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Natural Gas Industry B xx (2017) 1-11
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Characteristics of Dengying Fm sedimentary sequence in the central—western Sichuan Basin and their controlling effect on gas
accumulation
Wen Longa,b *, Yang Yuemingb, You Chuanqiangc, Zhang Xihuab, Peng Hanlinb, Wang Wenzhib,
Luo Bingb, Luo Wenjunb
a PetroChina Research Institute of Petroleum Exploration & Development, Beijing, 100083, China b Research Institute of Exploration and Development, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan, 610041, China c Capital Management Department, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan, 610051, China
Received 17 May 2016; accepted 1 September 2016
Abstract
A large gas reservoir in the Sinian Dengying Fm was recently discovered in Anyue area, central Sichuan Basin. In order to promote the oil and gas exploration in this area, it is of great significance to sustainably deepen the analysis on its high-quality reservoir distribution and source-reservoir assemblage. In this paper, a series of studies were performed on the sequence frameworks and sedimentary filling evolution characteristics of the Dengying Fm in central—western Sichuan Basin, and their controlling effects on the reservoir development and distribution and source-reservoir assemblage during its sedimentation. It is shown that the Deyang—Anyue paleo-rift is, on the whole, distributed in a NW direction, and is morphologically wide and steep in the north, but narrow and gentle in the south. There are two rift boundaries of the second and fourth members of Dengying Fm (Deng 2 Member and Deng 4 Member, respectively). The rift of Deng 2 Member is distributed in the shape of "U" in Deyang—Neijiang area. Dengying Fm is divided into four third-order sequences (SQ1—SQ4). In SQ3 of Gaoshiti—Moxi area, central Sichuan Basin, a complete sedimentation reflection structure of transgressive overlap and regressive progradation can be identified with early—middle Deng 4 Member platform margin. The paleo-rift was in embryo before the sedimentation of Sinian, so the sedimentary filling and lithofacies distribution of the Sinian were influenced by the inherited activities of pre-Sinian rifts. The favorable distribution zones of high-quality reservoirs are located at the rift margins of Deng 2 and Deng 4 Members that were formed respectively during the SQ2 and SQ4 and the platform margin of early Deng 4 Member that was formed during the SQ3. A laterally connected source-reservoir assemblage occurs at the margins of Deng 2 and Deng 4 Members around the Lower Cambrian source rocks of the paleo-rift. The northwestward extending margin zone along the eastern flank of the paleo-rift shows a good potential for hydrocarbon accumulation.
© 2017 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; Sinian; Gas accumulation; Sedimentary structure setting; Stratigraphic sequence; Sedimentary filling; Reservoirs; Source-reservoir assemblage; Controlling factors
The exploration of the Sinian Dengying Formation in the Sichuan Basin started in the 1950s. Since then, geologists have continuously worked on this most archaic carbonate gas-
* Corresponding author. PetroChina Research Institute of Petroleum Exploration & Development, Beijing, 100083, China.
E-mail address: wenlong@petrochina.com.cn (Wen L.).
Peer review under responsibility of Sichuan Petroleum Administration.
bearing formation. After 2011, giant gas fields have been discovered in the Lower Paleozoic—Sinian in the Anyue area of the Sichuan Basin and a large quantity of new geological viewpoints and recognitions have been formed regarding the tectonic, sedimentary, reservoir, accumulation and the likes [1 — 11]. The exploration practices have verified the great potential of Dengying Fm, which is considered as the key target of natural gas exploration in the Sichuan Basin. Further
http://dx.doi.org/10.1016/j.ngib.2017.02.004
2352-8540/© 2017 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/).
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understanding the distribution law of high-quality reservoirs and source-reservoir assemblage in the Dengying Fm will be significant for the expanding of exploration. The authors analyzed the sequence framework characteristics and the sedimentary filling evolution of the Dengying Fm in the central—western Sichuan Basin, in order to investigate the controlling effects of sedimentary process of the Dengying Fm—Lower Cambrian on the distribution and development of reservoirs, source-reservoir assemblage and other elements.
