Characteristics of the Permian coal-formed gas sandstone reservoirs in Bohai Bay Basin and the adjacent areas, North China

Journal of Petroleum Science and Engineering 78 (2011) 516–528

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l

Characteristics of the Permian coal-formed gas sandstone reservoirs in Bohai Bay Basin and the adjacent areas, North China Dawei Lv a,⁎, Zengxue Li a, Jitao Chen a, b, Haiyan Liu a, Jianbin Guo a, Luning Shang a a Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of Geological Sciences & Engineering, Shandong University of Science and Technology, Qingdao 266510, China b School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea

a r t i c l e

i n f o

Article history: Received 27 April 2010 Accepted 6 June 2011 Available online 15 June 2011 Keywords: coal-formed gas tight sandstone sedimentary facies Bohai Bay Basin Luxi area

a b s t r a c t This paper focuses on the physical properties and spatial distributions of the Permian sandstone reservoirs in Bohai Bay Basin and the adjacent areas in the northeast China. The coal-formed gas sandstone reservoirs occur in the Permian Shihezi Formation and the maturity of the sandstone in the lower part is gradually higher than that in the upper part. The high-maturity sandstones are mainly the channel lag deposits and widely distributed in the study area. Due to the low porosity and permeability, however, the sandstones formed only the medium-poor reservoirs. The sandstones contain clay matrix and commonly a small portion of calcareous, siliceous, and authigenic clay cement. On the other hand, severe compaction and cementation caused nearly complete destruction of the primary porosity during diagenesis. Therefore, the main space of the sandstone reservoirs comes from the secondary pores. The distributions of reservoirs are affected by several periods of faulting and denudation. The sandstones of the Lower Shihezi Formation are thicker in the Jizhong and Bozhong depressions, and Luxi areas, and those of the Upper Shihezi Formation are thicker in Jizhong and Jiyang depressions and thinner in Luxi areas. © 2011 Elsevier B.V. All rights reserved.

1. Introduction With respect to the study of coal-formed gas of China, there have been many achievements recently (Aizenshtat et al., 1998; Anna, 2003; Chen et al., 2003; Dai, 1979, 2007, 2009; 2001; Fu et al., 1990; Imam and Shaw, 1987; Karim et al., 2010; Lee, 1989; Peng et al., 2009; Purvis, 1992; Ritts et al., 2006; Song and Liu, 2008; Song et al., 2004; Waiter and Ayers, 2002; Wang et al., 1994; Wilkins and George, 2002; Wolela, 2009; Wu et al., 2003; Yang et al., 1987; Zhang et al., 2002; Zou et al., 2009). During exploration and exploitation of the coal-formed gas, tight sandstones were regarded as a particular type of gas reservoir. Coal-formed gas reservoirs of the tight sandstones bear special characteristics, and they are influenced by distribution and physical properties of source, reservoir, and cap rocks. The explorations have been restricted due to the complicated controlling factors. It is, therefore, of great theoretical significance and practical importance to carefully study the characteristics and formation mechanism of the tight sandstones. However, the formation mechanisms of the coal-formed gas preserved in the Permian tight sandstones have not been clearly figured out because it is usually difficult to discover it. Moreover,

⁎ Corresponding author at: 579 Qianwangang Road Economic & Technical Development Zone, Qingdao Shandong Province, 266510, China. Tel./fax: + 86 532 86057229. E-mail address: [email protected] (D. Lv). 0920-4105/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2011.06.018

many sandstone bodies are commonly present between the reservoirs and source rocks. The migration paths and mechanisms of gas are thus becoming hot topics. The previous studies in China focused only on some of the depressions in certain basins (e.g., Ordos Basin and Qinshui Basin) (Dai, 2007; Yang et al., 2008), which hampered understanding the migration, preservation, and formation mechanisms of coal-formed gas in the tight sandstones. For these reasons, this paper focuses on the characteristics of the Upper Paleozoic coal-formed gas reservoirs and illustrates the distribution of the tight sandstone reservoirs. It provides the theoretical and practical basis for the coal-formed gas exploration and predication in the northeast China. The Bohai Bay Basin is one of the most important areas for exploration of oil and gas in China. In spite of a great progress in oil exploration, it did not make a breakthrough in gas exploration of this area. However, there is a great potential of coal-formed gas exploration in Bohai Bay Basin and the adjacent areas because of many gas showings (e.g., Su50 Well in Jizhong Depression, GBG1 Well in Jiyang Depression, etc.). There have been several reports in the literatures discussing some aspects of the general geological setting and oil and gas of Bohai Bay Basin (Jin and Liu, 2008; Lee,1989; Li et al., 2006, 2007; Liu et al., 2007a, 2007b, 2009; Lv et al., 2008; Querol et al., 1999; Su et al., 2005; Yu et al., 2007; Zhang et al., 2010), but little specific information on coal-formed gas exploration is available. In order to bridge this gap, this paper aims to provide an update on our understanding of coal-formed gas

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

517

Fig. 1. Brief geological map of the Bohai Bay Basin and adjacent areas.

reservoirs characteristics and their distribution in Bohai Bay Basin and the adjacent areas.

