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Formation mechanism and oil-bearing properties of gravity flow sand body of Chang 63 sub-member of Yanchang Formation in Huaqing area, Ordos Basin

  • Jianbo Liao , Liwen Long EMAIL logo , Zedong Zhu , Sujuan Liang , Haoyuan Wei and Leyi Zhao
Published/Copyright: October 16, 2024
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Open Geosciences
From the journal Open Geosciences Volume 16 Issue 1

Abstract

Gravity flow sand body is an important reservoir for deep water deposition. The in-depth study of its formation mechanism and oil enrichment law can provide important guidance for oil and gas exploration in deep water areas. In this article, taking the deep-water gravity flow sand body of the Triassic Chang 63 sub-member in the Huaqing area as an example, the identification characteristics, formation mechanism, and oil-bearing characteristics of the gravity flow sand body are systematically discussed. The results show that sandy debris flow, turbidite flow, and mixed event flow present different superposition combination modes on vertical gravity flow deposition. The lithofacies of the gravity flow sandstone complex are mainly affected by lake level fluctuation, provenance supply, and hydrodynamic conditions. According to the formation conditions, sedimentary types. and distribution characteristics of deep water gravity flow sand bodies, a three-dimensional sedimentary mode of deep water gravity flow sand bodies is established. It is found that the physical and oil-bearing properties of the sandy debris flow sand body are better than those of the turbidity flow sand body. The reason is that the proportion of debris in the sand body of sandy debris flow is high, and the content of debris particles is generally greater than 70% according to the statistics of thin sections, which is conducive to the formation of pores. Second, sandy debris flow belongs to block transport, and the mixing of lithology is not sufficient in the process of block flow transport, so some pores will be preserved. Finally, gravity flows, which are dense at the beginning, become less dense later as detrital sediments unload, allowing them to be transported to distant regions. Therefore, the monolayer thickness of the sandy debris flow sand body is large (generally greater than 0.5 m), and the particle size of the debris ranges from 0.03 to 0.3 mm. Finally, the physical and oil-bearing properties of the sandy clastic flow sand body are better than those of the turbidity flow sand body.

Keywords: Huaqing area; gravity flow deposition; formation mechanism; pore structure; oil-bearing property

1 Introduction

The sedimentary types of gravity flow in deep water mainly include sliding rock, slump rock, liquefied flow, sandy debris flow, muddy clastic flow, and turbidite flow [1,2,3,4,5]. Among them, sandy debris flow and turbidity current deposits have better reservoir physical properties [6,7,8]. At present, most of the researches on deep-water gravity flow around the world focus on its genetic mechanism and identification markers. However, the researches on the micropetrology, differences in reservoir properties, and their effects on oil enrichment of these two types of sand bodies are very limited [9,10,11].

Gravity flow sand body is an important reservoir of deep water deposition. The distribution of sand body is closely related to the large-scale accumulation of oil and gas [12,13,14,15]. Generally, the distribution of gravity flow sand bodies is mainly controlled by internal and external factors [16,17,18,19]. External factors include tectonic activity, sediment supply, climate change, and sea level rise and fall. Tectonic activity mainly controls the collapse of sediments and the flow and velocity of water bodies. Sediment supply usually comes from provenance areas adjacent to drainage systems [20,21]. Internal factors mainly include water density and basin topography. Water density mainly refers to the difference in water density caused by flood flow. Basin topography includes topographic slope and basin landform [22,23,24]. The topographic slope mainly determines the differentiation rate of sand bodies in the fluid, while the basin geomorphology determines the distribution and sedimentary characteristics of the final gravity flow sand bodies [25,26,27,28].

The Ordos Basin is the second largest sedimentary basin in China, and the Chang 7 Member period of the Late Triassic Yanchang Formation is the largest lake flooding period, with deep and extensive water bodies, and the distribution of semi-deep lakes-deep lakes is up to 5 × 104 km2. Among them, the Huaqing area is located in the center of the semi-deep lake-deep lake (Figure 1). By the Chang 6 Member period, the basin basement began to rise, the peripheral deltaic sedimentary system continued to accumulate toward the central part of the lake basin, and the slope of the leading edge slope reached a critical angle. Triggered by external forces, the sandy-muddy soft sediments formed by rapid accretion became gravitationally destabilized, and a set of giant-thick gravity-flow composite sediments accumulated in the Huaqing area (lake-bottom plain) in the center of the lake basin. Due to the close proximity to the underlying Chang 7 Member hydrocarbon source rock, in the early Cretaceous end of the key reservoir period “near the water tower” priority to capture hydrocarbons, 1 billion tons of crude oil gathered in the Chang 6 Member. Therefore, the Huaqing area provides conditions for the coupled study of the elements of the source-sink system of gravity-flow sediments. In addition, different types of gravity flow deposition types are developed in the Chang 6 Member of the Huaqing area, which is the most ideal area for dissecting the genesis of gravity flow sand bodies and studying the oil-bearing characteristics of sand bodies of different genesis.

