AUTHORS

Wei Dong � School of Energy Resources,China University of Geosciences, Beijing,100083, China; present address: Department ofGeosciences, Virginia Polytechnic Institute andState University, Blacksburg, Virginia 24061;dongwei_21@163.com

Wei Dong is a Ph.D. candidate of petroleumgeology in the School of Energy Resources,China University of Geosciences in Beijing. Shereceived her M.S. degree in 2008 from theChina University of Geosciences in Beijing. Herresearch interests include sedimentology, basin

Depositional systems andsequence architecture of theOligocene Dongying Formation,Liaozhong depression, BohaiBay Basin, northeast ChinaWei Dong, Changsong Lin, Kenneth A. Eriksson,Xinhuai Zhou, Jingyan Liu, and Yubo Teng

analysis, and sequence stratigraphy. Her recentwork has focused on the sequence stratigraphyand depositional systems of rift basins innortheast China.

Changsong Lin � The Geological Lab Cen-ter, China University of Geosciences, Beijing,100083, China; lincs@cugb.edu.cn

Changsong Lin is a professor of geology in theDepartment of Energy Resources, China Uni-versity of Geosciences in Beijing. He receivedhis M.S. degree in 1984 and Ph.D. in 1988 fromthe China University of Geosciences in Beijing.His research interests include sedimentology,basin analysis, and basin process modeling. Hisrecent work has focused on the sequencestratigraphy and basin-filling dynamics of theMesozoic–Cenozoic basins in northeast andwest China.

Kenneth A. Eriksson � Department ofGeosciences, Virginia Polytechnic Institute andState University, Blacksburg, Virginia 24061;kaeson@vt.edu

Kenneth A. Eriksson is a professor of geology atVirginia Tech. He received his B.Sc. and M.Sc.degrees and Ph.D. (1977) from the University ofthe Witwatersrand, Johannesburg, South Africa.His current research interests include paleo-environmental analysis, sequence stratigraphy,paleoclimatology, detrital zircon geochronology,

ABSTRACT

The sequence-stratigraphic framework and depositional sys-tems of an Oligocene lacustrine rift succession in the Liaozhongdepression of Bohai Bay Basin in northeastern China were in-vestigated using seismic profiles, complemented by well logsand cores. Seven sequences are identified on the basis of un-conformities on basin margins and correlative conformities inthe basin center. Depositional systems are associated with sys-tems tracts within sequences. Lowstand systems tracts are dom-inated by sublacustrine fans; transgressive systems tracts aremainly composed of beach-bar deposits; and highstand systemstracts are characterized by deep lacustrine fan or braid-deltadeposits. Episodic tectonic movement was the principal factorthat controlled the development of the sequences. Lake-levelchanges resulting from climatic oscillations may have controlledthe development of these sequences. The balance betweentectonic subsidence and sediment supply controlled the type oflake basin. The Bohai Bay Basin was a balanced-fill basin in theearly and middle stages of the Dongying Formation and evolvedinto an overfilled basin at the end of deposition of the DongyingFormation. This study enhances the understanding of deposi-tional system configuration and systems tracts in a typical riftbasin and proposes that the most favorable sandstone reservoirsare developed in sublacustrine fan deposits. Relative lowstand

and biostabilization on the early Earth.

Xinhuai Zhou � Tianjin Branch of ChinaNational Offshore Oil Corporation Ltd., Tianjin,300452, China

Xinhuai Zhou obtained his Ph.D. in geologyfrom the China University of Geosciences. Heis now the chief geologist of the TechnologyDepartment of the Tianjin Branch of China

Copyright ©2011. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received April 30, 2010; provisional acceptance September 14, 2010; revised manuscriptreceived December 31, 2010; final acceptance January 28, 2011.DOI:10.1306/01281110074

AAPG Bulletin, v. 95, no. 9 (September 2011), pp. 1475– 1493 1475

National Offshore Oil Corporation Ltd. He hasconducted petroleum geology studies in theBohai Bay Basin for more than 10 yr. Hispublications include studies on petroleum gen-eration, migration, and accumulation in theoffshore area of the Bohai Bay Basin.

Jingyan Liu � School of Energy Resources,China University of Geosciences, Beijing,100083, China

Jingyan Liu is an associate professor in theSchool of Energy Resources, China University ofGeosciences, Beijing. She received her Ph.D.in 2001. Her research interests include sedimen-tary geology, basin analysis and modeling,and interpretation of geophysics data.

Yubo Teng � Tianjin Branch of China Na-tional Offshore Oil Corporation Ltd., Tianjin,300452, China

Yubo Teng works for the Tianjin Branch ofChina National Offshore Oil Corporation Ltd.,and he is now studying for a Ph.D. at theChengdu University of Technology.

ACKNOWLEDGEMENTS

This research is supported by the Natural Sci-ence Foundation of China (grant no. 40072039)and the National Key Scientific and Techno-logical Project (grant no. 2008zx05023-002). Wethank the Tianjin Branch of China NationalOffshore Oil Corporation Ltd. for their supportand for permission to conduct this research.The China Scholarship Council is thanked forsupporting the exchange program with VirginiaPolytechnic Institute and State University. Wethank Wei Wu, Zhifeng Rui, Weicheng Lai, andChenglong Li for their participation in the initialresearch. Ryan Grimm, JoBeth Carbaugh, andChristina Blue are thanked for their informativeand constructive comments. The authors bene-fited greatly from comments by Ashton F. Embryand Richard J. Moiola and an anonymousreviewer.The AAPG Editor thanks the following reviewersfor their work on this paper: Ashton F. Embry,Richard J. Moiola, and an anonymous reviewer.

