(Shandong Lubei Geological Engineering Survey Institute, Dezhou 2530 15)

About the author: Zou Zuguang (196 1-), a senior engineer from Penglai, Shandong Province, is engaged in the exploration and evaluation of hydraulic environmental geology.

Dongying city is located in the subsidence area of North China, with a large Cenozoic thickness. Due to the long-term exploitation of oil, natural gas and deep groundwater, the deep groundwater subsidence funnel has been formed, which makes the formation stress increase and compress, resulting in ground subsidence. Neotectonic movement and self-weight consolidation of underconsolidated soil also have some influence on land subsidence. This paper analyzes the present situation and forming factors of land subsidence in Dongying city, and puts forward the prevention measures.

Keywords: land subsidence; Factor analysis; Oil and gas; Groundwater; Dongying city

Dongying City, located in the Yellow River Delta, is an important economic development zone in Shandong Province and the main oil-producing area of Shengli Oilfield. The development of economy and the intensification of human activities have a serious impact on the ecological environment. Overexploitation of oil, natural gas and groundwater resources has led to geological disasters such as land subsidence and environmental geological problems. The terrain in the area is low and flat, with a small slope, close to the Bohai Sea. If land subsidence is further developed and superimposed with sea level rise, the flood discharge function and storm surge resistance of urban areas will be further reduced, which will easily cause serious disasters.

1 Overview of land subsidence

During the period of 1985, the Institute of Geophysics of the State Seismological Bureau and the Seismological Bureau of Shandong Province carried out modern deformation measurement in Dongying-Kenli, and the maximum ground subsidence was 80 mm In 2000, some elevation points in this area were re-measured, and the ground subsidence was 248~397 mm

From 2002 to 2003, the monitoring results of 43 land subsidence observation points in this area were analyzed. There are 7 points with settlement of 30 ~ 30~30mm, 20 ~ 29 mm or more 14 points, 5 points below 10mm, and the rest are1~19.

Land subsidence is widespread in this area, and the amount of land subsidence in the west is greater than that in the east. The average settlement of each observation point in Xicheng-Liu Hu-Niuzhuang West is 2 1 mm, and the settlement is more than 20mm, and the eastern coastal area is less than 10mm (Figure 1).

2 Geological background conditions

2. 1 terrain

The working area is located in the Yellow River Delta region, with the overall topography being high in the west and low in the east, with a topographic slope of110000 and an elevation of12 ~ 2 m. Affected by natural and human factors, especially the geological action of the Yellow River, a complex micro-geomorphic landscape with hills, slopes and depressions interlaced with each other has been formed. There are flood plain highlands, crevasse fan highlands, sloping fields, depressions and beaches.

Figure 1 Isogram of Land Subsidence in Dongying City

2.2 Hydrogeological conditions

The buried depths of the top and bottom of the deep groundwater aquifer in this area are 400 ~ 500m and 800 ~1000m respectively. Distributed in the south of Shikou-Guang Bei Reservoir. The accumulative thickness of aquifer is 20 ~ 40m, the thickness of single layer is 2 ~ 5m, the water inflow of single well is 500 ~ 1000m3/d, the salinity is generally less than 1g/L, and the hydrochemical types are mainly sodium bicarbonate and sodium bicarbonate. Long-term exploitation makes the groundwater level drop continuously, forming a groundwater funnel. An observation well was opened in Liuhu Town, and the average water level elevation decreased from 19 1.4m in June 2007 to -3 1.9m, with an average annual decrease of 3. 15m.

Fig. 2 Dynamic curve of deep groundwater

2.3 Engineering geological conditions

The shallow sediments in this area are Quaternary alluvial marine sediments, khaki-grayish yellow-grayish black silt, muddy silty clay and silty fine sand. Natural water content, void ratio and compressibility are large; The deep part is mostly Paleogene-Neogene alluvial-lacustrine deposits, with clay, silty clay and medium-fine sand interbedded. Its natural water content, void ratio and compressibility are relatively small. The shallow part is sandwiched with soft soil and saturated sand which is easy to liquefy. It is divided into three compression layers from top to bottom.

3 Analysis of land subsidence factors

The factors leading to land subsidence in this area include neotectonic movement, natural consolidation of soil, exploitation of oil, natural gas and groundwater, etc.

3. 1 Relationship between land subsidence and neotectonic movement

This area is located in the subsidence zone of North China. Since Neogene, under the influence of neotectonic movement, the crust has slowly subsided, and extremely thick Neogene and Quaternary systems have been deposited.

At the beginning of Neogene, the fault activity in this area weakened, the difference between depressions became smaller, and the boundary between uplift and depression tended to disappear. The sedimentary Guantao Formation was thin (300 ~ 400 m) and distributed stably. At this stage, the fault activity mode is thrust activity, forming a derived reverse traction anticline structure.

