Scholarly article on topic 'Sedimentary features and exploration targets of Middle Permian reservoirs in the SW Sichuan Basin'

Sedimentary features and exploration targets of Middle Permian reservoirs in the SW Sichuan Basin Academic research paper on "Earth and related environmental sciences"

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Natural Gas Industry B
Keywords
{"Sichuan Basin" / Southwest / "Middle Permian" / "High energy shallow water gentle slope" / "Reef flat facies" / "Unconformity karst weathering crust" / "Karst slope belt" / "Karst reservoir" / "Exploration target"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Guoming Xu, Gangping Xie, Ke Long, Xiaobo Song

Abstract The exploration direction and targets for the large-scale Middle Permian gas reservoirs in the Sichuan Basin are hot spots and challenges in current exploration researches. The exploration successes of large gas field of Cambrian Longwangmiao Formation in Gaoshiti-Moxi region, Central Sichuan Basin, indicated that prospective sedimentary facies belt was the basis for the formation of large gas fields. In this paper, based on seismic data, outcrop data and drilling data, the tectonic framework and sedimentary features of the Middle Permian in the SW Sichuan Basin were comprehensively studied. The following conclusions were reached from the perspective of sedimentary facies control: (1) during the Middle Permian, this region was in shallow water gentle slope belts with high energy, where thick reef flat facies were deposited; (2) the basement was uplifted during Middle Permian, resulting in the unconformity weathering crust at the top of Maokou Formation due to erosion; the SW Sichuan Basin was located in the karst slope belt, where epigenic karstification was intense; and (3) reef flat deposits superimposed by karst weathering crust was favorable for the formation of large-scale reef flat karst reservoirs. Based on the combination of the resources conditions and hydrocarbon accumulation conditions in this region, it was pointed out that the Middle Permian has great potential of large-scale reef flat karst gas reservoir due to its advantageous geological conditions; the Middle Permian traps with good hydrocarbon accumulation conditions were developed in the Longmen Mountain front closed structural belt in the SW Sichuan Basin and Western Sichuan Basin depression slope belt, which are favorable targets for large-scale reef flat karst reservoirs.

Academic research paper on topic "Sedimentary features and exploration targets of Middle Permian reservoirs in the SW Sichuan Basin"

Available online at www.sciencedirect.com

ScienceDirect

Natural Gas Industry B 2 (2015) 415-420

Research article

www.elsevier.com/locate/ngib

Sedimentary features and exploration targets of Middle Permian reservoirs

in the SW Sichuan Basin

Xu Guoming*, Xie Gangping, Long Ke, Song Xiaobo

Exploration and Development Research Institute of Sinopec Southwest Branch Company, Chengdu, Sichuan 610041, China

Received 18 March 2015; accepted 8 September 2015 Available online 19 February 2016

Abstract

The exploration direction and targets for the large-scale Middle Permian gas reservoirs in the Sichuan Basin are hot spots and challenges in current exploration researches. The exploration successes of large gas field of Cambrian Longwangmiao Formation in Gaoshiti-Moxi region, Central Sichuan Basin, indicated that prospective sedimentary facies belt was the basis for the formation of large gas fields. In this paper, based on seismic data, outcrop data and drilling data, the tectonic framework and sedimentary features of the Middle Permian in the SW Sichuan Basin were comprehensively studied. The following conclusions were reached from the perspective of sedimentary facies control: (1) during the Middle Permian, this region was in shallow water gentle slope belts with high energy, where thick reef flat facies were deposited; (2) the basement was uplifted during Middle Permian, resulting in the unconformity weathering crust at the top of Maokou Formation due to erosion; the SW Sichuan Basin was located in the karst slope belt, where epigenic karstification was intense; and (3) reef flat deposits superimposed by karst weathering crust was favorable for the formation of large-scale reef flat karst reservoirs. Based on the combination of the resources conditions and hydrocarbon accumulation conditions in this region, it was pointed out that the Middle Permian has great potential of large-scale reef flat karst gas reservoir due to its advantageous geological conditions; the Middle Permian traps with good hydrocarbon accumulation conditions were developed in the Longmen Mountain front closed structural belt in the SW Sichuan Basin and Western Sichuan Basin depression slope belt, which are favorable targets for large-scale reef flat karst reservoirs.

