Palaeogeographical development of two merging delta systems (Eocene Shahejie Formation) in the Bohai Bay Basin, E China and implications for hydrocarbon exploration

Ren-Cho Yng , Yng Li , Ai-Ping Fn ,A.J.Vn Loon (Tom),＊, Jin Li , Zuo-Zhen Hn , Jie Chen

a Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology,Qingdao 266071, China

c Department of Resource Science and Engineering, Shandong Institute of Petroleum and Chemical Technology, Dongying 257061, China

Abstract Two deltas developed simultaneously during the Eocene on the eastern side of a large lake that existed in the Dongying Sub-basin, which forms part of the Bohai Bay Basin in eastern China.The rivers that built the deltas had different catchment areas, which resulted in sediments with different permeability and porosity,due to differences in sorting and mud content.Both deltas prograded,and mass flows that originated frequently on their fronts formed lobes that expanded laterally.This eventually led to merging of both deltas,a feature that has rarely been described from ancient deposits thus far.Core analysis and seismic reflection data show that the merging of the two deltas took place in nine phases, determined by phases of slower progradation or even temporary retrogradation in between.The alternation of sediments from both deltas and their eventual mixing makes the architecture of the merged deltas much more complex than that of‘classical’single deltas.This affects the predictability of the spatial distribution of possible reservoir characteristics significantly, but detailed core analysis shows that the best hydrocarbon reservoirs consist of sand bodies formed in distributary channels on the delta plains, and of sandy turbidites formed in the deep-lacustrine environment in front of the merged deltas.

Keywords Delta development,Merging deltas,Palaeogeography,Bohai Bay Basin,Lacustrine environment,Eocene, Hydrocarbon exploration

1.Introduction

The sedimentology of deltas has been investigated in detail for a long time, which has resulted already more than half a century ago in some benchmark studies (e.g., Scruton, 1960; Visher, 1965; Van Loon,1972).The interest in prograding deltas is still significant(e.g.,Xu et al.,2012;Fan et al.,2017;Han et al.,2017;Lu et al.,2018),partly because they form large sedimentary bodies that are well preserved in the geological record, partly because the combined occurrence of thick sandstones and mudstones frequently has resulted in significant hydrocarbon occurrences (e.g., Gong et al., 2015, 2016; Chen et al.,2016), also in sediment gravity flow deposits, since these appear to have a huge potential for containing oil and gas accumulations and consequently are important targets for oil and gas exploration (Qiu et al., 2001; Yang et al., 2014, 2016, 2017a, 2017b,2017c; Yang et al., 2014, 2018).The networks of exploration wells in the deposits of prograding deltas have provided a detailed insight into the architecture of prograding deltas, whether marine (e.g., Schieber et al., 2007; Schieber and Southard, 2009; Kostic,2014) or lacustrine (Li et al., 2013, 2015, 2016, 2018;Zhang et al., 2015; Lv et al., 2016).

Deltas are common morphological elements along modern coasts, particularly in lowland areas where rivers from several source areas can merge, such as in The Netherlands, where the Rhine, Meuse and Scheldt rivers jointly built a deltaic complex(e.g.,Middelkoop et al., 2010; Martinus and Van den Berg, 2011;Stouthamer et al., 2011) by merging of originally separate deltaic systems.Another,but much less easily accessible example are the recent merging deltas in the shallow Daihai Lake, Inner Mongolia, which have been described earlier in English in this journal (Yu et al.,2013) and later also in Chinese (Shi et al., 2014).The present-day common merging of deltas is, remarkably,hardly known from the geological record.Sedimentary models of ancient converging deltas and analyses of their sediments(particularly gravity-flow deposits)are consequently scarce,and the processes that take place under such conditions in deep-water settings(whether marine or lacustrine)are still incompletely understood,so that there are only a few existing models of converging deltas.These earlier models are not really detailed;they concern the Triassic Yanchang Formation in the Ordos Basin(Yang et al.,2017a;Fan et al.,2018)and the Paleogene Wilcox Group in the Texas Gulf coast(Olariu and Zeng, 2018).These models are, however,still under debate.Additional studies of such delta developments might therefore shed more light on the process of converging and, eventually, merging, thus helping to deepen the insight into the precise interactive processes that play a role in these settings.

An excellent opportunity to study converging deltas in the sedimentary record is provided by the Paleogene succession in the Dongying Sub-basin,which forms part of the Bohai Basin in eastern China(Fig.1).This holds particularly for the Eocene Shahejie Formation,a lacustrine unit which is subdivided into four members.Particularly Member 3 of this formation is highly interesting from this point of view, because it accumulated with a high sedimentation rate, due to the abundant supply of particles from several provenance areas.Two large, more or less parallel, deltas prograded almost continuously toward the lacustrine center.Both developed thick sloping delta foresets(Rao et al.,2004).Sediment gravity-flows of different types, as well as bottom currents were the main depositional agents (Carvalho and Vesely, 2017; Yang et al., 2014, 2015; Liu et al., 2020).

Although earlier studies have contributed much to our understanding of the architecture of delta bodies and the types of sediments that built them (e.g.,Galloway, 1975; Miall, 1979; Fagherazzi, 2008;Anthony,2015),little is still known about the extent of the various units in prograding deltas, particularly regarding the extent and facies changes of gravity-flow deposits on delta fronts and in the basin plain in front of deltas (e.g., Haughton et al., 2009; Fan et al.,2018), which is also well known from the Bohai Bay Basin(e.g.,Xian et al.,2018).Based on the analysis of well logs, cores and seismic profiles, the present contribution analyzes the sedimentary characteristics of the succession in the area where the two deltas of the Shahejie Formation converged, and establishes a detailed sedimentary model for this feature.This study was carried out to better understand the processes involved in the convergence of prograding deltas, and to facilitate more effective sedimentary investigations into potential hydrocarbon reservoirs and their quality in front of deltas.

