Organic-rich Rocks Research Articles

Sulfur isotope composition of individual compounds in immature organic-rich rocks and possible geochemical implications

We applied compound-specific sulfur isotope analysis (CSSIA) to organic matter (OM) extracted from ancient and immature organic-rich rocks from the Cretaceous Ghareb (Shefela Basin locality, Israel) and Miocene Monterey (Naples Beach locality, California, USA) Formations. Large variations in the δ34S values of different organosulfur compounds (OSCs), that reach up to 28‰ and 36‰, were observed in the Ghareb and Monterey samples, respectively. Additionally, some common OSCs in both locations showed consistent 34S trends relative to each other. The consistent enrichment in 34S of C35 hopane thiophene relative to iC20 thiophene in the studied sections probably resulted from differences in the timing of OM sulfurization. Reactive organic precursors quickly consume the most 34S-depleted reduced S, while less reactive species incorporate the heavier residual S at a later time. Despite the differences in the depositional environments, ages, and the initial δ34S values of the reduced S (represented by the δ34S of pyrite) between the Ghareb and the Monterey Formations, the sulfurization order of common organic compounds seems to be similar. All of the δ34S values of OSCs are 34S enriched relative to that of the coexisting pyrite with the exception of the C25 highly branched isoprenoid (HBI) thiophene in several samples from the Monterey Formation. The existence of 34S-depleted sulfurized HBI may point to OM sulfurization that occurred at or near the sediment-water interface during the deposition of the Monterey. Moreover, the δ34S of steroid sulfides shows an inverse trend with the pristane/phytane ratio, which may indicate that the sulfurization mechanism of these OSCs are affected by redox conditions. Further investigation of CSSI values in immature rocks from other basins may help constrain the OM sulfurization process, timescale, and depositional conditions and their possible use as paleoenvironmental proxies.

Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities.

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C30 sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C27 sterane (cholestane) predominance among total C26 -C30 steranes is a common feature found for all samples investigated, with lower amounts of C28 steranes (ergostane and crysotane) also present. No traces of known ancient C30 sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

Effects of lacustrine organic-rich fine-grained sedimentary rocks on the accumulation of tight oil

In order to establish the distribution pattern of fine-grained sedimentary rocks and clarify the coupling relationship between lacustrine organic-rich fine-grained sedimentary rocks and tight oil reservoir, the Chang7 Member of Triassic Yanchang Formation in Longdong area was taken as an example to be studied by core description and log interpretation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Lacustrine organic-rich fine-grained sedimentary rocks were divided into shale and mudstone according to their lithology and texture. The results show that the shale is mainly developed in Chang73 Sub-member, while the mudstone is developed in each sub-member of Chang7 Member. The organic-rich fine-grained sedimentary rocks in Bazhu-Maling-Qingcheng-Huachi area are the thickest in Longdong area. During the Chang73 sedimentary period, the shale was widely distributed in the center of the lake. In the Chang72 sedimentary period, the mudstone and turbidite were interstratified and superimposed. During the Chang71 sedimentary period, the mudstone was only distributed as thin layers between the thick turbidite. The Chang7 tight oil is widely distributed in the turbidite sand bodies, and it is mainly sourced from the Chang73 shale. The distribution of tight oil is controlled by the development of organic-rich fine-grained sedimentary rocks.

Hydrocarbon Generation Potential of the Organic-Rich Naifa Formation, Say’un–Masila Rift Basin, Yemen: Insights from Geochemical and Palynofacies Analyses

