Scientific Drilling Project Research Articles

Hydrogen-rich gas discovery in continental scientific drilling project of Songliao Basin, Northeast China: new insights into deep Earth exploration

Hydrogen-rich gas discovery in continental scientific drilling project of Songliao Basin, Northeast China: new insights into deep Earth exploration

Discing behavior and mechanism of cores extracted from Songke-2 well at depths below 4,500 m

Understanding in-situ stresses is the foundation for investigating deep rock mechanics. In practice, the fracture morphology of a disced core can often reflect its in-situ environment and disturbance-induced fracture process to some extent. In this study, three-dimensional dynamic scanning technology is used to reconstruct the fracture morphology of the disced cores extracted from Songke-2 as well as part of the Chinese Continental Scientific Drilling Project . The roughness of various regions of the fracture surfaces of the cores is investigated. Additionally, a parametric system is developed for the quantitative study of the integrity coefficient of the disced rock samples. Furthermore, the mechanisms and characteristics of the fractures along the stress paths are investigated. According to the findings, when shallow cores undergo discing, they have notable saddle-shaped coarse fractures, which may be related to the properties of the cores at these depths. Deep cores demonstrate discing behavior, but the generated discs are relatively thick. Deep core fracture surfaces are mostly smooth and straight, possibly propagating from the outside to the inside. Under excavation disturbance in the hydrostatic state, the extreme values of the maximum tensile stress at the core stubs show a saddle-shaped distribution. In general, the results of this study can serve as a theoretical foundation for the quantitative assessment of the intactness of disced cores. Furthermore, because disc patterns and characteristics are associated with stress states, this approach can be used to evaluate in-situ stress environments.

Major Mineral Fraction and Physical Properties of Carbonated Peridotite (Listvenite) From ICDP Oman Drilling Project Hole BT1B Inferred From X‐Ray CT Core Images

AbstractWe quantified mineral proportions in listvenite (completely carbonated peridotite) in Hole BT1B drilled across the basal thrust of the Samail ophiolite by the International Continental Scientific Drilling Project Oman Drilling Project using 3D X‐ray Computed Tomography (XCT). We analyzed >250,000 XCT images from a continuous ∼200 m listvenite core. Histograms of the intensity of X‐ray attenuation of each XCT core‐slice image were fitted assuming that the listvenites are composed of magnesite, quartz, and dolomite. The XCT mineral peaks were confirmed by comparison with chemical mapping data obtained using an X‐ray fluorescence (XRF) core scanner. Listvenite matrix is composed almost entirely of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting the basal thrust acted as a source of Ca‐ and CO2‐rich fluids. The SiO2:MgO:CaO molar ratio in the entire core from BT1B is 42:52:6, similar to that of onboard XRF data for discrete samples (41:54:5), whereas average Oman peridotites have ratios of (39:60:1), indicating Ca addition perhaps during carbonation. P‐ and S‐wave velocities and density of listvenite are close to those of peridotite and are higher than those of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O. Listvenites formed in the mantle wedge above subduction zones may be an overlooked reservoir for carbon in the Earth's interior.

Greenland petroleum exploration history: Rise and fall, learnings, and future perspectives

In a period of less than 10 years, Greenland has seen a dramatic change in industry interest from being one of the hottest regions for investments to virtually no activities at all. As a result of detailed strategic planning and several successful licensing rounds offshore West Greenland, in Baffin Bay, and offshore North-East Greenland, activities culminated in the period 2010–2015 with large 2D and 3D seismic programs, seven exploration wells and lots of relevant science based on large field campaigns and drilling of cored holes. The history behind the licensing activities and research projects, and the most important results and models with implications for exploration are reviewed in this paper that also gives a status on present knowledge, main risks, and future needs. In the same period, Greenland experienced many political and administrative changes in relation to petroleum exploration. The massive relinquishment of licenses took place during a short period, and various key factors of importance for decision makers are discussed. It is not the resource potential nor the data background that is the main problem in Greenland exploration. It is mainly a combination of drastic fall in oil prices causing reduced exploration budgets in a critical period of decisions, high costs and technical challenges, rigid regulations, and lack of flexibility in relation to commitments that caused industry to leave Greenland. With the present industry outlook, it does not seem likely that industry will ever come back, especially not with the signals from the new Greenland Government. Data and knowledge will, however, be of importance for future scientific drilling and new research projects, especially on tectonic history combined with models of paleo climate and oceanography.

