The Eocene Nanpu depression is located in the north part of Bohai bay basin and is an EW-trending depression, filled with associated coarse clastic deposits in the northern margin. Two fan delta systems have been recognized in the Eocene deposits of the depression. Transgressive fan delta and regressive fan delta systems were deposited when the basin was actively subsiding and uplifting. The transgressive fan deposits comprise proximal fan, middle fan and distal fan, and regressive fan delta can be divided into fan delta plain, fan delta front and prodelta subfacies. Detailed lithofacies studies have been carried out in the Eocene Nanpu depression. Ten lithofacies are present: massive conglomerate (Gm), massive or weakly to poor stratified conglomerate (Gms), stratified gravely sandstone (Gt), medium to very coarse-grained massive sandstone (Sm), coarse-grained trough cross-bedded sandstone (St), medium- to coarse-grained planar cross-bedded sandstone (Sp), pipple cross-laminated sandstone (Sr), very fine- to medium-grained sandstone with flat bedding (Sh), parallel laminated siltstone and claystone (Fl), massive mudstone (Fm). The origin, characteristics, distribution and spatial arrangement of the various lithofacies represent sediments distribution characteristics of different subfacies. Fan delta depositional systems developed good reservoir, and becoming one of the main oil and gas exploration targets. Therefore, the study of the fan delta sedimentary characteristics is of great value.

Keywords: shahejie formation, nanpu depression, fan delta, lithofacies, sedimentary model

The term “fan delta” was first introduced into the geological literature over five decades ago by Holmes. Fan detas are common depositional systems in lacustrine and marine rift basin.^1–3 Now, fan deltas have begun to be recognized more widely in the rocks record and have become the focus of considerable geological interest.^2–3

This is primarily because the sedimentary record of fan deltas proves to be of great value to basin analysis and to the reconstruction of basin margin sedimentary model. It normally also provide much information about the source-area evolution as well and an additional reason for studying fan delta deposits is that they can act as hydrocarbon reservoirs, due to their abrupt lithologic changes.⁴ A great deal of researches on the sedimentary characteristics of fan delta systems around the world have been made using different data and various methods. The purpose of this paper is to document and interpret two types of geologically young (Eocene) fan delta systems in Shahejie formation of Nanpu depression and make a contribution to the understanding of the wide range of different fan delta types and geological setting associated. The Eocene fan delta sandstone of Nanpu depression is one of the most important sets of oil and gas reservoir. According to core observations and descriptions combined with electrical properties and regional sedimentary background, two fan delta systems have been recognized. It is of great academic value to study the sedimentary characteristics of the Eocene fan delta of Nanpu depression. Furthermore, the study could guide the oil and gas exploration in this area and have broad implications for other similar basins.

Nanpu depression is located in northern Huanghua depression of Bohai bay basin which experienced Mesozoic and Cenozoic fault block movement Figure 1 and it is a meso-cenozoic dustpan-like depression of faulting in north and overlapping in south on the north China platform basement.^5-6 This depression is a characteristic oil & gas depression in Bohai bay basin where oil and gas resources is extremely rich, and the oil and gas accumulation is very complex.⁷ Nanpu depression is close to Laowangzhuang uplift and Xinanzhuang uplift in north, Matouying uplift and Bogezhuang uplift in east, Shaleitian uplift in south, and Qikou depression in west, and it is bounded with Xinanzhuang fault, Bogezhuang fault, Shabei fault and Jiandong fault respectively. Nanpu depression mainly developed Shichang，Linque, Liunan and Caofeidian sub-depression, Gaosahngpu, Liuzan, Laoyemiao, south Bogezhuang and east Xinanzhhaung land structural belts and Nanpu No.1 to No.5 tectonic zones. Internal fracture is well developed and distributed in Gaoliu fault, Xinanzhuang fault and Bogezhuang fault area, and it is related to the evolution process of Xinanzhuang structural belt in north-east trending and Bogezhuang structural belt in north-west trending. Gaoliu fault is a syngenetic fault which is long-term inherited and developed.⁸

Figure 1 Location map of Gaoshangpu oilfield.

The drilling in Nanpu depression reveals that the formation includes Eocene Shahejie formation, Dongying formation, Guantao formation, Minghuazhen formation, Pingyuan formation from bottom to top. Kongdian formation is lost, and Shahejie formation is lack of the forth member. Shahejie formation can be divided into three sections, and the third member is divided into five sub members. The second-third sub members are the main exploration target stratum, which can be divided into two third-order sequences called SQ2 (sequence 2) and SQ3 (sequence 3). SQ2 and SQ3 were subdivided into six forth-order sequences.

