Distribution of Platform Edge Reefs and Beach As Well As Their Major Controlling Factors Over the Changxing-Feixianguan Formations in Northeast Sichuan Basin, China

+ MODEL Available online at www.sciencedirect.com ScienceDirect

Journal of Natural Gas Geoscience xx (2017) 1e10 http://www.keaipublishing.com/jnggs Original research paper Distribution of platform edge reefs and beach as well as their major controlling factors over the Changxing-Feixianguan formations in northeast Sichuan Basin, China

Mancang Liu*, Wei Yang, Hui Jin, Wulin Mo, Saijun Wu, Nan Su

Institute of Natural Gas Geology, Research Institute of Petroleum Exploration & Development, PetroChina, Langfang 065007, China

Received 9 February 2017; revised 12 April 2017 Available online ▪▪▪

Abstract

The platform facie, platform edge facie, slope, and basin facie were developed in the Chengkou-Western Hubei Trough zones during the Upper Permian Changxing Formation's Early Triassic Feixianguan period; during this time the platform edge had the most favorable sedimentary facie. The Changxing Formation was composed of three sections in which the second section was rich in framework reef, whereas the third one was rich in micro-reefs at the bottom. Recently, it has been discovered that abundant platform edge shoals are developed in the third member and that oolitic shoals with large thickness are widely distributed. This paper systematically investigated the inﬂuence of the changes in reef-building organism system on the development of organic reefs. The results show that the distribution of bank belts is determined through paleogeography framework. The rapid change of relative sea level affects the formation and movement of bank belts. In addition, the regional tectonic activity controls the distribution and transformation of the bank belts. Finally, the prospective zone in the Chengkou-Western Hubei area was evaluated and divided into four favorable zones, in which the Wanyuan-Yunyang area was deemed to be most advantageous. Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Organic reefs; Oolitic beach; Changxing Formation; Feixianguan Formation; Development and distribution; Chengkou-Western Hubei; Sichuan Basin

1. Introduction Wuxi, Fengjie, and Fengdu in Chongqing, Lichuan and Enshi in Hubei, and so on (Fig. 1). With the same formation mechanism, The Changxing and Feixianguan formations are rich in reefs the Chengkou-Western Hubei Trough and the Kaijiang- and shoals with a wide distribution range in the Sichuan Basin. Liangping Trough are worth exploring. Recently, most re- Thus far, a number of abundant big industrial gas pools have been searchers aimed to study the characteristics of reefs and shoals as discovered, such as the Longgang, Puguang, Tieshanpo, Luo well as their distribution patterns and controlling factors [4e9]. Jiazhai, and Wubaiti, which have become the most important Their research provides abundant materials for gas prospecting prospecting areas after the Carboniferous in the Kaijang- in Chenkou-Western Hubei Trough. These ﬁndings are all based Liangping Trough [1e3] within northeastern Sichuan. The on the investigations of outcrop and well observation using a study area, Chengkou-Western Hubei zone, is located in the variety of geological, logging, and seismic methods. north-eastern Sichuan Basin. It refers to the Zhenba County in We studied the controlling factors of platform edge reefs Shanxi, Wanyuan in Sichuan, Chenkou, Kai County, Yunyang, and shoals' distribution and the development of the Changxing-Feixianguan formations in Chenkou-Western * Corresponding author. Hubei Trough areas within Sichuan Basin. Furthermore, we E-mail address: [email protected] (M. Liu). analyzed the exploration prospect of this region which could Peer review under responsibility of Editorial ofﬁce of Journal of Natural Gas be helpful for oil-gas prospecting. Geoscience. http://dx.doi.org/10.1016/j.jnggs.2017.06.002 2468-256X/Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, Petro- China. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

frequent. It is rich in nutrient substance; therefore reef- building organisms were abundant. They lead to high productivity in carbonate rock. According to the features of reef- building organisms, it could be further divided into skeleton reef and lime-mud mound characterized by light gray to gray thick-blocky sponger limestone, algal limestone, reef dolostone, and microbialite. The types of the organisms in the sedimentary environment that were abundant include sponges, hydrozoans, corals, and other reef-attached organisms. Framework reefs mainly developed in the mid-bottom of the Changxing Formation (Fig. 4aed), while lime-mud was widely developed in the upper-mid section (Fig. 4eeh). The scale of the platform edge reefs was large on the plane, but not continuous in the horizon. It kept string beads distributed along the platform edge.

