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Reef-Bank Features and Their Constraint to Reservoirs of Natural Gas, from Permian Changxing Formation to Triassic Feixianguan Formation in Daxian-Xuanhan Area of Sichuan Province, South China
EARTH SCIENCE FRONTIERS Volume 14, Issue 1, January 2007 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2007, 14(1): 182–192.
RESEARCH PAPER
Reef-Bank Features and Their Constraint to Reservoirs of Natural Gas, from Permian Changxing Formation to Triassic Feixianguan Formation in Daxian-Xuanhan Area of Sichuan Province, South China MA Yongsheng1, *, MOU Chuanlong2, TAN Qinyin2, YU Qian2, WANG Ruihua2 1 Southern Exploration and Exploitation Filiale, China Petrochemical Corporation (SINOPEC), Kunming 650200, China 2 Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China
Abstract: Detail studies of the seismic section and certain wells, such as the Puguang-2, the Puguang-6, and the Maoba Wells, demonstrate that the reef facies from the Permian Changxing Formation to the Triassic Feixianguan Formation are expressed as the lens with medium-strong changing swings and chaotic images in the seismic section. The reef facies is made up of gray limestones and dolomites of both baffling sponge reef and the frame sponge reef. Moreover, the bank facies from the Permian Changxing Formation to the Triassic Feixianguan Formation is represented as the lens with medium-strong changing swings and discontinuity images in the seismic section, which is made up of several petrologic types, that is, the French gray thick-bedded to massive sparitic oolitic dolomite, the sparitic gravel-oolitic dolomite, the sparitic pisolitic-oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. Both the reef facies and the bank facies concomitant are developed and distributed in space along the platform margin, which form a special reef-bank facies zone. This facies zone controls the petrological features, the reservoir qualities, and the spatial distribution of the reservoirs of natural gas in the Daxian-Xuanhan area. Key Words:
1
reef-bank facies; reservoirs; from the Permian Changxing Fm. to the Triassic Fm; the Daxian-Xuanhan area
Introduction
The restriction of actual materials inhibits the systematic study on sedimentary facies in the study area especially for the detail analysis and description regarding the reef and bank facies. Certain geologists completed the study relating to sedimentary facies and its relative reservoir of natural gas[1–25]. The Permian Changxing Fm. to the Triassic Feixianguan Fm. is an important horizon for the exploration of oil and gas. With the continuous disclosure of certain wells and the good revealing of seismic materials, the depositional body of reefs and banks are observed through further exploration. Based on the former researching fruits, as well as, according to the materials of wells and seismic sections, the sedimentary features of reefs and banks are described in detail and their constraints to the reservoir are discussed in this study.
2 Sedimentary facies features of the Changxing Fm. to the Feixianguan Fm. In the depositional period from the Permian Changxing Fm. to the Triassic Feixianguan Fm., the Daxian-Xuanhan area is marked by both the carbonate platform in the eastern part and the shelf in the western part, where many types of sedimentary facies are developed. The types of sedimentary facies include the evaporation platform, the restricted platform, the open platform, the shallow bank of platform margin, the reef of platform margin, the ramp and the shelf facies. Generally, sedimentary facies of reefs and banks are developed in the Permian Changxing Fm. and shallow bank facies are highly developed in the Triassic Feixianguan Fm. 2.1
Shallow bank
Shallow bank facies in the transitional period between the
Received date: 2006-11-25. *Corresponding author: E-mail: [email protected] Foundation item: Supported by the National Key Basic Research Development Program “973” of China (No.2005CB422106). Copyright © 2007, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B. V. All rights reserved.
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Permian and the Triassic of the study area can be subdivided into the intra-platform shallow bank and the platform- margin shallow bank facies, and the former is more widespread than the latter. A few intra-platform shallow banks can be observed in the Permian Changxing Fm. within certain wells, such as the Puguang-4 Well and the Po-1 Well. This facies is developed in high energy locations within the open-platform facies zone and is made up of the sparite including the sparitic calcarenite and the sparitic oolitic limestone. According to sediment types, shallow bank can be further divided into the bioclastic bank, the arenite bank, and the oolitic bank, but the bioclastic bank is the chief type of bank. The platform-margin bank is chiefly developed in the second Member of the Permian Changxing Fm. and the first and second Members of the Triassic Feixianguan Fm. Spatially, this type of sedimentary facies can be found in many wells, such as the Po-1 Well, the Po-2 Well, the Puguang-2 Well, the Puguang-5 Well, and the Puguang-6 Well, and is made up of the thick-bedded to massive sparitic
Fig. 1
Sparitic oolitic dolomite in the Triassic Feixianguan
Fm. From the Well Puguang-2 Well (2.5×4)
Fig. 3
Mold pores and intra-grain dissolved pores of oolites
without filling and with asphalt in the pore wall, oolitic
oolitic dolomite (Fig. 1), the sparitic gravel oolitic dolomite (Fig. 2), the sparitic pesolitic oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. The oolite is the main type of sedimentary grain, and its content ranges from 65 % to 85 % in the sedimentary rock. There are several kinds of oolites, such as the high-energy oolite, the thin-skinned oolite, and the compound oolite. Certain oolites are deformed into the oolitic mold pore (Fig. 3) and the intra-grain dissolved pore (Fig. 4). The configuration of oolites is chiefly round and oval, and the diameter of most oolites is 1 mm. Other types of grains in the platform-margin bank facies include arenite, rudite, and pisolite. Micritic dolomite and the oolitic dilomite make up the chief composition of rudite with the configuration of the irregular sub-circularity and with the diameter ranging from 4 mm to 5 mm and even to 1 cm. Most of the pisolites are either circular or elliptical in shape with the diameter ranging from 3 cm to 4 cm. The pisolitic dolomite is marked by the grain supporting and the sparitic cementing, and its content of cements ranges from 15 % to 35 %.
