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Natural Gas Industry B 2 (2015) 127e135 www.elsevier.com/locate/ngib Research article Genesis mechanism of the Sinian-Cambrian reservoirs in the Anyue Gas Field, Basin*

Zhou Jingaoa,b, Yao Genshuna,b, Yang Guangc, Zhang Jianyonga,b, Hao Yia,b, Wang Fanga, Gu Mingfenga, Li Wenzhenga,*

a Hangzhou Branch of Research Institute of Geology, PetroChina, Hangzhou, Zhejiang 310023, China b CNPC Key Laboratory of Carbonate Reservoirs, Hangzhou, Zhejiang 310023, China c Exploration and Development Research Institute of Southwest Oil & Gasfield Company, PetroChina, , 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 tested a gas flow of 102 Â 104 m3/d at the 2nd member of Sinian Dengying Fm, marking a major breakthrough in the Located in Suining city, Anyue county of city, paleo-uplift gas exploration. In September 2012, Well Moxi 8 Sichuan province, and Tongnan county in Chongqing city, the tested a gas flow of 107 Â 104 m3/d at the Lower Cambrian Anyue Gas Field tectonically lies in the east of - Longwangmiao Fm, marking a historic breakthrough in nat- Longnu¨si paleo-uplift [1] (Fig. 1). In July 2011, Well Gaoshi 1 ural gas exploration of Cambrian. Consequently, fast explo- ration in Anyue Gas Field began. By the end of 2013, the Cambrian Longwangmiao Fm in Moxi area in central Sichuan * Foundation item: National Science and Technology MajorPro- had proved geologic gas reserves of 4403.83 Â 108 m3, ject(2011ZX05004-002), PetroChina Exploration and Production Major Proj- ranking it the largest single carbonate integral gas field ever ect(2012ZD01-02-03), PetroChina Special Project(2014e-32). * Corresponding author. discovered in China [2]. E-mail address: [email protected] (Li WZ). Major payzones in the Anyue Gas Field are Longwangmiao Peer review under responsibility of Sichuan Petroleum Administration. 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/). 128 Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135

Fig. 1. Location of the Anyue Gas Field.

Longwangmiao Fm is a set of grain dolomite and muddy to and evolution mechanisms of the three sets of reservoirs above micritic dolomite with gypsum interlayers, a three-order were examined, in the hope to provide research support for sequence on the whole with type I sequence boundary on the continuous exploration in the area. top [3]. The Dengying Fm in the Sichuan Basin is divided into four members from bottom up: the 1st member is dominated 2. Reservoir characteristics by dolomite poor in cyanobacteria in the lower part, the 2nd member is dolomite rich in cyanobacteria, the 3rd member is 2.1. Fractured-vuggy dolomite reservoir of grain shoal dominated by clastic rocks; the 4th member, the same with the facies in Longwangmiao Fm 1st member, is dominated by dolomite poor in cyanobacteria [4]. Vertically, the 1st member and the 2nd member, the 3rd The dominant lithology of Longwangmiao Fm is residual member and 4th member respectively form two third-order dolarenite (Fig. 2a), including oolitic dolomite, crystal dolo- sequences, and the top of each sequence is a type I sequence mite and porphyritic dolomite, in which dissolution vugs and boundary line. Exploration reveals that the Anyue Gas Field pores of 2e6 mm are the main reservoir space (Fig. 2b, c), and has three sets of industrial gas layers, Longwangmiao Fm, the residual intergranular pores, intracrystalline pores and frac- 4th and 2nd member of Dengying Fm. Research shows that the tures are secondary reservoir space [5]. three sets of gas layers are different in reservoir type. Ac- The intrusive mercury curve of grain shoal reservoir in cording to sedimentary microfacies, lithology and porosity, the Longwangmiao Fm shows that the initial mercury injection Longwangmiao Fm is identified as fractured-vuggy dolomite pressure of fractured-vuggy reservoir is mostly less than reservoir of grain shoal facies, the 4th member of the Den- 0.3 MPa, and the mercury saturation in the low pressure gying Fm as fractured-vuggy (cavernous) dolomite reservoir platform of less than 5 MPa ranges from 70% to 80%, while of cyanobacteria mound beach facies, and the 2nd member of the mercury saturation in high platform is 10%e20%, showing the Dengying Fm as fractured-vuggy dolomite reservoir of the feature of dual porosity medium with vugs in absolute cyanobacteria mound beach facies. Through comprehensive majority followed by micropores. analysis of geological and geophysical data, in combination Observation of core samples and thin sections shows that with experiment analysis, the main characteristics, formation dissolution pores are connected by residual intergranular Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135 129

