Wufeng Longmaxi Research Articles

Factors Controlling Organic Matter Accumulation in the Wufeng–Longmaxi Formations in Northwestern Hunan Province: Insights from Major/Trace Elements and Shale Composition

Organic matter (OM) is the material basis of shale gas accumulation, so understanding the mechanisms of OM accumulation plays a critical role in shale gas exploration. The marine shale from the Ord...

Hydrocarbon generation and storage mechanisms of deep-water shelf shales of Ordovician Wufeng Formation–Silurian Longmaxi Formation in Sichuan Basin, China

As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation- Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions; the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that: (1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil. (2) The biogenic silicon (opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time; moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores. (3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.

Lithofacies and depositional mechanisms of the Ordovician–Silurian Wufeng–Longmaxi organic-rich shales in the Upper Yangtze area, southern China

The Ordovician–Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty–shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of well-log data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a third-order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed isochronal stratigraphic framework, the characteristics and genetic mechanism of different shale lithofacies were studied. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician–Silurian transition when glaciation and mass extinction took place. The gray silty–shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry. A better understanding of the distribution pattern and depositional history of the Wufeng and Longmaxi Formations might provide guidance for future profitable shale gas exploration.

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

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

Geochemical Characteristics of Bentonite and Its Influence on Shale Reservoir Quality in Wufeng–Longmaxi Formation, South Sichuan Basin, China

On the basis of X-ray diffraction analysis, trace element test, rare earth element test, total organic carbon (TOC) analysis, total gas content, and porosity test, the characteristics of bentonite ...

Accumulation Conditions and an Analysis of the Origins of Natural Gas in the Lower Silurian Shiniulan Formation from Well Anye 1, Northern Guizhou Province

The origin of natural gas and the mechanisms that lead to gas accumulation in the marine calcareous mudstone of the Lower Silurian Shiniulan Formation in northern Guizhou province are special and complicated. According to a combination of qualitative and quantitative methods, including the reconstruction of hydrocarbon generative potential and gas content’s measurement, in the context of some geochemistry information—the origins of the natural gas of Shiniulan Formation is suggested to be mixed gas. Furthermore, the accumulation of the natural gas can be proposed combined with some geological information. Results indicated that the volume of the in-place gas content of Shiniulan samples, reinstated by the formulas’ computation, reaches a yield of 3.67 m3·t−1 in rock. The theoretical gas content for Shiniulan Formation mudstone ranges from 1.6 to 5.8 m3·t−1 using the indirect calculation of gas content, and the total gas contents of those samples range from 0.065 to 0.841 m3/t, according to the United States Bureau of Mines’ (USBM) direct method. According to the combination of the reconstructed in-place gas content and the gas content, even mudstone in the Shiniulan Formation has potential to generating gas but could not satisfy the actual gas content in Shiniulan Formation. In addition, according to the composition, the carbon and hydrogen isotope charts of gaseous hydrocarbons further indicate that the gas origin of Shiniulan Formation is a mixed gas, which also means that the gas is not just generated in the layer, but has partly migrated from other formations, such as the Wufeng–Longmaxi Formation. The lower Shiniulan Formation in the study area is characterized by frequent interbed of limestone and calcareous mudstone. The geological information shows that well-developed fractures of mudstone and faults can be used as main pathways for the upward migration of gases from the underlying strata to the Shiniulan Formation. The limestone with fairly low porosity and permeability hinders the migration of natural gas as much as possible and keeps that efficiently reserved in the horizontal fractures of calcareous mudstone. This migration pattern implies that the interbedded rock association is also favorable for gas accumulation in the Shiniulan Formation.

