Study on sedimentary microfacies and sand body distribution of Chang 8 oil reservoir group in Xunyi area, southern Ordos

Geological surveys were conducted in this area as early as the 1960s. Only a few wells were drilled in the 1970s, and there will be no large-scale exploration in the future. It is currently difficult to meet the need for further exploration. The provenance direction of the Chang 8 reservoir in this area is still controversial, the sedimentary microfacies characteristics and sand body distribution are unclear, and the development law is unclear. Therefore, the subsequent exploration and development process of Chang 8 reservoir in this area is greatly affected. Therefore, it is urgent to carry out research on sedimentary microfacies and sand body distribution. This paper studies the distribution of sedimentary microfacies and sand bodies of Chang 81 and Chang 82 oil layers, which provides an important basis for future reservoir research and analysis of accumulation factors, ascertaiN the distribution of oil reservoirs, predicting favorable zones, and evaluating and optimizing exploration targets.


Introduction
The study area is located in the southern Ordos Basin. across Shaanxi and Gansu provinces, with an area of about 12,000 km 2 , starting from Zhangjiawan in the north, Liulin in the south, Panke in the west, and Huangling in the east. The structure is located in two tectonic units of the northern Shaanxi slope and the Weibei uplift [1.2] .This paper studies the sedimentary microfacies and sand body distribution characteristics of Chang 8 reservoir in Xunyi area.Another section of your paper.

Regional sedimentary background
The Chang 8 sedimentary period of the Ordos Basin was controlled by four major provenance systems in the southwest, northwest, west and northeast. Among them, the northwest, northeast, and southwest are important source systems, and the west is a secondary source system. The Chang 8 sedimentary period is in the middle of the Chang 9 Lake flooding period and the Chang 7 Lake transgression period. The lake basin changes rapidly, and delta accumulation and degeneration occur in a short period of time. Therefore, the Chang 8 oil layer is divided into Chang 81 and Chang 82. Chang 82 is the process of base-level descent after the flooding period of Chang 9 Lake, and delta proliferation occurs [3][4] . Chang 81 is the period of base-level rise before the transgression of Lake Chang 7 and most of it is the period of delta regression.

Granularity characteristics
The particle size probability curve of length 8 can reflect that the debris particles are between 1.90 and 5.10. The cumulative probability curve shows a "three-level" pattern of traction flow samples, and the jump is mainly divided into "two stages". The particle size characteristics indicate the effect of two-way flow ( Figure 1). Generally speaking, the underwater distributary channel at the front edge of the shallow delta has such hydrodynamic characteristics.

Logging facies
The Chang 8 logging curves of 67 wells in this area have been studied. The results show that the SP and GR curves of Chang 8 wells are mainly shaped like a toothed bell, and the thickness is generally between 4 to 14m; the second is a toothed box shape. The thickness is usually 11~21m, and the shape and scale of the sand body are easily formed on the front edge of the shallow lake delta ( Figure 2).  Figure 2 The characteristics of the Chang 8 logging curve in the study area

Sedimentary microfacies and distribution characteristics of sand bodies
Based on the analysis of the color, lithology, sedimentary structure, grain size characteristics and logging curve shape of the Chang 8 oil layer group, Chang 8 is determined to be a delta-lacustrine sedimentary system, and the delta front is the main one. Two types of sedimentary facies, three types of sedimentary subfacies, and seven types of sedimentary microfacies are identified from the Chang 8 reservoir group, as shown in Table 1.  The underwater distributary channel flanks are distributed between the underwater distributary channel microfacies and the interdistributed bay microfacies, mostly in the southwest and west of the area, with a width of about 1.51-17.27km and a sand body thickness of 6-15.9m.

Chang 81 sedimentary microfacies and sand body distribution
It is controlled by provenance in the south, southwest and northeast. There are 4 underwater diversion channels entering the study area: N 201 Zheng 71 well area, 1.65 ~ 14.53 km wide and 15 ~ 23.90 m thick; N 51 well and N 41 well area, well width 1.74 ~ 3.49 km, sand thickness 15 ～17.70m; T1-N 73 well area, the well width is 1.1-19.3km, the sand thickness is 15-22.5m; the width of the T 19 to the north of Luoshanfu is 4.29-22.50km, and the sand thickness is 15.00-22.50m. There are two underwater distributary channels that enter the study area, which converge in the area from Well N 73 to Well N 39 north of Luoshan Prefecture. There are 5 irregular diversion bays, of which 3 are larger than 270km 2 and 2 are smaller, with 106.74km 2 and 68.53km 2 respectively.
The side of the underwater distributary channel is distributed between the underwater distributary channel microfacies and the interdistributed bay microfacies. The most developed area in the northcentral part of the study area is 1.4～20.74km wide and 5～14.6m sand thickness. , and the number of wells with a sand ratio less than 0.2 accounted for 53.61%; the cumulative thickness of the sand body was 2-30m, with an average of 11.2m. The number of wells with a thickness of 20m accounted for 8.32%, the number of wells with a thickness of 13-18m accounted for 21.46%, the number of wells with a thickness of 11-16m accounted for 24.22%, and the number of wells with a thickness of 10m accounted for 42.51%; the thickness of a single sand body was 1.60m-15.30m, with an average of 3.5m. In general, the average value of the total thickness of the sand body is slightly smaller than that of the single sand body, and the average value of the total thickness of the sand body is basically the same as that of the single sand body ( Table 2).