The concept of low-amplitude structure is basically the same as that of micro-amplitude structure or microstructure. In 1982, ZHAO^[1] put forward the concept of micro-amplitude structure and micro-amplitude structure group for the first time through detailed interpretation and comprehensive study of the seismic data collected in the main body of Sanzhao sag on the east side of Daqing placanticline, and discussed its origin and influence on exploration. In 1987, the scholar further defined the microstructure more clearly as a structural trap formed on a very gentle regional tectonic background with a small uplift (with closed height of 10-20 m) and weak appearance on time profile^[2]. In the same year, LI^[3,4] defined the microstructure as "the structural characteristics caused by the subtle fluctuation of the oil layer itself on the overall oilfield tectonic background" through in-depth analysis of the relationship between Shengtuo and Gudao oilfield microstructure and production dynamics, usually within 0.3 km² in area and 20 m in uplift amplitude. In the following years, new research results on low-amplitude structures have come up constantly, involving a wide range of oilfields and regions, with the boundaries of structural amplitude adjusted somewhat^[5]. But the research approach still relies primarily on fine processing and interpretation of seismic data^[5].

The Ordos Basin is a large overlapping craton basin, where the "Yishaan slope" in the middle gently dipping to the west, constitutes the main tectonic body of the basin^[6,7,8]. A large number of oil and gas exploration and development results show that the "Yishaan slope" has internal undulations and a large number of low-amplitude structures, which have a decisive impact on the distribution and enrichment of oil and gas and the development of oil and gas fields (reservoirs)^{[9,10,11,12,13,14,15]}. According to the petroleum geology^[10] of the basin, the structure of many oil and gas fields (reservoirs) on the Yishaan slope are generally less than 30 m in amplitude, but vary greatly in structure and trap area, from the minimum structure area of less than 1.0 km² to the maximum structure area of around 200 km², and the minimum closure area of less than 0.3 km² to the maximum closure area of about 19 km². In recent years, the concepts of continuous reservoir and tight reservoir^[16,17] have been merging continuously, and emphasizes the continuous controlling action of nonstructural trap and large-scale reservoir to hydrocarbon accumulation. Mesozoic strata in Yishaan slope were mainly composed of inland fluvial and lacustrine sediments; in particular, the delta front and deep water gravity flow deposits in the Yanchang Formation deposition period are large in scale and widespread, with many sandstone reservoirs^[18,19]; a number of large and medium-sized extra-low permeability oilfields, including Ansai, Yanchang, Zhidan, Jing’an, and Xifeng etc. are scattered and embedded in the system^[20,21], giving us the false image that the reservoirs are uniform and continuous. In addition, the low amplitude structures are unnoticeable and subtle, making them easy to be neglected in the research work.

The control and influence of low-amplitude structure on the large area oil and gas accumulation and development of large and medium-sized oilfields on Yishaan slope is an important geological issue related to the development of oil and gas accumulation theory, evidence and answers need to be searched from more examples of oilfield development and more detailed study on development geology. Previously, studies mainly focused on the corresponding analysis of regional geological characteristics and macro oil and gas distribution, etc.^{[22,23,24,25,26]}, the target areas were large, low in well control, thick in target interval, and coarse in analysis unit with little dynamic data and large isoline spacing, making it difficult for the detailed research, and reflecting the true appearance of low-amplitude structures, and reveal the important influence of low-amplitude structure on hydrocarbon accumulation and development dynamics.

To make up for the shortcomings of previous studies, taking the Chang 6 oil reservoir in the Yanwumao block of Zhidan oilfield and its development thin layers as research objects, based on a large number of drilling, geological logging, logging, production and dynamic monitoring and static data, the effects of low-amplitude structure on oil, gas and water distribution and development dynamics of ultra-low permeability reservoirs have been examined comprehensively by combining dense well pattern and multi-factor geological modeling, macroscopic comparison and local dissection, and static and dynamic analysis, in the hope to study the influence of low-amplitude structure on the large area oil and gas accumulation and development of large and medium-sized oilfields and guide the exploration and development of extra-low permeability reservoirs.