Since the Neogene, the area has generally shown a unified subsidence trend, with a sedimentary thickness of 900 ~ 1 100 m, of which the Minghuazhen Formation is 600 ~ 800 m; The thickness of Quaternary system is about 300 meters, with little change. At this stage, fault activity is characterized by inherited and new tremor movement. The differential activity of faults gradually weakened and transformed into crustal structural deformation characterized by horizontal torsion and shear fracture.

Due to the elevation difference caused by neotectonic movement, the regional change is small, which has certain influence on the temporal and spatial change of land subsidence.

3.2 Relationship between land subsidence and consolidation of underconsolidated soil

This area is located in the Yellow River Delta, and most of the strata within the depth range of 0 ~ 30m are Quaternary Holocene strata, and its sedimentary environment is influenced by the interaction or co-influence of the Yellow River and the ocean, forming a stratum dominated by fine particles, with silty soil being the most extensive lithology, followed by silty clay, silty sand and clay, and silty soil locally. The early consolidation pressure of soil is low, and the overconsolidation ratio is about 0.5, so it is underconsolidated soil. Under dead weight pressure, it will cause consolidation settlement. It shows that the proportion of self-weight consolidation settlement to total settlement is very small. The thickness of loose soil layer in this area is 25 ~ 3 1m, and the calculated natural ground subsidence rate is 3 mm/a ... The relationship with the ground subsidence funnel is not obvious. The self-weight consolidation settlement in coastal areas accounts for a large proportion.

3.3 Relationship between land subsidence and oil and gas exploitation

Located in the Yellow River Delta, this area is rich in oil and gas resources. Since its exploitation in 1960s, it has been the second largest oil field in China. Oil and gas reservoirs are mainly produced in sandstone, shale and limestone strata of Paleogene Shahejie Formation. Long-term exploitation has made the groundwater in Shahejie Formation form a huge descending funnel, and the groundwater level in the center of the funnel is about 700m m. The oilfields in this area mainly include Dongying Oilfield, Dongxin Oilfield, Shinan Oilfield, Liangjialou Oilfield, Niuzhuang Oilfield, Xianhezhuang Oilfield, Liu Hu Oilfield, Wangjiagang Oilfield and Guangli Oilfield.

Because of the long-term exploitation of oil and gas resources, it is easy to compress the stratum and lead to ground subsidence. The main oil and gas exploitation areas in this area are mostly in the subsidence funnel area, such as the subsidence funnel in Guangli Oilfield. It shows that oil and gas exploitation is one of the important factors causing land subsidence (see Figure 3).

3.4 Relationship between Ground Subsidence and Groundwater Exploitation

The city is located in the Yellow River Delta, where fresh water resources are extremely scarce and unevenly distributed. Affected by the seasons, the Yellow River water is often in short supply. Since 1970s, deep groundwater has been exploited. In the early stage of development, the deep groundwater is gravity flow, and the height of gravity flow is generally above 3 m. After long-term over-exploitation, the groundwater level has now formed a funnel.

Fig. 3 Relationship between land subsidence and oil and gas exploitation

Deep groundwater exploitation is distributed in Shikou and the south of the power plant, and concentrated in Niuzhuang and Liu Hu areas. The groundwater level in the center of the funnel is buried at a depth of 50 ~ 60 m, and the decline rate is 2 ~ 3 m/a.

The groundwater in the central area is mainly affected by the outside, and a groundwater drop funnel has been formed. Its funnel shape is similar to that of deep groundwater.

At present, the distribution of the ground subsidence funnel center in this area is basically the same as that of the groundwater level subsidence funnel area (see Figure 4). The center of the ground subsidence funnel is located in Niuzhuang and Liu Hu, and it is also the center of the deep groundwater subsidence funnel. Located at the edge of the underground water level drop funnel in the north and east of Dongcheng, the land subsidence is small. Therefore, the exploitation of deep groundwater is the main factor causing land subsidence at present.

Fig. 4 Relationship between land subsidence and groundwater exploitation

4 conclusion

The exploitation of groundwater, oil and natural gas in this area is closely related to land subsidence, which is the main external cause of land subsidence. Self-weight consolidation and neotectonic movement of underconsolidated soil are important factors that cause land subsidence, but their influence on land subsidence is relatively small.

Land subsidence is a gradual geological disaster, which is not easy to be detected, but once it leads to serious consequences, it will be difficult to remedy. Its hazards mainly include seawater intrusion, salt water intrusion, poor urban drainage, building cracking, equipment failure and so on. Strengthening the monitoring of groundwater dynamics and land subsidence, rationally exploiting groundwater, oil and natural gas resources and implementing artificial groundwater recharge project are important measures to protect geological environment and prevent land subsidence.

refer to

Fan Xiufeng et al. 2004. Analysis of land subsidence in Fuzhou hot spring area. Geological disasters and environmental protection, 6 (2): 89 ~ 92 pages.

Xue Chuandong et al. 2004. Mechanism analysis of land subsidence in Kunming. Chinese journal of geological hazard and control, 9 (3): 47 ~ 53.