EDITOR ’S NOTE

A color version of Figures 4–6 and 8–10 may beseen in the online version of this article.

1476 Depositional Systems and Sequence Arch

sublacustrine fan deposits capped by relative highstand prodeltaor deep lacustrine mudstones form the highest potential li-thostratigraphic traps.

INTRODUCTION

Research during the past decade has demonstrated thatsequence-stratigraphic concepts developed for passive conti-nental margin strata also can be applied to the study of lacus-trine basins (Vail et al., 1977; Aitken and Flint, 1995; Lin et al.,2001, 2005; Keighley et al., 2003; Folkestad and Satur, 2008).However, since the development and evolution of lacustrinebasins, especially rift lacustrine basins, which are much morecomplicated than marine basins (Gu et al., 1995), an under-standing of the sequence stratigraphy of ancient rift lacustrinebasins has remained limited. In recent years, improved high-resolution seismic exploration techniques have enhanced thebasin-scale analysis of rift basins and led to an improved un-derstanding of systems tract to reservoir-scale sequence strati-graphy and a more detailed delineation of depositional archi-tecture (Macurda, 1996; Lin et al., 2000, 2002). Recognitionof sequences in rift lacustrine basins (Helland-Hansen andHampson, 2009) has the same significance as in passive con-tinental margins insofar as leading to an improved under-standing of controls on sequence development (Borer andMcPherson, 1996; Eliet and Gawthorpe, 1996; Pietras andCarroll, 2006).

In China, lacustrine basin deposits host important reservesof coal, oil, and gas. Numerous studies have been conductedon the sedimentology and sequence stratigraphy of MesozoicandCenozoic lacustrine deposits in China (Li, 1988, 1996; Linet al., 1991, 1995), but little progress has been made in under-standing the depositional and systems tract configuration ofthese basins. The Bohai Bay Basin, located in northeasternChina(Figure 1), is a typical Cenozoic rift basin developed on thebasement of the north China block (Gardemal et al., 1996; Liand Lu, 2002; Xu et al., 2008). Multistage tectonic events re-sulted in complex sediment input patterns and depositionalarchitecture, and the purposes of this study are to discuss thecharacteristics of rift lacustrine sequences and the evolution ofdepositional systems in the Bohai Bay Basin. Three-dimensional(3-D) seismic data, complemented by well logs and cores, per-mit recognition of sequences and thereby a stratigraphicframework for Bohai Bay Basin. The data also allow for recon-struction of a regional sequence-stratigraphic framework andthe prediction of favorable reservoirs in the basin.

itecture, Lacustrine Basin, China

DATABASE AND METHODS

All data pertinent to this study were provided byTianjin Branch of China National Offshore Oil

Corporation Ltd. These data include several late1980s 2 × 2 km (1.25 × 1.25mi) or 2 × 4 km (1.25 ×2.49 mi) grid two-dimensional (2-D) seismic data,25 × 25 m (82 ft) grid spacing of 3-D seismic

Figure 1. Location of the Bohai Bay Basin in northeastern China. Note that the basin consists of several uplifts and depressions with theLiaozhong depression in the center. AA′, BB′, CC′, and DD′ are seismic profiles. 1 and 2 are structural cross sections. W = well.

Dong et al. 1477

volumedata, gamma-ray and resistivity logs, selectedcore samples from more than 140 exploration andproduction wells, and biostratigraphic and oil-gas distribution data. This subsurface study of theDongying Formation in the Liaozhong depressionis based primarily on regional 3-D seismic data cov-ering an approximately 5400-km2 (2083.25-mi2)area of northeastern Bohai Bay Basin (Figure 1). Theseismic data include a series of surveys acquiredduring the last several decades that were recentlyreprocessed and assembled into one volume. Theresolution that is equivalent to 15 to 30 m (49 to98 ft) from a 30-Hz dominant-frequency seismicdata set is sufficient for reconstruction of a sequence-stratigraphic framework in the region of seismiccoverage outside well control. Well data (wirelinelogs, cores) tied to the seismic survey are used tocomplement and calibrate seismic interpretationsbecause the seismic data may be ambiguous be-cause of their relatively low resolution. Interpre-tation of well logs is critical to the identification ofsequences and systems tracts.Well-log data also areused to interpret depositional systems on the basisof curve shapes. Core data provide the most directand accurate control on the stacking patterns ofdepositional facies. Biostratigraphic data are docu-mented in previous work and used for estimatingstratal age (Chen et al., 1981; Wang et al., 1989;Liang et al., 1992;Wu et al., 2005). Oil distributiondata provide a basic knowledge on factors respon-sible for oil occurrences and prediction of unknownoccurrences.

Themethods used in this study were conductedas follows. First, regional unconformities, which in-dicate depositional hiatuses, were recognizedbased on 2-D and 3-D seismic data. These inter-pretations provide an understanding of the struc-tural style and the composite sequence-stratigraphicframework. After a regionally consistent seismicsequence-stratigraphic framework was constructed,biostratigraphic data were used to calibrate theseismic interpretations by age interval. Second, de-positional cycles were identified based on bothseismic and check shot surveys and synthetic seis-mograms of well-log data. At this stage, systemstracts were defined in relation to maximum flood-ing surfaces that are apparent on seismic profiles as

1478 Depositional Systems and Sequence Architecture, Lacustr

downlap surfaces and as condensed sections in welllogs. Third, depositional systems and facies associ-ations were delineated within the established se-quence framework through the analysis ofwell logsand available core. Finally, controlling factors onsequence development were addressed as well asthe implications for subtle stratigraphic traps toprovide a framework for further exploration.

