The degree of organic matter enrichment in shale determines its oil and gas potential. To understand the factors controlling this OM enrichment, we have used petrological and geochemical analyses to study the Lower Cambrian Niutitang Formation Shale in the eastern shelf margin of the Yangtze Block. Our results reveal that the total organic carbon content of the Niutitang Formation Shale varies significantly throughout the vertical section. The lower part of the Niutitang Formation has a high TOC content, likely due to (...) its formation in a dysoxic/anoxic environment with intense upwelling that favored OM enrichment. The middle part of the Niutitang Formation has the highest TOC content, which can be attributed to its formation in an environment with more hydrothermal activity, moderately upwelling, high paleosalinity, dysoxic/anoxic conditions, and moderately restricted water contents. Finally, the upper part of the Niutitang Formation records the lowest TOC contents because it likely formed in an oxic to dysoxic environment with weakly restricted hydrographic conditions that did not favor the preservation of OM. Therefore, these data demonstrate that the factors controlling OM enrichment in the Niutitang Formation Shale on the shelf margin include intense upwelling, greater amounts of hydrothermal activity, high paleosalinity, dysoxic/anoxic conditions, and moderately restricted water contents. (shrink)

Because of the great potential for hydrocarbon generation, the Lower Cambrian Wangyinpu and Guanyintang Formations of the Jiangxi Xiuwu Basin have become the most important targets for shale-gas exploration in the Jiangxi province. We investigate the pore characteristics and main controlling factors of overmature shale using field emission-scanning electron microscopy, image-processing software, X-ray diffraction, and gas-adsorption experiments. The results show that the shales have a high abundance of organic matter, over maturity, and highly siliceous mineral content. The kerogen type is (...) identified as type I. OM pores are the most developed, followed by interparticle pores and intraparticle pores. We combine complementary image processing and gas-adsorption methods to reveal that micropores are mainly from OM pores; mesopores are from OM pores and interP pores; and macropores are from OM pores, interP pores, and intraP pores. Although the number of micropores is at a maximum, the total contribution of mesopores and macropores to the pore volume is larger than that of micropores. However, the specific surface area is mainly from the micropores. OM content and maturity are the main controlling factors for the development of pore structures. Because of overmaturity, OM loses its potential for hydrocarbon generation and new pores cannot be produced. Gas loss leads to reservoir pressure drop, and the pores generated during the mature stage collapse and even disappear because they lack support. Therefore, PV, SSA, and porosity decrease when the OM content is more than 10%. When the OM content is less than 10%, most of the OM pores are preserved because they are protected by the skeleton particles. (shrink)

Because of the great potential for hydrocarbon generation, the Lower Cambrian Wangyinpu and Guanyintang Formations of the Jiangxi Xiuwu Basin have become the most important targets for shale-gas exploration in the Jiangxi province. We investigate the pore characteristics and main controlling factors of overmature shale using field emission-scanning electron microscopy, image-processing software, X-ray diffraction, and gas-adsorption experiments. The results show that the shales have a high abundance of organic matter, over maturity, and highly siliceous mineral content. The kerogen type is (...) identified as type I. OM pores are the most developed, followed by interparticle pores and intraparticle pores. We combine complementary image processing and gas-adsorption methods to reveal that micropores are mainly from OM pores; mesopores are from OM pores and interP pores; and macropores are from OM pores, interP pores, and intraP pores. Although the number of micropores is at a maximum, the total contribution of mesopores and macropores to the pore volume is larger than that of micropores. However, the specific surface area is mainly from the micropores. OM content and maturity are the main controlling factors for the development of pore structures. Because of overmaturity, OM loses its potential for hydrocarbon generation and new pores cannot be produced. Gas loss leads to reservoir pressure drop, and the pores generated during the mature stage collapse and even disappear because they lack support. Therefore, PV, SSA, and porosity decrease when the OM content is more than 10%. When the OM content is less than 10%, most of the OM pores are preserved because they are protected by the skeleton particles. (shrink)

Shale heterogeneity is important for micro- to macroscale-quality reservoir prediction. We evaluated the multiscale heterogeneity in shale based on thin-section observation, field emission scanning electron microscope observation, low-pressure [Formula: see text] adsorption analysis, mercury intrusion porosimetry, organic carbon analysis, and bulk chemical analysis by X-ray powder diffraction. We evaluated the results in which the heterogeneity of minerals and organic matter in shale was shown by disorderly and unsystematically distributed mineral grains, lamina, and rapid lithology at multiple scales. The random arrangement (...) of minerals, seasonal climate changes, large changes of sedimentary environment, and the provenance supply was considered to be the origin of the heterogeneity from the micro- to the macroscale in shale. In addition, a multiscale heterogeneity distribution model of marine shale in southeastern Sichuan basin was established, which can be used to predict shale gas distribution from the micro- to the macroscale. (shrink)

