Tectonic Fractures Research Articles

Distribution Prediction of Shale Deformation Structures in Tectonically Complex Area Based on Relationship Between Geological Structures and Shale Deformation

The alteration of shale structures and properties induced by tectonic activities is an important factor restricting the efficient utilization of shale gas resources. Predicting the distributions of shale deformation structures is of significance for the potential evaluation and favorable area optimization of shale resources in tectonically complex areas. Taking the Wufeng–Longmaxi shale of the southern Sichuan Basin as the research object, deformation observations of shale outcrops and shale core samples were conducted to reveal the distribution patterns of shale deformation structures in fault and fold structures. On this basis, the distribution rules of shale deformation structures in the unexposed areas were predicted by considering the structural framework of the study area. Our research indicated that faults can cause structural deformation in a limited area and that the influences of reverse faults were relatively more significant. Shale near the fault planes of reverse faults usually showed intense folding deformations, with well-developed bedding-parallel and crumpled cleavages. Strong deformation structures (crumpled, mylonitized, scaly, fractured-crumpled, and flaky structure) were distributed. Structural deformations in shale near normal faults were mainly characterized by the increase of tectonic fractures, and shale usually showed cataclastic structure. In the areas affected by strike-slip faults, bedding-perpendicular fractures and the fractures high-angle oblique to bedding planes were well developed. Folds can cause shale to deform in a larger area than faults. Shale in core zones usually displayed strong deformation structures. In the core–limb transitional areas of folds, shale mainly developed bedding-parallel and bedding-perpendicular fractures, and shale usually displayed platy and cataclastic structure. The observed structural deformations in fold limbs were generally weak, and shale usually showed primary structure and weak brittle deformation structures. According to the structural framework of the study area, it is predicted that strong deformation structures are mainly distributed in the core zones of anticlines (especially the tight and closed ones) and near the fault planes of large-scale reverse faults, while medium-intensity brittle deformation structures (platy and cataclastic structure) are distributed in core–limb transitional areas of anticlines and near the normal and strike-slip faults. In the limbs of anticlines and the areas controlled by synclines (mostly wide and gentle), shale mainly shows primary structure and weak brittle deformation structures.

Lithofacies characteristics and controlling on volcanic reservoirs in the basement: A case study of the offshore Bohai Bay Basin, Eastern China

Volcanic rocks are important, but special reservoirs, for hydrocarbon exploration. Volcanic lithofacies play an important role in reservoir development, but the understanding of the relationship between lithofacies and reservoirs is still limited, especially for volcanic basements. In this paper, we characterized the intermediate-basic lithofacies features and discuss the relationship between lithofacies and reservoirs in the offshore Bohai Bay Basin. The results show that there developed volcanic conduit, eruptive (basic and neutral), explosive, and volcanogenic sedimentary facies in the study area. Among them, the explosive and eruptive facies are dominant. Different lithofacies present diverse well-logging and seismic responses, and the basic-eruptive (basalt), neutral-eruptive (andesite), explosive, and volcanogenic sedimentary facies are distributed from the bottom to top orderly fashion. This vertical superposition pattern represents a volcanic eruption process that has resulted in eruptive facies being more developed than explosive facies. The pores (intershard, dissolution, and devitrified pores) and tectonic fractures are the predominant spaces for explosive and neutral-eruptive facies (andesite), respectively. The lithofacies control the development of the primary spaces, fractures, and the diagenesis processes, leading to explosive and neutral-eruptive facies being the dominant facies for reservoir development. The understanding of high-quality reservoirs in the volcanic buried hills is not only controlled by weathering but also dominant lithofacies that will guide the volcanic exploration globally.

Metallogenesis of the Bawang Sn-Zn Polymetallic Deposit, Wuxu Ore Field, Guangxi, South China: U-Pb Dating and C-O-S-Pb Isotopic Constraints

