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Characteristics and genesis of the (Sinian) Dengying Formation reservoir in Central Sichuan, China
Journal of Natural Gas Science and Engineering 29 (2016) 311e321
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Characteristics and genesis of the (Sinian) Dengying Formation reservoir in Central Sichuan, China Zheng Zhou a, b, c, *, Xingzhi Wang a, *, Ge Yin c, Shuseng Yuan c, Shejiao Zeng b a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan China College of Geology & Environment, Xi0 an University of Science and Technology, Xi0 an, 710054, Shanxi China c Southeast Sichuan Geological Team of Chongqing Bureau of Geology and Minerals Exploitation, Chongqing 400038, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2015 Received in revised form 24 November 2015 Accepted 6 December 2015 Available online 7 January 2016
The Dengying Formation is an important stratum of exploration in the Gaoshiti-Moxi structure belt in the Sichuan Basin. Therefore, this study combines core, thin section, mud logging data and geochemical analysis (C, O and Sr isotopes) with seismic section data to investigate the basic characteristics and genesis of the Dengying Formation reservoir. Observations indicate that the reservoir rocks mainly consist of sand crumb dolomites, algae laminated dolomite and breccia. The reservoir space is dominated by porous dissolved holes, dissolved pores (or holes) along the direction of algal laminae, intra-grit holes and residual “grape lace” holes. The reservoir has the characteristics of low porosity and mediumpermeability. There are three types of reservoir: porous reservoir, fractured porous reservoir and cave reservoir. The reservoir was formed by the common action of deposition, diagenesis and tectogenesis. Sedimentation is the basis and presupposition for reservoir development, but the effective reservoir is mainly controlled by the algal flat microfacies; Furthermore, compaction, cementation and filling are the main causes of reservoir density; Recrystallization is the basis for the formation of porous reservoir; Supergene dissolution is the key to reservoir formation; Buried dissolution also promotes the formation of a high quality reservoir. The fractures formed by tectogenesis significantly improve the permeability of the reservoir and contribute very little to porosity. The reservoir gradually evolved and formed under the control of various geological and diagenetic activities. This study provides important information and references for oil and gas exploitation and development in central Sichuan. © 2016 Elsevier B.V. All rights reserved.
Keywords: Central Sichuan Dengying formation Reservoir Characteristics and genesis
1. Introduction The Dengying Formation of the Gaoshiti-Moxi structure belt is thick and extensive, with an average thickness of approximately 82 m and extends up to 320 m, and has abundant natural gas resources in central Sichuan (Hong et al., 2000); The GS1 well was completed in July 2011, and it is an important risk exploration well required to explore the oil and gas potential in the Sinian stratum of the Gaoshiti-Moxi structure belt. After the Dengying Formation was acidified, test results indicated that GS1 has produced a daily yield of one million cubic meters of natural gas. These results show the potential of having good oil and gas exploration prospects in the Dengying Formation of the study area. Therefore, the Dengying
* Corresponding authors. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China E-mail address: [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.jngse.2015.12.005 1875-5100/© 2016 Elsevier B.V. All rights reserved.
Formation is a notable successful stratum for oil and gas exploration in the Sichuan Basin. Previous geologists have conducted a tremendous amount of work on the Dengying Formation in the study area and have some understanding of the oil and gas geological conditions. The stratum is ancient, the burial depths vary widely and the diagenesis is variable (Wang et al., 1998; Liu et al., 2007), therefore, some debate about the characteristics and the genesis of the reservoir remains. These debates have led to uncertainty when making exploration and exploitation strategies. The oil and gas exploration and development of the Dengying Formation in the central Sichuan area is still in its infancy, and the degree of awareness about the stratum remains relatively low. Meanwhile, in the process of oil and gas exploration, elucidating the characteristics and genesis of the reservoir are not only important, but they also represent a key factor that directly impacts the success of the oil and gas exploration (Han et al., 2012). Therefore, we study the reservoir's genesis based on a comprehensive investigation of the reservoir's basic characteristics using
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new data from systematically coring wells of the Dengying Formation in central Sichuan and the surrounding area, including comprehensive observations of core, thin sections, cast thin sections, mud logging data and seismic section data with geochemical examinations (i.e., C, O and Sr isotopes). The aim of this paper is to investigate the basic reservoir characteristics, to evaluate the factors that influence reservoir quality and to provide theoretical support for oil and gas exploration and development in Central Sichuan. 2. Geological setting The area is under control of Longmen Mountain, uplift of the Chuanxi-central Sichuan basement and multi-Paleozoic tectogenesis, especially the Caledonian tectogenesis in the late Silurian period. The Leshan-Longnvsi paleo-uplift was formed in the Sichuan Basin and is an inheritance paleo-uplift. Whereas the Gaoshiti-Moxi structure belt (Fig. 1), as a latent structure belt of the Paleo-uplift, is in the east high-end area of the Paleo-uplift axial area (Zhang et al., 2004 and Fig. 1). The Sinian top boundary of the Gaoshiti-Moxi structure belt covers more than 1000 km2. Because of its location at the east high point area of the paleo-high shaft portion, conditions for hydrocarbon accumulation are favorable (Zhang and Tang, 1986; Hong et al., 2000; Chen, 2010). In the Upper Sinan, the Sichuan Basin was in a platform depositional environment and accumulated a set of dolomite domination strata with rich blue-green algae, the strata are the Dengying Formation. According to the algae content and lithology, the Dengying Formation was divided into four members from bottom to top (Fig. 1): Z2dn1, Z2dn2, Z2dn3 and Z2dn4. The Z2dn1 is poor in algae content, the Z2dn3 is poor in algae content with high mudstone content and the Z2dn2 and Z2dn4 are rich in algae content. The reservoir is mainly developed in Z2dn2 and Z2dn4 (Shi et al., 2010). During the depositional process, the Tongwan tectogenesis caused the Dengying Formation be exposed twice above sea level for a long time and be intensely transformed by meteoric fresh water (Wang et al., 2014). The Dengying Formation underwent a burial period after it was formed and has experienced seven major tectogenic events in the last 600 million years, the burial depths vary widely, and the diagenesis is variable making the reservoir's characteristics and genesis very complex.
3. Material and methods This study was based on samples from nine systematically cored wells in central Sichuan and the surrounding area, including GS1, Z2, Z6, Z4, MX8, GK1, W117, AP1 and MX9 wells. The cumulative length of the cored well sections is 821 m and consists primarily of two intervals, Z2dn2 and Z2dn4. A total of 1120 samples were selected for making thin sections, and some of them were impregnated with blue-dyed resin to examine the porosity characteristics of the rock more clearly. To investigate the reservoir origins, Eight samples (Table 2) from the MX8 well which was in different rocks (i.e. micritic dolomite, powder crystal dolomite, fine powder crystal dolomite, fine crystal dolomite and weathered residual breccia) were selected for carbon (C), oxygen (O) and Strontium (Sr) isotopic analysis, Six samples (Table 2) were selected for degree of rock order, we used the results to examined the influence of atmospheric fresh water and dolomite recrystallization on reservoir formation. To investigate the reservoir properties and types, Approximately 328 samples of horizontal core plugs whose diameter is about 2.5 cm from the GS1, the MX8, the AP1 and the GK1 wells were test educing the mercury intrusion method to analyze the reservoir properties, including porosity and permeability. We used the crossplot of porosity and permeability and mud logging data of the GS1, the MX8, the AP1 and the GK1 wells to study the reservoir types. Nearly 250 km of seismic section data were selected from central Sichuan and used to study the response characteristics of the weathering crust. According to these results, this study discusses the relationship between reservoir genesis and deposition, diagenesis and tectogenesis. 4. Results 4.1. Basic reservoir characteristics 4.1.1. Lithologies Visual inspection of more than 1100 thin sections from cores and core observations indicate that Dengying Formation rocks are mainly dolomites (including algae laminated dolomite, sand crumb dolomite, particle adhesion dolomite, mud crystal dolomite, powder crystal dolomite and fine crystal dolomite) and breccias (including weathered eluvial breccia, karst breccia and false karst
Fig. 1. Location of the study area and stratigraphic histogram of the Dengying Formation.
Z. Zhou et al. / Journal of Natural Gas Science and Engineering 29 (2016) 311e321
breccia). There are only a few mudstones rocks. The reservoir rocks are algal laminated dolomites, sand crumb dolomite and karst breccia. 4.1.2. Pore space Thin sections and core observations indicate that primary pores had disappeared today, the Dengying Formation reservoirs are formed after deposition and are dominated by porous dissolved holes (Fig. 2(a)), dissolved pores (or holes) along the direction of laminar algae (Fig. 2(b) and (c)), intra-grit holes (Fig. 2(d)) and residual.“grape lace” holes (Fig. 2(e)) comprise the main reservoir space, which accounts for 86.8% of the total reservoir space (Fig. 3). Intergranular dissolved pores (Fig. 2(f)), crystal pores (Fig. 2(f)), crystal dissolved pores and fracture dissolved holes account for 13.2% of the total reservoir space (Fig. 3). Additionally, there are some microfissures that have a certain reservoir performance, but the most important role of microfissures is to serve as oil and gas seepage channels to improve the permeability of the reservoir. 4.1.3. Reservoir properties We summarized the petrophysical properties (i.e., porosity and permeability) of 328 samples from the Dengying Formation (Fig. 4). The porosity averages3.86% and is mainly 2.00%e5.00%, accounting for 73.71% of all samples, whereas porosity over 5% only accounts for 12.50% of all of the samples. The permeability averages 27.07 103 mm2 and is mainly 10.0~100.0 103 mm2, accounting for 57.32%. These results indicate that the reservoir of Dengying
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Formation has the characteristics of low porosity and mediumpermeability.
