E-mail:billyu@cugb.edu.cn Bill Yu (1958-), male, professor, doctor, mainly engaged in oil and gas exploration and development research.

1. School of Energy, China Geo University, Beijing 100083.

2. Guangzhou Marine Geological Survey, Guangzhou 5 10760.

The quantitative simulation study on the history of tectonic subsidence in the deep water area of the Pearl River Mouth Basin shows that the tectonic subsidence in this area has gradually accelerated from north to south and from west to east since the late Miocene. From late Miocene to Pleistocene, the deep water area of the basin experienced a process of tectonic subsidence from weak to strong: late Miocene (1 1.6 ~ 5.3 Ma), with an average tectonic subsidence rate of 67 m/Ma, and Pliocene (5.3 ~ 1.8~0 Ma), with an average tectonic subsidence rate of 68 m. The main reasons for these changes are Dongsha movement in the late Miocene to the late Miocene and Taiwan Province movement in the early Pliocene to Pleistocene. Dongsha movement (10 ~ 5 Ma) caused the basin to rise and fall, uplift and denudation, and the movement intensity and structural deformation gradually weakened from east to west, which made the deep water area of the basin continue to settle steadily. The movement of Taiwan Province Province (3 Ma) completely changed the structural pattern of the deep water area of the basin. Due to the adjustment of gravity balance, the deep water area of the basin continues to sink, and the deeper it sinks to the south. Superimposed analysis of BSR development zone and subsidence rate plan shows that more than 80% of BSR area is distributed in the area where the tectonic subsidence rate is 75 ~ 125m/ma and the subsidence rate changes rapidly.

Key words: Zhu Ⅱ depression; Deep water area; Quantitative simulation; Tectonic subsidence; BSR

Characteristics of tectonic subsidence in deep water area of Pearl River Mouth Basin since late Miocene and its influence on BSR distribution

Yu Xinghe 1, Liang Jinqiang 2, Fang Jingnan 1, Jiang Liangyan 1, Cong Xiaorong 1, Wang Jianzhong 1

1. School of Energy, China Geo University, Beijing 100083.

2. Guangzhou Marine Geological Survey, Guangzhou 5 10760, China.

Abstract: Based on the quantitative basin simulation study on the history of tectonic subsidence in the deep water area of the Pearl River Mouth Basin, it is found that the tectonic subsidence in the study area has generally accelerated from north to south and from west to east since the late Miocene. From the late Miocene to the Pleistocene, the deep water area of the basin experienced a process from weak to strong tectonic subsidence effect: the average tectonic subsidence rate was 67m/Ma in the late Miocene (1 1.6~5.3 Ma) and 68m/Ma in the Pliocene (5.3~ 1.8 Ma). In addition, these changes are mainly caused by the Dongsha tectonic event from the end of Middle Miocene to the end of Late Miocene and the Taiwan Province tectonic event from Pliocene to Early Pleistocene: Dongsha tectonic event (10~5 Ma) caused the fault block to heave and sink, and the uplift area suffered erosion, and the movement intensity and structural deformation weakened from east to west, resulting in the stable settlement of the deep water area of the basin; The Taiwan Province tectonic event (3 Ma) completely changed the structural pattern of the deep-water area of the basin, and the deep-water area continued to sink and sank southward due to gravity balance. Superimposing the developed area of BSR with the plan of tectonic subsidence rate, it is found that more than 80% of BSR tends to be distributed in areas where the average tectonic subsidence rate changes rapidly from 75 m/Ma to 125m/Ma.

Key words: Zhu Ⅱ depression; Deep water area; Quantitative basin simulation; Tectonic subsidence; BSR

1 regional geological background

The term "deep water (sea)" usually refers to the deep-water environment (water depth > > 200 m) located on the seaward side of the continental shelf slope, including continental slope, continental uplift and deep-sea plain. After the middle Miocene, Dongsha Uplift quickly subsided and entered the uncompensated sedimentary period, which can accommodate the rapid growth of space.

Table 1 Stratigraphic division of the Pearl River Mouth Basin, so the approximate value of △SL is 0.

2.5 Calculation results of tectonic settlement

In this study, according to the above principles, using the data of stratum, lithology, water depth and sea level in the study area, and using PRA basin simulation software, the structural subsidence of the deep water area 172 virtual points in the Pearl River Mouth Basin was calculated, and the history of structural subsidence in the study area was quantitatively restored. The plane subsidence characteristics of four typical sections and three sets of strata are analyzed, and the structural evolution characteristics of the deep water area of the Pearl River Mouth Basin and its influence on the distribution of BSR are discussed. For example, after the subsidence history of Shen 72 structure is restored, the total subsidence rate and tectonic subsidence rate at different times at this point are obtained (Table 4).