The Bawang deposit, located in the west of Wuxu ore field, southern section of the Nandan-Hechi metallogenic belt, is a medium-sized tin–zinc polymetallic deposit. Its genesis has been a matter of debate because of lacking constraints from isotope geology. In this study, LA-MC-ICP-MS U-Pb dating of cassiterite and C-O-S-Pb isotope analyses of monominerals were used to investigate the mineralization age and source of the ore-forming minerals in the Bawang deposit. LA-ICP-MS U–Pb dating of cassiterite yielded ages of 93.1 ± 4.8 Ma and 85.3 ± 6.3 Ma, indicating that the mineralization occurred in the early Late Cretaceous. The δ13CPDB and δ18OSMOW values of calcites at the ore stage range between −0.41‰ and 0.44‰ (average = −11‰) and between 13.8‰ and 15.40‰ (average = 14.59‰), respectively. This shows that ore fluid sources were a mixture of those derived from magma and stratum. Pyrite and sphalerite have uniform δ34S values (−4.45‰~−2.20‰), indicating that sulfur is also derived from the mixing of magmatic hydrothermal and stratum fluids. The Pb isotopic composition of sulfide (206Pb/204Pb = 18.4055~18.7625, 207Pb/204Pb = 15.6745~15.7209, 208Pb/204Pb = 38.6232~39.0370) is consistent with the granite of the same age, indicating that ore-forming metals are mainly derived from magmatic hydrothermal solution. The Bawang deposit is a hydrothermal vein-type deposit in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal fluid and carbonate rock. There may be large skarn-type ore bodies in the deep part. The results of this study provide insights into the research and exploration of similar deposits in Nandan-Hechi metallogenic belt and in the Youjiang Basin.

Horizontal Fracture Formation in the Cenozoic Mudstone of the Western Qaidam Basin

Fractures can provide valuable information on the tectonic evolution of the Earth’s crust. Our field observation data, rock core data, and thin section observations reveal several horizontal fractures in the Cenozoic mudstone strata in the western Qaidam Basin, Qinghai Province, China. These horizontal fractures have rough surfaces and are filled by gypsum. The fractures are mainly curved; however, some straight fractures are also found. There are many intersections between adjacent curved fractures, which often adjoin to form “T” shape fractures. Fracture apertures are mainly between 1 mm and 1 cm, with a maximum of 3 cm. Both the size and density of fractures decrease as strata denudation thickness decreases and depth of burial increases. Horizontal fractures are only found in a core area of anticlines with severe strata denudation. Accordingly, these fractures are related to vertical extension due to residual stress in the process of rapid uplift and denudation. The horizontal fractures cut earlier vertical tectonic fractures, indicating their formation after tectonic extrusion. Electron spin resonance (ESR) dating of fracture-filling materials revealed that horizontal fractures appeared in two stages. The first stage was approximately 1.8 Ma ago, while the second stage was approximately 0.3 Ma ago. The size and density of the later-stage horizontal fractures are markedly greater than the earlier-stage fractures. Based on the driving stress when fractures are formed, which is obtained from fracture parameters, the denudation rate in the later stage was approximately 2∼3-fold greater than that in the earlier stage. According to the evidence, during the Quaternary, the western Qaidam Basin experienced pulsating tectonic extrusion, producing uplift and denudation and an increase in tectonic activity. Such findings suggest a gradual increase in episodic tectonic activity in the entire Tibetan Plateau region during the Quaternary.

Characteristics, controlling factors and petroleum geologic significance of fractures in the Archean crystalline basement rocks: A case study of the South Jinzhou oilfield in Liaodong Bay depression, North China

Fractures play an important role in hydrocarbon storage and migration in basement buried hills. Taking the South Jinzhou oilfield (JZS oilfield) as an example, based on the data from cores, thin sections, seismic, logging, and experimental analysis, we clarified the genetic types and characteristics of fractures in the JZS crystalline basement oilfield and discussed controlling factors and their significance to petroleum geology. Tectonic and non-tectonic fractures are present in the study area, in which tectonic fractures are predominant. Tectonic fractures show three dominant directions with E-W, NE-SW, and NW-SE trends; among them, fractures with E-W trends are best developed. Most of the fractures are oblique, with a dip angle between 45° and 60°. The shear-compression zone trigged by strike-slip faults in-plane is favorable for fracture development. The granitic rocks with low compressive strength are more prone to fracture than diabase. Meteoric water and organic acids promote dissolution along fractures, expanding the tectonic fractures further. The fractures can increase the permeability of crystalline basement rocks by 103 times compared with the matrix. The dissolution along fractures can effectively improve the storage capacity of the crystalline basement rocks. Furthermore, there are a large number of fractures developed below the oil-water contact, which are important lateral migration channels for hydrocarbons.

