Lacustrine Organic-rich Shale Research Articles

Formation conditions and enrichment mechanisms of the Jurassic lacustrine organic-rich shale in the East Fukang Sag, Junggar Basin, NW China: A reassessment based on organic geochemistry

Chemical composition of sediments is often used to evaluate paleoclimate condition, provenance, tectonic setting, depositional condition, and paleoproductivity. However, the validity of these proxies has long been questioned. The comprehensive use of organic and inorganic multi-indicators in combination when interpreting issues related to terrestrial shales should be advocated. The paleodepositional environment, origin of organic matter (OM) and factor controlling OM accumulation in the Early Jurassic Badaowan (J1b) and Sangonghe (J1s) as well as Middle Jurassic Xishanyao (J2x) lacustrine shales in the East Fukang Sag are reassessed by using organic geochemical characteristics of the OM. Some previous knowledge is updated, and some knowledge is further supported by more evidence. The typical clay-rich shale developed under a lacustrine sedimental environment, and the thermal maturity of these organic-rich shales has entered the oil window and formed economic hydrocarbon potential for the tight-oil and shale-oil reservoirs. The paleoclimate conditions of the study area were warm and humid from the Early to Middle Jurassic periods but were colder and drier after the Middle Jurassic period. The salinity of the water column ranged from freshwater to brackish conditions. The J2x Formation was deposited under oxic conditions, while J1b and J1s formations developed under suboxic and reducing environmental conditions. The J2x Formation OM mainly derived from higher plants was deposited in a terrestrial environment,while the OM of J1b and J1s formations was a mixed OM derived from higher plants and bacteria with little algae deposited under bay/estuary environments alternated with terrestrial environments. It is effective to reflect the paleoclimate by element index and judge the salinity by the updated element thresholds, but it is not effective to evaluate the paleoredox conditions by common elemental ratios and to evaluate the paleoproductivity by Ba in the study area.

Geological controls on the pore system of lacustrine unconventional shale reservoirs: The Triassic Chang 7 member in the Ordos Basin, China

The complex pore system of lacustrine organic-rich shales poses challenges to the assessment of shale oil and gas resources. To better understand the pore system and its controls of the lacustrine unconventional shale reservoirs, 12 oil mature shale samples with varying organic matter contents from the Triassic Chang 7 Member in the Ordos Basin were analyzed using elemental, petrographical, petrophysical, and fractal methods. The results showed that macro- and meso-pores are the main pore space in the lacustrine Chang 7 Shale. Pore diameter peaks are dominantly distributed around 0.7 and 37 nm. The intragranular pores in clays and intergranular pores between organic matter (OM) and minerals are well developed. Whereas the pores in OM are rarely observed. The fractal dimensions of the lacustrine shale's pore structure suggest that D2 is better than D1 in reflecting pore heterogeneity. OM shows negative effects on the total pore volume, while quartz promotes the development of macro- and meso-pores. Total pore volume shows an oscillation with clay mineral content (pore volume increases first up to 45% clay content and decreases thereafter). Both quartz and clays show negative effects on the heterogeneity of meso- and macro-pores, and positive effects on the micro-pore heterogeneity. A new critical maturity point of 0.8% Ro is observed for the change of the pore system of the lacustrine shale reservoirs during the early oil window. Both the fractal dimensions (D1 and D2) and pore structure proxies show the inverse trends before and after this critical value. These observations are probably controlled by the compaction processes coupled with hydrocarbon generation and expulsion, which helps better understand the pore system evolution in the lacustrine shales. The positive role of quartz, 45% clay mineral content and 0.8% Ro as the critical points contribute to the high-quality reservoir or sweet spot identification in lacustrine shales.