2. Geological setting The study area is situated in the middle-east part of the North China Platform, including Hebei, Shandong, and Henan provinces (Zhang et al., 2006). The east is Bohai Bay; the north, Yanshan folded belt; the west, Taihang mountains uplift; and the south boundary, FengPei fault (Fig. 1). The study area is about 300,000 km 2, mainly including Jizhong, Jiyang, Linqing, Huanghua, Bozhong depressions and Luxi uplift-depression. The relic area of the Carboniferous-Permian strata is about 130,111 km 2 and occupies 43.4% of the primary depositional area. Coal-formed gas fields, whose source rock is the Paleozoic coal-bearing strata, have been found in Gubei area of the Shengli Oilfield (Li et al., 2006, 2007), Suqiao area of the Huabei Oilfield, Wumaying area of the Dagang Gasfield, and Wenliu area of the Zhongyuan Oilfield since the exploration and study of coal-formed gas in 1988. Especially, the daily production of QG 1 well in Jiyang Depression can reach 41,347 m 3 (Liu et al., 2007a, 2007b), showing the great potentiality of coal-formed gas exploration in this region.

Table 1 Sedimentary facies of the Upper Shihezi Formation in study area. Sedimentary environments of various scales Continental Fluvial and facies lacustrine depositional system

Transitional Fluvialfacies controlled shallow-water deltaic depositional system

Fluvial facies

Sedimentary facies Channel Floodplain Levee

Channel lag, point bar Flood plain, flood lake, marsh Natural levee, crevassesplays Lacustrine Lacustrine Lake delta facies shore Shallow lake Semideep lake Deep lake Delta Delta Distributary channel, facies plain natural levee, crevase splay, interchannel, marsh Delta Subaqueous distributary front channel, subaqueous natural levee, subaqueous interchannel, river mouth bars, distal bar, front sheetsand Prodelta

518

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

3. Typical sedimentary facies The Permian tight sandstone reservoirs mainly occur in the lower and upper parts of the Shihezi Formation. The Lower Shihezi Formation was formed in a fluvial-dominated delta system with a shallow-water depth, whereas the Upper was deposited in continental environments. The upper Permian system in this area is discontinuous due to regional tectonic movements. The Shanxi and Shihezi formations consist mainly of five sedimentary facies

which represent various fluvial-deltaic environments (Table 1; Fig. 2). 3.1. Channel lag facies The channel lag deposits mainly consist of coarse bedload gravels on the channel scour surfaces (Fig. 3). The deposits are mainly composed of terrigenous gravels such as flint, quartz, and rock debris, and mudstones and coals as well. The gravels are generally poorly

Fig. 2. Brief stratigraphy, sedimentary facies, and well-logging (GBG2, Upper Shihezi Formation). 1. pebbly sandstone; 2. conglomerate; 3. sandstone; 4. sandy mudstone; 5. carbonaceous mudstone; 6. calcareous sandstone; 7. aluminum mudstone; 8. coal seam; 9. horizontal bedding; 10. ripple cross-bedding; 11. trough cross-bedding; 12. Planar cross-bedding; 13. erosion surface.

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

519

0 10

0

Q

0

10 0

25

75

50

50

25

75

Fig. 3. Conglomerate and sandstone of the channel lag deposits at the bottom of the Upper Shihezi Formation.

F 0

3.2. Point bar facies Point bars are the major fluvial sediments. The point-bar sand bodies occur mainly in the Shihezi Formation. Channel lag and point bar facies formed the main succession of the fluvial systems, which mainly consist of large-scale tabular, festoon, and wedge-shaped cross beddings (Fig. 4). The scales of the cross beddings become smaller and the grain size of sandstones becomes finer upward in the fluvial succession. The point-bar sandstones generally show a planar geometry and a scour base above which the channel lag deposits often occur.

3.3. Lacustrine delta and shoreland facies

100 L

75

Fig. 5. Triangular plot of the sandstone composition in the Lower Shihezi Formation in Bohai Bay Basin (after Zhang et al., 2010). Q = quartz, F = feldspar; L = lithic fragment.

3.4. Crevasse splay facies The crevasse splay deposits consist mainly of light-gray to gray or varicolored sandstone with fine to medium grain size. It is characterized by low maturity, poor roundness, and poor sorting. This facies often shows normal grading with an erosional base. The crevasse splay facies usually occurs in a set of fine-grained facies association, such as banks, flood plains, or flood basins, with lenticular shape in cross section and blanket shape in bedding plane. Generally, the crevasse splay sand body becomes thinner from fluvial to the flood plain. 4. Petrology characteristics of sandstone reservoirs Analyses of 352 reservoir cores from 42 wells reveal that reservoir rocks in the Lower Shihezi Formation and Upper Shihezi Formation of the Bohai Bay Basin consist mainly of various sandstones such as quartz sandstone, feldspathic-quartz sandstone, lithic-quartz sandstone, lithic sandstone, and tuff sandstone, and a small portion of quartz greywacke and feldspathic-quartz greywacke. Quartz sandstone is dominant in the Lower and Upper Shihezi Formation (Figs. 5 and 6). The reservoirs of the Lower Shihezi Formation are mainly coarsegrain quartz sandstones that are composed of quartz (72%–95%),

Q 10 0 75

F 0 Fig. 4. Point bar with wedge-shaped cross beddings in the Wanshan Section of the Upper Shihezi Formation.

0

1 00

25

75

50

50

25

This facies is characterized by large-scale sand bodies which are mainly lenticular in shape. It was formed by lateral accretion of the point bars. Partly blanket-shaped sand bodies occur and were probably formed in fluvial-lacustrine systems. Alluvial and crevasse splay sediments locally occur. This facies association is widely distributed in the Upper Shihezi Formation, which was formed in fluvial-delta-lake shoreland and large lacustrine delta systems. However, it cannot be commonly found due to the erosion during tectonic movement.

