(Guangzhou Marine Geological Survey Guangzhou 5 10760)

Brief introduction of the first author: Gao, female, 197 1 born, master, senior engineer, mainly engaged in comprehensive analysis and research of petroliferous basins.

Using Petrosys basin simulation system, the subsidence process of three main depressions in the Pearl River Mouth Basin is quantitatively and dynamically simulated, and the relationship between the change of subsidence rate and the development of source rocks, reservoirs and caprocks is discussed. It is considered that the tectonic subsidence history of the Pearl River Mouth Basin has the characteristics of episodic and multi-stage changes. The first and second acts of subsidence are the main periods of basin development, which laid the structural framework of the basin and formed the main sedimentary strata and oil and gas resources of the basin. The third act of subsidence is the stage of transformation and perfection of the basin and the main period of the development of regional caprock in the basin.

Quantitative simulation of episodic subsidence in Pearl River Mouth Basin

1 overview

Figure 1 Location Map of Pearl River Mouth Basin

Figure 1 Location Map of Pearl River Mouth Basin

The Pearl River Mouth Basin is located on the discrete continental margin of the northern South China Sea and the southern margin of the South China continent (Figure 1). The development of the Pearl River Mouth Basin experienced early fault depression and late depression. The Cenozoic strata have a typical double-layer structure with lower faults and upper depressions, and the strike is northeast, which consists of three depressions, namely, Zhu Yi depression (Figure 2), Zhu Yi depression (Figure 3) and Zhu San depression (Figure 4), separated by low uplift. Its Cenozoic strata are divided into upper and lower structural layers with T7 as the boundary. Paleocene, Eocene and Oligocene deposits are developed in the lower structural layer, which is continental deposits, while river-alluvial fan and shallow lake-swamp deposits are in the lower part and shallow lake-semi-deep lake deposits are in the upper part. Neogene, that is, the upper structural layer developed Miocene, Pliocene and Holocene, which were marine sediments. The geological characteristics of the upper and lower structural layers are completely different, showing different evolution processes.

Fig. 2 Seismic profile of Zhu Yi sag in Pearl River Mouth Basin shows that

Fig. 2 seismic profile of Zhu 1 sag in the Pearl River Mouth Basin

Fig. 3 seismic profile of Zhu 2 sag shows that

Fig. 3 Seismic Profile of Zhu2 Depression

After the Late Cretaceous, a series of tectonic movements led to the continuous thinning of the crust and lithosphere at the continental margin of South China, resulting in the subsidence of the crust. The subsidence of various mechanisms leads to the filling and burial of sediments and structural evolution, so studying the evolution process of basin subsidence is the basis of basin analysis. Analysis of subsidence history has always been an important technology in basin analysis and simulation. In this paper, the Petrosys basin simulation system is taken as the research object, and the extensional subsidence process of the Pearl River Mouth basin is quantitatively and dynamically simulated, and the changing trend and evolution history of the basin tectonic subsidence and total subsidence are quantitatively and dynamically analyzed.

2 Historical model and parameters of basin tectonic subsidence

The total subsidence of sedimentary basins is mainly related to factors such as tectonism, sediment compaction, balance, change of sedimentary datum level or change of ancient water depth (Lin, Zhang Yanmei, 1995).

The tectonic subsidence of sedimentary basin (pure water basin) can be expressed as

Tectonic settlement = total settlement-(water and sediment load settlement+sediment compaction settlement+lake level change)

2. 1 sediment compaction correction

The compaction process of sediments is influenced by lithology, overpressure, diagenesis and other factors, and lithology often plays a leading role. Under normal compaction conditions, the relationship between porosity and depth can be considered as exponential distribution (Athy, 1930):

φ=φ0 e-cy

Where φ is the porosity at the depth y; φ0 is the surface porosity; C is the compaction coefficient; φ0 and c are mainly related to lithology.

The water-filled pores in the rock stratum between the depth Y 1 and Y2 are

Geological research in the South China Sea. 2006

Skeleton volume of sediment: Vs=Vt-Vw

Then, its thickness: ys = y2-y1-φ 0 (e-cy2-e-cy1)/c.

Where: Vs is the skeleton volume of the deposit, that is, the volume of the deposit excluding pores; Vt is the volume of the whole rock stratum; Vw is the pore volume (assuming that the pores are filled with water); Ys is the thickness of rock stratum between the depth Y 1 and Y2 after compaction correction (the depth Y 1 is less than Y2).

