Organic Matter Pores Research Articles

Insights in the pore structure, fluid mobility and oiliness in oil shales of Paleogene Funing Formation in Subei Basin, China

Abstract Pore structure is an important factor influencing reservoir properties of oil shales. Routine core analysis, thin section, scanning electron microscope (SEM), mercury injection capillary pressure (MICP) tests, Nuclear Magnetic Resonance (NMR), and computed tomography (CT) were used to provide insights into the pore throat distribution in oil shales of member 2 of Paleogene Funing Formation (E1f2) in Subei Basin, China, with the special aim to unravel the effect of pore structure on fluid mobility and oiliness. The relationships between NMR parameters, petrophysical property and capillary parameters are investigated. The results show that rock composition in the oil shales consist of quartz, feldspar, carbonate particles, clay minerals and organic matters. Pore systems consist of large-scale interparticle pores, intragranular dissolution pores, and small-scale micropores within clay minerals and the organic matter pores. Four types of pore structure (Type Ⅰ, Type Ⅱ, Type Ⅲ and Type Ⅳ) are divided according to the patterns of capillary curves, capillary parameters, NMR T2 (transverse relaxation time) spectrum and BVI (bulk volume of immovable fluid) values. From Type Ⅰ to Type Ⅳ pore structure, the maximum mercury saturation (SHgmax) and mercury extrusion efficiency are decreasing, and the SHgmax are less than 50% averagely, indicating the oil shales are characterized by very poor pore connectivity. The T2 spectrum changes from bi-modal behavior to uni-modal and the right peaks become lower or even disappearing from Type Ⅰ to Type Ⅳ pore structure. Fluid mobility is not primarily controlled by pore size, but dependent on the content of short T2 components ( Pore structure controlled fluid mobility and determined the microscopic oiliness and macroscopic oil bearing property or hydrocarbon productivity. The comprehensive study above gains insights into the microscopic pore structure and their controls on fluid mobility and oiliness in shale oil reservoirs, and this may have implications for resource potential evaluation and effective exploitation of oil shales.

Analysis of the Distribution and Microscopic Characteristics and Disintegration Characteristics of Carbonaceous Rocks: A Case Study of the Middle Devonian Luofu Formation in Western Guangxi of China

Carbonaceous rock is a special soft rock containing TOC organic carbon 6%∼40%. In order to reveal the influence of engineering characteristics of carbonaceous rock on the engineering construction, firstly the stratigraphic distribution of carbonaceous Rocks in Guangxi is investigated, and the genetic mechanism and tectonic environment of carbonaceous rocks are discussed. Secondly, the influence of pore microstructure on the disintegration characteristics of carbonaceous rocks is analyzed. The role of geochemical characteristics of carbonaceous rocks (mineral composition, TOC total organic matter content, and type) on engineering properties is revealed. Finally, combined with the distribution, structure, microstructure, and microscopic characteristics of the previous studies, the disintegration mechanism of carbonaceous rocks in water swelling and heat dehydration is discussed. The results are as follows. (1) Carbonaceous rocks in Guangxi are mainly distributed in Devonian, Carboniferous, and Cambrian systems. It is mainly formed in anoxic and reductive deep water basins, slopes, and relatively confined coastal lagoons and swamps. The carbonaceous rocks in the Devonian Luofu formation are most typical. (2) The pores of carbonaceous rocks are divided into mineral pores, organic matter pores, and microfracture, which are mainly mineral pores. The more developed pores in mineral pores are intragranular dissolved pores. Secondly, mineral intergranular pores and a small amount of intergranular dissolution pores and less inner pores. Organic matter porosity increases with the increase of shale organic carbon content and maturity, but the shale porosity and adsorption capacity decrease when shale maturity reaches more than 2.4%. (3) Clay minerals in carbonaceous shale mainly consist of illite and illite/montmorillonite layer, which have water swelling and heat dehydration. The total average value of TOC (total organic matter content) is more than 1%, which belongs to carbonaceous rocks of medium high grade hydrocarbon source rocks. Organic matter is mainly dominated by type I and II1‐II2 type with large hydrocarbon generating potential, which is prone to oxidation‐reduction reaction and cause rock disintegration.

