Scholarly article on topic 'Characteristics and exploration prospects of Middle Permian reservoirs in the Sichuan Basin'

Characteristics and exploration prospects of Middle Permian reservoirs in the Sichuan Basin Academic research paper on "Earth and related environmental sciences"

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{"Sichuan Basin" / "Middle Permian" / Dolomite / "Porous reservoir" / "Karst reservoir" / "Exploration prospect" / "Thermal water deposition" / "Thermal sub-basin"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Guang Yang, Hua Wang, Hao Shen, Yuran Yang, Song Jia, et al.

Abstract In recent years, encouraging progress has been made in the Middle Permian natural gas exploration in the Sichuan Basin – numerous wells have obtained high-yield industrial gas flows in the dolomite porous reservoir sections in the Middle Permian Maokou Formation and the Qixia Formation, making the Middle Permian one of the most realistic replacement strata at present. In order to clarify the further exploration prospect of the basin, the Middle Permian depositional settings and prospective reservoir types were analyzed. The results show that: (1) the depositional environment was predominately open, shallow water; the paleogeomorphology was high in the west and low in the east, with bioclastic flat reservoirs more extensive in the middle-west part than the east part; crustal extension made Middle Permian deposits have good conditions for thermal water deposition and hydrothermal alteration; and (2) the prospect reservoirs for exploration were shoal facies dolomite porous reservoir of the Qixia Formation, thermal water dolomite porous reservoir and karst reservoir of the Maokou Formation, thus, dolomite porous reservoir was the most perspective target in the Middle Permian. It is concluded that Middle Permian limestone source rocks have large hydrocarbon generation intensity in the northern part of Western and Central Sichuan Basin and have the material basis for the formation of large-medium gas fields; the thick dolomite reservoir of the Qixia Formation in the Western Sichuan Basin is the most prospective reservoir in the Middle Permian; three types of reservoirs are distributed in the Central Sichuan Basin, and they are superimposed vertically with large exploration potential; Jiange–Nanchong–Fengdu region is located in the thermal sub-basin zone, where the thermal water dolomite reservoir of the Maokou formation is pervasive and superimposed on the karst reservoir of the Maokou Formation, showing good natural gas exploration prospects.

Academic research paper on topic "Characteristics and exploration prospects of Middle Permian reservoirs in the Sichuan Basin"

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Natural Gas Industry B 2 (2015) 399-405

Research article

www.elsevier.com/locate/ngib

Characteristics and exploration prospects of Middle Permian reservoirs in

the Sichuan Basin

Yang Guang, Wang Hua*, Shen Hao, Yang Yuran, Jia Song, Chen Wen, Zhu Hua, Li Yi

Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610041, China

Received 20 March 2015; accepted 8 September 2015 Available online 19 February 2016

Abstract

In recent years, encouraging progress has been made in the Middle Permian natural gas exploration in the Sichuan Basin — numerous wells have obtained high-yield industrial gas flows in the dolomite porous reservoir sections in the Middle Permian Maokou Formation and the Qixia Formation, making the Middle Permian one of the most realistic replacement strata at present. In order to clarify the further exploration prospect of the basin, the Middle Permian depositional settings and prospective reservoir types were analyzed. The results show that: (1) the depositional environment was predominately open, shallow water; the paleogeomorphology was high in the west and low in the east, with bioclastic flat reservoirs more extensive in the middle-west part than the east part; crustal extension made Middle Permian deposits have good conditions for thermal water deposition and hydrothermal alteration; and (2) the prospect reservoirs for exploration were shoal facies dolomite porous reservoir of the Qixia Formation, thermal water dolomite porous reservoir and karst reservoir of the Maokou Formation, thus, dolomite porous reservoir was the most perspective target in the Middle Permian. It is concluded that Middle Permian limestone source rocks have large hydrocarbon generation intensity in the northern part of Western and Central Sichuan Basin and have the material basis for the formation of large-medium gas fields; the thick dolomite reservoir of the Qixia Formation in the Western Sichuan Basin is the most prospective reservoir in the Middle Permian; three types of reservoirs are distributed in the Central Sichuan Basin, and they are superimposed vertically with large exploration potential; Jiange—Nanchong—Fengdu region is located in the thermal sub-basin zone, where the thermal water dolomite reservoir of the Maokou formation is pervasive and superimposed on the karst reservoir of the Maokou Formation, showing good natural gas exploration prospects. © 2016 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; Middle Permian; Dolomite; Porous reservoir; Karst reservoir; Exploration prospect; Thermal water deposition; Thermal sub-basin

