Research on Thin Layer Structure Identification and Sedimentary Facies of Middle and Deep Layers Based on Reflection Coefficient Inversion —By Taking Dongying Formation of CFD Oilfield in Bohai Offshore as an Example

The sand layer B of Dongying Formation of CFD oilfield in Bohai offshore belongs to the middle deep layer of buried hill overlap deposit. Its reservoir distribution has the characteristics of large burial depth, thin thickness and rapidly lateral change. Because of low resolution of seismic data and overlying sand layer. It is difficult to identify and interpret the structure of sand layer accurately. The uncertainty of structure and reservoir restricts the fine development of B sand layer. In order to identify the top surface of reservoir effectively. The seismic data are processed by using the reflection coefficient inversion method. The results show that the inversion resolution of reflection coefficient is significantly higher than that of original data. The top surface of sand layer B and its overlying sand layer can be well identified and traced. Carrying out structural interpretation of B sand layer based on reflection coefficient inversion data and the microstructure and the formation tip ex-tinction point are implemented. Based on the constraint of new interpretation level, the sedimentary facies plane distribution of B sand layer is described and make prediction of dominant reservoir development area in detail com-bining with sedimentary paleogeomorphology, along layer attribute section and limited drilling data. The research shows that the study area is mainly from the northwest material sources, the slope belt in the northwest is close to the lake shoreline with a gentle slope and shallow water depositional envi-ronment, which is located on the main transport and deposition channels. The shallow water gentle slope landform is suitable for forming large-area sand bar deposition, mainly composed of underwater distributary development well location effectively.


Background
The middle and deep reservoir layers as an important sedimentary which play a key role in oil producing layer within the Bohai oilfield. Generally, the formation is buried at a big depth and due to the particularity of offshore oilfield development, in addition to the lack of drilling data and low resolution of seismic data. All these reasons make the study and analysis of the reservoir very complex. The accurate understanding of structure and reservoir has become the key to restrict oilfield development. At present, scholars have carried out a lot of research work in the aspects of paleoseismic facies control and reservoir restoration [1]- [13]. However, due to the influence of formation absorption attenuation, interference and tuning, the signal-to-noise ratio of seismic data in the middle and deep layers are generally low, which leads to the problems of multi solution and low accuracy in the study of structures and reservoirs, especially in some thin layers of the middle and deep layers.
The inversion method based on reflection coefficient is developed in recent years, which can effectively distinguish thin layers and improve the resolution [14] [15] [16]. The reflection coefficient volume obtained by this inversion method can effectively eliminate the thin layer interference phenomenon in the original seismic reflection, and has the advantages of not depending on the well and requiring the initial model, which can improve the seismic data resolution to an extremely high level. However, the inversion results are easy to be disturbed by noise, and the matching between the results and well data is poor, which affects the application effect.
In addition, seismic sedimentology method has unique advantages in describing the plain distribution prediction of sedimentary body, but its results are often constrained by the accuracy of the structural horizon. Aiming at the problem that the structural top of sand layer B in Dongying Formation of CFD oilfield can not be identified and traced, the reflection coefficient inversion method of time-varying wavelet is used to inverse the broadband reflection coefficient body and calibrate the synthetic record. On this basis, the interpretation of stratigraphic structure and seismic sedimentology is carried out again to depict sedimentary facies belt and dominant reservoir area, and to guide the deployment of oilfield well location.

General Situation of Oilfield
CFD oilfield is geographically located in the Western Bohai offshore and struc-   2) The productivity of production wells is quite different. The daily oil production of high-yield wells is 80 -150 m 3 /d, while that of low-yield wells is only 10 -20 m 3 /d and some wells even have no production. Therefore, it is necessary to carry out the plane sedimentary facies distribution research to find the "sweet spot" area of reservoir.
In order to solve the two problems about the structure is not implemented and the reservoir research is not deep, the seismic data reflection coefficient inversion is carried out to improve the vertical resolution. Effectively identify the top surface of the target layer and the exposed area of the buried hill, and carry out the fine structure interpretation and Paleogeomorphology restoration again.
Combined with core and logging data, constrained by the new interpretation surface, the seismic attributes along the formation are extracted, and the sedimentary facies belt and favorable reservoir development area are depicted in combination with sedimentary model.