Elastic anisotropy and its influencing factors in organic-rich marine shale of southern China

Shale is observed to have strong anisotropy due to its unique mineralogy and microstructure, and this anisotropy property has significant impact on seismic and well-log data. The organic-rich marine shale in the southern and eastern Sichuan Basin is one of the most important shale-gas reservoir formations in China. To investigate the elastic anisotropy of this shale and its influencing factors, we performed ultrasonic velocity measurements, X-ray diffraction analysis, rock-eval pyrolysis and vitrinite reflectance measurement on the samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. The experimental results show the that: (1) the velocity anisotropy of the Wufeng-Longmaxi (WL) shale varies from 10% to 50%, and most samples have strong anisotropy; (2) the P- and S-wave anisotropy parameters (Thomsen’s e and γ) increase with clay contents, but this relationship can be greatly affected by the clay orientation index; (3) organic matter content (OMC) is found to have little influence in seismic anisotropy for the over mature WL shale, whereas the OMC determines the magnitude of anisotropy of immature/mature shales (e.g. the Bakken shale or the Bazhenov shale) according to the published literatures, because organic matters in shales of different maturity have different morphologies and distributions; (4) the OMC of WL shale has positive correlation with quartz content, and this weakens the correlation between OMC and the magnitude of anisotropy to a certain extent. The results of this study provide an important rock-physics basis and data support for seismic anisotropy exploration, quantitative interpretation and resource evaluation of the organic-rich marine shales in southern China.

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

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

Pore characteristics and evolution of Wufeng–Longmaxi Fms shale gas reservoirs in the basin-margin transition zone of SE Chongqing

At present, researches on the pore evolution of shale reservoir and its evolution mechanism are still at such a groping stage that a consensus has not yet reached. Based on core analysis and thermal simulation experiments, the pore types, pore structures and pore-size change rules of shale gas reservoirs of Upper Ordovician Wufeng–Lower Silurian Longmaxi Fms in the southeastern (SE) Sichuan Basin and its basin-margin transition zone (hereinafter referred to as the basin-margin transition zone of SE Chongqing) were studied by means of argon ion polishing–scanning electron microscopy (SEM) and atomic force microscopy. Then, the evolution characteristics of organic pores were discussed, and the influence of associated minerals on pore evolution was analyzed. Finally, a pore evolution model for the shale gas reservoirs in this area was established. And the following research results were obtained. First, three types of reservoir spaces are mainly developed in the high-quality shale reservoirs of Wufeng–Longmaxi Fms in this area, including fracture, inorganic pore and organic pore. And the organic pores provide the primary reservoir space of shale gas, which can be divided into four categories, i.e., amorphous kerogen pores, structured kerogen pores, asphaltene pores and paleontology fossil pores. Second, organic contracted fractures are related to the contraction of organic matters, first appearing on one side of the organic matters and then becomes wider and wider with the increase of temperatures. Third, organic pores are mostly the “spongy” pores distributed densely inside the organic matters. When Ro is in the range of 1.56–3.50%, macropores and mesopores are dominant. And when Ro exceeds 3.50%, macropores decrease while mesopores and micropores increase. Fourth, the types of organic matters and the content of associated minerals (e.g. clay minerals, siliceous particles and pyrite) play an important role in the development of pores. In conclusion, the pore evolution law of Wufeng–Longmaxi shale in the basin-margin transition zone of SE Chongqing is that with the increase of burial depth, inorganic porosity decreases significantly, organic porosity increases first and then decreases, and the total porosity shows a change trend of decreasing first, then increasing and finally decreasing continuously.

Organic Matter Pore Characterization of the Wufeng-Longmaxi Shales from the Fuling Gas Field, Sichuan Basin: Evidence from Organic Matter Isolation and Low-Pressure CO2 and N2 Adsorption

Organic matter (OM) pores are significant for shale gas accumulation and flow mechanisms. The pores of Wufeng-Longmaxi (W-L) shale in the Sichuan Basin, China have been extensively characterized, however, the proportion of OM pores in this shale have not been adequately discussed. In this study, the contribution of OM pores to the total pore volume of W-L shale was quantitatively studied through the analysis of OM isolation, field emission scanning electron microscopy (FE-SEM) and low-pressure CO2 and N2 adsorption (LPGA). FE-SEM images showed abundant OM pores, interparticle pores and intraparticle pores with various shapes and widths in the W-L shales. The pore size distribution (PSD) of the isolated OM from five shale samples showed a consistent, unimodal pattern. The pore volume of isolated OM was greater than that of the bulk shale samples, suggesting that OM is more porous than the inorganic compositions in shales. The average contribution of OM to the volumes of micropores, mesopores and macropores was 58.42%, 10.34% and 10.72%, respectively. Therefore, the pore volume of the W-L shale was dominantly related to inorganic minerals. This was probably due to the small weight ratio of OM in the shale samples (1.5 wt%–4.2 wt%). The findings of this study reveal the different effects of OM and minerals on pore development, and provide new insights into the quantitative contribution of OM pores to the total pore volume of the W-L shale.