50

0

sorted, although some are well sorted and imbricated. The channel lag sandstone bodies are usually lens-shaped, occurring at the bottom of fluvial sequences and underlying the point-bar and channel-fill sand bodies. Many lag sand bodies with lower scour surfaces occur in the Lower Shihezi Formation.

25

25

50

75

100 L

Fig. 6. Triangular plot of the sandstone composition in the Upper Shihezi Formation in Bohai Bay Basin (after Zhang et al., 2010). Q = quartz, F = feldspar; L = lithic fragment.

520

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

Fig. 7. Characteristics and types of the quartz. a. Jigsaw contact of quartz grains due to corrosion and secondary enlargement of quartz. Rock sample is from the sandstones of the Upper Shihezi Formation in Gubeigu2 Well, Jiyang Depression. The sample is 3458.1 m in depth. (perpendicular polarized light, field of view is 1.2 mm wide.). b. Volcanic type quartz in the lithic greywacke. The sample is from the sandstones of Lower Shihezi Formation in T92-1 Well, Tengxian Coalfield. The sampling depth is 336.50 m. (Perpendicular polarized light, the field of view is 0.8 mm wide.). c. Two reincarnation quartz in the fine-grained feldspathic lithic greywacke The sample is the sandstones from the Upper Shihezi Formation in T92-1 Well, Tengxian Coalfield. The sampling depth is 425.75 m. (Perpendicular polarized light, the field of view is 0.5 mm wide.). d. Secondary enlargement of quartz and argillaceous matrix. The sample is the sandstones from the Upper Shihezi Formation in L8-3 Well, Juye Coalfield. The sampling depth is 906.00 m. (plane polarized light; the field of view is 1.0 mm wide.).

feldspar (0–16%), and lithic fragments (5%–12%) (Fig. 5). The quartz is commonly monocrystalline. Lithic fragments contain mainly metamorphic and sedimentary rocks. The matrix (5%–10%) is composed of clay and siltstone. The cements consist of authigenic quartz, authigenic kaolinite, chlorite, illite, and carbonate (mainly ferrocalcite and calcite). Pyrite and authigenic quartz occasionally occur. The sandstone is characterized by poor sorting, moderate rounding, low textural maturity, and relatively high compositional maturity. The sandstone bodies in the Upper Shihezi Formation consist mainly of coarse sandstones, gravelly coarse sandstone, and partly fine-pebble conglomerate. The sandstones are composed mainly of quartz (60%–95%, ca. 80% on average) and lithic fragments (mainly quartzite, 5%–30%), and a small portion of feldspar (0–10%) (Fig. 6). The quartz is monocrystalline. The matrix (10%–15%) of the sandstone is mainly composed of clay and siltstone to fine sandstone. Cement consists of authigenic quartz, authigenic kaolinite, chlorite, and carbonate (mainly ferrocalcite and calcite). The sandstones are characterized by poor sorting, moderate rounding, low textural maturity, and high compositional maturity.

from sedimentary rocks, and wavy extinction, which does not contain fluid inclusions; (4) Polycrystalline quartz comes mainly from metamorphic rocks and quartz veins. The quartz from sedimentary rocks comes mainly from the remains of the non-siliceous-cemented sandstones, which is characterized by the residual secondary concrescence or poor roundness and secondary concrescence. 4.1.2. Feldspar Feldspar (5–15% on average and locally up to 25%) in the study area belongs to alkali feldspar group. The microcline, orthoclase, and acid plagioclase are commonly found except for the perthite and basic plagioclase. As a result of the secondary alternation such as kaolinization, sericitization and carbonatization, pseudomorphs of

4.1. Composition of clastic grains 4.1.1. Quartz The detrital quartz is mostly monocrystalline and less commonly polycrystalline, and rarely chert. The monocrystalline quartz was mainly derived from igneous rocks, and only a small portion from sedimentary and metamorphic rocks (Fig. 7). The characteristics are as follows: (1) The monocrystalline quartz of granite contains abundant gas and liquid inclusions; (2) The monocrystalline quartz of volcanic rocks shows embayment corrosion borders; (3) The monocrystalline quartz of metamorphic rocks is characterized by abrasion secondary concrescence of quartz edge, similar to the quartz

Fig. 8. The biotite was compacted and broken off and feldspar was taken the place of calcite due to metasomatism. The sample is from the sandstones of the Upper Shihezi Formation in Yi139 Well; Jiyang Depression. The sampling depth is 4075.9 m. (Perpendicular polarized light; the field of view is 1.0 mm wide.).

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

521

4.2. Fillings In the study area, the fillings of clastic rocks consist mainly of clay matrix and a small portion of siliceous, calcareous, and authigene cements. The degree of rock cementation is very high, especially when the cements are ferrocalcite, ankerite, and clay mineral. The strong cementation resulted in poor rock property.

Fig. 9. Mudstone rock detritus was pushed into the pore of rigid fragments, resembling matrix. The sample is from the sandstones of the Upper Shihezi Formation in Gubeigu1 Well; Jiyang Depression. The sampling depth is 4075.1 m. (Perpendicular polarized light; the field of view is 1.2 mm wide.).

feldspar detritus commonly occur (Fig. 8). Sometimes anorthose was changed with a high degree of carbonatization. The feldspar crystals were usually deformed by strong compaction. The deformation types include the bicrystal curving, rugged contact surface, and fragmentary fractures.