In 2014, numerous wells drilled in the central and western parts in the Sichuan Basin produced high-yield gas flow from the Middle Permian dolomite reservoirs in the Maokou Formation and Qixia Formation. This indicates an encouraging progress in the exploration of the Middle Permian, making the Middle Permian one of the presently most realistic replacement strata in the basin.

* Corresponding author.

E-mail address: wanghua1230@petrochina.com.cn (Wang H.). Peer review under responsibility of Sichuan Petroleum Administration.

1. Depositional setting of the Middle Permian

1.1. Paleo-geomorphologic height difference

In the Sichuan Basin, the Middle Permian unconformably overlies the Carboniferous, Devonian, Silurian and other older strata. Due to the impacts of the Caledonian and Hercynian movements, its deposition was inherited from and controlled by the paleo-geomorphology formed on the Leshan-Longniisi paleo-uplift. By tracking the stratigraphic truncation points of the strata underlying the Middle Permian on seismic section, the Sichuan Basin can be divided into two regions: the paleo-

http://dx.doi.org/10.1016/j.ngib.2015.09.015

2352-8540/© 2016 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/).

geomorphologically lower eastern region where the Silurian remains present, and the paleo-geomorphologically higher central-western region where the Silurian Basin has been eroded. The relative paleo-geomorphologic height difference accounts for the depositional difference of the Middle Permian. This is particularly evident during the regression (the mid-late Qixia and Mao-3 periods), leading to more broadly developed bioclastic shoal reservoirs in the central-western region than in the eastern region.

1.2. Extensional setting

The whole Yangtze Plate was within an extensional environment [1] during the Middle Permian, with intrusive and extrusive rocks developed in local areas. The Sichuan Basin is no exception. For example, in the eastern Sichuan region, Well YA006-X5 encountered 30 m-thick extrusive rocks in the P2q:b; and in the southern part of the western Sichuan region, Well DS001-X1 encountered two basalt beds totaling 180 m thick (Fig. 1-a). The extensional setting in which the Permian system was deposited not only controls the deposition of the Middle Permian, but also makes it possible to form thermal water deposition and hydrothermal alteration conditions in the Middle Permian.

1.3. Open platform shallow water carbonate depositional environment

There are various paleontological types in the Middle Permian including fusulinida, chlorella and foraminifera that can be commonly observed on thin sections, in addition to rhodophyta, echinoderm, brachiopoda, bivalve, gastropod, coral, sponge, ostracoda, trilobite, bryozoan, and so on (Fig. 1-b, 1-c and 1-d). They are mostly typical marine stenohaline organisms, indicating a depositional environment with relatively shallower water, rich oxygen and normal salinity [2]; that is, open platform shallow-water carbonate depositional environment. Lithologically, the Middle Permian Qixia Formation and Maokou Formation are dominated by a section of stably deposited limestone, consisting of the light-colored rocks of the Mao-3 and Qi-2 Members and the Qi-1a Sub-member (in the western Sichuan region) that were deposited in a relatively shallow-water open platform environment; the dark-colored rocks of the Mao-1 Member that are rich in organic matter and mud, contain augen limestone and were deposited in a relatively deep-water open platform environment; and the other members that are dark-colored were also deposited in this environment.

