Marine Carbonate Rocks Research Articles

Dissolved ammonia catalyzes proto-dolomite precipitation at Earth surface temperature

Marine carbonate rocks are the major reservoir of carbon through CaCO3 and CaMg(CO3)2 precipitation extracting CO2/HCO3- from the atmosphere/oceans; hence dolomite is one of the major sinks in the global carbon cycle. Most ancient dolomite has been considered as mainly precipitated under Earth surface conditions. Previous studies have demonstrated that microbes can mediate dolomite formation. However, the “microbial dolomite” model is not sufficient to explain the dolomite that shows no or little evidence of a microbial origin. Although attempted for decades, the synthesis of inorganically-produced dolomite at normal temperatures (<50 °C) has been relatively unsuccessful. Hence, this "dolomite enigma" has been one principal research focus this century. Here we demonstrate that NH3 catalyzes proto-dolomite precipitation inorganically with higher Mg/Ca molar ratios and CO32- activity at normal temperatures of 30 °C and 40 °C. NH3 dissolution increases the alkalinity of the solution and transformes into NH4+ ions, which prefer to bond with H2O on Mg[(H2O)6]2+ rather than free H2O, thus releasing Mg2+ to facilitate proto-dolomite nucleation. Furthermore, the low dielectric constant and low dipole moment allow NH4+ absorbed on crystal surfaces to lower the energy barrier of Mg[(H2O)6]2+ dehydration, promoting proto-dolomite nucleis growth. The system for proto-dolomite precipitation in our experiments closely simulates the natural aqueous environment. This study brings new insights to understanding the mechanisms of dolomite precipitation in natural waters.

Hydrocarbon accumulation and orderly distribution of whole petroleum system in marine carbonate rocks of Sichuan Basin, SW China

Based on the situation and progress of marine oil/gas exploration in the Sichuan Basin, SW China, the whole petroleum system is divided for marine carbonate rocks of the basin according to the combinations of hydrocarbon accumulation elements, especially the source rock. The hydrocarbon accumulation characteristics of each whole petroleum system are analyzed, the patterns of integrated conventional and unconventional hydrocarbon accumulation are summarized, and the favorable exploration targets are proposed. Under the control of multiple extensional-convergent tectonic cycles, the marine carbonate rocks of the Sichuan Basin contain three sets of regional source rocks and three sets of regional cap rocks, and can be divided into the Cambrian, Silurian and Permian whole petroleum systems. These whole petroleum systems present mainly independent hydrocarbon accumulation, containing natural gas of affinity individually. Locally, large fault zones run through multiple whole petroleum systems, forming a fault-controlled complex whole petroleum system. The hydrocarbon accumulation sequence of continental shelf facies shale gas accumulation, marginal platform facies-controlled gas reservoirs, and intra-platform fault- and facies-controlled gas reservoirs is common in the whole petroleum system, with a stereoscopic accumulation and orderly distribution pattern. High-quality source rock is fundamental to the formation of large gas fields, and natural gas in a whole petroleum system is generally enriched near and within the source rocks. The development and maintenance of large-scale reservoirs are essential for natural gas enrichment, multiple sources, oil and gas transformation, and dynamic adjustment are the characteristics of marine petroleum accumulation, and good preservation conditions are critical to natural gas accumulation. Large-scale marginal-platform reef-bank facies zones, deep shale gas, and large-scale lithological complexes related to source-connected faults are future marine hydrocarbon exploration targets in the Sichuan Basin.

Environmental microbiology explains the largest positive carbon isotope excursion in Earth history, the Lomagundi-Jatuli Event.

The geological record of stable carbon isotopes preserved in marine carbonate rocks spans nearly 4 billion years. Numerous perturbations mark this record, but one stands out for its magnitude, the Lomagundi-Jatuli Event, which spanned the transition of the Earth's surface from an anoxic to an oxic state. An Applied and Environmental Microbiology article by D. Y. Sumner (90:e00093-24, 2024, https://doi.org/10.1128/aem.00093-24) provides, for the first time, a biological explanation for its initiation, cessation, environmental specific restriction, and geological singularity.