2.2. Platform edge shoals

The platform edge shoals, often developed in the high energy environment of the platform edge, blocks the wind and faces the wide sea. Its depth ranged from 5 to 10 m above the sea level. In addition, it's also characterized by recycled well water, abundant oxygen, formal salt, and visible current. It contained grayewhite to light gray thick-blocky sparry bioclastic limestone, sparry intraclast limestone, and sparry grain limestone (Fig. 5aed). It is also accompanied by large middle cross bedding and wedge cross bedding. It was popular during Fig. 1. Location and local structural map showing the study area. the sea lever falling act of the Changxing-Feixiangaun phase, depositing bioclastic bank, intraclast bank, and oolitic bank, 2. Characteristic of platform edge reefs and shoals respectively. This is common in many outcrop profiles and well-drilling profiles, such as the Tianba, Manyue, Shuanghe, On the basis of petrography, sedimentary structure, and Miaoba, Well Feng 1, Well Jinzhu 1, Well Luojia 2, and Well ' paleontology feathers, in combination with an areas sediment- Jian 27. With the rapid change in relative sea level, the plat- tectonic background, we divided the Changxing Formation- form edge bank transformed observably. Feixianguan Formation into four facies belts (Figs. 2 and 3): platform, platform edge, slope, and basin. The platform edge 3. Deposition situation and distribution of reefs and shoals is rich in reefs and shoals. at the Changxing-Feixianguan formations The platform edge is located in the narrow area between the platform edge and slope where the terrain is higher than the 3.1. Reefs of the Changxing Formation others. The said area is intensely affected by waves and tides. Its boundary comes in contact with the wave base. Thus, Study areas suffered from transgression that leads to the developing platform edge reef-shoals on the side of the wide water deepening from west to east during the early to middle sea and forming the rimmed platform between the platform Changxing phase. In response to the change, it successively and the slope boundary. It exposes itself to a certain sea level developed the platform, platform edge, slope, and basin that causes it to suffer from atmospheric water eluviation and (Fig. 6). The area of the Tiechanghe, Fengtong, Panlongdong, e mixed dolomitization [10 12]. The typical sediments were and Jiantianba is regarded as the geological boundary of the reef limestone and grain limestone with fine grain sorting and sedimentary facies division, of which the west developed the roundness, as well as sparry cement, all of which reflects that open platform. The dominant sediment was intra-platform the sedimentary environment was of high energy. Based on the shoals or reefs in the higher geomography of the intra- sedimentary features, the platform edge could be divided into platform. The framework reefs could be seen in many places a reef and bank sub-phase. such as Fengtong in Wanyuan City, Panlongdong in Xuanhan County, Manyue in Kai County, Well Feng 1 and Jiantianba. 2.1. The platform edge reefs They kept the state of string beads and broken chain along the edge. The slope was located on the outside of the platform The platform edge reef was one of the important sedi- edge and it gradually became steep and narrow due to the mentary types on the carbonate platform edge. It faces the effect of the powerful facies of the platform edges. The basin wide sea where waves and upward current processes are facies mainly developed in the east of the slope facie,