Origin and significance of organic-matter pores in Upper Ordovician Wufeng-Lower Silurian Longmaxi mudstones, Sichuan Basin

The types of organic-matter (OM) pores in shale gas reservoirs were comprehensively investigated from the Wufeng-Longmaxi (W-L) mudstones in the Changning gas field, southern Sichuan Basin, China. The aim of this research is to provide the origin and significance of OM pores development in the nanometer-scaled pore structures within shale rocks. To understand the contribution of OM pores development to the W-L mudstones, the thin-section, OM extract from bulk cores, mineralogy, pore size distribution, and image analyses by scanning electron microscopy (SEM) and atomic force microscopy (AFM) were comprehensively carried out. Combining SEM photographs and AFM observations, two types of OM pores were identified, including structural OM pores and non-structural spongy OM pores. Through organic petrology analyses, the structural OM pores evolve from telalginites mainly sourced from phytoplankton and acritarch, while the non-structural spongy OM pores are derived from bacteria, benthic algae and certain graptolites. Three basic lithofacies were observed in the first member of the W-L shales, based on the results of the low-pressure N2 adsorption. We did not find any complex interaction between mineralogy and OM pores evolution. However, siliceous shales generate the most amount of bitumen which is the main OM pores producer, and the rigid siliceous particles can make the shelter of bigger OM pores. OM pores connectivity therefore increases within the siliceous shales. OM pores development, especially structural OM pores, has a crucial effect on shale gas exploitation.

Organic geochemical characteristics, mineralogy, petrophysical properties, and shale gas prospects of the Wufeng–Longmaxi shales in Sanquan Town of the Nanchuan District, Chongqing

Thick marine shales occur in the Wufeng Formation and Longmaxi Formation of Sanquan Town of the Nanchuan District, Chongqing Municipality, which is located on the southeast margin of the Sichuan Basin. However, few details of the characteristics of the Wufeng–Longmaxi shales in this area have been reported. In this study, a well approximately 100 m (∼328 ft) deep was drilled. A high-quality shale (total organic carbon [TOC] >2.0 wt. %, clay <40%) interval that was approximately 24 m (∼79 ft) thick with an average TOC value of 3.0 wt. % mainly occurs in the Ordovician Wufeng Formation (Katian and Hirnantian) and base of the Silurian Longmaxi Formation (Rhuddanian). Shales with higher TOC values commonly have a higher porosity and specific surface area. Tectonic movements may also have been very important factors that influenced the petrophysical properties of the shales. For example, a detachment layer that resulted from complex tectonic movements is extensive in the Wufeng Formation. The cracks and microcracks in the detachment layer can result in good pore connectivity. Consequently, the detachment layer can be an effective migration pathway. The Longmaxi–Wufeng shales of Sanquan Town are also compared with those of the famous Jiaoye 1 well in the Jiaoshiba shale gas field in the eastern Sichuan Basin. Although the shales in Sanquan Town have considerable shale gas generation potential, the shale gas resource potential in Sanquan Town is probably poor because the escape of shale gas may be accelerated by the detachment layer in destroyed anticlines and synclines.

Integrated assessment of thermal maturity of the Upper Ordovician–Lower Silurian Wufeng–Longmaxi shale in Sichuan Basin, China