Figure 1 Location of the study area (a) and the target layer is the Chang 63 sub-member (b) of Chang 6 Member, Yanchang Formation, Ordos Basin.
Figure 1

Location of the study area (a) and the target layer is the Chang 63 sub-member (b) of Chang 6 Member, Yanchang Formation, Ordos Basin.

In this study, taking the deep-water gravity flow sand body of the Triassic Chang 63 sub-member in the Huaqing area of the Ordos Basin as an example, the identification characteristics, formation mechanism, and oil-bearing characteristics of gravity flow sand body are systematically discussed. This study has positive significance for clarifying the difference between reservoir properties and oil enrichment law of gravity flow sand bodies.

2 Geological background

The study area is located in the Huaqing area in the middle part of the Ordos Basin (Figure 1a). During the sedimentary period of the Chang 63 sub-member, the study area was located in the center of the lake basin, and multiple provenance and rapid sedimentary facies changes resulted in strong heterogeneity of the reservoir. The Chang 7 Member oil shale is well developed in the central part of the lake basin. The gravity flow sand body of the Chang 63 sub-member is in direct contact with the source rock, and a good source–reservoir combination of lower generation and upper reservoir is formed. The research horizon in this study is the Chang 63 sub-member (Figure 1b). The Chang 63 sub-member consists of 1–2 positive and antiprosodic units, and its thickness is generally 29–56 m.

3 Methods

The experiments carried out in this study included thin sections, scanning electron microscopy (SEM), constant velocity mercury injection, and nuclear magnetic resonance. Thin section and SEM experiments were used to observe the microstructure of the internal mineral components of the Chang 63 sub-member sandstone. Furthermore, the observation results of the microscopic structure of mineral components and pores can provide a reference for the revelation of the genetic mechanism of the water gravity flow sand body [29,30,31].

An ASPE-730 mercury injection instrument was used to carry out the constant velocity mercury injection experiments. Constant velocity mercury injection is an important way to quantitatively calculate the pore–throat ratio. It accurately determines the size and quantity of different pore–throats through the pressure fluctuation during quasi-static mercury injection, and one fluctuation represents one pore–throat. Constant-rate mercury injection is done at a very low constant speed (usually 0.00005 ml/min). By injecting mercury into the throats and pores of rock samples, the quantity of throats can be measured, thus overcoming the defect of high-pressure mercury injection. Due to the low mercury intake rate, the quasi-static mercury intake process can be approximately maintained, and the pore and throat information can be obtained according to the pressure fluctuation of mercury intake.

To further clarify the controlling mechanism of physical properties on oil-bearing properties in sand bodies, NMR movable fluid saturation experiments were carried out. In this study, 13.90 ms was used as the T2 relaxation time limit of movable and bound fluids. The magnitude and percentage of the magnitude sum of all points on the T2 spectrum with the T2 relaxation time greater than 13.895 ms is the mobile fluid saturation. Conversely, bound fluid saturation is defined by the magnitude of each point on the T2 spectrum whose T2 relaxation time is less than 13.90 ms and the percentage of the magnitude sum of all points on the T2 spectrum.

4 Results

The deep water gravity flow deposits mainly include debris flow, turbidity current, and mixed event current. They have different genetic mechanisms and distribution characteristics. During the Chang 63 sub-member period, the sandy debris flow and turbidity flow sand bodies were widely developed in the middle part of the lake basin, while mixed event flow sand bodies were only developed in part of the lake basin.

4.1 Gravity flow types and identification markers

The Chang 6 Member represents a relatively stable development stage of lake basin. On the basis of the deep lacustrine facies of the Chang 7 Member, a group of zonal delta sedimentary bodies developed around the lake basin. Furthermore, a large number of turbidite fans developed under the lake slope, and good reservoir was formed. In general, the Chang 63 sub-member in the Huaqing area is dominated by the semi-deep lake gravity flow (slump gravity flow), followed by the delta front traction flow. Moreover, the slump gravity flow is mainly composed of sandy debris flow and turbidity current, and mixed event flow is rare.

4.1.1 Sandy debris flow sandstone

According to the observations of cores and outcrop profiles, the thickness of the sandy debris flow sand body ranges from 2.3 to 42.56 m, with an average thickness of 19.32 m, and most of the sand body thickness is concentrated in the range of 13–18 m. The sand body of sandy debris flow has fine particles and is mainly light gray in color (Figure 2a).

