Facies Rocks Research Articles

A comprehensive perspective on pore connectivity and natural fracture analysis in Oligo-Miocene heterogeneous carbonates, southern Iran

Ascertaining the pore connection criterion has always been a challenge for geoscientists. In the framework of this research, an attempt was made to study the pore connectivity in a heterogeneous carbonate rock by combining several techniques. Pore types and rock facies were investigated for 321 thin sections. The abundance of connected porosities, cementation type and intensity which significantly control the matrix pore connectivity were specified on thin sections and finally a connectivity log was extracted that showed an accurate result on poro-perm cross-plots. Most of the porosity volumes were related to interparticle, vuggy and moldic porosities which are microscale. The velocity deviation log, estimated in four wells, indicates the presence of micro-porosity, which is consistent with the results of thin-sections. Zero velocity deviation is usually associated with the predominant presence of micro-pores. Different permeability values in identical lithological facies, as well as micro-fracture markers, prompted us to conduct more extensive research on fractures. FMI analysis proved that the studied reservoir is intensively fractured. The recorded high permeability in a sample with low matrix connectivity is specifically a fracture indication. Fracture apertures vary from microscale (<50 μm) to 12.5 mm and calculated fracture density is raised up to 2 fractures per meter. Determined fractured intervals are very consistent with mud-loss information, for instance, a complete loss has been reported in the fault zone identified in well B. The detected fractures somewhat follow the main Zagros fault trend (NW-SE). Eventually, calculated connectivity was predicted in other wells using Artificial Neural Network (ANN) and a 3D model was constructed with respect to seismic attributes which gives a precise perspective of pore connection distribution throughout the studied field.

Lithotypes, Facies, and Sources of Sedimentary Rocks in the Barguzin Valley of the Baikal Rift System

Lithological, petrographic, and lithogeochemical data are given for sedimentary rocks of alluvial, lacustrine-marsh, and lacustrine facies in the Barguzin valley, uncovered by new boreholes. During petrographic studies of thin sections, the main lithotypes of stratons of rift basins were identified. According to the mineral composition of the rocks, petrochemical and microelement characteristics, a conclusion was made about the origin of sedimentary deposits as a result of erosion of the granite basement rocks.

ГЕОХІМІЧНІ ОСОБЛИВОСТІ РУДНО-ПОРФІРОВОЇ МІНЕРАЛІЗАЦІЇ У МАГМАТИЧНІЙ СИСТЕМІ ГОШГАРЧАЙ (МУРОВДАРСЬКИЙ РАЙОН, АЗЕРБАЙДЖАНСЬКА ЧАСТИНА МАЛОГО КАВКАЗУ)

The geochemical features of intrusive rocks and the surrounding rocks associated with porphyry copper mineralization in the Goshgarchay magmatic system were studied. Major factors of concentration of copper and copper-porphyritic mineralization in rocks of gabbro-diorite-granodiorite formation has been revealed. It has been established that the structure of this ore region was formed as a result of successive alteration of some deformation stages accompanied by development of fractures in intrusive massive, by dike intrusions, locks displacements along faults and the fractures composed of various mineral associations. Main ore components were defined and relations hips among elements were considered. Geochemical associations allowing the localization of the development area of mineralization have been analyzed in various formation stages of porphyry copper system according to the results of cluster analysis. In this study, Cu, Mo, Pb, Zn, Ag, Ni, Co, Mn, Ti, Cr, V, and Sr both in the surrounding rocks and in ore-bearing intrusive rocks were considered as stable elements. This group of elements can be regarded as indicator elements in porphyry copper. The results of the claster analysis show that the elements which are less characteristical for hydrothermal solutions (Mn, Ti, Cr, V, Sr) keep their freedom in all wallrocks and the existence of their grouping is probably associated with their transportation process from silicate rocks with surrounding basic-average content. The elements such as Cu, Mo, Ag, Pb and Zn which form free group are connected with the generation from the magmatic source in the course of crystallization differentiation process and exposure of rocks to the hydrothermal activity which is considered the source of these elements.The identification of reasons of the association of V with Cu and Zn and Pb with Fe group elements in the intrusive facies rocks is characterized with its uncertainty. The analysis of the schedule made according to the results of Cu and basic ore components (Mo, Ag, As, Bi, Gd, Co, Cr, Se, Ge, Li, Nb and etc.) in the ores of Goshgarchay deposit show their dependence on one another equally. The amount of Cu in the ores have Ag, As, Bi, Cd, Ce, Ge, Sr, Mo, Sb, Th and Ti positive correlation relationship, but it has negative correlation relationship with Co, Cr, Cs, La, Li, Pb, Rb, Ni, Nb and Zn. Direct dependence is observed for ore mass between concentrations of Cu, Mo, Pb, Zn, Ag, Se, Te and other elements in the Goshgarchay porphyry copper deposit. Three various mineral associations differing from one another participate in stock work type copper-porphyry mineralization: 1) primary sulfide minerals; 2) oxidized ores; 3) primary sulfide minerals significantly enriched with products of hydrothermal ore carrying solutions.

