Study Of Devonian Reef Development And Distributional Characteristics In West Sichuan Depression And Youjiang Basin

The Devonian reef has occupied an important position in quality and scale duringglobal geological history. The reef-type reservoirs had got considerable reserves. TheSouthern China region which is picked as the researching area of this paper is locatedin the most development area of Devonian reefs in China. The studying of reefcharacter not only has important theoretic meaning, but also provides a direction forthe reef-type reservoir exploration.This paper based on four measured reef-sections and five reef-observation placesin west Sichuan depression and Youjiang basin, by using detailed field description,sampling, indoor study of thinsections and comprehensive geochemical analysis oftrace elements, analyzing single reef form characters of reef-building andreef-dwelling organism, reef sedimentary environment and growth control factors,summarizing the controlling factors of reef development and exploration prospects ofreef-type reservoir.It is recognized that there are fringe reef, platform internal reef, platform marginreef, slope reef and basin reef developed in studying area. Platform internal reef,platform margin reef and basin reef are the main study objects. There are differenttypes of reef-building organisms during Devonian. Coral is the main organism in earlyDevonian and appeared in plexiform group. Stromatopora is the main organism inmiddle Devonian and appeared in irregular, bulbous, large block and coated blockshape. Algae is the main organism in late Devonian. Structure, paleogeography,paleoenvironment are three factors that control the development of reef. In earlyDevonian, reef developed in transgresive area of shallow water of DianQianGui andwestern Sichuan. In middle Devonian, reef developed considerably in isolatedcarbonate platform of DianQianGui shallow water and carbonate gentle slope marginof western Sichuan shallow water. In late Devonian, the distribution of reef reducedrapidly. After the F-F biological extinction event, there is no more stromatopora reef to be found in South China.With the DaChang reef ancient oil reservoir as an example, the high-qualityhydrocarbon source rocks in the sag besides the uplift, the reef limestone that rich inpore which existed on the uplift, the mudstone caprocks on the reef, are a goodcombination of source, reservoir and seal. These are the necessary conditions to be aoil-gas reservoir. Restricted platform margin, platform internal and trench margin arethe favorable places for reef mineralization.The reef rock types are mainly reef limestone and bioclasticlimestone. Thedegree of dolomitization is high in platform internal reef. Limestone is the main typein platform margin reef and basin reef. The pore types are biologic framework pore,biologic covered pore, intergranular pore, dissolution pore and microfracture. Themain diagenesis types are cementation, compaction, pressure solution,recrystallization, dolomitization, silicification, dedolomitization and infilling.