Lixue Cheng,Liangjun Wang,Yanping Luo

a Southwest Petroleum University,Chengdu,610500,China

b Sinopec Exploration Company,Chengdu,610000,China

ABSTRACT Through precisely measured data of field outcrops,wells logging data and samples analytical data in the research area,it is considered that the topography of the sedimentary period of Xiaoheba formation is relatively flat,the water flow is shallow and wide and the fluctuation is frequent,the supply of source is abundant,and it has favorable sedimentary conditions to forming shallow-river fluvial-dominated delta.The delta in the research area is a distal-basin sedimentary,and due to multiple branches and long extension of the river channel,there are large area of delta front sand deposits with fine grained development,high maturity and underwater environment existed within the research area.The Xiaoheba formation in the Jiannan-Fuling three-dimensional seismic region has the obvious characteristics of strong peak progradation reflection.Based on the above analysis,the sedimentary mode of shallow-river fluvial-dominated delta of Xiaoheba formation of Southeastern Sichuan Basin is established and the reservoir forming conditions are discussed.It is considered that the Xiaoheba formation has excellent hydrocarbon source,high-quality reservoirs and effective migration channel.This research has certain guiding significance for the next stage of tight marine clastic rocks oil and gas exploration of Southeastern Sichuan Basin.

Keywords:Southeastern sichuan basin Silurian Xiaoheba formation Distal-basin shallow-river fluvial-dominated delta

1.Introduction

There is rich oil and gas resources existed in Sichuan Basin,and good exploration results of Upper Triassic Xujiahe formation and Jurassic period have been achieved in the clastic rock exploration field within the basin,and the deep tight clastic rock of Silurian system Xiaoheba formation of lower association is one of the hot spots and important replacement targets in oil and gas exploration [1,2].A set of Longmaxi formation high quality shale is developed in the underlying formation of Xiaoheba formation in Southeastern Sichuan and a set of Hanjiadian formation thick clay-shale is developed in the top formation,then a good source-reservoir-caprock association [3,4]is formed and the study of sedimentary characteristics and model of Xiaoheba formation has important guiding significance for oil and gas exploration.

A series of studies targeting at the sedimentary facies of the Silurian system Xiaoheba formation in Southern Sichuan have been carried out on,but there have been disagreements,the Exploration & Development Research Institute of Jianghan Oilfield first came up with the conclusion that the Xiaoheba formation is the tide flat sedimentary model [5],then Chengdu University of Technology came up with the wave dominate delta model targeting at the distribution characteristics of Xiaoheba formation sandstone body [6]which thought the sandstone body is strongly modified by wave action and it has the characteristics of coastal and sheet-shape distribution.But as exploration continues,a large number of field outcrops in Southeastern Sichuan were observed and analyzed by Exploration Company of Sinopec,then it has been founded that some special sedimentary phenomena in Zhunzhong Area can't be explained by this conventional braided river delta sedimentary model.Therefore,through the fine measurements of 12 field outcrops,the logging data of 16 drilling wells and the system analysis data of collected 286 samples including sedimentary structure,grading analysis,geochemistry and other aspects,combining with the regional geological background,and analogying the Bashijiqike formation deep tight clastic rock Sedimentary characteristics of Kuqa of Xinjiang[7-12],then the Silurian system Xiaoheba formation in Southeastern Sichuan is finally determined as the distal shallow river control type delta sedimentary model,and he favorable reservoir forming conditions is discussed under the guidance of this sedimentary model to indicate the direction for further exploration in this area.

2.Geological setting

Southeastern Sichuan locates in the west of East Sichuan high fold belt,Southeast Sichuan low steep fold belt is the Huaying Mountain faulted belt extending southwesterly,and it is the brush-spreading echelon low anticline group.At present,the structure is a series of tectonic belts gradually branching from the anticline to the south of Huaying Mountain.The tectonic zones are high in the north and low in the south.In the northern tectonic zone,the fold is developed with a large number of faults.It is a long and narrow comb structure with many Triassic strata are exposed in the axial part; the southward folds gradually weakened,the faults is less and it is geniculate and domal structure.In south of Luzhou,it is an east-west tectonic distribution area affecting by the Loushan faulted belt in southern margin of the basin,there are mainly Gaomuding,Changyuanba,Naxi and other structural belts.Among them,the Changyuanba structural belt is typical one,it consists of a series of bead-arranging dome anticline structures and accompanied by east-west fault (Fig.1).

