Depositional characteristics of lake-floor fan of cretaceous lower ...

Journal of Earth Science, Vol. 20, No. 4, p. 731–745, August 2009 Printed in China DOI: 10.1007/s12583-009-0065-5

ISSN 1674-487X

Depositional Characteristics of Lake-Floor Fan of Cretaceous Lower Yaojia Formation in Western Part of Central Depression Region, Songliao Basin Xin Renchen* (辛仁臣) School of Ocean Sciences, China University of Geosciences, Beijing 100083, China Li Guifan (李桂范) School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China Feng Zhiqiang (冯志强), Liang Jiangping (梁江平) Daqing Oilfield Company, PetroChina, Daqing 163453, China Lin Changsong (林畅松) School of Energy Resources, China University of Geosciences, Beijing 100083, China ABSTRACT: Based on the integrated subsurface data, including those of over 600 m drilled cores, more than 30 drilled wells and 600 km2 three-dimensional (3D) seismic-reflection data of the study area, the characteristics of the lake-floor fan of lower Yaojia (姚家) Formation have been clarified. An evident lacustrine slope break and a steep slope belt developed in the west of Songliao (松辽) basin during depositional period of Qingshankou (青山口)-Yaojia formations (K2). The slope gradient was about 15 m/km. During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. The wedge-shaped sediment body, which is interpreted as the lowstand system tract of SQy1 (LSTy1), developed in the area below the slope-break. The LSTy1 is pinched out in the west of the study area. As to the thickness of LSTy1, it is thicker in the east with 50 m in its thickness than in the west. The LSTy1, rich in sandstone, can be divided into lower part LSTy1L and upper part LSTy1u based on two onlap seismic reflection phases, and core and logging data clearly. The various sediments’ gravity flow deposits developed and the complex of lake-floor fan formed in the LSTy1 under the slope-break in the western part of the central depression region. The lake-floor fan consists of various sediments’ gravity flow deposits, including: (1) turbidity deposits with characteristics of Bouma sequences; (2) sand-bearing muddy debrite dominated by mud and mixed by sand; (3) mud-bearing sandy debrites characterized by dominated sand and mixed by mud; (4) sandy debris laminar flow deposits with massive or inclined bedding, and (5) sandy slump deposits developed as deformaThis study was supported by the National Natural Science

tional sedimentary structure. During the lower

Foundation of China (No. 40772075) and Daqing Petroleum

lake-level period (LSTy1L), the western clino-

Institute, PetroChina.

form region was erosion or sediment pass-by

*Corresponding author: [email protected]

area; the terrigenous clastic was directly transported to deep-water area, converted to chan-

Manuscript received February 20, 2009.

nelized sandy debris flow, and combined with

Manuscript accepted April 25, 2009.

slump derived gravity flow deposited in the

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Xin Renchen, Li Guifan, Feng Zhiqiang, Liang Jiangping and Lin Changsong lower part of the slope and the deep depression region, which then formed lake-floor fans. During higher lake-level period (LSTy1u), terrigenous clastic deposited in the upper part of the slope, and formed deltaic depositional system; while in the lower part of the slope, the deep depression area was filled by lake-floor fans which were mainly slump deposits. The sandstone bodies of the lake-floor fan are the favorable targets of subtle oil-bearing traps in the western part of the central depression region. KEY WORDS: lacustrine slope-break, gravity flow deposit, lake-floor fan, Cretaceous, Songliao basin.