The pore characteristics and gas potential of the Jurassic continental shales in the middle-small basins, northwest China

Jurassic continental shales were extensively developed in the northwest of China. To characterize and analyze the pores and gas potential of these shales, geochemical, mineralogical, petrophysical, and methane adsorption capacity (MAC) experiments were performed on 10 samples. The results indicated that these shales have a high total organic carbon (TOC) content, low porosity and permeability. Interparticle pores between clay platelets are dominant. Organic matter (OM) pores cannot be observed using scanning electron microscope. The methane adsorption capacity is strong, and it is controlled by TOC content. Strong MAC indicated the shales have an excellent capacity of storing gas. In addition, type III organic matters can generate amount of gas at a lowly mature stage. Therefore, it is speculated that the studied shales have a good gas potential, which is supported by canister desorption results.

Distribution of adsorbed water in shale: An experimental study on isolated kerogen and bulk shale samples

Bakken shale samples were studied for distribution of adsorbed water using low-pressure nitrogen sorption. By comparing results between dry and wet samples, the distribution of adsorbed water in shale was determined. Two of the isolated kerogen samples show a striking change of pore size distribution (PSD) in large pores (>16 nm), indicating the pronounced distribution of adsorbed water in large pores of organic matter. As for the bulk shale, water can adsorb in both small (<16 nm) and large pores (>16 nm) depending on hydrophilic sites. However, hydrophilic sites in small pores are mainly contributed by inorganic matter, while hydrophilic sites in large pores are composed of inorganic or organic matter. The overall results therefore clarify the contribution of inorganic and organic matter to water adsorption in shale and provide a better understanding of the significance of adsorbed water in shale.

Porosity characteristics of different lithofacies in marine shale: A case study of Neoproterozoic Sinian Doushantuo formation in Yichang area, China

This study evaluates the pore network systems in the Neoproterozoic Sinian Doushantuo (Z1d) shale from the Yichang area in the Middle Yangtze region. All samples were classified based on their lithologies and examined by geochemical and petrographic analysis, assisted with carbon dioxide/nitrogen (CO2/N2) adsorption, mercury injection capillary pressure (MICP) and focused ion beam-scanning electron microscopy (FIB-SEM) to fully characterize the nanoscale porosity of the different shale lithofacies. The calcareous shale lithofacies group (I-C), mixed shale lithofacies group (II-M) and siliceous shale lithofacies group (III-S) were identified via ternary mineralogy diagrams. Clearly, different lithofacies had different porosities, pore volume, surface area and pore-size distribution (ranging from 0.3 nm to 36 μm). The mixed shale lithofacies group (II-M) had the highest total pore volumes, followed by the calcareous shale lithofacies group (I-C) and the siliceous shale lithofacies group (III-S). Peak values (2–20 nm, 20–50 nm, 50–60 nm and 90–200 nm) obtained from N2 adsorption were identified as organic matter (OM) pores, dissolution intraparticle (intraP) pores and intercrystalline pores (using SEM images analysis). Porosity shows a non-monotonic trend with TOC content and a maximum at 4.5–5.5 wt% TOC, similar relationships between TOC and mesopore/macropore volumes, due to pore collapse and compaction. The FIB-SEM images revealed solid bitumen and two types of kerogen in the examined shale samples; these different organic matter types had clearly distinct pore characteristics. While the smaller OM particles mobilized and converted into solid bitumen in which OM pores were larger and more abundant, the large-scale OM particles were most likely non-porous inert maceral and sponge-like porous amorphous kerogen. Overall TOC content and organic types are the key controlling factors to the nanoscale porosity development in the Doushantuo Shale, and the Sinian Doushantuo shale (in the Yichang area) has great potential for shale gas exploration and exploitation in South China.

