Organic-rich Rocks Research Articles

Carbon isotopic composition and organic petrography of thermally metamorphosed Antarctic coal: Implications for evaluation of δ13Corg excursions in paleo-atmospheric reconstruction

Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13Corg excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13Corg). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13Corg in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% Rmax). These highly altered coals and cokes can have random vitrinite reflectances (Rr) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with Rr > 2%. Values of δ13Corg range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13Corg and the level of organic maturity, organic carbon (Corg) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13Corg studies.

Distribution of Li, Ga, Nb, and REEs in coal as determined by LA-ICP-MS imaging: A case study from Jungar coalfield, Ordos Basin, China

Previous studies on modes of occurrence of Li, Ga, and Nb in coal are primarily based upon statistical analyses due to the low concentrations of these elements and the lack of methods of direct analysis. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) can be used for the in-situ determination of trace elements in minerals. The strong heterogeneity and diversity of macerals lead to a lack of suitable internal calibration elements required for LA-ICP-MS-based analysis of trace elements in coal, making the determination of trace elements in organic matter in coal challenging. In this study, the previously investigated high-Al and Li-Ga-Nb enriched No. 6 coal in the Haerwusu Mine of the Jungar Coalfield, Ordos Basin, China, was chosen for revealing the distribution and modes of occurrence of Li, Ga, Nb, and rare earth elements (REEs; REY if Y included) in coal, using the LA-ICP-MS method of mapping (imaging) technology. The results indicate that the majority of Ga and Nb present within the coal was concentrated within a specific collotelinite layer. Gallium and Nb have an organic association and are rare within minerals (i.e., clay minerals and boehmite). Lithium is homogeneously distributed throughout the clay minerals in the collodetrinite. Most of the La and Ce in the coal are concentrated in La- and Ce-enriched minerals, including monazite, bastnaesite, and lanthanite. The other REEs are concentrated in the Ga- and Nb-enriched collotelinite, along with small amounts of La and Ce. A similar characterization processing may be used to determine the modes of occurrences of trace elements in other organic-rich rocks or fine-grained sediments.

The deposition of type II-S Jordan oil shale in the context of Late Cretaceous source rock formation in the Eastern Mediterranean realm. Insights from organic and inorganic geochemistry and petrography

This study provides a detailed geochemical and petrographic investigation of 22 Upper Cretaceous Jordan Oil Shale (JOS) samples obtained from various wells distributed across Jordan. The JOS is generally characterized by high amounts of organic matter with total organic carbon (TOC) contents of 10–29 wt%, high carbonate contents of 15–75 wt%, and variable biogenic SiO2 contents of 1–50 wt%. The organic matter is oil-prone, reflected by a clear predominance of liptinite macerals and Rock-Eval Hydrogen Index (HI) values of 608–1078 mg HC/g TOC. In terms of elemental composition, bulk and molecular organic geochemistry and petrology, the JOS is similar to other age-equivalent organic-rich rocks from the Eastern Mediterranean. Primary bioproductivity was probably controlled by a Late Cretaceous upwelling event that affected the whole region. Low Pr/Ph ratios <0.5 indicate that organic matter preservation was not only controlled by the formation of an upwelling-related oxygen minimum zone but also by further oxygen depletion in isolated sub-basins. This setting led to vulcanization of kerogen; proven by a positive correlation between calculated organic sulfur contents and the amount of thiophenes generated during Curie-point pyrolysis. Although the thermal maturity is low (Tmax values < 430°C), significant amounts of hydrocarbons (up to 30 kg HC per ton of rock) are present within these rocks. The present bitumen fraction shows high concentrations of asphaltenes (10–65%) and NSO compounds (30–85%) with higher contributions in samples with the lowest thermal maturities. The relative contributions of aliphatic and aromatic compounds are correspondingly low, but increase with higher thermal maturities. Kinetic parameters suggest that isothermal pyrolysis at 350°C might be already enough to artificially convert 80% of the reactive kerogen within about one day, making the JOS a potential target for oil shale retorting.

