Yu-xi Zhng, Jin-wen Chen, Jing-yu Zhou, Yong Yun

a Faculty of Earth Resource, China University of Geosciences, Wuhan 430074, China

b Qingdao Institute of Marine Geology, China Geological Survey, Ministry of Natural Resources, Qingdao 266071, China

Keywords:

Lower Triassic

Sequence stratigraphy

Sea-level change

Carbonate rocks

Marine geological survey engineering

Yellow Sea

China

A B S T R A C T

Based on well logging and seismic data, combined with a comparative analysis of drilling data in the Lower Yangtze region, the sequence stratigraphic framework of the Qinglong Formation was established and divided into four third-order sequences.Each sequence is mainly composed of the transgress system tract (TST) and the high-stand system tract (HST).According to the lithology, logging curve and seismic reflection structure, the sedimentary filling characteristics and evolution law for the sequence are analyzed.The results show that each sequence is dominated by a half-cycle decline of sea level, and the rise-fall of sea level controls the distribution and evolution of sedimentary systems within the sequence.During the relative sea-level rise, sedimentation rates slow down and muddy sediments are developed.The characteristics of condensing intervals on the flooding surface are very obvious, and continental shelf and open platform deposition are mainly developed.During the relative decline of sea level, the thickness of sediments increased.The main developments were restricted platform and platform shoal environment,and locally developed evaporation platform environment.

1.Introduction

In the early 1990s, Carbonate sequence stratigraphy was significantly developed, in establishing the basic theory of carbonate sequence stratigraphy (Gnaccolini M and Jadoul F,1990; Mitchum RMJ and Wagoner JCV, 1991; Schlager W,1991; Ogg JG and Steiner MB, 1991; Hunt D and Maurice ME, 1992; Oloriz F et al., 1993) and analyzing the different sequence patterns of carbonate strata (Burchette TP et al.,1990; Gardulski AF et al., 1991; Vander ZCJ and Spaak P,1992; Maynard JR, 1992; Loucks RG and Sarg JF, 1995; Sarg JF, 2001; Catuneanu O et al., 2009).After years of continuous research (Gattolin G et al., 2015; Gutierrez PHC et al., 2017;Mei MX and Tucker ME, 2013; Hashmie A et al., 2016; Yi HS et al., 2011; Li SH et al., 2017; Jamaludin SNF et al.,2018) and have been widely used in the field of oil and gas exploration (Ma YS et al., 2008; Mei MX, 2015; Cui XH et al., 2010; Du W et al., 2013; Liu D et al., 2014; Chang JH et al., 2017; Beigi M et al., 2017; Zecchin M et al., 2017).In recent years, studies of carbonate sedimentary filling within the sequence framework has attracted much attention in oil and gas exploration (Lin CS et al., 2013; Zhang JL, 2017;Tavakoli A, 2017).

The South Yellow Sea Basin (SYSB) is a multi-cycle superimposed basin, which is located at the basement of the Sinian metamorphic rocks up of the Lower Yangtze Platform(Fig.1).The exploration of the basin is relatively low.In recent years, more research has been done on the marine Mesozoic-Paleozoic strata.The study found the Lower Triassic carbonate strata in the SYSB to be well developed and relatively stable in structure (Chen JW et al., 2016, Zhang YG et al., 2014).Although certain exploration achievements have been made, no significant industrial oil and gas discoveries have been obtained so far.

Fig.1.Location of the study area and division of the Lower Triassic strata (modified from Cai LX et al., 2017).

Many scholars have pointed out that the formation and evolution of the sequence is the result of sea level changes,regional tectonic changes, and sedimentary filling rates (Sarg JF, 2001, Loucks RG and Sarg JF, 1995).For marine carbonates, the main factors affecting the sequence deposition are sea level change and basement settlement (Zhao ZJ,2015).In the early Triassic, the settlement of the SYSB was relatively stable.It can be seen that the development of the Lower Triassic marine carbonate sequence in the SYSB is mainly controlled by sea level changes.Using drilling,logging, and seismic data, combined with the comparison of sub-continental basins in the Lower Yangtze region, the Lower Triassic carbonate strata sequence in the SYSB was analyzed and studied in order to establish the sequence framework and evolution of the area.The results provide the basis for further exploration of oil and natural gas in the basin.