Petrostructural and Geochemical Characteristics of the Metamagmatites in the External Zone of the Dahomeyides Belt: Case of the Kant&amp;#232; Serpentinites (Northern Togo)

Thanks to detailed field investigations, microstructural and geochemical analysis and relationship with enclosing rocks, microfabrics, magmatic typology and metamorphic evolution of the Kantè sepentinites have been specified for the first time. The Kantè serpentinites in northern Togo constitute a mega-lens of ultrabasic rocks tectonically intercalated in the sericite chlorite schists of the Atacora structural unit. The brecciated, schitotose or massive rock facies are strongly marked by an S1 schistocity plane superimposed by a flat C shear plane linked to a west vergence thrusting movement. The parageneses that compose the metamagmatites are essentially serpentinous, containing plagioclase, opaque minerals (magnetite, chromite, spinel) and pyroxene porphyroblasts. These microfabrics represent relics of a probable gabbroic protolith. In fact, the geochemical characteristics of the Kantè serpentinites suggest that their magmatic typology is that of komatiites or tholeiitic basalts with oceanic arc affinities. They would have been emplaced in an active margin environment. The retromorphic evolution of the protolith corresponds to the phase of involvement in a major tangential contact during the panafrican tectogenesis.

Xenoliths Evidence of Alkaline Magmatic Infiltrations Beneath Lake Nyos (Cameroon Volcanic Line, West Africa)

Xenoliths enclosed in Lavas of the Nyos volcano (Cameroon Volcanic Line, continental sector) range from fertile lherzolites to harzburgites. One spinel-free wehrlite has been also sampled. The occurrence of phlogopites and pargasites in some harzburgites together with specific textural rock-type (lherzolites transitional porphyroclastic to equigranular), including major and trace element compositions both in peridotites bulk rocks and minerals point out interactions between the mantle and basaltic magmas responsible for the formation of wehrlites beneath the Nyos volcano. Hydrous minerals (phlogopites and pargasites) and metasomatic events are their main petrogeochemical signatures different from group 1 samples which are characterized by spoon-shaped REE patterns. Later on, hydrous phases, Ti-rich Cpx, CaO rich Ol, Ti, and V rich Ol wehrlite precipitated from melt enrichments due to the percolation of the mantle by basaltic magmas of alkaline affinity. The metasomatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that Nyos mantle peridotites have experienced: 1) variable metasomatic events related to the percolating of the depleted mantle by a alkaline silicate liquid, 2) the spinel-free wehrlite is a group 2 sample corresponding to a cumulate of a similar melt, 3) amphibole may be a potassium-bearing mineral instead of or in addition to phlogopite at shallower levels of Nyos upper mantle and 4) transitional textural rock facies express also the fingerprint of rising mantle plume which were percolated by alkaline magma during their transit to the surface.

