(A) Songnan low uplift
1. Analysis of hydrocarbon migration and accumulation and matching conditions
Songnan low uplift is adjacent to four depressions (Figure 6-7), namely Lingshui Depression, Songnan Depression, Bao Dao Depression and Beijiao Depression, all of which are surrounded by depressions. Three sets of source rocks are mainly developed, the main source rocks are Eocene and Yacheng Formation, and the secondary source rocks are Lingshui Formation and Miocene. As mentioned above, Lingshui sag, Songnan sag and Baodao sag have large hydrocarbon generation amount, resource amount, hydrocarbon generation intensity and resource abundance, which meet the standards of hydrocarbon-rich sag and provide rich oil and gas resources for the structure of Songnan low uplift. According to the results of basin simulation in 2000, the peak of gas generation in Lingshui sag was the sedimentary period of Sanya Formation and Huangliu Formation. The peak period of gas generation in Songnan sag is the sedimentary period of Ling 2 member, Sanya formation and Huangliu formation. The peak of gas generation in Bao Dao sag is the sedimentary period of Sanya Formation and Liuhuang Formation. The peak period of gas generation in the northern suburb depression is the sedimentary period of Ling-2, Ling-3, Sanya and Huang Liu Formation. The gas production peaks of the four depressions are all late, which is beneficial to oil and gas accumulation.
Figure 6-7 T70 Structural Map of Songnan Uplift in Qiongdongnan Basin
2. Yongle 3- 1 structure
Yongle 3- 1 structure (Figure 6-8 and Figure 6-9) is located in the east of Songnan low uplift in the deep water area southeast of Qionghai, belonging to the range of 53/ 16 outsourcing block, with the water depth of 1000 ~ 2000m, which is the transitional part from Bao Dao sag to Songnan low uplift. According to the collected 3D seismic data of 1 100km2, the structure is carefully interpreted and implemented. The research shows that Yongle 3- 1 structure is a large fault block structure, which is divided into seven units by several nearly east-west faults, with a total structural area of 405km2. According to the comprehensive calculation, the structural potential natural gas resources are 2508× 108m3 or the potential oil resources are 12.56× 108t, so the comprehensive exploration potential is huge.
Yongle 3- 1 structure is located in the east of Songnan low uplift, surrounded by depressions on three sides, and has a superior position, which can gather oil and gas generated by three large hydrocarbon-generating depressions in Songnan, Bao Dao and Changchang, and has superior hydrocarbon source conditions. Eocene lacustrine facies and Oligocene Yacheng Formation shallow-sea facies source rocks with obvious reflection characteristics are developed in Bao Dao, Changchang and Songnan Depression, and the buried depth of source rocks can reach more than 4S (6000 ~ 8000 m), which is beneficial to the large-scale maturity of deep source rocks in the depression. The basin simulation results show that the Eocene source rocks in the depression reached the peak of hydrocarbon generation from late Oligocene to early Miocene. However, the source rocks of Yacheng Formation had two hydrocarbon generation peaks in the early Miocene and Yinggehai Formation so far, and the total hydrocarbon generation was huge, which provided sufficient oil and gas sources for the structure.
There are many large faults in the structure, which can be used as the main channel for deep oil and gas migration to the structure; The faults strike mainly in NE direction and near EW direction, and the NE direction faults are mainly active from Eocene to early Oligocene, while the near EW direction faults are active from early Oligocene to the end of late Miocene, which runs through the main hydrocarbon generation peaks of two sets of source rocks and plays a key role in oil and gas migration. The pressure potential analysis shows that the structure is located in the pressure transition zone, which is beneficial to the migration of oil and gas to traps.
Yongle 3- 1 structure is divided into seven independent units by many faults, and its trap preservation conditions are greatly affected by the lateral sealing of faults. When analyzing the fault sealing of several main faults in the structural position, it is considered that the fault distance is generally large, and the delta sandstone of Yacheng Formation is completely disconnected on the upper and lower walls of the fault, forming an effective lateral sealing of the fault, and it is difficult for oil and gas in the trap to leak through the fault, so the overall preservation condition is good.
