Journal of Earth Science, Vol. 25, No. 5, p , October 2014 ISSN X Printed in China DOI: /s

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1 Journal of Earth Science, Vol. 25, No. 5, p , October 2014 ISSN X Printed in China DOI: /s Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area of Qiongdongnan Basin, Northern South China Sea: Implications for Coal-Bearing Source Rocks Jinfeng Ren* 1, 2, Hua Wang 2, Ming Sun 3, Huajun Gan 2, Guangzeng Song 2, Zhipeng Sun 4 1. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan , China 2. Faculty of Earth Resources, China University of Geosciences, Wuhan , China 3. Guangzhou Marine Geological Survey, Guangzhou , China 4. Zhanjiang Branch of CNOOC Limited, Zhanjiang , China ABSTRACT: For unveiling coal-bearing source rocks in terrestrial-marine transitional sequences, the sequence stratigraphic framework and sedimentary facies of Lower Oligocene Yacheng Formation of Qiongdongnan Basin were investigated using seismic profiles, complemented by well bores and cores. Three third-order sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center, namely SQYC3, SQYC2 and SQYC1 from bottom to top. Coal measure in Yacheng Formation of Qiongdongnan Basin were deposited within a range of facies associations from delta plain/tidal zone to neritic sea, and three types of favourable sedimentary facies associations for coal measure were established within the sequence stratigraphic framework, including braided delta plain and alluvial fan, lagoon and tidal flat, and fan delta and coastal plain facies associations. Results shown that, in the third-order sequences, coal accumulation in landward areas (such as delta plain) of the study area predominantly correlates with the early transgressive systems tract (TST) to middle highstand systems tract (HST), while in seaward areas (such as tidal flat-lagoon) it correlates with the early TST and middle HST. The most potential coal-bearing source rocks formed where the accommodation creation rate (Ra) and the peat-accumulation rate (Rp) could reach a state of balance, which varied among different sedimentary settings. Furthermore, intense tectonic subsidence and frequent alternative marine-continental changes of Yacheng Formation during the middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. The proposed sedimentary facies associations may aid in predicting distribution of coal-bearing source rocks. This study also demonstrates that controlling factors analysis using sequence stratigraphy and sedimentology may serve as an effective approach for coal-bearing characteristics in the lower exploration deepwater area of South China Sea. KEY WORDS: Qiongdongnan Basin, Yacheng Formation, sequence stratigraphy, sedimentary facies, coal measure. 0 INTRODUCTION Deepwater area of Qiongdongnan Basin (QDNB) in the northern South China Sea has become a new focus of deepwater exploration due to a series of discovery of natural gas reservoirs (Zhu et al., 2012; Wang Z F et al., 2011; Mi et al., 2010; Zhang et al., 2007). The geochemical characteristics demonstrated that the gas field was closely related to coal-bearing source rocks within Yacheng Formation (Huang et a l., , *Corresponding author: jf_ren@163.com China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received September 18, Manuscript accepted May 15, ; Li et al., 2011; Dong and Huang, 2000). During the past decades, five coal accumulation theories have been achieved based on a great deal of successful applications (Shao et al., 2009, 2008; Allen and Fielding, 2007; Holz et al., 2002), including coal accumulation of continental facies, coal accumulation of transgressive process, coal accumulation of transgressive event, episodic coal accumulation and coal accumulation of different systems tracts. Coal accumulation of continental facies has been proposed in the peat swamp during the shrinking stage of lake, and coal-beds are formed in highstand systems tract (HST) during the descending period of sea level (Li et al., 2003; Diessel, 1992). Coal accumulation of transgressive process has been proposed in the littoral setting of marginal-sea basin, and coal-beds are formed in transgressive Ren, J. F., Wang, H., Sun, M., et al., Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area of Qiongdongnan Basin, Northern South China Sea: Implications for Coal-Bearing Source Rocks. Journal of Earth Science, 25(5): doi: /s

2 872 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun systems tract (TST) during the rising period of sea level (Li et al., 1999; Diessel, 1992). Coal accumulation of transgressive event has been proposed in the epicontinental basin, and coal-beds are formed at early TST and late HST during the transitional period of sea level (Li et al., 2006, 2001). Episodic coal accumulation has been proposed by the relationship between large-scale transgression and coal accumulation in the marine-terrigenous facies, coal-beds are formed in the rising cycle of base level at different order within the sequence stratigraphic framework (Hao et al., 2000; Shao et al., 1999; Li et al., 1995). In the theory for coal accumulation of different system tracts, in the shelf-margin delta and incised valley filling sedimentation, coal-beds are mainly formed in lowstand systems tract (LST); in the slope break belt of passive continental margin or epicontinental basin, coal-beds are formed in TST during the rising period