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Organic matter (OM) type critically controls the hydrocarbon generation potential and organic pore development in black shales. However, maceral variation in lacustrine shales and its control on hydrocarbon generation potential and organic pore development are not yet well understood. In this study, 15 Chang 7 Member shale samples of the Yanchang Formation, Ordos Basin, were investigated with organic petrography, Rock-Eval pyrolysis, and a scanning electron microscope to study the maceral composition, hydrocarbon generation potential, and organic pores in this black shale succession. The results show that the studied shales are in the oil window (Ro~0.70%). OM belongs to Type I and Type III kerogen, as demonstrated by Rock-Eval pyrolysis. Macerals in the Chang 7 Member shales are composed of amorphous OM, alginite, sporinite, liptodetrinite, vitrinite, inertinite, and solid bitumen. Amorphous OM and alginite are major hydrocarbon-generating macerals, and their content determines the hydrocarbon potential of shales. Secondary organic pores were not observed in the studied Chang 7 Member shales due to either a low thermal maturity or a dominance of terrigenous OM. Maceral variation can affect the reliability of using Rock-Eval Tmax as a thermal maturity indicator. This study provides important insights into maceral control on hydrocarbon generation and organic pore development in black shales, calling for a critical evaluation of OM in black shale successions with organic petrography. [ABSTRACT FROM AUTHOR]

The pore structure characteristics of shales are controlled by their mineral and organic matter compositions. However, the contributions of different components to the pore structure characteristics of lacustrine shales remain poorly understood. In this study, fifteen Chang 7 Member shales of the Yanchang Formation, Ordos Basin, were investigated through total organic carbon (TOC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-pressure N2 and CO2 adsorption analyses to study the control of shale composition on the pore structure characteristics of lacustrine shales. The results show that the average TOC content of the Chang 7 Member shales is 9.63 wt.%. XRD analysis shows that minerals in the Chang 7 Member shales consist of quartz, feldspars, clay minerals, and pyrite. The clay minerals were dominated by illite, chlorite, and interstratified illite/smectite. The mesopore characteristics of the Chang 7 Member shales and micropore characteristics of organic-lean shales are mainly controlled by clay minerals, whereas the micropore characteristics of organic-rich samples are controlled by both clay minerals and organic matter. SEM observations show that the phyllosilicate framework pores are the main pore type in the Chang 7 Member shales. The results of this study provide important insights into compositional control on the pore structure characteristics of lacustrine shales. [ABSTRACT FROM AUTHOR]

Organic matter (OM)-hosted pores play important roles in controlling the porosity and gas content of gas shales. However, the control of organic pore development and preservation remains poorly understood, partly because of the inability to distinguish OM types under the scanning electron microscope (SEM). In this study, seven over-mature Wufeng-Longmaxi Shale samples (equivalent vitrinite reflectance 1.93%–3.07%) were investigated with organic petrography and scanning electron microscope to study the origin of OM and development of organic pores in this black shale succession. Correlative light and electron microscopy was employed to examine pore development in individual macerals. Organic petrographic observations show that OM in these over-mature black shales is dominated by pyrobitumen and graptolites (> 90 vol% of total OM based on point counting). Pyrobitumen mainly occurs as matrix bitumen in the fine-grained matrix, and also as a mixture with clay minerals and fossil cavity infillings. Vitrinite-like particles are minor constituents of OM in the Wufeng-Longmaxi Shale, and their average reflectance, reflectance distribution, and morphology suggest that they may be derived from graptolite fragments lacking diagnostic features and acritarchs. OM type critically controls the development of organic pores. Organic pores primarily occur in pyrobitumen, whereas other macerals are mostly non-porous when examined under the SEM. Results of this study provided important insights into OM thermal evolution and organic pore development in source-rock reservoirs, calling for a critical appraisal of OM in black shales with organic petrography in addition to SEM. • Maceral composition in the over-mature Wufeng-Longmaxi Shale was studied. • Maceral control on organic pore development was analyzed. • Organic matter mainly consists of pyrobitumen and graptolites. • Oil-prone alginite does not exist in the Wufeng-Longmaxi Shale. • Organic pores are primarily hosted by pyrobitumen. [ABSTRACT FROM AUTHOR]