The Zhidan oil field is located in the hinterland of northern Shaanxi, with the Yishaan slope as the background structurally, and in the main part of the large delta front of the north wing of the lake basin depositing in the sedimentary period of the Mesozoic Triassic Yanchang Formation in facies belt^[27]. Yanwumao block is located in the west of Zhidan oilfield, with an area of about 50 km². From 1999 to 2010, it was developed by natural depletion, and was transferred to large- scale waterflooding in 2011.

There are many oil-bearing strata in the area with Triassic Yanchang 6 oil-bearing group as the major pay. The Chang 6 oil group is divided from bottom to top into 4 subgroups: Chang 6₄, Chang 6₃, Chang 6₂ and Chang 6₁, which are stable in thickness and slightly thickening from north to south. Chang 6₂ and Chang 6₁ are further subdivided into three thin layers, Chang 6₂³, Chang 6₂², Chang 6₂¹ and Chang 6₁³, Chang 6₁², Chang 6₁¹, respectively. Among them, Chang 6₁³ and Chang 6₂¹ have the largest reserves, highest producing degree, and highest production capacity, are the major pays of Chang 6 oil reservoir^[28].

The Chang 6 oil layer group developed mainly delta front subfacies, the ancient flow was from north to south in nearly north-south direction, and its west side was affected by the ancient water flow from the east of Wuqi delta from the NW during its deposition. According to the three-dimensional sedimentary facies model of the main thin layers under constraints of the tight well pattern and multiple conditions, the main body and flanks of the facies belt are mainly composed of widespread sand bodies such as distributary channels, mouth bars and distal sand bars. The interdistributary bay deposit is most developed in Chang $6_{2}^{\text{ }3}$ thin layer, while restricted in distribution in the other thin layers. Sandstone reservoirs are well developed, with an average sand-formation ratio of 0.47, an average single-sand thickness of 5.0 m, an average porosity of 10.3%, and an average permeability of 1.42×10^-3 μm², representing typical low porosity and ultra-low permeability reservoirs.

The basic features and development characteristics of low-amplitude structures in the area can be well revealed by using the interpolation method of dense well pattern and small-spacing contour line (1-5 m) (Fig. 1). Under the control of regional monoclinic background, the top surface structure of Chang 6 reservoir group on the whole dips west, with a dip angle of about 1°, there are a series of nose-shaped low-amplitude structures trending EW from south to north, the ridge lines of the structures are often slightly deflected, concave-convex, forked and subducted, deriving some new branches of highs and lows, forming a larger nose-like fold belt. Overall, the fold belt inclines in the west and upturns in the east, and is lower in the south and higher in the north, with a significant structural difference^[29]; the structure has an east-west height difference of about 70 m, an average slope of about 8 m/km; a north-south height difference of about 40 m, and an average slope of about 4.3 m/km. In the north, the low-amplitude structures are mostly positive bulges and denser, and larger in scale combined. In the south, the low-amplitude structures are mostly negative lows and gentle and wide, smaller in scale on the whole. Some contours at the top of the local nose-shaped structures close, forming some anticlinal traps of different sizes, with a closing height of generally 5-10 m, up to 15 m in local parts, and a closure area of 0.2-0.8 km². The top structures of all thin layers in Chang 6 oil group have good inheritance in morphology and combination characteristics from bottom to top.

Low-amplitude structures on the Yishaan slope are mostly nose-shaped, with low amplitude, wide variation in scale and no closure at the top, making it difficult to define the structural amplitude and scale. In this study, 24 low-amplitude structural units of positive bulges and 5 low-amplitude structural combination units (Fig. 1) were delineated in the area through in-depth study of the fine structural features on the top surface of the Chang 6 reservoir in Yanwumao block. By correcting the tectonic background of relative structural height difference (the difference between the structural base point and the high point), the true amplitude of the structure units were worked out at 0.4-19.8 m, with an average of 9.3 m.

The top structure of the Chang 6 reservoir in Yanwumao block has obvious trend differences in the east-west and south-north directions. The internal low-amplitude structures also have some local traps, with an average closure height of 5.2 m. The formation crude oil has a density of 0.8238 g/cm³, and the oil-water density difference of 0.1762 g/cm³. These differences indicate that the Chang 6 reservoir has the basic conditions for oil-gas-water differentiation and directional migration^[6].