Discussion on Deep Geothermal Characteristics and Exploration Prospects in the Northern Jiangsu Basin

The Northern Jiangsu Basin (NJB), located at the northeast edge of the Yangtze block, is not only rich in oil and gas resources but also contains abundant geothermal resources. Nevertheless, the distribution of geothermal resources at medium depth in the NJB is still unclear due to its complex geological structure and tectonic–thermal evolution process, which restricts its exploitation and utilization. The characteristics of the geothermal field and distribution of geothermal reservoirs within the NJB are preliminarily analyzed based on available temperature measurements and geothermal exploration data. The prospective areas for the exploration of deep geothermal resources are discussed. The analysis results show that (1) Mesozoic–Paleozoic marine carbonate rocks are appropriate for use as principal geothermal reservoirs for the deep geothermal exploration and development within the NJB; (2) the geothermal field is evidently affected by the base fluctuation, and the high-temperature area is mainly concentrated at the junction of the Jianhu uplift and Dongtai depression; (3) the southeast margin of Jinhu sag, Lianbei sag, the east and west slope zone of Gaoyou sag, the low subuplifts within the depression such as Lingtangqiao–Liubao–Zheduo subuplifts, Xiaohai–Yuhua subuplifts and the west of Wubao low subuplift, have good prospects for deep geothermal exploration.

Early‐Precambrian crystalline basement beneath the Upper Yangtze Block from regional aeromagnetic anomalies: Depth from extreme points approach

AbstractAlthough the early‐Precambrian crystalline basement is now only sporadically exposed in the northern and south‐western parts of the Yangtze Block, it is supposed to have a widespread distribution beneath its Neoproterozoic and Phanerozoic covers. Here we present results of regional aeromagnetic data processing in consideration of remanent magnetization to investigate the spatial distribution of the early‐Precambrian basement buried deep under the Upper Yangtze Block and surrounding areas. The direct analytic signal amplitude of the aeromagnetic anomalies, which is less affected by the magnetization direction, reveals a broader basement below the Sichuan Basin, extending far north to the Micang Mountain. A comparison between the direct analytic signal amplitude with the reduction to the pole aeromagnetic anomalies indicates that possible remanent magnetization exists beneath the Micang Mountain near the boundary between the Qinling Orogen and Sichuan Basin. The automatic depth from extreme points transform is then performed on the direct analytic signal amplitude to estimate the depth to the early‐Precambrian crystalline basement. A synthetic model of a magnetic interface with remanent magnetization and random noise shows that the depth from extreme points method is able to resolve variable basement depths. Application of the depth from extreme points method to the direct analytic signal amplitude of the Upper Yangtze Block presents meaningful results about the early‐Precambrian crystalline basement undulations. It is shallow and uplifted beneath the Sichuan Basin, extending north to the Qinling‐Dabie Orogen, probably corresponding to the ancient Chuanzhong palaeo‐uplift. Although it gradually deepens to the east, the deepest basement is buried under the Jiangnan Orogen, which is likely associated with the collision‐induced crustal thickening between the Yangtze and Cathaysia blocks during the assembly of the Columbia supercontinent. Large gas fields around the Sichuan Basin are found at the slopes or depressions between basement uplifts, indicating that the deep marine carbonate rocks in the south and east of Sichuan Basin, particularly those located at the slopes or depressions between ancient basement uplifts, are favourable targets for further petroleum exploration.

