Article pubs.acs.org/EF
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 Yue Wu,*,† Tailiang Fan,† Jinchuan Zhang,† Shu Jiang,‡ Yifan Li,† Junpeng Zhang,† and Chen Xie† †
School of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China Energy and Geoscience Institute, University of Utah, Salt Lake City, Utah 84108, United States
‡
ABSTRACT: 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.
1. INTRODUCTION The recoverable shale gas resources in China could amount to about 1115 trillion cubic feet (TCF) according to an initial assessment by the United States Energy Information Administration (U.S. EIA). This unconventional natural gas is expected to make a great contribution to the domestic energy demand of China in the future. The interest in shale gas has resulted in an increase of geological surveys and well drilling recently. The upper Ordovician Wufeng shale and lower Silurian Longmaxi shale in northwestern Guizhou province (NWG) of the upper Yangtze block is currently one of the most attractive shale gas plays for large net thickness, high organic matter abundance, and high maturity.1−5 However, shale gas exploration in this area is just in the early stage, and there is a lack of detailed data to characterize this black shale sequence. A good understanding to shale reservoir is the base to make proper assessment of undiscovered hydrocarbon resources, to carefully apply reservoir analogues, and to design the optimal exploration programs.6−8 In this paper, we conducted an integrated research on the upper Ordovician Wufeng shale and lower Silurian Longmaxi shale in NWG. The organic matter content, thermal maturity, trace element content, mineralogy, porosity, permeability, and gas adsorption capacity for this shale are characterized on the basis of the analysis of a suit of experimental data. All of our understandings will provide significant geological references for shale gas exploration and development in NWG of the upper Yangtze block in China. © 2014 American Chemical Society
2. GEOLOGICAL SETTING The Guizhou province is located in the southwest of China, with its main body in the southeast part of the upper Yangtze block tectonically. The study area for this paper is NWG bounded by three major faults, and a generalized lower Paleozoic stratigraphy in this area is shown in Figure 1. Well Xiye-1, with the geographical coordinate of 28° 15.049′ N and 106° 10.921′ E, is a typical national geological investigation shale gas well in NWG. The target drilling formations of this well are the Ordovician Wufeng formation and Silurian Longmaxi formation, with a completion depth of 706.76 m. Because of collision of the Yangtze block and Cathaysian block in the middle−late Ordovician, the upper Yangtze block gradually evolved from a passive continent margin carbonate platform into a foreland basin. By the surrounding of neighboring paleo highs, a restricted depositional environment was formed in NWG at the late Ordovician and early Silurian, which was located behind the back bulge of the upper Yangtze foreland basin. With the two large global transgressions at the late Ordovician and early Silurian, a thick succession of organicrich black shale was deposited in an intrashelf low in NWG, which has been studied before as a major source rock.9,10 The Ordovician formation and Silurian formation are of continuous deposition, with conformity contact between them Received: February 18, 2014 Revised: May 30, 2014 Published: June 2, 2014 3679
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Figure 1. Study area location map and stratigraphic column for the lower Paleozoic in NWG (modified with permission from the study by Chen et al.4).
in NWG.1−3 The organic-rich sedimentary rock sequence characterized in this study is the upper Ordovician Wufeng shale and Lower Silurian Longmaxi shale, named as Wufeng− Longmaxi shale, which are mainly composed of carbonaceous shale, siliceous shale, clay-rich shale, calcareous shale, and silty shale, with abundant pyrite, graptolite, and natural fractures. The sedimentary base of the Wufeng formation is a set of argillaceous limestone with tortoise-like cracks, and the upper section of the Longmaxi formation is alternated siltstone and carbonate rock (Figure 2). The Wufeng−Longmaxi shale is distributed widely in NWG, especially in the north part of Bijie−Jinsha−Zunyi areas (see these areas in Figure 1).