The strata from Sinian to Silurian are well developed in this region,multiple transgressions and regressions have occurred during the process of sedimentation,and it has the characteristics of polycyclic sedimentation longitudinally.Based on the sequence stratigraphic characteristics,the Xiaoheba formation is divided into 2 sections which successively are 1st section and 2nd section from old to new and they correspond to two three-order sequences respectively,the comparability of sequence interface area is strong.

3.Samples and analytical method

Sample preparation and analysis is based on rock samples from wells and field outcrops of southeastern Sichuan basin.There are 3 wells and 10 outcrops which have rock samples.The number of wells are as follows: JS1,LY1 and SX1.The name of outcrops are as follows:Shuangliuba,Lengshuixi,Sanquan,Huangcaochang,Mawu,Xiaohe,Huilong,Haokou,Xixin and Kapeng.The rock samples are from Xiaoheba formation and Longmaxi formation,especially from the main reservoir layers of the 2nd section of Xiaoheba formation.After being screened and classified,the samples were sent to the experimental center of Wuxi research Institute of Petroleum Geology,SINOPEC for analysis.The sedimentary environments are identified by the information recording of rock samples,such as lithological feature,geochemical feature and sedimentogenesis et al.Among them,the thin sections are made by rock samples and different sedimentary reservoir types are identified by microscope,which is the basic data for depositional description.And the distribution of sedimentary microfacies are judged by seismic profile and attribute plane.Besides,the hydrocarbon source condition,reservoir quality and source storage configuration are from experimental data combined with 3D seismic data.

4.Results

4.1.Provenance characteristics

Conduct the paleo current measurement on the wave mark (mostly are symmetrical wave mark)found on the sandstone top and bottom of field outcrops,the wave ridge trends concentrate on between 25° and 75°,the paleo current rose diagram indicates that the paleo current direction of Xiaoheba formation is South East - North West.According to the statistical result of sandstone rock debris content of Xiaoheba formation,the rock debris,feldspar and other unstable mineral content decrease gradually from South East to North West,and the Q/(F + R)relative stability index increase gradually from South East to North West (Table 1).According to the association characteristics of heavy minerals,the relative content of stable heavy minerals gradually increase from South East to North West (zircon,rutile,tourmaline and other)(Fig.2).Meanwhile the heavy mineral assemblages in the southeastern Sichuan basin reflect that the mother rock is a mixed zone,which consists of sedimentary rocks represented by the combination of zircon,leucoxene,rutile,tourmaline,garnet,the basic and ultrabasic intrusive rocks represented by the combination of pyroxene,ilmenite,apatite,and the intermediate-basic extrusive rocks represented by the combination of hornblende,apatite,pyroxene.The longitudinal observation on changes of heavy mineral assemblages indicate that the 1st section of Lower Xiaoheba formation is intermediate-basic extrusive rocks dominated by apatite (36.92%),pyroxene (1.35%)and amphibole(0.57%).To the 2nd section of Upper Xiaoheba formation,it becomes apatite (1.09%),pyroxene (0.51%).The result shows that the influence of the intermediate-basic extrusive rocks is gradually weakened.In contrast,the contents of ilmenite increased from 0.93% to 2.01%,which shows the influence of basic-ultrabasic intrusive rocks represented by iron ore on the provenances is increasing gradually[13,14].These characteristics are consistent with those basic-ultrabasic intrusive rocks and intermediate-basic extrusive rocks in the Upper Proterozoic Banxi group from the west side of Jiangnan - Xuefeng Mountain uplift in the south-east part.Moreover,the volcanic grain size of Banxi group is generally fine,which is consistent with the grain size characteristics of sedimentary sandstone in the study area.Combined with previous studies,it can be concluded that the sedimentary sandstone of Xiaoheba formation in Southeastern Sichuan mainly comes from the west side of Jiangnan - Xuefeng Mountain uplift.