Thermal Evolution and Hydrocarbon Generation Capacity of Typical Palaeozoic Marine Source Rocks in the South China Craton: Constrains from Semiclosed Artificial Thermal Maturation Experiments

Abstract Palaeozoic petroleum systems in the South China craton are prospecting frontiers for unconventional oil/gas. Understanding the hydrocarbon generation/retention capacity of source rocks has critical implications for reserve evaluation and targeting, which yet remains ambiguous in this region. In this study, six representative lithological types of organic-rich, immature-low-mature source rocks (two shales, two carbonates, a coal, and a solid bitumen) taken from the South China craton were treated with semiclosed hydrous pyrolysis (300–550°C). Pyrolysis results show that shale and carbonate source rocks have great hydrocarbon generation capacities, and immature solid bitumen can also generate appreciable quantities of hydrocarbons, whereas coal has the least hydrocarbon generation capacity. Both oil and gas are important hydrocarbon products of thermal maturation of shales, whereas gas generation capacity of carbonate source rocks is relatively moderate compared to their oil generation capacity. Therefore, marine shale is an important exploration target for unconventional oil and gas, yet marine carbonate is an important target for unconventional oils. The presence of appreciable carbonate minerals in shales can facilitate oil and gas generation likely by (1) enhancing oil expulsion, (2) facilitating decarboxylation of organic acid, and (3) catalysis. Pyrolysis data also support that molecular composition of thermogenic gas is not dominantly controlled by lithofacies of source rocks but mainly affected by maturation. Outcomes of this study demonstrate that marine organic-rich rocks within the Palaeozoic strata in the South China craton are critical prospecting targets for unconventional oil and gas.

A new vanadium species in black shales: Updated burial pathways and implications

Examining vanadium (V) geochemistry in organic-rich rocks might provide new insights into the redox evolution of paleo ocean and atmosphere through Earth’s history. Previous work, mainly based on experiments and thermodynamic predictions, suggested that V is mainly bound to oxygen (O) atoms and/or ON (nitrogen) groups with oxidation states ranging from +III to +V, when removed from the water column. This conceptual model represented our understanding of the burial processes of V in sedimentary archives. However, until our study, the speciation of V had not been investigated in ancient sediments. For the first time, micro-focused X-ray Absorption Near Edge Structure (µ-XANES) analysis is applied to characterize V speciation in the late Cambrian – Early Ordovician, organic-rich Alum Shale of the Scandinavian region. The result shows the presence of a new V(+IV)–S structure that largely dominates the V speciation (>80%), subordinated by a V(+III)–O structure (<20%) in our samples deposited under euxinic conditions. The V(+IV)–S/∑V ratio seems to be influenced by the intensity of euxinic conditions. We suggest that the new V(+IV)–S structure may have been formed under strongly sulfidic conditions. Organic matter is most likely to be the dominant host phase for this newly identified species. Additionally, we propose an updated model describing the processes involved during the burial of V under a wide range of redox conditions. This model shows that the V speciation has the potential to provide a more nuanced picture of the redox conditions that prevailed at the time of deposition.

Strain partitioning between ductile and brittle stratigraphy

The Sand Wash fault zone is a segmented and discontinuous fault system that strikes northwest to south east in the central part of the Uinta Basin. It is approximately 34 kilometers long with an uncommonly wide damage zone, typically 100 to 200 meters wide. Due to recent, rapid, and large-scale incision by the Green River and its tributaries, the Sand-Wash fault zone is well exposed in several closely spaced canyons. These canyon exposures allow mapping of the lateral relationships through panoramic photographs and surface kinematic descriptions. Most movement on the Sand Wash fault zone occurred in the late Eocene, but minor, more recent movement likely occurred. Evidence for fault timing includes strata-bound, syndepositional movement which occurred during Lake Uinta time (55 to 43 Ma BP) resulting in debris flows, slump blocks, and small (&gt;150 meters diameter) sag basins filled with poorly organized sediments. After lithification, elongate grabens formed with up to 33.5 meters of horizontal extension. Two styles of deformation are present. Brittle rocks, such as sandstone and limestone beds, are intensely fractured and faulted, whereas clay and organic-rich rocks are largely unfractured and unfaulted, with variably folded beds that have experienced some layer-parallel slip. Laterally, deformation is distributed up to 100 meters from the fault core, which is uncommonly large for faults with short lengths and little displacement. Vertically, displacement is concentrated in brittle sandstone and carbonate beds and rare in clay and hydrocarbon-rich units, such as the Mahogany oil-shale zone of the Eocene Green River Formation. The Mahogany oil-shale zone mostly displays ductile flow (granular flow) commonly forming small décollements between overlying and underlying units. Vertical displacement on separate fault segments is generally less than 5 meters and decreases down section, dying out completely around the top of the Mahogany oil-shale zone. In this paper we show evidence for syndepositional deformation along the Sand Wash fault zone, strain partitioning along décollement surfaces, fault surfaces that experience multiple deformational phases, pop-up blocks, and graben development. We also show that deformation on the fault zone is related to extension above a neutral surface of a larger fold. This larger fold is associated with a basement-rooted fault zone that moved during Laramide tectonism as the Uncompahgre uplift developed. The Sand Wash fault zone appears to have many similarities to the larger, and more deeply buried, Duchesne fault zone 25 kilometers to the north, and the more deeply eroded Cedar Ridge fault zone located 30 kilometers to the south. The high-resolution fault model, developed herein, is thus a good proxy for other complex fault zones in the Uinta Basin. Our model will be useful to oil and gas operators as they develop horizontal wells across this and other complex fault zones in the basin.