Basic Principles of Seismic Reflection Coefficient Inversion
Rayleigh criterion considers that the resolution limit of a reflection wave is 1/4 wavelength, which has been widely accepted in geophysical field. Widess thinks The difference between reflection coefficient inversion and conventional  inversion is the establishment of the objective function. In the inversion process, the effective optimization algorithm is used, and the logging data is used to constrain. The basic process is to remove the influence of time-varying wavelet according to the broadband seismic signal obtained by frequency extension processing, and the reflection coefficient obtained is taken as the initial reflection coefficient sequence, and then the objective function is solved to obtain the optimal target Reflection coefficient.

Practical Application Effect
The specific process is as follows: firstly, on the basis of seismic data denoising, the time-varying wavelet is extracted and the odd and even parts of the reflection coefficient are extracted. According to the method of calculating the initial reflection coefficient, the high frequency components and the odd and even reflection coefficients are inversed. Finally, the weight function is used to combine the whole broadband reflection coefficient volume. Compared with the conventional sparse pulse inversion, the reflection coefficient inversion is more sensitive to the thin layer tuning effect, which can effectively eliminate the thin layer tuning effect, and has strong anti noise ability. From the actual section comparison (Figure 5), the seismic resolution of the processed results is improved. The seismic reflection characteristics of a set of low-frequency complex wave troughs on the top surface of Dongying Formation II oil formation in the original data are transformed into two troughs and one peak reflection, showing two sets of reservoir reflection characteristics. After well seismic calibration and comparative analysis, the upper trough corresponds to the top surface of sand formation A in the II oil formation, and the lower trough corresponds to the top surface of sand formation B. The results show that there is a good corresponding relationship between well and seismic data. According to the calibration results, the fine interpretation of the top structure of sand layer B is carried out again, and the pinch out position of structure and stratum was determined.
From the comparison before and after the structural map, the map is interpreted according to the reflection coefficient inversion method. The overall structural Open Journal of Geology trend and shape are basically consistent with the old map, but the local structural description is more refined, and the understanding of the buried hill exposure range within the oilfield scope is more clear, so as to effectively avoid the structural risk ( Figure 6).

Restoration of Paleogeomorphology
The sand layer B is a middle deep buried hill overburden deposit, and provenance supply and paleogeomorphology have obvious control on the distribution of sand bodies and sedimentary facies. According to the principle of sedimentary compensation, without considering the differential compaction, the present residual thickness can roughly reflect the change of accommodation space, that is the relative height of paleotopography. Therefore, on the basis of stratigraphic framework division and fine structural interpretation of target horizon, the residual thickness method is used to restore the relative paleotopography.
During the sedimentary period of sand layer B, the paleogeomorphology of the study area is characterized by "high in the north and low in the South", and five types of paleogeomorphology are mainly developed: uplift, steep slope, gentle slope, valley and marsh land. Some ancient high points and slopes are developed in the oil field. The depositional margin is mainly from northwest, and the braided river delta front is formed after the oil field enters the lake (Figure 7).

Characteristics of Core and Single Well Facies
Combined with the lithology and logging data of coring wells, the lithofacies and  shallow lake mud of the pre Delta (Figure 9).      Based on this understanding, the oilfield deployed and implemented two horizontal production wells F7H and F9H in the west slope zone of the sand body in 2020. The average drilling encounter rate of the two wells in the horizontal section is 86%, and the initial oil production is 70 -90 m 3 /d, which has achieved good development adjustment effect, and also verified the geological understanding.

Summary
1) Based on the principle that even component reflection coefficient can improve the resolution of thin layer, the reflection coefficient profile can be inversed to improve the seismic resolution. On this basis, fine interpretation is carried out to implement the structure, which provides a new research idea and 2) The distribution of dominant reservoirs in the middle and deep layers is often controlled by paleogeomorphology and provenance supply. Based on the analysis of paleogeomorphology, single well facies and seismic attribute slice data, the provenance of sand layer B of Dongying Formation comes from the Northwest Shaleitian uplift, which forms the braided river delta front deposit in the study area. The northwest slope belt is close to the lake shoreline, and is lo-