Organic matter accumulation in lacustrine shale of the Permian Jimsar Sag, Junggar Basin, NW China

The lacustrine organic-rich shale in the Permian Lucaogou (LCG) Formation of the Jimsar Sag, Junggar Basin, is one of the main shale oil plays in China. In this paper, geological and geochemical research techniques were employed to evaluate the geochemical variability of the lacustrine shale and the production of organic matter and its preservation conditions. The LCG Formation is characterized by its complex mineral compositions and a wide range of organic matter richness and quality. The presence of high proportions of β-carotane and C29 steranes, indicates that the organic matter mainly originated from phytoplankton and aquatic algal-bacterial organisms, especially cyanobacteria. This study found that the productivity of the Lower LCG Member (P2l1) was highest, and the Middle LCG Member (P2l2) was the lowest. During the deposition of the Lower LCG Member, the lake's bottom water was predominantly a reducing environment, and the degradation of organic matter was largely a result of bacterial sulfate reduction. During the deposition of the Middle and Upper LCG members, the lake's bottom water was mainly oxidizing, and the degradation of organic matter was likely to be caused by aerobic processes. Based on a comprehensive analysis of the origin and production of organic matter, as well as its depositional environment and preservation conditions, two organic matter accumulation models were proposed to explain the distribution of the organic-rich shale. In model A, the high influx of volcanic ash released nutrients and brought abundant sulfate into the water, the accumulation of organic matter was mainly controlled by the preservation of organic matter, which was mainly controlled by BSR. In the model B, the influx of volcanic ash was small, organic matter was mainly degraded by oxygen and the accumulation of organic matter is mainly determined by the production of organic matter.

Characterization of Microstructures in Lacustrine Organic-Rich Shale Using Micro-CT Images: Qingshankou Formation in Songliao Basin

In order to explore the development characteristics and influencing factors of microscale pores in lacustrine organic-rich muddy shale, this study selected five shale samples with different mineral compositions from the Qingshankou Formation in the Songliao Basin. The oil content and mineralogy of the shale samples were obtained by pyrolysis and X-ray diffraction analysis, respectively, while the porosity of the samples was computed by micro-CT imaging. Next, based on the CT images, the permeability of each sample was calculated by the Avizo software. Results showed that the continuous porosity of Qingshankou shale in the Songliao Basin was found between 0.84 and 7.79% (average 4.76%), the total porosity between 1.87 and 12.03% (average 8.28%), and the absolute permeability was calculated between 0.061 and 2.284 × 10−3 μm2. The total porosity of the samples has a good positive correlation with the continuous porosity and permeability. This means higher values of total porosity suggested better continuous porosity and permeability. Both total porosity and continuous porosity are positively correlated with the content of clay minerals. Moreover, the oil content of the samples (the S1 peak from programmed pyrolysis) exhibits a good positive correlation with the total porosity, continuous porosity, permeability, and clay mineral content. Therefore, pores that are developed by clay minerals are the main storage space for oil and flow conduits as well. Clay minerals were found to be the main controlling factor in the porosity, permeability, and the amount of oil content in the pores in the study area.

Paleoenvironment evolution of the Permian Lucaogou Formation in the southern Junggar Basin, NW China

The Early-Middle Permian is regarded as a critical period for the Junggar Basin in marine to continental transition. Lacustrine organic-rich shale successions were deposited in the Lucaogou Formation in the southern Junggar Basin (SJB) at that time. Although the paleoenvironment conditions of the Lucaogou Formation have been discussed from diverse perspectives, their spatial and temporal evolution remains controversial. This study systematically investigated 11 outcrop sections and cores in three sags from the SJB, namely the Chaiwopu Sag, the Fukang Sag, and the Jimsar Sag, respectively. Paleoclimatic indicators (i.e., CIA values and Sr/Cu ratios) show that the Lucaogou Formation was generally deposited under semiarid to arid conditions, among which the Jimsar Sag was more arid than the other two sags. Meanwhile, paleoredox indices (V/Cr, V/(V + Ni) ratios, and MoEF, UEF values) illustrate dysoxic-anoxic conditions prevailing during its deposition. Paleosalinity indicators (Sr/Ba and B/Ga ratios) suggest high salinity levels in the three sags, among which the Jimsar Sag exhibited the highest salinity. Our results suggest that, relative to the Chaiwopu and Fukang sags, the Jimsar Sag experienced a more arid, more restricted, and saltier condition during deposition of the Lucaogou Formation. There is a generally lake transgression trend, with relatively less arid climate and less salty water columns in the upper Lucaogou in all the three sags. This transgression may be related to the deglaciation of Gondwanaland, which cause an increase in global temperature and more rainfall. The SJB was composed of a cluster of lakes that were relatively independent and rarely connected with each other. However, lake level rising may have coalesced to form a giant paleo-lake when lake transgression took place.