The SQ2 sequence is regressive system tract (RST). Sandstone thickness increases while mudstone becomes thin gradually, and the ratio of sand and mud increase. This system tract shows progradational characteristics and coarsening-upwards sequence. The two coarsing-upward sequences are corresponded to IV and V oil group of the third member of Shahejie formation respectively. The IV oil group developed grey and brown conglomeratic sandstone and coarse-medium sandstone with taupe gray mudstone, and the thickness of this stratum is about 105~235 meters. TheVoil group developed light grey medium-coarse sandstone and taupe gray mudstone interbedding, and the thickness of this stratum is about 90~220 meters Figure 2.

Figure 2 Kainozoic stratigraphy in Nanpu depression, Baohai Bay basin.

The main study unit and the lithologic character of G32-32 well (right)

The SQ3 sequence is a transgressive system tract (TST), in which sandstone thickness decreases while mudstone becomes thick gradually, and the ratio of sand and mud decrease. This system tract shows the retro gradation characteristics and fining-upwards sequence. The two fining-upward sequences are corresponded to II and III oil group of the third member and 0 and I oil group of the second member of Shahejie formation respectively. The 0 oil group developed dark grey and taupe gray mudstone with thin siltstone and fine sandstone, and the thickness of this stratum is about 80~140 meters. The I oil group developed glutenite, conglomeratic sandstone, and medium-coarse sandstone with grey-green and taupe gray mudstone at bottom, dark grey and taupe gray mudstone with thin siltstone and fine sandstone at central section, and the thickness of this stratum is about 150~225 meters. The II oil group developed brown glutenite, coarse-medium sandstone and taupe gray mudstone interbedding, and the thickness of this stratum is about 110~155 meters. The III oil group developed brown and grey conglomeratic sandstone, coarse sandstone, fine sandstone and shaly sandstones with grey-green and taupe gray mudstone interbedding, and the thickness of this stratum is about 120~200 meters Figure 2.

In this research, lots of work has been conducted to make successive core description. Sedimentary facies indicators such as rock constituent, rock texture, sedimentary structure are gathered. Then analysis has been made by combining logging information and geological data. A comprehensive understanding of sedimentary characteristics and sedimentary facies of Shahejie formation in Gaoshangpu oilfield is discussed below:

Provenance analysis

Provenance analysis is a very important part of basin analysis and it is significant for the analysis of sedimentary environments and ancient paleo geomorphology.⁹

There are various research methods on ancient provenance and paleo current analysis, such as lithic component analysis, compositional maturity analysis, heavy mineral analysis, dipmeter survey.¹⁰ In this study, two research approaches are chosen to investigate the veracious paleo current direction in Shahejie formation.¹¹

Heavy mineral analysis: Heavy minerals are an important sign of provenance. The ZTR index is often be used in heavy mineral analysis (The ZTR index refers to a ratio that is given by the content of stable heavy minerals / the content of transparent heavy minerals). The ZTR index is less than 0.1 of Well G4 in the northwestern study area; it is also less than 0.1 of Well G87, G66X5 and L11-16 in the northeastern study area. It ranges from 0.1 to 0.4 in the central portion of the area, and reaches values higher than 0.4 in the southwestern study area. The ZTR index increases from the northwest and northeast to the southwest Figure 3.

Figure 3 Contours of ZTR index in the third member of Shahejie formation.

Compositional maturity: Clastic compositional maturity refers to the extent to which a clastic sedimentary rock approaches the most stable final product during the transformation process associated with its long-term weathering, transportation, and deposition.¹² In this study, we use Ic = Q / (F + R) (Q, F and R are on behalf of the content of quartz, feldspar and rock debris respectively) to represent the compositional maturity of sandstone samples from the third member in Shahejie formation, then the content of quartz can represent compositional maturity, as quartz can keep away from weathering better than feldspar and rock debris, abrade less in the process of transport Figure 4. So, the compositional maturity of sandstones is lower in the northeast and northwest than in the southwestern study area. Specifically, the sediment in the study area came from northeast and northwest during the third member of Shahejie formation. Analysis of compositional maturity and heavy minerals suggests that the sediment within the study area came mainly from the northeastern Baigezhuang uplift and the northwestern Xinanzhuang uplift.

Figure 4 The content of quartz, feldspar and rock debris in the third member of Shahejie formation.

