2. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China;
3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
4. No.1 Institute of Geology and Mineral Resources of Shandong Province, Jinan 250014, China;
5. Geological Surveying & Mapping Institute of Shandong Province, Jinan 250011, China
The study of relative influence of climate, base-level change and tectonism on sedimentation is at the cutting-edge of basin analysis (Wang D D et al., 2016; Lü et al., 2015; Allen and Allen, 2005; Egger et al., 2002), especially the sedimentary facies and sequence stratigraphic patterns that fill a sedimentary basin (Song et al., 2014a; Barredo et al., 2012; Catuneanu and Eriksson, 2007). In marine basins, especially those on tectonically stable continental margins, the sedimentary facies and sequence stratigraphic petterns mainly result from eustatic sea level changes (Li et al., 2016; Lü et al., 2014; Song et al., 2014b; Wang and Li, 2000). Whereas, in tectonically active basins, tectonism increases or decreases accommodation, alters depositional base level, controls the distribution of source areas, and even influences local climatic patterns (Feng et al., 2015; Yang et al., 2010; Lin et al., 2001). Tectonism turns to be the major factor controlling sedimentary facies, sequence stratigraphic patterns (Wang G H et al., 2016; Leleu and Hartley, 2010; Sun et al., 2009; Deng et al., 2008; Lin et al., 2000). Being the most important geological event since Mesozoic in eastern China (Zhang et al., 2013), Yanshan Movement could largely affect sedimentation, so the sedimentary facies and sequence stratigraphic patterns of Yanshanian succession, including Early Yanshanian succession which is an inland compressional basin, will be very complex.
Since the sequence stratigraphy was introduced to China, it has been well applied to Cenozoic active rift basins, and sequence stratigraphic patterns were established to help prospect subtle reservoirs (Zhu et al., 2016; Liu et al., 2015; Song et al., 2013; Lin et al., 2005). However, in pre-Cenozoic inland compressional basins, as the accommodation space and sediment input were more sensible, the sedimentary facies and sequence stratigraphic patterns became more complex than the Cenozoic rift basins, but little previous attention has been paid.
In China, the current petroleum exploration mainly focuses on Cenozoic formations, which now is faced up with high exploration density and limited resources (Lin et al., 2004). Shortage of oil and gas is becoming the bottleneck restricting the sustainable development of Chinese economy. Liu (2002) pointed out that there still stored plenty potential petroleum in the pre-Cenozoic residual basins. Though it is difficult for exploring residual basins, more oil and gas can be found as long as making further study on geological research, especially the sedimentary facies and sequence stratigraphic patterns.
Combining the three above points, the authors concluded that the sedimentary facies and sequence stratigraphic patterns in pre-Cenozoic inland compressional basin is of much interest, so this study takes the Early Yanshanian succession (J1+2) in eastern Yihezhuang salient as an example to carry out this research.1 GEOLOGICAL BACKGROUND
Yihezhuang salient was located in the north of Jiyang depression, and separated from Zhanhua sag with Yinan fault trending EW in south and Yidong fault trending NE-SW in east as boundaries, respectively, and was close to Chezhen sag in the west and north with transitional slopes (Fig. 1). North to Chezhen sag, there is Chengnan fault, which is important for the formation and evolution of Yihezhuang salient. Yihezhuang salient belongs to the up-thrown wall of Yinan fault, and down-thrown wall of Chengnan fault.
Being influenced by Indosinian Movement, Yanshan Movement and Himalayan Movement, the Mesozoic succession in eastern Yihezhuang salient was severely denudated, with only Jurassic system developed, which consisted of, in ascending order, lower member of Fangzi Formation (Tg-Tmz3), upper member of Fangzi Formation (Tmz3-Tmz2), Santai Formation (Tmz2-Tgm) and Mengyin Formation (Tgm-Tr) (Table 1).
The study area has always been one of the important zones for oil exploration and development in Shengli Oilfield Co. Ltd., Sinopec. Large number of geology data, geophysical data and researching results, especially drilling data with high density and the three-dimensional seismic profiles with high resolution covering most of the study areas, have been accumulated after many years of exploration and development, which provided profitable conditions for researching in this study. Moreover, the authors carried out one week of field work to observe Early Yanshanian outcrops nearby the study area.
