Although organic-rich oil-producing mudrocks have been studied extensively during the last decade, kerogen, as one the main constituents, is not thoroughly understood. The unknowns about kerogen ...elevate when it comes to its modulus of elasticity. Since kerogen is not as stiff as inorganic minerals, its presence can have a significant impact on the initiation and propagation of fractures in kerogen-rich formations that should undergo stimulation.
This study proposes an approach to estimate modulus of elasticity of kerogen with different thermal maturities using Raman spectroscopy. Various shale samples from the upper and lower members of the Bakken Formation were picked from several wells within the Williston Basin in North Dakota, USA. These samples were analyzed using Rock-Eval (RE) pyrolysis and vitrinite reflectance (%Ro) for thermal maturity. In addition, Raman spectroscopic measurements were made on samples and followed by PeakForce AFM for Young's modulus estimation of the organic matter. First, the Raman responses were correlated with the thermal maturity and then, a correlation was established to show the potential relationship between elastic modulus of organic matter and its Raman response based on the maturity levels.
•In this study, Rock-Eval, %Ro maturity, Raman spectroscopy and AFM PeakForce measurements were done on samples from 6 wells in Bakken Formation.•We proposed the potential of understanding thermal maturity and elastic modulus of organic matter using Raman spectroscopy.•Using proposed method, we predicted the range of mechanical properties of organic matter from initial stages of maturation to late dry gas window.
Vitrinite maturity and programmed pyrolysis are conventional methods to evaluate organic matter (OM) regarding its thermal maturity. Moreover, vitrinite reflectance analysis can be difficult if ...prepared samples have no primary vitrinite or dispersed widely. Raman spectroscopy is a nondestructive method that has been used in the last decade for maturity evaluation of organic matter by detecting structural transformations, however, it might suffer from fluorescence background in low mature samples. In this study, four samples of different maturities from both shale formations of Bakken (the upper and lower members) Formation were collected and analyzed with Rock-Eval (RE) and Raman spectroscopy. In the next step, portions of the same samples were then used for the isolation of kerogen and analyzed by Raman spectroscopy. Results showed that Raman spectroscopy, by detecting structural information of OM, could reflect thermal maturity parameters that were derived from programmed pyrolysis. Moreover, isolating kerogen will reduce the background noise (fluorescence) in the samples dramatically and yield a better spectrum. The study showed that thermal properties of OM could be precisely reflected in Raman signals.
Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna ...Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin.
Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.
•This work presents a high-resolution geochemical and paleontological record of the Oceanic Anoxic Event 2 in South America.•The La Luna Sea was in many regards biogeochemically similar to the modern Cariaco Basin.•Low amounts of organic carbon were buried in the epicontinental seas located on the western side of the Proto-Atlantic.•The La Luna Sea did not play an important role in carbon cycling during OAE2.
Temperatures in tropical regions are estimated to have increased by 3° to 5°C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. ...We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.
The amount of organic sulfur influences the kinetic behavior of kerogen upon thermal maturation as well as the crude quality and sourness. There has always been an interest to specify the amount and ...different forms of sulfur in source and reservoir rocks. In this study, we analyzed 28 samples from the Bakken Formation having different maturities and 5 samples from other shales containing type I, type IS, or type IIS kerogen using the Rock-Eval 7S instrument. Total sulfur data were compared to those from LECO SC-632 and CHNS elemental analyzer, with excellent to very good correlation, respectively. The sulfur index is a fast and reliable method to differentiate between type I and type II kerogens against their S-rich counterparts. Organic sulfur in the Bakken is not massively associated with S2 but with the refractory component of total organic carbon (S4). Thus, not all total organic sulfur (TOS) is responsible for the early generation of liquid hydrocarbons in organic-rich shales. Also, TOS should not be used as an indicator of kerogens being “type S”. Residual Sorg in the Bakken increased with maturity, and a considerable amount was available to generate H2S at relatively low temperatures via aquathermolysis, a process that occurs in the Greater Permian Basin. High concentrations of residual TOS in the Bakken (average of 2.3 wt % on whole-rock basis) meet the minimum of 1.4 wt % S (on a kerogen basis) required for H2S production. This suggests a possible correlation between TOS and high H2S production in Williston and other basins.
Biogenic gas shales, predominantly microbial in origin, form an important class of organic-rich shale reservoirs with a significant economic potential. Yet large gaps remain in the understanding of ...their gas generation, storage, and transport mechanisms, as previous studies have been largely focused on mature thermogenic shale reservoirs. In this study, the pore structure of 18 Antrim Shale samples was characterized using gas adsorption (CO
2
and N
2
). The results show that most of the Antrim Shale samples are rich in organic matter content (0.58 wt.% to 14.15 wt.%), with highest values found in the Lachine and Norwood members. Samples from the Paxton Member, characterized by lower organic content, have smaller micropore surface area and micropore volume but larger meso-macro pore surface area and volume. The deconvolution results of the pore size distribution from the N
2
adsorption indicate that all of the tested Antrim Shale samples have similar pore groups. Organic matter in the Antrim Shale hosts micro pores instead of meso-macro pores, while clay minerals host both micro and meso-macro pores. Mineral-related pores play a primary role in the total porosity. The biogenic Antrim Shale, therefore, has different pore structures from other well-studied thermogenic gas shales worldwide.
Geochemical screening is routinely integrated into larger exploration (and sometimes development) programs that also include assessments of the geological setting, petrophysics, mechanical properties ...of the rock, etc. The Rock-Eval analytical equipment and its classical Basic/Bulk-Rock method have been developed mainly to characterize potential source-rock intervals in petroleum systems. However, with the increasing interest in unconventional plays, it has been recently demonstrated that the use of modified pyrolysis-temperature regimes improves the quantification of hydrocarbons still present in oil-impregnated samples. In spite of their availability, the use of such modified pyrolysis-temperature regimes still remains scarce among users of pyrolysis data (e.g., exploration geologists and geochemists, reservoir engineers, petrophysicists, and other geoscientists).
Several cases were selected to portray how different analytical programs are necessary to obtain less biased and more accurate answers to critical questions during prospect and play evaluations and appraisals. Samples originating from conventional and unconventional plays in the Greater Permian Basin of West Texas (Wolfcamp & Spraberry formations), the DJ Basin in Colorado (Niobrara Formation), the Williston Basin (Lower Bakken Shale), and source-rock reservoirs in the Middle East were analysed each using three known different pyrolysis methods, namely the Institut Franҫais du Pétrole's “Basic/Bulk-Rock”, “Reservoir”, and “Shale Play”. The Shale Play and Reservoir pyrolysis methods yield oil-in-place estimates 20–42% higher than those yielded by the Basic/Bulk-Rock method on the same sample (e.g., for the Niobrara Formation – 87bbl oil/ac-ft Bulk method, 118bbl oil/ac-ft Reservoir method, 119bbl/ac-ft Shale method; for the Lower Bakken – 194bbl oil/ac-ft Bulk method, 246bbl oil/ac-ft Shale method). In addition, a mature, source-rock interval believed to contain gas-prone organic matter (Type III) based solely on TOC and pyrolysis data, was re-interpreted as composed mainly of amorphous oil-prone kerogen, following a multi-component study (which included transmitted and reflected-light organic petrography). These results present unequivocal evidence that underestimating the importance of selecting the proper analytical program can change interpretations dramatically.
•We discuss the importance of choosing the right analytical protocol for geochemical screening of hydrocarbon plays.•We show case-studies from unconventional plays where using the right pyrolysis method increased oil-in-place estimates.•Examining the pyrograms is the only wayto QC pyrolysis results.•We show a case-study where a multi-technique approach was necessary to understand the organic matter type and composition.
•Multifractal analysis was applied on the gas adsorption isotherms to derive the pore heterogeneity information.•Middle Bakken and Upper Bakken have the largest micropore and meso-macropore ...heterogeneity, respectively.•Organic matter could reduce meso-macropore heterogeneity and increase meso-macropore heterogeneity.•No robust relationship exists between maturity and pore heterogeneity.
Understanding pore heterogeneity can enable us to obtain a deeper insight into the flow and transport processes in any porous medium. In this study, multifractal analysis was employed to analyze gas adsorption isotherms (CO2 and N2) for pore structure characterization in both a source (Upper-Lower Bakken) and a reservoir rock (Middle Bakken). For this purpose, detected micropores from CO2 adsorption isotherms and meso-macropores from N2 adsorption isotherms were analyzed separately. The results showed that the generalized dimensions derived from CO2 and the N2 adsorption isotherms decrease as q increases, demonstrating a multifractal behavior followed by f(α) curves of all pores exhibiting a very strong asymmetry shape. Samples from the Middle Bakken demonstrated the smallest average H value and largest average α10−-α10+ for micropores while samples from the Upper Bakken depicted the highest average α10−-α10+ for the meso-macropores. This indicated that the Middle Bakken and the Upper Bakken have the largest micropore and meso-macropore heterogeneity, respectively. The impact of rock composition on pore structures showed that organic matter could increase the micropore connectivity and reduce micropore heterogeneity. Also, organic matter will reduce meso-macropore connectivity and increase meso-macropore heterogeneity. We were not able to establish a robust relationship between maturity and pore heterogeneity of the source rock samples from the Bakken.
Diamondoids are a class of cage-like hydrocarbons that are structurally similar to small pieces of diamonds. Their formation from polycyclic precursors parallels their dynamic stability. Diamondoids ...occur naturally in virtually every oil and condensate as well as in source-rock extracts. Their ubiquitous nature in oils and extracts of any thermal maturity level, along with their proven stability with increasing maturity, makes them useful as thermal maturity proxies past peak oil window and in the identification of mixtures of low- and high-maturity fluids. Two relationships in particular, methyldiamantane index (or MDI) and 3- + 4-methyldiamantane concentration vs. Stigmastane 5α, 14α, 17α(H)-24-ethylcholestane 20R C29 sterane concentration, are repeatedly used to establish thermal maturity relationships between oils and between oils and extracts and to identify mixtures of fluids with different thermal maturities.
The present comparative study shows how methyldiamantane proxies from West Texas oils and extracts (sourced from the Woodford Shale) and in oil samples from northern South America (Colombia) and the Middle East show overall a very good agreement with classic maturity proxies, such as Tmax values from Rock-Eval pyrolysis and vitrinite reflectance (Vro). However, caution is advised when using methyldiamondoid-derived proxies in the presence of drilling additives and contaminants (e.g., oil-based mud or OBM) and of oils and extracts that have experienced biodegradation. The latter is probably a highly unrecognized issue when using methyldiamondoids, which can result in erroneous interpretations of thermal maturity and mixing. Based on the results of this study we recommend the necessity of a holistic, multi-proxy approach, based on the integration of geochemical screening, organic petrography, and molecular geochemistry techniques as the best possible solution when dealing with such geochemical conundrums.
•Diamondoids can be used to determine the thermal maturity of oils, extracts from oils, and condensates of high maturity•Methyldiamantane concentrations are affected by oil-based mud and mixing with low-maturity fluids•Methyldiamantane ratio (MDI) appears to be less-affected by OBM than absolute concentrations•Correlation of MDI to methylphenanthrene index and ratio (MPI-1 & MPR) and vitrinite reflectance (VRo) is promising
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