4.1.3. Lithic fragment Lithic fragments consist mainly of chert and mudstone detritus in the Lower Shihezi Formation (Fig. 9). The major lithic fragments in the Upper Shihezi Formation are intermediate-acid extrusive rocks and cinerite. Metamorphic rock detritus locally occur, which include epirock quartzite and shale detritus. When the fragments of mudstone and siltstone are pushed into the pores between rigid fragments by pressure, they resemble matrix.

4.2.1. Matrix Matrix (7% on average) is composed of illite and silt. Clay matrix belonging to the positive matrix is usually recrystallized, which displays scaly structures (Fig. 10a) and distributes around the clasts or parallel to the direction of clasts. The fine or finer sand found in some coarse-grained sandstone is also regarded as matrix according to the hydrodynamic force (Fig. 10b). 4.2.2. Cements Siliceous, calcareous, and chlorite cements often occur. The siliceous cements bear three features, i.e., secondary enlargement, granulous authigene quartz, and minicrystal to cryptocrystalline chalcedony. The cements include micrite muscovite quality, fine grain and granular interlocking texture. Authigenic chlorite distributes around the detrital grain liking skinny, which is the product of the early diagenesis (Fig. 10c). The crystal forms of authigenic kaolinite resemble book shapes or pseudo-hexagonal lamella under the scanning electron microscope (Fig. 10d). 4.3. Maturity grade and granulometric analysis 4.3.1. Sampling Sandstones of Kuishan Section in the Upper Shihezi Formation were sampled in the Zhanhua Sag of Jiyang Depression. The sample facies include the fluvial channel fillings, flood plain, the combination of river channel and marginal bank, and crevasse splay as well.

Fig. 10. Characteristics of fillings. a. Clay matrix, showing flake structure for recrystallization. The sample is from the sandstones of the Upper Shihezi Formation in the Gubeigu2 Well, Jiyang Depression. The sampling depth is 3519.1 m. (Plane polarized light; the field of view is 1.2 mm wide.). b. Fillings of sand matrix and calcite among the grains. The sample is the sandstones from the Upper Shihezi Formation in the Gubeigu3 Well, Jiyang Depression. The sampling depth is 4163.8 m. (Plane polarized light; the field of view is 6.0 mm wide.) c and d. Authigenic kaolinite in sandstone, which resembles hexagon schistose or a collection of books. The sample is from the sandstones of the Upper Shihezi Formation in the Gubeigu1 Well, Jiyang Depression. The sampling depth is 4075.1 m.

522

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

99.99

(a) fluvial channel fill deposition

99.99

99.95 99.9 99.8 99.5 99 98

(Gubeigu2)

99.95 99.9 99.8 99.5 99 98

95

95

90

90

80 70 60 50 40 30

80 70 60 50 40 30

20

20

10

10

5

5

2 1 0.5 0.2 0.1 0.05

2 1 0.5 0.2 0.1 0.05

3519.60 3520.50 3521.50

depth:

(b) flood plain (Gubeigu3)

depth:

4085.10 4088.85 φ

0.01

0.01 -2 99.99

0

2

4

6

0

8 99.99

(c) the assemble both river channel and

99.95 99.9 99.8 99.5 99 98 95

95

90

90

80 70 60 50 40 30

80 70 60 50 40 30

20

20

10

10

5

5

2 1 0.5 0.2 0.1 0.05

2 1 0.5 0.2 0.1 0.05

4090.00 4091.20 φ

0.01 0

2

4

4

6

8

(d) crevasse splay (Gubeigu3)

99.95 99.9 99.8 99.5 99 98

marginal bank (Gubeigu3)

depth:

2

6

8

depth:

4075.20 4076.00 4077.00 φ

0.01 -2

0

2

4

6

8

Fig. 11. Probability curve of different depositional facies (well GBG1, 2, and 3).

4.3.2. Analysis The reservoirs of the Kuishan Member in the Upper Shihezi Formation consist mainly of medium and coarse sandstones. The average sorting coefficient of the sandstones is 1.610–2.666. The sorting is related to the sedimentary facies and granularity; the

sorting of fine sandstones is better than that of coarse sandstones. The sorting of the sandstones in the river channel fillings are better than that in the distributary channel plain in delta. The sands are mainly subangular, and less commonly subrounded to subangular. The tight degree can be reflected by the contact between sandstone grains. The

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

523

Fig. 12. Wreckage feldspar was formed by corrosion, in which intragranular pores exist. The sample is from the Kuishan Member of the Upper Shihezi Formation in the Yi136 Well, Jiyang Depression.

fillings of the river channel are medium-coarse sandstones. The suspended population of the sandstones of the river channel is greatly large on the cumulative curve in the grain size arithmetic probability graph. The point of intersection between the leap content curve and the suspension content curve is 2.0φ–3.0φ, and the C.T of the leap content is 0–1.0φ. The inclined position of the leap content is between 60°–65°. The curve has positive skewness and sharp kurtosis (Fig. 11a. ①②③). The cumulative curve in the grain size arithmetic probability graph of the flood plain sandstone can be divided into two parts. The point of intersection between the leap content and the suspension is 2.0φ–3.0φ. The leap content is 0–0.5φ. The inclined position of the leap content is 45°–65°. The curve has positive skewness and sharp kurtosis (Fig. 11b. ①②). The sandstones of the river channel and marginal bank contain medium-coarse grains, whose curve of leap content can be divided into two parts. The point of intersection of the leap content and the suspension is 2.0φ–3.0φ, and the C.T of the leap content is 0–0.5φ. The inclined position of the leap content is 45°–65°. The curve has positive skewness and sharp kurtosis (Fig. 11c. ③④). The splay sandstones contain gravels, and the curve of the leap content can be divided into two parts. The point of intersection of the leap content and the suspension is 0–3.0φ, and the leap content is 0– −1.0φ. The inclined position of the leap content is 45°–65°. The curve has positive skewness and sharp kurtosis (Fig. 11d. ①②③).

