Scholarly article on topic 'Significance of new breakthrough in and favorable targets of gas exploration in the Middle Permian system, Sichuan Basin'

Significance of new breakthrough in and favorable targets of gas exploration in the Middle Permian system, Sichuan Basin Academic research paper on "Earth and related environmental sciences"

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{"Sichuan Basin" / "Well Shuangtan 1" / Risk-exploration / "Middle Permian" / "Natural gas exploration" / Breakthrough / "Favorable zone" / "Hydrocarbon source conditions" / Dolomite / Reservoirs}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Ping Shen, Jian Zhang, Jiarong Song, Haitao Hong, Dahai Tang, et al.

Abstract The Middle Permian system in the Sichuan Basin represents favorable conditions for the formation of natural gas reservoirs. However, only fracture-vuggy limestone gas reservoirs were discovered in early exploration, whose scales and results were limited. In 2014, a high-yield industrial gas flow was produced there in Well Shuangtan 1, the major risk-exploration well, which means a great breakthrough in natural gas exploration in the Middle Permian system, Sichuan Basin. This plays a strategically important and guiding role in oil and gas exploration in ultra-deep marine formations. This paper illustrates the deployment background of Well Shuangtan 1, major exploration achievements and its significance in the gas exploration in the Middle Permian system, Sichuan Basin. Moreover, exploration potential and subsequent exploration targets of Middle Permian are analyzed in terms of its hydrocarbon source conditions, sedimentary characteristics, reservoir characteristics and areal extension, resources and favorable exploration zones. It is concluded that the Middle Permian system in the Sichuan Basin is a continent-connected carbonate platform with carbonate deposits being dominant. There are three types of reservoirs, including pore dolomite, karst fracture-vuggy, and fracture, which present considerable natural gas resources and huge exploration potential. Favorable exploration zones classification of Middle Permian in the Sichuan Basin indicates that the most favorable exploration zones are mainly distributed in the central-western Sichuan Basin. In particular, the northwestern Sichuan Basin is the most favorable exploration area which is characterized by the most excellent hydrocarbon source conditions, developed dolomite reservoirs and zonal distribution of structures.

Academic research paper on topic "Significance of new breakthrough in and favorable targets of gas exploration in the Middle Permian system, Sichuan Basin"

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Research article

Significance of new breakthrough in and favorable targets of gas exploration in the Middle Permian system, Sichuan Basin

Shen Pinga, Zhang Jiana, Song Jiarongb, Hong Haitaob*, Tang Dahaib, Wang Xiaojuanb,

Wang Huab, Luo Wenjunb

a Exploration Department of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China b Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610041, China

Received 5 June 2015; accepted 8 September 2015 Available online 19 February 2016

Abstract

The Middle Permian system in the Sichuan Basin represents favorable conditions for the formation of natural gas reservoirs. However, only fracture-vuggy limestone gas reservoirs were discovered in early exploration, whose scales and results were limited. In 2014, a high-yield industrial gas flow was produced there in Well Shuangtan 1, the major risk-exploration well, which means a great breakthrough in natural gas exploration in the Middle Permian system, Sichuan Basin. This plays a strategically important and guiding role in oil and gas exploration in ultra-deep marine formations. This paper illustrates the deployment background of Well Shuangtan 1, major exploration achievements and its significance in the gas exploration in the Middle Permian system, Sichuan Basin. Moreover, exploration potential and subsequent exploration targets of Middle Permian are analyzed in terms of its hydrocarbon source conditions, sedimentary characteristics, reservoir characteristics and areal extension, resources and favorable exploration zones. It is concluded that the Middle Permian system in the Sichuan Basin is a continent-connected carbonate platform with carbonate deposits being dominant. There are three types of reservoirs, including pore dolomite, karst fracture-vuggy, and fracture, which present considerable natural gas resources and huge exploration potential. Favorable exploration zones classification of Middle Permian in the Sichuan Basin indicates that the most favorable exploration zones are mainly distributed in the central-western Sichuan Basin. In particular, the northwestern Sichuan Basin is the most favorable exploration area which is characterized by the most excellent hydrocarbon source conditions, developed dolomite reservoirs and zonal distribution of structures.