Figure 6-8 3700 Seismic Profile Crossing Yongle 3- 1 Tectonic Line
Figure 6-9 Yongle 3- 1 Structure Depth Structural Map of the Top of Yacheng Formation
The structural position where the early NE-trending fault and the late EW-trending fault meet is also a favorable place for the water system to meet. The large delta front facies sandstone developed in Yacheng Formation can be used as the main reservoir system of Yongle 3- 1 structure. The reservoir thickness is large, and there are obvious amplitude anomalies on the seismic profile, which overlap with the structural parts well. The sedimentary strata of the third member of the Upper Fuling Formation and the late Yacheng Formation are mainly shallow-sea mudstone with weak reflection, which forms a good reservoir-cap combination with the delta sandstone developed in the early stage.
The main reservoir-forming advantages of Yongle 3- 1 structure are good quality of seismic data near the structure (three-dimensional partial coverage) and large trap scale; The sag has obvious hydrocarbon source characteristics and abundant oil and gas sources. The main reservoir has deep burial, good sedimentary facies and obvious reflection characteristics, so it is speculated that the reservoir physical properties are superior. There are many long-term active faults in the structure, which can be used as the main channels for vertical migration of oil and gas; The amount of computing potential resources is enormous.
The main risk of the structure is that the trap type is fault block and the relative risk is high; At the same time, the main target layer is located in Yacheng Formation, which is a secondary exploration series, and the reservoir quality has not been confirmed by drilling.
(2) Lingnan low uplift
1. Oil and gas migration and accumulation and matching conditions
Lingnan low uplift is surrounded by depressions, which is the main place for oil and gas migration and accumulation. From the simulation analysis of pressure potential and oil and gas migration and accumulation, the southern deep water area is the main pressure relief area of ultra-high pressure body in the central depression and should be one of the main directions of oil and gas migration. There are differences between the eastern and western ends of Lingshui sag. The west is a double fault structure, and the east is a superstructure with faults in the north and superstructures in the south. The low uplift is located in Lingshui sag and Ledong sag, adjacent to the south slope of Lingshui sag. On the seismic profile, late structures and faults are developed, which connect deep hydrocarbon sources with shallow sand bodies. Oil and gas migration is driven by overpressure in the depression. Vertical fault cracks, contemporaneous depression-controlling faults and their adjacent fault slope fan sand bodies are vertical migration channels, while unconformity, delta and coastal sandstone on the slope uplift are horizontal migration channels.
The development horizons of structural traps in this structural belt are T70 to T40. Yacheng Formation began to deposit and some structures began to develop. Some structures began to develop in the sedimentary period of the third member of Ling, all of them were formed in the sedimentary period of the Formation (Late Miocene), and most of them were formed in the sedimentary period of Meishan Formation (Middle Miocene) or before. The gas production peaks in Ledong sag, Lingshui sag and Beijiao sag are all late, and the gas production peaks (or some gas production peaks) have a good matching relationship after or at the same time after the formation of this structural belt, which is conducive to oil and gas accumulation. It is predicted that there are the following reservoir-forming modes in this area: the hydrocarbon sources are mainly Eocene middle-deep lake facies, Yacheng Formation semi-closed shallow-sea mudstone and coastal plain facies around the marginal uplift of the depression, and the oil and gas generated in the depression migrate vertically along vertical faults, sag-controlling faults and their nearby fault slope fans, and then migrate laterally to the slope uplift along unconformity surfaces and delta and coastal sandstone, and accumulate in traps.
At present, three structural traps have been found in Lingnan low uplift, namely Lingshui 33- 1, 26-2 and 32- 1 structure (Figure 6- 10).
2. Lingshui 33- 1 structure
The Lingshui 33- 1 structure is close to the source area of the southern uplift, and the developed reservoirs include coastal and shallow-sea facies sand bodies of Lingshui Formation, coastal and shallow-sea facies sandstone of Meishan Formation and Huangliu Formation, and shallow-water platform. The reservoir development conditions are good, and the oil and gas detection in the target interval is abnormal. Its structural high point water depth is 1450m, and the maximum trap area is 74km2. 04E34036 and 04E44035 are two sections (Figure 6- 1 and Figure 6- 12) passing through Lingshui 33- 1 structure, showing that the reservoir-cap combination of Lingshui 33- 1 structure is good, and Liuhuang Formation is the caprock in its area, with Lingshui in some areas.
