Sequence-based lithofacies and paleogeography of Majiagou Formation in Ordos Basin

Abstract：The Majiagou Formation in Ordos Basin can be divided into 6 third-order sequences and 12 system tracts on the basis of field profile, core and well log. In this paper, characteristics of sequence stratigraphy of Majiagou Formation were analyzed, sequence-based lithofacies and paleogeography were also compiled by means of condensation of mapping units of sedimentary system tracts. It is indicated that the basin sedimentary evolution was controlled by the North China Sea and the Qilian Sea, with the former being an epeiric sea and the latter being an epicontinental sea. OSQ1, OSQ2, OSQ3 and OSQ5 are mainly restricted platforms in TST，and some areas are anointed lakes in HST; open platform is the main facies of OSQ4 and OSQ6 whether in TST or HST; reef and shoal are formed in HST of southwest margin. Dolomites of HST in OSQ1, OSQ3, OSQ5 and southwest reef and shoal might be important oil-gas reservoirs, and gypsum of HST in OSQ3 and OSQ5 forms 2 major regional caps of Ordos Basin.