Exploration and discovery of the Maokou Formation dolomite gas field in Longnüsi area of central Sichuan Basin and its significance

In 2023, a major breakthrough was achieved in exploration of the Member 2 of Maokou Formation in Longnüsi area of central Sichuan Basin. The proven reserves submitted to Ministry of Natural Resources of the People’s Republic of China exceeded 460×10⁸m³, marking the first large-scale gas reservoir discovered in the 60-year exploration history of Maokou Formation in Sichuan Basin. As of 2024, PetroChina Company Limited has proven geological reserves of natural gas exceeding 1 500×10⁸m³ in Maokou Formation of central Sichuan Basin, making the Maokou Formation an important new field for increasing reserves and production in the Sichuan Basin. The main understandings obtained are as follows. (1) It is innovatively proposed that the paleo-uplift of central Sichuan Basin has a significant controlling effect on the differentiation of ancient landforms in the early sedimentary period of the Member 2 of Maokou Formation. The slope break zone developed at the edge of the paleo-uplift is a favorable area for the development of high-energy shoals. The shoal thickness is large in Longnüsi-Hechuan-Nanchong area, which is a favorable breakthrough area for exploration. (2) The high-energy shoal facies in Longnüsi area of central Sichuan Basin has undergone multiple dolomitization processes such as early exposure, dissolution and shallow burial, resulting in the formation of high-quality dolomite reservoirs in the Member 2 of Maokou Formation and providing reservoir space conditions for large-scale accumulation. The lower submember of Member 2 of Maokou Formation in the study area develops fracture-pore type high-quality reservoirs, with thickness from 5 m to 30 m, core porosity from 2 % to 8 %, and permeability from 0.01 mD to 1.00 mD. (3) The large-scale hydrocarbon accumulation in Maokou Formation of the Longnüsi area was mainly controlled by multi-source relay hydrocarbon supply, early reservoir formation and trap formation. High-quality source rocks from the Cambrian Qiongzhusi Formation, the Silurian Longmaxi Formation, and the Member 1 of Permian Maokou Formation successively played an important role in sustained hydrocarbon supply after maturation, providing four-stage continuous oil and gas charging from Middle Triassic to Late Cretaceous in Maokou Formation. Early-stage hydrocarbon accumulation occurred in high-energy shoal facies reservoirs, which were laterally sealed by tight limestone, forming early lithologic traps favorable for the formation of large-scale gas reservoirs. (4) The Longnüsi-Hechuan-Nanchong area is the most favorable exploration area for the lower submember of Member 2 of Maokou Formation. The Ya’an-Dujiangyan area in western Sichuan Basin, the Yanting area in northern Sichuan Basin, and the Luzhou area in southern Sichuan Basin have similar sedimentary backgrounds and favorable accumulation conditions, and are potential favorable areas for the next exploration.