Fig. 2. Formation and sedimentary facies columnar the Changxing Formation section in the study area. including Miaoba and Xianyi in Chengkou, Longtai in Wuxi, falling, the platform edge shoals extensively developed at the Well Lou 1, and Enshi in Hubei Province. platform edge belt, including Tiechanghe in Tongjiang Due to the changes in the ocean environment during the County, Fengtong in Wanyuan City, Panlongdong in Xuanhan middle-late Changxing phase, the multicellular reef-build or- County, Manyue in Kai Country, Jianshan in Wuxi County, ganisms became extinct, reefs and lime-mud mound developed and Jiantianba in Lichuan City, they extended to the wide sea which dominated the microforms and partial low algae's, with progradation parasequence set. Based on the latter, we distributed in Fengtong in Wanyuan City, Panlongdong in predicted that it was likely to develop into platform edge Xuanhan County, and Jianshan in Wuxi County. We inferred shoals in the east of Well Zhuyuan 1. The slope facies were that platform edge reefs and the lime-mud mound was likely to mainly located outside of the platform edge that was distrib- develop in the west of Manyue in Kai County and Jiantianba in uted in Longtai in Wuxi Country e Xiyi in Chengkou County. Lichuan City. However, there were no shoals the platform Under the effect of the platform edge, it was steeper and edge reefs stayed bead-like along the platform edge. The slope narrower than any other phases. The basin facies mainly lay outside of the Manyue, Kai County-Longtai, Wuxi County- developed in the east of the slope. Jiantianba, Lichuan City line. The basin was in the east of the slope. 3.2. Shoals of the Feixianguan Formation Taking, for example, the area bordered by Tiechanghe, Fengtong, Panlongdong, Manyu, Jiansha, and Jiantianba, the Because of the extensive transgression in the earlier stages open platform and restricted platform mainly developed in the of the ﬁrst member of Feixianguan, the scales of the platform west. edge were obviously smaller than the Changxing phase. While the open platform was mainly located in the south of Combining the materials of the study areas, the platform only Well Luojia 2-Honghua Kai County zones, restricted platforms developed in the northern higher paleogeography surrounded and lagoons were in the north because of the barrier of plat- by deep water. The ﬁnding corresponds to the development of form edges. They all could be seen at the top of the Changxing slope facies and basin facies from east to southeast. Northern Formation outcrops, such as Fengtong, Yanggudong, Dukou, open platform mainly developed along the line of Well Panlongdong, and Manyue. However, brought about the lim- Zhuyuan 1-Well Ying 1-Well Zi 1-Well Zhujia1, while the itations of the drill-well materials, the north boundary of the restricted platform was located at the Well Jinzhu 1 zone. limited platform was not clear. With the relative sea level Platform edge facies mainly developed in the line of Dukou

Fig. 3. Formation and sedimentary facies columnar the Feixianguan Formation section in the study area.

Xuanhan-Honghua Kai County-Well Pusa 1. Hence, it might Affected by the large transgressive and paleogeography, the develop the same facies in the east of the Well Zhuyuan 1. The southern paleogeography is lower in the section. According to slope facies mainly developed in the western zones of Batai the document literature of this research, it was developed slope Wanyuan-Manyue Kai County-Jianshan Wuxi-Well Lou 1. facies deposition with an extensive belt and stable lithology in

Fig. 4. Field and microscope photos from the reefs of the Changxing Formation. (a) Sponge framestone, the Chang-2 member, Panlongdong in Xuanhan Country; (b) Sponge framestone, skeletal pore, the Chang-2 member, Jiantianba Lichuan City; (c) Sponge of reef dolostone, the Chang-2 member, Panlongdong in Xuanhan Country, 2.5 Â 10 (À); (d) Sponge of reef dolostone, the Chang-2 member, Panlongdong in Xuanhan Country, 2.5 Â 10 (À); (e) Lime-mud mound, the Chang-3 member, Fengtong in Wanyuan City; (f) Lime-mud mound, the Chang-3 member, Jianshan in Wuxi Country; (g) Bindstone, the Chang-3 member, Jianshan in Wuxi Country, 2.5 Â 10 (À); (h) Bindstone, the Chang-3 member, Jianshan in Wuxi Country, 2.5 Â 10 (À).

Fig. 5. Field and microscope photos from the shoals of the Feixianguan Formation. (a) Gray Sparry limestone, the Fei-3 Member, Shatuo in Yunyang Country; (b) Gray sparry bioclastic dolomite, the Fei-2 Member, Shiliang in Yunyang Country; (c) Sparry oncolites Limestone, the Fei-2 Member, Shiliang in Yunyang Country, 2.5 Â 10 (À); (d) Sparry oolitic Limestone, the Fei-3 Member, Linchang in Tongjiang Country, 4 Â 10 (À). the south of Shibaozhai-Jiannan zones connecting to the sediments transfer to deep water. The open platform developed northern Yunanchang-Well Lou 1. in many places such as Well Pusa 1, Well Macao 2, Well Men To the second member of the Feixianguan, the relative sea 5, Well Tiancheng 1, and Well Yunan 6; the restricted platform level was falling. As a result, the northern isolated carbonate was popular in a lot of zones including Well Zhuyuan 1, Well platform extended to deep water. The depth was getting higher Jinzhu 1, Dukou, and Well Ying 1. Affected by the changes in and higher from west to east, corresponding to the distributed the relative sea level, the platform edge facies and platform platform, platform edge, slope, and basin. While it was still the edge shoals microfacies mainly distributed in these zones, higher geomography, the high productivity carbonate made the including Batai in Wanyuan City, Manyue in Kai County,

Fig. 6. The plane distribution of platform edge reefs in the Changxing Formation.