Review of Recent Drilling Projects in Unconventional Geothermal Resources at Campi Flegrei Caldera, Cornubian Batholith, and Williston Sedimentary Basin

Unconventional geothermal resource development can contribute to increase power generation from renewable energy sources in countries without conventional hydrothermal reservoirs, which are usually associated with magmatic activity and extensional faulting, as well as to expand the generation in those regions where conventional resources are already used. Three recent drilling experiences focused on the characterization of unconventional resources are described and compared: the Campi Flegrei Deep Drilling Project (CFDDP) in Italy, the United Downs Deep Geothermal Power (UDDGP) project in the United Kingdom, and the DEEP Earth Energy Production in Canada. The main aspects of each project are described (geology, drilling, data collection, communication strategies) and compared to discuss challenges encountered at the tree sites considered, including a scientific drilling project (CFDDP) and two industrial ones (UDDGP and DEEP). The first project, at the first stage of pilot hole, although not reaching deep supercritical targets, showed extremely high, very rare thermal gradients even at shallow depths. Although each project has its own history, as well as social and economic context, the lessons learned at each drilling site can be used to further facilitate geothermal energy development.

An Unbroken Record of Climate During the Age of Dinosaurs

A scientific drilling project in China has retrieved a continuous history of conditions from Earth’s most recent “greenhouse” period that may offer insights about future climate scenarios.

Detection of the deep biosphere in metamorphic rocks from the Chinese continental scientific drilling.

It is generally accepted that there is a vast, well-populated biosphere in the subsurface, but the depth limit of the terrestrial biosphere has yet to be determined, largely because of the lack of access to the subsurface. Here as part of the Chinese Continental Scientific Drilling (CCSD) project in eastern China, we acquired continuous rock cores and endeavored to probe the depth limit of the biosphere and the depth-dependent distribution of microorganisms at a geologically unique site, that is, a convergent plate boundary. Microbiological analyses of ultra-high-pressure metamorphic rock cores taken from the ground surface to 5,158-meter reveal that microbial distribution was continuous up to a depth of ~4,850m, where temperature was estimated to be ~137°C. The metabolic state of these organisms at such great depth remains to be determined. Microbial abundance, ranging from 103 to 108 cells/g, was also related to porosity, but not to the depth and rock composition. In addition, microbial diversity systematically decreased with depth. Our results support the notion that temperature is a key factor in determining the lower limit of the biosphere in the continental subsurface.

Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

ABSTRACT Recent studies in the context of the International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides” have focused on the importance of the Seve Nappe Complex (SNC) for understanding the subduction history of the Baltoscandian margin during closure of the Iapetus Ocean. In the classical Åre area of western central Jämtland, granulite facies migmatites and leucogranites of the Åreskutan Nappe provide evidence of Early Silurian (c. 440 Ma) high temperature metamorphism and a previous prograde, ultra-high pressure history, with microdiamonds. New LA-ICPMS zircon isotope age investigations of the underlying amphibolite facies Lower Seve Nappes, reported here, have also identified an Early Silurian tectonothermal history with pegmatitic leucogranite (c. 443 Ma) and, at lower structural levels, another felsic intrusion of earliest Middle Ordovician age (c. 469 Ma). The latter intrudes isoclinally folded host rock amphibolites and calcareous psammitic paragneisses and is itself tightly folded. Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma). This new evidence of early Caledonian deformation and metamorphism indicates that, as farther north in the orogen, the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to Scandian continent-continent collision, with Siluro-Devonian emplacement of the SNC across the foreland basins onto the Baltoscandian platform.

Scientific Drilling ‐ to Construct the Telescopes that Inserting to the Earth Interior

