Depositional systems and sequence stratigraphy of mesozoic lacustrine rift basins in NE China: A case study of the Wulan-Hua sag in the southern Erlian Basin

Journal of Asian Earth Sciences 174 (2019) 68–98

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Depositional systems and sequence stratigraphy of mesozoic lacustrine rift basins in NE China: A case study of the Wulan-Hua sag in the southern Erlian Basin

T

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Sheng Fua, Zhen Liua, , Yi-ming Zhangb, Xin Wangb, Ning Tianb, Ning Yaoa, Ye Xionga, Pan Chena, Lei Lia, Hui-lai Wangb a b

State Key Laboratory for Petroleum Resource and Prospecting, China University of Petroleum-Beijing, China Huabei Oil Field Branch of PetroChina, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Sequence framework System tract Depositional systems Controlling factors Wulan-Hua sag Erlian Basin

The deltaic and lacustrine sediments, respectively in the syn- and post- rift stages, are recorded during the Cretaceous at the Wulan-Hua Sag, Erlian Basin, Inner Mongolia. In this study, the Mesozoic depositional system and sequence stratigraphic framework are recognized in the Wulan-Hua sag using core, well-log and seismic data. Five sedimentary facies have been identified including fan-delta, braided delta, shallow-lacustrine, deeplacustrine, and sublacustrine-fan during Cretaceous at the Wulan-Hua Sag. One first-order sequence (MSQ1), three second-order sequences (SSQ1, SSQ2, and SSQ3), and five third-order sequences (SQ1, SQ2, SQ3, SQ4, and SQ5) are classified by subaerial unconformities along the basin margins and correlative conformities in the central part of the basin. During the depositional period of SQ1, SQ2, and SQ3, the depocenter controlled by lake-level fluctuations migrated from southern to northern part companying with multiple- to single-provenance supplies. The characteristics of sedimentary facies and sequence stratigraphy in the Wulan-Hua sag are commonly controlled by faults, provenance and depocenters. The development of the third-order sequences is dominated by tectonism and sediment supplies, as well as lake-level changes driven by climate. The favorable targets for hydrocarbon located in the lowstand system tract of SQ1, SQ2 and SQ3 at the southern parts of the Wulan-Hua Sag.

1. Introduction

systems (Yang et al., 2010; Ge et al., 2017, 2018a). In recent years, improved high-resolution seismic exploration techniques have enhanced the basin-scale analysis of rift basins and led to enhanced understanding of system tract to reservoir-scale sequence stratigraphy and detailed delineation of depositional architecture (Macurda, 1996; Lin et al., 2000, 2002; Martins-Neto and Catuneanu, 2010; Scherer et al., 2015; Reuter et al., 2017). Identifying the sequences in lacustrine rift basins (Helland-Hansen and Hampson, 2009) is as important as identifying the sequences in passive continental margins and improves the understanding of controls on sequence stratigraphic architecture (Borer and McPherson, 1996; Eliet and Gawthorpe, 1996; Pietras and Carroll, 2006). However, the controlling effect of tectonism on stratigraphic architecture and sedimentation variability through the syn- and post-rift events remains poorly understood in lacustrine rift basins; further study on the sequence stratigraphy model of lacustrine rift basins is still needed (Martins-Neto and Catuneanu, 2010; Scherer et al., 2015; Reuter et al., 2017; Ge et al.,

Previous studies have demonstrated that sequence-stratigraphic concepts developed for passive continental margin strata can be applied to the study of lacustrine basins (Vail et al., 1977; Aitken and Flint, 1996; Lin et al., 2001a,b, 2005; Keighley et al., 2003; Folkestad and Satur, 2008). However, given that the development and evolution of lacustrine basins, especially rift basins, are more complicated than those of marine basins (Gu et al., 1995), knowledge on the sequence stratigraphy of ancient lacustrine rift basins has remained limited. Lacustrine rift basins show common characteristics of non-marine lacustrine basins, such as limited distribution, multiple sediment supplies, multi-depocenters, variable facies associations, narrow facies belt, limited water volume, and heterogeneous tectonic subsidence. These lacustrine rift basins show distinguishing characteristics, such as the controlling role of tectonism, especially fault activity during basin evolution for sequence stratigraphic framework and depositional

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Corresponding author. E-mail address: [email protected] (Z. Liu).

https://doi.org/10.1016/j.jseaes.2018.11.020 Received 12 May 2018; Received in revised form 14 November 2018; Accepted 17 November 2018 Available online 20 November 2018 1367-9120/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Structural units sketch map of the Erlian Basin, showing the location of the Wulan-Hua sag in the Erlian Basin. (b). General structure and well locations in the Wulan-Hua sag. Seismic sections AA′ to NN′ in Fig. 1b are presented in Figs. 7, 14–16, 18, 26, and 27.

it one of the largest continental sedimentary basins in China (Cui et al., 2011). The Erlian Basin lies on a basement deformed by NE, NEE, and WE-striking faults. The Erlian Basin consists of three uplifts and five depressions: the Sunite uplift in the central part, the Bayinbaolige uplift to the north, the Ondor Sum uplift to the south, the Chuanjing depression to the west, the Wulanchabu depression in the central-west, the Manite depression in the central-north, the Tengge’er depression in the southeast, and the Wunite depression to the northeast. The basin could be further divided into 53 sags and 21 uplifts (Lin et al., 2001a,b; Dou and Chang, 2003; Bonnetti et al., 2014; Meng et al., 2015) (Fig. 1a). The Mesozoic strata of the Erlian Basin consists of six lithostratigraphic units. The basal Middle-Upper Jurassic rocks overlie the Paleozoic metamorphic basement and consist mainly of alluvial and lacustrine deposits with thin coal seams and local volcanic rocks. This stratigraphic unit has limited distribution (observed only in some sags of the Erlian Basin). The overlying Qinganling Group is approximately 300–500 m in thickness and consists of basal volcanic rocks (tuffs and andesites) and coarse-grained alluvial deposits capped by shallow-lacustrine sandy mudstones and thin coal seams. The Qinganling Group is widely distributed across the basin and reflects the onset of the Late Jurassic rifting event in the eastern China (Li et al., 1995). The Lower Cretaceous Aershan Formation unconformably overlies the Qinganling Group. This formation is 600–800 m thick and fines upward from relatively coarse-grained alluvial-fan to fine-grained shallow-lacustrine deposits. The younger Tengge’er Formation is up to 1200 m thick and unconformably overlies the Aershan Formation. The Tengge’er Formation is dominated by deep-lacustrine mudstones and fan-delta, braided-delta, and sublacustrine-fan sandstones. Shallow-lacustrine deposits dominate the upper Tengge’er Formation. Most of the proven petroleum reservoirs in the basin developed in this unit. Thick lacustrine mudstones of the lower Tengge’er Formation and the upper Aershan Formation comprise the best source rocks in most of the sub-basins. The Tengge’er Formation is unconformably overlaid by the Lower Cretaceous Saihantala Formation, which consists of coarse-grained

2017). The Erlian Basin, which was affected by the Carboniferous movement, formed as a Mesozoic rift basin in the middle and north areas of Inner Mongolia Autonomous Region, North China; this basin contains several sags and salients (Jiao, 2003; Li et al., 2009). The Wulan-Hua sag is located in the Ondor Sum uplift of the southern Erlian Basin; it is a small-scale lacustrine rift sag formed during the Cretaceous, and enriched in hydrocarbon (Xing et al., 2014; Chen et al., 2014). This study aims to (1) characterize the lithofacies, depositional systems, and sequence stratigraphic framework of the Wulan-Hua sag based on the cored wells and seismic profiles; (2) discuss tectonic and provenance controls on sequence stratigraphy and depositional systems and identify favorable areas for oil and gas prospecting in the Wulan-Hua sag; and (3) present tectono-stratigraphic evolution during the syn- and post-rift periods. Our study provides not only a typical example for further exploration and development of other small-scale rift lacustrine sags in the Erlian Basin (such as the Saihantala and Bayindulan sags) but also a potential model for the sequence and sedimentation study in other similar rift basins worldwide.

2. Geological setting The Erlian Basin, which is located in the sutured zone between the Siberian Plate and North China block, is a Mesozoic-Cenozoic rift basin developed on the Paleozoic folded basement and is characterized by wide-scale and soft collision (Jia, 2001; Jiao, 2003; Fang et al., 2006; Wang, 2011). The Erlian Basin is situated in the middle and north parts of the Inner Mongolia Autonomous Region, spreading in the NE and NEE directions. The Erlian Basin is located adjacent to the Sainshand and Jolen basins of Mongolia, abuts the Greater Khingan uplift to the east, the Solonker Mountain uplift to the west, the Ondor Sum uplift to the south, and the Bayinbaolige uplift to the north (Jiao, 2003; Li et al., 2009). The basin is 1000 km in east-west length, and 20–220 km in north-south width, and it has a total area of 1.0 * 105 km2; which makes 69

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Fig. 2. Tectono-stratigraphic log of the Erlian Basin (modified after Cai et al., 1990; Graham et al., 2001; Lin et al., 2001a,b; Wang et al., 2006; Sha, 2007; Bonnetti et al., 2014). J. = Jurassic; U1 = Unconformity 1; U2 = Unconformity 2; In the column Sedimentology the main biostratigraphical ages of the Erlian Formation proposed by authors are represented. Ages based on dinosaur fauna.

