Dolomite Unit Research Articles

Reservoir heterogeneities due to diagenesis in Jurassic Samana Suk Formation Kahi section Nizampur Basin North West Himalayas Pakistan

The porosity and permeability determine the reservoir quality of sedimentary rocks, which is fundamentally influenced by both depositional and diagenetic processes. The Jurassic carbonates are targeted in many regions for oil and gas exploration. The current study is carried out to elaborate on the diagenetic alterations and their effect on reservoir properties. A thick outcrop of the Jurassic Samana Suk Formation is studied in the Kahi section of Nizampur Basin Northwest Himalayas, Pakistan, to study and relate the trend of lithological variations and depositional settings. Field investigation revealed that the Samana Suk Formation is extensively distributed in the area and primarily made up of interbedded limestone and dolomite units. The original unaltered limestone has a thick-bedded and oolitic to bioclastic nature. Different types of dolomites have been recognized based on colour contrast and sedimentary features. Moreover, saddle dolomite cement, calcite cementation, and mechanical and chemical compaction have also been observed. The petrographic studies show different types of diagenetic alterations that affected the Samana Suk Formation, including micritization, bioturbation, mechanical and chemical compaction in the form of fractures and stylolites, various calcite cementation, which includes the various cement types that range from isopachous, blocky, granular equant, fibrous, and dog tooth cementation, along with dissolution that occurred in different diagenetic realms. Pyritization was rarely observed. Moreover, different phases of dolomites were identified, ranging in size and shape, i.e., finely crystalline to coarse crystalline and planar euhedral to non-planar anhedral. The stable oxygen isotope values of these dolomites show depletion from original marine signatures and suggest burial-related fault-controlled dolomitization events. Overall, diagenetic processes like dissolution, fracturing and dolomitization increased the reservoir potential. On the contrary, the overburden and cement precipitation result in a decrease in the reservoir properties.

Integrated stable S isotope, microbial and hydrochemical analysis of hydrogen sulphide origin in groundwater from the Legnica-Głogów Copper District, Poland

Hydrogen sulphide (H2S) was first reported in 2010 in an underground mining area of the Legnica-Głogów Copper District (LGCD). However, during the deposit exploration and exploitation stage in 1970, H2S was not detected. Previous studies have not indicated the origin of H2S in LGCD mine waters. Indirect evidence has suggested that, most likely, H2S migrated with the groundwater from the Main Dolomite aquifer to the mine galleries through cracks in the separating anhydrite and halite layers resulting from long-term mining operations. Interdisciplinary studies have been conducted to confirm this hypothesis. Investigations of the δ34S in H2S of 12 groundwater samples taken from leakages from cracks in galleries and drilled boreholes showed high isotopic similarity to H2S in natural gas and S-organic compounds of oil from petroleum reservoirs in the Main Dolomite unit, where this gas was generated in high volumes mainly as a result of thermochemical sulphate reduction (TSR). Microbial investigations of mine water indicated the presence of Desulfitobacterales, Desulfobacterales, Desulfotomaculales, Desulfovibrionales, and Desulfuromonadia as possible indicators of contact between the analysed water and rocks inhabited by sulphur-reducing bacteria and Halothiobacillales, as an indicator of contact between water and salt. Hydrochemical analysis revealed that water flowing out of mine galleries and boreholes has a higher salinity than groundwater that is naturally associated with the horizon where mines operate; this phenomenon is associated with H2S occurrence. This investigation confirmed the hypothesis that the H2S in the LGCD mines originates mainly from the Main Dolomite aquifer. Furthermore, this study confirms that a multifaceted approach is necessary to answer complex geochemical questions in mining.