Table 4 Calculation results of structural settlement of Shen 72 in virtual well point

3 simulation results analysis

3. 1 historical characteristics of single well settlement

The settlement history is calculated and analyzed by using the virtual points selected on the two-dimensional seismic exploration line. Taking the imaginary point God -72 in Shunde sag, imaginary point God -23 in Liwan sag, imaginary point God-152 in Baiyun sag and imaginary point God-1 17 in southern uplift as examples, the burial history and subsidence history of the deep water area of the basin are quantitatively simulated. These points are basically located in the center of each structural unit and can be used to analyze the evolution characteristics of structural subsidence of each depression or uplift.

The tectonic subsidence rate of Shen 72 in the southwest margin of the deep-water area of the Pearl River Mouth Basin is quite different in different periods, with 104 m/Ma in the late Miocene, 43 m/Ma in Pliocene and only 23 m/Ma in Pleistocene (Figure 4A). This shows that the tectonic movement of Shunde sag is getting weaker and weaker with time.

Shen 23 (Figure 4B) in the southeastern margin of the basin has similar characteristics to Shen 1 17 (Figure 4C) in the southern margin: the late Miocene tectonic subsidence rate of Shen 23 was 87 m/Ma, Pliocene was 100 m/Ma and Pleistocene was 76 m/ma; ; The late Miocene tectonic subsidence rate of Shen-1 17 was 54 m/Ma, Pliocene was 63 m/Ma and Pleistocene was 45 m/Ma. This slow-fast-slow settlement rate shows that both Liwan sag and southern uplift experienced a settlement peak in Pliocene, and then the settlement weakened.

The Shen-152 point in the northeast of the deep water area of the basin is different from the first three points, and its tectonic subsidence rate has little change: 7 1 m/Ma in the late Miocene, 72 m/Ma in the Pliocene and 72m/Ma in the Pleistocene (Figure 4D). This shows that Baiyun sag has been in a stable subsidence period since the late Miocene, and the Neogene structural changes in this area are not too great.

The subsidence characteristics of each depression in deep water area are different, and they have their own unique burial history and subsidence history, but they generally show the characteristics of continuous and stable subsidence.

Combined with 2D seismic data, it can be concluded (Table 5) that the sedimentation rate of four imaginary points in each period is less than the sedimentation rate, which indicates that the deep water area of the basin has the function of under-compensated sedimentation recharge and the sedimentation rate is relatively high. Rapid subsidence and insufficient compensation led to the formation of deep water area in the basin.

Fig. 4 curves of burial history and settlement history of virtual points in the study area.

Table 5 Typical single-point settlement rate and sedimentation rate m/Ma in deep water area of Pearl River Mouth Basin since late Miocene

3.2 Comparison of settlement rate of structural units

By analyzing the subsidence rate values of various depressions in the deep water area of the Pearl River Mouth Basin since the late Miocene (Table 6), it is not difficult to find that there are obvious differences in subsidence rates of various structural units. The maximum subsidence rate in the late Miocene occurred in Baiyun sag, with a tectonic subsidence rate of 65438±0.20m/Ma/ma and a total subsidence rate of 208 m/ma. . The maximum settlement rate of Pliocene occurred in Liwan sag, with a tectonic settlement rate of 152 m/Ma and a total settlement rate of 200 m/ma. . The maximum subsidence rate in Pleistocene is still in Liwan sag, with the tectonic subsidence rate of 122 m/Ma and the total subsidence rate of 167 m/Ma. This shows the plane migration law of the subsidence center of the basin: in the late Miocene, the subsidence center was located in Baiyun sag in the north of the basin; The Pliocene-Pleistocene subsidence center moved to Liwan sag in the east.

Table 6 Settlement rate of each structural unit since late Miocene in deep water area of Pearl River Mouth Basin

3.3 Historical characteristics of basin subsidence

The basin simulation results show (Figure 5): In the basin subsidence process since the late Miocene, the structural subsidence in the T3-T2 subsidence period, that is, the late Miocene, is the weakest, with an average structural subsidence rate of 67 m/Ma. This is consistent with the Dongsha movement (10 ~ 5 Ma) from the end of Middle Miocene to the end of Late Miocene. Dongsha movement caused the basin to rise and fall, uplift and denudation during the rise and fall, accompanied by compressive fold faults and frequent evolutionary activities, and developed NWW-trending faults with strong tectonic activity. Therefore, in the late Miocene, various structural units of the basin have undergone different degrees of subsidence.

From the late Middle Miocene to Holocene, the basin experienced the change process of tectonic subsidence amplitude from small to large, tectonic subsidence amount from large to small, and tectonic subsidence rate from small to large, showing a gradient change trend, showing the dynamic background of the extensional basin. After the uplift and denudation of the basin at the end of Middle Miocene, the late Miocene basin entered the stage of fault block uplift, and the subsidence amplitude and rate began to increase, and the accommodation space increased. The average tectonic subsidence rate of Pliocene was 68 m/Ma, which was not obvious compared with that of Late Miocene. By the Pleistocene, the average tectonic subsidence rate was 765,438+0 m/ma, and the basin tectonic activity was enhanced.

