Fluid Rock Research Articles

AVO analysis for high amplitude anomalies using 2D pre-stack seismic data

Amplitude variation with offset (AVO) analysis is an 1 efficient tool for hydrocarbon detection and identification of elastic rock properties and fluid types. It has been applied in the present study using reprocessed pre-stack 2D seismic data (1992, Caulerpa) from north-west of the Bonaparte Basin, Australia. The AVO response along the 2D pre-stack seismic data in the Laminaria High NW shelf of Australia was also investigated. Three hypotheses were suggested to investigate the AVO behaviour of the amplitude anomalies in which three different factors; fluid substitution, porosity and thickness (Wedge model) were tested. The AVO models with the synthetic gathers were analysed using log information to find which of these is the controlling parameter on the AVO analysis. AVO cross plots from the real pre-stack seismic data reveal AVO class IV (showing a negative intercept decreasing with offset). This result matches our modelled result of fluid substitution for the seismic synthetics. It is concluded that fluid substitution is the controlling parameter on the AVO analysis and therefore, the high amplitude anomaly on the seabed and the target horizon 9 is the result of changing the fluid content and the lithology along the target horizons. While changing the porosity has little effect on the amplitude variation with offset within the AVO cross plot. Finally, results from the wedge models show that a small change of thickness causes a change in the amplitude; however, this change in thickness gives a different AVO characteristic and a mismatch with the AVO result of the real 2D pre-stack seismic data. Therefore, a constant thin layer with changing fluids is more likely to be the cause of the high amplitude anomalies.

A framework to obtain the formulas of the conductivity and aperture of rough fractures under thermohydromechanical conditions

In this investigation, a framework is proposed to find the equations to describe the fracture conductivity and fracture aperture as the functions of effective stress (the difference between geo-stress and fluid pressure), temperature difference (the difference between rock temperature and fluid temperature), and the rock mechanical properties. The framework consists of several lab experiments and a numerical model, which is time-saving to study the influences of the aforementioned influential factors on the fracture conductivity and aperture. After analyzing the lab experimental data, we find that it’s almost impossible to accurately measure the fracture aperture even with high-precision displacement sensors. Therefore, the numerical model coupling the fluid flow in the rough fracture, the solid deformation, and the heat transfer between the hot rock and the injected fluid is built. Three different distributions of the fracture surfaces are analyzed, and we find that the fracture surface roughness fits the normal distribution. After the data matching between the lab experimental data and the numerical results, a large number of numerical experiments using the numerical model are carried out to study the influences of the effective stress, temperature difference and the rock properties, including the elasticity and Poisson's ratio. When fitting the single variable analysis results, we find that the fracture conductivity and aperture are in different function forms of the aforementioned factors. But the reduction of fracture aperture and the natural logarithm of the normalized fracture conductivity can be described using the binary linear function of the effective stress and the temperature difference. Two key strengths of the research lie with the fact that it is time-saving to obtain the exact formulas of fracture conductivity and fracture aperture, and it can measure the dynamic fracture aperture accurately.

Reconstruction of oil charging history in the multi-source petroleum system of the Beidagang buried-hill structural belt in the Qikou Sag, Bohai Bay Basin, China: Based on the integrated analysis of oil-source rock correlations, fluid inclusions and geologic data

The Beidagang buried-hill structural belt (BBHSB), located in the central part of the Qikou Sag, Bohai Bay Basin, hosts a multi-source and structurally complex petroleum system, in which the interpretation of oil-source relationships and the definition of hydrocarbon accumulation features are challenging due to the highly variable composition of oils and complex oil migration pathways. In this study, we use biomarkers, fluid inclusions, and geologic data to address the complexities of petroleum system analysis in BBHSB. Based on hierarchical cluster analysis, four genetic groups of oils (A, B, C and D) with distinct molecular compositions were identified. Group A, B, and C oils are originated from the source rocks in the third member (Es 3 ) of the Eocene Shahejie Formation (Es), the lower (Es 1 x ) section of the first member (Es 1 ) of Es, and the middle (Es 1 z ) section of Es 1 , respectively. Group D oils are mixed oils contributed by the source rocks of Es 1 x and the second member (Es 2 ) of Es. Bounded by geological settings, the charging histories of the four groups of oils were different. Fluid inclusion analysis suggests that Group A oils have two charging stages, i.e., the late Oligocene and the Pliocene-present. During the first stage, Group A oils mainly accumulated in Es 3 with relatively low maturity. During the second stage, the Group A oils vertically migrated along the fault in a large scale, resulting in its accumulation in the middle-shallow formations near the fault. Due to the late maturity of source rocks, Group B, C, and D oils have one accumulation stage that occurred during the Pliocene-present. Due to the blocking effect of the reverse drag anticline on the migration of oils to the fault, the accumulation of Group B and C oils were limited to inner source-layers and the distribution of Group D oils in the northeastern BBHSB was limited in Es 1 x . • The crude oils can be separated into four genetic groups (A, B, C, and D) with distinct sources. • Group A oils have two charging stages and the other three group oils have only one. • Constrained by geological conditions, the four group oils have different migration features. • The charging history of the four groups of oils has been constructed.

