Deep Gas Discoveries and New Exploration Horizons in the Songliao Basin*

Abstract

The Songliao Basin is a Mesozoic-Cenozoic basin with structures that formed during two stages of development. Rift development in the Late Jurassic to Early Cretaceous period resulted in the deposition of thick clastic and volcanic sediments in isolated rift depressions, with average area around 1300-5800 sq. km, whereas the sag development in the Upper Cretaceous series led to several kilometers of freshwater lacustrine-deltaic sediments in a gigantic sag basin. A number of giant oil discoveries were made by early exploration of structural traps in the Upper Cretaceous strata. The depletion of oil reserves in this basin from over half of a century's production has been substantially offset by oil reserve addition from increased exploration of subtle stratigraphic-lithological traps toward the distal parts of the basin, and more importantly by major breakthroughs in gas exploration in the deep rift depressions underlying the current oil production zones. This presentation will use the giant gas discovery from volcanic reservoirs in the Xujiaweizi rift depression, as an example, to discuss the critical geological controls on volcanic reservoir properties and to highlight the exploration strategies and key technical factors that were responsible for the discovery. The deep discovery in volcanic reservoirs will not only broaden the exploration scope in the Songliao Basin, but also provide an excellent analog for exploration in other rift basins worldwide.