2. Types of the favorable Middle Permian reservoirs

In the Sichuan Basin, the Middle Permian reservoirs that contain lithologies of dolomite and limestone can be classified into four types: fracture, fracture-cave, fracture-pore, and composite fracture-cave-pore. There are different opinions with regard to the origin of dolomite, such as the mixed-water dolomitization [3,4], burial dolomitization [5], basalt leaching

dolomitization [6] and tectonic hydrothermal dolomitization [7,8]. With the continuous development of research, more and more scholars tend to agree on the mixed origin theory that combines various origin models. The author believes that, the Qixia Formation was formed as the result of hydrothermal dolomitization and hydrothermal alteration [9] in addition to the mixed-water dolomitization, and the Maokou Formation dolomite of thermal water origin was formed by hydrothermal dolomitization alteration [10-12]. The Middle Permian reservoirs that are considered favorable for exploration in the Sichuan Basin can be divided into the Qixia Formation shoal facies dolomite pore-type reservoir, the Maokou Formation thermal water dolomite pore-type reservoir, and the Maokou Formation karst reservoir, depending on their lithological characters and dolomite origins.

2.1. The Qixia Formation shoal facies dolomite poretype reservoir

The regression occurred during the mid-late Qixia Formation stage results in widespread deposition of numerous shoal bodies in the central-western parts of the Sichuan Basin, forming sparry grainstone comprising bioclastic limestone and a small amount of algal limestone. Dolomitization allows for the conversion from high-energy shoal facies limestone to dolomite, and then dolomite went through multiple periods of re-crystallization. The Qixia Formation dolomites are mostly crystal grain type, with intercrystalline pores and intercrys-talline dissolved pores being dominant. Porosity measured from a total of 350 dolomite samples obtained from field outcrops and drilled wells averages 2.22%. In addition to matrix pores, the well-developed karst caves in the dolomite section of the Qixia Formation (Fig. 1-e) shown by core observation and the strongly sandified saccharoidal dolomite of the Qixia Formation revealed by the Kuangshanliang profile (Fig. 1-f) both indicate the frequent fluid activity after the Qixia Formation has been deposited and the occurrence of various periods of dissolution and re-crystallization during the burial period. Dolomitization appears to be common in dolomite sections, forming numerous sub-horizontally distributed karst caves, as is shown by imaging logs; and in the limestone bed that lies between these dolomite sections, karst caves are poorly developed and mostly vertically distributed.

Controlled by bioclastic shoal facies, the Qixia Formation shoal facies dolomite pore-type reservoir in the Sichuan Basin is distributed primarily in the central-western region. Along the southwestern margin of the basin where platform margin shoal sediments are mainly deposited, thick-bedded shoal facies dolomite reservoir is developed in the vertical direction (the Qixia Formation dolomite reservoir reaches a maximum thickness of 55 m in Well HS1 and is up to 100 m thick on Longmenshan Nianziba profile). Stratigraphically, this reservoir is commonly developed in the middle to upper sections of the Qixia Formation: i.e., the Qi-2 Member and the Qi-1a Sub-member. In 2014, a 16 m-thick Qixia Formation dolomite reservoir was encountered in ST1, an exploration well drilled in the northern part of the western Sichuan region, and

Fig. 1. Deposition and reservoir characteristics of the Middle Permian in the Sichuan Basin.

revealed high production rate in testing. In the central part of the basin, the intra-platform shoal facies sediments are developed and reservoirs, although individually thin-bedded (several meters thick), are widespread. In the Gaoshiti-Moxi region, the Qixia Formation dolomite is mostly developed in the middle section of the Qi-1 Member, with the exception of those in the Qi-2 Member encountered by some wells. Well MX31-X1 penetrated a 5 m-thick dolomite reservoir in the Qi-2 Member and another 4 m-thick dolomite reservoir in the Qi-1 Member. These two reservoirs produce 36.69 x 104 m3 gas per day by acidification testing.