Organic geochemistry of possible Middle Miocene–Pliocene source rocks in the west and northwest Nile Delta, Egypt

An important step for any frontier basin source rock survey is to understand the natural variability of geochemical characteristics within the source intervals. No previous studies addressed the molecular characteristics of organic matter or the source rock potential in the west and northwest onshore/offshore portion of the Nile Delta province. For this reason, the current work used geochemical proxies, including Rock-Eval/TOC screening analyses, gas chromatography–mass spectrometry, and metastable reaction monitoring mass spectrometry to examine the type and origin of the disseminated organic matter, thermal maturity, and depositional environments of the Miocene–Pliocene sediments in this basin. Results show fair–very good organic content for the Miocene Sidi Salem and Abu Madi rock samples compared to the Pliocene Kafr El Sheikh and El Wastani samples that generally show low TOC with fair–good organic content. The low organic content recoded in the Pliocene rocks may be due to high rates of sedimentation associated with clastic dilution and microbial degradation, typical of deltaic environments. In general, kerogen in the samples has high proportions of gas or non-generating and recycled organic materials and is dominantly of fluvial or deltaic origin. The analyzed source rocks exhibit a wide range of organic matter quality varying from Type-III, Type-II/III, to Type-IV kerogens. The molecular findings suggest mixed-source input from planktonic–bacterial and land plants with significant algal contributions to the source rock facies. This is based on high C30 24-n-propylcholestanes, C27 cholestanes and C29 stigmastanes, moderate tetracyclic polyprenoid (TPP) ratios, and low oleanane and gammacerane. The most striking biomarkers are high bicadinane and 24-norcholestane ratios. Local differences related to facies variation in the source depositional environment result in significant variations in biomarker characteristics. Pyrolysis Tmax, sterane isomerizations %20S and %ββ, moretane/hopane ratios, and C32 homohopane %22S suggest immature to maturity near the beginning of the oil window. Five genetic families were identified among the extract samples using hierarchical cluster analysis (HCA) and principal component analysis (PCA) based on 15 source-related biomarker ratios. Some of these families are questionable because of low TOC, significant differences in maturity between the extracts and condensates, and possible contamination by diesel additive. However, five rocks from the Miocene Sidi Salem Formation contain elevated TOC (1.22–2.25 wt%) and HI (366–458 mg HC/g TOC) and the extracts show no evidence of significant contamination in the biomarker range of molecular weight and good correlation with four WDDM-14 (2739, 2854, 2804, and 2870 m) oils in the offshore Rosetta and Abu Qir oilfields.

Texture and Trace Element Composition of Rutile in Orogenic Gold Deposits

AbstractRutile from a wide range of orogenic gold deposits and districts, including representative world-class deposits, was investigated for its texture and trace element composition using scanning electron microscopy, electron probe microanalysis, and laser ablation-inductively coupled plasma-mass spectrometry. Deposits are hosted in various country rocks including felsic to ultramafic igneous rocks and sedimentary rocks, which were metamorphosed from lower greenschist to middle amphibolite facies and with ages of mineralization that range from Archean to Phanerozoic. Rutile presents a wide range of size, texture, and chemical zoning. Rutile is the dominant TiO2 polymorph in orogenic gold mineralization. Elemental plots and partial least square-discriminant analysis suggest that the composition of the country rocks exerts a strong control on concentrations of V, Nb, Ta, and Cr in rutile, whereas the metamorphic facies of the country rocks controls concentrations of V, Zr, Sc, U, rare earth elements, Y, Ca, Th, and Ba in rutile. The trace element composition of rutile in orogenic gold deposits can be distinguished from rutile in other deposit types and geologic settings. Elemental ratios Nb/V, Nb/Sb, and Sn/V differentiate the rutile trace element composition of orogenic gold deposits compared with those from other geologic settings and environments. A binary plot of Nb/V vs. W enables distinction of rutile in metamorphic-hydrothermal and hydrothermal deposits from rutile in magmatic-hydrothermal deposits and magmatic environments. The binary plot Nb/Sb vs. Sn/V distinguishes rutile in orogenic gold deposits from other geologic settings and environments. Results are used to establish geochemical criteria to constrain the source of rutile for indicator mineral surveys and potentially guide mineral exploration.

