Main Storage Space Research Articles

Characteristics of pore structures from the Lower Paleozoic shale gas reservoirs in northern Guizhou, South China

Pore structure in shale is an important factor influencing the storage capacity of a shale gas reservoir and the effect of shale gas development. Taking the core and outcrop shale samples from the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation as the object of the research, combined with the scanning electron microscope, high pressure mercury injection, CO2 and N2 gas adsorption experiments, we investigated the porosity, pore types, pore structure, and pore size distribution of the Lower Paleozoic shale gas reservoirs in northern Guizhou, South China. The results indicated that there are developed various genetic types of micro-nano pores in the Lower Paleozoic marine shale in northern Guizhou, which provide the main reservoir spaces for shale gas. The porosity of the Lower Paleozoic shale in northern Guizhou is 1.30%–13.37% in range, and the average is 4.56%. The inter-granular pore, organic matter pore and intra-granular pore are most common. Micropore (<2 nm) and mesopore (2–50 nm) are the main reservoir storage space for shale gas. The specific surface area of micropore plus mesopore in the Lower Paleozoic shale accounts for 94.3% of the total specific surface area, and their pore volume accounts for 59.1% of the total pore volume in northern Guizhou. The pore size of the Lower Paleozoic shale is mainly distributed in 0.4–0.7 nm, 3–10 nm and 0.05–200 μm. The pore structure is the parallel-plate pore with slit shape, two-ends-opened tube pore and ink-bottle shape pore. There are significant differences in the porosity and pore size between the Longmaxi Formation shale and the Niutitang Formation shale. The average porosity and average pore size of the Longmaxi Formation shale is 6.4% and 4.71 nm, while the Niutitang Formation shale is 5.3% and 4.23 nm, respectively. The pore structure of the Lower Paleozoic shale gas reservoir is mainly influenced by organic matter abundance, thermal maturity, mineral composition of shale. The porosity and micropore volume of shale show an increasing trend, with the increase of total organic carbon content and quartz content. The porosity and pore volume of shale have a decreasing trend, with the increase of thermal maturity level and clay mineral content.

The temperature distribution model and its application to reservoir depth prediction in fault-karst carbonate reservoirs

The fault-karst carbonate reservoirs in Tarim Basin have previously been determined to have promising prospects for exploration and development. Large karst caves are the main storage space and flow channel, which are separated from one another, yet connected by fractures. During the drilling processes, when large karst caves are encountered, drilling break and mud leakage often occur. Such occurrences make it impractical to carry out normal logging operations or determine the depths of the reservoirs. Therefore, in order to address the difficulties in determining the reservoir depths during the development fault-karst reservoirs, this study presented a method by which to accurately predict reservoir depths by means of combining the relationships between the flow temperatures and rest temperatures within the well structures. A mathematical model describing the heat transfer was established for the purpose of obtaining the temperature distributions of the fluid within the karst caves and wellbores during stable oil production, and the model was verified using numerical simulations. In addition, a temperature distribution method was applied to the temperature predictions of wells in Shunbei Block of Tarim Basin, and the reservoir depths were predicted at the same time. Then, the research results were compared with the seismic section of the fault-karst. It was found that good application effects and consistency had been achieved.

Pore characteristic responses to categories of depositional microfacies of delta-lacustrine tight reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, NW China

