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.
Wenming Jia,b, c, Yan Songa,b, d, Zhenxue Jianga,b, Mianmo Menga,b, Qingxin Liua,b, Fenglin Gaoa,b
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China. Unconventional Natural Gas Research Institute of China University of Petroleum, Beijing, 102249, China. School of Geosciences, China University of Petroleum, Shandong, 266580, China. PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China
Received 20 July 2016 Accepted 29 December 2016
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.
Shale gas have been one of hot topics for energy research in the world since the 21st Century. Innovation on geology of shale gas and development theory as well as progress in key technology for exploration and development, especially progress and widespread application of horizontal well technology and hydraulic fracturing technology, led to rapid development of shale gas in North America (Montgomery et al., 2005, Jarvie et al., 2007, Ross and Bustin, 2008, Chalmers and Bustin, 2008). On this basis, supply situation of global energy was changed, so that shale gas rapidly became an important new target for exploration and development of natural gas in the world.
Shale gas, mainly occurred in micro-nano pores of organic matter-rich shale in adsorption state and free state, was typical unconventional gas reservoir characterized by self-generation and self preservation continuous distribution (Curtis, 2002, Zhang et al., 2004; Jarvie et al., 2007, Zou et al., 2010, Ji et al., 2015). Characteristics of the shale reservoir were low porosity and low permeability, and dominated by nano-scale pore throat system with local development of millimeter- to micrometer-scale pores (Jia et al., 2014).
Structural characteristics of micro- to nano- pores in the shale reservoir not only influenced reserving and adsorption capacity of the gas (Yang et al., 2013, Hou et al., 2014, Ji et al., 2014, Yang et al., 2014, Xue et al., 2015), but also impacted gas migration (Javadpour et al., 2007, Zou et al., 2012). For the shale of Lower Silurian Longmaxi Formation in Chongqing, southeast Sichuan Basin as the research object, types and morphological characteristics of reserving space in marine shale reservoir were systematically studied to analyze structural characteristics of micro- to nano- pores in the shale reservoir and main control factors of microscopic pore structure of shale reservoir by means of Field-Emission Scanning Electron Microscope (FE-SEM) and Low-temperature Low-pressure Adsorption Experiment of CO2 and N2, so as to correctly evaluate gas reserving performance of shale, reveal enrichment law of shale gas and guide evaluation, exploration and development of marine shale gas resource in south area.
The study area, located in Chongqing, southeast Sichuan Basin (Fig. 1a), belonged to the tectonic unit of depression in the inner Upper Yangtze Platform at the intersection area of Paleo-Asiatic tectonic domain, Tethys tectonic domain and Pacific Rim tectonic domain. During the evolution process of geological history, due to separation and collision of plates, tectonics of this area was relative complex, and folds and faults were distributed alternatively (Wang et al., 1989).
The Upper Yangtze Platform was integrally subsided in the Early Cambrian, the sedimentary environment gradually evolved from deep shelf in the early period to shallow shelf and tidal flat in the late period, and a series of black shale deposited during the sedimentary period of deep shelf (Guo et al., 2004). Thrust-nappe fault, reverse fault, normal fault, transverse fault, hinge fault were developed in this area. Extension directions of faults were mostly consistent with strike of folds as NNE or NE, dominated by NNE-trending faults.
Trough-like folds, battlement-like folds and barrier folds were successively developed from southeast to northwest (Fig. 1b). In the studied area, a set of stable marine organic matter-rich shale was develop in the Lower Silurian Longmaxi Formation, and had favorable geological conditions for development of shale gas. The success of gas test on Well Qianye 1 indicated the developing potential of shale gas resource of Lower Silurian Longmaxi Formation in southeast Chongqing (Han et al., 2013).
Fig. 1. Location of the study area and well distribution.
Samples and experiments
Experimental samples were collected from the shale of Lower Silurian Longmaxi Formation in Well YC4, Well YC6, Well YC7 and Well YC8 in Chongqing area, southeast Sichuan Basin (Fig. 1b), and one sample was from the top and bottom of Longmaxi Formation respectively in each well. The outcrop lithology of Longmaxi Formation in the peripheral areas of Sichuan Basin mainly consisted of grey black-black carbonaceous shale and silty shale with well developing lamellation, high content of carbon and abundant graptolite fossils; the sedimentary environment mainly was the deep shelf, and the regional source rock of black shale with widespread distribution and large thickness were formed (Guo et al., 2004).
