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ELSEVIER Natural Gas Industry B 2 (2015) 127-135
Natural Gas Industry B
www.elsevier.com/locate/ngib
Research article
Genesis mechanism of the Sinian-Cambrian reservoirs in the Anyue Gas
Field, Sichuan Basin*
Zhou Jingaoab, Yao Genshunab, Yang Guangc, Zhang Jianyongab, Hao Yia,b, Wang Fanga,
Gu Mingfenga, Li Wenzhenga
1 Hangzhou Branch of Research Institute of Geology, PetroChina, Hangzhou, Zhejiang 310023, China
CNPC Key Laboratory of Carbonate Reservoirs, Hangzhou, Zhejiang 310023, China
' Exploration and Development Research Institute of Southwest Oil & Gasfield Company, PetroChina, Chengdu, Sichuan 610051, China
Received 25 November 2014; accepted 20 January 2015 Available online 31 August 2015
Abstract
The Lower Cambrian Longwangmiao Fm, the 4th and 2nd members of the Sinian Dengying Fm are the three major gas layers in the Anyue Gas Field of the Sichuan Basin. Their main characteristics and genesis mechanism were investigated, and the following three findings were obtained. First, according to sedimentary microfacies, lithology and porosity, the Longwangmiao Fm is identified as fractured-vuggy dolomite reservoir of grain shoal facies, the 4th member of the Dengying Fm as fractured-vuggy (cavernous) dolomite reservoir of cyanobacteria mound beach facies, and the 2nd member of the Dengying Fm as fractured-vuggy dolomite reservoirs of cyanobacteria mound beach facies. Second, the Longwangmiao Fm is mainly grain dolomite, with dissolution pores and vugs as major reservoir space, at an average porosity of 4.24% and an average thickness of 36 m. The 4th member of the Dengying Fm made up of cyanobacteria dolomite has dissolution pores, vugs and caverns as major reservoir space with an average porosity of 3.22% and an average thickness of 70 m. The 2nd member of the Dengying Fm composed of cyanobacteria dolomite has fractures and vugs as major reservoir space with an average porosity of 3.34% and an average thickness of 80 m. Third, those reservoirs experienced multiple evolutionary stages including porosity development, hydrothermal mineral filling, asphalt filling etc. Penecontemporaneous dissolution and supergene karstification are the key factors controlling the formation of the reservoir space and the evolution models of the reservoirs were figured out.
© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Sichuan Basin; Anyue Gas Field; Early Cambrian; Sinian; Reservoir types; Major control factor; Supergene karstification
1. Overview of Anyue Gas Field
Located in Suining city, Anyue county of Ziyang city, Sichuan province, and Tongnan county in Chongqing city, the Anyue Gas Field tectonically lies in the east of Leshan-Longniisi paleo-uplift [1] (Fig. 1). In July 2011, Well Gaoshi 1
* Foundation item: National Science and Technology MajorPro-ject(2011ZX05004-002), PetroChina Exploration and Production Major Proj-ect(2012ZD01-02-03), PetroChina Special Project(2014e-32).
* Corresponding author.
E-mail address: liwz_hz@petrochina.com.cn (Li WZ).
Peer review under responsibility of Sichuan Petroleum Administration.
tested a gas flow of 102 x 104 m3/d at the 2nd member of Sinian Dengying Fm, marking a major breakthrough in the paleo-uplift gas exploration. In September 2012, Well Moxi 8 tested a gas flow of 107 x 104 m3/d at the Lower Cambrian Longwangmiao Fm, marking a historic breakthrough in natural gas exploration of Cambrian. Consequently, fast exploration in Anyue Gas Field began. By the end of 2013, the Cambrian Longwangmiao Fm in Moxi area in central Sichuan had proved geologic gas reserves of 4403.83 x 108 m3, ranking it the largest single carbonate integral gas field ever discovered in China [2].
Major payzones in the Anyue Gas Field are Longwangmiao Fm in Cambrian and Dengying Fm in Sinian. The
http://dx.doi.org/10.1016/j.ngib.2015.07.001
2352-8540/© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Carboniferous Middle and Upper Ordovician City
Fig. 1. Location of the Anyue Gas Field.
