Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4–0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30–2.30 m3/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07–0.93 m3/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ13C1 and a decrease in the value of δ13C1-δ13C2. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales.

https://www.mdpi.com/2075-163X/9/7/428/pdf

Pyrite Morphology as an Indicator of Paleoredox Conditions and Shale Gas Content of the Longmaxi and Wufeng Shales in the Middle Yangtze Area, South China

Unconventional petroleum sedimentology: A key to understanding unconventional hydrocarbon accumulation

Depositional environment of shale in Wufeng and Longmaxi Formations, Sichuan Basin

Carboniferous organic-rich mudrocks are proven source rocks for conventional gas systems in northwestern Europe and have sourced most of the gas found in the Rotliegend sandstones (di Primio et al., 2008). They are therefore considered as one of the stratigraphic horizons in the North German Basin that potentially could host unconventional gas occurrences. Recent gas-in-place (GIP) estimates of the German Federal Institute for Geosciences and Natural Resources (BGR) attribute 3.1–13.9 Bcm of shale gas to fine-grained sediments in depths between 1,000–5,000 m of which ∼ 40 % are assigned to carbonaceous mudrocks of the Lower Carboniferous (Ladage et al., 2016). The research and exploration activities on Carboniferous sediments in Germany in the last decades led to an overall good understanding of the formation and the collection of vast amount of data. However, despite the numerous boreholes that have been drilled to explore for conventional hydrocarbons in these successions, the potential for shale gas resources remains unassessed. In this integrated study, the potential for shale gas occurrences in Lower and Early Upper Carboniferous sediments is evaluated based on data compiled from different sources. The aim of this study is to elucidate the interrelationship of depositional features, rock characteristics and diagenetic influences on the governing processes for shale gas occurrences by incorporating: • a comprehensive literature database that emerged from intense research on various aspects of petroleum systems in general and unconventional shale gas systems in particular, • field data, e.g., from well reports, and • new laboratory measurements. The results of the initial shale gas and reservoir quality assessment conducted through comparison with successful U.S. shale gas system parameters and the detailed geochemical, petrographic and petrophysical measurements reveal, that heterogeneity in the investigated sample set is manifold. In general it seems, that variations too small to be resolved by the methodology applied can make a

Sedimentology and diagenesis of lower and upper carboniferous sediments in the North German Basin