The Say’un–Masila rift, one of the most important basins of Yemen, contains unconventional resources of organic-rich rocks. In the present work, geochemical and palynofacies analyses were performed on the organic-rich source rock (argillaceous limestone) of the late Jurassic–early Cretaceous Naifa Formation in the Say’un–Masila Rift Basin to investigate its hydrocarbon generation potential. The geochemical analysis of the organic matter reveals that the argillaceous limestone unit has total organic carbon contents of 0.5–2.6%, which indicate fair to good hydrocarbon source potential. The organic matter in the analyzed samples is dominated by Type II/III and Type III kerogens, with minor Type II kerogen, referring to both oil- and gas-prone source rocks. The dominant kerogen Types II/III and Type III are indicated further from quantitative results based on an open pyrolysis–gas chromatography analysis. Biological markers and palynofacies analysis suggest that the kerogen was formed through a combination of mixed marine and high land plant organic matter and was preserved under sub-oxic sedimentary conditions. The different geochemical and optical maturity parameters indicate that the analyzed argillaceous limestone of the Naifa Formation is thermally mature, equivalent to early mature of oil-generation window. This implies that the burial depth of the analyzed samples is not deep enough; therefore, only the deeper parts of the Naifa source rocks at the basin have reached a satisfactory maturity level to be considered as potentially effective petroleum source rock. The biomarker source and depositional environment indicators presented and discussed in this study are useful for future exploration in the Say’un–Masila Basin for oil-to-oil and oil-to-source geochemical correlations and offer guidance for potential prospecting targets that may occur within the basin.

Comparison of influence on the formation by secondary and tertiary methods for the production of hydrocarbon compounds from oil source rocks with high oil-generating potential

The work presents the comparison results of the quantitative and qualitative composition of hydrocarbon compounds that can be obtained as a result of secondary and tertiary methods of influence on organic-rich rocks (on the example of the Bazhenov Formation rocks) with a high oil-generating potential. It is shown that as a result of extraction of bitumoids presented in open pores, realisation the generation potential and the production of synthetic oil, it is possible to produce hydrocarbon compounds, the amount of which reaches 35 kg and 20 kg per 1 m3 of rock, respectively. Products possess high maturity and are identical in composition to the oil extracted from these rocks by standard technology of development. It was found that with the development of appropriate technologies of subsequent influence on the formation by secondary and tertiary methods, oil production can be significantly increased in the future.

Sedimentary environment and the controlling factors of organic-rich rocks in the Lucaogou Formation of the Jimusar Sag, Junggar Basin, NW China

The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels. During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.

Sulfur isotopic composition of gas-phase organic sulfur compounds provides insights into the thermal maturation of organic-rich rocks

Volatile and gas phase organic sulfur compounds (VOSCs) are important components in subsurface reservoir fluids and despite their relatively low concentrations, can provide important information about organic matter origin, diagenetic transformation, thermal maturation, and oil and gas generation, expulsion, and migration. We present an approach for the utilization of VOSC concentration and compound specific S isotopes ratio (δ34S) data as a new geochemical tool to study natural gas origin and formation. We studied the formation pathways of VOSCs, their δ34S values, and interaction with an organic-rich sedimentary rock (or ‘source rock’) from the Ghareb Formation (Type II-S kerogen) deposited in an upwelling marine environment. The immature source rock was subjected to laboratory controlled thermal maturation. We used a semi-open, non-isothermal pyrolysis system heated between 200 and 440 °C, and analyzed the molecular composition and compound specific δ34S values of the evolved gases at various thermal maturity stages. Formation of VOSCs commenced at 206 °C, a temperature generally associated with less thermally mature systems, and typically before the onset of oil generation for similar organic-rich source rocks, allowing study of low thermal maturity conditions (∼0.3 %Roeq). Overall, the VOSCs obtained had δ34S values similar to the bulk kerogen values (mostly within 4‰). However, thiol (or mercaptan) δ34S values closely followed those of H2S (Δ34Sthiols-H2S = −1 ± 1‰) throughout the experiment. Ab-initio calculations for the S isotopic exchange between H2S and thiols were close to the experimental observations. This suggests that thiols rapidly attained equilibrium with H2S despite their very short residence time in the system (minutes to hours). Furthermore, throughout the experiment, the concentrations of the six different thiols were found to be proportional to the coexisting, concentration ratio of [H2]/[H2S]. The proportionality factors of the 6 different thiols strongly correlate with reported heats of formation (ΔHf°), further supporting the notion that the generation of thiols in the system rapidly reached equilibrium. At pyrolysis temperatures up to 350 °C, δ34S and concentration values of other VOSCs produced in the gas phase, including sulfides and thiophenes, probably represent generation from S-containing moieties in kerogen and bitumen with limited interaction with co-existing H2S. The reaction of hydrocarbons and H2S to produce VOSCs is inversely correlated with thermal stability, i.e. thiols > sulfides > thiophenes. Therefore, the δ34S values, concentrations and distributions of VOSCs can be utilized as a proxy for reaction extent and formation mechanism of petroleum and H2S, including primary generation by thermal maturation, or potentially other processes such as migration and thermochemical sulfate reduction. This work demonstrates a novel and useful geochemical tool to study the source and fate of natural gas in the subsurface.