Sedimentary composition analysis

Shahejie formation fan delta deposit a high proportion of conglomerate and conglomeratic sandstone in lithology profile. According to the samples of rock slices analyzed, the sandstones of Shahejie formation have low content of quartz and feldspar as well as high content of rock debris Figure 5. The types of the sandstones are mainly lithic arkose and feldapatic litharenite, with a small amount of arkose and litharenite. The content of quartz ranges from 17.1% to 37.1%, and 27.4% averagely. The content of feldspar fluctuates between 16.2-37.9% with 25.4% averagely. The content of debris ranges between 26.4-56.8%, and 38.4% averagely. Clastic particles are point-line contact, sorting is poor to moderate, psephicity is Subangular-subrounded, cementation type is pore cementation Table 1. In general, the content of stable components is less and sediment maturity is lower.

Figure 5 Lithology triangle figure of Shahejie formation in Gaoshangpu oilfield.

Lithofacies analysis

The Shahejie formation sandstone can be informally divided into several conglomerate, sandstone and clay stone units.¹² Ten lithofacies are recognized in the Shahejie formation fan delta system Figure 6.

Figure 6 Summary of the characteristic features of the lithofacies types encountered in Shahejie formation of Nanpu depression.

Massive conglomerate (Gm) consists of clast-supported conglomerate with massive bedding, it is ungraded to weakly graded, poorly sorted and represents lag deposits or high hydrodynamic conditions. Massive or weakly stratified conglomerate (Gms) consists of matrix-supported conglomerate, ungraded to weakly inversely graded, poorly sorted, it presents sub aerial plastic debris flows.¹³ Stratified gravely sandstone (Gt) consists of coarse-to very coarse- grained sandstone with trough cross-bedded sets and weakly stratified at set base. Geometrically, Gt consists of thick lenticular to tabular shaped bodies on the transverse. The lower contact is erosional and sharp and upper boundary is usually gradational. This facies suggest relatively rapid sedimentation in fluvial channels from high velocity flows in the deepest part of the stream.^14-16 Medium to very coarse-grained massive sandstone (Sm) consists of medium to very coarse-grained massive sandstone, clast-supported, moderately to poorly sorted.¹³ Coarse-grained trough cross-bedded sandstone (St) consists of medium- to very coarse-grained with small trough cross-bedded sets. Geometrically, this facies occurs as lenticular shaped bodies. The lower boundary is sharp and flat with facies Gt or is erosional with facies Fm of the underlying cycle and the upper contact is usually sharp with facies Sp. Facies St is interpreted to form a fluvial or low-angle-inclined fronts of bars in the upper part of lower flow regime.^13,17-20 Medium- to coarse-grained planar cross-bedded sandstone (Sp) consists of medium- to coarse-grained sandstone, sandstone arranged into lenticular or tabular, which are characterized with planar cross-bedding. The lower boundary of Sp is sharp with facies Gt or St and upper contact is sharp with facies Sr, or Fm. Facies Sp is interpreted to form medium parts of bed forms or channel bars. Ripple cross-laminated sandstone (Sr) consists of very fine- to fine-grained sandstone with siltstone lenses. The sandstone is thin lenticular or wedge-shaped bodies that contain ripple cross laminated and less amounts of small-scale planar. The cross strata in both facies Sp and Sr dip consistently in the same direction. This facies typically grades laterally and vertically into the finely laminated sediments of facies Sh and Fl. This facies likely represents the deposition of upper bars or in overbank areas [16,21,22].^16,21,22 It may be also attributed to the slow sedimentation within largely inactive channels as fill deposits.¹⁹ Very fine- to medium-grained sandstone with flat bedding (Sh) consists of very fine- to fine-grained, flat bedded and horizontally laminated.^28-30

Sandstone, with thin and a sheet-like geometry. Lower and upper contacts of this facies are sharp or rarely gradational with facies Sr or Fl. This lithofacies suggests deposition as bar-top and sand sheets as high-energy sheet floods that spilled into a lower energy environment from channels during flooding of the main fluvial channel system. Parallel laminated siltstone and clay stone (Fl) consists of planar laminated clay stone units interbedded with siltstone to form shale horizons. Its lower boundary is flat with facies Sp, Sr or Sh. This facies contains low-density bioturbation and some foraminifera trace fossils. Units of this facies appear to possess sheet-like or rare lenticular geometries. Facies Fl is interpreted to represent the deposits of waning stage flood deposition, chiefly in overbank areas.²³ Massive mudstone (Fm) consists of massive mudstone with sandstone and siltstone interbeds. It contains abundant bioturbation. The lower contact of this facies is erosional or sharp with facies Sp, Sr or St, whereas the upper contact is usually erosional with Gt. Units of this facies appear to possess thin or thick, massive-shaped geometries. Facies Fm is interpreted to represent deposition from flood plain distal to the main channel or prodelta.