1. Sedimentary—tectonic setting
In recent years, the hiatus of the Dengying Fm with varying degrees has been found in the western—southern Sichuan Basin. The scholars have different viewpoints on this geological phenomenon [3—5]. The authors hold that, under the
extensional background composed of the large-scale extension of China continent from the Sinian to Middle Cambrian and the Rodinia supercontinent split during the Ediacaran (Sinian), the palaeo-rift started from pre-Sinian was formed as a result of the basement rifting, sedimentary filling, and exposure and denudation caused by structural uplifting (Fig. 1a). The palaeo-rift played a prominent role in controlling the sedimentation, reservoir, and hydrocarbon accumulation of the Sinian Den-gying Fm in the central—western Sichuan Basin.
Characterizations with a large number of two- and three-dimensional seismic data demonstrate that the Deyang—Anyue palaeo-rift, on the whole, presents a NW distribution and is wide and steep in the north and narrow and gentle in the south, with rifting boundaries in Deng 2 and Deng 4 Members (Fig. 1a). To the north of Lezhi—Suining area, the Deng 2 Member rifting boundary rests in the west of the Deng 4 Member rifting
Fig. 1. Morphology and geomorphological features of palaeo-lift in the southern—western Sichuan Basin.
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boundary, with a distance of 50—100 km. This boundary is clear and steep. In northwestern Sichuan Basin, the depth of the Deng 2 Member rift exceeds 1000 m (500 ms) as revealed by seismic data. The eastern boundaries of the Deng 2 and Deng 4 Members intersect in Anyue area of central Sichuan Basin, and locally the Deng 2 Member rifting boundary is located in the east of the Deng 4 Member rifting boundary. To the north of Neijiang, the Deng 2 Member rifting boundary turns to Ziyang, presenting a U-shaped distribution. To the south of Neijiang—Dazu area, the Deng 4 Member rifting boundary extends southward to become gradually a gentle slope. According to the analysis of distribution features, the intensity and scale of the palaeo-rift extension towards northwestern Sichuan Basin increase significantly, and the initial extensional zone is believed to exist in the northwestern part of the Sichuan Basin.
Generally, the sedimentary geomorphology of the Dengy-ing Fm in central—western Sichuan Basin tends to be lower towards the northwest, showing a better correlation with the above-mentioned extensional morphology. In the 3000 km2 3D seismic area of the Gaoshiti—Moxi area in the central Sichuan Basin, the palaeo-geomorphology after the sedimentation of the Dengying Fm has been delineated by the thickness of Lower Cambrian Qiongzhusi Fm shale (Fig. 1b). It is found that the Lower Cambrian in the Gaoshiti and Longniisi areas in the southeast of the Sichuan Basin has relatively small acoustic time (thin), while it presents significantly large acoustic time (thick) in the northwest direction, suggesting
that the Dengying Fm is low in the northwest and high in the southeast. Moreover, the NW seismic profile reveals clearly the overlapping inside the Dengying Fm and at the top of Dengying Fm (Fig. 1c), but such phenomenon is not obvious in other directions. Therefore, in contrast to the northwestern Sichuan Basin, the central Sichuan Basin shows a higher palaeogeomorphology.
2. Sequence stratigraphic framework
2.1. Demarcation and correlation of stratigraphic framework
The top boundaries of Deng 2 and Deng 4 Members respectively correspond to the erosion surfaces formed in two episodes (I and II) of the Tongwan movement. As is revealed by the E'bian Xianfeng section and Well Wei 117 in Weiyuan structure, the Deng 3 Member pewter mudstone unconformably overlies on the Deng 2 Member dolomite and the d13C near the interface shows obvious negative shift (Fig. 2). In the Mox-i—Gaoshiti area of the central Sichuan Basin, the top boundaries of Deng 2 and Deng 4 Members show clearly abrupt contact both in lithologies and logging. The Dengying Fm cores recovered below the boundaries in Episodes I and II contain vugs and solution fractures, and also vadose siltstone in the microscopic observation, presenting an obvious regional erosion surface. According to outcrop and drilling data, the
Fig. 2. Division and correlation of sequence stratigraphic framework of Dengying Fm in the central—western Sichuan Basin.
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Dengying Fm can be divided into four Ill-order sequences. Specifically, the Deng 1 and Deng 2 Members before Episode I can be divided into two Ill-order sequences (SQ1, SQ2), and the Deng 3 and Deng 4 Members between Episode I and Episode II can be divided into two Ill-order sequences (SQ3, SQ4) (Fig. 2).