Petrochronology of high‐pressure granulite facies rocks from Southern Brasília Orogen, SE Brazil: Combining quantitative compositional mapping, single‐element thermometry and geochronology

AbstractWe use a combination of several in situ techniques to assess the P‐T‐t path of high‐pressure granulites from the Passos Nappe in the Southern Brasília Orogen (SE Brazil). Quantitative element mapping and single‐element thermometers (Zr‐in‐rutile and Ti‐in‐quartz) are coupled with P‐T pseudosections and monazite and rutile dating. Compositional and temperature maps, based on cathodoluminescence mapping and in situ analyses of Ti‐in‐quartz, are presented as a novel approach to evaluate crystallization temperature. The studied rocks have a pelitic protolith and record a peak pressure assemblage of garnet + kyanite + rutile + K‐feldspar + quartz + melt ± plagioclase that formed at ~830°C and 1.2 GPa. Retrograde conditions of ~560°C and 0.6 GPa are determined based on the grossular content of garnet and the crystallization of biotite and ilmenite. Metamorphic peak conditions occurred ca. 635 Ma, according to monazite dating, with a younger date of ca. 615 Ma associated with later kyanite crystallization. Rutile ages of ca. 590 Ma are linked to the late retrograde stage (at ~600°C). Results show that the distribution of Ti‐in‐quartz is heterogeneous, decreasing in abundance towards the rim of crystals, though the higher temperatures constrained with Ti‐in‐quartz thermometry are broadly consistent with peak conditions. The peak pressure conditions are consistent with continental collision setting in the Southern Brasília Orogen and were followed by an early cooling/decompression stage and then by a slow cooling during exhumation and transport to shallower crustal levels.

Machine-Learning Work Flow Identifies Brittle, Fracable, Producible Rock Using Drilling Data

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201456, “Machine-Learning Work Flow To Identify Brittle, Fracable, and Producible Rock in Horizontal Wells Using Surface Drilling Data,” by Ngoc Lam Tran, SPE, Ishank Gupta, SPE, and Deepak Devegowda, SPE, University of Oklahoma, et al., prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, Colorado, 5–7 October. The paper has not been peer reviewed. The complete paper demonstrates the application of an interpretable machine-learning work flow using surface drilling data to identify fracable, brittle, and productive rock intervals along horizontal laterals in the Marcellus shale. The results are supported by a thorough model-agnostic interpretation of the input/output relationships to make the model explainable to users. The methodology described here can be generalized to real-time processing of surface drilling data for optimal landing of laterals, placing of fracture stages, optimizing production, and minimizing frac hits. Modeling Approach In a multiwell field development, the ability to improve recovery efficiency per rock volume depends on well spacing, stacking, completion strategy, and avoidance of frac hits. In this study, the authors use surface drilling data to predict mechanical rock facies (generated using Poisson’s ratio and Young’s modulus) using several supervised classification methods. The objective is to use the trained model to predict the mechanical facies in real time using surface drilling data in future wells to optimize the well trajectory and placement of fracture stages. The highlighted modeling approach is conducted as follows: - Data preparation. Preliminary univariate (histograms, box plots) and bivariate analyses (cross plots) are performed, outliers removed, and missing values handled. The clean data are scaled, and a few additional derived variables such as mechanical specific energy (MSE) are added (also called feature engineering) that may boost predictive efficiency. - Unsupervised clustering. Clusters are generated using the derived variables of Young’s modulus and Poisson’s ratio. The number of clusters is optimized by silhouette width and within the sum of squares. The clusters group points of similar brittleness/fracability and are referred to as a geomechanical facies in this paper. - Supervised classification. A classifier is built using K-nearest neighbors, gradient boosting, random forests, and neural networks to identify geomechanical facies from surface drilling data. Seventy-five percent of the data are used for training and 25% for testing. Tenfold cross validation is performed on the training data to prevent overfitting. In 10-fold cross validation, the training data is subdivided randomly into 10 parts. The model is trained on nine parts and then validated on the remaining part. This process is repeated multiple times for each machine-learning technique. Only those models are averaged to provide the final model that gives good results for the validation data.