2.2. The Maokou Formation thermal water dolomite pore-type reservoir

In recent years, the detailed research of wells encountering the Middle Permian in the Sichuan Basin and the field profile survey of the Middle Permian in the Longmenshan, Huayingshan and Qiyaoshan have disclosed the presence of dolomite reservoir in the Mao-2 Member across some regions within the basin. A new wave of research on the giant fracture system in the eastern Sichuan region (W67 well system) has confirmed that. The W67 well system and the

Mao-2 Member share the same dolomite reservoir, and this reservoir can be classified as the type of pore medium-supported composite fracture-cave-pore [10]. Re-research of the most famous Z2 well system of the Middle Permian in the Sichuan Basin indicates that, numerous wells have encountered leopard limestone (described as porphyritic dolomitic limestone in well completion report, with thickness ranging from 20 to 30 m) in Maokou Formation and Qixia Formation. Also, it is mentioned in the completion report of Well Z7 that, "dolomite is commonly developed in the Permian system". In this well, the Maokou Formation and Qixia Formation contain abundant thermal-water and hydrothermal minerals, including quartz, fluorite, chlorite, garnet, magnetite, siderite, orthoclase, anorthose, tourmaline, hematite, pyrite and barite.

The stably distributed, layered dolomite reservoir that is developed primarily in the Mao-2 Member is considered as a significant discovery in exploration of the Maokou Formation in the basin. This dolomite is interpreted to be of thermal-water origin [11]. It experienced paleokarstification after deposition due to the impact of the Dongwu movement, forming a large number of dissolved pores, caves and fractures. It experienced the hydrothermal alteration during the Longtan period, enabling caves and fractures to be filled or semi-filled with numerous saddle dolomites. Similar to the Qixia Formation, the Maokou Formation mainly contains crystal grain dolomites, with intercrystalline pores and inter-crystalline dissolved pores being dominant. The features of the Maokou Formation dolomite are as follows:

(1) Mostly dark-colored: the color of dolomite is dark gray, dark brown gray or black gray, darker than dolomites present in other series (Fig. 1-g), indicating a relatively deeper depositional water body and non-shoal facies deposition.

(2) Mostly crystal grain texture: this dolomite partially contains biological residual texture, with commonly microscopically observed biological illusion.

(3) Mostly coarse texture: this formation contains mainly the medium-fine crystalline dolomite, and a small amount of powder-crystalline and coarse-crystalline dolomites (Fig. 1-h), indicating the occurrence of multiple re-crystallization at later stage.

(4) Association with silica: the silica-rich and magnesium-rich thermal fluids enable the thermal water dolomite to be associated with silica and magnesium, named "the silica—magnesium association", which: © is composed partially of siliceous dolomite; © shows interbedded dolomite and layered siliceous rock; © contains caves and fractures filled with siliceous materials. The production layer of Well NC1, for example, is overlain by a siliceous rock bed and contains the lithologies of dolomite and siliceous dolomite (Fig. 1-i).

(5) Availability of numerous thermal water and hydrothermal minerals carried by thermal fluids when thermal water deposition and hydrothermal alteration occurs, including quartz, fluorite, pyrite and magnetite.

(6) Well-developed dissolved pores and caves: the preexisting pore-fracture system can act as the conduit system for karst ground water during the tectonic uplifting stage of Dongwunian, enabling dissolution and then enlargement of the primary pores and earlier developed fractures to form intercrystalline dissolved pores, caves and fractures. The hydrothermal dissolution alteration occurred during the Longtan period allows for further development of dissolved pores, caves and fractures. For example, the Fengdu Fangniuba profile shows a surface cave ratio reaching up to 11.5%, illustrating the excellent physical property conditions of the Maokou Formation dolomite reservoir.

Controlled by the thermal sub-basin microfacies [9], the Maokou Formation thermal water dolomite was deposited near the basement faults, deep and large faults, and fracture systems that remain extensionally active during the deposition of the Maokou Formation. The presently drilled wells have confirmed the NW extension direction of this dolomite (Fig. 2, refer to Reference [13] after this paper for the basement faults of the Sichuan Basin). In April 2015, MX39, a well drilled in the central Sichuan region, produced 24.697 x 104 m3 gas per day from the Mao-2 Member by testing. Its production zone, as is indicated by sidewall cores, comprises fine-to medium-crystalline dolomite. In addition, Well NC1, located 34 km from Well MX39, encountered a section of fine-to medium-crystalline dolomite reservoir in the Mao-2 Member and produced 44.74 x 104 m3 gas per day from this section. The author therefore speculates that, drilling new wells will make possible the discovery of dolomite reservoirs that extend along faults trending other directions.