Geochemistry and sources of boron and strontium of geothermal waters from the Pearl River Delta region, South China: Implications for water-rock interactions

Pearl River Delta (PRD) region of South China is abundant of geothermal water resources. However, the development and utilization level of geothermal resources in this region is relatively low, due in part to the lack of fundamental geochemical research. To access water-rock interaction processes of the PRD geothermal system, we analyzed the geochemistry of geothermal waters by combining trace elements (B, Sr, and Br) and isotopes (δ11B and 87Sr/86Sr) with conventional tracers (major elements) that had been reported. The Cl/Br ratios (from 31 to 639) confirm the multi-source salinity of marine origin, precipitation, and a minor dissolution of halite. Major ions chemistry highlights the influence of ions exchange, the dissolution of carbonate, silicate, and sulphate minerals as well as the contribution of seawater. The Cl/B ratios (61 to 22,583) suggest interactions with carbonate rock and felsic rock, input of seawater, and groundwater mixing. Boron isotopic compositions (δ11B) range between −9.22 and +39.78 ‰. Sr contents and 87Sr/86Sr ratios are more homogeneous, falling between 0.06 and 32.26 mg/L and between 0.71239 and 0.72121, respectively. The B and Sr isotopic signatures show that three processes contribute to geochemistry of geothermal waters: 1) water/rock interaction involving marine carbonate rock, evaporite, and granitoid or/and gneiss, 2) seawater intrusion, and 3) shallow groundwater mixing. Major-trace element chemistry and these isotope systematics on their own indicates essential information on the aspects of fluid origin or water-rock interaction processes, and however provide a more comprehensive understanding of the geothermal system in the PRD region, South China.

Signs of crustal extension in Lower Jurassic carbonates from central Slovenia

The Lower Jurassic Podbukovje Formation represents a succession of shallow marine carbonate rocks deposited on the former Southern Tethyan Megaplatform and one of its successors, the Adriatic Carbonate Platform. Several outcrops of the Podbukovje Formation from central Slovenia (southern margin of the Ljubljana Moor) are presented, bearing possible evidence of Early Jurassic extensional tectonics. Peritidal facies of the lowermost, Hettangian – Sinemurian, part of the Podbukovje Formation locally interfingers with bodies of matrix supported pervasively dolomitized polymictic breccia, several metres to tens of metres thick and is locally cut by neptunian dykes some few decimetres to metres wide. The same or slightly younger part of the formation locally contains grabens/half-grabens metres to tens of metres deep and filled with poorly sorted pervasively dolomitized matrix supported polymictic breccia. Small miliolid foraminifera are present within the clasts and in the matrix. Finally, partly dolomitized blocky breccia tens of metres thick locally overlies the Pliensbachian – lowermost Toarcian limestone with lithiotid bivalves. Besides completely and partly dolomitized clasts, the breccia contains a variety of limestone clasts and preserves common radial ooids and some bioclasts within the partially dolomitized matrix. The Hettangian-Sinemurian breccias and dykes are presumably related to the early, diffused rifting stage of the Penninic (Alpine Tethys) Ocean, whereas Toarcian breccias relate to the main, focused rifting stage. Together with evolving biota and changing paleo-oceanographic conditions, the extensional tectonics may have been an important factor behind the facies changes observed within the Podbukovje Formation.

Mineralogy and Sr Isotope Characteristics of Dahua Stratified Tremolite Nephrite and Host Rocks, Guangxi Province, China

The tremolite nephrite deposit in Dahua county, Hechi City, Guangxi province, China is a new genetic type of nephrite deposit. It is hosed by Mg-poor limestone (~1.30 wt.% MgO) and intruded by diabase (~45 wt.% SiO2). The Mg and Si contents of these rocks are lower than those of the tremolite (58.18 wt.% SiO2, 13.18 wt.% CaO, 24.16 wt.% MgO), indicating an obviously insufficient source for the metallogenic material that generated the deposit. In particular, some tremolite nephrite ore bodies have no clear contact metamorphism between the host and intrusive rocks, which have the characteristics of stratified mineralization (stratified tremolite nephrite). The origin and mineralization of stratified tremolite nephrite remain poorly constrained. To address this shortcoming, the mineralogy, geochemistry and Sr isotopic of host rock, altered marble, stratified tremolite nephrite and intrusive rock in the Dahua stratified tremolite nephrite deposit were studied. The results show: stratified tremolite nephrite mainly consists of aggregates of microcrystalline-cryptocrystalline tremolites with content exceeding 95%. The in situ rare earth elements (REEs) distribution pattern of hydrothermal calcite in the contact position between stratified tremolite nephrite and marble is similar to that of marine carbonate rock, showing obvious enrichment of HREE, which is different from calcite in limestone and marble. 87Sr/86Sr of stratified tremolite nephrite is relatively uniform, with an average value of 0.7070, within the range of Permian seawater. The mean value of Y/Ho in the hydrothermal calcite is 51.24, indicating that the marine fluid has not been impregnated by terrigenous materials. In summary, the hydrothermal fluid rich in Ca and Si is formed after marine carbonate rocks are altered by marine fluids. Hydrothermal fluids alter diabase rocks formed by altered minerals like titanite, chamosite, zoisite, etc. This process leads to the formation of metallogenic hydrothermal fluids abundant in Si, Ca, Fe and Mg. The metallogenic hydrothermal fluids migrate in faults and fractures of marble and crystallize to form tremolite nephrite under suitable ore-forming conditions.