Environmental reconstruction and formation mechanisms for lacustrine source rocks of the third member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin, eastern China

The third (Es3) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es3 (Es3L) and upper Es3 (Es3U) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es3L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es3U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es3L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es3U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es3U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es3L and Es3U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.

Tectonic controls on lacustrine source rock occurrence in the Huizhou Sag, Pearl River Mouth Basin, China

ABSTRACTStudy of the factors controlling lacustrine source rock deposition is critical to predict potential source rocks and discover hydrocarbon reserves. Tectonic controls on source rock deposition have been a major research focus in recent decades. In this study, we integrated geochemical, geophysical, and microfossil data to investigate the effect of tectonic movements on lacustrine source rock deposition of the Wenchang Formation in Huizhou Sag, Pearl River Mouth Basin, southern China. Geochemical, geophysical, and microfossil analyses suggest that the lacustrine source rocks are characterized by high total organic carbon (TOC) contents, type I and type II1–2 kerogen, which primarily originated from aquatic organic matter (OM), and a peak oil-generation thermal maturity stage. Rift-related faults controlled the formation and distribution of two types of half-grabens in the Wenchang Formation. Synsedimentary faults created a large amount of accommodation space, making the half-grabens under-filled. Under warm and humid climatic conditions, plankton were abundant and organic matter supply was sufficient in the half-grabens of Huizhou Sag during the deposition of Wenchang Formation. Maximum flooding surfaces (mfs) created high accommodation spaces and strongly reducing conditions, which are favourable for organic-rich lacustrine source rock preservation. Source rocks close to the depocentre and slope of the half-grabens have high TOC contents. The depositional environment (i.e. the stable highly reducing conditions) of the lacustrine source rock was largely controlled by tectonics, therefore faulting activities were considered as the most important factor controlling the occurrence of lacustrine source rock. Models of lacustrine source rock deposition were established for the Wenchang Formation in the half-grabens using a multiple analysis model. This model is also suitable for investigating the deposition of other lacustrine source rocks in half-grabens with tectonic and climatic conditions similar to those of the South China Sea Basin.

Methods of step extraction of rocks of high-carbon formations for the study of the component distribution of bitumen and variability of their basic geochemical parameters

The article describes organic-rich rocks sequential extraction method for the bitumen composition difference. The method step-by-step extracts bitumen first from open and then from closed pores using three organic solvents with increasing polarity. The released bitumen differs in malthens/asphaltenes ratio, the distribution of n-alkanes, the content of terpanes and steranes, and, thus, the values of the biomarker parameters. These parameters show difference in the degree of organic matter thermal maturity, which is relates to different types of rock pore space. At the same time different parameters show different maturity ratio between bitumen from open and closed pores. Changes in the composition of bitumen allow to estimate the migration of hydrocarbon compounds through the well, to investigate the effect of sedimentation conditions on the pore space and the distribution of bitumen. The results show that investigation of bitumen received by one-step extraction from powder can lead to incorrect interpret of obtained results.