Early Triassic paleogeography of the northern Perth Basin, and controls on the distribution of source rock facies

The facies distribution of the Lower Triassic Kockatea Shale in the northern Perth Basin is an important factor for assessing the Lower Triassic source rock potential in the basin. Seismic interpretation and well analyses demonstrate that Permian-aged faults are responsible for the morphology of the basin in the Early Triassic and created remnant topography that controls the deposition of Lower Triassic sediments. Cores from petroleum wells and their correlated wireline logs show the lithological variation and lateral distribution of facies across the base of the Kockatea Shale. Darker coloured mudstones were deposited under anoxic conditions in the deeper part of the basin, while lighter coloured mudstones and tidally influenced sediments with bioturbation were deposited under oxic conditions in a shallow marine setting. Microbial mats developed in shallower water depths on structural highs (Beagle Ridge and Turtle Dove Ridge) in the anoxic parts of the basin.The distribution of microbial mats is controlled by two factors during the period of ecological recovery from the end-Permian mass extinction: 1) the presence of topographic highs and, 2) distance from the edge of the basin. The source rock potential of both darker coloured mudstone and microbial mat facies is demonstrated by TOC values between 0.3 and 3.4% and HI values between 39 and 579 mg/gTOC. On the other hand, the lighter coloured mudstones, deposited under more aerobic tidal settings, have poor source rock quality and TOC values between 0.2 and 0.5% and HI values between 59 and 127 mg/gTOC.

K-Mean Clustering Analysis and Logistic Boosting Regression for Rock Facies Characterization and Classification in Zubair Reservoir in Luhais Oil Field, Southern Iraq

Identifying rock facies from petrophysical logs is a crucial step in the evaluation and characterization of hydrocarbon reservoirs. The rock facies can be obtained either from core analysis (lithofacies) or from well logging data (electrofacies). In this research, two advanced machine learning approaches were adopted for electrofacies identification and for lithofacies classification, both given the well-logging interpretations from a well in the upper shale member in Luhais Oil Field, southern Iraq. Specifically, the K-mean partitioning analysis and Logistic Boosting (Logit Boost) were conducted for electrofacies characterization and lithofacies classification, respectively. The dataset includes the routine core analysis of core porosity, core permeability, and measured discrete lithofacies along with the well-logging interpretations include (shale volume, water saturation and effective porosity) given the entire reservoir interval. The K-Mean clustering technique demonstrated good matching between the vertical sequence of identified electrofacies and the observed lithofacies from core description as attained 89.92% total correct percent from the confusion matrix table. The Logit Boost showed excellent matching between the recognized lithofacies from the core description and the predicted lithofacies through attained 98.26% total correct classification rate index from the confusion matrix table. The high accuracy of the Logit Boost algorithm comes from taking into account the non-linearity between the lithofacies and petrophysical properties in the classification process. The high degree of lithofacies classification by Logit Boost in this research can be considered in a similar procedure at all sandstone reservoirs to improve the reservoir characterization. The complete facies identification and classification were implemented with the programming language R, the powerful open-source statistical computing language.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

Integration of well logs and seismic data for facies analysis and modelling of the El-Wastani Formation in the Sequoia Gas Field, offshore Nile Delta, Egypt