BASIN SETTING

Tectonic Setting

Bohai Bay Basin, a large oil and gas province inChina, developed in response to compound andcomplex tectonic events (Zhao and Zheng, 2005).Five secondary structural units with northeast ornorth-northeast strike are developed in the north-eastern Bohai Bay Basin, with the Liaozhong de-pression in the center (Figure 1). These structuralunits are bounded by deep master normal faultsand locally by strike-slip faults (Ye et al., 1985;Allen et al., 1998; Hsiao et al., 2004).

The most significant control on extension ofeasternChina iswidely accepted to be subduction ofthe Pacific plate beneath the eastern margin of Asia(Watson et al., 1987; Tian et al., 1992; Zhao andZheng, 2005). Rifting of eastern China persistedfrom the Late Cretaceous to the Oligocene (Chenet al., 1981; Ma et al., 1983; Ren et al., 2002; Zhaoand Zheng, 2005). In common with other rift ba-sins, the tectonic history of Bohai Bay Basin iscomplicated by episodic rifting events, which in-cludes block faultingwith associated rapid tectonicsubsidence and volcanism (Keen andDehler, 1993;Hsiao et al., 2004; Lin et al., 2005). In Bohai BayBasin, the initial rifting stage began in the earlyPaleocene based on a maximum 65-Ma age of ba-salts (Figure 2) (Chen et al., 1981; Yang et al.,2006). The second and maximum rifting episodeextended from the middle Eocene to the lateEocene (Figure 2). A third weakening episode ofrifting followed during the Oligocene (Figure 2).Basin inversion related to reactivation of normalfaults in a transpressional strike-slip regime tookplace after deposition of S1 and particularly following

ine Basin, China

deposition of D1. This style of faulting is especiallyapparent along the eastern margin of the basin in theform of flower structures (Figure 3). The predom-inant rifting style during the Oligocene, and parti-

cularly in the Liaozhong depression, was asym-metrical, leading to half-graben formation with anescarpment to the east and ramp to the west.However, this pattern varies along strike (Figure 3).

Figure 2. Sequence classification and depositional systems of Paleogene strata in the Liaozhong depression (modified from C. Lin,personal communication, 2008). SB = sequence boundary.

Dong et al. 1479

Regional Stratigraphy

The Paleogene stratigraphic record in Bohai BayBasin consists of, in ascending order, the Kongdian,Shahejie, andDongying formations (Figure 2). Ageconstraints on these stratigraphic units are pro-vided by K-Ar isotopic dating from volcanic rocks;biostratigraphy based on palynology, algae, andostracods; and paleomagnetism (Chen et al., 1981;Wang et al., 1989; Liang et al., 1992; Li, 1996; Linet al., 2005; Wu et al., 2005).

The basal Kongdian Formation that uncon-formably overlies Mesozoic basement has similarlithologic characteristics to the overlying S4 Mem-

1480 Depositional Systems and Sequence Architecture, Lacustr

ber; it is dominated by interbedded red-coloredcontinental mudstones, sandstones, sandy conglom-erates, and volcanics (Chen et al., 1981). This rockassemblage indicates alluvial and fan-delta deposi-tion in a relatively dry climatic setting. The overlyingS3Member consists of dark lacustrinemudstone inthe center of the basin and sandstone of fan-deltaorigin along the basinmargin. The overlying S2 andS1 members display fining-upward trends from rel-atively coarse-grained alluvial fan or braid-deltato fine-grained shoreline lacustrine deposits. TheDongying Formation (D3, D2, and D1 members)consists mainly of fluvial-deltaic and shallow todeep lacustrine facies.

Figure 3. Schematic structuralcross sections of the Liaozhongdepression in Bohai Bay Basin.Locations of cross sections areshown in Figure 1. Horizontaldatum is sea level (equivalent totwo-way traveltime = 0s).

ine Basin, China

The Dongying Formation is the subject of thisdetailed study because this formation is thickerin the Liaozhong depression compared with theShahejie and Kongdian formations. Moreover, theDongying Formation has become an important oiland gas producing interval in the Liaozhong de-pression in recent years, but its detailed sequencestratigraphy and depositional system frameworkare poorly understood.

SEQUENCE STRATIGRAPHY

Sequence stratigraphy establishes a genetic frame-work for the study of facies relationships andstratal architectures (Catuneanu et al., 2009), andthe recognition of sequence-stratigraphic surfacescontributes to an interpretation of the chronostrati-graphic framework (Embry, 2002). Several sequence-stratigraphic surfaces develop during a full cycleof base-level change. Two types of surfaces can berecognized: time barriers (or approximate timesurfaces) and time offsets (or diachronous sufaces)(Embry, 2009). The low diachroneity of time-barrier surfaces make them useful for correlationand building an approximate time framework forspecific sequence-stratigraphic units. In contrast,time offset surfaces are highly diachronous andthus are not suitable for use as bounding surfacesof sequence-stratigraphic units (Embry, 2009).Generally, unconformities and their correlativeconformities are used as sequence-bounding sur-faces (Mitchum, 1977;Haq et al., 1987; Posamentieret al., 1988) because they represent time-barriersurfaces.