Impact of thermal maturity on the diagenesis and porosity of lacustrine oil-prone shales: Insights from natural shale samples with thermal maturation in the oil generation window

Shale diagenesis controls the generation, migration and accumulation of hydrocarbons through time and has become a frontier area of study in the fields of sedimentology and petroleum geology. There have been only a few studies on the generation of organic pores in lacustrine shale oil reservoirs during the oil generation thermal evolution stage. In this study, lacustrine organic-rich shale samples at various stages of maturity, from the first member of the Songliao Basin Qingshankou Formation were investigated, and a thermal transformation law of macerals was established, based on organic petrology. The diagenetic alteration styles and sequences were investigated using a high-resolution field emission scanning electron microscope, and cathode luminescence. Furthermore, automated pore extraction technology enabled the examination of the characteristics and genetic process of organic and inorganic pores under the influence of thermal maturation. The results indicate that a small quantity of pre-oil bitumen is generated during the early oil mature stage (Ro: 0.7% - 0.9%), and that the generation of a large quantity of post-oil bitumen that occurs following the peak oil generation stage (i.e., late oil mature stage, Ro: 1.1% - 1.4%), is controlled by the unimodal distribution of the activation energy of Type I kerogen. Compaction intensity certainly influences pore volume reduction. Siliceous cementation primarily occurs during peak oil generation maturity and the subsequent thermal evolution stage. The pores formed in the oil generation window primarily arise from mineral dissolution and organic matter polycondensation. Following peak oil generation, the pores undergo iron dolomitization, smectite illitization, and bitumen cracking. The degree of development of inorganic pores, the most common pore type in shale oil reservoirs, appears to have a significant impact on the total porosity of a reservoir shale. During thermal evolution, the inorganic pore volumes show a decreasing trend. There is a secondary pore development stage, reached when Ro exceeds 1.2%, closely related to the transformation of clay minerals. According to the hydrocarbon generation kinetics and compaction model, organic pore volumes increase gradually at 0.7%Ro, reach their peak at 1.1%Ro, and then steadily decrease due to compaction. The organic pores are relatively well developed when Ro is >0.85% (at the start of the oil window). The shale oil reservoir's most favorable thermal evolution stage is late oil maturity stage that follows the oil generation peak.

Determination of Favorable Lithofacies for Continental Shale Gas: A Case Study of the Shahezi Formation, Changling Fault Depression, Songliao Basin