Fig. 2 Sparitic gravel oolitic dolomite in the Feixiang Fm. From the Well Puguang-2 Well (2.5×4)
Fig. 4 Intra-grain dissolved pores with partial filling of asphalt in pores resulting from the selective solution, oolitic dolomite
grainstonet Puguang-2 Well, molding thin section
(2.5×4) of the Feixianguan Fm. at the Puguang-2 Well,
(2.5×4) of the Triassic Feixianguan Fm.
molding thin section
Certain sedimentary structures, such as horizontal beddings and cross beddings develop in the oolitic dolomite of the shallow bank faices. The shallow bank facies is marked by the
vertical sedimentary succession of the upward increasing grain size and content, as well as, the thickening rock beds, which is a succession from micritic dolomites to oolitic dolomites. The
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
shallow bank facies in the study area can be subdivided into several subfacies and microfacies. The subfacies of the shallow bank facies include the oolite bank, the gravel oolite bank, the arenite bank, and little evaporitic tidal flat; the microfacies contains the sparitic oolitic dolomite, the sparitic gravel oolitic dolomite, the sparitic bioclastic dolomite, the lag deposit of breccia dolomite, the nonbedded dolomitic mudstone, and so on. As shown in Fig. 5, beds of No. 116 to No. 139 form the facies of platform-margin facies that is made up of French gray to off-white oolitic dolomies and sparitic oolitic dolomites interbedded with gray micritic or fine crystalline dolomites. Beds from No. 5 to No. 32 of the Permian Changxing Fm. at the Puguang-6 Well form exposed shallow bank deposits in the margin of the carbonate platform (Fig. 6) that are
Fig. 5
made up of French gray sparitic bioclastic dolomites with certain dissolved pores, residual bioclastic dolomites interbedded with rudite dolomites with dissolved pores, and powered-crystal dolomites with dissolved pores. Beds from No. 113 to No. 124 of the Permian Changxing Fm. at the Maoba-3 Well form the deposits of the platform-margin bank facies (Fig. 7) that are made up of French gray residual arenite dolomites with dissolved pores with interbeds of fine to powdered crystal dolomites. This type of deposits includes several subfacies, that is, the bioclastic bank, the evaporitic tidal-flat and so on, where the following microfacies develop: the sparitic arenite domomite, the arenitic dolomitic mudstone, the nonlaminitic dolomitic mudstone, the gypsum dolomitic mudstone, and so on.
Diagram showing the succession of sedimentary facies of beds from the No. 116 to the No. 139
which the Triassic Feixianguan Fm. at the Puguang-2 Well
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2.2
Platform-margin reef
The platform-margin reef facies mainly develop in the upper Submember of the first Member of the Permian Changxing Fm. at many wells, such as the Maoba-3 Well, the Puguang-5 Well, the Puguang-6 Well, and so on. This
Fig. 6
facies is made up of gray limestones and the dolomites of baffling sponge reef, and gray limestones and dolomites of frame sponge reef. Few reef-building organisms in the reef,
Sedimentary succession of the shallow bank in the Permian Changxing Fm. at the Well Puguang-6
Fig. 7
Sedimentary succession of the shallow platform-margin bank formed by beds from
No. 113 to No. 124 of Permian Changxing Fm. at the Well Maoba-3
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and the content of reef-building organisms ranges from 25 % to 35 %. Detailed observation demonstrates that certain sponges grow in the original pattern and certain grow in a chaotic pattern, and a few sponge form the reef framework with lots of calcite micrites and bioclastics. The content of the reef-secondary organisms ranges from 5 % to 10 %, and the content of filling matters ranges from 60 % to 70 %. The reef rock chiefly includes the dark gray dolomites and limestones of sponge-baffling reef, in which dissolved pores or voids develop especially for the reef dolomites. (1) Subfacies of sponge bafflestones Sponge bafflestones are mainly made up of branch and fasciculate sponges (Fig. 8), few bryozoan and stromatolite including certain reef-secondary organisms, such as brachia and forminifer. The content of reef-building organisms ranges from 25% to 35%. Detailed observation demonstrates that
Fig. 8
certain sponges grow in an original pattern and certain in a chaotic pattern, and a few sponge form the reef framework with lots of calcite micrites and bioclastics. The content of the reef-secondary organisms ranges from 5 % to 10 % but the content of filling matters ranges from 60 % to 70 %. A lot of frame voids develop in the reef bafflestone, and the wall of voids is formed by calcite peel shells but the central parts of the voids are filled by coarse calcite crystals. The bafflestone includes the gray dolomites and limestones of sponge bafflestones, in which dissolved pores and voids develop especially for the dolomite. (2) Subfacies of sponge framestones Sponge framestones (Fig. 9) are formed by the original growth of sponges and bryzoans, and these reef-building organisms are bound and enveloped by a lot of algae, which form the frame fabric of the reef.
Sponge bafflestones of the Permian Changxing Fm.
Photo A is a vertical section at the Puguang-5 Well and Photo B is the outcrop at the Tiechanghe section in Tongjiang
Fig. 9
Sponge-frame limestones of the Permian Changxing Fm.
Photo A is a vertical section at the Puguang-5 Well, and Photo B is the outcrop at the Panlongdong section in Xuanhan
The reef-building organisms are frequently filled by micritic calcites, bioclastics, and arenites. The content of reef-building organisms ranges from 50 % to 60 %, that of the reef-secondary organisms ranges from10 % to 20 %, and that of filling matters ranges from 30 % to 40 %. This framestone
includes limestones, dolimotes, and dolomitic limestones of sponge framestones, which are rich in frame voids with the void wall of peel shell calcite and the filling of calcite coarse crystals in the center of the voids.
