Sequence Stratigraphy and Sedimentary Facies of Feixianguan Formation in the Kaijiang-Liangping Area of Sichuan Basin, China

The Feixianguan formation in the Kaijiang-Liangping basin has been the focus of extensive research on multiple aspects. Based on field survey, core observation, laboratory analysis and seismic data interpretation, the sequence stratigraphy and sedimentary facies of the Kaijiang-Liangping area are studied. Four sequence boundaries and three maximum flooding surfaces of the Feixianguan formation are detected in this area. Three third-order sequences are identified as first sequence (SQ1), the second sequence (SQ2), and the third sequence (SQ3) in which SQ1 corresponds to the member 1 of the Feixianguan formation, SQ2 corresponds to the member 2, and SQ3 corresponds to the member 3 and member 4. Member 1, 2, 3 and 4 are lithologic sections divided by predecessors. On the basis of this sequence division and their sedimentary marks, the facies of the Feixianguan formation can be divided into open platform and evaporate platform categories. The open platform is composed of three subfacies identified as platform bank or basin marginal bank, interbank, and platform basin. Thus, a sedimentary evolution model is established. According to the sedimentary and seismic characteristics of the Kaijiang-Liangping area, we determine that two oolitic bank models, the aggradation model and the progradation model, are developed in this area. The aggradation model is developed in the Longgang region, which includes the basin marginal bank as a favorable exploring zone. The progradation model is developed in the Jiulongshan and Longhui areas, besides the basin marginal bank, the favorable exploration zones also include the oolitic bank developing areas of the inner basin. How to cite this paper: Xie, Q.B., Wang, Z.F., Ouyang, Y.L., Zhang, M.Y., Liu, B., Wang, L.L. and Liu, X.J. (2020) Sequence Stratigraphy and Sedimentary Facies of Feixianguan Formation in the Kaijiang-Liangping Area of Sichuan Basin, China. Open Journal of Geology, 10, 641-660. https://doi.org/10.4236/ojg.2020.106029 Received: March 12, 2019 Accepted: June 20, 2020 Published: June 23, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
The concept Kaijiang-Liangping trough was put forward in the end of the 20th century [1]. Since then numerous studies have been conducted on the sequence and sedimentary facies of the Kaijiang-Liangping Feixianguan Formation [2]- [7]. Thus far, two different perspectives have been reported on the division of the stratigraphic sequence of the Feixianguan Formation: two third-order sequences and three third-order sequences. Previous research on the sedimentary facies also shows different conclusions. Yang and Wen determined that the facies of the Kaijiang-Liangping Feixianguan Formation evolved from the trough to the continental shelf [2]. Wei suggested that the facies gradually evolved from the basin to the slopes, platform margin bank, open platform, restricted platform, and evaporation platform [3]. Ma deemed that the facies of the Kaijiang-Liangping area from the Changxing period to the Early Triassic period were produced in an environment of platform shelf in relatively deep waters of a carbonate platform [4]. In addition to these inconsistencies, little research has been conducted on the inner basin.
In the present study, the sequence stratigraphy of the sedimentary facies in the outcrop is examined, and laboratory tests, drilling and logging information, and seismic activity are analyzed. The results indicate that the sedimentary model of the early Triassic Feixianguan period, northeast Sichuan, is a shallow sea carbonate platform. This study is based on the sequence division in the Kaijiang-Liangping basin area. The objectives include examining the sedimentary facies of the Feixianguan Formation in the Kaijiang-Liangping area, building a sedimentary evolution model, and putting forward two development models for an oolitic beach involving progradation and aggradation, respectively. It is hoped that the results will provide a geologic basis for further exploration in the Kaijiang-Liangping basin.