Abstract The Wufeng-Longmaxi marine shale of the Upper Ordovician (O3w)-Lower Silurian (S1l) is the main source rock and target for shale gas exploration in Sichuan Basin, China. Maturity assessment is very important for shale gas evaluation, and such assessment is difficult because of the absent of vitrinite, which is the main objective for vitrinite reflectance analysis. Graptolite fragments and solid bitumens are the dominant forms of organic matter in the shales. In this article, optical reflectance analysis of solid bitumen and graptolite and Raman spectroscopy characterization of graptolite were investigated to reduce the levels of uncertainty in thermal maturity assessment. The aims were to (1) build the correlation between graptolite reflectance and solid bitumen reflectance, to (2) establish the relationship between graptolite reflectance and Raman spectroscopy, and (3) to re-build the maturity distribution of such gas shale in Sichuan Basin. 32 core samples and 22 outcrop samples have been collected for the optical reflectance analysis, and the mean graptolite random reflectance in Sichuan Basin ranges from 1.21% to 4.91%. The random reflectance of graptolite (GRo) is higher than that of solid bitumen (BRo), and graptolite random reflectance can effectively be used as a maturity indicator. Compared with graptolite maximum reflectance (GRomax), graptolite random reflectance is more precise owing to smaller standard deviation. A natural evolution of maturity series of graptolites have been chosen for Raman spectroscopy analysis. Based on the previous studies of the relationship between Raman spectroscopy and vitrinite reflectance, the correlations between the graptolite random reflectance and vitrinite reflectance (VRo) for over maturity stage were obtained through characteristic of the Raman spectroscopy. The results indicate relationship between GRo and VRo is not a single linear relationship. Through the calculation, the maturities of this gas shale interval range from 1.16% to 3.63%, which provides the basic important information for shale gas evaluation in Sichuan Basin.

Modes of Shale‐Gas Enrichment Controlled by Tectonic Evolution

AbstractThe typical characteristics of shale gas and the enrichment differences show that some shale gases are insufficiently explained by the existing continuous enrichment mode. These shale gases include the Wufeng–Longmaxi shale gas in the Jiaoshiba and Youyang Blocks, the Lewis shale gas in the San Juan Basin. Further analysis reveals three static subsystems (hydrocarbon source rock, gas reservoirs and seal formations) and four dynamic subsystems (tectonic evolution, sedimentary sequence, diagenetic evolution and hydrocarbon‐generation history) in shale‐ gas enrichment systems. Tectonic evolution drives the dynamic operation of the whole shale‐gas enrichment system. The shale‐gas enrichment modes controlled by tectonic evolution are classifiable into three groups and six subgroups. Group I modes are characterized by tectonically controlled hydrocarbon source rock, and include continuous in‐situ biogenic shale gas (I1) and continuous in‐situ thermogenic shale gas (I2). Group II modes are characterized by tectonically controlled gas reservoirs, and include anticline‐controlled reservoir enrichment (II1) and fracture‐controlled reservoir enrichment (II2). Group III modes possess tectonically controlled seal formations, and include faulted leakage enrichment (III1) and eroded residual enrichment (III2). In terms of quantity and exploitation potential, I1 and I2 are the best shale‐gas enrichment modes, followed by II1 and II2. The least effective modes are III1 and III2. The categorization provides a different perspective for deep shale‐gas exploration.

Effects of volcanic activities in Ordovician Wufeng–Silurian Longmaxi period on organic-rich shale in the Upper Yangtze area, South China

Abstract Based on the corresponding relationship between the paleoproductivity, redox conditions and volcanism within a chronostratigraphic framework, the effects of volcanic events in the Wufeng–Longmaxi period on organic abundance of shale were examined. Bentonite layers were mostly developed in the transgressive systems tract 1 (TST1, Wufeng Formation) and transgressive systems tract 2 (TST2, Longmaxi Formation), and the two systems tracts corresponded to favorite shale lithofacies with high silica and total organic carbon (TOC) contents. According to the stratigraphic characteristics of bentonite rich interval, TST1 is classified as the interval with dense bentonite layers with the frequency of bentonite layer (bentonite layers/time) of more than 1.5 layers/Ma and the cumulative thickness ratio of bentonite layers (thickness of bentonite layers/thickness of shale) of more than 1%; TST2 is classified as the interval with sparse bentonite layers (frequency

Progress, challenges and prospects of shale gas exploration in the Wufeng–Longmaxi reservoirs in the Sichuan Basin