Sequence Classification

Based on a combination of 2-D and 3-D seismic,well-log, and core data, the entire Paleogene systemof the Liaozhong depression can be divided intothree composite (second-order) sequences corre-sponding to threemajor tectonic stages (Figure 2).This study focuses on the Dongying Formation,which is part of the uppermost composite sequence.Within the Dongying Formation, seven sequences

can be recognized based on subaerial unconformi-ties and correlative conformities.

The base (T8) and the top (T2) of the Paleo-gene are defined by first-order unconformities thattypically are traceable throughout the basin. Thesesurfaces were generated by tectonics (begin and endof rifting) and are associated with major truncationsthat penetrate into the basin center (Figures 4, 5).

Within the Dongying Formation, seven sequence-stratigraphic cycles canbe recognized.Theboundingsurfaces of these sequences are defined by localsubaerial unconformities at the basin margin show-ing truncations or incisions (Figure 5). The erosionalcharacter of subaerial unconformities is also fea-tured in well logs by abrupt changes in lithology orsedimentary facies and the presence of nonmarinedeposits above the unconformities (Figure 6). Sub-aerial unconformities can be traced basinward andcorrespond to slope onlap surfaces (Embry, 1995,2002, 2009) (Figures 4, 5). Their correlative con-formities (Posamentier et al., 1988) (Figures 4, 5)correspond to bilateral downlap surfaces (Figure 4) orbasal surfaces of hummocky reflections (Figure 5).

Thus, sequence boundaries of the Liaozhongdepression encompass subaerial unconformities onthe basin margin, slope onlap surfaces on the basinslope, and bilateral downlap surfaces or basal sur-faces of hummocky reflections in the basin center.Sequences bounded by these surfaces have thick-nesses ranging from 100 to 300 m (328 to 984 ft)(Figure 6) and are considered to correspond to third-order sequences. Most of the sequences record a fullcycle of regression-transgression-regression, exceptthat some parts of cycles are missing locally becauseof erosion beneath unconformities.

Depositional Systems

Based on seismic data, well logs, and cores, anumber of depositional systems are identified inthe Liaozhong depression: (1) proximal fan-deltadeposits adjacent to the escarpment; (2) braid-delta and beach-bar deposits on the western rampand adjacent slope; (3) axial sublacustrine fan anddeep lacustrine deposits in the basin center; (4)incised-valley and fluvial-deltaic deposits developedduring the late stage of the Dongying Formation.

Dong et al. 1481

Representative vertical successions for the majordepositional systems are shown in Figure 7, andtheir characteristics are discussed briefly below.

Sublacustrine Fan DepositsSublacustrine fan deposits discussed in this articlerefer to subaqueous gravity-flow deposits formedin a sublacustrine environment. Such deposits aredeveloped widely in the Dongying Formation ofthe Liaozhong depression and can be recognizedon seismic profiles (Figures 4, 5) and in well logs(Figure 7A). The deposits typically display con-tinuous or discontinuous hummocky reflections(Figures 4, 5), with thicknesses of 30–200 m (89–656 ft). They also can be recognized in well logs bydistinctive low gamma-ray and high-resistivitysignals within background high gamma-ray andlow-resistivity sections. Gamma-ray patterns forthese deposits display both upward-coarsening andupward-fining trends (Figure 7A).

1482 Depositional Systems and Sequence Architecture, Lacustr

Discontinuous seismic reflections in sublacus-trine fan deposits are interpreted to indicate sand-stone facies, whereas continuous reflections areinterpreted as cohesive mud-rich facies with soft-sediment deformation. On the basis of verticaltrends in well logs, three parts of typical sublacus-trine fan deposits, as characterized by Normark(1978), can be identified: an upper fan leveedchannel, a middle fan consisting of suprafan lobes,and a flat lower fan lacking channels (Figure 7A).Upper fan deposits have distinctive bell-shapedwell-log patterns that are characteristic of channelfills (Figure 7A). Middle fan deposits are com-posed of sets of channelized lobes with charac-teristic bell-shaped well-log patterns and smoothlobes that characteristically cap funnel-shaped well-log patterns (Figure 7A). Lower fan deposits aredominated by mudstones with intercalated thinsandstone beds and commonly extend over largeareas into the basin center. Medium- to fine-grained

Figure 4. Annotated 3-D seismic profile AA′ across the Liaozhong depression showing sequence boundaries (SB) (see Figure 1 forlocation). T2 and T8 are regional unconformities bounding the Paleogene. SB5 and SB6 are correlative conformities (i.e., Posamentieret al., 1988) and are defined as basal surfaces of bilateral downlap surfaces. SB7 is a slope onlap surface. SB2, SB3, and SB4 arecharacterized by high-amplitude reflections. Arrows demarcate onlap or truncation.

ine Basin, China

Figure 5. Seismic lines across the Liaozhong depression (see Figure 1 for location). Sequence boundaries (SB) are local subaerialunconformities at the basin margin (SB2, SB4), slope onlap surface (SB4), or basal surfaces of hummocky reflections (SB3). Arrowsdemarcate onlap, downlap, or truncation.

Figure 6. Well-log crosssection (see Figure 1 for lo-cations of wells) showingsequence boundaries (SB).The subaerial unconformity(SU) is characterized byabrupt changes in lithologyor sedimentary facies indi-cating the presence of non-marine deposits. The slopeonlap surface (SOS) is bestdefined seismically andtypically is overlain by thinsandstones representedby interfingered well-logshapes. The correlative con-formity (CC) lies above acoarsening-upward trendand typically is overlain bydeep-water facies. Wirelinelogs are gamma ray (GR)and resistivity (R). MFS =

maximum flooding surface.