TOC estimation of shale oil reservoir by combining nuclear magnetic resonance logging and nuclear physics logging

Abstract The evaluation of source rock properties has become a vital step in logging interpretation. Total organic carbon (TOC) content is the key to estimating the quality and hydrocarbon generation potential of source rocks. In the shale oilfield of the Junggar Basin, the conventional method of calculating the TOC of hydrocarbon source rocks cannot satisfy logging evaluation requirements. This paper predominantly deals with a method for the quantitative estimation of TOC in source rocks via nuclear physics and nuclear magnetic resonance (NMR) logs. According to this method, the total hydrogen index of the source rock is the sum of the response of kerogen, clay minerals and fluid, expressed by corrected neutron porosity. The hydrogen index of fluid and clay minerals is indicated by the effective porosity of NMR and the estimated clay content, respectively. To eliminate the hydrogen index of fluid, the effective NMR porosity is subtracted from the corrected neutron porosity. On this basis, a new and overlapping method suitable for clay-rich rocks and oil reservoirs is proposed. This method was developed by overlaying the scaled clay content curve on the hydrogen index curve. In non-source rocks, the two curves regularly overlap. However, in organic-rich rocks the two curves will separate. The separation distance between the two curves was used to estimate TOC continuously. Possessing sound application and benefiting from the measured results of sweet spots, this method provides new insights for TOC quantitative prediction in shale oil reservoirs.

Petrology of the Parvadeh coals, Tabas Basin, Iran

The Tabas Basin is in the Tabas Block in the central part of Iran, and it contains Iran’s largest coal deposits, mostly in the Parvadeh Coalfield. The basin is structurally characterised by regional scale thrust faults and orthogonal strike-slip faults that segregate the coalfields into different areas namely Parvadeh (P) I to IV. The coals under study are in the upper Triassic Nayband Formation. Samples were collected from underground coal mines in PI, PII, PIII, and core from PIV, and prepared for petrographic analysis. The resulting vitrinite reflectance varies between 1.13% and 1.54%, which places the coals into medium volatile bituminous rank. In all samples, the vitrinite content is significantly higher than the inertinite followed by minor amounts of liptinite, mostly secondary exsudatinite. The samples also contain abundant pseudovitrinite, which can be above 40% (volume, mineral matter free basis). The maceral composition and moderate to high mineral matter with common framboidal pyrite and siderite corroborate previous interpretations of the Parvadeh coals forming in topogenous to transitional peatlands in paralic to fluvial palaeoenvironments. However, this study makes additional interpretations of the coalification history of the Parvadeh coals and the occurrence of pseudovitrinite. The coals commonly contain meta-cutinite and associated meta-exsudatinite, and fluorescing exsudatinite that permeates carbargillite lenses, lumina of inertinite macerals, and the slits of the pseudovitrinite. This suggests at least two stages of hydrocarbon generation occurred in the Parvadeh coals: the first phase during entry into the oil window and later devolatilisation to meta- exsudatinite in the early gas window; the second phase post-dates this and could be generated internally from the vitrinite else migrated from surrounding organic-rich rocks in the interburdens of the Nayband Formation. The fact that the pseudovitrinite slits are filled with exsudatinite but not meta-exsudatinite indicates that pseudovitrinite formed just before during partial devolatilisation or contemporaneous with the second generation of exsudatinite. Epigenetic mineralisation includes massive pyrite and dolomite in the fractures, indicative of potential hydrothermal circulation that may also be related to the second phase of hydrocarbon generation as exsudatinite is found trapped in the intracrystalline spaces of the carbonates.