Этапы формирования инженерно-геологических свойств пород красноцветной терригенной формации Прикамья

The paper analyzes the stages of formation of engineering-geological properties of rocks of the red-colored terrigenous formation. Two main stages are distinguished, which determined the features of their engineering-geological characteristics. The first stage – the stage of progressive lithogenesis was characterized by facies sedimentation environments and determined the main genetic types of rocks of the formation. The second stage – the stage of epigenesis (hypergenesis) was defining for the formation of the engineering and geological properties existing at the present time that must be taken into account when solving problems in the course of design and construction work. The role of local tectonic structures and tectonic fracturing within them is emphasized in the activation of hypergenesis processes and changes in rock properties.

The diagnostic criteria of borehole electrical imaging log for volcanic reservoir interpretation: An example from the Yingcheng Formation in the Xujiaweizi Depression, Songliao Basin, China

Electrical image log can be used to establish volcanic facies sequences, and thus used for reservoir interpretation, while the key is to set up available diagnostic criteria to guide lithology, structure and facies interpretation. A total of 50 datasets of borehole image logs, combined with wireline logs and cores, have been collected in the Yingcheng volcanic reservoir in the Xujiaweizi Depression, Songliao Basin, providing an opportunity for repeated image interpretation in the volcanic strata. The interpreted lithology includes rhyolite, dacite, andesite, basalt, welded ignimbrite, tuff, volcanic breccia and agglomerate, sedimentary tuff, sedimentary volcanic breccia and agglomerate. The volcanic structures are calibrated in terms of fluidal and deformed fluidal structure, vesicular and amygdaloidal structure, and massive structure. Based on the lithology and structures, volcanic facies including volcanic sedimentary facies, effusive facies, explosive facies and volcanic conduit facies, have been further determined. Layered structures of the weathered volcanic crusts are shown in borehole images and mainly developed at the top of the Yingcheng Formation. Borehole images indicate that SHmax direction in the area is consistent with the regional SHmax direction, with a preferable trending of 88°. Four sets of filled/open tectonic fractures are developed with approximately north, south, NNE and NW dipping. It infers that different volcanic facies can be well correlated between wells, and tectonic movements have a slight influence on the Yingcheng volcanic strata. The proposed diagnostic criteria and the workflow can be referred for a direct image interpretation in volcanic reservoirs or ocean drilling programs when cores are unavailable.

Pressure transient analysis for wells drilled into vertical beads-on-string caves in fracture-caved carbonate reservoirs: Field cases in Shunbei Oilfield

Tarim Basin, Western China has newly discovered fracture-caved carbonate reservoirs formed by large-scale fault tectonic activity and multiple-stage karstification. The large karst caves interconnecting with tectonic fractures in series to form vertical beads-on-string structure is one of the most common patterns. Moreover, the situations of wells drilled into caves often occur during drilling. The complex geological and engineering factors cause that traditional continuum-flow models are difficult to match the recorded pressure data. To present a novel well test model to reasonably estimate reservoir parameters, this work first presents a multi-cave multi-fracture series connection physical model by simplifying complex vertical beads-on-string structure. Four kinds of mediums are considered in physical model, including large caves, small vugs, high-angle tectonic fractures and rock matrix. Thereafter, the multi-cave-multi-region model for wells drilled into vertical beads-on-string caves is developed, among of which the flow in fracture regions obeys the Darcy's law, while the flow in large caves is assumed to obey free flow. Particularly, gravity effect is considered in fracture regions because the fluids flow along vertical structure. Then we analyze the typical flow regimes and conduct sensitivity analysis on crucial parameters. Results indicate that (a) the new cave storage regimes, characterized by unit-slope pressure-derivative curve, are recognized. This may help identify large karst caves in field case analysis; (b) The typical feature of vertical beads-on-string structure on type curves is that cave storage regimes and linear flow regimes alternately appear; (c) large cave and vug can be distinguished by the slope of pressure derivative on type curves; (d) gravity effect could lead to unit-slope pressure and pressure derivative curves, which is traditional recognized as the presence of a closed boundary when it may not be the case. Finally, three field cases from Shunbei Oilfield in which it is difficult to achieve good fitting quality using traditional models are interpreted to verify the practicability and feasibility of the proposed model.