Figure 2 Sedimentary characteristics of Chang 63 sub-member in the study area. Notes: (a) Field profile (Yaoqu area), there is no internal sedimentary structure, no gradation of grain sequence, and its thickness is greater than 50 cm; (b) field profile (Yaoqu area), “mud-coated gravel” structure; (c) field profile (Shanshuihe area), “mud-coated gravel” structure; (d) well X322, 1956.4 m, interbedded mud-sand structure; (e) field profile (Yaoqu area), groove cast; (f) field profile (Yaoqu area); flute cast (g) well L87, 2,225 m, H1 is massive fine sandstone, and H3 is lacerated deformed mudstone fragment; (h) well H56, 2424.3, H1 is massive fine sandstone, and H3 is lacerated deformed mudstone fragment; and (i) well L87, 2231.5 m, H1 is massive fine sandstone, H2 is a banded thin interbedded sand–mudstone combination.
Figure 2

Sedimentary characteristics of Chang 63 sub-member in the study area. Notes: (a) Field profile (Yaoqu area), there is no internal sedimentary structure, no gradation of grain sequence, and its thickness is greater than 50 cm; (b) field profile (Yaoqu area), “mud-coated gravel” structure; (c) field profile (Shanshuihe area), “mud-coated gravel” structure; (d) well X322, 1956.4 m, interbedded mud-sand structure; (e) field profile (Yaoqu area), groove cast; (f) field profile (Yaoqu area); flute cast (g) well L87, 2,225 m, H1 is massive fine sandstone, and H3 is lacerated deformed mudstone fragment; (h) well H56, 2424.3, H1 is massive fine sandstone, and H3 is lacerated deformed mudstone fragment; and (i) well L87, 2231.5 m, H1 is massive fine sandstone, H2 is a banded thin interbedded sand–mudstone combination.

The sandy debris flow has typical sedimentary structure features such as floating mud gravel and mudstone tearing debris. In addition, massive bedding is developed, there is no obvious grain sequence change in sandstone, and the top and bottom are in abrupt contact with mudstone. The upper part of the massive sandstone developed mud-gravel and mud-coated gravel along the bedding (Figure 2b). The gravel is usually in the shape of long strips. The “mud-coated gravel” has a double-layer structure with an outer layer of muddy or silty cladding not exceeding 3 cm thick (Figure 2c).

Most of the sandy debris flows have the characteristics of plastic rheology and belong to Bingham plastic fluid [32,33]. Under the action of various supporting mechanisms, such as dispersion pressure, buoyancy, and particle support, the mud and gravel are distributed along the bedding in a directional manner. The main difference between sandy debris flow sand bodies and other gravity flow sedimentary body is that it has massive structure, abrupt contact with mudstone, and floating mud debris and mud gravel inside [34,35].

4.1.2 Turbidity current sandstone

The typical sequence of turbidity deposition is the Bauma sequence, which consists of five tapering intervals from bottom to top. The bottom is composed of sandstone with graded bedding (section a), the bottom has groove mold and other scour casting mold, and the upper part is sandstone with parallel bedding; section c is siltstone with cross-bedding and deformed bedding with wavelet marks; section d is siltstone and silty mudstone with horizontal texture; and section e at the top is massive mudstone.

Turbidity current deposits have a wide distribution range in the Chang 63 sub-member of the Huaqing area, but the thickness of sand body is relatively thin, ranging from 0.02 to 13.67 m, with an average thickness of 8.47 m. The turbidity sand body has fine grains, and fine sandstone, siltstone, and mudstone are developed (Figure 2d). Among them, the content and color of mud are mainly determined by the depth of water body deposited by turbidity sand body. The deeper the water, the more muddy content, the more gray the color. According to the observation, the muddy content in turbidity sand bodies changes rapidly, and the sand-mud interbedding structure is obvious, which is a typical sign for identifying turbidity sand bodies (Figure 2d). The sedimentary structures of turbidity sand bodies include peeling lineation structure, trough mold, parallel bedding, sand lamination, and flame structure (Figure 2e and f). Turbidity current is mainly carried by suspension supported by turbulent flow, and the grain size of sediment becomes finer from bottom to top. Due to the relatively simple transport mode, turbidity current cannot transport or carry large clastic particles, so the sandstone particle size is fine and pure [36,37,38].

4.1.3 Mixed event flow

Mixed event flow refers to the mixed deposition of a variety of sedimentary sequences due to fluid transformation in a gravity flow event. The mixed event flow is formed under the condition of high sediment concentration and long transport distance, including the mixed sandstone of sandy debris flow and turbidite current, and the mixed sandstone of turbidite flow and muddy clastic flow [39,40,41].

For the mixed event flow sand bodies, massive sandstones (H1), banded thin interbedded sand-mudstone (H2), and deformed torn mudstone fragments (H3) develop. Among them, the H1 section is mainly fine sandstone with a thickness between 0.1 and 0.7 m, with an average thickness of 0.42 m. It has good oil content, massive bedding, and abrupt contact between top and bottom. In the H2 section, banded thin sand-mudstone is mainly developed. The H3 section is mainly argillaceous siltstone with thickness ranging from 0.1 to 0.5 m, with an average thickness of 0.26 m. Moreover, its interior contains scattered fragments of mud. The mixed event flow developed in the Chang 63 sub-member in the study area was mainly composed of H1 and H3 sections (Figure 2g–i). The reason for its formation is that with the increase in migration distance, the upper turbidity current is eroded by mudstone, and the flow pattern is transformed, which leads to part of the turbidity current being transformed into muddy clastic flow. The characteristics of argillaceous debris and the difference in lithology between upper and lower sandstone are the key evidence of turbidite flow transforming into argillaceous debris flow. It is found that the mixed event flow of the Chang 63 sub-member has low development degree and small thickness in the study area and only distributes sporadically in the semi-deep lake to the deep lake area.