4.2. Reservoir types Generally speaking, the permeability of porous reservoir increases when the Porosity increases, the correlation between them is good. The permeability of.fractured reservoir is high when the porosity is low, there is no correlation between them (Shi et al., 2010; Zhou et al., 2014). Therefore, we can qualitatively study the reservoir types according to the correlation characteristics between the porosity and permeability (Shi et al., 2010; Dai et al., 2007; Qiang, 2007). Fig. 5 illustrates a distribution pattern of porosity and permeability, mainly comprised of two regions: A and B. The region B shows clear positive correlation, highlighted by synchronous change of porosity (0.13%~9.88%) and permeability (0.01 103 mm2~8.78 103 mm2), the thin section analysis from samples indicates that the reservoir spaces are mainly crystal pores, crystal-dissolved pores and intergrannular dissolved pores that are evenly distributed and formed when the dolomites were recrystallized or dissolved. The region A features positive correlation too, the porosity distributed between 1.65% and 5.85%, with high permeability (9.80 103 mm2~8.78 103mm2), indicating the existence of fractures in the reservoir. The thin section and core analysis from samples indicates that the reservoir spaces are dominated by crystal pores, crystal-dissolved pores and
Fig. 2. Photomicrographs and core photos showing different reservoir space and reservoir types. (a) the Z4 well at a depth of 5486.00 m exhibited porous dissolved holes; (b) dissolved pores (or holes) along the direction of laminar algal developed in algal laminated dolomite, sample from the Z2 well at a depth of 5482.76 m; (c) the Z6 well at a depth of 5149.98 m exhibited dissolved pores (or holes) along the direction of laminar algae; (d) the MX8 well at a depth of 5406.00 m exhibited intra-grit holes. (e) the W117 well at a depth of 4986.60 m exhibited a residual “grape lace” hole half-filled by dolomite; (f) Intergranular dissolved pores developed in sand crumb dolomite, sample from GS1well at a depth of 4986.60 m; (g) crystal pores developed in powder crystal dolomite, sample from the Z2 well at a depth of 3706.20 m; (h) Crystal dissolved pores developed in fine crystal dolomite, sample came from the MX8 well at a depth of 5306.08 m; (i) the GK1 well at a depth of 5302.00 m exhibited fractured dissolved holes; (j) the MX8 well at a depth of 5113.05 m exhibited a fractured porous reservoir; (k) the AP1well at a depth of 5215.00 m exhibited a porous reservoir; (l) the Z6 well at a depth of 3790.58 m exhibited a cave reservoir.
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Fig. 3. Development frequency of different reservoir spaces within the Dengying Formation reservoir rock.
Fig. 4. The frequency distribution of porosity and permeability of reservoir rock of the Dengying Formation in central Sichuan.
In fact, there is cave reservoir which is composed of different seize dissolved holes in Dengying Formation. The samples can not possess cave reservoir because the diameter of horizontal core plugs is about 2.5 cm, in the progress of core observation, we found that there are some big dissolved holes that formed cave reservoir (Fig. 2(l)). Meanwhile, in the drilling progress, the slurry losses and drill pipe venting occurred, for example, the slurry losses when the GS1 were drilled in 5846.23 m, these indicates that there is cave reservoir in the study area. From the above analysis, we can see that pore space structures within the Dengying Formation reservoirs are quite complex and that reservoir genesis was influenced by many factors. Three types of reservoir occur in the Dengying Formation, comprised of fractured porous reservoir (Fig. 2(j)), porous reservoir (Fig. 2(k)) and cave reservoir (Fig. 2(l)).
5. Discussion Fig. 5. Relationship between porosity and permeability of the Dengying Formation reservoir rock in central Sichuan.
intergrannular dissolved pores, and contains a few micro fractures. Therefore, region A and region B may display different types of reservoir, indicative of fractured porous reservoirs and porous reservoirs respectively.
Generally speaking, the carbonate reservoir was formed by the common action of deposition, diagenes and tectogenesis (Brasher and Vagle, 1996; Wang et al., 1998; Qiang, 2007). In central Sichuan, the Sinian Dengying Formation reservoir is a typical carbonate reservoir (Xu et al., 2008; Wang et al., 2014, Chen et al., 2015), therefore, we respectively discuss the role of sedimentation, diagenesis and tectogenesis on the formation of reservoir.
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5.1. The impact of deposition on reservoir formation During the Dengying Formation sedimentation, only the sedimentation of Z2dn3 subfacies occurred on a deep water platform. The sedimentation of other subfacies occurred in a shallow water platform. The shallow water platform subfacies can be further divided into a shoal patch, an algal flat, a dolomitic flat and a mudflat microfacies (Fig. 1) (Zhou et al., 2014). Based on the observation of cores and thin sections, the different sedimentary microfacies have different reservoirs (Ma et al., 2006). The algal flat microfacies have more organic matter and many original pores, which is beneficial to late dissolution, easily forming various sizes dissolution holes, thereby forming cave reservoir. The dolomitic flat microfacies have many crystal pores where the dissolution is not good, the diameter of organic pores increased a little or not, thereby forming porous reservoirs or fractured porous reservoirs in conjunction with the fracture. Where the dissolution is good, there are many various sizes holes within the dissolution breccia, thereby forming cave reservoirs between the dissolution breccia. The shoal patch microfacies has many original pores, which are beneficial to late dissolution and easily formed various sizes dissolution holes, thereby forming cave reservoir. The mudflat microfacies are mainly distributed in Z2dn3. The lithology is mudstone rock, in which it is difficult for pores or holes to form during the dissolution process. Therefore, there is less oil and gas reservoir space. Based on physical property data analysis, porosity and permeability are markedly different in various deposits within microfacies. From the porosity histogram, the porosity and the permeability statistics of different deposits in central Sichuan (Table 1, Fig. 6) indicate that the maximum porosity of the shoal patch deposits is 8.59% and the minimum porosity is 0.13%, with an average of 3.34%, moreover, porosity of over 4% accounts for 33.34% of all of the samples; The maximum permeability of shoal patch deposits is 96.360 103 mm2, and the minimum is 1.110 103 mm2 with an average of 60.280 103 mm2. The maximum porosity of the algal flat deposits is 8.02%, and the minimum is 1.01%, with an average of 2.87%, porosity of over 4% accounts for 22.23% of all of the samples taken; The maximum permeability of algal flat deposits is 83.610 103 mm2and the minimum is 0.771 103 mm2 with an average of 58.252 103 mm2. The maximum porosity of the dolomitic flat deposits is 2.69%, and the minimum is 0.19%, with an average of 1.74%, and porosity of all samples are less than 4%; The maximum permeability of dolomitic flat deposits is 70.660 103 mm2, and the minimum is 0.001 103 mm2 with an average of 12.351 103 mm2. Simultaneously, in different sedimentary microfacies, the reservoir thickness and the range are different (Fig. 7). In the shoal patch microfacies, the reservoir thickness is bigger, and the reservoir distribution range is narrower and has a certain randomness. In the algal flat microfacies, the reservoir thickness is not as significant, and the reservoir distribution range is larger. In the dolomitic flat, the reservoir thickness is the smallest, and the reservoir distribution range is the largest. From the above analysis, we can see that the porosity and
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permeability of shoal patch deposits and the algal flat deposits are well developed, but the reservoir distribution range of shoal patch deposits is narrower and has a certain randomness. Overall, the effective reservoir of the Dengying Formation is mainly controlled by algal flat microfacies. 5.2. Compaction, filling and cementation and their impact on reservoir formation The porosity and permeability of reservoirs in the Dengying Formation are severely affected by compaction, filling and cementation (Dixon et al., 1989; Yang et al., 2004; Xu et al., 2008; Shi et al., 2010; Zhou et al., 2014). The Dengying Formation has been buried nearly thousands of meters deep. Under strong compaction and pre-solution, the original porosity of the reservoir dropped abruptly. Although presolution seams (Fig. 8(a)) were formed in the process of being buried, certain contributions were made to the reservoir. Generally, compaction causes the pores to decrease or disappear, which has become one of the most important contributors to reservoir density. The cementation process caused the chemical sediment to consolidate loose material in the primary pores. There were 2e3 stages of cementation in the Dengying Formation (Wang et al., 1998). The first stage occurred in the seafloor's diagenetic environment where fibrous dolomite cements (Fig. 8(b); Zhou et al., 2014); and cylindrical dolomite cements (Fig. 8(c); Zhou et al., 2014) were formed in the intergranular pores. The second stage occurred in a shallow buried environment where cements were mainly formed inside the residual primary pores that became filled by the early cements comprising fine crystal dolomite and powdered crystal dolomite (Fig. 8(d)) with obvious filling characteristics. The third stage occurred in a medium or deep buried environment, the dolomite cements are mainly inside the bigger holes, and the crystals of dolomite cements are larger (Fig. 2(e)). The cementation had a tremendous destructive effect on the Dengying Formation reservoir. The cements decreased reservoir porosity, hindered fluid flow into pores, and limited the growth of dissolution. Therefore, cementation also contributed to reservoir density. The filling is that secondary pores were filled mechanically and physically. there are many secondary pores, and holes were filled with quartz, asphalt and physical debris from overlying strata (Fig. 8(e)) in the Dengying Formation that caused the space to clearly narrow. Therefore, filling is the reason why secondary pores are difficult to find and are also a leading cause of reservoir density. 5.3. Recrystallization and its impact on reservoir formation In the process of studying carbonate rocks, we can add the oxygen and carbon isotope values (Table 2) into the following formula (Chen, 1994; Wang and Xiang, 1996):
Z ¼ 2:048
d13 C þ 50 þ 0:498 d18 O þ 50 ðPDBÞ
(1)
Table 1 Physical parameters of Dengying Formation reservoir rock. Sedimentary microfacies
Shoal Patch Algal Flat DolomiticFlat
Permeability/ 103mm2
Porosity/% Maximum
Minimum
Average
Quantity
Maximum
Minimum
Average
Quantity
8.59 8.02 2.69
0.13 1.01 0.19
3.34 2.87 1.74
21 18 19
96.360 83.610 70.660
1.110 0.771 0.001
60.280 58.252 12.351
17 20 15
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Table 2 Geochemical characteristics of the Dengying Formation in central Sichuan. NO.