Facies associations and depositional environments

The sedimentary microfacies can be identified by the combination characteristics of different types of lithofacies in the third member of Shahejie formation. Braided channel, front bar, underwater braided channel, channel bar, sand sheet, and interchannel bay microfacies can be identify in the study area Figure 7.

Figure 7 The combination characteristics of different types of lithofacies in the third member of Shahejie formation.

Braided channel: This facies association consists of poorly sorted massive conglomerates, indistinctly cross stratified conglomerate, medium to very coarse-grained massive sandstone, coarse-grained (facies Gm, Gt, Sm,) with intercalation of mudstone and thin-bedded siltstone or sandstone beds.^24-27 It is characterized by erosively-based units that cut down into fine-grained clay stone or mudstone sediments (facies Fl and Fm) of the underlying cycle. These surfaces mark the beginning of each fining-upward fluvial cycle within the succession. The poor internal organization and the lack of well-developed grading in the conglomerates are suggestive of sub aerial emplacement of debris flows. These facies associations represent deposition on a debris-flow-dominated braided channel. This interpretation is based on the consistent erosional relationship with underlying mudstone deposits. Channel bodies are incised into underlying sediments and then it may infers that avulsion of the rivers to new sites involved variable levels of excavation of previously deposited alluvium.^29-30

Underwater braided channel:The underwater braided channel facies association consists of weakly stratified conglomerate, stratified gravely trough cross-bedded sandstone and medium- to coarse-grained trough cross-bedded sandstone (facies Gms, Gt, St). The basal surface is generally sharp and flat and erosive features are often observed. This deposits of the facies association were formed by bed load of gravel and sand into a water body. Because of this high-flow velocity and the fine grain size of the sediment, their bottom is mainly eroding into the underlying mudstone.^31,32 Erosion surfaces and scours are common at the bottom of this facies association.

Front bar: Front bar is composed of stacked sets, arranged into coarsing-upward packages, and characterized by finer-grained cross- to horizontally-stratified sands (Sr, Sh), medium- to coarse-grained planar cross bedded sandstones (Sp) and very coarse-grained trough cross-bedded (St). Mottled or speckled silty-sandstone is usually developed at its bottom. Minor bounding surfaces separate sets and costs of facies St, Sp and Sr.³³ These elements represent the deposits of migratory dune-scale bed forms in fan delta front. Downstream reduction in the scale of the cross-bedded sets likely indicates downstream changes from dune- to ripple-scale bed forms, probably reflecting a local decrease in transport energy.²⁶ This interpretation is based upon their size and their orientation. The lens-, tabular- or wedge-shaped geometry of front bar might reflect the presence of straight-crested dunes deposited during high discharge events.³⁴

Channel bar: Channel bar is well developed in the succession that is identified as downstream accretion and lateral accretion elements. These microfacies are composed of medium- to coarse-grained sandstones arranged into trough cross-bedded, large-scale planar cross-bedded sets (facies St, Sp). Downstream accretion elements form the lower portions of the fining-upward cycles, either for the entire element or for individual units within a single element.²⁵ They are characterized by fining-upward packages composed of fine- to medium grained sandstones with siltstone interbeds in the upper parts. Low angle-inclined compound costs of planar cross-bedding are more abundant than trough cross-bedded sets and, within the inclined strata of these larger costs, smaller scale structures including ripple cross-lamination and small-scale trough cross-stratification are developed. The downstream accretion units are interpreted to have been deposited under a constant flow regime during normal river discharge and likely represent a succession of mid-channel transverse bars within the central parts of a sinuous channel.^26,35,36 The convex-up macro form bounding surfaces and lack of features otherwise indicative of lateral accretion suggest the lateral shifting of the down current-migrating sand dunes or channel bars.³⁴ The fining-upward grain size trend, the unidirectional dip direction of forests and internal bounding surfaces, and the downstream-dipping forests within these elements are together indicative of the downstream end of a channel bar.³⁷

