Lower Silurian Longmaxi Shale Research Articles

Investigation of pore structure and fractal characteristics of the Lower Silurian Longmaxi shales in western Hunan and Hubei Provinces in China

The pore structure characteristics of the Lower Silurian Longmaxi Formation shales collected from western Hunan and Hubei were investigated using high-pressure mercury injection and low-temperature N2 adsorption/desorption experiments. Total organic carbon values, thermal maturity values and mineralogical compositions were also obtained by relevant experimental methods. The TOC value varies from 0.71% to 4.75%, with a mean value of 2.58%, and their thermal maturities have reached the over-mature stage and dry gas generation window. Quartz and clay minerals are the major mineral compositions in the samples. In contrast to previous studies of other shales (Chalmers and Bustin, 2008; Han et al., 2013; Wang et al., 2013), there are no carbonates in the shale samples of this area. Due to their biogenic source, quartz content is positively correlated with TOC values. Pore size distributions obtained from mercury injection experiments were divided into three groups. Samples with higher TOC content always have smaller dominant pore sizes, and the pore sizes of samples with higher clay mineral content and lower TOC values may be larger, with a major peak between 30 nm and 1 μm. Through analyzing the N2 adsorption and desorption isotherms, we found that the morphology of pores in the shales rich in organic matter is narrow neck and wide body, and that of those rich in clay minerals is flat-shaped. BJH and DFT models were used to derive pore size distributions using N2 adsorption data, and both are representative. The differential distribution curves of pore volumes and surface areas show that all shale samples' dominant pore sizes are within the range of micropores and mesopores, and they are the major contributors to pore surface areas, while mesopores and macropores make more significant contributions to pore volumes. The specific surface area calculated by the BET method ranges from 6.12 to 28.42 m2/g, with an average value of 16.14 m2/g, and the total pore volume varies from 0.0105 to 0.0338 cm3/g, with a mean value of 0.0213 cm3/g. The correlational analysis between pore structure parameters and TOC value, quartz and clay mineral content indicates that organic matter and quartz are positively associated with the micropores and fine mesopores, though larger pores can also exist in organic matter, and clay minerals play an important role in mesopores and macropores generation. Due to the infeasibility of using mercury porosimetry data to obtain fractal dimensions, Dn calculated from N2 adsorption data are used, its value ranges from 2.6353 to 2.7694, with an average value of 2.7284. As we know, the more micropores contained in the shale, the more complex the pore structure will be, and as a result, the fractal dimension will be larger. Therefore, organic matter and quartz positively influence fractal dimension, and there is a negative relationship between clay mineral content and fractal dimension.

Nano-scale pore structure and fractal dimension of organic-rich Wufeng-Longmaxi shale from Jiaoshiba area, Sichuan Basin: Investigations using FE-SEM, gas adsorption and helium pycnometry

Much attention have been recently paid to the upper Ordovician Wufeng shale (O3w) and lower Silurian Longmaxi shale (S1l) in the Jiaoshiba area of Sichuan Basin, which is now the largest producing shale gas field in China. Field emission scanning electron microscopy (FE-SEM), low pressure gas (N2 and CO2) adsorption, helium pycnometry, X-ray diffraction and geochemical analyses were performed to investigate the pore structure and fractal dimension of the pores in O3w-S1l shale formation in the Jiaoshiba area. FE-SEM images show that organic matter (OM) pores are dominant in the organic-rich samples and these pores are often irregular, bubble-like, elliptical and faveolate in shape, while in organic-poor samples, limited and isolated interparticle (interP), intraparticle (intraP) and OM pores are observed. Reversed S-shaped isotherms obtained from nitrogen adsorption are type Ⅱ, and hysteresis loops indicate that the shape of micropore in the samples is slit-or plate-like. BET surface areas and total pore volume vary from 12.2 to 27.1 m2/g and from 1.8 × 10−2 to 2.9 × 10−2 cm3/g, with an average of 19.5 m2/g and 2.3 × 10−2 cm3/g, respectively. Adsorption volume from both N2 and CO2 adsorption increases with respect to TOC contents. Porosities obtained from helium porosimetry are comparable with these from gas (CO2 and N2) adsorption in O3w-S1l shale. However, porosity determined by quantitative FE-SEM analysis is much smaller, which is mainly related to limited resolution and the small areas of investigation.Based on the Frenkel-Halsey-Hill (FHH) model of low-pressure N2 adsorption, fractal dimensions of the pores varied from 2.737 to 2.823. Relationships between pore structure parameters and TOC content, mineral composition and fractal dimension reveal that the fractal dimension is mainly associated with micropores. Samples with higher TOC content, higher quartz content and lower clay content tend to contain more heterogeneous micropores, resulting in higher fractal dimensions and more complicated pore structure in shales. Therefore, fractal dimension is an effective parameter to reflect the complexity of pore structure and the degree of micropore development in O3w-S1l shale.