model the gas adsorption capacity, V = VLP/(PL + P), where V is the volume of absorbed gas, VL is the Langmuir volume (on the basis of monolayer adsorption), which is the maximum adsorption capacity of the absorbent, P is the gas pressure, and PL is the Langmuir pressure, at which the absorbed gas content (V) is equal to half of the Langmuir volume (VL).12
4. RESULTS AND DISCUSSION 4.1. Paleoredox Conditions. The redox condition classification is characterized by oxic, dysoxic, suboxic, and anoxic (including sulfidic and non-sulfidic).13 On the basis of previous studies,14−17 trace element concentrations and ratios are usually used as indicators to evaluate the redox conditions of a paleoenvironment. In the present study, trace element indices V/Cr, U/Th, and V/(V + Ni) were used to reconstruct the paleoredox conditions in NWG. Cr is not influenced by redox conditions, which is only associated with the detrital fraction.14,15 V in contrast is bound to organic matter by the incorporation of V4+ into porphyrins and is concentrated in sediments under reducing conditions.14,15 Therefore, the ratios of V/Cr are suggested as an index for the paleoredox conditions. The V/Cr ratios of 4.25 infer suboxic−anoxic conditions.14 Th is relatively immobile in the low-temperature surface environment and is concentrated in resistate minerals during weathering.14,15 U is soluble in the 6+ valence state, and insoluble in the 4+ valence state; thus, it may be fixed in sediments under reducing conditions.14,15 The U/Th ratios of 1.25 infer suboxic−anoxic conditions.14 Both Ni and V are preferentially preserved under reducing conditions, which are fixed in sediments by the formation of tetrapyrrole complexes.16 The proportionality of V and Ni in bitumen and oils remains unchanged during thermal evolution and, as such, records the redox conditions at the time of
3. SAMPLES AND EXPERIMENTAL METHODS 3.1. Samples. On the basis of drilling data, core observation, and logging response, a total of 30 core samples (CS01−CS30) were collected from the Wufeng−Longmaxi shale in well Xiye-1 (5 from the Wufeng shale and 25 from the Longmaxi shale). These samples were geochemically, mineralogically, petrophysically, and isothermally characterized by a relatively complete experimental program. 3.2. Methods. Trace element concentrations of a total of 15 samples was determined by inductively coupled plasma−mass spectrometry (ICP−MS) using sample powders dissolved in HF, HClO4, and HNO3. The detailed sample processing procedure for ICP−MS analyses and the analytical precision and accuracy for trace elements follows the methods described by Liu et al.11 Bulk mineralogy was measured by X-ray diffraction (XRD) using 18 powdered samples with the device of D/max-2600. The total organic carbon (TOC) content of 30 samples was measured through a Leco carbon−sulfur analyzer. Thermal maturity of 10 samples was determined according to the vitrinite reflectance, which was dominated on whole-rock particulate mounts by a MVP-3 microscope photomultiplier. Porosity and permeability of 10 samples from different burial depths were measured using the instruments of an ULTRAPORE-200A He porosimeter and ULTRA-PERMTM200 permeameter. These measurements were performed under the conditions of room temperature at 23 °C, moisture at 50%, and barometric pressure at 1025 hPa. Methane adsorption isotherms were determined on representative moisture-equilibrated samples with different TOC contents under reservoir temperature (30 °C). The Langmuir isotherm was applied to 3680
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Figure 2. Log curve, lithology, and properties for the Wufeng−Longmaxi shale in well Xiye-1. GR, γ ray in American Petroleum Institute (API) units; LLS, shallow laterolog in Ω m units; and LLD, deep laterolog in Ω m units.
deposition.16,17 It is suggested that the V/(V + Ni) ratios of >0.84 represent euxinic conditions (with free H2S in the water column), the V/(V + Ni) ratios of 0.54−0.82 represent anoxic conditions, and the V/(V + Ni) ratios of 0.46−0.60 represent dysoxic conditions.17 The variations of the paleoredox conditions in NWG during the accumulation of the Wugfeng−Longmaxi shale are clearly illustrated by the changes of trace element ratios of V/Cr, U/ Th, and V/(V + Ni) (Table 1 and Figure 3). The V/Cr radios