Identification of fractures in tight-oil reservoirs: a case study of the Da'anzhai member in the central Sichuan Basin, SW China

The Da'anzhai Member of the Jurassic Ziliujing formation in central Sichuan is a typical tight-oil reservoir with porosity and permeability less than 2% and 0.1 × 10–3 μm2, respectively. Fractures in this formation are well developed in micro- and nano-scale. However, the factors that control the fracture distribution are unclear. Additionally, the uncomprehensive and ineffective identification and evaluation of fractures in the early stage of tight-oil development makes it difficult to meet the requirements of tight-oil development. In our work, we used cores, thin sections, and a scanning electron microscope (SEM) to study the influence of the microscopic rock composition, including the shelly grains, calcite grains, and clastic grains, on the fracture development. We found that the microscopic composition of shelly grains and calcite grains separately control the development of inter-shelly fractures and shelly fractures, and intergranular fractures, and tectonic fractures. Except for a small number of dissolution fractures found in mudstone, the fractures are not well developed in the formations with clastic grains. According to the characteristics of the development degree of fracture and the resolution of the well-logs, the fractures are divided into large scale, small scale, and micro-scale. By a newly established level-by-level constraints method, we systematically identified the scale, occurrence, filling characteristics, and development degree of fractures in the Da'anzhai member by well-logs. Moreover, a quantitative model is also proposed for identifying the angles and development degree of fractures. The results show that the scale of fractures can be effectively identified by the shapes and values of resistivity logs; the occurrence, development, and filling characteristics of fractures can be semi-quantitatively evaluated by the relative amplitude difference between the matrix resistivity (Rb) and formation resistivity (RT). The results are consistent with the interpretation results by formation micro-resistivity imaging (FMI) log, which further demonstrates that the level-by-level constraint method by conventional well-logs can be used to systematically and effectively predict the fracture characteristics in tight-oil reservoirs.

Post-compactional porosity in a hydrocarbon-bearing carbonate succession: Potential timing and processes controlling porosity creation in the Aptian Shuaiba Formation, Oman

Porosity formed after compaction has been recognised for many years in carbonate rocks, but there is little consensus as to the origin of the fluids responsible for dissolution, and the processes by which pores are generated. In particular, it is often hard to identify the source of fluids that are below calcite saturation, and in sufficient volumes, to explain the apparent volume of porosity. This study directly addresses this challenge through analysis of core data from a number of oil fields across north Oman, which produce from the lower Cretaceous (Aptian) Shuaiba Formation. The Shuaiba Formation in all these fields exhibits high total porosity, comprising pores that range in size from less than a micron to centimetres in diameter. There is an increase in the effective porosity upwards within the succession; permeability is highest in the upper 15 m of the formation, beneath the top-Shuaiba unconformity. Since this unconformity represents a hiatus of over 5 million years (My), caused by platform emergence, previous studies often interpreted porosity modification to be due to dissolution by groundwater during penecontemporaneous exposure of the platform. Nevertheless, paragenetic relationships indicate that most of the secondary porosity that was formed early in the burial history was either occluded by drusy calcite cements, or destroyed by compaction. Furthermore, many of the drusy calcite cements are etched, indicating that dissolution took place after cementation. Consistent with this, many of the pores formed on the margins of tectonic fractures and stylolites, indicating that some pore-formation post-dates compaction.The top of the Shuaiba Formation in all fields is between 600 and 1600 m, following uplift in the late Cretaceous and early Oligocene by inversion along deep-seated, strike-slip faults. Possible causes of dissolution are cooling of upward-rising brines fluxed along faults and corrosion by organic acids prior to hydrocarbon emplacement. It is unclear, however, as to the extent to which these fluids could dissolve carbonate before reaching equilibrium. A third possible fluid is groundwater. Sulphur isotope data from paragenetically late pyrite, is consistent with bacterial sulphate reduction, indicative of relatively cool temperatures (<60-80 °C). Porosity enhancement might, therefore, have been facilitated by mixing of groundwater with highly saline reservoir fluids, resulting in undersaturation with respect to calcium carbonate. Overall, the data reveal a complex history of porosity modification, which relates to rock fabric, stratal architecture and tectonic history and suggests multiple controls on post-compactional porosity modification. Most significantly, the data strongly suggest that dissolution occurred during orogenic uplift and telogenesis, providing a new explanation for how porosity formation can occur after lithification, along stylolites and fractures, from a volumetrically significant source of carbonate-undersaturated fluid.