Marine refractory dissolved organic carbon and transgressive black shales

<p indent="0mm">The marine refractory dissolved organic carbon (RDOC) pool, derived from the microbial carbon pump (MCP), holds a tremendous amount of carbon, as much as that in today’s atmosphere; hence, an understanding of this pool is crucial for investigating carbon cycling and climate change at the global scale. The size of the RDOC pool dynamically evolves over time. In the geological past, particularly during the Neoproterozoic to Cambrian transition, the marine RDOC pool might have reached a hundred to a thousand times its present size. However, the patterns of the reduction in the RDOC pool through time and the means by which RDOC was transferred into the geological reservoir have been poorly known to science. Black shales, which are the source rocks of oil and gas reservoirs and are enriched in many rare metals, are economically important sedimentary rocks. Black shale is characterized by the enrichment of clay sediments and organic matter (total organic carbon (TOC) &gt;1%). At least a dozen depositional models have been proposed to interpret various types of black shales based on the depositional environments and three major controls on organic content, i.e., the production and supply of organic matter, the demand and consumption of oxygen, and the rate of sedimentation. Organo-clay interactions and potential organic sources of the bulk RDOC in ancient oceans have not been fully considered. Clay minerals, e.g., smectite and illite, are well known for their capability of adsorption and preservation of dissolved organic matter in significant quantities. Organic compounds adsorbed by clays are present dominantly within interlayer sites and thus are difficult to be decomposed or used by microbes. Accordingly, here, I propose a hypothetical mechanism of RDOC deposition and preservation, which also functions as an alternative model for the origin of transgressive black shales that always abruptly contact the underlying rock unit. The proposed mechanism is outlined as follows: During global-scale sea level rise driven by crustal subsidence or climate change, upwelling currents transport RDOC-rich water bodies from deep oceans to clay-rich shallow sea depositional environments, where clay sediments mix well with the RDOC; the mixture triggers the interplay between clay minerals and organic matter, and thus leads to large amounts of RDOC being absorbed by clay sediments; after deposition, the clay sediments are able to absorb organic matter from pore water; finally, the organic-rich transgressive clay sediments form black shales through diagenesis. Experiments indicate clays are capable of absorbing as much as 12% or more dissolved organic matter, far more than the minimal 1% TOC in black shales. Therefore, it is reasonable to assume that the RDOC is an important organic source in transgressive black shales. The new model provides a potential link between ancient marine RDOC and black shales; hence, it might offer a mechanism for transferring marine RDOC into the geological carbon reservoir and thereby present an additional path of marine carbon cycling. Unlike the previous model that invoked primary production as the major organic source, the present model emphasizes the contribution of RDOC to the organic matter of black shales. However, it does not refute the contributions of any other organic source or any other model for transgressive black shales. To test this hypothesis, future work may include enhanced experimental studies of the interplay between clay minerals and organic compounds under different settings of temperature, pressure, and redox state, establish a set of geological criteria to recognize RDOC from organic-rich rocks, e.g., black shales; and quantitatively model the evolution of RDOC over geological time. Each of these tests would provide critical information for understanding the global carbon cycles of the past, present, and future.

Characterization, Classification, and Evaluation of the Reservoir Pore Structure Features of Lacustrine Fine-Grained Sedimentary Rocks. A Case Study of the Fourth Member of the Shahejie Formation in the Chenguanzhuang Area of the Southern Gently Sloping Zone of the Dongying Depression, Bohai Bay Basin