We have used focused ion beam-helium ion microscopy and low-pressure [Formula: see text] adsorption to investigate the pore characteristics of Lower Silurian Longmaxi shale. These results, combined with the molecular potential energy theory, reveal the relationship between pore size and the occurrence state of shale gas. Our results reveal that the pore volume and the specific surface area of Lower Silurian Longmaxi shale are mainly contributed by the pores with diameters of less than 10 nm. Nanoscale organic-matter pores are predominant, (...) and the pore surface is not generally smooth and has fractal characteristics. When the distance between the methane molecule and the pore wall in organic-matter pores is limited to 2 nm, there is a strong interaction force between them, and the methane molecule is affected by the interaction force and is in the adsorbed state accordingly. When the distance between them is greater than 2 nm, the interaction force can be ignored, and the methane molecule is not affected by the interaction force and is in the free state accordingly. In the pores having a radius greater than 2 nm, the adsorption capacity is positively correlated with the specific surface area; whereas in the pores having a radius smaller than 2 nm, the average gas concentration is related to the pore radius. First, the adsorption-zone volume increases, and then it decreases with increasing pore diameter. When the pore diameter ranges from 1 to 6 nm, the adsorption-zone volume is significantly larger than the free-zone volume. Within this range, the adsorption volume nearly equals the pore volume. When the pore diameter ranges from 6 to 60 nm, the adsorption volume gradually decreases, whereas the free volume increases, which equals the adsorption volume when the diameter reaches 15 nm. (shrink)

The pore structure controls the formation processes of tight oil reservoirs. It is meaningful to study the characteristics and origin of the pore structure of the tight oil reservoir. We have analyzed the pore structure of the tight oil reservoir by thin sections, scanning electron microscopy, and mercury intrusion porosimetry. We analyze the origin of the pore structure based on sedimentological, diagenetic, and tectonism processes. The porosity of the tight oil reservoirs is mainly approximately 2%–10%, and the permeability is mainly (...) from 0.01 to 0.3 mD. The pores of the lacustrine tight oil reservoir can be classified into the primary pore and the secondary pore. The main pores are matrix micropores and clay intercrystalline pores, as well as a few dissolved pores. However, the primary residual intergranular pore has almost disappeared, leading to a poor connectivity with a general size between 20 and 50 μm. The pore throat is divided into three categories according to the porosity, permeability, and throat size and distribution. We determine that the pore structure of the lacustrine tight oil reservoir is related to sedimentary, diagenetic processes, and later tectonic events. The compaction and cementation are the main factors, whereas the dissolution and tectonic events have minor effects. (shrink)

Similar to mineral composition and organic geochemical features, laminae development significantly influences pore structure. Taking the lower third member of the Shahejie Shale, Zhanhua Sag, Eastern China as the research object, we introduced various methods to analyze the influence of laminae development on pore structure, including thin section observations, field emission scanning electron microscopy, gas adsorption, high-pressure mercury injection, nano-computed tomography, quantitative evaluation of minerals by scanning electron microscopy, and spontaneous imbibition. We draw the conclusions that various minerals present a (...) mixed distribution in nonlaminated shale, whereas laminated shale is characterized by alternating bright and dark laminae. Dark laminae comprise clay and quartz, whereas bright laminae consist of calcite. Microfractures are abundant at the edges of the bright and dark laminae. Nonlaminated shale possesses a pore volume of [Formula: see text] and a specific surface area of [Formula: see text]. In contrast, laminated shale has a PV of [Formula: see text] and an SSA of [Formula: see text] with good reservoir property. Pores, especially macropores and micropores, are much more developed in laminated shale than in nonlaminated shale. Interconnected pores in sheet form are extremely developed in laminated shale, whereas most of the interconnected pores in nonlaminated shale are distributed in isolated spherical and tubular forms. Because of the abundant interconnected pores and throats, laminated shale presents good connectivity. The slopes of the spontaneous imbibition curves in the first and second stages for laminated shale are greater than those for nonlaminated shale. Laminae development could provide microfractures as dominant pathways for fluid migration as well as promote the interconnection of pores, greatly increasing the connectivity of shale reservoirs. (shrink)