of sea level; in the peat swamp after sea level descending, coal-beds are formed in HST (Lü et al., 2010; Diessel, 2007; Jin et al., 2000; Bohacs and Suter, 1997). However, the marine-terrestrial transitional facies are developed in Yacheng Formation and the coal-beds are with the characteristics of multi-beds, thin single-bed, and rapidly lateral changes, it is difficult to explain the distribution pattern and the main controlling factors of coal measure based on conventional methodologies and theories. In this paper, the isochronous sequence stratigraphic framework of Yacheng Formation (Lower Oligocene) of QDNB in the northwestern South China Sea is established and favorable sedimentary facies associations for the marine-terrestrial coal measure from Yacheng Formation sedimentary succession are revealed within the sequence stratigraphic framework based on a combination of 2D and 3D seismic, well-log, and core data in the shallow water and deepwater areas. From these data coal accumulation patterns are proposed in different settings and the main controlling factors of the characteristics of coal-beds are analyzed. It is anticipated that this work will not only provide an integrated sequence stratigraphic and sedimentological method for coal-bearing accumulation in terrestrial-marine transitional settings, but it will also facilitate the prediction and assessment of coal-bearing source rocks (composed of coal, carbonaceous mudstones and dark mudstone) for the north deepwater area in South China Sea. 1 GEOLOGICAL SETTING The QDNB is a continental rift basin and stretches NE-SW in northern South China Sea (Gong and Li, 1997). It is separated from Yingehai Basin by the No. 1 fault in the west, Shenhu uplift by the No. 12 fault in the east, Hainan uplift by the No. 5 fault in the northern and southern uplift by the No. 11 fault in the south (Song et al., 2014; Su et al., 2013; Fig. 1a). The deepwater area is located in the south of the No. 2 fault and consists of Ledong, Lingshui, Beijiao, Songnan, Baodao and Changchang sags, covering approximately km 2 (Chen et al., 2012; Fig. 1a). The Paleocene rifting sediments contain, from bottom to top, the Eocene Lingtou Formation ( Ma), the Early Oligocene Yacheng Formation ( Ma) and the Late Oligocene Lingshui Formation ( Ma), which correspond to three major tectonic episodes respectively: the initial rifting stage, the main rifting stage, and transitional rifting stage (Yuan et al., 2008; Fig. 2). Two styles of structures, half-grabens and grabens on the interpreted seismic profiles, were developed with zonation from north to south and segmentation from east to west on the fault plane map (Fig. 1). Consequently, the terrain of QDNB during Lower Oligocene was characterized by muti-water systems, muti-uplifts and muti-sags. The depositional environment was terrestrial-marine transitional deposition and changes from braided delta in early periods to coastal plain then to tidal flat-lagoon in late periods (Figs. 1b, 2). In addition, lush ferns as well as hot and humid climate provided favorable conditions for coal-forming (Li et al., 2012). Therefore, the coal and carbonaceous mudstones of Yacheng Formation is one of the most important source rocks in the study area and the focus in this paper. 2 SEQUENCE STRATIGRAPHY In this study, the terminology about sequence stratigraphy of Van Wagoner et al. (1990) is applied. Due to the difficulties of identifying transgressive surfaces, however, the concepts of parasequence and parasequence set are not applicable. Nevertheless, it is possible to identify genetically system tracts based on first flooding surface (FFS) and maximum flooding surface (MFS). For the purpose of regional correlation and mapping, the sequence stratigraphic framework is constructed using a high-resolution sequence stratigraphic methodology, following Mitchum and Van Wagoner (1991). A sequence, bounded by subaerial unconformities and their marine correlative conformities, is defined as a succession of strata deposited during a full cycle of change in accommodation or sediment supply (Catuneanu et al., 2011) and refers to a succession of related sequences in which the individual sequences stack into lowstand, transgressive, and highstand system tracts (Mitchum and Van Wagoner, 1991). In this study area, long-term sequences (third-order sequence) are identified to comprise shorter term sequences (estimated as fourth-order sequences). 2.1 Sequence Boundaries Combining 2D and 3D seismic, well-log, and core data, sequence boundaries were identified. We used the following criteria for recognizing sequence boundaries in the study area. 1. Generally, subaerial unconformities along the basin margins and correlative conformities within the central basin are defined as sequence boundaries. Unconformable stratigraphic contacts are reflected on seismic profiles as truncations, surfaces of onlap, and downlap (Fig. 3). For example, S70 in Fig. 3 was identified as local erosional truncations with strong amplitude reflection characteristics in the sag margin and graded towards the central basin into conformable contacts shown as strong parallel reflectors. 2. Sequence boundaries are also represented by abrupt changes in physical characteristics such as lithology and sedimentary facies. Such boundaries can be identified using the shapes of well logs (Fig. 4) and the changes of seismic reflectors (Fig. 3).