The Sichuan Basin is a major target for shale gas exploration in present China because of its rich gas stored in abundant black shales with multiple bed series. For further guidance or reference, field exploration and development practices in the shale reservoirs Upper Ordovician Wufeng–Lower Silurian Longmaxi shale reservoirs were studied in terms of development stages and progress, favorable conditions for shale gas accumulation, bottlenecking issues on theories and technologies related to shale gas development, and so on. The following findings were obtained. (1) Shale with rich organic matters originated from the deep shelf has a good quality and great thickness in the continuous beds. The relatively stable wide buffer zones in synclines (anticlines) provides favorable conditions for shale gas accumulation and preservation with well-developed micro-fractures and overpressure as necessary factors for a great potential of high shale gas productivity. (2) The bottlenecking technical issues restricting the shale gas industrial development in this study area include the following aspects: understandings of rich-organic matter shale sedimentary facies and modes, shale reservoir diagenetic process and evaluation systems, shale gas generation and accumulation mechanism, geophysical logging identification and prediction of shale gas layers, low resource utilization rate, great uncertainty of shale gas development, no technological breakthrough in the exploration of shale gas reservoirs buried deeper than 3500 m. In conclusion, this study area will be the major target for the shale gas exploration and development in China in a rather long period in the future.

Optical characteristics of graptolite-bearing sediments and its implication for thermal maturity assessment

Graptolite reflectance was thought to be one of the most useful thermal maturity indicators for graptolite-bearing sediments, however, the relationship between graptolite reflectance and vitrinite reflectance is not well-established. Graptolites, especially in the Wufeng–Longmaxi Formations from the Ordovician to Silurian of South China, have been mistaken for vitrinite-like particles or solid bitumen, which results in inconsistent data on the thermal maturity. In this paper, we have employed optical microscope techniques to describe the detailed optical characteristics of graptolites and solid bitumen in the Wufeng–Longmaxi Formations. Laboratory simulation of maturation was used to determine the relationship between graptolite reflectance and vitrinite reflectance.The organic constituents in the Wufeng–Longmaxi Formations are mainly composed of graptolites and solid bitumen. Granular and non-granular graptolites were observed in the Wufeng–Longmaxi Formations, with non-granular as the most common texture. Solid bitumen can be distinguished from non-granular graptolites by its coarse surface, weaker anisotropy, and lower random reflectance. The combination of non-polarized and polarized light is very helpful to distinguish solid bitumen from graptolite. For comparison, organic material from the early Ordovician Alum Shale Formation of Sweden and Estonia was also studied. The macerals of the Alum shales are mainly composed of lamalginites, mineral-bituminous groundmasses, graptolites, and solid bitumen. The major textures of the graptolites in the Sweden and Estonia sediments are non-granular and granular, respectively.Both non-granular graptolite and vitrinite reflectances display a systematic increase with the increase of heating temperature and time. The granular graptolites in the Estonian sample were gradually changed to non-granular graptolites following laboratory simulated maturation, indicating that granular graptolites can transform into non-granular graptolites with maturation. Solid bitumen in the Wufeng–Longmaxi Formations was derived from the solid residue of kerogen and/or post-oil bitumen. The graptolite random reflectance is a better thermal maturity indicator than graptolite maximum reflectance and is more precise due to the smaller standard deviation. Several equations are proposed to determine the thermal maturity of the graptolite-bearing sediments based on graptolite random reflectance, graptolite maximum reflectance and solid bitumen random reflectance.

Discussion on the contribution of graptolite to organic enrichment and gas shale reservoir: A case study of the Wufeng–Longmaxi shales in South China

The graptolitic shale of the Wufeng–Longmaxi Formations is widely deposited across the Ordovician and Silurian transition in South China, which is the target of shale gas exploration and development within China. The contribution of graptolites to organic enrichment and reservoir of gas shale is discussed below based on the statistics of nearly 1000 shale samples from the Wufeng Formation and the bottom part of the Longmaxi Formation in the southern and northern margins of the Yangtze plate. The assessment involves graptolites abundance, the total organic carbon (TOC) content analyses, and the different scales of scanning electron microscopy analyses of related samples. The TOC content of the Wufeng–Longmaxi graptolitic shales (including graptolites and non-graptolites, i.e., the host shale) is mainly controlled by that of its host shale, while less affected by the graptolites abundance, indicating that the graptolites barely influence the organic enrichment. Graptolites consist of a large number of organic matter with reticular biological tissue structure; they account for 20%–50% of the graptolitic area. The aforementioned also developed honeycomb-shaped pores with pore sizes ranging 110 nm-1.7 μm (an average of about 500 nm), which are higher than those of the organic pores in the host shale (108–770 nm, average 330 nm), proving that graptolites have an important contribution to shale gas storage space. Since there are a large number of graptolites within the shales from the Wufeng Formation and the bottom part of the Longmaxi Formation, the laminated and stacked local pattern of their distribution provides abundant storage space for shale gas. Moreover, the feature also serves as the predominant channel for shale gas flow. Therefore, the widely developed graptolites should be considered as one of the essential factors controlling enrichment and high productivity of shale gas in the Wufeng–Longmaxi Formations.