Longwangmiao Fm is a set of grain dolomite and muddy to micritic dolomite with gypsum interlayers, a three-order sequence on the whole with type I sequence boundary on the top [3]. The Dengying Fm in the Sichuan Basin is divided into four members from bottom up: the 1st member is dominated by dolomite poor in cyanobacteria in the lower part, the 2nd member is dolomite rich in cyanobacteria, the 3rd member is dominated by clastic rocks; the 4th member, the same with the 1st member, is dominated by dolomite poor in cyanobacteria [4]. Vertically, the 1st member and the 2nd member, the 3rd member and 4th member respectively form two third-order sequences, and the top of each sequence is a type I sequence boundary line. Exploration reveals that the Anyue Gas Field has three sets of industrial gas layers, Longwangmiao Fm, the 4th and 2nd member of Dengying Fm. Research shows that the three sets of gas layers are different in reservoir type. According to sedimentary microfacies, lithology and porosity, the Longwangmiao Fm is identified as fractured-vuggy dolomite reservoir of grain shoal facies, the 4th member of the Den-gying Fm as fractured-vuggy (cavernous) dolomite reservoir of cyanobacteria mound beach facies, and the 2nd member of the Dengying Fm as fractured-vuggy dolomite reservoir of cyanobacteria mound beach facies. Through comprehensive analysis of geological and geophysical data, in combination with experiment analysis, the main characteristics, formation
and evolution mechanisms of the three sets of reservoirs above were examined, in the hope to provide research support for continuous exploration in the area.
2. Reservoir characteristics
2.1. Fractured-vuggy dolomite reservoir of grain shoal facies in Longwangmiao Fm
The dominant lithology of Longwangmiao Fm is residual dolarenite (Fig. 2a), including oolitic dolomite, crystal dolomite and porphyritic dolomite, in which dissolution vugs and pores of 2-6 mm are the main reservoir space (Fig. 2b, c), and residual intergranular pores, intracrystalline pores and fractures are secondary reservoir space [5].
The intrusive mercury curve of grain shoal reservoir in Longwangmiao Fm shows that the initial mercury injection pressure of fractured-vuggy reservoir is mostly less than 0.3 MPa, and the mercury saturation in the low pressure platform of less than 5 MPa ranges from 70% to 80%, while the mercury saturation in high platform is 10%-20%, showing the feature of dual porosity medium with vugs in absolute majority followed by micropores.
Observation of core samples and thin sections shows that dissolution pores are connected by residual intergranular
a. Well Moxi 13,4 607.54 m, Longwangmiao Fm, brown gray dolarenite dolomite with irregular circle (elliptic) dissolved pores and vugs, 2-10 mm in diameter, a small amount of filling materials of dolomite and asphalt, plane porosity of 14%.
b. Well Moxi 17, 4 655.63 m, Longwangmiao Fm, dolarenite, particle size 0.3-0.5 mm, mainly in ellipse shape, with a little granular dolomite cements between grains, rich in residual intergranular pores, blue cast thin section, plane-polarized light.
c. Well Moxi 13,4 613.15 m, Longwangmiao Fm, fine crystalline dolomite, 0.1-0.2 mm in diameter, hypautomorphic or allotriomorphic mosatic structure, rich in intergranular pores and dissolution pores, with asphalt filling, plane porosity 9%, blue cast thin section, plane-polarized light.
d. Well Gaoshi 1,4 975.29 m, the 4'" member of Dengying Fm, irregular wavy algal-laminated dolomite, rich in bedding caves, filled with a small amount of granular dolomite.
e. Well Gaoshi 1, 4 795 m, the 41" member of Dengying Fm, algae bonded dolomicrite, laminar algal in local area, rich in dissolution pores filled with a small amount of granular dolomite, and asphalt later, blue cast thin section, plane-polarized light.
f. Well Gaoshi 1, 4 983.1 m, the 4Ul member of Dengying Fm, karst breccia, rich in dissolution intergravel pores filled by vadose silt, crystalline dolomite and partially asphalt, blue cast thin section, plane-polarized light.
g. Well Xianfeng 1 profile, the 31" layer, the 2"a member of Dengying Fm, cyanobacteria stromatolitic dolomite, crack cutting through bacteria laminae filled by grape-lace dolomite cement later, plane-poiarized light.
h. Well Zi 4,4 533.01 m, the 2nd member of Dengying Fm, grain dolomite, rich in pinhole dissolved pores partially filled by asphalt.
i. The Profile of Xianfeng in Ebian, the 2nd member of Dengying Fm, gray cyanobacteria dolomite, dissolved pores and cracks filled by grape-lace dolomite ccment, residual pores and cracks are effective reservoir spacc.