Fig. 2. Reservoir characteristics of the Anyue Gas Field. pores, intracrystalline pores and microcracks, forming a good are usually found by drill break, lost circulation and imaging percolation system [5e7]. Analysis of large numbers of plug logging, for example, a well section of 5208.29e5210.21 m in samples indicate that the porosity ranges from 2% to 18.48%, Well Gaoshi 2 encountered a drill break of 1.92 m, on imaging and 4.28% on average (Fig. 3). The logging interpretation logging, this section and the section of 5195e5198 m and reveals that there are 2e3 sets of reservoirs developed verti- cally, with a thickness of 20e50 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 e e [7 9].Vugsof1 15 mm in diameter are commonly seen in Fig. 3. Porosity distribution histogram of Longwangmiao Fm reservoir outcrop and core samples, and caves of 0.5e5 m in diameter samples. 130 Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135

Fig. 4. Physical property histogram of whole-diameter core samples of the 4th member of Dengying Fm.

5085e5088 m appear as black in orange or bright orange permeability of 0.0026e2.08 mD, and 0.445 mD on average, background, showing the characteristics of karst caves. As an and samples over 0.1 mD in permeability account for 71% important part of reservoir space in 4th member of Dengying (Fig. 5). On imaging logging, the reservoir shows black Fm, caves are not only common in the Gaoshiti-Moxi area, but mottled structure in orange background, with a thickness of also in the Weiyuan Gas Field. According to the statistics on 50e80 m. 78 wells, drill break happened in 15 wells, accounting for 19.23 percent. If calculated by the times of drill break, the 3. Reservoir formation and evolution model drilling ratio reaches 25.64%. Full-hole core samples have a porosity of 2%e10%, 4.34% on average. Samples with 3.1. Reservoir formation and evolution model of porosity of over 4% account for 48.5%; with an average Longwangmiao Fm permeability of 0.59 mD, in which samples over 0.1 mD in permeability account for 48 percent (Fig. 4). The logging The research on diagenesis shows that Longwangmiao interpretation reveals that there are two (upper and lower) formation has experienced the following diagenetic sequences reservoir layers vertically, with a cumulative thickness of [10,11]: grain shoal deposition, submarine cementation, 30e80 m. penecontemporaneous dissolution, penecontemporaneous dolomitization, supergene karstification, burial filling, super- 2.3. Fractured-vuggy dolomite reservoir of gene karstification, hydrothermal mineral filling, burial cyanobacteria mound beach facies of the 2nd member of organic dissolution, hydrocarbon charging, hydrocarbon the Dengying Fm cracking, asphalt filling, dissolution of thermochemical sulfate reduction and natural gas charging, structural break, etc. The The 2nd member of Dengying Fm is cyanobacteria mound diagenetic sequences can be summarized to four stages: pore beach dolomite, typically grape-lace structural cyanobacteria formation in sedimentary and penecontemporaneous period, algal-laminated dolomite and cyanobacteria thrombolite supergene dissolution transformation in Caledonian, burial dolomite (Fig. 2g) and grain dolomite (Fig. 2g). The reservoir hydrothermal filling in Hercynian, burial dissolution and space is mainly residual karst fractures and vugs after grape- asphalt filling since Indo-Chinese epoch (Fig. 6). lace dolomite cement filling (Fig. 2i), with the vugs, mainly in the size of 0.5e5 cm, distributing along the beddings [7e9]. 3.1.1. Pore formation in sedimentary and The whole-diameter core samples range from 2.02% to 9.88% penecontemporaneous period in porosity, and 3.73% on average, and samples over 4% in This period included two phases of pore formationthe first porosity account for 31.5%; the samples have a vertical 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. Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135 131