Sand sheet: These discontinuous tabular thin sheets of fine-grained sandstone that are commonly inter-laminated (facies Sr and Sh). This microfacies is wide spread throughout the formation and its principal occurrence is in the upper portion of the fining-upward cycles and is laid down by nonchannelised flows.^38-39 Their thin, discontinuous, sheet-like geometry, together with the small-scale of sedimentary structures and the fine-grained lithology suggests deposition as a bar-top or bar-flank,²⁵ which accumulated in the shallower parts of channels, terminal of fan delta font or middle fan, profan delta or distal fan.^36,40-42 However, in places, the sand sheets are also deposited by overbank sheet flows rather than true sheet floods.³⁹ Alternatively, laminated sand sheets attributed to upper flow regime plane bed conditions may develop in shallow water on the upper parts of bars or low energy environment.^43-45

Interchannel bay: Interchannel area is characterized by fine-grained clay stone and mudstone with interbeds of siltstone and fine-grained sandstone. These microfacies can be traced laterally both parallel and perpendicular to channels and represent swamp deposits (facies Fl and Fm). The fine grain size and extensive, geometry of these elements indicates deposition over a wide area that was distal to the main channel.²⁷ Fine-grained units that show only weak palaeosol development are indicative of avulsion deposits that were subject to rapid sedimentation The origins of grey and greenish-grey colours suggests a perennially water-logged, vegetated mudflat setting, indicative of an oxygen-poor environment.

Single well facies analysis is based on the study of the core section. The sedimentary microfacies types is recognized through the observation and description of rock mineral composition, mineral structure, sedimentary structure, the change of sediment rhythm, analysis and comparison to the core top and bottom contact relation, sedimentary sequence cycle.⁴⁶

Well G82 is located in the central of the research region. It is shown that fan delta was developed in third member of Shahejie formation. Distal fan subfacies and middle fan front subfacies were developed in I and III oil group while profan delta subfacies and fan delta front were developed in IV oil group. As a whole, the content of sandstones is more than mudstones. The coring section I oil group is composed of light grey mudstone, thin bed silty mudstone, pelitic siltstone with ripple cross bedding, fine sandstone with trough cross bedding, siltstone with ripple cross bedding, sandstone with planar bedding, pebbly sandstone with trough cross bedding, and an isomerous sandstone with cross bedding. III oil group is composed of glutenite with cross bedding and grey mudstone. The core section showing a variety of sedimentary microfacies, including braided channel, underwater braided channel, front bar, channel bar, sand sheet and interchannel bay microfacies Figure 8.

Figure 8 Single-well facies analysis of Shahejie Formation (Well G82).

Fan delta sedimentary model is used to describe and reveal the sedimentary environment and sedimentary characteristics of Shahejie formation in Nanpu depression of Bohaiwan basin, according to the characteristics of sedimentary environment, lithology and sedimentary structure analysis. In different microfacies of the fan delta, the hydrodynamic conditions are distinctive, and the characteristics of sedimentary facies sequence also have obvious differences.⁴⁷

Fan delta deposition is affected by two kinds of system tracts in the study area. The lower LST shows a coarsening-upward sedimentary sequence, and thickness of sandstone increased from bottom to top while the ratio of sandstone and mudstone increased gradually, the upper TST suggests a fining-upward sequence, and thickness of sandstone decreased from bottom to top while thickness of mudstone increased gradually. Sedimentary model can be divide into two types in Nanpu depression-transgressive fan delta and regressive fan delta. Transgressive fan delta can be identified three subfacies: proximal fan, middle fan and diatal fan. Fan delta plain, fan delta front and profan delta subfacies can be identified in regressive fan delta.

When relative lake level drops, wave energy decreases and fluvial energy increases, the regressive fan delta is formed. Facies distribution is complete on plain with a coarsing-upward sequence. It can be divided into fan delta plain, fan delta front and front delta subfacies and it can also be identified as braided channel, front bar, interchannel bay and sand sheet microfacies further Figure 9. Fan delta plain is mainly composed of glutenite, course- to very coarse- grained sandstone, medium-grained sandstone and fine-grained grained sandstone in channel, grey, green and the variegated mudstone at interchannel area. The braided channel developed massive bedding, weakly trough cross bedding, spontaneous potential curve performed jugged box shape. The fan delta front deposits weakly stratified conglomerate, medium- to coarse-grained trough cross bedding or planar bedding sandstone with gray-green mudstone and a small amount of inferior oil shale, spontaneous potential curve performed box shape, funnel-bell shape and finger shape. The fore-delta entered a lake and developed depression green, dark grey mudstone.