According to the theory of oil-water two-phase flow and the change mechanism of water cut, the initial water cut is a function of the original water saturation, the lower the original water saturation is, the lower the initial water cut of the reservoir is, and the higher the initial oil production capacity is, that is, the initial water cut can reflect the original oil saturation and oil and gas enrichment degree^{[30,31,32,33]}. Therefore, finding out the relationship between low-amplitude structure and oil well productivity and water cut can effectively reveal the control and influence degree of low-amplitude structure for the oil-gas-water distribution and enrichment degree of ultra-low permeability reservoir.

Based on the production data of the main thin layers under the dense well pattern of the area, the initial water cut model of Chang 6 oil reservoir was established, and the model was superimposed with the low-amplitude structure of the oil reservoir and initial stable production of the oil wells (Fig. 2). The initial stable output is the output when the reservoir reaches a stable state after put into production for 3-5 months by fracturing, which was worked out according to the oil production curve of the well and represented by oil-water column, the initial water cut was calculated according to the initial stable liquid yield.

Fig. 2 shows that the initial stable yield and initial water cut of the thin layers in Chang 6 oil reservoir all follow the fluctuation and natural extension of low-amplitude structures. The wells at structural high have higher productivity and lower water cut, while wells at lower structural position have lower productivity and higher water cut. The larger the scale of structure combination, the larger the scope of its influence and action will be. The smaller the scale of structure combination, the smaller the scope of its influence and action will be. Meanwhile, the initial stable yield and initial water cut in the east and west, south and north of the block also have significant overall differences.

It can be seen that the low-amplitude structure and its com-bination scale have significant control and influence on the distribution and enrichment of original gas, oil and water in the ultra-low permeability reservoirs. Low-amplitude structures are not only the main direction of oil and gas migration, but also favorable places for oil and gas relative enrichment. The overall difference of the low amplitude structures controls the overall enrichment difference of oil and gas.

By using Petrol petrofacies and attribute modeling technology, the well-connected sedimentary facies and saturation profile in arbitrary direction can be obtained under multiple constraints. By combining these two types of profiles with reservoir characteristic profile, the control and influence of low-amplitude structure for the differentiation and enrichment of oil-gas-water on the profile were comprehensively analyzed. Fig. 3 shows a low undulation at the top of the reservoir profile, with overall elevation to the north and east; the local structure is developed but not prominent, oil and gas have the characteristics of widespread distribution, local concentration and great difference in enrichment degree. At higher structure position, there are more or better quality oil layers, at lower structure position, there are less or worse oil layers. For example, Wells in the north of Well YW5 (Fig. 3a), YW11 and YW13 (Fig. 3b) are higher in structural position, and have more and concentrated oil layers. Wells YW8 and YW10 are lower in structural position, and have less oil layers and significant rise in water cut. In addition, no matter in the south-north or east-west sections, Chang 6₃ has more oil layers at higher structural position, indicating that the structural differences of low amplitude structure and the whole block are closely related to the differentiation and enrichment of oil, gas and water.

Fig. 3 also shows that different sedimentary facies zones in the lithofacies profile are obviously different in property with strong heterogeneity. The "main body" and "flanks" of distributary channel in the delta front are richer in sand bodies of good continuity and connectivity. The interdistributary bay is dominated by dark argillaceous sediments and limited in distribution. This kind of sediment is mainly distributed in the lower part of Chang 6₂ and the middle and upper part of Chang 6₁. The interdistributary bay deposit in the lower part of Chang 6₂ appears in intermittent layers. The distribution of facies zones and their property differences not only make the reservoirs heterogeneous, but also affect the local distribution and enrichment of oil and gas. For example, the discontinuous interdistributary bay argillaceous rock in the lower part of Chang 6₂ in Fig. 3, together with the low-amplitude structure, have a significant shielding effect on the oil and gas reservoir in the underlying Chang 6₃ member. In Fig. 3b, the restricted interdistributary bay argillaceous rocks in the middle and upper part of Chang 6₁ in wells YW14 and YW15 also have a certain lateral sealing effect on the underlying oil and gas and the oil and gas in the updip direction on the west side.