2.1.1. Characteristics of SQ1
The bottom boundary of SQ1 accords with the bottom of Deng 1 Member. An intensive sea level fall occurred in the end of the deposition period of the Doushantuo Fm. In the Weiyuan area, there is gypseous dolomite on the top of Doushantuo Fm. As for the Xianfeng section in Ebian and the Nujijing section in the central Sichuan Basin, the bottom of Deng 1 Member is bounded by powder crystal dolomite and the breccia dolomite on the top of Doushantuo Fm. During the deposition period of the Dengying Fm, the transgression started and the Deng 1 Member is characterized by argillaceous dolomite and micritic dolomite with an increasing trend of d13C. In the lower part of Deng 2 Member, the transgression reached the apotheosis, and the d13C presented obvious positive shift peak at the depth of 3445 m in the Deng 2 Member of Well Wei 117 (Fig. 2). Thereafter, the relative seal level started to downgrade, recording the period of highstand system tract (HST). The initial occurrence of dolarenite, coagulated dolomite, and algae coagulated dolomite indicates that a shallowing water depth and an obvious increase in hydraulic dynamic. The d13C gradually appears to have a decreased and negative tendency. In the middle part of the Dengying Fm, the sea level reached the lowest point, and the turning point of d13C negative shift peak occurred at the depth of 3258 m in Well Wei 117, suggesting as the top boundary of SQ1.
2.1.2. Characteristics of SQ2
Although the thickness of SQ2 varies in the Xianfeng section of Ebian and Well Wei 117, the variation and fluctuation of the measured d13C values are well comparable. In the middle—upper part of the Deng 2 Member, the relative sea level started to rise. In the early stage of transgression, the lithologies were dominated by coagulated dolomite and algae dolomite. In the end of transgression period, the thickness of dolomicrite increased. In the Xianfeng section and Well Wei 117, the d13C values of the SQ1 top started to increase from the negative shift turning point gradually. At the depth of 3162 m in Well Wei 117, the second positive d13C shift peak occurred, indicating the apotheosis of transgression. From the SQ1 top to the depth of 3162 m, the serrated high GR value zone in Well Wei 117 is well correlated with that in the Gaoshiti area in central Sichuan Basin. After that, the d13C values present a tendency of decreased negative shift, thus recording the HST of SQ2. The interval is mainly dominated by coagulated dolomite and dolarenite which are intercalated with algae dolomite and dolomicrite. The GR curve is low and gentle. From the top boundary of the Deng 2 Member, affected by Episode I tectonic movement, the strata were exposed and denudated intensively, forming an obvious negative shift peak of d13C. Large quantity of karsts can be commonly observed in outcrops or drilling data.
2.1.3. Characteristics of SQ3
The sea water invaded again after intensive uplifting and exposure in Episode I of the Tongwan tectonic movement. The SQ3 bottom was observed with the Deng 3 pewter shale in southwestern Sichuan Basin and the Deng 3 dark shale and argillaceous dolomite in central Sichuan Basin, which were deposited due to the transgression of the weathering surface on the top boundary of the Deng 2 Member. In the Xianfeng section, the d13C values present gradually an increasing positive trend and it reached the turning point of positive shift in the argillaceous dolomite interval. In central Sichuan Basin, the maximum flooding surface is marked by the interval of black shale. Thereafter, it developed thick layers of massive dolarenite, algae laminae dolomite, and coagulated dolomite intercalated with dolomicrite, representing superimposed distribution longitudinally (Fig. 3). In Well Gaoshi 1 in central Sichuan Basin, it reached a thickness of 91 m and continued to develop to the middle of the Deng 4 Member. The GR curve is relatively low and gentle. This lithological assemblage indicates that the relative sea level kept to be stable and started to downgrade after the maximum flooding in the early Deng 3 Member period. The water became shallower and the energy significantly intensified. In the Xianfeng section, the SQ3 top presents an obvious negative shift peak of d13C value; in the central Sichuan Basin, it presents an abrupt increase of GR values and the development of dolomicrite in flat facies. Karst breccia was observed on the SQ3 top in Well Gaoshi 103 and large scale dissolution was discovered under the SQ3 top in Well Gaoshi 7.
