| In this study,the types,genesis,distribution,and composition of hydrocarbon precursors in the Wufeng Formation-Longmaxi Formation,which are from the Jiaoye 1 Well in Jiaoshiba Area of Sichuan Basin,are studied through the systematic organic petrology and paleontological analysis.The contribution of different hydrocarbon precursors to shale gas-generaion and reservation is discussed,which provides an important basis for studying the shale gas pooling mechanism and the formation of shale gas“dessert”.(1)Through the research of organic petrology and micropaleontology,the micro-components of Wufeng Formation and Longmaxi Formation in the Jiaoye 1 Well are divided,and their causes are analyzed.The hydrocarbon precursors of Wufeng Formation and Longmaxi Formation in the Jiaoye 1 Well can be divided into two parts:morphological components and non-morphological components.Among them,morphological components mainly include alginite and animal-organism while the non-morphological components include amorphous solid in the kerogen and micrinites,asphaltenes,and solid asphalt in the whole rocks.Alginite can be further divided into layered alginite,structural alginite 1,and structural alginite 2.Layered alginite and structural alginite 1 mainly come from planktonic algae,and structural alginite 1 contains abundant fossils of acritarchs,which can be divided into fossils preserving in the form of organic matter and fossils preserving in the form of framboidal pyrite according to the preservation form.The structural alginite 2 are mainly derived from benthic algae.The animal-organism includes spicules of sponges,scolecodont,conodonts,graptolites and chitinozoans.12 graptolite zones are established in the Wufeng Formation-Longmaxi Formation according to the species of the graptolites.By combining the graptolite zones with the variation in the species as well as the abundance of chitinozoans,different stratums can be spotted.The non-morphological components are not preserved well in fine shape,but are abundant in content as the main form of organic matter in the study area.(2)By the means of microscopic Fourier transform infrared spectroscopy and microscopic laser Raman spectroscopy,the differences in chemical composition and hydrocarbon generation potential of different hydrocarbon precursors were investigated.The results of Raman spectroscopy show that the peak shift difference between D peak and G peak of the solid asphalt is the largest,and the peak intensity ratio(D peak/G peak)is the smallest,indicating that the aromatic rings of the solid asphalt have the highest degree of condensation and aromatic structure order.The graptolite have the the smallest peak shift difference and the largest peak intensity ratio,indicating that its aromatic ring condensation and the aromatic structure order are not high.The Raman spectroscopic characteristics of the acritarchs range between the solid asphalt and the graptolite,but are closer to those of solid asphalt.The infrared spectrum analysis showes that the graptolite contains only a small amount of aliphatic components,and it is a high molecular polymer containing oxygenic groups;the wall of chitinozoans also contains only a very small amount of aliphatic components,but rich in oxygenic groups,such as carboxyl and ether bonds,as well as aromatic components.Despite being in an over-matured state,the acritarchs still contains highly-abundant aliphatic components.Based on the chemical composition analysis of different types of hydrocarbon precursors,it is inferred that the conodonts and graptolites,which are rich in the Jiaoye 1 Well,are weak in their hydrocarbon generation capacity,and their organic type should be Type III,while the organic type of the acritarchs is I.As the product of crude oil,solid asphalt have the highest degree of condensation and aromatic structure order,indicating that it has undergone drastic changes during the same thermal evolution with other organic matters,producing a large amount of hydrocarbons.Therefore solid asphalt has a high hydrocarbon generation potential as one of the hydrocarbon precursors of shale gas.(3)By using the methods of SEM and FIB-SEM analyses,a large number of micro-nano pores are found in the hydrocarbon precursors of the Wufeng Formation and Longmaxi Formation in the Jiaoye 1 well,which vary from one type of hydrocarbon precursor to another.The micro-nano-pores of the acritarchs are the most developed and generally range in size from a few nanometers to hundreds of nanometers.Solid asphalt also develops a large number of nanopores,but the abundance of nano-pores varies.Generally,animal-organism do not have well-developed pores.The conodonts develop a large number of nano-pores that are not connected to each other.Only a small amount of pores are found in the interior of the conodonts and graptolites,and almost no pores are developed in the walls of the scolecodont.The above results indicate that the reservoir space of hydrocarbon in the Wufeng Formation-Longmaxi Formation may come mainly from the acritarchs and solid asphalt.(4)By comparing the types and pore development characteristics of hydrocarbon precursors with the shale gas content,the relationship between the development of different hydrocarbon precursors and gas-bearing properties is discussed.The results show that the type of the acritarchs.which develop many pores with good connectivity,is I.And the acritarchs were both important hydrocarbon precursors and good reservoirs for the shale gas.While the animal-organism,such as scolecodont,conodonts,graptolites and chitinozoans,is rich in oxygenic groups and has a organic type of ?.The animal-organism has low hydrocarbon generation potential and poor pore development therefore contributes less to shale gas conten.However,the combination of graptolites and chitinozoans can be used to establish graptolite-chitinozoans belts,which have a corresponding relationship with shale gas content.For instance the WF2-WF3 graptolite belt and the LM2-LM6 graptolite belt correspond to a high gas content,providing an important indicator for shale gas“dessert”.Solid asphalt has a high degree of change in the thermal evolution process,and a large number of nanopores are developed.Therefore,the solid asphalt is not only the main contributor to shale gas,but also provides a lot of space for the storage of shale gas.The analysis of the differences in the contribution of different hydrocarbon precursors to the shale gas production and reservation provides an important basis for the fine evaluation of the shale gas content of the studied and even in the shale of Early Paleozoic in the south of China. |
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