Petrological Characteristics And Evaluation Of Marine Organic Shale Reservoirs In Longmaxi Formation, Southern Sichuan

The detailed study of petrophysical characteristics of shale is the basis for evaluating shale gas reservoirs,which can better explain the complex nature of shale reservoir parameters and provide support for subsequent research work,which is of great significance to the exploration and development of shale gas.Based on the observation of field outcrop and drilling core,this paper analyzes the petrological characteristics and establishes petrology type division scheme of organic matter shale in Longxaxi Formation,Lower Silurian,South China,combined with mineral composition,organic petrological parameters and rock slice observation.On the basis of this,this paper combines the pore-permeability characteristics,pore type and pore structure parameters to make a evaluation of the shale reservoir in Longmaxi Formation of the study area.The petrological characteristics of shale reservoirs in Longmaxi Formation of the study area were clarified by means of core and rock slices observation,all-rock X-ray diffraction and organic petrology test techniques.According to the mineral composition,the shale of the Longmaxi Formation in the study area is divided into clay shale(I),siliceous shale(II),calcareous shale(III),clayy siliceous mixed shale(IV)silicaous calcium mixed shale(V),clayey calcareous mixed shale(VI),and then with organic matter content of 2%and 4%as the sector,according to high,medium and low organic matter abundance of standards to do further division.The pore types of shale reservoirs in Longmaxi Formation of the study area were identified by argon ion polishing and field emission scanning electron microscopy,including organic matter pores,intergranular pores,intergranular pores and a small amount of microcracks,the intergranular pores are the most developed,followed by organic matter pores.Organic matter pores have good connectivity in the organic matter,and the grain boundary microgrids between the mineral particles can be used as the throat connection with other pores,which is the main throat space of the shale in the study area.The organic matter pore is mainly developed in the lower organic matter shale of the lower part of the Longmaxi~1,with the formation upwards,the organic matter pore tends to decrease,and the dissolved holes and the pores of the crystal is increasing.In the low organic shale,the inorganic pores become the main pore type.The physical properties of the shale reservoirs in the Longmaxi Formation were analyzed by the combination of physical properties test,liquid nitrogen gas adsorption method and high pressure mercury intrusion method.The results show that the physical properties show low porosity and low permeability,the pore volume is small and the nanometer pore is mainly developed.The average pore size of the pores of high and medium organic shale in the lower part of the Longmaxi~1 is the smallest,the average specific surface area is larger,the connectivity of the pores is better.With the decrease of burial depth,the average pore size becomes bigger,the specific surface area becomes smaller and the pore connectivity becomes worse;Consolidate the various geological conditions parameters,the high quality shale reservoirs of Longmaxi Formation in southern Sichuan are mainly distributed in the high organic shale and medium organic siliceous shale in the lower part of the Longmaxi~1.The secondary reservoir is the middle organic matter siliceous shale in the middle of the Longmaxi~1.The medium organic clayy siliceous mixed shale in the upper and lower part of Longmaxi~3 and low organic matter siliceous shale in the upper Longmaxi~1 are poor reservoirs.