Fig. 6. Reservoir evolution mode of Longwangmiao Fm in the Anyue Gas Field. large number of intergranular pores formed, resulting in an fracture-cavity systems, but it can be seen from cores taken original porosity of about 40%. Compaction and submarine from Well Moxi 17 that the karst caves have been filled by cementation, including fibrous cement of seawater in vadose mud, making reservoir properties poor; a large number of core zone and bladed cement of phreatic zone, happened in the observation and thin section statistics show that these two subsequent process of submarine cementation, and made phases of karstification added less than 5% of porosity, making porosity dramatically decrease to 0e10%; the second phase the reservoir porosity increase to 15%e35%. was the penecontemporaneous leaching, when a large number of dissolved pores and dissolved caves formed, making the 3.1.3. Hydrothermal mineral filling in Hercynian porosity increase by 0e20% to 10%e25%. Meanwhile, the The hydrothermal activity is closely related to the reservoirs experienced penecontemporaneous dolomitization Emeishan volcanic action [16]. This period of filling is in the in this period, in which the rock transformation into dolomite form of bright coarse dolomite and euhedral quartz mineral enhanced the rock strength and ability to resist pressure so- filling in cracks and caves formed earlier, and the homogeni- lution, conducive to the preservation of intergranular pores, zation temperature of fluid inclusions is 180e220 C [17,18]. dissolution pores and vugs formed earlier. The filling effect was uneven, stronger in some areas where most of the caves were partially filled or fully filled, dramat- 3.1.2. Supergene karstification transformation in ically decreasing the porosity; weaker in other areas where Caledonian only a small amount of euhedral dolomite precipitated along There were two phases of supergene karstification in this the cave side wall. From the available data, this period of period. The first phase, happened at the end of Longwangmiao filling reduces the porosity by 10% to 5%e20%. formation deposition [12], is related to the regional sea level falling; although this phase of karstification was wide in range, 3.1.4. Burial dissolution and asphalt filling since Indo- shorter in duration, and weak in intensity, it enlarged early Chinese epoch pores, and produced a small amount of dissolved fractures. Two phases of dissolution happened in this period. The first The second phase of supergene karstification taking place in phase was associated with hydrocarbon charging [19], while the late Caledonian [13e15], is related to tectonic uplift and the second dissolution was related to TSR effect [20,21]. The formation of Leshan-Longnu¨si paleo uplift; compared with the two phases of dissolution are relatively weak, resulting in less first phase supergene karstification, this phase, stronger in than 2% of porosity increase, but in this period, a lot of asphalt intensity in paleo-uplift areas, might give birth to large-scale was generated with the hydrocarbon cracking, which not only 132 Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135

fills pores, but also plugs throat, causing a porosity drop of Dengying Fm were exposed to the surface due to the influence 2%e5%. Therefore, the reservoir porosity reduced on the of the 2nd and 3rd episode of Tongwan Movement [23], and whole to 2%e16%. Meanwhile, the regional tectonic uplift at subject to erosion and leaching and dissolution of meteoric Himalayan period gave rise to micro-fractures [22], which fresh water, a large number of dissolved pores and caves were although have little effect on the reservoir porosity, do formed [24,25]. On the core scale, the core samples have improve the permeability of the reservoirs significantly. The mainly pores and caves of 1e6 mm, and a plane porosity of general high yield of Longwangmiao formation is closely 2%e15%. Caves haven't been seen in field, but drill break and related to the connection of dissolved pores and dissolved leakage are frequently encountered during the drilling of this caves by the structural fractures. reservoir, indicating the existence of fracture-cave systems. Imaging logging has also confirmed the presence of large 3.2. Formation and evolution model of the 4th member of caves [7], with a height of 0.5e6 m. The pores, vugs and caves Dengying Fm formed in this period take up the majority of the reservoir space, with an estimated total porosity of 5%e25%. The 4th member of Dengying Fm has experienced cyano- bacteria mound beach deposition, early burial compaction, 3.2.2. Hydrothermal mineral filling in Hercynian supergene karstification, burial hydrothermal mineral filling In this stage, the main hydrothermal filling minerals were (macrocrystalline dolomite and quartz filling), burial dissolu- granular quartz and macrocrystalline dolomite, and there was tion and asphalt filling, and structural break (crack), etc. The more quartz than dolomite filling, which is the main factor evolution of its reservoir space can be divided into three causing reservoir space decrease [26]. According to the log- stages: pore formation in supergene karstification in Tongwan ging evaluation, there is an obvious negative correlation be- movement, burial hydrothermal mineral filling in Hercynian, tween silica content and reservoir productivity. The statistics burial dissolution and asphalt filling since Indo-Chinese epoch. of thin slice indicates that the filling content accounts for 2%e Fig. 7 shows the evolution model of dissolution pores and 10% in this stage, resulting in a total porosity decrease of 3%e caves. 20%.