A quantitative model to predict hydraulic fracture propagating across cemented natural fracture

A quantitative model is proposed to predict hydraulic fracture propagating across cemented natural fracture based on the numerical simulation and statistical regression. The cohesive zone method is employed to capture fracture initiation and growth, and the model combining rock deformation and fluid flow is established to simulate hydraulic fracture propagation with pre-existing natural fracture. The numerical model is solved by using finite element method, and verified against the analytical solution on the classical fracture problem. The investigation of influencing factors on hydraulic fracture propagation shows that the horizontal principal stress difference, approaching angle, cementing strength of natural fracture, injection rate and fluid viscosity codetermine whether hydraulic fracture propagates across natural fracture. After identification of these influencing factors, a series of numerical cases are conducted based on different combination of factor levels. Then the logistic regression method is employed to get the quantitative relationship between the crossing result and the factors, and the accuracy of regression equation is above 99%. The crossing curves are drawn to give the threshold of injection rate for varying other influencing factors. The potential applications include the usage of crossing criterion in the numerical modeling of fracture network propagation and the optimization of injection rate in the real field.

ЗАСТОСУВАННЯ ТЕХНОЛОГІЇ АНКЕРНОГО КРІПЛЕННЯ ГІРНИЧИХ ВИРОБОК В ГАЗОНОСНИХ ТА ОБВОДНЕНИХ ПОРОДАХ

Purpose. To substantiate the use of roof bolting for mine workings as a technological way to reduce gas and water inflows into mine workings and to develop the method of reducing water inflow with the use of combined frames and bolts supporting. Methodology. Numerical modelling method for coupled processes of rock mass strain and fluid filtration has been applied. The model is based upon fundamental ideas of mechanics of solids and filtration theory. The problem has been solved using a finite element method. The influence of stress field on formation of the filtration area, the effect of changes in fluid pressure on stress state and decrease in the strength properties of rocks during soaking (in the case of water filtration) were taken into account at each time iteration. Results. A study of the influence of supporting mine workings on filtration permeability caused by its driving has been performed. It is shown that the use of roof bolting restrains the process of cracking in the area affected by mining. At the same time the sizes of the filtration area are considerably reduced, permeability in this area decreases. Roof bolting allows to reduce the average filtration permeability caused by the redistribution of the stress field during the driving of mine working by 15 times or more. The influence of bolts on the process of fluid filtration has been studied. It is established that filtration process is actively taking place around the mine workings with frame support. When using roof bolting, methane from undermined gas sources is not involved in the filtration process. On the walls of the mine working, methane filtration intensity from the coal seam is significantly reduced. Roof bolting reduces water inflows into the mine workings in water-resistant rocks by 2.5 times, in non-water-resistant rocks by 3.4 times. Scientific novelty. For the first time the study of the influence of supporting type in mine workings on formation of the filtration area in the host rocks and the course of filtration processes has been performed. A method of reducing water inflows into mine workings with the use of combined frames and bolts supporting has been developed. It is shown that roof bolting can be used as a technological way to reduce gas and water inflows into the mine workings. Practical significance. Application of the proposed method leads to a significant economic effect by reducing the cost of water pumping and repair work to restore mine workings after emergency water inflows.

