Abstract: As a kind of abnormal gas accumulation, deep-basin gas is trapped with particular geological conditions and has particular trapping mechanism and distribution pattern. The tectonic framework with the characteristics of continuous deposition provides favorable structural condition for deep-basin gas accumulation in Taibei sag. The natural gas generated during the first stage for the Middle- Lower Jurassic coal-measure source rock which still generating gas currently is the main source for deep-basin gas accumulation. The sandstone of Middle- Lower Jurassic is low-porosity and low-permeability. The porosity and permeability of Xishanyao formation and the deeper reservoirs are usually less than 10% and 4×10^-3μm²,respectively. The assemblage of 3 sets of regional cap rocks and some local cap rocks forms tight top-covering and bottom-covering. The most favorable region for deep-basin gas exploration in Taibei sag is located in Xiaocaohu sub-depression. The characteristics of source-accumulation being a unity, abnormally low pressure, gas bearing in large area, uncontrolled by a tectonic isobath, gas-water inversion indicate the existence of deep-basin gas accumulation.

Key words: deep-basin gas, geological condition, reservoir-forming feature, Turpan-Hami basin, Taibei sag