The original distribution of oil saturation on the profile is not only consistent with the development and distribution of oil layers in the reservoir profile, but also consistent with the local tectonic relief and the overall structural difference of the block. The higher structural position has higher oil content, the lower structural position has poorer oil content. The profile structure uplifts from west to east and from south to north, and the oil-bearing property gets obviously better toward the updip direction.

Apparently, apart from the local influence of facies belt and lithology, the difference of low-amplitude structure and overall structure have strong control and influence on the distribution and enrichment of original oil, gas and water in ultra-low permeability reservoirs. The larger the structure amplitude, the stronger the oil-water differentiation will be, and the higher the structural position, the better the oil-bearing property and the higher the saturation will be. The overall structure difference controls the migration and accumulation of oil and gas to the updip part of the structure.

It can be seen from the above analysis that the accumulation effect of low-amplitude structure on oil and gas mainly depends on the amplitude and scale of structure. Based on the data of oil wells at the top of each low-amplitude structural unit and the statistical average of multiple well points, the oil and gas effect parameters corresponding to the structural amplitude, such as reservoir thickness, oil saturation and stable productivity, were worked out, and the oil and gas accumulation effect model of low-amplitude structure was established (Fig. 4). The results show that the low-amplitude structure has significant hydrocarbon enrichment effect, and the structural amplitude has good correspondence with each control effect parameter. As the structure amplitude increases, the reservoir thickness gradually thickens (Fig. 4a), the original oil saturation increases correspondingly, the differentiation ability of oil and gas water increases correspondingly, and the degree of oil and gas enrichment increases correspondingly (Fig. 4b); oil well productivity also increases (Fig. 4c). On the other hand, the fact that many oil wells generally produce water to some extent in the initial production stage also indicates that oil, gas and water in ultra-low permeability reservoirs are usually difficult to reach full differentiation, rather relative differentiation and enrichment is a normal state.

Water injection started in the Chang 6 reservoir in 2011 with the diamond inverted nine spot injection-production pattern of 450 m×150 m, in which the actual well spacing between oil wells was about 300 m, the well spacing between water wells and edge wells was 300-350 m, and the well spacing between water wells and angle wells was 400-500 m. Comprehensive study shows that sand-rich facies zones are extensive in this area, the reservoirs are similar in sedimentary types, good in continuity, similar in lithology, and have no obvious difference in physical properties between the north and the south. The average reservoir connectivity ratio in the injection-production group was 65.6%; all oil wells were put into production after fracturing. At present, water cut can not only reflect the status and dynamic effect of reservoir development, but also reflect the control and influence degree of structure geological characteristics on reservoir development index and oil-gas-water dynamics. By using the production dynamic data of the main thin layers under the dense well network, the current output and water cut of the wells were calculated to establish the superposition model of the low-amplitude structure of Chang 6 reservoir and the current water cut and production distribution (Fig. 5). The control and influence of low-amplitude structure on the waterflooding performance of Chang 6 ultra-low permeability reservoir were comprehensively analyzed based on the changes of initial stable production and initial water cut of the reservoir (Fig. 2) : (1) Overall, the current water cut of the block increased, and the liquid production dropped significantly compared with the initial development stage. The overall difference between the north and the south still remains, and has increased gradually. No matter in the north or south of the block, both the initial and present water cut decrease gradually with the elevation rise and scale increase of the structure, and vice versa, while the fluid production shows the opposite variation pattern. (2) Local watering-out always starts from the low-lying part of the structure, and then gradually moves up to the higher part of the structure, the extent of watering-out continues to expand. The area with high oil content and low water cut shrinks gradually to the high part of the low amplitude structure, and the area with medium and high water cut and severe watering-out gradually expands.