Microfacies analysis of marine shale: A case study of the shales of the Wufeng–Longmaxi formation in the western Chongqing, Sichuan Basin, China

Abstract It is a great challenge for sedimentologists to perform the facies analysis of shales due to the lack of macro-structures and high heterogeneity in shale, and thus, conventional research methods are poorly applied in the field of shale sedimentology. To establish a typical depositional model for marine shales, a new method adopted from marine carbonate rocks was employed to perform the microfacies analysis of marine shales of the Wufeng and Longmaxi Formation in Sichuan Basin. This method emphasizes the fact that many skeletal constituents in marine shales have specific bathymetric distributions, allowing for a better understanding of the depositional process of shales. With this approach, a total of nine microfacies were identified and two depositional models for marine shales were proposed. The depositional models show that sea levels were high during the Middle to Late Wufeng period, which was followed by a rapid regression that led to a significant sea-level drop by over 50 m at the topmost of the Wufeng Formation, and consequently, widespread fine-grained sandy shales with large amounts of benthic bioclasts were deposited in the study region. The eustatic sea-level changes recovered by using the method of microfacies analysis are in line with the global trend, indicating that the method has promising applications in the field of marine shale sedimentology.

Zinc isotopes revealed the role of ore-hosting carbonate rocks in the formation of MVT deposits: A case study of the Huize Pb[sbnd]Zn deposit, SW China

Although carbonate rocks are important for carbonate-hosted PbZn deposits, there is no effective constraint on the role of carbonate rocks during the PbZn mineralization process. This paper uses the giant Huize PbZn deposit (>7 Mt. metals @ 25–35 % Pb + Zn) as a case study and employs zinc isotopes to gain new insights into such issue. The Huize deposit, located in the Sichuan-Yunnan-Guizhou (SYG) PbZn metallogenic province, is the largest carbonate-hosted deposit in the western Yangtze Block, SW China. Zinc isotope compositions of sphalerite, altered carbonate rocks and fresh carbonate rocks from the Huize deposit are of +0.39 ‰ to +0.68 ‰ (mean + 0.52 ‰, n = 20), −0.25 ‰ to +0.34 ‰ (mean + 0.07 ‰, n = 11) and + 0.35 ‰ to +0.71 ‰ (mean + 0.51 ‰, n = 6), respectively. The initial δ66Zn value of hydrothermal fluids calculated by the Rayleigh fractionation simulation is +0.4 ‰. Such zinc isotope signatures are significantly different from those of potential source rocks, such as the basement Proterozoic metamorphic rocks (δ66Zn = +0.10 ‰ to +0.34 ‰, mean + 0.22 ‰), the host Upper Ediacaran to Middle Permian sedimentary rocks (δ66Zn = −0.24 ‰ to +0.35 ‰, mean + 0.1 ‰) and the cover Upper Permian Emeishan basalts (δ66Zn = +0.30 ‰ to +0.44 ‰, mean + 0.35 ‰). Hence, it is likely that the carbonate rocks acted zinc source and provided heavy zinc isotopes for the PbZn mineralization. The ore-forming fluids extracted the heavy zinc isotopes of the ore-hosting carbonate rocks, resulting in the zinc isotopes of the altered carbonate rocks being far lighter than those of the marine carbonate rocks (δ66Zn = +0.24 ‰ to +1.32 ‰). The sulfur isotope compositions (+12.9 ‰ to +15.6 ‰, mean + 14.3 ‰, n = 20) of sphalerite from the Huize deposit also suggested that the source of reduced sulfur is the marine sulfate minerals within the ore-hosting carbonate rocks. Therefore, carbonate rocks play key roles in the formation of carbonate-hosted PbZn deposits: 1) provide ore-forming elements and ore-hosting space through dissolution; 2) cause isotope variation (e.g., zinc isotopes) by isotope exchange; 3) act as a carbonate buffer to adjust physicochemical conditions (e.g., pH) via cyclical carbonate dissolution, CO2 degassing and recrystallization. On the other hand, the zinc isotope mapping shows that the western part of the SYG PbZn metallogenic province has lighter zinc isotopes than those of the eastern part. This suggests that the migration pathway of hydrothermal fluids is likely west to east, which provides a new clue for regional PbZn ore prospecting. The outcomes have important benefits for understanding the genesis of PbZn deposits, the role of carbonate rocks and the application of zinc isotopes.