Organoporosity Evaluation of Shale: A Case Study of the Lower Silurian Longmaxi Shale in Southeast Chongqing

Organoporosity Evaluation of Shale: A Case Study of the Lower Silurian Longmaxi Shale in Southeast Chongqing

Mineralogy and fracture development characteristics of marine shale-gas reservoirs: A case study of Lower Silurian strata in southeastern margin of Sichuan Basin, China

Mineral contents and fractures of shale from well Yuye-1 and outcrops were examined mainly based on systematic description of the cores and outcrops, and data from experimental analyses. The data enabled us to thoroughly explore the mineralogy and developmental features of shale of the Lower Silurian Longmaxi Formation in the study area. The results show that, the Lower Silurian Longmaxi Shale (SLS) in the southeastern margin of Sichuan Basin, China, is primarily characterized by a high content of brittle minerals and a relatively low content of clay minerals. The total content of brittle minerals is approximately 57%, including 27% quartz, 12.2% feldspar, 11.2% carbonate and 2.4% pyrite. The total content of clay minerals reaches 41.6%, composed of illite (23.8%), mixed-layer of illite and smectite (I/S) (10.8%) and chlorite (7.0%). The SLS accommodates the widespread development of various types of fractures, including tectonic fractures, diagenetic fractures, inter-layer fractures and slip fractures. The developmental level of the fracture in the SLS is mainly influenced by faults, lithology, mineral contents and total organic carbon content (TOC) in study area.

The Fuling Shale Gas Field — A highly productive Silurian gas shale with high thermal maturity and complex evolution history, southeastern Sichuan Basin, China

The organic-rich Lower Silurian Longmaxi and Upper Ordovician Wufeng Shale (LSLUOWS) is one of the most important marine shale gas plays in southern China, with relatively high thermal maturity and complex structural evolution. The delineation of the Jiaoshiba shale gas play has been highly successful for the LSLUOWS in the Fuling area of southeastern Sichuan Basin. The drilling targets the basal part of the LSLUOWS, where the approximately 38–45 m of organic-rich (total organic carbon of greater than 2%) section corresponds to a maturity range around 2.2%–3.0% Ro, with high brittle mineral content (55%–65%). The produced shale gas displays a clear reversal in the stable carbon isotopes. The shale gas play zone is between the overlain mudstone in the Middle-Upper Longmaxi Formation (Fm) and the underlain Upper Ordovician tight limestone in the Jiancaogou Fm. The shale gas reservoir is overpressured, with an average pressure coefficient of 1.55 (relative to hydrostatic pressure). From December 2013 to late 2014, results of production tests determined relatively stable pressure and production curves. Fifty prolific gas wells have been completed to date in the Jiaoshiba Shale Gas Field in Fuling. Preliminary study revealed several characteristics of this structurally complex shale play that are distinctly different from those in the United States, including an anticlinal structure with mild deformation, abundant free gas, and very short gas migration through microfractures, the natural fracture network formed by the two fault systems, and the slip parallel to the layer in the basal LSLUOWS. Movement along strike-slip faults has gone through up into the Quaternary, which is considered to be a significant factor in the establishment and preservation of the overpressured region in the LSLUOWS.