4.2.Sedimentary characteristics

4.2.1.Lithological characteristics

Generally speaking,it is more mud and less sand for the Xiaoheba formation in Southeastern Sichuan,the sand-mud ratio is relatively low which generally less than 20%.The sandstone body of Xiaoheba formation mainly is coarse siltstone and fine sandstone can be seen locally,the rock type mainly are lithic quartz sandstone and feldspar quartz sandstone (Fig.3a).The sandstone grains are subangular - subcircular,it has a certain degree of roundness and the separation is medium - fine which indicates that it has experienced a long distance transportation,screening and grinding.The interstitial materials include the matrix and cement.The content of matrix is high,but the maturity of local laminate or block sandstone body is high.The main cement is carbonate cement,the content varies greatly in the plane,and the content is relative low in the Shuangliuba and Lengshuix in the north part (Fig.3b).The light gray and dark mudstone are inter-sedimented which reflects the water depth changes frequently and the diverging channels or underwater diverging channels are frequently diverted,it is a shallow water sedimentary environment.

4.2.2.Granularity characteristics

According to the grain size probability cumulative curve of Xiaoheba formation sandstone,it's similar to the fluvial facies,it is mainly the two-section type (Fig.4),and the two-section characteristic is the “one saltation,one suspension”,the slope of saltation component is relative high which indicate the particle separation is relative better,the rolling component is almost nonexistent.Among them,the content of suspension component can be up to 30%,it is higher than normal fluvial sediment,which indicates the energy of sedimentary water is weaker than that of fluvial facies,and it reflects the sedimentary environment closer to the estuary region [15-18].

4.2.3.Geochemical characteristics

Fig.1.Location diagram of tectonic units and study areas in Sichuan Basin.

Table 1 Sandstone detrital composition of Xiaoheba Formation in Southeastern Sichuan.

The geochemical salinity and water environment indicate that the reduction environment water body of marine sedimentation is dominated in Southeastern Sichuan.Through the analysis of geochemical parameters including paleo salinity,rare earth elements and microelements of multi profile samples,the Mg/Al value of Xiaoheba formation is mostly in the interval of more than 10 and less than 500(Table 2),it indicates that it's mainly the marine sedimentary environment,and the values of Ceanom and V/(V + Ni)reflect it's a weak reduction environment at that time.Comprehensive judgment shows that the shallow water reduction water body of marine sedimentation is dominated in the sedimentary stage of Xiaoheba formation in Southeast Sichuan.

4.2.4.Seismic characteristics

In the corresponding seismic stratigraphic units,different seismic facies units are divided according to the seismic facies signs,and the sedimentary facies can be interpreted and inferred from the seismic facies characteristics.With this simple and direct method (horizon flattening)of restoring paleogeomorphology,the relative paleogeomorphology of stratigraphic sedimentary period can be seen directly.Through the comparative analysis of the seismic profile characteristics of the target layer after levelling the whole study area (the silurian bottom boundary is flattened),it is found that the Xiaoheba formation in the study area shows the obvious reflection characteristics of the river-controlled delta in the seismic reflection profile.It has typical three-layer structure characteristic of delta sediment,it is prograding from South East to North West,the s-type,s-oblique compound type and oblique type progradation reflection are developed (Fig.5).Meanwhile,the sedimentation process is greatly influenced by waves.And the progradation reflection is advancing from the South East to the North West,reflecting the process of the sedimentary system advancing toward the basin and showing obvious constructive characteristics.It is a constructive delta.The underwater distributary channel is developed,and the estuary dam and front blanket sand are developed.In addition,the foresets of Xiaoheba formation in the study area are multi-phases,each period foreset shows a South East to North West banding distribution on the seismic attribute plan (Fig.6).It is a constructive delta.The underwater distributary channel is developed,and the estuary dam and front blanket sand are developed.