Dong et al. 1483

sandstone with dark mudstone boulders with soft-sediment deformation (Figure 8A) indicate slump-ing of sublacustrine fans. The Normark (1978)

1484 Depositional Systems and Sequence Architecture, Lacustr

model for fan growth emphasizes progradation andperiodic switching of depositional lobes termedsuprafans. As one lobe grows, its feeder channel

Figure 7. Vertical successions of (A) sublacustrine fan, (B) fan-delta, (C) braid-delta, (D) beach-bar, and (E) fluvial-deltaic deposits inthe Liaozhong depression. Wireline logs are gamma ray (GR) and resistivity (R).

ine Basin, China

aggrades and eventually initiates a levee break.Turbidity currents are diverted to a lower part of themid-fan surface to begin construction of a new lobe.When one suprafan lobe is active, abandoned lobesand their former distributary channels are cappedby mud. Evidence for these processes is shown onFigure 7A as upward-coarsening overlain by upward-fining gamma-ray patterns.

Fan-Delta DepositsFan-delta deposits are mainly developed along theeastern escarpment of the Liaozhong depression,associatedwith large-scale faults. These deposits aresmall in aerial extent but occur in large volumes. Incross section, fan-delta deposits are wedge shaped

and thin in a basinward direction (Figure 4). Inter-nal seismic reflections are medium to low ampli-tude. Fan-delta deposits are upward coarsening, asindicated by funnel-shaped gamma-ray patterns(Figure 7B). Limited core samples are dominatedby thick-bedded, coarse-grained, and conglomer-atic sandstone (Figure 8B). Scour surfaces are com-mon and are overlain by graded beds or conglom-erates (Figure 8B).

Fan deltas contain three distinctive compo-nents: subaerial, transitional, and subaqueous de-posits (Wescott and Ethridge, 1980; Ethridge andWescott, 1984). Sediment is contributed by prox-imal provenances with short transport distances(McPherson et al., 1987) because the deposits are

Figure 8. Core photographs of representative facies. (A) Soft-sediment deformation structures in sublacustrine fan deposits. (B) Coarse-grained, poorly sorted sandstones with scoured basal surfaces in fan-delta deposits. (C) Medium-grained sandstone in braid-delta de-posits. (D) Thin-bedded light-gray siltstone and argillaceous siltstone with horizontal laminations and massive dark mudstones withcalcareous shells in beach-bar deposits. (E) Small-scale cross-bedding in fluvial-deltaic deposits.

Dong et al. 1485

very coarse grained and poorly sorted (Figure 8B).There is a general tendency for the sediment tobecome progressively finer grained in an offshoredirection, as the prodelta region is approached.Progradation of fan deltas produces distinctivecoarsening-upward sequences (Figure 7B). Coarse-graded beds and conglomerate-rich beds with ba-sal scoured basal surfaces (Figure 8B) are inter-preted as distributary channel deposits.

Braid-Delta DepositsBraid-delta deposits in the study area are developedmainly on the ramp slope along the westernmarginof the Liaozhong depression. Inferred deltaic de-posits are also present along the axis of the centraldepression. In contrastwith fan-delta deposits, braid-delta deposits in cross sections on seismic profiledo not have a typical wedgelike geometry, but arecharacterized by a flatter seismic facies geometrywith medium to high amplitude and continuousforeset reflections (Figure 5; SB2-SB3). In com-mon with fan-delta deposits in the study area,braid-delta deposits are characterized by grosslycoarsening-upward successions (Figure 7C). Both

1486 Depositional Systems and Sequence Architecture, Lacustr

gamma-ray and resistivity logs show funnel-shapedcurves, with sets of bell-shaped curves intercalatedin the upper parts of coarsening-upward succes-sions (Figure 7C). Available cores are dominatedby medium-grained sandstones (Figure 8C).

A braid delta is a distinctive type of coarse-grained delta formed by the progradation of abraided fluvial system into a standing body ofwater(McPherson et al., 1987). Because braided riversoccupy broad flood plains, braid-delta depositsare laterally persistent and sheetlike in geometry,which contrasts markedly with the wedge-shapedgeometry of fan-delta deposits. Axial-delta depos-its developed in the central part of the Liaozhongdepression are interpreted as braid-delta deposits(Figure 9A) associated with longitudinal prograda-tion into topographic lows. Where fully preserved,braid-delta deposits, from bottom to top, consist ofprodelta, delta-front mouth bar, and braid-deltaplain (Galloway and Hobday, 1983; McPhersonet al., 1987). Funnel-shaped patterns in well logs areinterpreted as progradational prodelta and mouth-bar deposits, and overlying bell-shaped patterns areinterpreted as distributary channel deposits.

Figure 9. Seismic lines acrossthe southwestern (CC′) andnortheastern (DD′) Liaozhongdepression (see Figure 1 forlocation). Annotations are asfor Figure 4. Note fluvial-deltaicand incised-valley deposits (A, B)and axial-deltaic deposits (A).SB = sequence boundary.

ine Basin, China

Shallow-Lacustrine Beach-Bar DepositsBeach-bar deposits are widely developed along theramp slope of the Liaozhong depression. These de-posits are characterizedbymedium- to low-amplitude,discontinuous, subparallel sheetlike or tabular to len-ticular reflections on seismic profiles. They display acomposite coarsening-upward vertical succession inboth gamma-ray and resistivitywell logs (Figure 7D).Cores from drill well W4 show that beach-bar de-posits are dominated by thin-bedded light-gray silt-stone and argillaceous siltstone, with plane lamina-tion or small-scale cross-stratification, and massivedark mudstones with calcareous shells (Figure 8D).Siltstones typically are well sorted andwell rounded.