3 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 873 Figure 1. (a) Sketch of the study area, with a detailed bathymetry showing the major tectonic units and well. Inset shows location of study area in the northwestern South China Sea; (b) paleogeographical outline map of Yacheng Formation in the study area. 2.2 Systems Tracts and Fourth-Order Sequences Systems tracts, divided into three component units including LST, TST and HST, should be identified by the nature of their boundaries and stacking pattern of their internal stratigraphy (Catuneanu et al., 2011). This principle applied to their recognition from seismic data after the first flooding surface (FFS) and the maximum flooding surface (MFS) had been identified. In the study area, because LST and TST of the third-order sequences are combined in the landward areas, therefore, the FFS was difficult to be identified and tracked on seismic profiles while the MFS was relatively easy to be identified. MFS was often expressed as a downlap surface in seismic stratigraphic terms, as it was typically downlapped by the overlying highstand clinoforms which recorded progradation (Fig. 5). In addition, MFS within sequences are characterized by high gamma-ray signals as a result of high concentrations of organic matter and radioactive elements. Although in some sequence stratigraphic studies extensive and thick coal seams are used as MFS in the correlation of coal measure (Aitken, 1994; Hamilton and Tadros, 1994), we did not adopt this methodology here (see Section 5.2). Under the constraint of systems tracts, a fourth-order sequence is a unit bounded by litho-stratigraphic surfaces and an upward-coarsening or upward-fining succession of microfacies bounded by flooding surfaces. This boundary changed from sandstone below to shale/siltstone above, leading to abrupt change of log curve shapes. It is worth noting that the fourth-order sequences consist of regressive and transgressive

4 874 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun Figure 2. Sequence classification and sedimentary systems of Paleogene strata in QDNB. The age of the sequence boundary, the lithology and change of relative sea-level in the QDNB were provided by China National Offshore Oil Zhanjiang Ltd., Corporation. types of deposit, and display various stacking patterns within systems tracts. The fourth-order sequences are used to construct the third-order sequence stratigraphic framework. 2.3 Third-Order Sequences and Composite Sequence Using the preceding criteria, third-order sequences can be recognized within the study area. The entire succession of Yacheng Formation is considered to represent a composite (second-order) sequences and can be divided into three third-order sequences (Fig. 6). S100 is a regional angular unconformable contact and basement surface in QDNB. S80, an onlaped unconformity, is developed limitedly in the deep sag and overlapped basin basement (S100). S70 is one of the most widespread angular unconformable surfaces, Its distribution range significantly extended much more widely than the S80. At the edge of slope zones, the Lingshui Formation strata Figure 3. Annotated 3D seismic profile across the Beijiao sag showing sequence boundaries. The sequence boundaries are characterized by truncation, onlap and the distinct surfaces associated with facies changes (see location in Fig. 1a).

5 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 875 Figure 4. Types of sequences boundaries of Yacheng Formation in deepwater area of Qiongdongnan Basin. (a) Erosional surface in Well Y132; (b) surface of sudden basinward facies shift in Well S191; (c) paleosol or the base in Well Y132 (see wells location in Fig. 1a). Wire-line logs are natural gamma ray (GR) and deep investigate induction log (ILD). Figure 5. Seismic expression of the maximum flooding surface (MFS) in member 1 of Yacheng Formation. HST. Highstand systems tracts; TST. transgressive systems tract; LST. lowstand systems tract; MFS. maximum flooding surface (see location in Fig. 1a). overlapped S70 while Yacheng Formation strata was truncated by S70; high-amplitude and continuous reflection of S70 could be found at sag centers. Furthermore, the strata between S70 and S80 presented the characteristics of strong amplitude and medium continuous reflectors, which were different from the strata up or down. S72 and S71 are two sequence boundaries associated with significant facies changes in the deep sag, dividing the Yacheng Formation sequence into three members (Fig. 6). 3 SEDIMENTARY FACIES AND FACIES ASSOCIA- TIONS Based on seismic data, well logs, and cores, a number of sedimentary systems from terrestrial to marine were identified in Yacheng Formation of QDNB, and the transitional sedimentary facies sequences had a large proportion (Fig. 1b). The distribution features of the sedimentary facies in Yacheng Formation have been revealed under the constraint of the sequence stratigraphic framework. During the SQYC3 period, a number of sags, which were small and near to provenance, were developed alluvial fan, braided/fan delta, littoral facies and neritic facies (Fig. 7a). During the SQYC2 period, relative sea-level rose rapidly as a result of weakened tectonic activities and regional transgression, the study area mainly developed coastal plain, littoral and neritic sea (Fig. 7b). During the SQYC1 period, relative sea-level fell, tidal flat-lagoon, littoral and neritic sea facies were mainly developed in the study area (Fig. 7c). Three types of representative sedimentary facies for coal-beds in the sequence stratigraphic framework were analyzed as follows: (1) braided delta plain and alluvial fan; (2) tidal flat and lagoon; (3) fan delta and coastal plain. The main characteristics of these facies associations, especially the corresponding coal-accumulation potentials, will be discussed briefly as follows. 3.1 Braided Delta Plain and Alluvial Fan Facies Association Braided delta plain and alluvial fan facies association were mostly developed in the early Yacheng Formation (SQYC3) (Fig. 8a). The alluvial fan was developed on the downthrow side of boundary fault along the edge of uplifts (Fig. 8), with the characteristics of coarse-grained, poorly sorted sandstone, medium-amplitude and progradational reflection (Fig. 8a). The braid-delta systems are located on the gentle slope of half grabens (seismic profile of Fig. 8 is crossing section in the northwest Beijiao sag. Braided delta plain subfacies is the land part of the delta, composed mainly of distributary channel, distributary interchannel, flood plain and peat swamp microfacies and so on. Internal seismic reflectors are medium to low amplitude (Fig. 8a). In the core of Well Y821, coarse or conglomerate- rich sandstone characterized by large cross bedding, parallel bedding and scoured basal surfaces (Fig. 8b) are interpreted as distributary channel deposits. The trends of log shape response to the distributary channel are high amplitude and box-shaped gamma-ray patterns. Composition of gray green mudstone with siltstone and pelitic siltstone are interpreted as peat swamp deposits, indicated by fine serrated middle-high amplitude and box-shaped gamma-ray patterns. The succession of distributary channel-flood plain or interdistributary bay-peat swamp-distributary channel is typical

6 876 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun Figure 6. The seismic profile cross the Beijiao sag shows characteristics of the major boundary surfaces and typical seismic facies of Yacheng Formation in QDNB (see location in Fig. 1a). Figure 7. Log of borehole S191, S331, and Y821, with interpretation of lithofacies, sequence boundaries, depositional systems, and systems tracts. The log is based on wire-line logs supplemented by cores of selected intervals (see location of the wells in Fig. 1). Wire-line logs are natural gamma ray (GR) and density (DEN). LST. Lowstand systems Tract; TST. transgressive systems tract; HST. highstand systems tract; FFS. first flooding surface; MFS. maximum flooding surface. sedimentary sequence for coal accumulation in the study area (Fig. 8b). 3.2 Tidal Flat and Lagoon Facies Association Tidal flat and lagoon facies association are developed in the middle and late Yacheng Formation (SQYC2 and SQYC1) (Fig. 9a). Lagoon deposits are developed in the small sags of the basin edge. The lagoon was linked to open sea through tidal channel, but was separated from the open sea by structural barrier with the relatively stable sedimentary environment.