On the delimitation of the carbonate burial realm

AbstractOver the past decades, the burial realm, the most prolonged and arguably the least well‐understood diagenetic environment, has received significant research attention. Despite remarkable progress driven by exploratory drilling, outcrop analogue studies and experimental work, the scientific theories defining the burial sub‐domains are inconsistently presented in the literature. This paper reviews the concepts, processes and products that characterise the burial realm from the viewpoint of the carbonate geoscientist. Typical features of carbonate burial (fluid types, porosity evolution, diagenetic fabrics, patterns in isotope geochemistry) in epicontinental and marine basins are presented and discussed. A step towards an improved conceptual delimitation and a subdivision of the carbonate burial realm is taken, and an intuitive terminology is proposed. The very shallow limit of the burial realm is placed within the upper sediment column (redox boundary; centimetres to tens of metres). In the shallow (marine) burial domain (down to depths of many hundreds of metres), carbonate dissolution and reprecipitation, sediment dewatering and grain reorganisation take place. Interstitial waters are mainly marine (and subordinate meteoric) in origin and the system is fluid‐dominated. Under ongoing burial, physical and chemical compaction reduces pore space. At burial depths of ca 750 m, initial sediment porosities (40–80%) are reduced to about 30%. The intermediate burial domain (hundreds of metres to about 2 km; T < 100°C) is characterised by the transition from fluid‐buffered to rock‐buffered diagenesis. In the deep‐burial domain (ca 2–12 km; T > 100°C), marine formation fluids are increasingly modified by rock–fluid interaction and replaced by saline brines. The transition from the deep burial to the very low‐grade metamorphic domain is placed at depths of 12–15 km (T > 250°C). Here, carbonates undergo recrystallisation into meta‐carbonate and equigranular marble fabrics.

Use of Mohr Diagrams to Predict Fracturing in a Potential Geothermal Reservoir

Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during stimulation of a potential geothermal reservoir. In particular, Mohr diagrams are used to show under what fluid pressures and stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model, and predict the ranges of mechanical properties of rock, stresses, fluid pressures, and the resultant fractures that are likely to occur in the sub-surface. Here, the modelling is applied to folded and thrusted greywackes and slates, which are planned to be developed as an Enhanced Geothermal System beneath Göttingen.

Monitoring the formation kinetics of a bicontinuous microemulsion

HypothesesThe performance of bicontinuous microemulsions is usually assessed on the characteristics of the middle phase at equilibrium. However, applied to Enhanced Oil Recovery, such an evaluation would not be representative of the structure and composition of fluids in reservoir rocks. Studies on the properties of non-equilibrated microemulsions are still needed to better understand the formation of such complex systems, in particular to optimize input parameters of process simulation tools. ExperimentsFor this purpose, we monitored the formation of a microemulsion from contact with the oil to equilibrium when no mixing or convection is provided. Non-destructive methods such as Nuclear Magnetic Resonance, Micro-Computed Tomography, Dynamic Light Scattering and Small Angle X-ray scattering were used to extract the compositions, phase thicknesses, dynamics and structures of the system over time. FindingWe found that the system gets structured into several layers over time that include the transient presence of an oriented semi-crystalline phase. The growth of the bicontinuous middle phase results from a progressive reorganization of the liquid crystal. The compositional and structural gradients, observed along the sample height, are correlated and linked to the corresponding structures of the phase diagram of the quaternary system. Equilibrium is reached after the total transfer of the liquid crystal into the bicontinuous phase.

Assessing Crack‐Induced Compliance in Low Porosity Rocks Damaged by Thermo‐Hydro‐Chemo‐Mechanical Processes

AbstractFracture geometry, density, and distribution control fluid and thermal transport in low porosity rocks, as well as their seismic attributes. The creation of cracks throughout the matrix can result in large and disparate reductions in P and S wave velocities with negligible porosity development, which breaks the classic velocity‐porosity relationship. Properly interpreting these signatures in seismic surveys from stimulated geothermal, oil, and gas reservoirs or seismogenic areas requires a rock physics model that uses realistic elastic properties of fractures. Conventional models for cracked rocks employ specific inclusion shapes such as spheroids with small aspect ratios. However, all these exhibit the same normal‐to‐shear compliance ratio—regardless of aspect ratio—and therefore impact the bulk and shear moduli in the same qualitative way. As such they are inadequate for describing many of the complementary evolutions of P and S wave velocities that transpire with the generation of cracks. Using a differential model of dimensionless compliances, we analyze published velocities for granite, basalt, and limestone to quantify typical compliance ratios and find that they are often different from those assumed in conventional crack models, which then affects crack density estimates. We also investigate how the total normal and shear compliances change due to certain thermo‐hydro‐chemo‐mechanical processes that generate large crack densities, and conclude that the final ratios often still deviate from that of conventional models. Finally, we demonstrate how the model can be used to describe scatter in both P and S wave velocity‐porosity trends. Implications for energy extraction and seismic monitoring are discussed.