Lithology
87
sr/86sr
1 2 3 4 5 6 7 8
Micritic dolomite Powder crystal dolomite Powder crystal dolomite Fine powder crystal dolomite Fine crystal dolomite Fine crystal dolomite Weathered residual breccia Weathered residual breccia
0.70980 0.70958 0.70960 0.70955 0.70910 0.70906 0.71010 0.71012
d13C(‰,PDB)
d18O(‰,PDB)
Degree of order
Z
0.551 1.503 0.416 0.327 0.446 0.457 1.046 1.075
7.911 7.826 7.560 7.621 7.885 7.609 9.837 9.098
0.5021 0.6452 0.6953 0.7053 0.7526 0.7453 _ _
121.6175 126.4808 122.6832 124.1744 124.2867 124.4467 _ _
Fig. 6. The porosity histogram of different sedimentary microfacies In central Sichuan.
Fig. 7. The reservoir thickness in different sedimentary facies of the Dengying Formation in central Sichuan.
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Fig. 8. Photomicrographs and core photos showing diagenetic types of the Dengying Formation in central Sichuan; (a) The Z4 well at a depth of 4353.35 m exhibited a pre-solution seam; (b) intergranular pores of sand crumbs in dolomite were cemented by fibrous rim dolomite, the sample came from MX9 well at a depth of 5426.10 m; (c) intergranular pore of sand crumb dolomite was cemented by columnar dolomite, the sample came from the MX8 well at a depth of 5436.10 m; (d) intergranular pore were cemented by powdered crystals and fine crystal dolomite, the sample from the Z4 well at a depth of 5495.73 m; (e) the GK1 well at a depth of 5435.40 m exhibited weathered eluvial breccia, and the gravel hole was filled by mechanical debris from the overlying strata; (f) the GK1well at a depth of 5168.75 m exhibited dissolved pore formed by the corrosion caused by acidic fluid under burial dissolution.
Then add the Z values (Table 2) into Fig. 9 and study the dolomite formed in the diagenetic environment. Z values > 120 are useful for locating a seawater diagenetic environment. These indicate that the dolomite of the Dengying Formation was formed in seawater (Chen, 1994; Wang and Xiang, 1996). In upper Sinian, the dolomite of the Dengying Formation formed in seawater was mud crystal dolomite (Wang and Xiang, 1996). There is a lot of powder
crystal dolomite and fine crystal dolomite (Table 2, NO.2~6) in which the crystal grains are larger in the Dengying Formation. Table 2 shows six samples with different degrees of order (NO.1e6). The crystal grains gradually become larger, the degree of order gradually becomes higher and the value of 87sr/86sr is gradually reduced. This is because the powder dolomite and finegrained dolomite have been transformed from microcrystalline
Fig. 9. Carbon isotopic and Z values for the carbonate rocks from distinctive environments (Chen, 1994; Wang and Xiang, 1996).
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dolomite and micritic dolomite during recrystallization under burial conditions (N. Clauer et al., 1989; Wang and Xiang, 1996; Glumac and Spivak-Birndorf, 2002). The recrystallization not only changed the Dengying Formation's rock structural components but also made the dolomite crystals become euhedral and coarse with pore diameter increases and pore throats becoming straight and smooth (J. Barclay Ferm et al., 1993). Hence, the pores are effective reservoir spaces. In the late diagenesis process, the pores are beneficial for acidic fluid flow, which promoted karstification and the formation of dissolved pores, dissolved holes and dissolved seams, which are effective in restoring gas and oil. Therefore, recrystallization is the basis for the formation of porous reservoir. 5.4. Impact of supergene dissolution and burial dissolution on reservoir formation The Dengying Formation has mainly experienced dissolution three times: syngenetic-penecontemporaneous dissolution, hypergene dissolution and burial dissolution (Shi et al., 2010). Syngenetic-penecontemporaneous dissolution: In the syngenetic-penecontempor-aneous period, shoal patch deposits and algal flat deposits were intermittently exposed to the atmosphere and fresh water, and mainly occurred in selective dissolution, forming a small amount of inter-granular dissolved pores, that basically disappeared in late compaction, filling or other diagenesis that had no direct effect on any existing reservoir. Supergene karstification: Based on strontium isotopes analysis of the two weathered residual breccia samples (Table 2, No.7 and No.8), the 87sr/86sr (PDB) is 0.71010~0.71012, with an average of 0.7101. The 87sr/86sr (PDB) of bedrock (Table 2, No.1e6) is 0.70906~0.70980, with an average of 0.70945. Comparing the 87 sr/86sr value, the value from the weathered residual breccia higher. Simultaneously, in the weathered residual breccia samples (Table 2, No. 7 and No. 8), d13C (PDB) is 1.075‰~1.046‰, with an average of 1.061‰, and d18O (PDB) is 9.837‰~9.098‰, with an average of 9.468‰. In the bedrock samples (Table 2, No.1e6), d18O (PDB) is 7.911~7.560, with an average of 7.735‰, and d13C(PDB) is 0.551‰~1.503‰, with an average of 0.294‰. Comparing the carbon and oxygen isotopes, the weathered residual breccia is obviously negative. These results indicate that weathered residual breccia was significantly influenced by fresh water. The Tongwan tectogenesis caused the Dengying Formation to twice be exposed to above sea level for a long time at the end of Z2dn2 and Z2dn4 period (Shi et al., 2010; Wang et al., 2014); Hence, we find that, under the influence of atmospheric freshwater, strong supergene dissolution occurred at the top of Z2dn2 and Z2dn4 and formed a lot of weathered residual breccias (Fig. 8(e)) and karst breccias, which is one of the important indicators identifying the presence of paleo-karst (Gale and Gomez, 2007). The seismic performance of the supergene dissolution is very obvious. The seismic reflection profile of the weathering crust (Fig. 10) shows incoherence and random reflection, and the events are characterized by being alternatively strong and weak with abnormal complex waves intermittently or during activity. Meanwhile, the strata become thinner or disappear in the areas of the most developed supergene dissolution; thus (Fig. 10), they are easy to be identified. Therefore, because of strong supergene dissolution, many dissolved pores, holes and seams were formed at the top of Z2dn2 and Z2dn4 between the gravel. Although some were filled and reformed during late diagenesis, most were retained and became effective oil and gas migration channels and reservoir spaces. For example, several slurry losses and many drill pipe venting occurred when the GS1, the GS2, the GS3 and the GS6 wells were drilled in the cave site in the study area. Therefore, supergene dissolution has played a key
role in reservoir formation. Burial dissolution: Two stages of burial dissolution occurred in the Dengying Formation in the study area (Yang et al., 2014a, b; Zhou et al., 2014). The first occurred from the Late Silurian to before the Permian and was caused by a lot of acidic fluid from organic maturation; when the hydrocarbon source rocks (Lower Cambrian Qiongzhusi Formation) were buried approximately 4 km deep, organic matter releases large amounts of acidic fluids, and the acidic fluids dissolve the surrounding rock. The second stage occurred in the Jurassic, and the liquid hydrocarbon degraded, producing a lot of acidic fluid that naturally dissolved the surrounding rock. Burial dissolution is nonselective, does not increase the total porosity and plays an important role in reforming the early pore structures (Fig. 8(f); Zhou et al., 2014), greatly improving the permeability of the reservoir and promoting the formation of high quality reservoirs. 5.5. Impact of fractures formed by tectogenesis on reservoir formation The Dengying Formation in the Sichuan Basin has experienced seven major tectogenic events and has been affected by strong uplift, tension and squeeze-slip tectonic forces (Wang et al., 2014) resulting in many structural fractures with good connectivity. However, only the half-filled and unfilled fractures became conducive to oil and gas permeability and porosity in the reservoir space. There are approximately 150 m of core area and 615 branch angle fractures (Table 3) in the Dengying Formation that are related to the three major wells (GS2, GS1 and MX9), The unfilled fractures are represented by 111 branches and comprise 18.05% of the total area. The half-filled fractures represent 24 branches and comprise 3.9% of the total area. Full-filled fractures represent 480 branches and comprise 78.05% of the total area. Therefore, the half-filled and unfilled fractures effectively comprise 135 branches, encompassing 21.14% of the total area indicating that the effective fracture proportion is smaller. However, the effective fractures can hold oil and gas as a percolation space with a width of over 2 mm (Qiang, 2007) and a proportion of 20%. The effective fracture can only be used as an oil and gas percolation space with a width of less than 2 mm and a proportion of 80%. Therefore, the fractures formed by tectogenesis significantly improve reservoir permeability, but they do not contribute much to the overall reservoir space. 5.6. Impact of reservoir space evolution on reservoir formation Based on the above analysis and depending on the impact of diagenesis on reservoir formation, the reservoir formation process of the Dengying Formation in the Sichuan Basin can be divided into the following stages (Liu et al., 2007; Shi et al., 2010 and (Fig. 11)): a. Syngeneic stage: The process of deposition ended in the seafloor diagenetic environment, the sand crumb (gravel) dolomite and article adhesion dolomite accumulated in the northeast of the study area and contained several primary intergranular pores. The microcrystalline dolomite in the rest of the area has many crystal pores. Overall, the porosity of the reservoir is more than 60%. During cementation, the porosity of the reservoir decreased by 10e20% after the intergranular pores were cemented by fiber or columnar dolomite. Meanwhile, the early, loose sediments remained weak and semi-consolidated. b. The penecontemporaneous stage: Because of sea level change, the algal flat and shoal patch microfacies sediments were intermittently exposed to be above sea level and were selectively dissolved by freshwater in the atmosphere, sea water and
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Fig. 10. The weathered crust characters of the Dengying Formation from the seismic reflection profile in central Sichuan.