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3 Seismic facies from Permian Changxing Fm. to Triassic Feixianguan Fm. 3.1
Seismic recognizing marks of shallow bank facies
Wells indicates that the platform-margin shallow bank facies is developed in the first and second Members of the Triassic Feixianguan Fm. at many wells, that is, the Puguang-1 Well, the Puguang-2 Well, the Puguang-5 Well, the Puguang-6 Well, the Po-1 Well, the Po-2 Well, and wells from the Du-1 to the Du-4 Wells. Deposits of the shallow
Fig. 10
3.2
bank are characterized by certain particular image features, such as: 1) the general strong swing and the medium-to-low frequencies (Fig. 10 to Fig. 13); 2) most of them are discontinuous except a few (Fig. 10); 3) most of them are sub-parallel. The shallow bank is distributed between the restricted platform facies and the slope facies. The restricted platform facies is marked by the weak swing, the continued and sun-parallel reflection, and the medium-to-low frequencies, and the slope facies is marked by the seismic progradation fabric.
Section of the seismic facies crossing the Well Laojun-1
Seismic facies marks of the platform-margin reef
Organic reefs are particularly sedimentary bodies growing in the platform-margin, which are frequently in the shape of lens with massive structures. The deposits are made up of limestones and dolomites of reef that are different from the circumferences strata. Therefore, organic reefs are frequently in the lens shaped with strong or weak swings in seismic section. The reef at the Puguang-1 and the Puguang-2 Wells are marked by the strong swing (Figs.12 and 13), but the reef at the Laojun-1 Well is characteristic of the weak swing (Figs. 10 and 11). Most of the reefs are discontinuous with the medium-to-low frequencies and the weak progradational fabric. A few reefs are continuous, and certain reefs are sub-parallel and certain
chaotic. Organic reefs distribute between the open platform facies and the slope facies. The open-platform facies is marked by the weak swing, the continued and medium frequencies, the sub-parallel and weak progradational fabric; the slope facies is characteristic of the progradational fabric.
4 Sedimentary facies and reservoirs from Permian Changxing Fm.to Triassic Feixianguan Fm. 4.1
Spatial distribution of the platform-margin reef facies
Depositional units of the Permian Changxing Fm. in the northeastern part of the Sichuan Basin include the basin, the ramp, the open platform, the platform margin, and so on[25].
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Fig. 11
Seismic section crossing the Laojun-1 Well and the Well Chuanyue-83
Fig. 12
Seismic section crossing the Puguang-6 Well and the Well Puguang-2
Fig. 13
Seismic section crossing the Puguang-6 Well and the Well Puguang-5
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Fig. 14 Outline map showing the distribution of sedimentary facies for the Permian Changxing Fm. in the northeastern part of the Sichuan Basin
As shown in Fig. 14, the platform-margin facies is mainly distributed in the following regions; the margin of carbonate platform in the western part of the West-Hubei Basin and the Guangwang Basin, margin of carbonate platform in both the eastern and the western parts of carbonate platform of the Kaijiang-Liangping shelf. The facies zone of reef and bank facies in the western part of the West-Hubei Basin is outcropped in the following regions: the Panlongdong region, the Yanggudong region, the Honghua region, and the Macao region of the Kaixian County of Sichuan Province. The extending distance of this facies zone is about 150 km with the width of 20 km and thickness ranging from several meters to 100 m. The reef is frequently paragenerated with the bank, and the reef facies is dolomitizational or weak dolomitizational, but the oolitic bank is almost dolomitizational, which indicates that the dolomitization is as a result of exposure. The reef and bank deposits being rich in dissolved pores and voids become excellent reservoirs. The reef and bank of the Panlongdong rich in asphalt suggest that they are a residual paleo-oil pool[1,14,22]. The facies zone of reef and bank in the eastern parts of both the Guangwang Basin and the Liangping-Kaijiang Shelf are distributed along the following region: the Tiechanghe Forestry Centre, the Shutanghe region toward east to many other wells through the Hechiliang region, and the wells include the Maoba-3, the Puguang-5, the Puguang-6, the
Huanglong-1, the Huanglong-4, and the Tiandong-1 Wells. The extending distance of the reef bank facies is about 200 km with a width of more than 10 km. In this reef-bank assemblage, the scope or reef is small with a thickness of several meters to more than 10 m and with the discontinued distribution; certain reef bodies have been dolomitized. But the shallow bank facies that are made up by oolitic dolomites contain thicknesses that range from several tens of meters to more than 100 m and with thorough dolomitization. The deposits of the platform-margin reef and bank facies rich in various types of the dissolved pores and voids become excellent reservoir rocks. The facies zone of the western parts of both the Guangwang Basin and the Kaijiang-Liangping Shelf are distributed in the Tieshanpo region; it can be inferred that this facies zone extends to the Yuanba region of Bazhong County toward the west. As shown in Fig. 15, the platform-margin exposed shallow bank facies of the Triassic. Feixianguan Fm. is distributed along the two sides of the Liangping-Kaijiang Shelf. This facies zone is distributed along the following regions toward the east: the Tiechanghe region, the Hechiliang region, the Maoba-1, the Puguang-1, the Puguang-2, the Puguang-5, and the Puguang-6 Wells; it can be inferred that this facies zone extends to the Yuanba region of the Bazhong County toward the west. French gray thick-bedded to massive sparitic oolitic dolomites, sparitic gravel oolitic dolomites and sparitic arenite dolomites
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
constitute this facies. Rocks of this facies is marked by the high extent of crystallization, which are rich in inter-crystalline pores, inter-grained pores, and dissolved
pores and become useful reservoirs of natural gas. A great deal of industrially natural gases has been found in this kind of facies.