In this paper, we used the high-quantity seismic data and the latest geological documents as the basic data to establish the sequence framework and recognize internal sedimentation, then further discussed the evolution and distribution of sedimentary bodies inside the framework with borehole analysis and connecting-well analysis.3 ESTABLISHMENT OF SEQUENCE FRAMEWORK
Sequence stratigraphy establishes a genetic framework for the study of sequence architecture and internal makeup (Catuneanu et al., 2009), and the recognition of sequence boundaries contributes to building the chronostratigraphic framework. The sequence classification proposed for the Yihezhuang salient is similar to that of Vail (1977) and Posamentier et al. (1988).
Identification of sequence boundaries in the study area is based on analysis of 3-D seismic profiles, complemented with well logs and core data. We recognized the sequence boundaries on the basis of the following criteria.
1. Generally, unconformities and their correlative conformities are used as sequence-bounding surfaces (Catuneanu et al., 2009), because they represent time-barrier surfaces. Unconformable stratigraphic contacts are reflected on seismic profiles as truncations, and surfaces of onlap (Fig. 2).
2. Sequence boundaries are also represented by abrupt changes in physical characteristics such as lithology and sedimentary facies (Chen et al., 2012). Such boundaries can be identified through shapes of well logs (Fig. 3).
According to the preceding two criteria recommended, the sequence framework of the study area is established (Fig. 4), with two orders of unconformity-bounded sequences recognized. The entire Early Yanshanian succession (Tg-Tgm) is considered to be one first-order sequence, and the lower member of Fangzi Formation, upper member of Fangzi Formation and Santai Formation turn to be three third-order sequences (Table 1).4 SEDIMENTARY FACIES 4.1 Recognition of Sedimentary Facies
As influenced by Indosinian Movement, Yanshan Movement and Himalayan Movement, the Mesozoic basin turns to be a residual basin, and has become complicated for researching sedimentary facies. According to previous study, before Early Yanshanian, North China Platform covering Jiyang depression was nearly geomorphologic flat as being exposed for millions of years during Indochina epoch, Late Triassic (Song and Li, 2001), so the Early Yanshanian sedimentary facies changed little between nearby places (Zhang et al., 2009) and were mainly dominated by fluvial facies (Fig. 5). The study area in Jiyang depression was argued to develop similar sedimentary facies with nearby places such as Boshan (Fig. 5b).
In Boshan, close to Yihezhuang salient, there developed perfect outcrops of Early Yanshanian succession (Fig. 6), which can provide obvious evidences for recognizing sedimentary facies in study area. In outcrops, there developed channel floor lag deposits (Fig. 6a), trough cross bedding (Fig. 6b), scoured base (Fig. 6c) and multiple cycles of dual structure (Fig. 6d), which together represented meandering river deposit.
In study area, the sedimentary facies were delineated by analyzing seismic profiles (Fig. 7), boreholes and well logs (Fig. 8), and cores (Fig. 9). In Fig. 7, there can be observed lateral aggradation, the channel shape, two-way onlap inside the channel, and the channel marginal truncation, which are the relevant seismic reflection of meandering river. The dual structures, which are featured by bell-shaped log curve and a fining- upward deposition from conglomerate or sandstone at the base to mudstone at the top, often was superposed vertically and formed the distinctive characteristic of meandering river deposit in boreholes. In Fig. 8, four dual structures were recognized in Well D2, three dual structures were recognized in Well D5, respectively, four dual structures were recognized in Well D6. In Fig. 9, three dual structures were recognized through core observation in Well D43, although No. Ⅱ only developed the lower section as the up section was truncated out by No. Ⅲ.
Complemented with previous research and outcrops, the seismic profiles, borehole data and core observation contributed to recognition of meandering river deposited in study area.4.2 Sedimentary Characteristics
Based on the characteristical and fundamental components of meandering river, the dual structures developed during Early Yanshanian in study area can be divided into three kinds according to the logging curves and lithologic characteristics (Table 2). Logging curves of all the three kinds of dual structures are presented bell-shaped. However, from Type Ⅰ to Type Ⅲ, the bell shape turned to have changed specifically.