The organic matter was dominated by Type-I kerogen (Pu et al., 2010). Geochemical parameters and characteristics of mineral composition of samples were shown in Table 1. The relative experiment was conducted on eight shale samples from Longmaxi Formation in four wells. Results showed TOC was from 0.45% to 4.13%, and TOC of shale samples at the bottom of Longmaxi Formation were greater than that at the top; vitrinite reflectance of samples was determined by Leica DM4500P Polarizing Microscope and CRAIC Microspectrophotometer, and Ro of samples ranged from 1.93% to 2.78%, indicating thermal evolution degree of organic matter was from maturity to post maturity.
The mineral composition was determined by Bruker D8 DISCOVER automatic powder X-ray diffraction analyzer. Mineral contents of eight shale samples differed from each other. Overall, contents of clay minerals and quartz were the highest, and content of quartz ranged from 29% to 57% with an average of 39%, content of clay minerals varied from 21% to 58% with and averaged of 36%. Additionally, the samples also contained a certain amount of feldspar, calcite, dolomite and pyrite. Clay minerals mainly included illite, mixed layer of illite and montmorillonite, and a small amount of chlorite.
Table 1. Geochemical parameters and mineral compositions of shale samples.
Field-Emission Scanning Electron Microscope
FEI Helios NanoLab 650 Field-Emission Scanning Electron Microscope (FE-SEM) with the maximum resolution of 0.8 nm was selected as the experimental apparatus, but the resolution was generally only about several nanometers due to the poor electrical conductivity of shale samples. Core samples collected in wells were cut into sections with the size of 1 cm × 1 cm, and surfaces of the sections were mechanically smoothed by Germany Leica EM TXP automatic target surface processor.
Finally, the smoothed sections were placed into LJB-1A ion reduction device produced by Shenyang Huaye Company with proper-set working parameters, and the surface of samples was bombarded with argon ion. The samples polished by argon ion were fixed on the sample platform with conducting resin, and were sprayed with very thin layer of carbon on their surface in order to increase electrical conductivity of shale surface. The prepared samples were put into the sample room of FEI Helios 650 Scanning Electron Microscope for vacuum.
When the vacuum level of sample room met the device requirement, it was required to open the electron beam, and adjust the working distance of Electron Microscope to 4 mm, so as to observe the surface of shale under Transmission Electron Microscope (TEM) model and Compton Backscatter Scanning (CBS) model.
Compton Backscatter Scanning electron referred to a part of incident electron rebounded back by nucleus of solid samples, and imaging signals could not only reflect morphology features, but also reveal ordinal contrast of atom and qualitatively analyze material composition, e.g., compared with other minerals, color of organic matter was dark. When the scanning morphology of samples was observing, through X-ray energy spectrum, the micro-area elemental analysis of samples could also be conducted to determine the mineral types (Han et al., 2013).
Low-temperature and low-pressure gas adsorption
The method of low-temperature and low-pressure N2 adsorption was applicable to analysis of mesopores and some macropores, and the effective pore size ranged from 2 to 100 nm; the adsorption energy of micropores was higher than mesopores. The N2 molecule was relatively inert and difficult to enter the micropores, while CO2 molecule as the adsorbate had advantages of high analysis temperature, strong energy and rapid equilibrium. Additionally, such CO2 molecule could enter the micropores, was reasonably applicable to microporous measurement (Chalmers et al., 2012), and effective pore size for measurement varied from 0.35 to 2 nm.
Therefore, combination of both gas types characterized distribution characteristics of pore size in the shale (Tian et al., 2012, Pan et al., 2013). Micromeritics ASAP2020 automatic specific surface area and pore size analyzer was selected as the experimental instrument, and the adsorption capacity of gas in shale samples could be determined under conditions of different pressures by the static volumetric method of isothermal physisorption. The detailed method was shown as follows: samples of 3–5 g with pore size from 60 mesh to 120 mesh were weighed for vacuum degassing at 100 °C for 8 h, followed by the isothermal adsorption-desorption experiment of N2 under the liquid nitrogen environment (77.4 K).
Finally, the specific surface area of the shale was calculated by means of multi-point BET (Brunauer-Emmett-Teller) model, and BJH (Barrett-Johner-Halenda) theory and Kelvin Equation were used to calculate total pore volume and describe pore size distribution characteristics. After the vacuum degassing, the secondary degassing was performed for 4 h, the isothermal adsorption experiment of CO2 was completed in the ice-water bath environment (273.1 K); the specific surface area and pore volume of micropores were calculated by means of DA (Dubinin Astakhov) theoretical model (Dubinin and Stoeckli, 1980).