Effect of Shale Lithofacies on Pore Structure of the Wufeng–Longmaxi Shale in Southeast Chongqing, China

Total organic carbon (TOC), optical microscopy (OPM), field emission scanning electron mi-croscopy (FE-SEM), X-ray diffraction (XRD) and nitrogen gas adsorption (N2GA) analyses were performed on shale samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation (Wufeng-Longmaxi Formation) in order to explore the effect of shale lithofacies on pore structure. The results show that the Wufeng-Longmaxi shale consists of four types of pores: organic matter pores, interparticle pores, intraparticle pores and micro-fracture pores. A total of 8 kinds of shale lithofacies were identified on the basis of TOC and the ternary diagram of siliceous minerals (quartz and feldspar), clay and carbonate. The geological properties have substantial heterogeneity in these shale lithofacies. The volumes of organic matter (OM), clay-related and brittle mineral-related pores are 0.00119-0.01262 cm3/g, 0.00080-0.00260 cm3/g, and 0.00122-0.00215 cm3/g, with an average proportion of 51.85 %, 26.44 % ...

Fracture Characteristics and Change of Permeability Under the Influence of Natural Fractures: Experimental Study of Wufeng–Longmaxi Shale

Summary Currently, substantial shale gas reserves have been recognized in many countries, including China and the United States. An improved comprehension of the structure of these shale plays can lead to improved shale gas producibility. Gas is produced through natural fractures (NFs) and through hydraulically induced fractures, together creating the so-called stimulated reservoir volume (SRV). Although NFs are advocated as significantly influencing the size and shape of an SRV, the effects of NFs on fracturing and permeability changes are incomplete. This paper describes experimental studies performed on calcite-filled fractures from outcrops of Wufeng (O3w)-Longmaxi (S1l) shale. These samples are representative of Ordovician-Silurian productive shales in the Fuling Basin, near Chongqing City, China. Basic parameters, such as acoustic wave velocity, porosity, and permeability, were first measured on representative samples. Subsequently, permeability and acoustic emissions were measured during triaxial compression experiments through the post-peak domain. After these evaluations, the fracture modes and the fragment distribution were evaluated. In the measurements made, the natural calcite-filled macroscopic fractures reduced the confined compressive strength of these shale samples from 26 to 47%. Permeability is also affected. During the triaxial compression experiments, different NF orientations affected the overall permeability differently. We divided the collected samples into three groups. Group 1 samples had axial fractures (AFs). The gas permeability reduced from 0.70 to 0.12 md. When the sample was broken, permeability increased to 2.0 md, whereas the axial stress dropped to zero. Samples in Group 2 had transverse natural fractures (TFs). When new fractures were generated, permeability increased to greater than 0.70 md. Because samples in Group 3 had no obvious, natural macroscopic fractures (NFs), permeability was too low to measure before catastrophic failure during triaxial compression. When these samples did fail in compression, significant fractures formed, and permeability increased, resembling Group 2. There was a close relationship between the initial pattern of NFs and the subsequently generated fracture morphologies. For Group 1, the samples broke into several larger pieces along the original AFs. For Group 2, the samples broke into numerous small and relatively uniform pieces. For Group 3, the samples did not break completely, and the constituent fragments were nonuniform. Our results are helpful for understanding fracture-propagation mechanisms and the evolution of permeability under in-situ loading processes (long-term geologic loading, stress alteration during well construction and stimulation, and reservoir modification during depletion).