GEOLOGICAL OCCURRENCE CHARACTERISTICS AND ENGINEERING HAZARDS OF ULTRA SHALLOW-BURIED GASES IN NINGMING BASIN

Shallow gas is a natural resource and also can bring about geological hazard. The disturbance of engineering unloading and the effect of formation pressure cause shallow gas evolution, resulting in damages to engineering works and accidents. This paper examines the shallow gas geological disasters occurred during the construction of Nali Bridge in Ningming Basin in Guangxi. In this context, the occurrence characteristics of ultra-shallow gas in the research area and its damage to engineering works are studied using geological drilling sampling method combined with geochemical experiments. Results show that:(1)Due to the frequent regional tectonic activities and fracture development, the thin silt, siltstone and silty mudstone sandwiched by the mudstone had a thickness of varying from 2 mm to 200 mm, with a measured porosity of about 25.6%, and the permeability of about 37.2~46.8 μm². Shallow gas occurs in all kinds of reservoirs in the form of facies control, fracture control, shell control and their transition, showing the characteristics of independent, discontinuous and staggered distribution. (2)The deep and thick mudstone layer in the region with an organic carbon content(TOC) of 0.61% to 1.24% and abundant organic matter and high gas production capacity belongs to the source rock of shallow gas. The measured porosity of mudstone cap rock is 22.8% to 34%, with a permeability of (2.4~22.2)×10^-3μm² and ordinary sealing capacity. (3)Due to the geological occurrence characteristics of ultra-shallow natural gas in the area, it was hard to generate abnormal pressure within the range of engineering exploration and construction depth(10~100 m). Despite no occurrence of strong blowout accident during the construction of bridge cast-in-situ bored piles, it would cause gas invasion damage to the pile foundation concrete and affect pile body completeness. It is recommended to adopt advanced exhaust gas measures and to improve relevant technologies.