Seismic-sedimentary Facies Analysis of Es3x Formation in Southern Region, Laizhou Bay Depression, Bohai Bay Basin, China

The southern region is located in Laizhou bay Depression of Bohai bay Basin. Due to the research area is just in the early exploration phase, there are few drilling wells, low research degree of seismic data, incomplete sedimentary evolution, and no similar oil fields have been developed, the relevant researches on the formation, deposition of this area will mainly rely on seismic1 and well logging data. And previous studies on seismic facies and sedimentary facies in this area were relatively weak. For the study on seismic facies, Dong conducted in-depth discussion on the seismic facies in the Liaodong bay area of Bohai bay Basin, reclassified the seismic facies, and combined the seismic facies and sedimentary facies to identify favorable reservoir facies zones [1]. In the study of sedimentary facies, Liu analyzed the mixed sedimentary characteristics, development patterns and main controlling factors in the south of Laizhou bay Depression in detail, and identified the dominant reservoir sub facies[2]. There are some differences between the previous studies and the study area. Therefore, it is particularly important to further study the seismic-sedimentary facies based on some previous views. In this paper, the distribution of sedimentary facies in the study area is studied by using the regional sedimentary environment combined with logging data and seismic data, and thecharacteristics of seismic sedimentology are discussed and summarized, so as to provide guidance and basis for the research and application of seismic facies and sedimentary facies in the study area in the future.

The southern region is located in the southern sea area of Bohaisea, China (Figure 1A). It is a favorable zone for oil and gas migration and accumulation in Nan sag (Figure 1B). Structural traps are well developed and formed (Figure 1C). The structural area is close to the source material, the spatial location of hydrocarbon source rocks, faults and sandstone reservoirs is well matched, and oil and gas migration is unobstructed, which is a favorable exploration target in the southern slope area of Laizhou bay Sag.

 

Figure 1: Schematic diagram of the location of the study area.

The tectonic evolution of Laizhou bay Sag in Bohai bay Basin is mainly influenced by the four stages of tectonic activities. Cenozoic strata are well developed in the study area. The main oil-bearing strata are Shahejieformation, especially Es3x formation, which is revealed by drilling data. From top to bottom of stratum is Nm formation, Ng formation, Ed formation, Es formation. The lithology of Nm formation is medium sandstones, fine sandstones and mudstones. The lithology of Ng formation is medium sandstones, grits and mudstones. The lithology of Ed formation is fine sandstones and mudstones. The lithology of Es formation is siltstones, fine sandstones and mudstones. The color of siltstones and fine sandstones is light grey. The color of mudstones is gray. On the whole, it is a set of sedimentary sequence from semi-deep lake to shore-shallow lake delta [2], the sedimentary range gradually becomes larger from small, and the shoreline gradually recedes(Figure 2A). In the middle well areas of Y3, Y4 and Y5 wells, terrigenous clastic and carbonate mixed sediments are developed. Sand and gravel bodies are the main favorable reservoirs and typical mudstone reservoirs (Figure 2B).

 

Figure 2: Comprehensive stratigraphic section of the southern region in Laizhou bay Sag.

The study area is located in the sea, with relatively complex geological conditions, low resolution of seismic data (the data is old and not clear). The data collected includes log data from some wells, the 3d seismic data volume of the study area and some lithologic photographs. The distribution characteristics of sand body are studied by analyzing seismic profile and using seismic attribute analysis to identify seismic facies.

Seismic phase analysis is to qualitatively identify various types of seismic phase markers on the seismic section, divide seismic phase units, make a variety of seismic phase parameter maps, and finally conduct comprehensive analysis of these maps [3]. The external geometry and internal reflection structure of the reflected waves can reflect the overall deposition process, provenance direction and geological background of sediments [4]. In this paper, firstly, the research horizon [5-7] was identified on the seismic profile by means of synthetic record and well seismic combination, andthen the planar distribution characteristics of sedimentary bodies were identified by using seismic attributes, including verification of sediment sources. Finally, seismic facies were converted into sedimentary facies by combining different seismic reflection structures and sedimentary environments.

