The standard petrography test method for measuring air voids in concrete (ASTM C457) requires a meticulous and long examination of sample phase composition under a stereomicroscope. The high ...expertise and specialized equipment discourage this test for routine concrete quality control. Though the task can be alleviated with the aid of color-based image segmentation, additional surface color treatment is required. Recently, deep learning algorithms using convolutional neural networks (CNN) have achieved unprecedented segmentation performance on image testing benchmarks. In this study, we investigated the feasibility of using CNN to conduct concrete segmentation without the use of color treatment. The CNN demonstrated a strong potential to process a wide range of concretes, including those not involved in model training. The experimental results showed that CNN outperforms the color-based segmentation by a considerable margin, and has comparable accuracy to human experts. Furthermore, the segmentation time is reduced to mere seconds.
Organic matter (OM)-hosted pores are important constituents of the pore system of black shales and play a crucial role in determining their methane adsorption capacity and porosity. OM-hosted pores ...are generally observed and described with scanning electron microscope (SEM) on Ar ion-milled surfaces. However, SEM imaging is not able to reliably distinguish OM types and relate the observed pores to specific macerals. Partly because of this inability to relate organic pores to macerals, the evolution of organic porosity during thermal maturation remains poorly understood.
In this paper, we review the petrographic characteristics of dispersed organic matter (DOM) in black shales under the SEM. Organic petrographic classification of DOM developed for reflected-light microscopy is so far the most practical method when describing DOM in black shales under the SEM because this classification has information on the origin of DOM. Therefore, correlative microscopy (combination of reflected-light and electron microscopy) is the most effective method to identify both OM types and OM-hosted pores. This method, however, is not readily available to most researchers. Although identifying OM on the basis of SEM observations is a challenging task, it is achievable provided there is a good understanding of the studied shales, especially their thermal maturity and original OM composition. Therefore, the overall objective of this paper is to review petrographic characteristics of DOM in black shales under the SEM to provide some guidelines for identifying DOM from SEM observations.
We also review factors that control the formation and preservation of OM-hosted pores. OM-hosted pores consist of primary and secondary organic pores. Primary organic pores are pores inherited from the biological structure of the original OM. Secondary organic pores develop during hydrocarbon generation and expulsion from oil-prone OM and are hosted by solid bitumen or pyrobitumen. The development of secondary organic pores is controlled by thermal maturity and OM type, and their preservation is subject to thermal maturity, OM content, and mineralogical composition.
The presented view of the evolution of micropore and mesopore characteristics of OM with thermal maturity is based on data from the literature. The specific surface area and pore volume of OM in black shales follow parabolic patterns with increasing thermal maturity (quantified via vitrinite reflectance, Ro). The initial increase reflects development of OM-hosted pores, and the subsequent decrease is due to denser stacking of aromatic units in the macromolecular structure of OM, with maximum values (specific surface area ~ 300 m2/g and pore volume ~ 0.3 cm3/g) reached at Ro values in the 2.5–3.5% range. The contribution of OM-hosted pores to the pore characteristics of black shales depends on OM content, OM type, and thermal maturity.
•Petrographic characteristics of dispersed OM in black shales under the SEM were reviewed.•Factors controlling the formation and preservation of OM-hosted pores were discussed.•Differentiating OM type under the SEM is challenging, but achievable.•Specific surface area and pore volume of OM follow parabolic patterns with increasing thermal maturity.
•The Huronian hosts the earliest appearance of terrestrial red beds at ∼2.31 Ga.•Petrography shows detrital quartz has hematite dust rims encased by quartz cement.•Fe(III)-(oxy)(hydr)oxides formed in ...oxidizing, meteoric conditions prior to burial.•Reddening coincides with loss of S mass-independent fractionation in the Transvaal.•Bolstered evidence for red beds reflecting development of an oxidizing atmosphere.