With the development of unconventional oil and gas exploration “from sea to land,” lacustrine fine-grained sedimentary rocks (FSR) have gradually attracted the attention of scholars and become an important topic in the field of unconventional oil and gas, but the research is still in its initial stage. In this study, lacustrine FSR in the Dongying Depression of the Bohai Bay Basin are used as the research object, and nuclear magnetic resonance (NMR) and quantitative image characterization are used to characterize the pore structure of the reservoir in the study area on multiple scales, analyze the reservoir characteristics control factors, and classify and evaluate the reservoir. The results show that: 1) the favorable petrographic phases of the FSR reservoir can be classified into six types of organic-rich lime mudstone, organic-rich laminoid lime clay rock, organic-rich laminoid clay micritic limestone, organic-bearing banding clay micritic limestone, organic-rich banding lime clay rock, and organic-bearing lumpy clay micritic limestone. With an average porosity of 12.3% and an average permeability of 10.58 mD, the overall reservoir is a typical low-porosity-low-permeability type; 2) the reservoir space types are diverse, with strong microscopic inhomogeneity; pores with a pore size of less than 2 nm almost have no contribution to the reservoir space; the pore volume and pore area are mainly provided by organic matter pores at the 100 nm level, mineral intergranular pores, and clay mineral shrinkage pores/slits. The FSR reservoirs in the study area are classified into three categories, and the pore structure of the reservoirs from categories I to III deteriorates in turn. This study provides a basis for the microscopic characterization, classification, and evaluation of lacustrine FSR reservoirs and their exploration.

Predicting marine organic-rich deposits using forward stratigraphic modelling: The Jurassic Najmah source rock – Case study

Predicting the distribution and heterogeneity of marine Mesozoic organic-rich rocks is a challenging task that requires multi-disciplinary data integration supported by innovative numerical modelling. This study aims at investigating the factors controlling marine organic matter production, accumulation, and preservation along the eastern Arabian intrashelf basins. Such types of basins exhibit an enhanced lateral and vertical facies variability caused by the interplay between shallow and deeper water sedimentation. The work focuses on characterizing the Mid-Upper Jurassic Najmah Formation that presents source rock, reservoir, as well as seal characteristics along Kuwait. It comprises high maturity levels (>0.6% Ro) and TOC values ranging between 2 and 30%. Forward stratigraphic modelling that includes organic matter production, transport, and preservation (i.e., degradation and dilution) was utilized. Models corresponding to two end-member basin types (restricted and fully open marine) have been simulated to mimic the organic matter deposition within a 3rd-order sequence stratigraphic framework. Simulation outputs provide a qualitative and quantitative assessment of organic matter characteristics (TOC, HI, and OI) at the time of deposition. Results show that (1) a combined variable eustatic sea level change and tectonic subsidence are the main driving mechanisms for sediment accommodation during the Mid-Upper Jurassic time, (2) strong anoxic conditions in the restricted basin simulations promote the preservation of organic-rich deposits in the distal northern sectors while (3) the influence of open marine bottom current oxygenation is not expected to impact the relatively shallower conditions affecting the intrashelf basin (<200 m depth), (4) organic matter dilution can be observed along the southernmost sectors of Kuwait due to high sedimentation rates related to the development of gravity driven deposits. Sensitivity analysis on input parameters (i.e., primary productivity vs water depth) supports the partially restricted basin conditions with occasional connection to the open ocean during eustatic sea level rise. This study contributes to the improvement of basin models in conventional as well as unconventional petroleum system exploration.

Introduction to this special section: Unconventionals

Unconventional reservoirs are made up of organic-rich source rocks in which the organic matter has matured thermally but the hydrocarbons formed during maturation have not migrated. Hydrocarbons trapped in tight source rocks are challenging to extract, requiring nontraditional methods such as directional drilling and reservoir stimulation via hydraulic fracturing. This special section is dedicated to various geophysical aspects of unconventionals.

No effect of thermal maturity on the Mo, U, Cd, and Zn isotope compositions of Lower Jurassic organic-rich sediments

AbstractThe isotope ratios of redox-sensitive metals in organic-rich rocks are critical tools for quantifying the timing and severity of deoxygenation and nutrient cycling in Earth's past. The resilience of isotopic data to thermal alteration of the host sediments over millions of years of burial is, however, largely unknown. We present molybdenum, uranium, cadmium, and zinc stable-isotope data from two stratigraphic successions of the same Lower Jurassic Posidonienschiefer unit in the Lower Saxony Basin of northern Germany that were affected by different burial histories. We show that thermal maturity had no effect on the isotopic compositions of these elements but does appear to have increased their concentrations in the rock. The data corroborate the results of laboratory-based maturation studies and provide constraints on the Mo, U, Cd, and Zn isotopic compositions of ca. 182 Ma seawater in the Lower Saxony Basin.