Peculiarities of the Rock–Fluid System of the Subduction Zone in the South Caspian Basin

Peculiarities of the Rock–Fluid System of the Subduction Zone in the South Caspian Basin

Effect of Fluid Properties on Contact Angles in the Eagle Ford Shale Measured with Spontaneous Imbibition.

Models of fluid flow are used to improve the efficiency of oil and gas extraction and to estimate the storage and leakage of carbon dioxide in geologic reservoirs. Therefore, a quantitative understanding of key parameters of rock–fluid interactions, such as contact angles, wetting, and the rate of spontaneous imbibition, is necessary if these models are to predict reservoir behavior accurately. In this study, aqueous fluid imbibition rates were measured in fractures in samples of the Eagle Ford Shale using neutron imaging. Several liquids, including pure water and aqueous solutions containing sodium bicarbonate and sodium chloride, were used to determine the impact of solution chemistry on uptake rates. Uptake rate analysis provided dynamic contact angles for the Eagle Ford Shale that ranged from 51 to 90° using the Schwiebert–Leong equation, suggesting moderately hydrophilic mineralogy. When corrected for hydrostatic pressure, the average contact angle was calculated as 76 ± 7°, with higher values at the fracture inlet. Differences in imbibition arising from differing fracture widths, physical liquid properties, and wetting front height were investigated. For example, bicarbonate-contacted samples had average contact angles that varied between 62 ± 10° and ∼84 ± 6° as the fluid rose in the column, likely reflecting a convergence–divergence structure within the fracture. Secondary imbibitions into the same samples showed a much more rapid uptake for water and sodium chloride solutions that suggested alteration of the clay in contact with the solution producing a water-wet environment. The same effect was not observed for sodium bicarbonate, which suggested that the bicarbonate ion prevented shale hydration. This study demonstrates how the imbibition rate measured by neutron imaging can be used to determine contact angles for solutions in contact with shale or other materials and that wetting properties can vary on a relatively fine scale during imbibition, requiring detailed descriptions of wetting for accurate reservoir modeling.

Convergence of micro-geochemistry and micro-geomechanics towards understanding proppant shale rock interaction: A Caney shale case study in southern Oklahoma, USA

As a direct outcome of economic development coupled with an increase in population, global energy demand will continue to rise in the coming decades. Although renewable energy sources are increasingly investigated for optimal production, the immediate needs require focus on energy sources that are currently available and reliable, with a minimal environmental impact; the efficient exploration and production of unconventional hydrocarbon resources is bridging the energy needs and energy aspirations, during the current energy transition period. The main challenges are related to the accurate quantification of the critical rock properties that influence production, their heterogeneity and the multiscale driven physico-chemical nature of rock–fluid interactions. A key feature of shale reservoirs is their low permeability due to dominating nanoporosity of the clay-rich matrix. As a means of producing these reservoirs in a cost-effective manner, a prerequisite is creation of hydraulic fracture networks capable of the highest level of continued conductivity. Fracturing fluid chemical design, formation brine geochemical composition, and rock mineralogy all contribute to swelling-induced conductivity damage. The Caney Shale is an organic-rich, often calcareous mudrock. Many studies have examined the impact that clay has on different kinds of shale productivity but there is currently no data reported on the Caney Shale in relation to horizontal drilling; all reported data on the Caney Shale is on vertical wells which are shallow, compared to an emerging play that is at double the depth. In this work we develop geochemical–geomechanical integration of rock properties at micro-and nanoscales that can provide insights into the potential proppant embedment and its mitigation. The novel methodology amalgamates the following: computed X-ray tomography, scanning electron microscopy, energy dispersive spectroscopy, micro-indentation, and Raman spectroscopy techniques. Our results show that due to the multiscale heterogeneity in the Caney Shale, these geochemical and structural properties translate into a variation in mechanical properties that will impact interaction between the proppant and the host shale rock.