Reconstructing the trajectory of Earth’s initial rise of atmospheric oxygen (i.e., the Great Oxidation Event, GOE) remains a significant but important challenge due to the intricate connections between oxygen and life. Further refinement in our understanding of the GOE requires establishing tighter links between geochemical and mineralogical oxygenation proxies specifically in terrestrial environments where signals reflect oxygen accumulation beyond realms of localized production. The appearance of terrestrial red beds in the Paleoproterozoic rock record is oft-cited evidence for the GOE; however, there is a lack of robust evidence that establishes Fe(III)-(oxy)(hydr)oxides (now hematite) as a primary clastic sedimentary feature, and often insufficient stratigraphic and geochronological context to link red beds to other oxygenation proxies. This study revisits the transition from the youngest detrital pyrite- and uraninite-hosting terrestrial (alluvial-fluvial) strata to the oldest reddened fluvial strata in the ca. 2.45–2.22 Ga Huronian Supergroup, with the aim to directly link the mineralogy of the latter deposits to environmental oxygenation and thus the GOE. Key fluvial sandstone units preserve hematite as rims of “dust” on detrital quartz encased by epitaxial quartz overgrowth cements, providing unequivocal evidence for Fe(III)-(oxy)(hydr)oxide adhesion to detrital quartz during early meteoric diagenesis, and thus indicating terrestrial Fe oxidation pathways were more widespread than oxidized paleosols formed at this time. Geochronological constraints place the appearance of these terrestrial red beds at ∼2.31 Ga, timing that closely matches with the S-isotope evidence for the GOE in correlative strata of the Transvaal Supergroup. The S-isotope and red bed proxy records show promise for a closely coupled oxygenation threshold, with the advantage that they are typically preserved in different depositional environments.
To better understand evolution of oil-prone sedimentary organic matter to petroleum and expulsion from source rock, we evaluated organic petrographic features of Leonardian Wolfcamp A repetitive ...siliceous and calcareous mudrock and fine-grained carbonate lithofacies cycles occurring in the R. Ricker #1 core from Reagan County, Midland Basin, Texas. The objectives of the petrographic investigation were to estimate thermal maturity, identify organic matter types and abundances, and identify the presence or absence of migrated hydrocarbons in organic-lean carbonate layers. An integrated analytical program included geochemical screening total organic carbon (TOC) content by LECO, programmed pyrolysis by hydrocarbon analyzer with kinetics (HAWK) including analysis of solvent-extracted samples, X-ray diffraction mineralogy, organic petrography, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) including correlative light and electron microscopy (CLEM), and micro-Fourier transform infrared spectroscopy (μ-FTIR) analyses of solid bitumen. The data indicate all samples are early to middle oil window thermal maturity with solid bitumen reflectance (BRo) values of 0.55–0.86% and Tmax of 440–455 °C. Organic matter is predominantly solid bitumen (as identified by optical microscopy) in all lithofacies with minor contributions from inertinite. Solid bitumen abundance decreases from siliceous mudrock (TOC >3.0 wt%) to calcareous mudrock (TOC 1.0 to 3.0 wt%) to fine-grained carbonate (TOC <1.0 wt%) lithofacies. Interpretations of petrographic data suggest siliceous and calcareous mudrocks are source rock lithofacies and contain solid bitumen (with petroleum generation potential) that is residual (what remains) from conversion of an original Type II sedimentary organic matter. In turn, fine-grained carbonates are interpreted as reservoir lithofacies which contained little or no original oil-prone sedimentary organic matter and at present-day contain only a minor component of migrated solid petroleum sourced from adjacent siliceous and calcareous mudrock lithofacies. This work helps to document petroleum generation and migration processes, improve unconventional reservoir characterization and better define areas of oil window thermal maturity in an area critical to United States hydrocarbon production.
•Organic petrography of Wolfcamp A mudrocks and carbonates from Midland Basin was studied.•Thermal maturity is early to middle oil window.•Organic matter is predominantly solid bitumen in all lithofacies with minor contributions from inertinite.•Solid bitumen is residual from thermal conversion of oil-prone sedimentary organic matter.•Mudrocks are source rock lithofacies whereas carbonates are reservoir lithofacies.