Process-based and dynamic 2D modeling of shale samples: Considering the geology and pore-system evolution

Accurate characterization of shale samples is of great importance for predicting and evaluating their intrinsic properties as they undergo various geological processes. As such, constructing shale models not only entails accounting for the numerous components and pores, but also requires considering the effects of thermal maturation on the evolutions of the pore systems and components. In this study, a new dynamic and process-based modeling for considering the geological evolutions occurred in pore systems is developed, which makes a significant improvement in reproducing the complex geological features in shale samples. Three types of pores, including organic matter (OM), interparticle (interP), and intraparticle (intraP) pores, and several other components including kerogen, pyrite, clay minerals, globigerinids, calcite, feldspar, quartz, and petroleum products, are all considered in our process-based modeling. In order to demonstrate the performance of our process-based method, two-dimensional (2D) dynamic shale models of multiple cases with different geological processes are constructed. Then, to characterize the constructed samples, the occurrences and distributions of all components are studied using a two-point correlation function. Aside from statistical comparisons, the gas flow in these shale models is also simulated to exhibit the permeability variations produced in the process-based models. Moreover, the degree of cementation, the fractions of interP, intraP, and OM pores are varied to mimic new geological scenarios, which aims to further evaluate the performance of the proposed technique. The results indicate that our process-based model presents an excellent performance. Furthermore, the presented modeling technique can be extended to investigating the effects of various components and pores on the intrinsic properties of shales in the future.

A comprehensive re-understanding of the OM-hosted nanopores in the marine Wufeng–Longmaxi shale formation in South China by organic petrology, gas adsorption, and X-ray diffraction studies

The overmature marine Ordovician–Silurian Wufeng–Longmaxi (WF–LMX) formation shale is the most important exploration target for shale gas in South China. In this study, WF–LMX shale samples from Well XY-1 in the Northwestern Guizhou Province (NGP) were selected to comprehensively investigate organic matter (OM) assemblages and OM-hosted nanopores. Petrographic observations under an optical microscope and a scanning electron microscope (SEM) revealed that the OM in the WF–LMX shale is predominately solid bitumen (pyrobitumen), followed by organic zooclasts (predominantly as graptolites), with a minor amount of micrinites (as the heavier conversion products of the oil-prone kerogen macerals). Additionally, the degraded alginites were traced by their bright halo feature in reflected light and blurred fluorescence in blue light when enhancing the exposure intensity, which demonstrates that the previously deposited alginites in the WF–LMX shale were basically converted to hydrocarbons. SEM imaging revealed that the secondary OM pores associated with hydrocarbon generation are pervasive in pyrobitumen, while undeveloped in organic zooclasts and micrinites. Conversely, the good linear relationship between TOC contents and methane absorbed amounts indicates that there must be numerous SEM-invisible pores existed in both the solid bitumen and organic zooclasts in addition to the secondary pores.Aiming to certify and quantify the SEM-invisible pores, the OM were separated from the WF-LMX shale and then were subjected to a series physical and chemical measurements. The combined N2 and CO2 adsorption on the OM isolates of the WF–LMX shale further revealed two pore concentration sections, i.e., pores of 0.33–0.5 nm and 1.2–2.0 nm within the micropore regime, which contributed approximately 82% and 18% to the total surface area and pore volume of the OM, respectively. X-ray diffraction testing further correlated the micropores to the spacing of aromatic rings and macromolecular structural units that constitute the OM. As such, it is the micropores related to the chemical structure of the OM that essentially determine the sorptive capacity of OM and also the WF–LMX bulk shale, whereas the main contribution of SEM-visible OM pores is affording the effective porosity for gas storage. Overall, the clarification of OM assemblages, carriers of secondary OM pores, and existence and origin of OM micropores in the WF–LMX shale provide a further understanding of the generation and occurrence of shale gas in South China.

The architecture of organic matter and its pores in highly mature gas shales of the lower Silurian Longmaxi Formation in the upper Yangtze platform, south China

