STUDY ON THE RESERVOIR CHARACTERISTICS AND GAS-BEARING PROPERTY OF LONGMAXI FORMATION IN PY-1 WELL, SOUTHEAST CHONGQING

The southeast of Chongqing is a key area for shale gas exploration and development in China. Taking the shale of Longmaxi Formation in PY-1 well, Pengshui area as the object, combined with the drilling data, whole rock analysis, thin section observation, scanning electron microscope(SEM), nuclear magnetic resonance and nitrogen adsorption experiments, the paper systematically analyzes the rock-mineral characteristics, organic geochemistry, reservoir characteristics and gas-bearing property of the shale. The study shows that the shale is about 80 m thick with high brittle mineral content, mainly quartz accounting for about 48%, followed by feldspar and carbonate minerals. The clay minerals account for 29%, mainly illite accounting for 65% of the total, similar to the mineral composition and content of American Barnett shale, which is conducive to the later fracturing. The organic matter type of shale is II₂ with TOC of 1%-3% (averagely 1.84%) and maturity of 2.5%-4.4%. The reservoir space is mostly nano-micron scale pore, mainly organic matter pore and intergranular pore. The intragranular dissolution pores, microfissures, intergranular pores of clay mineral and a few intercrystalline pores are seen with average porosity of 2.43%, micro-pore dominated (pore diameter of 2-80 nm), and permeability of 0.005-0.01 mD. The gas-bearing property is moderate-low with average desorbed gas of 1.15 m³/t, and average saturated adsorption capacity (VL) of 2.56 m³/t, both lower than the gas content of the Barnett shale and JY-1 well. The organic matter abundance and reservoir porosity are the main controlling factors of shale gas content in the area, followed by the contents of brittle and clay minerals.