3.4 Settlement History of Configuration Files

Four typical sections located in different positions in the deep water area of the basin are selected to calculate the structural settlement, and the structural evolution characteristics in the longitudinal and transverse directions are analyzed. Generally speaking, the tectonic subsidence rate tends to increase from land to sea, and gradually becomes faster from west to east, which is consistent with the plane subsidence characteristics of the deep water area of the basin.

Section A is located in the southwest of the study area, passing through Kaiping sag, Shenhu uplift, Shunde sag and southern uplift from northwest to southeast. In the late Miocene, from Kaiping sag to Shenhu uplift, the tectonic subsidence rate decreased until Shunde sag dropped to 42 m/Ma, and then increased until the southern uplift reached more than 100 m/Ma. In Pliocene, the tectonic subsidence rate first increased from about 50 m/Ma to 73 m/Ma in Kaiping sag-Shenhu uplift-Shunde sag, then experienced a weak decline process in Shunde sag, and finally decreased to 60 m/Ma at the junction of Shunde sag and southern uplift, and then began to rise sharply until it was above 90 m/Ma. The characteristics of tectonic subsidence in Pleistocene are similar to those in Pliocene and have good inheritance. After rising from 45 m/Ma to 76 m/Ma, it fell to 72 m/Ma at the intersection of Shunde sag and southern uplift, and then the tectonic subsidence rate rose rapidly, reaching more than 105m/Ma (Figure 6).

Section B is located in the eastern part of the central part of the study area, passing through Panyu low uplift, Baiyun sag, Baiyun low uplift, Liwan sag and southern uplift from north to south. The change rules of the three periods tend to be consistent: in Panyu low uplift-Baiyun sag, the tectonic subsidence rates of late Miocene, Pliocene and Pleistocene increased from about 60 m/Ma, 32 m/Ma and 39 m/M a to about 80 m/Ma, 78 m/Ma and 79 m/Ma respectively, while in Baiyun sag-Baiyun low uplift-Liwan sag, the tectonic subsidence rates were different.

Section C is located in Dongsha Uplift in the southeast of the study area. The tectonic subsidence rate of Dongsha Uplift increased slowly from land to sea in three periods, and the tectonic subsidence rate of Late Miocene, Pliocene and Pleistocene increased from 100m/ma, 165, 438+05 m/ma and 120m/ma to 135m/ma and respectively.

Fig. 5 Histograms of settlement amplitude (a), settlement amount (b) and settlement rate (c) at different times in the deep water area of the Pearl River Estuary.

Section D runs through the whole study area from southwest to northeast, and passes through Shenhu uplift, Shunde sag, southern uplift, Baiyun sag, Baiyun low uplift and Dongsha uplift. In the late Miocene, the tectonic subsidence of Shenhu uplift slowly decreased to 40 m/Ma in Shunde sag, then quickly increased to about 55 m/Ma, and then stabilized. After the southern uplift slowly dropped to 45 m/Ma, it began to rise rapidly from the intersection of the southern uplift and Baiyun sag, and began to decline after Dongsha uplift reached the highest value of 93 m/Ma, which was related to Dongsha movement, which made Dongsha uplift rise and denude, with the following characteristics. The change trend of Pliocene and Pleistocene tectonic subsidence rate in section D is similar to that in late Miocene, but the difference is that the tectonic subsidence rate of Shenhu uplift-Shunde sag experienced a rapid increase from about 43 m/Ma and 38 m/Ma to about 72 m/Ma and 80 m/Ma from northwest to southeast, and then quickly decreased to about 54 m/Ma and 60 m/Ma. Then it conforms to the characteristics of late Miocene tectonic subsidence: after a relatively stable subsidence period, it slowly decreased to 43 m/Ma and 42 m/Ma in the southern uplift area, then quickly increased to 100m/Ma, 95 m/Ma and then decreased to 56 m/Ma and 85 m/Ma (Figure 9).

Comparative profile of tectonic subsidence rate in different periods.

Fig. 7b B is a comparative profile of tectonic subsidence rate in different periods.

Fig. 8 C Comparative sectional view of tectonic subsidence rate in different periods.

Fig. 9D comparison profile of tectonic subsidence rate in different periods

3.5 Historical analysis of plane subsidence and its relationship with BSR

Natural gas hydrate usually has a strong reflection wave on seismic profile, which is roughly parallel to the seabed, so it is called BSR. It is a strong seismic reflection formed by the interaction between the high impedance of hydrate sediments and the low impedance of underlying sediments, and it is the main geophysical indicator of gas hydrate enrichment and deposition. At present, it is considered that BSR has become an important evidence to judge the existence of natural gas hydrate in the ocean and find its distribution [3 1].