© 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; Well Shuangtan 1; Risk-exploration; Middle Permian; Natural gas exploration; Breakthrough; Favorable zone; Hydrocarbon source conditions; Dolomite; Reservoirs

1. Overview of natural gas exploration

1.1. General situation

With a long natural gas exploration history, the Middle Permian has always been an important stratum for natural gas exploration and development in the Sichuan Basin. The exploration of this stratum started over 60 years ago, when the

* Corresponding author. E-mail address: xfhht@petrochina.com.cn (Hong HT.). Peer review under responsibility of Sichuan Petroleum Administration.

first well (Long 10) produced gas from the Middle Permian Maokou Formation in 1955. At early stage, exploration activities in the southern Sichuan region mainly focused on the Middle Permian, aiming to find karst fracture-cave gas reservoirs. To date, proved natural gas reserves of the Middle Permian over the whole basin total 852.03 x 108 m3, of which 87.7% (746.9 x 108 m3) is produced in the southern Sichuan region. These reserves are distributed within a total of 325 fracture-cave systems of varying sizes. Scale layered reservoirs are absent in this region.

Field workers have long noted that, the Middle Permian Qixia Formation in local area of the northern part of the western

http://dx.doi.org/10.1016/j.ngib.2015.09.014

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/).

Sichuan region contains layered "saccharoidal" dolomite and "leopard" limestone, which are 30—40 m thick, exhibit good physical properties and can act as a favorable reservoir. Yet, a long-standing question is whether or not this dolomite can be present at subsurface. Kuang 2, a well drilled in the northern part of the western Sichuan region targeting the Kuangshanliang structure, reveals the presence of thick-bedded pore-type dolomite inside the basin. This triggered considerable interest in prospecting the Middle Permian pore-type large-scale gas reservoirs. Then Well Kuang 3 was subsequently drilled to the north of Well Kuang 2, targeting the Baolunzhen structure in the Kuangshanliang structural belt. According to the completion well-log interpretation, the Qixia Formation reservoir is absent in this well. This has initially demonstrated that the Qixia Formation pore-type dolomite reservoir in the northern part of the western Sichuan region is characterized by rapidly changed lateral distribution and high heterogeneity.

In order to achieve breakthrough in natural gas exploration of marine strata in the western Sichuan region, PetroChina Southwest Oil & Gas Field Company (hereinafter referred to as PetroChina Southwest) accelerates the exploration process of the Middle Permian in the western Sichuan region, and places much emphasis on integrated geological study and 2D seismic survey during the 10th and 11th Five-Year Plan. Although some wells have been drilled to test some favorable structures and the Middle Permian reservoir facies belt, including some small-scale high-pressure and extra-high-pressure gas reservoirs (e.g., Hewanchang and Jiulongshan) and gas-bearing structures (e.g., Shejianhe, Zhangjiabian and Xiasi), the discoveries are considered less significant, because there is no substantial breakthrough.

1.2. New breakthrough in recent natural gas exploration

1.2.1. Deployment of Well Shuangtan 1

Continuous drilling and geological study results at early stage suggest that the Middle Permian in the northern part of the western Sichuan region possesses a favorable condition for natural gas accumulation, and numerous wells producing gas from the Middle Permian Maokou Formation through testing have preliminarily proved to have the superiority of preservation condition of this region [1—7]. All these jobs and understandings have further strengthened our confidence and determination to find large- to medium-sized gas fields in the northern part of the western Sichuan region.

In the western Sichuan region, PetroChina Southwest has selected the Shuangyushi-Hewanchang structural belt (Fig. 1) in the northern part to seek a breakthrough in prospecting the marine strata, deployed 1140 km of 2D seismic survey in 2010 and 2011, and invited numerous companies to systematically conduct the integrated geological study and seismic processing & interpretation since the 11th Five-Year Plan, in order to determine the reservoir development degree and gas-bearing capacity of the Permian and Triassic strata and open up a new territory for natural gas exploration. Based on these results, the exploration Well Shuangtan 1 was designed and drilled in 2012, targeting the Shuangyushi structure (Fig. 1). This well is located in the northwestern part of the Sichuan Basin, within Jiange County, Guanyuan City. Regionally, it structurally lies at the joint part of the Longmenshan nappe structural belt and the Zitong syncline, which is bordered by the Ganzi-Songpan structural belt to the west, to the east by the front of the Micangshan platform marginal uplift belt, and

Fig. 1. Structural map of the Upper Permian base floor in the Shuangyushi structure in the northwestern part of the Sichuan Basin.

to the southeast by the Zitong sag. Locally, it has close proximity to the structural high of the Shuangyushi buried structure of the Longmenshan fault-fold belt at the base surface of the Upper Permian (Fig. 1).