Table 3 Fracture statistics of Dengying Formation core in the central Sichuan Basin/Branch. Type
Members Z2dn4 Z2dn3 Z2dn2 Total Percent/%
Classification according to filling conditions
Classification according to effective fracture width
Unfilled
Half-filled
Full-filled
Total
>2 mm
0.2~2 mm
<0.2 mm
Total
4 5 102 111 18.05
4 e 20 24 3.9
49 7 424 480 78.05
57 12 546 615 100
1 1 25 27 20.00
4 1 32 37 27.40
3 3 65 71 52.60
8 5 122 135 100
mixed water causing intergranular and enlarged pores to form and increasing the porosity of the reservoir by 5%e10%. c. The first shallow burial stage: the lower part of the sediments went into the first shallow burial stage under the stacking of overlying sediments. The various primary pores were formed in the process of deposition, except the selective intergranular dissolved pores, which were formed under the control of atmospheric fresh water during the penecontemporaneous period; Under the control of cementation and compaction diagenesis, the pore space suffered serious damage, and the porosity of the reservoir was reduced to below 5%. d. The supergene dissolution stage: The Dengying Formation was uplifted twice and exposed to long-term exposed atmospheric freshwater and was strongly affected by the strong supergene dissolution, after which the weathering crust was formed at the top of the Zndn2 and Zndn4 creating a large number of gravel holes. Meanwhile, the atmospheric fresh water migrated downward along the gravel pores, and strong dissolution occurred in the overlying stratum, thereby producing more dissolution breccias, dissolution pores, dissolution holes and dissolution seams, although physical debris of the overlying strata and other materials filled the spaces. Nevertheless, most spaces were preserved and simultaneously became important reservoirs and permeation spaces in the Dengying Formation; hence, the total porosity of the reservoir increased by 10%e20%. e. The second shallow burial stages: After the TongWan movement, with the accumulation of the overlying Cambrian strata, the Dengying Formation went into the second shallow burial stage. On one hand, the compaction and pressure solution in the Dengying Formation produced a small amount of collapsed physical matter, which led to many early pores disappearing. On the other hand, a weakestrong recrystallization occurred producing a certain amount of crystal pores in the reservoir space.
Overall, the effective reservoir space was greatly reduced to 6%e 8% f. The middle-deep diagenesis stage: The Dengying Formation was a medium-deep burial diagenetic stage with a depth of 3000e4000 m where compaction and cementation caused much of the reservoir space to disappear. However, the rock had a certain resistance to compaction, but the recrystallization and buried karstification produced a lot of secondary pores. Overall, the increasing reservoir space volume and the destruction of the reservoir space volume were roughly equivalent, and changes in total effective porosity were not obvious. g. The deep buried stage: when the burial depth exceeded 4 km, the effective porosity was decreased to approximately 0.5~3.0%, but the organic matter produced an amount of acidic fluid during the maturation and degradation process. This acidic fluid dissolved the surrounding rocks and reformed the early pore structures, the total porosity did not increase, but the effective porosity increased to be about 1.0%~5.0%. Meanwhile, in this stage, coarse crystal dolomite and quartz filled the reservoir space, but their effect on the reservoir was minimal. h. The tectonic stages: The Dengying Formation has experienced seven tectogenic events, and there are many fractures in the reservoir. However, the fractures contribute little to the reservoir porosity; therefore, the effective porosity remains1.0%~5.0% (Shi et al., 2010). Overall, the permeability of the reservoir has greatly increased.
6. Conclusions (1) The reservoir rocks in the Dengying Formation in central Sichuan mainly consist of sand crumb dolomite, algae laminated dolomite and breccia. The reservoir space is dominated by pore dissolved holes, dissolved pores (or holes) along the direction of algal laminae, intra-grit holes and residual
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Fig. 11. Schematic diagram of reservoir space evolution and diagenesis of the Dengying Formation in central Sichuan (according to Shi et al., 2010).
“grape lace” holes. Three types of reservoir comprised of fractured porous reservoir, porous reservoir and cave reservoir. (2) The reservoir was formed under the common actions of deposition, diagenesis and tectogenesis. Sedimentation is the basis and presupposition for reservoir development, but the effective reservoir is mainly controlled by the algal flat microfacies. The compaction, cementation and filling played an important role in reservoir densification. Recrystallization is the basis for forming the porous reservoir. Supergene karstification plays a key role in reservoir formation, but the burial karstification promotes the formation of a high quality reservoir. The fractures play an important role in improving reservoir permeability and do not contribute that much to reservoir porosity. The reservoir is gradually evolved and formed under the control of various geological and diagenetic processes.
Acknowledgments This study was supported by National Natural Science Foundation (No.51204133) and (No.41502159) and Sichuan provincial key discipline construction project (No.SZD0414)
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Z. Zhou et al. / Journal of Natural Gas Science and Engineering 29 (2016) 311e321 natural gas exploration in thesinian and cambrian systems in Sichuan Basin. J. Chengdu Univ. Technol. 27, 143e146. http://dx.doi:1005-9539(2000)-0143-04. J. Barclay Ferm, Ehrlich, Robert, Allan Crawford, G., 1993. Petrographic image analysis and petrophysics: analysis of crystalline carbonates from the Permian basin, west Texas. Carbonates Evaporites 8, 90e108. http://dx.doi:10.1007/ BF03175166. Liu, S.G., Ma, Y.S., Huang, W.M., et al., 2007. Densification process of upper simian Dengying Formation, Sichuan Basin. Nat. Gas. Geosci. 18, 485e496. http://dx. doi:16722-1926(2007)04-0485-12. Ma, Y.S., Mou, C.L., Guo, X.S., 2006. Sedimentary facies and distribution of reservoir rocks from the Feixianguan Formation in the Daxian-Xuanhan region, NE Sichuan. Acta Geol. Sin. 80 (1), 137e151. Qiang, Z.T., 2007. Carbonate Reservoir Geology. China University of Petroleum Press, Dongying, pp. 46e49. Shi, Z.J., Peng, J., Wang, Y., et al., 2010. Reservoir features and controlling factors of Dengying Formation in Southeast Sichuan, China. J. Chengdu Univ. Technol. Sci. Technol. Ed. 37, 1e8. http://dx.doi:1671-9-727(2010)01-0001-08. Wang, S.F., Xiang, F., 1996. Study on the dolomite genesis of (Sinian) Dengying Formation in Zi Yang area. Sediment. Facies Palaeogeogr. 19, 21e27. http://dx. doi:1004-7824(1999)03-0021-09. Wang, X.Z., Fang, S.X., Hou, F.H., et al., 1998. A study of the cement in primary pores in carbonate reservoirs of Dengying Formation, Sichuan Basin. J. Southwest Pet. Inst. 20, 1e3. http://dx.doi:1000-0550(2000)04-0549-06.