According to the north-south seismic profile EF of well Y5 in the research area (Figure 3 and the profile position is shown in figure 1C), obvious foreset and truncated reflection structures can be seen in the profile, which is speculated to be a delta sedimentary environment [8-9], and the overall formation is thinning to the south, indicating that a south-to-north provenance sedimentary system near the profile. By analyzing the characteristics of the seismic profile MN in Es3x Formation of the study area, it is found that both parallel-subparallel reflection structures and fore-integrating reflection structures (Figure 4 and the position of the section is shown in figure 1C). In other sections, the characteristics of seismic facies are not only wedge-shaped and fill-shaped, but also sheet-shaped and other low-energy sedimentary environments, which fully reflect the characteristics of rapid deposition in fan delta, great difference in water energy, and rapid sedimentary phase transition. In the classification of seismic facies in the study area, the seismic profiles of several survey lines are dissected and analyzed one by one. It is considered that there are mainly four seismic facies in the study area (Figure 5): parallel to subparallel seismic facies, foreset seismic facies, wedge-shaped seismic facies and filling seismic facies.

 

Figure 3: Cross-Y5 well seismic profile (Inline461).

Figure 4: Seismic profile of Y2 well and Y5 well (other well locations are projection wells) (Xline1890).

Figure 5: Regional seismic legend of the study area.

Parallel to the horizontal seismic facies: In the study area, it mainly distributed in the central of Y3 well, Y4 well and Y5 well. Internal reflection structure is high amplitude, high continuity, nearly parallel to the shape characteristic. The seismic reflection external form for sheet, the lower level is parallel or nearly parallel to the contact, the thickness of relatively stable. And the lithology mainly mudstone purpose layer, sand ratio is less than 0.2 by analyzing core samples, it reflecting the weak hydrodynamic conditions of deposition as a result, reflects the uneven subsidence of delta platform characteristics

Foreset seismic facies: The front end of the foreset reflection layer presents a downward superstate toward the pelvic floor, while the proximal end presents a top superstate toward the basin edge. The wedge-shaped foreset in the study area is characterized by wedge-shaped distributionalong the same direction axis and forward accretion. It is mainly distributed in the south of Y4 well and Y8 well. According to the logging data of Y8 well, it indicates the fan delta deposition on the slope 

Wedge-shaped seismic facies: The whole profile is wedge-shaped, and the internal reflection features are composed of medium-strong amplitude and medium-high continuous reflection in the same phase axis, which is the result of different settlement and deposition rates around the lake basin. In the study area, it is mainly distributed in the southwest of Y2 well and located in the slope belt. According to the results of coring interval calibration, the wedge-shaped seismic facies in this area belongs to the sedimentary seismic response of braided river delta 

Filling phase: It is characterized by filling shape of the unit. There are two facies in gulch filling phase: open filling (low-energy environment) and partial filling (high-energy environment), the filling in the study area for filling deposit in valleys. In the seismic section, in local dent, rendered in valleys. For the waveform characteristics of the region, medium strong amplitude and continuity well and parallel reflection can be found. It mainly reflect the shore and shallow lake, half deep lake, braided delta and delta front sedimentary environment

Different types of sedimentary environments have different characteristics in the seismic profile, which can also be shown in the attribute map. Analyzing attribute maps to study sedimentary environment and other information can better and more comprehensively reflect the planar distribution characteristics of sedimentary facies. Only by extracting seismic attributes in an appropriate way according to the exploration degree of the research area and the research object can the expected effect be achieved [10].

Under the guidance of seismic sedimentology, the target layer properties are accurately extracted. Appropriate time window layer is selected to extract seismic attributes, and seismic attributes are used to identify the planar distribution characteristics of the sedimentary system, and the sensitivity of the sedimentary system to seismic attributes is comprehensively considered to determine the characteristics of seismic facies [11]. We extracted RMS (root-mean-square) amplitude, mean instantaneous frequency and other attributes for analysis. Finally, it was found that the root-mean-square amplitude attribute had a guiding effect on the sedimentary plane distribution in the study area. By extracting the seismic attributes of RMS amplitude in Es3x formation in the 3D- seismic working area of the study area, it is found that there is a strong amplitude area in the south and southeast area of the study area (Figure 6ii), indicating that sand bodies in these areas are relatively developed and may be the main developed area of the delta. In the western part of the study area (Figure 6i), there is a band of mid-strong amplitude area, which is presumed to be braided river delta deposition. In the central area of the depression, that is, the south of Y8 well and around the northeast (Figure 6iii) are all weak amplitude areas, indicating that these areas are lake mudstone sedimentary areas and lacustrine sediments.