Beach-bar sand is mainly transported longshorefrom proximal deltas or nearshore shallow-watersand bodies and accumulates as coastline-parallelbodies resulting from wave processes (Wang et al.,2008). The composite coarsening-upward verticalsuccessions observed in well logs (Figure 7D) areinterpreted to record a transition from offshore toshoreline deposits. Low-angle cross-beds (Figure 8D)are common in beach-bar deposits and result fromswash processes. Massive dark mudstones with cal-careous shells (Figure 8D) formed as a result of bio-turbation in relatively quiet offshore environments.

Deep Lacustrine Fine-Grained DepositsDeep lacustrine deposits are the predominant faciesin the center of the Liaozhong depression. They arecharacterized on seismic profiles by medium- tolow-amplitude continuous parallel reflections andless common hummocky reflections (Figure 4, SB2-SB5; Figure 5, SB2-SB6). Deep lacustrine depositsare tens to hundreds of meters thick and havecharacteristic high gamma-ray and low-resistivitywell-log patterns. Lithologies consist mainly of thick-bedded black mudstones with horizontal lamina-tions. Deep lacustrine mudstones are interpretedas the hemipelagic fallout of the fine-grained loadof turbidity currents (cf. Walker, 1984), althoughsome eolian contribution cannot be ruled out.

Incised-Valley DepositsIncised-valley deposits are developed above se-quence boundaries and are best exemplified aboveSB2 (Figure 9B). On seismic profiles, incised-valleys

are characterized by relatively large-scale concave-up bases. Internally, the incised-valley fills displaysubparallel variable-amplitude reflections and ex-hibit onlap onto valley margins (Figure 9B).

Incised valleys are formed in a subaerial envi-ronment by downcutting of fluvial systems associ-ated with a fall in base level (Zaitlin et al., 1994). Inthe study area, the base of the incised valley repre-sents a significant unconformity eroded into under-lying braid-delta deposits and coincides with a se-quence boundary. Incised valleys represent feedersystems to lowstand fluvial deltas and typically arebackfilled with coarse-grained fluvial deposits dur-ing a rise in base level (Posamentier et al., 1992).

Fluvial-Deltaic DepositsFluvial-deltaic deposits are widely developed inthe upper Dongying Formation and are charac-terized on seismic profiles by sets of relatively flator low-relief reflections with variable amplitude(Figure 9A, B). They also can be recognized in welllogs as upward-coarsening and thickening verticalsuccessions capped by cylindrical or bell-shapedpatterns (Figure 7E). Sandy facies at the top ofupward-coarsening successions are dominated bysmall-scale cross-bedding (Figure 8E).

In contrast with coarse-grained fan-delta andbraid-delta systems, fluvial deltas are characterizedbymud, silt, and sand (McPhersonet al., 1987;Ortonand Reading, 1993; Reading and Collinson, 1996).Fluvial-deltaic deposits in the upper Dongying For-mation developed along moderate- to low-gradientcoastal plains and are amalgamated to form a setof relatively flat reflections rather than diagnosticmound-shaped geometries described by Sangree andWidmier (1978). Components of fluvial deltas arethe delta plain that is partly subaerial and partlysubaqueous and the delta front and prodelta that areentirely subaqueous (Reading andCollinson, 1996).The funnel-shaped upward-coarsening well-log pat-terns are interpreted as prodelta to delta-front depos-its, and the cylindrical or bell-shaped patterns at thetop are interpreted asdelta-plaindeposits (Figure 7E).

Maximum Flooding Surface DepositsMaximum flooding surfaces within sequences inthe Dongying Formation are recognized on seismic

Dong et al. 1487

profiles as surfaces of downlap (Figure 5) or sub-parallel reflections (Figure 4) and are characterizedby high gamma-ray signals. Maximum flooding sur-face deposits can be equated with condensed sec-tions related to very slow depositional rates (Vailet al., 1984). Condensed sections typically consistof fine-grained hemipelagic to pelagic sediments,starved of sediment from basin margins, and depos-ited at times when shorelines reach maximum land-ward positions. Downlap surfaces in the DongyingFormation are related to prograding deltas (Figure 5),but in the center of the basin or where prograda-tional deltaic deposits are absent, maximum flood-ing surfaces tend to exhibit parallel or subparallelreflections (Figure 4). High gamma-ray signals are aresult of high concentrations of organic matter andradioactive elements.

Systems Tracts

Depositional systems in Bohai Bay Basin are asso-ciated within specific systems tracts. The types ofdepositional systems and systems tracts vary sig-

1488 Depositional Systems and Sequence Architecture, Lacustr

nificantly between sequences that formed at dif-ferent stages of basin evolution. During the earlystage of basin development, the stacking pattern ofsystems tracts was dominated by lowstand subla-custrine fans and highstand mudstones (Figure 10,between SB4 and SB7). Each of these sequencesbegan with lowstand sublacustrine fan deposits inthe center of the basin, rare braid-delta deposits onthe ramp slope, and fan-delta deposits adjacent to thetectonically active eastern escarpment (Figures 4,10). Basinward progradation of these depositionalsystems indicates a regression of the shoreline. Aslake level rose, lowstand systems tract deposits wereoverlain by thick intervals of prodeltaic or lacustrinemudstones (Figure 10). During this stage, beach-bar deposits developed along the western margin(Figures 5, 10). The thick intervals of fine-graineddeposits indicate deepening of the lake basin andtransgression of the shoreline. At highstand, mud-stones developed in the basin center as well as onthe ramp slope (Figure 10). At late highstand,distinctive axial deltas prograded southward intotopographic lows in the basin center (Figure 10);