2.Geological background

The SYSB is located at the eastern end of the Lower Yangtze Platform.From the distribution map of the ancient continent nucleus of the Yangtze Platform, we can see that the ancient continent nucleus of the Lower Yangtze is continuous from the Subei basin to the SYSB, and its main body is in the SYSB.The magnetic anomaly map shows that the magnetic anomalies are also continuous from the Subei basin to the SYSB, and the anomalous body is in the SYSB.Therefore,the SYSB is an extension of the Lower Yangtze bock to the sea along the direction of the NE, and it is the main body of the Lower Yangtze block.

At present, industrial oil flow is obtained in the wells of R2 and R3 in the Jurong area of the Subei basin (Hua CX,2014), and the wells of N4 and N5 in the Huangqiao area also receive a small amount of crude oil and natural gas.These results show that there are hydrocarbon accumulation processes in the Lower Triassic of the Lower Yangtze platform.In recent years, predecessors have done more research on the Mesozoic-Paleozoic marine strata of the SYSB.It is believed that there are three sets of source rocks and three source-reservoir-cap assemblages of the Mesozoic-Paleozoic marine strata in the SYSB, with huge potential for oil and gas resources (Chen JW et al., 2016).The CSDP-2 well, which is located in the Laoshan uplift, has crude oil exuded in the Lower Triassic strata, and the discovery of the Ophiceratidae molecule can be compared with the Early Triassic in the northern Jiangsu basin (Guo TL, 2011).In the early Triassic, most areas of the SYSB experienced a hydrocarbon generation.The carbonate rocks of the Lower Triassic Qinglong Formation in the SYSB are well developed and relatively stable in structure (Chen JW et al., 2016; Zhang YG et al., 2014).Although certain exploration results have been obtained, no significant industrial oil and gas discoveries have been obtained so far.

The SYSB is a structure pattern of “two depressed with three heave” from north to south, followed by the Qianliyan uplift, Yantai depression, Laoshan uplift, Qingdao depression,and Wunansha uplift.The Lower Triassic strata are mainly distributed in the Qingdao depression and the Wunansha uplift.Except for partial bulge erosion, the basic contiguous distribution is generally 500-1500 m in thickness and the maximum thickness exceeds 2000 m (Chen JW et al., 2018).The Yantai depression and the Laoshan uplift are relatively limited in distribution.The Kachi-1 well revealed a thickness of only 33 m, and the CSDP-2 well revealed a thickness of about 470 m.At the bottom of the Early Triassic Qinglong Formation in the SYSB, there was a good source of oil, with an effective source rock thickness of 160-200 m, which has good oil and gas prospects (Liang J, 2011).The Qinglong component is composed of two upper and lower sections.From the bottom up, there is a continental shelf deposition, an open platform deposition, and a limited platform deposition(Liang J et al., 2017; Fig.1)

3.The sedimentological sequence

The identification, tracking and comparison of key interfaces are key to the study of sequence stratigraphy.The key interface in the carbonate sedimentary sequence is mainly the sequence boundary and the maximum flooding surface(MFS).Since there is little drilling data in the sea, the identification of the interface is mainly based on well logging and seismic data.A small amount of drilling core is used as an aid and compared with the field profile of the land.

The change of lithology has a corresponding response on the logging curve.Better continuity and higher resolution logging data provide a good basis for the identification of the sequence boundary.The sensitivity of the Gamma ray curve to the shale content in carbonate formations can identify the sequence boundary, the MFS and the unconformity surface.