Paleoenvironmental conditions and organic matter accumulation in Upper Paleozoic organic-rich rocks in the east margin of the Ordos Basin, China

Marine-continental transitional shale gas contains significant resources, but scant areas have been developed economically. The evolution of paleoenvironmental and potential of high-quality shales generation from the Carboniferous to Permian (mainly of Benxi, Taiyuan and Shanxi formations) in the east margin of Ordos Basin, China, was systematically studied by integrated analysis including rock-mineral composition, trace elements and organic geochemical testing. The results show that the Benxi Formation consisted principally by marine deposits, while the co-existence of marine and marine-continental transitional deposits are found in the Taiyuan Formation. The Shanxi and its upon Xiashihezi formations are mainly constituted by continental deposits. The Taiyuan Formation has been affected by frequent transgression and regression processes, while the sedimentary environment of the Shanxi Formation was relatively stable. The degree of oxidation gradually increased from the Benxi to the Xiashihezi Formation, and the paleoclimate transformed from humid and warm climate to aridity. The paleosalinity decreased gradually, even though the environment was generally maintained in a salt water environment with considerable rainfall and relatively active hydrodynamic conditions. The paleoproductivity increased gradually, and a generally anoxic environment in the surface water contributed to the enrichment of organic matter. The rock-mineral compositions of the shales are dominated by clay minerals and quartz, and the organic matter content of the shales is relatively high (averaging of 1.28%). The shales primarily contain type III kerogen, and the shales have generally entered a high maturity stage with good gas generation potential. Furthermore, the gas generated from the interbedded coal seams in the Upper Paleozoic is a stable gas supply for interbedded shales. The results provide a geochemical basis for further study of marine-continental transitional shales in the Ordos Basin and supply recommendations for the optimization of high-quality shales production in similar basins around the world.

Hydrothermal dedolomitisation of carbonate rocks of the Paleoproterozoic Zaonega Formation, NW Russia — Implications for the preservation of primary C isotope signals

The Paleoproterozoic Zaonega Formation in Karelia, NW Russia, has played a key role in understanding the environmental conditions postdating the Great Oxidation and Lomagundi-Jatuli Events. Its carbonate- and organic-rich rocks (shungite) define the postulated Shunga Event representing an accumulation of very organic-rich sediments at c. 2 Ga and are central in ideas about changing ocean-atmosphere composition in the wake of those worldwide biogeochemical phenomena. Our work focussed on a key interval of carbonate rocks in the upper part of the Formation to: (i) obtain new high-resolution carbon, oxygen and strontium isotope data complemented by detailed petrography and mineralogical characterisation and (ii) expand upon previous studies by using our data to constrain geochemical modelling and show in greater detail how magmatic hydrothermal fluids induced dedolomitisation and altered geochemical signals. Our findings show that the δ13Ccarb of calcite-rich intervals are the most altered, with values between −16.9 to 0.6‰, whereas the dolomite-dominated parts retain the best-preserved (i.e. most original) values. Those define a trend of steadily increasing δ13Ccarb, from −6 to +0.5‰, which we interpret as a return to normal marine conditions and carbonate‑carbon values following the Lomagundi-Jatuli Event.

Geochemical Characteristics of Oil from Oligocene Lower Ganchaigou Formation Oil Sand in Northern Qaidam Basin, China

Oil from the Oligocene oil sands of the Lower Ganchaigou Formation in the Northern Qaidam Basin and the related asphaltenes was analyzed using bulk and organic geochemical methods to assess the organic matter source input, thermal maturity, paleo-environmental conditions, kerogen type, hydrocarbon quality, and the correlation between this oil and its potential source rock in the basin. The extracted oil samples are characterized by very high contents of saturated hydrocarbons (average 62.76%), low contents of aromatic hydrocarbons (average 16.11%), and moderate amounts of nitrogen–sulfur–oxygen or resin compounds (average 21.57%), suggesting that the fluid petroleum extracted from the Oligocene oil sands is of high quality. However, a variety of biomarker parameters obtained from the hydrocarbon fractions (saturated and aromatic) indicate that the extracted oil was generated from source rocks with a wide range of thermal maturity conditions, ranging from the early to peak oil window stages, which are generally consistent with the biomarker maturity parameters, vitrinite reflectance (approximately 0.6%), and Tmax values of the Middle Jurassic carbonaceous mudstones and organic-rich mudstone source rocks of the Dameigou Formation, as reported in the literature. These findings suggest that the studied oil is derived from Dameigou Formation source rocks. Furthermore, the source- and environment-related biomarker parameters of the studied oil are characterized by relatively high pristane/phytane ratios, the presence of tricyclic terpanes, low abundances of C27 regular steranes, low C27/C29 regular sterane ratios, and very low sterane/hopane ratios. These data suggest that the oil was generated from source rocks containing plankton/land plant matter that was mainly deposited in a lacustrine environment and preserved under sub-oxic to oxic conditions, and the data also indicate a potential relationship between the studied oil and the associated potential source rocks. The distribution of pristane, phytane, tricyclic terpanes, regular steranes and hopane shows an affinity with the studied Oligocene Lower Ganchaigou Formation oil to previously published Dameigou Formation source rocks. In support of this finding, the pyrolysis–gas chromatography results of the analyzed oil asphaltene indicate that the oil was primarily derived from type II organic matter, which is also consistent with the organic matter of the Middle Jurassic source rocks. Thus, the Middle Jurassic carbonaceous mudstones and organic rock mudstones of the Dameigou Formation could be significantly contributing source rocks to the Oligocene Lower Ganchaigou Formation oil sand and other oil reservoirs in the Northern Qaidam Basin.