Fig.2.Distribution diagram of mineral composition of Xiaoheba Formation in Southeastern Sichuan and Western Hunan area.

4.2.5.Sedimentary structure characteristics

A lot of field outcrops observations have been conducted targeting at the Xiaoheba formation in Southeastern Sichuan,it has been found that the strong hydrodynamic sedimentary structure (Fig.7)can be seen in the Xiaoheba formation sandstone,the development of sandstone is controlled by the distributary channel or underwater distributary channel.Taking the Shizhu Shuanugliuba profile as example,the development of delta front underwater distributary main channel is dominated; a lot of channel sandstone bodies are developed in both 1st and 2nd section of Xiaoheba formation which mainly is thick - block rock,the lithology is mainly powdery sandstone with thin layer mudstone,pebbly fine sandstone can be seen locally,and obvious undercut erosion surface (Fig.7a)can be seen in the bottom of sandstone body,the parallel bedding and cross bedding in the middle of sandstone body are developed,the wave-built symmetrical wave mark can be seen from middle to top,the overall grain size of the sandstone body deforms upward,it is dominated by positive grain sequence stratification,the characteristics of grain size probability curve indicates that it is characterized as the typical two-section structure of delta channel and it reflects that the fluvial sedimentation with stronger hydrodynamic force is existed in this region.

But the Huilong,Huangcaochang and other profiles are far away from main channel comparing to Shizhu Shuanugliuba,the thickness of sandstone is thinner,the granularity is finer and the shale content increases.It is thin - medium stratified structure,the main lithology are siltstone and argillaceous siltstone,and fine sandstone can be seen locally; the parallel bedding and small cross bedding still are developed,the undercut erosion surface can be seen (Fig.7b and c); the wave-built symmetrical wave mark is developed in the top sandstone; both the positive grain sequence and negative grain sequence stratification are developed.

The characteristics of Xiaoheba formation of Lengshuixi profile in the north of Shuanugliuba profile is different with other profiles,the development of delta front estuary dam microfacies sedimentation characterized by negative grain sequence is dominated in the 1st section of Xiaoheba formation,then it evolves to delta front interdistributary bay microfacies to the 2nd section of Xiaoheba formation,it is dominated by clay sediments and with a little bit of sandy silt.It indicates the channel has migrated here and it evolves to the interdistributary bay sedimentary microfacies.

Among,the hail mark,wormtrail and other structures are existed in multi profiles (Fig.7d),it reflects the Xiaoheba formation is a shallow river control type flow environment at that time from the side.

Fig.3.Lithology photos of Xiaoheba Formation in Southeastern Sichuan basin.

Fig.4.Cumulative graph of particle size probability.

Table 2 Statistics table of paleo salinity,rare earth elements and microelements.

Fig.5.Seismic profile characteristics of SE-NW seismic profile delta in Fuling 3D working area (profile location seen in Fig.1).

4.3.Xiaoheba formation distal shallow river control type delta sediment microfacies and distribution characteristics

4.3.1.Delta sedimentary environment

The basin is in the depression stage during the Silurian system sedimentary stage,the tectonic activity is weak and the sedimentation is slow [19-31].The restoration result of basin paleo geomorphology indicates that the formation slopes of most areas from Xiushan to Fuling are small which is generally less than 1.5°,it has the characteristics of typical gentle slope.The shallow water biological fossils and coalified plant rhizome fragments are found in the outcrop profiles which indirectly proves that the water in the study area is shallow during the Xiaoheba formation sedimentary stage and it has intermittent exposures along with the change of lake level.Through the observations of multi outcrop profiles in the study area,it indicates that the development of Xiaoheba formation sandstone body in the direction of Nanchuan -Pengshui -Shizhu is relative better,the sand-mud ratio value is relative high and the average cumulative thickness of sandstone body is 20-70m,but the transverse continuity is relative strong,the nonhomogeneity of sandstone body is strong,it indicates that not only the Xuefeng Mountain provenance has provided relatively sufficient land source materials to Sichuan Basin at the Xiaoheba formation sedimentary stage which provides the material basis to the extensive development of delta,but also the material transport channels are intertwined.Therefore,the development of Silurian system Xiaoheba formation delta in Southeastern Sichuan has the sedimentary environment of slow slope,shallow water,sufficient provenance and developed channel.

