| Ordovician pre-salt dolomite is an important natural gas producing layer in Ordos Basin, which has great
exploration potential. However, the distribution law of favorable facies of dolomite reservoir is not clear, which restricts
the next exploration process. Therefore, based on the new progress of paleogeographic pattern research, combined with
data of field outcrop, drilling core, rock slice and physical property, the sedimentary characteristics and the distribution
law of favorable facies of Ordovician pre-salt strata in the cetral-eastern Ordos Basin are studied. The results show that:
(1) There are mainly two types of sedimentary facies (eight types of sedimentary microfacies) in Ordovician pre-salt
layer, which are restricted platform (dolomite flat, intra platform mound-shoal complexes, dolomitic lagoon and
calcareous lagoon) and evaporation platform (mud-dolomite flat, gypsiferous dolomite flat, gypsum-dolomite flat and
gypsum-salt lagoon).（2）Under the control of paleoclimate, sea level fluctuation and paleogeographic pattern of “three
uplifts and three depressions”, the Ordovician pre-salt paleogeography presents the plane distribution characteristics of
“uplift controlled intra platform mound-shoal-flat and depression controlled lagoon”. In periods of the 10th submember of
Member-5 of the Lower Ordovician Majiagou Formation(O1m510) and O1m58, Mizhi depression and Taolimiao depression
are gypsum-salt lagoon and calcareous lagoon respectively; Hengshan uplift and residual areas of central paleo uplift are
gypsum-dolomite flat and mud-dolomite flat respectively. In periods of O1m59 and O1m57, Mizhi depression and Taolimiao
depression are mainly calcareous lagoon and dolomite lagoon; Hengshan uplift and residual areas of central paleo uplift
are mainly intra platform mound-shoal complexes and dolomitic flat. In periods of O1m56, Mizhi depression is gypsumsalt
lagoon; Hengshan uplift is gypsum-dolomite flat, and the residual area of central ancient uplift is mud-dolomite flat.
（3）The lithology of intra platform mound-shoal complexes is consisted of arenaceous dolomite, oolitic dolomite and
agglomerate dolomite, with dissolution holes, intergranular dissolution holes, intergranular pores and intragranular pores
developed, which has good relatively reservoir performance. It is a favorable facies belt for the development of Ordovician
pre-salt high-quality dolomite reservoirs. It is concluded that Sulige-Chengchuan, Wushenqi-Zhidan and Shenmu-Mizhi
areas are favorable reservoir development areas and an important direction of natural gas exploration. |