Primary depositional variations interact with diagenetic modifications to exert strong control on pore networks in tight hydrocarbon reservoirs. However, the linkage between pore characteristics and depositional microfacies remains enigmatic, which restricts the prediction of high-quality reservoir distributions. The continental tight reservoirs of the Yanchang Formation in the Ordos Basin are targeted to clarify and estimate the sensitivity of pore characteristics to depositional microfacies. A multi-methodical approach utilizing petrographic assessments, logging evaluations, digital imaging analyses and high-pressure mercury intrusion experiments enables us to reveal the high microheterogeneity of pore types, pore-size distribution and pore fractal features. The results identified the microfacies including distributary channels (DCs), mouth bars (MBs), interdistributary bays (IBs), sheet sands (SSs), turbidite fans (TFs), shallow lacustrine mudstones (SLMs) and semi-deep lacustrine mudstones (SDLMs) from the Chang 8 to Chang 6 members. Dominated by residual intergranular pores and microfractures, the main storage spaces of the DC and MB microfacies are within mesopores, which provide 62%–92% and 81% of the total pore volume, respectively. However, the micropores that are mainly composed of clay intercrystalline pores and feldspar dissolved pores provide 55%–70%, 52%–62% and 53%–80% of the total pore volume for the IB, SLM and SDLM microfacies, respectively. In addition, the pore fractal dimension results indicate that the mesoporous structure of the argillaceous microfacies (IB, SLM and SDLM) is more heterogeneous than that of the arenaceous microfacies (DC, MB and TF). Therefore, the dominant reservoir spaces of the different depositional microfacies are distributed over diverse pore-radius ranges, and the varying degrees of mesoporous development are a crucial factor affecting the reservoir capacity. Moreover, the fractal dimensions have a significant negative correlation with pore volume in the meso-micropore range. Characterized by lower pore sorting coefficients, broader skewness and more abundant rigid siliceous grains, the arenaceous microfacies with lower fractal dimensions and higher pore volume have better reservoir capacity than argillaceous microfacies that are rich in ductile minerals. Microscopic observations show that the muddier reservoirs of interdistributary bays and lacustrine mudstones are more susceptible to pore-filling chlorite and illite, which can deteriorate the reservoir quality. These results indicate that the controlling effect of depositional microfacies on pore characteristics is fundamentally caused by differences in mineralogical composition and depositional texture, and these results could provide new insights into the mechanisms of microfacies controls on reservoir quality.

Study on Influencing Factors of Microscopic Occurrence of Tight Oil in Nanoscale Pore Throat.

Tight oil is widely distributed around the world, and industrialized exploitation has been developed and applied in North America, greatly alleviating the contradiction between supply and demand of energy. The main storage space of tight oil reservoirs is micro-nano pores, and a reliable method for studying the occurrence of tight oil in micro-nano pores has not been studied. In this paper, the microscopic occurrence of tight oils in the Songliao Basin, Ordos Basin, and Tarim Basin in China are studied in combination with nuclear magnetic resonance (NMR) and field emission scanning electron microscopy (FE-SEM) scanning techniques. A method for characterizing the storage space, occurrence state, and influencing factors of tight oil based on SEM and NMR was proposed. The results show that the storage space of tight oil reservoirs is nanoscale cracks and pores. Mainly, these are distributed on the scale of less than 1 μm and more than 10 μm, the volume of porecracks is less than 1 μm, which account for more than 80% of the total volume. This indicates that the tight oil mainly exists in the nanoscale pore throat. The scattered distribution of crude oil in the black shape of oil spot is mainly in the micro-nano pores, and the crude oil content in the micro-cracks is small, indicating that nanoscale cracks are the main space for the occurrence of tight oil. However, some micro-cracks can penetrate oil spots or oil stains, and can connect the oil spots with scattered distribution, which will greatly improve the mobility of crude oil. In addition, oil spot accumulation areas often develop illite and illite/smectite mixed-layer, and clay minerals have developed nanoscale pores. The crude oil and clay minerals have certain associated characteristics. This study is of great significance for understanding the law of the occurrence and migration of tight oil and the basic theoretical research and technological innovation of the development of tight oil reservoir.

A unified apparent porosity/permeability model of organic porous media: Coupling complex pore structure and multi-migration mechanism