Timing and distribution of bedding-parallel veins, in evaporitic rocks, Bouhedma Formation, Northern Chotts, Tunisia

Bedding-parallel veins, ‘BPV’, are widespread worldwide especially within sediments of low permeability such as mudstones, marls and evaporitic formations. Gypsum, calcite or quartz fibrous minerals compose these geological features as well as less common components such as bitumen. These structures are heavily studied as they provide significant information about the paleo-stress states, host rock deformation and fluid flow. In the Bouhedma Formation, Northern Chotts Range, Southern Tunisia, although gypsum BPV are clearly observed there is particularly a dearth of published data concerning their characterization. Petrographic examination, tectonic and micro-tectonic analysis of both of the BPV and host rocks were conducted to determine their origin and the formation mechanism of the bedding-parallel fibrous gypsum veins in the Bouhedma Formation.Field studies indicate that the host rocks for the BPV are mostly mudstones and evaporites. The BPV are especially localized within the lower part of the Bouhedma Formation, at boundaries between these sedimentary beds. The deformed gypsum and anhydrite fibers composing the BPV indicate vein growth during the shortening of the basin from the Miocene through Villafranchian times. We argue that the BPV are the result of fluid overpressure and natural hydraulic fracturing in low-permeable sediments due to mineralogical transformations and tectonic compression.

Coupled thermal–hydraulic–mechanical model for an enhanced geothermal system and numerical analysis of its heat mining performance

The operation of an enhanced geothermal system (EGS) involves a complex thermal–hydraulic–mechanical (THM) coupling process, which is important for exploiting the geothermal energy contained in hot dry rocks. In this study, the THM coupling mechanism in an EGS reservoir is explained, and a fully coupled THM mathematical model with local thermal nonequilibrium of a dual media is established, considering the dynamic changes in the properties of the rock matrix, fractures, and fluid during EGS operation. The model is verified through an example of the thermoelastic consolidation of a saturated soil. The heat transfer, fluid flow, and mechanical characteristics of a geothermal reservoir over 40 years are studied via a numerical simulation of a 2D geometric model. The effects of fracture occurrence, coupling conditions, and model parameters on the mining performance are analyzed. The results show that the dual-media model can reflect the anisotropy in the temperature, pressure, and stress due to the presence of fractures. An area with a more complex fracture network and smaller spacing is found to be more conducive for heat transfer. The inlet–outlet pressure differences, thermal expansion coefficient of the rock matrix, and initial fracture permeability have significant effects. When the inlet–outlet pressure difference is increased from 4 to 10 MPa, the heat extraction ratio increases from 56% to 89% at 40 years, and the initial output thermal power is increased from 9.2 to 24.9 MW. When the coefficient of thermal expansion is increased from 1 × 10−6 to 7 × 10−6 K−1, the heat extraction ratio is increased from 54% to 84%, and the initial output thermal power is increased from 8.5 to 19.7 MW. When the initial permeability is increased from 1 × 10−11 to 2.5 × 10−11 m2, the heat extraction ratio increases from 74% to 92%. These results provide a reference for optimizing the mining effect of the EGS.

Development of microcracks in granitic rock by liquid CO2 fracturing

Hydraulic fracturing by liquid carbon dioxide (LCO2) generates fracture and cracking patterns that vary from those generated by water injection. The use of LCO2 as a fracturing fluid can minimize water usage and potentially sequester CO2. In this study, hydraulic fracturing by LCO2 and water in a granitic specimen was performed, and the micro-scale characteristics of generated microcracks were investigated using an X-ray imaging technique and thin-section analysis. The results revealed that LCO2 and water injection produced different fracture characteristics. The injection of LCO2 with less viscosity and high compressibility required a greater fluid volume for fracture initiation to generate a lower breakdown pressure, and also generated higher crack-density zones located near the borehole hole possibly because of facilitated permeation amount into the rock matrix as compared to the water-injection case. In both the LCO2- and water-injection cases, the fractures developed along the rift cleavage plane, and an increase in microcrack density was observed in regions within 6 mm from the borehole. It was confirmed that the statistical and spatial distributions of developed microcracks were affected by the fracturing fluid and anisotropic properties of granitic rocks. The results of this study could be applied to fracturing that employs less water, CO2 sequestration, and recovery of geothermal energy.

Introduction to this special section: Quantitative interpretation

Quantitative interpretation (QI) is the geophysicist's endeavor to go beyond reservoir architecture. It is the effort to use geophysical measurements in understanding reservoir properties such as rock type, porosity, and fluid composition. QI often refers to the use of seismic amplitude analysis to predict lithology, porosity, and pore fluids away from the wellbore in oil and gas reservoirs. However, we can generalize and expand the concept of QI beyond seismic methods and beyond oil and gas reservoirs. In this special section, we feature five papers and cover not only seismic and well-log data, but also gravity and magnetic data. We address a hydrothermal reservoir in addition to several oil and gas reservoirs.