3.1.Sequence boundary

In the seismic profile, the sequence boundary usually appears as onlap and downlap (Fig.2).The research area mainly develops TST and HST.The TST is characterized by the onlap, which is superior to the sequenced interface.In the HST, the onlap reflection is above the MFS, and there is a local oblique pre-product reflection.The top interface is mostly the sequence boundary with strong amplitude reflection (Fig.2).In the log curves, the sequence boundary corresponds to the sudden change surface of the Gamma ray curve or the resistivity curve (Fig.2).In the outcrop profile,there are also obvious lithological changes.

Fig.2.Identification signs of the Lower Triassic critical interface in the Lower Yangtze area.

3.2.The maximum flooding surface

The maximum flooding surface (MFS) is the top surface of the TST.In the case of transgression, the depth of water during deposition deepens, and the mud content increases,resulting in an increase in the value of the Gamma ray curve(Fig.3).Taking the well WX5-ST1 as an example, the progressively thinner progressive quasi-sequences on the Gamma ray curve show an obvious retrogradation sequence,with the top boundary being the MFS (Fig.3).In the keep up transgressive system interval, the Gamma ray curve is relatively flat and lacks the obvious characteristics of retrogradation, so the 4-parasequences define a progradational curve pattern.Each progradational unit end in the forward mutation.The curve mutated out of the MFS (Fig.3b).The spike-like Gamma ray curve appears locally, reflecting the tendency of each upper sedimentary unit to become thicker upwards.In lithology, a small amount of shale and siltstone appear at the bottom of the progradational unit, and its bottom is the MFS (Fig.3c).

Fig.3.Identification signs of the Lower Triassic critical interface in Lower Yangtze area.

For the identification of the MFS, the drilling and outcrop data on land also have obvious characteristics.In the N5 well of the Subei Basin, there was a clear retrogradation sequence of GR curves at the time of transgression.The corresponding core was a thin layer of micrite and mudstone interlayer (Fig.4a).At the time of HST, there was a progradational sequence of Gamma ray curves, corresponding to the core of muddy limestone and stylolite development (Fig.4b).The interface characteristics of the system tract are also very obvious in the outcrop of the Lower Yangtze, showing an increase in the thickness of the bottom-up sequence (Fig.4c).

Fig.4.Typical sedimentary characteristics of the Lower Triassic in the Lower Yangtze area.a, b-identification signs of the critical interface;c-e-identification signs of the field photo; f-nodular limestone, well N5; j- gray limestone and deep gray mudstone with different thicknesses,well N5; h-dark gray with “worm-like” limestone, well N5; i-brilliant crystal grain limestone, well WX5-ST1 (Liang J, 2011); j-bright crystal aggregate limestone, well WX5-ST1 (Liang J, 2011); k-dolomitic limestone, well csdp-2.

3.3.Stratigraphic sequence framework

In the Lower Yangtze land area, the former did a lot of detailed research on biostratigraphy and chronostratigraphy.The land of the Lower Yangtze was divided into four thirdorder sequences, corresponding to four sub-orders (Tong JN and Yin HF, 1997, 2015).In terms of stratigraphic division,the Lower Triassic in Anhui Province was divided into the Yinkeng Formation, the Helongshan Formation and the Nanlinghu Formation, and the Jiangsu region was divided into Hushan Section and the Cangbomen Section.The SYSB is divided into the upper-Qinglong section and the lower-Qinglong section (Table 1).

Table 1.Comparison of the Lower Triassic in the Lower Yangtze area.

Elemental geochemical characteristics can be used to distinguish paleo-salinity, paleo-water depths, the relative rise-fall of sea level (Ming CD et al., 2015; Xu ZJ et al., 2012)and the identification of sequence boundaries (Yu Y et al.,2014).The values of Sr/Ba and B/Ga are used as the sensitive elements for the determination of paleo-salinity, and they are positively correlated with the paleo-water depth.The values of Sr/Ba and B/Ga increase, the paleo-water depth increases,and the sea level rises relatively.Based on the logging curve and elemental geochemical data, combined with the cyclic change of lithology section, the well Chang-1 in the Lower Yangtze land area was divided into four third-order sequences(SQ1 to SQ4) (Fig.5).