1.2.2. Drilling result of Well Shuangtan 1

In addition to the Permian Qixia Formation and Maokou Formation (the major targets), the Permian Changxing Formation and the Triassic Feixianguan Formation are also the exploration targets for Well Shuangtan 1. This well was spudded on November 27, 2012, and completed at a total depth of 7308.65 m in the Middle Permian Liangshan Formation on February 8, 2014.

Thin section analysis of rocks in Well Shuangtan 1 has shown that, the Qixia Formation reservoir rocks comprise crystal grain dolomite, limy dolomite and dolomitic limestone (Fig. 2), and contain reservoir spaces dominated by inter-crystalline pores and intercrystalline dissolved pores and well-developed fractures. Four sections of reservoirs with 4.3% average porosity and 45.1 m in thickness are interpreted using well logs. Of these reservoirs, three are gas layers with 4.8% average porosity totaling 29.3 m thick. The completion test for these three gas layers show the tubing pressure rising from 57.4 MPa to 58.9 MPa, the per-day gas production of 87.6 x 104 m3 and H2S content of 4.85 g/m3. The "single test" open-flow of natural gas calculated using well test model is 316.23 x 104 m3/d.

For the Maokou Formation, the completion test shows the stable tubing pressure of 80.33 MPa, per-day gas production of 126.77 x 104 m3, H2S content of 0.226 g/m3, formation pressure of 123.2 MPa and pressure coefficient of 1.8. The open-flow of natural gas for Well Shuangtan 1 from the Maokou Formation calculated using "single test" method is 701 x 104 m3/d.

1.3. Significance of exploration breakthrough in Well Shuangtan 1

Well Shuangtan 1 has produced a high-yield industrial gas flow from two formations of the Middle Permian and hence kicks off the large-scale exploration of the Middle Permian in the Sichuan Basin. This well is considered to be of great strategic

significance: © it discovers the Middle Permian dolomite poretype high-quality gas reservoir in the Sichuan Basin for the first time, which has milestone significance in natural gas exploration; © it facilitates the change of exploration idea, strengthens prospectors' confidence of finding large-sized pore-type gas field in the Middle Permian, and strongly promotes the progress in exploration deployment of the Middle Permian in the Sichuan Basin; and © it urges natural gas prospectors' to re-study and reassess the resources potential and exploration prospects of the Middle Permian, which is expected to become another important replacement domain for exploration and development in the Sichuan Basin following the Sinian and Cambrian Long-wangmiao Formation.

2. Favorable targets of natural gas exploration in the Middle Permian

2.1. Lithological association and distribution of the Middle Permian

The Middle Permian in the Sichuan Basin disconformably overlies the Carboniferous system [1-9] and, from bottom to top, can be divided into three formations: Liangshan, Qixia and Maokou. The Liangshan Formation contains grey, grey black carbonaceous shale and green grey bauxitic shale intercalated with coal seam and pyrite. This formation ranges from 5 to 20 m in thickness in the Sichuan Basin and is absent in local areas (e.g., it was not deposited in some local Gaoshiti areas). The Qixia Formation can be divided into two units: a lower section of grey, grey black, medium- to thick-bedded, layered, fine- to powder-crystalline, algal and bioclastic limestone intercalated with argillaceous limestone and finely laminated black shale, rich in organic matter and mud; and an upper layer of grey, light grey, grey white, thick-bedded and massive sparry algal limestone, bioclastic limestone and leopard dolomitic limestone, with crystal grain dolomite developed locally in its upper-middle parts. This 80-200 m thick formation is relatively thick in the northern and eastern parts of the basin and thins to less than 120 m in the central and western parts [3-7]. The Maokou Formation can be divided into two units: a lower section of black grey, dark grey, medium- to thick-bedded bioclastic limestone, rich in organic

a. Fine-medium crystalline dolomite. Qixia Formation b. Dolomitic bioelastie limestone. Qixia Formation

Fig. 2. Photos of cutting thin sections from Well Shuangtan 1 in the northwestern part of the Sichuan Basin.