Fig. 15 Outline map showing the distribution of sedimentary facies for the second Member of the Triaccic Feixianguan Fm. in the northeastern part of the Sichuan Basin
4.2
Platform-margin reef, bank facies and reservoirs
Detailed studies show that the development of gas reservoirs of the Triassic Feixianguan Fm. in Daxian-Xuanhan area are controlled by multiple factors, among which the sedimentary facies is the principle controlling factor that is the chief constraint for the petrological features, the quality, and the spatial distribution of the natural-gas reservoirs. (1) Sedimentary environment and reservoir qualities Porosity is an important parameter that is used to evaluate the quality of reservoirs. The statistic result indicates that an obvious relationship existed between the quality of reservoirs and the facies including the subfacies and the microfacies. The best reservoir horizons are located in the oolitic shallow-bank subfacies, and the value of the porosity ranges from 3.75 % to 28.86 % with an average value of 8.12 %. And the changing scope of the permeability for the best reservoirs is very small, that is, the changing scope ranges from 0.0115 × 10–3 μm2 to 3354.6965×10–3 μm2. The average value of porosity and permeability for the residual oolitic dolomites of the Triassic Feixianguan Fm. are 8.47 % and 252.1267×10–3μm2, respectively. For the oolitic dolomite, the average value of porosity is 8.47 % and that of permeability is 1.0815×10–3μm2; for the sugar-granule residual oolitic dolomite, the average value of porosity is 13.39 % and the value of permeability is larger than the value of
100×10–3μm2 with the largest value of 3354.6965×10–3μm2; for the rudite oolitic dolomite, the average value of porosity is 10.56 %, and the value of permeability is smaller than 1.0×10–3μm2 with the changing scope ranging from 0.02×10–3μm2 to 0.25×10–3 μm2. Thus, the order of reservoir quality of petrological types is as follows: sugar-granule residual oolitic dolomite, the rudite oolitic dolomite, residual oolitic dolomite, and oolitic dolomite. And the order of reservoir quality of microfacies is as follows: microfacies of sparitic oolitic dolomite, microfacies of sparitic residual oolitic dolomite, the microfacies of sparitic arenite dolomite, and other microfacies are generally bad. As shown in Table 1, 77 analysis examples of the Permian Changxing Fm. demonstrate the following result: 1) 33 examples belong to the shallow reef-back bank facies accounting for 43 % of the total number of examples, their average value of porosity is 4.72 %; the numbers of both the reef-front facies and the slope facies account for 1.36 % and 1.28 %, respectively. For the 26 examples with the porosity larger than 3 %, example number of the reef-back is 16 accounting for 62 % of the total number, and their average value of porosity is 8.75 %; example number of the reef facies is 8 accounting for 31 % of the total number, and their average value of porosity is 4.6 %; example number of the reef-front slope is only 2 with average value or porosity of 5.23 %. Therefore, the best reservoir is that of the reef-back shallow-
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Table 1 Porosities in different sedimentary facies of the Permian Changxing Fm. Porosity
The example number with porosity value that is larger 3
Total number of examples and porosity (%) Example
Facies
Lowest-Highest
Average
33
0.24–18.05
4.72
24
0.24–6.46
20
0.19–3.58
number Reef-back shallow bank Reef (Reef core) Reef-front slope
% and their porosity values (%) Example
Lowest-Highest
Average
16
3.04–18.05
8.75
1.36
8
3.00–6.46
4.60
1.28
2
3.58–6.87
5.23
bank rock. (2) Petrological types and their reservoir qualities As shown in Table 2, different petrological types in the same sedimentary environment have different features of the porosity. The porosity of dolomites is clearly larger than that of limestones, in which most of them belong to the reef-back bank facies. Among 77 examples of the Changxing Fm., dolomite examples with the average porosity value of 6.1 % account for 34 %; among the 26 examples with the porosity of larger than 3 %, the dolomite examples account for 85 %, and their average value of the porosity is 6.88 %. Limestone
number
examples account for 66 % of the total example number, and their average value of the porosity is only 1.58 %; the examples with the porosity of larger than 3 % account for 15 % among the total limestone examples. And the examples with average porosity value of 10.32 % account for only 5 % of the total limestone examples, in which most of them belong to the reef-back shallow bank facies. For limestone examples of the reef facies and the reef-front slope facies, there are no examples with the porosity of larger than 3 %. In summary, the porosity of dolomites is the highest, and most of them belong to the reef-back shallow bank facies.
Table 2 Porosity values of different petrological types in the Permian Changxing Fm. Example number and porosity (%) with the porosity
Example number and porosity (%)
value larger than 3 %
Example Facies
Lithology
Example
Number
number of
Lowest
of
dolomite
value/Highest
examples
or
value
Average porosity
Number
number of
Lowest
of
dolomite
value/Highest
examples
or
value
limestone Reef-back
dolomite
bank
16
limestone Reef
Reef
dolomite
1.19/18.05
6.57
Reef
Reef-front
dolomite
slope
limestone
12
3.04/18.05
8.22
4
3.11/14.07
10.32
8
3.00/6.46
4.60
0
/
/
2
3.58/6.87
5.23
0
/
/
16 17
0.24/14.07
2.99
10
2.47/6.46
4.20
24
limestone
porosity
limestone
33
shallow
Average
8 14
0.27/1.02
0.60
2
3.58/6.87
5.23
20
2 18
0.19/2.69
Certain dissolved limestones of the reef-back shallow bank facies also have high porosity. That is, only those dolomitized reefs have the porosity larger than 3 %. Dolomites with porosity larger than 3 % include the algae-mat microcrystallinedolomite, the residual oolitic dolomite, the semi-automorphic to automorphic crystalline domomite, and the reef dolomite, which is the result of the strong
1.19
reconstruction caused by dolomitization, diagenetic solution, and corrosion. For limestones, those dissolved limestones and oolitic grainstones become good reservoirs.
5
Conclusions Detail studies including the outcrop sections, wells, and
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
seismic sections indicate that deposits of both the reef and the shallow bank are widespread and are distributed in strata from the Permian Changxing Fm. to the Triassic Feixianguan Fm. along the carbonate platform in the Daxian-Xuanhan area. These deposits form a facies zone of ramp-margin reef and bank, which provides an important basis for the development of good reservoirs of natural gas. The lithologic types of the platform-margin reef and bank facies chiefly include the following types: the french gray thick-bedded to massive sparitic dolomite, the sparitic rudite oolitic dolomite, the sparitic psiolitic and oolitic dolomite, the sparitic bioclastic dolomite and the sparitic arenitic dolomite. And facies zone is a main controlling factor to the reservoir, which direct controls the lothology and the quality of reservoirs and their spatial distribution.