Results and discussions
Pore types and morphological characteristics
Abundant micro-to nano- pores, including abundant organic pores, microfractures and a small amount of inorganic pores, were well developed in the shale of Longmaxi Formation through a large number of field-emission environmental scanning electron microscopy experiments; inorganic pores were dominated by intergranular pores, intragranular pores, intercrystalline pores and dissolution pores, indicating a certain similarity to previous research results (Guo et al., 2014, Wang et al., 2014, Xue et al., 2015).
Organic pore was a kind of intragranular pore within organic matter. In the shale of Longmaxi Formation of the study area, abundant organic pores mainly occurred in the shape of bubble, oval, crescent, slit and other irregular shapes with pore size range from several nanometers to hundreds of nanometers.
The previous research results indicated that organic pores were only developed in the shale when the maturity of organic matter in a certain degree (Jarvie et al., 2007), indicating that organic pores were closely related with process of thermal evolution and hydrocarbon generation of organic matter, likely were derived from consumption of organic matter in the hydrocarbon generation from kerogen (Xue et al., 2015), or from bubbles generated from the accumulated liquid or gas (Jarvie et al., 2007, Yang et al., 2014), or shrinkage fractures produced by consumption of water in hydrocarbon generation (Xue et al., 2015).
The formation of organic pores was mainly controlled by types of kerogen, abundance of organic matter and degree of thermal evolution (Yang et al., 2014). The organic matter in the shale of Longmaxi Formation commonly occurred in the adsorption, inclusion and filling with clay minerals, clastic minerals and pyrite, etc (Zhang et al., 2013).
In different occurrence of organic matter, inner pores had different development characteristics as following: the irregular organic matter was filled among mineral particles and abundant organic pores occurred in the band-shape distribution (Fig. 2a); a plenty of oval and punctual organic pores were mainly developed in the organic matter which was filled in chlorite bedding (Fig. 2b); the organic matter mixed with illite to form the organic-argillaceous complex, and the round organic pores were mainly developed in the organic matter (Fig. 2c); the slit-shape pores occurred in the contact edge between the banded organic matter and mineral particles, and organic pores in the inner part of organic matter were not developed (Fig. 2e); the organic matter dispersively was filled among mineral particles, the oval and irregular organic pores were developed in the inner part of organic matter (Fig. 2f); a plenty of irregular organic pores were developed in the organic matter which was filled in framboidal pyrite particles (Fig. 2h); the slit-shape organic pores occurred in massive organic matter which was filled among mineral particles (Fig. 2i).
Enrichment of organic matter was commonly accompanied with pyrite (Figs. 2d–i), indicating that enrichment degree of organic matter and development characteristics of pores had a relationship with the oxidation-reduction conditions of sedimentary water during the formation process of the shale (Hou et al., 2014).
Organic pores were particularly well developed in the inner part of organic matter which was related to clay minerals, especially the illite or the mixed layer of illite and montmorillonite (Figs. 2b–d), this was because that the catalytic activity of source rocks was mainly derived from the conversion process from montmorillonite to illite, thus the transitional mixed-layer minerals of illite and montmorillonite with very strong catalytic activity were formed, and provided excellent conditions for reaction of hydrocarbon generation and formation of organic pores (Huang and Shen, 2015). Organic pores, largely developed in the shale of Longmaxi Formation, had a certain level of connectivity with intergranular pores and intragranular pores, providing microcosmic seepage channels for occurrence and migration of gas in the shale reservoir.
Fig. 2. Distribution and morphological characteristics of organic pores. (a) Abundant organic pores occurred in the band shape and were developed in irregular organic matter which was filled among mineral particles, the sample was from Well YC4 at the depth of 749.2 m; (b) a plenty of oval and punctual organic pores were mainly developed in the organic matter which was filled in chlorite beddings, the sample was from Well YC4 at the depth of 749.2 m; (c) the rounded organic pores mainly were developed in the organic-argillaceous complex which was formed by the mixture of organic matter and illite, the sample was from Well YC4 at the depth of 749.2 m; (d) the vesicular organic pores were developed in the organic matter, the organic-argillaceous complex was associated with pyrite, the sample was from Well YC4 at the depth of 749.2 m; (e) the slit-shape pores were developed in the contact edge between the banded organic matter and mineral particles, the sample was from Well YC4 at the depth of 749.2 m, the red arrows indicated the intergranular pores; (f) the oval and irregular organic pores were developed in the discrete organic matter which was filled in mineral particles, the sample was from Well YC4 at the depth of 749.2 m; (g) a plenty of oval organic pores well developed within the organic matter, the organic matter and pyrite were filled in the chlorite beddings, the sample was from Well YC4 at the depth of 709.4 m; (h) a plenty of irregular organic pores were developed in the organic matter which was filled between framboidal pyrite particles, the sample was from Well YC4 at the depth of 709.4 m; (i) the slit-shape organic pores were developed in massive organic matters which was filled in mineral particles, the sample was from Well YC4 at the depth of 709.4 m.
