Study on Sedimentary Facies and Reservoir Characteristics of Paleogene Sandstone in Yingmaili Block, Tarim Basin

Block Yingmai 7 is structurally located in the western segment of the southern margin of the Kuqa Depression in the Tarim Basin. In the foreland basin, huge continental Mesozoic and Cenozoic strata have been deposited. In recent years, the Paleogene bottom sandstone section has been the main oil and gas exploration horizon. In order to further improve the oil and gas exploration e ﬀ ect of the Paleogene bottom sandstone in the study area and improve the accuracy of reservoir prediction, based on the related theory of sedimentology and petrology, this paper analyzes and studies the cores obtained by drilling in the Paleogene through laboratory core analysis experiments. The study shows that the Paleogene bottom sandstone is a braided river delta sedimentary system, and the sedimentary microfacies are mainly underwater distributary channels. The sandstone type is mainly light gray lithic feldspar ﬁ ne sandstone, the sandstone debris particle sorting degree is medium-good, and the roundness is sub-edge-sub-circular. The microscopic characteristics of the reservoir are large pore throat radius, low displacement pressure, mainly intergranular pores, and good pore connectivity. It is a thin-layered mesoporous, medium-permeable, large-throat, and strongly heterogeneous reservoir. Finally, a classi ﬁ cation evaluation table of reservoirs in block Yingmai 7 is established, and it is considered that the reservoirs of types I and II are enriched areas of oil and gas resources. The research results provide evidence for the prediction of oil and gas reservoirs.


Introduction
The Tarim Basin is the largest hydrocarbon-bearing basin in China. It is a superimposed composite basin which is located between the Tianshan Mountains, Kunlun Mountains, and Altun Mountains in Xinjiang Uygur Autonomous Region [1]. The basin area is 56 × 10 4 km 2 . The Proterozoic-Cenozoic was developed from bottom to top in the basin, with a maximum thickness of more than 10 000 m, containing rich oil and gas resources. In the past 30 years, largescale exploration has been carried out around the Tabei uplift and the Tazhong uplift in the platform basin, the Kuqa Depression, and the southwest depression in the foreland area [2][3][4]. Important discoveries have been made in three areas of clastic rocks in the basin area. The clastic rock in the platform basin is the key area for oil and gas exploration in the Tarim Basin [5]. In recent years, the exploration has mainly focused on the Meso-Cenozoic continental clastic rocks in the northern Tabei area and the Silurian-Carboniferous marine clastic rocks in the platform area [6][7][8]. The types of oil and gas reservoirs are complex, including anticlines, faulted anticlines, fault blocks, structural-stratigraphic, and structural-lithologic reservoirs [9]. The oil and gas source analysis results of the discovered oil and gas reservoirs show that the oil and gas from the Kuqa Depression are mainly distributed in the Neogene, Paleogene, Cretaceous, and Jurassic in the Luntai uplift and the Yingmaili low uplift in the western Tabei uplift and Silurian buried hill formations [10,11].
The Paleogene condensate gas reservoir in the Yingmaili area is one of the earliest oil and gas reservoirs discovered in the Cenozoic in the Kuqa Depression [12][13][14]. Well YM9 is the first well in the area. It was completed and tested in the 4683-4690 m well section in April 1992. The 11.11 mm nozzle was used to obtain production, with a daily oil production of 43.6 m 3 and a daily gas production of 15:7 × 10 4 m 3 , which proves that the Paleogene in the Yingmaili area has the conditions for forming oil and gas reservoirs [15,16]. The Yingmai 7 fault structural belt is located in the western segment of the frontal uplift belt of the Kuqa Depression in the Tarim Basin, adjacent to the Yangtake anticline to the north, and the Karayuergun structural belt to the west. The structural belt is NE-SW trending, about 70 km long and 10 km wide ( Figure 1) [17]. The entire fault tectonic belt has experienced three development periods: the Hercynian-Indosinian period, which is dominated by strong reverse fault activities and developed a series of large thrust faults below the Cretaceous; Yanshan-Xishan early stage: after the deposition of the Genian system, fault activity was the mainstay, and a series of north-dipping normal faults developed, forming beaded anticlines and fault noses on the upper wall of the fault; Himalayan period: due to the change in the form and direction of fault activity, the Neogene strata developed thrust faults with different properties from those formed earlier, resulting in the uneven thickness of the Neogene strata on the upper and lower walls of the fault [18].
With the continuous deepening of the exploration of the Paleogene condensate gas reservoirs in the Yingmaili area, the problems of increasing difficulty in reservoir prediction and difficult reservoir identification have emerged, which have brought great challenges to the oil and gas exploration in this area. Sedimentary facies are stratigraphic units that reflect the characteristics of paleo-sedimentary environments and have certain lithology and paleobiological markers. From the lithology, structure, and paleontology of sediments (rocks), the environment and process of deposition can be judged. The concept of sedimentary facies was first put forward by Swiss A·Gressly in 1838. The analysis of sedimentary facies for oil and gas reservoirs is an important research process in oil and gas exploration. In the early stage of exploration, laboratory experiments were carried out on the Paleogene reservoir cores. However, due to the large differences in reservoir characteristics between different strata and different regions in the Yingmaili block, it is still necessary to conduct detailed analysis and research on key exploration areas. In this paper, taking the Yingmai 7 fault structural zone in the Yingmaili block as the research area, the core samples obtained through a large number of drillings and the laboratory core test method are used to analyze the sedimentary facies and reservoir characteristics of the sandstone at the bottom of the Paleogene in detail. Oil and gas exploration provides a research basis.