4.3.3. Results Based on the petrographic characteristics, the Kuishan Member in the Upper Shihezi Formation is characterized by: (1) quartz sandstone, greywacke, fine sandstone, sandy mudstone, and pebbly sandstone; (2) moderate to poor sortings; (3) subrounded to

Fig. 14. Porosity and depositional facies of sandstone reservoir in the Upper Shihezi Formation of Chegu1 well (After Zhang et al., 2009a, 2009b).

subangular shapes, and mainly the pore-throats support; (4) low compositional maturity, moderate textural maturity. 5. Microscopic pore structures of sandstone reservoirs According to the casting sheet image, thin section, and electron micrographs, the reservoir spaces are mainly secondary pores. The main types of the reservoir pore spaces of the study area are detailed beneath. 5.1. Intergranular pores The primary intergranular pores are rarely preserved due to the intensive diagenetic alternation of the sandstones. Most of the intergranular pores were formed after the diagenesis due to the corrosion of the matrix and the cement between the primary intergranular pores. The primary pores can be classified into three groups based on the pore diameters.

Fig. 13. Fractures. a. Structural fracture among the sandstone grains of CG31 Well. The sampling depth is 2220.1 m (the field of view is 1.0 mm wide.); b. Microfracture formed by the diagenesis of GBG3. The sampling depth is 4087.5 m.

524

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

permeability(10-3 µm-2)

1.2

y = 0.0566x - 0.1183 2 R = 0.2896

1 0.8 0.6 0.4 0.2 0

0

2

4

6

8

10

12

14

porosity(%) Fig. 15. The relation between permeability and porosity (samples from the Permian strata in Bohai Bay).

(1) Intergranular dissolved pores were distributed between particles and formed by the corrosion of the intergranular material (matrix and cement). The sizes, shapes, and structures of these pores are same as the primary pores because the granules were not affected by corrosion. The borders of these pores are normal and the radiuses of the pore throats are small. (2) The architecture of necked intergranular dissolved pores resembles the primary pores. These pores formed during further diagenesis. During this period, the shapes of the granules changed as a result of the chemical compaction and cementation at the contact points of granules. Consequently, the normal intergranular dissolved pores and the primary pores easily became necked. (3) Enlarged intergranular dissolved pores were formed by corrosion of the interstitial material and the particle margins. Although it was mainly distributed in the intergranular spaces, these pores influenced the granules. Evolved from the preexisting pores, these kinds of pores were characterized by

Fig. 16. The distribution of the sandstones in the Lower Shihezi Fomation.

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

broad pore throats and continuous distribution. According to the interstitial materials, these pores can be classified into three types, i.e., intergranular calcite-dissolved pores, intergranular dolomite-dissolved pores, and argillaceous matrix pores. 5.2. Intragranular pores Intragranular pores are all secondary and mainly formed by corrosion of the inner part of unstable particles such as feldspar, mica, lithic fragments, and siderite. Intragranular pores developed in the feldspars were mainly formed by corrosion of the particles along the cleavage planes (Fig. 12). The moldic pores were formed by dissolution of particles or crystals, which retained the same shape as the preexisting particles. Muddy incrusts occur around the pores, and insoluble residues often remain inside the pores.

525

0.162 × 10 −3μm −2. In the Luxi areas, the porosity of sandstones is below 5% and the permeability is below 1 × 10 − 3 μm 2. Second, the pores are typically narrow void and their structures are very complicated (Liu et al., 2007a, 2007b). In the reservoirs of the Lower and Upper Shihezi Formation, the average throat diameter is 2.92 μm and the widest is 7.79 μm (Zhang et al., 2009a, 2009b). Third, the material characteristics of reservoirs are related to the rock texture and depositional environment, and the permeability is with positive correlation to the porosity (Fig. 15). When the sandstone thickness is over four meters, the reservoirs are more efficient (Zhang et al., 2009a, 2009b). However, the materiality of single-stage river sandstones, whose depositional environment is the facies of abandoned channel, is poorer than that of multi-stage, whose depositional environment is folded channel. Above all, although the influencing factors are complicated, the type of river channel, the tectonic movement, and the water dissolution degree are the main controlling conditions.

5.3. Micropores and cracks

6.2. Controlling factors of reservoirs

Micropores exist between kaolinites after the corrosion of feldspars, goeschwitzites, and didrimits. Cracks are all secondary, including large-scale intrastone cracks and small-scale intragranular cracks; the latter are mostly microcracks (Fig. 13).

Generally, the poor characteristics reservoirs cannot form coalformed gas field. In the study area, how could good gas fields form? Although the conditions of coal-formed gas reservoirs are generally very poor, the conditions in certain areas could be better due to particular geological setting. Then, what are the special geological settings? Three main reasons are concluded. First, the solution pores were formed by the chemical dissolution. Compaction barely influences the materiality of sandstone reservoirs in areas with a deep burial depth (e.g. Jiyang Depression, Linqing Depression and Jizhong Depression) (Zhang et al., 2009a, 2009b). The cements among the debris particles of sandstones are dissolved to form the secondary interstices and vuggy pores. Thus, the materiality is improved. Second, many fractures and intergranular pores were formed by the tectonic evolution and filling processes. Taking Luxi Areas as an example, the thickness of fractured strata reaches to 80 m and the width of these fractures is 1–4 mm. These fractures and pores can improve the conditions of storage and permeability (Li et al., 2006). Meanwhile, the surfaces of unconformity formed by the regional tectonic movements greatly influence the materiality of reservoirs. Liu et al. (2007a) studied the reservoirs of Suqiao Gas Field in the Jizhong Depression. He found that the reservoir, medium- to coarse-grained sandstones of the Upper Shihezi Formation, was buried in depth between 3100 and 3900 m, which is only 20–130 m below the surfaces of unconformity of the Mesozoic or Cenozoic group. Due to

