Fine Single Channel Identification Controlled by Sedimentary Facies

The Neogene fluvial reservoir in the Bohai oilfield is one of the leading development horizons for increasing reserves and production in the Bohai oilfield. However, the development of offshore fluvial reservoirs is faced with the problems of thin reservoir thickness, narrow plane width, rapid lateral change, and thin well pattern. Taking the KLA oilfield as an example, this paper discusses the nuanced characterization and configuration of a single channel controlled by sedimentary facies to guide developing offshore river facies’ narrow channel main control oilfield. Firstly, based on a large number of core data, the acceptable sedimentary facies identification is realized, the sedimentary model of the study area is established, the delicate calibration of logging facies and seismic facies is realized under the constraint of the sedimentary model, and a set of technical methods for nuanced reservoir characterization guided by seismic sedimentology is summarized, to realize the boundary identification of composite channel configuration and further realize the nuanced characterization of the single narrow channel. Based on this set of technology, it guides the smooth implementation of horizontal wells in the oilfield. The drilling encounter rate of the reservoir in the horizontal section of the single well exceeds 90%, ensuring the injection production connectivity and increasing the reserve production rate by more than 10%.


Introduction
With the gradual development of oil and gas field exploration and development, the research on sedimentary facies is becoming more and more detailed at home and abroad, and the accuracy of sedimentary facies division is becoming higher and higher [1]. Sedimentary microfacies are the most basic constituent unit in the sedimentary system, which reflects the sedimentary rocks formed under the same sedimentary conditions [2]. The viewpoint that the sedimentary characteristics of different microfacies are reflected and expressed in logging data is the basis for using logging data to identify sedimentary microfacies [3] [4].
A large number of studies show that the role of sedimentary facies in oilfield exploration and development cannot be ignored [5]. Especially at present, most domestic oilfields have entered the late stage of development, and the potential replacement and stable oil and water control are inseparable from fine sedimentary facies control [6]. With the understanding of delicate sedimentary geology, we can more clearly understand the reservoir distribution and heterogeneity [7].
We can more accurately understand the distribution of potential sand bodies [8]. Jianwei and others believe that the characteristics and combination of reservoir sedimentary microfacies are the main factor determining the heterogeneity of sand body and the fundamental reason for producing a large amount of remaining oil [1]; Yu Xinghe put forward a new reservoir characterization method of "microfacies guidance, phase sequence guidance, genetic quantification and combination of plane and section" through the study of domestic oilfields in the later stage of development, so as to better reflect the function and function of reservoir characterization, that is to identify the genetic characteristics and distribution laws of different reservoirs (small layers or single sand bodies for development) and solve the contradiction of oilfield development [12].
The detailed study of sedimentary facies control can understand the geometric shape, size, distribution, and vertical and horizontal connectivity of sand bodies, point out the development direction and distribution law of favorable reservoirs, guide the layout of block development well pattern and the subsequent rolling expansion and tapping potential [13] [14]. Taking the KLA oilfield as an exam- According to this technology, 11 horizontal production wells have been implemented in the fine production of composite narrow channel sand bodies, which have been put into operation for more than one year, and the average productivity of a single well can reach 180 m 3 /d, which provides a strong guarantee for the stable development of the oilfield. as an example, this research method is described.

Sedimentary Facies Identification
During the period from Guantao formation to Minghuazhen Formation of KLA oilfield, the structure of Huanghekou sag was generally uplifted. During the lower Minghuazhen Formation (lower Minghuazhen Formation), it was mainly fluvial facies deposition. Affected by Paleoclimate and paleogeomorphology, during the deposition period of the low stand region, the lake water body was shallow, rivers controlled the regional sedimentary sand body, and the sand body structure was mainly channel-shaped sand body. The sedimentary evolution characteristics of the lower Ming section of the KLA oilfield can be attributed to the continuous retrogradation process from the meandering plain deposition of the lower Ming Section V oil formation to the lacustrine shallow-water delta deposition of the lower Ming Section IV oil formation. In the lower Ming Dynasty, the oil formation V is a meandering plain deposit. The material source comes from Kendong uplift in the southwest. The river direction is mainly southwest-northeast. Some river channels cut each other. The sediments are mainly deposited by water distributary channel, crevasse fan, and flood plain. As the depression structure continues to rise, the lake level rises, and the lake area expands. The oil formation IV in the lower Ming formation is mainly affected by the southern provenance. Subparallel seismic facies dominate the seismic section cut along the provenance direction, and there are no "S-type" progradation reflection seismic facies on the classical delta seismic section, which reflects the gentle macro stratigraphic framework of the study area. Combined with the color combination of algae and mudstone, in the later stage of oil formation IV in the lower Ming Dynasty, KLA oilfield was in a shallow water environment, and the river controlled shore shallow lake shallow-water delta sedimentary facies was formed after the River entered the lake.