The nanometer-scale architecture of organic matter (OM) and associated pores in highly mature gas shales from the lower Silurian Longmaxi Formation in the upper Yangtze platform of south China were investigated using field emission scanning electron microscopy (SEM), focused ion beam SEM, and low-pressure gas (N2 and CO2) adsorption bulk pore characterization. The Longmaxi shale comprises fine-grained siliciclastic rocks deposited in a marine shelf environment, which was dominated by quartz and clay minerals. Four porous OM types were found in the Longmaxi shale on the basis of the chemical composition and spatial occurrence of OM, including (1) isolated original OM particles, (2) OM–clay mineral complexes, (3) OM–heavy mineral complexes, and (4) secondary migrated bituminous OM. The pores in the particulate OM are not homogeneously distributed, and the processes leading to different pores depend on the specific OM type. The nature of OM-hosted pores is a result of several factors, such as primary porous kerogen, mechanical compaction, organic–inorganic interactions, gaseous and liquid hydrocarbon generation, retention, and expulsion. Pore volumes and specific surface areas of the Longmaxi shale derived from low-pressure N2 and CO2 adsorption experiments reveal positive linear relationships with total organic carbon contents, which indicates that the pore systems in the highly mature Longmaxi shale are dominated by OM-hosted pores. Additionally, the OM-hosted pores appear connected compared to pores in the mineral matrix. Therefore, the OM-hosted pore systems offer the preferential storage space and primary migration pathways for natural gas in the Longmaxi shale reservoir.

Effect of thermal maturity on shale pore structure: A combined study using extracted organic matter and bulk shale from Sichuan Basin, China

The thermal maturity is of great significance in the evolution of pores in gas shales. In this work, the comparison of pore structures between Wufeng-Longmaxi and Niutitang shales and its corresponding extracted organic matter (OM) is made. The Wufeng-Longmaxi shales have higher total pore volumes, surface areas and porosity than the Niutitang shales and OM pores are the main pore types in both shales. With the thermal maturity (Ro) increases, the OM porosity decreases. Combined with literature results, it is further proved that the evolution of OM pores was related to the gas generation with increasing thermal maturity. Compared with Niutitang shales, the lower thermal maturity of Wufeng-Longmaxi shales is beneficial for preserving OM pores and providing better connectivity. It can be concluded that the thermal maturity is the main controlling factor on the development of OM pores, which are important for the shale gas generation and preservation.

Nanoscale pore structure heterogeneity and its quantitative characterization in Chang7 lacustrine shale of the southeastern Ordos Basin, China

To gain a better understanding of nanoscale pore structure characteristics in Chang7 lacustrine shale, Ordos Basin, China, 33 shale samples from 15 wells are examined and analyzed by total organic carbon (TOC), X-ray diffraction (XRD), mercury porosimetry, helium pycnometry, low-pressure N2 adsorption experiments and fractal method. The results show that Chang7 lacustrine shale pores diameter is mainly from 1.5 to 5 nm, and mesopores give the largest contribution to the total pore specific surface area (SBET) and volume (VBJH), and micropores have the second contribution to SBET but little contribution to VBJH, macropores have very little contribution to SBET but make a good contribution to VBJH. According to the occurrence and origin of shale pores, micro-pores can be divided into mineral intragranular pore, minera intercrystalline pore and organic pore. Two fractal dimensions (D1 and D2) are obtained from N2 adsorption isotherms analysis using FHH method. Relationships between fractal dimensions and shale compositions, pore structure parameters are investigated, which show that both D1 and D2 have good positive relationships with SBET and VBJH, but obvious negative correlations with the average pore diameter. That is, the smaller the pore diameter, the more complex the pore structure is, resulting in the larger pore surface and volume. D1 and D2 are both positively correlated with total clay, illite, mixed-layer of illite-smectite, whereas no obvious relationships with chlorite, indicating that the layer and flocculent structure increase the complexity of nanoscale pore structure. TOC content has positive correlation with D1 but no obvious relationship with D2, indicating that the pores in organic matter are mostly micropores. Fractal dimensions are negatively correlated with total brittle mineral, quartz and feldspar content, that is because the surface of brittle minerals is smoother and more homogenous than that of clay minerals.