Geochemical and petrographic analyses of new petroleum source rocks from the onshore Upper Jurassic and Lower Cretaceous of Lebanon

The recent interest in the Lebanese petroleum potential led to new studies mainly concerning marine petroleum source rocks, without taking in consideration the dysodiles and other associated facies from the Upper Jurassic and the Lower Cretaceous. These organic-rich outcrops of metric thickness (max. 3 m) are of limited lateral extension and often found next to volcanic deposits. Sixty-three samples were collected from 25 different localities, and were subdivided into three facies that are presented in this paper: (i) dysodiles formed by the superposition of millimetric layers that are easy to separate; (ii) dysodiles-like with distinct, but more compacted layers; (iii) non-dysodiles where the layering is not visible. The aim of this work is to assess the origin of these organic-rich rocks, and to evaluate their source rock potential. We used herein Rock-Eval pyrolysis, elemental analysis, palynofacies observation, organic petrography and molecular geochemical analyses. Rock-Eval pyrolysis reveals a very good to excellent source-rock potential for all above cited rock types, with average total organic carbon (TOC) of 14 ± 7% for the dysodiles, 19 ± 12% for the dysodiles-like and 13 ± 9% for the non-dysodiles. Hydrogen index (HI) and oxygen index (OI) values indicate a type I to II kerogen for the dysodiles (average HI = 649 ± 162 mg HC/g TOC), type I to II kerogen for the dysodiles-like (average HI = 340 ± 182 mg HC/g TOC) and type III kerogen for the non-dysodiles (average HI = 145 ± 42 mg HC/g TOC). The dominance of fluorescent amorphous organic matter (AOM) and alginite macerals confirm an algal/bacterial source of organic matter for the dysodiles. The presence of opaque AOM, phytoclasts, sporinites and cutinites macerals, suggests a variable terrestrial organic matter input for the dysodiles-like and non-dysodiles. Maturity parameters such as Tmax values (on average 428 ± 7 °C for dysodiles, 429 ± 6 °C for dysodiles-like and 424 ± 10 °C for non-dysodiles), vitrinite reflectance (<0.5%) and Carbon Preference Index (>1) indicate that the organic matter is still thermally immature. The present study sheds light on the long forgotten dysodile shales and similar material, and their importance as potential source rocks or oil shales. A comparison with local and regional source rocks is provided in order to emphasize the specific character of the dysodile deposits.