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RESEARCH PAPER
Reef-Bank Features and Their Constraint to Reservoirs of Natural Gas, from Permian Changxing Formation to Triassic Feixianguan Formation in Daxian-Xuanhan Area of Sichuan Province, South China MA Yongsheng1, *, MOU Chuanlong2, TAN Qinyin2, YU Qian2, WANG Ruihua2 1 Southern Exploration and Exploitation Filiale, China Petrochemical Corporation (SINOPEC), Kunming 650200, China 2 Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China
Abstract: Detail studies of the seismic section and certain wells, such as the Puguang-2, the Puguang-6, and the Maoba Wells, demonstrate that the reef facies from the Permian Changxing Formation to the Triassic Feixianguan Formation are expressed as the lens with medium-strong changing swings and chaotic images in the seismic section. The reef facies is made up of gray limestones and dolomites of both baffling sponge reef and the frame sponge reef. Moreover, the bank facies from the Permian Changxing Formation to the Triassic Feixianguan Formation is represented as the lens with medium-strong changing swings and discontinuity images in the seismic section, which is made up of several petrologic types, that is, the French gray thick-bedded to massive sparitic oolitic dolomite, the sparitic gravel-oolitic dolomite, the sparitic pisolitic-oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. Both the reef facies and the bank facies concomitant are developed and distributed in space along the platform margin, which form a special reef-bank facies zone. This facies zone controls the petrological features, the reservoir qualities, and the spatial distribution of the reservoirs of natural gas in the Daxian-Xuanhan area. Key Words:
1
reef-bank facies; reservoirs; from the Permian Changxing Fm. to the Triassic Fm; the Daxian-Xuanhan area
Introduction
The restriction of actual materials inhibits the systematic study on sedimentary facies in the study area especially for the detail analysis and description regarding the reef and bank facies. Certain geologists completed the study relating to sedimentary facies and its relative reservoir of natural gas[1–25]. The Permian Changxing Fm. to the Triassic Feixianguan Fm. is an important horizon for the exploration of oil and gas. With the continuous disclosure of certain wells and the good revealing of seismic materials, the depositional body of reefs and banks are observed through further exploration. Based on the former researching fruits, as well as, according to the materials of wells and seismic sections, the sedimentary features of reefs and banks are described in detail and their constraints to the reservoir are discussed in this study.
2 Sedimentary facies features of the Changxing Fm. to the Feixianguan Fm. In the depositional period from the Permian Changxing Fm. to the Triassic Feixianguan Fm., the Daxian-Xuanhan area is marked by both the carbonate platform in the eastern part and the shelf in the western part, where many types of sedimentary facies are developed. The types of sedimentary facies include the evaporation platform, the restricted platform, the open platform, the shallow bank of platform margin, the reef of platform margin, the ramp and the shelf facies. Generally, sedimentary facies of reefs and banks are developed in the Permian Changxing Fm. and shallow bank facies are highly developed in the Triassic Feixianguan Fm. 2.1
Shallow bank
Shallow bank facies in the transitional period between the
Received date: 2006-11-25. *Corresponding author: E-mail: [email protected] Foundation item: Supported by the National Key Basic Research Development Program “973” of China (No.2005CB422106). Copyright © 2007, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B. V. All rights reserved.
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Permian and the Triassic of the study area can be subdivided into the intra-platform shallow bank and the platform- margin shallow bank facies, and the former is more widespread than the latter. A few intra-platform shallow banks can be observed in the Permian Changxing Fm. within certain wells, such as the Puguang-4 Well and the Po-1 Well. This facies is developed in high energy locations within the open-platform facies zone and is made up of the sparite including the sparitic calcarenite and the sparitic oolitic limestone. According to sediment types, shallow bank can be further divided into the bioclastic bank, the arenite bank, and the oolitic bank, but the bioclastic bank is the chief type of bank. The platform-margin bank is chiefly developed in the second Member of the Permian Changxing Fm. and the first and second Members of the Triassic Feixianguan Fm. Spatially, this type of sedimentary facies can be found in many wells, such as the Po-1 Well, the Po-2 Well, the Puguang-2 Well, the Puguang-5 Well, and the Puguang-6 Well, and is made up of the thick-bedded to massive sparitic
Fig. 1
Sparitic oolitic dolomite in the Triassic Feixianguan
Fm. From the Well Puguang-2 Well (2.5×4)
Fig. 3
Mold pores and intra-grain dissolved pores of oolites
without filling and with asphalt in the pore wall, oolitic
oolitic dolomite (Fig. 1), the sparitic gravel oolitic dolomite (Fig. 2), the sparitic pesolitic oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. The oolite is the main type of sedimentary grain, and its content ranges from 65 % to 85 % in the sedimentary rock. There are several kinds of oolites, such as the high-energy oolite, the thin-skinned oolite, and the compound oolite. Certain oolites are deformed into the oolitic mold pore (Fig. 3) and the intra-grain dissolved pore (Fig. 4). The configuration of oolites is chiefly round and oval, and the diameter of most oolites is 1 mm. Other types of grains in the platform-margin bank facies include arenite, rudite, and pisolite. Micritic dolomite and the oolitic dilomite make up the chief composition of rudite with the configuration of the irregular sub-circularity and with the diameter ranging from 4 mm to 5 mm and even to 1 cm. Most of the pisolites are either circular or elliptical in shape with the diameter ranging from 3 cm to 4 cm. The pisolitic dolomite is marked by the grain supporting and the sparitic cementing, and its content of cements ranges from 15 % to 35 %.