(1) Intergranular pores. The intergranular pores were mainly the residual spaces among mineral particles reformed by sedimentation or late diagenesis. The intergranular pores in the shale of Longmaxi Formation were irregularly and dispersedly distributed in the rock matrix with multiple morphologies dominated by triangular, polygonal and linear pores (Figs. 3b–e, g and i–l) due to effect of particle morphology, contact relationship and arrangement mode. In the study area, some brittle particles were easily broken to form the intergranular pores, e.g., potash feldspar (Fig. 3j), albite and quartz. The intergranular pores could provide great seepage channels for methane due to their better connectivity.
Fig. 3. Morphology and distribution characteristics of inorganic pores. (a) Intergranular pores occurred among quartz particles, the sample was from Well YC6 at the depth of 748.6 m; (b) dolostone intragranular pores were filled by quartz, chlorite and pyrite, dissolution pores occurred in dolostone, periphery of dolostone was replaced by ankerite, the sample was from Well YC4 at the depth of 655.8 m; (c) dolostone dissolution pores, periphery of dolostone was replaced by ankerite, the sample was from Well YC4 at the depth of 655.8 m; (d) potash feldspar dissolution pores, the sample was from Well YC6 at the depth of 748.6 m; (e) albite dissolution pores, the sample was from Well YC6 at the depth of 748.6 m; (f) calcite dissolution pores and a small number of quartz dissolution pores, intergranular embayment-shape dissolution pores occurred in the contact edge of quartz and calcite, the sample was from Well YC4 at the depth of 743.3 m; (g) quartz dissolution pores, the sample was from Well YC4 at the depth of 655.8 m; (h) interbedded intragranular pores occurred in flocculent illite, the sample was from Well YC4 at the depth of 743.3 m; (i) interbedded intragranular pores occurred in booklet-like chlorite, the sampled was from Well YC6 at the depth of 748.6 m; (j) intergranular pores occurred in cracked potash feldspar, the sample was from Well YC4 at the depth of 743.3 m; (k) tectonic fractures, the sample was from Well YC6 at the depth of 748.6 m; (l) diagenetic shrinkage fractures, the sample was from Well YC4 at the depth of 655.8 m. The red arrows indicated intergranular pores.
(2) Intragranular pores. The intragranular pores mainly occurred within mineral particles, and the morphology was mostly irregular. The intragranular pores in the interbeds of clay minerals were widely developed in the shale of Longmaxi Formation, like the interbedded intragranular pores in flocculent illite (Fig. 3h), booklet-like chlorite (Fig. 3i), but other minerals had less interbedded intragranular pores (Fig. 3b); those interbedded intragranular pores were commonly infilled by organic matter or pyrite (Figs. 3b–d and g).
Occasionally, the intragranular pores in dolomite were infilled by quartz, chlorite and pyrite (Fig. 3b), that was likely related to the late diagenetic evolution. The intragranular pores could provide large storage spaces for gas and form pore network with intergranular pores and organic pores to increase seepage capacity of the shale.
(3) Intercrystalline pores. The intercrystalline pores mainly referred to pores among crystals in the mineral aggregates. Pyrites were widely developed in the shale of Longmaxi Formation, and occurred in form of microsphere and framboidal druse.
The framboidal druses had the diameter of about several microns, it was composed of many pyrite crystals, and a certain number of nano-size pores formed by untight-packed accumulation during the crystals growth process usually were developed among these pyrite (Figs. 3b, c, f, j and l), the inner part of the framboidal druse was of certain connectivity. These intercrystalline pores of pyrites were generally infilled by organic matter with development of organic pores (Fig. 2h).
(4) Dissolution pores. During the geological evolution process, along with increase of buried depth of strata and enhancement of diagenesis, dissolution pores were formed due to dissolution of unstable minerals when chemical property of diagenetic fluid could not reach a chemical equilibrium with each component of the rock. The development of dissolution pores was generally related to hydrocarbon generation process of organic matter; organic acid formed from soluble organic matter in source rocks during the thermal evolution process could increase solubility of minerals, affect their stability and then dissolve them by providing H+ and complex metallic elements (Liu et al., 2006).
The main stage for formation of short-chain hydroxy acid from thermal decomposition of kerogen was the process of the paleo-geotemperature from 60 °C to 140 °C (Surdam et al., 1989). Dissolution pores, e.g., potash feldspar dissolution pores (Fig. 3d), albite dissolution pores (Fig. 3e), calcite dissolution pores (Fig. 3f), dolomite dissolution pores (Figs. 3b-c), quartz dissolution pores (Figs. 3g and l) and so on, were extremely developed in the shale of Longmaxi Formation which was in the high-mature stage and experienced large buried depth and hydrocarbon generation process.