Shuanghe in Wanzhou City (Fig. 7), as well as the zone Well sea level dropped evidently, the platform grew quickly. The Zhujia 1-Well Luojia 1. The slope was mainly distributed in platform edge shoals extended to the west with a large scale. the northern and southern zones, such as Miaoba in Chengkou It was mainly distributed along the Zhenba-Xianyi-Ningchang County, Xianyi in Chengkou County, and Well Feng 1, while Wuxi line and southern Well Zizhu 1-Well Jian 27-Well the eastern zones were basin facies such as Kongjiagou in Jian12 line. Shiyan. Since the early phase of the third member of Feix- Affected by the relative sea level falling, most of the zones ianguan, it suffered from transgression, with the water depth developed the restricted platform, including a lagoon and tidal becoming greater from west to east, deposited on the platform, ﬂat deposition, while Kongjiagou Shiyan mainly developed platform edge, slope, and basin, respectively. platform edge oolitic shoals. The platform edge deposition Taking the Jianchi-Batai-Manyue-Jianshan-Well Macao 1- presented an apparent progradation. Jiannan line as a boundary, in the west of it developed the open platform and affected by the sea level rising, the hy- 4. The main factors controlling the development of drodynamic condition of intra-platform was weak, and the platform edge reefs and shoals chief deposited shoals interact with the sea. On the other hand, the platform edge mainly deposited mortar, while the 4.1. The inﬂuence of ecosystem' change in the platform edge shoals were only developed in the Miaoba development of reefs Chengkou and the bank body with a mortar and argillaceous was thin. Outside the line of Gaolou-Ningchang were devel- Modern reefs, including ancient reefs, usually develop oped slope facies. While the basin mainly developed in the under clean and warm tropical ocean condition. According to east, namely the Kongjiagou Shiyan zone. In the middle-late previous research, the controlling factors were mainly tectonic phase of the third member of the Feixianguan Formation, the movement and relative changes of sea level [4,8,13,14].

Fig. 7. The plane distribution of platform edge beach in Feixianguan Formation.

Moreover, we believed that the main controlling factors multicellular reefs death, bacteria, and algae flooding. When should include changes in the ecosystem. The reef-building the framework reefs became extinct during the middle-late organisms were abundant in the early Changxing phase, Late Changxing phase, the Late Permian bacteria's and algae's Permian, and South China such as sponges, bryozoans, and ecosystem developed extensively, thus taking up an ecological hydras. They formed the large-scale platform edge framework space and deposited lime-mud mound. The multi-stage cycle in reefs or intra-platform framework reefs. Researchers believed the interior of micro reefs reflected that the ecosystem was that large extent of the reefs was largely due to transgression getting terrible and it presented the pattern multi-stage screen in the late Changxing phase within the north-eastern Sichuan type with the unstable state (Fig. 8). Basin zones [13]. It proved the existence of reefs during the middle-late Changxing phase that large scale platform edge 4.2. The factors controlling the development of platform lime-mud mound was discovered in Jianshan Wuxi, late edge shoals Changxing phase. The ecological evolution process of framework reefs and micro bioherm reflected the change of the The shoal's body deposition boundary was near the base ocean environment in the Late Permian. wave that was directly controlled by paleogeography and sea As we all know, the ocean environmental deterioration be- level, while the factors that affected paleogeography and sea tween the Permian and Triassic led to the massive extinction of level could indirectly affect the development of the shoal's ocean organisms in the Phanerozoic. It also caused a complete body. In recent decades, many scholars domestically and collapse of the multicellular reef ecosystem around the world, abroad had made further studies on the development of shoals which lasted about 10 Ma. These all began before the line P-T. on both sides of the Kaijiang-Liangping Trough and put for- This change might contain greenhouse effect, eutrophication, ward the factors that affected the shoal's development [3,7,15]. high salinity, and so on. For the intense unstable environment The author believed that the main controlling factors included during the late Permian, the biotic crisis and the change of paleogeography framework, structure adjustment, and sea level environment presented themselves episodically. eustacy to the both sides of the Kaijiang-Liangping Trough. Research on abundant carbon isotopes showed that it had The paleogeography framework was controlled by tectonic formed intensive negative bias near the line P-T. It reflected extension that formed the deposition framework of the plat- hypoxia, water upwelling [14]. Once this water type reaches form alteration with the trough. The said alteration further the shallow sea, the carbonate ions of the water and the car- affected and controlled the distribution of sedimentary facies bonate ions of the sea would combine with calcium and mag- resulting in the shoal's distribution in the plane obviously nesium, forming the carbonate deposition. The carbon isotope controlled by the paleogeography framework. Meanwhile, the in the dissolved carbon dioxide that came from the deep ocean intra-platform shoals distributed in the high geomography was so light that carbon isotope of carbonate deposition near belonged to the shallow platform between the Kaijiang- the boundary became intensively negative. In addition to the Liangping Trough and the Chengkou-Western Hubei Trough, effect on the carbonate isotope value, it could also transfer the the distribution range was also controlled by it. For example, nutrient elements to the shallow sea, which may lead to the shoals developed above the intra-platform shoals with the

