Thermal Evolution Of Organic Matter Research Articles

Diagenetic evolution of tight sandstone and the formation of an effective reservoir in the lower member 3 of the Shahejie Formation, Bohai Bay Basin, East China

Heterogeneity in the physical properties of tight sandstones is mainly caused by complex diagenetic interactions in various combinations during the burial process. The tight sandstone oil in the Linnan Sag of Bohai Bay Basin has great development potential but it is unclear how effective reservoirs were formed, and this is a significant reason why exploration in the deep reservoir is so difficult. Here we studied the diagenetic evolution of the tight sandstone of Lower Member 3 of the Shahejie Formation, Linnan Sag, Bohai Bay Basin, by using cores, thin sections, XRD, SEM, grain-size analysis, high-pressure mercury intrusion porosimetry, fluid inclusions, and carbon and oxygen stable isotopes. Compaction, replacement, cementation and dissolution diagenesis are identified; all four processes mainly occur in the A phase of mesodiagenesis. The interaction of formation burial, thermal evolution of organic matter and dehydration of gypsum leads to the alternating presence of three stages of alkaline fluids and two stages of acidic fluids. The average dissolution porosity of the dissolution sandstone facies is greater than the lower limit of the porosity for the effective reservoir in the study area, which is key to the formation of effective reservoirs. By determining differential diagenetic evolution sequences of tight sandstone reservoirs under the constraints of different lithologies where sandstone is interbedded with mudstone, we are able to elucidate the formation of the effective reservoir as follows: (1) In the eodiagenetic stage, a large amount of early carbonate cement developed in the sandstone adjacent to the mudstone, filling the primary pores and forming a cementation sandstone facies. Smaller amounts of early carbonate cement developed in the interior of the sandstone, which “fixed” the primary pores, and has a positive effect on the formation of a large number of secondary pores in the dissolution stage. (2) In the early stage of mesodiagenesis (A1), due to the filling of primary pores, it is difficult for acidic fluids to enter the cementation sandstone facies adjacent to mudstone, and the dissolution is weak. But dissolution within the sand body was strong, producing a large number of dissolution pores and forming a dissolution sandstone facies that clearly enhanced the physical properties. (3) In a second early stage of mesodiagenesis (A2), dissolution still played a dominant role in the dissolution sandstone facies, due to the development of dissolution pores in the previous stage (it is easy for acidic fluids to enter the dissolution sandstone facies). The physical properties of the dissolution sandstone facies were once again enhanced and an effective reservoir gradually began to form. (4) At the end of mesodiagenesis stages A1 and A2 (26.2–24.6 Ma; 7.5–2.8 Ma), the reservoir experienced two phases of petroleum charging. The charging of the cementation sandstone facies developed at the margins of the sandstones was difficult due to the filling of primary pores. The dissolution sandstone facies are oil-bearing, with higher porosity, which indicates an effective reservoir. By coupling the thermal evolution of organic matter and the dehydration of gypsum with differential diagenetic evolution and the porosity evolution of reservoir, we are able to establish a reservoir genetic model that involves alternating seepage of organic acids and alkaline brines.

In-situ hydrocarbon formation and accumulation mechanisms of micro- and nano-scale pore-fracture in Gulong shale, Songliao Basin, NE China

By conducting experimental analyses, including thermal pyrolysis, micro-/nano-CT, argon-ion polishing field emission scanning electron microscopy (FE-SEM), confocal laser scanning microscopy (CLSM), and two-dimensional nuclear magnetic resonance (2D NMR), the Gulong shale oil in the Songliao Basin was investigated with respect to formation model, pore structure and accumulation mechanism. First, in the Gulong shale, there are a large number of pico-algae, nano-algae and dinoflagellates, which were formed in brackish water environment and constituted the hydrogen-rich oil source materials of shale. Second, most of the oil-generating materials of the Qingshankou Formation shale exist in the form of organo-clay complex. During organic matter thermal evolution, clay minerals had double effects of suppression and catalytic hydrogenation, which expanded shale oil window and increased light hydrocarbon yield. Third, the formation of storage space in the Gulong Shale was related to dissolution and hydrocarbon generation. With the diagenesis, micro-/nano-pores increased, pore diameter decreased and more bedding fractures appeared, which jointly gave rise to the unique reservoir with dual media (i.e. nano-scale pores and micro-scale bedding fractures) in the Gulong shale. Fourth, the micro-/nano-scale oil storage unit in the Gulong shale exhibits independent oil/gas occurrence phase, and shows that all-size pores contain oils, which occur in condensate state in micropores or in oil-gas two phase (or liquid) state in macropores/mesopores. The understanding about Gulong shale oil formation and accumulation mechanism has theoretical and practical significance for advancing continental shale oil exploration in China.