Fig.5.Sedimentologic sequence of the well Chang-1 in the Early Triassic.

The logging curves and lithology data of the wells CZ35-2-1 and WX5-ST1, which are located in the Lower Yangtze sea area, were used to identify the key interfaces and further stratigraphic sequence divisions.The same 4 third-order sequences were divided.In contrast with the well Chang-1,the sequence stratigraphic framework of the Early Triassic in the Lower Yangtze region was established (Fig.6).

Fig.6.The division of the Lower Triassic sequence in the Lower Yangtze area.

The SQ1 is equivalent to the lower part of the lower part of the Qinglong Formation.In the Lower Yangtze land area,Permian-Triassic biotopes and strata continued to transition,and significant sedimentary discontinuities existed only at the edge of the basin and uplift on the shelf (Wuxi, Jiangsu Province; Tong JN and Yin HF, 1997).The well CZ35-2-1 and well WX5-ST1 are continuously deposited between the Lower Triassic Qinglong Formation and the Upper Permian Dalong Formation.During the Early Triassic, a large-scale transgression was inherited from the end of the Permian, and SQ1 was composed of the early TST and the later HST.Well Chang-1, the initial geochemical elements showed a positive drift followed by a negative drift, which showed a complete cycle.The Gamma ray curve of well WX5-ST1 presents a complete degenerative-progressive cycle, indicating a complete transgression-regression process, with the dominant of a half-cycle decline of sea level.The well Chang-1 and well CZ35-2-1 lacked an obvious degenerative-progressive cycle, and the sediment thickness was small, indicating the well was located in the basin.At the same time, this coincides with the conclusion that the Lower Yangtze plate tends to the southwest during the Early Triassic (Fig.6).

The SQ2 is equivalent to the upper part of the lower part of the Qinglong Formation.It consists of an early TST and a late HST.The elements of well Chang-1 generally show a cycle of positive drift to negative drift.In the period of transgression, the increase of Gamma ray value is particularly obvious (Chang-1 and CZ35-2-1).Well Chang-1 has a gradually increasing funnel shape, and well CZ35-2-1 shows a box shape.The GR curve of the well WX5-ST1 shows a zigzag shape, which is a combination of a box shape and a funnel shape.In the HST stage, the GR curve is a gentle low value, and the increase and decrease of the GR value indicates a complete transgression-regression process (Fig.6).

The SQ3 is equivalent to the lower part of the upper part of the Qinglong Formation.It consists of an early TST and a later HST.With the exception of Sr/Ba, the variation of other geochemical elements in the Well Chang 1 is relatively small,and the log curves show a cycle behavior.In the early stage of TST, all of the Gamma ray values increased as fingers and were abrupt (Chang-1, CZ 35-2-1, and WX5-ST1).In the later HST, the Gamma ray curve is a gentle, jagged, low value,indicating a transgression-regression process, which is relatively small compared to the previous period of transgression (Fig.6).

The SQ4, which is equivalent to the upper part of the upper part of the Qinglong Formation, consists of TST and HST.In the early stage of transgression, the well CZ35-2-1 and well WX5-ST1 had obvious degenerative sequences, and the GR curve showed a funnel-shaped increase.The GR curve of well Chang-1 showed a box-type increase.In the period of HST, the well Chang-1 and well CZ35-2-1 are presents progradation sequences, while the well WX5-ST1 exhibits a serrated mid-high value, and the resistivity curve presents a low value as box-type, indicating a transgression-regression process (Fig.6).