Fig. 2 Sparitic gravel oolitic dolomite in the Feixiang Fm. From the Well Puguang-2 Well (2.5×4)
Fig. 4 Intra-grain dissolved pores with partial filling of asphalt in pores resulting from the selective solution, oolitic dolomite
grainstonet Puguang-2 Well, molding thin section
(2.5×4) of the Feixianguan Fm. at the Puguang-2 Well,
(2.5×4) of the Triassic Feixianguan Fm.
molding thin section
Certain sedimentary structures, such as horizontal beddings and cross beddings develop in the oolitic dolomite of the shallow bank faices. The shallow bank facies is marked by the
vertical sedimentary succession of the upward increasing grain size and content, as well as, the thickening rock beds, which is a succession from micritic dolomites to oolitic dolomites. The
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
shallow bank facies in the study area can be subdivided into several subfacies and microfacies. The subfacies of the shallow bank facies include the oolite bank, the gravel oolite bank, the arenite bank, and little evaporitic tidal flat; the microfacies contains the sparitic oolitic dolomite, the sparitic gravel oolitic dolomite, the sparitic bioclastic dolomite, the lag deposit of breccia dolomite, the nonbedded dolomitic mudstone, and so on. As shown in Fig. 5, beds of No. 116 to No. 139 form the facies of platform-margin facies that is made up of French gray to off-white oolitic dolomies and sparitic oolitic dolomites interbedded with gray micritic or fine crystalline dolomites. Beds from No. 5 to No. 32 of the Permian Changxing Fm. at the Puguang-6 Well form exposed shallow bank deposits in the margin of the carbonate platform (Fig. 6) that are
Fig. 5
made up of French gray sparitic bioclastic dolomites with certain dissolved pores, residual bioclastic dolomites interbedded with rudite dolomites with dissolved pores, and powered-crystal dolomites with dissolved pores. Beds from No. 113 to No. 124 of the Permian Changxing Fm. at the Maoba-3 Well form the deposits of the platform-margin bank facies (Fig. 7) that are made up of French gray residual arenite dolomites with dissolved pores with interbeds of fine to powdered crystal dolomites. This type of deposits includes several subfacies, that is, the bioclastic bank, the evaporitic tidal-flat and so on, where the following microfacies develop: the sparitic arenite domomite, the arenitic dolomitic mudstone, the nonlaminitic dolomitic mudstone, the gypsum dolomitic mudstone, and so on.
Diagram showing the succession of sedimentary facies of beds from the No. 116 to the No. 139
which the Triassic Feixianguan Fm. at the Puguang-2 Well
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2.2
Platform-margin reef
The platform-margin reef facies mainly develop in the upper Submember of the first Member of the Permian Changxing Fm. at many wells, such as the Maoba-3 Well, the Puguang-5 Well, the Puguang-6 Well, and so on. This
Fig. 6
facies is made up of gray limestones and the dolomites of baffling sponge reef, and gray limestones and dolomites of frame sponge reef. Few reef-building organisms in the reef,
Sedimentary succession of the shallow bank in the Permian Changxing Fm. at the Well Puguang-6
Fig. 7
Sedimentary succession of the shallow platform-margin bank formed by beds from
No. 113 to No. 124 of Permian Changxing Fm. at the Well Maoba-3
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and the content of reef-building organisms ranges from 25 % to 35 %. Detailed observation demonstrates that certain sponges grow in the original pattern and certain grow in a chaotic pattern, and a few sponge form the reef framework with lots of calcite micrites and bioclastics. The content of the reef-secondary organisms ranges from 5 % to 10 %, and the content of filling matters ranges from 60 % to 70 %. The reef rock chiefly includes the dark gray dolomites and limestones of sponge-baffling reef, in which dissolved pores or voids develop especially for the reef dolomites. (1) Subfacies of sponge bafflestones Sponge bafflestones are mainly made up of branch and fasciculate sponges (Fig. 8), few bryozoan and stromatolite including certain reef-secondary organisms, such as brachia and forminifer. The content of reef-building organisms ranges from 25% to 35%. Detailed observation demonstrates that
Fig. 8
certain sponges grow in an original pattern and certain in a chaotic pattern, and a few sponge form the reef framework with lots of calcite micrites and bioclastics. The content of the reef-secondary organisms ranges from 5 % to 10 % but the content of filling matters ranges from 60 % to 70 %. A lot of frame voids develop in the reef bafflestone, and the wall of voids is formed by calcite peel shells but the central parts of the voids are filled by coarse calcite crystals. The bafflestone includes the gray dolomites and limestones of sponge bafflestones, in which dissolved pores and voids develop especially for the dolomite. (2) Subfacies of sponge framestones Sponge framestones (Fig. 9) are formed by the original growth of sponges and bryzoans, and these reef-building organisms are bound and enveloped by a lot of algae, which form the frame fabric of the reef.
Sponge bafflestones of the Permian Changxing Fm.
Photo A is a vertical section at the Puguang-5 Well and Photo B is the outcrop at the Tiechanghe section in Tongjiang
Fig. 9
Sponge-frame limestones of the Permian Changxing Fm.
Photo A is a vertical section at the Puguang-5 Well, and Photo B is the outcrop at the Panlongdong section in Xuanhan
The reef-building organisms are frequently filled by micritic calcites, bioclastics, and arenites. The content of reef-building organisms ranges from 50 % to 60 %, that of the reef-secondary organisms ranges from10 % to 20 %, and that of filling matters ranges from 30 % to 40 %. This framestone
includes limestones, dolimotes, and dolomitic limestones of sponge framestones, which are rich in frame voids with the void wall of peel shell calcite and the filling of calcite coarse crystals in the center of the voids.
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3 Seismic facies from Permian Changxing Fm. to Triassic Feixianguan Fm. 3.1
Seismic recognizing marks of shallow bank facies
Wells indicates that the platform-margin shallow bank facies is developed in the first and second Members of the Triassic Feixianguan Fm. at many wells, that is, the Puguang-1 Well, the Puguang-2 Well, the Puguang-5 Well, the Puguang-6 Well, the Po-1 Well, the Po-2 Well, and wells from the Du-1 to the Du-4 Wells. Deposits of the shallow
Fig. 10
3.2
bank are characterized by certain particular image features, such as: 1) the general strong swing and the medium-to-low frequencies (Fig. 10 to Fig. 13); 2) most of them are discontinuous except a few (Fig. 10); 3) most of them are sub-parallel. The shallow bank is distributed between the restricted platform facies and the slope facies. The restricted platform facies is marked by the weak swing, the continued and sun-parallel reflection, and the medium-to-low frequencies, and the slope facies is marked by the seismic progradation fabric.