Fig.6.Arc length attribute plane graph of foreset of Xiaheba formation in study area.

4.3.2.Main microfacies types and sedimentary characteristics

The microfacies types of shallow water delta development of Xiaoheba formation in Southeastern Sichuan mainly are underwater distributary main channel,underwater branch channel,estuary dam,interdistributary bay and so on.Most of delta plain distributary channel are developed in the basin peripheral,it is not common in the study area,so it will not be discussed as the main sedimentary microfacies type in this paper.

(1)Underwater distributary main channel microfacies

The underwater distributary main channel is major sedimentary microfacies type of Xiaoheba formation delta development [32-34].Affected by the climate,provenance and paleo geomorphology,the channel continues to extend into the basin and it is distributed in band along the direction of provenance.After the river enters the sea basin,the flow rate reduces,the channels frequently diverge,migrate and mutually incise,the sedimentary sequence is mostly expressed as the multi-phase incomplete positive cycle superimposition longitudinally,and the complete fluvial sedimentation unit superimposition can be seen locally.The sandstone body of underwater distributary main channel mainly is fine sandstone with a little pebbly fine sandstone and fine sandstone,the structure mainly is positive grain sequence,the channel erosion surfaces are often seen in the bottom,the parallel bedding and trough cross bedding are developed (Fig.7a),the shape of GR logging curve is mainly represented as the bell characteristics(Fig.8).

(2)Underwater branch channel microfacies

Fig.7.Field sedimentary structure of Xiaoheba formation.

Fig.8.Composite histogram of Xiaoheba formation in JY7.

The underwater branch channel is located away from the main channel,the thickness of sandstone becomes to be thin,the granularity becomes to be fine and the shale content increases.It is dominated by the thin - medium stratified structure sandstone,the lithology mainly are siltstone and argillaceous siltstone,and the fine sandstone can be seen locally; the parallel bedding and small cross bedding are developed,the undercut erosion surface can be seen; the wave-built symmetrical wave mark is developed in the top sandstone; both the positive grain sequence and negative grain sequence are developed.The channel undercut erosion surfaces can be seen in the Xiaohe,Sanquan,Huangcaochang and other profiles.The response of GR logging curve is mainly represented as the finger-box characteristics (Fig.8).

(3)Estuary dam sedimentary microfacies

Because the Xiaoheba formation delta is dominated by fluvial process,the hydrodynamic is strong,the modification effect of ocean waves on the estuarine sediments are weakly,the typical estuary dam sediment is not easy to be developed; even the estuary dam sediment is formed by the intermittent attenuation of river hydrodynamic,but it will still suffer the erosion and modification of branch channels latterly,it can't be well preserved or only the lower sedimentary sequence of estuary dam can be preserved to form the sedimentary association type of lower-residual-dam-upper-river.Through the observations of outcrops,the estuary dam sediment is intensively developed in the Lengshuixi profile,the sedimentary characteristics is represented to be dominated by siltstone,the negative grain sequence characteristics is obvious,the lower part is often argillaceous,the ripple bedding is developed and the GR logging curve is represented as the tooth-funnel characteristics (Fig.8).