Figure 10. Depositional systems and systems tracts in lacustrine sequence framework of the Liaozhong depression. During the earlystage of basin development (from SB4 to SB7), sequences consist of lowstand sublacustrine fan and highstand mudstone deposits; duringthe middle stage (from SB2 to SB4), sequences consist of lowstand sublacustrine fan and highstand delta deposits; during the late stage(from T2 to SB2), sequences consist of fluvial-deltaic deposits.

ine Basin, China

sediment may have been supplied by an easterndrainage system. The occurrence of highstandsystems tract records another regression of theshoreline.

During the middle stage of basin development,stacking patterns of depositional systems in systemstracts was dominated by lowstand sublacustrinefans and highstand deltas (Figure 10, between SB2and SB4). The spatial relationships of depositionalsystems in these sequences resembles the modelfor the early stage of basin development in thatthe lowstand systems tracts are dominated by sub-lacustrine fan deposits above sequence boundaries(Figures 5, 10). Highstand systems tracts, in con-trast, are dominated by braid-delta deposits andpossibly synchronous fan-delta deposits along theeastern escarpment margin (Figures 5, 10). Axialdeltas developed during highstand (Figure 10). Theevolution of systems tracts records a full cycle ofregression-transgression-regression.

During the late stage of basin development,systems tracts were dominated by fluvial deltas(Figures 9A, 10, between T2 and SB2). Sedimentwas fed from basin margins to both transverse andaxial deltas. Axial deltas prograded into topo-graphic lows from the northeast to the southwest(Figure 9A). Under these circumstances, most ofthe highstand systems tracts were removed by flu-vial erosion, and only lowstand regressive and trans-gressive deposits are preserved.

RESERVOIRS

Within the Dongying Formation in the Liaozhongdepression, major oil reservoirs are developed inlithologies that range from siltstone, fine-grainedsandstone, and coarse-grained sandstone, to con-glomeratic sandstone. The economically most im-portant reservoirs are in large sublacustrine fandeposits (Figures 4, 5, 10), some of which havebeen proven by exploration. Deltaic sandstonedeposits represent another favorable reservoirtype. Braid-delta deposits along the downthrownside of the ramp slope break (Figures 4, 10) con-stitute potential reservoirs, especially where thickdelta-front sandstones are stacked to form strati-

graphic overlap or complex structural-stratigraphictraps (Figures 4, 10). Sandstone reservoirs aredeveloped in fluvial-deltaic deposits, especially inthe incised-valley fills (Figures 9B, 10) of deltaplain, and in distributary channel or mouth-barsandstones of the delta front. Coarse-grained fan-delta deposits (Figures 4, 10) have relatively poorsorting and rounding, which is detrimental to res-ervoir quality.

Sandstones in sublacustrine fan, braid-delta,fan-delta, and fluvial-deltaic depositional systemsaremostly litharenites and feldspathic arenites. Theporosity of these sandstones ranges from10 to 40%,and the permeability is between 1 and 1800 md,more commonly, between 20 and 110 md. Dis-solution of feldspar and lithics produced second-ary pores that have contributed to a higher porosityand permeability.

In a temporal framework, the most favorablesandstone reservoirs in the lower Dongying Forma-tion are developed in deep lacustrine fan depositsand in the middle and upper Dongying Formationare located in delta-front deposits (Figure 10). Ina spatial framework, the relative lowstand sub-lacustrine fan deposits capped by relative highstandprodelta or deep-water mudstones constitute litho-stratigraphic traps with the greatest potential.

CONTROLS ON THE DEVELOPMENT OFSEQUENCES AND DEPOSITIONAL SYSTEMS

The development and distribution of sequencesand systems tracts reflect paleogeographic changesthat were strongly influenced by tectonics, sedi-ment supply, and climate changes. The balance be-tween rates of potential accommodation generatedby tectonic subsidence versus sediment supply in-fluences the type of lake basin (Carroll and Bohacs,1999). Most lacustrine basin fills display an evo-lution of facies associations (Neal et al., 1997) thatreflect overfilled, balanced-fill, or underfilled lakes(Carroll and Bohacs, 1999). The following discus-sion evaluates controls on systems tract and se-quence evolution in the Dongying Formation interms of these controls.

Dong et al. 1489

Balance between Tectonic Subsidence andSediment Supply

The Dongying Formation developed during a latephase of rifting in the Bohai Bay Basin (Figure 2)and records an evolution in basin fill from predom-inantly deep-water sublacustrine fans to predomi-nantly shallow-water fluvial deltas. The evolution ofdifferent lacustrine facies associations correspondsto three lake basin types. During the initial riftingstage (Paleocene and early Eocene, Figure 10; se-quences between SB4 and SB7), evaporative faciestypified basin fill that was dominated by inter-bedded red-colored continental alluvial fan strata.This relatively dry climatic setting resulted in anunderfilled lake basin with lake levels that rarelyreach the sill. During the late Eocene and earlyand middle Oligocene, the predominance of sub-lacustrine fans suggests that the basin was probablya balanced-fill basin with accommodation approx-imately equal to sediment and water fill. Duringthis stage, rapidly fluctuating lake levels resulted ina significant change in depositional facies at thescale of sequences (Figure 10; sequences betweenSB2 and SB4).During the lateOligocene, decreasedbasin subsidence and reduction of tectonic drainagedivides led to infill of Liaozhong depression fromnorth to south by through-going fluvial-deltaic sys-tems (Figure 10; sequences between T2 and SB2).These fluvial-deltaic systems carried sufficient sedi-ment to ultimately overfill the lake basin. Duringthis stage, the overfilled freshwater lake was hy-drologically open (Olsen, 1990), and climaticallydriven lake-level fluctuationswereminimal aswaterinflows nearly always equaled outflows.