6. Materiality characteristics and controlling factors of reservoirs 6.1. Materiality characteristics The reservoir rocks of coal-formed gas are mostly tight sandstones. Micropore and secondary interstice are the main reservoir pore space; the primary porosity was not developed. Although poor poroperm characteristics compared to the sandstones of conventional gas, these tight sandstones may form the unconventional gas resources of high industrial value. Due to intensive long-term compaction and cementation during the late diagenesis, most primary pores were nearly completely filled (Fig. 14). There are several characteristics showing the poor reservoir materiality. First, the primary pores disappeared and secondary pores become the major spaces. According to the analysis of the core materiality characteristics, in the Bohai Bay Basin distribution, the porosity is 1.2%–11.6%, with an average porosity value about 5.76%. Most samples' porosities are below 6%. The permeability is 0.01 × 10 − 3–0.675 × 10 − 3 μm − 2, with an average value below

Fig. 17. The A–A' cross section showing distribution of the Lower Shihezi Formation sandstones (Linqing Depression).

526

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

adjacent areas not only adds a great amount of gas reserve in North China, but also strongly indicates the overlook potential of both tight sandstone reservoirs rock and sweet spots in it. So we accomplished the study of the reservoirs distribution in order to point to the direction of coal-formed gas explorations.

the chemical eluviation near the weathered crust, many feldspar particles were dissolved and formed the intergranular dissolved pores, which makes better reservoirs characteristics. Third, the sedimentary environments have a great influence on the reservoir characteristics. Thick sandstones of overlain channel floor lag facies have their own materiality in different positions; the central part is better than the upper and lower parts.

7.1. Lower Shihezi Formation 7. Distribution of reservoirs The sandstone reservoirs facies of the Lower Shihezi Formation consist of channel floor lag facies, marginal bank facies, and levees and crevasse-splay facies. The sandstone reservoirs of the Lower Shihezi Formation are generally restrictedly distributed, except that the thick sandstone reservoirs are widely distributed. Especially in Jizhong and Bozhong depressions, the thickness can reach up to 120 m (Figs. 16 and 17). Thus, these areas can be the exploration targets. In Luxi Areas, as the thickness of sandstone reservoirs is less than 50 m and the thinnest down to 10 m, the place is not suitable for exploration.

After exploration and research for a long period, we found that the reservoirs are sandstones of the Upper Shihezi Formation, and early exploration and development in the research area mainly concentrated on Jiyang Depression and Jizhong Depression. We have not found the gas reservoirs showing gas of the Lower Shihezi Formation. Many of the potential producing areas were not available because of the lack of knowledge of the gas reservoirs in the Lower and Upper Shihezi Formation. Now, the discovery of the Bohai Bay Basin and

N

Beijing

China

Ya

ns

Beijing

ha

t

plif

n

Liaodong Uplift

Up

lif

t

Tangshan

d-U Fol

n

Tianjin

tai

n ou

Baoding

0

40

ih

an

g

60

M

Ta

40

Shijiazhuang

0

Huanghai Sea Bohai Bay 0

40

0

A

0

0 40 60 80 100

60 40 40

0 40

60

80 60

40

80

0

Xingtai

40 60

80

80

80

60 40 0

80 60 40

0 40

40

Zibo Weifang

100 80

Ludong Uplift

40

60

Handan A'

100

80

Liaocheng

Jinan 40

20

100

40

80

40

60

Laiwu

80

0

0

40

20

Taian 40 100 80

40

60 30

Puyang

20

Wenshang Ningyang

0

20

40

60

Taian

60 80

Sandstone contour

20

60

20

60 40

40

Jining 40

20

Heze

60 40

0

40

Inferred sandstone contour

City

20

60

40

80

60

0

60

20

Zaozhuang

Shangqiu

Above 60m

Fault

Fig. 18. The distribution of the Upper Shihezi Formation sandstones.

Below 40m

the boundary of sea or river

40-60m

stratigraphic boundary

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

527

Fig. 19. The A–A' cross section showing distribution of the Upper Shihezi Formation sandstones (Linqing Depression).