渝东南两套富有机质页岩的孔隙结构特征

Abstract We used a Focused Ion Beam Scanning Electron Microscope (FIB-SEM) to observe the pore structures of the Lower Cambrian Niutitang shale and the Upper Ordovician Wufeng to Lower Silurian Longmaxi shale, sampled in the southeast of Chongqing. Three-dimensional distributions of the nanometer-sized organic-matter pores were reconstructed and their pore structure parameters were analyzed quantitatively. Our work indicated that microfractures between strata, interparticle and intraparticle pores in the mineral matrix, and organic-matter pores were well-developed in the Wufeng-Longmaxi formations. Pores within organic matter form a well-distributed honeycomb like structure with good connectivity, and the pore radius is between 3 and 100 nm. Porosity of organic matter calculated using the FIB-SEM 3-D reconstruction is between 9.13% and 18.42%,and the contribution of organic-matter pores in the total porosity is correlated with the total organic carbon (TOC) content. Dissolution pores and interparticle pores are developed in the Niutitang Formation while organic-matter pores are unevenly distributed. The organic-matter pores are tabular or pinhole with the pore radius between 3 and 25 nm and have poor connectivity. The calculated organic-matter porosity is below 1.59% and has little relationship with the TOC content. So the total porosity in the Niutitang shale mainly comes from the mineral matrix pores. The FIB-SEM results have shown significant differences in the pore structure of the two shales.

The shale characteristics and shale gas exploration prospects of the Lower Silurian Longmaxi shale, Sichuan Basin, South China

The Lower Silurian Longmaxi Formation is a key target for shale-gas exploration in South China and comprises organic-rich black shale unit deposited in an anoxic deep-water setting. Mineral components are dominated by quartz and clay minerals with minor amounts of plagioclase, potassium feldspar, calcite and pyrite. The brittle mineral content ranges from 45% to 80% by weight (average: 62.5 wt. %). Organic geochemical analyses (TOC, organic matter type, Ro and gas content) show that the shale has good source rock potential. Matrix pores include organic pores; inter-particle pores between clay floccules, faecal pellets, organic matter particles and quartz and framboidal pyrite; and inter-crystalline pores within clay minerals and pyrite framboids. Fractures are also present. The development of storage space is controlled by the TOC content, the thermal maturity and type of organic matter present, by mineral composition and structural deformation. Factors which support the occurrence of shale gas accumulations in the study area include a high TOC content together with thermal maturity and organic matter type; the presence of thick and widespread shales; high rock fragility; and a stable, anoxic deep-water environment during shale deposition.

Characterization of the Upper Ordovician and Lower Silurian Marine Shale in Northwestern Guizhou Province of the Upper Yangtze Block, South China: Implication for Shale Gas Potential

A detailed study of the upper Ordovician Wufeng shale and lower Silurian Longmaxi shale in northwestern Guizhou province of the upper Yangtze block was conducted on the basis of systematic analyses of a series of experimental measurements on core samples. Trace element ratios V/Cr, U/Th, and V/(V + Ni) reveal that variable paleoredox conditions existed during the deposition of the shale and the degree of anoxia decreased upward. The shale has high thermal maturity with an average Ro value of 3.38% (ranging from 2.94 to 3.65%) and high amounts of organic matter with an average total organic carbon (TOC) content of 2.02% (ranging from 0.36 to 6.73%), the type of which is sapropelic (I) to humic–sapropelic (II1). This indicates that the shale is good gas-producing shale in terms of geochemistry. The mineralogy of the shale is composed of 44.7% quartz and 32.6% clay minerals on average. The shale is characterized by low porosity ranging from 0.6 to 4.4% (averaging 1.8%) and low permeability varying from 0.0066 × 10–3 to 0.1098 × 10–3 μm2 (averaging 0.0378 × 10–3 μm2). The correlations of porosity with the TOC content, quartz content, and clay mineral content demonstrate that the shale interval rich in quartz and organic matter has higher porosity than the clay-rich interval. The gas adsorption capacity increases with the increasing of the TOC content, indicating that organic matter is responsible for adsorbing gas in this shale. Comprehensive analyses through experimental data show that the studied Wufeng–Longmaxi shale succession, especially the lower section, holds the potential for shale gas exploration and development because of TOC enrichment, high thermal maturity, good fracability, and potentially large gas content.