have a wide range of values and show variable decreases upward, suggesting that the shale was deposited in varying conditions, from anoxic, to dysoxic, to marginally oxic. The U/ Th ratios show a similar changing tendency, inferring predominantly anoxic to dysoxic conditions for the lower section and dysoxic to oxic conditions for the upper section. There is not good agreement between these two environmental indicators and V/(V + Ni); the latter ratios infer consistent anoxic conditions; and sometimes, euxinic conditions prevailed 3681
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Table 1. Trace Element Ratios of V/Cr, U/Th, and V/(V + Ni) for the Wufeng−Longmaxi Shale in NWG code
depth (m)
V/Cr
U/Th
V/(Ni + V)
CS03 CS04 CS05 CS08 CS09 CS10 CS11 CS14 CS16 CS21 CS24 CS25 CS27 CS28 CS30
569.3 578.7 585 594.1 601.4 606.2 613 615.7 621.3 633 638.7 639 643.5 645 647.6
1.50 1.51 1.45 1.49 1.49 1.82 2.09 1.94 3.99 3.36 2.52 3.56 6.31 9.52 6.72
0.43 0.38 0.36 0.45 0.28 0.59 0.66 0.88 1.45 0.86 0.92 0.96 2.93 9.46 5.02
0.76 0.77 0.75 0.73 0.72 0.78 0.80 0.80 0.74 0.76 0.78 0.83 0.78 0.93 0.92
Table 2. Organic Matter Type Classification Based on the Maceral and Carbon Isotope of Kerogena kerogen type sapropelic (I) humic−sapropelic (II1) sapropelic−humic (II2) humic (III)
carbon isotope of kerogen (δ13C) (‰)
maceral of kerogen (TI value)
< −28 from −28 to −26
from >80 to 100 from 80 to 40
from −26 to −24
from 40 to 0
> −24
from 2%) is at least 15 m, mainly distributed at the bottom part. 3682
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Table 4. Geochemical Characteristics of the Wufeng−Longmaxi Shale in NWG and Major Shales in North Americaa
a
basin/region
period
shale
kerogen type
TOC (%)
Ro (%)
Appalachian Michigan Illinois Fort Worth San Juan NWG
Devonian Devonian Devonian Lower Carboniferous Upper Cretaceous Upper Ordovician and Lower Silurian
Ohio shale Antrim shale New Albany shale Barnett shale Lewis shale Wufeng−Longmaxi shale
II I II II II II1
0−4.7 0.3−24 1−25 4.5 0.45−2.5 0.36−6.73
0.4−1.3 0.4−0.6 0.4−1.0 1.0−1.3 1.6−1.88 2.94−3.65
The geochemical data for major shales in North America are from the study by Curtis.23
Table 5. Relative Mineral Abundances for the Wufeng−Longmaxi Shale in NWG code
depth (m)
quartz (%)
K-feldspar (%)
plagioclase (%)
calcite (%)
dolomite (%)
pyrite (%)
total clay (%)
CS01 CS02 CS04 CS05 CS07 CS09 CS11 CS15 CS18 CS19 CS20 CS21 CS22 CS25 CS26 CS27 CS28 CS30
558.7 566.4 578.7 585 592.5 601.4 613 618.4 627 629 631 633 636.5 639 641 643.5 645 647.6
37.9 38.9 40.2 33.2 32.7 31.9 35.5 35.7 38.7 39 40 39.8 45.0 55.7 69.4 69.9 57.6 63.5
3 1.8 2.4 1.6 0.3 1 1.5 4.2 1.4 1.3 0.8 2.2 1.8 1.6 1.1 0.0 1.1 0.0
7.8 9.3 9.4 4.5 6.9 7.3 13.6 10.2 8.6 12.9 13.7 16.9 5.8 3.6 2.5 2.2 4.6 3.3
6 10.4 5.8 12.7 1.8 10.8 4.5 3.6 3.8 1.8 1.2 1.1 9.7 7.9 3.4 7.5 6.4 5.1
3.9 8.1 4.4 6.5 2.8 3.6 4.3 2 3.5 3.9 4.2 4.5 3.2 7.6 2.6 3.2 10.4 6.7
1.3 1.6 1.5 4.1 2.3 3.8 4.6 3.4 1.6 3.1 3.6 3.3 3.7 3.5 2.1 1.8 2.0 2.6
40.1 29.9 36.3 37.4 53.2 41.6 36 40.9 42.4 38 36.5 32.2 30.8 20.1 18.9 15.4 17.9 18.8
Figure 4. Percentage of mineral composition for the Wufeng−Longmaxi shale in NWG.
Because of the lack of vitrinite in this earlier Paleozoic marine shale, the thermal maturity prediction for this shale is based on the measurements of pyrobitumen reflectance (Rob). Zhong and Qin22 established the relationship between pyrobitumen reflectance (Rob) and equivalent vitrinite reflectance (Ro) as shown in the following equations:
R o = 1.042R ob + 0.052
(0.30 < R ob < 1.40%)
R o = 4.162R ob − 4.327
(1.40 < R ob < 1.60%)
R o = 2.092R ob − 1.079
(1.60 < R ob < 3.0%)
On the basis of the above arithmetic equations, the vitrinite reflectance (Ro) of the Wufeng−Longmaxi shale was calculated 3683
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Figure 5. Quantitative electron microscope scanning (QEMSCAN) images for selected representative core samples from the Wufeng−Longmaxi shale in well Xiye-1. (a) Quartz-rich shale at 643.5 m depth. Note that quartz is dominant, and little clays and carbonate acting as matrix or cement are scattered throughout. (b) Clay-rich shale at 594.1 m depth. Note that illite is the major mineral in this shale, and these natural fractures identified are filled with calcite.