4.Depositional filling and evolution

4.1.Depositional filling characteristics and facies

In the Lower Yangtze land area, the major rock types of the Lower Triassic are mudstone, gray mudstone, argillaceous limestone, nodular limestone, muddy limestone, igneous limestone, vermicular limestone, siltstone, and dolomite.The types of sedimentary facies include continental facies, slope facies, open platform facies, and restricted platform facies.In the sea area, sparry oolitic limestone, and sparry granular limestone are commonly seen in the well WX5-ST1 (Fig.4i,Fig.4j).The thickness of the Triassic Qinglong Formation is about 470 m in the CSDP-2 well, and the lithology is entirely limestone.The fractures are developed and filled with calcite(Fig.4c-e).

The bottom of the SQ1 is the interface between the Permian and Triassic, integrated contact, and is of a type II-sequence.During transgression, the well WX5-ST1 was a large set of limestone layers, and the progressively thinning of progressive quasi-sequences from the bottom to the top determined a distinct pattern of retrogradation on the GR curve.Chang 1 well is an interbedded layer of mudstone and mudstone.The main component of the core is calcite, with a content of more than 90% and a particle content of 25%-30%.The well CZ35-2-1 is dominated by mudstone and argillaceous limestone deposits with a small thickness.In the lower Yangtze land area, the lower part of the Yinkeng Formation is gray mud shale in the Chaohu area (Fig.5a), and the N5 well is a limestone-muddy belt (Fig.5e), which is gradually turned into a thin layer of limestone and mudstone interbedding (Fig.4a).This shows that during this period, a large-scale transgression was inherited from the end of the Permian.Affected by paleostructural patterns, the Chaohu area is dominated by continental shelf-slope sediments, and the Jiangsu area is dominated by carbonate platform sediments.In the SYSB, the well CZ35-2-1 is surrounded by basin-shelf sediments.The appearance of siltstone in the well WX5-ST1 indicates the beginning of progradation and the sea level is high.The HST are mainly composed of mudstone limestone, interbedded mudstone and limestone, siltstone and limestone.After the MFS, sediments in seawater gradually increase to saturation, the deposition rate increases, and the thickness of the sequence significantly increases (Fig.4c).Through the seismic section of the well WX5-ST1, the bottom of the SQ1 is a parallel strong-amplitude reflection, with a series of downlap and partial S-type progradation structures above the MFS at the top.There is a localized mound-like reflection (Fig.7).It is considered that the SQ1 mainly develops continental shelf sedimentary and open platform sedimentary.

Fig.7.The stratigraphic division and typical seismic facies of the Early Triassic in the SYSB.

The SQ2 is equivalent to the upper part of the lower-Qinglong Formation.During the transgression period, the well CZ35-2-1 and well Chang-1 were dominated by mudstones, with thin-bedded limestone and marlstones, and there was a pronounced condensation section deposition near the MFS (Fig.4a).In the HST, the well WX5-ST1 is a large set of limestone sections, in which large sections of pure dolomite are developed.The changes in the carbon isotope and Z values of the well Chang-1 reflect a large change in sea level.After the MFS, the V/Ni value decreases and the relative sea level decreases.Through the seismic section of the well WX5-ST1, it can be seen that the TST is above the sequence boundary.During the HST, the seismic reflections were skewed and progradation above the MFS, and the mound-like reflections migrated to the south, indicating that there was a lateral migration of the shallows during the transgression and regression.According to the analysis, the SQ2 is mainly composed of open platform sedimentary, and with the significant decline of sea level, restricted platform sedimentary and platform shallow sedimentary are locally developed.

The SQ3 is equivalent to the lower part of the upper-Qinglong Formation and is mainly dominated by limestone.The GR curve showed an increase in finger-like mutations at the sequence boundary, and then decreased to a smooth, lowvalue jagged pattern, indicating that the transgression was relatively rapid and small compared to what it was previously.Well CZ35-2-1 shows a clear thin layer of mudstone and interbedded limestone near the MFS.According to the seismic profile of the well WX5-ST1, the TST is above the sequence boundary, in the HST stage, below the MFS, the mound-like reflections continues to migrate toward the basin (Fig.7).According to the analysis, SQ3 mainly develops open platform sedimentary and locally develops shallow shoals sedimentary.