Shale gas resources are widely distributed and abundant in China, which is an important field for strategic replacement and development of oil and gas resources. Shale gas reservoirs has adsorption gas, free gas. The structure of different scale media, such as organic pores, are difficult to describe. Therefore, flow behavior cannot be simulated by conventional method. In this paper, the micro-scale fluid migration in shale gas reservoirs was established in a single pore, which coupled surface diffusion, slip flow, and viscous flow. On this basis, the fractal scale relationship was applied to describe the distribution of pore radius, tortuosity, and surface roughness. Based on the comprehensive characterization of static structure haracteristics of porous media, such as pore size distribution, pore shapes, tortuosity and surface roughness, and the dynamic pore size influenced by various stresses, the apparent porosity/permeability model of organic matter considering single-phase multi-migration mechanism was established. The gas migration in organic porous media was analyzed with the apparent porosity/permeability model. The results show that the small pores in organic matter are the main storage space of gas (more than 95% of the gas is stored in pores less than 10 nm), and the large pores are gas flow channel. At the same time, the apparent porosity/permeability model combined with conventional Darcy equation can be used to describe the single-phase gas flow in shale gas reservoirs. Cited as : Sheng, G., Su, Y., Zhao, H., Liu, J. A unified apparent porosity/permeability model of organic porous media: Coupling complex pore structure and multi-migration mechanism. Advances in Geo-Energy Research, 2020, 4(2): 115-125, doi: 10.26804/ager.2020.02.01

A Distributed Storage and Computation k-Nearest Neighbor Algorithm Based Cloud-Edge Computing for Cyber-Physical-Social Systems

The k-nearest neighbor (kNN) algorithm is a classic supervised machine learning algorithm. It is widely used in cyber-physical-social systems (CPSS) to analyze and mine data. However, in practical CPSS applications, the standard linear kNN algorithm struggles to efficiently process massive data sets. This paper proposes a distributed storage and computation k-nearest neighbor (D-kNN) algorithm. The D-kNN algorithm has the following advantages: First, the concept of k-nearest neighbor boundaries is proposed and the k-nearest neighbor search within the k-nearest neighbors boundaries can effectively reduce the time complexity of kNN. Second, based on the k-neighbor boundary, massive data sets beyond the main storage space are stored on distributed storage nodes. Third, the algorithm performs k-nearest neighbor searching efficiently by performing distributed calculations at each storage node. Finally, a series of experiments were performed to verify the effectiveness of the D-kNN algorithm. The experimental results show that the D-kNN algorithm based on distributed storage and calculation effectively improves the operation efficiency of k-nearest neighbor search. The algorithm can be easily and flexibly deployed in a cloud-edge computing environment to process massive data sets in CPSS.

Downsizing Without Downgrading: Approximated Dynamic Time Warping on Nonvolatile Memories

In recent years, time-series data have emerged in a variety of application domains, such as wireless sensor networks and surveillance systems. To identify the similarity between time-series data, the Euclidean distance and its variations are common metrics that quantify the differences between time-series data. However, the Euclidean distance is limited by its inability to elastically shift with the time axis, which motivates the development of dynamic time warping (DTW) algorithms. While DTW algorithms have been proven very useful in diversified applications like speech recognition, their efficacy might be seriously affected by the resolution of the time-series data. However, high-resolution time-series data might take up a gigantic amount of main memory and storage space, which will slow down the DTW analysis procedure. This makes the upscaling of DTW analysis more challenging, especially for in-memory data analytics platforms with limited nonvolatile memory space. In this paper, we propose a strategy to downsample time-series data to significantly reduce their size without seriously affecting the precision of the results obtained by DTW algorithms (downsizing without downgrading). In other words, this paper proposes a technique to remove the unimportant details that are largely ignored by DTW algorithms. The efficacy of the proposed technique is verified by a series of experimental studies, where the results are quite encouraging.

Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China

Shale gas reservoir performance and canister desorption experiments of the Lower Cambrian organic-rich shales in the eastern Upper Yangtze Platform reveal a significant difference in shale gas content between the Dabashan arc-like fold-thrust belt in northeastern Chongqing (Deformed Zone) and the slightly folded area in southeastern Chong (Non-deformed Zone). Integrated pore characterization methods including scanning electron microscopy (SEM), low-temperature N2 and CO2 adsorption, and mercury injection capillary pressure (MICP) analyses were comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment.The Lower Cambrian shales in both Deformed Zone (DZ) and Non-deformed Zone (NDZ) were deposited in the deep-water shelf and show similar organic matter richness and thermal maturity. The majority of organic matter (OM)-hosted pores in DZ samples are in nanoscale size range with the dominance of micro-fractures within the OM or at the interface of OM and minerals. In contrast, OM-hosted meso-(2–50 nm) to macropores (>50 nm) are the dominant pore types in the NDZ samples. OM-hosted micropores (<2 nm) are abundant in both zones. Helium ion microscopy observations further confirm the presence of OM-hosted micropores in the studied samples. Mineral-hosted pores in carbonate minerals are abundant in both zones, while dissolution rims around carbonate minerals are more abundant in NDZ samples.The Dabashan arc-like fold-thrust belt took place by the end of the Late Triassic, while the Lower Cambrian shales have reached thermal maturity peak. OM-hosted meso-(2–50 nm) to macropores (>50 nm) in DZ samples are most probably collapsed during structural deformation related to tectonic compression, while micropores due to their smaller size survived the tectonic stress. The OM-hosted micropores are the main storage space for adsorbed gas in the DZ area. The dominance of micro-pores in DZ and lack of connection between those pores and matrix pores led to higher gas content in DZ samples. On the contrary, the well-connected OM-hosted pore network in NDZ allows easier gas flow in the rock matrix that eventually led to significant gas leakage during uplift and exhumation and lower gas content in this zone. The results of this study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in south China where had experienced complex tectonic movements.

Organic nanopore structure and fractal characteristics of Wufeng and lower member of Longmaxi shales in southeastern Sichuan, China

Abstract Organic nanopores in shale gas reservoirs are the main storage space for free gas and adsorbed gas and directly affect the occurrence mode and movement of shale gas. In this study, field emission-scanning electron microscopy (FE-SEM), low-pressure gas (N2 and CO2) adsorption and mercury injection capillary pressure (MICP) analyses were performed and interpreted with statistical and fractal analyses to investigate the organic pore structure in the Upper Ordovician Wufeng shale (O3w) and the Lower Silurian lower member of Longmaxi shale (S1l). It was found that organic pores in 15 samples from 8 layers of the Wufeng-Longmaxi shale section are usually developed in discrete organic matter particles, organic matter associated with clay minerals and organic matter associated with pyrite framboids. The size, shape and quantity of organic pores vary greatly. The organic pores in Wufeng Formation (layer 1) are densely developed in organic matter particles with relatively small sizes and irregular shapes. The organic pores in the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5) are less developed and have relatively larger pore sizes with an elliptical shape. In contrast, the organic pores in the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) are the least developed, with elliptical shapes and pore sizes in between the two cases above. A higher value of fractal dimension refers a more complex form and this value of whole organic pores with full range of sizes (Dwop) is the largest and followed by organic macropores (Dop>50), organic pores with pore sizes between 4 nm and 50 nm (Dop4-50), and organic pores with pore sizes less than 4 nm (Dop

A model of gas-induced effective expansion strain for porous carbon materials in irradiation environments

The application of fully ceramic microencapsulated (FCM) fuels to light water reactors has attracted increasing attention. As the main storage space for fission gases and some other gases in FCM fuels, the microstructure and thickness of porous carbon buffer layer should be optimally designed. In this study, a model of gas-induced effective expansion strain rate for porous carbon materials is developed based on meso-mechanical method and homogenization theory. By combining theoretical analysis with finite element (FE) simulation, the resulting model is verified, which considers the irradiation-induced creep deformation of dense PyC skeleton, the gas contained in the pores of the buffer layer, the accumulated effective volumetric strain and the external pressure. The effects of temperature, fission rate, kernel materials, initial porosity, external hydrostatic pressure and creep coefficient of dense PyC skeleton are investigated. This work is expected to provide a theoretical reference for optimization design of a porous carbon buffer layer, and the developed model can be used for further simulation of irradiation-induced thermo-mechanical behavior in FCM fuels.