Evolution of organic pores in Permian low maturity shales from the Dalong Formation in the Sichuan Basin: Insights from a thermal simulation experiment

Shale oil and gas exploration is greatly impacted by the development and evolution of organic pore structures. In order to study the evolution of shale pore structures over maturities, we conducted thermal simulation experiments on low-mature organic-rich shale (Ro = 0.73%, TOC = 9.57%) obtained from the Dalong Formation. Various maturity stages were covered in these experiments, from low to over-maturity. By analyzing mineral composition, organic geochemistry, and characteristics of pore structures at various temperatures used for thermal simulation, we examined how organic matter pores generate and evolve during the formation of shale hydrocarbons. As a result, an integrated model of the evolution of pore structures in organic-rich shale was developed. Our study revealed the following insights: With increasing thermal simulation temperature, shale pore development exhibited three distinct stages: initial development, rapid development, and slow development. These stages correspond respectively to the three stages of organic matter thermal evolution. The distribution of pores in the shale transitioned from a unimodal structure dominated by larger pores to a bimodal structure with both micropores and macropores. Decomposition of organic matter and pressure compaction play a controlling role on organic porosity. Pore structure parameters and fractal dimensions of the pores were primarily influenced by organic matter decomposition, residual liquid hydrocarbons, and diagenetic processes. Additionally, kerogen type, morphology of organic matter debris, symbiosis of minerals-organic matter also impacted pore development.

Comparison of Evaporite-Related Source Rocks and Implications for Petroleum Exploration: A Case Study of the Dongying Depression, Bohai Bay Basin, Eastern China

The Dongying Depression (Bohai Bay Basin, eastern China) was widely filled with evaporite (anhydrite and halite) layers during the Paleogene period, especially the middle of the fourth member of the Shahejie Formation (Es4). Most evaporite layers are interbedded with mudstone strata. The strata of Es4 are divided into three sections, referred to as the upper layers, evaporite layers, and lower layers, respectively. The analysis of elemental concentrations, elemental ratios, and Pr/Ph suggests that the lower layers were deposited in an intermittent saline lake environment within a relatively dry climate. The evaporite layers were formed in a highly saline lake environment, whereas the upper layers were formed in a brackish-saline to fresh-water environment. Organic matter (OM) abundance indices, including total organic carbon (TOC), chloroform extracts, total hydrocarbon content (HC), hydrocarbon generation potential (S1 + S2), and OM type, show that the source rock potential for petroleum generation in the upper layers is best, that in the evaporite layers is fair, and in the lower layers it is poor. Carbon isotopes (δ13C) of hydrocarbons in the evaporite and lower layers were heavier than those in the upper layers. Thermal maturity parameters show that the thermal evolution process of OM in the upper layers was faster where evaporite were present compared with evaporite-free areas, while the thermal evolution of OM in the lower layers was slower in these regions. The high thermal conductivity of evaporites may have accelerated the thermal evolution of source rocks in upper layers and allowed hydrocarbon generation at a shallower burial depth. This resulted in the earlier appearance of the petroleum generation window compared to in evaporite-free areas. Meanwhile, the thermal evolution of OM in the lower layers was restrained, and consequently the hydrocarbon generation window was widened, which implies the potential for petroleum exploration in deep strata under the evaporite sequence. This is a common phenomenon in evaporite-bearing basins, according to previous and present studies.