Genesis of the Large-Scale Kamado Magnesite Deposit on the Tibetan Plateau

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical “Kraubath-type” magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata.

Chemical and boron isotope composition of tourmaline from Koktokay pegmatite, Altay Orogenic Belt, Northwest China: Implications for metallogenic mechanism and prospecting indicator for rare-metal pegmatites

The metallogenesis of Li-rich pegmatites remains contentious and an innovative approach for exploring such deposits is needed. The well-known Koktokay pegmatites in the Altay Orogenic Belt exhibit various mineralization degrees and patterns. In this study, we conducted systematic elemental and boron isotope analyses on five tourmaline samples to decipher the metallogenic mechanism and prospecting indicators of rare metal pegmatites. KKTH2 and KKTH3 were collected directly from the magmatic stage and magmatic–hydrothermal stage of the No. 3 pegmatite, KKTH1 was collected from the Aikoz mine pit, and KKTH4 and KKTH5 were collected from a small operational mine near the No. 3 pegmatite. Unlike the No. 3 pegmatite, this mine produces gem crystals but lacks Li mineralization. All tourmalines belong to the alkali group, whereby KKTH1 and KKTH2 are classified as fluor-elbaite, whereas KKTH3 and KKTH4 are schorl. The tourmaline in KKTH5 shows high Al content (up to 45.30 wt%), classifying as mostly rossmanite–elbaite series, with minor amounts of olenite and liddicoatite.A Rayleigh fractionation model yields a shift in △11B value of 4.8‰ between Zone IV and VI involved more than just magma, as B-isotope fractionation between tourmaline and melt would be <1‰ when temperature drop from 650 °C to 500 °C. It further indicates that different parts of No. 3 pegmatite may have originated from diverse sources.After comparing the δ11B values of tourmaline in this study with other fertile and barren pegmatite globally, we suggest that δ11B values of tourmaline higher than −10‰ is a potentially useful indicator for Li-rich pegmatites because of the likely relationship with marine evaporite or carbonate rocks in the source. Furthermore, we propose that tourmaline with a V/Sc value lower than 1 and a Li/Sr value higher than 100 can be used as indicators for Li exploration. The extent of Li migration from magma into the hydrothermal stage and mineralized in the residual melt correlates with the salinity of magma, which may reasonably explain the less intense wallrock alteration near the No.3 pegmatite. This finding provides new insight into the use of trace elements and boron isotope composition as a guide for exploring Li-rich pegmatites.