alluvial deposits and gray or greenish shallow-lacustrine and fluvialflood-plain siltstones with some coal seams. The regional unconformity between the Saihantala and Tengge’er formations represents a breakup unconformity. Upper Cretaceous strata are absent in most parts of the Erlian Basin, thus, Lower Cretaceous sedimentary rocks are unconformably overlaid by thin Tertiary or Quaternary sandy conglomerates. In this study, the Lower Cretaceous Aershan, Tengge’er and Saihantala formations are the target interval (Fig. 2). The Wulan-Hua sag is a petroliferous lacustrine rift sag that formed in the Ondor Sum uplift in the Cretaceous Erlian Basin, with an area of 600 km2 and a maximum burial depth of 3000 m (Xing et al., 2014; Chen et al., 2014). The sag can be divided into four secondary structural units: south sub-sag, central uplift, north sub-sag, and north slope areas (Fig. 1b). The Wulan-Hua sag is an NE-striking residual sag after extensive uplifts associated erosions induced by the Jurassic-Mesozoic Orogeny (Wang et al., 2013). The south sub-sag of the Wulan-Hua sag is double-faulted, while the north sub-sag is a dustpan-like geometry structure with east-faulting and west-overlapping (Yuan et al., 2016; Piao, 2016). The Wulan-Hua sag experienced two major tectonic phases: the synrift and post-rift phases. The syn-rift phase is subdivided into four episodes (named 1, 2, 3 and 4 episodes). Episode 1 of the syn-rift

represents the depositions of the Jurassic successions. Episodes 2, 3, and 4 of the syn-rift resemble the depositions of the Aershan Formation, the first member of the Tengge’er Formation, and the second member of the Tengge’er Formation, respectively. The post-rift phase represents the deposition of the Saihantala Formation. Episodes 2, 3, and 4 of the synrift correspond to the initial rifting stage, rift basin expanding stage, and rift basin stable developing and shrinking stage, respectively. The post-rift phase is related to the rift basin dying stage of the Wulan-Hua sag. 3. Data and methods 3.1. Data acquisition The data sets used in this study comprise seismic profiles, well data, and cores. Most sags were covered with 2D seismic profiles with a spacing of 0.5–1.0 km. 3D seismic surveys were conducted in major oil prospecting districts. The data of the Wulan-Hua sag used in this study include 3D seismic data covering an area of 347 km2, well logs of 34 wells, and approximately 1000 photographs of cores. The signal-to-noise ratio of the seismic data is more than 3, their dominant frequency is 15 Hz; the seismic bandwidth can reach 40 Hz. 70

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Fig. 3. Seismic formation near well and log horizon and synthetic record marked of key well LD1 in the Wulan-Hua sag. (see Fig. 1 for the location of this well).

3.2.2. Sequence stratigraphic framework establishment A sequence stratigraphic framework is established based on seismic reflection terminations (truncation, onlap, downlap, and toplap) combined with abrupt changes in log curves and logging lithology, seismicto-well correlation through calibration of well synthetic seismogram, phase scanning and seismic reflections extraction. These interpretations can serve as a basis for correlating the stratigraphic sequence boundaries in the Wulan-Hua sag. Well synthetic seismogram calibration, phase scanning, seismic wave extraction, Michiko polarity discrimination and spectrum analysis are used to study the response characteristics of each sequence in seismic section. The seismic events, which are easily contrasted and interpreted in the region, can be selected as sequence boundaries in establishing the seismic reflection characteristics of each sequence (Fig. 3). System tracts are defined based on the seismic reflection configurations and logging geometric stacking patterns, which are divided by first and maximum flooding surfaces (FFS and MFS) on well logs and seismic data. The proposed system tract classification for the WulanHua sag, is similar to that of previous studies (Brown and Fisher, 1980; Vail, 1987; Galloway, 1989; Hunt and Tucker, 1992, Plint and Nummedal 1997; Posamentier and Allen, 1999).

Well log data used in this study includes spontaneous potential log data, gamma ray log data, apparent resistivity log data and logging lithologic data. The cores are photographed by using a Canon DSLR camera. All of the data used in our study is obtained from the Huabei Oil Field Branch of PetroChina.

3.2. Methods Four comprehensive methods, namely, depositional system recognition, sequence stratigraphic framework establishment, system tract division, and tectonic subsidence history analysis are used in this study.

3.2.1. Depositional system recognition The core and well log data provides first-hand evidence for characterizing facies architecture. Well-log geometries and seismic reflections can be used for depositional system identification. Detailed description and interpretation of these data can define sedimentary facies associations and deposits partition in the Wulan-Hua sag. Seismic facies analysis is also used for facies architecture interpretation and depositional system analysis. 71

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Table 1 Summary of lithofacies observed in the Wulan-hua sag (based on Paula-Freitas (2010) and data collected in this work). Facies

Description

Interpretation

Gm

Massive or crudely-stratified, clast-supported conglomerates; coarse sandstone matrix; mud intraclasts are dominant, granite and quartz clasts are also present; 10–30 cm thick beds. Sandy conglomerates; granule to pebble clasts; medium to coarse sandstone matrix, trough cross-stratified; 10–40 cm thick sets. Fine- to very coarse-grained sandstones; moderately- to well-sorted; massive; 20–80 cm thick beds Medium-grained to granular sandstones; poorly- to well-sorted; extraformacional clasts of quartzite, granite and quartz and mud intraclasts at the base of sets, or parallel to stratification; trough cross-stratification with 20 cm to 2 m thick sets; sometimes burrows in the topmost part of, or throughout, the bed. Medium-grained to granular sandstones; poorly to well-sorted; rare extraformational granules and pebbles of granite and quartz at the base of sets, or parallel to stratification; planar cross-stratification; 10–50 cm thick sets. Medium to coarse-grained sandstones; moderately sorted; sigmoidal crossbedding; 20–30 cm thick sets. Medium-grained to granular sandstones; poorly- to well-sorted; granule and pebble of granite and quartz (b2 cm) parallel to stratification; low-angle crossstratification; 20 cm to 1.90 m thick beds. Very fine- to fine-grained sandstones; moderately- to well-sorted; horizontal lamination; 10–40 cm thick beds. Fine to medium-grained, micaceous sandstones; ripple cross-lamination, set thickness a few cm, forming up to 1.5 m thick cosets; supercritical to subcritical climbing angle. Mudstones to very fine-grained sandstones; massive; sometimes fissile in weathered surfaces; reddish to reddish brown; diffuse horizons of color variations; mottle and block structures; 20 cm to 3.5 m thick beds. Mudstones to very fine-grained sandstones; massive; medium gray, brownish gray and greenish gray; sometimes burrowed; 20 cm to 8 m thick beds.

Bedload deposition as diffuse gravel sheets (Hein and Walker, 1977) or lags deposits by high-magnitude flood flows (Miall, 1977; Nemec and Postma, 1993).

Gt Sm St

Sp

Ss Sl

Sh Sr

Fmr

Fmg

Flg Florg

Mudstones; thin parallel lamination; medium gray, brownish gray and greenish gray; 20 cm to 6 m thick beds. Organic-rich mudstone; laminated; dark gray to black

Gravel dunes (Rust, 1975; Todd, 1996). Rapid deposition of hyperconcentrated flows, fluidization (Miall, 1978, 1996). Subaqueous sandy dunes (upper flow regime) (Allen, 1963; Harms et al., 1982; Todd, 1996; Collinson et al., 2006)

2D subaqueous sandy dunes (lower flow regime); (Allen, 1963; Harms et al., 1982; Todd, 1989; Collinson et al., 2006) Upper-flow regime transitional bedform (Wizevich, 1992). Washed-out dunes and humpack dunes (transition between subcritical and supercritical flows) (Harms et al., 1982; Bridge and Best, 1988). Planar-bedded deposits originated via upper flow regime (Miall, 1977; Best and Bridge, 1992). ripples (lower flow regime) (Allen, 1963; Miall, 1977).

Suspension setting on floodplains; later modified by desiccation or pedogenetic processes; post-depositional reddening under oxidizing conditions (Miall, 1977; Foix et al, 2013). Suspension setting from weak currents or standing water; lack of lamination due to (i) flocculation of clay suspension or (ii) loss of lamination associated intensive bioturbation; post-depositional graying under reducing conditions (Miall, 1977; Foix et al., 2013). Suspension setting dominantly from standing water; post-depositional graying under reducing conditions (Turner, 1980; Jo and Chough, 2001). Suspension setting of sediments under anoxic or poorly-oxidized conditions (Chakraborty and Sarkar, 2005; Foix et al., 2013)

5.1.1. Subaqueous channel, mouth bar, and sheet sand facies associations 5.1.1.1. Description. These facies associations consist of lithofacies including imbricated, graded to massive, framework-supported conglomerates (Gm) with intercalated massive or parallel sandstones (Sp), well-sorted conglomeratic sandstones (Sm) and fine-grained to coarse-grained sandstones with high compositional maturity, coarsegrained sandstone, and muddy sandstone with massive bedding and ripple cross-lamination (Sr). Lithofacies association 1 comprises imbricated, graded to massive, framework- supported conglomerates (Gm) with intercalated massive or parallel laminated sandstones (Sp), has low muddy content, and is well-sorted and well-rounded (Fig. 3b). The lithological associations evolve from parallel laminated sandstones in the bottom to frameworksupported sandy conglomerates in the middle and to the massive coarse-grained sandstones in the top (Fig. 3b). Single well features such as weak jugged box-shaped GR curve, bellshaped SP curve, and yellow sandy conglomerates abruptly contacting with gray thin-bedded mudstone of this lithofacies association are recognized in well L5 (Fig. 4a). Lithofacies association 2 is composed of well-sorted conglomeratic sandstone (Sm) and fine- to coarse-grained sandstone with a high compositional maturity. The sand layer in this lithofacies association is middle-bedded to thick-bedded with coarsening-upward successions (reverse graded bedding) and wedge-shaped cross-bedding, and horizontal bedding (Sh; middle interval of cores section in Fig. 5b). The lithological association evolves from black muddy siltstone in the bottom to siltstone with ripple cross-lamination in the middle and to the massive sandstone in the top (middle interval of core section in Fig. 5b). Single well features such as jugged funnel-shaped GR curve, funnelshaped SP curve, and yellow conglomeratic sandstone abruptly contacting with gray thin-bedded mudstone are recognized in well L1