Sequence framework, internal distribution, and evolution of sedimentary bodies all result in the sequence stratigraphic patterns (Li et al., 2013). Based on the sequence framework established and internal sedimentary facies recognized, the evolution and distribution of sedimentary bodies were revealed through analyzing single-well and connecting-wells, in order to establish the sequence stratigraphic patterns.5.1 Sequence Stratigraphic Patterns in Single-Well (in Point)
Single-well analysis can provide abundant lithological data and longing data to carry out sequence stratigraphic patterns analysis with high precision (Li et al., 2004), including recognizing the assembly and evolution of parasequences, parasequence sets and depositional system tracts inside every sequence.
Analyses of the sequence stratigraphic patterns in Well D7 (Fig. 10), showed that the sequence can be totally divided into three sedimentary cycles with relatively large scale, i.e., three system tracts, which included lowstand system tract (LST), expanded system tract (EST) and highstand system tract (HST) in ascending order. Dual structures of meandering river constituted the fundamental component of each system tract, and one dual structure actually formed one parasequence.
In LST, six dual structures were superposed vertically. The dual structures developed belong to Type Ⅰ. From bottom to top, in ascending order, the thickness of sand-body and the sand-content in each parasequence both turned to increase, leading the sediment presented pro-gradation.
In EST, there developed two cycles of dual structures superposed vertically, which belong to Type Ⅱ. Different from LST, the sand-body in single parasequence became thinner. Moreover, the thickness of sand-body and the sand-content in each parasequence both turned to decrease from bottom to top, leading the sediment presented retro-gradation.
In HST, three cycles of dual structures can be recognized, with each cycle presented by Type Ⅲ bell-shaped log curve. The sand-body thickness in single dual structure reached largest among the three system tracts, and even reached almost 10 m. In ascending order, from bottom to top, the sand-body thickness and sand-content in each parasequence turned to increase, leading the sediment presented pro-gradation vertically.5.2 Sequence Stratigraphic Patterns in Connecting-Wells (in Line)
On basis of analyzing sequence stratigraphic patterns in point, the patterns in connecting-wells can be built up (Fig. 11), so as to recognize not only the vertical, but also the horizontal patterns.
From Fig. 10, it can be concluded that, the sand-body in LST distributed widely with superposing each other and well connectivity; while the EST was dominated by flood plain, inside which, the sand-body showed isolate distribution with poor lateral connectivity and thin thickness; differently, in HST, the lower part was dominated by mud deposit, whereas, the upper part was dominated by sand-body deposit, and the thickness and connectivity of sand-body both increased upward.6 DISCUSSION
From the analyses of the above single-well and connecting-wells, the model of Early Yanshanian sequence stratigraphic patterns in eastern Yihezhuang salient could be established (Fig. 12). In Early Yanshanian, there developed three third- order sequences, including lower member of Fangzi Formation (SQ-Fz1), upper member of Fangzi Formation (SQ-Fz2) and Santai Formation (SQ-St). Every third-order sequence developed LST, EST and HST, with meandering river deposited inside each system tract. Each system tract and inside sedimentation showed different evolutionary history.
During the period of lowstand forced regression (LFR) (Fig. 12a), as the underlying HST presented over compensation of sedimentation, the sedimentary base-level declining made no room for accommodation, which resulted in that the river eroded and truncated the underlying strata to form an unconformity surface as a sequence boundary, such as Tg in Fig. 10. LFR was lack of sedimentation as river truncation and none accommodation space increase. During the period of lowstand normal regression (LNR) (Fig. 12b), the sedimentary base-level began to rise, the accommodation space increased, but the speed of increasing still cannot be compared with the velocity of sediment input. Sufficient sediment supply made the obvious vertical superposition of channel deposit, which formed that multiple cycles of dual structures superposed vertically and sand-bodies overlaid each other. As truncated by the upper channel, the lower channel sand-body presented unapparent dual structure, and even lacked the top muddy deposit, such as No. Ⅱ in Fig. 9. From bottom to top, the thickness of sand-body in channel turned to increase obviously. During the late period of LST, as the compensated deposition of sediment, the accommodation space left for sediment accumulation was limited, which resulted in that the sediment supply was relatively abundant and likely to break through the channel to form flood fan.