Characteristics of Sequence Stratigraphy
The key to sequence classification is the recognition of sequence boundaries. A sequence boundary is defined as an unconformity or its correlative conformity [9]. In this study, four sequence boundaries and three maximum flooding surfaces were identified in the Feixianguan Formation in the study area ( Figure 2    SB1 is the interface of the Changxing Formation and the Feixianguan Formation and is also an important lithology trans-Formation interface. The lithology turns from thick, massive chert limestone and reef limestone at the top of the Changxing Formation, or siliceous rock and carbonaceous shale in the Dalong Formation, into thin layers of argillaceous limestone and micrite limestone of the Feixianguan Formation ( Figure 3). The electrical behavior turns from low gamma ray (GR) and high resistivity of the true formation (Rt) into high GR and low Rt. Moreover, the Permian extinction led to obvious decreases in species in the Feixianguan period, particularly in Hindeodus parvus, Otoceras, and Claraiawangi; this biological mutation phenomenon is indicated by the fossil presence at the boundary.
The seismic reflection characteristics of each interface are obvious ( Figure 4). The SB1 shows moderate reflection characteristics and obvious onlap phenomena at the boundaries of the basin. SB2 is the interface of the member 1 and member 2 of the Feixianguan Formation, which is often characterized by mudstone overlapping the limestone and by onlap at the bottom and truncation at the top of the member 2 of the Feixianguan Formation. SB3 is the interface of the member 2 and member 3 of the Feixianguan Formation, with onlap evident at the bottom of the member 3 of the Feixianguan Formation. This interface is also an important because it shows mutations in lithology and electrical behavior ( Figure 5). SB4, the interface of the Feixianguan and Jialingjiang Formations ( Figure 6(a)), shows strong reflection characteristics. The lithology is mainly tan or fuchsia mud stone ( Figure 6 According to the recognition features of sequence boundaries in the study area, the Feixianguan Formation was divided into three third-order sequences in the Kaijiang-Liangping area including SQ1, which corresponds to the member 1 of the Feixianguan Formation; SQ2, which corresponds to the member 2; and SQ3, which corresponds to the member 3 and member 4. Both the transgressive systems tract (TST) and high-stand systems tract (HST) are developed in SQ1, SQ2, and SQ3 on the platform. In the basin, TST and HST are developed in SQ1 and SQ3; low-stand system tract (LST), TST and HST are developed in SQ2 ( Figure 7).

Characteristics of Sedimentary Facies and Sedimentary
Evolution Model

Types and Characteristics of Sedimentary Facies
The types and characteristics of the sedimentary facies of the Feixianguan      Table 2).

1) Evaporate platform facies
This facies is developed mainly in HST of the SQ3 sequence, corresponding to the member 4 of the Feixianguan Formation ( Figure 6(a)). It formed in arid climate conditions, depositing a set of regional fuchsia mud stone ( Figure 6

b) Interbank
This facies is between platform banks with limited water circulation, dark-colored deposits, and fine-grained sediments in a wide distribution range. It exhibits a set of gray to dark gray thin-medium micrite and particles of micrite limestone interbedded with thin micritic particles of limestone. Horizontal beddings and sand grain beddings are present ( Figure 9). Biology is active and includes include gastropods, lamellibranches, ostracods, and blue-green algae with burrows.
c) Platform basin This facies lies inside the open platform. The depth of the water is from a few meters to hundreds of meters with normal water circulation; development is in the SQ1 or SQ2 sequence of the Feixianguan Formation. From the platform basin slope to the inner basin, remarkable differences in sedimentary characteristics are noted. In the platform basin slope zone, the water level is deeper, and deposits include a set of thin to medium dark gray micrite with slump breccia (Figure 10(a)) accompanied by a sliding deformation structure including gastropod and lamellibranch fossils. Within the platform basin, under adverse hydrodynamic conditions, a set of dark-colored, fine-grained dark gray micritic limestone, marlstone, and calcareous mudstone is present (Figure 10(b)). Horizontal beddings are developed, including oceanic plankton assemblages.

Sequences and Sedimentary Facies of the Connecting-Well Section
To determine the development law of sedimentary facies and the distribution of the oolitic beach in the Feixianguan Formation, a stratigraphic correlation of the third-order sequences in the Feixianguan Formation of the Kaijiang-Luanping area was made. Seven connecting-well sections covering the entire area ( Figure  1) were selected to conduct a comparative study of the basin formation in parallel and perpendicular directions. Due to the paper length limitation, we selected only two representative sections for the present analysis: section 2 and section 7. Section 2 ( Figure 11) is a connecting-well section of the Feixianguan Formation that crosses the basins of the Kaijiang-Liangping area. Among the facies of the entire section, a strong similarity is shown in which oolitic beaches are obviously controlled by facies. In the platform basin zone, the oolitic beaches are barely developed in wells Lg10 and Sm1. In the basin margins, wells Pg2 and Lg2 show several stages of basin marginal bank development with large thickness noted in the SQ2 sequence and TST of the SQ3 sequence, which is the member 3 of the Feixianguan Formation. Wells Lg11 and Lg9 in the open platform show partially developed oolitic beaches of various thickness. In the HST of SQ3 sequence, the platform is a homogenized evaporate platform, and the deposits include a typical set of regional purple calcareous mudstone, argillaceous dolomite, and micrite interbedding with gypsum. Section 7 ( Figure 12) is a connecting-well section of the Feixianguan Formation running parallel with the margin of the Kaijiang-Liangping basin and extending through Jiulongshan. In the Longgang and Longhuichang areas, the changes in sedimentary facies are essentially revealed in the basin margins. Wells L4 and Lh3 are located at Jiulongshan and the Longhuichang area respectively. The thickness of the Feixianguan Formation is relatively small, and oolitic beaches are deposited in the TST of wells L4 SQ3 sequence and well Lh3 SQ1, SQ2, and SQ3 sequences, which indicate shallow water with strong hydrodynamic power in both places in the Feixianguan period. These represent two high points in the shallow water region. Wells Yb 4 and Lg 10 are located in the basin near the Longgang area. The thickness of the Feixianguan Formation is large, which suggests deep water is with week hydrodynamic power. Little or no particle deposition is noted in the Feixianguan period.