3.2.3. Tectonic subsidence history analysis In this study, the tectonic subsidence history is determined by using back-stripping analysis (balanced cross-section). Numerous seismic lines are used to illustrate the basin structure and stratigraphic variability across the basin. 4. Lithofacies The Wulan-Hua sag is composed of sandstones, mudstones and subordinate conglomerates. Texturally, the sandstones are very fine- to coarse-grained (mostly medium-grained) and moderately-sorted, subangular to rounded grains. The mudstones commonly appear to massive- or laminated-shape. The massive- or stratified-shaped sandy conglomerates are composed of sub-angular to sub-rounded granules to pebbles (dominantly quartz, and subordinately feldspar and granitic fragments). Thirteen lithofacies in the Wulan-Hua sag are summarized in Table 1. 5. Depositional systems On the basis of the cores, well log, and seismic data, depositional systems in the Wulan-Hua sag are identified as fan-delta, braided-delta, shallow-lacustrine, deep-lacustrine and sublacustrine-fan deposits. The characteristics of these depositional systems are discussed briefly in this section. 5.1. Fan-delta deposits This depositional system is widely developed in the upper part of the Aershan Formation and first member of the Tengge’er Formation. The system is located mostly adjacent to basin margin-faults. 72

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Fig. 4. Single well and core photographs section of lithofacies association 1 of fan-delta front subfacies. a. single well sedimentary facies map of Well L5, indicating single well features of lithofacies association 1 of fan delta front subfacies (See Well location in Fig. 1); b. core photographs section of well L5 in 929.4–933.57 m depth, its upper interval is massive coarse grained sandstone (Sm), middle interval is framework-supported sandy conglomerate (Gm), and the lower interval is parallel laminated sandstones, indicating lithofacies association 1 of fan-delta front subfacies (The location of these cores are indicated in Fig. 4a). SQ3 = sequence 3.

subaqueous channel microfacies of fan-delta front subfacies with coarse granularity; structures including massive and parallel bedding; and well log marks such as weak jugged box-shaped GR curve, bell-shaped SP curve, yellow sandy conglomerates abruptly contacting with gray thinbedded mudstone, and silty mudstone in logging. Lithofacies association 2 is regarded as mouth bar deposits of fandelta front subfacies with typical features including relative fine granularity, high compositional maturity, reverse graded bedding, wedgeshaped cross bedding and horizontal bedding; and well log marks such as jugged funnel-shaped GR curve, funnel-shaped SP curve, and yellow

(Fig. 5a). Lithofacies association 3 consists of muddy sandstone with massive bedding and ripple cross-lamination (Sr) (lower interval of core section in Fig. 6b). Single well features such as figure- or low-amplitude thin boxshaped GR curve, low-amplitude finger-shaped SP curve, and yellow conglomeratic sandstone abruptly contacting with thick-bedded gray silty mudstone are recognized in the well LD1 (Fig. 6a).

5.1.1.2. Interpretation. Lithofacies association 1 can be interpreted as 73

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Fig. 5. Single well and core photographs section of lithofacies association 2 of fan-delta front subfacies. a. Single well sedimentary facies map of Well L1, indicating single well features of lithofacies association 2 of delta-front subfacies (See Well location in Fig. 1). SQ3 = sequence 3. b. core photographs section of well L1 in 932.5–936.1 m depth, its upper interval consists of the sandy conglomerate with fining upward graded bedding and directive property (Gt), middle interval is massive conglomeratic sandstone with reverse graded bedding, namely coarsening upward sedimentary succession (black muddy siltstone (Florg) in the bottom, siltstone with ripple cross-lamination (Sr) in the middle, and massive sandstone (Sm) in the top), and the lower interval is conglomeratic sandstone (Sm) with obvious scoured surface, and the middle interval indicates lithofacies association 2 of fan-delta front subfacies (The location of these cores are indicated in Fig. 5a).

content, massive bedding and ripple cross-lamination, and well log marks such as low-amplitude finger-shaped SP curve and yellow conglomeratic sandstone abruptly contacting with thick-bedded gray silty mudstone. Lithofacies association 3 is interpreted as sheet sand deposits formed on a delta-front slope (Postma, 1984; Prior and Bornhold, 1988). Subaqueous channel, mouth bar, and sheet sand facies associations

conglomeratic sandstone abruptly contacting with gray thin-bedded mudstone in logging. Lithofacies associations 1 and 2, which are intercalated with dark mudstone, comprise the majority of the delta-front deposits; these deposits are comparable to subaqueous channel deposits and water-lain mouth bar described by Wood and Ethridge (1988). Lithofacies association 3 is interpreted as sheet sand deposits of fandelta front subfacies characterized by fine granularity, high shale 74

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Fig. 6. Single well and core photographs section of lithofacies association 3 of fan delta front subfacies. a. single well sedimentary facies map of Well LD1, indicating single well features of lithofacies association 3 of fan delta front subfacies (See Well location in Fig. 1). SQ3 = sequence 3. b. core photographs section of well LD1 in 859.32–863.27 m depth, Its upper interval is conglomeratic massive sandstone with scoured surface (Sm), middle interval is muddy siltstone with horizontal bedding (Sh), and the lower interval is muddy sandstone with massive bedding and ripple cross lamination, and the lower interval indicates lithofacies 3 of fan-delta front subfacies (The location of these cores are indicated in Fig. 6a).

and silty mudstone with parallel bedding and oil immersion (Florg). Lithofacies association 1 comprises massive yellow and grayishyellow sandy conglomerates and conglomerates (Gt); it has a high shale content, low-compositional granularity, scoured surface, and directive property. The sandstone of the lithofacies association 1 is poorly-sorted and poorly-rounded (Fig. 8b). Single well features, such as half-jugged box-shaped or bell-shaped GR curve, smooth bell-shaped SP curve, and grayish-yellow sandy conglomerate abruptly contacting with brown-red or silty mudstone, are observed in this association (Fig. 8a). Lithofacies association 2 consists of red-brown and silty mudstone with parallel bedding and oil immersion (Florg). The middle interval of core photograph section is shown in Fig. 8b.

resemble the fan-delta front subfacies. Logging recognition marks include a large set of yellow to gray sandy conglomerate and coarse sandstone layers interbedded by gray thin-bedded mudstone mainly due to its underwater depositional environment and its seismic features including sigmoid progradational seismic facies. The typical seismic cross section is shown in Fig. 7c. This subfacies distributes in the middle and north areas of the south sub-sag and can be recognized in 14 wells, such as L1, L5, and L6X. 5.1.2. Channel fill and interchannel facies associations 5.1.2.1. Description. These facies associations are composed of the lithofacies associations including massive yellow, and grayish-yellow sandy conglomerates and conglomerates (Gt), red-brown mudstone, 75

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Fig. 7. Seismic facies recognized in the Wulan-Hua sag. a. Low-amplitude seismic facies, located in the southeast steep slope area of the Teng 1 member, corresponding to Fan delta-plain and shore-shallow lacustrine subfacies; b. Wedge type progradational seismic facies, located in the Aershan Formation, corresponding to main channel micorfacies of inner fan subfacies in sublacustrine fan; c. S type progradational seismic facies, located in the Aershan Formation, corresponding to fandelta front subfacies; d. Mounded seismic facies, located in the east steep slope area of Teng1 member and Aershan Formation, corresponding to fan-delta front subfacies; e. Subtle progradational seismic facies, located in the Aershan Fomation, corresponding to braided delta front subfacies; f. Sheet seismic facies, located in the middle sub-sag areas of Teng1 member and Aershan Formation, corresponding to shore-shallow lacustrine, semi-deep lacustrine subfacies; g. Filled seismic facies, located in the Aershan Fomration, corresponding to main channel micorfacies of inner fan subfacies in Sublacustrine fan (The location of typical seismic corsssections of each seismic facies are shown in Fig. 1).

5.1.3. Muddy facies associations These associations mainly develop in the north area of the south sub-sag of the Wulan-Hua sag.

Single well feature, including low-amplitude jugged GR curve, is observed in this association (Fig. 8a). As shown in Fig. 8a, these facies associations develop in the south of the Wulan-Hua sag, and can be recognized in seven wells (such as L10, L42, and L45X) as reflected by the logging features, including thinlayered brown-red mudstone bedded in a large set of conglomerates.

5.1.3.1. Description. These associations mainly include gray to black mudstones (Florg) and sandy mudstones containing thin-bedded sandstones (Fmg) (logging lithology in Fig. 9) and can be recognized in LD3, LD2, L21X, and L2X by the low-amplitude jugged GR curve features in well log. The single well features are indicated in Fig. 9.

5.1.2.2. Interpretation. Lithofacies association 1 can be interpreted as channel fill deposits of fan-delta plain subfacies with a coarse granularity, yellow and grayish-yellow color, high shale content, massive beddings, scoured surfaces, conglomerate with directive property; its well log marks are commonly related to the half-jugged box-shaped or bell-shaped GR curve, smooth bell-shaped SP curve, and grayish yellow sandy conglomerate abruptly contacting with brown-red or silty mudstone. Lithofacies association 2 is regarded as interchannel microfacies of fan-delta plain subfacies characterized by very-fine granularity silty or muddy sandstone, brownish-red color, and low-amplitude jugged GR curve in well log. The coarse-grained facies association (facies association 1) is observed as channel fills on the delta plain, whereas facies association 2 is considered to represent interchannel deposits (McPherson et al., 1987). Channel fill and interchannel facies associations is related to the fandelta plain subfacies. This subfacies mainly develops in the south of the Wulan-Hua sag and can be recognized in seven wells (including L10, L42, and L45X) as reflected by the low-amplitude jugged GR curve features in the well logs (Fig. 8a).

5.1.3.2. Interpretation. The fine-grained muddy facies associations are interpreted as prodeltaic facies with distinguishing features, such as middle-layered conglomeratic sandstone interbedded in a large set of silty mudstone in logging. Prodeltaic facies has limited distribution as reflected by the less penetrated mudstones in the wells of the WulanHua sag.