In order to determine the optimum lithofacies for continental shale, the reservoir characteristics of different lithofacies types were studied based on a series of experiments. The lacustrine organic-rich shale of the Shahezi Formation is divided into siliceous (ORS), argillaceous (ORA), calcareous (ORC), and mixed (ORM) shales. The ORS, ORA, ORC, and ORM shales all carried out comprehensive reservoir comparative analysis. The results showed that the moderate content of clay minerals (45%) can significantly improve porosity, and high and low clay mineral contents are not conducive to the improvement of porosity. The ORM shale tends to have better pore connectivity than the ORS and ORA shales, and the ORC shale has the poorest pore-throat connectivity in micron-size. Internal pores in bitumen and clay shrinkage cracks are the dominant pore type and are well developed in ORS and ORM shales. The two types of pores are less developed in the ORA shale; however, dissolution pores are better developed than those of ORS and ORM shales. Inorganic pores are well-developed in the ORC shale, but organic pores are not. Organic and inorganic pores tend to be better connected in the ORM shale than those of the other three types, which could help improve the gas storage capacity. The ORM shale may have more irregular surfaces and lower liquid/gas surface tension. The higher capillary condensation on pore surfaces is more likely to occur in the ORA shale. The ORS shale offers fewer adsorption sites for CH4 and has lower adsorption capacity. The ORC shale has weak heterogeneity of the pore structure. Therefore, the ORM shale may be the most favorable lithofacies for shale gas enrichment and development, which has high porosity, good pore connectivity, moderate brittleness, and strong gas adsorption and storage capacity, followed by the ORS and ORA shales. The ORC shale is the worst.

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.

Competitive adsorption of methane and ethane on organic-rich shale at pressure up to 30 MPa: Experimental results and geological implications

Methane and ethane are the two most abundant hydrocarbon species in shale gas, while the adsorption behavior of their binary mixture in shale is still ambiguous. In this study, the adsorptions of methane, ethane, and their binary mixture on twelve dried mature lacustrine organic-rich shale samples were measured at 60 °C and variable pressures ranging from 0.001 to 30 MPa. The Langmuir and supercritical Dubinin–Radushkevich (SDR) models combined with the ideal adsorbed solution theory (IAST) and Extended Langmuir (EL) model were used to describe the raw adsorption isotherms. By contrast, the Langmuir model with consistent adsorbed phase volume can better describe the ethane excess adsorption. The IAST and EL models matched binary gas excess adsorption equally well. The adsorption capacity of pure ethane was about 1.6 times as large as pure methane, while the Langmuir pressure for ethane was lower. For the adsorption of pure and binary gas, organic matter was the primary contributor, followed by clay minerals. The adsorption selectivity of ethane over methane ranged from 3.49 to 8.85, which strongly depended on total organic carbon (TOC), and weakly negatively correlated with clay mineral content. The estimated gas-in-place (GIP) of binary gas ranged from 4.57 cm3/g to 8.08 cm3/g, which was about 30% larger than that of pure methane. A two-stage trend, first falling and then rising, was observed in relationship between binary gas GIP and TOC. This phenomenon could be attributed to the opposite effects of residual bitumen on shale porosity and gas adsorption. This study provides better insight in the storage mechanisms of shale gas and a basis for the reliable estimation of shale gas potential.

Pore structure evolution of lacustrine organic-rich shale from the second member of the Kongdian formation in the Cangdong Sag, Bohai Bay Basin, China

Pyrolysis experiments were conducted on lacustrine organic-rich shale from Cangdong Sag in Bohai Bay Basin, China, to investigate the impact of hydrocarbon generation on shale pore structure evolution. Thermal evolution is found to control the transformation of organic matter, hydrocarbon products characteristics, and pore structure changes. Furthermore, pore volume and specific surface area increase with increasing maturity. In low-mature stage, the retained oil content begins to increase, pore volumes show slight changes, and primary pores are occluded by the generated crude oil of high molecular weight and density. In the oil-window stage, the retained oil content rapidly increases and reaches maximum, and pore volumes gradually increase with increasing thermal maturity. At high mature stage, the retained oil content begins to decrease, and the pore volume increases considerably owing to the expulsion of liquid hydrocarbon. In over mature stage, natural gas content significantly increases and kerogen transforms to asphalt. Numerous organic pores are formed and the pore size gradually increases, resulting from the connection of organic pores caused the increasing thermal stress. This study lays a foundation for understanding variation of hydrocarbon products during the thermal evolution of lacustrine shales and its relationship with the evolution of shale reservoirs.