7 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 877 Figure 8. Sedimentary characteristics of braided delta plain and alluvial fan facies association for coal accumulation. (a) Seismic reflection characteristics on the profile crossing the Well Y821 of braided delta plain and alluvial fan facies association; (b) the spatial model and typical sedimentary microfacies sequence of braided delta plain and alluvial fan facies association (modified from Li et al., 2010). SB. Sequence boundary; MFS. maximum flooding surface. Medium amplitude, subparallel and continuous seismic reflectors in the Fig. 9a are interpreted as lagoon facies. Tidal flat deposits were developed around the lagoon, composed mainly of tidal channel, sublittoral zone, intertidal zone and supralittoral zone subfacies. In the core of Well Y191, coarse sandstone characterized by cross bedding, trough plate cross bedding and scoured basal surfaces are interpreted as tidal channel (Fig. 9b). The gamma-ray logs display slightly serrated box-shaped trend (Fig. 9b). Gray green silty mudstones are interpreted as supralittoral zone, with the coal-beds developed in the swamp sedimentary microfacies (Fig. 9b). 3.3 Fan Delta and Coastal Plain Facies Association Fan delta and coastal plain facies association are mostly developed in the transitional zone of the whole Yacheng Formation (SQYC3, SQYC2 and SQYC1) from paleo-uplift to shallow sea, and mainly located on the steep slope zone of grabens (Fig. 10a). Internal seismic reflections of fan delta are medium to high amplitude, progradational, wedge-shape reflectors (Fig. 10a). Thin-bedded light-gray siltstone with lenticular bedding and dark mudstones with bioturbation structures are interpreted as coastal plain deposits; coarse-grained, poorly sorted sandstone and wave cross-bedding are interpreted as fan-delta front deposits. Fan plain and coastal plain are developed at the front of the fan delta with the characteristics of medium amplitude, subparallel and continuous seismic reflectors. The terrain of fan delta plain and coastal plain zone was relatively flat, which was advantage for germinating paleovegetation and accumulating peat. The coal accumulation was mainly formed in the inter-fan hollow, fanfront and coastal plain marsh (Fig. 10b). Due to instability of fan delta environments, the thickness of coal-beds is very thin and the coal-beds are split. Therefore, the coal-beds of braided delta plain and tidal flat-lagoon systems are thicker and more stabilized than that of littoral fan delta system. 4 RESULTS 4.1 Coal Development Characteristics within Sequence Stratigraphic Framework According to observation of the logging shape of coal-beds, the rule of logging shape response to coal-beds in the study area is shown as low natural gamma ray (GR), low density (DEN), low spontaneous potential (SP), high acoustic (AC), high formation resistivity (RT), high neutron porosity (NPHI) and expanded hole diameter (CAL) namely three high, three low, and one expand. Based on this rule, coal-beds were identified by artificial logical decision, and the 1/3 amplitude thickness of logging trace is proximal with the true thickness of coal-bed (Shen et al., 2010). In 3 member of Yacheng Formation in the Well Y132, 26 coal-beds were identified and their cumulative thickness were 17.5 m (Fig. 11a); in 1 member of Yacheng Formation in the Well Y821, 16 coal-beds were identified and their cumulative thickness were 7.55 m (Fig. 11b); in 2 member of Yacheng Formation in the Well Y821, 4 coal-beds were identified and their cumulative thickness were 2.8 m (Fig. 11c). These results show that the average thickness of single bed is about m and the maximum thickness of single bed is less than 1 m. So, the coal beds of Yacheng Formation are with the characteristics of multi-beds, thin single-bed, and rapidly lateral changes, which cause more difficulties to explain their distribution patterns using conventional methodologies and theories in the sequence stratigraphic framework. 4.2 Coal Distribution within Sequence Stratigraphic Framework Maximum total organic carbon (namely Maximum TOC ), source potential determination (S 1 +S 2 ) of equally spaced samples of cuttings and accumulative total coal thickness (namely total coal ) within fourth-order sequences were quantitatively analyzed in Fig. 11. It is apparent from