Dissolution pores occurred in different host minerals had diversified morphology, e.g., regular rectangular shape (Fig. 3d), rhombic shape (Fig. 3f), round shape (Fig. 3g), embayment shape (Fig. 3f), punctiform shape (Fig. 3d–g), irregular shape (Figs. 3b, c, and e) and so on. Dissolution pores sometimes were partly or totally infilled again by quartz and clay (Fig. 3b), while the unfilled dissolution pores could provide storage space for gas. This kind of pores was relatively isolated due to poor connectivity. However, dissolution pores could connect with intragranular pores and organic pores to become seepage channels for shale gas when the dissolution was stronger (Fig. 3f).
The formation of microfractures was related to stress change caused by sedimentary structure or late diagenesis, and the diameter generally was micron scale. Microfractures, mainly including tectonic fractures and diagenetic shrinkage fractures, were greatly developed in the shale of Longmaxi Formation.
The tectonic fracture was formed due to one or several times of tectonic stress destruction of the shale, and was main fracture type occurred in any place of shale, the fracture surfaces were straight and smooth (Fig. 3k). The diagenetic shrinkage fractures were formed due to internal stress produced by dehydration, uniform shrinkage, crack and recrystallization of shale under the pressure of overlying strata during the diagenetic process, the microfractures were of morphological bending and good ductility.
The width of the microfractures was mainly less than 200 nm under scanning electron microscope (Fig. 3l). The microfractures were generally open, and were connected with other pore types to form the anfractuous steric pore network which not only was favorable for enrichment of free gas, but also provided main channels for seepage and migration of shale gas, and played a critical role in development of shale gas.
Structural characteristics of pores
The results from scanning electron microscope indicated that the pore network developed in the marine shale of Longmaxi Formation was complex and varied with various pore type assemblages. Organic pores, interbedded intragranular pores of clay minerals and dissolution pores were mostly developed. Considering limitation of observation range of electron microscope, gas adsorption methods of CO2 and N2 were used to quantitatively describe the structural characteristic of micron- to nano-pores in the shale.
The division of pores in the shale still had no uniform cognition due to their complex structure and wide distribution range of pore size. In this study, the classification method of pores proposed by International Union of Pure and Applied Chemistry (IUPAC) based on the physisorption property and agglomeration theory of capillary was adopted to divide pores of porous matter into micropores (diameter less than 2 nm), mesopores (diameter ranging from 2 to 50 nm) and macropores (diameter greater than 50 nm) according to pore size (Gregg and Sing, 1982).
Characteristics of gas isothermal adsorption-desorption curves
Fig. 4 showed the N2 adsorption curves of shale samples from Longmaxi Formation during the pressurization process at low temperature and the desorption curves during the decompression process. The adsorption isotherms of each shale sample had the reversed S-shape characteristics with slight difference in morphology. The N2 adsorption isotherms of shale samples belonged to Type II according to classification schema of International Union of Pure and Applied Chemistry (IUPAC) (Brunauer et al., 1938).
The first half of adsorption curves rose quickly and then slowly with upward slight convex after reaching a certain value, while the second half of adsorption curves rose sharply and continued to the relative pressure close to 1.0, and still no saturated adsorption appeared, indicating that a certain number of macropores occurred in the shale. N2 adsorption capacity of shale samples collected from well bottom (samples of Group B) was obviously greater than that of samples from the top (samples of Group A) of the same well, suggesting that content of organic carbon was likely the important control factor for development of pores in the shale.
The isothermal adsorption curves of the shale started separating from the desorption curves at the relative pressure of 0.4, thus the adsorption hysteresis loops were formed, indicating that the phenomenon of capillary condensation emerged during the process of adsorbing N2 and a certain number of mesopores occurred in the shale.
These hysteresis loops mainly were of characteristics of Type H4 and Type H3 according to the classification schema of IUPAC. The loops were the superposition of multiple standard-type loops, and it were synthetic reflection of pore morphology, indicating that the nano-size pores occurred in the shale of Longmaxi Formation were mainly the open cylinder-shape pores or slit-shape pores.
Fig. 4. N2 adsorption-desorption isotherms of shale samples. (a) YC4-A; (b) YC4-B; (c) YC6-A; (d) YC6-B; (e) YC7-A; (f) YC7-B; (g) YC8-A; (h) YC8-B. p represented the experimental pressure, Pa; p0 represented the saturated vapor pressure of N2, Pa.