| the Ordovician Majiagou Formation can be divided into six members in Ordos Basin, The fifth member,
Majiagou Member-5 (Ma-5), can be further divided into ten submembers, including Ma-51 to Ma-510, in which the
Ma-51+2, Ma-54, Ma-56, Ma-58 and Ma-510 submembers deposited during relative regressions, and the Ma-53, Ma-55,
Ma -57 and Ma -59 submembers deposited during relative transgressions. The formers are commonly composed of
gypsum and salt while the latters are commonly of limestone. Based on the observation of the outcrops, drill cores and
thin sections, as well as well logging and seismic interpretation, 16 sedimentary microfacies sequences are identified by
the single-factor integrated method and then the associated models of transgression and regression are established. The
distribution of intraplatform grain beach is sketched in the Middle Assemblage (Ma-55 to Ma-510) on the east side of
paleouplift and 10 Majiagou submembers of lithofacies palaeogeography are reconstructed and mapped. The plane
distribution of favorable sediment facies belts is predicted for the weathering -crust karst reservoirs in the Upper
Assemblage (Ma-51 to Ma-54) and the dolostone reservoirs in the Middle Assemblage of Majiagou Formation. |
