Abstract:

 Lithofacies paleogeographic mapping is a key to paleogeography reconstruction.The mapping has various interpretations in foreland areas because of strong deformation, strata hiatus, facies-break and pseudomorphs of paleo-uplift.This paper puts forward a five-steps method to reconstruct paleo-structure in the foreland areas.First, it is necessary to reconstruct structures at 3 key stages: pre-depositional, syn-depositional and post-depositional paleo-structures.Secondly, the distribution and mapping of sedimentary systems should be studied in isochronous stratigraphic framework by the integrated analyses of drilling, log and seismic data.Thirdly, the source area analysis is a tie of distribution of sedimentary systems between basin and orogenic belts.Fourthly, the original strata distributions are restored by reasonably choosing reference points. Finally, the reasonable lithofacies and paleogeographical mapping can be made after all these works mentioned above. This paper takes the Permian Shanxi Formation and Xiashihezhi Formation in the west margin of Ordos Basin as examples to reconstruct lithofacies paleogeography on the basis of paleo-structure restoration and source area analyses by the above five steps.The study  shows that there was a slide-tension fault depression in the north of west margin of Ordos Basin at the early and middle Carboniferous periods, there were three source areas at the Permian period, and the Mesozoic thrusting resulted in 20% to 47% strata shortage. The distribution of sedimentary systems and sandbodies after the restoration of lithofacies and paleogeography differs greatly from that before the restoration, and this will be significant to guide the Upper Palaeozoic gas exploration in the west margin of Ordos Basin.

Key words: paleo-structure reconstruction, lithofacies paleogeography, balanced crosssection, source area, the Permian System, the west margin of Ordos Basin