When the strata are stripped back to the total height, the sediment part Vs remains unchanged, but only the water in the pores changes (Vw). Therefore, the thickness of the rock stratum at the stripping position is given by the following formula (Allen et al., 1990):

Geological research in the South China Sea. 2006

The surface porosity of sandstone, mudstone and other single lithology has mature empirical values, and the mixed lithology can be obtained by weighting the values provided in table 1 in proportion.

Table 1 table of common compaction parameters under normal conditions 1 compaction coefficient under normal conditions

(According to Sclater and Christie, 1980)

2.2 Load correction of sediments

If the porosity of the rock formation is φ, the average density of the sedimentary layer is:

Geological research in the South China Sea. 2006

Where: φi is the porosity of each single layer; S is the corrected thickness; ρs is the average density of sedimentary layer; ρw is the density of water body; ρsgi is the particle density of each monolayer sediment; Is the thickness of each layer.

If the tectonic subsidence is Y (water filling), and the thickness of the water in the basin is S after being replaced by sediments, only considering the local balance, there are:

Y=S(ρm-ρs)/(ρm-ρw)

Where ρm is the mantle density.

2.3 water depth correction

When the water depth of sedimentary basin is large, it is necessary to correct the water depth to get the correct tectonic subsidence. The estimation of ancient water depth can be carried out by sedimentary facies analysis and paleontology combination. Generally speaking, alluvial-fluvial facies can be ignored when calculating water depth. The shallow lake water depth is 0 ~ 10 m, and the sedimentary depth of semi-deep lake-deep lake is above10 ~100 m. Shallow water depth is 0 ~ 50m, shallow water depth is 50 ~ 200m, and semi-deep sea-deep sea depth is over 200m. On the seismic profile, the large prestack is actually a slope sedimentary system. The shape of the ancient slope can be restored by de-compaction, so as to estimate the distribution of the ancient water depth.

Table 2 shows the change of ancient water depth in the basin.

Table 2 Estimated Paleo-water Depth Distribution Table 2 Tertiary Water Depth Table of Pearl River Mouth Basin

Fig. 4 seismic profile of Zhu 3 sag shows that

Fig. 4 Seismic Profile of Zhu3 Depression

After estimating the ancient water depth, it is easy to deduct the influence of water depth from the total settlement.

2.4 Equalization correction

Because the crust (especially the upper crust) is elastic, it has a certain bending stiffness. Therefore, the balance of lithosphere is actually a flexural balance. The ratio of deflection balance to local balance is

C=(ρm-ρs)/[ρm-ρs+(2π/λ)4D/g]

Where: c is the equilibrium ratio; D is the bending stiffness, which mainly depends on the effective elastic thickness. Generally speaking, the effective elastic thickness is 3 ~ 5km (Kusnir, 1992). The thinning of the lithosphere in the South China Sea is mainly pure shear thinning, and the brittle deformation depth of the surface layer is small, and the effective elastic thickness should be small. In the above formula, 2π/λ is called wave number, and λ is twice the width of the basin. If the basin is very wide or the effective elastic thickness is very small, that is, (2π/λ)4D/g approaches zero and c approaches 1, this is a local equilibrium. At the same time, if the rifting is not instantaneous, the crust will be gradually heated during the stretching process, which will greatly reduce the effective elastic thickness.

If only local equilibrium is considered, the local equilibrium settlement of water body is Wd [ρ m/(ρ m-ρ w)], where Wd is the ancient water depth. Therefore, tectonic subsidence can be expressed as

y = S(ρm-ρS)/(ρm-ρw)-Wdρm/(ρm-ρw)+Wd

3 simulation results analysis

According to the simulation results of basin subsidence history (Figure 5, Figure 6 and Figure 7), the tectonic subsidence curve of the basin showed a gradient change trend from Paleocene to Holocene, showing the subsidence characteristics of the extensional basin and showing the dynamic background of the extensional basin in the Pearl River Mouth Basin. By analyzing the histogram and curve of subsidence rate in the basin, it is found that the subsidence rate and subsidence amount in different geological periods in the Pearl River Mouth Basin are obviously different, showing obvious subsidence heterogeneity. In the formation and development of Cenozoic basins, the subsidence caused by tectonic factors has always been the main factor of basin subsidence, and tectonic subsidence plays a leading role in basin subsidence and controls the change of total basin subsidence. In the development and evolution of the basin, there are three main subsidence peaks, namely, late Paleocene, late Oligocene and Miocene. These three subsidence peaks divide the tectonic subsidence history of the basin into three episodes.

The first scene is from the late Paleocene to the early Oligocene, which is divided into three stages, and the subsidence rate changes from large to small.