In the Moxi area, the lithological assemblage of SQ3 has different features. The Deng 3 Member is dominated by dark-gray, black shale and dolomitic siltstone intercalated with arenous shale, and occasionally massive siliceous rocks in some wells, which were deposited with the second flooding of sea water after the extensive exposure of the top boundary of the Deng 2 Member (Fig. 3). Dark shale reaches an accumulative thickness of 60 m, which is significantly larger than that (20 m) in the Gaoshiti area. In the Longniisi area in the southeast of the study area, the middle—lower part of the Deng 3 Member gradually changes to arenous dolomite, indicating the water body in the Moxi area is much deeper with water flooding trending northwest; geomorphologically, it shows a gradually elevated tendency from the Moxi area to the southeastern direction. From the upper part of the Deng 3 Member to the lower part of the Deng 4 Member, the water body is relatively shallower, and the lithologies are dominated by dolomitic sandstone, arenous dolomite, and dolomicrite—powder crystal dolomite. In the lower part of the Deng 4 Member, there is a set of dolomitic siltstone with significantly increased GR values. Upwards to the middle part of the Deng 4 Member, there is a set of argillaceous dolomite. Compared with the Gaoshiti area, the Moxi area has no thick massive arenous dolomite, laminated dolomite, and coagulated dolomite. The top boundary of SQ3 is the interface bounded by the powder crystal dolomite and the dolomitic siltstone in the lower part of the Deng 4 Member, belonging to an interface of lithologies and lithofacies. The HST of SQ3 is low geomorphologically in the Moxi area with
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Fig. 3. Sedimentary sequence framework features of Dengying Fm in the Gaoshiti—Moxi area in the central Sichuan Basin.
Cambrian by coagulated dolomite. In the Moxi area, the upper part of the Deng 4 Member is characterized by arenous dolomite and the intercalated stacking of coagulated dolomite; the top is bordered with the Cambrian by a set of karst breccia dolomite (Fig. 2).
In the palaeo-rift zone, the lithological and electrical characteristics of the Dengying Fm in Well ZY1 are similar to that of the bottom of the Deng 1—Deng 2 Member in central—southwestern Sichuan Basin. The dolomicrite—powder crystal dolomite is predominant, intercalated with algae debris dolomite and argillaceous dolomite. The GR values are higher and the dissolution was observed. It is indicated that the scale and coverage of Episodes I and II of the Tongwan tectonic movement were extremely large. There was comprehensive regression of sea water and the rifting zone was affected by two stages of karst denudation with severe erosion.
small depositional thickness (35—60 m), but high in the same interval in the Gaoshiti area with stronger water energy. In the later position, there are mainly laminated, stromatolitic dolomite, arenous dolomite, and coagulated dolomite, with large depositional thickness ranging between 100 m and 200 m.
2.1.4. Characteristics of SQ4
As for the Xianfeng section in Ebian, the arenous dolomite at the top of SQ3 was overlaid by the argillaceous dolomite in the lower part of SQ4. Upwardly, there is the laminated algae dolomite and the d13C shows an increased positive shift, which indicates the beginning of another transgression. The d13C values present a turning point after being kept stable at a positive peak, indicating that it reaches the maximum transgression point. In the Gaoshiti area, central Sichuan Basin, the transgression of SQ4 presents developed dolomicrite and laminated dolomite; the Moxi area is dominated by dolomitic siltstone and argillaceous dolomite. In the middle—upper part of the Deng 4 Member, the HST of SQ4 is initially observed; a large amount of algae stromatolitic, coagulated dolomite, and arenous dolomite was developed in the Xianfeng section, indicating much shallower water bodies and intensified energy. The d13C values present a tendency of decreasing negative shift. The d13C values don't reach the negative shift peak until at the Tongwan Episode II unconformity at the top boundary of Deng 4 Member. Correspondingly, the upper part of the Deng 4 Member in the Gaoshiti area, central Sichuan Basin, also contains coagulated dolomite and arenous dolomite. The top boundary of the Deng 4 Member is bordered with the
2.2. Seismic responses of sequence boundary
There is a large quantity of high-quality 3D seismic data in the Moxi—Gaoshiti area, central Sichuan Basin. On the basis of division and correlation of sequence stratigraphic framework in the Gaoshiti—Moxi area, together with the 3D seismic data, and through calibration of synthetic seismic recording, the reflecting features of sequence boundary were identified and traced (Fig. 3a) and the internal reflecting structures of SQ3 and SQ4 were delineated. According to well drilling and seismic reflecting structure, the sedimentary sequence strati-graphic framework (Fig. 3b) of the Deng 4 Member was
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established. The top boundary (SQ2) of the Deng 2 Member, the erosion surface of top boundary (SQ2) of the Deng 4 Member, and the top boundary of sequence (SQ3) serve as the strong reflecting interface regionally comparable. Each system tract and seismic reflection structure in SQ3 and SQ4 are well correlated. In the depositional period of TST in SQ3, at the rift margin, the reflection structure of the Deng 3 Member overlaying the top boundary of the Deng 2 Member can be observed (Fig. 3a). In the depositional period of the HST in SQ3, multiple foreset reflecting structures were identified in the Gaoshiti area, which are the typical reflection feature of progradation in sea water regression.