matter and mud, with augen structure developed; and a upper layer of grey, light grey, thick-bedded and massive, fine- to powder-crystalline bioclastic limestone and sparry algal limestone, and bioclastic limestone intercalated with dolomitic limestone or limy dolomite, with small amounts of chert nodule present locally. This formation contains dark grey, fine-to powder-crystalline chloralgal limestone, bioclastic limestone, medium- to thick-bedded siliceous rock, siliceous mudstone and organic mudstone on its top. The thickness of the Maokou Formation varies greatly due to the erosion during the Dongwu Movement. With thickness ranging from 120 to 350 m, this formation is generally thick in the southwest and southeast, thinning to the northeast, and is fairly thin in the adjoining area of the Luzhou paleo-uplift [8,9].

2.2. Gas source condition

Most of the gas accumulated in the Middle Permian came from the mixed sources. The current research results suggest that hydrocarbon generation took place in the Middle Permian, Silurian, Cambrian and the Upper Permian Longtan Formation [1 -4]. The principal source rock is the Middle Permian dark to dark grey organic-rich argillaceous limestone in the Mao-kou and Qixia formations, with the hydrocarbon generating center located in the northern part of the western Sichuan region and the central Sichuan region, reaching a gasgenerating intensity of 26 x 108-44 x 108 m3/km2 (Fig. 3). The Lower Silurian Longmaxi Formation source rock shows a hydrocarbon generating center at the eastern Sichuan and southern Sichuan regions, reaching a gas-generating intensity of 280 x 108 m3/km2, and this rock, however, is absent in the central and western parts of the Sichuan Basin. The hydrocarbon generating center of the Cambrian is distributed in the southern Sichuan region, the Anyue-Deyang intra-platform

rift formed during the late stage of the Tongwan Movement and other regions to the north. In particular, the Lower Cambrian Qiongzhusi Formation ranges from 322 to 742 m in thickness in the northern part of the western Sichuan region, and Well Zeng 1 penetrated the 742 m thick Qiongzhusi Formation in the 1970s, including a section of carbonaceous shale reaching up to 100 m in thickness. This formation is preliminarily considered the principal source rock in the northern part of the western Sichuan region (Fig. 4) and is absent along the southwestern margin of the basin. The Longtan Formation contains rich hydrocarbon source in the central Sichuan region and the northeastern part of the basin, showing a hydrocarbon generating intensity of (30-55) x 108 m3/km2, while other regions within the basin have reported to have a hydrocarbon generating intensity of less than 30 x 108 m3/km2.

The organic carbon content of the Middle Permian limy source rock samples obtained from Well Shuangtan 1 averaging 5.67% indicates its strong hydrocarbon generating capability. According to the statistics of wells and surface outcrop profiles, the Middle Permian contains 100-200 m thick carbonate source rock, 23-54 m thick grey black siliceous rock, siliceous mudstone and carbonaceous mudstone, 66-262 m thick dark grey to black grey bioclastic limestone, and 40-214 m thick grey bioclastic limestone. Organic matter is predominately sapropelic and sapropelic-prone mixed kerogen in the Middle Permian source rock, which contains 0.4%-4.6% average organic carbon content, much higher than the lower limit (0.2%) of carbonate rock for hydrocarbon generation. The Middle Permian Liangshan Formation consisting of black mud shale, limy mud shale and carbonaceous mud shale intercalated with coal seam is likely to act as the source for coal-type gas. In addition, coal-type gas may also come from the Upper Permian coal measure strata in the

Fig. 3. Hydrocarbon generating intensity contour of the Middle Permian in the Sichuan Basin (Modified from Chen Shengji et al. [2]).

Fig. 4. Gas-generating intensity contour of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin.

Longtan Formation. The Upper Permian carbonate source rock ranges from 50 to 100 m in thickness and has an organic carbon abundance of 1.0%-1.2%; and the argillaceous source rock ranges from 20 to 40 m in thickness and has an organic carbon abundance of 2%-4% and a gas-generating intensity of 15 x 108 m3/km2. Therefore, the Middle Permian in the northern part of the western Sichuan region is considered to have favorable source rock conditions and the material basis for the formation of large- to medium-sized gas fields.

