The domain of normal pressure shale gas in the Middle and Upper Yangtze region is a new focus of attention

with a typical case of the Lower Cambrian shale gas in the Xiangzhong Depression.Based on two measured outcrop sections

eight observation outcrop sections

one shallow-drilling and seismic data

we studied the geological characteristics of the Lower Cambrian shale gas in the Xiangzhong Depression.The study area is favorable for the generation of shale gas.The Lower Cambrian black-carbonaceous and siliceous shales were deposited under deep-water environments for favorable slope and basinal facies

with a continuous thicknesses of more than 150 m and a high-quality shale thickness of 50-90 m.The TOC of the black shales ranges between 1.12% and 22.07%

averaging at 4.64%;and the reflectance of vitrinite (R

o

) averages 2.78%.The shale reservoir space consists of organic-matter nanopores

dissolved pores

intergranular pores of pyrites and clay minerals

high-angle fractures

bedding-parallel fractures

and so on

but the organic-matter nanopores are dominant.The organic-matter nanopores occur in strips or irregular polygons due to pressure relief and exposure to the surface

with aperture size ranging from 21.8 nm to 328.4 nm.Samples from the surface are observed to be filled with many secondary dissolved pores of meteoric water origin

and cracks due to tectonic stress.The porosity ranges between 2.0% and 8.7%

averaging at 6.05%.Quartz predominates the mineral composition of shales

averaging at 61.2%.Clay minerals rank second

averaging at 23.5%.Brittle mineral content averages 73.38%.Large number of isothermal adsorption experiments have indicated that its average adsorption capacity is 5.95 m

3

/t.Favorable zones of Type I shale gas in the new interval of Lower Cambrian were predicted to distribute in an area of 8 766.1 km

2

by comprehensive geological methods.It is proven that the exploration potential of shale gas is significant in the both sides of the Longshan uplift

where the burial depth of the Lower Cambrian is mostly between 3 000 m and 4 000 m

a favorable condition for preservation.