Abstract
In order to finely characterize the physical properties and seepage characteristics of underwater distributary channel sand bodies in a braided river delta, the particle size distribution, mercury injection, and withdrawal of low, medium, high, and extra-high permeability core samples from the TIII reservoir from 15 core wells in the Sangtamu Oilfield were compared. The pore-throat characteristics of the rocks with different deposition modes and their effects on the fluid seepage were analyzed from the perspective of the deposition processes. The statistical results revealed that the rocks formed in a continuous and stable strong hydrodynamic environment were coarse grained. The seepage capacities of the rock increased as the maximum throat radius connecting with the pores increased. The mercury withdrawal efficiency decreased as the difference between the throat radius and pore radius increased. The percentage of the throat volume without effective seepage to the total volume of the pore-throat system increased as the permeability of the samples increased. Therefore, when characterizing reservoirs based on the porosity and permeability, it is important to differentiate the energy of the hydrodynamic depositional environment of a monogenic sand body in order to improve the accuracy of the understanding of the reservoirs.
1. Introduction
The continuity of the depositional process of clastic rocks is controlled by the duration of the stable state of the dynamic strength of the transport medium. When the dynamic strength of the transport medium changes from weak to strong, the preexisting sediments can be corroded and destroyed. Conversely, when the dynamic strength of the transport medium changes from strong to weak, the deposition of the earlier moving particles will occur. Taking flowing water as an example, the strength of the hydrodynamic force controls the size of the deposited particles. The percentage of each particle fraction can be determined using the grain size analysis data for the core samples, which can be used to determine the transport mode of the particles by the water and the corresponding sedimentary environment [1–3]. In addition, the changes in the water energy during a sedimentation stage can be determined from the vertical variations in the particle size in the different areas, providing a basis for analyzing climate change in large regions [4–6]. At present, grain size analysis, as a mature method, has also been used in studies of wind erosion and desert deposition [7–10]. However, these studies all focused on the indicative significance of the deposition process and environment, and they did not further analyze the differences in the pore-throat system characteristics of samples with different particle size distributions. A pore-throat system composed of pores and throats between sedimentary grains provides space for fluid storage and seepage [11–13]. The more complex the pore-throat system is, the lower the flow capacity of the internal fluid is, and the worse the physical property development degree of the rock is. The complexity of a pore system is controlled by the sedimentary and late diagenetic processes in a specific research area with the same source rock lithology with different particle size distributions. When the regional tectonic evolution and ground stress are the same, depositional periods characterized by different particle sizes at different positions can be regarded as the root cause of the significant differences in the pore system development [14, 15].The development degree of the pore-throat system and the maximum throat radius connecting the pores can be characterized using the mercury injection curve morphology and displacement pressure, and the pore-throat characteristics of a sand body can be interpreted from the microscopic perspective based on mercury injection and core thin section data. In previous studies on reservoir characterization, some scholars focused on the macroscale sedimentary facies, sedimentary microfacies, and describing the physical properties of single-genesis sand bodies [16–19]. Other scholars focused on the characterization of pore-throat characteristics at the microscale [20–22]. However, the weak relationship between the macro- and microdimensions is not conducive to gaining a comprehensive understanding of the reservoir. In this study, particle size analysis data, mercury injection data, and core thin section data were comprehensively analyzed in order to analyze the degree of difference of the pores and throats between samples with different particle sizes from the perspective of the original deposition mode. Then its influence on fluid flow was determined, and the relationship between the sedimentary hydrodynamics and reservoir physical properties was established in order to provide relevant insights for understanding reservoirs in other areas with similar a sedimentary background.
2. Study Area
The study area is located in the Sangtamu fault horst belt in the middle of the Lunnan low uplift of the Tabei uplift, Tarim Basin, which is the biggest inland basin in China. It is bounded by the Caohu sag and Halahatang sag to the east and west, respectively, and by the Manjiaer sag to the south. Tabei uplift is located in the north of Tarim Basin, adjacent to Kaqu Depression in the north and to Beibu Depression in the south [23].The fault horst belt is composed of the Sangtamu fault and its derived Sangtamu southern fault. The two faults are about 1.5 km apart, and their trends are nearly east-west and parallel. The TIII reservoir in the Sangtamu Oilfield has an oil-bearing area of 24 km2. It is divided into six production blocks by several NNE trending secondary faults. The location and tectonic background of the study area are shown in Figure 1.
3. Petrophysical Characteristics
The TIII oil formation is a braided river delta deposit. According to the cyclic characteristics of the lacustrine surface changes, the TIII oil formation can be divided into three sand formations: TIII1, TIII2, and TIII3. In this study, the oil-rich TIII1 sand formation was the target layer.