Sedimentary Subfacies Identification
There are two coring wells in KLA oilfield. According to the core photos of well kla-2 in the target interval ( interbedding of grayish-green mudstone and reddish-brown mudstone can be seen locally. The logging data shows that the mudstone interlayer in the sand body above each well also includes four colors: green-gray with reddish-brown, green-gray with yellowish-brown, red-brown with green-gray and yellowish-brown with green-gray.
Based on the macro stratigraphic framework of the study area and the sedimentary background of the coexistence of reductive and oxidized mudstone, it can be inferred that the target stratum in the study area is the sedimentary subfacies of the lower delta plain, which is located between the average high water level and the average low water level, exposed in the dry season and submerged in the flood season. Therefore, the distributary Bay in the reductive environment and the flood plain in the oxidizing environment are developed.

Sedimentary Microfacies Identification
Distributary channel and natural dike: the distributary channel is rich in strong hydrodynamic structures, including uneven scouring surface, parallel bedding, oblique bedding, wedge-shaped and plate-shaped cross-bedding, etc. (Figure 2).

Study on Identification and Characterization of Composite Channel
The lower member of Neogene Minghuazhen Formation in Bohai Sea area is a set of sand mudstone interbedded sediments in shallow lake basin, which has a low sand to ground ratio as a whole, and the sand body identification is good in earthquake [14]. In the target interval of the oilfield, the continuity of seismic data The depiction of river boundary by seismic facies is proved to be reliable by real drilling. However, due to the influence of seismic resolution and formation structure factors such as the development of thin interbedding and interlayer, some areas are inconsistent with well earthquakes due to the development of gas layer, thin narrow channel sand body and abnormal velocity above the target layer, It is necessary to correct the depicted composite channel boundary with the help of logging data.

Fine Depiction of Single Channel
The composite channel shows multiple positive rhythm characteristics vertically, 1) The two phases of single channel overlap vertically, but the two phases of channel do not contact, and the middle is separated by mudstone interlayer; 2) The two stages of single channel overlap vertically, the late single channel cuts the early single channel, and there is no mudstone interlayer in the middle; 3) The two phases of rivers overlap laterally, but the two phases of rivers do not contact, and the middle is separated by mudstone interlayer; 4) The two stages of river courses overlap laterally, the late single channel cuts the early single channel, and there is no mudstone interlayer in the middle part. Comprehensive research and practice show that the superposition characteristics of single channel and its corresponding seismic response characteristics can be divided into two types.
1) The corresponding seismic waveform is simple and symmetrical when there is no multi-stage channel superposition in the first stage single channel; 2) When the two phases of single channel overlap, the corresponding seismic waveform is complex wave or the waveform is asymmetric; In conclusion, when the two phases of single channel overlap, the seismic reflection waveform often becomes "fat" or complex wave and the frequency decreases, which is usually reflected as a low-frequency distribution area on the plane. Through the combination of well and earthquake, the plane layout of two stages of single channel is depicted.

Technical Achievements and Application
Using the fine description of single channel and composite channel, we can have partially covered by the crevasse fan. According to the river channel characterization results, the well location is optimized. In order to increase reserve production, the principle of single well through multi-channel development is adopted to maximize the benefit of single well. The horizontal well crosses multiple river channels obliquely on the plane and highly inclined wells cross multi-stage river channels vertically, Ensure the drilling encounter rate and reserve production degree of the reservoir, and the injection and production efficiency.
The actual drilling confirmed that the thickness difference between the river course intersection area and the single-stage river course was large, which con- It further shows that the sand body is deposited in two different river channels.
At the same time, well a8h and upper channel A7h in the same period is an effective injection production well group. The production performance also confirms that the two wells are connected to form effective injection production; For well cluster A2H-A3H-A6H, wells A2H and A3H are optimized to the river channel intersection area. The two wells are drilled through the two-stage longitudinal superimposed river channel to improve the degree of reserve production.
The a6h well is drilled to the horizontal water injection well in the lower river channel to use the reserves of a single river channel to supplement the formation  energy, and the production performance confirms the connection between wells to form effective injection and production. The production of production wells exceeds 150 m 3 /d (Figure 3). and increase the reserve production rate of the oilfield by at least 10%.