Research on shale lithofacies is important for shale oil and gas production. This study focused on the lower third member of the Shahejie Formation in the Luo-69 well in the Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, eastern China. Several methods, including thin section observations, total organic carbon analysis, X-ray diffraction analysis, quantitative evaluations of minerals by scanning electron microscopy, major and trace-element analyses, and field emission-scanning electron microscopy, are used to investigate the effect of sedimentary environment on the type (...) and distribution of shale lithofacies. Our research indicates that 36 types of shale lithofacies can be classified based on the TOC content, mineral composition, and sedimentary structure, of which five types are identified in the study area. The [Formula: see text] shale has a high calcareous mineral content, low clay and siliceous minerals contents, a high TOC content, and well-developed horizontal bedding. The sedimentary environment during the deposition of the [Formula: see text] shale in the Zhanhua Sag had a warm and moist climate, limited provenance, saline water, and strong reducibility. The sedimentary environment in the early stage had a drier climate, more limited provenance, higher salinity, and stronger reducibility than that in the later stage. Shale lithofacies can reflect a certain sedimentary environment and depositional process; similarly, a depositional environment controls the type and distribution of shale lithofacies. Due to the characteristics of the [Formula: see text] sedimentary environment, organic-rich massive mixed shale, organic-rich bedded mixed-calcareous shale, organic-rich laminated calcareous shale, and organic-fair laminated calcareous shale are developed in the [Formula: see text] formation from top to bottom. (shrink)

To figure out porosity evolution mechanisms of marine shales inside and at the margin of the Sichuan Basin, South China, we measured samples selected from Lower Cambrian Qiongzhusi shales and Lower Silurian Longmaxi shales by a combination of X-ray diffraction, geochemistry measurement, focused ion beam milling and scanning electron microscopy imaging, and [Formula: see text] adsorption. It was suggested that shales of the upper Qiongzhusi Formation and the Longmaxi Formation possessed larger pore volumes and larger pore surface areas than those (...) of the lower Qiongzhusi Formation. Pores, in terms of pores of mineral frameworks, pores associated with clay minerals, dissolved pores, and organic matter -hosted pores, were all observed in the upper Qiongzhusi Formation and Longmaxi Formation, whereas none of the four types of pores developed well in the bottom of the Qiongzhusi Formation. Moreover, migrated OM is superior to depositional OM in the contribution of the pore space, in terms of pore productivity and pore protection. Good sealing abilities of the upper Qiongzhusi Formation and Longmaxi Formation allow more migrated OM and gaseous hydrocarbon retention than the lower Qiongzhusi Formation with an unconformity beneath acting as a channel of liquid and gaseous hydrocarbon migration, which bring about better pore properties. Finally, through the above analysis, the pore evolution mode has been established to gain insights for mechanisms of destruction, formation, and preservation of pores ranging from original sedimentary to metamorphic stage. Mechanisms of pore destruction contain mechanical compaction, chemical cementation, and OM carbonation. Mechanisms of pore generation comprise thermal pyrolysis of OM, transformation of clay minerals, and dissolution of soluble minerals. Mechanisms of pore preservation include mechanical stability of rigid grains, chemical stability of hydrophobic OM, and gas supporting through overpressure. (shrink)

To figure out porosity evolution mechanisms of marine shales inside and at the margin of the Sichuan Basin, South China, we measured samples selected from Lower Cambrian Qiongzhusi shales and Lower Silurian Longmaxi shales by a combination of X-ray diffraction, geochemistry measurement, focused ion beam milling and scanning electron microscopy imaging, and [Formula: see text] adsorption. It was suggested that shales of the upper Qiongzhusi Formation and the Longmaxi Formation possessed larger pore volumes and larger pore surface areas than those (...) of the lower Qiongzhusi Formation. Pores, in terms of pores of mineral frameworks, pores associated with clay minerals, dissolved pores, and organic matter -hosted pores, were all observed in the upper Qiongzhusi Formation and Longmaxi Formation, whereas none of the four types of pores developed well in the bottom of the Qiongzhusi Formation. Moreover, migrated OM is superior to depositional OM in the contribution of the pore space, in terms of pore productivity and pore protection. Good sealing abilities of the upper Qiongzhusi Formation and Longmaxi Formation allow more migrated OM and gaseous hydrocarbon retention than the lower Qiongzhusi Formation with an unconformity beneath acting as a channel of liquid and gaseous hydrocarbon migration, which bring about better pore properties. Finally, through the above analysis, the pore evolution mode has been established to gain insights for mechanisms of destruction, formation, and preservation of pores ranging from original sedimentary to metamorphic stage. Mechanisms of pore destruction contain mechanical compaction, chemical cementation, and OM carbonation. Mechanisms of pore generation comprise thermal pyrolysis of OM, transformation of clay minerals, and dissolution of soluble minerals. Mechanisms of pore preservation include mechanical stability of rigid grains, chemical stability of hydrophobic OM, and gas supporting through overpressure. (shrink)