Lingshui 33- 1 structural sandstone and fault development. The F 1 fault in Lingshui 33- 1 structure is controlled by sedimentation, and the activity in the west section is stronger than that in the east section, and the fault ends before (2 1Ma). However, the eastern fault ended its activity before (23Ma), and the activity in the east ended earlier than that in the west. The F 1 fault is mainly connected with the shallow-sea mudstone of the second member of Xiapanling in the third member of Ling, and the lateral sealing condition is good. The fault distance of F4 fault activity is not large as a whole, and it is in the nature of tension and torsion, and the fault activity ended at the end of Lingshui period. F4 fault is butted with the coastal mudstone in the lower part of Pan Ling 1 lower member of Lingxia Formation, and the lateral sealing condition may be good. The southern section of the fault is butted with mudstone in the shallow sea of Ling-2 member, and the lateral sealing condition is good. The fault activity ended early, and there was no fault and tectonic movement damage in the later period after the trap was formed, so the trap was well preserved.
Figure 6- 10 Structural Location Map of Lingshui 33- 1, Lingshui 26-2 and Lingshui 32- 1
Figure 6- 1 1 Line Migration Profile of Qiongdongnan Basin 04e34036
The pressure coefficient of the main target layer of Lingshui 33- 1 structure is between 1. 1 ~ 1.3, which is the main direction of overpressure oil and gas migration in Lingshui sag and has good vertical and lateral migration conditions for oil and gas. Its migration mode is mainly along the longitudinal direction of the fault, but there are also transverse migration. The two key moments of oil and gas accumulation are 17.5Ma and 2.4Ma, and the structure is basically shaped in Miocene, which has a good matching relationship with oil and gas generation and migration.
The total resources of the main target layer of Lingshui 33- 1 structure are1150×108m3.
Fig. 6- 12 migration profile of line 04e44035 in Qiongdongnan basin
Fig. 6- 13 reservoir-cap assemblage characteristics of Lingshui 33- 1 structure
3. Lingshui 26-2 Structure
Lingshui 26-2 structure is located in the west low uplift of the reef in the south and north of Ledong-Lingshui sag, on the rising plate of the south of the sag 13 fault, adjacent to the hydrocarbon-generating sag.
The structural trap of Lingshui 26-2 is an overthrust anticline and anticline structure (Figure 6- 14 and Figure 6- 15), with a large scale and several traps vertically superimposed.
Lingshui 26-2 structure is far away from the source area of the southern uplift, and the main developed reservoirs are coastal shallow-sea sand bodies of Lingshui Formation, coastal shallow-sea sandstone of Meishan Formation and shallow-water platform of Huangliu Formation, and the reservoirs are relatively developed. 04E4402 1 AVD inversion profile shows that there are abnormal areas with high AVD, which are favorable gas-bearing areas. 04E4402 1 DR inversion profile also shows an abnormal area with high DR, which is a favorable gas-bearing area. Attached Figure 6- 16 shows that the reservoir-cap combination of LS26-2 structure is good, Liuhuang Formation is the caprock of this area, and there are mudstone of the second member of Lingshui Formation locally, and mudstone of Sanya Formation and Huangliu Formation is also a good caprock.
Fig. 6- 14 migration profile of line 04e34032 in Qiongdongnan basin
Fig. 6- 15 migration profile of line 04e440 19 in Qiongdongnan basin
In Lingshui 26-2 structure, the main fault F 1 is a subsidence-controlling fault in depression period, and it is active in Yacheng period, with the maximum fault distance of 1800m, which slows down in Lingshui period and basically ends at the end of Lingshui period. The F 1 of the third member of the target layer is connected with the shallow sea mudstone of the second member of the downstream layer; Ling-1 member and Sanya-2 member are butted with shallow mudstone, and the lateral sealing condition of the fault is good. The F2 fault activity ended early. At the end of Lingshui period, the first member of Lingshui Formation was connected with the second member of Sanya Formation in the footwall, which may have good lateral sealing conditions. There may be a risk of side sealing when the third member of Ling is docked with the coastal stratum of the first member of Downstream. Lingshui 26-2 structural fault activity ended early, and there was no fault and tectonic movement damage in the later period, so the preservation conditions were good.
Figure 6- 16 Reservoir-Cover Combination Diagram of Lingshui 26-2 Structure
Lingshui 26-2 is close to the center of Lingshui sag, with a migration distance of about 10km, which has very favorable oil and gas capture conditions, fault communication and good hydrocarbon source and migration conditions.