Reservoir rock facies modelling helps in determining the degree of reservoir variability for enhanced management of the heterologous facies’ reservoirs, both at borehole scale and more regionally at seismic scale. The Pliocene Offshore Sequoia Gas Field is located near the boundary between the Rosetta Concession and the West Delta Deep Marine Concession (WDDM) on the western side of the Nile Delta. It is characterised by canyon systems of complex turbidite channel-levee deposits that are multi-stacked. In order to create various depositional facies that are compatible with a deep marine environment, an advanced facies model was built from the combination of the seismic, core, and formation micro imager (FMI). Sandstones (laminated and massive), shale (laminated and massive mud-rock), silt, and thin sandstone beds were identified as four major facies. In the Sequoia field area, the El-Wastani Formation is known as a heterogeneous sequence of sandstones and mudstones arranged into a broad upward-fining succession. High-quality, blocky sands exist at the base, while the upper portion is characterised primarily by isolated sand bodies intercalated with mudstones in the presence of some thin-bedded sands (their thickness less than 1 m).

Paleoweathering, Provenance and tectonic setting of gold-bearing neoproterozoic rocks from the Atacora Structural Unit, Pan-African Dahomeyide belt, northwestern Bénin

The Atacora Structural Unit (ASU) is part of the Pan-African belt located in Northwestern Bénin Republic. It consists of metasedimentary rocks of greenschist facies and composed of quartzites, mica schists and chlorite-sericite-quartz schists. A petrochemical study of these rock types was undertaken, geared at inferring their paleoweathering, provenance and tectonic setting. Texturally, the rocks are, medium to coarse-grained with angular to rounded clasts. Detrital component statistics classified them as sublitharenite/quartzarenite. Geochemically, they can be classified as arkose/subarkose and sublitharenite rocks. The average values for PIA, CIA, and CIW for the quartzites, mica schists and schists range from 75% to 98%, reflecting an intensive degree of chemical weathering. The LREE enrichment (LaN/YbN = 9.56), negative europium anomaly (Eu/Eu* = 0.77), and fractionated HREE patterns of chondrite-normalized, are comparable to melts generated from crustal continental materials. The geochemical studies show that the studied rocks were sourced from ancient quartzose sedimentary rocks. Trace element ratios Cr/Th, Zr/Sc, Th/Sc, Th/Co and La/Th and rare earth elements of the source components indicate that these mature materials were chiefly derived from felsic rocks, though the involvement of basic rocks (basalts) cannot be ruled out. Chondrite-normalized REE patterns indicate that the metasedimentary rocks are sourced from pre-existing sedimentary rocks in the ASU, which rocks were mainly derived from the Dahomeyide basement and probably from the Amazonia craton. The major and trace elements discriminant diagrams suggest these metasediments were deposited in a passive-margin setting, though continental island arc setting signature was also identified.

High microbial gas potential of Pleistocene lacustrine deposits in the central Qaidam Basin, China: An organic geochemical and petrographic assessment

Thick Pleistocene fine-grained rocks in the central Qaidam Basin, China, are regarded as the principal source rocks for microbial gas there. Here, for the first time, a detailed investigation on the organic geochemistry and petrology of this sedimentary sequence is presented. Two different, immature, lacustrine source rocks facies are present in the study area. Organic facies A samples with high TOC (4.1–25.3 wt%) and TS (1.5–3.7 wt%) contents mainly developed under a suboxic to anoxic freshwater column at the lake margins. This shallow water fluvio-lacustrine facies, accounts for only a small proportion of the whole sedimentary sequence, but has the highest petroleum generation potential. In contrast, organic facies B samples having lower TOC (0.5–1.1 wt%) and TS (0.14–1.0 wt%) contents were deposited in a more oxic, brackish-lacustrine water body. However, exceptionally, anoxic conditions were present in sediments that were sealed by algae mats.The majority of the interval is comprised of gas prone, mixed type II-III kerogen derived mainly from aquatic plants. Organic facies A is interpreted to be derived from abundant macrophytes (e.g. non-marine algae, submerged angiosperms) around the lake margin. By contrast, the organic matter in organic facies B mainly stems from saltwater algae, with some additional bacterial contribution. Terrestrial higher plants are subordinate in both organic facies. The organic carbon accumulation rate was high compared to organic matter-rich Quaternary marine sediments. Cold and dry climate conditions and high burial rates favored methanogenesis via carbonate reduction in the sediments.