Diagenesis and Pore Formation Evolution of Continental Shale in the Da’anzhai Lower Jurassic Section in the Sichuan Basin

As an unconventional oil and gas reservoir, the diagenesis and evolution of continental shale controls the formation and occurrence of inorganic and organic pores. In order to quantitatively characterize the pore characteristics of a continental shale reservoir and their influence on the evolution of the diagenesis stage, the characteristics of organic and inorganic pore types of continental shale in the Da’anzhai section of the lower Jurassic Ziliujing Formation were identified by means of X-ray diffraction mineral composition analysis and argon ion polishing scanning electron microscope measurements and observations, and the influence control of the diagenesis stage on the pore development of the continental shale reservoir and its control were clarified. The results show the following: ① The organic matter pores in continental shale are developed in large quantities, including organic matter pores in the mineral asphalt matrix and organic matter pores in the kerogen; the pore types of inorganic minerals are very rich, the main pore types are linear pores between clay minerals, intergranular (intergranular) pores, and intragranular corrosion pores, and microcracks are also developed. ② When affected by compaction diagenesis, the inorganic pores of continental shale decrease with an increase in the burial depth and diagenesis degree. ③ The burial depth of continental shale is 2000–3000 m in the middle of diagenetic stage A, and a large number of organic matter pores and dissolved inorganic pores develop at this depth, meaning that the total porosity of shale increases significantly. The burial depth of continental shale is 3000–4000 m at diagenetic stage B, where kaolinite and other clay minerals are dehydrated and converted into illite, the brittleness of shale is increased, and the interior of the shale is subject to external stress, causing microcracks to form. In the late diagenetic stage, when the buried depth of the continental shale is more than 4000 m, the organic matter is subject to secondary cracking and hydrocarbon generation, the organic pores of shale increase in number again, and the inorganic pores decrease in number due to compaction. In conclusion, we found that the burial depth is the main control factor for the development of pores and microfractures in continental shale reservoirs; diagenesis caused by burial depth is the main factor affecting the development of pores and microfractures in continental shale reservoirs; and the shale burial depth in this case is more than 3500 m, which is in the middle of diagenetic stage B. Inorganic porosity in shale is reduced, and the number of microfractures is increased. When the shale is buried more than 4000 m deep in the late diagenetic stage, the thermal evolution of organic matter in shale is high, and methane gas is generated in large quantities, which is conducive to the formation and development of organic matter pores in continental shale.

Fine characterization of pore evolution of oil shale during thermal simulation

Oil shale is a kind of organic‐rich fine‐grained sedimentary, whose pore evolution is affected not only by the evolution of inorganic minerals, but also by the thermal evolution of organic matter (OM). Meanwhile, the interaction between inorganic minerals and OM makes the pore evolution of oil shale more complex. Therefore, it is more important to fine the evolution characteristic of pores in different maturity stages and explore the pore size range affected by each influencing factor. This study selects immature oil shale (Tmax = 430°C, TOC = 8.2%) of Nenjiang Formation in Songliao Basin as a research object, and samples with different maturity are obtained through a series of thermal simulation experiments. Combined with petromineralogy, organic geochemistry and reservoir physical property analysis, the evolution process of pores is determined. The volume and specific surface area evolution of mesopores is different from that of macropores in the immature to mature stage, the volume and specific surface area evolution of which is same in the mature to over‐mature stage. There is a strong positive correlation between volumes of macropores and porosity (R2 = 0.94), between volumes of mesopores and specific surface area (R2 = 0.96). The influencing factors mainly affect the pore size less than 400 nm. Hydrocarbon generation of OM affects the whole pore sizes. Residual oil is stored in pores with pore size of 5–25 nm. The effect of carbonate dissolution and pyrite pyrolysis on mesopores is stronger than that of macropores. The transformation of illite/smectite to illite mainly affects the pore size greater than 100 nm. In the whole process of thermal simulation, the OM thermal evolution, the inorganic minerals dissolution and pyrolysis and the clay minerals transformation are the main influencing factors of pore evolution.

Diagenetic Evolution Mechanism of the Jurassic Tuffaceous Sandstone Reservoir in Qikou Sag, Bohai Bay Basin, East China

