydrothermal fluid, which is supported by REY pattern of concomitant calcite, surrounding limestone and fluorite, and fluid inclusion microthermometric data of concomitant calcite and fluorite. Fluorine-bearing hydrothermal fluid was responsible for the concomitant C2 calcite of high Y/Ho ratio. Therefore, the superchondritic Y/Ho ratio of concomitant calcite can be probably used as proxy for identifying distribution of minor fluorine-bearing hydrothermal fluid. For thick fluorite bed, influx of fluorite-rich fluid resulted in surrounding limestone dissolution and altered zones between fluorite and limestone. This process simultaneously improves porosity and permeability of high-energy sedimentary facies reservoirs. In contrast, the fluorine-rich hydrothermal fluid through the low-energy sedimentary facies mainly enhanced permeability of the tight carbonate reservoirs rather than porosity. For widespread tight reservoir of high-energy and low-energy sedimentary facies, even minor fluorite-bearing fluids still enhance permeability. The fluorine-bearing hydrothermal fluid may influence carbonate reservoirs away from hydrothermal-altered zones. Significance of minor fluorine-bearing hydrothermal fluid for widespread tight carbonates has probably been underestimated for deep burial carbonate reservoir.

Petrographic, mineralogical and geochemical constraints on the fluid origin and multistage karstification of the Middle-Lower Ordovician carbonate reservoir, NW Tarim Basin, China

The Middle-Lower Ordovician carbonate formation is the most important hydrocarbon-bearing unit in the Tarim Basin, China. Previous studies have predominately attributed the fracture-cave carbonate reservoirs to epigene karstification, and few involved polygenic karstification. Until now, mechanism and evolution model of the fracture-cave reservoirs has not been very robust. Based on detailed petrographic, mineralogical, karstological and geochemical analysis at the outcrop, this research provides new insights into the characteristics of the karst reservoir, origin of diagenetic fluids and multistage karstification in the Middle-Lower Ordovician. The results indicate that matrix porosity and permeability of the reservoir rock are low, ranging from 0.03% to 1.35% (mean 0.46%) and from 0.000864 mD to 9.15 mD (mean 0.33 mD), respectively, and that karst cave and fracture systems account for the dominant secondary reservoir porosity in the Middle-Lower Ordovician. Meteoric water and hydrothermal fluids are the two most important diagenetic fluids in the Middle-Lower Ordovician, and two stages of dissolution are responsible for the development of the fracture-cave reservoirs in the region. Extensive dissolution by meteoric water resulted in the formation of large volumes of fracture-cave reservoirs during the Middle Caledonian. Hydrothermal dissolution induced by the hydrothermal event during the Early Permian formed additional fracture-cave reservoirs in the Middle-Lower Ordovician carbonate as well. The meteoric dissolution is primarily controlled by the karst unconformity, and the fracture-cave system resulting from meteoric dissolution has a stratiform distribution below the unconformity. The fracture-cave system formed by hydrothermal fluids is mainly controlled by the deep-seated faults and the previous permeable layers, which determine that its distribution is closely related to faults. Moreover, hydrothermal dissolution may occur at high temperature and at great depths, while mineral precipitation may be the dominate process at relatively low temperature and at shallow depths. In this study, a reservoir development model of multistage karstification was established that could further enhance the knowledge of karstification process and assist in petroleum exploration in deep carbonate strata. • Two types of diagenetic fluids are revealed by integration of Geochemical, Petrographic and Mineralogical analysis. • The effects of multistage karstification on the carbonate reservoir are determined. • Hydrothermal karstification plays a positive role in the deeply burial environment. • A development model of the carbonate reservoir covered by the overlying impermeable strata is established.

The Diagenetic Alteration of the Carbonate Rocks from the Permian Qixia Formation as Response to Two Periods of Hydrothermal Fluids Charging in the Central Uplift of Sichuan Basin, SW China

The hydrothermal fluid–carbonate rock reaction is frequently regarded to occur in deep-burial diagenesis, and the hydrothermal dissolution is usually distributed and takes place along the faults. Previous studies have suggested that there was hydrothermal fluid activity locally in the Permian Qixia Formation in Sichuan Basin, likely related to the Emeishan basalt eruption. However, the effect of hydrothermal fluids on the carbonate rocks of the Qixia Formation in the central uplift of Sichuan Basin is still unclear. Based on the characteristics and geochemical parameters of the diagenetic minerals, this study aims to reveal the diagenetic alteration related to the hydrothermal fluid–rock reaction in the Qixia Formation and reestablish the diagenetic evolution by using the timing of diagenetic mineral precipitation. The methods include petrographic observation; trace and rare earth element (REE) analysis; C, O and Sr isotope measurement; fluid inclusion temperature measurement and cathodoluminescence analysis. According to the petrographic characteristics, the dolostones are mainly of crystalline structure, namely fine-medium crystalline dolostone, meso-coarse crystalline dolostone, and coarse crystalline dolostone, with the cathodoluminescence color becoming brighter in that order. The limestones from the Qixia Formation are of the bioclastic limestone type, with no cathodoluminescence color. Compared with dolostones, limestones have higher Sr content, lower Mn content, and heavier oxygen isotopes. With the crystalline size of dolostone becoming coarser, the oxygen isotopes of dolostones tend to become lighter. The meso-coarse crystalline dolostone has the highest Mn content and negative carbon isotope. Both limestones and dolostones have an obvious positive Eu anomaly in the Qixia Formation. However, the REE patterns of fine-medium crystalline dolostones are very different from those of meso-coarse crystalline dolostones. It is credible that there were two periods of hydrothermal fluid charging, with different chemical compositions. The first period of hydrothermal fluids could laterally migrate along the sequence boundary. Fine-medium crystalline dolostones were almost completely distributed below the sequence boundary and were dolomitized during the shallow burial period. As products of the hydrothermal fluid–dolostone reaction, the saddle-shaped dolomites in the meso-coarse crystalline dolostones were the evidence of the second period of hydrothermal fluids. As a result, the dolomitization model was established according to the timing of diagenetic mineral precipitation, which can improve that the geological understanding of the effect of hydrothermal fluid activities on the carbonate rocks in the Qixia Formation.