The contribution of elastic geothermobarometry to the debate on HP versus UHP metamorphism.

Characterizing the pressure and temperature (P–T) histories of eclogite facies rocks is of key importance for unravelling subduction zone processes at all scales. Accurate P–T estimates provide constraints on tectonic and geochemical processes affecting subduction dynamics and help in interpreting the geophysical images of present‐day converging plates. Conventional equilibrium geothermobarometers are challenged in ultra high pressure (UHP) metamorphic terranes, as minerals may undergo re‐equilibration along their exhumation path. Elastic geobarometry applied to host‐inclusion systems is a complementary method to determine P–T conditions of metamorphism independent from chemical equilibrium. Because only a single measurement, the inclusion strain, is made, only a line in P–T space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known. An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence such as being in the same garnet growth zone, to have been entrapped at the same time. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P–T conditions for the host rock to be determined. Here, we combine Zr‐in‐Rutile thermometry and thermodynamic modelling with micro‐Raman measurements on quartz and zircon inclusions trapped in garnet to obtain pressures and temperatures of equilibration of a quartz–garnet vein from the Proterozoic Ulla gneiss basement and of garnet–kyanite gneiss from the Caledonian Blåhø nappe, both in the Fjørtoft UHP terrane, Norway. We find that the quartz–garnet vein formed at high pressure (1.5–2.5 GPa and 750–800°C) and recrystallized at ~1.2 GPa and 880°C. In contrast, the garnet–kyanite gneiss followed an anticlockwise path with peak P–T at 1.2 GPa and 880°C: these estimates are consistent with previous thermodynamic modelling and suggest that the Ulla gneiss and the Blåhø nappe came into contact at these last conditions. We also discuss a new method to detect hydrostatic versus Non‐hydrostatic stresses near quartz and zircon inclusions in garnet.

Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

Abstract Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures.

Unravelling the evolution of regional-scale shear zones with the aid of microstructures and textural analysis: an example from the central Himalaya

Microstructure and texture data on the Main Central Thrust zone (MCTz) in central Himalaya, on two nearly N-S oriented valleys in the Manaslu range (Western Nepal), are provided. Kinematic indicators at the meso and microscale support a south-directed flow. From south to north, greenschist facies paragneiss in the garnet-zone pass to amphibolite facies rocks in the kyanite-zone, suggesting an up-section metamorphic temperature increase. Combining microstructures and data from two different texture analysers (X-ray Texture goniometry and crystal-fabric microanalyzer) allowed to link the temperature path (from 460°C to 600°C up-section) and kinematic vorticity estimates. The “orientation of oblique grain shape fabrics” method was applied, pointing out a general shear flow (40-56% of pure shear) at the MCTz top. Results are in line with a “decelerating strain-path” model, as relatively high simple shear components are recorded at deeper structural levels. This work highlights that the combination of X-ray texture goniometry and crystal-fabric microanalyzer techniques is a good strategy to ensure quartz full-fabric.