Pore characteristics of the lower Sinian Doushantuo Shale in the Mid-Yangtze Yichang area of China: Insights into a distinct shale gas reservoir in the Neoproterozoic formation

The Lower Sinian Doushantuo Shale comprises the oldest Neoproterozoic shale gas reservoir in the Middle Yangtze Yichang area of China, which makes the study of their pore characteristics critical to future shale gas exploration and exploitation. Geochemical, petrographical, and pore characteristics are studied in detail, with focused ion beam scanning electron microscopy (FIB-SEM) image analysis to further support and confirm the interpretations of the nano-scale pore structure characteristics. High total organic carbon (TOC) content, high thermal maturity, and considerable brittle mineral content indicate promising potential of the Doushantuo shale. High TOC samples suggest elevated micro-meso porosity, and H2(b) and H3 type hysteresis loops indicate inkbottle, slit-like, and flake-like pore types. Significant inorganic pores are commonly observed in dolomite, pyrite, and apatite, which are characterized by diamond or irregular shapes, with pore diameters ranging from 5 nm to 1000 nm as a result of the interaction between the minerals and the acidic organic fluids. Clustering organic matter (OM) pores are observed with pore diameters ranging from 5 nm to 50 nm; they are characterized by sponge-like, elliptical, and alveolate shapes, which indicate minor organic matter (OM) pores. More substantial organic matter (OM) pores can also be found around or between the pyrite and apatite. Fractures can be clearly observed between the organic matter and minerals with lengths of approximately 10 μm and 50 nm in width, which serve as conduits for shale gas migration. The pores in the Doushantuo Shale are dominated by inorganic pores and minor organic matter (OM) pores, which are different from the pore structure of the Wufeng-Longmaxi Shales in the Sichuan Basin. The Doushantuo Shale organic matter (OM) pores are much smaller and more densely distributed than those in the Wufeng-Longmaxi Shales. The different organic matter origins of these two shales have caused distinct organic matter (OM) pore-development mechanisms. Organic matter (OM) pores and inorganic pores could contribute to the volume of the adsorbed gas and free gas, respectively, which provide significant reservoir space in the Lower Sinian Doushantuo Shale in the Yichang area.

The effect of thermal maturity on porosity development in the Upper Devonian –Lower Mississippian Woodford Shale, Permian Basin, US: Insights into the role of silica nanospheres and microcrystalline quartz on porosity preservation

Organic-matter pores are considered to be the dominant contributor to total porosity in many shale reservoirs, and consequently much attention has been paid to investigate the factors influencing their generation, especially thermal maturity. Recent studies have shown that in addition to maturation of organic matter, other diagenetic processes also play important roles in evolution of pore systems, including compaction, mineral dissolution, and quartz cementation. This study investigates the effect of thermal maturity on porosity development in the siliceous Woodford Shale by integrating multiple techniques including helium pycnometry, low pressure gas (N2) adsorption, mercury injection capillary pressure, and field emission scanning electron microscopy (SEM) analyses. We compare samples from two long cores from Woodford Shale at different levels of thermal maturity, one in the early oil window (average calculated Ro = 0.69%) and the other in the wet gas window (average calculated Ro = 1.34%).Porosity, pore size, type, and volume vary systematically with the thermal maturity increasing from early oil window to wet gas window. Porosity measured by helium pycnometry averages 4.1% for early oil window samples, and increased by approximately 50% for the wet gas window samples (average porosity =6.3%). Both pore size and pore throat size distributions display decreasing trends from early oil window to wet gas window. Petrographic examination of SEM images shows that pores developed between clay platelets dominate in the early oil window samples, and secondary organic-matter pores are the dominant pore type in wet gas window samples. Porosity shows no obvious correlation to TOC content in both oil window and gas window samples, but correlates positive to silica content, suggesting that silica content largely controls variations in porosity. Both silica nanospheres and microcrystalline quartz were observed and appear to be significant in preserving pores from compaction by forming rigid frameworks that prevent pores from collapsing. Generation of secondary organic-matter pores and preservation of primary pores by silica nanospheres account for the net porosity increase from early oil window to wet gas window.