2.3. The Maokou Formation karst reservoir

The deposited Maokou Formation was weathered and eroded due to the impact of the Dongwu movement, forming karst reservoir at its top (hereinafter referred to as the Top Mao) (Fig. 1-j, 1-k and 1-l). The Maokou Formation karst reservoir is considered to be the most important type of reservoir during the early exploration stage of the Middle Permian in the basin. In the southern Sichuan region, there are frequent events such as drilling break, lost circulation and well blowout when encountering the Maokou Formation. In particular, lost circulation is the most common logging show, accounting for 76.2% of total shows. The majority of these are recorded in the Mao-2 Member, which is the primary production layer and is distributed within the 50—150 m interval at Top Mao. In recent years, there are also frequent logging shows in the Gaoshiti-Moxi region when the Middle Permian strata were drilled, including lost circulation, gas cut and abnormal gas logging. A large number of dissolved pores and caves are visible on cores and field outcrops. The imaging logs in this area confirm the presence of caves at Top Maokou Formation. The Top Maokou Formation, as is indicated by cores obtained from Well GS1 (the only well with cores obtained from the Top Maokou Formation), contains a 6 m-thick

Fig. 2. Overlapped map of thickness contour and faults of the Middle Permian Maokou Formation dolomite in the Sichuan Basin.

karst breccia, which is underlain by a 20 m-thick rock bed with well-developed fractures and caves. These caves, however, are totally filled with: © collapsed materials from the overlying Longtan Formation, including dark argillaceous rock and pyrite; © calcites, including the powder-, medium-& coarse-crystalline calcites that are microscopically visible; and © bitumen: large amounts of bitumen visible in cores obtained from the Mao-2 Member in Well GS-1 indicates the fact that considerable liquid hydrocarbons were accumulated in this area.

3. Gas exploration prospects of the Middle Permian

The 60 years of exploration of the Middle Permian in the Sichuan Basin has shown that, although the fracture-type and fracture-cave-karst-type gas reservoirs have been explored for a long time, proved reserves as of 2014 are as low as 852 x 108 m3 in the Middle Permian. In particular, the Z2 and W67 well systems that represent the feature of pore-type reservoirs together account for tens of billions of cubic meter of pressure drop reserves. The actual development has confirmed that, the Middle Permian limestone fracture-cave-type gas reservoirs are highly heterogeneous, and single dynamic system contains very low reserves of gas. For example, in the YGS region where the Middle Permian gas reservoirs contain numerous fracture-cave systems and hold 36.41 x 108 m3 of proved gas resources, the Yang 7 well system, which is the largest gas reservoir, contains 24 x 108 m3 of pressure drop reserves, and the Yang 46 well system, which is the smallest gas reservoir, contains 0.08 x 108 m3 of pressure drop reserves. Thus, the author believes that the layered pore-type reservoir is the most

favorable and profitable target for the exploration of the Middle Permian in the Sichuan Basin.

Carbon isotope values of gas samples obtained from ST1 and NC1, two wells drilled in 2014, indicate that there is highly mature to over-mature mixed source gas. Current analysis suggests that the Middle Permian gas reservoirs are sourced by the Middle Permian, Silurian, Cambrian and Upper Permian Longtan Formation [14,15]. In particular, the Middle Permian Maokou Formation and Qixia Formation contain the dark to dark gray organic-rich argillaceous limestone. Analysis of limy hydrocarbon source rock samples of Qixia Formation and Maokou Formation has shown that, the organic carbon content averages 5.67% in Well ST1 and 3.68% in Well NC1, indicating the strong capability of this source rock in generating hydrocarbon. The Middle Permian strata across the western and central Sichuan regions are considered to have excellent condition of hydrocarbon source rock and, particularly, with gas-generating intensity ranging from 26 x 108 to 44 x 108 m3/km2, the northern part of the western Sichuan region and the central Sichuan region are believed to possess the material basis for forming medium-to large-sized gasfield.