INTRODUCTION The main purpose of the study on depositional characteristics of lake-floor fan of Cretaceous lower Yaojiao Formation in the western part of the central depression region of the Songliao basin is promoting lithologic reservoir exploration. The lithologic reservoirs are the main targets for oil and gas exploration after more than 50 years’ exploration in the Songliao basin. The area developing gravity flow deposits in deep water (i.e., sea-floor fan) is the favorable region for lithologic reservoirs (Stow and Mayall, 2000). Lake-floor fan and sea-floor fan are generally called basin-floor fan. The methodology of basin-floor fan study is generally geological and geophysical analysis based on data of core, logging and seismic, and analogy to modern sea-floor fan. Over the last 5–10 years, the significance of submarine fan reservoirs to the hydrocarbon industry has substantially improved our understanding of these depositional systems in particular. Much of our knowledge has come from the observations and interpretations made possible by the increasingly available high quality 3D seismic data, particularly from West Africa (e.g., Navarre et al., 2002). Interpretation of the high-resolution seismic data is being supported by increasing well log and core data. With the subsurface data, outcrop analogue and modern and Pleistocene channel system studies have resulted in an extensive and mounting literature on the submarine fan reservoirs (Giresse et al., 2009; Mayall et al., 2006; Adeogba et al., 2005; Beaubouef, 2004; Abreu et al., 2003; Eschard et al., 2003; Gardner et al., 2003; Kneller, 2003; Mulder et al., 2003; Pirmez and Imran, 2003; Posamentier, 2003; Posamentier and Kolla, 2003; Prather, 2003; Samuel et al., 2003; Weber et al., 2003; Babonneau et al., 2002; Browne and Slatt, 2002; Hickson and Lowe, 2002; Johnson et al., 2001; Spinell and Field, 2001; Cronin et al., 2000; Peakall et al.,

2000; Cronin and Kidd, 1998; Pickering and Hilton, 1998). These detailed and comprehensive studies focused on submarine fan classification, specific aspects of submarine fan morphology, depositional processes, as well as detailed studies of individual submarine fan and regional systems. The ancient basin floor fans, which are usually interpreted as mostly belonging to lowstand system tracts, were deposited during or shortly after an episode of relative low sea level (Pickering et al., 1995; Mitchum, 1985; Mutti, 1985). Deposition of coarse-grained deep-water sediments (i.e., basin-floor fan, such as submarine fan, sublacustrine fan or lake-floor fan) formed depocenters of complex structures and has received increasing attention over the last decade. It was driven largely by the exploration and exploitation of oil and gas resources in associated settings (Li et al., 2008; Liu et al., 2008; Peng et al., 2008; Wynn et al., 2007; Anderson et al., 2006; Adeogba et al., 2005; Xin et al., 2004; Dasgupta, 2002; Stow and Mayall, 2000; Lonergan and Cartwright, 1999; Prather et al., 1998). Although the knowledge of submarine fan is used for reference of study on lake-floor fan, the characteristics of lake-floor fan are different from submarine fan (Li et al., 2008; Liu et al., 2008; Peng et al., 2008; Xin et al., 2004; Buatois and Mángano, 1994). Based on the integrated subsurface data, including those of 600 m of drilled cores, more than 30 drilled well and three-dimensional (3D) seismic-reflection data from 600 km2 of the study area, the characteristics of lakefloor fan of Cretaceous lower Yaojia Formation at the western part of the central depression region in Songliao basin controlled by slope-break, have been clarified in this article. GEOLOGICAL SETTING Songliao basin is superposed by deep-rift basin (J3–K1) and depression basin (K2). The strata of

Depositional Characteristics of Lake-Floor Fan of Cretaceous Lower Yaojia Formation

Songliao depression basin are composed of Quantou, Qingshankou, Yaojia, Nenjiang, Sifangtai, and Mingshui formations. Horizontally, Songliao depression basin has been divided into the western clinoform region, the northern incline region, the central depression region, the northeastern uplift region, the south-

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eastern uplift region, and the southwestern uplift region (Gao and Cai, 1997). The oil pools of Songliao basin mainly developed in the central depression region, and the study area is located in the western part of the central depression region (Fig. 1).

Figure 1. Tectonic units divided in Songliao depression basin and stratigraphic sequence of Songliao depression basin (modified after Xin et al., 2008; Gao and Cai, 1997). Much research indicates that during the deposition of the Qingshankou to Nenjiang formations (K2qn–K2n), two large-scale lake expanding (i.e., transgression) occurred in the Songliao basin (Gao and Cai, 1997; Wei et al., 1996; Ye and Wei, 1996; Gao and Xiao, 1995), and consequently the Songliao basin developed into a deep-water depressional lacustrine basin. During the two periods, the lake level was characterized by two rises and one fall on large scale and the lacustrine area changed greatly.