AbstractMankind live in the earth for countless years, but until now; people do not really understand the connotation of the Earth. We know that the earth composition including the lithosphere, the asthenosphere, mantle and core. Of course, the lithosphere supports all the life on Earth. For a long time, geoscientists trying to use all kind of methods such as geological, geophysical and geochemical methods to detect and study the earth, but the knowledge about earth are mostly indirect. Through the direct observation to the lithosphere, people can understand and recognize the plate movement of ocean and the mainland, crustal stress, earthquakes, volcanic processes, deep resources, the origins of life, global climate change and biodiversity. They are all the basis of a series of geosciences problems (Su and Yang, 2010).Geological specimens, especially the true samples from deep of the Earth, are the most directly study subjects for geologists. But the only way to access the true samples from deep of the earth is drilling. The most directly relevant evidence always originated from the deep of the earth, such as core, cuttings, fluid samples and other physical samples.Continental scientific drilling has been demonstrated which is an efficient technique for directly obtaining information from the Earth's surface to the deep crust, and is acknowledged as “to build a telescope inserting to the interior of the Earth”, as well as “a key for opening the door of the Earth”. Over the last four decades, continental scientific drilling has achieved great success in enhancing our knowledge of the Earth, and in providing information on mineral resources, large engineering projects and global change. SinoProbe‐05 is a new scientific drilling venture, which builds on the success of the Chinese Continental Scientific Drilling Project (CCSD), and is similar to the current major scientific drilling project on the Wenchuan earthquake fault. SinoProbe‐05 will focus on 6 critical tectonic and mineral resource regions, including the Jinchuan Cu‐Ni sulphide deposits in Gansu, the Luobusa chromite deposits in Tibet, the Tengchong volcano‐thermal tectonic zone in Yunnan, the Yudu‐Ganxian polymetallic deposits in South China, the Tongling polymetallic deposit and the Luzong volcanic basin and mineral deposit district in Anhui. As of the end of 2013, all of these pilot holes have been completed, all of them have achieved the desired scientific objectives.The construction of another ICDP project, Songke No.2 well, has come to an end. Current well depth is 5929m. Drilling throughout the Cretaceous strata is just around the corner (The design well depth is 6400m.). This will be the first complete Cretaceous stratigraphic profile in the world.The deep exploration project which will be stared soon will build a large number of different depths of scientific drilling holes. The deepest hole depth will reach to 13000m. We believe that the construction of these scientific coring drilling holes will provide geologists with a lot of real core samples. These cores can meet the needs for different geoscience research areas. No doubt, the research results based on these cores will promote China's geological science research to a new height, of course; will also contribute to the progress of the world's earth science. This is also a good opportunity to promote China's drilling technology.So, we know that no advanced drilling technology, no enough high quality samples from the deep of the Earth, the in‐depth studies for geosciences will be restricted of course (Zhang et al., 2013).

Overview of the Wenchuan Fault Scientific Drilling Project in China

Overview of the Wenchuan Fault Scientific Drilling Project in China

Potential mobilizable Fe from secondary phases of differentially altered subsurface basaltic rock– a sequential extraction study on ICDP site Hawaii

The potential for the mobilization of Fe from secondary phases within subsurface basaltic rocks of the Hawaii Scientific Drilling Project Phase2 (HSDP2) drill core was investigated to elucidate the possible contribution of volcanic islands to the Fe budget of nearby ocean surface waters. Rock specific parameters governing Fe mobilization, such as Fe redox state, specific surface area (SSA), and connected porosity were determined. A four-step sequential extraction procedure using solutions with increasing strength of the extractants was applied to characterize different states of chemical bonding of Fe in secondary phases of the basaltic rocks, a controlling parameter for its release to oceanic water. The sequential extraction results were then used as a measure for the reactivity of secondary Fe-bearing phases and the mobilizable Fe from these rocks. Basaltic rocks with different degrees of weathering showed elevated Fe(III) contents up to 58% total Fe as Fe(III), compared to 11–18% in fresh basalts. SSAs increased with depth, with maximum values of 70 m2/g observed for hyaloclastites. Both parameters depended mainly on the alteration state of the basalt, which was more strongly affected by the fluid chemistry (freshwater ↔ seawater) than by the age of the rocks. The sequential extractions revealed the presence of highly reactive secondary Fe-bearing phases in submarine rocks exposed to seawater whereas observations for rocks altered in freshwater point to better crystallized phases with lower mobilizable Fe contents. In seawater, aging of secondary Fe-bearing phases was most probably suppressed by the adsorption of silica and multivalent anions. Comparing different types of rock, hyaloclastites and pillow basalts showed the highest mobilizable Fe with up to 19% and 16%, respectively, of the total Fe of the bulk rock. The potential for high amounts of mobilizable Fe from basaltic rocks altered under seawater dominated conditions suggests that the submarine part of volcanic ocean islands represent an underestimated source of Fe supply to ocean surface waters.