8 878 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun Figure 9. Sedimentary characteristics of lagoon and tidal flat facies association for coal accumulation. (a) Seismic reflection characteristics on the profile crossing the Well Y191 of lagoon and tidal flat facies association; (b) the spatial model and typical sedimentary microfacies sequence of lagoon and tidal flat facies association (modified from Li et al., 2010). SB. Sequence boundary; MFS. maximum flooding surface. Figure 10. Fan delta and coastal plain facies association for coal accumulation. (a) Seismic reflection characteristics on the profile crossing the edges of Changchang sag; (b) the spatial model and typical sedimentary microfacies sequence of fan delta and coastal plain facies association (modified from Li et al., 2010). SB. Sequence boundary; MFS. maximum flooding surface. Fig. 11 that coal measure (coal and carbonaceous mudstones) are with the characteristics of high TOC and S 1 +S 2. Therefore, the maximum TOC within fourth-order sequences can be used to represent the most favorable development position for coal measure. In addition, a systems tract is divided into three fourth-order sequences (e.g., early TST, middle TST and late TST), which will be facilitate to statistical analysis of coal-bearing potential. This quantitative study of high-quality coal-bearing source rocks within fourth-order sequences from this study and Mi et al. (2010) confirmed in the study area that coal measure typically developed from the early TST to HST (Fig. 11; Diessel, 2007; Holz et al., 2002). In the seaward area, the greatest coal accumulation occurred in the early TST and middle HST, while few coal accumulation occurred within the late or middle TST (Figs. 11c, 11c ). On the contrary, coal-bearing accumulation within the middle TST account for a considerable percentage of overall coal-bearing in the landward areas of the study area (Figs. 11a, 11a ). In conclusion, apparent differences occur among the landward (delta plain setting) (Fig. 11a), transitional (tidal flat setting) (Fig. 11b) and seaward (lagoon setting) areas (Fig. 11c). Furthermore, the coal measure in the transitional zone follows the general pattern (Figs. 11b, 11b ), but some differences are apparent in the other two settings. 5 DISCUSSION 5.1 Coal Distribution Pattern in Sequence Stratigraphic Framework According to Fourier s theorem, any time series, no matter what shape it is provided it has some oscillations and no infinite values, can be recreated by adding together regular sine and cosine waves having the correct wavelengths and amplitudes (Weedon, 2003). So, in Fig. 12, the change of base-level can be recreated by adding together third-order cosine waves and fourth-order cosine waves having their corresponding wavelengths and amplitudes. The changes of third-order base-level can be assumed as 2π B3rd A1 cos( t) T (1) 3rd

9 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 879 Figure 11. Graphs showing composition of coal-bearing source rocks within fourth-order sequence in different settings. Maximum TOC. the Maximum total organic carbon of debris within 4th SQ; Total coal. cumulative total coal thickness within 4th SQ; 4th SQ. four-order sequence; ST. systems tracts.

10 880 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun where A 1 is the amplitude of third-order base-level, T 3rd is the period of third-order base-level. The changes of fourth-order base-level can be assumed as 2π B4th A2 cos( t) T 4th where A 2 is the amplitude of fourth-order base-level, T 4th is the period of fourth-order base-level. We note that composite changes of base-level (B 3rd+4th ) adding together third-order base-level and fourth-order base-level can be written as 2π 2π B A cos( t) A cos( t) T 3rd+4th 1 2 T3rd where A 1 >>A 2, T 3rd > T 4rd. Hence, the rate of composite changes of base-level (Rb 3rd+4th ) is the 1st derivation of B 3rd+4th. 1 2 b3rd+4th sin( ) sin( ) T3rd T3rd T4th T4th 4th (2) (3) 2πA 2π 2πA 2π R t t (4) Due to the subsidence rate at the margins is lower than the basinward, the Rb 3rd+4th of landward settings (such as delta plain) is less than Rb 3rd+4th of seaward settings (such as tidal flat-lagoon), the accommodation creation rate of landward settings (Ral) (such as delta plain) is less than the accommodation creation rate of seaward settings (Ras) (such as tidal flat-lagoon), namely Ral<Ras. But the sedimentation rate of landward settings (Rsl) is apparently greater than the sedimentation rate of seaward settings (Rss), namely Rsl>Rss, the peat-accumulation rate of landward settings (Rpl) is also greater than the peat-accumulation rate of seaward settings (Rps), namely Rpl>Rss. So, The relationship between the rate of composite changes of base-level (Rb 3rd+4th ) and the sedimentation rate (Rs) among different sedimentary settings can be expressed in Fig. 12. Assuming sedimentary compaction rate is considered as a constant because of the small changes of sedimentary compaction among different coal-forming settings, the accommodation creation rate (Ra) is equal to the rate of composite changes of base-level (Rb 3rd+4th ) (Catuneanu et al., 2009). The relationship between the rate of accommodation creation (Ra=Rb 3rd+4th ) and the peat-accumulation rate (Rp<Rs) among different sedimentary settings can be expressed in the Fig. 12. Within a sequence stratigraphic framework, the relationship between the accommodation creation rate (Ra) and the peat-accumulation rate (Rp) is a fundamental control on the coal-forming patterns (Fig. 13). The thickest and most