5.2. Braided-delta deposits Braided-delta deposits are considerably finer grained than fan-delta deposits (McPherson et al., 1987). This type of depositional system is widely developed in the Aershan Formation and is located on gentle slope of half graben. The fine-grained muddy facies associations interpreted as prodeltaic facies of braided-delta, which are similar to the aforementioned prodeltaic facies of fan-delta deposits. 76

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Fig. 8. Single well and core photographs section of lithofacies association 1 and lithofacies association 2 of fa- delta plain subfacies. a. Single well sedimentary facies map of Well L42, indicating single well features of proximal subfacies (See Well location in Fig. 1). SQ3 = sequence 3, SQ2 = sequence 2. b. core photographs section of well L42 in 931.6–934.1 m depth, all three intervals contains yellow sandy conglomerates and red-brown conglomerate to coarse grained sandstones associated in fining-upward trend, while middle interval contains interbedded sandstones and red-brown mudstones, and silty mudstone with oil immersion, indicating facies association of proximal fan-delta subfacies (The location of these cores are indicated in Fig. 8a).

core photograph section in Fig. 10b). Lithofacies association 2 is composed of fine sandstone and siltstone displaying sediment deformation structures and gray muddy sandstone with cross and parallel bedding and has a scoured surface (the middle interval of the core photograph section in Fig. 10b). Lithofacies association 3 consists of upward-fining, fine sandstone and siltstone with erosional base (the upper interval of the core photograph section in Fig. 10b). The three facies are 2.13 m

5.2.1. Underwater distributary channel–delta-front facies associations 5.2.1.1. Description. These facies associations include massive brownish-gray conglomeratic sandstone (Sm) with oil immersion, fine sandstone and siltstone displaying sediment deformation structures, and gray muddy sandstone with cross and parallel bedding (Sp). Lithofacies association 1 comprises massive brownish gray conglomeratic sandstone (Sm) with oil immersion (the lower interval of the 77

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5.2.1.2. Interpretation. From Table 1, these lithofacies associations can be interpreted as underwater distributary channel-delta-front deposits with coarse granularity, gray color, low shale content, massive bedding, cross bedding, parallel bedding, sourced surface, well log features (such as weak jugged box-shaped GR curve), and seismic features (such as subtle progradational seismic facies proposed by Zeng et al., 2015) (Galloway and Hobday, 1983; McPherson et al., 1987; Rhine and Smith, 1988, Zeng et al., 2015). These associations are mainly observed in the southwest gentle slope areas of the Wulan-Hua sag. 5.3. Shallow-lacustrine and deep-lacustrine fine-grained deposits Shallow-lacustrine and deep-lacustrine fine-grained deposits are widely developed in the Wulan-Hua sag. 5.3.1. Fine-grained shallow-lacustrine facies associations 5.3.1.1. Description. These facies associations include lithofacies association 1 with deep gray mudstone, lithofacies association 2 with gray muddy siltstone, and lithofacies association 3 with yellow siltstone and sandy conglomerate. As shown in the logging in Fig. 11, thinbedded gray muddy siltstone, yellow siltstone and sandy conglomerate interbedded with thick-bedded dark mudstones. These facies associations can be recognized in most wells of the Wulan-Hua sag as obtained from the finger-shaped GR curve features in the well log. Fig. 11 shows the single-well characteristics of shallowlacustrine subfacies in well L3. 5.3.1.2. Interpretation. From Table 1, these lithofacies associations can be interpreted as shallow-lacustrine fine-grained deposits with distinguishing features, such as thin-bedded gray muddy siltstone, yellow siltstone, and yellow sandy conglomerate interbedded with thick-bedded dark mudstones in logging; and seismic features, such as low-amplitude and sheet seismic facies (Bustillo and Alonso-Zarza, 2007). These associations are relatively thin but widespread in the Wulan-Hua sag. 5.4. Sublacustrine-fan deposits Sublacustrine-fans refer to depositional systems composed of subaqueous gravity-flow deposits formed in a sublacustrine environment (Sullwold, 1960). Fans may be fed by subaqueous channels in front of deltas or directly from basin margins. Sublacustrine-fan deposits have been described from many modern lake environments and identified in ancient lacustrine basin fills (Scholz and Rosendahl, 1990; Nelson et al., 1995, 1999). Sublacustrine-fan lithofacies associations include lithofacies association 1 of conglomeratic sandstone with graded bedding (Sm), lithofacies association 2 of grayish-brown oil immersed fine-grained sandstone with parallel bedding (Sp), lithofacies association 3 of sandstone and siltstone with wavy cross-bedding (Sr), lithofacies association 4 of mudstone with horizontal bedding (Sh), and lithofacies association 5 of massive mudstone (Fl). These lithofacies associations correspond to the A, B, C, D, and E sections of the Bouma sequence. 5.4.1. Main channel facies associations 5.4.1.1. Description. These facies associations develop incomplete Bouma sequence, and have many poorly-sorted and poorly-rounded superimposed massive conglomeratic sandstones and structures such as graded, wavy bedding and scoured structure in the bottom (Fig. 12b). The core photograph section of L12X in Fig. 12b shows that the lithological association evolves from conglomeratic sandstone with graded bedding, sandstone and siltstone with wavy cross bedding, and massive mudstone (ACE of Bouma sequence) in the bottom to conglomeratic sandstone with wavy and lenticular bedding, and massive mudstone (CE of Bouma sequence) in the middle and to sandstone with load structure, and mudstone with horizontal bedding (CD of Bouma

Fig. 9. Single well sedimentary facies map of Well L21X, indicating single well features of prodeltaic subfacies of fan-delta deposition system (See Well location in Fig. 1). SQ2 = sequence 2.

thick, have upward-coarsening parasequences with low shale content, and are well-sorted. This facies association can be observed in three wells, namely, LD1, L1, and L42, as reflected by weak jugged box-shaped GR curve features in well log (Fig. 10a).

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Fig. 10. Single well and core photographs section of sublacustrine channel microfacies of braided-delta front subfacies. a. single well sedimentary facies map of Well L5, indicating single well features of braid-delta facies (See Well location in Fig. 1). SQ1 = sequence 1. b. core photograph of well L5 in 1752.48–1754.61 m depth, its upper interval is massive brownish grey conglomeratic sandstone (Sm) with oil immersion, middle interval is fine sandstone and siltstone displaying sediment deformation structures, grey muddy sandstone with cross bedding and parallel bedding, and scoured surface, and the lower interval is upward-fining, fine sandstone, siltstone with erosional base, indicating facies association of sublacustrine channel microfacies in braided-delta front subfacies (The location of these cores are indicated in Fig. 10a).

progradational seismic facies in the section along the provenance direction (Scholz and Rosendahl, 1990; Nelson et al., 1995, 1999).

sequence) in the top. These facies associations can be recognized in L12X as observed from the middle- to high-amplitude jugged box-shaped GR curve and middle-amplitude jugged box-shaped AC curve (Fig. 12a).

5.4.2. Braided channel facies associations 5.4.2.1. Description. These facies associations are mainly characterized by incomplete Bouma sequence (A, B, C, and E sedimentary intervals), coarse and conglomeratic sandstones with low shale content, good physical property, massive and oblique bedding, and obvious scoured surface (lower interval of core photograph section in Fig. 13b). These facies associations can be observed in five wells including LD3, L12X, and L17X, as observed from the high jugged box-shaped GR curve and weak amplitude jugged box-shaped AC curve (single well

5.4.1.2. Interpretation. As shown in Table 1, these lithofacies associations can be interpreted as the main channel microfacies of inner fan subfacies with distinguishing characteristics, such as an incomplete Bouma sequence; middle- to high-amplitude jugged boxshaped GR curve and middle-amplitude jugged box-shaped AC curve in the well log; and seismic features such as filled seismic facies in seismic section transecting the provenance direction and wedge-shaped 79

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inner and middle fan subfacies. 5.5. Seismic facies interpretation The term “seismic facies” comes from sedimentary facies, and can be interpreted as the seismic reflection of sedimentary facies. Sheriff and Geldart (1982) defined seismic facies as seismic characteristics formed from depositional environments such as marine or non-marine environment. Seismic facies analysis “indicates interpreting the environmental settings and lithofacies by using seismic data” (Vail, 1977). On the basis of the seismic facies division and representation method proposed by Yu in 2008 (Table 2), five seismic facies of the Wulan-Hua sag in seismic sections are identified, namely, low-amplitude, mounded, progradational, sheet and filled seismic facies. 5.5.1. Low-amplitude seismic facies 5.5.1.1. Description. This facies is characterized by parallel contact relation in the top and downlap contact relation in the bottom. It shows a wedge external form, foreset internal reflection configuration, and continuous low-amplitude reflection attribute (Fig. 7a). 5.5.1.2. Interpretation. The magnitude of the apparent amplitude can be interpreted as the difference in lithologies between the strata overlying and underlying the interfaces. The upper part of fan-delta plain subfacies is situated above lake level and largely affected by external factors. It displays mixed mud and sands, low acoustic impedance difference between lithology of the strata overlying and underlying the interfaces, low reflectance and low reflection wave amplitude of seismic interface (Mchargue and Webb, 1986). Therefore, we interpret the low-amplitude seismic facies corresponds to fan-delta plain deposits. 5.5.2. Mounded seismic facies 5.5.2.1. Description. This facies is featured by truncation contact relation in the top and downlap contact relation in the bottom and mound external form. It displays a bidirectional downlap internal reflection configuration and middle-amplitude middle-continuity middle-amplitude reflection (Fig. 7d). 5.5.2.2. Interpretation. This seismic facies can be considered the products of sedimentation in a high-energy environment, representing the rapid unloading of sediments. The inner part of a large-scale 3D mound is commonly characterized by bidirectional downlap reflection, which is a characteristic of delta transverse section. Therefore, the corresponding deposits of mounded seismic facies is fan-delta front deposits.

features of well LD3 in Fig. 13a).

5.5.3. Progradational seismic facies 5.5.3.1. Description. This facies is characterized by parallel contact relation in the top and downlap contact relation in the bottom and wedge external form. It shows a sigmoidal foreset internal reflection configuration and middle-amplitude middle-continuity low-amplitude reflection. This facies consists of S-type, wedge-type, and subtle progradational facies (Fig. 7b, c, and e).