Sedimentary Evolution and Models
According to the characteristics of the sedimentary facies in the study area,   Entering the TST period of the SQ3 sequence, the sea water was shallow, and sedimentation was enhanced. The Kaijiang-Liangping basin was filled quickly until it finally disappeared, and the entire study area evolved into the open platform facies, with oolitic beaches deposited extensively in the platform and the basin area. During this period, the sedimentary framework changed completely.
In addition to the filling of the Kaijiang-Liangping basin, the Feixianguan Formation was essentially filled at the regional scale. In HST of the SQ3 sequence, the entire Kaijiang-Liangping area evolved into a homogenized evaporate platform facies and developed a set of regional purple mud shale and marl with gypsum layers (Figure 15, Figure 16).
According to the evolution process of plane facies of the Kaijiang-Liangping area, we established a sedimentary facies model of the study area (Figure 17), showing the complete sedimentary filling process of the Feixianguan Formation.

Prediction of Favorable Exploration Areas in the Kaijiang-Liangping Basin
Before   Figure 19(a)). The bottom interface reflection axis of the member 1 of the Feixianguan Formation bends slowly from the margin to the inner area after a certain distance, then extends closely to the bottom interface reflection axis of the Changxing Formation. The bottom interface reflection axis of the member 2 of the Feixianguan Formation advances slowly from the margin to the inner area after a certain distance, then bends downward to the bottom interface reflection axis of the member 1 of the Feixianguan Formation. It then extends to the basin center, where bright spot reflection is shown on the propelling extension axis. The bottom interface reflection axis of the member 3 of the Feixianguan Formation is almost an extended parallel from the margin to the inner area. The reflection characteristics of the Jiulongshan seismic section indicate that oolitic beaches are developed mainly at the basin margin and near the top interface of the member 1of the Feixianguan Formation. Well logging data shows that well Jianmen 1 includes mainly deposits of argillaceous limestone and thick layers of limestone, with thin layers of oolitic limestone occurring only in HST of the SQ1 sequence and TST of the SQ2 sequence. At the location of well Longgang 16, the oolitic limestone is developed mainly in the SQ1 sequence and TST of the SQ3 sequence. The development of oolitic beach reservoirs migrates from the margin to the inner basin in the entire area. The migration distance from the margin to the inner basin in Jiulongshan is approximately 34 km; the local area maximum is 40 km. The Jiulongshan model was established according to the sedimentary characteristics of the Feixianguan Formation in the Jiulongshan area (Figure 19(b)).
In summary, the two types of oolitic beach development models revealed that favorable exploration zones of the Feixianguan Formation in the Longgang area occur mainly in the margin zone of oolitic beaches development. The favorable exploration zones in Jiulongshan and Longhui advanced further to the inner basin.

Conclusions
1) According to the characteristics of sequence interfaces, the Feixianguan Formation of the Kaijiang-Liangping area can be divided into three third-order sequences: SQ1, SQ2, and SQ3. SQ1 and SQ2 correspond to the member 1 and the member 2, respectively, and SQ3 corresponds to the member 3 and the member 4. In the platform, both TST and HST are developed in the SQ1, SQ2, and SQ3 sequences. However, in the basin, TST and HST are developed in the SQ1 and SQ3 sequences, and LST, TST, and HST are developed in the SQ2 sequence. Open Journal of Geology 2) The sedimentary facies of the Feixianguan Formation in the Kaijiang-Liangping area can be divided into open platform facies and evaporate plateform facies. The open platform includes three subfacies: platform bank, interbank, and platform basin. During the first and second periods of the Feixianguan Formation, the study area was mainly platform bank, interbank, and platform basin subfacies. In the third period, the basin was filled and leveled, and the major facies were platform bank and interbank subfacies. In the fourth period, the entire area evolved into evaporated platform facies.
3) On the basis of the seismic reflection characteristics and the sedimentary facies, two types of development models for oolitic beaches at the west margin of the Kaijiang-Liangping basin are proposed. The aggradation model shows development mainly in the Longgang area. Oolitic beaches are developed in the SQ1 and SQ2 sequences and in TST of the SQ3 sequence of the basin margin, which are favorable exploration areas. The progradation model shows development in the Jiulongshan and Longhui areas with oolitic reservoirs occurring mainly in HST of SQ1 and TST of SQ3. With the progradation to the inner basin, the basin margin advanced gradually, the oolitic banks migrated into the basin along with the progradation.