The first stage is the late Paleocene subsidence period, which is the highest tectonic subsidence rate in the whole Pearl River Mouth Basin. During this period, the basin subsidence was severe, and the tectonic subsidence rate was generally up to 100 ~ 180 m/ma, and the total subsidence rate was generally 200 ~ 300 m/ma. The tectonic subsidence is generally 600 ~ 1000 m, and the total subsidence is 800 ~ 1600 m, which is the initial stage of rift development in the early stage of basin development and can correspond to the period of Shenhu movement in the northern South China Sea. Among all structural units in the basin, Zhu-2 Depression is the largest subsidence center in the basin, with the largest structural subsidence, the fastest subsidence speed and the strongest rift intensity.

The second stage is the early and middle Eocene subsidence period. During this period, rifting continued, and the extension of the crust caused the differential uplift movement between fault blocks in the basin, which led to the tectonic subsidence of the basin again. The tectonic subsidence is obviously increased, generally 700 ~ 1500m, and the total subsidence can reach 1000 ~ 4000m. The tectonic subsidence rate began to decrease obviously, generally 40 ~ 100 m/ma, and the total subsidence rate was generally 100 ~ 200 m/ma. This stage corresponds to the Zhu Qiong movement scene in the regional tectonic movement. The basin has cracked, the area is expanding and the accommodation space is greatly increased. Deep lake and semi-deep lake facies are developed, forming several deep-water lake basins, which are the main development periods of source rocks in the Pearl River Mouth Basin, and the most important source rocks of Wenchang Formation are deposited in the basins. The largest subsidence center of the basin is still located in Zhuer sag, and its subsidence amount and subsidence rate are much greater than those of the other two sags.

The third stage is the late Eocene-early Oligocene subsidence period. The extension of the basin continued, but the intensity weakened. The tectonic subsidence began to decrease, generally 200 ~ 400 m, and the total subsidence was generally 400 ~1000 m. The tectonic subsidence rate decreased again, generally 20 ~ 35 m/ma, and the total subsidence rate was generally 50 ~ 85 m/ma. During this period, there were two Zhu Qiong movements. The basin first uplifted and denuded, then cracked, and the area continued to expand. The lake basin began to become shallow, but there was still room. Shallow lakes and swamp mudstone developed, which was also an important development period of source rocks in the basin. There is little difference in subsidence among the depressions in the basin, and the subsidence centers are scattered in each depression.

The second subsidence of the basin occurred in the late Oligocene to the middle Miocene and went through three stages.

In the late Oligocene, the curvature of tectonic subsidence and total subsidence curves increased, and the rate of tectonic subsidence rebounded, which opened the curtain of the second subsidence of the basin. The tectonic subsidence rate is generally 30 ~ 65m/ma, and the total subsidence rate is generally 60 ~ 135m/ma. The tectonic subsidence is generally 150 ~ 400 m, and the total subsidence is generally 400 ~ 880 m. During this period, the South China Sea movement took place. At first, the basin experienced regional uplift and denudation, and the crust became thinner, then it began to subside, and the tectonic subsidence strengthened again, from fault depression to depression, and the basin evolution entered the post-fracture development stage.

Fig. 5 Histogram (a) and settlement curve (b) of settlement rate in Zhu Yi sag of Pearl River Mouth Basin.

Fig. 5 sedimentation rate diagram (a) and sedimentation curve diagram (b) of Pearl River Pearl 1 sag.

By the early Miocene, the subsidence speed of the basin began to slow down. The tectonic subsidence rate decreased to 15 ~ 35m/ma, and the total subsidence rate decreased to 30 ~ 75m/ma. The tectonic subsidence is generally 80 ~ 280 m, and the total subsidence is generally 250 ~ 550 m. There is little difference in the subsidence of each structural unit in the basin, and the subsidence of Zhu 2 sag is slightly stronger. With the weakening of basin subsidence, the accommodation space is obviously reduced, and the sedimentary system of the basin has also changed obviously. Sandstone deposits such as deltas and coastal shallow seas and carbonate rocks, reefs and beaches are extremely developed. Therefore, this stage is the main development period of reservoirs in the Pearl River Mouth Basin.

Histogram (a) and settlement curve (b) of settlement rate in Zhuer sag, Pearl River Mouth Basin.

Fig. 6 Settlement rate (a) and curve (b) of Zhu2 sag.

The subsidence characteristics of the basin changed in the middle Miocene. The subsidence rate of Zhu 1 sag decreased slightly, the subsidence rate of Zhu 2 sag and Zhu 3 sag increased slightly, and the accommodation space in some areas of the basin increased slightly.

