203 / 2022-02-28 22:42:32

Lithofacies paleogeography and its geological significance of oil and gas exploration of Sub-member Ma54 in the Ordovician Majiagou Formation, Northwestern Ordos Basin

Paleogeographic restoration; Natural gas distribution; Carbonate strata; Majiagou Formation; Ordos Basin

Restoration of lithological paleogeography is a important link in exploration of oil and gas of carbonate reservoir. In order to clarify the relationship between paleogeography and distribution of natural gas, this paper studied the characteristics of lithology and lithological paleogeography of the Sub-member Ma54 of Majiagou Formation in the northwestern Ordos Basin by core observation, thin section identification, multiple mineral logging interpretation, and “four steps mapping method”. Then the significance of lithological paleogeography restoration was discussed by combing with the distribution of natural gas layer thickness calculated by logging interpretation and actual gas yield datas. The result shows that:(1) The main rock types of the Sub-member 54 mainly include argillaceous dolostone, gypsum dolostone, dolostone containing calcite and dolostone. (2) The lithofacies paleogeography environment can be divided into two subfacies including evaporate platform and restricted platform, and the former mainly includes two microfacies such as dolomitic flat and gypsum-containing dolomitic flat, while the latter mainly develop gypsiferous lake. (3) Dolomitic flat and gypsum-containing dolomitic flat are mainly distributed in the northwest and middle of the study area, respectively, while the gypsiferous lake is mainly developed in the central and east, and southeast. The gypsum-containing dolomitic flat developed in evaporate platform and gypsiferous lake developed in restricted platform are the favorable areas for natural gas exploration in the study area.