Reservoir characteristics of the middle Eocene Avanah Formation in Erbil governorate, northern Iraq: Integration of outcrop and subsurface data

The current work investigate the petrophysical characteristics of the Middle Eocene Avanah Formation in the Erbil Governorate using IP software to analyze the well data, integrated with the 
 petrographic investigation of the formation in the nearest outcrop in the Gomaspan section. Well logging data revealed that the main lithology of the formation is limestone and dolostone while the lithology in the Gomaspan section is composed of limestone, dolomitic limestone, marly dolomitic limestone, and thin beds of shale. The lower dolomitic unit in the subsurface section of the formation is believed to be the most suitable reservoir unit due to good petrophysical characteristics including, low water saturation, high porous medium, and the presence of movable hydrocarbon. To measure porosities, a variety of well logging techniques were used in this study. The investigated formation was divided into Avanah dense (limestone unit) and Avanah porous (dolostone unit) based on their porous components. The petrographic study shows that most pore types of the formation are secondary and represented mainly by vuggy, moldic, intercrystalline, and fracture types. Most of the fractures in the upper limestone units are filled by calcite cement. The study claimed that the lower part of the formation (dolostone unit) in both sections is considered a good reservoir.

Dolomitization of the Lower Part of the Sargelu Formation, Northeastern Iraq

The prominent characteristic of the lower part of the Sargelu Formation of the Middle Jurassic is the occurrence of destructive dolomitization. This dolomitized unit, cropped out in the high folded and Imbricated zones in Northern Iraq, is situated in the Gotnia Basin and has been distinctly identified and are documented in previous studies. Dolomite crystals are euhedral in shape; hence, they are classified as saccharoidal dolomite, resulting in the elimination of all biocontent and the destruction of the original rock fabrics. Despite the abundance of intercrystalline pores in this unit and their role in the quality of reservoirs in the oil fields, the origin or mechanism of dolomitization is not well understood yet. The aim of this investigation is to find out a suitable model for the dolomitization and to propose possible mechanisms that led to the dolomitization. At the top of the Lower Jurassic Sehkaniyan Formation, an extensive mass of destructive dolomitization has also occurred. The dolomitic unit of the Sargelu Formation, which extends for hundred kilometers constantly. Stratigraphic correlation reveals that the Sargelu Formation is juxtaposed with different successions, such as the Alan and Sehkaniyan formations, that show evidence about peritidal and sabkha environments, where the dolomitization units were associated with solution collapse breccia and microbial stromatolite. This study proposes that the reflux and burial dolomitization models for the lower part of the Sargelu Formation by comparing the paleoenvironmental conditions of the late Lower Jurassic and early Middle Jurassic in the Gotnia basin with a modern analogue model.

Predictive insights for copper recovery: A synergistic approach integrating variability data and machine learning in the geometallurgical study of the Tizert deposit, Morocco

This work involved the application of machine learning (ML) to predict copper recovery through variability data in the context of the geometallurgical study of the Tizert deposit in Morocco. For this purpose, geological and metallurgical variability investigations were conducted on drilling samples from Tizert ore, leading to the consolidation of a robust database for ML algorithms. The dataset comprised 345 observations across seven variables, namely GPS coordinates, geological units, lithological facies, the main copper deportment, and metallurgical responses from flotation tests, as well as the copper grade and the oxidation rate. The geological and mineralogical variability revealed two geological units: The Basal Series and Tamjout Dolomite unit, hosting copper mineralization dominated by chalcocite and malachite. Besides, the metallurgical evaluation demonstrated recovery variability, primarily influenced by oxidized copper. Copper oxides exhibit less efficient flotation than sulfides, with recovery decreasing as the oxidation rate increases. The prediction of copper recovery was performed using the variability data through the XGBoost algorithm. The resulting model showed good performance on the test dataset, with a coefficient of determination R2 of 0.83 and a RMSE of 2.47, improving the accuracy of copper recovery predictions. In addition, the model elucidated the most significant variables impacting Tizert ore behavior, notably the casing, copper-bearing mineral, and oxidation rate. These variables were considered key factors for advancing Tizert ore valuation and geometallurgical model development. Overall, metallurgical modeling based on the ML approach provided an innovative way to integrate variables into mine planning and processing. The use of such a predictive model gave valuable insights for improving efficiency, understanding the metallurgical behavior, optimizing production, and reducing costs along the value chain of the Tizert deposit. Further data processing efforts are underway, to improve the assessment of copper recovery quality for the Tizert ore and the development of a complete geometallurgical model on a plant scale.