Depositional condition of Paleoproterozoic Francevillian carbonate rocks revisited from rare earth element contents

The Paleoproterozoic Francevillian Group in the eastern Gabonese Republic has been recently attracting increasing attention because it includes distinctive macroscopic structures interpreted as eukaryotic fossils. Therefore, its depositional setting and associated redox conditions need to be clearly understood. To clarify these, petrological studies and rare earth element (REE) abundances of carbonate rocks of the Francevillian Group in the Lastoursville basin were determined with ICP-MS coupled with a laser ablation sampling technique. Detailed microscopic observations indicate that dolostones in this basin underwent the complicated diagenetic history, including sparitization, decomposition of organic matter, reduction of ferromanganese oxides, silicification, and later calcification. The dolostones of lower Francevillian B (FB) Formation show heterogeneous textures formed by alterations during diagenesis. By the alterations, changes of REE abundances and degrees of positive Y anomalies appear to have occurred. This indicates that REE signatures in carbonate rocks can vary during diagenesis. Characteristically high Mn contents in the dolostones of upper FB Formation were derived from the reduction of ferromanganese oxides in sediments. The signatures of REE abundance in dolostones can be explained by the mixing of seawater and ferromanganese oxide components. In that sense, the dolostones inherited the trace element characteristics of precursor ferromanganese oxides even after the diagenesis. Y/Ho values of the most primary parts of each sample exceed 33, the newly determined threshold value for marine carbonate rocks based on our compilation. The magnitude of the La anomaly values also falls within the range of that of Holocene reefal microbialites. These indicate that a precursor of the dolostone of the Francevillian Group was deposited in a marine environment. In addition, presence of Ce anomalies in the upper FB Formation suggests that the Paleoproterozoic ocean was oxidized to such an extent as for Ce to be preferentially absorbed by ferromanganese oxides.

Geology, fluid inclusions and C−O−S−Pb isotopic compositions of the Chahmileh Pb-Zn deposit, Central Iran: Implications for ore genesis

AbstractThe Chahmileh Pb–Zn deposit, located northwest of the Central Iran Zone, is a sediment-hosted Pb–Zn deposit in the ‘Yazd-Anarak Metallogenic Belt’. It is hosted in Middle Triassic carbonate rocks and is mainly controlled by NW-trending faults. The main ore minerals are galena and sphalerite with minor chalcopyrite, pyrite, and quartz, dolomite, along with minor calcite and baryte as gangue minerals. Cerussite, hemimorphite, wulfenite, mimetite, smithsonite, malachite and iron oxy-hydroxides are the main non-sulphide ore minerals. The main styles of mineralization are vein-veinlet, breccia, disseminated and replacement in association with silicification and dolomitization. Microthermometry of fluid inclusions in dolomite and quartz indicates that the ore precipitated from a warm to hot basin-derived saline fluid. Dolomite samples have δ13CVPDB and δ18OVSMOW values of −0.99 to +1.99‰ and +20.74 to +25.48‰, respectively, and are plotted in the marine carbonate rocks field. These isotopic values suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rock. The δ34S values range from +6.3 to +8.2‰ for galena, +5.9 to +6.2‰ for sphalerite, +1.4 to +3.4‰ for chalcopyrite and +15.0 to +17.4‰ for bayite are compatible with a predominant thermochemical sulphate reduction process, and with sulphur sourced from Triassic seawater. Galena samples have a homogeneous Pb isotopic composition that is indicative of a continental crustal reservoir as the main source of lead and probably for the other ore metals. Based on geology, mineralogy, texture and fluid characteristics, the Chahmileh deposit is classified as a carbonate-hosted Mississippi Valley-type deposit.

FIRST JURASSIC EVIDENCE OF A POSSIBLE SPINOSAURID PEDAL UNGUAL, FROM THE JAISALMER BASIN, INDIA

We describe an isolated, almost complete pedal ungual phalanx from the Middle Jurassic marine carbonate rocks of the Jaisalmer Basin, Rajasthan, north-western India. The ungual bone is triangular shaped, pointed, elongated, asymmetrical, dorsoventrally compressed, ventrally flat, bearing a shallow semi-circular excavation, and almost straight in lateral view. The morphological features, as well as its proportions closely resemble the pedal ungual phalanges of spinosaurid theropods, presently known dominantly from the Cretaceous. The affinity to spinosaurids is supported by bivariate and multivariate analyses. The ungual phalanx is tentatively identified as a basally branching Jurassic spinosaurid under Megalosauroidea. Considering the stratigraphical and geographical provenances, this contribution may represent the oldest record of a spinosaurid.