The pressure coefficient of the main target layer of Lingshui 26-2 structure is between 1. 1 ~ 1.5, which is the main direction of overpressure oil and gas migration in Lingshui sag. Oil and gas have good vertical and horizontal migration conditions. Its migration mode is mainly along the longitudinal direction of the fault, but there are also transverse migration. The two key moments of reservoir formation are 17.5Ma and 2.4Ma, but the structure was formed in Miocene and basically shaped, which has a good matching relationship with oil and gas generation and migration. However, the structure is far from the southern uplift, and the reservoir has certain risks. Its total resources are 1049× 108m3.
4. Lingshui 32- 1 structure
Lingshui 32- 1 structure is located in the western low uplift in the northern suburb of southern Lingshui sag, and its formation mechanism is similar to that of 33- 1.
The structural trap of Lingshui 32- 1 is an overthrust anticline and stratigraphic overlapping structure (Figure 6- 17 and Figure 6- 18). The scale is smaller than the first two structures, but the buried depth is shallow, and the Neogene Meishan Formation and Sanya Formation are larger. Lingshui 32- 1 reservoir is composed of coastal and shallow-sea facies sand bodies of Lingshui Formation, coastal and shallow-sea facies sandstone of Meishan Formation and Huangliu Formation, and shallow-water platform. Lingshui 32- 1 structure is characterized by abnormally high AVD and DR and low wave impedance in Meishan Formation, Sanya Formation, Lingshui Formation and its basement. Lingshui 32- 1 also has obvious AVD, DR and three AVO high anomalies in the shallow layer of 2.76s, which are likely to contain oil and gas, and are the representatives of the oil and gas system in Ledong sag.
Lingshui 32- 1 sandstone and faults are developed, which has good vertical and horizontal migration conditions for oil and gas. The F 1 fault in Lingshui 32- 1 structure is characterized by sedimentary-controlled contemporaneous faults. The early fault activity was strong, and the late fault activity gradually weakened, and the fault activity basically ended at the end of Sanya. The early fault activity was strong, and the Lingshui formation and the second member of Sanya in the high part of the structure were partially missing. The structural position of Ling-3 member is butted with the shallow-sea mudstone of Sanya-2 member in the falling plate, and the lateral sealing condition is good. The first member of Ling is connected with the lower stratum of the first member of Sanya. The side seal is in good condition; The northern block of the structure is a stratigraphic overlap trap developed on the inherited uplift, and the Ling 1 and Ling 3 members are laterally closed by basement granite. The first section of Sanya, adjacent to the shallow sea of the descending Panmeishan Formation, may have good side sealing conditions. There is a flame chimney in the late stage of the northern block of the structure, which may have an influence on the early accumulation of oil and gas in the underlying strata; The south block structure has no late fracture damage and good preservation conditions (Figure 6- 19).
The pressure coefficients of the main target layer of Lingshui 32- 1 structure are 1. 1 ~ 1.5 respectively, and the main target layer pressure of Lingshui 32- 1 structure is positive pressure, which is the main direction of overpressure oil and gas migration in Ledong and Lingshui sag. Lingshui 32- 1 structure took a long time to form, and Paleogene, fault block and Neogene fault anticline traps developed on the basement uplift. There are two critical moments of oil and gas accumulation, 17.5Ma and 2.4Ma, and there is a good time-space matching relationship among all the factors. The resource is 856× 108m3.