Exploring for hydrocarbons in a pyroclastic-affected reservoir is an important research topic. Previous studies have mainly focused on laminated pyroclastic. A large number of dispersed pyroclastic is present in sedimentary rocks, and dispersed volcanic ash strongly influences the diagenetic evolution of sandstone reservoirs. However, these aspects remain understudied. We studied the mechanism of the diagenetic evolution of the Jurassic tuffaceous sandstone reservoir in Qikou Sag of the Bohai Bay Basin by performing inclusion temperature measurements, rock slice identification, and scanning electron microscopy, and using electron microprobes and microzone isotopes. We determined the mechanism of water-rock interaction. Based on microscopic observations, we determined that the main diagenesis included two-stage dolomite cementation, two-stage calcite cementation, quartz cementation, and transformation and dissolution of clay minerals. The hydrolysis and chemical transformation of pyroclastic during burial not only provided an alkaline environment in the early stage of diagenesis but also supplied ions for the formation of microcrystalline quartz and early dolomite and the transformation of clay minerals. Leaching and denudation generated early dissolution caused by a tectonic uplift. Following the epigenetic stage, microbial activity stimulated the formation of early calcite during the shallow burial stage. When the burial temperature of the stratum was 80 °C, the acidic fluid discharged from the thermal evolution of organic matter was neutralized by the soluble components in the pyroclastic, which prevented the formation of a large-scale acidic environment. When the burial temperature exceeded 100 °C, the acidic fluid generated by thermal catalytic decarboxylation of organic matter formed a large quantity of dissolution. The dissolution of plagioclase promoted the overgrowth of quartz and the growth of kaolinite.

Hydrocarbon yield evolution characteristics and geological significance in temperature-pressure controlled simulation experiment

A semi-open system temperature-pressure controlled hydrocarbon generation and expulsion simulation experiment was carried out to explore the hydrocarbon generation and expulsion potential and mechanism of deep environment “coal measure” source rocks in the Qaidam Basin. A WYNN-3 high temperature and high pressure (HTHP) simulator and Middle Jurassic source rocks (III type organic matter, carbonaceous mudstone, and coal, RO is 0.67% and 0.64%, respectively) of Well DMG1 in the northern margin of the Qaidam Basin were used during the investigation. The results demonstrated that: (1) The maximum total oil yields of carbonaceous mudstone and lignite, which respectively measured at 79.38 mg/gTOC and 37.30 mg/gTOC, revealed a “double peaks” evolution law as a whole. (2) In the lower evolution stages (T ≤ 300 °C, P ≤ 42.0 MPa), the expelled/discharged oil yields of the two types of source rocks were lower than those of the residual oil, and the hydrocarbon expulsion efficiencies were low. However, at 400 °C (51.0 MPa), they significantly increased reaching 76.84% and 83.72%, respectively. (3) The main group components of the discharged oil were resin and asphaltene, and the component yields were generally comparable to those of liquid hydrocarbons. The yields of expelled oil group components of carbonaceous mudstone were higher than those of coal. (4) The simulated gas was primarily composed of hydrocarbon gas and non-hydrocarbon gas (CO2, N2), and with the elevated thermal evolution, the yields of total hydrocarbon gas/gaseous hydrocarbon increased. The two types of source rocks had maximum hydrocarbon gas yield of 116.46 mL/gTOC and 36.85 mL/gTOC, respectively. (5) The vitrinite reflectance (RO) increased as temperature and pressure conditions enhanced, and it exhibited good temperature consistency. The results of this temperature-pressure controlled simulation experiment showed that temperature was still the dominant factor in the thermal evolution of organic matter, fluid pressure had a “dual” control on the formation of type III organic hydrocarbon products, and “coal measures” source rocks still had a strong potential for hydrocarbon generation in the later stages of evolution. This research provided a certain data reference for the hydrocarbon generation and expulsion law of Jurassic deep “coal measures” source rocks in the northern margin of the Qaidam Basin.

Pore Structure Characteristics and Permeability Stress Sensitivity of Jurassic Continental Shale of Dongyuemiao Member of Ziliujing Formation, Fuxing Area, Eastern Sichuan Basin