3.2.1. Pore formation stage during supergene karstification 3.2.3. Burial dissolution-asphalt filling since Indo-Chinese in Tongwan Movement epoch After deposition or a short period of burial compaction, the It is inferred from the weak dissolution of partial hydro- cyanobacteria dolomite and dolomicrite of the 4th member of thermal 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. Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135 133 member of Dengying Fm experienced two kinds of dissolution 3.3.1. Pore formation stage during the deposition and since Late Permian: (1) organic acid dissolution associated penecontemporaneous period with hydrocarbon charging, (2) dissolution related to TSR The main reservoir space of the 2nd member of Dengying effect, both of which were weak in intensity and produced few Fm includes cyanobacteria mound framework pores, intra- pores. However, the asphalt filling associated with hydrocar- granular pores, intergranular pores and dissolution cracks and bon cracking of pores in this stage is significant, and the im- vugs. Among them, framework pores were formed during the aging logging and thin section observation show that asphalt building of cyanobacteria mounds, and were enlarged by filling can reduce the porosity by 2%e7%. Therefore, in this penecontemporaneous freshwater later; intergranular and stage the reservoir has an overall densification trend, with a intragranular dissolution pores were generated by pene- porosity decrease of 2%e13%. Although opening cracks contemporaneous freshwater dissolution; while the dissolution formed in Himalayan added little to porosity, they played an cracks and vugs, formed a bedding plane system, in two steps, important role in improving the reservoir permeability. first, shrinkage joints or drying cracks grew after exposure of high frequency cyanobacteria mounds, and then the cracks and 3.3. Formation and evolution model of the 2nd member pores were enlarged and dissolved once more by fresh water, of Dengying Fm and this system already has the karst characteristics. The reservoir storage space mentioned above accounts for The 2nd member of Dengying Fm has experienced cya- 10e40%, however, experiencing intensive penecontempora- nobacteria mound beach deposition-submarine cementation, neous cementation after formation, the pores are mostly filled dissolution fracture-vug formation during Penecontempora- completely. For example, dissolution cracks and vugs have an neous period and grape-lace cement filling, hydrothermal obvious porosity reduction, the residual crackevug plane dolomite filling (macrocrystalline dolomite), hydrocarbon porosity drops to 2%e20% after grape-lace dolomite filling filling, burial dissolution and asphalt filling, and structural [27]. break (crack), etc. Evolution of its reservoir space can be divided into three stages: pore formation in supergene kar- 3.3.2. Hydrothermal mineral filling in Hercynian stification, hydrothermal mineral filling in Hercynian, burial The filling mineral is mainly luminous macrograin dolo- dissolution and asphalt filling since Indo-Chinese epoch mite, and a small amount of automorphic quartz in this stage, (Fig. 8). resulting in a porosity drop by 3%e8%, to 2%e12%.

Fig. 8. Reservoir evolution mode of the 2nd member of Dengying Fm in the Anyue Gas Field. 134 Zhou JG et al. / Natural Gas Industry B 2 (2015) 127e135

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