The TST of SQ4 in the Moxi area can also be clearly identified in the Moxi area, presenting multiple overlapping reflections overlying the strong axial on the top boundary of SQ3 (Fig. 3a). However, the foreset was barely found in the high system tract. It is analyzed that in the Episode II of the Tongwan tectonic movement, the system has experienced denudation and it was caused by the incomplete preservation in current geological recording history.
According to the interpretation of 3D seismic data, the strong reflection axis in the interior (SQ3 top) of the Deng 4 Member is closer to the top boundary of Deng 4 Member and is further to the top boundary of the Deng 2 Member. However, in the Moxi area, the strong reflection axis of the SQ3 top boundary is further to the top boundary of Deng 4 Member and is closer to the strong reflection axis of Deng 2 Member top boundary (Fig. 3a). This phenomenon shows that there are geomorphological differences in SQ3 between the Gaoshiti area and the Moxi area. In the early depositional period of the Deng 4 Member, the Gaoshiti area was a carbonate platform and the Moxi area was seated in the slope zone. The seismic reflection structure of both sides of platform margin slope differs greatly. In the Moxi area, it is characterized by the overlying of TST in SQ3; in the Gaoshiti area, it is characterized by the progradation reflection structure of platform.
3. Sedimentary filling and evolution model
Currently, the Chengjiang Fm in the Proterozoic Nanhua system is considered to be the earliest sedimentary caprock preserved in the Sichuan Basin. However, the sedimentary rocks of the Chengjiang Fm have not been encountered in the basin. The thickness of Doushantuo Fm in Wells Laolong 1, Wei 28, Wei 117, and Nuji ranges from 8 m to 24 m, and the lithologies are mainly arenous dolomite, gypseous dolomite, dolomitic sandstone locally intercalated with quartz sandstone and mudstone, which directly overly the granite (Fig. 4a). The Rb—Sr isotopic dating of granite in Well Wei 28 is 740.99 Ma to present [12], belonging to the product of the volcanic activity in the Chengjiang period. These four wells are located in the southwestern and central parts of the Sichuan Basin, which are distant to each other. However, all these wells encountered granites, indicating the intense tectonic movements accompanied by large-scale faults and volcanic activities, a typical epeirogenic movement. It was named by Luo Zhili [12] as the Chengjiang movement.
Well ZY1 located in the rift zone encountered the Doush-antuo Fm, with the drilled thickness of 54 m. Although the formation was not penetrated, its thickness increased significantly. Its lithologies are obviously different from those of other four wells. The upper part contains sandstone, the middle part is dominated by dolomicrite powder dolomite intercalated with black shale, and the lower part is mainly claret shale with a thickness of 27 m intercalated with silt-stone. Some literatures indicate that, in the central—western Sichuan Basin, there are thick sedimentary rocks in the giant granite intrusions formed by the faults and volcano activities in the Chengjiang period [6]. The authors propose that the occurrence of strata thickness in the rift zone, especially the significant increase of shale thickness, was formed by the sedimentary filling of the pre-Cambrian extensional rift in central—western Sichuan Basin. The prototype of palaeo-rift in the western—southern Sichuan Basin was formed before the Cambrian sedimentation (Figs. 4b and 5a).