 

Figure 6: Root-mean-square amplitude attribute map of Es3x Formation.

The seismic units were divided by analyzing multiple vertical provenance, parallel provenance profiles and seismic phases, and according to the frequency, amplitude and continuity characteristics of waveform features, the study zones were divided into 4 areas, and the seismic phase distribution map of Es3x formation of the target layer was obtained, which was named as I, II, III and IV (Figure 7).

Figure 7: Characteristics of the plane distribution of the seismic facies in Es3x formation.

According to the plane distribution characteristics of seismic facies in the study area and combined with the regional sedimentary environment [12-15], the corresponding transformation relationship between seismic facies and sedimentary facies was constructed (Table 1).

Phase type	Facies marks		Area
mixed sedimentary platform	parallel subparallel seismic facies	mid-low frequency mid-strong amplitude middle continuity	III
		mid frequency mid amplitude middle continuity
braided river delta	wedge-shaped seismic facies	mid-low frequency mid amplitude middle continuity	I
		low frequency mid-low amplitude low continuity
fan delta	foreset seismic facies	mid-low frequency mid-low amplitude low continuity	II
		mid frequency mid-low amplitude middle-low continuity
		mid frequency strong amplitude good continuity
		low frequency low amplitude middle-low continuity
shore shallow lake	parallel subparallel seismic facies	mid-low frequency mid-low amplitude middle continuity	IV
		low frequency mid-low amplitude good continuity

In Y3-Y4-Y5 wells area, it is still difficult to identify the mixed sedimentary characteristics from the seismic facies alone. The lithologic composition analysis of the raised part of similar platform will be conducted by using the single-well facies analysis to guide the sedimentary system. The sedimentary facies were analyzed by drilling data, and through well seismic combination, the over-well profile and single well phase were established to analyze the sedimentary pattern of the target horizon. Due to the differences in mixed sediments, the Ex3x formation can be divided into three oil groups (mainly oil groups II and III) (Figure 8).

 

Figure 8: Waveform characteristics and single well phase diagram of Y5 well.

From single well can be found in the phase diagram, III-2 of oil group to II-2 of oil group deposit gray powder sandstone with thin layers of mudstone, through the core can be found in the development of cross-bedding, wedge cross bedding, wavy bedding and other bar sedimentary characteristics, response on seismic profile characteristics of weak amplitude, medium-low frequency, medium continuously. II-1 of oil group to I-2 of oil group deposit thin layer powder sandstone and argillaceous siltstone with a large set of mudstone. Bioclastic can be found in the core photos. Sedimentary characteristics have parallel bedding, tabular and trough cross-bedding delta front. Waveform response on seismic profile for weak amplitude, low frequency, medium continuously; According to the lithologic combination characteristics, there are both carbonate rocks and terrigenous clastic rocks, it can be judged that the Y5 well area has mixed sedimentary morphology, which reveals the characteristics of the sand body being transformed by waves, and it is the lakeside sedimentary environment with shallow water.

It is found that the braided river delta deposits from the far Kendong uplift are developed in the western Y2 well area, while the fan delta deposits from the Weibei uplift are mainly developed in the eastern Y6-Y7-Y8 wells area. However, affected by the bottom uplift and faults, Y3-Y4-Y5 wells in the middle blocks the source materials from the Kendong and Weibei uplift and deposits the mixed sediments belonging to lacuvial facies (Figure 9).

Figure 9: Sediment map of Es3x formation in southern region.

In Y3-Y4-Y5 wells area develop with mixed sedimentary mode - terrigenous clastic rock and carbonate rock mixed deposits, to the difficulty of mixed deposits has been a research on its mixed sedimentary area, surrounded by two kinds of material sources. One is in the study area of fan delta to the south and east, the other one is from the braided river delta deposits in western area, the terrigenous detrital material is as the waves were taken to mixed sedimentary area. The carbonate rocks were repeatedly changed by sea level at that time, which resulted in the protrusion exposed to the sea level or in the shallow water environment, resulting in the development of carbonate rocks.

In the study area, the fan delta seismic facies and braided river delta seismic facies can be identified, which are the main sedimentary facies types in this area. However, the mixed sedimentary part in the middle is not easy to be identified from the seismic facies, which is the focus of the following research, trying to find the basis of mixed sedimentary from the seismic facies.