4.3.3.Delta distribution characteristics

The sandstone body of Xiaoheba formation in Southeastern Sichuan mainly comes from the west side of Jiangnan - Xuefeng Mountain uplift in the Southeast.Affected by the sea level eustacy and provenance supply quantity,the sandstone body of 1st section of Xiaoheba formation in the southern region of Pengshui - Jiaoshiba near the provenance region is mostly developed (Fig.9a),the development position of sandstone body moves upwards gradually with the increase of advance distance; in the Fuling - Jiannan region,the sandstone body is mainly developed in the 2nd section of Xiaoheba formation,and the advance distance can be up to 200 km (Fig.9b).As a whole,the sandstone body distribution is obviously affected by distributary channels,it has the characteristics of “multi-branch distributary channel,wide channel distribution range,long extending distance,fine sandstone granularity”.The multi-phase delta sandstone body superimposition is developed vertically,it continues to advance from South East to North West in the plane.

4.4.Shallow water delta sedimentary model

Through the studies of Xiaoheba formation delta sedimentary environment,sedimentary microfacies characteristics and plane distribution law,the study area has the characteristics similar to the typical shallow water delta such as the Bashijiqike group of Kuqa Basin and the cretaceous system of Songliao basin [35,36],then the distal river-control shallow water delta sedimentary model of Silurian system Xiaoheba formation in Southeastern Sichuan is established.This shallow water delta development has the sedimentary environment of sufficient provenance supply,gentle sedimentary slope and shallower water body.The shallow water delta is divided into the shallow water delta plain and shallow water delta front according to the sea level change,the front development area of shallow water delta is wider comparing to normal delta (Fig.10)and it mainly locates below the lake level during flood period; but the area of shallow water delta plain is relative small and it mainly locates above the lake level during flood period,there is no obvious slope break belt existed between those two.The shallow water delta front facies belt can be subdivided into the internal front and external front.The internal front is mainly developed between the flood period lake level and the drought period lake level,and because of the gentle slope and lake level change,the surface area periodic exposure by delta is big,then the broad internal front facies belt is formed and the coal bed is extensively developed.The internal front underwater branch channel is well developed and they are multiparallel,the hydrodynamic is strong,then the channel sand dominated by medium and fine sand has formed the skeleton of shallow water delta.The sandstone body is distributed in band along the advance direction of shallow water delta,the channel migration frequently diverges and mutually incises with the increase of advance distance and change of lake level,then the discontinuous sedimentary sequences are developed vertically.The external front is developed below the lake level of drought period and the area is small,because the hydrodynamic is weaker than the internal front,the lithology is mainly the silty-fine sand,the estuary dam sediment can be developed locally affected by lake waves,but it is not common due to it is vulnerable to later erosion of branch channel.

Fig.9.Distribution diagram of sedimentary facies of Xiaoheba formation in Southeastern Sichuan.

4.5.Implications for oil and gas exploration

The shallow river control type delta of Silurian system Xiaoheba formation in Southeast Sichuan has the sedimentary characteristics of both normal delta facies and fluvial facies which provide the favorable reservoir forming conditions to form the marine giant gas field.

4.5.1.Favorable hydrocarbon source condition

Through the gas composition and isotope analysis of Jianshen 1 Well,the natural gas of Xiaoheba formation mainly come from the Longmaxi formation of Lower Silurian Series,it belongs to oil-type cracking gas.The underlying Longmaxi formation of Silurian system Xiaoheba formation has excellent hydrocarbon generation potential,the Longmaxi formation exploration and exploitation has formed the largest shale gas reservoir in China,the thickness of hydrocarbon source is 60-160m (Fig.11),the content of organic carbon mostly exceeds 2%and the average value is 2-4%,the hydrocarbon-generating intensity is 20-120 × 108m3/km2; the kerogen type is mainly I type and there is a little Ⅱ1type.The average value of Ro is 2-3.5%,it has reached the over-mature stage,the dry gas is dominated,it is high quality source rock.Due to the basin slope of Xiaoheba formation at the sedimentary stage is gentle,the sandstone body of Xiaoheba formation sandstone body is distributed widely and the distribution area of sandstone body is almost completely covered on the top of development area of Longmaxi formation high quality source rock (Shizhu - Pengshui region); at the same time,the oil is became to be generated during the late permian- early triassic for the Longmaxi formation,the middle triassic - early jurassic is the peak period of oil generation,the late jurassic - early cretaceous is the peak period of gas generation [37].The study area locates in the east side of Central Sichuan uplift during late caledonian period and locates in the slope of ancient Kaijiang uplift during the Indo-China period,it has the paleotectonic background which is favorable for the hydrocarbon accumulation period.It is beneficial to the near source accumulation of oil and gas.

