Lithofacies Paleogeographic Characteristics of Ma3 Member in Eastern Ordos Basin

Abstract

The Ordovician Magnolia Formation in Odors Basin is one of the core horizons of oil and gas exploration. The third member of Magnolia Formation ( Ma3 member ) is an important part of this formation, and its facilitates choreographically pattern profoundly controls the development and distribution of high-quality reservoirs. In recent years, the predecessors have made many new understandings on the study of the third member of the Madman Formation, but the overall research degree is weak, and the accumulation law and main control factors of the natural gas in the third member of the Madian Formation have not been clarified, resulting in difficult well deployment and high risk, which restricts the exploration and development process of deep natural gas under salt to a certain extent.In this paper, the main research goal is to restore the lithofacies paleogeography of the Masan Formation in the Ordos Basin.Based on the geological, logging, and seismic data, the' trichotomy ' of the Masan Formation was established. The pre-depositional paleogeomorphology was restored by the impression method. Combined with sedimentology, paleogeomorphology restoration and lithofacies paleogeographic analysis, the lithofacies paleogeographic characteristics of each layer of the Masan Formation were systematically described, and the changes of the internal uplift and depression of the Masan Formation and the resulting sedimentary microfacies evolution were analyzed. The study shows that the internal area of the platform in the sedimentary front of the Masan member is also relatively flat, accounting for about 70 % of the area, of which the low-lying area in the eastern platform accounts for about 45 % of the area. The southern platform margin area and the western denudation ancient land slope area account for about 30 % of the area ; the Ma3 member is dominated by the evaporative platform facies under the regression system, mainly developing the intra-platform lagoon and the evaporative tidal flat subfacies. The intra-platform lagoon is mainly the salt rock lagoon microfacies, and the evaporative tidal flat subfacies is further divided into ash dolomite flat, dolomite flat, mud dolomite flat, gypsum dolomite flat and other microfacies. During the deposition period of the third member of the Mashan Formation, the eastern Ordos Basin belonged to the transgressive system tract of the second stage of the Mashan Formation to the regressive system tract of the third stage of the Mashan Formation, and the seawater depth became shallow, but the low-lying areas along the Shenmu-Yichuan area still existed. Coupled with the strong evaporation of paleoclimate conditions, the low-lying areas became sulfates, that is, the favorable place for the deposition of gypsum-salt rocks, thus forming a relatively closed platform lagoon environment. The lagoon area is very large, from the Shenmu area Shuang 176 well-Shen 74-M 1 to the southern Yu 152-Yi 68 well line, becoming a salt rock sedimentary area ; from the lithofacies paleogeographic distribution map of each sub-layer of the Masan Member, the favorable microfacies intra-platform shoals are mainly distributed in the Masan 1 and Masan 2 sections, and the distribution of Masan 3 is limited.