2.Geological setting

The development of the Eocene converging deltas in the Dongying Sub-basin can be understood well only if the various controlling factors are all taken into account.These concern the structural setting, the stratigraphic context and the sedimentological development.

Fig.1 Geographical and structural setting of the Dongying Sub-basin.A) Location of the Bohai Bay Basin in China; The map of China is modified after the Standard Map Service of the National Administration of Surveying,Mapping and Geoinformation of China(http://bzdt.ch.mnr.gov.cn/)(No.GS(2022)4307);B)Location of the study area within the Dongying Sub-basin;C)Schematic structural map of the Dongying Sub-basin and its surroundings.The crossing red lines indicate the seismic-reflection sections shown in Figs.9－11.Scale bar (upper left)concerns Fig.1C.

2.1.Structural setting

The Dongying Sub-basin is located in the southeastern part of the Jiyang Depression, which forms part of the Mesozoic and Cenozoic Bohai Bay Basin(Ni et al.,2011).The sub-basin measures 90 km from east to west,and 65 km from north to south,thus covering an area of about 5700 km2.It is surrounded by the Chenjiazhuang Structural High in the north, the Qingtuozi Structural High in the northeast, the Weibei Structural High in the southeast, the Guangrao Structural High in the south,and the Binxian Structural High in the northwest (Fig.1C).It is a lacustrine rift basin,bounded by a fault in the north and with stratigraphically overlapping sediments in the south(Fig.1C).The basin is tectonically subdivided into units that represent a steep northern slope, a gentle southern slope, the Lijin Low,the Niuzhuang Low,the Lizezhen Low, the Boxing Low and the Central Fault Belt(Fig.1C).The lake in the Dongying Sub-basin was deeper in the west than in the east,and also deeper in the north than in the south,resulting in a depositional center in its northwestern part.Obviously, these differences in depth controlled, in combination with the sediment supply from the highs around the basin, the sedimentary patterns within the lake (cf.Liu et al.,2020).

2.2.Stratigraphic characteristics

The Paleogene in the Dongying Sub-basin is built,from bottom to top, by the Kongdian Formation, the Shahejie Formation and the Dongying Formation(Fig.2).

The Shahejie Fm.,which is the focus of the present contribution,comprises four units,from bottom to top indicated as the Es4, Es3, Es2and Es1Members (Zhou et al., 2010; Liu et al., 2020).The lower part of Member 4 (Es4) consists mainly of alternating mudstones and siltstones, whereas the upper part is dominated by oil shales.The lower part of Member 3(Es3) is dominated by fine sandstones and mudstones;the middle part is built by mudstones and oil shales,and the upper part consists of siltstones and mudstones with intercalated oil shales.Member 2 (Es2) is characterized by alternating mudstones and sandstones,with a small number of conglomerates.The lower part of Member 1 (Es1) consists of alternating mudstones and siltstones,whereas the upper part is dominated by mudstones and oil shales.

Fig.2 Stratigraphy and depositional environments of the Paleogene in the study area.

Member 3 is an important source rock of hydrocarbons(e.g.,Guo et al.,2011;Li et al.,2019).It has been sub-divided into three sub-members: a lower, a middle and an upper one, coded Es3l, Es3mand Es3u,respectively (Fig.2).Our study indicates that nine phases of delta development can be distinguished in the Es3mSub-member; they represent changing environmental conditions and consequent sedimentary processes, forming cyclical transitions from mudstone to sandstone to mudstone.Sub-member ES3l, which is some 400 m thick, consists mainly of alternating mudstones and oil shales,with minor intercalations of limestones.Sub-member ES3m, which is 400－500 m thick, is composed of mudstones, argillaceous siltstones and calcareous mudstones, with thin intercalations of sandstones; this sub-member is the main subject of the present contribution.Submember ES3uis dominated by mudstones but contains also argillaceous limestones; it is ～400－450 m thick.

2.3.Sedimentary evolution

The Dongying Sub-basin represents overall a lacustrine system in which deltas and fan-deltas developed.The largest ones are the Dongying Delta in the east and the Yong'an Delta in the north-east.Investigation of particularly these two deltas has provided a wealth of data regarding both their sedimentological and their reservoir characteristics(Rao et al.,2004; Chen et al., 2016).

The Dongying Delta extended from east to west along the long axis of the Dongying Sub-basin.The sediment was supplied mainly by rivers with a catchment area east and southeast of Weibei, on the Guangrao Structural High (Fig.1).The Yong'an Delta extended from northeast to southwest, and was built by sediments derived from the Qingtuozi Structural High (Fig.1).These two deltas converged and eventually merged during the early Eocene.The area where this happened is almost entirely situated in the southern part of the Central Fault Belt and in part of the Niuzhuang Low (Fig.1).

3.Methods and data collection

Logs of 330 wells, discontinuous but mutually overlapping cores from 30 wells(with a total length of approx.1200 m) and 15 seismic profiles (with a total length of some 450 km) have been analyzed.The lithofacies were interpreted mainly on the basis of the cores in combination with borehole logs from noncored wells.This allowed obtaining a complete picture of the lateral and vertical development of this sub-member.