Test Materials and Equipment.
The test material comes from a large number of core samples obtained from drilling in the sandstone section at the bottom of the Paleogene in block Yingmai 7. The core samples were prepared according to the experimental method to prepare the core slices to be tested. In the thin section production process, we first select representative rock samples, wash the samples with oil, keep the vacuum state during the production process, and cut and number the produced cast thin sections. After rough grinding, fine grinding, and fine grinding, the thickness is 0.03 mm, and it is ready for use after sticking the label.

Characteristics of Sedimentary Facies
3.1.1. Reservoir Petrological Characteristics. The target layer of block Yingmai 7 is the sandstone section at the bottom of the Paleogene. The rock type is mainly lithic feldspar sandstone, accounting for 54.7% of the rock type, followed by feldspar lithic sandstone and quartz sandstone, accounting for 23.2% and 22.1% of the rock type, respectively ( Figure 2).
The Paleogene sandstone reservoirs in Yingmai 7 block have high quartz content, concentrated at more than 70%, feldspar content at 15-20%, and debris content at 10-15%, reflecting that the Paleogene rock composition maturity is relatively high. The overall grain size of sandstone is mainly fine sand, accounting for 69.03% of the rock particles, followed by silt, accounting for 19.12% of the rock particles. The sandstone reservoirs in the Paleogene target layer are mainly loose in density, with rounded edges and corners from subedge to subround, and the sorting grade is medium-good ( Figure 3). For oblique bedding, oblique bedding is developed in the sandstone in the 4676.4 m section of Well YM702, and the interface between layers is relatively straight. The layer system is composed of many thin layers that are inclined in the same direction, and the thin layer and the layer system interface are oblique. If the adjacent layers are parallel to each other, the medium and fine layers in each layer are inclined in one direction, which is called monoclinic bedding. It is formed when water flows in a single direction, and the sloping direction of its thin layers indicates the downstream direction of water flow and is commonly found in river deposits and other sediments with flowing water (Figure 4(b)).
For wavy bedding, wavy bedding is formed when the hydrodynamic conditions alternate between strong and weak. It is more developed in delta fronts, rivers, tides, and other environments (Figure 4(c)).
For erosion surface, the undulating structural surface formed by the erosion and erosion of the underlying sediment due to the sudden increase of the flow velocity reflects the characteristics of strong hydrodynamics. The sediments on the scour surface are coarser than the underlying sediments and are more developed in the braided river delta environment (Figure 4(d)).
According to the particle size analysis data of Yingmai 7 block, the C-M map of the Paleogene was established. The samples of the C-M map of the Paleogene were concentrated in the QR section, and the reaction sediments were mainly transported by the gradient suspension of the tractive flow ( Figure 5). The grain size probability curve of Paleogene braided river delta facies has two segments. The two-stage grain size probability curve is composed of jumping and suspending populations, and the jumping population is the main one, reflecting the transport and deposition mechanism of tractive flow, mainly developed in siltstone and fine sandstone ( Figure 6).

Sedimentary Microfacies Division.
Through the analysis of rock color, rock mineral composition, rock structure, grain size, and comprehensive analysis of a large number of sedimentary structures in coring wells, the target layer of the Paleogene in the study area is divided into a lakebraided river delta sedimentary system, and the subfacies is divided into a shallow lake and braided river delta fronts. The microfacies are divided into shallow lake mud, underwater distributary channels, and underwater interdistributary bays ( Table 2).

