Sedimentary Characteristics And Favorable Reservoir Facies Prediction Of Ma 5 1+2 In The Northeastern Ordos Basin

This paper takes Ordovician Ma5₁₊₂ Sub-member in northeastern Ordos Basin as an example,it is found that during deposition of Ma5₁₊₂ Sub-member,the study area was a site of deposition of restricted-evaporate carbonate platform facies by observing and analyzing cores and thin sections of 32 cored wells.Five subfacies types are recognized,which represents about five sedimentary environment in the following:grain shoal,mud mound,platform inner flat,restricted-evaporate lagoon and shoal interact sea,and they can be further divided into a number of microfacies.These subfacies make up a typical sedimentary sequence in vertical,shoal interact sea-grain shoals-platform inner flat,restricted-evaporate lagoon-grain shoals-platform inner flat complete-incomplete upward shoaling sedimentary sequence,and it develops by the superposition of multiple sedimentary cycles.Based on the cross correlation of sedimentary facies,it is found that the grain shoals owns a stable distribution and good continuity,interbedding with platform inner flat,the horizontal extension of restricted-evaporate lagoon is relatively good,interbedding with grain shoals,while mud mound and shoal interact sea distributed sporadically.On the plane,grain shoals owns a stable distribution,and the stdy area with a nearly NS-striking distribution,the other areas consist mainly of shoal interact sea,restricted-evaporate lagoon,gypsum flat.By comparing the reservoir properties of different lithology-lithofacies,it is found that microtopography landform and grain shoal are the most favorable reservoir subfacies belt,karst palaeogeomorphology is a comparatively favorable reservoir subfacies belt,and they mainly distribute in the north and central,which has provided the geological basis for the furture exploration of Ma5₁₊₂ Sub-member in the study area.