The temporal evolution of the carbon isotope composition of calcite in the presence of cyanobacteria

Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite (δ13CCalcite) and dissolved inorganic carbon (δ13CDIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO3 saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of 12C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75 × 10−14 to 1.71 × 10−13 mol 13C/m2/s. These rates are sufficiently fast such that the δ13CDIC value of aqueous fluids in calcite-rich rocks would be buffered by the δ13CCalcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local 12C sink such as photosynthetic microorganisms or 12C source such as decomposition of organic material.

Time-Lapse Integration at FWU: Fluids, Rock Physics, Numerical Model Integration, and Field Data Comparison

We present the current status of time-lapse seismic integration at the Farnsworth (FWU) CO2 WAG (water-alternating-gas) EOR (Enhanced Oil Recovery) project at Ochiltree County, northwest Texas. As a potential carbon sequestration mechanism, CO2 WAG projects will be subject to some degree of monitoring and verification, either as a regulatory requirement or to qualify for economic incentives. In order to evaluate the viability of time-lapse seismic as a monitoring method the Southwest Partnership (SWP) has conducted time-lapse seismic monitoring at FWU using the 3D Vertical Seismic Profiling (VSP) method. The efficacy of seismic time-lapse depends on a number of key factors, which vary widely from one application to another. Most important among these are the thermophysical properties of the original fluid in place and the displacing fluid, followed by the petrophysical properties of the rock matrix, which together determine the effective elastic properties of the rock fluid system. We present systematic analysis of fluid thermodynamics and resulting thermophysical properties, petrophysics and rock frame elastic properties, and elastic property modeling through fluid substitution using data collected at FWU. These analyses will be framed in realistic scenarios presented by the FWU CO2 WAG development. The resulting fluid/rock physics models will be applied to output from the calibrated FWU compositional reservoir simulation model to forward model the time-lapse seismic response. Modeled results are compared with field time-lapse seismic measurements and strategies for numerical model feedback/update are discussed. While mechanical effects are neglected in the work presented here, complementary parallel studies are underway in which laboratory measurements are introduced to introduce stress dependence of matrix elastic moduli.

Origin of the Xitieshan Pb-Zn deposit, Qinghai, China: Evidence from petrography and S-C-O-Sr isotope geochemistry

The Xitieshan Pb-Zn deposit in the North Qaidam orogenic belt, contains a geologic resource of 64 million tons grading 4.86 percent Zn and 4.16 percent Pb. The mineralized zones are hosted by the Tanjianshan Group, a metamorphosed succession of volcanic-sedimentary rocks in a Middle to Late Ordovician back-arc basin. Lower greenschist to middle amphibolite facies metamorphism and deformation of the original Xitieshan Pb-Zn ores and host rocks resulted in zones hosted in marble and schist. Sulfides in the marble consist of pyrite, sphalerite, galena, and marcasite with accessory chalcopyrite with undeformed textures that reflect the original carbonate replacement mineralization. Pyrrhotite, pyrite, sphalerite, and galena are the major sulfides in the schist-hosted ores, which occur as laminated sulfide layers that pinch, swell, and conform to the foliation of the schist. The schist-hosted mineralization occurs as a remobilized paragenetic assemblage that reflects the recrystallization and physical migration of the original sulfides across the brittle-ductile boundaries of these minerals during dynamic metamorphism. The sulfur isotopic compositions of ore-stage sulfides cluster around values of −1‰ to +5‰, consistent with sulfur derived from leaching volcanic rocks in the host sequence, or magmatic-related fluids. Carbon, oxygen, and strontium isotope patterns of the altered marble (δ13C = −3.8 to −1.5‰, δ18O = 8.7 to 12.2‰, and 87Sr/86Sr = 0.713092 to 0.713503) indicate extensive isotopic exchange with hydrothermal fluids. Gangue minerals, including ankerite and siderite, intergrown with sulfides at Xitieshan have C, O, and Sr isotopic compositions (δ13C = −1 to +1.7‰, δ18O = 2.3 to 7.8‰, and 87Sr/86Sr = 0.715707 to 0.722918) and textures indicative of replacement reactions between an originally marine carbonate rock and magmatic-related hydrothermal fluids. Considering the Xitieshan ore fluids replaced carbonate in a volcanoclastic sequence together with the similarities of host rocks, mineralogy, alterations, and sulfur isotope composition, the most likely genetic model for the deposit is an intrusion-related carbonate replacement deposit that underwent subsequent metamorphism and deformation.

Petroleum rock mechanics: An area worthy of focus in geo-energy research

Petroleum rock mechanics: An area worthy of focus in geo-energy research