The thicknesses of the sandstone bodies and the sandstone/mudstone ratios were calculated and statistically handled on the basis of the obtained lithofacies interpretations.By combining these data with the interpretations of the seismic profiles, isopach maps of the various lithofacies units were constructed and the spatial extent of the sedimentary facies was determined for each of the nine developmental phases of Sub-member ES3m, thus representing the delta development of this sub-member from its beginning to its end.Finally, a 3-D sedimentary model of the two converging prograding deltas was constructed.This all is presented here in a somewhat schematic way,because a more detailed reconstruction of the development on the basis of all logs, core descriptions and seismic profiles would require far more than a complete issue of this journal.

4.Depositional environments

Numerous studies (e.g., Qiu et al., 2001; Wang et al., 2015; Chen et al., 2016) have already made clear that the Paleogene in the Bohai Bay Basin represents a lacustrine setting with delta development.Sections from representative cores that provide more insight into the precise environmental conditions have been selected for the present contribution.

The core analysis and seismic data indicate that the sediments in the study area represent mainly delta plains, delta foresets, a shallow-lacustrine area in front of the delta that is still under the influence of the inflow of fresh river water (here indicated as the prodelta) and an intermediate to deep lake farther away from the delta, where the influence of the river is negligible,although this area could still be reached by delta-derived turbidity currents, which results in alternations of autochthonous muds and(commonly fine sandy and muddy)turbidites.Gravity-flow deposits are consequently not only common on the delta slopes but also in the basin in front of the deltas and alongside the delta lobes.

It is emphasized here that only a combination of all well data made it possible to obtain a detailed insight into the distribution and architecture of the various lithological units.This allowed unraveling the complex architecture of the interfingering facies of the two merging deltas.

4.1.Subaerial delta-plain facies

Delta-plain deposits can be distinguished in several cores.An example are the sediments in well W120 at depths between 2004 and 2008 m, which consist mainly of fine-grained sandstones of 0.5－1.3 m thick that scour the underlying layers and show small troughs (Fig.3); they are characterized by three upward-fining units.Thin layers(approx.10－15 cm)of low-quality coal in situ are intercalated (Fig.3);similar coal occurrences are also found in several other wells; a well-developed example is present in well X301 (Fig.S1 in the supplementary material).

The sandstones with increasing mud content toward the top reflect the classical gradual infilling of distributary channels.The scoured lower boundaries,the trough cross-bedding in the sandstones and the laminae in their muddy top parts reflect a decrease of the current energy.The coal reflects very shallow water or even subaerial conditions,but no indications of roots in situ have been found.All these features are characteristic of a delta plain (Fig.3), with distributary channels, interdistributary bays and a peat bog.

Fig.3 Representative section through a subaerial delta plain dominated by distributary channels (stage S3).Well W120.P1 cores:2003－2006.5 m; P2 cores: 2007－2010 m.

Relatively coarse deposits between finer-grained overbank or swamp deposits that penetrate the natural levees are crevasse-splay deposits(Fig.4);they are dominated by argillaceous siltstones, silty mudstones and fine sandstones,occasionally with an upward-fining tendency.The natural levees alongside the channels consist mainly of argillaceous siltstone and silty mudstone.They show normal grading, horizontal lamination and current ripples.Levees,crevasse splays and peat bogs are well-developed in well X301(Fig.4).

Alternations of sandstones deposited in distributary channels and of silty mudstones and muddy siltstones building natural levees are present in well L64(Fig.S2), indicating the meandering character of the river branches on the subaerial delta plain.

4.2.Subaqueous delta-plain and delta-front facies

Characteristic subaqueous delta-plain facies are present in well S126(Fig.5).The core is dominated by argillaceous siltstones, siltstones and fine sandstones,intercalated with mudstones, shales and silty mudstones.The succession contains three upwardcoarsening sequences, each representing the deltaic sequence of a river arm.Ripple cross-bedding and flaser bedding occur in the intermediate-to-thick sandstones.The alternation of sands and muds frequently caused instabilities, resulting in numerous soft-sediment deformation structures (Fig.S3).These are not restricted to this well(S126),but occur also in the cores of many other wells (see, for instance,Fig.S4).

Fig.4 Representative section through a subaerial delta plain with channels, peat bogs and crevasse-splay deposits (stages S3－S4).Well X301.P1 cores: 2733－2736.7 m; P2 cores: 2736.7－2740.3 m.A: core surface; B－D: cross-sections through cores at some representative intervals.

Well S126 shows a well-developed example of the joint occurrence of the various facies that build the subaqueous delta plain: distributary channels, levees and depressions with interdistributary bays with lakes and marshes can be distinguished (Fig.5).The distributary channels form a system of subaerial channels passing into subaqueous ones.Due to the decreasing current velocity, the sandstones gradually fine upwards and the content of mud increases (Fig.5).

The subaqueous delta plain changed gradually into delta foresets (delta front).Although the well shows three coarsening-upward sequences, the overall vertical succession shows upward fining, but also thickening of the sandstone bodies within each upward coarsening cycle,which suggests that the progradation of the delta front was occasionally interrupted; an alternative explanation is migration of the delta-front lobes during their construction.

Most of the subaqueous delta plain is characterized by upward fining sandstones that were deposited in distributary channels, by upward coarsening sandstones that formed during the development of mouth bars,and by fine-grained deposits that accumulated in depressions.In some sand bodies, the lower upwardfining part is interpreted to have formed in subaqueous distributary channels, whereas the upper coarsening-upward part was deposited as a mouth bar,thus indicating progradation of the delta and an upward shallowing delta front.

At the transition to the prodelta plain, where the inclination of the sedimentary surface diminished,some sediment gravity flows halted.Turbidites and debrites with intraclasts are intercalated here between the other delta-front sediments, sometimes as fairly thick and chaotically-looking levels consisting almost entirely of intraclasts(Fig.S5),sometimes with only a few intraclasts,which commonly are deformed,however (Fig.S6).In addition, slumps are present(Fig.S7).