Fig. 6- 17 Line migration profile of Lingshui 32- 1 structure 04e3402 1
Figure 6- 18 Line migration profile of Lingshui 32- 1 structure 04e4403 1
Figure 6- 19 Lingshui 32- 1 Structural Reservoir-Cover Combination Diagram
(3) southern uplift
1. Reservoir and caprock conditions
The third member of Ling (T70~T62) is a southern uplift area with high terrain, shallow water and thin sediments. Seismic facies is characterized by medium-strong amplitude parallel continuous reflection, mainly coastal facies deposition, and fan delta is developed locally. The second member of Ling (T62 ~ T6 1) is mainly developed with shallow marine sediments, coastal sediments and fan delta sediments in the eastern margin, and coastal sediments in the west. The return began after Ling Er. The first member of Ling (T6 1~T60 ~ T60) has small buried depth, thin stratigraphic deposits and extensive coastal facies deposits. The northeastern part of the uplift is dominated by shallow-sea facies, and the development range of shallow-sea facies is smaller than that of Ling-2 member, and coastal facies sediments are developed locally in the eastern part. After T60, the sea level has been rising continuously, but the topography of the uplift area is higher. In the early stage of paleouplift, the second member of Sanya Formation was exposed as denudation area. The strata on the uplift are thin, and coastal facies deposits are widely developed, and only a small area of shallow sea facies deposits are developed in the northeast. During the sedimentary period (T52~T50), the water bodies in most areas of uplift became shallow, and the sedimentary thickness was thin, belonging to coastal facies. During Meishan Formation (T50~T40), a large area of reefs developed in the eastern part of the bulge, and the reefs covered all the bulges together with a large area of reef deposits. After the deposition of Liuhuang Formation, the basin subsided rapidly, which is a typical shelf-slope system with more mudstone and smaller particle size. Semi-deep-sea sediments are developed in a large area in deep water area, and all of them are covered by semi-deep-sea strata from depression to low uplift. There are three main types of reservoirs in Yongle uplift, namely coastal sandstone, delta sandstone, reef and beach limestone. * * * Four sets of reservoir-cap assemblage are developed: ① reservoir-cap assemblage of coastal sandstone, fan delta sandstone and shallow sea mudstone in Ling-3 member; ② Reservoir-cap combination of coastal sandstone in Ling-1 member and shallow-sea mudstone in Sanya-2 member; ③ Reservoir-cap assemblage of interbedded sandstone and mudstone in Sanya Formation; ④ Reservoir-cap assemblage of Meishan Formation reef, reef beach limestone and Liuhuang Formation semi-deep marine mudstone.
2. Oil and gas migration and accumulation and matching conditions
Yongle uplift is adjacent to Changchang sag in the east and Beijiao sag in the west. Located in the southern uplift area with high terrain and low potential energy, it is a favorable place for oil and gas migration and accumulation. The northern suburb sag is faulted in the south and overtopped in the north, and Yongle uplift is located in the steep slope direction of the eastern part of the northern suburb sag, which is one of the directions of oil and gas migration in the sag. Graben-type Bao Dao sag is located in the north of the uplift, separated by Lingnan low uplift, and there are two steps from Bao Dao sag and Songnan sag to the south uplift. Driven by overpressure, the oil and gas generated in Bao Dao sag crossed two steps of Lingnan low uplift and reached Yongle uplift with higher terrain to form oil and gas reservoirs. Changchang sag is a graben sag, which is connected with Yongle uplift. Yongle uplift is one of the main directions of oil and gas migration in this depression because of its high terrain and low potential energy. On the seismic profile, we can see the inherited development of faults formed in the early stage, which links deep hydrocarbon sources with shallow sand bodies. Oil and gas migration is driven by overpressure in the depression, faults are longitudinal migration channels, and unconformity surfaces on the bulge and delta and coastal sandstone are transverse migration channels. It is predicted that there are the following reservoir-forming modes in this area: the hydrocarbon sources are mainly Eocene middle-deep lake facies in the depression, semi-closed shallow sea mudstone of Yacheng Formation and coastal plain facies around the edge uplift of the depression, and the oil and gas generated in the depression migrate longitudinally along the fault, then laterally to the slope uplift, and then migrate to the trap along the unconformity surface and delta and coastal facies sandstone to form reservoirs.
(4) Changchang sag
1. Oil and gas migration and accumulation and matching conditions
The internal faults in Changchang sag are relatively developed, and oil and gas can migrate and accumulate in short distance, which is a favorable place for oil and gas migration and accumulation. There are two main ways of oil and gas migration in Changchang sag: one is to communicate directly with underlying source rocks through fault structures, and oil and gas directly reach traps through vertical migration of faults to form oil and gas reservoirs; The other is that oil and gas migrate vertically through faults, and then migrate laterally through unconformity and transverse communication sand bodies to reach traps to form oil and gas reservoirs. Due to the development of faults in the depression, most of the structural formation is related to the growth normal faults formed in the early stage, so the first migration mode is the main migration mode. At present, the structures and traps found in this structural belt are T70~T50. Since the deposition of Yacheng Formation, some structures began to develop, and some structures began to develop during the deposition of Ling-3 Member. All structures were formed during the deposition period of Meishan Formation (Middle Miocene). The gas production peaks in Changchang sag are all late. After the formation of this structural belt, the gas production peaks or some peaks have a good matching relationship, which is beneficial to oil and gas accumulation.