A shale condensate gas reservoir with a high clay content and a high formation pressure was found in the Jurassic shale of the Dongyuemiao Member in the Fuxing area of the eastern Sichuan Basin. Reservoir characteristics and formation pressure have a significant influence on optimal development. The present study investigated the continental shale of the Dongyuemiao Member in Well F. The petrological properties, physical properties, and pore structure of the Dongyuemiao Member were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N2 adsorption, and mercury intrusion porosimetry (MIP). The permeability stress sensitivity characteristics of the shale reservoirs are discussed based on the change in shale porosity and permeability under overburden pressure. The tested shale samples yielded total organic carbon (TOC) and S1 + S2 values ranging mainly from 1.0 wt.% to 1.5 wt.% and from 0.39 to 2.28 mg/g, respectively, which was in the high maturity stage of the thermal evolution of organic matter (OM). The shales of the Dongyuemiao Member were found to contain high average clay mineral contents (more than 50%) of calcite and quartz, as well as albite, pyrite, dolomite, and halite. The main developments were identified as silica-rich argillaceous shale lithofacies, argillaceous shale lithofacies, and mixed argillaceous shale lithofacies. The pores were found to mainly be plate-like and flake-like interlayer pores of clay minerals and OM pores with various shapes. The pore size was mainly concentrated below 110 nm, and the pore volume increment increased in flakes with pore diameter. The average porosity and permeability of shale were found to be 4.827% and 0.243 mD, respectively. Clay minerals and quartz are beneficial for improving the porosity and permeability of reservoirs, while carbonate minerals have the opposite effect. The permeability of the shale showed a negative exponential change with increasing effective stress under overburden pressure. When the effective confining pressure was greater than 20 MPa, the decline rate of the shale permeability decreased with increases in the effective stress. The higher the clay mineral and TOC content, the stronger the stress sensitivity of shale permeability. The higher the carbonate mineral content, the weaker the stress sensitivity of shale permeability. The porosity sensitivity exponent indicates that matrix pores and micro-fractures are both developed in the Dongyuemiao Member, and the development of internal fractures is the main factor in the strong stress sensitivity of the shale permeability in the study area.

Organic matter occurrence and pore-forming mechanisms in lacustrine shales in China

The evolution of pore structure in shales is affected by both the thermal evolution of organic matter (OM) and by inorganic diagenesis, resulting in a wide variety of pore structures. This paper examines the OM distribution in lacustrine shales and its influence on pore structure, and describes the process of porosity development. The principal findings are: (i) Three distribution patterns of OM in lacustrine shales are distinguished; laminated continuous distribution, clumped distribution, and stellate scattered distribution. The differences in total organic carbon (TOC) content, free hydrocarbon content (S1), and OM porosity among these distribution patterns are discussed. (ii) Porosity is negatively correlated with TOC and plagioclase content and positively correlated with quartz, dolomite, and clay mineral content. (iii) Pore evolution in lacustrine shales is characterized by a sequence of decreasing-increasing-decreasing porosity, followed by continuously increasing porosity until a relatively stable condition is reached. (iv) A new model for evaluating porosity in lacustrine shales is proposed. Using this model, the organic and inorganic porosity of shales in the Permian Lucaogou Formation are calculated to be 2.5%–5% and 1%–6.3%, respectively, which correlate closely with measured data. These findings may provide a scientific basis and technical support for the sweet spotting in lacustrine shales in China.

Origin of Dolomite in Lacustrine Organic-Rich Shale: A Case Study in the Shahejie Formation of the Dongying Sag, Bohai Bay Basin

In most organic-rich shale reservoirs, dolomite is widely distributed and has different types and crystal sizes. However, the characteristics and formation mechanism of the dolomites in organic-rich shale are still poorly understood. Petrographic and geochemical analyses were performed to interpret the formation of dolomite in the lacustrine organic-rich shale of the Shahejie Formation, Dongying Sag, Bohai Bay Basin. Four types of dolomites, which represent episodic recrystallization, were classified based on crystal size and shape: 1) micritic dolomite (Dol-1), 2) sub-to euhedral (cloudy cores with clear rims) dolomite (planar-e) (Dol-2), 3) anhedral dolomite (coarse planar-s to nonplanar crystals) in phosphatic particles (Dol-3), and 4) fracture-filling anhedral dolomite (Dol-4). Dol-1 has nonplanar mosaic micritic crystals with irregular intercrystalline boundaries and dull cathodoluminescence (CL), suggesting dolomitization during the early burial stage. Dol-1 tends to occur under high paleosalinity and warm conditions. Furthermore, the syngenetic relationship, with abundant framboidal pyrite and gypsum, suggests that bacterial sulfate reduction (BSR) may influence the formation of Dol-1. The high content of Sr and low content of Mn/Sr also indicate less influence on burial. The Dol-2 crystals show cloudy cores with clear rims attributed to progressive dolomitization during burial. Dol-2 is always associated with the organic matter within the organic matter-rich lamina. The anhedral crystals and undulate (sweeping) extinction of Dol-3, which is usually encased by phosphatic particles in the organic-rich lamina, reflect the recrystallization affected by bacteria and the subsequent thermal evolution of organic matter. Dol-4 fulfills the abnormal pressure fractures crosscutting the earlier phases (Dol-1 and Dol-2) with undulate (sweeping) extinction and different rare earth element (REE) patterns. Dol-4 may be affected by hydrothermal fluids, which are influenced by the thermal evolution of organic matter. The 87Sr/86Sr values of the four types of dolomites similarly demonstrate the same dolomitizing fluids. Dol-1 to Dol-3 have similar REE patterns but are different from Dol-4, suggesting that Dol-4 likely resulted from circulation through basinal sediments instead of different fluids. Focusing on the origin of dolomite has been instrumental in understanding the diagenetic evolution, fluid flow, and organic-inorganic interactions in organic-rich shale and, hence, the reservoir formation of shale oil.

