A Study On Hydrocarbon Accumulation Pattern Of Lithologic Reservoir Controlled By Dual Slope Break Belts Of Wide And Low-gradient Shelf In Northern South China Sea

With the degree of oil and gas exploration constant improvement, the structural reservoir which is easy to find is less and lithologic reservoirs that have been found are more and more. In our country, lithologic reservoir has become the important exploration target. High exploration degree regions in the eastern part of our country have a continuous success in lithologic reservoir exploration of sandstone, and carbonate rock of the west of lithologic reservoir exploration has been making a breakthrough in recent years. Therefore, the exploration area of lithologic reservoir is broad and the exploration potential is tremendous.Pearl River Mouth Basin of northern South China Sea is the key basin of domestic offshore oil and gas exploration, and the basin is rich in oil and gas resources and has great exploration potential.In neogene stratum, wide and low-gradient shelf in northern South China Sea has depositional slope break belt in Huizhou Sag and shelf-margin slope break belt in north slope of Baiyun Sag. Due to the control of dual slope break belts to depositional process, the type and distribution of depositional system have certain particularity, which affect the development of favorable oil and gas reservoir and prediction of oil and gas accumulation zone.Based on theory of sedimentology and petroleum geology, this paper makes full use of the geology methods, choses the tectonic evolution and sedimentary characteristics of Pearl River Mouth Basin at the beginning, systematically analyzes the basic petroleum geological conditions such as hydrocarbon source rock, reservoir, cap rock, trap and oil and gas migration condition. Therefore, we can get the characteristic of hydrocarbon accumulation and law of lithologic reservoir controlled by dual slope break belts.Through a comprehensive analysis of the main controlling factors of hydrocarbon accumulation in the basin, we can conclude the pattern of lithologic reservoir controlled by dual slope break belts and finally point out the favorable exploration zone. Through the research, we have got the following understanding:1. Pearl River Mouth Basin is a typical representative of the continental margin of northern South China Sea, the basin has structural characteristic of typical fault depression double-layer or three-layer. In Cenozoic era, the basin universally is filled with paleogene continental deposit of fault depression and neogene and quaternary marine deposit of depression.2. Wenchang Formation of lacustrine mudstone and Enping Formation of river limnetic facies sand-mudstone and interval coal strata is the main hydrocarbon source rocks in Pearl River Mouth Basin, and the hydrocarbon source rock has three characteristics:better organic type, high organic abundance, matural state and a large amount of petroleum are generated in the stage; The main reservoirs are Zhuhai Formation sandstone of oligocene and Zhujiang Formation sandstone of miocene; from top to bottom,Pearl River Mouth Basin has three sets of regional cap rocks, which are developed in the upper Zhujiang Formation, the Hanjiang Formation and the interior of Wenchang Formation, among them, the upper part of Zhujiang Formation of the regional cap rock of basin in neogene plays a main role of controlling and blocking the distribution of oil and gas layer. What’s else, there is a good matching combination relationship about hydrocarbon source rock, reservoir and cap rock.3. The type of lithologic trap in Pearl River Mouth Basin contains sandstone lens, unconformity screened, reefs and buried hill, etc.; Hydrocarbon migration includes vertical migration and migration on the surface:vertical migration is given priority to the migration along faults, plane migration is given priority to the migration along construal ridge and unconformity surface, these two parts complement each other.4. Main controlling factors of hydrocarbon accumulation in the basin are: sedimentary facies control the plane distribution of lithologic trap, stratigraphic sequence controls the vertical distribution of lithologic trap, fault is the principal way of bridging the hydrocarbon source rock and the lithologic trap, physical property of reservoir controls the effectiveness of lithologic trap. Based on the research of lithologic reservoir exploration in the Pearl River Mouth Basin, we think three combinations which have good hydrocarbon accumulation conditions are favorable exploration zones of Huizhou area,they are:(1) the favorable zone between up and down:Wenchang sedimentary area of Huizhou Sag along the fault boundary; (2) the favorable zone near deep strata of paleogene:namely Enping sedimentary area; (3) the favorable zone near deep strata of neogene:the updip pinch-out belts of Ancient Pearl River Delta front sandbody on the tectonic background in the southern edge of Huizhou Sag. The favorable zone of neogene exploration area in Baiyun Sag are: southern of Baiyun Sag and the southeastern slope area, favorable exploration formation mainly are bottom of the Zhujiang Formation and Zhuhai Formation.