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Wang, Z.C., Jiang, H., Wang, T.S., et al., 2014. Paleo-geomorphology formed during Tongwan tectogenesis in Sichuan Basin and its significance for hydrocarbon accumulation. Pet. Explor. Dev. 41, 305e312. http://dx.doi:10.11698/PED.2014. 03.05. Xu, Z.Y., Zhang, X.Y., Wu, S.H., et al., 2008. Genesis of the low permeability reservoir bed of upper Triassic Xujiahe Formation in Xinchang gas field, western Sichuan. Pet. Sci. 5, 230e237. http://dx.doi:10.1007/s12182-008-0035-z. Yang, X.F., Wang, X.Z., Zhang, S.N., et al., 2014a. Reservoir characteristics and main controlling factors of oil sand in Houba area, northwestern Sichuan Basin, China. Arabian J. Geosci. 7, 2118e2119. http://dx.doi:10.1007/s12-517-013-09853. Yang, P., Xie, Y., Wang, Z.J., et al., 2014b. Fluid activity and hydrocarbo accumulation period of sinian Dengying formation in northern Guizhou, South China. Pet. Explor. Dev. 41, 313e322. http://dx.doi:10.11698/PED.2014.03.06. Zhang, S.Y., Tang, C.M., 1986. RegionaI geology conditions of Dengying Formation (sinian) in Sichuan Basin and its gas prospects. Nat. Gas. Ind. 6, 3e9. Zhang, L., Wei, G.Q., Wang, Z.C., et al., 2004. Analsis of formation mechanism of sinian Dengying Formation in Gaoshiti-Moxi structure belt in Sichuan Basin. Nat. Gas. Geosci. 15, 584e589. http://dx.doi:1672-1926(2004)06-0584-06. Zhou, Z., Wang, X.Z., XieL., et al., 2014. Reservoir features and physical influences of the Sinian Dengying Formation (Sinian) in central Sichuan, China. Nat. Gas. Geosci. 25, 701e708. http://dx.doi:10.11764/j.issn.1672-1926.2014.0701.
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Characteristics and genesis of the (Sinian) Dengying Formation reservoir in Central Sichuan, China Zheng Zhou a, b, c, *, Xingzhi Wang a, *, Ge Yin c, Shuseng Yuan c, Shejiao Zeng b a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan China College of Geology & Environment, Xi0 an University of Science and Technology, Xi0 an, 710054, Shanxi China c Southeast Sichuan Geological Team of Chongqing Bureau of Geology and Minerals Exploitation, Chongqing 400038, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2015 Received in revised form 24 November 2015 Accepted 6 December 2015 Available online 7 January 2016
The Dengying Formation is an important stratum of exploration in the Gaoshiti-Moxi structure belt in the Sichuan Basin. Therefore, this study combines core, thin section, mud logging data and geochemical analysis (C, O and Sr isotopes) with seismic section data to investigate the basic characteristics and genesis of the Dengying Formation reservoir. Observations indicate that the reservoir rocks mainly consist of sand crumb dolomites, algae laminated dolomite and breccia. The reservoir space is dominated by porous dissolved holes, dissolved pores (or holes) along the direction of algal laminae, intra-grit holes and residual “grape lace” holes. The reservoir has the characteristics of low porosity and mediumpermeability. There are three types of reservoir: porous reservoir, fractured porous reservoir and cave reservoir. The reservoir was formed by the common action of deposition, diagenesis and tectogenesis. Sedimentation is the basis and presupposition for reservoir development, but the effective reservoir is mainly controlled by the algal flat microfacies; Furthermore, compaction, cementation and filling are the main causes of reservoir density; Recrystallization is the basis for the formation of porous reservoir; Supergene dissolution is the key to reservoir formation; Buried dissolution also promotes the formation of a high quality reservoir. The fractures formed by tectogenesis significantly improve the permeability of the reservoir and contribute very little to porosity. The reservoir gradually evolved and formed under the control of various geological and diagenetic activities. This study provides important information and references for oil and gas exploitation and development in central Sichuan. © 2016 Elsevier B.V. All rights reserved.
Keywords: Central Sichuan Dengying formation Reservoir Characteristics and genesis
1. Introduction The Dengying Formation of the Gaoshiti-Moxi structure belt is thick and extensive, with an average thickness of approximately 82 m and extends up to 320 m, and has abundant natural gas resources in central Sichuan (Hong et al., 2000); The GS1 well was completed in July 2011, and it is an important risk exploration well required to explore the oil and gas potential in the Sinian stratum of the Gaoshiti-Moxi structure belt. After the Dengying Formation was acidified, test results indicated that GS1 has produced a daily yield of one million cubic meters of natural gas. These results show the potential of having good oil and gas exploration prospects in the Dengying Formation of the study area. Therefore, the Dengying
* Corresponding authors. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China E-mail address: [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.jngse.2015.12.005 1875-5100/© 2016 Elsevier B.V. All rights reserved.
Formation is a notable successful stratum for oil and gas exploration in the Sichuan Basin. Previous geologists have conducted a tremendous amount of work on the Dengying Formation in the study area and have some understanding of the oil and gas geological conditions. The stratum is ancient, the burial depths vary widely and the diagenesis is variable (Wang et al., 1998; Liu et al., 2007), therefore, some debate about the characteristics and the genesis of the reservoir remains. These debates have led to uncertainty when making exploration and exploitation strategies. The oil and gas exploration and development of the Dengying Formation in the central Sichuan area is still in its infancy, and the degree of awareness about the stratum remains relatively low. Meanwhile, in the process of oil and gas exploration, elucidating the characteristics and genesis of the reservoir are not only important, but they also represent a key factor that directly impacts the success of the oil and gas exploration (Han et al., 2012). Therefore, we study the reservoir's genesis based on a comprehensive investigation of the reservoir's basic characteristics using
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new data from systematically coring wells of the Dengying Formation in central Sichuan and the surrounding area, including comprehensive observations of core, thin sections, cast thin sections, mud logging data and seismic section data with geochemical examinations (i.e., C, O and Sr isotopes). The aim of this paper is to investigate the basic reservoir characteristics, to evaluate the factors that influence reservoir quality and to provide theoretical support for oil and gas exploration and development in Central Sichuan. 2. Geological setting The area is under control of Longmen Mountain, uplift of the Chuanxi-central Sichuan basement and multi-Paleozoic tectogenesis, especially the Caledonian tectogenesis in the late Silurian period. The Leshan-Longnvsi paleo-uplift was formed in the Sichuan Basin and is an inheritance paleo-uplift. Whereas the Gaoshiti-Moxi structure belt (Fig. 1), as a latent structure belt of the Paleo-uplift, is in the east high-end area of the Paleo-uplift axial area (Zhang et al., 2004 and Fig. 1). The Sinian top boundary of the Gaoshiti-Moxi structure belt covers more than 1000 km2. Because of its location at the east high point area of the paleo-high shaft portion, conditions for hydrocarbon accumulation are favorable (Zhang and Tang, 1986; Hong et al., 2000; Chen, 2010). In the Upper Sinan, the Sichuan Basin was in a platform depositional environment and accumulated a set of dolomite domination strata with rich blue-green algae, the strata are the Dengying Formation. According to the algae content and lithology, the Dengying Formation was divided into four members from bottom to top (Fig. 1): Z2dn1, Z2dn2, Z2dn3 and Z2dn4. The Z2dn1 is poor in algae content, the Z2dn3 is poor in algae content with high mudstone content and the Z2dn2 and Z2dn4 are rich in algae content. The reservoir is mainly developed in Z2dn2 and Z2dn4 (Shi et al., 2010). During the depositional process, the Tongwan tectogenesis caused the Dengying Formation be exposed twice above sea level for a long time and be intensely transformed by meteoric fresh water (Wang et al., 2014). The Dengying Formation underwent a burial period after it was formed and has experienced seven major tectogenic events in the last 600 million years, the burial depths vary widely, and the diagenesis is variable making the reservoir's characteristics and genesis very complex.
3. Material and methods This study was based on samples from nine systematically cored wells in central Sichuan and the surrounding area, including GS1, Z2, Z6, Z4, MX8, GK1, W117, AP1 and MX9 wells. The cumulative length of the cored well sections is 821 m and consists primarily of two intervals, Z2dn2 and Z2dn4. A total of 1120 samples were selected for making thin sections, and some of them were impregnated with blue-dyed resin to examine the porosity characteristics of the rock more clearly. To investigate the reservoir origins, Eight samples (Table 2) from the MX8 well which was in different rocks (i.e. micritic dolomite, powder crystal dolomite, fine powder crystal dolomite, fine crystal dolomite and weathered residual breccia) were selected for carbon (C), oxygen (O) and Strontium (Sr) isotopic analysis, Six samples (Table 2) were selected for degree of rock order, we used the results to examined the influence of atmospheric fresh water and dolomite recrystallization on reservoir formation. To investigate the reservoir properties and types, Approximately 328 samples of horizontal core plugs whose diameter is about 2.5 cm from the GS1, the MX8, the AP1 and the GK1 wells were test educing the mercury intrusion method to analyze the reservoir properties, including porosity and permeability. We used the crossplot of porosity and permeability and mud logging data of the GS1, the MX8, the AP1 and the GK1 wells to study the reservoir types. Nearly 250 km of seismic section data were selected from central Sichuan and used to study the response characteristics of the weathering crust. According to these results, this study discusses the relationship between reservoir genesis and deposition, diagenesis and tectogenesis. 4. Results 4.1. Basic reservoir characteristics 4.1.1. Lithologies Visual inspection of more than 1100 thin sections from cores and core observations indicate that Dengying Formation rocks are mainly dolomites (including algae laminated dolomite, sand crumb dolomite, particle adhesion dolomite, mud crystal dolomite, powder crystal dolomite and fine crystal dolomite) and breccias (including weathered eluvial breccia, karst breccia and false karst
Fig. 1. Location of the study area and stratigraphic histogram of the Dengying Formation.
