Abstract:

The Lower Silurian Longmaxi Formation marine shales, with high organic matter abundance and high thermal maturity, have been known as the main shale gas reservoirs in southern China. Pore characteristics and the influencing factors for pore development were investigated by samples collected from the Well Xiye1 and Well Tongye1 in northwestern Guizhou, China. FESEM implied that nearly all pores are nanometerscaled. Pores could be divided into mineral matrix pores (including interparticle pores and intraparticle pores), organic matter (OM) pores and microfracture pores. OM pores are the most prevalent ones within these shale samples. Results of other experiments show that the TOC content averages 3.80%; the Ro value averages 2.73%; and the total porosity averages 1.56%. The shales are rich in clay and detrital minerals (quartz and feldspar), with average contents of 32.42% and 43.97%, respectively. The average BET specific surface area is 16.98 m2/g; the BJH total pore volume is 0.0137 mL/g; and the pore diameter is 33.75 nm. Regression analyses suggest that the TOC content is the key influencing factor for pores development. The total porosity is positively correlated with TOC and detrital mineral content, but is negatively correlated with clay mineral content. Besides, saturated adsorption amount averages 2.42 m3/t and there is a strong positive linear correlation between saturated adsorption amount and the TOC content, which infers that OM pores may provide a main space for gas storage. Saturated adsorption amount also shows strong positive linear correlation to total porosity, detrital minerals content, the BET surface area, and the BJH total pore volume.

Key words: Northwestern Guizhou, Longmaxi Formation, pore types, influencing factors, gas storage