3.1. Core Sedimentary Characteristics
The types of sand bodies corresponding to the TIII1 sand group were mainly deposited in an underwater distributary channel at a braided river delta front [24], with a wide range of water energy variations during the deposition period, and the lithology is mainly medium sandstone and fine sandstone, followed by siltstone. Core photos of the different lithologies are shown in Figure 2. There is scouring phenomenon at the bottom of the distributary channel. Conglomerate deposits are common in the scour intervals, and the thickness of the scour intervals is about 1 m. The occurrence frequency of gravel in the coarse sandstone, medium sandstone, and fine sandstone decreases successively, and the gravel in the local position of the stable sandstone sedimentary section is distributed in a single layer, which reflects the sudden appearance and rapid disappearance of strong water flow caused by a channel swing event. In the middle section of the channel, where the water energy was relatively stable, the deposits are mainly medium sandstone and fine sandstone. As the water energy decreased, massive siltstones were deposited in the upper part of the channel and between the distributary channels. Affected by the organic matter content and the source supply, horizontal dark laminae developed locally. The laminae formed occasionally or repeatedly in successive single layers, and the thickness at some locations is about 5 cm. Massive light green silty mudstone and light green to dark mudstone are present in the interdistributary channel, indicating a shallow water weakly reducing environment.
(a)
(b)
(c)
(d)
(e)
(f)
The logging curve morphology and core sample from the sediment section of the TIII1 sand formation in the study area are shown in Figure 3. The logging curves are bell-shaped and box-shaped, reflecting frequent changes in the water energy in this area. The response characteristics of the gamma ray (GR) value, i.e., increasing by different amplitudes in adjacent positions, correspond to the position of the hydrodynamic adjustment interfaces. Many types of abrupt lithologic contacts, including medium sandstone-fine sandstone, fine sandstone-mudstone, and coarse sandstone-mudstone contacts, were observed in the core. The average thickness of the TIII1 sand group is 18.3 m, while the thickness of the mudstone interlayer in 15 coring wells is less than 0.5 m. Science coring wells had been deployed in all of the production blocks; it can be concluded that the preservation degree of the mudstone interlayer in the study area is generally low. According to the degree of shielding of the vertical flow, the superimposed interface can be subdivided into a physical interface and a logical interface. The physical interface refers to the interface with a complete shielding ability, and the logical interface refers to the interface with an incomplete vertical shielding ability due to complete denudation of the mudstone deposits on the top of the early channel caused by the later channel cutting. Therefore, the TIII1 sand group is composed of overlapping multistage distributary channels, and the position where the channels overlap in each stage cannot completely block the vertical fluid flow, resulting in a vertical difference in the oil and gas enrichment. Taking position 4772.1 m in well LN44 as an example, the gravelly sandstone formed by the late hydrodynamic enhancement is superimposed on the previously deposited fine sandstone, and the superimposed interface cannot effectively block the vertical migration of the oil and gas. The oil grade of the gravelly sandstone deposited in the upper late water body is oil immersion, while the oil grade of the fine sandstone deposited in the lower early water body is only fluorescent.
(a)
(b)
(c)
(d)
(e)
(f)
3.2. Grain Size Characteristics
Cumulative grain size probability curves were created for samples with different lithologies and from different vertical positions in the distributary channels, as shown in Figure 4. The transport modes of the coarse medium sandstone particles at the bottom of the channel represented by the sample from 4774.5 m in well LN44 include rolling, jumping, and suspension. The total content transported by rolling is 23.5%, that transported by jumping is 70.9%, and that transported by suspension is only 5.6%. Owing to the low sorting degree caused by the hydrodynamic changes, the cumulative grain size probability curve exhibits a six-section pattern. The transport modes of the fine sandstone particles in the middle of the channel, represented by the sample from 4633.3 m in well LN22, include jumping and suspension. The total content transported by jumping is 85.2%, and that transported by suspension is 14.8%. The cumulative grain size probability curve exhibits a four-section pattern. The fine siltstone at the top of the channel is represented by the sample from 4645.2 m in well LN23, as formed by two types of particle transport: jumping and suspension. The total contents transported by jumping and suspension are 67.7% and 32.3%, respectively. The sorting degree of each is high, and the cumulative grain size probability curve exhibits a two-section pattern. The cumulative grain size probability curve characteristics of the samples from different locations in the study area indicate that the detrital deposition mode was traction flow deposition, and the hydrodynamic force changed frequently.