2. Changchang 33- 1 structure
Changchang 33- 1 structure is located in the middle of Changchang sag, and it is in a anticline state with four sides dipping, which is related to fault development. The structure has developed T60, T6 1, T62, T70, T 100 multi-layer traps from top to bottom, and the depth domain area is 36 ~ 122km2. The trap types are faulted anticline and anticline respectively (Figure 6-20, Figure 6-2 1). The structure is divided into two high points, the east high point of each trap layer is developed, and the west high point is only developed at T6 1 and later, with a small area. Traps mainly extend in the northwest-southeast direction. However, T60 trap is NE-oriented in the west and NW-oriented in the east. The east and west wings of the structure are controlled by two groups of faults with different strike, and the top is cut by faults, but the whole structure is not cut into many blocks.
Figure 6-20 Seismic Profile of Chang 33- 1 Structural High Point
Fig. 6-2 1 Chang 33- 1 Oil and gas accumulation pattern diagram
Gas chimneys are developed at the west high point, and there are bright spots in the shallow layer. Before Miocene, the fault poles developed near the structure, and the fluid was easy to discharge, so it was difficult to form the fluid diapir. Therefore, its formation time may be after the formation of the regional cap layer, that is, after T40; The anticline is developed in the basement of the east high point, and the structural amplitude is strong, which weakens and disappears to the west, and the strong amplitude is within the structural range. The east high point structure is formed by extrusion. The compression occurred in the Yacheng period of Oligocene. It shows that the eastern part of the structure is high, but the Eocene and Yacheng Formation are thick, with low reactive deposition and rising in the later stage.
Late faults are developed in the 1) structure, which connects the hydrocarbon source layer with the deep target layer.
The structure is located near the middle part of the depression, and there are many late faults, which connect hydrocarbon sources and deep target layers. Vertical migration of oil and gas is the main way, and the vertical migration of oil and gas in the middle of the depression is active and lasts for a long time, which has favorable conditions for near-source reservoir formation. The pressure prediction results show that the formation pressure in Chang 33- 1 anticline structure is low, which is beneficial to the migration and accumulation of oil and gas generated by high-pressure hydrocarbon source beds in the depression.
2) The formation and development of structures are well matched with the time and space of oil and gas generation, migration and accumulation.
Faults are mainly active in Yacheng period, Lingshui period and Sanya period, and as a channel, they are beneficial for traps formed in Yacheng period, Lingshui period and Sanya period to capture oil and gas. In addition, there is a gas chimney at the west high point as a channel, which can capture oil and gas from the deposition period of Yinggehai Formation to the peak of hydrocarbon generation and expulsion in the second phase (Figure 6-2 1).
3) Good storage conditions
Chang 33- 1 is located in the center of the depression, with anticline shape and downward dip on all sides, and the overall preservation conditions are good. The fault strata in the northeast gradually become higher and flatter, which may lead to the leakage of oil and gas to the north slope here. However, from the structural map, the terrain on the west side of the fault is higher, while the strata on the east side are inclined downward, so even if two sets of sandstone are close to oil and gas, there will be no large-scale leakage. The later faults basically did not affect the main part of the structure.
4) The oil-gas bearing characteristics are obvious.
Chang 33- 1 west high point shows strong amplitude anomaly, which may be gas-bearing anomaly through hydrocarbon absorption detection and AVO attribute intersection analysis. Changchang 33- 1 East high alluvial fan has strong amplitude. After cluster analysis, the structural position is abnormal, indicating that it may contain oil and gas.
The reservoir-forming conditions of Chang 33- 1 structure have the following characteristics: the characteristics of oil-generating layers are obvious, and the total hydrocarbon generation is huge; Good trap type, multi-layer and large scale; Vertical oil and gas migration channels are developed; The potential resources are huge; Because of the long history of the reservoir, the reservoir of Lingshui Formation may be thin. Alluvial fan reservoir may be affected by buried depth, resulting in a certain loss of reservoir quality, and due to the large water depth and high geothermal gradient, the bottom hole temperature is high, and the overall well depth is too large, which requires higher drilling engineering.
Chang 33- 1 has a large amount of structural resources and has the potential to become the first breakthrough point in deepwater exploration. The calculated potential natural gas resources are 1925× 108m3, or the potential oil resources are 9.5× 108m3.