This paper is a study of grain provenance, depositional processes, and the relationship of those processes to composition and diagenesis of fine-grained rocks, based on high-resolution petrography. The marly successions deposited in siliciclastic flysch and related to the paleotectonics of the northern domain of the Tethys Ocean (Outer Carpathian Basin) were examined. The following marls were studied: Upper Jurassic marls from the Vendryně Formation (Goleszow Marls); Upper Cretaceous Siliceous-Fucoid, Wegierka, Weglowka, Frydek, Jasienica, and Żegocina Marls; Eocene Łącko, Budzow, Zembrzyce, and Leluchow Marls; and Oligocene Sub-Cergowa and Grybow Marls. These marls are products of the redeposition of shelf sediments into deeper parts of the basin. Knowledge of their diagenetic history can contribute to the evaluation of the petroleum potential of the Polish Outer Carpathians. Thin-section analysis by optical and electron back-scattered microscopy (FESEM-BSE) highlights the similarity of all these marls with respect to composition, grain size, sedimentary features, and diagenetic changes. They differ only in the quantitative proportion of constituents, the relative extent of key microfacies, and diagenetic effects. The diagenetic modifications recognized in the rocks studied are characteristic of the processes operating in the sedimentary environment and during burial diagenesis. Thin-section petrography of the marls reveals fine, mostly silt-size siliciclastic material and bioclasts supported by matrix. Siliciclastic particles comprise mostly quartz, micas, feldspars, glauconitic nodules, and lithoclasts. Calcareous and siliceous bioclasts, mostly microfossils (foram and sponge spicules, and a few radiolarians and diatoms) and coccospheres, are variably preserved; they may occur as calcite, silica, and clay pseudomorphs. Additional components include peloids and stringers of brown organic matter. Millimeter-scale fabric analysis provides detailed insight into the processes of transport and deposition. Five microfacies are identified: parallel-laminated, wavy and lenticular-laminated, homogeneous exhibiting preferred orientation of elongated components, bioturbated, and bioclast-bearing. The range of microfacies suggests that the marl sediment was deposited while currents were active. The depositional settings identified range from traction, reworking by bottom currents, supply by nepheloid flow (indistinctly bounded laminae), possible eolian input (laminae a few grains thick), and biogenic reworking of sediments. Back-scattered electron images reveal that the matrix consists of coccolithophorid remains with varying abundances of clay, silica, and carbonate cements. Five varieties of matrix are distinguished: 1) carbonate–clay–silica bearing, 2) carbonate–clay supported, 3) carbonate–silica supported, 4) clay–silica supported, and 5) clay dominated. Clay appears as rosette and lamellar aggregates, silica as nanoquartz (microspheres, clusters, coatings), quartz overgrowths, and occasionally opal CT as lepispheres and aggregates moss-like in appearance, and calcite as overgrown coccoliths, pseudomicrite, micrite, and microspar. Pyrite framboids are present in some samples. Intrabiotic and intergranular cements are developed. Also present are partially dissolved coccolith placoliths, silica- and clay-replaced micarb, and grains similar in appearance to volcanic glass shards that now consist of calcite and quartz. Bioclasts may have been a source for calcite and silica cements. Volcanic glass may have been a source for authigenic silica and clay minerals; this is suggested by the close spatial juxtaposition of silica microspheres and authigenic clay (cauliflower-like aggregates). The authigenic origin of the clay in these marls is also suggested by the presence of overgrown coccoliths because recrystallization of shields is reduced when clay is present, so that the overgrowths must have been formed in an environment where clay minerals were scarce initially. Despite diagenetic changes, the presence of pyroclasts is still recognizable. There are quartz crystal chips, shard-shaped grains, volcanic rock fragments, and minerals (biotite, apatite, zircon). The marls studied could be described as impure fine-grained detrital limestones chalky in type, with evidence of redeposition and possible volcanic-ash input. The marl depositional system within the Outer Carpathian Basin was associated with tectonic movements which initiated chalk-type facies, and this was particularly widespread in the Upper Cretaceous and Paleocene.

Insights Into Marls From Optical and Back-Scattered Electron Petrography: An Example From the Outer Carpathians (Poland)

Carbonate-rich lithologies of the gas-producing Upper Mississippian Barnett Shale, Fort Worth Basin, Texas, are diverse and include lithologies with carbonate components that are primarily authigenic, as well as those that have carbonate components dominated by skeletal debris and other allochems such as peloids and intraclasts. Compositionally, carbonate-bearing lithologies of the Barnett Shale (including the informal unit known as the Forestburg Limestone) can be viewed as mixtures of authigenic or allochemical carbonate and siliciclastic sediment derived mostly from outside the basin. With the exception of the Forestburg Limestone, these varied carbonate lithologies dominate only in local zones, at the scale of a hand specimen or thin section, and do not constitute a volumetrically significant part of the gas-producing reservoir rock. Carbonate lithologies are significant, however, for clues they provide on environmental and early diagenetic conditions during accumulation of the Barnett Shale. Carbonate lithologies dominated by skeletal components contain distinct and impoverished marine faunas that are consistent with low oxygenation levels. The generally early timing of carbonate cement precipitation is supported by the reworking of diagenetic carbonate as silt- to sand-size intraclasts, sediment infilling of fractures in cemented beds and concretions, displacive fabrics, and highly random orientations of phyllosilicate grains within carbonate units. In some cases, detrital allochemical carbonates provided nucleation substrates for precipitation of highly displacive authigenic carbonate that was extensively reworked into microspar-size sediment particles. The elemental and isotopic chemistries of authigenic carbonates are consistent with near-sea-floor authigenesis driven by microbial cycling of organic carbon into carbonate minerals under generally reducing and low-temperature conditions.

Carbonate Lithologies of the Mississippian Barnett Shale, Fort Worth Basin, Texas

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Carbonate Lithologies of the Mississippian Barnett Shale, Fort Worth Basin, Texas