Petrophysical Characterization and Fractal Analysis of Carbonate Reservoirs of the Eastern Margin of the Pre-Caspian Basin

Petrophysical properties including pore structure and permeability are essential for successful evaluation and development of reservoirs. In this paper, we use casting thin section and mercury intrusion capillary pressure (MICP) data to investigate the pore structure characterization, permeability estimation, and fractal characteristics of Carboniferous carbonate reservoirs in the middle blocks of the eastern margin of the Pre-Caspian Basin. Rock casting thin sections show that intergranular and intragranular dissolution pores are the main storage spaces. The pore throats greater than 1 μm and lower than 0.1 μm account for 47.98% and 22.85% respectively. A permeability prediction model was proposed by incorporating the porosity, Swanson, and R35 parameters. The prediction result agrees well with the core sample data. Fractal dimensions based on MICP curves range from 2.29 to 2.77 with an average of 2.61. The maximum mercury intrusion saturation is weakly correlated with the fractal dimension, while the pore structure parameters such as displacement pressure and median radii have no correlation with fractal dimension, indicating that single fractal dimension could not capture the pore structure characteristics. Finally, combined with the pore types, MICP shape, and petrophysical parameters, the studied reservoirs were classified into four types. The productivity shows a good correlation with the reservoir types.

Characterization of pore structure and its impact on methane adsorption capacity for semi-anthracite in Shizhuangnan Block, Qinshui Basin

To determine the pore structure characteristics and its effect on the adsorption capacity of semi-anthracite, high pressure mercury injection (HPMI), Low pressure nitrogen gas adsorption (LP-N2GA), Low pressure carbon dioxide gas adsorption (LP-CO2GA) and isotherm adsorption experiments were carried out on eight coal samples collected from the Shizhuangnan Block. The results show that the semi-anthracite reservoirs predominantly developed semi-open pores with poor connectivity and various pore morphology. The combined pore size distribution (PSD) from results of HPMI, LP-N2GA and LP-CO2GA indicated that the super-micropores in semi-anthracite reservoirs are most developed, providing the main storage space, accounting for 80.75% of the total TPV and 99.69% of the total SSA, followed by macropores. Additionally, the pore volume and SSA distributions of semi-anthracite reservoirs are unimodal with peak values present at 0.5–0.6 nm, indicating that pores with pore diameter between 0.5 and 0.6 nm are the largest contributors to TPV and SSA, accounting for 40.09% of the TPV and 56.95% of the total SSA. The super-micropores have a controlling factor on the adsorption capacity of semi-anthracite reservoirs. Vitrinite-rich coals developed stronger adsorption capacity as the vitrinite is rich in super-micropores whereas there are no obvious correlations between inertinite content with super-micropore SSA and VL. Additionally, mineral has a negative effect on the adsorption capacity of coals by inhibiting the development level of super-micropores. The combination application of HPMI, LP-N2GA and LP-CO2GA can more accurately reflect the pore structure of coal reservoir, especially for super-micropores.

Distribution of gob empty space for methane drainage during the longwall mining: A case study

In China, there are a large number of coal mine gobs have been formed as a result of longwall mining processes. These mine gobs contain an enormous amount of methane storage spaces within the gob formations. The methane storage spaces as well as guide methane drainage via vertical boreholes require quantitative measurements. In this study, physical simulation experiments were conducted for the purpose of obtaining digital images of gobs at various working face advance distances. Then, a Matlab-based digital image processing and analysis program was developed to process the obtained high-definition images, extract the geometrical parameters, and calculate the areas of methane storage spaces within the gobs. The results showed that the heights of the methane storage spaces first increased, and then stabilized as the gob lengths increased. The area sizes of the gobs increased gradually with the lengths, while the methane storage space areas were observed to first increase steadily to a maximum, and then decrease sharply. The main methane storage spaces exhibited a decreasing trend stage as the gob lengths increased. Whereas, the secondary methane storage spaces showed the characteristics of increasing trends. These research findings provide a theoretical foundation for the accurate predictions of both the methane storage capacities and methane drainage efficiency of longwall mining gobs.