and illustrated in Table 3. The Ro values range from 2.94 to 3.65% (averaging 3.38%), indicating that the shale has evolved into a dry gas window, with the organic matter in the overmature stage. The comparison of geochemical characteristics between the major gas-producing shales in North America and the Wufeng− Longmaxi shale in NWG shows that the organic matter type and abundance for these shales are similar, while the thermal maturity of the Wufeng−Longmaxi shale is much higher (Table 4). 4.3. Bulk Mineralogy. Quartz and clay minerals are the major mineral composition of the Wufeng−Longmaxi shale, and there are also minor amounts of feldspar, calcite, dolomite, and pyrite (Table 5 and Figure 4). Specifically, quartz is the most abundant, with measured contents between 31.9 and
69.9% (44.7% on average), followed by clay minerals with an average content of 32.6% (ranging between 15.4 and 53.2%). The content of quartz increases with the burial depth, while it is opposite for that of the clay minerals (Figure 4). The average contents of calcite and dolomite are 5.8 and 4.7%, respectively, which are mainly from cement and natural fracture filler in this shale (Figure 5). The average contents of K-feldspar and plagioclase are 1.5 and 8.0%, respectively. The content of pyrite is no more than 5%, with an average of 2.8%. There are more than 67.4% brittle minerals (chiefly including quartz, feldspar, calcite, and dolomite) on average, indicating a high brittleness of this shale interval, which is favorable for hydraulic fracturing to produce shale gas. Ternary diagrams of mineralogy between those hot shales in North America and the Wufeng−Longmaxi shale in NWG are 3684
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Figure 6. Ternary diagrams showing the mineralogy comparison between (a) typical shales in North America (data from the studies by Han et al.5 and Jarvie et al.24) and (b) Wufeng−Longmaxi shale in NWG.
shown in Figure 6. A meaningful comparison was made between the two diagrams, revealing that the mineralogy of the Wufeng−Longmaxi shale in NWG is similar to that of the Barnett shale, whose quartz and feldspar contents are higher than 40%, clay mineral content is less than 50%, and carbonate content is lower than 25%. 4.4. Porosity and Permeability. The porosity of the Wufeng−Longmaxi shale in NWG ranges between 0.6 and 4.4%, with a mean value of 1.8%, increasing toward the base, and the permeability ranges between 0.0066 × 10−3 and 0.1098 × 10−3 μm2, with a mean value of 0.0378 × 10−3 μm2 (Table 6). There is no statistically significant correlation between the Table 6. Rock Density, Porosity, and Permeability for the Wufeng−Longmaxi Shale in NWG code
depth (m)
rock density (g/cm3)
porosity (%)
permeability (×10−3, μm2)
CS01 CS04 CS09 CS11 CS18 CS21 CS22 CS26 CS28 CS30
558.7 578.7 601.4 613 627 633 636.5 641 645 647.6
2.68 2.67 2.66 2.68 2.68 2.67 2.65 2.41 2.46 2.39
0.9 1.2 1.6 1.6 0.6 1.5 1.8 2.1 2.1 4.4
0.0069 0.0071 0.1098 0.0136 0.0367 0.0066 0.0994 0.0617 0.0111 0.0253
Figure 7. Correlation plot between porosity and permeability for the Wufeng−Longmaxi shale in NWG. This poor correlation may result from the heterogeneity of this shale.
porosity and permeability (Figure 7), although higher permeability is generally associated with more porous sediments, indicating a complex pore system in the shale. In comparison to those typical shales around the world, the Wufeng−Longmaxi shale in NWG has relatively low porosity but high permeability, indicating a good mobility of natural gas in this shale (Figure 8). Many researchers reported that there was a strong association between porosity and mineral composition in shales.24−33 For example, when studying the Devonian shale and lower Jurassic Gordondale shale in the western Canadian sedimentary basin (WCSB), Ross and Bustin demonstrated that the porosity was closely related to the ratios of quartz and clay minerals.29 To illustrate the relationship between rock composition and porosity in the studied shale, we analyzed
Figure 8. Porosity and permeability for typical shales all over the world (data from the studies by Loucks et al.25 and Joel et al.26).