4.2 Petrological characteristics

The gravity flow rock types in the study area are mainly feldspar lithic sandstone and lithic sandstone (Figure 3). The debris content of quartz, feldspar, and debris is 28.66, 8.61, and 31.87%, respectively. Moreover, the debris mainly includes volcanic debris, metamorphic debris, and sedimentary debris. According to statistics, the average content of volcanic, metamorphic, and sedimentary rock debris is 2.45, 12.5, and 18.9%, respectively.

Figure 3 Triangle diagram of sandstone composition of Chang 63 sub-member. (a) sandy debris flow sand body and (b) turbidity sand body. Notes: Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅳ, Ⅵ, and Ⅶ represent quartz sandstone, feldspar quartz sandstone, lithic quartz sandstone, feldspar sandstone, lithic feldspar sandstone, feldspar lithic sandstone, and lithic sandstone, respectively.
Figure 3

Triangle diagram of sandstone composition of Chang 63 sub-member. (a) sandy debris flow sand body and (b) turbidity sand body. Notes: Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅳ, Ⅵ, and Ⅶ represent quartz sandstone, feldspar quartz sandstone, lithic quartz sandstone, feldspar sandstone, lithic feldspar sandstone, feldspar lithic sandstone, and lithic sandstone, respectively.

The clastic composition of sandy debris flow is significantly different from that of turbidite sandstone. The clastic rock is mainly quartz, while the turbidite sandstone is mainly debris. The content of feldspar and quartz in sandy debris flow sandstone is relatively high. The average content of quartz, feldspar, and debris is 31.2, 11.7, and 28.3%, respectively. In addition, the average content of metamorphic, volcanic, and sedimentary rock debris is 13.6, 2.6, and 12.1%, respectively.

Based on the statistical analysis of the grain size of 80 samples, it is found that the lithology of gravity flow sand body in the study area is mainly fine sandstone, accounting for 85.39%; silty sand, medium sand, and clay accounted for 6.18, 4.12, and 5.24%, respectively; however, the coarse sand content is the least, accounting for only 1.12% (Figure 4).

Figure 4 Histogram of grain size of Chang 63 sub-member in the study area.
Figure 4

Histogram of grain size of Chang 63 sub-member in the study area.

The particle size of sandy debris flow and turbidite sand body is similar, and both are mainly fine sandstone, accounting for 83.27 and 87.51%, respectively. Among them, the total content of fine sand, medium sand, and coarse sand in sandy debris flow is high, accounting for 93.18%. In contrast, the content of silt and clay in turbidite sandstone is high, accounting for 11.93%.

4.3 Pore structures

The gravity flow sand body of the Chang 63 sub-member mainly developed two types of reservoir space: pores and fractures (Figure 5). The pore morphology of gravity flow sandstone in the study area is mostly in the shape of long strip, grid, and honeycomb. The surface porosity is generally greater than 0.5% and up to 8.0%, and the pore diameter is generally 0.02–0.50 mm (Figure 5a–c). The pore morphology of intergranular pores in the cast thin section is triangular, quadrilateral, and irregular. Good connectivity constitutes the main percolation channels within the reservoir (Figure 5b). The dissolution pores are mainly feldspar and debris dissolution pores (Figure 5c–e). Feldspar dissolution pores are relatively developed in low permeability reservoirs in the middle of the lacustrine basin and are the main storage space of gravity flow sand bodies. Feldspar dissolution pores are often formed by selective dissolution along the cleavage direction of feldspar grains and the relatively easy dissolution parts of feldspar grains [42,43]. A large number of debris dissolution pores provide favorable reservoir conditions, with pore radius ranging from 5 to 35 μm (Figure 5f and g). The generation of dissolution pores has an important effect on optimizing pore structures and improving the connectivity of pore throats. A large number of intercrystalline pores are mainly distributed between clay and matrix components. They are mainly intercrystalline pores formed between quartz secondary enlargement, illite, and calcite crystals or between autogenous feldspar (Figure 5g–l).