© 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; Southwest; Middle Permian; High energy shallow water gentle slope; Reef flat facies; Unconformity karst weathering crust; Karst slope belt; Karst reservoir; Exploration target

After over six decades of exploration, several medium and large natural gas fields (e.g. Puguang, Longgang, and Yuanba) have been discovered and built in the Upper Permian Changxing Formation in the Sichuan Basin. High-productivity natural gas flows have also been tapped in Middle Permian. For example, in the Northeastern Sichuan Basin, Well Long 4 obtained 20.97 x 104 m3/d gas in the Maokou Formation, and Well Yuanba 3 obtained 160 x 104 m3/d gas in the Maokou Formation; in the Southern Sichuan Basin, Well Tai 4 obtained 202 x 104 m3/d gas in the Maokou Formation, and Well Wei 3

* Corresponding author. E-mail address: xgm6363@163.com (Xu GM).

Peer review under responsibility of Sichuan Petroleum Administration.

obtained 8.94 x 104 m3/d gas in the Maokou Formation. Recently, Well Shuangtan 1 drilled by CNPC in Shuangyushi closed structure in the NW Sichuan Basin, tapped 100 x 104 m3/d gas in the Qixia Formation and Maokou Formation, indicating huge natural gas potential in Permian. However, no large-scale gas field has been discovered in the Middle Permian yet, and the large-scale gas pools discovered so far are all carbonate solution fissure and cave reservoirs related to fractures. Controlled by fracture karst system, these reservoirs are poor in continuity, complex in gas and water distribution, and limited in scale.

The exploration experience obtained from the discovery of large-scale gas fields in the Permian Changxing Formation in Puguang, Longgang and Yuanba in the NE Sichuan

http://dx.doi.org/10.1016/j.ngib.2015.09.017

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/).

Basin, and in the Cambrian Longwangmiao Formation in Gaoshiti-Moxi region in the Central Sichuan Basin shows that: favorable sedimentary facies belt is the basis for forming large gas fields. Puguang, Longgang and Yuanba large gas fields are all distributed in large-scale reef flat belt facies in the platform margin in the Changxing period in the Kaijiang-Liangping area, and Gaoshiti-Moxi large gas fields are all located in large-scale flat facies inside the platform in the Longwangmiao period; the formation of reservoir beds in large-scale gas fields are mainly controlled by reef flat deposition, dolomitization and denudation [1-7]. Inspired by successful experiences and cognitions, some researchers studied the origin of Middle Permian reservoir beds from the viewpoints of "sedimentary facies control" and "diagenetic facies control". The viewpoint of sedimentary facies control held that in the background of Middle Permian large gentle slope during Hercynian, large-scale reef flats are the base for forming high-quality reservoir beds, and coupling of dolo-mitization and dissolution in reef flat facies is the condition for forming high-quality reservoir beds [8,9]. The viewpoint of diagenetic facies control maintained that Emei taphrogeny during Hercynian led to mantle magma invasion along the basement tensional faults, and thus the hydrothermal sedimentary environment (hot sub-basin) during the deposition of Middle Permian or hydrothermal diagenetic environment during the Middle Permian diagenetic process is the major controlling factor for forming large-scale dolomite reservoirs [10-12]. The studies of the above two viewpoints tried to explore the major controlling factors for forming large-scale dolomite reservoirs in Middle Permian, and to guide exploration in the Middle Permian. Mainly based on the study on the deposition features of Permian in the SW Sichuan Basin, we advanced some findings on exploration targets from the viewpoint of sedimentary facies control in this paper.

1. Deposition features of Middle Permian

1.1. Development of high-energy reef flat deposits in shallow water gentle slope

Before the deposition of the Middle Permian, the "big uplift and big depression" framework caused by the Caledonian tectonic cycle in the Sichuan Basin experienced "planation and compensation" because of large-scale whole-slab uplifting of the basin basement during early Hercynian, forming "gentle slope type" basement with the feature of "peneplane" during the deposition of the Middle Permian [13,14]. During the deposition of this "gentle slope type" basement, influenced by the uplifting of Emei mantle plume on the southwestern edge of this basin during the initial Dongwu Movement, underwater dome-like uplift was generated on the depositional basement in SW Sichuan Basin [15-18], which gradually descended to the east and northeast directions.