Effects of rice straw biochar on sorption and desorption of di-n-butyl phthalate in different soil particle-size fractions

Sorption of organic contaminants by biochar greatly affects their bioavailability and fate in soils. Nevertheless, very little information is available regarding the effects of biochar on sorption and desorption of organic contaminants in different soil particle-size fractions. In this study, di-n-butyl phthalate (DBP), a prevalent organic contaminant in agricultural soils, was taken as a model contaminant. The effects of biochar on DBP sorption and desorption in six particle-size fractions (i.e., coarse sand, fine sand, coarse silt, fine silt, clay, and humic acid fractions) of paddy soil were investigated using batch sorption-desorption experiments. A straw-derived biochar with high specific surface area (116 m2/g) and high content of organic matter (OM) rich in aromatic carbon (67%) was prepared. Addition of this biochar (1% and 5%) significantly promoted the sorption and retention of DBP in all the paddy soil particle-size fractions at environmentally relevant DBP concentrations (2–12 mg/L) with 1.2–132-fold increase of the Kd values. With increasing addition rates of biochar, DBP retention by the biochar enhanced. The biochar’s effectiveness was remarkably influenced by the physicochemical properties of the soil particle-size fractions, especially, the OM contents and pore size showed the most striking effects. A parameter (rkd) reflecting the biochar’s effectiveness showed negative and positive correlations with OM contents and pore size of the soil particle-size fractions, respectively. Accordingly, strong effect of the biochar was found in the soil fractions with low OM contents and high pore size. The findings of this study gave insight into the effects and influencing factors of biochar on sorption and desorption of organic contaminants in soils at scale of various particle-size factions.

Multiscale connectivity characterization of marine shales in southern China by fluid intrusion, small-angle neutron scattering (SANS), and FIB-SEM

Abstract An evaluation of shale pore connectivity is essential for predicting the production behavior and optimizing the development plan of shale gas. To systematically investigate the multiscale connectivity characteristics and controlling factors, complementary tests were conducted on four overmature marine shale samples from southern China (one each from Longmaxi Formation and Wufeng Formation, and two from Niutitang Formation) with different maturity and composition. The methods included mercury intrusion capillary pressure (MICP), spontaneous imbibition with deionized water and n-decane, saturated diffusion distribution with seven nano-sized tracers, N2 physisorption, small-angle neutron scattering (SANS), and focused ion beam-scanning electron microscopy (FIB-SEM) tomography. Moreover, a novel repeated MICP measurement technique was developed and used to evaluate the pore connectivity from the distribution behavior of residual mercury. Results indicate that the connectivity loss of hydrophilic pore networks owing to an increased maturity and the compaction causes pore shrinkage and bond breakage between pore networks. According to the three-dimensional reconstruction and pore network extraction of FIB-SEM images, pores in organic matter (OM) have a good pore connectivity, but the minerals surrounding the OM reduce the overall pore connectivity of the shale matrix. By evaluating the pore connectivity using different methods, the hydrocarbon migration pattern in overmature marine shales was established. Our results imply that in addition to creating induced fracture networks, effectively connecting induced fractures with preexisting microcracks or joint sets between OM pore networks and transport pores can also be used to enhance overall pore connectivity. Microwave heating in shale reservoirs may be an effective way of further improving the production and recovery of hydrocarbons.

Reservoir characteristics of and potential analysis to Lower Palaeozoic shale in Xiuwu Basin, Lower Yangtze Plate

ABSTRACT In order to evaluate reservoir characteristics and gas potential of organic-rich shale in Bottom-Lower Cambrian Hetang Formation and Upper Ordovician Xinkailing Formation, Xiuwu Basin, Jiangxi Province, Lower Yangtze Plate of China, researches regarding reservoir characteristics of organic-rich shale thickness, moderate burial depth, abundance and maturity of organic matter, fracturability, organic-matter pores and absorption performance have been conducted to Hetang Formation and Xinkailing Formation through field survey, shallow drilling, analysis to hydrocarbon-generation potential, scanning electron microscopy, measuring specific surface area and isothermal adsorption experiments. The results show that (1) Hetang Formation reservoir, although with extremely rich material basis, is of unfavorable hydrocarbon generation conditions due to late-stage thermal maturity, besides of undeveloped micropore structure, reservoir space development mainly by macroscopic crack, so further exploration should focus on locating favorable areas of low maturity and developed cracks, which is considered distributing in the core of Yanshan-Lingangshan-Shiziyan Mountains syncline; (2) Xinkailing Formation reservoir is of better organic matter abundance, pore development and absorption performance than Xinkailing Formation and hence further exploration should focus on surveying favorable areas of better sedimentary facies belt and stable structure for major breakthrough in shale-gas exploration, which is considered distributing in Wuning-Lixi and Jinkou-Tuolin areas.