Figure 5 Cast thin section and SEM images of Chang 63 sub-member in the study area. Notes: (a) well S126, 1,807 m, intergranular pore, sandy debris flow sand body; (b) field profile, Xunyi area, intra- and intergranular dissolution pores, sandy debris flow sand body; (c) well B127, 1836.94 m, intergranular pore and debris dissolution pore, sandy debris flow sand body; (d) well N68, 1776.5 m, microfracture and intergranular pore, sandy debris flow sand body; (e) well L47, 1897.7 m, intergranular pore, turbidity sand body; (f) field profile, Xunyi area. Dense structure with visible small amounts of dissolved pores, turbidity sand body; (g) well L122, 2310.57 m, feldspar dissolution pore; (h) well B120, 2119.17 m, intergranular pores filled with Chlorite; (i) well B140, 2184.67 m, intercrystalline pore (quartz); (j) well B140, 2184.87 m, Intragranular dissolution pore; (k) well B411, 2123.45 m, intercrystalline pore (quartz and ferridolomite); and (l) well C211, 1955.18 m, intercrystalline pore (illite alteration).
Figure 5

Cast thin section and SEM images of Chang 63 sub-member in the study area. Notes: (a) well S126, 1,807 m, intergranular pore, sandy debris flow sand body; (b) field profile, Xunyi area, intra- and intergranular dissolution pores, sandy debris flow sand body; (c) well B127, 1836.94 m, intergranular pore and debris dissolution pore, sandy debris flow sand body; (d) well N68, 1776.5 m, microfracture and intergranular pore, sandy debris flow sand body; (e) well L47, 1897.7 m, intergranular pore, turbidity sand body; (f) field profile, Xunyi area. Dense structure with visible small amounts of dissolved pores, turbidity sand body; (g) well L122, 2310.57 m, feldspar dissolution pore; (h) well B120, 2119.17 m, intergranular pores filled with Chlorite; (i) well B140, 2184.67 m, intercrystalline pore (quartz); (j) well B140, 2184.87 m, Intragranular dissolution pore; (k) well B411, 2123.45 m, intercrystalline pore (quartz and ferridolomite); and (l) well C211, 1955.18 m, intercrystalline pore (illite alteration).

According to statistics, the gravity flow sand body of the Chang 63 sub-member is dominated by intergranular pores, intergranular dissolution pores, intragranular dissolution pores, feldspar dissolution pores, and debris dissolution pores, accounting for 33.26, 15.21, 18.26, 15.87, and 12.39%, respectively; however, microfractures and intercrystalline pores account for 4.13 and 0.65%, respectively (Figure 6).

Figure 6 Statistics of pore types of sandstone of Chang 63 sub-member in the study area.
Figure 6

Statistics of pore types of sandstone of Chang 63 sub-member in the study area.

According to the mercury injection results, the pore type of the Chang 63 sub-member gravity flow sand body is a small pore–fine throat type (Figure 7). The pore radius is concentrated in the range of 30–50 μm, the minimum pore radius is 0.89 μm, and the maximum pore radius is 124.2 μm. The throat radius is concentrated in 0.5–1 μm, the minimum throat radius is 0.11 μm, and the maximum throat radius is 11.8 μm. The pore–throat ratio is distributed in 10–20, with a minimum value of 1.43 and a maximum value of 913.2.

Figure 7 Distribution of pore radius (a), throat radius (b), and pore–throat ratio (c) of gravity flow sandstone in Chang 63 sub-member.
Figure 7

Distribution of pore radius (a), throat radius (b), and pore–throat ratio (c) of gravity flow sandstone in Chang 63 sub-member.

According to the correlation analysis of constant velocity injection mercury throat, pore inlet mercury saturation, and total pore inlet mercury saturation, it was found that there was an obvious intersection of the two correlation lines (Figure 8). Below this intersection, the contribution of laryngeal mercury saturation to total pore–throat mercury saturation exceeds that of pore–throat mercury saturation. Above this intersection, the contribution of pore–entry mercury saturation to total pore–throat mercury saturation exceeds that of throat mercury saturation. The worse the porosity, the greater the mercury saturation; the better the physical property, the greater the mercury saturation [44,45]. Therefore, the mercury intake in the total pore–throat of sandstone reservoir in the study area is the result of the joint action of the throats and pores. The mercury saturation in the pore of rock sample plays a leading role. However, in a certain range of values, the contribution of mercury saturation to total pore–throat mercury saturation is slightly higher.

Figure 8 Correlation of constant velocity mercury injection parameters.
Figure 8

Correlation of constant velocity mercury injection parameters.

5 Discussion

5.1 Formation mechanism of gravity flow deposition

5.1.1 Sandy debris flow sandstone

Outcrop and core data show that the supporting characteristics of gravity flow sand bodies in the study area show that large debris such as mud-gravel and “mud-coated gravel” are suspended by the sandy matrix formed in the quasi-syngenic sedimentary stage. In the process of mass transport, sandstone with strong shear resistance will be involved in soft clay layer with relatively weak shear resistance (argillaceous clastic mudstone) due to gravitational shear action in the bulk flow complex, and then sedimentary structures such as tearing mud gravel and mud cladding are formed [46,47]. The argillaceous crust of floating argillaceous gravel and “mud-coated gravel” structure is characterized by a shallow water environment near shore, partial oxidation, and low paleo-salinity. It is closely related to the accompanying argillaceous clastic flow mudstone but is significantly different from the bottom bed semi-deep lacustrine stratified mudstone. In addition, the formation process of debris flow sand body retains the characteristics of block transport, and there is a material exchange in the sandy argillaceous soft sediment under the action of shear stress, resulting in a certain degree of lithology mixing [48,49].

In the formation process of sandy debris flow sandstone complex, the superimposed form of a single sand body is mainly related to slope, provenance supply, and hydrodynamic strength. When the slope break zone has a large slope or a stepped high-angle slope, the provenance supply is small and the water power is weak, and the isolated lenticular sand body complex is formed along the provenance direction; when the source supply is large and the hydrodynamic force is strong, the superimposed lenticular sand body complex is easily formed; when the slope is small and the source supply is large, it is easy to form sheet sand body complex (Figure 9).

Figure 9 Several typical superimposed patterns of sandy debris flow sand bodies.
Figure 9

Several typical superimposed patterns of sandy debris flow sand bodies.

5.1.2 Turbidity current sandstone

By dissecting the complete Bauma sequence of turbidite sand bodies in the outcrop, it was found that positive grain order changes were observed in section A (Figure 10a and b). The results show that the energy and velocity of turbidity current decrease continuously during deposition. The sedimentation of the suspended coarse and fine sediments due to their different falling velocity indicates the turbulent support mechanism of the sediments. The directional arrangement of sheet mica and long strip minerals can be observed in B and C segment depositions (Figure 10c). This indicates that turbidity current changes to traction current due to further energy depletion after the deposition of segment A, and the fluid flows in a directional way. Furthermore, sedimentary structures such as parallel bedding and sand bedding were formed (Figure 10c).

Figure 10 Images of thin sections of typical turbidity deposits. Notes: (a) Turbidity sand body in Shanshuihe area. The complete Bouma sequences developed in Ta, Tb, and Tc segments. (b) In the Ta segment, the oriented thin sections have visible grain sequence bedding and a few pores are developed. (c) Tb segment, slightly oriented arrangement, and its surface porosity are very low.
Figure 10

Images of thin sections of typical turbidity deposits. Notes: (a) Turbidity sand body in Shanshuihe area. The complete Bouma sequences developed in Ta, Tb, and Tc segments. (b) In the Ta segment, the oriented thin sections have visible grain sequence bedding and a few pores are developed. (c) Tb segment, slightly oriented arrangement, and its surface porosity are very low.

5.1.3 Mixed event flow sand body

During the Chang 63 sub-member period, the development degree of mixed event flow sandstone was mainly determined by the internal characteristics of source-sink system [50]. Since the provenance of this period was mainly from the northeast, and the sandstone experienced a long period of migration, the content of argillaceous material in the sandstone was low, and it was unlikely to have the conditions to form the upper argillaceous debris flow [51,52]. On the other hand, since the northeastern sand body itself is relatively well sorted and rounded during collapse, the gravity flow sand body formed during collapse usually does not carry a large amount of mud debris [53]. However, the mixed gravity flow sand body in the study area developed a large number of argillaceous debris. Therefore, the rarely found mixed gravity flow sandstone in the study area is mainly formed due to a large number of argillaceous debris carried by flooding [53,54]. The distribution range of mixed gravity flow sand bodies is limited due to the seasonality and sweep range of flood.

Sandy debris flow and turbidity current can be transformed into each other in time and space. Several typical models of the transformation of sandy debris flow and turbidity current have been established by the previous (Figure 11a). It can be seen from Figure 11a that in time the debris flow can be transformed into a turbidity current; in space, the flow stratification of debris flow can occur in the upper part of the flow layer to form a turbidity current.

Figure 11 Development characteristics of deep water gravity flow deposits in the Yanchang Formation. Notes: (a) spatio-temporal transformation mode of sandy debris flow and turbidity current (modified after Shanmugam, [34]) and (b) combination mode of sandy debris flow and turbidity current deposition in Chang 63 sub-member, Huaqing area.
Figure 11

Development characteristics of deep water gravity flow deposits in the Yanchang Formation. Notes: (a) spatio-temporal transformation mode of sandy debris flow and turbidity current (modified after Shanmugam, [34]) and (b) combination mode of sandy debris flow and turbidity current deposition in Chang 63 sub-member, Huaqing area.

From the core observation, the three types of sediment gravity flows often interbedded in the longitudinal direction, and the combination of sandy debris flow and turbidity current is most common (Figure 11b). From the outcrop profile, the gravity flow of the Yanchang Formation usually develops in semi-deep lake mudstone. It shows that it was formed in a semi-deep lake environment and controlled by a deep water slope break zone.

Therefore, the transformation model of gravity flow formation in the Yanchang Formation sediments was established. In the vicinity of the deep water slope break zone, the loose sediments in the delta front will randomly slip and collapse under the condition of rapid deposition instability or triggered by some accidental mechanism (wave, volcano, earthquake, storm, etc.). As the slump body moves down the slope, the mixing of sediment with water and the decomposition of material increases. The slump flow forms a clastic flow in which the sediment flows as a loose viscous mass flow. As the debris flow continues to flow down the slope, the fluid composition in the flow increases, and the plastic debris flow may evolve into turbulent fluid and form turbidity current.