widespread coal measures are formed where those two reach a state of balance (Bohacs and Suter, 1997). However, this balance varies among different sedimentary settings (Shao et al., 2009, 2008). Since Rp is only capable of coping with a narrow range of Ra (0.5<Ra/Rp<1.53; Diessel, 2007; Bohacs and Suter, 1997), the peat-forming potential would be reduced in the seaward settings during the middle TST (around the R point in Fig. 12b). During this interval, a high Ra would prevent the terrestrial organic material from flourishing, or drown and cease peat formation (Diessel, 2007; Shao et al., 2003; Bohacs and Suter, 1997). Conversely, in more landward settings, a lower Ra even at the middle of the TST (around the R point in Fig. 12a) can balance Rp and would therefore generate the potential for peats to persist throughout the whole transgression (Wang H et al., 2011). Typically, during the late HST, peat accumulation and preservation are unfavorable due to oxidation or erosion following low base levels (Diessel, 2007; Aitken, 1994). The results can be used to interpret the coal-forming patterns among different transitional sedimentary facies sequences. This rule will be also favorable for forecasting the vertical and planar development range of coal-bearing source rocks in study area. 5.2 Controls of Coal Development Characteristics in Yacheng Formation According to the analysis of basin subsidence history, the Paleogene QDNB presents significant features of episodic tectonic evolution. The synrift stage can be divided into the initial rifting stage (46 32 Ma), the main rifting stage ( Ma), and transitional rifting stage ( Ma), respectively corresponding to Eocene Lingtou Formation, Lower Oligocene Yacheng Formation and Upper Oligocene Lingshui Formation (Fig. 13). The successions of Lower Oligocene Yacheng Formation were developed in the main rifting stage. During this period, tectonic subsidence rate was a major part of total subsidence rate, which indicated the existence of intense tectonic activities in the deposition process of Lower Oligocene Yacheng Formation. During SQYC3 period, 12 coal-beds were developed and total thickness was 6.5 m, the coal beds were with the characteristics of multi-beds, thin single-bed, and rapidly lateral changes (such as Well Y821; Fig. 13a). During SQYC2 period, 4 coal-beds were developed and total thickness was 2.8 m, the coal beds were with the features of less coal-beds but thick single-bed (such as Well Y821; Fig. 13a). During SQYC1 period, 16 coal-beds were developed and total thickness was 7.55 m, the characteristics of the coal-beds were quite similar to that in the SQYC3 period (such as Well Y821; Fig. 13a). Furthermore, it is apparent from the cross-sections of Yabei Sag (Fig. 13a) that the most favorable zone for coal-bearing accumulation in the study area migrated along the coastline within each third-order sequence, and was generally controlled by the second-order transgression. Especially, coal characteristics were intensively controlled by rapid changes of fourth-order sea level. In conclusion, intense tectonic subsidence and frequently alternative marine-terrestrial changes combined to produce rapid changes of the accommodation creation rate, leading to reach a short-term balance between the accommodation creation rate (Ra) and the peat-accumulation rate (Rp) and formed the thin coal-beds. For this reason, it is suggest that intense tectonic subsidence and frequent alternative marine-continental changes in the process of transgression is a major control of coal-bearing accumulation in the study area, which interpret the characteristics of multi-beds, thin single-bed, and rapidly lateral changes in deepwater area of QDNB in the northern South China.

11 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 881 Figure 12. Relation between ratio of accommodation rate/peat production rate and coal-accumulation point. Time of HST, TST and LST within a base-level changing cycle, as defined by the interplay of base-level change rate and sedimentation rate at the shoreline; R refers to the middle of the TST and accords to the highest base-level rising rate. SB. Sequence boundary; LST. lowstand systems tract; TST. transgressive systems tract; HST. highstand systems tract; FFS. first flooding surface; MFS. maximum flooding surface; Rs. the sedimentation rate; Rp. the peat-accumulation rate. Figure 13. The intense tectonic subsidence and relevant variable sea-level in Yacheng Formation of Yabei sag. (a) Aerial extent of marine bands (mudstone and muddy limestone beds) Yacheng Formation of Yabei sag in NW Qiongdongnan Basin that can be used to define the fourth-order marine transgressions, showing interpolated second-and third-order sequences; the distribution of major coals in the well Y821 is also schematically presented; (b) tectonic evolution in Yabei sag that can be used to reflect intensive tectonic subsidence in the main rifting episode of Qiongdongnan Basin.