4.3.Prodelta and deep-lacustrine facies

As indicated above, the inclination of the sedimentary surface on the deltaic foresets gradually decreases, and where this inclination becomes negligible, the prodelta starts.This area is, however,still under influence of the supply of fresh water.With increasing distance from the delta, the lake becomes deeper and the influence of the delta is there present only in the form of turbidites and debrites that reach this area where continuous autochthonous mud deposition took place.

Fig.5 Representative section through a subaqueous delta plain with channels, levees and mouth bars (stage S3).Well S126.P1 cores:3427－3435 m; P2 cores: 3435－3443 m; P3 cores: 3443－3447 m.

In well W541, the sediments (Fig.6) consist of alternating gray,deep-gray,light-brown and off-white fine-grained sandstones, siltstones, muddy siltstones and silty mudstones.The sandstone layers are less than 1 m thick.Both the sandstones and the mudstones show upward-fining intervals (Fig.S8), and climbing ripples, flasers, horizontal lamination and various types of soft-sediment deformation structures.

A comparable succession is present in well L64,which shows even more deformations, particularly load casts (Fig.S9).The common presence of carbonized plant fragments(Ma et al.,2015)in the relatively rare thick sandstones in this core (Fig.S10) indicates that hyperpycnal flows, carrying fresh-water plant debris,reached the prodelta area;the plant fragments settled on top of the hyperpycnites.Farther away from the coast (well H166), gravity flows reached the prodelta area only occasionally (Fig.7).

Floating mud intraclasts are common in many sandstones from well S115 (Fig.8), suggesting transport by debris flows(cf.Li et al.,2016).Such transport is supported by the occurrence of sand balls with tails(Fig.S11) that are interpreted as pseudonodules originated in sediment that was later broken-up during downslope mass flow.

All cores described above are interpreted as gravity-flow deposits, such as turbidites and debrites,intercalated between autochthonous mudstones.They form lobes that extend in front of the delta systems,but that also are directed sideward,thus widening the deltaic complexes and consequently contributing to the merging of the two delta systems under study.

The turbidites consist of commonly fine sandstones and mudstones, mostly grading into silt; they show groove casts and flute casts(Fig.S12)at their base(cf.Callec et al.,2010;Li et al.,2016).Locally they show severe soft-sediment deformations (Fig.S13).The autochthonous mudstones are horizontally laminated;they do not contain any sand, indicating a quiet depositional environment that is interpreted as deep lacustrine.The dark color of these mudstones suggests reducing conditions.

5.Depositional conditions

The sediments in the cores are mainly fine-grained,implying that either the energy in the environment was too low to transport coarse grains, or that the transport distance from the source area to the depositional sites was long.The mainly off-white to deep-gray color of the mudstones, occasionally with intercalated bronze-colored levels, indicates that the conditions were commonly reducing,but that the deeper parts of the lake occasionally became oxidizing due to the influx of huge amounts of oxygen-rich river water by hyperpycnal flows during high-discharge phases.

The high sedimentation rate(see also Yang and Van Loon, 2016; Yang et al., 2017a,b; Liang et al., 2018)allowed the two delta systems to prograde, thanks to ongoing supply from the Weibei High and the Qingtuozi High.The deltas developed westwards and southwestwards, respectively, and eventually merged.

6.Seismic data and interpretation

Seismic-reflection data reveal the internal architecture of the delta complex.Both the lower and the upper boundaries of Sub-member Es3mof the Shahejie Formation in the study area form strong and continuous seismic reflections.In the western part of the basin, the layers between the middle and the top reflections tend to be parallel and sheet-shaped; in the eastern part (left side of Fig.9), these reflections(Fig.9A and B) indicate a thicker sedimentary development in the more central part of the basin.The seismic profile shows eight boundaries; these reflect the boundaries between the nine developmental stages of the deltas (S9 to S1).The boundaries in the seismic profile suggest that the delta progradation was controlled by tectonic activity,basin topography,lakelevel fluctuations, and sediment supply.

Along the line from well W231 to the lake center,the seismic reflections show a break in the sedimentary slope, and the sedimentary succession becomes significantly thicker at the foot of the break zone(Fig.9).This indicates that the accumulation of the lobes at the delta front took place only below the slope-break zone.This must be ascribed to the depth of the lake under the slope break, the larger accommodation space and the large sediment supply.Under the slope-break zone, not only reaches Sub-member Es3mits largest thickness, but also the sand body of the delta front is the best developed: it has more layers, and both the individual layers and their cumulative succession are thickest.For example, the total thickness and the thicknesses of the individual sandstone layers are significantly larger in well N481(Fig.9C and D) than in the adjacent wells N32 and G101.

Fig.6 Representative section through a prodelta plain with turbidites and debrites dominating over autochthonous mudstones (stage S7).Well W541.P1 cores: 3042－3049 m; P2 cores: 3049－3057 m; P3 cores: 3057－3065 m.

Fig.7 Representative section through a prodelta plain reached only occasionally by debris flows and turbidity currents(stages S4－S5).Well H166.P1 cores: 3117.2－3121.1 m; P2 cores: 3121.1－3126 m.

The delta plains contain numerous distributary channels and estuarine bars, as is common in this environment.The sand layers thin toward the end of the delta plain, and the logging curves of the sediments here commonly show thin sand bodies, sometimes represented by turbidites, such as in well N481(Fig.9C).