Pore network characteristics of lacustrine shales in the Dongpu Depression, Bohai Bay Basin, China, with implications for oil retention

Abstract Pore development and the location of retained hydrocarbons are key topics to understand the shale oil system. In an attempt to investigate pore network characteristics and oil retention in lacustrine shales, 24 samples from the Es3 shales in the Dongpu Depression, Bohai Bay Basin were subjected to mineralogical, petrological, geochemistry and pore structure investigations using X-ray diffraction (XRD), thin section, field emission-scanning electronic microscope (FE-SEM), N2 adsorption-desorption. After removing retained oil by solvent extraction, 12 selected samples were again analyzed by Rock-Eval pyrolysis and N2 adsorption-desorption experiment. According to the mineral compositions and petrological characteristics, six types of lithofacies are identified in the Es3 shales in the Dongpu Depression: laminated calcareous shales, lenticular calcareous shales, laminated argillaceous shales, massive argillaceous shales, laminated siliceous shales, and massive siliceous shales. Geochemical characteristics of these shales are quite different, as calcareous/laminated shales have higher TOC contents than massive argillaceous and siliceous shales. Interparticle pores, intraparticle pores, organic matter pores and fracture pores are observed in the studied samples, while bitumens were found retaining in well-connected pore network. The N2 adsorption-desorption experiments on samples before and after solvent extraction reveal the different pore structure characteristics in six lithofacies and the occurrence of oil in lacustrine shales. In general, pore volumes within pore diameter range of 1.7–10nm and 10–200nm of samples show increase after solvent extraction, indicating that large and small pores in shales both play important roles in oil retention. Oil retained in shales can be divided as free and heavy fractions, as from the pyrolysis results. In lacustrine shales, larger pores are main storage space for hydrocarbon. Larger pores in different lithologies have similar capacity for free oil retention, while those in calcareous and argillaceous shales are more efficient for heavy oil retention. For both free and heavy oil, small pores in argillaceous shales are efficient for their storage.

Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China

The Lower Cambrian Niutitang shale is reported to be the oldest strata for large‐scale shale gas reserves. Based on outcrop observations and laboratory analyses, this study used a comprehensive approach that integrated sedimentology, mineralogy, petrology, organic geochemistry, and reservoir geology to estimate the prospects for exploring for shale gas in the Lower Cambrian Niutitang Formation, southern China. Whole rock and clay mineral X‐ray diffraction, scanning electron microscope (SEM) observations, and petro‐physical property measurements indicate that the Niutitang shale is dominated by quartz and clay minerals with minor amounts of plagioclase, potassium feldspar, calcite, dolomite, and pyrite. The organic matter in the Niutitang shale is dominated by type‐1 kerogen, with TOC levels ranging from 1.35% to 14.3% (average 4.46%) and Ro values ranging from 1.86% to 4.18% (average 2.79%). The three primary types of pores in the shale are organic pores, microfractures, and micropores. The micropores are primarily found in the clay mineral crystals and pyrite, and the microfractures are found along the boundaries of quartz grains. Most of the pore diameters are less than 20 nm, mainly concentrate at about 10 nm, which is consistent with that the organic pores act as the main storage space. The pore volume and gas content are strongly controlled by the mineral composition and lithofacies, and TOC content. The factors favouring shale gas accumulation of the Niutitang shale in the study area include (1) an effective thickness and widespread distribution, (2) a high organic carbon content and appropriate organic matter maturity, and (3) a favourable depositional environment.