Figure 9 Sedimentary model of Shahejie formation in Nanpu depression.

Transgressive fan delta is a subaqueous depositional body formed during the transgression phase with a fining-upward sequence. This type of fan delta appears in the early filling stage of monocyclic basin or transgression phase of polycyclic basin. Based on the study of depositional environment, lithology, sedimentary sequence and lithofaies characters, it can be further divided into three sedimentary subfacies: proximal fan, middle fan and distal fan. Proximal fan is formed by intermittent water flow and braided channel sedimentation with conglomerate of poor sorting and layered property. It appears as imbricate conglomerate and gravelly sandstone with rough parallel bedding at the top and, sometimes, massive bedding and large trough cross bedding at the bottom. The lower boundary of proximal fan is usually erosional and sharp with underlying mudstone deposition. Underwater braided channel, channel bar, interchannel bay and sand sheet microfacies can be further identified. Middle fan formed the main part of the fan delta with highly-developed braided channel, which can be divided into undertaker braided channel, channel bar, sand sheet and interchannel bay microfacies. The clastic rocks of the middle fan braided channel are mainly medium- to very coarse-grained sandstone and rarely gravelly sandstones. Its sedimentary structure includes small trough cross bedding, planar bedding as well as an erosional basal surface at the bottom sometimes. The braided channel extension towards basin forms the mid-fan front and changed into finer grains with better sorting and more cross bedding. The channel souring decreased and wave energy increased. The interchannel bay is characterized by celadon to dark grey massive or weakly laminated mud with low-density bioturbation and thin sandstone with small-scale ripple bedding and deformational structures. Distal fan is formed in semi-deep lacustrine and the lithologies are mainly green or dark grey mud with low-density bioturbation.

The analysis of the samples of rock slices shows that the sandstones of Shahejie formation have low content of quartz and feldspar as well as high content of rock debris. The types of the sandstones were mainly lithic arkose and feldapatic litharenite with a small amount of arkose and litharenite. In general, the content of stable components is less and sediment maturity is lower. The textural maturity is relatively low by comparing the maturity of different samples, and deposits are transported nearly.

Core observation shows that the sedimentary structures have plenty types with various combining form, and are widely distributed. These types mainly include scour erosion surface, trough-shaped cross-bedding, parallel bedding, ripple bedding, graded bedding lenticular bedding, horizontal bedding, massive bedding, and other sedimentary structures in Shahejie Formation, which reflects sedimentary characteristics of fan delta sediments.

The Shahejie formation Sandstone can be informally divided into several conglomerate, sandstone, clay stone and mudstone units. Ten lithofacies are present: massive conglomerate(Gm), massive or weakly to poor stratified conglomerate(Gms), stratified gravely sandstone(Gt), medium to very coarse-grained massive sandstone(Sm), coarse-grained trough cross-bedded sandstone(St), medium- to coarse-grained planar cross-bedded sandstone(Sp), pipple cross-laminated sandstone(Sr), very fine- to medium-grained sandstone with flat bedding(Sh), parallel laminated siltstone and claystone(Fl), massive mudstone(Fm).

The comprehensive analyses of sedimentary and sequence-stratigraphic characteristic show that sedimentary model of the third member of Shahejie formation was a fan delta deposition formed in RST and TST environment. Two types of fan detal, transgressive fan delta and regressive fan delta, are identified through sedimentary environment, lithology and sedimentary structure and lithofacies identification. Regressive fan delta developed a coarsing-upward sequence under the control of the RST, which can be divided into fan delta plain, fan delta front and profane delta subfacies. Transgressive fan delta is formed at the transgression phase with a fining-upward sequence, and it can be further divided into three sedimentary subfacies: proximal fan, middle fan and distal fan.

This research was funded by Beijing Normal University through the Fundamental Research Funds for the Central Universities, 2015KJJCB11. And the authors also wish to thank China National Petroleum Corporation Jidong Oilfield Exploration and Development Research Institute for kindly assistance with the core provision.

The author declares no conflict of interest.