5.4.2.2. Interpretation. The developed A, B, C, and E sedimentary intervals of Bouma sequence in these facies associations are interpreted as braided channel microfacies of middle fan subfacies with an incomplete Bouma sequence (A, B, C, and E sedimentary intervals); relative coarse granularity, low shale content, good physical property; structures such as massive, oblique bedding, and obvious scoured surface; and well log features such as high jugged box-shaped GR curve and weak amplitude jugged box-shaped AC curve. (Scholz and Rosendahl, 1990; Nelson et al., 1995, 1999). These facies associations develop in the middle fan subfacies of sublacustrine deposits. The sublacustrine-fan of the Wulan-Hua sag can be divided into the

5.5.3.2. Interpretation. Progradational seismic facies is a recognizable seismic reflection configuration and has obvious environmental importance. It is composed of a set of seismic events dipping toward the same direction, which have angle or tangent contacts with underlying horizontal seismic events. This seismic facies commonly represents delta deposition in continental basins and belongs to the seismic response of foreset deposits during the migration process of the delta depositional system to the basin direction (Sangree and Widmier, 1978; Brown and Fisher, 1982). The corresponding deposits of this seismic facies includes fan-delta front, braided-delta front, and sublacustrine-fan deposits.

Fig. 11. Single well sedimentary facies map of Well L3, indicating single well features of shallow lacustrine subfacies (See Well location in Fig. 1). SQ3 = sequence 3.

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Fig. 12. Single well and core photographs section of main channel microfacies of inner fan subfacies. a. Single well sedimentary facies map of Well L12X, indicating single well features of inner fan subfacies in sublacustrine-fan depositional system (See Well location in Fig. 1). SQ1 = sequence 1. b. core photograph of well L12X in 2144.1–2147.41 m depth, upper part develops CD sedimentary intervals (Bouma sequence), middle part develops CE sedimentary intervals, and lower part develops ACE sedimentary intervals, indicating lithofacies of inner fan subfacies in sublacustrine-fan depositional system (The location of these cores are indicated in Fig. 12a).

5.5.5. Filled seismic facies 5.5.5.1. Description. This facies is characterized by parallel and subparallel contact relations in the top and downlap contact in the bottom and filled external form. It shows wavy internal reflection configuration, and middle-amplitude middle-continuity reflection (Fig. 7g).

5.5.4. Sheet seismic facies 5.5.4.1. Description. This facies is characterized by relative stable thickness, parallel and subparallel contact relations in the top and bottom, and sheet external form. It displays parallel and subparallel internal reflection configurations, and middle-amplitude middlecontinuity middle- and low-amplitude reflection (Fig. 7f).

5.5.5.2. Interpretation. Filled seismic facies typically indicates a localized underwater erosional channel and can commonly indicate a submarine canyon and turbidity current channel scouring formed during the decline of the sea or lake level (Yin et al., 2004). The corresponding deposits of this seismic facies is fan-delta front deposits.

5.5.4.2. Interpretation. Sheet seismic facies has a much larger horizontal extent than its stratigraphic thickness and belongs to the products of sediments vertical aggradation. A parallel sheet shape commonly indicates stable sedimentary environments, such as deep marine (lacustrine) and semi-deep marine (lacustrine) (Yin et al., 2004). While subparallel sheet shape commonly represents unstable environments, such as shallow-shore marine (lacustrine), alluvial fan, and delta plain (Yin et al., 2004). The corresponding deposits of this seismic facies are shore-shallow and semi-deep lacustrine deposits.

6. Sequence stratigraphic interpretation On the basis of former studies on lithofacies, seismic facies, and depositional systems of the study area obtained by sequence 81

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Fig. 13. Single well and core photographs section of braid channel microfacies of middle fan subfacies. a. single well sedimentary facies map of Well LD3, indicating single well features of middle fan subfacies in sublacustrine-fan depositional system (See Well location in Fig. 3). SQ1 = sequence 1. b. core photograph of well LD3 in 777.59–784.07 m depth, developing ACE sedimentary intervals (Bouma sequence), indicating lithofacies of middle fan subfacies in sublacustrine-fan depositional system (The location of these cores are indicated in Fig. 13a). Table 2 Seismic facies division and representation method (Yu, 2008). Contact relationship of sequence top or bottom

Recognition marks of seismic facies Geometric parameter

Physical parameter

Top contact

TC

Bottom contact

BC

External form

EF

Internal reflection configuration

IRC

Reflection attribute

Parallel Top lap

P T

Parallel On lap

P O

Sheet Drape

S D

Parallel Divergent

P D

Amplitude

D

Wedge Bank

W B

Wavy Foreset

W F

Continuity

Mound Lens Fill

M L F

Hummocky Clinoform Bidirectional Downlap Eyeball

Hc Bd E

Frequency

Truncation Tr Downlap Example of Seismic facies code: P-D/WF-MMM P-Parallel top contact D-Downlap bottom contact WF-Wedge foreset reflection configuration MMMMiddle-amplitude middle-continuity middle-frequency

High Middle Low High Middle Low High Middle low

H M L H M L H M L

6.1. First- and second-order sequence identification

stratigraphy and artificial synthetic seismogram of forward model, six sequence boundaries are found in wells and their corresponding seismic sections. Accordingly, the Wulan-Hua sag is divided into one first-order sequence, three second-order sequences and five third-order sequences.

The first-level unconformity interface is the top and bottom interfaces of the first-order sequence, which represents long-term depositional discontinuity from several to tens of million years with obvious differences in seismic reflection and lithology between the upper and lower interfaces. We consider the entire rift basin evolution stage of the 82

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Fig. 14. Sequence boundary identification in well L42 and its corresponding seismic section AA′, indicating the MSB1/SSB1/SB1 as lithologic changing interface in wells and its upper strata shows progradational and onlap phenomenon in seismic The location of well L42 and seismic section AA′ are shown in Fig. 1.

Fig. 15. Sequence boundary identification in well L4 and its corresponding seismic section BB′, indicating the SSB3/SB4 as lithologic changing interface in wells and its upper strata shows onlap phenomenon in seismic, and onlap phenomenon also exist in SSB2/SB2, SB3, SSB4/SB5 (The location of well L4 and seismic section BB′ are shown in Fig. 1).

Wulan-Hua sag as one first-order sequence, namely Megasequence 1 (MSQ1), which is bounded by the bottom boundary of the Arshan Formation (MSB1, namely, Tg seismic interface, which represents the start of initial faulting) and the top boundary of the entire sag (MSB2, namely, T2 seismic interface, which represents the end of faulting, uplift, and erosion) with a duration of 40 Ma. MSB1 is the interface between the Lower Jurassic strata and Aershan Formation (Tg) and is recognized in well L42 of the study area as lithologic changing interface; the lithology of strata underlying the surface is andesite and tuff thereby representing the volcanic eruption and the sedimentary rock in the strata overlying the surface (Fig. 14). MSB2 is the interface between the Saihantala Formation and upper Cretaceous strata. This interface suffers strong erosion and is absent in most areas of the Wulan-Hua sag. On the basis of the limited logging data of shallow strata, we consider the well top boundary as SSB2. MSB1 and MSB2 are in angular unconformity contact with their upper and lower strata, and the overlying strata of MSB1 have onlap and

downlap seismic reflection features (Figs. 14 and 15). The secondary unconformity interface is the second-order sequence boundary, which is a lower-level interface within the first-order sequence boundary. Second-order sequence boundaries are mostly caused by the periodic evolution of the basin. The unconformity interfaces do not have long-term discontinuities and have similar lithologies and sedimentary facies between the upper and the lower strata of interfaces. Rift basin evolution stages of the Erlian Basin can be divided into: initial rifting (deposition of the Aershan Formation), rift basin expanding (deposition of the first member of the Tengge’er Formation), rift basin stable developing and shrinking (deposition of the second member of the Tengge’er Formation), and rift basin dying (deposition of the Saihantala Formation) stages. The start and end of each stage are bounded by obvious unconformities. Given these obvious unconformities (including SSB1, SSB2, SSB3, and SSB4), we subdivide one first-order sequence into three second-order sequences: SSQ1, SSQ2, and SSQ3. SSQ1 corresponds to the initial rifting stage, SSQ2 83

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Fig. 16. Sequence boundary identification in well LD1 and its corresponding seismic section BB′, indicating the SB3 as a shape change interface of SP curve in wells, its upper strata shows onlap phenomenon in seismic, and lower strata shows truncation phenomenon in seismic, and onlap phenomenon also exist in SSB4/SB5, downlap phenomenon recognized in MSQ1/SSQ1/SQ1 (The location of well LD1 and seismic section CC′ are shown in Fig. 1).

phase sequence and combination, change in cycle and superimposition pattern, and change in formation combination and contact relationship (Embry, 1995; Zhu, 2008). SB3 is the top boundary of the lower part of the Teng 1 member in the Tengge’er Formation and is observed in well LD1 as a shape change interface of the SP curve. The SP curve is featured by high base value, apparent jugged shape, and large amplitude under the interface; whereas the SP curve is featured by low base value, weak jugged shape, and small amplitude on the interface (Fig. 15). The overlying strata of the interface exhibit onlap in seismic section (Fig. 15). SB4 is the top boundary of the Teng 1 member in the Tengge’er Formation, is recognized in the well L4, and is a change interface of mudstone color. The color of mudstone, under the interface is brown, which indicate exposure, and is gray and dark-gray on the interface, which indicates an underwater environment (Fig. 16).

corresponds to the rift basin expanding, stable developing, and shrinking stage, and SSQ3 corresponds to the rift basin dying stage of the Wulan-Hua sag (Fig. 15). SSB1 corresponds to the aforementioned MSB1. SSB2 is the interface between the Aershan and Tengge’er formations (T8) and is a color change interface of dolomitic mudstone. The color of dolomitic mudstone is grayish brown under the interface, whereas the color of dolomitic mudstone is dark gray on the interface (Fig. 14). This interface has good continuity and onlap and progradational phenomenon in several seismic profiles in its overlying strata (Fig. 16). SSB3 is the interface between the Tengge’er and Saihantala formations (T3) and is a change interface of lithology. The stratum on the interface is sandstone, which reflects a relative shallow-water environment, and that in the lower part of the interface is dark-gray mudstone, which reflects a deep-water environment. The existing denudation of this interface has a limited distribution in the basin and local onlap of overlying strata (Fig. 15).