The Dengying Fm in Well ZY1 has a residual thickness of only 63 m and is dominated by dolomicrite—powder crystal dolomite. In the platform on both sides of the rift, the Dengying Fm contains microbialite, with a thickness of more than 600 m (Fig. 4b). In the depositional period of the Deng 2 Member, the microorganism of Cyanobacteria thrived and the progradation in Weiyuan and central Sichuan Basin in the platform margin was obvious. Thick microbialite with "algae-rich interval" can be uniformly observed [13]. The palaeo-rift continued to subside slowly and the depositional thickness of microbialite was small (Fig. 5b). In the platform margin on both sides of the palaeo-rift, the microbialite was centrally developed with a great depth, which was accompanied by arenous and grain-stone, forming mound and shoal bodies [8—10]. After the deposition of Deng 2 Member, the Episode I of the Tongwan tectonic movement caused large-scale uplifting and denudation (Fig. 5c), allowing the regional karstification. The reservoir was ubiquitously developed and the mound and shoal karst reservoirs in the platform margin belts were best in quality.
After Episode I of the Tongwan tectonic movement, the sea level rose in the early depositional period of Deng 3 Member. The marine transgression started and the provenance was not completely submerged in the early stage of transgression. The supply was terrestrial debris and the sand mudstone of Deng 3 Member was deposited. According to the drilling data, there is obvious facies variance in the lithologies of the Deng 3 Member. As the transgression reached the apotheosis, at the part being geomorphologically lower and with greater depth in the slope-shelf of Deng 2 Member, a set of dark-colored shale was deposited, forming a set of effective source rocks. However, in the geomorphologically higher location on the Deng 2 Member platform, the facies transformed to arenous dolomite gradually. After that, the relative sea level kept stable and the depositional period of HST in SQ3 started. The carbonate depositional rate in geomorphologically higher area was higher and the depositional thickness was larger. In the middle—late stage of this system tract, the relative sea level started to downgrade, with the geomorphologically higher Gaoshiti area starting to prograde to the Moxi area in the lower area, and on
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b. Thickness correlation of Sinian-Lower Cambrian Fig. 4. Sinian—Lower Cambrian correlation in the central—western Sichuan Basin.
the seismic profile, the transgression progradation reflection structure formed, together with the early transgression in the early period of Deng 3 Member comprising an integrated transgression and regression sedimentary sequence. Due to regression, the SQ3 top boundary of the geomorphologically
higher Gaoshi area was exposed and experienced leaching and dissolution. However, in the geomorphologically lower Moxi area, the sea water didn't retreat and its top boundary was a transformation plane of lithologic and lithofacies. The authors consider that the SQ3 eventually evolved into a platform
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Dolomite .^S3? Limestone Mounaana Fault Unconformity Fig. 5. Sinian—Lower Cambrian sedimentary filling and evolution model in the central—western Sichuan Basin.
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margin in the early Deng 4 Member. The practical drilling data indicates that the mound shoal bodies and reservoirs in the platform margin belts in early stage of Deng 4 Member were better developed (Fig. 5d).
During the middle—late Deng 4 Member (SQ4), on the geomorphological background formed in SQ3, with the slow rise of sea level, the stratigraphic overlapping on the platform margin in the early Deng 4 Member was formed. As the gradual stabilization of transgression, in the HST of SQ4, there were mound shoal bodies in the palaeo-rift margin (Fig. 5d). After that, the Episode II of the Tongwan tectonic movement in the end of Dengying Fm caused large-scale uplifting and omnipresent exposure, and it experienced intensive denudation. The Deng 4 Member in the southwestern Sichuan Basin experienced severe denudation, with a residual thickness of only tens of meters (Fig. 5e). In the palaeo-rift zone, the denudation of structural movement was the most intensive and there was a large-scale denudation of Deng 3 and 4 Members. The Dengying Fm in Well ZY1 was only 63 m and the dissolution caverns were found.