Pore evolution characteristic of shale in the Longmaxi Formation, Sichuan Basin

Through the field emission scanning electronic microscope (FESEM) and the nitrogen adsorption test, pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied. Result showed that the pore type includes organic pore, intercrystalline pore, dissolution intracrystalline pore and interparticle pore, and the organic pore was one of major pore types; among the organic pore, the micropore had large pore volume and specific surface area, and was the main storage space of shale gas. Through study on effect of total organic carbon (TOC), organic matter maturity (Ro), diagenesis and tectonism on shale porosity, influence of TOC on porosity could be divided into four stages: the rapid increasing stage (TOC from 0 to 2%), the slow decreasing stage (TOC from 2 to 3%), the rapid increasing stage (TOC from 3 to 4% or 6%) and the rapid decreasing stage (TOC > 4% or 6%); influence of the maturity on porosity of shale could be divided into three stages: the rapid decreasing stage (Ro from 1.5 to 2.2%), the rapid increasing stage (Ro from 2.2 to 2.7%) and the rapid decreasing stage (Ro > 2.7%); during the high thermal evolution stage, the organic diagenesis was stronger than the inorganic diagenesis; the tectonism had a great impact on porosity, and the more intense the tectonism was, the smaller the porosity would be. The evolution of shale porosity of the Longmaxi Formation underwent five stages: the immature rapid compaction stage (Ro<0.7%), the mature hydrocarbon generation and dissolution stage (Ro from 0.7 to 1.3%), the high mature pore closed stage (Ro from 1.3 to 2.2%), the overmature secondary pyrolysis stage (Ro from 2.2 to 2.7%) and the overmature slow compaction stage (Ro>2.7%); among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages.

Evolution of the deeply buried Jurassic reservoirs in the southern Junggar Basin, NW China: Evidences from the Well DS-1

The southern Junggar Basin has enormous hydrocarbon mainly from the Jurassic and Permian source rocks, which indicated the importance of exploration of the deeply buried Jurassic reservoirs, therefore, the study of the deeply buried Upper Jurassic Qigu Formation (J3q) reservoirs in Well DS-1 in the Dushanzi anticline was carried out through microscopic observation and measurement, nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and high pressure mercury injection. Results showed that the main reservoir storage spaces in the deeply buried Upper Jurassic Qigu Formation reservoirs were fractures and dissolved pores. The J3q reservoirs with low porosity and permeability values of 0–12% and (0–5) × 10−3μm2 respectively, were generally tight. According to the mercury injection data, heterogeneity existed in the deeply buried Qigu Formation reservoirs with inhomogeneous pores and changeable sizes of pores throats. Pores and pore throats in the silt-fine sandstones of the lower Qigu Formation (J3q1) were more big and wide respectively than that in the argillaceous siltstones of the upper Qigu Formation (J3q3). Reservoirs in J3q1 had more mobile fluid and better conductivities than reservoirs in J3q3. The strong compaction and multistage diagenesis resulted in the tight J3q reservoirs. Anhydrite and quartz cementation, and various authigenetic minerals (e.g. hematite, kaolinite, illite/smectite formation, illite, chlorite and zeolite) filled in the pores. The homogenization temperature of brine inclusions (63.1–161.7 °C) in quartz overgrowth indicated the quartz had grown since the late Eocene. Due to the Tian Shan reactivity in the Late Cenozoic, the structure fractures were developed and promoted dissolution by oil and formation water in the reservoirs of J3q1.

Micron-to nano-pore characteristics in the shale of Longmaxi Formation, southeast Sichuan Basin

For shale of Lower Silurian Longmaxi Formation in Chongqing, southeast Sichuan Basin, characteristics of micro-nano pores in marine shale reservoirs were well studies by means of Field-Emission Scanning Electron Microscope and Low-temperature Low-pressure Adsorption Experiment of CO2 and N2. Results showed that six types of pore were developed in the shale of Longmaxi Formation, i.e., organic pores, intergranular pores, intragranular pores, intercrystalline pores, dissolution pores and microfractures, among which the organic pores and intragranular pores in interlayers of clay minerals were most developed, and a plenty of dissolution pores were also well developed because of high thermal evolution degree. BET specific surface area of the shale in Longmaxi Formation ranged from 3.5 to 18.1 m2/g, BJH total pore volume was from 0.00234 to 0.01338 cm3/g, DA specific surface area of micropores vaired from 1.3 to 7.3 m2/g, and DA pore volume ranged from 0.00052 to 0.00273 cm3/g. The specific surface area of micropores in the shale accounted for 23.1%–80.2% of total specific surface area with an average of 50.3%, and the pore volume of micropores accounted for 12.1%–48.5% of total pore volume with an average of 32.3%. Micropore was the main storage space in shale reservoir for methane adsorption, that because capacities of specific surface area provided by micropores were considerably greater than those provided by mesopores and macropores. Pore size distribution of the shale was complex, and multiple different peaks occurred in the pore size curves, showing two or three peaks in the range from 0 to 100 nm and four peaks occasionally. TOC had a good linear relationship with pore structure parameters of micropores, mesopores + macropores and total pores in the shale, indicating that TOC was the most important control factor for micron-to nano- pore structure in the shale. After normalization of pore structure parameters to TOC, the pore structure parameters of total pores and mesopores + macropores, had positive linear relationships with content of clay minerals but negative linear relationships with content of brittle minerals, indicating that clay minerals and brittle minerals mainly controlled development of mesopores and macropores in the shale.