Sandy debris flow, turbidite flow, and mixed event flow show different combination modes in the vertical direction, which reflects the difference in the sedimentary evolution process of different gravity flow sandstone complexes. As shown in Figure 11b, two typical gravity flow sand body assemblage styles are developed in the study area. One is dominated by sandy debris flow sandstone, with a single sandy debris flow sand body having a large thickness and intermingled with thin layer turbidite and mixed event flow sand bodies. The other type is turbidity current, and the thickness of the single turbidity current sand body is relatively small, mixed with thick sandy debris flow and thin layer mixed event flow sand body. The lithofacies of these two gravity-flow sandstone complexes are mainly affected by lake level fluctuation, provenance supply, and hydrodynamic conditions.

5.2 Sedimentary mode of gravity flow sand body in deep water

According to the formation conditions, sedimentary types, and distribution characteristics of deep-water gravity flow sand bodies, their three-dimensional sedimentary mode was established (Figure 12).

  1. Gravity flow is developed on both steep slope and gentle slope in the Ordos Basin. The type and scale of gravity flow are controlled by provenance and slope break zone. During the sedimentary period of the Chang 63 sub-member, the northeastern part of the study area had a steep slope and abundant provenance, and meander river deltas were mainly developed. Under the action of earthquake, volcano, gravity, and flood, the sediments collapse, and then, large-scale sandy debris flow and turbidite sandstone formation. In the southwest of the study area, the slope is slow, the provenance is insufficient, and braided river delta deposits are mainly developed. Therefore, the collapse of turbidite sandstone is easy to form. The sand body is small in scale and the small-scale slump body is formed at the foot of slope.

  2. From the shore to the middle of the lake basin, there were three stages of large-scale sandy debris flow, turbidity current, and mixed event flow. In the near end, the multi-phase superimposed sandy debris flow sand body is the main body with large thickness and good transverse continuity. The middle part is dominated by sandy debris flow and turbidity flow sand bodies with large thickness and relatively good transverse continuity. At the far end, turbidity current and mixed event current are the main body, with small thickness and poor transverse continuity.

Figure 12 Sedimentary mode of deep water gravity flow sand body in Yanchang Formation, Ordos Basin.
Figure 12

Sedimentary mode of deep water gravity flow sand body in Yanchang Formation, Ordos Basin.

5.3 Evaluation of reservoir and oil-bearing properties

According to the NMR fluid saturation tests, the oil-bearing properties of sand bodies of the Chang 63 sub-member are divided into the following categories: class I (good), with mobile fluid saturation >65%; class II (relatively good), with mobile fluid saturation distribution between 50 and 65%; class III (moderate), with mobile fluid saturation between 35 and 50%; class IV (poor), with mobile fluid saturation between 20 and 35%; class V (very poor), with mobile fluid saturation <20%.

The results show that the better the physical properties of sandstone, the higher the mobile fluid saturation, and the better the oil-bearing property. The comparison shows that permeability has more influence on the saturation of mobile fluid. For example, in B259 and B269 wells, there is little difference in sand porosity, but the permeability difference is more than ten times. The mobile fluid saturation of well B259 with higher permeability reaches 46.55%, while that of well B269 with lower permeability is only 2.16% (Figure 13).

Figure 13 T2 spectra of moving fluid in sandstone samples based on NMR experiments. Notes: (a) sample collected from well 259, 2187.3 m; (b) sample collected from well 269, 1943.9 m; (c) sample collected from well 470, 2148.79 m; (d) sample collected from well 480, 2004.4 m; (e) sample collected from well S141, 2165.48 m; and (f) sample collected from well 463, 1984.34 m.
Figure 13

T2 spectra of moving fluid in sandstone samples based on NMR experiments. Notes: (a) sample collected from well 259, 2187.3 m; (b) sample collected from well 269, 1943.9 m; (c) sample collected from well 470, 2148.79 m; (d) sample collected from well 480, 2004.4 m; (e) sample collected from well S141, 2165.48 m; and (f) sample collected from well 463, 1984.34 m.

According to the correlation analysis of porosity, permeability, and mobile fluid saturation (Figure 14), the oil-bearing property of the sand body is mainly controlled by permeability. The correlation between mobile fluid saturation and permeability is high. The higher the permeability, the higher the mobile fluid saturation and the better the oil content of the sand body (Figure 14).

Figure 14 Relationship between petrophysical properties and mobile fluid saturation of the target layer. Notes: (a) With the increase of porosity, the mobile fluid saturation tends to decrease slightly and (b) with the increase of permeability, the mobile fluid saturation tends to increase gradually, and the correlation coefficient R 2 is 0.68.
Figure 14

Relationship between petrophysical properties and mobile fluid saturation of the target layer. Notes: (a) With the increase of porosity, the mobile fluid saturation tends to decrease slightly and (b) with the increase of permeability, the mobile fluid saturation tends to increase gradually, and the correlation coefficient R 2 is 0.68.