Petrologic and geochemical evidence for the formation of organic-rich siliceous rocks of the Late Permian Dalong Formation, Lower Yangtze region, southern China

Abstract Black shales are economically and scientifically important because they are commonly associated with high-quality hydrocarbon source rocks and metal accumulations formed during periods of abrupt environmental change. Organic-rich siliceous rocks of the Late Permian Dalong Formation developed extensively within Middle–Upper Yangtze region (western Yangtze platform); however, the formation mechanism of its formation in the Lower Yangtze region (eastern Yangtze platform) remains a subject of debate due to a lack of outcrop exposure. Petrologic, mineralogic, and geochemical data are presented for three sections of the Dalong Formation within the Lower Yangtze region. Geochemical ratios including V/(V + Ni), V/Cr, Ni/Co, U/Th, and V/Sc suggest that the Dalong Formation was deposited in an oxygen-poor environment. The elevated concentration of Mo (>100 ppm) further suggests that free H2S existed in the bottom water, which was favorable for the accumulation of large amounts of organic matter. Mo/TOC ratios reveal that water flow in the Lower Yangtze Basin was moderately to strongly restricted during deposition of the Dalong Formation. Flow within the upper–middle parts of the Yangtze Basin was also restricted, suggesting that northern marginal basins on the Yangtze Platform formed part of a single ocean during the Late Permian (i.e., Changhsingian). Cherts of the Dalong Formation reflect high primary productivity, similar to that of modern-day upwelling zones, which increased the amount of organic matter. These results, combined with previous work, suggest that the long-term oxygen-poor conditions during deposition of the Dalong Formation were caused by restricted water circulation in a unique paleogeographic environment characterized by low latitude and submarine paleo-highlands. Such settings of low latitude characterized by tropical climate favored the growth of siliceous zooplankton in the zones with submarine paleo-highlands having created topographic barriers to water circulation. Restricted bottom-water flow and oceanic anoxia during the Late Permian were likely the main controls on the accumulation of organic-rich rocks. Elevated nutrient levels caused by frequent oceanic upwelling along the western margin of the South China Craton and global volcanism at this time facilitated siliceous organism productivity.

Separation of macerals in organic-rich source rocks and their geochemical characteristics

Separation of macerals in organic-rich source rocks and their geochemical characteristics

Formation and occurrence of organic matter-hosted porosity in shales

Porosity within organic matter (OM) is considered to be the main site for gas storage (via fluid phase saturation of the pores and sorption on the pore walls) and thus its presence or absence is very critical for in-place gas assessment for organic-rich rocks. Numerous workers have suggested that OM-hosted porosity increases with thermal maturity mainly related to the process of bituminized organic matter cracking to gas. Comprehensive reviews of published literature enable us to conclude that organic porosity dominantly develops within bituminized organic matter (i.e., that portion that is petrographically identified, mainly based on its morphology, as solid bitumen (Mastalerz et al., 2018)) and primary (i.e, structured or amorphous) organic matter (kerogen) is mostly deficient in porosity. We show that in the same shale sample, structured kerogen shows no porosity whereas solid bitumen contains abundant porosity. It has been previously reported that sample preparation for SEM by ion milling may alter the organic matter. In this study, some adverse effects of ion milling have been observed by comparing SEM-visible pores of the same sample prepared by mechanical grind polish (MGP) and ion milling (IM) methods. Since solid bitumen is more labile, and probably more prone to chemical/physical alterations, we recommend that SEM observations of ion milled samples to be conducted with more attentiveness.We show that 1) MGP is a suitable sample preparation method for SEM observations, 2) the overwhelming majority of SEM-visible organic matter-hosted pores develop within solid bitumen and not in structured organic matter (kerogen), and 3) we reaffirm that shale mineral content is a key factor in preservation or destruction of solid bitumen-hosted porosity.Finally, we propose a conceptual model for the formation, preservation and destruction of OM-hosted porosity for shales. Accordingly, we suggest that, all other factors being equal, a quartz-rich brittle shale will preserve more SEM-visible solid bitumen-hosted (meso and micro) porosity compared to that of a clay-rich ductile shale. Therefore, gas storage capacity of a quartz-rich shale will be relatively high compared to that of a clay-rich shale.