GEOCHEMISTRY OF OILS IN THE TEREK‐CASPIAN FOREDEEP AND PRIKUMSK SWELL, NE GREATER CAUCASUS, SOUTHERN RUSSIA

Hydrocarbon reserves of the order of 1140 MM brl oe have been identified in the northern foreland of the eastern Greater Caucasus, principally in the Terek‐Caspian fold‐and‐thrust belt and the Prikumsk Swell in the north of the Terek‐Caspian foredeep. Despite the great economic significance of these areas and their long exploration history, the origin of the hydrocarbons is still poorly understood. In the present paper, geochemical data from 73 oil samples representing 28 fields are used to investigate the presence of oil families and to correlate the oils with potential source rocks.Biomarker composition of oils in Cretaceous and Miocene reservoirs in the Terek‐Caspian fold‐and‐thrust belt is mainly controlled by reservoir depth (100‐5700 m) and maturity (0.70‐1.15 %Ro), and it is therefore difficult to separate maturity and facies effects. For example, a downward increase in diasterane/sterane ratios may indicate a change in source rock facies or may be attributed to increasing maturity. Some shallow oils are biodegraded. The presence of short‐chain n‐alkanes in biodegraded oils indicates recent hydrocarbon migration. Biomarker data (e.g. the presence of oleanane) and compound‐specific isotope data suggest that the Khadum Formation in the lower part of the Maikop Group is the main source rock. However data from Cretaceous and Paleogene organic‐rich rocks, which may also have contributed to the accumulated oils, are urgently needed in order to quantify their possible input.In the Prikumsk Swell, at least two oil families, characterized by low and high C28/C29 sterane ratios respectively, can be distinguished in reservoir rocks of Triassic to Cretaceous age. Most oils are characterized by low C28/C29 sterane ratios and the absence of oleanane (“Group B oils”). These characteristics suggest a pre‐Upper Cretaceous source for the oils, which is also supported by the geological setting. Hierarchical cluster analysis suggests the presence of four sub‐groups (Sub‐Groups B1 to B4). Typically, biomarker ratios in oils in Cretaceous reservoirs are more uniform than those in Triassic and Jurassic reservoirs. Potential source rocks include Lower Triassic deep‐water clayey limestones and shales as well as Middle Jurassic and Aptian‐Albian marine shales. Three oil samples from Triassic and Cretaceous reservoirs form a separate oil family (“Group A”), which is genetically related to oils from the Terek‐Caspian fold‐and‐thrust belt. Group A oils have high C28/C29 sterane ratios and in general contain at least some oleanane. A contribution by Cenozoic source rocks to Group A oils is likely.

CO 2 mineral trapping comparison in different regions: predicted geochemical reactivity of the Precipice Sandstone reservoir and overlying Evergreen Formation

Injected CO 2 streams may have geochemical reactivity to different rock types in a CO 2 storage complex depending on solubility and formation water chemistry. The Precipice Sandstone and Evergreen Formation are a low-salinity reservoir–seal pair in the Surat Basin, Australia, targeted for potential CO 2 storage. The kinetic geochemical CO 2 reactivity of different rock facies from three regions were predicted over 30 and 1000 year time periods. No material CO 2 mineral trapping in the quartz-rich Precipice Sandstone reservoir was predicted, owing to the low rock reactivity. Predicted CO 2 mineral trapping in the Evergreen Formation was more variable due to different amounts of more reactive feldspars, clays, calcite and siderite. Predicted mineral trapping as siderite and ankerite was between 0.03 and 8.4 kg m −3 CO 2 , and mainly depends on chlorite and plagioclase content. Predicted pH was between 5 and 7.5 after 1000 years. Pyrite precipitation was also predicted with SO 2 present in the injectate. QEMSCAN and SEM-EDS (scanning electron microscopy and energy-dispersive spectroscopy) spot imaging of samples from the seal containing natural fractures filled by siderite, pyrite, clays, ankerite, calcite, barite and apatite represent a natural analogue for natural mineral trapping. These are in good agreement with our model predictions. This study suggests that, from a geochemical perspective, the Precipice Sandstone is a suitable storage reservoir, whereas mineral trapping would occur in the overlying Evergreen Formation. Supplementary material: Additional model inputs, characterization and model images, and an excel file of QEMSCAN mineral and porosity components, are available at https://doi.org/10.6084/m9.figshare.c.5395393 Thematic collection: This article is part of the Geoscience for CO 2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage