Influence of fractures in tight sandstone oil reservoir on hydrocarbon accumulation: A case study of Yanchang Formation in southeastern Ordos Basin

Zaiyu Zhang; Weiwei Liu; Xiaodong Wu

doi:10.1515/geo-2022-0509

Article Open Access

Influence of fractures in tight sandstone oil reservoir on hydrocarbon accumulation: A case study of Yanchang Formation in southeastern Ordos Basin

Zaiyu Zhang , Weiwei Liu and Xiaodong Wu

Published/Copyright: July 22, 2023

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From the journal Open Geosciences Volume 15 Issue 1

Abstract

The southwestern Ordos Basin is located at the junction of several stable plates, where faults and fractures are relatively developed, and the influence of fractures on the distribution of tight oil reservoirs in the Yanchang Formation is not clear. To solve this problem, the characteristics of fracture development of the Yanchang Formation, southwestern Ordos Basin and its control on hydrocarbon accumulation have been systematically studied using the core, thin section, well logging, productivity, fault and sand body distribution data. The results show that vertical and horizontal bedding fractures are much more developed in the Chang 8 Member compared with the Chang 6 and 7 Members. For horizontal bedding fractures, they are mainly developed in fine sandstone, followed by siltstone, while no horizontal bedding fractures are observed in medium sandstone. This is because horizontal bedding fractures are more common in fine-grained sediments. For vertical fractures, they are also mainly developed in fine sandstone. The controlling factors of fractures include lithology, sand body thickness, sedimentary microfacies and fault–fracture coupling relationship. Fractures are well developed in the fine-grained sandstone of the wing parts of the main river channel due to small compacted space. In the area where multiple river channels intersect, the sand is pure and easy to break. Based on the comprehensive study of sedimentation, structure and fracture, the classification criteria of sweet point reservoir of the Yanchang Formation are determined: sedimentary microfacies of the main river channel and its wing, tectonic location within 1.5 km from the main fault, plane arrangement of right-lateral and right-order faults and developed fractures. The sweet point reservoir can be identified effectively using the developed sweet point screening criteria.

Keywords: Ordos Basin; tight sandstone oil reservoir; fracture; fault; deposition; tectonics

1 Introduction

Natural fractures have important influence on the prediction of tight oil reservoir sweet point and the adjustment of development of well pattern [1–3]. Generally, the quantitative characterization of natural fractures in tight reservoirs is based on the comprehensive evaluation of geology, well logging and dynamic monitoring [4,5]. Natural fractures of different types and sizes have different occurrences, widths, lengths and apparent porosity, so they have different effects on the reservoir quality and the development adjustment measures [6,7]. If the characteristics of natural fractures are unknown, the hydrocarbon accumulation law and development adjustment measures cannot be targeted, which will seriously affect the ultimate degree of oil recovery [8,9].

The Ordos Basin is a typical craton basin with a large number of strike-slip faults at the edge of the basin. The strike-slip fault system at the periphery of Ordos Basin is different, and the main strike-slip system includes “X” conjugate strike-slip fault system and single shear strike-slip fault system [10,11]. Faults have similar genetic mechanism to tectonic fractures in nature and they are associated with each other. How the fault and its associated fracture affect the accumulation of oil and gas has always been the concern of petroleum geological researchers. Fracture is closely related to rock mass deformation, which affects the development effect of oil and gas reservoir [12,13].

The Upper Triassic Yanchang Formation in Ordos Basin has well developed tight oil reservoirs with porosity less than 10% and permeability less than 1 mD [9,10]. The tight oil reservoir has strong heterogeneity and the hydrocarbon mainly comes from the source rocks of the Chang 7 Member. With the acceleration of tight oil exploration in the Yanchang Formation in recent years, people gradually realize that there are a lot of fractures in the Yanchang Formation at the edge of basin, and the existence of fractures is a good indicator of tight oil sweet point. The Ordos Basin is located on the stable craton basement. Many research studies have been carried out on the influencing factors of reservoir fracture development in the Yanchang Formation under the low amplitude tectonic background [14,15]. These studies have focused on areas with stable structures and undeveloped faults. For the southwestern Ordos Basin, the area is at the junction of several plates. A series of fourth-order faults and a large number of fractures are developed in the low amplitude tectonic setting, which indicates that this area has stronger tectonic activity than the inner Ordos Basin. There are few studies on the controlling factors of fracture development in such areas. In the southwestern margin of the Ordos Basin, fractures divide the matrix reservoir into a series of blocks. The fractures have high conductivity, while the matrix reservoir has low permeability. Fractures promote the accumulation of oil and gas, but are often an important cause of low oil recovery. Therefore, it is very important to explore the controlling factors of fracture development in this kind of special area and special type of reservoir.

Taking the tight oil reservoir of the Yanchang Formation (Chang 6–8 Members) in the southwest Ordos Basin as an example, the characteristics of fracture and its control on hydrocarbon accumulation have been systematically studied using the core, thin section, well logging, productivity, fault and sand body distribution data. The main innovation of this article is that the control factors of fracture reservoir in the southwest Ordos Basin are proposed, including lithology, sand body thickness, sedimentary microfacies and fault–fracture coupling relationship. This study has important guiding significance for tight oil “sweet point” prediction.

2 Geological background

The Ordos Basin is a large-scale polycyclic craton basin. Looking at the present tectonic form, the basin is divided into six first-order structural units, namely Yimeng Uplift, Weibei Uplift, Jinxi Flexural Fold Belt, Yishan Slope, Tianhuan Depression and Western Margin Thrust Fault Belt [8,9]. Folds and faults are developed along the edge of the basin, while the internal structure is simple, and the structural fluctuation per 1 km is between 7 and 10 m. The study area is located in the southwest of the Ordos Basin (Figure 1).

Figure 1

Location of the study area (modified after ref. [22]).

Chang 6 to Chang 8 Members are the main oil production horizons of Yanchang Formation. During the Chang 8 period, the lake basin was always in a state of subsidence. In the Chang 7 period, the continental lake has the largest water flood range and the deep lacustrine source rocks were formed. In the Chang 6 deposition period, the continental lacustrine basin was gradually shrinking [3–5]. Then, the main stage of the construction of the lacustrine basin delta formed. The microfacies contain underwater distributary channels, interdistributary bays, estuary bars, sheet sand and semi-deep to deep lake mud microfacies [6,7].

The study area experienced three tectonic movements: Indosinian, Yanshanian and Himalayan Movements. In the early Mesozoic period (Late Triassic–Early Middle Jurassic [220–175 Ma]), affected by the Indosinian Movement [16,17], the southwestern Ordos Basin was retrograde and gradually changed into inland fluvi-lake deposits. Then, the Zhifang and Yanchang Formations are developed. In the Middle and late Triassic, collision orogeny occurred in the Qinling Mountains, and the southwest margin was reformed to some extent [18,19]. At the end of Late Triassic, uneven uplift occurred in the southwest margin. The Upper Triassic suffered denudation and continued to the Early Jurassic, and the Early Jurassic strata were missing [20,21]. After the short transition at the beginning of the Yan'an period in the Middle Jurassic, the lake basin expanded rapidly and entered the peak period again, and the Yan'an Formation, Zhiluo Formation and Anding Formation developed [18,19].

Regional tectonic uplift occurred continuously from Late Jurassic to Early Cretaceous (165–118 Ma) in the southwest margin. In the Late Jurassic (the main stage of Yanshanian), the working area was affected by the strong compressive/shear stress in the NE direction, the southwest margin developed strong thrust nappe structure to the east, and the early thrust fault combination in the NWW direction and NEE direction of Yanshanian formed, and a group of faults may be mainly developed in some areas [18,19]. In the Early Cretaceous (middle and late Yanshanian), tectonic negative inversion began to occur in early thrust faults under the action of local strike-slip pull-apart.

From Late Cretaceous to Early Cenozoic (95–40 Ma), regional depositional loss in Late Cretaceous lasted until Paleocene, and continuous denudation and erosion occurred in the southwest margin. In addition, extensional tectonic activity and negative inversion of faults occurred continuously [20,21]. Until the late Cenozoic (14–7 Ma), some NE-trending strike-slip tensile faults were developed mainly due to the NW trending compression-shear stress and NW trending weak tensile action. From the west to the east, the compressive action is weakened but the shear action is gradually strengthened. The Liupanshan Arcuate Thrust system has a certain influence on the eastern stable craton basement [20,21]. The Liupanshan Arcuate Thrust-tectonic belt is dominated by thrust-system under extrusion background, and gradually transitioned to strike-slip pull-apart and normal fault system in the east (southeast margin of Ordos Basin) (Figure 2).

Figure 2

Comprehensive histogram of the Mesozoic strata in the study area (modified after ref. [22]), where C2 represents Chang 2 Member, C3 represents Chang 3 Member… and C10 represents Chang 10 Member. The red boxes represent the target layers (C6–C8) of this study.

3 Methods

Core data of the target layers from more than 20 wells were used to systematically study fracture types and fracture development characteristics of different oil groups. Furthermore, the influence of fractures on hydrocarbon accumulation is analyzed in terms of lithology, river channel, sedimentary microfacies and fault–fracture coupling. Figure 3 shows the technical route of this study. The data used included common thin sections, well logs, productivity, fault and sand body distribution. Common thin sections are used to identify the lithology of rocks and their internal mineral composition. Moreover, productivity, fault and sand body distribution data are used to analyze the effect of fractures on hydrocarbon accumulation.

Figure 3

Technical route of this study.

4 Results

4.1 Comparison of fracture types

In this area, the content of feldspar in Yanchang Formation sandstone is usually higher than 50% (feldspar sandstone). Feldspar is divided into potassium feldspar and plagioclase. There is no significant difference in the percentage of quartz, feldspar and debris in the Chang 6, 7 and 8 Members. The lithic compositions of the three horizons are mainly volcanic rock and granite.

Figure 4 shows the images of core fractures. Vertical fractures and high-angle fractures are well developed (Figure 4a–c). Vertical fracture surfaces are mostly half-filled and unfilled, which represent good fracture effectiveness (Figure 4a). The flat fracture characteristics indicate that these fractures are mainly formed under the background conditions of regional compressive and shear tectonic stress (Figure 4b). Strong horizontal compression is an important reason for the development of near-vertical fractures and high-angle shear fractures in the target layer (Figure 4b and c).

$Figure 4 Fracture development characteristics of cores of Yanchang Formation in the study area. Notes: (a) Well J63, vertical fracture, unfilled, crude oil is seen at the fracture surface, 1393.04–1393.27 m, Chang 8 Member; (b) Well J63, the vertical fracture developed in the middle of the core divides the core into two parts, and there is no filling material on the fracture surface, 1373.3–1373.54 m, Chang 8 Member; (c) Well H74, high angle fracture, 2354.13–2354.43 m, Chang 8 Member; (d) Well J55, horizontal bedding fractures, which have good oil-bearing property, 1408–1408.15 m, Chang 6 Member.$

Figure 4

Fracture development characteristics of cores of Yanchang Formation in the study area. Notes: (a) Well J63, vertical fracture, unfilled, crude oil is seen at the fracture surface, 1393.04–1393.27 m, Chang 8 Member; (b) Well J63, the vertical fracture developed in the middle of the core divides the core into two parts, and there is no filling material on the fracture surface, 1373.3–1373.54 m, Chang 8 Member; (c) Well H74, high angle fracture, 2354.13–2354.43 m, Chang 8 Member; (d) Well J55, horizontal bedding fractures, which have good oil-bearing property, 1408–1408.15 m, Chang 6 Member.

In addition, horizontal bedding fractures are also well developed in the cores (Figure 4d). Horizontal bedding fractures are different from horizontal structural fractures. Horizontal tectonic fractures are formed by large-scale horizontal slip of rock mass under regional tectonic activities. This type of fracture usually has a very small opening but a significant displacement. However, it is observed that horizontal tectonic fractures are not developed in the target strata of the study area. In addition, bedding fractures are usually formed by sedimentary interfaces in rocks under the combined action of diagenesis and tectonic activities. Their formation is closely related to diagenesis and tectonic activity, and they are usually highly developed, with dozens of bedding fractures per meter. Moreover, this type of fracture usually has no significant horizontal displacement.

Horizontal bedding fractures are mostly of oil spot level. The fracture surface observed in the core is mostly straight and smooth, and the tail end of the fracture has folding tail, rhomboidal ring and rhomboidal bifurcation [23,24]. The linear density of fractures in the target layer is usually less than 2 fractures/m. Since vertical fractures are mainly developed, we indicate the degree of fracture development by designating the fracture development segment of a single well.

4.2 Comparison of fracture characteristics in different layers

According to the core observation, the development degree of vertical fractures in the Yanchang Formation is lower than that of bedding fractures on the whole (Figure 5a). The development degree of vertical and horizontal bedding fractures of the Chang 8 Member is slightly higher than that of the Chang 6 and 7 Members (Figure 5b and c). For horizontal bedding fractures, the fractures are mainly developed in fine sandstone, followed by siltstone, while no horizontal bedding fractures were observed in medium sandstone (Figure 5b). This is because horizontal bedding fractures are more common in fine grained sediments. For vertical fractures, they are also mainly developed in fine sandstone (Figure 5c). Only vertical fractures were found in the Chang 7 Member of siltstone, while fractures were found in the Chang 6 and Chang 8 Members of medium sandstone.

$Figure 5 Histogram of dip angle and lithology of fractures. Notes: (a) development degree of different types of fractures in the Chang 6–8 Members; (b) development characteristics of horizontal bedding fractures in sandstone of different lithology in the Chang 6–8 Members; (c) development characteristics of vertical fractures in different lithologic sandstones of the Chang 6–8 Members.$

Figure 5

Histogram of dip angle and lithology of fractures. Notes: (a) development degree of different types of fractures in the Chang 6–8 Members; (b) development characteristics of horizontal bedding fractures in sandstone of different lithology in the Chang 6–8 Members; (c) development characteristics of vertical fractures in different lithologic sandstones of the Chang 6–8 Members.

The fracture observed by the cores is only the fracture feature of a point on the plane, so it cannot reflect the actual fracture characteristics. Real fractures have more complex developmental characteristics underground. Therefore, the fracture frequency we calculated is the development frequency of different fracture types at a single well point. This is what we know based on the limited data we have. With the continuous enrichment of subsequent research data, our understanding of fracture development characteristics will be further improved.

The fracture of tight sandstone also has a certain response to conventional logging parameters. For example, the presence of vertical fractures in Figure 6 mainly causes the increase of acoustic wave time difference and resistivity, and these single sand bodies are interpreted as oil–water layers. After perforating the Yanchang Formation in the 1,400 m interval, the current daily oil production is 10.4 t. Therefore, fractures play a positive role in reservoir enrichment. Fractures are also related to porosity, permeability and saturation of sand bodies [25,26]. This is because, on a microscopic scale, microfractures usually form around the pores. Therefore, the higher the degree of fracture development, the higher the physical property and oil saturation of the corresponding sand body will be improved to a certain extent [27–30].

Figure 6

Fracture development characteristics of Yanchang Formation in Well JH55. Notes: GR, SP, CAL, AC, DEN, ILD, ILM and CNL are all logging symbols, which, respectively, represent natural gamma, spontaneous potential, well diameter, acoustic wave time difference, rock density, deep lateral resistivity, medium lateral resistivity and compensated neutron.

5 Discussion

5.1 Lithology of fractured sandstone and its control on hydrocarbon accumulation

There is no obvious oil–water interface in the Chang 6–8 tight reservoirs, and lithologic occlusion will form lithologic traps. The crude oil is mainly derived from the source rocks of the Chang 7 Member (Figure 7). Due to poor physical properties of matrix reservoirs, the existence of fractures has become a good drainage system for efficient migration and accumulation of hydrocarbons [31,32]. In terms of the relationship between fracture development degree and lithology, it can be found that fractures are mainly developed in medium and low thickness fine sandstones with thickness in the range of 2–6 m. For thick (greater than 6 m) sand bodies, fractures are underdeveloped. Fractures are generally developed in the sand body at the intersection of the river channel or near the estuary bar, while the thickness of the front facies sheet sand microfacies sand body is large and the fractures are relatively underdeveloped. According to the location of the sand body, the fractures mainly developed in the wing of the main river channel. The middle part of the main river channel has good physical property and low fracture development. From the middle part to the wing part of a certain river channel, the particle size will gradually change from medium to fine scale, which is easy to fracture [33,34].

$Figure 7 Diagram of Yanchang Formation fractures as a drainage system.$

Figure 7

Diagram of Yanchang Formation fractures as a drainage system.

Figure 8 shows the fracture characteristics of the Yanchang Formation in the southwest Ordos Basin revealed by FMI imaging logging, cores and outcrops. The extension scale of outcrop fractures can be tens of meters. Vertical fractures formed due to regional tectonic activity are well developed. These vertical fractures provide good drainage channels for vertical migration and accumulation of oil and gas [35,36].

$Figure 8 Fracture development characteristics of Chang 8 Member of Yanchang Formation in Jinghe Area, southwest Ordos Basin. (a) FMI imaging results of fractures in Chang 8 Member of Well JH13 [37], (b) photo of fracture in Well JH41 [38] and (c) vertical fractures that developed in the outcrops of Yanchang Formation in the southwest Ordos Basin [38].$

Figure 8

Fracture development characteristics of Chang 8 Member of Yanchang Formation in Jinghe Area, southwest Ordos Basin. (a) FMI imaging results of fractures in Chang 8 Member of Well JH13 [37], (b) photo of fracture in Well JH41 [38] and (c) vertical fractures that developed in the outcrops of Yanchang Formation in the southwest Ordos Basin [38].

5.2 Sedimentary microfacies–fracture coupling and its control on hydrocarbon accumulation

In the study area, multi-stage channel sandbodies and mudstone superposition are developed in the Chang 6–Chang 8 Members longitudinally, forming multi-stage effective lithologic traps. “Multi-stage channels” equal to “multi-stage sand bodies,” they refer to single sand bodies formed at different deposition times. They are formed due to the change of lithology or thickness or even loss of sand body with the change of paleosedimentary environment. Therefore, during the sedimentary history, there were multiple sets of sand bodies developed in the Chang 6 to Chang 8 Members. Multi-stage effective lithological traps represent the coupling system composed of tight sandstone reservoir and its overlying mudstone cap at different stages.

Oil and gas are charged into multi-stage traps through relatively high permeability (＞1 mD) sandstone and fracture drainage system. Fracture drainage system refers to a belt of developed fractures. The fractures in these areas are usually 1–2 fractures/m, and have high fluid drainage capacity, so they are important channels for oil and gas migration. Figure 9 shows the plane distribution of the Chang 8₁ ² layer in Block X36 of the study area. The provenance of the Chang 8 Member in this area is mainly from the northwest and southwest areas. The river water will converge in the middle area, and it has strong scouring ability.

$Figure 9 Relationship between sand thickness and fracture distribution of Chang81 2 layer in X36 working area. The well trajectories have two colors of black and red. The well path represents the horizontal section of the target zone, which is developed using horizontal wells. Black represents abandoned wells, while red represents wells that are still producing oil. However, all of these wells have complete logging data. The fracture index is the ratio of the thickness of the fractured sandstone segment to the cumulative thickness of the sand body.$

Figure 9

Relationship between sand thickness and fracture distribution of Chang8₁ ² layer in X36 working area. The well trajectories have two colors of black and red. The well path represents the horizontal section of the target zone, which is developed using horizontal wells. Black represents abandoned wells, while red represents wells that are still producing oil. However, all of these wells have complete logging data. The fracture index is the ratio of the thickness of the fractured sandstone segment to the cumulative thickness of the sand body.

The Chang 6 to Chang 8 members in the study area all belong to delta front facies. The river channel swing is relatively frequent, the river water in the intersection or diversion area is repeatedly scour, and the sandstone is well sorted and brittle, and prone to rupture (Figure 9). Since vertical fractures are mainly developed in the target layer, the fracture index in this article refers to the ratio of the thickness of the sandstone in the fractured section to the thickness of the total sand body. The thickness of fractured sandstone is determined by core, imaging logging and drilling fluid loss. Core and imaging logging can clearly reflect the fracture development characteristics. In addition, if drilling fluid is lost during drilling, this indicates that fractures have developed in the sand body.

The coupling relationship between distribution of the Chang 8₁ ² sand body and hydrocarbon also shows that the channel intersection or diversion area is prone to large-scale accumulation (Figure 9). In addition, lateral river erosion and fractures have improved the physical properties of tight reservoirs to a certain extent [39–42].

The fractures in Block X36 are mainly distributed along the river channel, and the thickness of the sand body in the northeast area is relatively large, usually larger than 8 m. At this time, the fracture development degree tends to decrease. It can be seen that the fractured zone is mainly distributed in the river channel wing [43,44]. However, for the argillaceous deposits in the interdistributary bay, the fractures do not develop. In general, if the fractured zone cuts the reservoir vertically, the up-dip disk trap on the side of the fracture zone is more conducive to hydrocarbon enrichment. Obviously, the relationship between the fracture zone and the sand body is “parallel sand body drainage,” i.e., the traps on both sides of the fracture zone can accumulate oil and gas on a large scale [44,45]. On the whole, the formation and distribution of reservoirs in the target layers are closely related to the strike and distribution of fractured zones.

5.3 Control of hydrocarbon accumulation by fault–fracture coupling

The deep strike-slip faults in the study area usually cut through the Triassic strata, while the top of the faults can reach the Quaternary strata [11]. Therefore, the formation period of the strike-slip fault zone is later than the formation period of the deep Triassic system, and the latest active period is earlier than the Quaternary system [7,8]. During the Indosinian Movement, the dextral strike-slip of strike-slip fault occurred on the basis of deep preexisting basement fault under the condition of weak compression of NS, and no branch fault was obviously developed [7,8]. During the Yanshanian Movement, the Ne-trending tectonic activity was strong [11]. The fault develops further, and two sets of branch fault systems are developed in the deep layer and shallow layer. The main fault types are flower and vertical fault. In the Himalayan period, the intensity of tectonic activity was weak, and the main faults continued to expand under the action of strike slip [11]. At the same time, some small-scale derived faults and detachment faults were generated. For the Chang 8 Member, the main faults indicate the Indosinian period, while the secondary faults are formed in the Yanshanian and Himalayan periods [36–40]. Lithologic and structural traps are mainly developed in the Chang 6–Chang 8 Members of the Yanchang Formation in the study area. The low-amplitude structural traps are small in scale and mainly distributed in 1–8 km². Taking the reservoir profile shown in Figure 10 as an example, it can be seen that nasal structures are developed in local areas. Plane fault distribution analysis shows that the right-lateral right-order fault combination represents the stretching area, which is conducive to oil and gas accumulation. On the other hand, the combination of right-lateral and left-step faults represents the extrusion area, which is not conducive to the large-scale accumulation of oil and gas.

$Figure 10 A reservoir profile across Wells H3–H7 in the study area. In the definition of oil- and water-bearing layers, the results of oil tests are used to define the properties of oil and water-bearing layers in different types of reservoirs. In turn, the reservoir segments are divided into oil, poor oil, dry and water layers according to resistivity and acoustic wave time difference. For oil layer, the resistivity is greater than 30 Ω m and the acoustic wave time difference is greater than 225 μs/m; for poor oil layer, the resistivity is between 26 and 30 Ω m and the acoustic wave time difference is between 215 and 225 μs/m; for dry layer, the resistivity is between 45 and 100 Ω m and the acoustic wave time difference is less than 215 μs/m; for water layer, the resistivity is less than 26 Ω m and the acoustic wave time difference is between 210 and 245 μs/m. Wells H5 and H6 meet all the requirements for the reservoir sweet spot we defined. According to the shape of sand body in Figure 10, Well H5 is located in the wing of the river channel, while Well H6 is located in the main river channel. They are located at a distance of 1 km from the main fault. Meanwhile, they have a plane arrangement of right-lateral and right-order faults. In addition, these two wells have a fracture index greater than 0.45, while the other three wells have a fracture index less than 0.2.$

Figure 10

A reservoir profile across Wells H3–H7 in the study area. In the definition of oil- and water-bearing layers, the results of oil tests are used to define the properties of oil and water-bearing layers in different types of reservoirs. In turn, the reservoir segments are divided into oil, poor oil, dry and water layers according to resistivity and acoustic wave time difference. For oil layer, the resistivity is greater than 30 Ω m and the acoustic wave time difference is greater than 225 μs/m; for poor oil layer, the resistivity is between 26 and 30 Ω m and the acoustic wave time difference is between 215 and 225 μs/m; for dry layer, the resistivity is between 45 and 100 Ω m and the acoustic wave time difference is less than 215 μs/m; for water layer, the resistivity is less than 26 Ω m and the acoustic wave time difference is between 210 and 245 μs/m. Wells H5 and H6 meet all the requirements for the reservoir sweet spot we defined. According to the shape of sand body in Figure 10, Well H5 is located in the wing of the river channel, while Well H6 is located in the main river channel. They are located at a distance of 1 km from the main fault. Meanwhile, they have a plane arrangement of right-lateral and right-order faults. In addition, these two wells have a fracture index greater than 0.45, while the other three wells have a fracture index less than 0.2.

Based on the comprehensive study of sedimentation, structure and fracture, the classification criteria of sweet point reservoir of the Yanchang Formation in the study area are determined [46,47]: sedimentary microfacies of the main channel and its wing, tectonic location within 1.5 km from the main fault, plane arrangement of right-lateral and right-order faults, and developed fractures.

Based on this criterion, the sweet point reservoir in Figure 10 is identified. The thick sand body represents the main river channel (near Well H4 and Well H6 ). While Wells H3, H5 and H7 represent the wing position of the river channel. Due to the influence of tectonic activity, Well H4 is located in the low part of the structure, while the other wells are located near the low amplitude uplift areas. Moreover, the fracture index of the main producing interval (Chang 8₁ ² layer) in Wells H3, H4, H5, H6 and H7 is 0.1, 0.05, 0.46, 0.48 and 0.12, respectively. There is a positive correlation between the daily oil production and fracture index of the Chang 8₁ ² layer in Wells H3 to H7 (Figure 11). In addition, Wells H5 and H6 are located in the right-lateral right-order fault combination area, while the other wells are located in the right-lateral and left-step fault combination area.

$Figure 11 Relationship between daily oil production and fracture index in Chang 81 2 layer in Wells H3–H7.$

Figure 11

Relationship between daily oil production and fracture index in Chang 8₁ ² layer in Wells H3–H7.

The oil-bearing reservoir of the Yanchang Formation in this area is developed in the Chang 8₁ ¹ and Chang 8₁ ² reservoirs. The oil producing horizon is the Chang 8₁ ² layer. The production results indicated that the H5 well in the area produced 2.5 tons of oil per day, while the H6 well produced 2.5 tons of oil per day. Other wells, however, are producing less than 1 t per day. Therefore, the sweet point reservoir can be effectively identified using the developed screening criteria.

6 Conclusions

Taking the tight oil reservoir of the Yanchang Formation (Chang 6–8 Members), southwestern Ordos Basin, as an example, the characteristics of fracture development and its control on hydrocarbon accumulation have been systematically studied using core, thin section, well logging, productivity, fault and sand body distribution data.
The development degree of vertical fractures in the Chang 8 Member (37%) is slightly higher than that in the Chang 6 (32) and Chang 7 (35%) Members. For horizontal bedding fractures, they are mainly developed in fine sandstone, followed by siltstone, while no horizontal bedding fractures are observed in medium sandstone. This is because horizontal bedding fractures are more common in fine grained sediments. For vertical fractures, they are also mainly developed in fine sandstone.
The controlling factors of fractures in the Yanchang Formation include lithology, sand body thickness, sedimentary microfacies and fault–fracture coupling relationship. Fractures are mainly developed in the wings of the main river channel due to the small compacted space. In the area where multiple channels intersect, the sand is pure and the fractures are relatively developed, which is conducive to the large-scale accumulation of hydrocarbons. Single sand bodies within 6 m thickness have a high degree of fracture development. Beyond this thickness, fractures usually do not develop. The linear density of fractures in the target layer is usually less than 2 fractures/m.
Based on the comprehensive study of sedimentation, structure and fracture, the classification criteria of sweet point reservoir of the Yanchang Formation in the study area are determined: sedimentary microfacies of the main channel and its wing, tectonic location within 1.5 km from the main fault, plane arrangement of right-lateral and right-order faults and developed fractures. The sweet point reservoir can be identified effectively using the developed sweet point screening criteria.

Conflict of interest: Authors state no conflict of interest.

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Received: 2023-04-10

Revised: 2023-06-13

Accepted: 2023-06-13

Published Online: 2023-07-22

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0509

Keywords for this article

Ordos Basin; tight sandstone oil reservoir; fracture; fault; deposition; tectonics

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