Palaeoenvironmental and tectonic controls on organic matter enrichment in the Middle Jurassic Dameigou Formation (Qaidam Basin, North China)

Lacustrine organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. These fine-grained sediments have been proven to have good hydrocarbon generation potential; therefore, understanding the sedimentary palaeoenvironment of these shales and organic matter enrichment mechanism is essential to increase prospects for shale oil and gas resources. The mineralogy, elemental geochemistry, and organic geochemical data combined with lithology of ZK23-1 well from the Yuka Depression, Qaidam Basin have been comprehensively analyzed to determine the hydrocarbon potential, palaeoenvironmental changes, and tectonic setting of the lacustrine organic-rich shales in the seventh member of Middle Jurassic Dameigou Formation (J 2 d 7 ). The profile of the ZK23-1 well is composed of three stages (Stages I to III) based on variations in bulk geochemical parameters and lithological observations. Mineral compositions of studied samples are dominated by clay minerals and quartz, with small amount of pyrite, calcite, dolomite, and plagioclase. Interestingly, all samples contain siderite, which is closely related to palaeoenvironment changes. Total organic carbon (TOC) content and hydrogen index (HI) vary significantly in the various lithologies, suggesting that oil shale holds the best hydrocarbon potential, followed by shales and mudstones. A warm-humid palaeoclimate prevailed in the Qaidam Basin during the Middle Jurassic; however, significant palaeoclimatic fluctuations occurred from Stages I to III in the J 2 d 7 , especially in Stage II. The climate varies from hot-arid to warm-humid in Stage II, likely resulting in changes in palaeowater column, chemical weathering intensity, as well as palaeoproductivity. The results of this study suggest that the Yuka Depression in the Middle Jurassic could be located at an active continental margin. Warm and humid conditions, as well as the tectonically quiescent setting with stable subsidence rate, are considered as two primary drivers for the enrichment and preservation of OM in the sediments of the J 2 d 7 . Ultimately, a comprehensive depositional model is proposed to interpret the evolution of deposition of the J 2 d 7 , providing new insights into the deposition of lacustrine organic-rich sediments linked to palaeoenvironment conditions and tectonic setting. • Three stages have been identified from ZK23-1 in the J 2 d 7 . • An obviously environmental fluctuation occurred from stage I to III of the J 2 d 7 . • Depositional models have been proposed to interpret the enrichment of OM. • The warm and humid condition and tectonically quiescent setting are popular. • Palaeoclimate and tectonic setting are primary drivers for OM enrichment.

Study on the pore structure, fluid mobility, and oiliness of the lacustrine organic-rich shale affected by volcanic ash from the Permian Lucaogou Formation in the Santanghu Basin, Northwest China

Lacustrine organic-rich shale with extremely heterogeneous pore structure is widespread in the second member of the Permian Lucaogou Formation (P2l2) of the Santanghu Basin. However, information on the pore structure, fluid mobility, and oiliness characteristics is still lacking, as well as the interaction between them in the organic-rich shales remains controversial. To address these problems, this study analyzed the influencing factors of pore structure, oiliness, and fluid mobility of the P2l2 shale and constrained the lower limit of movable fluid pore throat radius based on thin sections, routine core analysis, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), constant-rate mercury injection (CMI), and nano-CT scanning data. The P2l2 shale was characterized by fine-grained minerals of felsic (tuffaceous material) and carbonate (dolomite), with poor reservoir physical properties and strong oil-bearing heterogeneity. Micro- and nano-scale pores are the main type of storage space. Among them, most of the microfractures emit green to yellow hydrocarbon fluorescence regardless of whether they are filled or not, as do the edges of dolomite grains. While the edges of the felsic minerals are characterized by stronger orange yellow fluorescence of organic matter. According to the morphology and parameters of HPMI and NMR curves, the pore structure can be divided into four types. From type 1 to type 4 pore structure, the maximum mercury saturation decreases from 95 to 47 % (avg. 70 %), indicating poor pore connectivity, and the pore throat radius that mainly contributes to permeability is > 0.04 μm. Meantime, the NMR T2 spectrum gradually transitions from right unimodal to left unimodal behavior, with a gradual decrease in the saturation of movable fluid and a transition of the mineral type from carbonate to felsic. Fluid mobility is not only influenced by macropores, but also related to the content of small pores (T2 < 1 ms) and the mineral type. On this basis, the lower limit of the movable fluid pore throat radius in the P2l2 organic-rich shale is finally determined to be 40 nm.