Effect of Volcanic Events on Hydrocarbon Generation of Lacustrine Organic-Rich Shale: An Example of the Upper Triassic Galedesi Formation in the Hala Lake Depression, South Qilian Basin, China

The thermal evolution process of organic matter is associated with the complete hydrocarbon generation and expulsion process in shale, however, the thermal evolution of organic matter is a long process and cannot be realized without experimental simulations. Although several scholars have substantially studied the thermal evolution of organic matter, it remains a challenging and much debated issue in the studies of organic geochemistry. Volcanic events are crucial in the enrichment of organic matter, and appropriate heating accelerates the thermal evolution of organic matter. However, how strong-rock baking restricts the evolution of organic matter in shale has not been specifically studied. The South Qilian Basin in China is a typical superimposed basin where complex tectonic movements have induced multiple volcanic events, which makes it a favorable location to perform the aforementioned research. This study used the Galedesi Formation shale in the Hala Lake Depression of the South Qilian Basin as an example for investigating the constraints of the volcanic events related to the thermal evolution of organic matter by integrating the results obtained using the geochemical and petrological methods. Our results demonstrate that the lacustrine Galedesi Formation shale of the Hala Lake Depression in the Late Triassic is a typical deep-lake facies deposit with good hydrocarbon generation potential. However, because of the influence of regional tectonic evolution, the burial depth of shale is not deep and the thermal evolution of organic matter is insufficient. Due to the influence of multiple volcanic thermal events in the later stages, the thermal maturity of organic matter in the Galedesi Formation shale generally exceeds 3.0%, which is abnormally high. The apparent carbonization of organic matter can be observed via scanning electron microscopy. Rapid magma baking typically cannot effectively promote the hydrocarbon generation of shale organic matter. Finally, the burial depth of lacustrine shale of the Galedesi Formation in the Hala Lake Depression of South Qilian Basin is too shallow. Organic matter hydrocarbon generation and later shale preservation conditions are not conducive to the enrichment, accumulation, exploration, and development of shale gas.

Process and Controlling Factors of Pore Evolution in Marine-Continental Transitional Black Shale—An Example From Permian Shanxi Formation in the Eastern Margin of Ordos Basin

Pore and pore network evolution of shale is critical for the evaluation the pore system in shale gas reservoirs. Thermal maturation effect acts as an indispensable role in porosity evolution. In this paper, high-temperature and high-pressure in-situ thermal simulation experiments were conducted to investigate the process and controlling factors of pore evolution in marine-continental transitional shale. Multiple methods, including scanning electron microscopy (SEM), X-ray diffraction, helium porosimetry and low-pressure N2 and CO2 adsorption were used to investigate the evolution of mineral composition and pore structure at different stages of thermal maturity. The results showed that type III organic matter (OM) generated petroleum with the thermal maturity increasing. The total organic carbon (TOC) decreased by 13.3% when temperature reached 607°C. At the same time, it produced numerous organic and mineral pores during hydrocarbon generation. Besides, some changes in mineral composition have occurred, especially in illite (from 11% to 31%) and kaolinite (from 89% to 69%). In general, it can divide into 3 stages (maturity, high maturity and over maturity) for shale pore system evolution with the thermal maturity increasing. In the low maturity period, large amounts of pyrolytic bitumen and oil generated to fill the pores, causing the pore system to diminish; in the high maturity period, a large number of pores were generated when oil is cracked into gas, resulting in a rapid expansion of the pore system; in the over-maturity period, the cracking of pyrolysis and hydrocarbon slowed down, allowing the pore system to stabilize. Shale pore evolution is primarily controlled by the thermal evolution of OM, and the conversion of inorganic minerals contributes less to pore evolution compared to organic matter. The high maturity period (1.2% < Ro < 2.0%) was the period when extensive pyrolysis and hydrocarbon generated in the Shanxi Formation shale, which contributed mostly for the pores generation and accumulation of shale gas.