Petrographic analysis is based on the microscopic description and classification of rocks and is a crucial technique for sedimentary and diagenetic studies. When compared to hand specimens, thin ...sections provide better and more accurate means for analysis of mineral proportion, distribution, texture, pore space analysis, and cement composition. Most petrographic analysis relies on visual inspection of rock thin sections under a microscope, a task that is laborious even for experienced geologists. Large projects with a tight time frame requiring the analysis of a large amount of thin sections may require multiple petrographers, thereby risking the introduction of inconsistency in the analysis. To address this challenge, we explore the use of deep convolutional neural networks (CNN) as a tool for acceleration and automatization of microfacies classification. We make use of transfer learning based on robust and reliable CNN models trained with a large amount of non-geological images. With a relatively small number of labeled thin sections used in “fine-tuning” training we are able to adapt CNN models that achieve low error levels (<5%) for the classification of microfacies from the same dataset, and moderate results (<40%) for the classification of microfacies of thin sections from different datasets. These alternate datasets differ from the training data on two independent factors: the thin sections are from different formations and are prepared by different laboratories. While becoming widely accepted as a useful tool in the biological and manufacturing disciplines, CNN is currently underutilized in the geoscience community; we foresee an increase of use of such techniques to help accelerate and quantify a wide variety of geological tasks.
•Workflow for microfacies classification with convolutional neural networks.•Methodology can be used to quickly organize large amounts of thin section images.•High accuracy (>95%) for several tests performed (using data from same source).•Accuracy decreases when model is used to classify data processed by different labs.
Salt giants are large-scale, basin-wide deposits formed sporadically in the geological past, from the early Paleozoic to the late Cenozoic. Their role as sinks for seawater dissolved ions is well ...known, however the biogeochemical conditions that accompany salt giant formation and their effects on carbon cycling remain poorly constrained. Here we show that massive gypsum deposits of the Mediterranean salt giant – the youngest salt giant on Earth – formed in a particularly dynamic biogeochemical environment controlled by orbitally-driven climate oscillations at the precessional scale. Using multiple sulfur isotopes combined with a steady-state sulfur cycle model, we show that, prior to gypsum precipitation, more than 80% of its constituting sulfate was first microbially reduced into sulfide, possibly stored as elemental sulfur, and then almost completely microbially reoxidized back to sulfate. This “cryptic” sulfur cycling contemporaneous to gypsum precipitation implies both negligible net sulfate consumption and sulfide production, despite a significant benthic flux of organic carbon remineralized through microbial sulfate reduction. This is the first known evidence of cryptic sulfur cycling in the geological past.
The magnetic susceptibility is a reflection of minerals like the iron oxide that affect the pore system. This work applied the magnetic susceptibility as an effective indicator of reservoir quality. ...A total of (38) sandstone samples were collected from the Nubia Group exposed in Northeast Aswan, representing the Sabaya, Abu Agag and Um Bramil Formations. The samples were petrophysically examined in terms of porosity, grain density, bulk density, permeability and magnetic susceptibility (κ). Reservoir quality index (RQI) and distribution of the sample within the Global Hydraulic Elements (GHEs) were determined. Several relations constructed between (κ) as a function of iron oxides and the measured properties. The results showed Sabaya and Abu Agag samples are characterized by the highest average values of κ, (287 & 219SI) respectively, matched with the lowest average values of porosity, permeability, RQI and the highest average values of grain & bulk density, hence most samples lie in the lower quality zones (GHE-1, 2, 3 and 4). On the contrary, the lowest average value of (κ) is (37.9 SI) for Um Bramil samples, matched with the highest average values of porosity, permeability, RQI and the lowest average values of grain & bulk density, hence most samples lie in the higher quality zones (GHE-6, 7 and 8). Petrographic description showed the tight and wide pore system relation to the magnetic susceptibility values. The novelty here is utilizing the magnetic susceptibility as an effective pointer for the reservoir quality expectation and classification into hydraulic units.