Z. Zhou et al. / Journal of Natural Gas Science and Engineering 29 (2016) 311e321
breccia). There are only a few mudstones rocks. The reservoir rocks are algal laminated dolomites, sand crumb dolomite and karst breccia. 4.1.2. Pore space Thin sections and core observations indicate that primary pores had disappeared today, the Dengying Formation reservoirs are formed after deposition and are dominated by porous dissolved holes (Fig. 2(a)), dissolved pores (or holes) along the direction of laminar algae (Fig. 2(b) and (c)), intra-grit holes (Fig. 2(d)) and residual.“grape lace” holes (Fig. 2(e)) comprise the main reservoir space, which accounts for 86.8% of the total reservoir space (Fig. 3). Intergranular dissolved pores (Fig. 2(f)), crystal pores (Fig. 2(f)), crystal dissolved pores and fracture dissolved holes account for 13.2% of the total reservoir space (Fig. 3). Additionally, there are some microfissures that have a certain reservoir performance, but the most important role of microfissures is to serve as oil and gas seepage channels to improve the permeability of the reservoir. 4.1.3. Reservoir properties We summarized the petrophysical properties (i.e., porosity and permeability) of 328 samples from the Dengying Formation (Fig. 4). The porosity averages3.86% and is mainly 2.00%e5.00%, accounting for 73.71% of all samples, whereas porosity over 5% only accounts for 12.50% of all of the samples. The permeability averages 27.07 103 mm2 and is mainly 10.0~100.0 103 mm2, accounting for 57.32%. These results indicate that the reservoir of Dengying
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Formation has the characteristics of low porosity and mediumpermeability.
4.2. Reservoir types Generally speaking, the permeability of porous reservoir increases when the Porosity increases, the correlation between them is good. The permeability of.fractured reservoir is high when the porosity is low, there is no correlation between them (Shi et al., 2010; Zhou et al., 2014). Therefore, we can qualitatively study the reservoir types according to the correlation characteristics between the porosity and permeability (Shi et al., 2010; Dai et al., 2007; Qiang, 2007). Fig. 5 illustrates a distribution pattern of porosity and permeability, mainly comprised of two regions: A and B. The region B shows clear positive correlation, highlighted by synchronous change of porosity (0.13%~9.88%) and permeability (0.01 103 mm2~8.78 103 mm2), the thin section analysis from samples indicates that the reservoir spaces are mainly crystal pores, crystal-dissolved pores and intergrannular dissolved pores that are evenly distributed and formed when the dolomites were recrystallized or dissolved. The region A features positive correlation too, the porosity distributed between 1.65% and 5.85%, with high permeability (9.80 103 mm2~8.78 103mm2), indicating the existence of fractures in the reservoir. The thin section and core analysis from samples indicates that the reservoir spaces are dominated by crystal pores, crystal-dissolved pores and
Fig. 2. Photomicrographs and core photos showing different reservoir space and reservoir types. (a) the Z4 well at a depth of 5486.00 m exhibited porous dissolved holes; (b) dissolved pores (or holes) along the direction of laminar algal developed in algal laminated dolomite, sample from the Z2 well at a depth of 5482.76 m; (c) the Z6 well at a depth of 5149.98 m exhibited dissolved pores (or holes) along the direction of laminar algae; (d) the MX8 well at a depth of 5406.00 m exhibited intra-grit holes. (e) the W117 well at a depth of 4986.60 m exhibited a residual “grape lace” hole half-filled by dolomite; (f) Intergranular dissolved pores developed in sand crumb dolomite, sample from GS1well at a depth of 4986.60 m; (g) crystal pores developed in powder crystal dolomite, sample from the Z2 well at a depth of 3706.20 m; (h) Crystal dissolved pores developed in fine crystal dolomite, sample came from the MX8 well at a depth of 5306.08 m; (i) the GK1 well at a depth of 5302.00 m exhibited fractured dissolved holes; (j) the MX8 well at a depth of 5113.05 m exhibited a fractured porous reservoir; (k) the AP1well at a depth of 5215.00 m exhibited a porous reservoir; (l) the Z6 well at a depth of 3790.58 m exhibited a cave reservoir.
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Fig. 3. Development frequency of different reservoir spaces within the Dengying Formation reservoir rock.
Fig. 4. The frequency distribution of porosity and permeability of reservoir rock of the Dengying Formation in central Sichuan.
In fact, there is cave reservoir which is composed of different seize dissolved holes in Dengying Formation. The samples can not possess cave reservoir because the diameter of horizontal core plugs is about 2.5 cm, in the progress of core observation, we found that there are some big dissolved holes that formed cave reservoir (Fig. 2(l)). Meanwhile, in the drilling progress, the slurry losses and drill pipe venting occurred, for example, the slurry losses when the GS1 were drilled in 5846.23 m, these indicates that there is cave reservoir in the study area. From the above analysis, we can see that pore space structures within the Dengying Formation reservoirs are quite complex and that reservoir genesis was influenced by many factors. Three types of reservoir occur in the Dengying Formation, comprised of fractured porous reservoir (Fig. 2(j)), porous reservoir (Fig. 2(k)) and cave reservoir (Fig. 2(l)).
5. Discussion Fig. 5. Relationship between porosity and permeability of the Dengying Formation reservoir rock in central Sichuan.
intergrannular dissolved pores, and contains a few micro fractures. Therefore, region A and region B may display different types of reservoir, indicative of fractured porous reservoirs and porous reservoirs respectively.
Generally speaking, the carbonate reservoir was formed by the common action of deposition, diagenes and tectogenesis (Brasher and Vagle, 1996; Wang et al., 1998; Qiang, 2007). In central Sichuan, the Sinian Dengying Formation reservoir is a typical carbonate reservoir (Xu et al., 2008; Wang et al., 2014, Chen et al., 2015), therefore, we respectively discuss the role of sedimentation, diagenesis and tectogenesis on the formation of reservoir.