Tectonics

Episodic tectonicmovementwas theprincipal factorthat controlled the development of different kindsof sequences in theDongying Formation. Sequencesin the lower Dongying Formation (Figure 2, be-tween SB4 and SB7) developed during an earlystage of active rifting related to transtension (Hsiaoet al., 2004). Onlap at the base of these sequences(Figures 4, 5) is best exemplified on the ramp slopeand is related to initial rapid subsidence. Intense

1490 Depositional Systems and Sequence Architecture, Lacustr

faulting occurred on both margins of the Liaoxi andLiaodong uplifts (Figures 1, 3), resulting in signif-icant topographic contrast that significantly lim-ited sediment input into the Liaozhong depres-sion. Consequently, the sequences are dominatedby nearshore fan deltas, sublacustrine fans, and rareaxial deltas that may have been sourced from localtopographic highs.

The sequences in the middle Dongying For-mation (Figure 2, between SB2 and SB4) developedduring a stage of diminishing rifting, with mosttectonic subsidence localized along the easternbasin margin. Seismic profiles (e.g., Figure 9A) in-dicate that the western margin of the basin wastilted at this stage of basin evolution. Tilting of thebasin floor caused differential subsidence and re-sulted in truncation of the lowest sequence andrelocation of depocenters toward the south. Dur-ing this stage of basin evolution, the topography ofthe basin was high in the north and west, and lowin the south and east. As a result, depositionalsystems originated from different directions, andprograded southward into topographic lows alongthe basin center.

Sequences in the upper Dongying Formation(Figure 2, between T2 and SB2) developed fol-lowing the end of rifting, at which stage compart-ments partitioned by uplifts and depressions hadlargely disappeared. Fluvial deltas with incised chan-nels dominated these sequences. Rivers from dif-ferent catchments built out into the lake to formlarge deltaic complexes.

Climate

It is generally accepted that in both marine andnonmarine basins, sequences generated in responseto lake-level changes may be a result of climaticoscillations (Juhász et al., 1997). Climate changes,in turn, influence both lake water volumes and sed-iment yields (Ayhan and Nemec, 2005). Sequencesin the Dongying Formation record the full cycle of,or part of, regressive-transgressive-regressive events,but the causes of these drowning and shoalingevents are complex. Based on the consistent stack-ing patterns of systems tracts in the sequences, wepropose that climate change influenced relative lake

ine Basin, China

level that, in turn, impacted on sediment flux to thebasin. In this scenario, sublacustrine fans built intothe basin at lowstands of lake level. Transgressivesystems tracts developed under wetter climaticconditions that promoted an elevated lake level andlow sediment flux to the basin. In contrast, high-stand systems tracts developed under drier climaticconditions that promoted a drop in lake level with aresultant increase in sediment flux to the basin. Thisscenario is consistent with the findings of Juhászet al. (1997) that lake-level changes follow therhythm of climate changes but that sediment fluxlags behind climate changes. The detailed relation-ship between regressive-transgressive-regressive cy-cles and their controlling factors is the focus of on-going investigations.

CONCLUSIONS

1. The Oligocene Dongying Formation in theLiaozhong depression of the Bohai Bay Basincan be classified into seven sequences boundedby local unconformities and correlative confor-mities. Unconformities are best developed alongthe basin margin, whereas correlative conformi-ties encompass slope onlap surfaces on the basinslope and bilateral downlap surfaces or basalsurfaces of hummocky reflections in the basincenter.Most of the sequences record a full or partof a regressive-transgressive-regressive cycle.

2. The main depositional systems identified inthe Liaozhong depression include sublacustrinefan, fan-delta, braid-delta, fluvial-deltaic, beach-bar, incised-valley, and deep lacustrine deposits.These depositional systems are associated withinsystems tracts. Within sequences developed dur-ing early and middle Dongying Formation de-position, lowstand systems tracts are dominatedby sublacustrine fans; transgressive systems tractsaremainly composed of beach-bar deposits; andhighstand systems tracts are characterized bydeep lacustrine deposits or different kinds ofdeltaic deposits. Sequences developed duringthe lateDongying Formationwere dominated byfluvial deltas.

3. The most favorable sandstone reservoirs in thelower Dongying Formation are developed insublacustrine fan deposits and in themiddle andupper Dongying Formation are located in delta-front deposits. Relative lowstand sublacustrinefan deposits capped by relative highstand pro-delta or deep lacustrinemudstonesmay compriselithostratigraphic traps with the highest potential.

4. Episodic tectonic movement was the principalfactor that controlled development and distri-bution of the sequences and depositional sys-tems. Lake-level changes resulting from climaticoscillations may have controlled development ofthese sequences. The balance between tectonicsubsidence and sediment supply controlled thetype of lake basin. The Bohai Bay Basin was re-garded as a balanced-fill basin in the early andmiddle stage of the Dongying Formation andultimately turned into an overfilled basin at theend of the Dongying Formation.

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