The Characteristics and Main Controlling Factors for the Formation of Micropores in Shale from the Niutitang Formation, Wenshuicun Section, Southwest China

The characteristics of shale micro-pore development and its main influencing factors have important theoretical guiding significance for shale gas exploration and resource evaluation. In order to clarify the micro-pore development characteristics of lower Cambrian shale and the main controlling factors of micro-pore development, we used the lower Cambrian Niutitang formation shale, in the Wenshuicun section of the Guizhou Province in southwest China. The micro-pore development characteristics of the shale in the region were studied by argon ion profile field emission scanning electron microscopy and a low-temperature liquid nitrogen adsorption and desorption experimental system. The relationship between micro-pore and kerogen maceral composition, total organic carbon (TOC) content and different mineral content was analyzed in combination with mineral and geochemical characteristics. Inorganic pores (clay mineral pores, dissolution pores and pyrite intergranular pores) and micro-fractures (clay mineral shrinkage crack, tectonic fractures and overpressure fractures) were the main type of pore developed in the shale of the Niutitang formation in the Wenshuicun section, and no organic pores had developed. The pore size of shale is usually 2–50 nm, accounting for 58.33% of shale pores, e.g. mesopores. Clay mineral content has an obvious positive correlation with macropore volume and average pore diameter, and an obvious negative correlation with micropore volume. In addition, the content of feldspar in brittle minerals has a strong negative correlation with macropore volume and average pore diameter, and a strong positive correlation with micropore volume and BET-specific surface area. TOC content and the content of different kerogen macerals have no obvious correlation with the development of shale micropores in this region. It is concluded that inorganic mineral composition is the main controlling factor of micro-pore development within lower Cambrian shale, and organic matter abundance and maceral content have little influence on the micro-pore development. This study provides a case study for the characteristics of micropores in lower Cambrian shale in China.

Integrated geological interpretation of remote sensing data (Boysun structural-tectonic zone)

In recent years, remote sensing data are increasingly used in the practice of oil and gas prospecting. This article discusses the main methodological aspects of identifying oil and gas promising structures by using materials for interpreting remote sensing data and a complex of geological and geophysical data. Remotely sensed data exhibit a regional review of the various geological formations and tectonic fracture zone and faults that are otherwise not possible detection by human eyes on the ground. The method of structural interpretation space image allows you to: detail the internal structure of oil and gas regions; to reveal the position and features of the tectonic blocks, structures of the second and third (anticlines, synclines, monoclines, etc.) orders; identify major disruptive violations; identify chains of local structures; fix the transverse structural elements that determine tectonic fragmentation. By deciphering the remote sensing data, the distribution and nature of the lineament network marking disjunctive dislocations and zones of increased fracturing are revealed and analyzed, as well as ring structures are detected, which in most cases indicate local structures of the sedimentary cover at different depth sections. The lithology and lineament interpreted from these multi-level data were integrated with data collected from the ground.

Role of overlying and underlying limestones in the natural hydraulic fracturing of shale sections: The case of marine–continental transitional facies in the Southern North China Basin

A few limestone layers usually underlie and/or overlie the shale layers/sections in marine facies, especially in marine–continental transitional facies (MCTF). The role of limestones in the natural hydraulic fracturing of shale sections will help in selecting the fracturing location, improving the stimulation power, and clarifying fracture propagation law to product more shale oil and gas. Taking an MCTF in the Southern North China Basin as an example, the differences in natural fractures and mechanical properties between limestones and shales were compared to examine the role of limestone in hydraulic fracturing. Our analyses show that shales developed more non-tectonic fractures than limestones, usually interconnected with tectonic fractures to form a fracture network. Limestones generally have a higher Young’s modulus, compressive strength, in situ stress, and fracturing pressure and a more difficult fracture initiation than shales and sandstones. Overlying and underlying limestones with less fractures and higher mechanical properties can act as an effective barrier bed, which prevents induced fractures originating from the shale sections from penetrating their mechanical interfaces into the adjacent limestone sections. This action maintains a high stimulation power in the shale sections to form a complicated induced fracture network.