12 882 Jinfeng Ren, Hua Wang, Ming Sun, Huajun Gan, Guangzeng Song and Zhipeng Sun 6 CONCLUSIONS (1) The Lower Oligocene Yacheng Formation of the QDNB can be classified into three third-order sequences bounded by local unconformities and correlative conformities, and nine fourth-order sequences were bounded by flooding surfaces within the system tracts. Within the sequence stratigraphic framework, three types of favorable facies associations for coal measure were revealed: braided delta plain and alluvial fan, tidal flat and lagoon, fan delta and coastal plain facies associations. Results show that braided delta and tidal flat is the most potential sedimentary environment for coal-bearing source rocks. (2) Within the sequence stratigraphic framework, it is suggested that coal-bearing source rocks developed preferentially during the early TST to middle HST. However, variations are observed in the landward and seaward settings, where coal measure were more likely to have developed in seaward areas during the early TST and middle HST, and in landward areas during the middle TST to middle HST. The relationship between the accommodation creation rate (Ra) and the peat-accumulation rate (Rp) is a fundamental control on the coal-forming patterns. The thickest and most widespread coal measure formed where those two reached a state of balance. However, this balance varies among different sedimentary settings. (3) The most favorable zone for coal-bearing accumulation in the study area migrated along the coastline within each third-order sequence, and the preservation of coal measure in the early TST to middle HST was most likely related to the rise of the regional base level throughout the transgression of Yacheng Formation. Furthermore, unstable tectonic subsidence and relevant variable sea-level in middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. ACKNOWLEDGMENTS This study was supported by the Zhanjiang Branch of CNOOC Ltd., the National Science and Technology Projects (No. 2011ZX ), Natural Science Foundation of China (NSFC) (Nos and ) and the Key Laboratory of Tectonics and Petroleum Resources (CUG) of Ministry of Education Open Issue (No. TPR ). The authors thank the management and technicians in the Zhanjiang Branch of the CNOOC Research Center for preparing the data and geological discussions. We thank Dr. Guanzhong Shi s improvement for the manuscript. Helpful reviews by two anonymous reviewers are gratefully acknowledged. REFERENCES CITED Aitken, J. F., Coal in a Sequence Stratigraphic Framework. Geoscientist, 4(5): 9 12 Allen, J. P., Fielding, C. R., Sequence Architecture within a Low-Accommodation Setting: An Example from the Permian of the Galilee and Bowen Basins, Queensland, Australia. American Association of Petroleum Geologists Bulletin, 91(11): Bohacs, K., Suter, J., Sequence Stratigraphic Distribution of Coaly Rocks: Fundamental Controls and Paralic Examples. American Association of Petroleum Geologists Bulletin, 81(10): Catuneanu, O., Abreu, V., Bhattacharya, J. P., et al., Towards the Standardization of Sequence Stratigraphy. Earth-Science Reviews, 92(1): 1 33 Catuneanu, O., Galloway, W. E., Kendall, C. G. S. C., et al., Sequence Stratigraphy Methodology and Nomenclature. Newsletters on Stratigraphy, 44(3): Chen, H., Zhu, X., Zhang, G., Seismic Facies in a Deepwater Area of a Marine Faulted Basin: Deepwater Area of the Paleogene Lingshui Formation in the Qiongdongnan Basin. Acta Geologica Sinica, 86(2): Diessel, C. F. K., Coal-Bearing Depositional Systems Coal Facies and Depositional Environments. Springer-Verlag, Berlin Diessel, C. F. K., Utility of Coal Petrology for Sequence Stratigraphic Analysis. International Journal of Coal Geology, 70(1): 3 34 Dong, W. L., Huang, B. J., Identification Marks and Source Discrimination of the Coal Type Gas in YGH and QDN Basins of South China Sea. Natural Gas Industry, 20(1): (in Chinese with English Abstract) Gong, Z. S., Li, S. T., Continental Margin Basin Analysis and Hydrocarbon Accumulation of the Northern South China Sea. Science Press, Beijing (in Chinese) Hamilton, D. S., Tadros, N. Z., Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins An Example From the Gunnedah Basin, Australia. American Association of Petroleum Geologists Bulletin, 78(2): Hao, L. M., Shao, L. Y., Shi, Z. B., et al., The Application of Depositional Cycle Frequence Cure to Episodic Coal Accumulation: With an Example from the Upper Permian in Southwest China. Journal of Paleogeography, 2(4): (in Chinese with English Abstract) Holz, M., Kalkreuth, W., Banerjee, I., Sequence Stratigraphy of Paralic Coal-Bearing Strata: An Overview. International Journal of Coal Geology, 48(3): Huang, B. J., Li, X. S., Wang, Z. F., et al., Source Rock Geochemistry and Gas Potential in the Deepwater Area, Qiongdongnan Basin. China Offshore Oil and Gas, 24(4): 1 7 (in Chinese with English Abstract) Huang, B. J., Xiao, X., Li, X. S., Geochemistry and Origins of Natural Gases in the Yinggehai and Qiongdongnan Basins, Offshore South China Sea. Organic Geochemistry, 34(7): Jin, G. F., Gong, S. L., Zhang, C. X., et al., Sequence Stratigraphic Model of Coal Accumulating Process. Coal Geology & Exploration, 1(1): 1 5 (in Chinese with English Abstract) Li, B. F., Wen, X. R., Li, G. D., High Resolution Sequence Stratigraphy Analysis on Permo-Carboniferous in North China Platform. Earth Science Frontiers, (S1): (in Chinese with English Abstract) Li, W. H., Zhang, Z. H., Li, Y. C., et al., Geochemical Characteristics and Hydrocarbon Generation of Paleogene Oligocene Source Rocks in Qiongdongnan Basin. Natural