In EST (Fig. 12c), as the sedimentary base-level increased speedily, the accommodation space increase exceeded sediment input, which led the vertical superposition of sediment dominated by retro-gradation. Moreover, the groundwater table rise caused by the speedy increase of sedimentary base-level made favorable conditions for forming flood plain, lake and swamp. Compared with LST, the dual structure of meandering river was dominated by upper muddy deposit and became Type Ⅱ, inside which, there distributed isolate channel sand-bodies with smaller scale and poorer connectivity than the ones in LST (Fig. 11), which could not form the well reservoirs. In the late period of EST, the velocity of accommodation space growth decreased and turned to be balanced with the speed of sediment input, which was favorable for depositing peat swamp. The peat swamp distributed widely and presented mostly isochronism to act as MFS (Fig. 10).
In HST (Fig. 12d), there developed multiple sedimentary cycles. As the velocity of accommodation space increase became smaller than sediment input, the superposition of sediment was dominated by pro-gradation. Moreover, the channel sand-body and sand-content in single sedimentary cycle turned to increase from bottom to top, which was similar with the patterns in LST. In late period of HST, as the compensated sedimentation, the accommodation space left for accumulation became restricted, which resulted in flood fan deposited.
In LST and HST, there mainly deposited sand-bodies with well-connectivity. However, developing larger thickness than the ones in LST (Table 2, Fig. 10), the sand-bodies in HST may have the potential to form favorable reservoirs for hydrocarbon. Moreover, the peat swamp in EST below the sand-bodies in HST could act as source rocks to provide hydrocarbon upward, and the muddy-sedimentation above the sand-bodies in HST could act as the cap rocks (Fig. 10). Peat swamp in EST, sand-bodies and above muddy-sedimentation in HST could together form advantageous source-reservoir-cap associations, which would be helpful for the second startup of petroleum resources in Early Yanshanian succession.7 CONCLUSIONS
During Early Yanshanian, there developed meandering river in study area, which was characterized by vertical multiple superpositions of dual structures, each dual structures actually constituted one parasequence. Based on the lithology characteristics and logging curve shapes, the dual structures can be divided into three types, i.e., Type Ⅰ, Type Ⅱ, and Type Ⅲ.
Succession deposited in Early Yanshanian Period (J1+2) constituted one first-order sequence and three third-order sequences. Each third-order sequence turned to consist of lowstand system tract (LST), expanded system tract (EST), and highstand system tract (HST). Furthermore, each system tract can be divided into several parasequences and parasequence sets.
In LST, Type Ⅰ dual structure developed, the sand-body developed large thickness and well connectivity, and the sedimentation presented pro-gradation vertically; in EST, Type Ⅱ dual structure developed, the scale and connectivity of sand-body became poor, and the sedimentation presented retro- gradation, respectively; in HST, Type Ⅲ dual structure developed, with thick sand-body developing well-connectivity, and the sedimentation presented pro-gradation.
In the establishment of sequence stratigraphic patterns, sand-bodies in HST may have the potential to form the well reservoirs for hydrocarbon, as the below peat swamp in EST could act as the source rocks, and the above muddy- sedimentation could act as the cap rocks. The established patterns would be helpful to predict reservoirs and well source- reservoir-cap associations, which would contribute to the second startup of hydrocarbon in pre-Cenozoic residual basins.ACKNOWLEDGMENTS
This research was supported by the National Natural Science Foundation of China (NSFC) (No. 41472216), the Natural Science Foundation of Shandong Province (No. ZR2016DB29), the Project supported by Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology (No. DMSM2017015), the Water Conservancy Scientific Research and Technology Promotion Projects of Shandong Province (No. SDSLKY201808), and the Natural Science Foundation of Jinan University (No. XBS1647). We thank Petroleum Development Centre of the Shengli Oilfield Company for their support and permission to use industry data for this research. We appreciate the help from Professor Dawei Lü for good advices on this paper. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0867-4.
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