An operational classification system for the South Atlantic pre-salt rocks

Abstract The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic pre-salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the pre-salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual yet extremely important pre-salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs, and mud matrix. The resedimented rocks are classified according to the original volume of &amp;gt; 2 mm particles and the proportion between sand and mud, using calcirudite, calcarenite, and calcilutite as unbiased names. Such an approach can also be applied to the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective, and intuitive proposed classification system shall contribute to the understanding, exploration, and production of the extraordinary South Atlantic pre-salt petroleum province.

Mineralogy and geochemistry of the Middle Jurassic coal from the Hexi Mine, Shenfu Mining Area, Ordos Basin: With an emphasis on genetic indications of siderite

Siderite is a common carbonate mineral in coal and usually contains geochemical indications (elemental associations and isotopic compositions) for sedimentary environments and regional geological history, but the origin and enrichment mechanism of geochemically-pure siderite in coal is not fully understood. A better understanding of siderite origin in coal can help to decipher the depositional environment of peat and related geological processes of rank advances and epigenesis. This paper analyzed the petrological, mineralogical, and geochemical characteristics, particularly the content, occurrence, and origins of siderite in the Middle Jurassic coal from the Hexi mine of the Shenfu mining area, Ordos Basin, using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometry, and isotope ratio mass spectrometry. The results indicate that the No. 4 Coal is rich in inertinite and is defined as an ultra-low to low sulfur, high volatile bituminous coal. Siderite has contents varying from 5.8% to 54.4% (low temperature ash basis) and occurs in radial or concentric circle nodules in the coal and is mainly distributed within the clay matrix. Syngenetic siderite in the Hexi coals is relatively pure in chemical compositions, and was formed in a brackish water environment affected by salt lakes under arid conditions. The high content of inertinite, high Sr/Cu value, and positive Ce anomaly at the uppermost of the coal seam indicate that the escalating intensification of oxidation and evaporation. The Al2O3/TiO2 ratio and the diagrams of Al2O3/TiO2 vs. Zr/ TiO2 and Al2O3/TiO2 vs. Nb/Yb indicate that the sedimentary source rock in the study area is primarily intermediate-felsic. The δ13CPDB-δ18OSMOW diagram shows C and O for the siderite formation in this study were derived from marine carbonate rock alteration of the underlying Ordovician marine carbonate rock basement. The distribution patterns of rare earth elements and Y (Middle-type enrichment and Gd anomalies) in the coal further verify this hypothesis. Furthermore, siderite in the coal exhibits an enrichment in 13C suggest that bacterial fermentation of organic matter as another possible source of CO2. The iron of the siderite in the study coal was probably derived from the leaching of the magnetite-bearing granite.

Characterization of the Govanda Formation limestones: chemostratigraphy and tectonic setting of the last marine carbonate rocks in the Arabia–Eurasia suture zone, NW Zagros fold-thrust belt

Characterization of the Govanda Formation limestones: chemostratigraphy and tectonic setting of the last marine carbonate rocks in the Arabia–Eurasia suture zone, NW Zagros fold-thrust belt

Distribution and isotopic compositions of phenanthrene and methylphenanthrenes in crude oils: Implications for source input delineation and grouping oil families