3D Geological Modeling for Characterizing the Bahariya Formation and Abu Roash G Dolomite Unit, Horus Field, North Western Desert, Egypt

The Horus field is considered one of the most important hydrocarbon producers within the Alamein basin, Western Desert of Egypt. This area was affected by different tectonic movements throughout geological history. The re-evaluation and characterization of the Horus area, especially its main reservoirs of Lower Bahariya, middle Bahariya, upper Bahariya, and AR/G Dolomite units, because of its low production of oil tend (only 1100 BBD). Despite this, the Alamein basin is large and very rich in hydrocarbon. This study depends on the interpretation of thirty 2D seismic lines in addition to open-hole logs in the form of Resistivity, SP, GR, Density, Neutron, and Sonic logs for the five studied wells in order to delineate the subsurface structure and reservoir petrophysical characteristics of the Horus field. The data is tried to the formation tops and seismic lines by using synthetic seismograms. The main surfaces were mapped to proceed with the further static modeling. The work flow of the 3D geological modeling in the Horus area for characterization of the Bahariya Formation and AR/G Dolomite Unit. The processes that were carried out during the construction of the geological structure in the study area respectively, fault modeling, pillar gridding, skeleton grid, horizon modeling, zonation, and layering, and then distributed the facies and petrophysical parameters (Vsh, Sw and Φe) in this model. The main objective of this study is to delineate the seismic phenomena for structural, petrophysical, and facies modelling of the Bahariya Formation (Lower, Middle, and Upper) and AR/G Dolomite unit. The depth structure contour map on top of the Bahariya formation and the AR/G Dolomite unit indicated that the Horus area affected by NW-SE and WNW-ESE faults related to the lower Cretaceous. The study discussed the possibility of new structural traps that benefit in increasing the production of the area. All the results were consistent and confirmed that the Bahariya Formation and AR/G Dolomite are excellent reservoirs. Integrating multidisciplinary methods of seismic interpretation, static modeling, property modeling and well log analysis to design 3D structural and property models, helped predict new exploratory promising areas and structural traps to increase in production and the reserves in the study area, also decreasing the uncertainty of hydrocarbon exploration.

Late Cambrian climate change recorded by a shift from an arid carbonate platform to a storm‐dominated cool‐water platform at the Gondwana margin (Alborz Zone, Iran)

The Deh‐Molla Formation of the Alborz Zone in northern Iran is a key unit to understanding global sea‐level fluctuations, and likely climate changes during the Furongian Epoch. The sedimentological characteristics of the formation indicate that the succession was deposited on a gently sloping epeiric platform on the northern margin of Gondwana. The Paibian Member 1 of the Deh‐Molla Formation was deposited on a carbonate platform with stable environmental conditions, probably during a (semi)arid climate condition, as indicated by microbial–sponge patch reefs, thick‐bedded units formed in broad facies belts, and a thick dolomite unit formed under evaporative conditions. The overlying Member 2, considered to span the Jiangshanian–Age 10, includes thin‐bedded wavy, and rippled strata known as ribbon rocks. This sequence records considerable environmental fluctuations. Thin‐bedded strata with flat‐pebble conglomerates, hummocky cross‐stratification, lenticular, flaser, and wavy bedding, ripple cross‐lamination, and graded bedding signify that deposition of Member 2 took place in a depositional environment repeatedly affected by tidal currents, and storm waves. A comparison of the Deh‐Molla Formation, and particularly its ribbon rocks with coeval strata in Laurentia and North China reveals secular controls on their development. In contrast to the Furongian strata of the Great American Carbonate Bank, deposited under tropical conditions, the ribbon rocks of the Deh‐Molla Formation suggest deposition in a temperate latitude zone with dominant cool‐water conditions. However, the Deh‐Molla platform was seasonally subjected to monsoons that brought warm tropical waters to the region, and led to rapidly changing environments. Ooids, peloids, stromatolites, and other criteria suggestive of a tropical climate are frequent in Laurentian successions, but absent in the Deh‐Molla Member 2. However, both Laurentian and Gondwanan examples share characters that indicate comparable controls on their development. Significant environmental changes during the Furongian are indicated by the SPICE and HERB events, and the latter is taken as showing biotic and physico‐chemical changes in the ocean and atmosphere, possibly concurrent with the formation of ribbon rocks on different continental blocks.