6.2.2. Division of third-order sequence SSQ1 is the supersequence formed in the initial rifting bounded by SSB1 and SSB2 (corresponding to T11 and T8 seismic interfaces). It represents the strata of the Aershan Formation, and can be divided further into one third-order sequence including SQ1. SQ1 mainly develops an upward-fining sedimentary rhythm from relatively coarsegrained alluvial-fan to fine-grained shallow-lacustrine deposits. SQ1 is formed in the initial period of the rifting basin fill-up phase. The occurrence, lithologic combination, and seismic reflection of SQ1 greatly differ from the underlying strata. LST corresponds to the bottom of the Aershan Formation, and displays progradation and aggradation sedimentary cyclicity in the section. TST corresponds to the middle of the Aershan Formation and mainly displays regradation and aggradation cyclicity. MFS corresponds to a section of dark mudstone deposited in semi-deep lacustrine subfacies. HST corresponds to the top of the Aershan Formation, and displays progradation sedimentary cyclicity with the water body shallowing upward (the color of mudstone in this section changes from dark to gray). SQ1 is not only a main reservoir stratum, but is also a main source rock layer in the Wulan-Hua sag. LST and HST form the buck of the sequence, whereas TST is poorly developed (Fig. 17). SSQ2 is the supersequence formed during the rift basin expanding and bounded by SSB2 and SSB3 (corresponding to T8 and T3 seismic interfaces), represents the strata of the Tengge’er Formation, and can be further divided into three third-order sequences, namely, SQ2, SQ3, and SQ4. SQ2 represents the lower part of the Teng 1 member in the Tengge’er Formation. This sequence mainly develops light-gray conglomeratic sandstone from fan-delta, braided-delta, and sublacustrine-

6.2. Division of the third-order sequence and boundary characteristics The third-order sequence comprises isochronous strata limited by the third-order unconformity interfaces and their corresponding conformity. The third unconformity interface belongs to the local unconformity surface, which represents sedimentary discontinuities (Embry, 2002). On the basis of the above-mentioned depositional systems, a combination of wells and seismic method is used to divide the third-order sequence of the Wulan-Hua sag and identify its boundary features. The features are discussed as follows. 6.2.1. Third-order sequence boundaries The third-order sequence boundaries comprise unconformity interface or the corresponding conformity interface limited by the secondorder sequence boundaries. In addition to the four secondary sequence boundaries identified in the study area (SSB1/SB1, SSB2/SB2, SSB3/ SB5, and SSB4/SB6), six third-order sequence boundaries are recognized by rock electrical and seismic reflection characteristics (Fig. 17). The boundary types of third-order sequence boundaries in faultdepressed lake basins include local unconformity, sedimentary transformed surface, transformed interface between progradation and retrogradation, and corresponding conformity. The formation of sequence boundaries represents a sudden change in the combined effects of sequence controlling factors on sequence stratigraphy units and stratigraphic superimposed patterns over a period. This abrupt change in sedimentary and stratigraphic characteristics can be summarized as vertical change in single-phase physical property, vertical change in 84

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fan depositional systems and siltstone, interbedded with light-gray and grayish-green mudstone from shallow and deep-lacustrine depositional systems. LST corresponds to the bottom lower part of the Teng 1 member in the Tengge’er Formation and displays progradation and aggradation sedimentary cyclicity in the section. TST corresponds to the middle lower part of the Teng 1 member in the Tengge’er Formation, and it mainly displays regradation and aggradation cyclicity. MFS corresponds to a segment of gray mudstone deposited in fan-delta front subfacies. HST corresponds to the top lower part of the Teng 1 member in the Tengge’er Formation and displays progradation sedimentary cyclicity, and has water body shallowing upward. SQ2 is a main source rock layer in the Wulan-Hua sag. LST and TST form the buck of the sequence, whereas HST is poorly developed and nearly absent. SQ3 represents the upper part of the Teng 1 member in the Tengge’er Formation. This sequence mainly develops light- and deepgray mudstone from shallow and deep-lacustrine depositional systems interbedded with muddy dolomite, muddy limestone, and calcareous sandstone from shore and shallow-lacustrine deposition systems. LST corresponds to the bottom upper part of the Teng 1 member in the Tengge’er Formation and displays progradation and aggradation sedimentary cyclicity in the section. TST corresponds to the middle upper part of the Teng 1 member in the Tengge’er Formation and mainly displays regradation and aggradation cyclicity. MFS corresponds to a segment of grayish-green mudstone deposited in fan-delta front subfacies. HST corresponds to the top upper part of the Teng 1 member in the Tengge’er Formation and displays progradation sedimentary cyclicity, with the water body shallowing upward. SQ3 is a main source rock layer in the Wulan-Hua sag. LST and HST form the buck of the sequence, whereas TST is poorly developed (Fig. 17). SQ4 represents the Teng 2 member in the Tengge’er Formation. This sequence mainly develops gray and deep-gray mudstone from relative deep-lacustrine depositional system in a reducing environment. LST corresponds to the bottom of the Teng 2 member in the Tengge’er Formation and displays progradation and aggradation sedimentary cyclicity in the section. TST corresponds to the middle of the Teng 2 member in the Tengge’er Formation and mainly displays regradation and aggradation cyclicity. MFS corresponds to a section of gray mudstone. HST corresponds to the top of the Teng 2 member in the Tengge’er Formation, displays progradation sedimentary cyclicity, and has water body shallowing upward. SQ4 is a main reservoir stratum in the Wulan-Hua sag. LST and HST form the buck of the sequence, whereas TST is poorly developed (Fig. 17). SSQ3 is the supersequence formed during the rift basin shrinking stage and bounded by SSB3 and SSB4 (corresponding to T3 and T2 seismic interfaces), represents the strata of the Saihantala Formation, and can be further divided into one third-order sequence, namely, SQ5. SQ5 corresponds to the Saihantala Formation and mainly develops light-gray sandy conglomerate; conglomeratic sandstone from fan-delta depositional system; and sandstone, light-gray, purple, and grayishgreen massive mudstone from shallow-lacustrine. LST corresponds to the bottom of the Saihantala Formation and displays progradation sedimentary cyclicity in the section. TST corresponds to the middle of the Saihantala Formation and mainly displays regradation cyclicity. MFS corresponds to a section of gray mudstone. HST corresponds to the top of the Saihantala Formation, displays progradation sedimentary cyclicity, and has a water body shallowing upward. HST forms the buck of the sequence, whereas LST and TST are poorly developed (Fig. 17). 6.3. Key wells correlation After all the sequence boundaries have been defined, we select the key well sections of structural belts (such as subsidence center, depocenter, south sub-sag, and north sub-sag) to establish an inter-well sequence stratigraphic correlation section. The development characteristics of system tract under each third-order sequence are analyzed by examining the superimposition style.

Fig. 17. The general sequence stratigraphic figure of the Wulan-Hua sag, south of Erlian basin. The sequence stratigraphy hierarchy is mainly based on the changes episodic tectonic activities, lithology and detailed work of this paper. SB = Sequence Boundary, SQ = Sequence, SSQ = upersequence, SSQB = Supersequence Boundary, MSQB = Megasequence Boundary.

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Fig. 18. Inter-well section of well LD1-L20X-L22X-LD3-L2X-LD2-L21X-L3-L31 in study area connects the south sub-sag with north sub-sag of the Wulan-Hua sag, indicating development characteristics of its third order sequence (see Fig. 1 for the location of this section, marked by MM′).

third-order sequences (SQ1, SQ2, SQ3, SQ4, and SQ5) of the WulanHua sag are determined (Fig. 18). Each third-order sequence corresponds to a depositional cycle, which is composed of LST, TST, and LST. On the basis of contiguous interpretation of 3D seismic data, we classify the first- and second-order sequence boundaries as regional unconformity interfaces, and the third-order sequence boundary as local unconformity interfaces and corresponding conformity interfaces characterized by toplap/transitional and syndepositional truncating surfaces in the middle and outer areas of the slope belt and conformity surface in the sedimentary center.

The inter-well section of wells LD1, L20X, L22X, LD3, L2X, LD2, L21X, L3, and L31 (Fig. 18) in the study area connects the south sub-sag with the north sub-sag of the Wulan-Hua sag and develops five thirdorder sequences, namely, SQ1, SQ2, SQ3, SQ4, and SQ5. SQ1 deposits in the north sub-sag are generally thicker than that in the south sub-sag. Two depocenters are located in the north and south sub-sags. This phenomenon indicates the initial development of the rift sag. SQ2-4 deposits in the south sub-sag are generally thicker than that in the north sub-sag. The depocenter and subsidence center of the Wulan-Hua sag move southward. The developmental periods of these sequences can be interpreted as the rift sag expansion, during which the north and south sub-sags connected to form an entire lacustrine basin. SQ5 of the section has limited development, the sequences in most wells are incompletely developed, and SQ5 in the north sub-sag area is less developed than the sequence in the middle and south sub-sag areas. This phenomenon indicates the rift sag shrinking stage, during which the entire sag undergoes intensive uplift and erosion, and the north sub-sag is more eroded than the south sub-sag.

7. Distribution of depositional systems 7.1. Provenance analysis The directions of sediment supply can be defined by the variation of sediment granularity, mudstone color, and diplog interpretation. Sediment granularity, which decreases from the source area to the deposition area, can indicate provenance directions. Mudstone color can indicate sedimentary environments; gray and dark colors indicate underwater reducing environments, and red and brown colors indicate land oxidation environment. Provenance is due to migration from a land environment to an underwater environment. Diplog interpretation indicates directions of paleocurrents, which can also be regarded as provenance directions. The sediment granularity data in SQ1 indicates that five provenance directions have been identified including the south, southwest, northeast, northwest, and east, which correspond to the directions obtained from diplog interpretation (Fig. 19). Results also show that the provenance directions of SQ2 and SQ3 are south and east, however, the provenance directions of SQ4 are south, east, and west (Figs. 20–22).