The progradation of deltas during their nine developmental stages in a fairly western part of the study area(from just SE of well W53 to just NW of well N111) can be traced in a seismic profile (Fig.10) that shows foresets and changes in the thicknesses.A more proximal seismic profile (Fig.11) shows faultcontrolled deposition and progradation (in the form of foresets) during the same nine developmental stages from SE of well W7 (where the profile is subparallel to the current direction) to NW of well X158(where it is perpendicular to current direction).

Although the numerous faults near well X 158 obscure the overall picture, it is obvious that the thicknesses of the various units change,which must be ascribed to the fact that the section is here perpendicular to the main current direction in subaqueous distributary channels.This implies a fairly unique feature:different current directions are reflected in a single seismic profile which follows a straight direction.These different current directions must be explained by the interfingering of two deltas: one prograding from east to west, and another one from northeast to the southwest.This forms indisputable evidence of the gradual merging of the two delta systems,which occurred side by side rather than head to head.

7.The merging process

Fig.8 Representative section through a prodelta plain reached by debris flows and turbidity currents (stage S2).Well S115.P1 cores:3043.3－3047.3 m; P2 cores: 3047.3－3051.2 m.

The merging process of the two deltas can be reconstructed on the basis of the numerous cores from exploration wells, in combination with seismic data and logging data.The merging took place in nine stages,named S9－S1 from bottom to top.These stages can be recognized both in the seismic profiles and in the cores, presumably because of (relatively short)phases of non-deposition or even erosion,which might be explained by phases without delta progradation or even some(limited)retrogradation.The nine stages of merging are shortly described in section sections 7.1－7.9; the data behind this short description are provided in section 7.10 in order not to interrupt the description of the merging process.

It is, obviously, impossible to describe or even to depict all cores from all boreholes here.Consequently,we must summarize the development of the delta amalgamation on the basis of cores from representative wells, which are indicated in red in Fig.12 and(more easily findable) Fig.13.The amalgamation process could be reconstructed in detail by correlation of the successions in the numerous boreholes, in combination with seismic-reflection data that provide a complete picture of the delta architecture.We restrict ourselves for the sake of brevity to the development of the merging deltas along two longitudinal seismic profiles (Fig.9) and two profiles perpendicular to the main current direction (Figs.10－11; for location, see Figs.1 and 13).

7.1.Stage S9

When stage S9 of the merging process started(Fig.12A), the lacustrine basin was large and deep,gradually shallowing toward the margins.In the easternmost part of the study area, in well L64 (Figs.12 and 13), where eventually a subaqueous delta plain (Fig.5) and later, when the delta propagation had succeeded farther, a subaerial delta plain would develop with deep distributary channels between levees (Fig.S2), with crevasse-splay deposits (Fig.4)and peat bogs resulting in coal seams (Fig.S1), the position was still far away from the river mouths and deltas.

Fig.9 E－W running sections through the merging delta complexes.A)Seismic-reflection profile showing the sedimentary succession of the deltas in nine stages(S9－S1);B)Interpretation of seismic profile A;C)A second seismic-reflection profile;D)Interpretation of seismic profile C.For locations, see Fig.1C.

Fig.10 Seismic profile(A)and its interpretation(B),showing the progradation of the two deltas during their 9 stages from the right of well W53 (SE part of the section) to the left of well N111 (NW part of the section).The lower and the upper boundaries of Sub-member Es3m are both shown as thick light-brown lines.For location, see Fig.1C.

Farther to the west, the situation was different:during stage S9,the area around well L64 still formed part of the extensive, commonly quiet, deeplacustrine environment where mud could settle.The mostly quiet conditions were frequently interrupted,however, by turbidity currents that left fine-sandy turbidites that caused loading into the autochthonous muds (Fig.S9).In spite of the large distance from the then delta, the basin around well L64 was also reached by hyperpycnal flows.These carried plant remains with them, which were deposited on top of the hyperpycnites (Fig.S10), indicating that the influence of the rivers reached far into the deep-lacustrine realm.

Fig.11 Seismic profile(A)and its interpretation(B),showing the faults and progradation(in the form of foresets)during their 9 stages from the left of well W7(SE part of the section)to the right fault(sub-parallel to the current direction of the southern delta)to well X158 in the NW of the section, where the section runs perpendicular to the current direction of the northern delta.The lower and the upper boundaries of Sub-member Es3m are both shown as thick green lines.Although the numerous faults near well X 158 obscure the overall picture,it appears that the thicknesses of the various units change there, which must be ascribed to the migration of the subaqueous distributary channels, in combination with the lateral merging of the two deltas.For location, see Fig.1C.

Tectonically, the Dongying Sub-basin remained fairly stable,resulting in a limited supply of sediment,mainly from the elevated areas in the east.The Dongying Delta in the east and the Yong'an Delta in the northeast developed only slowly and only in the eastern part of the basin, with delta fronts that were built for a significant part by gravity flows.The two deltas remained still clearly separated morphological units, with (mostly subaerial) delta plains (green in Figs.12 and 13) that were separated from each other by some 10 km or more, while their subaqueous parts(yellow in the figures) were also still clearly apart,with several kilometers of lake bottom sediments(blue in Figs.12 and 13) in between.Some mouth bars developed in the areas between channels, and some turbidite fans were built in the lake in front of the delta.

7.2.Stage S8

Also during stage S8(Fig.12B),the subaerial plains of the two delta systems remained still far (～10 km)away from each other.The subaqueous parts of the delta prograded relatively little, but sideward directed gravity flows caused these deltaic parts to approach each other to some 2 km in the proximal part,though‘only’to some 5 km more distally.Mouth bars continued to develop, and new turbidite fans developed.