This study aimed to identify the origins of crude oils by analyzing the δ13C values of phenanthrene (Phen) and methylphenanthrenes (9-MP and 1-MP) in some selected oils from the Tarim Basin (NW China), Termit Basin (Niger), Bongor Basin (Chad), and Fushan Depression (South China Sea). The study showed that oils with low 9-MP/1-MP ratios (<2.0), phenanthrene (Phen) δ13C values below −30.0‰, and Δ13C(9MP-Phen) and Δ13C(1MP-Phen) values above zero are marine-originated oils, while oils with high 9-MP/1-MP ratios (>2.0), phenanthrene (Phen) δ13C values above −20.0‰, and Δ13C(9MP-Phen) and Δ13C(1MP-Phen) values below zero are terrestrial-originated oils. Oil samples with high 9-MP/1-MP ratios (>2.0), phenanthrene (Phen) δ13C values ranging from −30.0‰ to −20.0‰, and Δ13C(9MP−Phen) and Δ13C(1MP-Phen) plots below zero are mixed-originated oils with higher terrestrial contribution. In contrast, oils with low 9-MP/1-MP ratios, phenanthrene (Phen) δ13C values ranging from −30.0‰ to −20.0‰, and plotting above and below Δ13C(9-MP−Phen) and Δ13C(1-MP−Phen) vice versa, are mixed-originated oils with higher contribution from marine sources.In combination with the geological settings, the results indicated that the oils from the Tarim Basin have three distinct origins. The majority of the oils are marine-sourced oils derived from the Lower-Paleozoic marine carbonate rocks. Oil samples *T2, *T3, and *T4 are mixed-originated oils with higher terrestrial contribution generated from the Mesozoic lacustrine shales/mudstones in the Tarim Basin. The oil samples *T1 (XH1) and *T9 (S94) are from a mixed origin with higher marine material, are derived from Upper-Paleozoic marine shale source rocks in the Tarim Basin. In Termit Basin, most of the selected oils are mixed oils with higher marine organism contribution from YSQ1 and YSQ2 source rocks from the Mesozoic Yogou Shale Formations, except for *T27 oil sample (Douwani-1), which is a mixed oil with higher terrestrial contribution from YSQ3 source rocks of the Mesozoic Yogou Shale Formations. As for the selected oils from Bongor Basin and Fushan Depression, they are mixed oils with higher contributions from aquatic organisms generated from the Mesozoic and Cenozoic lacustrine shale rocks, respectively.This approach allowed to identify for the first time the mixed origin with higher contribution of marine organic matter of the oil samples *T1 (XH1) and *T9 (S94) from the Tarim Basin, and the mixed origin with higher contribution of terrestrial organic matter of the oil sample *T27 (Douwani-1) from the Termit Basin.

EOR technologies for fractured-vuggy carbonate condensate gas reservoirs

There are abundant marine carbonate rock resources in China, which are dominated by fractured-vuggy carbonate oil and gas reservoirs accounting for over two thirds of proved reserves. The fractured-vuggy carbonate condensate gas reservoir in the Tarim Basin is at a burial depth of 4500–7000 m with the characteristics of extremely strong reservoir heterogeneity, complicated and diverse reservoir seepage media, complex fluid properties and quite difficult static characterization, which brings many challenges to reserve evaluation, development plan design and performance analysis. In order to improve the recovery factor of this type of oil and gas reservoir, this paper takes the dynamic characterization to supplement the static characterization and combines each other to improve the accuracy. In addition, based on many years' of dynamic and static research and development practice, the key enhanced gas recovery (EGR) technologies for fractured-vuggy carbonate condensate gas reservoirs are innovatively developed, such as the high-accuracy dynamic characterization technology with dynamic and static iteration for fractured-vuggy body in the strongly attenuated area of desert by taking seismic inversion information as the basis and 3D numerical well test as the core, the multi-target 3D development technology in one well to improve reserve production of fractured-vuggy condensate gas reservoirs, and the gas-lift depressurization EGR technology for fractured-vuggy condensate gas reservoirs. In conclusion, this technological system better solves the key difficulties in the fine characterization of fractured-vuggy body, reserve production improvement and abandonment pressure reduction of well. What's more, it realizes the fine reservoir characterization of fractured zones and feather-shaped fractured zones and the accurate prediction of key development indexes, and increases the deployment success rate of effective wells and efficient wells by 26% compared with that in the initial stage of large-scale productivity construction. To sum up, these technologies provide powerful technical support and reference experience for the scientific and efficient development of fractured-vuggy carbonate condensate gas reservoirs.