Amorphous organic matter (AOM or bituminite) is a major oil-prone liptinite maceral in black shales that can be classified into two main types: low-reflectance AOM and micrinized AOM. In this study, we investigated the petrographic and chemical structure characteristics of these two types of AOM and discussed their origin based on 15 marine black shale samples from the Illinois Basin, including 5 samples from the Pennsylvanian shales and 10 samples from the Upper Devonian New Albany Shale. The studied samples are at early mature stage, with mean vitrinite reflectance ranging from 0.64 to 0.74%. Low-reflectance AOM is the dominant organic matter (OM) in all samples and micrinized AOM is common only in the Pennsylvanian strata but is very rare in the New Albany Shale. There are significant differences in reflectance, petrographic characteristics, and chemical structure characteristics between low-reflectance and micrinized AOM. Low-reflectance AOM has a reflectance of 0.30 to 0.57%, whereas the reflectance of micrinized AOM is typically >1.0%, which makes micrinized AOM brighter than low-reflectance AOM under an optical microscope. Both types of AOM have heterogeneous texture, occur as structureless organic streaks or lenses, and have abundant clay-sized mineral inclusions. Compared to micrinized AOM, low-reflectance AOM has lower aromaticity and higher aliphatic chain length (inferred from the CH 2 /CH 3 ratio). Based on the petrographic and chemical functional groups characteristics, we proposed evolutionary pathways for the formation of low-reflectance and micrinized AOM in marine black shales during early diagenesis. AOM is originally derived from microbially degraded phytoplankton. Low-reflectance AOM formed in the fermentation zone through microbial degradation of phytoplankton by anaerobic bacteria. In comparison, micrinized AOM is a product of an advanced degree of microbial degradation of phytoplankton by aerobic bacteria that preferentially consume aliphatic functional groups over aromatics in the oxidation zone. Different depositional environments and subsequent diagenetic alterations resulted in the difference in micrinized AOM content between the New Albany Shale and Pennsylvanian shales. This study provides a better understanding of the origin, formation, and thermal maturation of dispersed AOM in black shale successions. • Low-reflectance and micrinized amorphous organic matter were identified. • Petrographic and chemical characteristics of amorphous organic matter were studied. • An evolutionary pathway for the formation of amorphous organic matter was proposed. • Amorphous organic matter formed during early diagenesis. [ABSTRACT FROM AUTHOR]

The study of pore network evolution is an important aspect of tight shale reservoir characterization. Pore spaces in shales consist of three major types: interparticle pores and intraparticle pores associated with mineral matrix and organic-matter pores. In this study, the evolution of pore networks for each pore type in lacustrine shales during thermal maturation was studied based on a suite of 19 Triassic Yanchang Formation shale samples from the Ordos Basin across a maturation gradient from immature to late mature (vitrinite reflectance (R o) from 0.36 to 1.30%). The results show that different pore types follow systematic evolutions during thermal maturation and that the evolution of pore type and pore size is critically controlled by multiple diagenetic processes such as mechanical compaction, cementation, mineral dissolution, clay mineral transformation, and organic matter thermal maturation. Specifically, at immature and early mature stages (R o < 0.55%), pores associated with mineral matrix are abundant and have moderate pore sizes. Interparticle pores exhibit preferential orientation along clay platelets. At the mature stage (0.55% < R o < 1.15%), pores associated with mineral matrix have the smallest size, and interparticle pores become less common and show reduced preferential orientation because of mechanical compaction and cementation. Organic-matter pores start to develop at a maturity level of R o 0.77% and higher. The total porosity and BET specific surface area show minimum values at R o 1.12%, mainly because pre-existing matrix-related pores are destroyed by mechanical compaction and filled by migrated bitumen and liquid hydrocarbons, and then they increase when R o exceeds 1.12%, which likely results from the development of secondary organic-matter nanopores due to the generation and expulsion of gaseous hydrocarbons. An increase in pore size of matrix-related pores at the late mature stage (1.15% < R o < 1.30%) could be a result of dissolution by organic acid generated during organic matter thermal maturation. • Pore network of lacustrine shales from immature to late mature was studied. • Pore evolution of different pore types was discussed separately. • Pore size distribution of different pore types with increasing thermal maturity was studied. • Organic-matter pores start to develop at a maturity level of R o 0.77% and higher. • Total porosity evolution of shales is more complex than a monotonous trend. [ABSTRACT FROM AUTHOR]