Modeling free gas content of the Lower Paleozoic shales in the Weiyuan area of the Sichuan Basin, China

Two sets of Lower Paleozoic organic-rich shales develop well in the Weiyuan area of the Sichuan Basin: the Lower Cambrian Jiulaodong shale and the Lower Silurian Longmaxi shale. The Weiyuan area underwent a strong subsidence during the Triassic to Early Cretaceous and followed by an extensive uplifting and erosion after the Late Cretaceous. This has brought about great changes to the temperature and pressure conditions of the shales, which is vitally important for the accumulation and preservation of shale gas. Based on the burial and thermal history, averaged TOC and porosity data, geological and geochemical models for the two sets of shales were established. Within each of the shale units, gas generation was modeled and the evolution of the free gas content was calculated using the PVTSim software. Results show that the free gas content in the Lower Cambrian and Lower Silurian shales in the studied area reached the maxima of 1.98–2.93 m3/t and 3.29–4.91 m3/t, respectively (under a pressure coefficient of 1.0–2.0) at their maximum burial. Subsequently, the free gas content continuously decreased as the shale was uplifted. At present, the free gas content in the two sets of shales is 1.52–2.43 m3/t and 1.94–3.42 m3/t, respectively (under a current pressure coefficient of 1.0–2.0). The results are roughly coincident with the gas content data obtained from in situ measurements in the Weiyuan area. We proposed that the Lower Cambrian and Lower Silurian shales have a shale gas potential, even though they have experienced a strong uplifting.

The characterization and quantitative analysis of nanopores in unconventional gas reservoirs utilizing FESEM–FIB and image processing: An example from the lower Silurian Longmaxi Shale, upper Yangtze region, China

In contrast to the millimeter-to-micron scale of pore systems found in conventional oil and gas reservoirs, unconventional oil and gas reservoirs have nanoscale pore systems. However, the methods used to characterize conventional reservoirs are not normally effective for studying nanopores. In the present study, a dual-beam field emission scanning microscope–focused ion beam (FESEM–FIB) device was used to investigate nanopores in a core plug sample of the Longmaxi Shale from Pengye Well #1, Chongqing, China. A large number of nanopores were observed, which can be divided into three types: intraparticle pores within mineral particles (intraP pores), interparticle pores between mineral particles (interP pores), and organic-matter pores (OM pores). The latter type is the most abundant. The Pores (Particles) and Cracks Analysis System (PCAS) was used to identify and analyze pores in high-resolution SEM images. The results show differences in pore size, pore area, probability entropy, form factor, and fractal dimension between the three pore types, especially between organic-matter pores and the other two pore types. The pore size of organic-matter pores ranges from 4 to 483nm (average 37nm), whereas the pore sizes of intraP pores and interP pores are 57–1136nm (average 210nm) and 31–1976nm (average 219nm), respectively. Results for pore area also reflect the smaller pore sizes of organic-matter pores. The probability entropy, mean form factor, and fractal dimension of organic-matter pores are 0.90, 0.72, and 1.08 respectively, suggesting disordered pore directions and limited morphological complexity. The form factor and fractal dimension of intraP and interP pores indicate more complex pore shapes for these pore types. InterP pores have more ordered alignments, as indicated by their lower probability entropy. Differences in matrix composition and/or the mode of occurrence of pores give rise to differences in pore size and shape.

Organoporosity evaluation of shale: a case study of the Lower Silurian Longmaxi Shale in southeast Chongqing, China.

The organopores play an important role in determining total volume of hydrocarbons in shale gas reservoir. The Lower Silurian Longmaxi Shale in southeast Chongqing was selected as a case to confirm the contribution of organopores (microscale and nanoscale pores within organic matters in shale) formed by hydrocarbon generation to total volume of hydrocarbons in shale gas reservoir. Using the material balance principle combined with chemical kinetics methods, an evaluation model of organoporosity for shale gas reservoirs was established. The results indicate that there are four important model parameters to consider when evaluating organoporosity in shale: the original organic carbon (w(TOC0)), the original hydrogen index (I H0), the transformation ratio of generated hydrocarbon (F(R o)), and the organopore correction coefficient (C). The organoporosity of the Lower Silurian Longmaxi Shale in the Py1 well is from 0.20 to 2.76%, and the average value is 1.25%. The organoporosity variation trends and the residual organic carbon of Longmaxi Shale are consistent in section. The residual organic carbon is indicative of the relative levels of organoporosity, while the samples are in the same shale reservoirs with similar buried depths.

