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Graptolite reflectance was thought to be one of the most useful thermal maturity indicators for graptolite-bearing sediments, however, the relationship between graptolite reflectance and vitrinite reflectance is not well-established. Graptolites, especially in the Wufeng–Longmaxi Formations from the Ordovician to Silurian of South China, have been mistaken for vitrinite-like particles or solid bitumen, which results in inconsistent data on the thermal maturity. In this paper, we have employed optical microscope techniques to describe the detailed optical characteristics of graptolites and solid bitumen in the Wufeng–Longmaxi Formations. Laboratory simulation of maturation was used to determine the relationship between graptolite reflectance and vitrinite reflectance. The organic constituents in the Wufeng–Longmaxi Formations are mainly composed of graptolites and solid bitumen. Granular and non-granular graptolites were observed in the Wufeng–Longmaxi Formations, with non-granular as the most common texture. Solid bitumen can be distinguished from non-granular graptolites by its coarse surface, weaker anisotropy, and lower random reflectance. The combination of non-polarized and polarized light is very helpful to distinguish solid bitumen from graptolite. For comparison, organic material from the early Ordovician Alum Shale Formation of Sweden and Estonia was also studied. The macerals of the Alum shales are mainly composed of lamalginites, mineral-bituminous groundmasses, graptolites, and solid bitumen. The major textures of the graptolites in the Sweden and Estonia sediments are non-granular and granular, respectively. Both non-granular graptolite and vitrinite reflectances display a systematic increase with the increase of heating temperature and time. The granular graptolites in the Estonian sample were gradually changed to non-granular graptolites following laboratory simulated maturation, indicating that granular graptolites can transform into non-granular graptolites with maturation. Solid bitumen in the Wufeng–Longmaxi Formations was derived from the solid residue of kerogen and/or post-oil bitumen. The graptolite random reflectance is a better thermal maturity indicator than graptolite maximum reflectance and is more precise due to the smaller standard deviation. Several equations are proposed to determine the thermal maturity of the graptolite-bearing sediments based on graptolite random reflectance, graptolite maximum reflectance and solid bitumen random reflectance. [ABSTRACT FROM AUTHOR]

This paper reviews the significance of solid bitumen with emphasis on source-rock reservoirs. We discuss difficulties and discrepancies with terminology, especially those terms related to the origin of solid bitumen and its physical and chemical properties. Various definitions of solid bitumen have their own justifications and can be used provided there is clarity about which defining criteria are being considered. Difficulties in conforming to chemical-, solubility-, or origin-related definitions lead us to suggest adapting the reflectance of solid hydrocarbon as a practical choice for placing the boundary between solid bitumen and pyrobitumen, and 1.50% is proposed as the boundary value. It has to be noted that this boundary may be shifted down to 1.3% for sulfur-rich kerogen. Recently, much progress has been made by combining imaging and physical adsorption techniques in porosity studies, and so the porosity of solid bitumen is given special emphasis. Comparing pore characteristics obtained from SEM versus those generated by gas adsorption, mercury intrusion, or neutron scattering techniques indicates that the SEM pore inventory fails to account for the smallest pores (<5 nm in size) present in organic matter. Therefore, low-pressure CO 2 adsorption is still the most effective technique to assess microporosity (pores <2 nm in diameter) in shales. We conclude that combining observational in situ techniques with techniques based on physical principles is necessary to make progress toward a better understanding of porosity systems in organic matter, including solid bitumen. We review the implications of the abundance of solid bitumen on reservoir quality, porosity, permeability, and producibility, based on examples of selected sequences. One of the difficulties in predicting the influence of solid-bitumen-bearing horizons on reservoir quality arises from the problems with detecting organic phases using various logging techniques. The use of specialized techniques such as NMR logging that allows two-dimensional T 1 and T 2 measurements should be expanded, and other potential techniques need to be further researched and tested. Certain aspects of the properties of solid bitumen that are not as well understood, such as its hydrocarbon generation potential or its role in hydrocarbon migration are also discussed with the aim of identifying further research that could lead to a better understanding of the role that solid bitumen plays in unconventional reservoirs. [ABSTRACT FROM AUTHOR]

Organic-rich deposits of Qalikuh locality, Zagros Basin, Southwest of Iran are unconventional hydrocarbon resources considered as an oil shale, which is hosted within a carbonate succession with the age range of Middle Jurassic to Lower Cretaceous. Leco carbon, Rock–Eval VI and modified Fischer Assay analysis were performed on some outcrop samples of these resources to investigate the occurrence of oil shale. In the studied samples, mean total organic carbon (TOC) is 17.3 wt.% and the average of petroleum genetic potential (S 1 + S 2 ) is 98 kg HC/t rock. Furthermore, TOC has direct relationship with petroleum genetic potential. Hydrogen index (HI)–T max diagram indicates that the organic matter corresponds to Type II kerogen of probable marine origin with high quality of oil generation. According to Rock–Eval data, and bitumen reflectance (BR r ), the samples are thermally early mature. Based on the studied samples, oil yield of the Qalikuh oil shales ranges from 6.1 to 7.9 wt.%. Mean oil–gas ratio is 2.7 and mean oil yield is 56.7 l/t rock. According to the observations, the production value of Qalikuh oil shale is more or less similar in the entire studied regions. Whereas the total production is below the expected values, the produced oil is considerably lighter (higher API) than that of most other known oil shales. [ABSTRACT FROM AUTHOR]

Bitumen reflectance (BRo) is an alternative maturity indicator to vitrinite reflectance (VRo) in Lower Paleozoic and marine rocks where few higher plant macerals can be found. In unconventional petroleum rocks, locally exuded and/or migrated oil are retained in the tight rocks and thermally and diagenetically consolidated into solid bitumen and pyrobitumen. Finding suitable bitumen macerals for BRo measurements pose a challenge in indicating the proper thermal maturity. This study reports the effects of surface imperfections caused by nanoporosity, surface granularities, and impurities on measuring BRo in core samples from the Jurassic Nordegg Shale (shale oil) and the Triassic Montney siltstone (tight gas) in the Western Canada Sedimentary Basin. The degree that bitumen's surface imperfections influence any given reflectance measurement was controlled by changing the probe size area. The larger the measuring probe, the higher the degree of surface imperfections is included in BRo readings. The effect is quantified in this paper, showing up to 27% diminishment of mean BRo values when the probe size increased from 0.31 to 5.85 μm 2 . The mode in which bitumen accumulates within the rock has a strong influence on the degree of surface imperfections. Bitumen which is disseminated within clay matrix introduces a high degree of nanoporosity and mineral interference and hence will result in lowest BRo. In contrast, inclusions of high reflecting micrinite or diagenetic minerals such as pyrite result in higher BRo. These effects can vary at the microscopic scale leading to multiple populations of reflectance readings and difficult interpretations of thermal maturity. [ABSTRACT FROM AUTHOR]

Abstract: In their 2012 paper discussing the character of pores and organic matter in the Mississippian Barnett Shale, Bernard et al. (2012) identified only nonporous kerogen and nonporous solid bitumen in oil-window-maturity samples, but nanoporous pyrobitumen in gas-window-maturity samples. Our examination of samples from the same wells shows that pores are also present in organic matter from oil-window-maturity samples. We also argue that some of the pores we identified in gas-window-maturity samples are developed in organic matter that is unlikely to be pyrobitumen. [Copyright &y& Elsevier]