For ultra-low permeability reservoir, heterogeneity is an important factor to determine its oil content. If the heterogeneity of reservoir is weak, the oil content is good. The higher the heterogeneity, the worse the oil content. It is found that the larger the permeability range and standard deviation is, the lower the test oil production will be. This indicates that the strength of reservoir heterogeneity also controls the level of oil production (Figure 15).

Figure 15 Relationship between permeability heterogeneity and oil test production. Notes: (a) permeability range and (b) standard deviation of permeability.
Figure 15

Relationship between permeability heterogeneity and oil test production. Notes: (a) permeability range and (b) standard deviation of permeability.

Combined with the sedimentary and reservoir characteristics of gravity flow sand body, the physical properties and oil content of sandy debris flow sand body are better than turbidity flow sand body. There are mainly three reasons for this:

  1. First, the proportion of debris in the sand body of sandy debris flow is high, and the content of clastic particles is generally more than 70% according to thin section statistics. The content of mica in mechanical deposition is low, which is conducive to the formation of reservoir pores. The proportion of debris in turbidity current sediments is relatively low, and the content of debris particles is generally less than 65% according to thin section statistics. The content of fine-grained material and flake mica in mechanical deposition is high, which is not conducive to the formation of pores.

  2. Second, sandy debris flow belongs to mass transport. In the process of bulk flow transport, the sandy debris flow sedimentary sand is involved in some adjacent mudstone layers under the action of plastic shear, resulting in lithologic mixing. However, the lithologic mixing of sandy debris flow is not sufficient, so some pores will be preserved. Especially for massive sand bodies with large thickness, the mixing at the top and bottom is more sufficient. In the middle part, the homogenization mixing is weak and more pores are retained, which also leads to the high oil content of the central sand body. Turbidity deposition, however, belongs to fluid transport (a kind of liquid flow, whose sediment is supported by liquid turbulence). Fine particles and impurities are evenly dispersed and filled in the pores, resulting in serious pore damage.

  3. Finally, gravity flows, which are dense at the beginning, become less dense later as detrital sediments unload, allowing them to be transported to distant regions. Compared with sandy debris flow, turbidity current deposits carry longer distances and are relatively thin. Therefore, the single layer thickness of turbidity sand body is relatively thin, generally less than 0.5 m, and the particle size is relatively fine. According to thin section statistics, the particle size of the debris is between 0.03 and 0.15 mm. The thickness of single layer of sandy debris flow sand body is large, generally greater than 0.5 m, and the particle size of the debris is between 0.03 and 0.3 mm. Therefore, the physical property of turbidity flow sand body is worse than that of sandy debris flow sand body.

6 Conclusions

  1. Sandy debris flow, turbidite flow, and mixed event flow present different superposition combination modes on vertical gravity flow deposition of the Chang 63 sub-member in the Huaqing area. The lithofacies of the gravity flow sandstone complex is mainly affected by lake level fluctuation, provenance supply, and hydrodynamic conditions. According to the formation conditions, sedimentary types and distribution characteristics of deep water gravity flow sand bodies, their three-dimensional sedimentary mode is established.

  2. It is found that the physical and oil-bearing properties of the sandy debris flow sand body are better than those of the turbidity flow sand body. The reason is that the proportion of debris in the sandy debris flow is high, and the content of debris particles is generally greater than 70%, which is conducive to the formation of pores. Second, sandy debris flow belongs to block transport, and the mixing of lithology is not sufficient in the process of block flow transport, so some pores will be preserved. Finally, gravity flows, which are dense at the beginning, become less dense later as detrital sediments unload, allowing them to be transported to distant regions.

  3. The physical and oil-bearing properties of the sandy debris flow sand body are better than those of the turbidity flow sand body.

  1. Funding information: This research was financially supported by the Forward Looking Basic Strategic Technology Research Projects of China National Petroleum Corporation (Study on Reservoir Forming Law and the Key Technology of Lithologic Stratigraphic Oil and Gas Reservoirs) (No. 2021DJ0406).

  2. Author contributions: Jianbo Liao is responsible for the idea and organization of this manuscript. Liwen Long are responsible for Formal analysis. Zedong Zhu, Sujuan Liang, Haoyuan Wei, and Leyi Zhao are responsible for Data curation.

  3. Conflict of interest: There is no conflict of interest in this study.

  4. Data availability statement: The original contributions presented in the study are included; further inquiries can be directed to the corresponding authors.

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Received: 2024-01-12
Revised: 2024-03-12
Accepted: 2024-05-01
Published Online: 2024-10-16

© 2024 the author(s), published by De Gruyter

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

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  149. Geosite assessment as the first step for the development of canyoning activities in North Montenegro
  150. Urban geoheritage and degradation risk assessment of the Sokograd fortress (Sokobanja, Eastern Serbia)
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https://doi.org/10.1515/geo-2022-0690
Keywords for this article
Huaqing area; gravity flow deposition; formation mechanism; pore structure; oil-bearing property
Creative Commons
BY 4.0

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