Low geomagnetic field strength during End-Cretaceous Deccan volcanism and whole mantle convection

Knowledge about long-term variation of the geomagnetic dipole field remains in its nascent stage because of the paucity of reliable experimental data over geological periods. Here, we present the first robust experimental data from the largest Cretaceous flood basalt province on Earth, the ~65–66 Ma Deccan basalt within a thick (1250 m) unbiased stratigraphic section down to the basement, recovered from a drill hole of the Koyna Deep Scientific Drilling Project in the Western Ghats, India. Critical analysis of the result along with similar results of the Cretaceous age find that (i) the dipole moment during the end Cretaceous Deccan eruption is the lowest in whole of Cretaceous (ii) dipole moment at the onset/termination of the Cretaceous Normal Superchron is apparently lower relative to that in mid-superchron, however, such differences cannot be deciphered in shorter polarities probably because of insufficient time to develop recognizable variations (iii) inverse relation between dipole moment and reversal rate is lacking and (iv) a cause and effect relation between core-mantle boundary heat flux and low dipole moment that appears to be the principle governing factor in forming the Large Igneous Provinces on the surface of earth.

Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High‐Pressure Experiments Using Input Site Sediments

AbstractDepth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm‐1K‐1 and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples.

ICDP workshop on the Lake Tanganyika Scientific Drilling Project: a late Miocene–present record of climate, rifting, and ecosystem evolution from the world's oldest tropical lake

Abstract. The Neogene and Quaternary are characterized by enormous changes in global climate and environments, including global cooling and the establishment of northern high-latitude glaciers. These changes reshaped global ecosystems, including the emergence of tropical dry forests and savannahs that are found in Africa today, which in turn may have influenced the evolution of humans and their ancestors. However, despite decades of research we lack long, continuous, well-resolved records of tropical climate, ecosystem changes, and surface processes necessary to understand their interactions and influences on evolutionary processes. Lake Tanganyika, Africa, contains the most continuous, long continental climate record from the mid-Miocene (∼10 Ma) to the present anywhere in the tropics and has long been recognized as a top-priority site for scientific drilling. The lake is surrounded by the Miombo woodlands, part of the largest dry tropical biome on Earth. Lake Tanganyika also harbors incredibly diverse endemic biota and an entirely unexplored deep microbial biosphere, and it provides textbook examples of rift segmentation, fault behavior, and associated surface processes. To evaluate the interdisciplinary scientific opportunities that an ICDP drilling program at Lake Tanganyika could offer, more than 70 scientists representing 12 countries and a variety of scientific disciplines met in Dar es Salaam, Tanzania, in June 2019. The team developed key research objectives in basin evolution, source-to-sink sedimentology, organismal evolution, geomicrobiology, paleoclimatology, paleolimnology, terrestrial paleoecology, paleoanthropology, and geochronology to be addressed through scientific drilling on Lake Tanganyika. They also identified drilling targets and strategies, logistical challenges, and education and capacity building programs to be carried out through the project. Participants concluded that a drilling program at Lake Tanganyika would produce the first continuous Miocene–present record from the tropics, transforming our understanding of global environmental change, the environmental context of human origins in Africa, and providing a detailed window into the dynamics, tempo and mode of biological diversification and adaptive radiations.

Design and applications of drilling trajectory measurement instrumentation in an ultra-deep borehole based on a fiber-optic gyro

Abstract. The working environment in hot dry rock boreholes, encountered in deep geothermal investigation drilling and ultra-deep geological drilling (up to 5000 m), is very difficult at the present stage. We have developed a drilling trajectory measuring instrumentation (DTMI), which is based on the interference fiber-optic gyro (FOG). This can work continuously, for 4 h, in an environment where the ambient temperature does not exceed 270 ∘C and the pressure does not exceed 120 MPa. The DTMI is mainly divided into three parts: an external confining tube, a metal vacuum flask, and a FOG measurement probe. Here, we focus on the mechanical design, strength, and pressure field simulation analysis for the external tube, the structural design and temperature field simulation analysis for the vacuum flask, and the FOG Shupe error analysis and compensation in the temperature field. Finally, through the engineering applications of the SK-2 east borehole of the China Continental Scientific Drilling (CCSD) project and the geothermal well of Xingreguan-2, the data measurements of the drilling trajectory were used to analyze the stability of the DTMI. The instrument realizes long-duration, high-stability work in the process of making trajectory measurements in an ultra-deep hole. The instrument has the characteristic of anti-electromagnetic interference and enables work to be carried out in the blind zone of existing technologies and instrumentation. Therefore, DTMI has great potential in the promotion and development of geological drilling technology.