The palaeogeomorphic framework of the basin basement and regional sea level changes jointly controlled the distribution of sedimentary facies belts in the Middle Permian of

the Sichuan Basin. The general sedimentary framework of this basin during the Middle Permian was carbonate gentle slope, but affected by underwater paleo-uplift, the southwestern region (west of Yibin-Ziyang-Mianyang) of this basin was located in shallow water gentle slope with high energy, where large-scale deposits of reef flat facies in high energy environment developed. The seismic sections in Sinopec prospect area in the Longmen Mountain front show that the Middle Permian reflection layers have apparent instable, discontinuous, chaotic and moundy phase energy reflection features of high-energy deposition environment (Figs. 1 and 2).

The Middle Permian outcrop profiles in some areas (e.g. Yongqing in Beichuan county, Tianchi and Gaoqiao in Mianzhu city, Dafeishui in Dayi county, and Shuimo in San-jiang Village) in the Longmen Mountain front show that the lithologies in Middle Permian Qixia Formation and Maokou Formation are mainly gray calcsparite bioclastic limestone and psammitic limestone, with coral reef limestone and sponge reef limestone (Fig. 3). Wells drilled in the SW Sichuan Basin also reveals that thick bioclastic limestone and algal psam-mitic limestone developed in the Maokou Formation. For example, particle rocks in Well Dashen 1, Well Ziyang 1 and Well Jinshi 1 are more than 150 m thick. Some wells (e.g. Nuji, Langzhong 1, and Guanshen 1) drilled in broad region of this basin to the east of Yibin-Ziyang-Mianyang show that the lithology of the Middle Permian is mainly dark gray micrite and microcrystalline limestone. Seismic reflection structure also shows apparent parallel, continuous and stable features, which indicates that the Middle Permian gradually transited to deposits in gentle slope, deeper water and low energy environment. Previous studies show that Mao 2 and Mao 3 are two important flat-forming periods during the deposition of the Maokou Formation [9,19,20], and the flats generated are mainly distributed in southwestern region of this basin and consist of primarily light gray massive and calc-sparite cemented red alga and green alga limestone. Compared with the Qixia period, with shallower water and stronger water energy, the Maokou period had an upper zone of shallow gentle slope extending far east, and more developed organic reef flats (Figs. 4 and 5).

1.2. Located in karst slope belt, the "Maokou Formation top" has favorable conditions for forming reef flat karst reservoirs

The Dongwu Movement during Hercynian led to differential uplift of the basement of the Sichuan Basin. Differential denudation of the Middle Permian Maokou Formation top (briefly called "Mao top") gave rise to a large-scale unconformity weathering crust. In the southwestern region of this basin, the 2nd member of the Maokou Formation outcrops in some areas (Gaoqiao in Mianzhu-Yongqing in Bei-chuan), and the 3rd member of the Maokou Formation outcrops in most of the rest areas, with erosion thickness of 100-154 m. Research results of palaeogeomorphology indicate that the Mao top weathering crust was located in paleo-

Fig. 1. Seismic section of instantaneous phase of line NE229 in the Western Sichuan Basin.

karst slope belt [21] (Fig. 6), where overland flow and groundwater flow hydrodynamic force were strong, resulting in the development of vertical vadose zone and horizontal groundwater flow zone. Thus, it can be inferred that the Mao top in southwestern region of this basin experienced violent weathering and karstification, and large-scale "reef flat karst reservoirs" were very likely to form. Moreover, according to analysis, the Dongwu Movement was not only favorable for forming weathering crust karst reservoirs at Mao top, but also had important controlling effect on hydrothermal dolomite reservoirs related to basement faults in the Middle Permian [10,11]. Since the Dongwu Movement, multiphase intense tectonic movements (Indosinian Movement, Yanshan Movement and Himalayan Movement) happened in the Longmen Mountain front, so there are many paleo-faults breaking from the Middle Permian to the basement, and hydrothermal dolomite karstified reservoirs may have developed in buried structural belt of the mount front. Granular dolomite reservoirs were encountered in the Qixia Formation in Well Kuang 2 in the Northwestern Sichuan Basin [22,23], with rich intercrystal pores, intercrystalline dissolved pores and dissolved caverns. Therefore, hydrothermal origin dolomite reservoirs are also the exploration highlight in the Middle Permian at present.