Fig. 2. Reservoir characteristics of the Anyue Gas Field.
pores, intracrystalline pores and microcracks, forming a good percolation system [5—7]. Analysis of large numbers of plug samples indicate that the porosity ranges from 2% to 18.48%, and 4.28% on average (Fig. 3). The logging interpretation reveals that there are 2—3 sets of reservoirs developed vertically, with a thickness of 20—50 m, and 36 m on average [7].
2.2. Fractured-vuggy (cavernous) dolomite reservoirs of cyanobacteria mound beach facies in the 4th member of Dengying Fm
The 4th member of Dengying Fm is made up of cyano-bacteria stromatolitic dolomite, cyanobacteria algal-laminated dolomite, cyanobacteria thrombolite dolomite (Fig. 2d) and dolomicrite, with residual intergranular pores, residual karst-fractures (Fig. 2e, f) and caverns as major reservoir space [7—9]. Vugs of 1 — 15 mm in diameter are commonly seen in outcrop and core samples, and caves of 0.5—5 m in diameter
are usually found by drill break, lost circulation and imaging logging, for example, a well section of 5208.29—5210.21 m in Well Gaoshi 2 encountered a drill break of 1.92 m, on imaging logging, this section and the section of 5195—5198 m and
Fig. 3. Porosity distribution histogram of Longwangmiao Fm reservoir samples.
Fig. 4. Physical property histogram of whole-diameter core samples of the 4th member of Dengying Fm.
5085-5088 m appear as black in orange or bright orange background, showing the characteristics of karst caves. As an important part of reservoir space in 4th member of Dengying Fm, caves are not only common in the Gaoshiti-Moxi area, but also in the Weiyuan Gas Field. According to the statistics on 78 wells, drill break happened in 15 wells, accounting for 19.23 percent. If calculated by the times of drill break, the drilling ratio reaches 25.64%. Full-hole core samples have a porosity of 2%-10%, 4.34% on average. Samples with porosity of over 4% account for 48.5%; with an average permeability of 0.59 mD, in which samples over 0.1 mD in permeability account for 48 percent (Fig. 4). The logging interpretation reveals that there are two (upper and lower) reservoir layers vertically, with a cumulative thickness of 30-80 m.
2.3. Fractured-vuggy dolomite reservoir of cyanobacteria mound beach facies of the 2nd member of the Dengying Fm
The 2nd member of Dengying Fm is cyanobacteria mound beach dolomite, typically grape-lace structural cyanobacteria algal-laminated dolomite and cyanobacteria thrombolite dolomite (Fig. 2g) and grain dolomite (Fig. 2g). The reservoir space is mainly residual karst fractures and vugs after grape-lace dolomite cement filling (Fig. 2i), with the vugs, mainly in the size of 0.5-5 cm, distributing along the beddings [7-9]. The whole-diameter core samples range from 2.02% to 9.88% in porosity, and 3.73% on average, and samples over 4% in porosity account for 31.5%; the samples have a vertical
permeability of 0.0026-2.08 mD, and 0.445 mD on average, and samples over 0.1 mD in permeability account for 71% (Fig. 5). On imaging logging, the reservoir shows black mottled structure in orange background, with a thickness of 50-80 m.
3. Reservoir formation and evolution model
3.1. Reservoir formation and evolution model of Longwangmiao Fm
The research on diagenesis shows that Longwangmiao formation has experienced the following diagenetic sequences [10,11]: grain shoal deposition, submarine cementation, penecontemporaneous dissolution, penecontemporaneous dolomitization, supergene karstification, burial filling, supergene karstification, hydrothermal mineral filling, burial organic dissolution, hydrocarbon charging, hydrocarbon cracking, asphalt filling, dissolution of thermochemical sulfate reduction and natural gas charging, structural break, etc. The diagenetic sequences can be summarized to four stages: pore formation in sedimentary and penecontemporaneous period, supergene dissolution transformation in Caledonian, burial hydrothermal filling in Hercynian, burial dissolution and asphalt filling since Indo-Chinese epoch (Fig. 6).