Thermal maturity of source rocks indicated by vitrinite reflectance (R o) is an important and reliable parameter to determine the petroleum potential of sedimentary basins. Originally, R o was used to characterize the degree of coalification of coals and thus works well for coal seams, but it has limitations when used to determine the thermal maturity of black shale successions. A suite of six coal-shale pairs from the Upper Devonian black shale formations in the eastern United States (New Albany Shale, Ohio Shale, Dunkirk Shale, and Rhinestreet Shale) was selected to study the applicability of R o in assessing the thermal maturity of black shale successions. The results show that vitrinite in the studied coal samples is dominated by collotelinite, whereas vitrinite in black shales occurs as small dispersed particles (~5 μm) in the mineral matrix. When comparing the size and morphology of dispersed vitrinite particles in shales and collotelinite fragments in coals, vitrinite in shales and adjacent coals should have the same origin. The measured mean random R o of vitrinite in coals ranges from 0.51–0.68%, and is 0.04–0.11% (average 0.07%) lower than that of dispersed vitrinite particles in enclosing shales. This observation contrasts with previously reported R o suppression in liptinite-rich black shales. A high standard deviation of R o measurements indicates a highly heterogeneous nature of vitrinite in black shales. Three mechanisms can be envisioned to contribute to the higher R o of dispersed vitrinite particles in black shales relative to enclosed coals. First, small vitrinite particles may become more oxidized because small particles are more frequently suspended during transport to the site of deposition and reworking of bottom sediments after deposition than large pieces of driftwood that will turn into enclosed coal lenses during burial diagenesis. Second, misidentification of zooclast (e.g., chitinozoan) fragments as vitrinite due to loss of diagnostic morphology. Third, various macerals in the vitrinite group (e.g., corpogelinite and collotelinite) may have had different original reflectance values. Based on the petrographic characteristics of vitrinite and vitrinite-like particles, caution should be applied when using the R o values of dispersed vitrinite and vitrinite-like particles in black shales as indicators of thermal maturity. • R o values of Devonian black shales and enclosed coals were compared. • Vitrinite in black shales is more heterogeneous than that in enclosed coals. • R o of black shales is on average 0.07% higher than that of enclosed coals. • Oxidation of vitrinite, misidentification of zooclast fragments, and variation of vitrinite macerals could cause higher R o. [ABSTRACT FROM AUTHOR]

Abstract Organic matter quantity and type are important parameters in conventional source rock evaluation and unconventional shale oil/gas reservoir characterization. Understanding the stratigraphic distribution of organic matter content and type in black shale successions is critical for identifying potentially productive intervals, because organic matter can adsorb large amounts of oil and gas. Detailed examination of total organic carbon content, organic petrographic composition, and high-resolution (8 cm spacing) geochemical proxies in a sequence stratigraphic framework were conducted on an early mature (0.55% R o) New Albany Shale core of the Illinois Basin to study the influence of relative sea-level fluctuations on paleoproductivity, clastic supply, bottom-water redox conditions, and their combined control on total organic carbon distribution pattern and organic maceral variation. Marine organic matter including amorphous organic matter and alginite (mainly derived from Tasmanites cysts) is the dominant organic matter in the New Albany Shale. Terrestrial organic matter accounts for <10% of total organic matter. Within a sequence stratigraphic context, total organic carbon content increases in transgressive systems tracts, reaches a maximum before the maximum flooding surface, and shows relatively low values in highstand systems tracts. Comparatively, low total organic carbon contents at maximum flooding surfaces reflect a combination of low sedimentation rates, elevated bottom-water oxygenation, and high biogenic silica dilution. Stratigraphically, amorphous organic matter content increases in transgressive systems tracts, reaches a maximum near the maximum flooding surface, and decreases in highstand systems tracts. Enrichment of broken Tasmanites cysts and their detrital infills are an indicator of high-energy environments and could possibly indicate lowstand systems tracts. The stratigraphic distribution of organic matter content and type may result in cyclic stratigraphic variations of hydrocarbon generation potential and oil saturation, and influence the development of secondary organic pores when the New Albany Shale is within the oil and gas windows. Highlights • Stratigraphic variation of total organic carbon content was studied in the Upper Devonian New Albany Shale. • Organic maceral variation was examined in the sequence stratigraphic context of the New Albany Shale. • Relative sea-level fluctuations influence paleoproductivity, clastic supply, bottom-water redox conditions. • Clastic supply and paleoproductivity have dual impacts on organic matter accumulation. [ABSTRACT FROM AUTHOR]