Depositional Setting of Khurmala Formation (Paleocene-Early Eocene) in Nerwa section, Berat Anticline, Kurdistan Region, Northern Iraq

The depositional environment of the Khurmala Formation (Paleocene-Early Eocene) has been investigated in the Nerwa section, the southern limb of the Berat anticline in the High Folded Zone of Iraq. The thickness of the formation in the studied section was about 113m and can be divided into three units; fossiliferous bedded limestone unit (Unit A), massive dolomitic limestone unit (Unit B) and bedded marly dolomitic limestone unit (Unit C). The petrographic study of the 81 thin sections of the Khurmala carbonates revealed that the majority of skeletal grains are shallow marine derivative faunas and non-skeletal grains include peloids, cortoids and intraclasts. Depending on detailed field observations and petrographic study of the carbonate rocks, five main microfacies were recorded, these are; lime mudstone, lime wackestone, lime packstone, grainstone and boundstone in addition to one lithofacies which is an intraformational conglomerate. Based on the integration of the recognized microfacies and the field criteria of the Khurmala Formation, two facies associations were identified which elucidate deposition in a lagoonal environment including reef patches. Accompanying facies indicate that the formation's sedimentary model is a ramp carbonate platform.

Petrophysical analysis of well logs and core samples for reservoir evaluation: a case study of southern Issaran Field, Gulf of Suez province, Egypt

Understanding basic petrophysical properties may enhance the recovery of residual oil saturation and help in reservoir management. Reservoir oil saturation is the fraction of the porosity of a zone occupied by oil. The trapping may increase with the increase of porosity. Oil reservoirs represent a significant fraction of the reservoirs in the world-wide. We try to make a reservoir evaluation using petrophysical analysis of well logs and core samples at Issaran Field, South Gharib Formation, West of the Gulf of Suez, southeast Egypt. This area was heavily influenced by the structural setting of the Gulf of Suez’s central province, which is characterized by major SW dipping faults with a regional stratigraphic dip toward the NE. The high structural blocks are located in the center of the research area, and the low structural region is located in the southwest. South Gharib Formation is primarily composed of carbonates with few sulfates that grade upwards into sulfate-rich beds in the upper part. Laminated dolomitic limestone, with evaporites grades, changes downward into laminated dolomitic limestone and marly limestone, in the lower part. The anhydrite and dolomite units near the bottom have spotty oil stains. The structural setup is continued by the spread and deposition of this formation. We try to make analytical examination, of some reservoirs, accomplished by analyzing both well log data and core rock samples, vertically and laterally. According to the study of the horizontal permeability to vertical permeability ratio (Permeability anisotropy; lambda_{k}), the reservoir is laminated rock (lambda_{k} = 1.1 - 5), with a few data samples nearly isotropic (lambda_{k} = {1 mathord{left/ {vphantom {1 {1.1}}} right. kern-nulldelimiterspace} {1.1}}–1.1) and fractured (lambda_{k} = {1 mathord{left/ {vphantom {1 {2.5}}} right. kern-nulldelimiterspace} {2.5}}–1/1.1). Reservoir data histograms show that the most typical values of the data are 20–30% oil saturation and 7–10% porosity. The net-pay zone varies from four to 71 feet at different depths (897–1414 feet). Due to low shale content, water saturation, high effective porosity values, hydrocarbon saturation, may show higher net-pay thickness. These results may help to enhance the oil recovery. The area in the middle of the research area is suggested for future hydrocarbon development and using more petrophysical analysis.