Re-recognition of “unconventional” in unconventional oil and gas

Taking the Wufeng–Longmaxi shale gas in the Sichuan Basin as a typical example, based on the new progress in exploration and development, this study re-examines the “unconventional” of unconventional oil and gas from two aspects: oil and gas formation and accumulation mechanisms, and main features of oil and gas layers. The oil and gas of continuous accumulation and distribution from integrated source and reservoir is unconventional oil and gas, and the study focusing on shale oil and gas in comparison with conventional oil and gas has made progress in five aspects: (1) Unconventional oil and gas have source-reservoir-in-one and in-situ accumulation; according to the theory of continuous oil and gas accumulation, the accumulation power of oil and gas is overpressure and diffusion; for conventional oil and gas, the source and reservoir are different formations, the trapping accumulation is its theoretical foundation, and the accumulation power is characterized by buoyancy and capillary force. (2) The unconventional oil and gas reservoirs are mainly formed in the low-energy oxygen-anaerobic environment, dominantly semi-deep to deep shelf facies and the semi-deep to deep lake facies, simple in lithology, rich in organic matter and clay minerals; conventional oil and gas mainly occur in coarse-grained sedimentary rocks formed in high-energy waters with complex lithology. (3) The unconventional oil and gas reservoirs have mainly nano-scale pores, of which organic matter pores take a considerable proportion; conventional oil and gas reservoirs mainly have micron-millimeter pores and no organic matter pores. (4) Unconventional shale oil and gas reservoirs have oil and gas in uniform distribution, high oil and gas saturation, low or no water content, and no obvious oil and gas water boundary; conventional oil and gas reservoirs have oil and gas of complex properties, moderate oil and gas saturation, slightly higher water content, and obvious oil, gas and water boundaries. (5) Organic-rich shale is the main target of unconventional oil and gas exploration; the sedimentary environment controls high organic matter abundance zone and organic matter content controls oil and gas abundance; positive structure and high porosity control the yields of shale wells; bedding and fracture development are important factors deciding high yield.

Nanopores of the productive and non-productive sections within the Lower Silurian Longmaxi Shale Reservoir in the Southern Area of Sichuan Basin, China

This study focuses on nanopores of the productive and non-productive sections within the Lower Silurian Longmaxi shale reservoir in the southern area of Sichuan Basin. 4 rock samples from the productive section and 12 rock samples from the non-productive section were collected from a new exploration well in Changning Area. TOC, XRD and gas adsorption measurements were conducted on these samples. The productive section contains more organic matters, biogenic quartz, but less carbonates and feldspar and is much more porous relative to the non-productive section. Organic matters and clays have important impacts on the development of nanoporosity within the Lower Silurian Longmaxi shale reservoir in study area. Nanoporosity of the productive section is affected mainly by the organic matter contents, and hence implying the predominance of organic-matter pores within this section. Clay minerals have the primary controlling effects on the occurrence of nanopores within the non-productive section, and then indicating the predominance of mineral matrix pores in it. This study should be helpful for us to make a better understanding of in-situ shale gas resources and to optimize targets in this region.

Organic geochemical and petrophysical characteristics of saline lacustrine shale in the Dongpu Depression, Bohai Bay Basin, China: Implications for Es3 hydrocarbon exploration