Figure 10 Late Miocene (A), Pliocene (B), Pleistocene (C) and the superposition of structural subsidence rate and BSR in the deep water area of the Pearl River Mouth Basin since the late Miocene (D).

Generally speaking, the tectonic subsidence rate in the deep water area of the Pearl River Mouth Basin is gradually weakening from east to west and from south to north (Figure 10). In the late Miocene, BSR was distributed in the deep sea (generally the water depth was greater than 2 000 m), and the tectonic subsidence rate was mainly 75 ~ 1 15 m/ma (Figure 10(a), Table 7). In Pliocene, BSR was distributed in the area with dense tectonic subsidence rate curve and basin boundary, and the corresponding tectonic subsidence rate was 45 ~ 135 m/ma (Figure 10(b), Table 7). In the Pleistocene, BSR did not exist (Figure 10(c), Table 7). In a word, it is found that more than 80% of BSR distribution tends to the area where the tectonic subsidence rate is mainly 75 ~ 125m/ma and the subsidence rate changes rapidly (Figure 10(d)).

Relationship between structural subsidence and BSR in deep water area of Pearl River Mouth Basin

4 discussion

After the late Miocene, the basin entered the stage of neotectonic movement and thermal subsidence depression, and the East Philippine plate subducted and napped in NNW direction, resulting in the Dongsha movement in the late Miocene and early Pliocene. Dongsha movement is the root cause and power source of fault block fracture, uplift and denudation, compression fold fracture and magmatic activity in the basin. In the process of basin subsidence, a series of faults dominated by NWW tension and torsion were produced. From east to west, the intensity and structural deformation of Dongsha movement gradually weakened, resulting in the ups and downs of the eastern Pearl River Mouth Basin and the late activities of faults. During the Pliocene-Early Pleistocene (3 Ma) movement in Taiwan Province Province, due to the adjustment of gravity balance, the deep water area of the Pearl River Mouth Basin continued to sink, and the deeper it went to the south, the greater the settlement became.

In each geological period, the tectonic subsidence of the basin accounts for more than 1/2 of the total subsidence, which shows that tectonic subsidence always controls the change of the total subsidence of the basin, thus controlling the change of the basin's accommodation space, further controlling the sedimentary filling of the basin, and finally affecting the formation of source rocks and the distribution of reservoirs in the basin.

5 conclusion

The denser the isoline of settlement rate, the easier it is to develop BSR. This is because isolines are generally concentrated at the boundary of the basin or at the intersection of depression and uplift. The subsidence rate in these places changes rapidly, and faults and folds are developed, which may form special tectonic environments and structures such as fault zone, mud diapir, rapid accumulation body, slump body and accretionary wedge. The high sedimentation rate zone can provide a large accommodation space, which is beneficial to the rapid accumulation of sediments and the formation of BSR. The reason why BSR did not exist in Pleistocene is that after the tectonic movement stopped, the tectonic activity in the basin weakened, the tectonic subsidence rate changed little, the accommodation space was small, the sedimentation rate was low, and the organic debris could not be buried quickly, so it was easy to be oxidized and decomposed directly on the seabed.

The subsidence characteristics of each depression in the deep water area of 1) basin are different, and they have their own unique burial history and subsidence history, but they generally show continuous and stable subsidence characteristics.

2) The deep water area of the basin has the function of under-compensated sediment replenishment, and the settlement rate is relatively high. This shows that rapid subsidence and insufficient compensation led to the formation of deep water area in the basin.

3) Late Miocene, the subsidence center was located in Baiyun sag in the north of the basin; The Pliocene-Pleistocene subsidence center moved to Liwan sag in the east.

4) Dongsha movement from the middle Miocene to the end of Late Miocene (10 ~ 5 Ma) caused the fault block fracture, uplift and erosion in the deep water area of the basin, and the tectonic activity was strong, which made the deep water area of the basin continue to settle in the late Miocene. The movement of Taiwan Province Province from Pliocene to Early Pleistocene (3 Ma) completely changed the structural pattern of the deep-water area of the basin, and the deep-water area of the basin continued to sink, sinking deeper and deeper to the south.

5) Tectonic subsidence controls the change of the total subsidence of the basin, thus controlling the change of the accommodating space of the basin, thus controlling the sedimentary filling of the basin, and finally affecting the formation of source rocks and the distribution of reservoirs in the basin.

6) The area with high deposition rate can provide larger accommodation space, which is beneficial to the rapid accumulation of sediments and the formation of BSR.

Acknowledgement: Sha Zhibin and Nanchizi of Guangzhou Geological Survey provided relevant information and help for this study. Thank you!

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