Microfacies Analysis and Depositional Environment of Khurmala Formation (Paleocene-Early Eocene) in Kalati Section, Akre Area, Kurdistan Region, Iraq

The depositional environment of the Khurmala Formation (Paleocene-Early Eocene) in the Kalati village, Akre area, northern Kurdistan region of Iraq was carried out from its microfacies analysis. The total thickness of the formation in studied section is about 220 m and it is divided into three lithostratigraphic units: Brecciated dolomitic limestone unit, Medium to thick bedded dolomitic limestone unit and Thin to medium bedded limestone unit. Based on the detailed microfacies studies of 33 thin sections, three microfacies and ten sub microfacies were distinguished in the carbonate rocks of Khurmala Formation and they are subdivided depending on their environmental interpretation, into two facies association: back reef /lagoon and intertidal. Field observation and microfacies studies suggest that the Khurmala Formation in Kalati outcrop was deposited in the back reef lagoonal environment in the lower part and intervals of the middle part and changed to an intertidal environment toward the upper part.

Proterozoic carbonate-hosted Morro Agudo sulfide Pb-Zn district, Brazil: Mineralogical and geochemical evidence of fluid mixing during the ore stage

The Proterozoic Vazante-Paracatu mineral belt hosts distinct styles of mineralization in carbonate rocks, including world-class hypogene silicate-zinc deposits in the south and sulfide Zn-Pb deposits in the north. This study focuses on the Morro Agudo District (Morro Agudo deposit and Bento Carmelo, Sucuri, and Morro do Capão occurrences). The genesis of the Morro Agudo sulfide deposit is controversial (i.e., syn-diagenetic or syn-orogenic), and little was known about the other occurrences. Ore is stratigraphically controlled within distinct dolomitic units of the Morro do Calcário and Lapa formations and structurally controlled at the Morro Agudo deposit along a NW normal fault system. Mineralization is characterized by multiple generations of sphalerite and galena, coeval with hydrothermal Fe-rich dolomite, pyrite, and quartz, which overprint micritic and sparitic dolomite and diagenetic pyrite. The mineralized zones comprises Cd and Hg within the high-grade Zn-Pb ore and Ag, As, Cu, Fe, In, Se, Sb, and Tl extending into the hydrothermal halos surrounding the orebodies. The sphalerite compositions encompass variable proportions of Zn (58.75–66.92 wt%), Fe (0.15–5.03 wt%), and Cd (0.18–0.94 wt%) grouped in five populations based on Cd-Fe ratios and principal component analysis. Sphalerite from various generations and locations in the orebodies and occurrences do not show any specific signature, suggesting episodic contribution of multiple fluids with distinct Fe and Cd contents. The whole rock lithogeochemistry revealed similar trace element signatures for the orebodies of Morro Agudo deposit and surrounding occurrences indicating, despite the textural differences, that these orebodies formed from similar mineralizing fluids and processes, except for the Bento Carmelo occurrence. This study shows clear evidence that the mineralization at the Morro Agudo deposit, Sucuri and Morro do Capão occurrences is epigenetic, forming after the diagenesis of the host carbonate succession, and may have involved progressive mixing of multiple fluids with distinct compositions that interacted with sulfur reservoirs in the dolomitic rocks, similar to other MVT districts worldwide. The Morro Agudo district is one of the rare examples of Proterozoic MVT districts, and this study brings new insights to the exploration of this type of mineralization in similar basins.

Biosignatures Associated with Organic Matter in Late Paleoproterozoic Stromatolitic Dolomite and Implications for Martian Carbonates.

The documentation of biosignatures in Precambrian rocks is an important requirement in the search for evidence of life on other ancient planetary surfaces. Three major kinds of biosignatures are crucially important: primary microbial sedimentary textures, diagenetic organomineral assemblages, and stable isotope compositions. This study presents new petrographic, mineralogical, and organic geochemical analyses of biosignatures in dolomitic stromatolites from the Pethei Group (N.W.T., Canada) and the Kasegalik Formation of the Belcher Group (Nunavut, Canada). Both are approximately contemporary late Paleoproterozoic stromatolite-bearing dolomitic units deposited after the Great Oxidation Event. Micro-Raman and optical microscopy are used to identify and characterize possible diagenetic biosignatures, which include close spatial association of diagenetic materials (such as ferric-ferrous oxide and anatase) with disseminated organic matter (OM), dolomitic groundmass textures, and mineralized balls. Many of these petrographic relationships point to the oxidation of OM either biotically or abiotically in association with iron reduction and chemically oscillating reactions. Oxidation of OM in these stromatolites is consistent with the widespread oxidation of biomass during the late Paleoproterozoic Shunga-Francevillian Event. Biosignatures identified in this study are also compared with possible carbonate outcrops on Mars, and thereby contribute a basis for comparison with potential biosignatures in ancient martian terrains. Similarities are drawn between the paleoenvironments of the studied units to the Isidis and Chryse planitia as locations for potential extraterrestrial dolomitic stromatolites.