Diagenesis and hydrocarbon charging period in the Lower Cretaceous Bayingebi Formation, Hari sag, Yin`E Basin, Northern China

The Lower Cretaceous Bayingebi Formation at the Hari sag, Northern China is a significant bearing oil-gas tight reservoir in the Yin`E Basin. In order to understand petrophysical properties of the reservoirs, the diagenesis and hydrocarbon accumulation phases were investigated using casting thin sections, scanning electron microscopy (SEM), X-ray diffraction and fluid inclusion analysis. Additionally, the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge are investigated. The results showed that the pore types include primary pore, intergranular pore and dissolution intragranular pore. The diagenesis of sandstone reservoirs in the process of burial and diagenesis mainly includes compaction, fracture, cementation, metasomatism and dissolution. The comprehensive analysis of various diagenetic stage indexes shows that Bayingebi Formation is in the middle diagenetic stage A-B. The homogenization temperature and salinity of fluid inclusions are low temperature and low salinity (120 °C ∼ 130 °C, 0% ∼ 9%), medium temperature and high salinity (140 °C ∼ 150 °C, 10% ∼ 17%) and high temperature and medium salinity (160 °C ∼ 170 °C, 9% ∼ 11%) respectively. According to the homogenization temperature of the inclusions and the history of burial evolution, the Bayingebi Formation has mainly three phases of hydrocarbon accumulation (100 ∼ 97Ma, 93 ∼ 87Ma, 84 ∼ 81Ma).

Thermal history of the Sabinas - Piedras Negras Basin (Northeastern Mexico): Insights from 1D modelling

The thermal and burial history of the Sabinas-Piedras Negras basin was studied by 1D modelling using vitrinite reflectance values measured on core samples from nine wells. Three heat flow scenarios led to model calibrations, including a simple one and a complex one integrating a thermal anomaly following the maximal burial phase. The differences in hydrocarbon generation curves due to these two scenarios were studied.The studied thermal anomaly induces two distinct phases of primary cracking of kerogen. The first phase is clearly due to burial, whereas the second seems to be due to the thermal event. The increase in the proportion of converted organic matter attributed to this second phase varies depending on the source rocks considered, e.g., for the La Casita source rock, there is a 10% TR increase, whereas that of the La Peña and Eagle Ford source rocks, is 35 and 15% respectively.The thermal evolution of organic matter in the Sabinas-Piedras Negras Basin is mainly due to the burial phase. In contrast, the thermal event considered only causes a temporal shift of hydrocarbon generation. This hydrocarbon generation, primordially gas, occurred mainly before and during the compressive phase (“late Laramide” time step 40-35 Ma [middle-late Eocene]) that affected the Sabinas-Piedras Negras Basin.

Origin of organic matter and organic pores in the overmature Ordovician-Silurian Wufeng-Longmaxi Shale of the Sichuan Basin, China

Organic matter (OM)-hosted pores play important roles in controlling the porosity and gas content of gas shales. However, the control of organic pore development and preservation remains poorly understood, partly because of the inability to distinguish OM types under the scanning electron microscope (SEM). In this study, seven over-mature Wufeng-Longmaxi Shale samples (equivalent vitrinite reflectance 1.93%–3.07%) were investigated with organic petrography and scanning electron microscope to study the origin of OM and development of organic pores in this black shale succession. Correlative light and electron microscopy was employed to examine pore development in individual macerals. Organic petrographic observations show that OM in these over-mature black shales is dominated by pyrobitumen and graptolites (> 90 vol% of total OM based on point counting). Pyrobitumen mainly occurs as matrix bitumen in the fine-grained matrix, and also as a mixture with clay minerals and fossil cavity infillings. Vitrinite-like particles are minor constituents of OM in the Wufeng-Longmaxi Shale, and their average reflectance, reflectance distribution, and morphology suggest that they may be derived from graptolite fragments lacking diagnostic features and acritarchs. OM type critically controls the development of organic pores. Organic pores primarily occur in pyrobitumen, whereas other macerals are mostly non-porous when examined under the SEM. Results of this study provided important insights into OM thermal evolution and organic pore development in source-rock reservoirs, calling for a critical appraisal of OM in black shales with organic petrography in addition to SEM.