3.3. Pore-Throat Characteristics
3.3.1. Thin Section Petrology and Pore-Throat Characteristics
According to the fluorescence thin section and cast thin section observations of the different lithologic samples from well ST2-8 J, multiphase hydrodynamic adjustment occurred, as shown in Figure 5. The samples from the different hydrodynamic deposition zones in the TIII1 sand group exhibit medium-good sorting, and the particles are subangle to subrounded, with point and line contacts. Primary intergranular pores, expanded intergranular pores, and dissolved pores are developed. The argillaceous interstitial material has an enveloping distribution of particles. The pore connectivity is good, and the network of pores has a high degree of development, with medium pores and medium-fine pore throats. The pore-throat coordination numbers of the small conglomerate, medium sandstone, and fine sandstone samples all range from 0 to 4, and the main pore radii range from 0.02 to 0.175 mm, 0.02 to 0.15 mm, and 0.01 to 0.075 mm, respectively. The maximum pore radii are 0.2 mm, 0.2 mm, and 0.1 mm, respectively. The sandstone pore radius decreased as the sedimentary hydrodynamic force weakened. The capillary force is a seepage resistance force on the injected water under the oil-wet condition of the particles in the reservoir. When the injection pressure is lower than the capillary force, the injected water cannot enter the throats. Then this throat radius can be regarded as the throat radius under the critical seepage condition, and its size can be considered to be a fixed value under a certain pressure field. This means that the difference between the pore radius and throat radius of the sandstone is also related to the hydrodynamic force at the time of deposition; that is, the difference between throat radius and pore radius of the sandstone in a strong hydrodynamic sedimentary environment is large, while the difference is small in a weak hydrodynamic sedimentary environment.
3.3.2. Characteristics of Mercury Injection
The pore-throat system between the rock particles can be analyzed based on the morphology of the mercury injection and withdrawal curves created from mercury injection data for core samples in order to determine the size and distribution of the pore throats [25–27]. In mercury injection experiment, mercury is the nonwetting phase, the external pressure is the power of the mercury entering the pore-throat system in the mercury injection stage, and the capillary force and friction between the mercury and rock particles are the resistance to the mercury seepage. When the external pressure exceeds the displacement pressure, the mercury begins to enter the pore-throat system, and as the external pressure increases further, the mercury gradually enters the smaller throats until maximum mercury saturation is reached. In the mercury withdrawal stage, the capillary force is the driving force of the mercury outflow from the throats when the external pressure is absent, while the friction between the mercury and the rock particles is still the resistance to the mercury seepage. At the junction between the throat and the pore, when the throat radius increases to the point where the corresponding capillary force is lower than the friction force, the mercury stops flowing, and the corresponding mercury saturation is the minimum mercury saturation. When the difference between the throat radius and the pore radius is small, the change in the capillary force is relatively stable, mercury is easy to flow out, the minimum mercury saturation is low, and a large mercury withdrawal efficiency is reached. In contrast, when the pore radius and the throat radius are very different, the capillary force at the joint between the throat and the pore is suddenly greatly reduced. Thus mercury remains in the throat, and the minimum mercury saturation is large, resulting in a low mercury withdrawal efficiency. Therefore, the difference between the pore radius and throat radius can be determined from the mercury withdrawal efficiency, and the percentage of the throat volume without effective seepage to the total volume of the pore-throat system can be analyzed. A high mercury withdrawal efficiency indicates a small difference between the throat radius and pore radius, and it also indicates a low volume of mercury flowing out of the pore-throat system; that is, the proportion of the throat volume that cannot produce effective seepage to the total volume of the pore-throat system is small.
Scatter plots were created based on the statistical relationship between the permeability and porosity and the relationship between the mercury withdrawal efficiency and the porosity, permeability, and maximum throat radius for 38 samples from the study area, as shown in Figure 6. There is an obvious correlation between the permeability and porosity. When the porosity is less than 20%, the permeability changes with a low amplitude as the porosity increases, and when the porosity is greater than 20%, the permeability increases exponentially with increasing porosity. For sedimentary sandstones with a medium degree of diagenesis, the smaller the volume percentage of the pore-throat system is, the worse the seepage capacity is. In the pore-throat system, the throat radius is the main parameter determining the magnitude of the permeability of the rock [28–30]. Therefore, it can be concluded that the region with a high pore-throat volume percentage has larger throat radii. The mercury withdrawal efficiency decreases with increasing porosity, permeability, and maximum throat radius, but the variation ranges are slightly different. The mercury removal efficiency decreases linearly with increasing porosity, that is, when the percentage of the total volume of the pore-throat system is 5-30%, the higher the percentage of the total volume of the pore-throat system is, the lower the mercury withdrawal efficiency is, reflecting the fact that the percentage of the volume of the throats without effective seepage increases as the total volume of the pore throats increases. The mercury removal efficiency values vary significantly from 39.5 to 14.4% in the low to ultralow permeability range, and the maximum throat radius is less than 25 μm, while the mercury removal efficiency mainly concentrated within 14.4-6.7% in the medium permeability and high to ultrahigh permeability ranges, and the maximum throat radius is 25-250 μm. Based on the mercury injection data and the porosity and permeability of the rocks, it can be concluded that the less developed parts with a low porosity and permeability have smaller throat radii, and the difference between the pore radius and the throat radius is small. However, as the porosity and permeability increase, the part with high physical property development has larger throat radii, and the difference between the pore radius and throat radius is relatively high. This results in a high proportion of throats in which fluid cannot effectively flow, leading to flow around, which is not conducive to improve the sweep coefficient.