Evaluation of Reservoir Fluids Mobility, Mauddud Reservoir (Albian), Khabbaz Oil Field, Kirkuk Area, NE Iraq

The Albian Mauddud reservoir of the Khabbaz Oil Field is consisting of 170 m alternating shelf carbonates and pervasive dolomite horizons of coarse to fine crystalline mosaic. Core analysis and log measurements reveal the occurrence of three electrofacies units (A, B, and C) with variable petrophysical properties. Unit A with good reservoir quality shows average porosity of 18.8 % and average permeability of 27.5 md. The other two units (B and C) are less attractive and have an average porosity of 9.6 % and 9.2 % consequently. Pore size ranges between macro to meso types and related mainly to vugs, fractures and intercrystalline porosity, especially in the dolomite units. The reservoir fluids saturation, bulk volume, and mobility are evaluated using resistivity logs measurements and porosity logs (Neutron-Density porosities) in addition to other reservoir laboratory data. Calculations and cross data plotting of the related petrophysical parameters were applied to the three units of the Mauddud reservoir in seven wells of the field. It shows an overall good reservoir fluids mobility. Results indicate that the formation water of Khabbaz Oil Feld is a non-movable type especially for the crestal wells which make most of these wells produce water-free hydrocarbon. Variability within well’s hydrocarbon mobility is noticed and related to units and subunits lithology and porosity variation. Other variations seem to be related mainly to permeability, pores geometry and variability of water saturation in addition to the location of well with respect to oil pool within the field structure.

Geological and Geophysical Study in Udheri Khola Area, Nalgad Hydroelectric Project, Jajarkot District, Lesser Himalaya, Western Nepal

Subsidence in carbonate rock is one of common and challenging action in terms of engineering construction. Geological study and geophysical investigation carried out in the intake area of Nalgad Hydroelectric Project Jajarkot, western Nepal Lesser Himalaya. The main objective was to identify the cause of subsidence in the intake area of Nalgad Hydroelectric Project, Jajarkot. Geological study of the area was carried to understand the lithology, thickness and structure of the area.
 The study area comprises two distinct rock units, namely, Dolomite Unit followed up by the Slate Unit. The Dolomite Unit is composed of light grey to grayish white stromatolitic dolomite which is thrusted over the Slate Unit near to Laikham village and Sepu Khola area. The Slate Unit is made up of grayish black to graphitic slate. A thin prominent calcareous horizon wasconfined between Slate Unit.
 2D-Electric Resistivity Tomography (ERT) measurements were deployed in four different lines to investigate the cause of the subsidence in the carbonate terrain. A concentric very high resistivity patch shown by Tomogram ER-D-01 survey line was identified and interpreted as dry cavity. The result of the 2D- ERT survey was correlated with core log data of geotechnical exploration in the suspicious point to ensure the presence of karst in the Dolomite Unit at right bank of Nalsyagu Khola near dam axis of Nalgad Hydroelectric Project. The 2D – ERT survey together with geotechnical investigation is capable of identifying subsurface karst feature as the cause of surface collapse in the area.

