The Development Characteristics Of Pyrite In The Longmaxi Formation In Southeastern Sichuan And Its Oil And Gas Geological Significance

Pyrite is widely developed in marine shales of the Longmaxi Formation in the Sichuan Basin.The content of pyrite is generally between 1%and 6%,with the average content more than 2%.The morphology and microstructure of pyrite are of great significance for the sedimentary environment interpretation of marine shale.Pyrite is one of the important components in the shale and has good correlation with high-quality shale reservoirs,however,there is few researchs on pyrite in shale.Therefore,the Longmaxi marine shale in the southeastern Sichuan Basin is selected as the research object in this paper.Based on outcrops,cores,thinsection observation and various analytical data,the occurrence of pyrite in the high-quality shale interval of the Longmaxi Formation in the study area,the indication significance of pyrite to the environment and the geological significance of pyrite for shale gas exploration have been systematically studied.The results are as follows:(1)The high-quality shale of the Wufeng-Longmaxi Formation in southeastern Sichuan is generally about 20?45m thick;the organic matter type is mainly type ?-?1,the maturity is generally higher(Ro>2%);the TOC content is more than 2%;the average gas content is 2.84m3/t,and the porosity is between 0.41%and 7.22%,with an average of 3.43%,which indicates that the Longmaxi Formation has a good exploration potential in the study area.(2)The pyrite is well developed in the Longmaxi Formation shale with diverse types.(a)Macroscopically,the occurrences are as follows:pyrite strips;pyrite nodules;dispersed granular pyrite.(b)The morphology of pyrite is abundant in microscopy.Normal pyrite framboids is the most common,followed by partially recrystallized pyrite framboids and semi-automorphic granular pyrite.(3)The size of pyrite framboids shows that the marine shale of the S1L1 in the study area is developed in a sulfur reduction environment as a whole,and the partial area is sub-oxidized sedimentary environment,which is beneficial to the preservation of organic matter.(4)The content of pyrite in the study area is inversely proportional to S1(residual hydrocarbon content in rock);The content of pyrite is proportional to the content of S2(gaseous hydrocarbon content),indicating that pyrite has a catalytic effect on the formation of shale gas.The high content of pyrite can lead to a decrease in activation energy.Since the C-S bond energy is low,the C-S bond is easily broken at a lower temperature to generate hydrocarbons in advance.(5)The content of pyrite in the Longmaxi Formation shale has a good positive correlation with the organic matter in the study area,and the correlation coefficient can reach to 0.51.Combined with the previous research results,the content of pyrite in shale has a positive correlation with the organic content when the sedimengtary environment are reduction conditions.Therefore,in the deep-water shed deposition environment,the optimization and prediction of high-quality shale development section can be carried out by the content of pyrite.(6)The intercrystalline pore and pyrite-organic matter complex organic pore are the main pore types related to the pyrite in the Longmaxi Formation shale.The contribution rate of pyrite framboids aggregate to the pore is between 1.09%and 5.66%,with an average of 3.21%,indicating that the pore developed in the pyrite in shale have a favorable contribution to the shale reservoir pore system.These pores not only increase shale reservoir space and specific surface area,but also serve as the pores of free gas.Therefore,pyrite can promote the integration of shale gas in the Longmaxi Formation from both adsorption and accumulation of free gas.(7)There is a significant positive correlation between the content of pyrite and methane adsorption gas in marine shale of the Longmaxi Formation in the southeast Sichuan Basin.The more pyrite content is,the more methane adsorption gas is.The average content of the pyrite in the southeast Sichuan Basin is between 2.08%and 5.2%,which can increase the brittleness index of the shale reservoir by 2%?5%.The higher the brittleness index of shale is,the better the pore-fracture system can be formed after fracturing.Under the action of acid and oxidant composite fracturing fluid in shale reservoir,pyrite will be dissolved,oxidized,consumed and released heat,resulting in pore formation,which will increase the micro-nano pore system of reservoir,improve the transmission performance of shale reservoir,stimulate the analysis of adsorbed gas and alleviate the damage of reservoir,and improve the recovery of shale gas.