Structural evolution of southern Sichuan Basin (South China) and its control effects on tectonic fracture distribution in Longmaxi shale

We aimed to reveal the structural styles and evolution sequences of the superimposed folding zone in the southern Sichuan Basin, South China, and to predict the distribution rules of tectonic fractures in the Longmaxi shale. The field survey and geological section analysis showed that the structural pattern is characterized by the superposition of near-N–S-trending folds and near-E–W-trending compressive structures. The two structural systems experienced multistage development, and the finalization of the E–W structures was later than the N–S ones. By combining these findings with the back-stripping method and fluid inclusion analysis, the burial, thermal, and structural evolution histories were reconstructed. The results indicate that the E–W-trending compressive structures were finalized at the end of the Late Cretaceous whereas the N–S-trending folds were formed by the Oligocene Epoch. A comparative analysis was conducted to reveal the relationship between tectonic fractures and fold structures. The analysis revealed that the Longmaxi shale in the core zones of the N–S-trending anticlines and the cores and near-core zones of the E–W folds is more likely to show a well-developed natural fracture system, which was verified by fracture observations on shale cores and outcrops. These places have potential for the exploration and development of shale gas resources.

Quantitative prediction of multi-period tectonic fractures based on integrated geological-geophysical and geomechanics data in deep carbonate reservoirs of Halahatang oilfield in northern Tarim Basin

Multi-period tectonic fractures are common in deep carbonate reservoirs. It's difficult to identify and has the characteristics of stochastic spatial distribution. The quantitative prediction of fractures is critical for unravelling their spatial distribution and for understanding tectonic evolution and stress field distribution. Focusing on the Halahatang oilfield in northern Tarim Basin, outcrop surveys, drilling cores, thin sections, well logs, and seismic data were comprehensively used to better determine the development characteristics, formation time, and spatial distribution of multi-period tectonic fractures based on finite element tectonic stress field simulation. The strike-slip fault system was characterised based on post-stack seismic structural interpretation. By simulating the tectonic stress field in three different tectonic movement periods, taking the fracture density as the evaluation parameter, the spatial distribution characteristics of multi-period fractures were quantified. The numerical simulation results indicated that the development of fractures is controlled by the strike-slip fault, and the Middle Caledonian period is the main formation period of these fractures. This method can accurately predict multi-period complex fractures and can also be applied to other deep carbonate reservoirs.

Tectonic fracture prediction and its implication for hydrocarbon development with a case study in ultra-deep reservoirs of Kuqa Depression, Tarim Basin, NW China

Tectonic fractures can be important reserving space and the main contributor to permeability in ultra-deep reservoirs of Kuqa Depression. In this study, a workflow based on geomechanical method is presented. Firstly, a 3D heterogeneous mechanical field is constructed by core-logging-seismic data; then the finite element in-situ stress simulation is carried out, which is established using heterogeneous mechanical parameters instead of uniform parameters. Based on strain energy theory and energy conservation principle, the relationship between tectonic fracture parameters and in-situ stress field is established by a suitable rock fracture criterion for ultra-deep tight sandstone. Characteristics of tectonic fractures are predicted and revealed by the above stress field simulation and the established relationship between stress and tectonic fracture parameters. The results show that the maximum horizontal principal stress (SH) orientation is generally N-S-trending in Bozi-1 gas reservoir. The present-day in-situ stress magnitudes are generally high with the minimum horizontal principal stress (Sh) is about 110∼160MPa, and the stress difference is over 35MPa. The characteristics of present-day in-situ stress largely vary among different structural styles. The in-situ stress magnitude and burial depth generally indicates a linear relationship. The in-situ stress gradient is stratified with burial depth and is consistent with fold stress stratification. Controlled by the effect of fold bending and protection of salt layer, the development of tectonic fracture in pop-up structures is better with larger fracture aperture and higher permeability, and fracture strikes are generally consistent with the SH orientation. Tectonic fractures in imbricated structures indicate great heterogeneity because of the development of faults and folds, and the rebuilding effect (permeability reduction) under the present-day in-situ stress field. Optimization of favourable zone for in-situ stress and tectonic fracture is of practical effects during well drilling and completion works.