Fig. 8. The model graph of reefs evolution and microscope photos (A. Sponge reef; B. Lime-mud mound).

Fig. 9. Sedimentary model and evolution of the Feixianguan Formation.

Fig. 10. Comprehensive evaluation of favorable exploration belts in Chenkou-Western Hubei Trough.

Controlling regional tectonics formed the influence on the Favorable exploration in Wanyuan-Yunyang is the platform development of shoals through controlling paleogeography edge reefs' complex reservoir development zones. The gener- and paleogeography. Controlled by the regional extension ation intensity was (20e80) Â 108 m3/km2 in areas such as tectonic in the Changxing phase, the Chengkou-Western Hubei Jinzhuping, Pusadian, and so on. In recent years, the explo- formed. The platform edge shoals were mainly distributed ration in Fengjie zones had proved the existence of platform along the boundary of the trough affecting the development of edge belts. Organic reefs of the Changxing Formation were the trough. The regional tectonic activity was weak during the found in Well Feng 1. The reservoir thickness in Well Macao 1 Feixianguan phase, with the Chengkou-Western Hubei trough was more than 10 m, in which the 1e3 members of the atrophy and transference to the eastern zones, the platform Feixianguan Formation were explained as gas-bearing strata. edge continuously transferred to the wide sea, the development The reservoir thickness of Well Macao 2 was nearly 15 m in of stratigraphy gradually took shape. the Feixianguan Formation and 17 m in the Changxing For- To sum up, in the Changxing-Feixianguan phase, the paleo- mation. The most favorable exploration platform edge zones geography framework determined the plane distribution. The in Chengkou-Western Hubei showed a good prospect in reef changes in the relative sea level affected the formation and exploration in the Sichuan Basin. transformation of the shoals. The regional tectonics and evalu- Favorable exploration in Zhuyaunba-Wenfeng temple is the ation controlled the distribution and transformation of the shoals. platform edge reefs' complex reservoir development zones. The generation intensity was less than 20 Â 108 m3/km2.This 5. Exploration fields and prospects belt was located in front of Daba Mountain. Preservation conditions were the key to gas reservoir formation in this belt, The Chengkou-Western Hubei zones have well petroleum it shows multilayer-section gas logging that's abnormal in Well geologic conditions and are also important exploration fields Zhuyuan 1. The test result was 756 m3/d in the Feixianguan in the future, but the degree of exploration is low. Favorable Formation. The finding proved that the belt has a good sedimentary facies are based on favorable reservoir rock type, exploration potential, it is the favorable zone needed for hydrocarbon source condition, and well oil gas in drilled wells further study and exploration. and other factors [15,16]. Four favorable exploration fields Favorable exploration in Eastern Chongqing is the platform could be divided into the following (Fig. 10), Wanyuan- edge reefs' complex reservoir development zones. The gener- Yunyang, Zhuyuanba-Wenfeng temple, eastern Chongqing, ation intensity was (20e70) Â 108 m3/km2, the commercial and Zhong County. gas well and oil gas shows were less. However, the gas field

Please cite this article in press as: M. Liu, et al., Distribution of platform edge reefs and beach as well as their major controlling factors over the Changxing- Feixianguan formations in northeast Sichuan Basin, China, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.06.002 + MODEL 10 M. Liu et al. / Journal of Natural Gas Geoscience xx (2017) 1e10 exploration and development in Shibaozhai, the structure of platformal through in Feixianguan Formation of Lower Triassic in Dachigan and Jiannan, especially the breakthrough of Well northern Sichuan basin and its evolution, Acta Sedimentol. Sin. 22 (2) e Jianzhu 1 in the structure of Jiannan gas field showed great (2004) 254 260. [2] Guoqi Wei, Gengsheng Chen, Wei Yang, Yu Yang, Dong Jia, Lin Zhang, exploration potential. 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