The complicated source-reservoir-assemblage characteristics of lacustrine tight oil sand in China are the main controlling factors of tight reservoir oiliness. Several studies have focused on qualitative description of source-reservoir-assemblage characteristics without quantitative assessment. In this study, reservoir-source-assemblage has been evaluated quantitatively by fitting the RSA log in the evaluation of Qijia Depression in the Songliao Basin. Total organic carbon and sand volume logs are used to fit the RSA log in three steps: TOC and Vs log fitting and normalization, RSA (...) log fitting, and extraction of root-mean-square amplitude and frequency ) information from the RSA log. The rms represents the reservoir capability and hydrocarbon potential, and Frq represents the interbedding frequency that changes with the lake level. Positive values correspond to a low lake level. Based on RSA log values, we defined the parameter RSAsuf, a product of rms and Frq, to quantitatively evaluate the tight oil sweet spot. RSAsurf serves as tight oil sweet spot indicator and correlates positively to oil richness. As a result, four types of effective reservoirs, two types of effective sources, and three types of RSAs are identified based on cores and RSA logs. High RSAsuf values on the isoline map indicate the sweet spot zones around the G933 and J392 well areas, which correlates very well with the oilfield test data. The approach is appropriate for lacustrine basins with complicated RSA, in which RSA logs serve as indicator for the sedimentary rhythm, reservoir capability, and hydrocarbon potential. (shrink)

The complicated source-reservoir-assemblage characteristics of lacustrine tight oil sand in China are the main controlling factors of tight reservoir oiliness. Several studies have focused on qualitative description of source-reservoir-assemblage characteristics without quantitative assessment. In this study, reservoir-source-assemblage has been evaluated quantitatively by fitting the RSA log in the evaluation of Qijia Depression in the Songliao Basin. Total organic carbon and sand volume logs are used to fit the RSA log in three steps: TOC and Vs log fitting and normalization, RSA (...) log fitting, and extraction of root-mean-square amplitude and frequency ) information from the RSA log. The rms represents the reservoir capability and hydrocarbon potential, and Frq represents the interbedding frequency that changes with the lake level. Positive values correspond to a low lake level. Based on RSA log values, we defined the parameter RSAsuf, a product of rms and Frq, to quantitatively evaluate the tight oil sweet spot. RSAsurf serves as tight oil sweet spot indicator and correlates positively to oil richness. As a result, four types of effective reservoirs, two types of effective sources, and three types of RSAs are identified based on cores and RSA logs. High RSAsuf values on the isoline map indicate the sweet spot zones around the G933 and J392 well areas, which correlates very well with the oilfield test data. The approach is appropriate for lacustrine basins with complicated RSA, in which RSA logs serve as indicator for the sedimentary rhythm, reservoir capability, and hydrocarbon potential. (shrink)

We have evaluated continental shale oil enrichment via experiments. Rock pyrolysis, nuclear magnetic resonance, and pulse permeability tests were conducted to establish the pore saturation index, which comprehensively evaluates the enrichment of shale oil features using the characteristics of self-generation and self-preservation, the parameters of which include the pyrolysis-free hydrocarbon and total organic carbon of source rocks as well as the porosity and permeability of reservoir rocks. The correlation of the oil content and PSI values indicated that PSI values greater (...) than 50 generally indicate good oil enrichment plays, whereas those smaller than 50 imply poor conditions for oil enrichment. This conclusion was successfully applied to the Zhanhua Sag with shale oil plays in the Jiyang Depression, Bohai Bay Basin, China. In addition, the shale oil plays of the Dongying Sag in the Jiyang Depression were investigated to verify the effectiveness and reliability of the new method. (shrink)

Quantitative assessment is still lacking on the pore-sizes distribution of micropores and mesopores, and the major controlling factors remain enigmatic for the continental Chang 7th shale reservoirs in southeastern Ordos Basin. In this study, scanning electron microscopy, organic geochemical analysis, low-temperature gas adsorption, high-pressure mercury injection, and X-ray diffraction analysis were conducted on 29 core samples from representative eight wells in the Xiasiwan-Yanchang area, yielding a particular investigation on characteristics and controlling factors of the structure in the Chang 7th shale. (...) The Chang 7th shale is mainly characterized by organic and secondary micropore and mesopore, respectively, ranging the peaks of pore size from 0.35 to 0.65 nm, 0.75 to 1.00 nm, 1.10 to 1.35 nm, and from 4 to 13 nm. Whole-aperture characterization of the pore structure shows that meso- and macropores demonstrate a major contribution to total pore volume, and the surface area is mainly provided by micro- and mesopores with pore radii less than 20 nm. A weak positive variation was identified between the PV of micromesopores and total organic carbon content, implying that micropores are currently in the process of developing. The micromesopore specific surface area and volume, respectively, yield a significant positive and weak variation with the quartz and clay mineral content, suggesting that siliceous minerals are favorable for micromesopore development. The infusion of siliceous biogenic debris and silica-replaced carbonate biogenic debris from the northeastern and southwestern source areas, respectively, is likely responsible for development of biogenic siliceous minerals in the Chang 7th shale. Furthermore, the hollow cavities in siliceous organisms have considerably contributed to the preservation of primary pores owing to their characteristic skeleton-supported framework. (shrink)