Distribution Characteristics of Sedimentary Microfacies.
During the Paleogene sedimentary period, the provenance came from the southeast, and a set of braided river delta frontal deposits developed, and the overall performance from bottom to top was a water-transgressive sedimentary sequence.
During the E2 depositional period, a braided river delta sedimentary system was developed in the Yingmai 7 block. The provenance came from the southeast. The sand bodies were thin, but the lateral continuity was good. As a whole, the sand bodies in the southeast were more developed than those in the northwest. The planar facies distribution is dominated by underwater distributary channels, with mouth bars developed at the front of the underwater distributary channels, subaqueous interdistributary bays developed 5 Geofluids locally, and front sheet sand deposits developed outside the mouth bars (Figure 7).
During the E1 depositional period, the provenance came from the southeast. Compared with the E2 period, the water inflow occurred. Affected by water flow in paleosedimentary environment, the main body of the work area was the lake facies coastal shallow lake subfacies, mainly developed shallow lake mud (Figure 8).

Microscopic Characteristics of Reservoir.
According to the analysis data of the core casting thin section, the pore types of the E2 layer are mainly intergranular pores, and the intergranular porosity ranges from 16.33 to 20.25% (Table 3).
Through the analysis of the mercury intrusion data of Yingmai 7 block, the capillary pressure curve can be roughly divided into four types: Type A is dominated by high porosity and high permeability, the curve is platform-shaped, the pore throat is well sorted, the displacement pressure low, and the median pore throat radius is greater than 2 μm. The porosity and permeability of type B are still high, the curve is platform-shaped, the displacement pressure is low, and the median pore throat radius is greater than 1 μm. The C-type curve is slope-shaped, the displacement pressure is 0.06-0.1 MPa, and the median pore throat radius is less than 1 μm. Type D is dominated by low porosity and low permeability, the curve is slope-shaped, the pore throat sorting is poor, the displacement pressure is 0.5-3 MPa, and the median pore throat radius is less than 0.5 μm (Figure 9, Table 4).

Distribution Characteristics of Reservoir Physical
Properties. According to the statistics of the physical property analysis data of 64 Paleogene cores in Yingmai 7 block, the core analysis porosity is concentrated between 18 and 26%, with an average value of 15.44%. The permeability is concentrated in the range of 80 to 8000 × 10 −3 μm 2 ; the average value is 192:37 × 10 −3 μm 2 . The Paleogene target layers belong to mesoporous and medium permeability reservoirs (Figures 10 and 11; the reference standard is SY/T 6285-1997).
According to the statistics of porosity and permeability explained by logging in Yingmai 7 block, the variation range of Paleogene porosity is 14.50-24.60%, with an average value of 20.46%. The variation range of permeability is 58.60-2482:50 × 10 −3 μm 2 , and the average value is 599:45 × 10 −3 μm 2 , which indicates that the plane porosity and permeability of the Paleogene reservoirs are quite different, and the physical properties are highly nonuniform ( Table 6).
The average porosity of the E1 layer is 19.46%, and the average permeability is 432:71 × 10 −3 μm 2 . The minimum values of porosity and permeability are 14.53% and 58:63 × 10 −3 μm 2 of Well YM701, respectively. The plane distribution of porosity and permeability contours is controlled by sedimentation and sand body distribution shape, and the plane contours are densely distributed, reflecting the large difference in plane physical properties of this layer ( Figure 12).
The average porosity of the E2 layer is 21.46%, and the average permeability is 766:19 × 10 −3 μm 2 . The minimum values of porosity and permeability are 16.47% and 155:73 × 10 −3 μm 2 in Well YM16, respectively. The plane distribution of porosity and permeability contours is controlled by sedimentation and sand body distribution, and the plane contours are unevenly distributed, reflecting the large difference in   plane physical properties of this layer, but the difference is smaller than that of E1 ( Figure 13).