Guest Editorial: Growth in Unconventional Resources Adds to Technical Literature

Guest editorial Unconventional resources require technologies typically not used to produce conventional oil and gas reservoirs. In particular, starting in the late 1990s, the combination of horizontal wells and multistage hydraulic fracturing has been key to rapidly increasing success in exploiting unconventional resources. While the well completions in unconventional resources may be more expensive than those typically applied in conventional reservoirs, the advantage is access to the very large volumes of these resources, which include tight (low-permeability) gas in sandstone or carbonate rock, coalbed methane, and organic-rich source rocks classed as shale gas and tight oil. The exploitation of shale gas, along with tight gas and coalbed methane, enabled a stunning increase in US natural gas production of approximately 4 Tcf/D from 2005 to 2010. A similar story followed a few years later for US crude oil production. Tight oil production ramped up nearly 5 million B/D over approximately 5 years, starting in 2008, and continues to rise. And it is not just the US that is benefiting—unconventional resource development is now beginning to spread to other countries around the world. The technologies that accomplished these near miracles were developed, implemented, and continue to be improved largely by SPE members, and this has been reflected in an upsurge in conference presentations and journal publications related to unconventional resources over the past decade. This is illustrated by the chart (Fig. 1), which shows a dramatic increase in the percentage of peer-reviewed papers related to unconventional resources published in SPE Reservoir Evaluation &amp; Engineering (SPEREE) beginning in 2011. Before 2011, there were only a handful of publications related to unconventional resources each year, primarily dealing with coal-bed methane and fundamentals of fluid flow in tight pore systems. Beginning in 2011, publications related to the analysis and prediction of shale gas reservoir production performance showed a sharp increase, followed a few years later by publications related to tight oil reservoir performance analysis and prediction. In addition, enhanced oil recovery from tight oil reservoirs has become an increasing area of focus. This year, over half of the publications in SPEREE will be related to unconventional resources. As further indication of the increasing importance of unconventional resources, in July SPE released Unconventional Gas and Tight Oil Exploitation, the first SPE monograph devoted exclusively to the topic.

Multiple sulphur isotope records tracking basinal and global processes in the 1.98 Ga Zaonega Formation, NW Russia

The exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation deposited in the Onega Basin, NW Russia, have refined our understanding of Earth System evolution during the Paleoproterozoic rise in atmospheric oxygen. These rocks were formed in vent- or seep-influenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13C-depleted organic matter. Here we report new isotopic (δ34S, Δ33S, Δ36S, δ13Corg) and mineralogical, major element, total sulphur and organic carbon data for the upper part of the Zaonega Formation, which was deposited shortly after the termination of the Lomagundi-Jatuli positive carbon isotope excursion. The data were collected on a recently obtained 102 m drill-core section and show a δ13Corg shift from −38‰ to −25‰. Sedimentary sulphides have δ34S values typically between +15‰ and +25‰ reflecting closed-system sulphur isotope behaviour driven by high rates of microbial sulphate reduction, high sulphate demand, hydrothermal activity and hydrocarbon seepage. Four intervals record δ34S values that exceed +30‰. We interpret these unusually 34S-enriched sulphides to be a result of limited sulphate diffusion into pore waters due to changes in sedimentation and/or periods of basinal restriction. Additionally, there are four negative δ34S excursions that are interpreted to reflect changes in the open/closed-system behaviour of sulphate reduction or availability of reactive iron. Our findings highlight the influence of basinal processes in regulating sulphur isotope records and the need for care before interpreting such signals as reflecting global conditions.

Measuring martian organics and methane

Planetary Science The Curiosity rover has been sampling on Mars for the past 5 years (see the Perspective by ten Kate). Eigenbrode et al. used two instruments in the SAM (Sample Analysis at Mars) suite to catch traces of complex organics preserved in 3-billion-year-old sediments. Heating the sediments released an array of organics and volatiles reminiscent of organic-rich sedimentary rock found on Earth. Most methane on Earth is produced by biological sources, but numerous abiotic processes have been proposed to explain martian methane. Webster et al. report atmospheric measurements of methane covering 3 martian years and found that the background level varies with the local seasons. The seasonal variation provides an important clue for determining the origin of martian methane. Science , this issue p. [1096][1], p. [1093][2]; see also p. [1068][3] [1]: /lookup/doi/10.1126/science.aas9185 [2]: /lookup/doi/10.1126/science.aaq0131 [3]: /lookup/doi/10.1126/science.aat2662