Lithofacies And Characteristics Of Organic Matter In Transitional Facies Shale Of The Longtan Formation In Eastern Sichuan And Northern Guizhou

The recoverable geological reserves of shale gas in the transitional facies are huge,but the shale composition is complicated,the lithofacies are changeable,and the content of clay minerals is high.As well as it is difficult to develop.So far,no breakthrough has been made in the exploration and development of transition facies shale gas.Research on marine shale gas has shown that the classification of lithofacies based on mineral types is helpful for the evaluation of shale oil and gas content and development performance.However,due to the complex composition of the marine-terrestrial transitional shale,the varied lithofacies and the high content of clay minerals,the use of the division scheme of marine shale facies is not conducive to highlighting the characteristics of the transitional shale,and it is difficult to guide the prediction of dominant reservoir shale.Based on this,this article takes the shale of Longtan Formation in the Eastern Sichuan-Northern Guizhou marine facies M1 well,the transition facies DC1 well,and the DY3 well as the research object.Through whole rock mineral analysis,scanning electron microscopy analysis,oil objective lens analysis,ordinary thin section observation and low temperature Nitrogen adsorption experiment analyzes to the characteristics of inorganic minerals,organic matter,reservoir characteristics and depositional environment,compares marine and land-sea transitional shale,and proposes a division plan for the transitional shale facies.The research has achieved the following understandings and conclusions:(1)Well DC1 in Northern Guizhou developed a delta-tidal flat-lagoon sedimentary system,with distributary channels,interdistributary bays,quagmire marshes,natural dikes and lagoon subfacies.The depositional subfacies alternate longitudinally with rapid changes.The average content of clay minerals is 53.10%,the average content of quartz is 34.60%,the average content of calcite is 9.70%,the average content of pyrite is 3.50%,the average content of TOC is 19.90%,and the average vitrinite reflectance is 1.62%.It belongs to the high-mature stage and is type II kerogen;Well DY3 in Chongqing area is a tidal flat-lagoon sedimentary system.The average content of clay minerals is 68.80%,the average content of quartz is 13.00%,the average content of calcite is 1.10%,the average content of dolomite is 0.90%,the average content of pyrite is5.10%,and the average content of TOC is 15.64%,with an average vitrinite reflectance of 2.25%,which belongs to the over-mature stage and is type III kerogen.In contrast,the marine well M1 in the eastern Sichuan region is a platform basin sedimentary system,with carbonate platform and deep-water shelf sedimentary subfacies developed.Carbonate platform and shelf sediments alternately appear.The average content of clay minerals is 46.44%,which is higher than the bottom.At the top,the average content of quartz is 28.00%,the average content of calcite is 11.70%,the average content of dolomite is 8.70%,the average content of pyrite is 6.90%,and the average content of TOC is 4.60%.The organic matter belongs to the over-mature stage,and the upper part of the organic matter type is type II kerogen.The lower part is type III kerogen;(2)Based on the composition of clay minerals,a set of lithofacies classification standards have been established for the transitional facies shale: taking smectite + kaolinite,iodine mixed layer + green smudge layer,illite + chlorite as the three end members of lithofacies division.The smectite + kaolinite content of more than 50% is called Montmorillonite clay shale facies shale.The iodine mixed layer + green smudge layer content of more than 50% is called mixed clay shale facies shale.The illite + chlorite content of more than 50% is called yiluite clay shale facies shale.If the content of the three does not exceed 50%,it is called mixed clay shale facies shale.Finally,according to this standard,shale in the marine well M1 were classified into two types of clay shale facies: mixed-bed clay shale facies shale,and Yi-green clay shale facies shale;Shale in DC1 and DY3 wells were classified into four main types: High Mongolian clay shale facies shale,mixed clay shale facies shale,yilu clay shale facies shale and mixed clay shale facies shale.(3)Carry out comparative studies on organic matter,pyrite and pore characteristics of different lithofacies shale,and identify dominant reservoir facies: There are two dominant facies in the transitional shale facies,which are the Yi-green clay shale facies shale and mixed clay shale facies shale.The pyrite content of the two dominant facies changes greatly.The pyrite in the shale with more content is mostly strawberry-like distribution,and the pyrite in the shale with less content is mostly distributed in monomers;they develop in the diversion bay and peat swamp respectively,tidal flat-lagoon deposits;both minerals have developed intergranular pores and intragranular pores.The content of organic matter in the shale facies of yilu clay shale is relatively high,but the organic matter pores are not developed.The pore size distribution is 4nm,30 nm,35nm,which are single Peak,bimodal,and multimodal distribution,with an average TOC of17.16%;mixed clay shale facies shale has relatively high organic matter,with relatively developed organic pores,with a pore size distribution of 5nm,which is a single peak,with an average TOC of 9.14%.(4)There is also a corresponding relationship between different sedimentary facies and lithofacies: for marine sedimentary environments,there are mainly two types of clay mineral facies,which are mixed-bed clay shale facies and yilu clay shale facies,while the marine facies M1 well in the Longtan Formation,the mixed clay shale corresponds to the siliceous deep-water shelf and the argillaceous deep-water shelf,and the yilu clay shale corresponds to the siliceous deep-water shelf and the carbonate platform;four types of clay minerals are mainly distributed in the marinecontinent transition facies.The lithofacies are high-mongolian clay shale facies,mixed-layer clay shale facies,yi-green clay shale facies,and mixed clay shale facies.The high-mongolian clay shale corresponds to lagoon deposits,mixed-layer clay shale facies,and Iraqi shale facies.Green clay shale facies and mixed clay shale correspond to diversion bays,quagmire marshes and tidal flatlagoon deposits.