After Episode II of the Tongwan tectonic movement, deposition of the Cambrian started. The palaeogeomorphology of the Dengying Fm laid a foundation for the filling of the Lower Cambrian sediments and the geomorphological background of distribution. On both sides of the rift in central—western Sichuan Basin, the thickness of Lower Cambrian is 300—540 m, while in the rifting zone, the thickness of Lower Cambrian exceeds 800 m. Specifically, the thickness of Lower Cambrian in Well ZY1 significantly increases to 1038 m. The palaeo-rift of the Lower Cambrian Maidiping Fm is a platform basin zone containing siliceous shale and siliceous rocks, with a large thickness of being up to 160 m. Both sides of the palaeo-rift are platform facies zone containing shallow water carbonate, with a thickness of only 10—20 m. The platform facies are mainly composed of sandy dolomite, siliceous dolomite, and phosphate in southwestern Sichuan Basin, and micritic limestone and arenous limestone in central Sichuan Basin. There was large-scale subsiding in the Qiongzhusi Fm period and the black carbonaceous shale in the palaeo-rift had a thickness of being up to 500 m, but only 160—300 m in both sides of the platform facies zone. The tension and deposition in the Canglangpu Fm started to slow down, but the palaeo-rift has not been filled. The sedimentary thickness was still larger than that of platforms in both sides of the palaeo-lift and the palaeo-geomorphology has been continuing until the end of the Canglangpu and it even affected the distribution of li-thologies and lithofacies in the Longwangmiao Fm (Fig. 5f).
4. Control of sedimentary sequence on hydrocarbon accumulation
4.1. Control of sedimentary sequence on reservoir distribution
The successive development of palaeo-lift gave rise to the palaeogeomorphological background and facies distribution of the Dengying Fm. Practical drilling data indicates that the
carbonate platform margin with strong energy in higher geo-morphology controlled the development and distribution of favorable reservoirs, mainly composed of arenous dolomite and clotted dolomite. Currently, three marginal belts have been discovered, i.e. the palaeo-rift margin belt of Deng 2 Member (SQ2), the geomorphological break marginal belt in early Deng 4 Member (SQ3), and the Palaeo-rift marginal belts of Deng 4 Member (SQ4). According to the drilling data, in the early platform marginal belts (the Well Gaoshi 1 area) in central Sichuan Basin, the reservoir thickness of lower—middle Deng 4 Member ranged from 15 m to 50 m; however, in the gentle slope in the north of platform margin, the reservoir of lower—middle Deng 4 Member was not developed. The three marginal belts are the favorable zones of mound and shoal bodies in the Dengying Fm.
The top boundaries of SQ2 and SQ4 corresponding to Episodes I and II of the Tongwan tectonic movement are both Type I sequence boundaries. The top boundary of SQ3 in the Gaoshiti area is an exposed and denudated surface. Judged from the available data, the exposed time and scale are less than that of Type I sequence boundary, but it is still a key stage for the development of karst which dissolved and reworked the vertically and cyclically developed reservoirs. Finally, the strata-controlled weathering karst reservoirs were formed. Especially in the mound and shoal developmental zone of the three platform margin belts, the further reworking of dissolution formed the best reservoirs in the Dengying Fm [14,15]. It needs to be noted that, the karst denudation was the largest in Episode II of the Tongwan tectonic movement. The platform margin mound and shoal reservoir in SQ4 might experience a certain denudation, so attention should be paid to the reservoir heterogeneity caused by this.
In general, if the platform marginal belt in early Deng 4 Member (SQ3) is superimposed with the marginal belt of palaeo-rift, the superposed zone would be best for the development of most high-quality reservoirs. Under this condition, the mound and shoal bodies were uniformly developed in the Deng 4 Member and the karstification of Deng 2 Member (SQ3, SQ4) was superimposed in this member, which formed the longitudinal superposition of multiple sets of high-quality reservoirs in the Deng 4 Member upwardly, presenting thick distribution. Actually, Well Gaoshiti 1 in the Gaoshiti area currently was equipped with favorable conditions. The reservoirs were developed in the upper and lower parts of the Deng 4 Member, with an average cumulative thickness of 97 m.
