Abstract:

Qujing Basin is one of several Tertiary sedimentary basins with the discovered biogenic gas resources in Yunnan Province. Through tectonic and sedimentary interpretation using two-dimensional seismic lines combining with geological data and well logs, this article discusses the tectonic characteristics and evolution of this basin, and analyzes the effect of tectonic evolution on the accumulation elements of biogenic gas. During the subsidence of faulting stage within the Early to Mid Oligocene, the major gas source-rocks of the large-scale dark mudstone of Caijiachong Formation were deposited. From the Late Oligocene to the Early Pliocene, as a result of cooling due to the entire uplift and erosion of this basin, the overconsumption of the organic matters in Caijiachong Formation by organic maturation and hydrocarbon generation or biogas generation significantly was avoided. In the period of down-warping stage of the Late Pliocene, the secondary gas source rocks of the coal-bearing, sandstone reservoirs, mudstone cap-beds and lithological traps in Ciying Formation were formed. In the end of Pliocene, the basin underwent the stage of compression-shearing uplift and shrink, structural traps of fault nose and fault-anticlines as well as structural-lithological combination traps were formed; The steady subsidence stage of the Quaternary was the period of biogas accumulation.

Key words: Qujing Basin, tectonic characteristic, tectonic evolution, biogas accumulation