Section of the seismic facies crossing the Well Laojun-1
Seismic facies marks of the platform-margin reef
Organic reefs are particularly sedimentary bodies growing in the platform-margin, which are frequently in the shape of lens with massive structures. The deposits are made up of limestones and dolomites of reef that are different from the circumferences strata. Therefore, organic reefs are frequently in the lens shaped with strong or weak swings in seismic section. The reef at the Puguang-1 and the Puguang-2 Wells are marked by the strong swing (Figs.12 and 13), but the reef at the Laojun-1 Well is characteristic of the weak swing (Figs. 10 and 11). Most of the reefs are discontinuous with the medium-to-low frequencies and the weak progradational fabric. A few reefs are continuous, and certain reefs are sub-parallel and certain
chaotic. Organic reefs distribute between the open platform facies and the slope facies. The open-platform facies is marked by the weak swing, the continued and medium frequencies, the sub-parallel and weak progradational fabric; the slope facies is characteristic of the progradational fabric.
4 Sedimentary facies and reservoirs from Permian Changxing Fm.to Triassic Feixianguan Fm. 4.1
Spatial distribution of the platform-margin reef facies
Depositional units of the Permian Changxing Fm. in the northeastern part of the Sichuan Basin include the basin, the ramp, the open platform, the platform margin, and so on[25].
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Fig. 11
Seismic section crossing the Laojun-1 Well and the Well Chuanyue-83
Fig. 12
Seismic section crossing the Puguang-6 Well and the Well Puguang-2
Fig. 13
Seismic section crossing the Puguang-6 Well and the Well Puguang-5
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Fig. 14 Outline map showing the distribution of sedimentary facies for the Permian Changxing Fm. in the northeastern part of the Sichuan Basin
As shown in Fig. 14, the platform-margin facies is mainly distributed in the following regions; the margin of carbonate platform in the western part of the West-Hubei Basin and the Guangwang Basin, margin of carbonate platform in both the eastern and the western parts of carbonate platform of the Kaijiang-Liangping shelf. The facies zone of reef and bank facies in the western part of the West-Hubei Basin is outcropped in the following regions: the Panlongdong region, the Yanggudong region, the Honghua region, and the Macao region of the Kaixian County of Sichuan Province. The extending distance of this facies zone is about 150 km with the width of 20 km and thickness ranging from several meters to 100 m. The reef is frequently paragenerated with the bank, and the reef facies is dolomitizational or weak dolomitizational, but the oolitic bank is almost dolomitizational, which indicates that the dolomitization is as a result of exposure. The reef and bank deposits being rich in dissolved pores and voids become excellent reservoirs. The reef and bank of the Panlongdong rich in asphalt suggest that they are a residual paleo-oil pool[1,14,22]. The facies zone of reef and bank in the eastern parts of both the Guangwang Basin and the Liangping-Kaijiang Shelf are distributed along the following region: the Tiechanghe Forestry Centre, the Shutanghe region toward east to many other wells through the Hechiliang region, and the wells include the Maoba-3, the Puguang-5, the Puguang-6, the
Huanglong-1, the Huanglong-4, and the Tiandong-1 Wells. The extending distance of the reef bank facies is about 200 km with a width of more than 10 km. In this reef-bank assemblage, the scope or reef is small with a thickness of several meters to more than 10 m and with the discontinued distribution; certain reef bodies have been dolomitized. But the shallow bank facies that are made up by oolitic dolomites contain thicknesses that range from several tens of meters to more than 100 m and with thorough dolomitization. The deposits of the platform-margin reef and bank facies rich in various types of the dissolved pores and voids become excellent reservoir rocks. The facies zone of the western parts of both the Guangwang Basin and the Kaijiang-Liangping Shelf are distributed in the Tieshanpo region; it can be inferred that this facies zone extends to the Yuanba region of Bazhong County toward the west. As shown in Fig. 15, the platform-margin exposed shallow bank facies of the Triassic. Feixianguan Fm. is distributed along the two sides of the Liangping-Kaijiang Shelf. This facies zone is distributed along the following regions toward the east: the Tiechanghe region, the Hechiliang region, the Maoba-1, the Puguang-1, the Puguang-2, the Puguang-5, and the Puguang-6 Wells; it can be inferred that this facies zone extends to the Yuanba region of the Bazhong County toward the west. French gray thick-bedded to massive sparitic oolitic dolomites, sparitic gravel oolitic dolomites and sparitic arenite dolomites
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
constitute this facies. Rocks of this facies is marked by the high extent of crystallization, which are rich in inter-crystalline pores, inter-grained pores, and dissolved
pores and become useful reservoirs of natural gas. A great deal of industrially natural gases has been found in this kind of facies.