The development is well shown by cores from wells that are located some 15 km more to the west than well L64.These are(see Figs.12 and 13)well X301,in the area that was during stage S8 situated in front of the Yong'an Delta,and wells W541,W231 and W120 in front of the Dongying Delta.The N－S trending zone of these wells (perpendicular to the delta progradation)would eventually form part of the delta plain(see Figs.3, 4 and 13) with significant plant (later coal) accumulation(Fig.S1),but was during stage S8 still part of the prodelta area where debrites and turbidites(Fig.6 and S8) were deposited in a succession in which the slow accumulation of autochthonous mud (Fig.6) was most common in time, but where sediment gravity flows deposited much higher volumes of sediment,thus building lobes that caused the deltas to approach each other.

Fig.12 Palaeogeographical development of the study area during stages S9－S2.A)Stage S9;B)Stage S8;C)Stage S7;D)Stage S6;E)Stage S5; F) Stage S4; G) Stage S3; (H) Stage S2.

7.3.Stage S7

During stage S7 (Fig.12C), the subaerial parts of the two deltas remained still clearly separated from each other, but the subaqueous delta plains merged already in the most proximal part.Due to differences in the sediment supply,the southern delta prograded a bit faster than the northern one, thus extending farther westwards than the northern delta.The number of mouth bars increased in the southern delta,whereas they disappeared almost completely in the northern one.In both deltas the sizes of the interdistributary bays increased significantly because levees became well-developed along the river arms.In front of the deltas, turbidites extended ever farther westward into the prodelta plain.

Fig.13 Final palaeogeography of the study area during stage S1.

It is remarkable that the westward progradation of the deltas resulted in deposits of a fairly regular thickness during this stage for the entire distance from the easternmost part of the study area(well L64)to the N－S trending belt described for stage S8,but changed west of this belt.The longitudinal section(Fig.9)shows a clearly decreasing thickness of the deposits of this stage toward the west,from well G101 via well N32 to well N481 (Fig.9).A similar fairly sudden thinning is visible for stages S9 and S8 (Fig.10).This must be ascribed to the large distance from the delta front,where most sediment gravity flows originated,so that it must be deduced that most flows stopped already east of this zone(cf.Li et al.,2016).When the deltas prograded farther, the thinning stopped, because thick delta fronts could build up.

7.4.Stage S6

During stage S6 (Fig.12D), both deltas prograded more rapidly westwards and the lobes of the delta front, built largely by gravity-flow deposits, spread also ever more sideward.This led to merging of the subaerial parts of the deltas in the eastern part of the study area,for instance visible in the cores of wells L64(Fig.S2),W120(Fig.3)and even well X301(Fig.4),and to further merging of the subaqueous part.Yet, a remnant of the original lake was still present as a large interdistributary bay.In contrast, the other interdistributary bays became smaller, probably because the apparently increased sedimentation rate reflects periods of higher discharge, so that the levees could be breached and/or overflown and sediment could be deposited in the bays.The merging led in several parts of the deltas to interfingering of the sediments supplied by the two river systems.The sediments of the turbidite fans deposited in front of the two deltas were, however, still compositionally characteristic of the different source areas.

The development is shown by the different facies in the cores from different locations, particularly in the Dongying Delta deposits.The foresets of this delta reached the N－S trending zone with wells Y691,H166,N11 and,some 3 km to the west,well S134.Cores from the northernmost of these wells (Y691), just south of the remaining bay between both deltas, show slump structures at the foot of the delta slope(Fig.S7).More to the south, well H166 shows alternating autochthonous mudstones, turbidites and debrites(Fig.7),well N11 shows debrites and slumps(Fig.S6),and well S134 shows debrites at the foot of the delta slope (Fig.S5) and (deeper) turbidites with welldeveloped groove casts (Fig.S12), thus proving the gradual but irregular forward progradation and sideward building out of the Dongying Delta, mostly by deposition from sediment gravity flows that originated on the delta fronts due to failure of the quickly steepening sedimentary surface.

7.5.Stage S5

Stage S5 showed less rapid changes, though the deltas continued growing, particularly sideward(Fig.12E).The two originally separated subaerial delta plains started to form an entity, with interfingering sediments, but did not yet form a morphological unit with a typical delta shape,as the already existing large interdistributary bay still remained present between the two subaerial delta plains.Due to continuing sideward directed gravity flows, the subaqueous parts of the deltas almost merged, but a last remnant of the lake bottom remained present in between,so that the subaqueous deltas still formed separate entities.The subaerial and subaqueous parts of the delta plain became ever more developed, with clearly different subenvironments, and some sandy turbidites were deposited on the upper part of the delta fronts, suggesting that the inclination of the sedimentary surface was locally so small that the flows could come to a halt.

The development is well illustrated by the cores from well S126 (Fig.12).They show fairly chaotic debris flows (Fig.S3) but also the subaqueous part of the delta plain with channels, levees and mouth bars(Fig.5),suggesting a position close to the delta front.This implies that the outer end of the delta complex had already reached this area.At the same time, the more sideward positioned areas remained parts of the prodelta plain, where turbidites and debrites were deposited between autochthonous muds (Fig.8).The sediment gravity flows show several types of softsediment deformation (Fig.S11).Also, the area west of the deltaic tongue (well S122) was not yet reached by the delta itself,but by gravity flows(Fig.S13)that originated on the delta front to the east.The delta plain, with thick channel deposits (Fig.3), was situated at this time some 20 km to the west, in the N－S trending belt described for stage S8.