3. Wanning 3- 1 structure
Wanning 3- 1 structure is located in the middle of Changchang sag and belongs to block 53/30. The structure is located on the background of a large slope rising eastward, which is relatively high in the east. The trap types belong to structural+lithologic trap, T52 anticline trap, T52 fan body and T52U trap at the top of fan body. The top or updip direction of T52U and T52 traps is cut by waterways, and the bottom boundary of waterways in different parts is basically horizontal, indicating that the depth of channel undercut has little change in the structural area. The deeper undercut horizon of the East Waterway is related to the shallow T52U and T52 traps in the east. T52U and T52 are southeast channel side-sealed traps with moderate burial depth and large scale, which are mirror images of depression and uplift, and were formed by later extrusion. From bottom to top, the structural amplitude and trap area become larger, which can also reflect the characteristics of later compression (Figure 6-22).
Wanning 3- 1 structure is located in the middle of Changchang sag, a large hydrocarbon-generating sag, where huge Eocene lacustrine facies and Oligocene Yacheng Formation marine source rocks are widely developed, with obvious characteristics of source beds. The basin simulation results show that the overall maturity of source rocks in Changchang Depression is high, and the total hydrocarbon generation amount can reach 801.1×108t, which has good hydrocarbon source conditions.
Wanning 3- 1 Structure Sanya Formation has multiple sets of reservoir-cap combinations, which can be formed by turbidite fan reservoir developed in the second member of Sanya Formation and its overlying shallow-sea mudstone, and channel sand reservoir developed in the second member of Sanya Formation and its overlying shallow-sea mudstone. The basin floor fan developed in the first member of Sanya Formation and the overlying shallow sea mudstone can form a reservoir-cap combination. Figure 6-23 shows that Wanning 3- 1 structural reservoir-cap combination is good. Turbidite fan and channel sand developed in the second member of Sanya Formation and basin floor fan developed in the first member of Sanya Formation are all good reservoirs. The mudstone of Liuhuang Formation is a regional caprock, and the overlying shallow sea mudstone of Sanya Formation is also a good caprock.
Figure 6-22 Wanning 3- 1 Structure T52U (Top of Fan) Isochronous Diagram
Figure 6-23 Typical Characteristic Map of Wanning 3- 1 Structural Reservoir-Cap Combination
The whole structure is in a muddy sedimentary background environment, surrounded by muddy surrounding rocks, and the preservation conditions are good. However, the structure is cut by waterway, so it is necessary to analyze the sealing performance of waterway in detail. Drilling in the western section of the waterway (YC35- 1- 1, 2) reveals the development of sandstone in the upper part of the waterway, which shows continuous strong reflection on the seismic profile, while the mudstone-dominated interval (including thin sand layer) in the lower part shows discontinuous weak reflection. The seismic survey lines collected along the waterway show that the sandstone reflection in the upper part of the waterway is a strong reflection axis with good continuity and extends far away. The lower part is weak reflection discontinuous mudstone. Generally speaking, it is considered that the reflection characteristics of the filling deposits in the waterway at the east and west ends are similar, and they should have similar lithologic distribution characteristics, so they can be compared interactively. Based on the above understanding of lithologic distribution in the river channel, through the analysis of seismic profiles of cutting fans one by one and wave impedance inversion, it is concluded that the thick mudstone at the bottom of the river channel has formed a good seal on the sandstone of the basin bottom fan, which has good preservation conditions.
Faults are developed in the structural parts, which communicate hydrocarbon sources with deep and shallow target layers. Oil and gas generated from the deep source rocks in the depression can form effective vertical migration through faults, which has favorable conditions for near-source accumulation (Figure 6-24).
The pressure coefficient of the main structural target layer is between 1.0 ~ 1.5, which is the main direction of overpressure oil and gas migration in Changchang sag. Oil and gas have good vertical and horizontal migration conditions. Its migration mode is mainly vertical migration along the fault, which is active and lasts for a long time, and there are also lateral migration. The two critical moments of oil and gas accumulation are 23Ma and 7.5Ma respectively, and the structure is basically shaped in Miocene, which has a good matching relationship with oil and gas generation and migration.
Figure 6-24 Wanning 3- 1 Structural Oil and Gas Accumulation Model