3.1.1. Pore formation in sedimentary and penecontemporaneous period
This period included two phases of pore formationthe first phase happened when grain shoal deposited, during which a
Fig. 5. Physical property histogram of whole-diameter core samples of the 2nd member of Dengying Fm.
Fig. 6. Reservoir evolution mode of Longwangmiao Fm in the Anyue Gas Field.
large number of intergranular pores formed, resulting in an original porosity of about 40%. Compaction and submarine cementation, including fibrous cement of seawater in vadose zone and bladed cement of phreatic zone, happened in the subsequent process of submarine cementation, and made porosity dramatically decrease to 0—10%; the second phase was the penecontemporaneous leaching, when a large number of dissolved pores and dissolved caves formed, making the porosity increase by 0—20% to 10%—25%. Meanwhile, the reservoirs experienced penecontemporaneous dolomitization in this period, in which the rock transformation into dolomite enhanced the rock strength and ability to resist pressure solution, conducive to the preservation of intergranular pores, dissolution pores and vugs formed earlier.
3.1.2. Supergene karstification transformation in Caledonian
There were two phases of supergene karstification in this period. The first phase, happened at the end of Longwangmiao formation deposition [12], is related to the regional sea level falling; although this phase of karstification was wide in range, shorter in duration, and weak in intensity, it enlarged early pores, and produced a small amount of dissolved fractures. The second phase of supergene karstification taking place in the late Caledonian [13—15], is related to tectonic uplift and formation of Leshan-Longnüsi paleo uplift; compared with the first phase supergene karstification, this phase, stronger in intensity in paleo-uplift areas, might give birth to large-scale
fracture-cavity systems, but it can be seen from cores taken from Well Moxi 17 that the karst caves have been filled by mud, making reservoir properties poor; a large number of core observation and thin section statistics show that these two phases of karstification added less than 5% of porosity, making the reservoir porosity increase to 15%—35%.
3.1.3. Hydrothermal mineral filling in Hercynian
The hydrothermal activity is closely related to the Emeishan volcanic action [16]. This period of filling is in the form of bright coarse dolomite and euhedral quartz mineral filling in cracks and caves formed earlier, and the homogeni-zation temperature of fluid inclusions is 180—220 °C [17,18]. The filling effect was uneven, stronger in some areas where most of the caves were partially filled or fully filled, dramatically decreasing the porosity; weaker in other areas where only a small amount of euhedral dolomite precipitated along the cave side wall. From the available data, this period of filling reduces the porosity by 10% to 5%—20%.
3.1.4. Burial dissolution and asphalt filling since IndoChinese epoch
Two phases of dissolution happened in this period. The first phase was associated with hydrocarbon charging [19], while the second dissolution was related to TSR effect [20,21]. The two phases of dissolution are relatively weak, resulting in less than 2% of porosity increase, but in this period, a lot of asphalt was generated with the hydrocarbon cracking, which not only
fills pores, but also plugs throat, causing a porosity drop of 2%-5%. Therefore, the reservoir porosity reduced on the whole to 2%-16%. Meanwhile, the regional tectonic uplift at Himalayan period gave rise to micro-fractures [22], which although have little effect on the reservoir porosity, do improve the permeability of the reservoirs significantly. The general high yield of Longwangmiao formation is closely related to the connection of dissolved pores and dissolved caves by the structural fractures.
3.2. Formation and evolution model of the 4th member of Dengying Fm
The 4th member of Dengying Fm has experienced cyano-bacteria mound beach deposition, early burial compaction, supergene karstification, burial hydrothermal mineral filling (macrocrystalline dolomite and quartz filling), burial dissolution and asphalt filling, and structural break (crack), etc. The evolution of its reservoir space can be divided into three stages: pore formation in supergene karstification in Tongwan movement, burial hydrothermal mineral filling in Hercynian, burial dissolution and asphalt filling since Indo-Chinese epoch. Fig. 7 shows the evolution model of dissolution pores and caves.