7.6.Stage S4

Stage S4 shows a further gradual growth of both deltas (Fig.12F), and the fronts of both deltas joined completely, with sediments from both rivers forming occasionally interfingering but more often mixed sediments.However, the shapes of the two original delta complexes are still discernible in the subaerial part,where numerous peat bogs started to develop,particularly in the southern delta.

The delta fronts profited from the apparently again increased supply of sediment by both rivers, resulting in comparably thick deposits largely formed by gravity flows.The source of the individual turbidite fans in front of the deltas can no longer be recognized on the basis of their location,but must be determined on the basis of their composition.This marks the true merging process,which reflects a perfect ancient equivalent to what is known from the Quaternary deltaic development in The Netherlands, where the Rhine and Meuse rivers merged, and where the influence of both rivers in the regional stratigraphy can be distinguished well only on the basis of their differences in the content of heavy minerals(Kasse,1990;Westerhoff et al.,2008).

7.7.Stage S3

The ongoing forward and lateral growth of both deltas during stage S3 resulted in a sedimentary body that ever more started to look as a single delta complex(Fig.12G)as far as the subaerial and subaqueous parts are considered as an entity.This points at a significant ongoing sediment supply.It is therefore remarkable that the subaerial parts of both deltas had not merged truly more than in the previous stage,and that the large interdistributary bay between the two delta systems still had an almost unchanged size.On the other hand, the subaerial parts of the delta plain prograded relatively rapidly, resulting in deposits as far westwards as well S126.

Whereas the turbidite fans in front of the merged delta complex diminished in size and number,possibly because the sediment gravity flows became directed more sideward than forward, the peat bogs in the subaerial part increased in number and size (Fig.13).This must be ascribed to the large sediment supply,which was mostly‘trapped’on the subaqueous part of the delta.This also explains the thick succession that was formed during this stage.

7.8.Stage S2

During stage S2, the Dongying and Yong'an deltas joined completely (Fig.12H).Because the sediment supply to the Dongying Delta was largest, this part of the joint delta complex showed the fastest westward progradation, but in spite of the apparently considerable sediment supply,the subaerial part of both deltas remained still separated from each other, due to the survival of the large interdistributary bay at their mutual border; this long survival can be explained satisfactorily only by the absence of river branches in both delta systems that reached this area.

Turbidites accumulated again, more than during stage S3, in front of the deltas, probably because the full mergence of the delta fronts left no more space for sideward transport.

7.9.Stage S1

The final stage of the deltaic development during accumulation of the Es3mSub-member of the Shahejie Fm.can,obviously,be reconstructed best on the basis of cores from the most westernmost wells.During this final stage (S1), the subaqueous part of the merged delta system reached a ‘classical’ delta shape(Fig.13).The subaerial parts remained separated,however, but extended farther basinward.A large number of mouth bars developed, particularly in the area that was predominantly fed by the southern river,and elongated interdistributary bays originated also mostly in this part of the delta.This stage may be considered as the final stage of the merging, which caused complex interfingering and mixing of sediments supplied by the two river systems.

7.10.Complex character of the delta development

The above sketch of the delta development could,for the sake of brevity,only indicate the main trends.Like in modern deltas,phases of net deposition and net erosion alternated, complicating the overall picture.The cored successions show these alternations of progradation and retreat of the delta clearly.This is,as an example, well visible in the cored succession of well S126.During stage S3, which represents a stage of significant progradation,the area around the well was situated in the transitional zone between a subaqueous delta plain and the prodelta basin floor.This is expressed in the cores (Fig.5) by the dominance of sediments that represent the delta plain, together with(much less frequent)prodelta deposits.The delta foresets, where the turbidites most probably originated,are hardly recognizable,probably because they were exposed to severe wave erosion.This is supported by the fact that the delta had retreated temporarily at the same location during stage S1, so that debrites could be deposited(Fig.S3)at the foot of the delta foresets.

Similar complicating developments took place during all stages of delta development.They could be reconstructed on the basis of thorough analysis of the approx.1200 m of cores that were available for the present study.Because the present study aims at providing insight into the amalgamation process of the two deltas involved,we consider the complications in the delta development, however interesting from a sedimentological point of view, out of scope here.

8.Depositional model

A 3-D depositional model has been established(Fig.14) with the objective to help understand the sedimentary setting and its development (cf.Yang et al., 2010; Liu et al., 2016; Pang et al., 2017;Zhang et al., 2017).The main source areas of the deltas in Sub-member ES3min the study area were the Weibei High in the southeast and the Qingtuozi High in the northeast (Fig.1).The region was high in the southeast and northeast, but low in the central and western parts of the study area where the development was controlled largely by a series of northeastwards dipping normal faults and by the ENE-WSW trending Central Fault Belt of the Dongying Sub-basin(Fig.1).The area between the Weibei High, an ancient land mass in the southeast,the higher part of the Qingtuozi High in the northeast and the westward extending basin,formed the main part of a delta plain(Figs.12 and 13).From the source areas to the center of the basin, this configuration resulted, schematically, in a subaerial delta plain, a subaqueous delta plain in a shallow littoral lake, a delta front, a turbidite-dominated fan built mainly by mass-flow deposits (cf.Liu et al., 2017), and a prodelta area within a deep lake (Figs.12 and 13).Changes in the tectonic activity and sediment supply caused fluctuations of the lake level;it was highest during the middle part of Sub-member ES3m(Fig.2).Because the accommodation space in the fairly shallow water around the delta front was limited, the front of the delta could prograde rapidly(Meng et al., 2015).