2.3. Reservoir conditions

2.3.1. Lithofacies paleo-geographic features of the Middle Permian Qixia Formation and Maokou Formation

The Middle Permian Qixia Formation in the Sichuan Basin is composed mainly of carbonate platform facies sediments [10,11]. The deep-water basin facies is deposited to the west of the Longmenshan fault and a platform marginal shoal development area is present to the east. The water body deepens to the east, representing a transition from the platform margin to the open platform facies (Fig. 5). The platform marginal shoal in the northern part of the western Sichuan region is distributed along the Tongkou-Tianjingshan-Kuangshanliang-Chejiaba during the late Qixia Stage. This is considered an important stage in the Middle Permian period for shoal-forming. Reservoirs are therefore developed mainly in the middle and upper sections of the Qixia Formation, and platform marginal shoal rocks were mostly dolomitized and hence have good reservoir conditions [3,4]. The Middle Permian Maokou Formation in the basin is also composed of carbonate platform sediments, with open platform facies being dominant. During its

deposition, thermal sub-basin microfacies were developed in local areas [8,9] in the basinal extension setting, and the presence of hydrothermal fluids results in the formation of thermal water dolomites, which can form the crystal grain dolomites after multiple re-crystallizations.

2.3.2. Reservoir development characteristics of the Middle Permian Qixia Formation and Maokou Formation

The Middle Permian in the Sichuan Basin mainly contains three types of reservoirs: dolomite, karst and fracture reservoirs [6-9,12-15]. Exploration activities at early stage targeted the fracture and karst reservoirs. Well Shuangtan 1 encountered the shoal facies dolomite reservoir in the Qixia Formation and the fracture dolomite reservoir in the Maokou Formation. Dolomite and karst reservoirs are the primary targets of current exploration.

2.3.2.1. Dolomite reservoir. Both Qixia Formation and Maokou Formation of the Middle Permian contain dolomite reservoirs [6-9]. The Qixia Formation dolomite reservoirs are mostly crystal grain dolomite, with intercrystalline pores and intercrystalline dissolved pores being dominant. In addition, reservoirs of the Qixia Formation contain well-developed karst caves formed by burial hydrothermal dissolution. The porosity of the dolomite section, as shown by analysis of 350 samples obtained from drilled wells and surface outcrops, averages 2.22%. Reservoir distribution is controlled primarily by the paleo-geomorphological high. Shoals are formed at higher position, providing favorable material basis, and burial (hydrothermal) dissolution allows shoal facies dolomite reservoirs with well-developed dissolved pores, caves and fractures to form at later stage. Dolomite reservoirs of the Maokou

Fig. 5. Depositional facies map of the Qixia Formation in the Sichuan Basin.

Formation are mostly dark-colored crystal grain dolomite and residual bioclastic dolomite, with intercrystalline pores and intercrystalline dissolved pores being dominant. Moreover, a large number of dissolved pores and caves were formed in these reservoirs due to the effect of epigenetic karstification during the Dongwunian. Reservoirs commonly have low porosity and low permeability, with exceptional moderate to high porosity sections present locally. The average porosity of the dolomite, as is revealed by numerous coring wells, ranges

from 2.83% to 4.6%. The Maokou Formation dolomite is controlled by the thermal sub-basin microfacies and trends towards the northwest [8,9].

2.3.2.2. Karst reservoir. Weathering crust karst reservoirs are developed in the Maokou Formation, due to the crustal uplift induced by the Dongwu Movement. This type of reservoir contains well-developed dissolved pores, caves and fractures. The cored wells have shown that, the top of the Middle

Fig. 6. Distribution map of karst geomorphology during the Early Permian in the Sichuan Basin.

Permian in the basin, which was eroded, is underlain by several meters thick breccia and then by the zone where dissolved pores, caves and fractures are concentrated (i.e., the principal development zone of karst reservoir). Karst slope belt is the area with the strongest karstification. The distribution map of the karst geomorphology in the basin during Early Permian (Fig. 6) shows that, the northern part of the western Sichuan region, the majority of the central Sichuan region and the southern Sichuan region, and the Dachigan-Wolonghe-Linbei in the eastern Sichuan region are located within the range of the karst slope belt with the strongest karstification and the most developed karst reservoirs.