the correlations of porosity with the TOC content, quartz content, and clay mineral content. Organic matter, quartz, and 3685
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moisture capacity in shales is closely related to clay minerals.29−31,34 For example, Ross and Bustin31 reported that silica-rich shales have a lower moisture content than clayrich shales because of the non-adsorptive nature of quartz. They also reported that smectite−illite, illite, and kaolinite could cause a marked increase of the moisture content in shales.29,30 McCutcheon and Barton34 investigated the relationship between the clay mineral content and moisture capacity in a suite of bituminous coals, reporting that the moisture content was 2.3−2.8 times greater for clay minerals than that of the organic matter. To explore the role of organic matter in gas adsorption capacity for the studied shale, five representative core samples with different TOC contents were isothermally measured (Figure 10). The results show that the large adsorbed gas
clay minerals are the major composition for this shale, and little carbonate is present. The correlation results show that the porosity correlates positively with the TOC and quartz contents but negatively with the clay mineral content (panels a, b, and c of Figure 9; R2
Figure 10. Methane adsorption isotherms (at 30 °C) for representative core samples with varying TOC contents from the Wufeng−Longmaxi shale in well Xiye-1. Samples show a general increase in gas adsorption capacity with the TOC content.
content is generally associated with the organic-rich shale, indicating that organic matter is responsible for adsorbing gas in this shale. This understanding is consistent with the research results of many scholars that organic matter in high mature marine shales is microporous, with an internal surface area onto which shale gas can adsorb.29−31,35−37
5. CONCLUSION A thick succession of black shale was developed in NWG of the upper Yangtze block at the late Ordovician and early Silurian, which holds great shale gas potential because of high TOC content, high thermal maturity, high brittle mineral content, and potentially high gas content. Five detailed properties for this shale were concluded on the basis of experimental data analyses shown as follows: (1) Trace element ratios of V/Cr, U/Th, and V/(V + Ni) reveal that the Wufeng−Longmaxi shale deposited in variable paleoredox conditions, with the degree of anoxia decreasing upward. (2) The kerogen type of the Wufeng−Longmaxi shale is sapropelic (I) to humic− sapropelic (II1), which has strong hydrocarbon generation potential. TOC contents range from 0.36 to 6.73% (averaging 2.02%), increasing with the burial depth. Ro values are between 2.94 and 3.65% (averaging 3.38%), indicating an overmature stage of the shale. (3) Quartz and clay minerals are the major mineral composition of the Wufeng−Longmaxi shale, with an average content of 44.7 and 32.6%, respectively. The average contents of K-feldspar and plagioclase are 1.5 and 8%,
Figure 9. (a−c) Correlation plots of porosity with the TOC content, quartz content, and clay mineral content for the Wufeng−Longmaxi shale in NWG. Moderate correlations suggest that other factors influence the relationship between porosity and rock composition.
= 0.75, 0.52, and 0.56, respectively). This reveals that the shale interval rich in quartz and organic matter has a higher porosity than the clay-rich interval for the Wufeng−Longmaxi shale in NWG. 4.5. Gas Adsorption Capacity. Organic matter and clay minerals with a huge specific surface area are thought to have a great influence on gas adsorption capacity in shales.29−31,34−37 In this study, Methane adsorption experiments were performed on moisture-equilibrated shale powders under reservoir temperature (30 °C). The contribution of clay minerals to the gas adsorption capacity in this shale may be irrelevant because of the presence of moisture, which would block pore throats and remove a large proportion of potential gas adsorption sites.29−31 Previous studies demonstrated that 3686
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respectively, and those of calcite and dolomite are 5.8 and 4.7%, respectively. The mineralogy of this shale is comparable to that of the Barnett shale in North America. (4) The porosity of the Wufeng−Longmaxi shale is 1.8% on average (ranging from 0.6 to 4.4%), increasing toward the base, and the permeability is 0.0378 × 10−3 μm2 on average (ranging from 0.0066 × 10−3 to 0.1098 × 10−3 μm2). Correlation analyses between porosity and rock composition indicate that the shale interval rich in organic matter and quartz has higher porosity than the clay-rich interval. (5) The gas adsorption capacity of the Wufeng− Longmaxi shale increases with the increasing TOC content, indicating that organic matter is responsible for adsorbing gas in this shale.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 001-3852527541. E-mail: wuyue0906@gmail. com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Oil and Gas Strategic Investigation Program (Grant 2009GYXQ-15), the National Natural Science Foundation Research (Grant 40672087), and the Shale Gas Resources Investigation and Evaluation Program, Guizhou Province (Grant 2012GYYQ-01). The authors also sincerely appreciate the support from the China Scholarship Council (CSC) and the Energy and Geoscience Institute (EGI) of the University of Utah. Associate Editor Ryan P. Rodgers and all anonymous reviewers are gratefully acknowledged.
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