(a)
(b)
(c)
(d)
4. Influence of Deposition Mode on Pore-Throat Distribution and Fluid Seepage
For detritus deposition with flowing water as the transport medium, the hydrodynamic force determines the particle size, movement mode, and deposition mode of the rock particles [31, 32]. Strong hydrodynamic conditions can transport large rock particles away from their provenance via rolling and jumping. The longer the transport duration, the farther the particles are transported from the provenance and the higher the degree of separation of each particle component. The distribution of the pore-throat radii is more concentrated for sedimentary rocks formed under continuously weakening hydrodynamic force. In sedimentary rocks formed in areas with frequent hydrodynamic changes, the degree of sorting of each particle size component is low, and the pore-throat radii with different sizes are developed, which increases the difference between the pore and throat radii. In sedimentary rocks formed in areas with frequent changes in the hydrodynamic direction, the particles are arranged in a disorderly manner, which enhances the complexity of the pore-throat system. Therefore, there is a certain correlation between the pore-throat system and the hydrodynamic conditions during the depositional period.
The physical property data, mercury injection data, and percentages of the suspended component for typical samples with different seepage capacities from the study area are presented in Table 1. The maximum throat radii of the rock samples with higher seepage capacities are larger, and the suspended component contents are lower. This reflects the fact that the throat radius of a clastic sedimentary rock formed in a stable and weak hydrodynamic environment is small due to the combined influence of the suspended component content and particle size of the rock. The mercury removal efficiencies of samples with the same seepage capacities are significantly different, indicating that the development complexities of the pore-throat systems of the samples with the same seepage capacities are different, and the ratios of the maximum to minimum mercury withdrawal efficiencies of the low permeability, medium permeability, high permeability, and ultrahigh permeability samples are 2.42, 3.68, 2.23, and 1.96, respectively. It can be concluded that the percentage of the ineffective seepage volume to the pore-throat volume is the highest for samples with medium seepage capacities, and the percentage of ineffective seepage volume to pore-throat volume decreases as the seepage capacity increases when seepage capacity reaches the medium permeability level.
The cumulative probability curves and mercury injection curves for typical samples with different seepage capacities from the study area are shown in Figure 7. By comparing the cumulative grain size probability curves of the different samples, it was found that the slopes of jumping component curves of the low permeability samples are the lowest among the samples with the same particle transport mode. In other words, the smaller the slope of the jumping component curve is, the weaker the flow energy is, the lower the degree of particle sorting is, the worse the throat development degree is, and the weaker the seepage capacity of the sample is. For samples with given jumping component curve slope, the lower the suspended component content is, the stronger the seepage capacity is. Since the content of the suspended component can reflect the amount of water energy and the duration of the deposition by traction flow to a certain extent, the longer the duration of the strong hydrodynamic force conditions is, the lower the content of the suspended component is. Conversely, the longer the duration of the weak hydrodynamic force, the higher the content of the suspended component is. For samples with rolling, jumping, and suspension transport modes, the higher the content of the suspended component is, the shorter the duration of the strong current is, and the suspended components fill the pores between the coarse particles, which can increase the complexity of the pore-throat system and reduce its seepage capacity.
(a)
(b)
(c)
(d)
By comparing the mercury injection curves of core samples with the same seepage capacities, it was found that the morphologies of the mercury injection curves and the mercury withdrawal curves are quite different, which reflects the differences in the pore-throat systems of the core samples. The porosity and permeability parameters can explain the percentage of the total volume of the pore-throat system and the seepage capacity of the pore-throat system, but it cannot explain the internal complexity of the pore-throat system and cannot be used to characterize the internal fluid flow. Therefore, it is one sided to use only the porosity and permeability parameters to characterize the reservoir’s physical properties, and the influence on the fluid flow should be judged based on the sedimentary process. For the low permeability samples shown in Figure 7(a), the sorting degrees are basically the same, but the particles of the sample from well LN22 are slightly coarser, and the water energy was slightly higher than those of the samples from well LN39. However, the particle size distribution range of the sample from well LN39 is narrow. That is, small variations in the water energy during the deposition of the sample will result in a small difference between the throat radius and the pore radius, which will result in a higher mercury withdrawal efficiency, and the proportion of the effective seepage contributed by the pore-throat volume is larger under the same pressure conditions. For the medium permeability samples shown in Figure 7(b), the water energy of the sample from well LN22 was significantly higher than that of the sample from well LN39 at the time of deposition, but the hydrodynamic conditions were stable for a short time, and the degree of sorting was low, leading to a larger difference between the throat radius and pore radius and a larger proportion of the throat volume that could not effectively flow. For the high permeability samples shown in Figure 7(c), the hydrodynamic strength was relatively consistent during the deposition of the two samples. The slope of the cumulative probability curve of the jumping component of the sample from well LN22 is slightly higher than that of the sample from well LN23, and the degree of sorting is relatively high. The mercury removal efficiency is lower than that of the sample from well LN23 sample, that is, the percentage of the effective seepage contributed by the throat volume is smaller under a certain pressure. For the ultrahigh permeability samples shown in Figure 7(d), the variation ranges of the hydrodynamic strength and the particle size during deposition are basically the same, and the cumulative probability curves of the jumping components are approximation parallel. This indicates that even though the particle separation degrees are the same, the suspended component contents of the samples from well LN23 are slightly higher than those of the samples from well LN39, resulting in a larger difference between the throat radius and pore radius and a larger percentage of ineffective throat volume.
During the flow of water, the energy of the water is the highest, and the particle size is the largest along the midstream line. In the channel section, the water transport capacity decreases gradually as the distance from the midstream line of the channel increases, which is manifested as planar differences in the particle size fractions. Since the development degree of the pore-throat system is influenced by the deposited particles, the development degree of the pore-throat system within a monogenic sand body changes with the position in relation to the midstream line of the channel. For a clastic reservoir developed using water injection, the location of the throats without effective seepage is controlled by the position of the midstream line within the sweep range of the injected water. In the braided river delta front environment, the sudden and temporary strong water flow on the coastal side results in mixing of the particle size components. The degree of particle sorting is low, the internal pore-throat system exhibits complex development, and the percentage of the throat volume with effective seepage is low. The characterization of the sand body deposition process according to the distribution of the midstream line can improve the accuracy of understanding the reservoir to help determine the location of the area without effective seepage within the injected water sweep range and help predict the position of the remaining oil enrichment after water flooding.
5. Conclusions
Porosity and permeability are the main parameters to characterize the reservoir physical properties. However, influenced by the sedimentary conditions, even sedimentary sections with the same porosity and permeability have differences in their internal pore-throat systems due to different sedimentary process. These differences also affect the internal fluid seepage paths, especially for reservoirs developed by water injection. The low injected water sweep efficiency caused by injected water bypass is one of the reasons for the formation of remaining oil. Therefore, the following is what the study has obtained through comprehensive testing: (1)The hydrodynamic strength of the sedimentary environment influences not only the particle size of the sedimentary rock, but also the difference between the pore radius and throat radius. The permeability of a fine-grained sedimentary rock formed in a weak hydrodynamic environment is lower than that of a coarse-grained sedimentary rock formed in a strong hydrodynamic environment, and the difference between the pore radius and throat radius is also smaller than that of a sedimentary rock formed in a strong hydrodynamic environment(2)The maximum pore-throat radius connecting the pores is larger in the rock samples with a higher porosity and permeability, but the difference between the pore radius and throat radius is more obvious, and the percentage of the throat volume without effective seepage to the total volume of the pore-throat system is higher, which leads to the fluid flow around the throats being more seriously affected(3)The complexity of the pore-throat system of a rock with given porosity and permeability depends on the stability of the fluvial sedimentary environment in which the rock as deposited, including the variations in the water energy and the duration of the stability of the hydrodynamic conditions. When the duration is short and the water energy varies greatly, the distribution range of the rock’s throat radii becomes larger and the pore-throat system becomes more complex(4)From the perspective of the particle process, the reservoir can be characterized according to the movement mode of the sandstone particles rather than by dividing the seepage units only according to the porosity and permeability in order to reduce the deviation of the results from the actual situation, to gain a better geological understanding of the actual situation, and to improve the accuracy of the understanding of the reservoir properties
Data Availability
All data included in this study are available upon request by contact with the corresponding author.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the CNPC Innovation Foundation (2021DQ02-0106).