1. Koster EH, Steel RJ. Sedimentology of Gravels and Conglomerates. Can Soc Pet Geol Mem. 1984;10:411 p.
2. Nemec W, Steel RJ. Fan Deltas: Sedimentology and Tectonic Settings. Blackie, Glasgow, 1988. 441 p.
3. Colella A, Prior DB. Coarse–Grained Deltas. Int. Assoc. Sedimentol Spec. Publ. 1990. 10:357.
4. Ethridge FG, Wescott WA. Tectonic setting, recognition and hydrocarbon reservoir potential of fan–delta deposits. In:EH Koster, RJ Steel (Eds), Sedimentology of Gravels and Conglomerates. Can Soc Pet Geol Mem. 1984;10:217–235.
5. Ying Zhao, Wencai Zhang, ChunhuiDu, et al. Deep reservoir characteristics and influence factors analysis of Gaoliu area in Nanpu depression. Petroleum Geology and Engineering. 2009;23(3):23–26.
6. HaiminZhou, ZechengWang, HaiyingGuo. Tectonic Controls on Sequence Stratigraphy in Tertiary Period in Nanbao Depression. Journal of China University of Mining &Technology. 2000;29(3):326–330.
7. LibinWang, XianyuDuan. Subsidence history analysis of the Paleogene in Gaoliu region of Nanpu Depression. Reservoir Evelution and Development. 2012;2(3):6–22
8. BaijiangFan, ChenglinLiu, GuangdiLiu. Study of fracture system formation mechanism and tectonic evolution in Nanpu depression. Journal of Xican Shiyou University. 2010;25(2):13–17.
9. Zhao HG, Liu CY. Approaches and prospects of provenance analysis. Acta Sedimentary Sin. 2003;21(3):409–415.
10. Hu ZQ, Zhu XM, Peng YM. Analysis of provenance and palaeocurrent direction of Jurassic at Chepaizi Region in northwest edge of Junggar Basin, J Paleogeogr. 2001;3(3):49–54.
11. Zhao CL, Zhu XM. Sedimentary petrology. Beijing: Petroleum Industry Press; 2001.
12. Chaoqun Shi, Zhidong Bao, Jinliang Zhang, et al. Sedimentary characteristics of braided stream–braided fan of Shawan Formation in Chunfeng Oilfield, northwest margin of Junggar Basin, China. Arab J Geosci. 2015;8(7):4469–4484.
13. Sylvie Bourquin, Christophe Rigollet, Philippe Bourges. High–resolution sequence stratigraphy of an alluvial fan–fan delta environment:stratigraphic and geodynamic implications–An example from the Keuper Chaunoy Sandstones, Paris Basin. Sedimentary Geology. 1998;121:207–237.
14. Allen JRL. Studies in fluviatile sedimentation:a comparison of fining upwards cyclothems, with particular reference to coarse member composition and interpretation. J Sediment Petrol. 1970;40(1):298–323.
15. Miall AD. Architectural–element and bounding surface in fluvial deposits:anatomy of the Kayenta Formation (Lower Jurassic), southwest Colorado. Sediment Geol. 1988;55:233–262.
16. Collinson JD. Alluvial Sediments. In: Reading HG ed, Sedimentary Environments and Facies. 3rd ed. Blackwell Publishing, Oxford. 1996. p.37–82.
17. Casshyap SM. Sedimentary cycles and environment of deposition of the Barakar coal measures of Lower Gondwana. India J Sed Petrol. 1970;40(4):1302–1317.
18. Cant DJ, Walker RG. Development of a braided fluvial facies model for the Devonian Battery Point Sandstone, Quebec. Can J Earth Sci. 1976;13(1):102–119.
19. Miall AD. The Geology of Fluvial Deposits:Sedimentary Facies, Basin Analysis, and Petroleum Geology. Springer–Verlag. New York, USA, 1996. 582 p.
20. Capuzzo N, Wetzel A. Facies and basin architectural of the Late Carboniferous Salvan–Dorénaz continental basin (Western Alps, Switzerland/France). Sedimentology. 2004;51(4):675–697.
21. Kirk M. Bar development in a fluvial sandstone (Westphalian “A”). Scotland. Sedimentology. 1983;30(5):727–742.
22. Bose PK, Chakraborty PP. Marine to fluvial transition:Proterozoic Upper Rewa Sandstone, Maihar, India. Sediment. Geol. 1994;89:285–302.
23. Hjellbakk A. Facies and fluvial architecture of a high–energy braided river:the Upper Proterozoic Seglodden Member, Varanger Peninsula, northern Norway. Sediment Geol. 1997;114:131–161.
24. Jun Tabaka, Wataru Maejima. Fan–delta sedimentation on the basin margin slop of the Cretaceous, strike–slip Izumi Basin, south western Japan. Sedimentary Geology. 1995;98:205–213.