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5.1. The impact of deposition on reservoir formation During the Dengying Formation sedimentation, only the sedimentation of Z2dn3 subfacies occurred on a deep water platform. The sedimentation of other subfacies occurred in a shallow water platform. The shallow water platform subfacies can be further divided into a shoal patch, an algal flat, a dolomitic flat and a mudflat microfacies (Fig. 1) (Zhou et al., 2014). Based on the observation of cores and thin sections, the different sedimentary microfacies have different reservoirs (Ma et al., 2006). The algal flat microfacies have more organic matter and many original pores, which is beneficial to late dissolution, easily forming various sizes dissolution holes, thereby forming cave reservoir. The dolomitic flat microfacies have many crystal pores where the dissolution is not good, the diameter of organic pores increased a little or not, thereby forming porous reservoirs or fractured porous reservoirs in conjunction with the fracture. Where the dissolution is good, there are many various sizes holes within the dissolution breccia, thereby forming cave reservoirs between the dissolution breccia. The shoal patch microfacies has many original pores, which are beneficial to late dissolution and easily formed various sizes dissolution holes, thereby forming cave reservoir. The mudflat microfacies are mainly distributed in Z2dn3. The lithology is mudstone rock, in which it is difficult for pores or holes to form during the dissolution process. Therefore, there is less oil and gas reservoir space. Based on physical property data analysis, porosity and permeability are markedly different in various deposits within microfacies. From the porosity histogram, the porosity and the permeability statistics of different deposits in central Sichuan (Table 1, Fig. 6) indicate that the maximum porosity of the shoal patch deposits is 8.59% and the minimum porosity is 0.13%, with an average of 3.34%, moreover, porosity of over 4% accounts for 33.34% of all of the samples; The maximum permeability of shoal patch deposits is 96.360 103 mm2, and the minimum is 1.110 103 mm2 with an average of 60.280 103 mm2. The maximum porosity of the algal flat deposits is 8.02%, and the minimum is 1.01%, with an average of 2.87%, porosity of over 4% accounts for 22.23% of all of the samples taken; The maximum permeability of algal flat deposits is 83.610 103 mm2and the minimum is 0.771 103 mm2 with an average of 58.252 103 mm2. The maximum porosity of the dolomitic flat deposits is 2.69%, and the minimum is 0.19%, with an average of 1.74%, and porosity of all samples are less than 4%; The maximum permeability of dolomitic flat deposits is 70.660 103 mm2, and the minimum is 0.001 103 mm2 with an average of 12.351 103 mm2. Simultaneously, in different sedimentary microfacies, the reservoir thickness and the range are different (Fig. 7). In the shoal patch microfacies, the reservoir thickness is bigger, and the reservoir distribution range is narrower and has a certain randomness. In the algal flat microfacies, the reservoir thickness is not as significant, and the reservoir distribution range is larger. In the dolomitic flat, the reservoir thickness is the smallest, and the reservoir distribution range is the largest. From the above analysis, we can see that the porosity and
315
permeability of shoal patch deposits and the algal flat deposits are well developed, but the reservoir distribution range of shoal patch deposits is narrower and has a certain randomness. Overall, the effective reservoir of the Dengying Formation is mainly controlled by algal flat microfacies. 5.2. Compaction, filling and cementation and their impact on reservoir formation The porosity and permeability of reservoirs in the Dengying Formation are severely affected by compaction, filling and cementation (Dixon et al., 1989; Yang et al., 2004; Xu et al., 2008; Shi et al., 2010; Zhou et al., 2014). The Dengying Formation has been buried nearly thousands of meters deep. Under strong compaction and pre-solution, the original porosity of the reservoir dropped abruptly. Although presolution seams (Fig. 8(a)) were formed in the process of being buried, certain contributions were made to the reservoir. Generally, compaction causes the pores to decrease or disappear, which has become one of the most important contributors to reservoir density. The cementation process caused the chemical sediment to consolidate loose material in the primary pores. There were 2e3 stages of cementation in the Dengying Formation (Wang et al., 1998). The first stage occurred in the seafloor's diagenetic environment where fibrous dolomite cements (Fig. 8(b); Zhou et al., 2014); and cylindrical dolomite cements (Fig. 8(c); Zhou et al., 2014) were formed in the intergranular pores. The second stage occurred in a shallow buried environment where cements were mainly formed inside the residual primary pores that became filled by the early cements comprising fine crystal dolomite and powdered crystal dolomite (Fig. 8(d)) with obvious filling characteristics. The third stage occurred in a medium or deep buried environment, the dolomite cements are mainly inside the bigger holes, and the crystals of dolomite cements are larger (Fig. 2(e)). The cementation had a tremendous destructive effect on the Dengying Formation reservoir. The cements decreased reservoir porosity, hindered fluid flow into pores, and limited the growth of dissolution. Therefore, cementation also contributed to reservoir density. The filling is that secondary pores were filled mechanically and physically. there are many secondary pores, and holes were filled with quartz, asphalt and physical debris from overlying strata (Fig. 8(e)) in the Dengying Formation that caused the space to clearly narrow. Therefore, filling is the reason why secondary pores are difficult to find and are also a leading cause of reservoir density. 5.3. Recrystallization and its impact on reservoir formation In the process of studying carbonate rocks, we can add the oxygen and carbon isotope values (Table 2) into the following formula (Chen, 1994; Wang and Xiang, 1996):
Z ¼ 2:048
d13 C þ 50 þ 0:498 d18 O þ 50 ðPDBÞ
(1)
Table 1 Physical parameters of Dengying Formation reservoir rock. Sedimentary microfacies
Shoal Patch Algal Flat DolomiticFlat
Permeability/ 103mm2
Porosity/% Maximum
Minimum
Average
Quantity
Maximum
Minimum
Average
Quantity
8.59 8.02 2.69
0.13 1.01 0.19
3.34 2.87 1.74
21 18 19
96.360 83.610 70.660
1.110 0.771 0.001
60.280 58.252 12.351
17 20 15
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Table 2 Geochemical characteristics of the Dengying Formation in central Sichuan. NO.
Lithology
87
sr/86sr
1 2 3 4 5 6 7 8
Micritic dolomite Powder crystal dolomite Powder crystal dolomite Fine powder crystal dolomite Fine crystal dolomite Fine crystal dolomite Weathered residual breccia Weathered residual breccia
0.70980 0.70958 0.70960 0.70955 0.70910 0.70906 0.71010 0.71012
d13C(‰,PDB)
d18O(‰,PDB)
Degree of order
Z
0.551 1.503 0.416 0.327 0.446 0.457 1.046 1.075
7.911 7.826 7.560 7.621 7.885 7.609 9.837 9.098
0.5021 0.6452 0.6953 0.7053 0.7526 0.7453 _ _
121.6175 126.4808 122.6832 124.1744 124.2867 124.4467 _ _
Fig. 6. The porosity histogram of different sedimentary microfacies In central Sichuan.
Fig. 7. The reservoir thickness in different sedimentary facies of the Dengying Formation in central Sichuan.
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Fig. 8. Photomicrographs and core photos showing diagenetic types of the Dengying Formation in central Sichuan; (a) The Z4 well at a depth of 4353.35 m exhibited a pre-solution seam; (b) intergranular pores of sand crumbs in dolomite were cemented by fibrous rim dolomite, the sample came from MX9 well at a depth of 5426.10 m; (c) intergranular pore of sand crumb dolomite was cemented by columnar dolomite, the sample came from the MX8 well at a depth of 5436.10 m; (d) intergranular pore were cemented by powdered crystals and fine crystal dolomite, the sample from the Z4 well at a depth of 5495.73 m; (e) the GK1 well at a depth of 5435.40 m exhibited weathered eluvial breccia, and the gravel hole was filled by mechanical debris from the overlying strata; (f) the GK1well at a depth of 5168.75 m exhibited dissolved pore formed by the corrosion caused by acidic fluid under burial dissolution.
Then add the Z values (Table 2) into Fig. 9 and study the dolomite formed in the diagenetic environment. Z values > 120 are useful for locating a seawater diagenetic environment. These indicate that the dolomite of the Dengying Formation was formed in seawater (Chen, 1994; Wang and Xiang, 1996). In upper Sinian, the dolomite of the Dengying Formation formed in seawater was mud crystal dolomite (Wang and Xiang, 1996). There is a lot of powder
crystal dolomite and fine crystal dolomite (Table 2, NO.2~6) in which the crystal grains are larger in the Dengying Formation. Table 2 shows six samples with different degrees of order (NO.1e6). The crystal grains gradually become larger, the degree of order gradually becomes higher and the value of 87sr/86sr is gradually reduced. This is because the powder dolomite and finegrained dolomite have been transformed from microcrystalline
Fig. 9. Carbon isotopic and Z values for the carbonate rocks from distinctive environments (Chen, 1994; Wang and Xiang, 1996).