Sedimentary environment and organic matter enrichment of black mudstones from the upper Triassic Chang-7 member in the Ordos Basin, Northern China

The black mudstone of the Chang-7 Member is the most important source rock in the Ordos Basin. In order to understand the paleo-climate, weathering, redox conditions, salinity, water temperature, water depth, and primary productivity under which black mudstone of the Chang-7 Member was deposited, we have analyzed total organic carbon (TOC), rock pyrolysis parameters, major elements, and trace elements of 60 black mudstone samples from the Chang-7 Member of the Yanchang Formation in the Ordos Basin. Geochemical indicators such as TOC, volatile hydrocarbon and residual hydrocarbon (S1 + S2), hydrogen index (HI), chloroform asphalt “A”, maximum pyrolysis peak temperature (Tmax), Vitrinite reflectance (Ro), kerogen macerals, and other organic indicators, show that the kerogen type of the Chang-7 Member was type I–II1, the thermal evolution of organic matter was in the mature stage, and it has good hydrocarbon generation potential. The major and trace element characteristics, such as C-value, Sr/Cu, Rb/Sr and Mg/Ca ratios, chemical alteration index(CIA), Sr/Ba, B, Ga-B-Rb trace element triangle, B/Ga, 1000 × Rb/K2O, V/(V + Ni), pristane/phytane ratio (Pr/Ph), foraminiferal Mg/Ca, and Co values indicate that the sedimentary environment of the Chang-7 Member was semi-arid and semi-humid, with continental freshwater, a reductive sedimentary environment, and an average paleotemperature of 16 °C. Based on the analysis of Baxs and Ba/Al ratio, the Chang-7 Member has medium–high primary productivity, which together with the sedimentary environment, played an important role in the enrichment and preservation of organic matter.

Coexistence and Development Model of Multi-Minerals Dominated by Multilayer Magma Intrusion

The diversity of coal measure determines the occurrence state and spatial distribution complexity of mineral resources. Abundant resources have become an important part of geological resources and have attracted more and more attention. Coal measure and their overlying and underlying strata often coexist with various mineral resource types, and there is a certain relationship between their genesis and occurrence. In order to further enrich the theory of comprehensive exploration and coordinated development of multi-mineral resources, this paper takes the Huanghebei Coalfield as an example to systematically study the genesis mechanism and occurrence law of coal seam, coalbed methane, and coal-measure shale gas in Late Paleozoic and rich iron ore in Ordovician limestone underlie coal measure. The research is that: 1) The Late Paleozoic Carboniferous-Permian Marine facies, terrestrial facies, and transitional facies all developed in the coal-bearing area in the Huanghebei Coalfield, and the coal seams and mud shales developed well in Shanxi Formation and Taiyuan Formation. 2) Yanshanian magmatic intruded into Ordovician limestone. Contact metasomatism occurred between the ore-bearing hydrothermal fluids and the surrounding rocks, which led to skarn formation. The magnetite mineralization occurred in the metasomatism alteration process, and finally, the contact metasomatic iron deposit was formed; 3) Yanshanian magma intrusion has a significant impact on the generation of coal from coalbed methane and shale gas in the coal measures of Late Paleozoic. The magma carries a lot of heat by baking the coal seam and overlying shale, which is reflected explicitly in the increasing metamorphism degree of coal. Under the action of high temperature, the secondary gas of coal seam and coalbed methane increase sharply. The maturity and thermal evolution of organic matter in shale beds increased, and the shale gas entered a favorable range. The intrusion of magma greatly enhances the thermal evolution of organic matter in coal and shale, forming a variety of coals and promoting the generation and accumulation of coalbed methane and shale gas. At the same time, Mesozoic magmatic intrusion also controlled the formation of rich iron ores. According to the characteristics of mineral development and distribution in the study area, a multi-mineral development and distribution model of “coal - coalbed methane - shale gas - rich iron ore” coexists in the Huanghebei Coalfield, which is referred to as the “Huanghebei model”.

Discussion of “Quantitative evaluation of thixotropy-governed microfabric evolution in soft clays”

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.

Effects of montmorillonite charge reduction on the high-temperature/high-pressure pyrolysis of organic matter

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.