Abstract:

This study demonstrated the differences in depositional sequence evolution, sandbodies distribution and reservoir microstructure and their controlling factors in different structures of foreland in the southwestern Ordos basin. Detailed information has been acquired through the analyses of tectonic evolution of foreland basin in the southwestern Ordos basin, the observation and correlation between the outcrops from RuiheLongxianLinyou profile of Yanchang Formation in foreland thrust belt and the outcrops from Yanhe profile of Yanchang Formation in the foreland slope belt, and the analyses of thin sections of rocks. The main conclusions are as follows. Firstly, three transgressive system tracts and three regressive system tracts are divided from Chang 10 to the lower part of Chang 2 oil layers of Yanchang Formation in the southwestern Ordos Basin. Moreover, sands mainly distribute in the early transgression and the whole regression. Secondly, the foreland thrust belt is near the Qinqi fold orogenic belt provenance, and develops a steep slope. Consequently, the sedimentation of sandbodies is controlled by tectonization. Therefore, the characteristics of proximal provenance braided river delta front can be observed as follows. The braided river delta enters the lake swiftly, so the particle size of thick sandbodies is large. However, the content of detritus and matrix is high, for this reason, the physical property of reservoirs can be comparatively poor. Thirdly, foreland slope belt is far away from Yinshan fold belt provenance in the north, and develops a gentle slope. As a result, the sedimentation of sandbodies is controlled by the shoreline shift. Accordingly, the characteristics of distal provenance meandering river delta can be observed as follows. The sands spread wide, and the facies change frequently. Further, the particle size is small, and the content of matrix is low. Besides, diagenesis produces secondary interstices. As the above data show, good reservoirs can be formed.

Key words:  Ordos Basin, foreland basin, Yanchang Formation, delta, systems tract