A break in the depositional slope developed on the delta front(Fig.9).The steeper slope downwards of the break facilitated the rapidly deposited, watersaturated slope sediments to become unstable (cf.Mulder et al., 2003; Zou et al., 2012; Sumner et al.,2013; Baas et al., 2014; Sawyer et al., 2014; Sun et al., 2014; Gong et al., 2015, 2016; Fan et al.,2018), resulting in sediment gravity flows (Figs.6－8).These mass flows slowed down mostly at the base of the slope or within the more distal parts of the sub-basin,where the slope was no longer sufficiently steep(Fig.9).At these places,the sediments carried along by the flows accumulated rapidly, resulting in a large amount of particularly sandy turbidites in the lobes of the delta front(Figs.12 and 13)(Wang et al.,2013).

Fig.14 Sedimentary model of the converging and eventually merging deltas (modified after Chen et al., 2016).Top of the figure is east;bottom is west.

The continuous accumulation of sediments decreased the depth of the lake,particularly along the lake margins, with the consequence that the vertical succession shows a transition from deep-lacustrine fine-grained sediments (Figs.6 and 8) to shallowlacustrine and delta sediments, including those accumulated in distributary channels and interdistributary bays (Figs.4, 5 and 13).The lateral and forward growth of the deltas and the resulting shallowing of the lake also caused that the hydrodynamic conditions transformed into higher-energy ones.The delta fronts changed occasionally positions (Figs.12 and 13), due to fluctuations in sediment supply and tectonic activity that affected the lake level.

9.Implications for hydrocarbon exploration

Much sandy sediment was deposited on and near the delta plains, particularly in the form of sand bars and channel fills of subaqueous distributary channels.Like in all ‘classical’ deltaic sequences, the average grain size increased gradually from bottom to top.This picture became complicated when the lateral lobes of the two deltas reached the same area: the resulting sediment of the joined deltas shows,obviously,mixed characteristics that represent the different properties of the two source areas(Figs.5 and 6).Also due to the mutually independent fluctuations in water discharge,sediment supply, and changing positions of the channels in the delta plain, lobes developed into different directions (Figs.12 and 13).

Consequently,the architecture of the deltaic body in the area where the two deltas converged is much more complicated than in deltas formed by one single river:the sand bodies in the merging delta lobes which have a more complex architecture than in most deltas,are thinner (Fig.5) and have a wider distribution(Fig.12).However, where several sand layers are stacked upon each other, the resulting sand body can be quite large, and such a sand body may form a favorable oil and gas reservoir (cf.Stow and Johansson, 2000; Fan et al., 2017).Such sand bodies are often in direct contact with muds deposited in interdistributary bays (Fig.5) that contain much organic material and that may well be good source rocks.Moreover, such muds may seal the sandy reservoir rocks.Consequently, the zone where two deltas merged forms a potentially important target for oil and gas exploration.The palaeogeographical development of delta bodies, particularly if several deltas occurred closely together, is therefore of prime interest in the context of hydrocarbon exploration.

10.Discussion

Little is still known about the convergence and eventual merging of deltas in the geological past.It seems therefore worthwhile to discuss here what the precise conditions were, and how the sedimentary development depended on these conditions.

The distribution of the sediments in the subaqueous area where the two deltas merged was affected by several factors,particularly the tectonic activity,basin topography, water depth, supply of sedimentary particles from the two main catchment areas, and the migration of the deltaic lobes.The development of both deltas from east to west show significant thickening(Fig.9)because the basement of the basin slowly subsided during accumulation.This facilitated both forward and laterally expanding deltas, leading eventually to merging of the sides of the deltas(Fig.9).The configuration of the sedimentary surface of the basin was mainly controlled by tectonic activity and sedimentary filling.The central part of the sub-basin was relatively deep,so that the deltas that developed in the east and the northeast could prograde into this direction, until they eventually merged in the relatively shallow marginal lake.The large supply of sediment allowed the delta fronts to expand both forward and laterally.The two delta fronts could thus merge.

Delta deposits are numerous in both the geological record and modern environments.Several recent deltas have merged by developing sideward directed lobes.This must have happened also frequently in the geological past, and the example described here has provided by far the most detailed data, thanks to the large amount of exploration wells in the relatively small area under investigation.On the basis of cores from this dense network of exploration wells, a sedimentary model could be established that provides an analogue for basin analysis of other basins,and that helps predicting the extent of sand bodies in merged deltas that may be promising targets for hydrocarbon exploration.This emphasizes that it is not only of interest from the point of view of fundamental sedimentology to investigate in detail the palaeogeographical development of delta bodies,but also because of its potentially large economic implications.

11.Conclusions

Deltas in the Dongying Sub-basin could prograde slowly toward the lake center during the Eocene,thanks to a continuous supply of clastics from nearby elevated areas.Two deltas, supplying particles from different source areas and running into slightly different directions eventually merged.

Various types of sand bodies developed on the delta plain and the delta front,as well as in the shallow and deep parts of the lake.Sediment gravity flows ran from the deltaic foresets into the basin plain.They included sandy turbidites and debrites.The thickest sand bodies tend to occur on the delta front,but the distribution of the lobes of the delta fronts varied with time due to changes in the supply of clastics, the tectonicsinfluenced topography, the lake level and the accommodation space.These sand bodies form promising targets for hydrocarbon exploration.The findings presented here are particularly important because they show not only the hydrocarbon potential of lacustrine deposits, but also the potential of tight reservoirs in deltaic settings,as well as the importance of detailed unraveling of the architecture of sediment bodies built by simultaneous supply of sediment from different source areas.As shown by the investigated deltas of the Shahejie Formation, a detailed reconstruction of the palaeogeographical development is one of the best tools that can help unraveling delta architecture.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this contribution.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (grant 41972146).