Distribution and Thermal Maturity of Devonian Carbonate Reservoir Solid Bitumen in Desheng Area of Guizhong Depression, South China

The distribution of solid bitumen in the Devonian carbonate reservoir from well Desheng 1, Guizhong Depression, was investigated by optical microscope and hydrocarbon inclusions analysis. Vb and chemical structure indexes measured by bitumen reflectance, laser Raman microprobe (LRM), and Fourier transform infrared spectroscopy (FTIR) were carried out to determine the thermal maturity of solid bitumen. Based on the solid bitumen thermal maturity, the burial and thermal maturity history of Devonian carbonate reservoir were reconstructed by basin modeling. The results indicate that the fractures and fracture-related dissolution pores are the main storage space for the solid bitumen. The equivalent vitrinite reflectance of solid bitumen ranges from 3.42% to 4.43% converted by Vb (%) and LRM. The infrared spectroscopy analysis suggests that there are no aliphatic chains detected in the solid bitumen which is rich in aromatics C=C chains (1431–1440 cm−1). The results of Vb (%), LRM, and FTIR analysis demonstrate that the solid bitumen has experienced high temperature and evolved to the residual carbonaceous stage. The thermal evolution of Devonian reservoirs had experienced four stages. The Devonian reservoirs reached the highest reservoir temperature 210–260°C during the second rapid burial-warming stage, which is the main period for the solid bitumen formation.

Qualitative and quantitative characterization of a transitional shale reservoir: A case study from the Upper Carboniferous Taiyuan shale in the eastern uplift of Liaohe Depression, China

The paper takes the Upper Carboniferous Taiyuan shale in eastern uplift of Liaohe depression as an example to qualitatively and quantitatively characterize the transitional (coal-associated coastal swamp) shale reservoir. Focused Ion Beam Scanning Electron Microscope (FIB-SEM), nano-CT, helium pycnometry, high-pressure mercury intrusion and low-pressure gas (N2 & CO2) adsorption for eight shale samples were taken to investigate the pore structures. Four types of pores, i.e., organic matter (OM) pores, interparticle (InterP) pores, intraparticle (IntraP) pores and micro-fractures are identified in the shale reservoir. Among them, intraP pores and micro-fractures are the major pore types. Slit-shaped pores are the major shape in the pore system, and the connectivity of the pore-throat system is interpreted to be moderate, which is subordinate to marine shale. The porosity from three dimension (3D) reconstruction of SEM images is lower than the porosity of helium pycnometry, while the porosity trend of the above two methods is the same. Combination of mercury intrusion and gas absorption reveals that nanometer-scale pores provide the main storage space, accounting for 87.16% of the pore volume and 99.85% of the surface area. Micropores contribute 34.74% of the total pore volume and 74.92% of the total pore surface area; and mesopores account for 48.27% of the total pore volume and 24.93% of the total pore surface area; and macropores contribute 16.99% of the total pore volume and 0.15% of the total pore surface area. Pores with a diameter of less than 10 nm contribute the most to the pore volume and the surface area, accounting for 70.29% and 97.70%, respectively. Based on single factor analysis, clay minerals are positively related to the volume and surface area of micropores, mesopores and macropores, which finally control the free gas in pores and adsorbed gas content on surface area. Unlike marine shale, TOC contributes little to the development of micropores. Brittle minerals inhibit pore development of Taiyuan shale, which proves the influence of clay minerals in the pore system.