13 Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area 883 Gas Geosciences, 22(4): (in Chinese with English Abstract) Li, Z. X., He, Y. P., Liu, H. Y., et al., Sedimentology Characteristics and Coal-Forming Models in Yacheng Formation of Qiongdongnan Basin. Acta Petrolei Sinica, 31(4): (in Chinese with English Abstract) Li, Z. X., Li, S. C., Wei, J. C., Episodic Transgressions and Coal Accumulation: An Example from the Late Carboniferous Coal Rich Units in the Western Shandong Coal Fields. Sedimentary Facies and Paleogeography, 15(1): 1 9 (in Chinese with English Abstract) Li, Z. X., Wei, J. C., Han, M. L., Coal Formation in Transgressive Events: A New Pattern of Coal Accumulation. Advance in Earth Sciences, 16(1): (in Chinese with English Abstract) Li, Z. X., Yu, J. F., Guo, J. B., et al., Analysis on Coal Formation under Transgression Events and Its Mechanism in Epicontinental Sea Basin. Acta Sedimentologica Sinica, 21(2): (in Chinese with English Abstract) Li, Z. X., Yu, J. F., Guo, J. B., et al., Deposition of Coal-Bearing Measures and Coal Accumulation under Marine Transgression Events in North China Epicontinental Sea Basin. Journal of Coal Science & Engineering, 1(6): Li, Z. X., Zhang, G. C., Li, Y., et al., The Paleogene Coal-bearing Basin and Coal-Measures Distribution of China Sea Area. Earth Science Frontiers, 18(4): (in Chinese with English Abstract) Lü, D. W., Li, Z. X., Wei, J. C., et al., Sequence Division of Coal Strata Based on Different Coal Accumulation Theory. Journal of China University of Petroleum, 4(4): (in Chinese with English Abstract) Mi, L. J., Wang, D. D., Li, Z. X., et al., High Resolution Sequence Stratigraphic Framework and Coal Forming Features in Yacheng Formation of Qiongdongnan Basin. Acta Petrolei Sinica, 31(4): (in Chinese with English Abstract) Mitchum, J. R. M., Van Wagoner, J. C., High-Frequency Sequences and Their Stacking Patterns: Sequence- Stratigraphic Evidence of High-Frequency Eustatic Cycles. Sedimentary Geology, 70(2): Shao, L. Y., Lu, J., Wang, H., et al., Advances in Sedimentology and Sequence Stratigraphy of Paralic Coal Measures. Journal of Palaeogeography, 10(6): (in Chinese with English Abstract) Shao, L. Y., Lu, J., Wang, H., et al., Developments of Coal Measures Sequence Stratigraphy in China. Acta Sedimentologica Sinica, 27(9): (in Chinese with English Abstract) Shao, L. Y., Zhang, P. F., Gayer, R. A., et al., Coal in a Carbonate Sequence Stratigraphic Framework: The Upper Permian Heshan Formation in Central Guangxi, Southern China. Journal of the Geological Society, 160(2): Shao, L. Y., Zhang, P. F., Dou, J. W., et al., New Considerations on Coal Measures Sequence Stratigraphy: A Case Study from the Late Paleozoic Coal Measures in South Hebei. Journal of China University of Mining & Technology, 28(1): (in Chinese with English Abstract) Shen, H. L., Qin, C. W., Wang, D. D., et al., Distinguishing Methods for Coal Beds in Yacheng Formation of Qiongdongnan Basin. Acta Petrolei Sinica, 31(4): (in Chinese with English Abstract) Song, G. Z., Wang, H., Gan, H. J., et al., Paleogene Tectonic Evolution Controls on Sequence Stratigraphic Patterns in the Central Part of Deepwater Area of Qiongdongnan Basin, Northern South China Sea. Journal of Earth Science, 25(2): , doi: /s Su, M., Xie, X. N., Xie, Y. H., et al., The Segmentations and the Significances of the Central Canyon System in the Qiongdongnan Basin, Northern South China Sea. Journal of Asian Earth Sciences, 79: 1 12 Van Wagoner, J. C., Mitchum, R. M., Campion, K. M., et al., Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops. In: Exxon Production Research Co., ed., Methods in Exploration Series, Tulsa. 7: 55 Wang, H., Shao, L. Y., Hao, L. M., et al., Sedimentology and Sequence Stratigraphy of the Lopingian (Late Permian) Coal Measures in Southwestern China. International Journal of Coal Geology, 85(1): Wang, Z. F., Li, X. S., Sun, Z. P., et al., Hydrocarbon Accumulation Conditions and Exploration Potential in the Deep-Water Region, Qiongdongnan Basin. China Offshore Oil and Gas, 23(1): 7 13 (in Chinese with English Abstract) Weedon, G., Time-Series Analysis and Cyclostratigraphy: Examining Stratigraphic Records of Environmental Cycles. Cambridge University Press, Cambridge Yuan, Y. S., Yang, S. C., Hu, S. B., et al., Tectonic Subsidence of Qiongdongnan Basin and Its Main Controlling Factors. Chinese Journal of Geophysics, 51(2): (in Chinese with English Abstract) Zhang, G. C., Mi, L. J., Wu, S. G., et al., Deepwater Area: The New Prospecting Targets of Northern Continental Margin of South China Sea. Acta Petrolei Sinica, 28(2): (in Chinese with English Abstract) Zhu, W. L., Zhong, K., Li, Y. C., et al., Characteristics of Hydrocarbon Accumulation and Exploration Potential of the Northern South China Sea Deepwater Basins. Chinese Science Bulletin, 57(20):