The layered pore-type reservoir of the Qixia Formation is distributed primarily in the central and western parts of the Sichuan Basin (Fig. 3), with a platform marginal shoal present along the southwestern margin of the basin, forming vertically thick-bedded shoal facies strata that are considered the most favorable prospect. For the Sichuan Basin as a whole, the western margin has better hydrocarbon source condition than the southern margin. In the central Sichuan region where intra-platform shoal is developed, and shoal reservoirs are areally widely distributed, although thin-bedded, are considered the secondary favorable prospects. These reservoirs are frequently

Fig. 3. Thickness contour of the Middle Permian Qixia Formation dolomite in the Sichuan Basin.

encountered by wells in recent years, often with good shows from the Middle Permian, and the development of dolomite reservoirs are interpreted from numerous wells.

The Maokou Formation layered pore-type reservoir trends northwest (Fig. 2). It has been encountered by numerous wells along the Jiange—Nanchong—Fengdu. Some Middle Permian gas reservoirs such as the Wolonghe, Dachigan and Bandong have been discovered in the eastern Sichuan region, and GJ, NC1 and MX39 are some wells in the central-western Sichuan region that produced industrial gas flow from the Maokou Formation dolomite pore-type reservoir. All of these may demonstrate the exploration prospect of this area, which is considered the most favorable prospect for the present exploration of the Middle Permian Maokou Formation dolomite pore-type reservoir.

Karst slope belt is the most developed zone of karst reservoirs. The southern Sichuan region with a long exploration history and the Gaoshiti-Moxi region that is recently frequently drilled are located in a karst slope belt [16,17]. In addition to these regions, the northern part of the western Sichuan region, the majority of the central Sichuan region, and the northern part of the Dachigan—Wolonghe—Linshui in the eastern Sichuan region are the areas with well-developed karst reservoirs.

Therefore, emphasis should be placed on the three regions for the next exploration of the Middle Permian in the Sichuan Basin.

(1) The northern part of the western Sichuan region. In this region, the Qixia Formation platform marginal shoal is developed, and thick shoal facies dolomite reservoirs are broadly distributed. Moreover, it is located within the

thermal sub-basin where the Maokou Formation thermal water dolomites are developed.

(2) The central Sichuan region. In this region, three sections of reservoirs (i.e., the Qixia Formation shoal facies dolomite, the Maokou Formation thermal water dolomite, and the Maokou Formation karst reservoir) are developed and superimposed vertically, indicating a great potential of exploration.

(3) The Jiange—Nanchong—Fengdu region. This region is located within the thermal sub-basin where the Maokou Formation thermal water dolomite is developed and superimposed with the broadly distributed Maokou Formation karst reservoir, and hence is considered to have a good exploration prospect.

4. Conclusions

(1) During the deposition of the Middle Permian, the paleo-geomorphology of the Sichuan Basin is high in the west and low in the east, leading to more broadly developed shoal reservoirs in the central-western Sichuan region than in the eastern region; the crustal extension makes it possible for the Middle Permian to possess the condition for thermal water deposition and hydrothermal alteration; and the Middle Permian is deposited primarily in an open platform shallow-water carbonate environment.

(2) The Qixia Formation shoal facies dolomite pore-type reservoir, the Maokou Formation thermal water dolomite pore-type reservoir and the Maokou Formation karst reservoir are prospective reservoirs for the exploration of the Middle Permian in the Sichuan Basin.

(3) The dolomite pore-type reservoir is the most prospective target for the exploration of the Middle Permian in the Sichuan Basin.

(4) The western Sichuan region, where the thick-bedded dolomite reservoir of the Qixia Formation was deposited, is the most favorable area for the exploration of the Middle Permian in the Sichuan Basin. The central Sichuan region, where three sections of reservoirs are developed and superimposed vertically, has a great potential of exploration. The Jiange—Nanchong—Fengdu region, where the Maokou Formation thermal water dolomite reservoir is developed and superimposed with the Maokou Formation karst reservoir, shows a good exploration prospect.

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