Abstract
Forty-five years after the Apollo and Luna missions returned lunar samples, China's Chang’E-5 (CE-5) mission collected new samples from the mid-latitude region in the northeastern Oceanus ...Procellarum of the Moon. Our study shows that 95% of CE-5 lunar soil sizes are found to be within the range of 1.40–9.35 μm, while 95% of the soils by mass are within the size range of 4.84–432.27 μm. The bulk density, true density and specific surface area of CE-5 soils are 1.2387 g/cm3, 3.1952 g/cm3 and 0.56 m2/g, respectively. Fragments from the CE-5 regolith are classified into igneous clasts (mostly basalt), agglutinate and glass. A few breccias were also found. The minerals and compositions of CE-5 soils are consistent with mare basalts and can be classified as low-Ti/low-Al/low-K type with lower rare-earth-element contents than materials rich in potassium, rare earth element and phosphorus. CE-5 soils have high FeO and low Mg index, which could represent a new class of basalt.
The CE-5 sample is consistent with weathered mare basalts in mineralogy and petrochemistry, and is classified as low-Ti/low-Al/low-K type with lower REE (rare earth element) contents than KREEP (potassium, rare earth element, and phosphorus). This new sample characterized by high FeO and low Mg index could represent a new lunar basalt.
We here review what is known about the dunefields and fluvial systems of the Kalahari Basin in terms of geological setting and Quaternary dynamics and set out what has been hypothesized about the ...provenance of Kalahari sand so far. Previous work has tackled this problem by applying a limited number of techniques (mostly sediment textures and heavy minerals) to parts of the large dryland. The generally highly quartzose mineralogy of aeolian dunes and their compositional variability have been only broadly evaluated and several sedimentological issues have thus remained controversial, including the relative role played by fluvial processes versus aeolian reworking of older sediments and weathering controls. This reveals a need for a systematic provenance study that considers the entire basin. For this reason, here we combine original petrographic, heavy-mineral, and detrital-zircon geochronology data with previously published clay-mineral, geochemical, and geochronological information to present the first comprehensive provenance study of the vast Kalahari sand sea.
Our multi-proxy dataset comprises 100 samples, collected across the Kalahari Basin from 11°S (NW Zambia) to 28°S (NW South Africa) and from 15°E (Angola) to 28°30′W (Zimbabwe). Kalahari aeolian-dune sand mostly consists of monocrystalline quartz associated with durable heavy minerals and thus drastically differs from coastal dunefields of Namibia and Angola, which are notably richer in feldspar, lithic grains, and chemically labile clinopyroxene. The western Kalahari dunefield of southeastern Namibia is distinguished by its quartz-rich feldspatho-quartzose sand, indicating partly first-cycle provenance from the Damara Belt and Mesoproterozoic terranes. Within the basin, supply from Proterozoic outcrops is documented locally. Composition varies notably at the western and eastern edges of the sand sea, reflecting partly first-cycle fluvial supply from crystalline basements of Cambrian to Archean age in central Namibia and western Zimbabwe. Basaltic detritus from Jurassic Karoo lavas is dominant in aeolian dunes near Victoria Falls.
Bulk-sediment petrography and geochemistry of northern and central Kalahari pure quartzose sand, together with heavy-mineral and clay-mineral assemblages, indicate extensive recycling via aeolian and ephemeral-fluvial processes in arid climate of sediment strongly weathered during previous humid climatic stages in subequatorial Africa. Distilled homogenized composition of aeolian-dune sand thus reverberates the echo of paleo-weathering passed on to the present landscape through multiple episodes of fluvial and aeolian recycling.
Intracratonic sag basins such as the Kalahari contain vast amounts of quartz-rich polycyclic sand that may be tapped by rivers eroding backwards during rejuvenation stages associated with rift propagation. Such an event may considerably increase the sediment flux to the ocean, fostering the progradation of river-fed continental-embankments, as documented by augmented accumulation rates coupled with upward increasing mineralogical durability in the post-Tortonian subsurface succession of the Zambezi Delta.
The Central Kalahari is not a true desert. It has none of the naked, shifting sand dunes that typify the Sahara and other great deserts of the world. In some years the rains may exceed twenty — once even forty — inches, awakening a magic green paradise.” Mark Owens, Cry of the Kalahari.
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