4.5.2.Relative high quality reservoir

The sandstone body of Xiaoheba formation shallow river control type delta sediment system in the study area mainly is the channel sand,the overall granularity is fine affected by the provenance,but the sedimentary thickness is big and the distribution is wide,they are mutually incised and with good connectivity,it has the congenital advantage to form relative high quality reservoir.Especially the accumulative thickness of Xiaoheba formation sandstone body in the Shizhu Shuanugliuba outcrop can be up to over 100m,the average porosity is near 5%.Although it is different to the deep-lake semi-deep lake mudstone in the high reduction environment of normal delta development,the shallow water delta distributary channel and shallow lake mudstone developed in this system also have characteristics of fine granularity,wide distribution and big thickness,it has covered the channel sandstone body vertically,it still has good sealing condition and it can be the direct seal; at the same time,the argillaceous rock and gypsum-salt rock of lower Permian and middle and lower Triassic are developed,the wide and gentle synclinal area and high and steep structural belt have been well preserved.Good reservoir and seal strata combination is formed.

4.5.3.Dominant migration channel

Fig.10.Sedimentary model of Xiaoheba formation in Southeastern Sichuan.

Fig.11.Hydrocarbon source degree contour of Longmaxi formation in Sichuan Basin.

Fig.12.Asphalt filling of Xiaoheba formation.

The sandstone body of Xiaoheba formation shallow river control type delta front sandstone body advances along the direction of provenance,it is broadly connected transversely; the asphalt distribution can be seen in the Xiaoheba formation outcrop profiles of Shuanugliuba,Huangcaochang (Fig.12a); according to the fluorescence test and energy spectrum analysis under microscope,the asphalt distribution can be verified (Fig.12b)which indicates that during the oil and gas filling period,the Xiaoheba formation sandstone body has not been buried deeply,the primary porosity is developed.First,it has good conditions for early reservoir formation; secondly,the sandstone body can be used as a conductive layer for oil and gas transversal migration.Because the Longmaxi formation source rock of Shizhu region is developed,after the oil and gas migrated vertically to the Xiaoheba formation,the oil and gas can transversely migrate through this set of delta sandstone body to erich and accumulation in various traps in the upper strata (Fig.13).

5.Conclusions

(1)At the Xiaoheba formation sedimentary stage,most areas of Southeast Sichuan Basin have the sedimentary environment of gentle slope,shallow water and sufficient provenance which is beneficial to the formation of shallow water delta,the main microfacies types are underwater distributary main channel,underwater branch channel and estuary dam,the distribution of sandstone body is strongly affected by the distributary channel,it has the characteristics of “multi-branch distributary channel,wide channel distribution range,long extending distance,fine sandstone granularity”.The multi-phase delta sandstone body superimposition is developed vertically,it continues to advance from South East to North West in the plane.

Fig.13.Schematic diagram of accumulation evolution model of Xiaoheba formation in Southeastern Sichuan.

(2)The Silurian system Xiaoheba formation river-control shallow water delta has favorable reservoir forming conditions,this delta channel sandstone body system is distributed widely,it also has good hydrocarbon source,cap condition and hydrocarbon migration channel,it has favorable reservoir forming conditions of "strong source feeding" and "continuing preservation",the search of developed relative high quality sandstone reservoir in the high energy facies belt is the key for the next breakthrough in the exploration of Silurian system marine facies clastic rock.

Acknowledgements

Funded by National Science and Technology Major Project of China（No.2016ZX05002004-004).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.petlm.2020.05.001.