During the early depositional period of the Qingshankou Formation, the lacustrine area was about 9×104 km2. During the late depositional period of the Qingshankou Formation (K2qn) and the early depositional period of Yaojia Formation (K 2 y), the large-scale lake-level falling and the decreasing of lake area took place in Songliao basin (Xin et al., 2004; Xin and Wang, 2004; Gao and Cai, 1997), which resulted from the local tectonic uplift of Songliao basin and combined with the large scope global sea-level

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Xin Renchen, Li Guifan, Feng Zhiqiang, Liang Jiangping and Lin Changsong

falling during the Upper Cretaceous of Turonian (Haq et al., 1987). In the depositional period of the Member 1 of Yaojia Formation (K2y1), the smallest lake area was less than 1×104 km2 (Gao and Cai, 1997), and the second-order sequence boundary between the Cretaceous SSQq-qn and SSQy-n was formed in this period. This sequence boundary had obvious corresponding characteristics of seismic, logging, lithologic and paleontological data (Wang et al., 2005; Xin et al., 2004; Liu et al., 2002; Ye et al., 2002; Gao and Cai, 1997; Wei, 1996; Wei et al., 1996; Gao and Xiao, 1995; Gao et al., 1994). During the deposition of the members 2 and 3 of Yaojia Formation (K2y2, K2y3), the lake expanded again, and during the early depositional period of the Nenjiang Formation (K2n), the lake level of the basin reached its second peak and the lake area was over 12×104 km2.

Figure 2. Slope-break between the central depression and the western clinoform regions of Songliao basin. Upwardly, from the Member 1 of Qingshankou Formation to Member 1 of Nenjiang Formation, 7 third-order sequences and 2 second-order sequences

Figure 3. Depositional sequence of SQy1 (Well Y88).


were divided according to detailed analyses of paleontological, lithologic, logging and seismic data. Sequences SQq4-qn1, SQqn2, SQqn3 and SQqn4 belonged to the second-order sequence SSQq-qn, and SQy1, SQy2, SQy3 and SQn1 belonged to secondorder sequence SSQy-n. During the depositional period of upper Qingshankou Formation and lower Yaojia Formation, the lake shrank and the shore line of the western slope area shifted toward the central sagging areas on a large scale. The lowest part of Yaojia For-

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mation is the lowstand system tract of the SQy1 third-order sequence. The SQy1 third-order sequence is the lowest part of the SSQy-n second-order sequence (Xin et al., 2008). The western part of the central depression region, which abuted western clinoform region, included three second-order structural units such as LonghupaoDa’an terrace, Qijia-Gulong sag and Changling sag, developed a clear lacustrine slope-break. The slope gradient was more than 15 m/km (Fig. 2) during

Figure 4. Sequence stratigraphic framework of Yaojia Formation in the western part of the central depression region of Songliao basin. Below the lacustrine slope-break developed wedge-shaped sediment body, which is interpreted as the lowstand system tract of SQy1 (LSTy1). The LSTy1 represents two onlap seismic reflection phases clearly, and could be divided into two parts of LSTy1L and LSTy1u. The site of the seismic profile is marked in Fig. 5.

Figure 5. Isopach map of LSTy1 in the western part of the central depression region, Songliao basin. 1. Well and position; 2. pinch-out line of LSTy1; 3. isopach of thickness (m).

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depositional period of Qingshankou-Yaojia formations (K2). During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. Most of the area above the slope-break was exposed, which led to the erosion of the Qingshankou strata, the sediment pass-by, and the absence of lower Yaojia Formation. In the area below the lacustrine slope-break, wedge-shaped sediment body developed. The wedge-shaped sediment body is interpreted as the lowstand system tract of SQy1 (LSTy1). The LSTy1 can be divided into two parts of LSTy1L and LSTy1u based on logging and seismic data (Figs.