The SQ4 is equivalent to the top of the upper-Qinglong Formation and its top is the ablation surface.The wells WX5-ST1 developed oolitic limestone and gangue limestone of restricted platform sedimentary, and the limestone color became lighter.There is a weathering crust at the top of well CZ35-2-1.The well Chang-1 is dominated by limestone and the top is dolomitic limestone.From the seismic profiles, it can be seen that the transgression is small and the regression is dominant, and the mound reflection continues to migrate towards the basin.In the Chaohu area, there is an ointment rock layer on it.Based on comprehensive analysis, during this period, there were mainly restricted platform sedimentary,locally developed evaporative platform sedimentary and platform shoal sedimentary.

Through the combination of single well phase and seismic sedimentary facies analysis, it is believed that in the early period of the Lower Triassic, the well CZ35-2-1 was dominated by continental f sedimentary, and an open platform sedimentary developed around well WX5-ST1.In the late Lower Triassic, the well CZ35-2-1 was dominated by open platform sedimentary.Well WX5-ST1 is mainly dominated continental deposit sedimentary, accounting for more than 90% of limestone and a set of dolomites in the middle.At the same time, the wells are generally seen oolitic sparry limestone and pelagic sparry limestone of platform shoal sedimentary.The CSDP-2 is dominated by gray limestone in the Lower Triassic.with local inclusions of gray dolomite and dolomitic limestone.In many sections, nodular limestone has developed with a horizontal bedding of restricted platform sedimentary.In the early Triassic, the SYSB was dominated by regression and in the process of frequent transgressionregression, the sedimentary of the carbonate platform gradually migrated to the basin (Fig.8).

Fig.8.Distribution of the sedimentary facies of the Lower Triassic at the bottom of the SYSB.

4.2.Depositional evolution

In the early Triassic, the Lower Yangtze region was a migrating subsidence basin that settled on the basis of the Permian platform-trace.The coupling relationship between the sedimentary filling characteristics and the rise-fall of the sea level of the Lower Triassic Qinglong Formation in the SYSB shows that the rise-fall of sea level has a controlling effect on the spatial distribution of the carbonate rock deposition system.

In the early Lower Triassic, the transgression of the Lower Permian continued in the SYSB.Well CZ35-2-1 reveals that the southern part of the basin is dominated by thin-bedded mudstone and interbedded limestone sediments of continental shelf sedimentary.The shale composition gradually decreased from the bottom to the up and the limestone increased.Well WX5-ST1 reveals that the eastern part of the basin is dominated by limestone of carbonate platform sedimentary.During the period of sea level decline, the platform shallows migrate toward the basin and the direction of the basin is thickened.Between each sequence, the seismic facies show a sequence boundary with strong amplitude reflection (Fig.7).Along with the rise-fall of the sea level, the thickness of the sediment changes from a thin layer to a thick layer.On the lithology, the interbeds of mudstone and limestone gradually transform into large sets of limestone.The scope and scale of transgression in the later period of the Early Triassic was also significantly smaller than before.The overall performance was a sedimentary environment with regression, and at the end of the Triassic, marine sediments exited the SYSB (Fig.9).

Fig.9.The evolution model of the Lower Triassic sequence in the SYSB.

5.Discussions

5.1.Sea level change and the migration of platform sediments

Carbonate platform sedimentation is affected by sea level changes and has obvious mobility (Chen HD et al., 2009),especially in the marginal zone of the platform and the oolitic beach (Tan XC et al., 2009).Its migration is very obvious in seismic profiles.(Fig.10).For the development characteristics and stacking style of the oolitic beach, there have been many studies.The shoal in Feixianguan Formation of the Lower Triassic of the Upper Yangtze was characterized by lateral migration and vertical overhang (Duan JB et al.,2008).The oolitic beach in the Yudongzi area of Jiangyou Oilfield is affected by sea level and hydrodynamic forces, and there is a lateral migration of the oolitic beach.There is a different composition in each oolitic beach sequence, and it is easy to form high-quality reservoirs by stronger hydrodynamic conditions (Rong H et al., 2010).