Reservoir Heterogeneity between Layers.
In this study, the following parameters are mainly used to evaluate the heterogeneity of reservoir physical properties.
Sand ratio refers to the ratio of the total thickness of sandstone on the vertical section to the total thickness of the formation, expressed as a percentage, equivalent to the ratio of sand to ground (%). According to the stratum thickness of Yingmai 7 block and the statistical results of sand body thickness interpreted by logging, the sand-to-ground ratio is calculated. The average total thickness of the two Paleogene layers in block Yingmai 7 is 63.09 m, the average total sandstone thickness is 33.55 m, and the average sandto-ground ratio is 0.51 (Table 7).
For interlayer permeability heterogeneity, the physical property statistics show that the minimum permeability of each sublayer of Paleogene in Yingmai 7 block is 58:60 × 10 −3 μm 2 , the maximum value is 2482:50 × 10 −3 μm 2 , and the average value is 645:50 × 10 −3 μm 2 . The permeability difference between layers is larger, indicating that the degree of heterogeneity between layers is strong.
Formula (1) and formula (2) show the calculation of permeability coefficient of variation (VK): In the formula, K is the average permeability of the core, 10 -3 μm 2 ; k i is the permeability of each core sample, 10 -3 μm 2 ; δ is the permeability variance, 10 -3 μm 2 ; and n is the number of core samples.
Generally, when V K < 0:5, it means that the degree of heterogeneity is weak; when V K is 0.5 to 0.7, it means that the degree of heterogeneity is moderate; when V K > 0:7, it means that the degree of heterogeneity is strong.
The calculation of the rush factor (TK) is shown in formula (3):       10 Geofluids In the formula, K max is the maximum permeability of the core sample, 10 -3 μm 2 .
When T K < 2, it means that the degree of heterogeneity is weak; when T K is 2 to 3, it means that the degree of heterogeneity is moderate; when T K > 3, it means that the degree of heterogeneity is strong.
The difference (J K ) calculation is shown in formula (4): In the formula, K min is the minimum permeability of the core sample, 10 -3 μm 2 .
When J k < 20, it means that the degree of heterogeneity is weak; when J k is 20-30, it means that the degree of heterogeneity is moderate; when J k > 30, it means that the degree of heterogeneity is strong.
Through the calculation of the heterogeneity parameters of the interlayer permeability in the Yingmai 7 block, the coefficient of variation of the Paleogene permeability is 1.46, the inrush coefficient is 3.85, and the gradient is 42.36, reflecting the characteristics of strong interlayer heterogeneity (Table 8).

Reservoir
Heterogeneity. According to the vertical variation of intralayer permeability, the intralayer heterogeneity in this area can be divided into four types, namely,   11 Geofluids positive rhythm type, inverse rhythm type, compound rhythm type, and homogeneous rhythm type. Generally, the permeability heterogeneity of the compound rhythm type is higher, followed by the positive rhythm type, the inverse rhythm type has relatively good homogeneity, and the homogeneous rhythm type has weak permeability heterogeneity.
For positive rhythm type, permeability increases vertically from bottom to top, and the highest permeability segment is distributed in the lower or middle-lower part. The natural gamma curve presents a box-bell shape. The acoustic transit time decreases slightly from bottom to top. The curve does not change much from bottom to top.
For inverse rhythm type, permeability increases from small to large vertically from bottom to top, the highest permeability segment is distributed in the upper or middleupper part, the natural gamma curve presents a funnel-box shape, the acoustic time difference increases slightly from bottom to top, and the resistivity curve decreases slightly from bottom to top.
For compound rhythm type, compound rhythm mainly occurs in thick sand layers (thick single sand layer or thick superimposed sand layer); there are four types of compound rhythms: one is multiphase positive rhythm combination type, the second is multiphase antiarrhythmic combination type, the third is the combination type of inverse rhythmpositive rhythm, and the fourth is the combination type of positive rhythm-inverse rhythm. Multiphase positive rhythm combination is a common type of permeability combination in thick sand bodies. It is composed of multiple positive permeability rhythms. Natural gamma logging often shows a box-bell combination type, and the resistivity curve slightly reduced from bottom to top, and the sonic curve also slightly reduced.
For homogeneous rhythm type, if the permeability changes vertically without rhythm, it is called homogeneous rhythm.
The Paleogene in Yingmai 7 block developed compound rhythm and inverse rhythm, among which compound rhythm was the main one, which indicated that it had the characteristics of strong intralayer heterogeneity. The intralayer permeability heterogeneity evaluation of the Paleogene reservoir shows that the intralayer permeability gradient is 75.76, the inrush coefficient is 4.09, and the variation coefficient is 2.25, showing a strong degree of heterogeneity (Table 9).   (Table 10). Among them, type I and II reservoirs are high-quality reservoirs for key drilling.

Conclusion
The Paleogene of Yingmai 7 block is a set of braided river delta sedimentary system, the microfacies is mainly underwater distributary channel, the reservoir rocks are mainly light gray lithic feldspar fine sandstone, and the rock density is loose-medium. The reservoir has average sand-to-ground ratio of 0.51%, strong interlayer and plane heterogeneity, well-developed interlayers, large thickness, and strong intralayer heterogeneity. The average porosity of the reservoir is 15.44%, and the microscopic characteristics of the reservoir are large pore throat radius, low displacement pressure, mainly intergranular pores, and good pore connectivity. The reservoir is a thin-layered mesoporous, medium permeability, large throat, and strongly heterogeneous reservoir.

Data Availability
The figures and tables used to support the findings of this study are included in the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.