3 and 4). The LSTy1 is rich in sandstone (Fig. 3) and represents two onlap seismic reflection phases clearly (Fig. 4). The LSTy1 is pinched out in the west of the study area. As to the thickness of LSTy1, it is thicker in the east, about 50 m in the thickness, and thinner in the west (Fig. 5). The various sediments’ gravity flow deposits developed in the LSTy1 under the slope-break in the western part of the central depression region according to core data. TYPES AND CHARACTERISTICS OF SEDIMENTS’ GRAVITY FLOW There are five types of sediments’ gravity flow

Figure 6. Depositional sequence of sediments’ gravity flow (Well Y35, 1 757–1 765 m, K2y1).


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deposits which have been recognized by the core data in lower Yaojia Formation in the western part of the central depression region of Songliao basin. The 5 types of sediments’ gravity flow deposits are: (1) turbidity current deposit with characteristics of Bouma sequences; (2) sand-bearing muddy debris flow deposits with characteristics of dominated mud and mixed by sand; (3) mud-bearing sandy debris deposits with characteristics of dominated sand and mixed by mud; (4) sandy debris flow deposits with characteristics of massive or inclined bedding, and (5) sandy slump deposits with characteristics of deformed sedimentary structure. The depositional sequence of sediments’ gravity flow deposits in Well Y35 is shown in Fig. 6. The sandstones of the lake-floor fan are the main reservoirs of oil in the western part of the central depression region.

Sand-Bearing Muddy Debris Flow Deposits Sand-bearing muddy debris flow deposits consist of sandy mudstone. The distribution features of sand in mudstone are as follows: (1) sandy mudstone, which was featured by sand grains and dispersed in mudstone (Fig. 8a), probably resulted from sandbearing muddy debris plastic flow deposited; (2) rigidity sand block distributed in mudstone (Fig. 8b), which probably resulted from muddy debris plastic flow containing little rigidity sand shiver deposited.

Turbidity Current Deposits The term turbidite used here is restricted to a deposit which displays the vertical sequence (partial or complete) of Ta to Te divisions (as defined by Bouma, 1962) plus at least a minimal amount of upward-fining, graded bedding. There is usually a sharp basal contact with the underlying muddy layer and less well defined upper contact with the overlying muddy layer. The turbidity current deposits (i.e., turbidite) in the study area consist of Tb-Tc-Td or Tc-Td (Fig. 6). The Ta-Tb-Tc-Td assemblage was mainly Tc interval with ripple or convolute lamination silt or fine sand and Td interval sandy mud (Fig. 7), Tb interval, fine-moderate sand, with parallel bedding or cross bedding, which generally was thinner, with thickness less than 20 cm (Fig. 6), and Tc-Td assemblage was more common in core data. Compared with typical Bouma turbidite sequence (Bouma, 1962), there was no Ta sand interval with normal graded bedding that had been found in core data, the reason was that the turbidity current was derived from debris flow resulted from delta front sand slumping, but there was no gravel or coarse sand in the delta front sand, or the gravel or coarse sand had deposited by debris flow during the evolution process from the debris flow to turbidity current.

Figure 7. Core photograph of Tb-Tc-Td assemblage of turbidite deposits (Well Y35, 1 735 m, K2y1). Mud-Bearing Sandy Debris Deposits Mud-bearing sandy debris flow deposits have features of muddy sandstone (Fig. 7). The distribution shape of mud in sandstone is usually represented by: (1) muddy sandstones, (2) sandstones featured by irregular mud lamination or agglomerate, (3) mud clast-bearing sandstone, and (4) sandstone with convolute bedding displayed by mud. The muddy sandstones characterized by mud particulates are disperse in sandstone, without detectable graded bedding (Fig. 9a), and probably resulted from sand debris plastic flow deposited (Giresse et al., 2009). The sandstones are featured by irregular mud lamination or agglomerate (Fig. 9b). In sand and mud intermixed debris flow, the shear-resisting intensity of sandy part is about 6–12 times that of muddy part (Johnson, 1970), so that the sandy part reveals rigidity, but the muddy part is plastic and easy to develop into intense plastic deformation and to form convolute bedding in the process of debris flow. The mud clast-bearing sandstone characterized by various shapes of mud clasts irregularly is distributed in sandstones, especially, the mud clast whose long axis is perpendicular to bedding plane (Fig. 9c),

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Figure 8. Core photographs of sand-bearing muddy debris flow deposits. (a) Sandy mudstone, dispersed sand grain in mudstone, Well Y83, 1 841.5 m, K2y1; (b) rigidity sand block distributed in mudstone, Well Y35, 1 773.3 m, K2y1.

