The Changes In Porosity And Permeability Layer And Genetic Characteristics Of The Neogene Biohermal Carbonates In The Western Of The South China Sea

Reef reservoir has good physical properties, which is easy to form a large oil andgas reservoir, therefore, the oil and gas reef-reservoirs plays a very important positionin the oil and gas reserves of the world.The South China Sea is rich in oil and gasresources as the largest marginal sea of China. The author studied the part of thecreatures of reef-carbonate type, the changes of porosity and the characteristics of thecauses in western of south China sea and expected to get certain theoreticalunderstanding and have a important application value for the carbonate reservoirexploration and development of the western south China sea.This study area is Located in thecontinental slope of northwestern South ChinaSea, which can be divided into the eastern reefs areas and the western basin area inaccordance with the basement structure and the characteristics of the overlyingsedimentary strata.Eastern reefs area is in the higher parts of the the Xisha uplift,where sea water are relatively shallow,and the reef sedimentary strata are coveringon the basement directly. Since the Miocene, the reef facies carbonate rocks in theXisha waters were widely developed. In the eastern reefs District, pure Miocene reefcarbonate rocks directly cover the crystalline basement in the former Precambrian andcontinued until now. In this paper, the main object of study is the eastern reefsDistrict, in which samples were taken from the Xisha islands and reefs,including theYongle Atoll, Antelope reef, Island Oasis, Island Quanfu, Silver Islet,Jin Qing Island,Chenhang Island, Guangjin Island, Yagong Island, the Panshi Atoll far away from thesouth of the Huaguang atoll15km, the Shi island which is in the disk of a reefwhich the southeast of the Xuande Atoll and Yongxing Island,the silver gravelbeach which lurks in the southwest of underwater, the North Island and South Islandand the Zhaoshu island at its western reef aerodromes,as well as the East Island at theEast island atoll and surf atoll.We taken a total of43surface samplesfrom from these19reefs, as well as the coring data in the Xichen-1well and the part of the coringdata in the Xiyong-2well to study the carbonate type, the change of porosity andpermeability layer and genesis. The ideas and the scientific basis of research in eastern reefs and the Xisha Islands reefs is mainly based on the spirit of the land andsea contrast and this on the ancient principles. In view of that, the Neogene reefs in oiland gas field reservoir systems in the South China Sea are mostly Miocene reefs,Then,the Miocene reefs in Xisha Islands and basins have the same depositionalenvironment, but the differences in post-tectonic movement led to the differences ofthe basin area formed by reef-carbonate platform tectonic subsidence from thepresent distribution and occurrence. In other words, from the Miocene reefs in XishaIslands we can get the information of the Miocene reefs in the deep basin, which is ashortcut to study the porosity and permeability characteristics of the deep basin areabefore drilling the deep basin area.Similarly, the sedimentary genesis of the modernXisha islands and reefs are self-evident meaning of the basin area of ancient Miocenereefs.In this paper, the author studies the reef carbonate rock by surface samples anddrill sampling data of the western South China Sea islands and reefs,which weretaken for the polarized light microscopy analysis, whole rock analysis, SEM and EDSanalysis, electron microprobe analysis, and porosity and permeability layeranalysis.And then,we analyzed several typical samples: the main surface sedimentsare the reef sbioclastic carbonate in the every islands of Xisha Islands; the carbonaterocks in the drilling cores are mainly dolomite and calcite; the result of porosity andpermeability test for Xichen-1well drilling core showes that the main combinationof pore types are intergranular dissolution porosity+intragranular dissolved pores+intergranular hole, which are the products of strong dolomitization; the Neogenecarbonate reservoirs genetic model of Xisha Islands maybe Mixed water model+hydrothermal model-microbial model.