25. Miall AD. Architectural–element analysis: a new method of facies analysis applied to fluvial deposits. Earth Sci Rev. 1985;22:261–308.
26. Halfar J, Riegel W, Walther H. Facies architecture and sedimentology of a meandering fluvial system: a Palaeogene example from the Weisselster Basin, Germany. Sedimentology. 1998;45(1):1–17.
27. Mrinjek E, Sremac J, Velic J. Pliocene alluvial sediments in the Drava Depression of the Virovitic–Slantina area, Northern Croatia. J Geol Cro. 2006;59(1):65–84.
28. Fielding CR, Webb JA. Facies and cyclicity of the Late Permian Bainmedart coal measures in the northern Prince Charles Mountains, MacRobertson Land, Antarctica. Sedimentology. 1996. 43(2):295–322.
29. Bridge JS. Fluvial facies models:recent developments. In:Posamentier H, Walker RG (Eds.), Facies Models Revisited:Soc Eco Pal Min Spec Pub. 2006. 84:85–170.
30. Gibling MR. Width and thickness of fluvial channel bodies and valley fills in the geological record:A literature compilation and classification. J Sediment Res. 2006;76(5):731–770.
31. Belevich EF. About the vertical growth of islands in the lower zones of the Volga delta. Rep Oceanogr Commun Acad Sci USSR. 1960. 6.
32. Polonski VF, Mikhaylov VN, Kir’yanov SV. The Volga River Mouth Area:Hydrological–Morphological Processes, Regime of Contaminants and Influence of Caspian Sea Level Changes, GEOS. State Oceanographic Institute and Faculty of Geography of Moscow State University, Russian, 1998. 280 p.
33. Roberts EM. Facies architecture and depositional environments of the Upper Cretaceous Kaiparowits Formation. southern Utah Sediment Geol. 2007. 197:207–233.
34. Bordy EM, Catuneanu O. Sedimentology of the lower Karoo Supergroup fluvial strata in the Tuli Basin, South Africa. J African Earth Sci. 2002;35:503–521.
35. Smith ND. The braided stream depositional environment: comparison of the Platte River with some Silurian clastic rocks:north–central Appalachians. Geol Soc Am Bull. 1970;81:2993–3041.
36. Smith ND. Some sediment logical aspects of planar cross–stratification in a sandy braided river. J Sediment Pet. 1972;42:624–634.
37. Sharma M, Sharma S, Shukla KU, et al. Sandstone body architecture and stratigraphic trend in the Middle Siwalik Succession of the Jammu area. India J Asian Earth Sci. 2002;20:817–828.
38. Tunbridge IP. Facies model for a sandy ephemeral stream and clay playa complex: the Middle Devonian Trentishoe Formation of North Devon, UK. Sedimentology. 1984;31:697–716.
39. Olsen H. The architecture of a sandy braided–meandering river system: an example from the Lower Triassic Solling Formation (M. Buntsandstein) in WGermany. Geol Rundschau. 1988;77:797–814.
40. Jackson RG. Large scale ripples of the lower Wabash River. Sedimentology. 1976;23:593–632.
41. Cant DJ, Walker RG. Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada. Sedimentology. 1978. 25(5):625–648.
42. Nijman W, Puigdefabregas C. Coarse grained pointbar structure in a molasses–type fluvial system, Eocene Castisent SandstoneFormation, South PyreneanBasin. In:Miall AD (Ed), Fluvial Sedimentology. Canb Soc Petrol Geol Mem. 1978;5:487–510.
43. Harms JC, Mackenzie DB, McCubbin DG. Stratification in modern sands of the Red River, Louisiana. J Geol. 1963;71:566–580.
44. Plint AG. Sandy fluvial point bar sediments from the Middle Eocene of Dorset, England. In: Collinson. JD, Lewin J , editors. Modern and Ancient Fluvial System. Int Assoc Sediment Spec Pub. 1983;6:355–368.
45. Wakelin King GA, Webb JA. Upper flow regime mud floodplains, lower flow regime sand channels:sediment transport and deposition in a dry land mud aggregate river. J Sediment Res. 2007;77(9):702–712.
46. Smith ND, Cross TA, Dufficy JP, et al. Anatomy of an avulsion. Sedimentology. 1989;36(1):1–23.
47. Kraus MJ, Gwinn B. Facies and facies architecture of Palaeogene floodplain deposits, Willwood Formation, Bighorn Basin, Wyoming, USA. Sediment Geol. 1997;114:33–54.