Influence of late Quaternary climate on the biogeography of Neotropical aquatic species as reflected by non-marine ostracodes

Abstract. We evaluated how ranges of four endemic and non-endemic aquatic ostracode species changed in response to long-term (glacial–interglacial cycles) and abrupt climate fluctuations during the last 155 kyr in the northern Neotropical region. We employed two complementary approaches, fossil records and species distribution models (SDMs). Fossil assemblages were obtained from sediment cores PI-1, PI-2, PI-6 and Petén-Itzá 22-VIII-99 from the Petén Itzá Scientific Drilling Project, Lake Petén Itzá, Guatemala. To obtain a spatially resolved pattern of (past) species distribution, a downscaling cascade is employed. SDMs were reconstructed for the last interglacial (∼120 ka), the last glacial maximum (∼22 ka) and the middle Holocene (∼6 ka). During glacial and interglacial cycles and marine isotope stages (MISs), modelled paleo-distributions and paleo-records show the nearly continuous presence of endemic and non-endemic species in the region, suggesting negligible effects of long-term climate variations on aquatic niche stability. During periods of abrupt ecological disruption such as Heinrich Stadial 1 (HS1), endemic species were resilient, remaining within their current areas of distribution. Non-endemic species, however, proved to be more sensitive. Modelled paleo-distributions suggest that the geographic range of non-endemic species changed, moving southward into Central America. Due to the uncertainties involved in the downscaling from the global numerical to the highly resolved regional geospatial statistical modelling, results can be seen as a benchmark for future studies using similar approaches. Given relatively moderate temperature decreases in Lake Petén Itzá waters (∼5 ∘C) and the persistence of some aquatic ecosystems even during periods of severe drying in HS1, our data suggest (1) the existence of micro-refugia and/or (2) continuous interaction between central metapopulations and surrounding populations, enabling aquatic taxa to survive climate fluctuations in the northern Neotropical region.

Research and application of high−temperature drilling fluid designed for the continental scientific drilling project of Songliao Basin, China

ABSTRACT Well Songke−2 is the main hole of the Continental Scientific Drilling Project of Songliao Basin, China, and its bottom hole temperature is estimated to exceed 240°C in the fourth spud at the well depth of 5800 m. To ensure that the drilling fluid could meet the demand of high temperature resistance, different components, including clay, filtration reducers and anti−collapse additives, of the high–temperature drilling fluid system were optimized through thermal stability tests. Comprehensive test results suggested that the optimal bentonite content to be 4% (w/v). Compared with other materials of the same type, polymer filtration reducers SO−1 and HLW−1, as well as sulfonation additive LOCKSEAL were demonstrated to have stronger resistance to high temperature. The proposed high temperature drilling fluid formula exhibited good thermal stability, rheological behaviour, anti−contamination capacity and shale inhibition property. Finally, a great success of the formula was practically obtained in the field application during the following−up core drilling project.

The Schandelah Scientific Drilling Project: A 25-million year record of Early Jurassic palaeo-environmental change from northern Germany