7.2. Upper Shihezi Formation The sandstone reservoirs distribution of the Upper Shihezi Formation, zoned in north–south and distributed in east–west, is significantly different from the one of the Lower Shihezi Formation. The facies of sandstone reservoirs include channel floor lag and shore lacustrine facies. The sandstone reservoirs in the Jiyang Depression are the thickest (up to more than 60 m) (Figs. 18 and 19). The sandstone reservoirs in Luxi areas are thinner than that of Bohai Bay Basin due to the late tectonic movement. Only in the north of the Jiaxiang Depression and Jining area the thickness of sandstone reaches 60 m, and thickness of sandstones in other areas is between 20 and 60 m. On the whole, sandstones with best quality are in the Lower Shihezi Formation. 8. Conclusions (1) The Permian coal-formed gas reservoir sandstones in the study area were mainly deposited in the fluvial and lacustrine depositional systems, including the typical sedimentary facies such as channel lag, point bar, lacustrine delta and shoreland, and crevasse splay facies. (2) The main reservoir type is sandstone, and its maturity gradually increases from the Lower Shihezi Formation to the Upper Shihezi Formation. The sandstones of channel lag facies in the Shihezi Formation comprise the main reservoir. The medium to coarse sandstones are composed of subangular sands, with medium textural maturity. This type of sandstone reservoir is distributed extensively. The fillings of the sandstones are clay matrix with a small portion of calcareous and siliceous cement. (3) The primary intergranular pores are very minor due to the intensive diagenetic alternation. The reservoir spaces are mainly secondary pores. Most of intergranular pores are formed by the late dissolution. These secondary intergranular pores, and the matrix and cement in the intergranular pores formed during the dissolution. The sandstones formed low porosity and poor permeability reservoirs due to the poor physical property. (4) The reservoirs of the Lower Shihezi Formation, which are mainly distributed in the northern area of the Jizhong and Bozhong depressions, and become smaller in the Luxi Areas to the south. The sandstone reservoirs of Upper Shihezi Formation, zoned in north–south and distributed in east–west, are

significantly different from those of the Lower Shihezi Formation. The sandstone reservoirs in the Jiyang Depression are the thickest, whereas the sandstone reservoirs in the Luxi areas are thinner than those of the Bohai Bay Basin due to late tectonic movement. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 40872100), and the National Basic Research Program of China (“973” Program) (Grant No. 2003CB214608). We are grateful to Sheli Oil Field for providing fund for this work. We thank H. Zhao and Q. Jia for useful discussions. We warmly acknowledge the two anonymous reviewers for the critical comments to the early version of the manuscript. References Aizenshtat, Zeev, Feinstein, Shimon, Miloslavski, Irena, Yakubson, Zoya, Yakubson, Christof I., 1998. Oil–oil correlation and potential source rocks for oils in Paleozoic reservoir rocks in the Tataria and Perm basins, Russia. Org. Geochem. 29 (1–3), 701–712. Anna, L.O., 2003. Groundwater flow associated with coalbed gas production, ferron sandstone, east-central Utah. Int. J. Coal Geol. 56 (1–2), 69–95. Chen, B., Xu, B., 2003. Identification of fracture and gas-bearing bed in hypercompact sandstone reservoir. Well Logging Technology 27, 136–139 (in Chinese with English abstract). Dai, J., 1979. Petroleum and natural gas generation in coalification. Pet. Explor. Dev. 6 (3), 10–17 (in Chinese). Dai, J., 2007. Potential areas for coal-formed gas exploration in China. Pet. Explor. Dev. 34 (6), 641–645 (in Chinese with English abstract). Dai, J., 2009. Major developments of coal-formed gas exploration in the last 30 years in China. Pet. Explor. Dev. 36 (3), 264–279. Dai, J., Qi, H., Wang, S., Song, Y., Guan, D., Xu, H., 2001. Geochemical Features of Hydrocarbon from Coal-measure, Formation and Resource Evaluation of Coalformed Gas Reservoir in China. The Petroleum Industry Press, Beijing. (in Chinese). Fu, J., Liu, D., Sheng, G., 1990. Geochemistry of Coal-generated Hydrocarbons. Geological Puhlishing House, Beijing, pp. 348–355 (in Chinese with English abstract). Imam, M.B., Shaw, H.F., 1987. Diagenetic controls on the reservoir properties of gas bearing Neogene Surma Group sandstones in the Bengal Basin, Bangladesh. Mar. Pet. Geol. 4 (2), 103–111. Jin, Z., Liu, C., 2008. Quantitative study on reservoir diagenesis in Northern Dagang Structural Belt. Huanghua Depress. Pet. Explor. Dev. 35 (5), 581–587 (in Chinese with English abstract). Karim, A., Pe-Piper, G., Piper, J.D.W., 2010. Controls on diagenesis of Lower Cretaceous reservoir sandstones in the western Sable Subbasin, offshore Nova Scotia. Sediment. Geol. 224 (1–4), 65–83. Lee, K.Y., 1989. Geology of petroleum and coal deposits in the North China Basin, Eastern China. Department of the Interior Manuel Lujan, Jr., Secretary, pp. 1–35. Li, Z., Liu, H., Yu, J., Fang, Q., Li, J., Liu, X., 2006. Sedimentary study of coal-formed gas reservoir sediment of Carbonic-Permian system in Jiyang Shandong. Acta Sedimentol. Sin. 24 (4), 505–510 (in Chinese with English abstract).