2.4. Analysis of favorable targets for natural gas exploration in the Middle Permian

2.4.1. Potential of natural gas resources

The 60 years of exploration in the Sichuan Basin indicates that, although fracture gas reservoirs and karst fracture-cave gas reservoirs have been explored for a long time, with over 1000 wells drilled, the proved natural gas reserves of the Middle Permian in the basin as of 2014 total only 852 x 108 m3 [1,2]. Besides the significant discovery reported in Well Shuangtan 1, numerous wells drilled within the basin in recent years have produced high-yield gas flow from the Middle Permian dolomite reservoirs in the Maokou and Qixia Formations. The distribution area of the Qixia Formation dolomite reservoir is calculated to be 9110 km2 with a thickness of 10 m and, with 2.66 x 108 m3/km2 in average resources abundance in Well Shuangtan 1 and Well MX31X1, the natural gas resources of the Qixia Formation are estimated to be 2.42 x 1012 m3. With 2.98 x 104 km2 distribution area of the dolomite reservoir and 1.00 x 108 m3/km2 resources

abundance of Well NC1, the natural gas resources of the Maokou Formation are estimated to be 2.98 x 1012 m3. Accordingly, the Middle Permian in the Sichuan Basin is considered to hold considerable natural gas resources and huge exploration potential.

2.4.2. Favorable targets of natural gas exploration

The Qixia Formation dolomite reservoirs are mainly distributed in the western Sichuan Basin, where platform marginal shoal facies sediments are deposited. These dolomite reservoirs are commonly thick-bedded and broadly distributed, thinning gradually from the margin to the interior of the basin. The northern part of the western Sichuan region, however, contains the overlapped development zones of the Maokou Formation thermal sub-basin thermal water dolomite and Qixia Formation shoal facies dolomite, and is therefore considered the most favorable area for exploration of dolomite reservoirs in the Middle Permian due to the availability of superior hydrocarbon source conditions. Within this region, structures were developed in zones and rows and the area of high-lying zone totals 7710 km2, indicating huge exploration potential. In the central Sichuan region, the Maokou karst reservoir is developed in addition to the Qixia Formation intra-platform shoal facies dolomite reservoir and the Maokou Formation thermal water dolomite reservoir. These reservoirs are vertically superimposed and considered to have great exploration potential. The karst reservoirs that have been discovered are distributed mainly in the southern Sichuan and Gaoshiti-Moxi regions. Other regions with karst reservoirs include the northern part of the western Sichuan region, the majority of the central Sichuan region and the Dachigan-Wolonghe-Linbei in the eastern Sichuan region (Fig. 7).

Fig. 7. Evaluation map of the Middle Permian prospects in the Sichuan Basin.

References

3. Conclusions

(1) Drilling of Well Shuangtan 1 suggests that the Qixia Formation dolomite reservoir is well-developed and broadly distributed in the northern part of the western Sichuan region. This pore-type reservoir, with reservoir space dominated by intercrystalline pores and inter-crystalline dissolved pores, contains well-developed fractures and has good lateral connectivity.

(2) Drilling of Well Shuangtan 1 also confirms that the Qixia Formation pore-type dolomite reservoir in the northern part of the western Sichuan region has good gas-bearing property and is considered the primary target for next-step exploration. It also facilitates the change of exploration idea from the "fracture-cave-type" gas reservoir at earlier stage to the Qixia Formation porous dolomite gas reservoir. This results in the change of exploration deployment idea for the Middle Permian in the Sichuan Basin.

(3) The Middle Permian in the western and central Sichuan regions are believed to have superior hydrocarbon source rock conditions, particularly the northern part of the western Sichuan region — the central Sichuan region, with gas-generating intensity ranging from 26 x 108 to 44 x 108 m3/km2, have the material basis for the formation of large- to medium-sized gas fields.

(4) With superior hydrocarbon source conditions, the northern part of the western Sichuan region, where the Qixia Formation shoal facies dolomite reservoir with good porosity is developed and the Maokou Formation is within the favorable development zone of thermal sub-basin dolomite, is considered the most favorable area for exploration of the Middle Permian in the Sichuan Basin.

(5) The preliminary research results indicate the huge exploration potential of the Middle Permian in the Sichuan Basin. 3D high-precision seismic study of reservoir and further characterization of the distribution of porous dolomite are required in order to realize the conversion from resources to high-quality reserves. It is possible that carbonate rocks could act as reservoirs under extra-deep conditions, allowing for accumulation of natural gas and high production rate. This has a great significance in guiding the natural gas exploration in China's deep to extra-deep carbonate rocks.

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