Sequence Stratigraphy of Organic-Rich Rocks in the Niger Delta

Sequence Stratigraphy of Organic-Rich Rocks in the Niger Delta

Depositional Heterogeneities and Brittleness of Mudstone Lithofacies in the Marcellus Subgroup, Appalachian Basin, New York, U.S.A.

Organic-rich rocks of the Marcellus subgroup in the study area consist of a diverse suite of mudstone lithofacies that were deposited in distinct facies belts. Lithofacies in the succession range in composition from argillaceous to siliceous, calcareous, and carbonaceous mudstone. Heterogeneities in the succession occurs in the form of varying mineralogical composition, slightly bioturbated to highly bioturbated chaotic matrix, organic-rich and organic-lean laminae, scattered fossil shells in the matrix, and fossils acting as lamination planes. Lithofacies were deposited in three facies belts from the proximal to the distal zone of the depositional system. Bedded siliceous mudstone (BSM) facies occur in the proximal facies belt and consists of a high quartz content in addition to clay minerals and pyrite. In the medial part of the facies belt lies the laminated argillaceous mudstone (LAM), bedded calcareous mudstone (BCaM), and bedded carbonaceous mudstone (BCM). The size of detrital mineral grains in the lithofacies of the medial facies belt is larger than bedded argillaceous mudstone (BAM) of the distal facies belt, characterized by clay-rich matrix with occasional fossil shells and horizontally aligned fossils. Two types of horizontal traces and one type of fecal string characterize the proximal mud-stone facies, whereas only single horizontal trace fossil is found in the mudstones of the medial and distal facies belt. Parallel alignment of fossil shells and fossil lags in lithofacies indicate that bed-load transport was active periodically from the proximal source of the depositional system. Bioturbation has heavily affected all of the lithofacies and presence of mottled burrows as well as Devonian fauna indicate that oxic to dysoxic conditions prevailed during deposition. The deposition of this organic-rich mudstone succession through dynamic processes in an overall oxic to dysoxic environment is different from conventional anoxic depositional models interpreted for most of the organic rich black shales worldwide. Total organic content (TOC) varies from top to bottom in the succession and is highest in BCM facies. The brittleness index, calculated on the basis of mineralogy, allowed classification of the lithofacies into three distinct zones, i.e., a brittle zone, a less brittle zone, and a ductile zone with a general proximal to distal decrease in the brittle behavior due to a decrease in the size of the sediments.

Application of nanoindentation creep tests to characterize viscoelasticity behavior of deep Longmaxi shales

The development of deep shale gas reservoirs has become economically and commercially viable through novel horizontal well completion and multistage hydraulic fracturing technologies in recent years. In the Sichuan Basin, the deep Longmaxi shale is a major target formation that has attracted increasing interest. Shale is generally characterized as a sedimentary rock with fine grains, high clay, and organic content, low permeability and porosity, and high heterogeneity and anisotropy. Geomechanical characterization of these organic-rich rocks usually includes an estimation of the elastic modulus, strength properties, hysteresis behavior, and creep deformation. Understanding and characterizing the viscoelastic behaviors of shale is essential because they affect the elastic and fracture properties of rock and cause a series of drilling problems related to the wellbore stability, such as borehole shrinkage, pipe sticking, and casing collapse. The viscoelastic behavior of specimens retrieved from a deep well in the Sichuan Basin was examined using nanoindentation creep tests. Nanoindentation is a technique based on a hard tip, which is pressed into the surface of a sample. The advantage of this test is the use of a small sample volume, such as drilling cuttings or fragments, which are obtained easily during drilling. The results indicated that the creep modulus parallel to bedding and normal to bedding is similar, while the creep modulus of drilling cuttings is the lowest. A linear relationship was both observed between the creep modulus and hardness, Young’s modulus for deep Longmaxi shale. The Burgers creep model showed good agreement with the experimental data. The creep behavior of Longmaxi shale exhibited anisotropy, and the effect of drilling fluids could aggravate the creep displacement. These results have elucidated the creep properties in engineering problems of shale gas formation, drilling, and stimulation.