6.4. Sequence stratigraphic framework On the basis of former comprehensive methods, six sequence boundaries (SB1-SB6) of the Wulan-Hua sag are identified, namely, SB1 (the bottom boundary of the Aershan Formation), SB2 (the bottom boundary of the Tengge’er Formation), SB3 (the middle interface of the first member of the Tengge’er Formation), SB4 (the top boundary of the first member of the Tengge’er Formation), SB5 (the top boundary of the second member of the Tengge’er Formation), and SB6 (the top boundary of the Saihantala Formation with erosional characteristics). After these sequence boundaries are identified, one first-order sequence, three second-order sequences (SSQ1, SSQ2, SSQ3), and five 86

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Fig. 19. Provenance analysis map of SQ1, indicating provenance directions of the Wulan-Hua during the deposition of SQ1 are South, Southwest, Northeast, Northwest, and East directions. SQ1 = Sequence 1.

considerably enlarged, thereby indicating the shrinking of lacustrine in the north sub-sag; and the subaqueous-fan has a large distribution. Plane distributions of depositional systems in SQ1, SQ2 and SQ3 also indicate basin evolution. During the period from SQ1 deposition to SQ3 deposition, provenance changes from multi-directions to single direction, depositional system types change from multi-types to single type, water body changes from deep to shallow, sand bodies change from small-scale distribution to large-scale distribution, the depocenter migrates southward, and the subsidence center migrates northward.

7.2. Distribution of depositional systems The distributions of depositional system in SQ1, SQ2, and SQ3 are defined based on previously identified depositional systems according to lithofacies associations and seismic facies in combination with the results of provenance analysis and geological setting of the Wulan-Hua sag (Figs. 23–25). In SQ1, semi-deep lacustrine depositional system commonly develops in the entire sag. Fan-delta mainly develops near provenance areas, and the fan-delta in the south sag is more developed than that in the north sag. Shore and shallow lacustrine develop in the middle sag, and limited subaqueous-fan develops in the east area of the north sub-sag. The distributions of deposition systems in SQ2 are listed as follows: a semi-deep lacustrine depositional system develops only in the north and small areas of the middle sag; fan-delta develops only in the south and some areas of the middle sag; shore and shallow lacustrine deposits develop in the north sag; and limited subaqueous-fan develops in the east area of the north sub-sag (with a larger distribution than that in SQ1). The distribution of depositional systems in SQ3 is similar to that in SQ2 with only the following differences: the distribution extent of the semi-deep and deep lacustrine system is evidently reduced; the distribution of the shore and shallow lake system is

8. Discussion Controls on stratigraphic sequence, depositional systems and favorable areas for oil and gas prospecting in the Wulan-Hua sag are discussed in this section. 8.1. Controls on stratigraphic sequence of the Wulan-Hua sag The stratigraphic architecture of the Wulan-Hua sag can be controlled by four allogenic factors, namely, tectonics, provenance, climate, and relative lake level, tectonics being the main controlling 87

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Fig. 20. Provenance analysis map of SQ2, indicating provenance direction of the Wulan-Hua during the deposition of SQ2 is South direction. SQ2 = Sequence2.

During the deposition of SQ1, the boundary faults of the Wulan-Hua sag were activated by NW-SE regional stress. In addition, a rift basin emerged, accompanied by the formation of a lake within short period. In this period, the sag was initially subdivided into two sub-sags by the Tumur uplift. The north sub-sag formed as a single-faulted dustpan-like structure with east-faulting and west-overlapping due to the east boundary fault, and the south sub-sag formed as a double-faulted structure because of two boundary faults. Accommodation increased gradually, A/S ratio was more than 1, and the sag was dominated by subsidence with limited sedimentation of SQ1 (Fig. 26). Fig. 26a indicates that two boundary faults (referred as first-level faults) in the south sub-sag control the paleogeomorphology of the Wulan-Hua sag and thus control the development characteristics of SQ1. Four second-level faults directly control the development characteristics of SQ1, particularly in highly developed downthrown wall strata of these normal faults. Fig. 26b shows the typical tectonic development of a single-faulted dustpan-like structure. The east boundary fault controls the development characteristics of SQ1. The east area has the largest thickness of SQ1, which decreases considerably from east to west.

factor. 8.1.1. Tectonics Tectonism, increases or decreases accommodation, alters the depositional base level, controls the distribution of source areas, and influences local climatic patterns (Williams, 1993; Katz and Liu, 1998; Ravnas and Steel, 1998). Furthermore, tectonic movements control the A/S ratio during the accumulation of the Wulan-Hua sag. Decreasing A/ S ratio, which is responsible for the generation of unconformities, can be linked to tectonic uplift of the sag. In turn, increasing A/S ratio is linked to subsidence in the sag. LST represents a low subsidence context, whereas HST is related to widespread increase in subsidence. The tectonic evolution of the Wulan-Hua sag has two major phases: syn-rift and post-rift phases. The syn-rift phase can be divided further into four stages, namely, episodes 1, 2, 3 and 4 (Fig. 17). Episode 1 of syn-rift represents the period during the deposition of the Jurassic strata. The Jurassic formation is not developed in the Wulan-Hua sag. Thus, we do not discuss this period here. (1) Episode 2 of syn-rift phase

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Fig. 21. Provenance analysis map of SQ3, indicating provenance direction of the Wulan-Hua during the deposition of SQ3 is South direction. SQ3 = Sequence 3.

characteristics of single-faulted dustpan-like structure successively develop on the paleogeomorphology of deposited SQ1 (Fig. 26b).

(2) Episode 3 of syn-rift phase During the depositions of SQ2 and SQ3, many NW-SN arc transtensional fault systems formed in the south sub-sag with the change in the direction of regional stress. All these arc transtensional faults were activated intensively, thereby controlling the sequence development. Meanwhile, early-formed boundary faults were still active, thus, the accommodation increased rapidly, as well as the lake basin expanded. During this period, the extent of the lacustrine basin expanded, the lake became deeper than it previously was, and the northwest sag started to uplift gradually during the deposition of SQ2 (Fig. 26). The two first-order faults in the south sub-sag successively controlled the paleogeomorphology of the Wulan-Hua sag after the deposition of SQ1 thereby controlling the development of SQ2 and SQ3. The four second-order faults continuously develop and control the development characteristics of SQ2 and SQ3, particularly in the highly developed downthrown wall strata of these normal faults. Two thirdorder faults (adjusting faults) partly affect the development of SQ2 and SQ3 (Fig. 26a). Apart from the main controlling effects of successively developed east boundary fault in the north sub-sag, two second-order faults can also control the development of SQ2 and SQ3. Typical

(3) Episode 4 of syn-rift phase Many inverse tectonic units formed during the deposition of SQ4. The activity of the main faults weakened due to the change in regional stress. The strata experienced erosion because of intensive uplift in the south and north boundaries, and SQ4 was absent in some areas of the Wulan-Hua sag. The north part of the Wulan-Hua sag experienced intensive fold deformation, thereby forming shear and reversal anticlines (Fig. 26). In the south sub-sag, fault activities become intensive. Moreover, the controlling effects from two first-level faults, four second-level faults and two third-level faults on the development characteristics of SQ4 became evident. After the deposition of SQ4, the west area of the south sub-sag suffers erosion, and the thickness of SQ4 in this area became thinner than it previously was (Fig. 26a). In the north sub-sag, the above-mentioned east boundary fault and two second-level faults successively developed and controlled the development characteristics of SQ4. Prior to the deposition of SQ5, the SQ4 experienced greater 89

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Fig. 22. Provenance analysis map of SQ4, indicating provenance direction of the Wulan-Hua during the deposition of SQ4 are South and East directions. SQ4 = Sequence 4.

area control the types of sediments in basin. Provenance analysis results indicate that SQ1 has three provenance directions, namely, south, southwest, and east. The provenance direction of SQ2 and SQ3 is south, and the provenance directions of SQ4 are south and west (Figs. 18–21). Tectonic controls accommodation creation and provenance controls sediment supply. A/S (the ratio of accommodation to sediment supply), which considers the two main controlling factors of sequence stratigraphy, can explain the evolution of the Wulan-Hua sag and other related rift lacustrine basins. During episode 2 of syn-rift stage (rift basin initially forming stage), A/S is positive and less than 1, and LST is more developed than HST and TST. During episode 3 of the syn-rift stage (rift basin expanding stage), A/S is more than 1, and TST is more largely developed than LST and HST. During episode 4 of the syn-rift stage (rift basin shrinking stage), A/S is positive and less than 1, and LST is more developed than HST and TST. During the post-rift stage (rift basin dying stage), A/S is less than 1 or negative, LST is largely developed, and some areas have erosion or depositional break.

erosion in the west part than in the south part (Fig. 26b). (4) Post-rift phase After the deposition of SQ5, the entire sag experienced uplift and erosion, and SQ5 was absent in most areas of the Wulan-Hua sag. In the south sub-sag, the activities of all the aforementioned faults weakened. These weak-activated faults combined with three newly developed third-level faults adjusted the development characteristics of SQ5. SQ5 experienced great erosion and exhibited considerable thickness in the middle part, and its thickness decreased gradually in both sides (Fig. 26a). In the north sub-sag, SQ5 had large thickness in the east and middle areas, and its thickness decreased considerably from the middle to the west area (Fig. 26b). 8.1.2. Provenance The secondary controlling factor is provenance. Provenance scale controls the scale of the sand body under the sequence stratigraphic framework. The distribution of provenance determines the distribution range of the sand body, and the types of sediments in the provenance 90

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Fig. 23. Plane distribution of depositional systems in SQ1, Wulan-Hua sag, indicating semi-deep lacustrine depositional system commonly develops in the whole sag; fan-delta mainly develops in near provenance areas and south of sag is more developed than its north; shore and shallow lacustrine develops in middle of sag; with limited developed subaqueous-fan in east area of north sub-sag.

fluctuated between a mainly balanced-fill and periodically overfilled basin since the early Holocene (Carroll and Bohacs, 1999). At times of overfill and throughflow of water, lake level fluctuations are controlled by changes in the elevation of spillover points; elevation of spillover points is influenced by uplift, erosion, and stream piracy (Carroll and Bohacs, 1999). The influence of spillover on lake level changes and the generation of third-order sequence boundaries in the Erlian Basin are the focus of ongoing investigations.