Fig.10.Sea level change and the migration of platform sediments.

With the rise and fall of the sea level, the Qinglong Formation is migrating toward the center of the basin,especially the shallow shoal of the platform.With the change of sea level, its migration is more apparent (Fig.10).The sealevel cycle fluctuates, resulting in periodic depositional cycles, and log curves appear as parallel-type accretion.Periodic lateral depositional migration also occurs on the seismic section, resulting in the lateral migration of platform shoal facies.

5.2.Hydrocarbon exploration significance

Wells CZ35-2-1, well CZ24-1-1, well WX5-ST1, and well WX4-2-1 reveal that the source rock of the Lower Triassic Qinglong Formation is marl and the effective source rock thickness is about 160-200 m, and organic carbon content is 0.12% to 0.55%, with hydrocarbon generation capacity (Chen JW et al., 2016).The CSDP-2 well revealed that Ro was 0.682%, and the TOC was 0.362% in the Lower Triassic Qinglong Formation (Cai LX et al., 2017).The lower part of the Qinglong Formation is a relatively good oil producing zone and is also located in a more favorable oil producing zone.During the Yinzhi-Yanshan movement, sedimentary discontinuities caused severe weathering and erosion of the Qinglong Formation, thus forming an ancient karst reservoir with good reservoir conditions (Liang J et al., 2017).

The discovery of large and medium-sized natural gas fields such as Puguang, Yuanba and Longgang reveal the platform margin reefs and beach belts on both sides of the Feixianguan Formation on the Upper Triassic of the Upper Triassic of the Upper Yangtze region have great oil and gas exploration potential.At the same time, there are significant differences in the characteristics of reefs, beaches, and reservoirs associated with the platform margins of different sites, and different types of reservoirs have formed (Guo TL et al., 2011).The Lower Yangtze and the Upper Yangtze have a similar Middle-Paleozoic marine sedimentary evolution history.Therefore, studying the sedimentary sequence of the Early Triassic in the South Yellow Sea Basin, establishing a sequence stratigraphic framework and sedimentary evolution model, it is possible to identify favorable reservoirs such as stratigraphic traps and shallow shoals and to better predict potential hydrocarbon accumulation belts.

6.Conculsions

(i) In the Early Triassic of the SYSB, the Qinglong Formation underwent multiple transgression-regression processes, and the boundary between the sequence and system was well-characterized on log curves and seismic profiles.Combined with drilling and seismic data, the carbonate strata of the Qinglong Formation was divided into four third-order sequences.Each sequence was dominated by the fall of sea level, and the overall sea level decline.

(ii) During the early Triassic, the western and southern parts of the SYSB developed mainly continental shelf sedimentary and open platform sedimentary.In the east, there are open platform sedimentary and restricted platform sedimentary, and locally developed shallow shoals sedimentary.After the large-scale regression, sedimentary of the evaporative platform occurs.

(iii) The rise-fall of sea levels has a significant control over the sedimentary filling of carbonate sequences in the Early Triassic of the SYSB.During the transgression period,the water depth deepens, the sedimentation rate slows down,typical condensing layers appear, and the lateral migration characteristics of the platform shallows are obvious.At the end of the transgression, during the period of HST, the sediment thickness increased, the characteristics of progradation were obvious, and the edge of the platform gradually migrated towards the basin.With the multiple risefall of sea level dominated by regression, the marine sediments eventually exited the SYSB.

Acknowledgement

This study was supported by the Oil and Gas Resource Survey in the South Yellow Sea Basin (DD20160512).The authors thank Sinopec East China Oil and Gas Company for some data preparation.