Laminar characteristics of lacustrine organic-rich shales and their significance for shale reservoir formation: A case study of the Paleogene shales in the Dongying Sag, Bohai Bay Basin, China

• Six types of the organic-rich shale laminae are reported in the Dongying Sag. • Discontinuous laminae were formed under high-energy hydrodynamic conditions. • Shale laminae containing favorable reservoir storage space are in the order of wavy laminar > lenticular laminar > weak laminar. • Thick and wavy laminae are the most favorable types for shale oil exploration. Shale oil exploration in China has demonstrated that laminated shales are favorable lithofacies for producing shale oil effectively. It is of great significance to study the type and reservoir properties of shales with different laminar structures. In this study, the laminae of Paleogene organic-rich shales in the Dongying Sag, Bohai Bay Basin were divided into two categories and six types based on thin section petrography, scanning electron microscopy (SEM), and geochemical characterization. The laminar types include thin parallel laminae, thick parallel laminae, wavy laminae, lenticular laminae, weak laminae, and sandy laminae. The first three types of laminae have good continuity and were formed under low-energy hydrodynamic conditions. In comparison, the latter three types are discontinuous and were formed under relatively high-energy hydrodynamic conditions with a strong terrigenous input. Significant differences were observed in the reservoir space types, microcracks, and oil-bearing properties among the different laminae. Analyses of porosity and oil saturation further revealed that laminated shales with high laminar continuity exhibit higher porosity, permeability, and oil saturation values than shales with poor laminar continuity. Shales with thick parallel laminar shale and wavy laminar shale possess favorable petrophysical parameters. Thus, from the perspectives of reservoir and oil-bearing properties, laminated shales with high laminar continuity are better than shales with poor laminar continuity for shale oil exploration in the Dongying Sag. The new findings provide a reference for the optimization and evaluation of favorable continental shale oil reservoirs in eastern China.

Geological Characteristics and Exploration Prospect of Black Shale in the Dongyuemiao Member of Lower Jurassic, the Eastern Sichuan Basin, China

China has yielded huge commercial production from the marine organic-rich shale but shows a slow exploration process in the lacustrine organic-rich shale. Multiple lacustrine shales in the Lower Jurassic of the eastern Sichuan Basin, rich in organic matters, are potential targets for shale hydrocarbon exploration and development. An investigation of the Dongyuemiao member, Lower Jurassic, was firstly conducted utilizing the macroscopic and microscopic analyses on outcrops and drilling cores to reveal the characteristics of sedimentary subfacies, mineral compositions, organic matter content and types, thermal maturity level, and reservoir quality. The dark shales in the Dongyuemiao member can be grouped into four general categories: shore, shallow, semi-deep, and deep lacustrine shales. The semi-deep and deep lacustrine shales generally have higher values in thickness (&amp;gt;20 m), average total organic carbon (TOC) content (&amp;gt;1.5 wt.%), and average porosity (&amp;gt;2%) relative to shore and shallow lacustrine shales. All four categories of shales primarily consist of type II kerogen and have thermal maturity levels exceeding the vitrinite reflectance value of 0.9–1.0% (or the Tmax of ∼440°C). Thermally powered pore generation generally promoted the pore system as indicated by the positive correlation between porosity and Tmax. Notably, the semi-deep lacustrine shale in the vicinity of the Qiyueshan Fault Zone shows abnormally high porosity and low oil saturation index (OSI) at Tmax&amp;gt;∼465°C potentially due to the promoted hydrocarbon expulsion through multiscale fractures. Except for the vicinity of the Qiyueshan Fault Zone, the semi-deep and deep lacustrine shales generally show the better exploration prospect relative to the shore and shallow lacustrine shales. Additionally, the high content of clay minerals (&amp;gt;40 wt%) reduced the brittleness of the semi-deep and deep lacustrine shales which may challenge the artificial hydraulic fracturing.