Quantitative characterization of reservoir space in the Lower Silurian Longmaxi Shale, southern Sichuan, China

Based on the drilling data of the Upper Ordovician Wufeng Shale and the Lower Silurian Longmaxi Shale in southern Sichuan Basin, the construction of matrix pores and the development condition of fractures in a marine organic-rich shale are quantitatively evaluated through the establishment of the reservoir petrophysical models and porosity mathematical models. Our studies show that there are four major characteristics of the Longmaxi Shale confirmed by the quantitative characterization: (1) the pore volume of per unit mass is the highest in organic matter, followed in clay minerals, finally in brittle minerals; (2) the porosity of the effective shale reservoir is moderate and equal to that of the Barnett Shale, and the main parts of the shale reservoir spaces are interlayer pores of clay minerals and organic pores; (3) the porosity of the organic-rich shale is closely related to TOC and brittle mineral/clay mineral ratio, and mainly increases with TOC and clay mineral content; (4) fractures are developed in this black shale, and are mainly micro ones and medium-large ones. In the Longmaxi Shale, the fracture density increases from top to bottom, reflecting the characteristics with high brittle mineral content, high Young’s modulus, low Poisson’s ratio and high brittleness at its bottom.

Exploration Tries To Answer Questions About the Lower Silurian Longmaxi Shale in China

This article, written by Editorial Manager Adam Wilson, contains highlights of paper IPTC 14487, ’Sichuan Basin Shale Gas, China: Exploring the Lower Silurian Longmaxi Shale,’ by Claudia Hackbarth, SPE, Danny Soo, and Navpreet Singh, SPE, Shell, prepared for the 2011 International Petroleum Technology Conference, Bangkok, Thailand, rescheduled to 7-9 February 2012. The paper has not been peer reviewed. Shale-gas exploration in China is already under way and rapidly increasing in activity level. The Sichuan basin, one of the most prolific petroleum basins in China, is at the forefront. However, exploration and assessment of an untested shale-gas play is typically lengthy and expensive. Compared with conventional oil and gas projects, shale-gas projects may require a number of early wells to prove the existence and distribution of the gas but then many more wells to test and optimize engineering techniques until a commercially viable combination is found. This process could be summarized into two overarching questions: “Is there a material amount of gas?” and, if yes, “Can it be produced economically?” Sichuan Basin Structure The topographic Sichuan basin today is a remnant of the originally much larger Upper Yangtze cratonic sedimentary basin (Fig. 1). In the southern Sichuan basin, structures are characterized by thin-skinned faulting and thrusting, often above an Upper Cambrian detachment surface, likely an evaporitic ductile layer that now cores the folds. The Sichuan-area stratigraphic column contains several potential source-rock intervals. Industry has so far focused attention on the Lower Cambrian Qiongzhusi and the Ordovician-Silurian boundary as potential shale-gas reservoirs. The Upper Ordovician Wufeng shale and Lower Silurian Longmaxi formation are generally regarded as belonging to the same black-shale succession, but an erosional surface and regional disconformity separate the two intervals. The Longmaxi formation is not as widespread as the Wufeng formation and is restricted to the Yangtze region. From the bottom up, the Ordovician/Silurian succession consists of A lower graptolite-bearing, black or dark-gray, carbonaceous/siliceous shale (Wufeng formation), generally several meters thick The Guoyinchao limestone or calcareous siltstone with abundant Hirnantian fauna and other shelly fossils, typically 0.5–3 m thick The Lower Longmaxi black, graptolite-bearing, carbonaceous/siliceous shale with cherty slate The Upper Longmaxi gray calcareous mudstones and siltstones, interbedded with an increasing number of thin limestone layers toward the top of the formation Regional studies indicate that Wufeng and Longmaxi shales were buried deeply enough over a large area of the Sichuan basin to have reached the gas-generation window.