3.2.1. Pore formation stage during supergene karstification in Tongwan Movement
After deposition or a short period of burial compaction, the cyanobacteria dolomite and dolomicrite of the 4th member of
Dengying Fm were exposed to the surface due to the influence of the 2nd and 3rd episode of Tongwan Movement [23], and subject to erosion and leaching and dissolution of meteoric fresh water, a large number of dissolved pores and caves were formed [24,25]. On the core scale, the core samples have mainly pores and caves of 1 -6 mm, and a plane porosity of 2%-15%. Caves haven't been seen in field, but drill break and leakage are frequently encountered during the drilling of this reservoir, indicating the existence of fracture-cave systems. Imaging logging has also confirmed the presence of large caves [7], with a height of 0.5-6 m. The pores, vugs and caves formed in this period take up the majority of the reservoir space, with an estimated total porosity of 5%-25%.
3.2.2. Hydrothermal mineral filling in Hercynian
In this stage, the main hydrothermal filling minerals were granular quartz and macrocrystalline dolomite, and there was more quartz than dolomite filling, which is the main factor causing reservoir space decrease [26]. According to the logging evaluation, there is an obvious negative correlation between silica content and reservoir productivity. The statistics of thin slice indicates that the filling content accounts for 2%-10% in this stage, resulting in a total porosity decrease of 3%-20%.
3.2.3. Burial dissolution-asphalt filling since Indo-Chinese epoch
It is inferred from the weak dissolution of partial hydrothermal dolomite filling in dissolution pores that the 4th
Fig. 7. Reservoir evolution mode of the 4th member of Dengying Fm in the Anyue Gas Field.
member of Dengying Fm experienced two kinds of dissolution since Late Permian: (1) organic acid dissolution associated with hydrocarbon charging, (2) dissolution related to TSR effect, both of which were weak in intensity and produced few pores. However, the asphalt filling associated with hydrocarbon cracking of pores in this stage is significant, and the imaging logging and thin section observation show that asphalt filling can reduce the porosity by 2%—'7%. Therefore, in this stage the reservoir has an overall densification trend, with a porosity decrease of 2%—13%. Although opening cracks formed in Himalayan added little to porosity, they played an important role in improving the reservoir permeability.
3.3. Formation and evolution model of the 2nd member of Dengying Fm
The 2nd member of Dengying Fm has experienced cya-nobacteria mound beach deposition-submarine cementation, dissolution fracture-vug formation during Penecontempora-neous period and grape-lace cement filling, hydrothermal dolomite filling (macrocrystalline dolomite), hydrocarbon filling, burial dissolution and asphalt filling, and structural break (crack), etc. Evolution of its reservoir space can be divided into three stages: pore formation in supergene kar-stification, hydrothermal mineral filling in Hercynian, burial dissolution and asphalt filling since Indo-Chinese epoch (Fig. 8).
3.3.1. Pore formation stage during the deposition and penecontemporaneous period
The main reservoir space of the 2nd member of Dengying Fm includes cyanobacteria mound framework pores, intra-granular pores, intergranular pores and dissolution cracks and vugs. Among them, framework pores were formed during the building of cyanobacteria mounds, and were enlarged by penecontemporaneous freshwater later; intergranular and intragranular dissolution pores were generated by pene-contemporaneous freshwater dissolution; while the dissolution cracks and vugs, formed a bedding plane system, in two steps, first, shrinkage joints or drying cracks grew after exposure of high frequency cyanobacteria mounds, and then the cracks and pores were enlarged and dissolved once more by fresh water, and this system already has the karst characteristics. The reservoir storage space mentioned above accounts for 10—40%, however, experiencing intensive penecontempora-neous cementation after formation, the pores are mostly filled completely. For example, dissolution cracks and vugs have an obvious porosity reduction, the residual crack—vug plane porosity drops to 2%—20% after grape-lace dolomite filling [27].
3.3.2. Hydrothermal mineral filling in Hercynian
The filling mineral is mainly luminous macrograin dolomite, and a small amount of automorphic quartz in this stage, resulting in a porosity drop by 3%—8%, to 2%—12%.