2. Natural gas exploration potential and targets

2.1. Favorable basic petroleum geological conditions and great exploration potential

There are several sets of fine source rocks in Middle Permian and marine formations below it in the SW Sichuan Basin, with wider distribution, bigger thickness and better quality. Among them, the carbonate source rocks in the Middle Permian distributed in the whole SW Sichuan Basin, 100-180 m thick (186 m in Well Heshen 1, 170 m in Jinhe, Shifang), with a hydrocarbon generation center in the areas around Zitong, Mianyang and Deyang, has an average gas generation intensity of 39.93 x 108 m3/km2; the Middle Permian muddy source rocks are thinner (10-25 m thick), with a hydrocarbon generation center in the areas around Dayi and Qionglai, and gas a generation intensity of (3-5) x 108 m3/km2. According to preliminary calculation, the total natural gas resources in source rocks in the Middle Permian and marine formations below it in the Western Sichuan Basin are up to 2 x 1012 m3 [24]. Currently, sugary grained dolomite reservoirs in the Qixia Formation were discovered with major storage space of dolomite intercrystal pores and intercrystal dissolved pores; limestone and dolomite

Fig. 2. Sections of high-resolution sequence analysis and geologic interpretation of line NW 151 in the Western Sichuan Basin.

Fig. 3. Lithologic features of the Middle Permian in the Western Sichuan Basin.

dissolved pore and fissure reservoirs in reef flat facies were also discovered in the Maokou Formation. Huge thick marine and terrestrial formations were developed in the Middle Permian and measures above them with fewer "breaking to surface" faults, forming good regional sealing and preserving conditions. On the whole, the Middle Permian in the SW Sichuan Basin has favorable petroleum geological conditions and great exploration potential.

2.2. High possibility to discover large-scale reef flat karst gas reservoirs

With an advantageous hydrocarbon resource condition, and large-scale reef flat karst reservoirs in the Middle Permian, Permian in the Southern Sichuan Basin has the basis for forming giant oil and gas fields. Effective traps are the key for hydrocarbon accumulation. At present in Sinopec prospect

_J Biogenic limestone

Fig. 4. Sedimentary facies of Middle Permian Qixia Formation in the Sichuan Basin.

limestone

Fig. 5. Sedimentary facies of Middle Permian Maokou Formation in the Sichuan Basin.

area in the Western Sichuan Basin, clues of larger Middle Permian lithological-structural traps have been found in the Dayi-Pengzhou-Anxian buried structural belt in the Long-men Mountain front by seismic prospecting, these traps are mainly controlled by structures, and lithological elements secondarily (reef flat karst reservoir). Moreover, clues of larger Middle Permian stratigraphic-lithological traps mainly controlled by lithological elements have been found in

Guanghan—Zhongjiang—Xindu slope belt in western Sichuan Depression. Further studies on these trap clues show that the Middle Permian structural traps in the buried structural belt in the Longmen Mountain front were formed in the Yanshanian period, which matches with gas generation peak period of the Permian source rocks [25]. There developed many hydrocarbon source conducting faults in the buried structural belt in the Longmen Mountain front, and very thick marine and terrestrial

Fig. 6. Paleo-karst zoning map of the Sichuan Basin during the Dongwu Movement.

formations above the source rock act as good seal, which together make up favorable conditions for forming large-scale lithological-structural gas pools. Lithological traps of mainly Middle Permian reef flat karst reservoirs developed in Guan-ghan-Zhongjiang-Xindu slope belt in the western Sichuan Depression, where the overlying very thick marine and terrestrial formations can serve as good seal, stratigraphic physical property difference can form lateral sealing, natural gas generated in Permian source rocks during the Yanshanian period could migrate along the unconformity surface at the Mao top and accumulated. In summary, it is very likely to find large-scale reef flat karst gas pools in the trap structural belt in the Longmen Mountain front in SW Sichuan Basin and in the slope belt in the western Sichuan Depression.

3. Conclusions

The Middle Permian in the SW Sichuan Basin, located in shallow water and gentle slope belt with high energy, developed large-scale reef flat deposits, which under the effect of weathering karstification, is likely to form large-scale reef flat karst reservoirs. The Middle Permian traps developed in the buried structural belt in the Longmen Mountain front in the SW Sichuan Basin and in the slope belt in the western Sichuan Depression, with good hydrocarbon accumulation conditions, are the prospective targets for searching large-scale reef flat karst gas pools.

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