In the first and second acts of basin subsidence, the tectonic subsidence rate experienced two changes from large to small, which showed the evolution process of basin rift from strong to weak until it turned into depression. The changes of tectonic subsidence in different geological periods are slightly different, and they have experienced the process from small to large and from large to small, showing the evolution history of initial rapid rift-strong rift expansion-stable rift expansion-stopping rift. The early subsidence of the Pearl River Mouth Basin is the strongest in Zhuer sag, followed by Zhu San sag. Tectonic subsidence controls the change of the total subsidence of the basin, so it controls the change of the basin accommodation space, and the sedimentation and filling of the basin also change, and a series of sedimentary layers with good source rocks and reservoirs are developed.

Fig. 7 Histogram (a) and settlement curve (b) of settlement rate in Zhu San sag.

Fig. 7 Settlement rate (a) and curve (b) of Zhu3 sag.

By the late Miocene, the subsidence gradually strengthened, and the third act of basin subsidence began. The amount and rate of subsidence have been enhanced, which can be divided into two stages: late Miocene and Holocene. The late Miocene tectonic subsidence is generally 150 ~ 600 m, the total subsidence is generally 350 ~ 1000 m, the tectonic subsidence rate is 30 ~ 60 m/ma, and the total subsidence rate is generally 70 ~ 150 m/ma. By the Holocene subsidence stage, the subsidence intensity of Zhu 1 sag was weak, while that of Zhu 2 sag and Zhu 3 sag was strong. The tectonic subsidence is 80 ~ 300m, the total subsidence is generally 150~750m ~ 750m, the tectonic subsidence rate is 15 ~ 40m/ma, and the total subsidence rate is generally 30 ~ 1 10m/ma. The third subsidence of the basin was accompanied by Dongsha movement and thermal subsidence in the South China Sea. Tectonic movement caused uplift and denudation of uplift area, but it had little effect on depression. Regional thermal subsidence is the main cause of basin subsidence in this period, which created new accommodation space for the basin and created good conditions for the formation of regional caprock.

4 conclusion

Based on the above analysis, the tectonic subsidence history of the Pearl River Mouth Basin shows episodic and multi-stage changes of the extensional basin, which reflects the heterogeneity and multi-stage characteristics of the basin rift and the multi-stage characteristics of regional tectonic activities. The first act and the second act of subsidence are the main stages of basin development, which laid the structural framework of the basin and formed the main sedimentary strata and oil and gas resources of the basin. The third act of subsidence is the stage of transformation and perfection of the basin and the main period of the development of regional caprock in the basin. Episodic and multi-stage subsidence of the basin has created favorable conditions for the formation, development and evolution of source rocks, reservoirs and caprocks. Therefore, the Pearl River Mouth Basin is rich in oil and gas resources, and Zhuer sag has the strongest early subsidence in the whole Pearl River Mouth Basin, which is suitable for the development of source rocks and may have a good oil and gas prospect.

refer to

Chen Changmin, Shi, Xu et al. 2003. Formation conditions of tertiary oil and gas reservoirs in the Pearl River Mouth Basin (east). Beijing: Science Press, 1 10 ~ 120.

Lin, Zhang Yanmei. 1995。 Theoretical basis and new progress of extensional basin simulation. Frontier of Earth Science, 2 (3 ~ 4): 79 ~ 88.

Allen J Allen Private Company Limited, zip code 1990. Basin analysis: principle and application. London: BP

Athy L F. 1930。 Density, porosity and compactness of sedimentary rocks. Bull Petroleum Geological Association, 14: 1~24

Christie's auction house, 1980. An explanation of continental extension for the late Cretaceous settlement in the central Beihai Basin. Journal of Geophysics, 85,3711~ 3739

Quantitative simulation and analysis of subsidence history of Pearl River Mouth Basin in South China Sea

Gao Hongfang Dudley Zhong Guangjian

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: Using Petrosys basin simulation system, the subsidence history of three main depressions in the Pearl River Mouth basin is quantitatively simulated, and then the relationship between subsidence rate and the development of source rocks, reservoirs and caprocks is discussed. The results show that the subsidence history of the Pearl River Mouth Basin has the dynamic characteristics of episodic multi-stage evolution. The first and second episodic subsidence are the main stages of the evolution of the Pearl River Mouth Basin, which established the structural framework of the Pearl River Mouth Basin and formed the main sedimentary and oil and gas resources. Under the action of the third act of subsidence, the basin was transformed and shaped, and regional caprocks were produced during this period.

Keywords: quantitative simulation of episodic subsidence in the Pearl River Mouth Basin