4.2. Control of sedimentary filling on source—reservoir assemblage
It is indicated in the sedimentary filling and evolution of the Dengying Fm—Lower Cambrian that the possible high-quality argillutite source rocks formations of the Dengying Fm in the central—western Sichuan Basin include the Doushantuo Fm in the palaeo-rift zone, residual Deng 3 Member near the palaeo-rift, and the Lower Cambrian Maidiping Fm, Qiongzhusi Fm [16—18]. The finally formed major source—reservoir assemblage includes the lower-source & upper-preservation type and
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laterally-abutted type. According to the exploration practice of the Anyue giant gas field, the Deng 4 Member shows a large oil-bearing area and high saturation. The laterally-abutted source—reservoir assemblages in the up-dipping direction are the extremely prominent controlling factor for accumulation. The marginal belts around the Deng 2 Member and Deng 4 Member have laterally-abutted source—reservoir assemblages. The best Lower Cambrian source rocks directly contact with the predominant development belts of the Dengying Fm reservoirs laterally. However, in the Anyue gas field, the gas reservoirs in Deng 2 Member show characteristics of bottom water, seemingly restricting the gas-bearing area and scale. The authors hold that four factors need to be considered in the accumulation of the Deng 2 Member. First, the distance to the palaeo-rift should not be too long; otherwise, sources might be scarce, and the laterally-abutted source—reservoir assemblage type is preferable. Second, generally, the reservoirs are well developed in the Deng 2 Member, and more stringent trap conditions need to be considered laterally. Third, if the source rocks are not developed in the Deng 3 Member, the facies change to arenous sandstone with a thickness of about 20 m. In the zone with well-developed reservoirs in the Deng 4 Member, the Deng 2 Member is less filled. After all, the Dengying Fm experienced many tectonic movements, where factures are well developed, and the Deng 2 and 4 Members might not well accumulate. And fourth, the inheritance and stability of tectonic movements need to be taken into consideration. If there was a great displacement between ancient and current locations, the gas reservoirs in the Deng 2 Member are more liable to be destroyed and dissipated.
According to the geomorphological features of palaeo-lift, the marginal belts extending to the northwest of the Sichuan Basin in the east side of the palaeo-rift should have a better potential for accumulation. Despite the greater burial depth in this region, the marginal belts of the Deng 2 Member are located in the west flank of the Deng 2 Member margin. The Cambrian directly overlies the Deng 2 Member and the depth of the palaeo-rift increases. The marginal slope of the Deng 2 Member becomes steeper and the thickness of the Cambrian source rocks increases, directly contacting with the Deng 2 Member in a laterally-abutted source—reservoir assemblage type and showing better source rock conditions. Both the top and the west-dipping directions have sealing conditions. The hydrocarbons might accumulate and be enriched in appropriate structural traps or structural—lithologic composite trap conditions.
5. Conclusions
1) Generally, the Deyang—Anyue palaeo-lift presents a NW distribution and it is steep and wide in the north and narrow and gentle in the south. There are two rifting boundaries in the Deng 2 and Deng 4 Members. The rift in the Deng 2 Member shows a U-shaped distribution in the Deyang—Neijiang belt. The rift in the Deng 4 Member strides the basin trending NS, but it gradually becomes gentle to the south of Neijiang—Dazu belt.
2) The Dengying Fm can be divided into four III-order sequences. Before Episode I of the Tongwan tectonic movement, there are SQ1 and SQ2 sequences; between Episode I and Episode II, there are SQ3 and SQ4 sequences. In the SQ3 period, the intact transgression overlapping and depositional reflection structure of transgression progradation can be identified in central Sichuan Basin and the platform margin belts in early—middle period were developed.
3) The prototype of the palaeo-rift was formed before the Sinian deposition. The successive activities of Pre-Sinian rift affected the sedimentary filling and lith-ofacies distribution of the Cambrian. Two periods of tectonic movements caused large-scale uplifting and denudation. Finally, the distribution morphologies of the Dengying Fm in the palaeo-rift were formed, laying a geomorphological foundation for the sedimentary filling of thick high-quality source rocks in the Cambrian.
4) The palaeo-rift margins respectively formed in the periods of SQ2 and SQ4 and the platform margin belts in early Deng 4 Member are the favorable distribution zones of high-quality reservoirs. Around the palaeo-rift, the margin belts of Deng 2 and Deng 4 Members both have a laterally-abutted source—reservoir assemblage in the zone of Cambrian source rocks. The margin belts extending northwest in the east side of the palaeo-rift show a better accumulation potential.
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