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dolomite and micritic dolomite during recrystallization under burial conditions (N. Clauer et al., 1989; Wang and Xiang, 1996; Glumac and Spivak-Birndorf, 2002). The recrystallization not only changed the Dengying Formation's rock structural components but also made the dolomite crystals become euhedral and coarse with pore diameter increases and pore throats becoming straight and smooth (J. Barclay Ferm et al., 1993). Hence, the pores are effective reservoir spaces. In the late diagenesis process, the pores are beneficial for acidic fluid flow, which promoted karstification and the formation of dissolved pores, dissolved holes and dissolved seams, which are effective in restoring gas and oil. Therefore, recrystallization is the basis for the formation of porous reservoir. 5.4. Impact of supergene dissolution and burial dissolution on reservoir formation The Dengying Formation has mainly experienced dissolution three times: syngenetic-penecontemporaneous dissolution, hypergene dissolution and burial dissolution (Shi et al., 2010). Syngenetic-penecontemporaneous dissolution: In the syngenetic-penecontempor-aneous period, shoal patch deposits and algal flat deposits were intermittently exposed to the atmosphere and fresh water, and mainly occurred in selective dissolution, forming a small amount of inter-granular dissolved pores, that basically disappeared in late compaction, filling or other diagenesis that had no direct effect on any existing reservoir. Supergene karstification: Based on strontium isotopes analysis of the two weathered residual breccia samples (Table 2, No.7 and No.8), the 87sr/86sr (PDB) is 0.71010~0.71012, with an average of 0.7101. The 87sr/86sr (PDB) of bedrock (Table 2, No.1e6) is 0.70906~0.70980, with an average of 0.70945. Comparing the 87 sr/86sr value, the value from the weathered residual breccia higher. Simultaneously, in the weathered residual breccia samples (Table 2, No. 7 and No. 8), d13C (PDB) is 1.075‰~1.046‰, with an average of 1.061‰, and d18O (PDB) is 9.837‰~9.098‰, with an average of 9.468‰. In the bedrock samples (Table 2, No.1e6), d18O (PDB) is 7.911~7.560, with an average of 7.735‰, and d13C(PDB) is 0.551‰~1.503‰, with an average of 0.294‰. Comparing the carbon and oxygen isotopes, the weathered residual breccia is obviously negative. These results indicate that weathered residual breccia was significantly influenced by fresh water. The Tongwan tectogenesis caused the Dengying Formation to twice be exposed to above sea level for a long time at the end of Z2dn2 and Z2dn4 period (Shi et al., 2010; Wang et al., 2014); Hence, we find that, under the influence of atmospheric freshwater, strong supergene dissolution occurred at the top of Z2dn2 and Z2dn4 and formed a lot of weathered residual breccias (Fig. 8(e)) and karst breccias, which is one of the important indicators identifying the presence of paleo-karst (Gale and Gomez, 2007). The seismic performance of the supergene dissolution is very obvious. The seismic reflection profile of the weathering crust (Fig. 10) shows incoherence and random reflection, and the events are characterized by being alternatively strong and weak with abnormal complex waves intermittently or during activity. Meanwhile, the strata become thinner or disappear in the areas of the most developed supergene dissolution; thus (Fig. 10), they are easy to be identified. Therefore, because of strong supergene dissolution, many dissolved pores, holes and seams were formed at the top of Z2dn2 and Z2dn4 between the gravel. Although some were filled and reformed during late diagenesis, most were retained and became effective oil and gas migration channels and reservoir spaces. For example, several slurry losses and many drill pipe venting occurred when the GS1, the GS2, the GS3 and the GS6 wells were drilled in the cave site in the study area. Therefore, supergene dissolution has played a key
role in reservoir formation. Burial dissolution: Two stages of burial dissolution occurred in the Dengying Formation in the study area (Yang et al., 2014a, b; Zhou et al., 2014). The first occurred from the Late Silurian to before the Permian and was caused by a lot of acidic fluid from organic maturation; when the hydrocarbon source rocks (Lower Cambrian Qiongzhusi Formation) were buried approximately 4 km deep, organic matter releases large amounts of acidic fluids, and the acidic fluids dissolve the surrounding rock. The second stage occurred in the Jurassic, and the liquid hydrocarbon degraded, producing a lot of acidic fluid that naturally dissolved the surrounding rock. Burial dissolution is nonselective, does not increase the total porosity and plays an important role in reforming the early pore structures (Fig. 8(f); Zhou et al., 2014), greatly improving the permeability of the reservoir and promoting the formation of high quality reservoirs. 5.5. Impact of fractures formed by tectogenesis on reservoir formation The Dengying Formation in the Sichuan Basin has experienced seven major tectogenic events and has been affected by strong uplift, tension and squeeze-slip tectonic forces (Wang et al., 2014) resulting in many structural fractures with good connectivity. However, only the half-filled and unfilled fractures became conducive to oil and gas permeability and porosity in the reservoir space. There are approximately 150 m of core area and 615 branch angle fractures (Table 3) in the Dengying Formation that are related to the three major wells (GS2, GS1 and MX9), The unfilled fractures are represented by 111 branches and comprise 18.05% of the total area. The half-filled fractures represent 24 branches and comprise 3.9% of the total area. Full-filled fractures represent 480 branches and comprise 78.05% of the total area. Therefore, the half-filled and unfilled fractures effectively comprise 135 branches, encompassing 21.14% of the total area indicating that the effective fracture proportion is smaller. However, the effective fractures can hold oil and gas as a percolation space with a width of over 2 mm (Qiang, 2007) and a proportion of 20%. The effective fracture can only be used as an oil and gas percolation space with a width of less than 2 mm and a proportion of 80%. Therefore, the fractures formed by tectogenesis significantly improve reservoir permeability, but they do not contribute much to the overall reservoir space. 5.6. Impact of reservoir space evolution on reservoir formation Based on the above analysis and depending on the impact of diagenesis on reservoir formation, the reservoir formation process of the Dengying Formation in the Sichuan Basin can be divided into the following stages (Liu et al., 2007; Shi et al., 2010 and (Fig. 11)): a. Syngeneic stage: The process of deposition ended in the seafloor diagenetic environment, the sand crumb (gravel) dolomite and article adhesion dolomite accumulated in the northeast of the study area and contained several primary intergranular pores. The microcrystalline dolomite in the rest of the area has many crystal pores. Overall, the porosity of the reservoir is more than 60%. During cementation, the porosity of the reservoir decreased by 10e20% after the intergranular pores were cemented by fiber or columnar dolomite. Meanwhile, the early, loose sediments remained weak and semi-consolidated. b. The penecontemporaneous stage: Because of sea level change, the algal flat and shoal patch microfacies sediments were intermittently exposed to be above sea level and were selectively dissolved by freshwater in the atmosphere, sea water and
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Fig. 10. The weathered crust characters of the Dengying Formation from the seismic reflection profile in central Sichuan.
Table 3 Fracture statistics of Dengying Formation core in the central Sichuan Basin/Branch. Type
Members Z2dn4 Z2dn3 Z2dn2 Total Percent/%
Classification according to filling conditions
Classification according to effective fracture width
Unfilled
Half-filled
Full-filled
Total
>2 mm
0.2~2 mm
<0.2 mm
Total
4 5 102 111 18.05
4 e 20 24 3.9
49 7 424 480 78.05
57 12 546 615 100
1 1 25 27 20.00
4 1 32 37 27.40
3 3 65 71 52.60
8 5 122 135 100
mixed water causing intergranular and enlarged pores to form and increasing the porosity of the reservoir by 5%e10%. c. The first shallow burial stage: the lower part of the sediments went into the first shallow burial stage under the stacking of overlying sediments. The various primary pores were formed in the process of deposition, except the selective intergranular dissolved pores, which were formed under the control of atmospheric fresh water during the penecontemporaneous period; Under the control of cementation and compaction diagenesis, the pore space suffered serious damage, and the porosity of the reservoir was reduced to below 5%. d. The supergene dissolution stage: The Dengying Formation was uplifted twice and exposed to long-term exposed atmospheric freshwater and was strongly affected by the strong supergene dissolution, after which the weathering crust was formed at the top of the Zndn2 and Zndn4 creating a large number of gravel holes. Meanwhile, the atmospheric fresh water migrated downward along the gravel pores, and strong dissolution occurred in the overlying stratum, thereby producing more dissolution breccias, dissolution pores, dissolution holes and dissolution seams, although physical debris of the overlying strata and other materials filled the spaces. Nevertheless, most spaces were preserved and simultaneously became important reservoirs and permeation spaces in the Dengying Formation; hence, the total porosity of the reservoir increased by 10%e20%. e. The second shallow burial stages: After the TongWan movement, with the accumulation of the overlying Cambrian strata, the Dengying Formation went into the second shallow burial stage. On one hand, the compaction and pressure solution in the Dengying Formation produced a small amount of collapsed physical matter, which led to many early pores disappearing. On the other hand, a weakestrong recrystallization occurred producing a certain amount of crystal pores in the reservoir space.
Overall, the effective reservoir space was greatly reduced to 6%e 8% f. The middle-deep diagenesis stage: The Dengying Formation was a medium-deep burial diagenetic stage with a depth of 3000e4000 m where compaction and cementation caused much of the reservoir space to disappear. However, the rock had a certain resistance to compaction, but the recrystallization and buried karstification produced a lot of secondary pores. Overall, the increasing reservoir space volume and the destruction of the reservoir space volume were roughly equivalent, and changes in total effective porosity were not obvious. g. The deep buried stage: when the burial depth exceeded 4 km, the effective porosity was decreased to approximately 0.5~3.0%, but the organic matter produced an amount of acidic fluid during the maturation and degradation process. This acidic fluid dissolved the surrounding rocks and reformed the early pore structures, the total porosity did not increase, but the effective porosity increased to be about 1.0%~5.0%. Meanwhile, in this stage, coarse crystal dolomite and quartz filled the reservoir space, but their effect on the reservoir was minimal. h. The tectonic stages: The Dengying Formation has experienced seven tectogenic events, and there are many fractures in the reservoir. However, the fractures contribute little to the reservoir porosity; therefore, the effective porosity remains1.0%~5.0% (Shi et al., 2010). Overall, the permeability of the reservoir has greatly increased.
6. Conclusions (1) The reservoir rocks in the Dengying Formation in central Sichuan mainly consist of sand crumb dolomite, algae laminated dolomite and breccia. The reservoir space is dominated by pore dissolved holes, dissolved pores (or holes) along the direction of algal laminae, intra-grit holes and residual
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Fig. 11. Schematic diagram of reservoir space evolution and diagenesis of the Dengying Formation in central Sichuan (according to Shi et al., 2010).
“grape lace” holes. Three types of reservoir comprised of fractured porous reservoir, porous reservoir and cave reservoir. (2) The reservoir was formed under the common actions of deposition, diagenesis and tectogenesis. Sedimentation is the basis and presupposition for reservoir development, but the effective reservoir is mainly controlled by the algal flat microfacies. The compaction, cementation and filling played an important role in reservoir densification. Recrystallization is the basis for forming the porous reservoir. Supergene karstification plays a key role in reservoir formation, but the burial karstification promotes the formation of a high quality reservoir. The fractures play an important role in improving reservoir permeability and do not contribute that much to reservoir porosity. The reservoir is gradually evolved and formed under the control of various geological and diagenetic processes.
Acknowledgments This study was supported by National Natural Science Foundation (No.51204133) and (No.41502159) and Sichuan provincial key discipline construction project (No.SZD0414)
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