Prediction of Shear-Wave Velocities in Carbonate Reservoirs

The purpose of this paper was to propose a method to predict shear-wave velocities in the process of AVO (amplitude variation with offset) inversion, to interpret seismic converted-wave data, and to match reservoir engineering information with seismic data in regions without shear-wave velocities. During CO2 flooding and sequestration, the shear-wave velocity varies with the pressure that is caused by fluid injection. Predictions of how the shear-wave velocity changes with pressure can be used to interpret 4D seismic monitoring data during different stages of CO2 injection. To verify the calculations, the authors used the characteristic parameters of the Marly dolomite and Vuggy limestone units of the Weyburn Oilfield reservoir in Canada. The Digby’s model and Gassmann’s equation were used to evaluate the dependence of the shear-wave velocity on the pressure in the carbonate rock formation. The authors have developed a new method for calculating the coordination number of the model and supplement the results with a shale correction, enabling the prediction to match the measured shear-wave velocities more closely. Our method is verified by dipole sonic logging data and petrophysical test data from the Weyburn Oilfield. When applied to petrophysical test data, the average error is below 5%. When the method is applied to well log data, the average error in the porous media is 3.014% in the Marly unit and 6.288% in the Vuggy unit.

Early Triassic Pachycladina fauna newly found in the southern Lhasa Terrane of Tibet and its palaeogeographic implications

The Lhasa Terrane of Tibet is critical to understanding Early Triassic crustal convergence of Gondwana and Eurasia. However, its palaeo-position during this period remains obscure. This paper considers the disputed tectonic nature of the Lhasa Terrane during Triassic time. The debate has taken the form of two schools of thought regarding the Lhasa Terrane. One holds that the terrane was a stable carbonate platform during Triassic time. The opposing view maintains that the Lhasa Terrane occupied a compressional orogenic belt. These contrasting explanations stem mainly from the absence of a sedimentary record, especially in the southern part of the Lhasa Terrane of Tibet. We report on results of a biostratigraphic investigation of a dolomite unit considered to be of Guadalupian age in the Comai area, southern Lhasa Terrane. This work yielded the ellisonid conodont species Pachycladina rendona n. sp. Wu and Ji suggesting an Early Triassic age and a low latitude depositional setting of the dolomite. Further, these findings favor correlation of the dolomite with the upper member of the Upper Permian to Lower Triassic Mujiuco Formation. Results of the present study, especially the newly collected Pachycladina fauna, suggest that the Lhasa Terrane was an element of a low-latitude carbonate platform that had separated from Gondwana during the Early Triassic. It is hoped that our findings will promote further research of the Triassic sedimentary history and paleogeographic evolution of the Lhasa Terrane.

ASTER and WorldView-3 satellite data for mapping lithology and alteration minerals associated with Pb-Zn mineralization

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and WorldView-3 (WV-3) satellite remote sensing data were used for mapping lithological units and hydrothermal alteration zones associated with Pb-Zn mineralization in the Kerman–Kashmar Tectonic Zone (KKTZ), Iran. The visible near infrared (VNIR), shortwave infrared (SWIR) and thermal infrared (TIR) bands of ASTER were used to map iron oxide/hydroxides, Al-OH minerals, Fe,Mg-OH minerals, quartz and carbonate minerals. The VNIR bands of WV-3 were used to discriminate Fe3+ and Fe2+ absorption intensities. Selective Principal Component analysis (SPCA), Spectral Angle Mapper (SAM), Linear Spectral Unmixing (LSU) and Automatic Lineament Extraction techniques were implemented. Lithological units were discriminated based on Al/Fe-OH, Fe2+/Fe3+ and Mg-Fe-OH/CO3 absorption properties. The spatial distribution of hematite, goethite, jarosite, gypsum, calcite, dolomite, kaolinite and muscovite were comprehensively detected. Some prospective zones were identified in the intersection of N-S, NW-SE and NE-SW trending fault systems, gossan, argillic/phyllic and dolomitic units.