The Neglected Involvement of Organic Matter in Forming Large and Rich Hydrothermal Orogenic Gold Deposits

Orogenic gold deposits have provided most of gold to humanity. These deposits were formed by fluids carrying dissolved gold at temperatures of 200–500 °C and at crustal depths of 4–12 km. The model involves gold mobilization as HS− complexes in aqueous solution buffered by CO2, with gold precipitation following changes in pH, redox activity (fO2), or H2S activity. In this contribution, the involvement of carbonaceous organic matter is addressed by considering the formation of large and/or rich orogenic gold deposits in three stages: the source of gold, its solubilization, and its precipitation. First, gold accumulates in nodular pyrite within carbonaceous-rich sedimentary rocks formed by bacterial reduction of sulfates in seawater in black shales. Second, gold can be transported as hydrocarbon-metal complexes and colloidal gold nanoparticles for which the hydrocarbons can be generated from the thermal maturation of gold-bearing black shales or from abiotic origin. The capacity of hydrocarbons for solubilizing gold is greater than those of aqueous fluids. Third, gold can be precipitated efficiently with graphite derived from fluids containing hydrocarbons or by reducing organic-rich rocks. Black shales are thus a key component in the formation of large and rich orogenic gold deposits from the standpoints of source, transport, and precipitation. Unusual CO2-rich, H2O-poor fluids are documented for some of the largest and richest orogenic gold deposits, regardless of their age. These fluids are interpreted to result from chemical reactions involving hydrocarbon degradation, hence supporting the fundamental role of organic matter in forming exceptional orogenic gold deposits.

Predicting S-Wave Velocity from Wire-Line Logs for Organic-Rich Rocks

Predicting S-Wave Velocity from Wire-Line Logs for Organic-Rich Rocks

The Structure of Cambrian Deposits of the Vilyui Hemisyneclise, Based on an Integrated Analysis of Drilling and Seismic Data

Abstract —We present a model of the stratigraphic and lateral distribution of Cambrian deposits in the Vilyui hemisyneclise, based on an analysis of drilling data and interpretation of seismic data. The study shows a series of formations and sequences penetrated by wells (Syugdzher saddle, Khorgochum monocline, Ygyatta depression, Tyukyan–Chybyda monocline, Arbai–Sinyaya megaswell, etc.). In the areas where the Cambrian was not penetrated by wells, the distribution of Cambrian deposit was inferred based on the available seismic data. The distribution of the Kuonamka Horizon formed by Cambrian organic-rich rocks is characterized in detail. These are the Kumakh and Sinyaya–Kutorgina sequences and the Inikan and Kuonamka formations. It has been found that the Kuonamka Horizon was deposited during two stages, Botomian and Toyonian–early Mayan. The horizon is overlain by younger deposits of the Mayan Stage, with characteristic cross-bedding structures. Schemes of facies zoning of the Cambrian for Botomian, Toyonian–early Mayan, and middle Mayan times were constructed based on the most recent geological and geophysical understanding of the Vilyui hemisyneclise.

Kerogen composition and origin, oil and gas generation potential of the Berriasian Wealden Shales of the Lower Saxony Basin

This study evaluates the organofacies and petroleum generation potential of kerogen of Early Cretaceous Wealden Shales in the Lower Saxony Basin, Germany. Methods applied include open-system-and Curie-point-pyrolysis, attenuated total reflectance infrared spectroscopy, x-ray diffraction and x-ray fluorescence spectroscopy. The geochemical results give a comprehensive elemental characterization of the Wealden source rocks and indicate an anoxic to oxygen-depleted lacustrine depositional setting in a rather warm climate. The kerogen composition is characterized by high abundance of aliphatic compounds with many alkylated low molecular weight oxygen-bearing compounds, but only a minor aromatic contribution. Differences between organic matter-rich lamalginite facies A and Botryococcus-bearing, less organic matter-rich facies B occur. For both facies, the total hydrocarbon generation potential (Rock-Eval HI) of facies A is higher than that of facies B. TOC-normalized cumulative hydrocarbon gas yields generated during pyrolysis-GC are similar for both facies and account for only 8–16% of the total hydrocarbon generation potential. In contrast, the cumulative yield of molecular hydrogen generated during open-system pyrolysis of facies B is twice as high than for facies A. Overall Wealden Shales generate 55 mg HC per g rock on average, they generate roughly 6 mg H2 per g TOC. Although these H2 yields are rather low compared to other organic-rich rocks or coals with aromatic kerogen structures, this study shows that detecting H2 during open-system-pyrolysis is important for the understanding of kerogen transformation processes during pyrolysis in both natural and laboratory settings.