With the aim to understand prolonged and repeated marine anoxia after the Triassic-Jurassic mass- extinction event, a continuously cored, 338 metre thick succession of Rhaetian to Toarcian sediments was retrieved close to the village of Schandelah near Braunschweig (Lower Saxony, northern Germany). Here, preliminary biostratigraphical, lithological, sedimentological, geochemical, and geophysical borehole data are presented and discussed. Based on the presence of ammonites, ostracods, benthic foraminifers, calcareous nannofossils, and palynomorphs, all major Late Triassic and Early Jurassic stage boundaries and many of the standard Lower Jurassic ammonite zones could be defined. The deltaic Rhaetian sand- and siltstone succession (Exter Fm) contains evidence for seismic activity probably related to large-scale geodynamic processes. The Hettangian (Lias Alpha) is represented by a thick heterolithic succession composed of shallow marine sandy and silty beds with intercalated organic-rich shale, representing deposition on a shoreface with frequent storm activity. Progressive deepening during the Sinemurian to Toarcian resulted in repeated deposition of laminated organic-rich facies. Periods of relative sea-level fall likely occurred during the Late Hettangian, the Late Sinemurian, and Early Pliensbachian, where a series of hardgrounds occur indicating erosion on the sea floor followed by sea level rise leading to omission, and increased reworking. One of the most conspicuous features of the Lower Jurassic in the Schandelah-1 core is the presence of abundant authigenic carbonates (glendonites,concretions, beef-calcite) within the Upper Pliensbachian with partly very negative C-isotope values (down to –37‰ V-PDB) suggesting the anaerobic oxidation of methane. A high-resolution organic carbon isotope record based on 485 analyses shows two major negative carbon isotope excursions (CIEs) within the lowermost Hettangian and Lower Toarcian, respectively. Both excursions coincide with the onset of black shale deposition and are well-known features of these time-intervals in other regions. Despite the fact that black shale deposition also characterizes parts of the Sinemurian and Pliensbachian in similar facies, no large negative CIEs are apparent. The Schandelah-1 core thus provides an unique archive of sedimentary, biotic and geochemical records of long-term Triassic-Jurassic palaeo-environmental change in the European Epicontinental Seaway.

Delineation of Damage Zones From 3 km Downhole Geophysical Logs in the Koyna Seismogenic Zone, Western India

AbstractDelineation of subsurface faults and damage zones is a major goal of scientific drilling projects in seismically active areas. Geophysical logs acquired in a 3‐km deep scientific borehole KFD1 in the Koyna seismogenic zone, a site of recurrent reservoir triggered seismicity over the past 55 years, provide an unprecedented opportunity to investigate the rock properties and delineate the fault zones. KFD1 passed through 1,247‐m thick Deccan traps and continued for 1,767 m into the underlying granitic basement rocks that host the seismic activity in the depth range 2–10 km. We have studied the physical properties and acoustic behavior of basement granitoids from the analysis of geophysical logs from 1,500 to 3,000 m. Salient results are as follows. (1) Seven anomalous zones are identified below 2,100‐m depth based on electrical resistivity, caliper, density, neutron porosity, self‐potential, and sonic data. (2) The anomalous zones are characterized by significant shear wave velocity anisotropy (up to 25%), as revealed by cross‐dipole sonic data. (3) Dispersion analysis of dipole flexural modes confirms that the anisotropy is primarily stress induced; fast polarized shear wave azimuth (FSA) therefore indicates the orientation of maximum horizontal compressive stress SHmax. (4) Comparison of FSA with independent estimates of SHmax orientations obtained from drilling induced tensile fractures and strike of inclined fractures in the anisotropic zones shows that FSA is controlled mainly by the stress regime. Therefore, the stress rotations inferred from anisotropy analyses in the anomalous zones indicate their association with subsurface fault damage zones.

Fabrics and water contents of peridotites in the Neotethyan Luobusa ophiolite, southern Tibet: implications for mantle recycling in supra-subduction zones

The preservation of ultra-high-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibet suggests their deep origin near the mantle transition zone. Dunite and harzburgite core samples from the Luobusa Scientific Drilling Project show supra-subduction zone geochemical signatures and equilibration temperatures of c. 950–1080°C. Olivine shows A-, B-, C- and E-type fabrics, and combinations of A- and E-type or B- and E-type fabrics. Transmission electron microscopy observations show straight dislocations and the activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. The mean water content in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples was 16 ± 5, 90 ± 21 and 492 ± 64 ppm, respectively, which is different from the water content of hydrated peridotites above the mantle wedge. The trace element compositions of Cpx exclude significant metasomatism after melt extraction. The high hydrogen partition coefficient between Cpx and Opx ( D H Cpx/Opx = 5.56 ± 0.96) implies equilibrium at high pressures and rapid exhumation. Based on deformation experiments, the B- and C-type fabrics could be formed in a subduction zone at depths >200 km, whereas the A- and E-type fabrics were produced in the shallow mantle. In a process triggered by slab rollback, the Luobusa peridotites may have been rapidly exhumed within a subduction channel and mixed with the lithospheric mantle of the forearc. Supplementary material: Major oxide contents in Opx, Cpx and spinel, trace element concentration in Cpx, micrographs and TEM images of peridotite are available at https://doi.org/10.6084/m9.figshare.c.4307828 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways