Hydrous Pyrolysis Experiments Research Articles

Formation mechanism of Gulong shale oil: Insights from semiclosed hydrous pyrolysis

As a typical lacustrine shale oil, Gulong shale oil has enormous resource potential. The current lack of understanding regarding the formation process of this oil has affected its development. This study uses a low-mature shale sample from the first member of the Qingshankou Formation in the Songliao Basin and analyzes the hydrocarbon generation, expulsion, and retention characteristics by conducting hydrous pyrolysis in a semiclosed system. The results show that the formation process of the Gulong shale oil can be divided into three stages. The slow hydrocarbon generation stage occurs when organic matter maturity (Ro) < 0.8%, and only a small amount of hydrocarbons is generated by kerogen cracking. The products are mainly heavy components (C15+), with retained oil composing the majority of them. The rapid oil generation stage occurs when Ro is of 0.8%–1.1%, where a large amount of kerogen is cracked to generate hydrocarbons and reaches its peak when Ro is 1.1%. Most of generated light component oil (C6–C14) is expelled, and retained oil is mostly composed of heavy components (C15+); Ro > 1.1% indicates the stage of oil cracking and rapid gas generation. Light component oil (C6–C14) and gaseous hydrocarbons (C1–C5) are formed when heavy components oil (C15+) begins to crack. The light-to-heavy ratio of the hydrocarbon products (C1–C14/C15+) rapidly increases with maturity. The hydrous pyrolysis experiment in a semiclosed system is more in line with actual geological conditions of Qingshankou Formation compared to other experimental systems and can simultaneously consider the influence of geological factors such as temperature, pressure, hydrocarbon expulsion, and formation water.

Generation Potential and Characteristics of Kerogen Cracking Gas of Over-Mature Shale

To investigate the characteristics and generation potential of gas generated from over-mature shale, hydrous and anhydrous pyrolysis experiments were carried out on the Longmaxi Formation in the Anwen 1 well of the Sichuan Basin of China at temperatures of 400–598 °C and pressures of 50 Mpa, with (hydrous) and without (anhydrous) the addition of liquid water. The results show that in the presence of water, the total yield of carbon-containing gases (i.e., the sum of methane, ethane, and carbon dioxide) was increased by up to 1.8 times when compared to the total yield from the anhydrous pyrolysis experiments. The increased yield of carbon dioxide and methane accounted for 89% and 10.5% of the total increased yield of carbon-containing gases. This indicated that the participation of water could have promoted the release of carbon from over-mature shale, like we used in this study. The methane generated in the hydrous pyrolysis experiments was heavier, with a δ13C value of −21.27‰ (544 °C) compared to that generated in the anhydrous pyrolysis experiments, which showed a lighter δ13C of −33.70‰ (544 °C). It is noteworthy that the δ13C values of the methane from hydrous pyrolysis at &gt;500 °C were even heavier than that of the kerogen from the over-mature shale, although the δ13C values of the methane show an overall increasing trend with increasing temperature both in hydrous and anhydrous pyrolysis. The carbon dioxide from hydrous pyrolysis was less enriched in 13C relative to that from anhydrous pyrolysis. Specifically, the δ 13C values of the carbon dioxide increased with the increasing temperature in anhydrous pyrolysis, whereas they remained nearly constant with increasing temperature in hydrous pyrolysis. The overall lighter δ13C values of the carbon dioxide generated in the hydrous pyrolysis experiments likely indicate that water tends to prompt the release of lighter carbon and/or suppress the release of heavier carbon from over-mature shale in the form of carbon dioxide, especially at higher temperatures, for example, of &gt;510 °C.

The influence of hydrocarbon generation on the sealing capability of mudstone caprock rich in organic matter

To investigate the sealing capability of mudstone caprock during the evolution of organic matter (OM)-rich mudstone, a series of hydrous pyrolysis experiments were first conducted to examine the impact of hydrocarbon generation. The pore type, pore structure, porosity, and gas breakthrough pressure of pyrolytic residual samples were analyzed by field emission scanning electron microscopy, low pressure nitrogen adsorption measurements, porosimetry, and gas breakout core experiments. To model the environment at different depths, these six experiments on hydrous pyrolysis were performed at different temperatures, lithostatic pressures, and hydrodynamic pressures, while other experimental factors such as the original sample, heating time, and rate were kept constant. The results showed that during the thermal evolution process, hydrocarbons were generated from OM in mudstone, resulting in the formation of pores within the OM. Organic acids produced by hydrocarbon generation effectively dissolved minerals, leading to the creation of numerous dissolution pores. Changes in pore type led to changes in pore structure and porosity. The volume of micropores and macropores showed an increasing trend before reaching a Ro value of 1.41%. However, after passing this threshold, they began to decrease. The volume of mesopores showed a decreasing trend before reaching a Ro value of 1.32%. After 1.32%, they began to increase. The porosity was mainly affected by the pore volumes of the mesopores and macropores. The porosity exhibited two peaks: the first occurred at a Ro value of 0.72%, with a porosity level of 4.6%. The second occurred at a Ro value of 1.41% and a porosity level of 10.3%. The breakthrough pressure was a comprehensive reflection of these influences, and its trend exhibited a negative correlation with porosity (R2 = 0.886). For two high values of porosity, the breakthrough pressure corresponded to two low values. Smaller values of the breakthrough pressure indicated a poorer sealing capability of the mudstone caprock. Overall, hydrocarbon generation in the mudstone affected the sealing capability. The mudstone in the studied area exhibited good sealing at Ro below 1.32%. However, once above the 1.32% threshold, the fluctuations of the breakthrough pressure values exhibited considerable variability, requiring a comprehensive evaluation to assess its sealing capability.

Characteristics of bound biomarkers released from pyrobitumens generated from the cracking of different types of crude oils

Pyrobitumen has attracted significant attention from petroleum geologists and geochemists due to its close relationship with crude oil and natural gas. Generally, the analysis of free biomarkers in pyrobitumen is of limited use, because these biomarkers can be easily modified by biodegradation and thermal alteration. The utilization of bound biomarkers in pyrobitumen appears to be a viable solution to address these issues. However, the compositions and concentrations of bound biomarker moieties in pyrobitumen can also be influenced by external factors such as thermal stress during its formation and the original abundance of asphaltenes in oil. The goal of this study is to examine the evolutionary history of free and bound biomarkers during the formation process of pyrobitumen from different types of crude oil. This is beneficial for the interpretation of covalently bound biomarkers released from natural pyrobitumens in applications such as oil/gas–solid bitumen-source correlation and oil/gas migration studies. Anhydrous and hydrous pyrolysis experiments were conducted on three types of crude oil (condensate, black oil, and heavy oil) to simulate the formation process of pyrobitumen. The comparison of free biomarkers in the liquid residuals of the oil pyrolysis and the bound biomarkers released by the hydropyrolysis of the corresponding pyrobitumen products indicated that the differences between free and bound biomarkers vary with the types of original oils. At elevated thermal maturity (Easy %Ro ≥ ca. 1.0%), the higher the absolute abundance of the asphaltene fraction within a particular oil, the greater the similarity between the distribution of bound biomarkers (especially for steroids) released from pyrobitumen and the corresponding original oil. Thus, the use of pyrobitumen-bound biomarkers formed by heavy oil (asphaltene-rich) can obtain relatively accurate biogenic information on paleo-reservoirs. However, caution should be exercised when employing pyrobitumen-bound biomarkers in paleo-reservoirs formed by light oil (asphaltene-depleted).

Semi-closed hydrous pyrolysis of type I source rock using formation water: Implications for biomarker behaviour under supercritical conditions

Hydrous pyrolysis is a useful artificial method to simulate hydrocarbon generation process and investigate the geochemical behaviour of biomarkers with the presence of water, which has great importance for conventional and unconventional oil-gas exploration. Formation water is ubiquitous saline water in oilfields, whereas not commonly utilized in hydrous pyrolysis. To determine the influence of formation water on the expulsion/retention behaviour of commonly utilized oil-source correlation indictors (Pr/Ph, Pr/n-C17, Ph/n-C18, and gammacerane index, C22/C21, C24/C23 and C26/C25 tricyclic terpane ratios, C31R/C30hopane, C31(22S/22S + 22R) hopane, moretane/hopane and Ts/Ts + Tm), two series of semi-closed hydrous pyrolysis experiments were conducted in 300–550 °C for the typical type I Green River Shale source rocks using distilled water and formation water. Such comparative study can also improve the understanding of whether supercritical fluids affect the biomarker behaviour because the temperature for major hydrocarbon generation in hydrous pyrolysis approaches the critical temperature for distilled water. The studied oil-source correlation indictors of immiscible oils generated in both experiments using distilled water and formation water exhibit similar patterns with the increase of thermal maturity levels. Both retained and expelled immiscible oils generated in the experiments using formation water and distilled water are situated in similar area of empirical genetic diagrams (e.g. Pr/n-C17vs Ph/n-C18, C31R/C30 hopanes vs C26/C25 tricyclic terpanes, and ternary plot of C27–C28–C29 regular steranes), suggesting that the presence of formation water/supercritical water does not significantly compromise the utility of these diagrams for oil-source correlation. Outcomes of this study also demonstrate that semi-closed hydrous pyrolysis is a useful artificial method for characterizing biomarker compositions of oils generated by type I source rocks and understanding geochemical behaviour of biomarkers under supercritical conditions.

Modified open-system hydrous pyrolysis

As an alternative to classic batch hydrous pyrolysis experiments, this proof-of-concept study presents a version of open-system hydrous pyrolysis (OSHP) in which the design of the apparatus allows for a small, continuous flow of only liquid-phase water to pass through the reactor during artificial maturation. This allows volatile, liquid, and soluble materials to migrate freely through and out of the system, whereas insoluble mineral matter and kerogen remain within the reactor. This apparatus differs from previous OSHP designs in the ability to maintain true open-system conditions, thus minimizing any cracking or back reaction of the products with the residual kerogen.Using the OSHP apparatus, immature Anna Shale (Pennsylvanian age, Illinois Basin) containing Type II marine kerogen was matured artificially at experimental temperatures ranging from 300 to 360 °C for up to 72 h. Petrographically, the immature Anna Shale is dominated by a micrinitic groundmass, and contains lesser amounts of vitrinite (both collotelinite and collodetrinite), inertodetrinite, bituminite, solid bitumen, and alginite. Under run conditions, mean random reflectance increased from 0.46 to 1.81% for vitrinite and from 0.66% to 1.50% for solid bitumen. Alginite fluorescence changed from a bright yellow to dull red-orange and decreased in intensity until all fluorescence was lost in shales pyrolyzed at temperatures ≥350 °C. Rock-Eval Tmax increased from 425° to 520 °C, TOC decreased (e.g., from 12.5% to ∼9% at a run temperature of 360 °C), S1 decreased from 0.71 to ∼ < 0.1 mg HC/g shale, S2 decreased from ∼40 to <1.0 mg HC/g shale, and HI decreased from ∼300 to <10 mg HC/g TOC. These changes are consistent with the maturation of the immature shale through the oil window and into the gas window. With increased maturity, the shale showed a loss in mass: shorter runs (≤ 24 h) typically lost <10%, whereas longer runs (72 h) lost 12–20%. These losses were likely due to the loss of moisture, liberation of hydrocarbons and other gases, and depletion in total sulfur from 2.03 to ∼0.1 wt% (much of which was pyritic sulfur); higher mass losses may have included retention of material in the reactor, tubing, or frits. Petrographic observations revealed that pyrite framboids, which were abundant in the immature shale, decreased in samples pyrolyzed at temperatures >330 °C and showed evidence of alteration to hematite.The OSHP design was intended to mimic normal burial maturation. However, based on the significant depletion in sulfur (i.e., pyrite), the current design and run parameters may more closely replicate conditions associated with hydrothermal alteration, possibly those associated with intrusion of organic-rich rocks. It is also possible that the OSHP approach could lead to a better understanding of organic-mineral interactions and mobility of elements, especially trace elements, during maturation.

Characterization of free and bound bitumen fractions in a thermal maturation shale sequence. Part 2: Structural evolution of kerogen and bitumen during shale oil generation, expulsion and retention

Knowledge of the formation and enrichment mechanisms of shale oil is essential to understanding its mobility and hence a critical aspect for evaluating shale oil exploration and production. Semi-open hydrous pyrolysis experiments were performed on an organic-rich shale to simulate the petroleum generation and expulsion processes of Type II kerogen. The residual rocks matured to different levels (Easy%Ro = 0.50–1.37) were thereafter treated with stepwise extraction to obtain free and bound bitumen fractions. This study focuses on the structural characteristics of kerogen, bitumen fractions, and expelled oil during shale maturation using Fourier transform infrared (FTIR) spectroscopy and quantitative flash pyrolysis–gas chromatography (Py–GC). The results reveal that the composition of expelled oils changes with increasing maturity, whereas an opposite trend in structural composition was observed for kerogen after significant oil expulsion from the shale occurs, beginning at Easy%Ro = 0.79. A notable change is that aliphatic carbon chains of kerogen became shorter and/or more branched while those of expelled oil appeared to be more aliphatic with longer carbon chains at this stage. However, the overall compositional structures of shale bitumen fractions, both in free and bound phases, show only slight changes within the oil window stage. The bound bitumen is generally enriched in oxygenated functional groups as compared to free bitumen, indicating that the bound bitumen is related mainly to the interaction between the mineral surface and the bitumen–kerogen matrix. During maturation, the cracking (including defunctionalization) and condensation of kerogen and bitumen fractions, as well as a compositional fractionation due to oil expulsion, are likely to be the main factors influencing the opposite trend observed in structural compositions between kerogen and expelled oil. These processes may control the quality and content of expelled oil and bitumen in a petroleum system. Nevertheless, kerogen, expelled oil and bitumen show a gradual trend of decarboxylation during shale maturation in general.

The initiation stage of thermochemical sulfate reduction: An isotopic and computational study

Thermochemical sulfate reduction (TSR) is one of the most important organic-inorganic reactions in petroleum reservoirs which significantly affects production and processing risks. The initiation (non-catalyzed) stage of TSR is critical in constructing a reliable kinetic model to predict this reaction. There is a large gap in our understanding of the mechanisms involved in this stage. In the present study, we used hydrous pyrolysis experiments with Na2SO4 and 1-dodecene as model compounds at pH= 1, and temperatures of 250 °C and 300 °C to simulate this early TSR stage. We measured and calculated the sulfur isotopic fractionations of sulfate, H2S and individual organic sulfur compounds (OSC). A thermodynamic model was used to determine the concentrations of the different sulfate species (H2SO4, HSO4¯, SO42-) at the experimental conditions. We then used ab initio calculations to determine possible radical and non-radical reaction mechanisms to explain the experimental results. During all the experiments, sulfate was consumed gradually while H2S and OSC were produced. The residual sulfate was 34S enriched up to 10.7‰ relative to the initial sulfate as pyrolysis time advanced. The produced H2S and OSC were typically about 21‰ 34S depleted relative to co-existing sulfate. The formation of OSC, such as C24 sulfides and C12 thiophenes, during the experiments and their similar δ34S values, suggest a common precursor and that radical mechanisms are involved in their formation. Calculated activation energies and free energies (Ea and ∆G‡, respectively) for the radical TSR mechanisms (20.6–34.1 and 32.3–44.4 kcal mol−1, respectively) yielded lower values than the non-radical TSR mechanisms (53.5–68.5 and 59.7–78.1 kcal mol−1, respectively). However, the calculated kinetic fractionation factors for the non-radical mechanism correlated better with the 11–12‰ experimental fractionation factor of current and previous studies. This similarity between the experimental and computational results indicates that these non-radical rather than radical mechanisms were likely involved in the reduction of sulfate in our experiments. Despite the lower activation energies, the radical TSR mechanisms are not dominant in our experimental system, probably due to competing reactions of the active radical species (alkyl and allyl radicals). Nevertheless, under natural conditions, the significantly lower amount of alkenes is expected to reduce the competition on the active alkyl and allyl radicals for alkylation, making these radical mechanisms competing pathways for the initiation stage of TSR.

Fractal characteristics of artificially matured lacustrine shales from Ordos Basin, West China

Nanometer scaled pores are critical to studying gas shale reservoirs. In order to obtain the information of the evolution mechanism of nanoscale pore within lacustrine organic-rich shales, artificially matured shale samples from the Ordos Basin were treated using hydrous pyrolysis experiment. Low-temperature nitrogen adsorption, inductively coupled plasma atomic emission spectrometry (ICP-AES), and field emission scanning electron microscopy (FE-SEM) experiments were used to investigate the nanopore evolution with migration and precipitation of materials. The results show that the pore sizes were distributed from 1.1 to 500 nm, and the overall porosity tends to increase first and then decrease. The micropores and fine mesopores (< 10 nm) increased gradually from the 250 to the 350 °C, calcite appeared dissolution following a small peak of feldspar dissolution at this stage, and the CO2 reaches a partial pressure peak at 350 °C. The micropores, mesopores and macropores increased steeply from the 370 °C to the 450 °C. Organic pores were not developed until 350 °C, and well developed at 370 and 400 °C. Organic pores, intergranular pores of clay and intragranular pores of pyrite were well developed at 370 °C. The cumulative specific surface areas increased at 400 °C caused by the dehydration and transformation reaction of clay minerals. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.

Effect of thermal maturation on the isotopic compositions of volatile organic sulfur compounds released from a kerogen by hydrous and anhydrous pyrolysis

Volatile organic sulfur compounds (VOSCs) may provide an alternative and improved approach in the evaluation of source and thermal maturity of natural gas. However, the role of water in the generation and decomposition of VOSCs is still ambiguous. Hydrous pyrolysis experiments were conducted on the isolated type Ⅱ-S kerogen of a source rock collected from the Ghareb Formation, Israel at 200–360 ℃ for 72 h (Easy%Ro = 0.32–1.20). The evolved gases were quantified for C1-C5 hydrocarbons, CO2, H2S and VOSCs. Compound specific isotope analysis were conducted both on the VOSCs and H2S (δ34S), and also on the C1-C5 hydrocarbons and CO2 (δ13C, δ2H). The results show that thiols concentration increased from 200 ℃ to 300 ℃ and then decreased from 300 ℃ to 360 ℃. The formed thiols isotopically co-vary with H2S from 200 ℃ to 340 ℃ and present the opposite trend with H2S from 340 ℃ to 360 ℃. The δ34S values of thiols are usually 3–7‰ lighter than the parent kerogen and 0.5–4‰ heavier than H2S. The concentrations of thiophenes generally increased with thermal stress with peak generation at 360 ℃, the δ34S values of thiophenes are mostly < 2–4‰ lighter than the parent kerogen and 2–8‰ heavier than H2S through the experiments. The δ13C and δ2H values of gaseous hydrocarbons are gradually enriched in heavy isotopes with increased thermal maturity. VOSCs dominated by thiols and thiophenes are generated by two ways: 1. the interactions between H2S and hydrocarbon molecules (synthetic pathway), 2. thermal cracking of OMs including kerogen and associated bitumen primary cracking and hydrocarbon secondary cracking (thermal cracking pathway). The synthetic pathway played the major role in the generation of thiols at the immature and early mature stages (200–300 ℃, Easy%Ro = 0.32–0.71), the cracking pathway has the dominant effect on the occurrence of thiols at higher thermal maturity levels (300–360 ℃, Easy%Ro = 0.71–1.20). The main pathway of thiophenes formation throughout the experiments is by the thermal cracking pathway. In the hydrous experiments, water promotes the synthetic pathway that generated thiols and retarded the thermal cracking pathway that forms thiophenes, but had no significant impact on δ34S values of individual VOSCs, δ13C and δ2H values of gaseous hydrocarbons. Therefore, combining with carbon and hydrogen isotopic compositions of gaseous hydrocarbons, molecular and sulfur isotopic compositions of individual VOSCs in the gas phase can provide valuable clues for petroleum systems in sedimentary basins including gas sources, generation pathways, thermal maturity levels and petroleum secondary transformations, such as thermochemical sulfate reduction in the subsurface reservoirs.

Hydrocarbon Generation Mechanism of Mixed Siliciclastic–Carbonate Shale: Implications from Semi–Closed Hydrous Pyrolysis

Affected by the complex mechanism of organic–inorganic interactions, the generation–retention–expulsion model of mixed siliciclastic–carbonate sediments is more complicated than that of common siliciclastic and carbonate shale deposited in lacustrine and marine environments. In this study, mixed siliciclastic–carbonate shale from Lucaogou Formation in Junggar Basin was selected for semi–closed hydrous pyrolysis experiments, and seven experiments were conducted from room temperature to 300, 325, 350, 375, 400, 450, and 500 °C, respectively. The quantities and chemical composition of oil, gases, and bitumen were comprehensively analyzed. The results show that the hydrocarbon generation stage of shale in Lucaogou Formation can be divided into kerogen cracking stage (300–350 °C), peak oil generation stage (350–400 °C), wet gas generation stage (400–450 °C), and gas secondary cracking stage (450–500 °C). The liquid hydrocarbon yield (oil + bitumen) reached the peak of 720.42 mg/g TOC at 400 °C. The saturate, aromatic, resin, and asphaltine percentages of bitumen were similar to those of crude oil collected from Lucaogou Formation, indicating that semi–closed pyrolysis could stimulate the natural hydrocarbon generation process. Lucaogou shale does not strictly follow the “sequential” reaction model of kerogen, which is described as kerogen firstly generating the intermediate products of heavy hydrocarbon compounds (NSOs) and NSOs then cracking to generate oil and gas. Indeed, the results of this study show that the generation of oil and gas was synchronous with that of NSOs and followed the “alternate pathway” mechanism during the initial pyrolysis stage. The hydrocarbon expulsion efficiency sharply increased from an average of 27% to 97% at 450 °C, meaning that the shale retained considerable amounts of oil below 450 °C. The producible oil reached the peak yield of 515.45 mg/g TOC at 400 °C and was synchronous with liquid hydrocarbons. Therefore, 400 °C is considered the most suitable temperature for fracturing technology.

Hydrocarbon generation and expulsion modeling of different lithological combination source rocks from the Funing Formation in the Subei Basin

The oil expulsion efficiency and retention efficiency of shale affect the enrichment and preservation of shale oil. Two series of semi-closed hydrous pyrolysis experiments were performed under in situ geological conditions on a Paleogene shale sample as a comparable analog to evaluate the generation and preservation potential of shale oil in the Funing Formation shale in the Subei Basin. The results show that 1) the oil-generation capacity evolution of different lithological combination source rocks in the Funing Formation of the Subei Basin can be roughly divided into four stages: a) relatively slow oil-generating and slow gas-generating, b) relatively fast oil-generating and slow gas-generating, c) oil cracking into gas, and d) kerogen cracking into gas; 2) different lithological combinations have different hydrocarbon generation, expulsion, and retention efficiencies. The total oil generation rate and gas generation rate of pure shale are higher than those of shale with a silty interlayer, and the exchange point between the oil expulsion rate and retention rate of pure shale is earlier than that of shale with the silty interlayer, which indicates that the pure shale experienced the expulsion and retention process earlier. Oil retention mainly occurs at an EqVRo of 0.84%–1.12%, while oil is mainly discharged to the adjacent siltstone at an EqVRo of 1.12%–1.28%. Based on the simulation under geological conditions, it is recognized that for shale oil exploration in the Subei Basin, the favorable thermal maturity is at an EqVRo of 0.84%–1.12%, and the favorable lithology is the shale with the silty interlayer. On one hand, the siltstone interlayer can provide pore space for the early generated oil, and the concentration difference of hydrocarbons between the shale and the interlayer can be formed so that the generated shale can continuously enter the interlayer. On the other hand, the shale above the interlayer can be used as a cap rock to preserve shale oil. The favorable area for shale oil exploration in the Subei Basin is the area with relatively high maturity (at a VR value of about 1.1%)

Hydrocarbon generation and potential of marine source rocks in supercritical water

Water, in ultra-deep layers of the earth and in layers receiving abnormally high heat, can exist in a supercritical state. Supercritical water (SCW) can participate in the transformations of organic compounds not only as a solvent but also as a reactant, influencing petroleum formation and the evolution of sedimentary organic matter. Here, we carried out hydrous pyrolysis experiments in both SCW and water vapor (WV) using two organic-rich marine rocks under closed conditions, to quantitatively evaluate the generation potential of hydrocarbons in a supercritical state and to clarify the effect of water phase on hydrous pyrolysis experiments. The results showed that SCW promoted gaseous and liquid hydrocarbon generation and facilitated the cracking of aliphatic hydrocarbons. For gaseous hydrocarbons, the action of SCW became stronger as the temperature increased. For liquid hydrocarbons, the peak yields of bitumen were enhanced by the SCW, and the temperature corresponding to peak yield in SCW was lower than that in WV. These results were attributed to the supply of hydrogen and oxygen from SCW for the petroleum formation and cracking. The δ13C and δD values of gases were also influenced by SCW. However, these values obtained in SCW did not always become positive as their yields increased. Generally, methane (CH4) was enriched in 12C and 1H. The influence of SCW on the isotopic fractionation of ethane (C2H6) and propane (C3H8) was more complex. The water phase is an important factor affecting the experimental results of hydrous pyrolysis. From these findings, it can be concluded that SCW allowed for increased conversion of sedimentary organic matter to gaseous and liquid hydrocarbons in ultra-deep layers and layers affected by volcanic-hydrothermal activity.

Physical properties of lacustrine shale oil: A case study on the lower member of the Lucaogou Formation (Jimusaer Sag, Junggar Basin, NW China)

Shale oil generated by the lower member of the Lucaogou Formation, which was deposited under saline lacustrine conditions within the Jimusaer Sag of the Junggar Basin (NW China), is generally characterized by high density and viscosity. However, a strong spatial heterogeneity of physical properties is observed as well. In this paper, these spatial variations and their controlling mechanisms are evaluated from the perspective of the primary depositional environment, source rock thermal evolution and fractionation effects of primary migration. Based on a detailed organic petrological and geochemical characterization of the target interval within the key well JHBE, hydrous pyrolysis experiments conducted on representative source rock samples from well JHBE, as well as established methods of oil-source correlation subjected to oil samples from several regionally distributed producing shale oil wells, a predictive model of density and viscosity changes of crude oil was established. According to the findings of this study, the thermal evolution and sedimentary environment of source rocks are the main factors affecting the quality of crude oil, and the fractionation effect of primary migration has little effect. Crude oil density and viscosity at first increase up to peak oil maturity, followed by a decrease with further increasing thermal evolution of the source rock. Furthermore, if the source rocks experienced similar thermal histories, it was discovered that relatively low density and corresponding viscosity values are characteristic for crudes from source rocks which were formed in a relatively high salinity environment. In contrast, hydrocarbon generation of source rocks formed under relatively low salinity environments is slightly retarded in direct comparison, and the density and viscosity of the generated crude oil are both relatively high. This highlights that a detailed organic petrographical and geochemical evaluation of the target formation is crucial for the assessment of shale oil potential and production sweet spots in alginite-dominated lacustrine sedimentary environments.

Generation and decomposition of low-molecular-weight organic acids in hydrous pyrolysis experiments with Opalinus Clay rock

Increasing temperature in claystone formations accelerates geochemical reactions between organic matter, water and mineral phases, promoting the generation and decomposition of organic compounds such as low-molecular-weight organic acids (LMWOA). LMWOA presence in claystone formations can enhance mineral dissolution, stimulate gas generation through thermal decomposition reactions, affect metal-ion complexation, alter sorption processes, and provide feedstock for microbial life. Hydrous pyrolysis experiments (80–200 °C and 20 MPa) using Dickson-type flexible gold-titanium reaction cells were conducted to investigate the thermal generation and decomposition of LMWOA in organic matter poor (0.64 wt%) Opalinus Clay. Maximum concentrations of dominant species were 44 µmol/kg clay formate (120 °C), 310 µmol/kg clay acetate (200 °C), and 74 µmol/kg clay oxalate (120 °C). Calculated half-live ranges of the dominant compounds were between 8.88 × 1010 and 2.81 × 10−3 years for the experimental conditions. Comparison of observed and calculated reaction rates suggests that the thermal generation and decomposition of LMWOA proceed simultaneously. Therefore, the observed generation rates in these and other previously published experiments are minimum estimates that need to be corrected for parallel decomposition reactions. The new quantitative reaction rate data presented here facilitate modelling of production/decomposition and local pore water concentrations of LMWOA by the extrapolation of kinetic data to temperatures encountered, for example, in natural heating during subsidence or by thermal intrusions.

Relating systematic compositional variability to the textural occurrence of solid bitumen in shales

This study presents Raman spectroscopic data paired with scanning electron microscopy (SEM) images to assess solid bitumen composition as a function of solid bitumen texture and association with minerals. A series of hydrous pyrolysis experiments (1–103 days, 300–370 °C) using a low maturity (0.25% solid bitumen reflectance, BRo), high total organic carbon [(TOC), 14.0 wt%] New Albany Shale sample as the starting material yielded pyrolysis residues designed to evaluate the evolution of solid bitumen aromaticity with increasing temperature and heating duration. Solid bitumen was analyzed by Raman spectroscopy wherein point data were collected from accumulations that ranged in size and degree of association with the mineral matrix. Raman spectroscopy results show that with increasing temperature and experimental duration, solid bitumen aromaticity increases and compositional variability decreases. With regards to texture and composition, coarser-grained solid bitumen (>1.3 μm from nearest mineral grain) has consistently higher, but less variable aromaticity than thinner, wispy solid bitumen which is more intimately associated with the mineral matrix. Collocated scanning electron microscope images were used to provide qualitative assessments of porosity hosted by the organic matter. These paired data sets suggest that solid bitumen porosity development and molecular composition are linked, and these parameters are related to the textural relationships of the organic matter within the whole rock. These results are discussed with perspective towards understanding how rock fabric and texture can influence organic matter evolution during thermal maturation of organic-rich marine shales and inform our broader understanding of these important energy resources.

Carbon-hydrogen isotopic systematics of gases generated in semi-closed hydrous pyrolysis of type I source rock with the presence of formation water: Implications for gas isotope partitioning under supercritical conditions

Hydrocarbon gas is a significant type of clean energy, and understanding its carbon-hydrogen (C–H) isotopic behaviour during hydrocarbon generation is essential for determining the gas genesis. Hydrous pyrolysis is an artificial method to simulate natural gas generation process with the presence of water. Formation water is a type of ubiquitous saline water in oilfields, whereas not commonly utilized in hydrous pyrolysis. To determine the influence of formation water on C–H isotopic systematics of hydrocarbon gas (methane, ethane and propane), two series of semi-closed hydrous pyrolysis experiments were conducted in 275–550 °C for the type I Green River Shale source rock using distilled water and formation water. Hydrocarbon gases generated in two series of experiments have similar δ13C ratios yet slightly different δD ratios at individual temperatures, which is likely attributed to the water-organic interaction as formation water has a different δD ratio compared to distilled water. The δ13C and δD ratios of gas increase with the rising of thermal maturity level, and exhibit quadratic correlations with vitrinite reflectance (VR) values in both series of experiments. The presence of supercritical water in hydrous pyrolysis does not significantly affect the gas C–H isotopic partitioning behaviour because gases collected below and above the critical temperature (374.2 °C) display consistent δ13C (δD)-VR patterns. Partial C–H isotopic reversals (δ13C2>δ13C3, δD2>δD3 or δD1>δD2) are observed during the pre-oil to oil generation peak and gas generation stages, which are likely mainly affected by gas mixing as evidenced by rigorous correlations between δ13Cn (δDn) ratios and reciprocals of the gas proportion (1/Cn).

Natural gas origin of Langgu Sag in Jizhong Depression, Bohai Bay Basin: Insight from hydrous pyrolysis experiments of gold tube

Two mudstone samples in low-maturity from the E2s3 source rock interval were used to hydrous pyrolysis experiments of gold tube with isothermal heating in order to better understand natural gas origins of Langgu Sag. The results displayed that the E2s source rocks in Langgu Sag attained the peak yield of nearly 350 mg/g TOC at about 0.9% of Ro for C6+ compounds due to thermal cracking occurrence of them if Ro higher than 0.9%, and C6-14 compounds would reach the maximum yield at about 1.3% of Ro owed to C14+ compounds preferentially cracking to them. Hydrocarbon gases were dominated by kerogen cracking gases at Ro lower than about 1.3% (referred to oil associated gases) and higher than about 1.8% (interpreted to residual kerogen cracking gases) but by secondary gases (belonged to early-generated liquid hydrocarbon cracking gases) if Ro at about 1.3%–1.8% for E2s source rocks. Furthermore, the yield of kerogen cracking gases would reach 90–100 ml/g TOC at 1.3% of Ro, far greater than 20 ml/g TOC as the lower limit standard of an effective gas source rock, even the kerogen cracking gas yield could reach 20 ml/g TOC if Ro at about 0.8%, therefore, as long as the E2s source rocks in Langgu Sag reach mature level, they can become effective gas source rocks.Considering their calculated Ro mainly distributing between 1.0% and 1.3% except Ordovician buried-hill reservoirs of the D2 oil-gas field, the proved natural gases in Langgu Sag mainly originated from kerogen cracking gases at Ro lower than 1.3%. And the gases from Ordovician buried-hill reservoirs of the D2 oil-gas field, with calculated Ro ranging about 1.3%–1.6%, essentially derived from residual liquid hydrocarbon cracking gases of the E2s4 source rocks, because the Ro of E2s3 source rock is mainly lower than 1.1% but it reaches about 1.8% for E2s4 source rock at present. The natural gases in shallow reservoirs experienced different degrees of bacterial degradation and formed methane-dominated gas reservoirs, consequently, the C1/C1-5 values displayed a range of 0.96–0.99 for bacterial degradation gas reservoirs and of 0.67–0.89 for natural gas reservoirs without bacterial reworking.

Soaking and hydrous pyrolysis of Indian oil shale: Identification of produced hydrocarbons and moieties

In the present study, soaking and hydrous pyrolysis experiments were performed on Indian oil shale in the presence of de-ionized (DI) water and brine solution under isothermal conditions of 300, 350, and 400 °C, and residence time of 72 h to simulate the effect of underground aquifers on the expulsion and composition of hydrocarbons from source rocks. During soaking pyrolysis, the oil yield increased minutely with the increase in soaking time from 1 to 4 weeks but was not influenced by the presence of brine solution (8.82 wt/wt%) or DI water (8.46 wt/wt%). A similar trend in the generation of oil yield was observed during hydrous pyrolysis in both DI water and brine solution. GC analysis of the gas samples showed methane (CH4) as the prominent gas produced during soaking and hydrous pyrolysis. During soaking pyrolysis, the concentration of C2 to C5 gases decreased with increased pyrolysis temperature. However, during hydrous pyrolysis, the concentration of C4 and C5 gases increased with an increase in pyrolysis temperature. GC-SimDist analysis of the oil samples obtained from soaking pyrolysis at 400 °C showed the compounds ranged from C8 to C38 in brine solution and from C6 to C34 in DI water. Nevertheless, during hydrous pyrolysis in both DI water and brine solution, the carbon number distribution of the oil samples was observed to be in the range of C8 to C38. GC–MS analysis of the oil samples obtained from the soaking and hydrous pyrolysis experiments showed that the molecular weight of the identified compounds ranged from 86 to 380 g/mol. GC–MS, ATR, and 1H NMR analyses showed that the oil samples obtained from soaking and hydrous pyrolysis experiments were rich in aliphatic compounds. The organic acids content appeared to be higher in the presence of DI water as compared to the brine solution.

Predicting isotopologue abundances in the products of organic catagenesis with a kinetic Monte-Carlo model

Thermal cracking of complex organic matter can occur under many geological settings. However, the role of thermal cracking vs. other chemistries (e.g., metallic catalysis or Fischer-Tropsch-type reactions) in petroleum formation remains controversial. Realistic modeling of isotope effects in chemical reaction networks involving thermal cracking might shed light on this problem, especially given the recent progress on measurements of intramolecular stable isotope distributions in organic compounds. Previously published models of thermal cracking in petroleum formation are incapable of predicting the intramolecular isotope patterns of products because they do not incorporate realistic precursors, elementary reactions and patterns of inheritance. In this study, we develop a kinetic Monte-Carlo (kMC) model to address this problem. We simulate thermal breakdown of different types of organic matter that is represented by molecular models. At the onset of each simulation, we initialize the model parent organic molecules with isotopic substitutions, and then subject them to free radical chain reactions in a many-step process. Every simulation captures a possible route of thermal degradation and tallies the numbers of each unique isotopologue of all product molecules at the end. Although this model produces data that contain information for all molecules and isotopic forms, in this study we focus on the proportions of many of the isotopologues of all of the C1-C7n-alkanes. We use two chemistry schemes that differ in complexity. The basic scheme (scheme A) includes only homolytic cleavage and capping of metastable radicals by hydrogen atoms. The more sophisticated scheme (scheme B) includes most reactions of importance in the free radical chain mechanism of thermal cracking. We find significant differences between the schemes A and B in predicted molecular and isotopic compositions of products. Scheme B is more consistent with natural data than scheme A, suggesting that full radical mechanisms should be considered in models of thermal cracking in natural hydrocarbon formation. Our model is further validated by reproducing results from hydrous pyrolysis experiments. For simulating natural petroleum formation, we found that the kMC model is acceptable at low maturity, but cannot match compositions and time estimates of mature gas formation, suggesting the influence of alternative mechanisms in late-stage, high-thermal-maturity catagenesis. Using our model, we provide mechanistic explanations for some of the existing observations, such as the evolution of intramolecular and intermolecular carbon isotope compositions with thermal maturity. Our study also makes predictions of intramolecular isotope compositions for higher-order alkanes (C4+) that have been little studied to date but present attractive targets for future measurements.