Figure 9. Core photographs of mud-bearing sandy debris flow deposits. (a) Muddy sandstone, mud particulate dispersed in sandstone, Well Y83, 1 840 m, K2y1; (b) sandstone with irregular mud lamination or agglomerate, Well Y38, 2 377.5 m, K2y1; (c) mud clast-bearing sandstone with various shape mud clasts irregularly distributed in sandstones, specially, the mud clast, whose long axis is perpendicular to bedding plane, is the evident symbol of debris flow, Well Y35, 1 761 m, K2y1; (d) sandstone with convolute bedding displayed by mud, Well Y89, 1 946 m, K2y1. is the evident symbol of the upper part debris flow. The upper part of debris flow is debris plastic flow because of the shearing strength, which was caused by the weight of covered water body, less than its shear-resisting intensity, so that the deposits of upper part of debris flow are usually chaotic mud clastbearing sandstone (Shanmugam, 2002). The sandstone with convolute bedding displayed by mud (Fig. 9d) probably resulted from mud-bearing sand debris plastic flow deposited. Sandy Debris Flow Deposits with Inclined Bedding Sandy debris flow deposits with inclined bed-

dings are characterized by moderate to fine sandstone with inclined bedding (Fig. 10a) under the condition of the lamination displayed by mud or carbon materials, or characterized by massive moderate to fine sandstone (Fig. 10b) under the condition of no mud or carbon materials displayed in the lamination. The cause of formation of inclined bedding in sandy debrites had been discussed by Johnson (1970), Shanmugam (2000) and Marr et al. (2001). Debris flow was commonly divided into lower and upper parts due to the difference of its interior hydromechanics (Johnson, 1970). The inclined bedding sandy debris flow deposits resulted from the debris laminar


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Figure 10. (a) Sandstone with inclined bedding, Well Y35, 1 762 m, K2y1; (b) massive moderate sandstone, Well Y36, 1 284 m, K2y1. flow of lower part of debris flow. Debris laminar flow was formed by the shearing strength when the weight of covered water body and upper part debris exceeded their shear-resisting intensity. Based on experimental study, Shanmugam (2000) and Marr et al. (2001) found that the internal layers developed in sandy debris flows are from postdepositional movement along failure planes (or secondary glide plane) during remobilization of flow.

These layers could be misidentified as parallel lamination in the rock record. Sandy Slump Deposits The sandy slump deposits are mainly sandstones with deformation lamination (Fig. 11a) and stones interweaved with various sandy clasts and muddy clasts (Fig. 11b).

Figure 11. (a) Sandstone with deformation lamination, Well Y35, 1 777.5 m, K2y1; (b) stones interweaved with various sandy clasts and muddy clasts, Well Y81, 1 903 m, K2y1. The sandstones with deformation lamination indicate that: (1) there are non-concretion, stability-less sand bodies with lamination in original area, (2) sand was transported by the manner of block (sand slump), and preserved the continuity of inhere lamination, (3) in the process of transportation, sandy block behaved plasticly, and plastic deformation occurred. The stones interweaved with various sandy clasts and muddy clasts indicate that primary sandy and muddy intervals were all broken and intermixed in the process of slumping (Garziglia et al., 2008). DISTRIBUTION OF SEDIMENTS’ GRAVITY FLOW The sediments’ gravity flow deposits are first of

all sandy and muddy debris flow deposit, and then slump deposit, however, there is less true turbidite. Different gravity flow deposits commonly coexisted and formed lake-floor fan. The complex of lake-floor fan was mainly distributed in the lower part of the slope and the deep depression. Figure 12 is the section across wells in dip direction of the slope, which reveals that the LSTy1 was wedge-shaped sediment body and the architecture of sediments. The LSTy1L is the lower part of LSTy1, which pinched out in the east of Well Y37, and a complex of lake-floor fan consisting of various sediments’ gravity flow deposits. The shape of wireline logs of LSTy1L is complex serrate open finger shape or tight finger shape. The LSTy1u is the upper part of LSTy1,

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Figure 12. Sequence stratigraphy and depositional profile of the west slope of the central depression region of Songliao basin (the position of the profile marked in Fig. 13a).