By using the oil well data at the top of each low-amplitude structural unit (Figs. 1 and 2), the development dynamic response parameters corresponding to the structural amplitude of each unit, such as initial water cut, current water cut, water cut rise rate, water breakthrough time of the well, were obtained through statistics and average of several well points. The development dynamic response model of low-amplitude structure was established by statistical mapping (Fig. 6). The results show that the low-amplitude structures have a good correlation with the dynamic parameters and a significant influence on the development dynamics. With the rise of structure amplitude, the initial water cut (Fig. 6a), the current water cut (Fig. 6b), and the rise rate of water cut (Fig. 6c) all show an obvious drop trend, and the decline rate of current water cut was faster than in the initial stage. The water-breakthrough time of the oil well (Fig. 6d) gets longer with the rise of the structure amplitude, the higher the structural position, the later the water-breakthrough time of the oil well will be. In the northern part of the block, there was still no sign of water breakthrough in individual well groups after nearly 5 years of waterflooding.

It can be seen that the low-amplitude structure not only has an important control effect on the original oil-gas-water distribution and initial production of the reservoir, but also has an important control on the development dynamic evolution of the reservoir, the current development status and even the distribution of remaining oil^[32,33].

W0 well group and W81 well group are located in the north and the south of the block respectively, representing the typical injection-production well groups with higher and lower production in the region. Taking the two well groups as examples, the influences of low-amplitude structure on production dynamics, especially on the dominant direction of water displacement have been investigated.

(1) Basic situation of the injection-production well groups. W0 injection-production well group is located in a low-lying area of the low-amplitude structural nose in the north of the block (Figs. 2 and 5). The structure is 2-10 m in elevation difference and lower in the northeast and southwest, and the injection well is located in the middle saddle (Fig. 7a) higher in elevation. The pay zone of the well group is Chang 6₁³ thin layer, which is consistent in physical properties and oil-bearing property, and good in oil layer connectivity in the well group. W81 injection-production well group is located in the west wing of a low-amplitude nose uplift structure in the southern block, with a slight bending in the middle and a structural amplitude difference of 2-10 m (Fig. 7b), where the main pays are Chang 6₁³ and Chang 6₂¹ thin layers. The water injection well is located at the relatively high position of the flexure, among the surrounding oil wells, all wells are in the position driven downward by injected water except the one in the southeast corner that is higher in structure position^[34].

(2) Waterflooding performance of the injection-production well groups. After W0 well group was transferred to waterflooding production, the wells had a stable fluid production of 40.0-149.2 m³/month, 89.9 m³/month on average, and a water cut during stable yield of 9.2%-23.0%, 15.7% on average. The stable yield period lasted for about 2 years. At present, the oil wells have a production of 21.0-77.0 m³/month, 47.6 m³/month on average, and a water cut of 9.2%-78.0%, 30.4% on average (Fig. 8a).

After W81 well group was transferred to waterflooding production, the oil wells had a stable fluid production of 15.0-31.0 m³/month, 21.6 m³/month on average, and a water cut of 36.0%-53.6%, 46.7% on average during stable yield period of about 1 year. Currently, the oil wells have a production of 8.0-52.0 m³/month, 29.6 m³/month on average, and a water cut of 52.0%-98.1%, 84.4% on average (Fig. 8b).

(3) The controlling effect of low-amplitude structure on water injection performance. As shown in Fig. 8, on one hand, the injection-production effect of the W0 injection-production well group in the north is much better than that of the W81 well group in the south, which is obviously related to the overall structural difference between the north and the south of the block. On the other hand, the production differences of oil wells in well groups widened gradually after stable production, and the specific effect differences are closely related to the structural location and the relative position between the oil well and the injection well. For example, wells W2, W3, W6 and W8 in W0 well group are located at higher positions of the structure, due to the upward drive of water, their water cuts have kept stable at the same level with their initial water cuts or even lower, and there is still no sign of water channeling. In contrast, wells W1 and W4, located in the relative low position of the structure, have seen water breakthrough successively, varied in water cut, and severe watering-out due to the downward drive of injected water (Fig. 8a). Well W1 was forced to shut down due to the sharp increase of water cut after water breakthrough in February 2013. After the well was opened again, its water cut still rose constantly. The water cut of well W4 went up linearly from June 2013, although stabilized temporarily after water plugging, it climbed again soon after.