Rock mechanical properties of tight shale reservoirs are important parameters in the exploration and development of unconventional shale oil and gas. Understanding the stratigraphic variability of rock mechanical properties in black shale successions is important for identifying promising intervals for hydraulic stimulation. High-resolution hardness tests (6 cm spacing) with an Equotip Bambino 2 hardness tester were conducted on a New Albany Shale core to study the stratigraphic variability of rock mechanical properties. Combined with high-resolution geochemical analyses (8 cm spacing) acquired with a portable X-ray fluorescence spectrometer, it is possible to relate hardness variability to shale composition. The results of our study suggest an average quartz content of about 28% for the entire succession. Approximately half of that amount (13%) is interpreted to be of biogenic origin. Biogenic quartz, mainly derived from dissolution of radiolaria, occurs as authigenic quartz precipitated in Tasmanites cysts, and as microcrystalline quartz precipitated in the shale matrix. Petrographic observations indicate that the hardness of the New Albany Shale is critically controlled by biogenic quartz content, not by total quartz content as suggested in multiple prior studies. Biogenic quartz forms an interconnected stiff framework which increases the hardness of shales. In contrast, dispersed "hard" grains (e.g., detrital quartz, feldspar, and dolomite) do not contribute to brittle behavior. When examined under a scanning electron microscope, "hard" shales are rich in biogenic quartz and contrast visibly with "soft" shales that are characterized by high contents of clay minerals. Negative correlation between hardness and Al 2 O 3 content, as well as differential compaction of clay minerals around "hard" grains, indicate that clay minerals are an important ductile component in the New Albany Shale. Although the general softness of kerogen suggests that there should be an influence of organic matter on hardness, this effect is counteracted by biogenic quartz cementation which has a comparable stratigraphic distribution pattern. We propose a new mineral-composition-based brittleness index on the basis of biogenic quartz content to characterize the brittleness of tight shale reservoirs. Within a sequence stratigraphic context, the hardness of the New Albany Shale increases in transgressive systems tracts, reaches a maximum at maximum flooding surfaces, and decreases in highstand systems tracts with the exception of the Blocher Member. Maximum flooding surface intervals have the highest potential to develop brittle fractures and should be potential targets for hydraulic stimulation. • Stratigraphic variability of hardness was studied within the New Albany Shale. • The control of rock composition on the hardness of black shales was studied. • Hardness of the New Albany Shale is controlled by biogenic quartz content. • Dispersed grains such as detrital quartz do not generate brittle behavior. • A new mineral-composition-based brittleness index was proposed. [ABSTRACT FROM AUTHOR]

Marine organic-rich shales generally have high uranium content associated with sedimentary organic matter. However, organic matter in black shales is not homogeneous, but instead a mixture of different macerals. The uranium content and organic petrographic composition of 27 New Albany Shale samples indicate differential contributions of macerals to the uranium content in the Upper Devonian New Albany Shale of the Illinois Basin. Specifically, uranium is mainly associated with amorphous organic matter (microbially degraded phytoplankton, zooplankton, and bacterial biomass), where an average uranium content of approximately 550 ppm has been estimated. The amorphous organic matter's lack of structure, as well as admixed with mineral grains (<7.8 μm) suggest that it originated via microbial degradation in surficial sediments. Uranium may have been incorporated into amorphous organic matter during phytoplankton degradation and amorphous kerogen formation early in diagenesis. Pore-water redox conditions and pH control the reduction of uranium (VI) and its incorporation into amorphous organic matter. Alginite derived from Tasmanites algae (average uranium content <10 ppm) contributes minimally to uranium content. Terrestrial organic matter in the New Albany Shale contains on average ~ 20 ppm uranium, and because it accounts for <10% of total organic matter in the New Albany Shale, its contribution is equally negligible. • Maceral variation in the New Albany Shale was studied. • Association of U with organic matter and varying macerals was examined. • U primarily associates with amorphous organic matter/bituminite. • U was incorporated into amorphous organic matter during early diagenesis. • Alginite and terrestrial organic matter have negligible contribution to U content. [ABSTRACT FROM AUTHOR]