Fig. 15 Outline map showing the distribution of sedimentary facies for the second Member of the Triaccic Feixianguan Fm. in the northeastern part of the Sichuan Basin
4.2
Platform-margin reef, bank facies and reservoirs
Detailed studies show that the development of gas reservoirs of the Triassic Feixianguan Fm. in Daxian-Xuanhan area are controlled by multiple factors, among which the sedimentary facies is the principle controlling factor that is the chief constraint for the petrological features, the quality, and the spatial distribution of the natural-gas reservoirs. (1) Sedimentary environment and reservoir qualities Porosity is an important parameter that is used to evaluate the quality of reservoirs. The statistic result indicates that an obvious relationship existed between the quality of reservoirs and the facies including the subfacies and the microfacies. The best reservoir horizons are located in the oolitic shallow-bank subfacies, and the value of the porosity ranges from 3.75 % to 28.86 % with an average value of 8.12 %. And the changing scope of the permeability for the best reservoirs is very small, that is, the changing scope ranges from 0.0115 × 10–3 μm2 to 3354.6965×10–3 μm2. The average value of porosity and permeability for the residual oolitic dolomites of the Triassic Feixianguan Fm. are 8.47 % and 252.1267×10–3μm2, respectively. For the oolitic dolomite, the average value of porosity is 8.47 % and that of permeability is 1.0815×10–3μm2; for the sugar-granule residual oolitic dolomite, the average value of porosity is 13.39 % and the value of permeability is larger than the value of
100×10–3μm2 with the largest value of 3354.6965×10–3μm2; for the rudite oolitic dolomite, the average value of porosity is 10.56 %, and the value of permeability is smaller than 1.0×10–3μm2 with the changing scope ranging from 0.02×10–3μm2 to 0.25×10–3 μm2. Thus, the order of reservoir quality of petrological types is as follows: sugar-granule residual oolitic dolomite, the rudite oolitic dolomite, residual oolitic dolomite, and oolitic dolomite. And the order of reservoir quality of microfacies is as follows: microfacies of sparitic oolitic dolomite, microfacies of sparitic residual oolitic dolomite, the microfacies of sparitic arenite dolomite, and other microfacies are generally bad. As shown in Table 1, 77 analysis examples of the Permian Changxing Fm. demonstrate the following result: 1) 33 examples belong to the shallow reef-back bank facies accounting for 43 % of the total number of examples, their average value of porosity is 4.72 %; the numbers of both the reef-front facies and the slope facies account for 1.36 % and 1.28 %, respectively. For the 26 examples with the porosity larger than 3 %, example number of the reef-back is 16 accounting for 62 % of the total number, and their average value of porosity is 8.75 %; example number of the reef facies is 8 accounting for 31 % of the total number, and their average value of porosity is 4.6 %; example number of the reef-front slope is only 2 with average value or porosity of 5.23 %. Therefore, the best reservoir is that of the reef-back shallow-
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
Table 1 Porosities in different sedimentary facies of the Permian Changxing Fm. Porosity
The example number with porosity value that is larger 3
Total number of examples and porosity (%) Example
Facies
Lowest-Highest
Average
33
0.24–18.05
4.72
24
0.24–6.46
20
0.19–3.58
number Reef-back shallow bank Reef (Reef core) Reef-front slope
% and their porosity values (%) Example
Lowest-Highest
Average
16
3.04–18.05
8.75
1.36
8
3.00–6.46
4.60
1.28
2
3.58–6.87
5.23
bank rock. (2) Petrological types and their reservoir qualities As shown in Table 2, different petrological types in the same sedimentary environment have different features of the porosity. The porosity of dolomites is clearly larger than that of limestones, in which most of them belong to the reef-back bank facies. Among 77 examples of the Changxing Fm., dolomite examples with the average porosity value of 6.1 % account for 34 %; among the 26 examples with the porosity of larger than 3 %, the dolomite examples account for 85 %, and their average value of the porosity is 6.88 %. Limestone
number
examples account for 66 % of the total example number, and their average value of the porosity is only 1.58 %; the examples with the porosity of larger than 3 % account for 15 % among the total limestone examples. And the examples with average porosity value of 10.32 % account for only 5 % of the total limestone examples, in which most of them belong to the reef-back shallow bank facies. For limestone examples of the reef facies and the reef-front slope facies, there are no examples with the porosity of larger than 3 %. In summary, the porosity of dolomites is the highest, and most of them belong to the reef-back shallow bank facies.
Table 2 Porosity values of different petrological types in the Permian Changxing Fm. Example number and porosity (%) with the porosity
Example number and porosity (%)
value larger than 3 %
Example Facies
Lithology
Example
Number
number of
Lowest
of
dolomite
value/Highest
examples
or
value
Average porosity
Number
number of
Lowest
of
dolomite
value/Highest
examples
or
value
limestone Reef-back
dolomite
bank
16
limestone Reef
Reef
dolomite
1.19/18.05
6.57
Reef
Reef-front
dolomite
slope
limestone
12
3.04/18.05
8.22
4
3.11/14.07
10.32
8
3.00/6.46
4.60
0
/
/
2
3.58/6.87
5.23
0
/
/
16 17
0.24/14.07
2.99
10
2.47/6.46
4.20
24
limestone
porosity
limestone
33
shallow
Average
8 14
0.27/1.02
0.60
2
3.58/6.87
5.23
20
2 18
0.19/2.69
Certain dissolved limestones of the reef-back shallow bank facies also have high porosity. That is, only those dolomitized reefs have the porosity larger than 3 %. Dolomites with porosity larger than 3 % include the algae-mat microcrystallinedolomite, the residual oolitic dolomite, the semi-automorphic to automorphic crystalline domomite, and the reef dolomite, which is the result of the strong
1.19
reconstruction caused by dolomitization, diagenetic solution, and corrosion. For limestones, those dissolved limestones and oolitic grainstones become good reservoirs.
5
Conclusions Detail studies including the outcrop sections, wells, and
MA Yongsheng et al. / Earth Science Frontiers, 2007, 14(1): 182–192
seismic sections indicate that deposits of both the reef and the shallow bank are widespread and are distributed in strata from the Permian Changxing Fm. to the Triassic Feixianguan Fm. along the carbonate platform in the Daxian-Xuanhan area. These deposits form a facies zone of ramp-margin reef and bank, which provides an important basis for the development of good reservoirs of natural gas. The lithologic types of the platform-margin reef and bank facies chiefly include the following types: the french gray thick-bedded to massive sparitic dolomite, the sparitic rudite oolitic dolomite, the sparitic psiolitic and oolitic dolomite, the sparitic bioclastic dolomite and the sparitic arenitic dolomite. And facies zone is a main controlling factor to the reservoir, which direct controls the lothology and the quality of reservoirs and their spatial distribution.
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