Fractal characteristics of organic-rich shale pore structure and its geological implications:A case study of the Lower Silurian Longmaxi Formation in the Weiyuan block,Sichuan Basin
Different from medium and shallow shales,the deep organic-rich shales from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the southern Sichuan Basin have experienced intricate diagenesis,resulting in diverse pore structures that have a significant impact on shale gas occurrence.To explore the pore structure of deep organic-rich shale and its fractal characteristics and impact on shale gas enrichment,this paper quantitatively characterizes the basic parameters and pore structures of deep organic-rich shale reservoirs in the Longmaxi Formation in the Weiyuan block,southern Sichuan Basin by means of organic carbon analysis,X-ray diffraction,high-resolution scanning electron microscopy and low-temperature nitrogen adsorption.Then,based on the Frenkel-Halsey-Hill(FHH)fractal theory and low-temperature nitrogen adsorption experiments,the pore fractal dimension of shale is calculated,and the influencing factors and geological significance of the pore structure are discussed.The results show that the deep shale samples of the Longmaxi Formation in the Weiyuan block have a high total organic carbon(TOC)content(averaging 2.04％)and contain brittle minerals such as quartz(accounting for 38.00％-73.00％).The deep shale reservoirs of the Longmaxi Formation universally develop inkpot-like organic pores and slot-type or tabulate inorganic pores,with a BET specific surface area of 1.252-28.676 m2/g and a BJH pore volume of 0.013 192-0.063 874 cm3/g.The pores in the deep shales present obviously segmented fractals,with the fractal dimension D2＞D1.The pore structure is controlled by the composition of shales,while the fractal dimension is controlled by the specific surface area and pore diameter,and is closely related to the TOC content.The organic-rich siliceous shales deposited in the LM1-LM3 graptolite zones have a high TOC content,high fractal dimension D,and D2 values,and the largest BET specific surface area and BJH pore volume of mesopores.Therefore,they have many gas adsorption sites to store adsorbed gas and also a large pore space to store free gas,making them"sweet spots"of deep shale gas enrichment.It is concluded that the integrated shale quality parameters and fractal dimension characteristics can evaluate the sweet spots of shale gas,which is of practical significance to incremental reserves/production of deep shale gas.
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