528

D. Lv et al. / Journal of Petroleum Science and Engineering 78 (2011) 516–528

Li, Z., Zhang, S., Liu, H., Fang, Q., Han, M., 2007. Deposition, stratigraphic sequences and spatial distribution of coal gas reservoirs—a case study of the Gubei block in Jiyang. Geol. Chin. 34 (4), 612–621 (in Chinese with English abstract). Liu, C., Jiang, W., Sang, S., Zhu, Y., Huang, H., 2007a. Depositional controlling of PermoCarboniferous source rocks in Bohaiwan Bay Basin. Coal Geol. Chin. 19 (1), 4–8 (in Chinese with English abstract). Liu, G., Zheng, L., Zhang, Y., Qi, C., Chen, Y., Peng, Z., 2007b. Distribution and mode of occurrence of As, Hg and Se and Sulfur in coal Seam 3 of the Shanxi Formation, Yanzhou Coalfield, China. Int. J. Coal Geol. 71, 371–385 (in Chinese with English abstract). Liu, D., Yao, Y., Tang, D., Tang, S., Che, Y., Huang, W., 2009. Coal reservoir characteristics and coalbed methane resource assessment in Huainan and Huaibei coalfields, Southern North China. Int. J. Coal Geol. 79, 97–112. Lv, D., Li, Z., Fang, Q., Wu, L., Liu, H., Li, W., Liu, X., 2008. Coal gas sandstone reservoir study of Upper Shihezi formation of Permian in Jiyang Depression, Shandong Province. J. Chin. Univ. Min. Technol. 37 (3), 389–395 (in Chinese with English abstract). Peng, P., Zou, Y., Fu, J., 2009. Progress in generation kinetics studies of coal-derived gases. Pet. Explor. Dev. 36 (3), 297–306 (in Chinese with English abstract). Purvis, Kevin, 1992. Lower Permian rotliegend sandstones, southern north sea: a case study of sandstone diagenesis in evaporite-associated sequences. Sediment. Geol. 77 (3–4), 155–171. Querol, X., Alastuey, A., Lopezscoler, A., Zeng, Rongshu, Zhao, Jiehui, Zhuang, Xinguo, 1999. Geological controls on the quality of coals from the west Shandong mining district, Eastern China. Int. J. Coal Geol. 42, 63–68. Ritts, Bradley D., Keele, Dustin, Darby, Brian J., et al., 2006. Ordos Basin gas reservoir outcrop analogs: Permian braided fluvial sandstone of the Zhuozi Shan and Helan Shan, China. Int. Geol. Rev. 48, 573–584. Song, Y., Liu, S., 2008. A transfer of thinking in natural gas exploration in China. Nat. Gas Ind. 28 (2), 12–16 (in Chinese with English abstract). Song, Y., Chen, M., Qin, S., Qiu, M., 2004. Progress in the study of natural gas in China. Pet. Explor. Dev. 31 (1), 8–11 (in Chinese with English abstract). Su, X., Lin, X., Liu, S., Zhao, M., Song, Y., 2005. Geology of coalbed methane reservoirs in the Southeast Qinshui Basin of China. Int. J. Coal Geol. 62, 197–210. Waiter, B., Ayers, J.R., 2002. Coalbed gas systems, resources and production and a review of contrasting cases from the San Juan and powder river basins. AAPG 86 (11), 1853–1891. Wang, Q., Song, Y., Zhang, S., 1994. The study on the methods used to delineate the deep tight gas bearing sands in Daqing Oil Field. J. Daqing Pet. Inst. 18 (2), 22–25 (in Chinese with English abstract).

Wilkins, Ronald W.T., George, Simon C., 2002. Coal as a source rock for oil: a review. Int. J. Coal Geol. 50, 317–361. Wolela, A., 2009. Sedimentation and depositional environments of the BarremianCenomanian Debre Libanose Sandstone, Blue Nile (Abay) Basin, Ethiopia. Cretac. Res. 30 (5), 1133–1145. Wu, Z., Tang, H., An, F., 2003. Causes of bedding fractures of tight sand gas-reservoir in Xinchang, West Sichuan region. Pet. Explor. Dev. 30 (2), 109–111 (in Chinese with English abstract). Yang, J., Hu, C., Shi, B., 1987. Symposium on Geological Research of Coal-formed Gas. The Petroleum Industry Press, Beijing. (in Chinese with English abstract). Yang, H., Fu, J., Wei, X., Liu, X., 2008. Sulige field in the Ordos Basin: geological setting, field discovery and tight gas reservoirs. Mar. Pet. Geol. 25 (4–5), 387–400. Yu, J., Li, Z., Wang, M., Liu, H., Wang, Y., Li, J., 2007. Correlation and analysis of well log sequences of epicontinental sea sediments of Permo-carboniferous in Luxi and Jiyang. J. Chin. Univ. Min. Technol. 36 (1), 85–89 (in Chinese with English abstract). Zhang, X., Zhung, J., Zhang, S., 2002. Coalbed Methane Geology and Resource Evaluation in China. Science Press, Beijing. (in Chinese). Zhang, Z., Long, S., Xu, H., 2006. Prospect of primary Palaeozoic oil–gas pools in eastern exploration region, Sinopec. Pet. Explor. Dev. 33 (4), 420–424 (in Chinese with English abstract). Zhang, G., Chen, S., Wang, H., 2009a. Characteristics and controlling factors of Permian reservoirs in Jiyang Depression. Pet. Explor. Dev. 36 (5), 575–582 (in Chinese with English abstract). Zhang, S., Mi, J., Liu, L., Tao, S., 2009b. Geological features and formation of coal-formed tight sandstone gas pools in China: Cases from Upper Paleozoic gas pools, Ordos Basin and Xujiahe Formation gas pools, Sichuan Basin. Pet. Explor. Dev. 36 (3), 320–330 (in Chinese with English abstract). Zhang, S., Tang, S., Tang, D., Pan, Z., Yang, F., 2010. The characteristics of coal reservoir pores and coal facies in Liulin district, Hedong coal field of China. Int. J. Coal Geol. 81 (2), 117–127. Zou, C., Tao, S., Zhu, R., Yuan, X., Liu, W., Zhang, G., Zhang, X., Gao, X., Liu, L., Xu, C., Song, J., Liu, G., 2009. Formation and distribution of “continuous” gas reservoirs and their giant gas province: a case from the Upper Triassic Xujiahe Formation giant gas province, Sichuan Basin. Pet. Explor. Dev. 36 (3), 307–319 (in Chinese with English abstract).