Saline lacustrine shales are widely developed in the Es3 strata of the Dongpu Depression. Recently, a high-yield oil flow from the upper sub-member of Es3 (Es3U) of the PS18-1 well first confirmed the presence of shale reservoirs. However, the detailed difference of geochemistry, hydrocarbon generation potential, and reservoir quality between the upper (Es3U), middle (Es3M), and lower (Es3L) sub-members of Es3 saline shales remained unclarified. Both geochemical and petrophysical analyses were performed to answer above questions in this study. The results indicate that compared with Es3U and Es3M shales, Es3L shale has a higher TOC content (mean 1.94%). Oil-prone type I-II kerogen is the main kerogen type of Es3U and Es3L shales, while Es3M shale is dominated by type II-III kerogen. Es3L shale shows a relatively higher original hydrocarbon generation potential (mean 10.26 mg HC/g Rock) with an average original total organic carbon (TOCo) value of 1.62%, and a higher generated hydrocarbon (QHC) than other two sets of shales. Compared with the dual organic matter source of marine and terrestrial organisms of Es3M shales, the main organic matter of Es3U and Es3L shales derived from aquatic organisms, and developed in saline water and brackish–saline water, respectively, with strong reducing environment. Organic-matter pores are undeveloped in Es3 shales, whereas interparticle and intraparticle pores are easily to be found. Higher brittle minerals are found in Es3U and Es3L shales, which are beneficial to form fracturing fractures during the development process, and widely developed bedding plane fractures can provide good conditions for hydrocarbon migration. Based on discussion, Es3L shales with present-day TOC over 1.5%, higher brittle minerals and gypsum salt around can be considered to be the most promising area for further shale oil exploration in the northern Dongpu Depression.

Structure characteristics and evolution mechanism of nanopore in transitional coal-bearing shale

Nanopore plays a crucial role in shale gas adsorption, and its characteristics and evolution are significant for the evaluation of shale reservoirs with strong heterogeneity. In this study, transitional coal-bearing shale samples from the south margin of North China were selected for investigation of pore structure characteristics and evolution mechanism. Results indicated that nanopores of the studied shales are fractal with strong heterogeneity. The higher fractal dimension leads to larger surface area and micropore volume of shale samples. The abundant terrigenous material and limited hydrocarbon generation and expulsion are favorable for the development of intraP pores in clay, so that the Organic matter (OM) pores are poorly developed and are partially filled with clay minerals and pyrite framboids. Shale samples having higher levels of clay content and lower levels of quartz content tend to develop more heterogeneous micropores, producing a more complex pore structure in shale. The evolution of nanopore in studied shales is simultaneously determined by the coupling relationship between clay content and the thermal maturation of OM. In the early oil and low clay content stage, the low degree of the thermal evolution of OM leads to generation of less acidic fluid during the decarboxylation of kerogen, resulting in a weak physical combination and chemical corrosion between OM and clay minerals, with the macropores taking the main place in the pore structure. In the main to late oil and medium clay content stage, hydrocarbon generation of OM leads to the development of pores in kerogen and the gradual generation of more acidic fluids that dissolve clay minerals, so that the mesopores are dominant. In the wet gas and high clay content stage, increases in hydrocarbon generation intensity and clay mineral content cause development of more complexes of OM and clay, with a large number of micropores produced in shale reservoirs.

Characterization of pore microstructure and methane adsorption of organic-rich black shales in northwestern Hunan, South China

The gas adsorption of shale is one of the most important factors influencing shale gas accumulation and production. Different from other shale gas fields, the shale in Northwestern Hunan province with strong structural deformation has especially high portion of adsorbed gas. The adsorption abilities of shale reservoir play a significant role in the success of exploration and production of shale gas in Northwestern Hunan. Therefore, based on many experiments including field emission scanning electron microscopy, N2/CO2 adsorption, excess CH4 adsorption with constant temperature and others, this paper found that: (1) the shale pores were identified with mineral matrix pores, organic-matter pores and fracture pores, and the pores can be further classified according to their shape features and forming mechanism; (2) after analyses of the distribution of the pore size, mesoporous is the largest contributor and widely distributed, and micropores provide more contribution on surface area but limited contribution to pore volume. Meanwhile, depth has little effect on the properties of shale reservoirs; (3) based on the fractal geometry theory, surface roughness is positively correlated to microstructure irregularity, which means the effects between the gas adsorption by Van der Waals force and by multilayer or capillary condensation in single pore system are consistent during the N2 adsorption process. (4) The abilities of gas storage are apparently sensitive to the surface roughness of shale pores and nonsensitive to microstructure irregularity. Total organic carbon has strong positive correlation with the surface roughness, specific surface area, total pore volume, and Langmuir volume, which means TOC can improve the adsorption of shale reservoir in many ways. Pyrite can offer much contribution to the ability of gas adsorption.