BALANCED CROSS-SECTION FOR RESTORATION OF STRUCTURAL EVOLUTION IN A FORELAND THRUST BELT:TAKING THE NORTHWESTERN MARGIN OF JUNGGAR BASIN FOR EXAMPLE

The balanced cross-section is an effective tool for quantitative restoration of structural evolution under the guidance of a constant geometry. The northwestern margin of the Junggar Basin is a typical foreland thrust belt. Due to the complexity of geological conditions and the horizontal variation of seismic wave velocity,the study of tectonic evolution and its dynamic mechanism is not deep enough in the basin that constraints effective oil exploration. In order to obtain the reliable results, adequate methods should be adopted on the basis of actual geological conditions. For the reliable restoration of structural evolution by balanced cross-section in foreland thrust belt in northwestern China, 4 critical steps are suggested from technical point of view in this paper: Firstly it is necessary to select a suitable seismic line; Secondly, use variable velocity parameters in time-depth conversion to have reliable geologic profiles; Thirdly, adopt the flexural-slip mechanism for middle and upper strata, and then apply the vertical mechanism to deeper strata; Finally, decompact the strata by segment where the lithologic characters vary intensively in the horizon to decrease the decompaction errors. The northwestern margin of the Junggar Basin in Northwest China, which is a typical foreland thrust belt, is discussed in this paper as an example. The result suggests that the study area experienced a complete tectonic cycle consisting of three tectonic movements of compression- extension- compression. There is a rapid low amplitude compression stage in late Permian, and the strongest compression of tectonic evolution in Triassic. All the above has important impact on the formation of structural framework and the migration and accumulation of hydrocarbons.