Algal-Microbial Community, Paleoenvironment, and Shale Oil Potential of Lacustrine Organic-Rich Shale in the Upper Cretaceous Nenjiang Formation of the Southern Central Depression, Songliao Basin (NE China)

Algal-Microbial Community, Paleoenvironment, and Shale Oil Potential of Lacustrine Organic-Rich Shale in the Upper Cretaceous Nenjiang Formation of the Southern Central Depression, Songliao Basin (NE China)

Salinized lacustrine organic-rich shale influenced by marine incursions: Algal-microbial community, paleoenvironment and shale oil potential in the Paleogene Biyang Depression, East China

Salinized lacustrine organic-rich shale (SLORS) in the Paleogene has been considered as one of the most important shale oil reservoirs in China. Although deposition of the Paleogene SLORS in East China is generally concomitant with marine incursions, the influence of marine incursions on the algal-microbial community, paleoenvironment and shale oil potential of SLORS is still controversial. Here we examine this issue using a case of the Paleogene Biyang Depression (Nanxiang Basin) in East China, based on the integration of bulk geochemical, mineralogical, organic petrographic, biomarkers, elemental geochemical, and shale reservoirs analyses. Marine incursions were occurred during deposition of the SLORS in the Paleogene Biyang Depression as evidenced by the presence of marine organisms and marine-derived biomarkers (24-n-propylcholestanes). Lake transgressions, global sea-level rise, and extensional tectonic setting were responsible for the occurrence of marine incursions. The algal-microbial community change is associated with marine incursions, from dominance of green algae with a weak influence of marine incursions to dominance of red (brown) algae and coccolithophores with a strong influence of marine incursions, accompanied by enhanced bacterial activity. With the stronger influence of marine incursions, higher salinity, more reducing conditions, higher productivity, and lower detrital influx were present in the paleo-lake, resulting in the deposition of SLORS with higher shale oil potential, which was characterized by higher free shale oil contents, more abundant pores and fractures in nano- to micro-meter scale, and higher brittle mineral contents. In summary, our data suggest that marine incursions have a positive influence on the development of biodiversity in paleo-lakes, and the deposition of SLORS with high shale oil potential. Therefore, the influence of marine incursions should be fully considered during the palaeoecological reconstruction and the shale oil exploration of SLORS, especially during high sea-level stages. In addition, the results provide guidance for future profitable shale oil exploration in East China and other similar areas.

Factors controlling organic-rich shale development in the Liushagang Formation, Weixinan Sag, Beibu Gulf Basin: Implications of structural activity and the depositional environment

The mechanisms of lacustrine organic-rich shale formation have attracted attention due to its association with global shale oil and shale gas exploration. Samples of general-quality and excellent-quality source rocks, and oil shale from the Beibu Gulf Basin were analyzed to investigate their organic geochemistry, palynofacies, and trace elements. Hydrocarbon potential was higher in the oil shale (29.79 mg/g) than in the general-quality source rock (3.82 mg/g), and its kerogen type was I-II 2 . Hydrogen-rich liptinite (cutinite and sporinite) components derived from terrigenous higher plants provided most of the hydrocarbon potential of excellent-quality source rock and oil shale. Under the influence of depression-controlling fault activity, a deeper subsidence center promotes the deposition of excellent-quality source rock and oil shale in brackish-hypoxic bottom water. A shallower subsidence center, due to subsag-controlling fault activity, promotes the formation of excellent-quality source rock under fresh-brackish and weak oxidation-weak reducing conditions. The local uplift and shallow-slope led to the formation of general-quality source rock, under freshwater weak-oxidation conditions. A model was established for organic matter (OM) accumulation in organic-rich shales, accounting for fault activity, terrigenous hydrogen-rich OM, and the preservation conditions, to predict the development of excellent-quality source rock from areas with low levels of exploration.