Figure 13. (a) Depositional system distribution plane map of LSTy1L; (b) depositional system distribution plane map of LSTy1u. LLF. Lake-floor fan; MB. mouth bar; DB. distal bar. 1, 5. Well and position; 2, 6. sandstone thickness (m); 3. pinch-out line of LSTy1L; 4, 10. outline of LLF; 7. pinch-out line of LSTy1u; 8. outline of MB; 9. outline of DB. composed of delta sediments and lake-floor fan sediments of lower slope. The delta developed in upper

part of the slope near Well Y87 and the western part. The wireline logs of delta are serrate bell-shaped. The


lake-floor fan developed in lower part of the slope in the east of Well Y87. The wireline logs of the lakefloor fan are also complex serrate open finger shape or tight finger shape. Figures 13a and 13b are depositional system distribution plane map of LSTy1L and LSTy1u, respectively, which is based on integrated analysis of data of core, logs and the spectrum decomposition of 3D seismic data. It clearly reveals the lake-floor fan and mainly distributes in the area of lower part of the slope and the deeper depression. Spectrum decomposition technique is a frequency-based reservoir interpretation method. As an advanced high definition imaging technique of discrete geologic bodies, it can be used to detect the discreteness of thin layers and geologic bodies in the 3D survey areas by using seismic data. Seismic data in time domain can be converted into frequency domain through discrete Fourier transform (DFT). The converted amplitude spectrum can be used to detect the variability of temporal bed thickness, while the phase spectrum can be used to indicate the lateral discontinuity of geologic bodies (Partyka et al., 1999). The base map of Fig. 13a is 50 Hz tuning energy distribution map in 20 ms middle-time-window based upon migration 10 ms from the bottom of LSTy1L. The tuning energy of the red area is the highest, and the tuning energy of light yellow area is the lowest. The higher tuning energy areas with relatively higher thickness of sandstone are the response to lake-floor fan. The depositional body of LSTy1L pinched out in the east of the line from Well Y32 to Well Y35, and the complexity of lake-floor fan developed in the area of lower part of the slope and the deeper depression in the east of the line along where wells Y3 to Y68 are located. The changes of thickness of sandstone of lake-floor fan resulted from the various sediments’ gravity flows (Fig. 13a). The base map of Fig. 13b is 50 Hz tuning energy distribution map in 20 ms middle-time-window based upon migration 10 ms from the bottom of LSTy1u. The tuning energy of the dark orange area is the highest, and the tuning energy of light yellow area is the lowest. The higher tuning energy areas with higher thickness of sandstone are the response to lake-floor fan. The depositional body of LSTy1u pinched out in

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the west of Well Y62. The delta depositional system developed in the upper part of the slope, and was identified as mouth bar and distal bar. The mouth bar featured by moderate-fine sandstone with cross bedding (Fig. 14a), which was distributed in the uppermost area of the slope, has the highest thickness of sandstone, and the distal bar featured by fine sandstone and siltstone with ripple bedding interbedded with mudstone (Fig. 14b), which was distributed in the front of mouth bar, has relatively lower thickness of sandstone. The lake-floor fan developed in the area of lower part of the slope and the deeper depression in the east of the line along where wells Y3 to Y68 are located. The thickness of sandstone of lake-floor fan changed obviously due to the various sediments’ gravity flows (Fig. 13b).