Reservoir diagenesis-porosity evolution history of the 2nd member of Dengying Fm in Gaoshiti-Moxi area, Sichuan Basin
Diagenesis stage Syngenetic diagenesis Eogenetic Middle-late diagenesis
Diagenesis environment 111 i |l 11 Shallow burial Middle-deep burial
Depositi on-di agenesi s Mound beacli
cémentation
Pcnccaicmporanccuí fcolition
C3nipe4ace
Pcnccortcnijwrarccus ddoniliailion
Supergene karstification
Compaction
Pressure solution
I Ivdrotlicnnal mineral filling O
dissolution - ■ <C>
Asphalt filling m
Stmctural thictiirc <®> O «effi»
Tectonic cycle Penecontemporaneous (time scale magnifying) Caledonian Hercvnian kWlb» Yanshan Jttmtp
Geological age Zdny. 2 II -e 1 o 1 s 1 D c P, IP [TJTJ j 1 K ^
Before prcscnt'Ma 551 54 542 50 450 400 50 3C 0 0 20 0 150 100 50
Burial history/m N
2 000 S (el Moxi 9
5 000 \
Key point: Pc iccontc s dolí ization supers ncd Ol Mn i in the !'"' mcmh
De lgying super enek ari ificalio in (lie *ep id of lougwa ig mover HI
Porosity evolution history 35% -30%-25% -20% -15% -10%-5% - V (el Moxi 9
a. Dissolution crack-vug formation during penecontemporaneous period, and half-filled by grape-lace dolomite cement ; b. Hydrothermal macrocrystalline dolomite filling in Hercynian ; c. Asphalt filling and microcrack formation in Yanshan- Himalayan
al. A large amount of pores were generated in penecontemporaneous dissolution of mound-shoal, a2. Grape-lace dolomite cements filled the fractures/vugs.bl. Hydrothermal dolomite filled in the center of fractures/vugs.cl. Some hydrothermal dolomite filling materials were dissolved and later filled with bitumen. In Himalayan period, micro-fractures occurred.
Fig. 8. Reservoir evolution mode of the 2nd member of Dengying Fm in the Anyue Gas Field.
3.3.3. Burial dissolution-asphalt filling since Indo-Chinese epoch
In this stage, the reservoir evolution is similar to the 4th member of Dengying Fm. Asphalt filling finally makes the reservoir denser, decreasing the porosity to 2%-10%, and fracture opened in Himalaya improves reservoir permeability to some extent.
4. Conclusions
(1) The Longwangmiao Fm grain dolomite fracture—vuggy reservoir of grain shoal facies mainly consists of residual dolarenite and crystal dolomite, with vugs and dissolution pores as the main storage space, and residual intergranular pores, intracrystalline pores and fractures as secondary storage space. The reservoir has a porosity of 2%—8%, 4.28% on average, a thickness of 15—60 m, and 36 m on average. The development of the reservoirs is controlled by shoal facies and penecontemporaneous dissolution, and has gone through four evolution stages, in which penecontemporaneous dissolution and deposition are the key factors affecting reservoir space. Supergene karstification and burial dissolution make some contribution to the improvement of the reservoir physical property, whereas hydrothermal mineral filling and asphalt filling are the main factors making reservoir quality worse by seriously blocking reservoir pores.
(2) The 4th member of the Dengying Fm is a fractured-vuggy (cavernous) dolomite reservoir of cyanobacteria mound beach facies, made up of cyanobacteria stro-matolitic dolomite, cyanobacteria algal-laminated dolomite, cyanobacteria thrombolite dolomite and dolomicrite, with different scales of dissolution pores and large caves as the main reservoir storage space, and fracture as secondary storage space. It ranges from 2% to 10% in porosity, 4.34% on average, and 30—80 m in cumulative thickness. It has experienced three evolution stages: pore formation stage during supergene karstification in Tongwan Movement, hydrothermal mineral filling in Hercynian, and burial dissolution-asphalt filling since Indo-Chinese epoch.
(3) The 2nd member of the Dengying Fm is a fractured-vuggy dolomite reservoir of cyanobacteria mound beach facies, similar to the 4th member of the Dengying Fm in lithology. With the residual karst dissolved fractures and caves after grape-lace cement filling as the main reservoir space. It has a porosity of 2.02%—9.88%, and 3.73% on average, and a reservoir thickness of 50—80 m. It has also experienced three evolution stages: pore formation stage during deposition and pene-contemporaneous period, hydrothermal mineral filling in Hercynian, and burial dissolution-asphalt filling since Indo-Chinese epoch, among which, the pores generated in deposition and penecontemporaneous dissolution of mound-shoal are the key part in effective reservoir space.
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