8.1.3. Climate and lake level fluctuation Lake level fluctuation controls the facies association and distribution under the sequence framework, and climate controls sediment supplies and types. These points are ignored in this study because of limited data. We discuss the two controlling factors only on the basis of former studies. Under arid climatic conditions, underfilled lacustrine basins experience lake level falls related to evaporation (Carroll and Bohacs, 1999). However, humid climatic or semi-humid conditions have existed throughout the depositional history of Erlian Basin. Mao (2015) found that the paleoclimate of Erlian Basin is semi-humid during the deposition of SQ1 and SQ2, humid during the deposition of SQ3 and SQ4, and semi-humid paleoclimate during the deposition of SQ5. The Erlian Lake is a balanced-fill basin (Carroll and Bohacs, 1999) for most of its history, and has accommodation approximately equal to sediment and water fill. Under the prevailing humid climatic conditions, considerable lake level changes are unlikely in such a lacustrine system. To generate third-order sequence boundaries, the lake must be periodically overfilled. Similarly, Lake Malawi in East Africa has

8.2. Controls on depositional systems Controls on depositional systems include sequence framework, tectonics, and provenance. A sequence stratigraphic framework is established by recognizing different unconformity levels and their correlative conformity control vertical association and plane distribution of depositional systems. The established sequence stratigraphic framework shows that sandbodies in shore-lacustrine facies, sublacustrine-fan, fan-delta plain and fan-delta front facies mainly develop in LST of third-order sequence. As shown in 91

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Fig. 24. Plane distribution of depositional systems in SQ2, Wulan-Hua sag, indicating semi-deep lacustrine depositional system commonly develops only in north and a small areas of middle sag; fan-delta only develops in south and some areas of middle sag; shore and shallow lacustrine develops in north of sag; with limited developed subaqueous-fan in east area of north sub-sag.

change from fan-delta plain to fan-delta front to lacustrine facies and sublacustrine-fan, and sediments change from coarse-grained conglomerate to very fine-grained mudstone.

Fig. 27a, sandbodies in shore-lacustrine facies mainly develop in the west part of LST in SQ1, and sublacustrine-fan mainly develops in the east and west areas of LST in SQ1 and the middle area of LST in SQ2. Fan-delta plain and fan-delta front are widely developed in section in SQ2 and SQ3 and display progradational and retrogradational characteristics. Fig. 26b shows that sandbodies in shore-lacustrine facies mainly develop in the west of LST in SQ1, and sublacustrine-fan mainly develops in the middle area of LST in SQ1. Fan-delta plain and fan-delta front are widely developed in the section, mainly in SQ2, SQ3, and SQ4 and display progradational and retrogradational characteristics. Tectonic activities especially fault activities, control the types, scale and distributions of sandbodies that develop in various depositional systems (Ge et al., 2018a,b). Sandbodies mainly extend along the fault plane to the downthrown wall direction, and syndepositional faults enlarge the distribution and thickness of sandbodies by increasing accommodation during sedimentation. Provenance controls the sediment partition and development characteristics of depositional systems. From the provenance area to the depocenter of the Wulan-Hua sag shown in Fig. 27b, deposition systems

8.3. Distinguishing characteristics of the Wulan-Hua sag The Wulan-Hua sag is a small-scale lacustrine rift sag with the following distinguishing characteristics. 1) it has two sub-sags, namely, the north sub-sag, which is characterized by a single-faulted dustpanlike structure with east-faulting and west-overlapping, and the south sub-sag with large areas characterized by a double-faulted structure. 2) five depositional systems develop in the Wulan-Hua sag including fandelta, braided-delta, shallow-lacustrine, deep-lacustrine, and sublacustrine-fan. Fan-delta deposits are widely observed in areas adjacent to basin margin faults in SQ2 and SQ3. Braided-delta deposits are located on the gentle slope of the north sub-sag in SQ1. Shallow- and deeplacustrine deposits are mostly extensively developed depositional systems distributed in the centers of the two sub-sags. Sublacustrine-fan deposits mainly develops in SQ1 of the Wulan-Hua sag. 3) SQ1 is more 92

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Fig. 25. Plane distribution of depositional systems in SQ3, Wulan-Hua sag, indicating the similar distribution of depositional systems in SQ2 with shrank distribution extent of semi-deep and deep lacustrine system and enlarged distribution extent of shore and shallow lake system.

8.4. Favorable areas for oil and gas prospecting

developed in the north sub-sag than in the south sub-sag due to tectonism; SQ2, SQ3 and SQ4 are more developed in the south sub-sag than in the north sub-sag; SQ5 is more developed in the north sub-sag than in the south sub-sag due to differential uplift and erosion. 4) SQ1 has three provenance directions, namely, south, southwest, and east; the provenance direction of SQ2 and SQ3 is south; and the provenance directions of SQ4 are south and west. And 5) post-rift stage and three episodes of syn-rift stage affect the sequence development, sediment partition, and evolution of the Wulan-Hua sag. In general, this sag is characterized by two typical types of fault depression, multi-depositional systems controlled by different faults levels, variable development characteristics of third-order sequences affected by tectonism and provenance, multi-provenances, and multidepocenters. Therefore, the analysis of depositional systems and establishment of a sequence stratigraphic framework in the small-scale rift lacustrine Wulan-Hua sag provide not only a typical example for further exploration and development of other small-scale lacustrine rift sags in the Erlian Basin (such as the Saihantala and Bayindulan sags), but also a detailed model for studying sequence and sedimentation in other similar rift basins worldwide.

The Aershan Formation and the first member of the Tengge’er Formation (namely, SQ1, SQ2, and SQ3) are target strata for petroleum exploration in the Wulan-Hua sag. On the basis of plane distribution, and vertical development of depositional systems under the established sequence stratigraphic framework, favorable areas for oil and gas prospecting in the Wulan-Hua sag are predicted. The middle and south areas of the south sub-sag, which belong to distributary channel and sheet sand of fan-delta plain subfacies, are favorable areas for the formation of the lithologic reservoirs in SQ1; the central area of south sub-sag, which belongs to the distributary channel and sheet sand of fan-delta plain subfacies are areas that are conducted to the formation of lithologic reservoirs in SQ2 and SQ3 (Figs. 23–25, 27a, and 27b). These areas are near provenances with developed sand bodies. 9. Conclusions (1) The major depositional systems in the Erlian Basin are fan-delta, 93

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Fig. 26. Balanced section illustrating sequence evolution characteristics of the Wulan-Hua sag. a. seismic line CC′, indicating the sequence development characteristics of the south sub-sag in the Wulan-hua sag; b. Seismic line NN′, indicating the sequence development characteristics of the north sub-sag in the Wulan-hua sag. SQ1 = Sequence 1; SQ2 = Sequence 2; SQ3 = Sequence 3; SQ4 = Sequence 4; SQ5 = Sequence 5 (see seismic lines’ location in Fig. 1).

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Fig. 26. (continued)

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Fig. 27. Depositional systems development sections along provenance direction, a. section DD′, indicating the development characteristics of depositional systems in NW-SE direction; b. section EE′, indicating the development characteristics of depositional systems in NE-SW direction (see seismic lines’ location in Fig. 1).

Moreover, SQ5 suffers strong erosion and is absent in some areas of the north sub-sag due to unbalanced uplift. (4) The characteristics of sedimentary facies and sequence stratigraphy in the Wulan-Hua sag are commonly controlled by faults, provenance and depocenters. The development of the third-order sequences is dominated by tectonism and sediment supplies, as well as lake-level changes driven by climate. The favorable targets for hydrocarbon located in the lowstand system tract of SQ1, SQ2 and SQ3 at the southern parts of the Wulan-Hua sag.

braided-delta, shallow-lacustrine, deep-lacustrine, and sublacustrine-fan. These depositional systems are arranged into system tracts within different lacustrine sequences. Lowstand-transgressive and highstand systems tracts are separated by major lacustrine flooding surfaces. During the period from SQ1 deposition to SQ3 deposition, provenance changed from multi-directions to single direction, depositional system types changed from multi-types to single type, water body changed from deep to shallow, sand bodies changed from small-scale to large-scale, the depocenter migrated southward, and the subsidence center migrated northward. (2) One first-order sequence (MSQ1, which represents the entire rift basin evolution stage of the Wulan-Hua sag), three second-order sequences (SSQ1, SSQ2, and SSQ3), and five third-order sequences (SQ1, SQ2, SQ3, SQ4, and SQ5) of the Wulan-Hua sag in the southern Erlian Basin are identified based on the characteristics of sequence boundaries and sedimentary successions in wells and seismic sections. (3) The established sequence stratigraphic framework of the WulanHua sag has the characteristics of residual basin, namely, thick in the early stage and eroded in the late stage. The north sub-sag first develops with thick SQ1, and then the south sub-sag develops with a wider distribution than the north sub-sag. SQ2, SQ3, and SQ4 are more develops in the south sub-sag than in the north sub-sag.

Acknowledgement This study benefited considerably from the assistance provided by experts from Huabei Oil Field Branch of PetroChina. We appreciate all the efforts and valuable comments from the reviewers and editors for this paper. This study was supported by Natural Science Foundation of China [grant number 41672124]; and State Key Project of Oil and Gas [grant number 2016ZX05047001-002-002]. References Aitken, J.F., Flint, S.S., 1996. Variable expressions of interfluvial sequence boundaries in the Breathitt Group (Pennsylvanian), eastern Kentucky, USA. Geol. Soc., Lond., Spec. Publ. 104 (1), 193–206.

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