After a short period of stable production, oil wells in well group W81 gradually saw water breakthrough and quickly entered high water cut stage. Wells W84 and W83, located in the lower part of the structure, saw abrupt watering-out successively, with the water cut soaring to more than 95% and the liquid production increasing significantly compared to the initial stage. Subsequently, wells W82, W87 and W85 were also flooded successively and entered discontinuous state with high water cut. Well W80 in the southeast corner of this well group is located at a higher position of the structure, its sudden flooding is mainly caused by the downward water drive of two adjacent injection wells located at higher parts of the structure in the east and southeast directions of the well (Fig. 5). Whereas wells W86 and W88, located in the northwest corner, are still in the state of natural depletion due to the non-corresponding injection and production layers, with monthly liquid yield of only 8-15 m³ with little change in water cut.

The analysis of the performance of the two injection-production well groups above shows that the low-amplitude structure has an obvious control on the preferred water drive direction and dominant watering-out area in the injection- production well group. Under the action of water flooding pressure difference and gravity, no matter in the south or the north of the block, the lower position of the structure will always be the preferred flowing direction of the injected water and watering-out area.

Under the condition of stable water flooding, when the water cut exceeds 40%^[35], there is a semi-logarithmic linear relationship between cumulative water production and cumulative oil production of a reservoir or a single well^{[33, 35]}, which can comprehensively reflect the dynamic characteristics in the actual water flooding process. Without adjustment measures, the slope of the line would not change^[36].

Fig. 9 shows the water drive characteristic curves of two adjacent oil wells (Wells W1 and W8) in well group W0, in which the water flooding characteristic curve of well W1 inflects after water breakthrough, forming two obvious straight lines, and the slope increased significantly after water breakthrough and the water flooding effect became worse (Fig. 9a). Well W8 is 286 m from well W1. In the past five years, this well hasn’t been treated by any adjustment measures, but its Chang 6₁³ oil layer has kept a high and stable yield, its water drive characteristic curve takes on a uniform and continuous rising state on the whole, its rising trend declined somewhat with the redistribution and new enrichment of oil and gas in the process of water flooding, and even approached a straight line, the slope of the straight line declined instead of rising, forming a general inflection point with the smooth curve before, and the water cut also kept stable with a little drop, which is completely opposite to the dynamic response of well W1.

The volume method and Type A water drive characteristic curve method were used to calculate the static proved reserves and water-driven dynamic reserves controlled by wells W1 and W8 respectively (Table 1). The results show that the dynamic reserves of well W8 exceeds its static reserves abnormally under the current water drive state, while the dynamic reserves of well W1 are much lower than its static reserves.

In view of the similarity of reservoir characteristics of W0 injection-production well group, according to the principle of water flooding, and the comprehensive analysis of the interaction and equilibrium of several driving forces such as water flooding pressure difference, oil production pressure difference, gravity of injected water and oil and gas buoyancy in the injection-production well group, the abnormal dynamic development response can be well explained. Under the control of low-amplitude structure, as the injected water converged to the low-lying area preferentially, the oil enriched and increased in saturation in the water flooding front, and became better in phase continuity and seepage state, the oil differentiated more from water and the oil and gas migrated more by buoyancy. Therefore, the adjacent updip direction of the structure turned out to be the favorable direction for the relative migration of oil and gas. At this point, if the oil well adjacent to the dominant waterway in the low-lying area has a large production pressure difference at the bottom of the well due to its high structural position, low water supply, high producing degree and imbalanced injection and production, etc. If there is a certain depletion in the formation energy at the bottom of the well, the saturated oil and gas in the water flooding front of the low-lying area will be partially diverted to the high part of the structure under the induction of the lateral effective displacement pressure difference and the enrichment effect of low-amplitude structure. That is, in the process of waterflooding, low-amplitude structure will lead to the transfer of some crude oil in the low-lying structural part of the reservoir to the oil well in the structural high part^[37], or the redistribution of oil, gas and water, and the new oil and gas enrichment, thus making the dynamic reserves of oil wells at high structural position increase and their production capacity keep stable continuously.

