| Northern Guizhou as a key area of Marine oil and gas exploration in southern China,in which the Strata from Sinian to Silurian developed several source-reservoir-cap longitudinally,has a broad prospect for exploration.Recently the China geological survey achieved a great breakthrough in gas exploration in Anye 1 well that located in Anchang syncline of Wuling mountain area,and the breakthrough confirmed its extensive exploration prospects.In order to study the gas reservoir conditions of Anchang syncline,this article took field geological survey and indoor study,geology and geochemistry,macroscopic and microcosmic study combination method,focusing on the characteristics of the Upper Ordovician—Lower Silurian Wufeng-Longmaxi Formation and the Lower Silurian Shiniulan Formation.In this paper,we collected samples of shale at the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formation,and of carbonate at middle Silurian in the two wings of Anchang syncline.By the means of microscope,SEM,XRD,nitrogen adsorpotion experiments,porosity and permeability test,and organic geochemical analysis,as the result,we acquired the characteristics of organic geochemistry,petrology,physical properties and micropore structure of the samples.Wufeng-Longmaxi formation shale,is mainly black shale,carbonaceous shale,silty sand shale;the total organic carbon content is between 0.81%and 5.71%,the mean value is 3.07%;the maturity is 1.394-2.416%,the average is 1.805%,which is at the peak of gas production.The content of brittle minerals(quartz,pyrite and feldspar)is 56.2%?71.9%,and the fracturing is good for the shale gas exploitation.According to the test results of overburden porosity test,the physical properties of the Wufeng—Longmaxi formation is poor,the porosity was 3.299%?8.016%,and the average value was 5.547%.,the permeability is 1.17 x 10-4--1.86 x 10-4md,with an average of 1.48*10-4md.The Shiniulan Formation can be divided into several types:laminated marl,nodular limestone,microcrystalline limestone,grain limestone,algal limestone,etc.,that means its heterogeneity stronger compared with shale.Calcite,quartz and clay are the main mineral composition,calcite content 16.6%,quartz content 14.1%,illite 11.2%,feldspar 11.8%.The porosity is 0.125%?0.401%,and the mean value is 0.26%.The permeability is 0.72 x 10-5?1.86 x 10-3nd,which belongs to the tight reservoir.The scanning electron microscopy(SEM)image showed that the storage space of Longmaxi Formation can be divided into intragranular pores,intergranular pores,organic pores and micro-fracture,among which the organic matter pores were the most significant ones for the storage space.However,the reservoir space ofShiniulan Formation is mainly microcracks and dissolution pores,and the micro-fractures are connected to form a micro-fracture network,which not only improves the storage space but also improves the permeability of the reservoir.The results of nitrogen adsorption experiment showed that the mesoporous pores were primary in the shale and the carbonate rocks,they are the main contributors to the surface area and the pore volume.The pore structure of Wufeng—Longmaxi Formation is mainly ink bottle-like pores and parallel plate-like pores,while the pore structure of Shiniulan Formation dominated by slit type pore.The characteristic of slit type pore is that the pore and pore throat is not obvious,this kind of pore structure greatly reduces the capillary resistance in the course of gas migration and seepage,which is beneficial to the gas migration.High quality reservoir of the Shiniulan Formation is located in the lower part,which is mainly thin to medium silty limestone and argillaceous limestone.The upper part of Shiniulan Formation is bioclastic limestone,and it has poorreservoir performance due to recrystallization,compaction and bituminous filling.According to the literature research,geochemistry experiment and SEM analysis,the source of natural gas in Shiniulan Formation is most likely from Wufeng-Longma Formation. |
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