A comparative study of two Mississippian dolostone reservoirs in the Volga-Ural Basin, Russia

A significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies. The present paper focuses on Tournaisian and Visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the Volga-Ural Basin, Russia. In contrast to many dolostone-hosted hydrocarbon reservoirs worldwide, the Tournaisian and Visean dolostones in the Demkinskoye and the Onbiyskoye oil fields plot near the low end of the porosity–permeability range. Replacive and porosity-destructive burial dolomitization has deteriorated the reservoir properties of these units. Dolomitization is related to the ascent of hot (170–215 °C) and saline (22–24 wt%) fluids as based on fluid inclusion data. This temperature range points to an abnormally high temperature background thermal regime of the Volga-Ural basin. The onset of burial dolomitization predates oil charge as documented by the fact that oil-bearing inclusions are only present in the outermost zones of porosity-occluding dolomite cements. Isotope geochemical data, dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the Demkinskoye and the Onbiyskoye oil fields. Based on these data and the detailed cement stratigraphy presented here, it is argued that dolomite bodies in both oil fields formed as the result of one main, Alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons.

Hydrogeology of desert springs in the Panamint Range, California, USA: Geologic controls on the geochemical kinetics, flowpaths, and mean residence times of springs

AbstractOver 180 springs emerge in the Panamint Range near Death Valley National Park, CA, yet, these springs have received very little hydrogeological attention despite their cultural, historical, and ecological importance. Here, we address the following questions: (1) which rock units support groundwater flow to springs in the Panamint Range, (2) what are the geochemical kinetics of these aquifers, and (3) and what are the residence times of these springs? All springs are at least partly supported by recharge in and flow through dolomitic units, namely, the Noonday Dolomite, Kingston Peak Formation, and Johnnie Formation. Thus, the geochemical composition of springs can largely be explained by dedolomitization: the dissolution of dolomite and gypsum with concurrent precipitation of calcite. However, interactions with hydrothermal deposits have likely influenced the geochemical composition of Thorndike Spring, Uppermost Spring, Hanaupah Canyon springs, and Trail Canyon springs. Faults are important controls on spring emergence. Seventeen of twenty‐one sampled springs emerge at faults (13 emerge at low‐angle detachment faults). On the eastern side of the Panamint Range, springs emerge where low‐angle faults intersect nearly vertical Late Proterozoic, Cambrian, and Ordovician sedimentary units. These geologic units are not present on the western side of the Panamint Range. Instead, springs on the west side emerge where low‐angle faults intersect Cenozoic breccias and fanglomerates. Mean residence times of springs range from 33 (±30) to 1,829 (±613) years. A total of 11 springs have relatively short mean residence times less than 500 years, whereas seven springs have mean residence times greater than 1,000 years. We infer that the Panamint Range springs are extremely vulnerable to climate change due to their dependence on local recharge, disconnection from regional groundwater flow (Death Valley Regional Flow System ‐ DVRFS), and relatively short mean residence times as compared with springs that are supported by the DVRFS (e.g., springs in Ash Meadows National Wildlife Refuge). In fact, four springs were not flowing during this campaign, yet they were flowing in the 1990s and 2000s.

Palaeoredox link with the late Neoproterozoic–early Cambrian Bilara carbonate deposition, Marwar Supergroup, India

This study performed a stable isotope and redox-sensitive trace-element investigation on the Gotan Limestone and Pondlo Dolomite of the Bilara Group, Marwar Supergroup to decipher the palaeodepositional redox condition. A multivariate-statistical analysis of geochemical data set (δ13C-carb, δ18O-carb, δ13C-org, TOC, V, Ni, Cu, Zn, Mo, Pb, and U) reveals that limestone and dolomite units of the Bilara Group were deposited under two different redox regimes. Dolomite has an average δ13C-org values ~ − 30.0 ‰ and shows a statistically significant negative correlation with δ13C-carb which indicates an oxygen-depleted closed palaeodepositional environment. Dolomite δ13C-carb also shows a statistically significant negative correlation with redox-sensitive trace elements V, Ce, Pb, and U which infer higher secondary productivity during the dolomite deposition. The limestone has an average δ13C-org values ~ − 28.9 ‰ and δ13C-carb shows a significant positive correlation with δ18O-carb and Sr, whereas δ13C-carb shows a significant negative correlation with V and U. These statistical relationships among the geochemical parameter are indicative of suboxic-to-oxic water column with a relatively higher depth that received a significant input of water discharge from the terrestrial region. This discharge may have brought nutrients and other weathering-derived ions into the basin.