Idunn Mons: Evidence for Ongoing Volcano-tectonic Activity and Atmospheric Implications on Venus

Abstract In 2010 the ESA Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument first observed 1 μm emissivity anomalies over the top and eastern flank of Idunn Mons (46° S; 146° W), a 200 km wide volcano located in Imdr Regio, a volcano-dominated large volcanic rise of Venus. The anomalies suggest the presence of chemically unweathered and fresh volcanic deposits, which provided the first hint that volcanism in this area may have been active during the past few million years. Subsequent studies have investigated the geologic and atmospheric evolution at Idunn Mons, but no study has comprehensively investigated the evolution and the implication for recent activity in Idunn Mons. Previous work, using both VIRTIS data and Magellan radar emissivity data, confirmed the occurrence of unaltered basaltic lava flows at Idunn Mons. Building on that previous work, experimental laboratory studies have revealed that chemical weathering on Venus may act much faster than previously expected, which suggests very young ages for these flows. This inference has been supported by investigations of the tectonic fracturing surrounding Idunn Mons. Finally, atmospheric data from VIRTIS also show regional anomalies in the speed of the winds in the lower atmosphere over Imdr Regio, which may be related to very recent or ongoing volcanism. In this paper, we take a comprehensive approach, using atmospheric to surface measurements, including recent laboratory experiments, to constrain the evolution of Idunn Mons. Our work suggests that Idunn Mons may be geologically both volcanically and tectonically active today.

Geological and structural characteristics of deep-level rock mass of the Udachnaya pipe deposit

For hard rock massifs, structural disturbance is a key indicator of mining structure stability. The presence of intersecting structural elements in the massif reduces rock strength and leads to formation of potential collapse structures. In addition to that, disjunctive deformations that penetrate rock strata serve as channels for fluid migration and connect aquifers into a single system. It was established that the largest of them –faults of east-northeastern, northeastern and northwestern directions – form the kimberlite-bearing junction of the Udachnaya pipe. These faults represent zones of increased fracturing, brecciation and tectonic foliation, distinguished from adjacent areas by increased destruction of the rock mass. Specifics of tectonic fracture distribution within structural and lithological domains are determined by the presence of multidirectional prevailing systems of tectonic fracturing, as well as by differences in their quantitative characteristics. With some exceptions, the main systems form a diagonal network of fractures (northeastern – northwestern orientation), which is typical for larger structural forms – faults. Despite the differences in dip orientation of the systems, most of them correspond to identified directions, which is typical for both kimberlites and sedimentary strata. Overall disturbance of the massif, expressed in terms of elementary block volume, reaches its peak in the western ore body. For such type of deposits, friction properties of fracture structures have average values. Consideration of geological and structural data in the design and development of new levels of the deposit will allow to maintain the necessary balance between efficiency and safety of performed operations.

The Accumulation Characteristics of the Paleozoic Reservoir in the Central-Southern Ordos Basin Recorded by Organic Inclusions

The Permian tight clastic reservoir and Ordovician carbonate reservoir were developed in the central-southern Ordos Basin. This study investigated the fluid inclusion petrography, diagenetic fluid characteristics, formation process of natural gas reservoir, source rock characteristics, and reservoir accumulation characteristics of these Paleozoic strata by petrographic observations, scanning electron microscope imaging, fluid inclusion homogenization temperature, salinity, laser Raman spectrum, and gas chromatograph analyses. The results have suggested two phases of fluid inclusions in both the Permian sandstone and the Ordovician Majiagou Formation dolomite reservoirs, and the fluid inclusions recorded the history from the early thermal evolution of hydrocarbon generation to the formation, migration, and accumulation of natural gas. The early-phase inclusions show weak yellow fluorescence and recorded the early formation of liquid hydrocarbons, while the late-phase inclusions are nonfluorescent natural gas inclusions distributed in the late tectonic fractures and recorded the late accumulation of natural gas. The brine systems of the Permian and Ordovician fluid inclusions are, respectively, dominated by CaCl2-H2O and MgCl2-NaCl-H2O. The diagenetic fluids were in the ranges of medium-low temperature and moderate-low salinity. The natural gas hydrocarbon source rocks in the Ordos Basin include both the Permian coal-bearing rocks and the Ordovician carbonates. The process of the early-phase liquid hydrocarbon formation and migration into the reservoir corresponded to 220 Ma (Late Triassic). The late large-scale migration and accumulation of natural gas occurred at 100 Ma (early Late Cretaceous), which was close to the inclusion Rb/Sr isochron age of 89.18 Ma, indicating that the natural gas accumulation was related to the Yanshanian tectonic movement.

Quantitative prediction of complex tectonic fractures in the tight sandstone reservoirs: a fractal method

Quantitative prediction of complex tectonic fractures in the tight sandstone reservoirs: a fractal method