Paleoenvironment evolution of the lacustrine organic-rich shales in the second member of Kongdian Formation of Cangdong Sag, Bohai Bay Basin, China: Implications for organic matter accumulation

The second member of Kongdian Formation (Ek2) of Cangdong Sag represents an excellent hydrocarbon source rocks interval target for crude oil and shale oil production in the Bohai Bay Basin. To reconstruct the palaeoenvironmental conditions and reveal the mechanisms of organic matter enrichment of the Ek2 shales, a series of analyses (total organic carbon (TOC), Rock-Eval pyrolysis, major and trace element analyses, and X-ray diffraction (XRD)) were performed on samples collected from the borehole of Cangdong Sag. The Ek2 consists of fine-grained rocks deposited in a lacustrine environment, and it can be further subdivided into four members (Ek24 to Ek21). The results show that the Ek2 contains a high abundance of thermally mature Type I and II kerogen, and most of the samples show good to excellent generative potential. The mineralogical compositions, dominated by quartz, feldspar, and dolomite, with subordinated calcite, clay minerals, and analcime. Specific major and trace elements were employed to analyze the changes in paleoclimate conditions, lake water chemistry, and the detrital influx of Ek2. The vertical changes of paleoenvironmental indicators suggest that during the sedimentary period of Ek2, the climate changed from semi-humid to arid, with the input of terrigenous detrital gradually decreased, and the salinity of the water increased. The relationship among the primary productivity, redox conditions, detrital influx indicators and TOC content suggests that primary productivity was the most critical factor in controlling organic enrichment, and the input of terrigenous debris promotes the increase in productivity. The high primary productivity, together with anoxic bottom water conditions, resulted in the greatest enrichment of organic matter in the lower part of Ek2 shale (Ek23). The upper part (Ek21) is mainly composed of moderate-TOC calcareous shale, which is deposited under arid paleoclimate, with low detrital input, moderate productivity, and favorable preservation conditions.

Organic matter-driven electrical resistivity of immature lacustrine oil-prone shales

Organic matter (OM) and minerals are major particle components in lacustrine organic-rich shales. Their association and distribution control the development of the primary pore space. The response of OM-driven conduction by modifying the pore-space volume and structure in organic-rich shales of the virgin zone is still unclear. Based on a detailed study of geochemical, mineralogical, and geophysical properties from immature lacustrine oil-prone shales of the Songliao Basin (Northeast China), we have observed a novel continuous variation of electrical resistivity driven by large ranges of total-organic-carbon (TOC) content (0.64–24.51 wt%). The reduced resistivity at low TOC content ([Formula: see text]) and then enhanced resistivity at high TOC content ([Formula: see text]) are present in our immature shales. These variations in electrical resistivity are confirmed by fluid (S1) and solid organic compounds (S2). Furthermore, clay and detrital minerals in shales contribute to the variation of electrical resistivity as well as OMs at low and high TOC content. The electrical resistivity of shales is closely related to the pore-space volume and structure for the electrical flow pathway. Two resistivity trends are highlighted by pore parameters such as the pore volume, throat/pore ratio, pore diameter, and bulk density. Although reduced amounts and the arrangement of large pores for low TOC content cannot decrease the conduction, the enhanced additional clay conduction and low OM concentration reduce the resistivity of shales. Moreover, increased amounts of nonconductive fluid and solid organic compounds and the effect of OM filling on pore space during high TOC content enhance the resistivity of shales. Thus, modified minerals and pore space driven by various OMs affect the electrical resistivity of immature shales. These results improve the understanding of OM-driven conduction in shales and contribute to the evaluation of source rocks using a well-log method.