198 / 2019-06-13 21:53:50

Rock Types and Analysis of Sedimentary Environment of Upper Assemblage of Ordovician Majiagou Formation in the Sulige area, Ordos Basin, China

Sulige area; Ordovician; Rock types; Sedimentary environment;

The Sulige area, which located in the north-central part of the Ordos Basin, formed a set of strata consisting mainly of carbonate rocks and evaporite rocks during the Majiagou period in the Ordovician. Among them, the strata of the upper assemblage of Majiagou Formation were superimposed diagenesis under the influence of Caledonian movement, which resulted in different degrees of reformation of the original rock fabric and was not conducive to the identification and restoration of the original sedimentary environment. Therefore, the original rock fabric was restored in this paper by observing the rock core and using methods such as thin section identification and cathodic luminescence. The rocks were divided into four rock types: grain dolostone, crystal grain dolostone, microbial dolostone, and strongly reformed rock whose original sedimentary fabric could not be restored. Among them, grain dolostone was mainly composed of dolarenite and oolitic dolomite; the crystal grain dolostone is mainly composed of powdered dolomite and putty crystal dolomite; the microbial dolostone is mainly composed of Algal frame dolomite, algal-laminated dolomite and algae-bonded dolarenite. According to sedimentary lithology and combination of the research region features, sedimentary structure and phase analysis, combined with regional geological background, it is considered that the upper assemblage strata of Majiagou Formation mainly develop restricted platform facies, and can be subdivided four subfacies of (half) limited lagoon subfacies, Mound shoal subfacies, platform subfacies, and limited sea level subfacies. Based on the above understanding, combined with the development characteristics and identification marks of facies in the study area, the sedimentary sub-facies of the restored reservoir rock types are analyzed by using the analysis method of carbonate sedimentary sub-facies. It is concluded that grain dolostone and microbial dolostone are developed in Mound shoal subfacies, formed in shallow water environment near and above the normal wave base surface, and the energy exchange of sedimentary water body is relatively active in the study area. The range is relatively small, mainly distributed in the strata of Ma53. The crystal grain dolostone developed in the platform subfacies and formed in the sedimentary environment where frequent exposure resulted in strong evaporation. The study area has a relatively large scope and is mainly distributed in the strata of Ma54, while a small amount is distributed in the strata of Ma51+2.