Figure 14. (a) Moderate-fine sandstone with cross bedding, mouth bar of delta front, Well Y64, 1 618.7 m, LSTy1u; (b) fine sandstone and siltstone with ripple bedding interbedded with mudstone, distal bar of delta front, Well Y64, 1 620 m, LSTy1u. CONCLUSIONS (1) An evident lacustrine slope-break and a steep slope belt developed in the west of Songliao ba-

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sin during depositional period of Qingshankou-Yaojia formations (K2). The slope gradient was about 15 m/km. During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. In the area below the slope-break, wedge-shaped sediment body developed. The wedge-shaped sediment body is interpreted as the lowstand system tract of SQy1 (LSTy1). The LSTy1 rich in sandstone can be divided into two parts of LSTy1L and LSTy1u. The LSTy1 represents two onlap seismic reflection phases clearly. The various sediments’ gravity flow deposits developed and the complexity of lake-floor fan formed in the LSTy1 under the slope-break in the western part of the central depression region. (2) There are five types of sediments’ gravity flow deposits recognized in the lower part of the slope and near the deep lake area controlled by the slope. They are: ① turbidite; ② sand-bearing muddy debrite; ③ mud-bearing sandy debrite; ④ sandy debris laminar flow deposits, and ⑤ sandy slump deposits. These five types of sediments’ gravity flow deposits formed complex lake-floor fans. The complex lake-floor fans are mainly debrite and slump deposits, with less turbidite. The sandstones of the lake-floor fan are the main reservoirs of oil in the western part of the central depression region. (3) The depositional body of LSTy1L developed in the area of lower part of the slope and the deeper depression, and it was all composed of lake-floor fans. During lower lake-level period (LSTy1L), the western clinoform region and the upper part of the slope were erosion or sediment pass-by areas, and terrigenous clastic matters were directly transported into deep-water area under the slope-break, and then converted to channelized sandy debris flow. It was combined with slump derived gravity flow deposited in the lower part of the slope, the deep depression region, and developed complex lake-floor fans. (4) The distribution area of depositional body of LSTy1u expanded obviously. The delta depositional system developed in the upper part of the slope, and the lake-floor fan developed in the area of lower part of the slope and the deeper depression. During higher water level period (LSTy1u), terrigenous clastic mat-

ters were deposited in upper part area of the slope formed delta depositional system, and the deep water area was filled by lake-floor fans which resulted from sediments’ gravity flows caused by slump. ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (No. 40772075) and Daqing Petroleum Institute, PetroChina. We would like to thank the Daqing Petroleum Institute, PetroChina, for providing core, well data and seismic data. We are appreciative of the collaboration and enthusiastic support from Wu Heyong, Huang Wei and Wei Xuerui from the Daqing Petroleum Institute, PetroChina, and Zhao Pengda and Liu Hao from China University of Geosciences. REFERENCES CITED Abreu, V., Sullivan, M., Pirmez, C., et al., 2003. Lateral Accretion Packages (LAPs): An Important Reservoir Element in Deep Water Sinuous Channels. Marine and Petroleum Geology, 20(6–8): 631–648 Adeogba, A. A., McHargue, T. R., Graham, S. A., 2005. Transient Fan Architecture and Depositional Controls from Near-Surface 3-D Seismic Data, Niger Delta Continental Slope. AAPG Bulletin, 89(5): 627–643 Anderson, K. S., Graham, S. A., Hubbard, S. M., 2006. Facies, Architecture, and Origin of a Reservoir-Scale Sand-Rich Succession within Submarine Canyon Fill: Insights from Wagon Caves Rock (Paleocene), Santa Lucia Range, California, USA. Journal of Sedimentary Research, 76: 819–838 Babonneau, N., Savoye, B., Cremer, M., et al., 2002. Morphology and Architecture of the Present Canyon and Channel System of the Zaire Deep-Sea Fan. Marine and Petroleum Geology, 19(4): 445–467 Beaubouef, R. T., 2004. Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation, Upper Cretaceous, Southern Chile. AAPG Bulletin, 88(11): 1471–1500 Bouma, A. H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam. 168 Browne, G. H., Slatt, R. M., 2002. Outcrop and Behind-Outcrop Characterization of a Late Miocene Slope Fan System, Mount Messenger Formation, New Zealand. AAPG Bulletin, 86: 841–862 Buatois, L. A., Mángano, M. G., 1994. Lithofacies and Deposi-


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