It is precisely because of this development dynamic response of low-amplitude structure that oil and gas near the bottom of well W8 became more and more saturated, the water-drive dynamic reserves increased significantly, and the slope of the water-drive characteristic curve decreased accordingly. Meanwhile, injected water gathered near the bottom of well W1, and the water saturation there went up constantly. When the water saturation exceeded a certain limit (51%), the seepage capacity of crude oil worsened significantly, and the water cut increased rapidly^[38], the water-drive dynamic reserves decreased, and the slope of the water-drive characteristic curve increased.

Clearly, oil wells water injection dynamic response and efficiency difference in the injection-production well group is closely related to the relative difference of elevation between oil and water wells. If the structure position of oil well is relatively low, and in a main water drive direction, and water injection drives downward, then the oil well will become the preferential target of water flooding even violent watering-out. If the well is higher in elevation and located in the upstream direction of the injection well, it will often become the direction of redistribution and new enrichment of oil and gas in the water flooding front of the adjacent low-lying well area, so it will improve in production performance and keep low in water cut for a long time.

According to the comprehensive interpretation results, the small layers of Chang 6 extra-low permeability reservoir have an average original oil saturation of 42.3%-61.9%. Due to the overall structural elevation difference high in the north and low in the south, the north of the block has an average original oil saturation of about 4% higher than the south, although this difference is not prominent, but the south and north parts differ widely in development dynamic response.

(1) Elevation difference of the structure in the block controlled the distribution of original oil, gas and water in the south and north reservoirs, and oil wells in the north have much higher initial stable output and much lower water cut than oil wells in the south on the whole (Fig. 2).

(2) After more than 10 years of discontinuous development by natural depletion and nearly 5 years of development by large-scale waterflooding, the production difference between the north and the south of the block not only still existed, but even widened to some extent (Figs. 2 and 5). In the south with lower elevation, the water cut increased rapidly, most of this part has entered medium to high water cut stage now, and some wells have been flooded seriously. Most of the oil wells have dropped in liquid production, but local part rose in liquid production rather than fell because of high water cut. In the north part higher in elevation, although the liquid yield also declined significantly, the water cut rose slowly, and most of this part is still medium-low in water cut. In the area of wells YW3-YW25 larger in structural amplitude and scale, the water cut now is still close to the initial water cut, and the production remains at a high level.

(3) According to the production performance curves of W0 and W81 well groups (Fig. 8) and the earliest water-breakthrough time of oil wells, it can be preliminarily estimated that the moving velocity of injected water in the north part and the south part of the block are about 12.5 m/month and 25 m/month respectively. Based on the production performance curve of the south and north parts of the block (Fig. 10), the control and influence of the overall structural elevation difference on the development performance of the south and north parts of the block can also be basically defined. In a word, the south of the block has much faster water flooding speed, higher watering-out degree, bigger watering-out range, earlier water breakthrough time and shorter stable production duration than the north.

The low-amplitude structure has a significant control and influence on the distribution and enrichment of the original gas, oil and water in the ultra-low permeability reservoir and the evolution trend of waterflooding dynamics. The low-amplitude structure is not only the direction of hydrocarbon migration but also a favorable place for hydrocarbon enrichment.

The controlling effect of low-amplitude structure on ultra-low permeability reservoir mainly depends on its amplitude and scale. With the increase of structure amplitude and elevation of structure position, the oil layer becomes thicker, the oil saturation becomes higher, the oil and water differentiation becomes stronger, the oil and gas become more enriched, the oil well productivity becomes higher, the water cut becomes lower, the rise rate of water cut becomes slower, and the water breakthrough time of the oil well becomes longer. The larger the scale of the structure, the larger the scope of its control and influence will be. Water cut and oil well output always fluctuate orderly with the elevation of the low-amplitude structure.

The dynamic response of waterflooding development is closely related to the relative structural position of the injection and production wells. The injected water always advances to the low-lying parts of the structure first, and then gradually moves up to the high-lying parts of the structure, with the flooded area expanding continuously. Meanwhile, some oil and gas in the low-lying parts of the structure are redistributed to the high parts of the structure to form new oil and gas accumulation. Oil wells at high structural locations would thus increase in dynamic reserves and keep stable in production for a long time.