Reconstruction of karst paleo-geomorphology and paleo-water system on the top of Maokou formation in Yuanba area, northern Sichuan Basin

The carbonate karst reservoir of Maokou formation is developed in the Yuanba area, northern Sichuan Basin. However, it is difficult to predict this set of reservoir because of its strong heterogeneity. Therefore, for the guidance of future oil and gas exploration and development, it is necessary for us to further restore karst paleo-geomorphology and paleo-water system so as to master the law of reservoir distribution. In order to better depict the micro karst landform, the ''the trend surface of residual thickness and impression residual" are combined to restore the ancient landform. According to the characteristics of karst paleo-geomorphology on the top of Maokou Formation in Yuanba area and the thickness from the top of Maokou formation to the top of its first section in Yuanba area, an index system for the classification of karst paleo-geomorphology types is established. Based on the thickness mentioned above, the paleogeographic environment and paleo-hydrodynamic conditions, the study area is divided into four second-tier geomorphic units—karst platform, gentle karst slope, karst plain and karst basin. Then, in terms of modern karst classification, a third-tier geomorphic units are subdivided into six types—micro hill depression, hill-cluster depression, micro hill trough, cluster-peak depression, hill-cluster ridge valley and monadnock plain, according to the micro geomorphic combination form of karst paleo-geomorphology on the top of Maokou Formation in Yuanba area. Based on the characteristics of paleo-topography as well as the plane distribution and mutual configuration relationship of the six third-tier geomorphic units, the surface water system of the paleo-karst surface at the top of Maokou formation in Yuanba and its adjacent areas is constructed. In general, controlled by the ancient topography, the surface runoff is mainly the scattered runoff from the karst platform to both sides. The ancient water system is not well developed with no centralized runoff system, and only small gullies and karst lakes are locally developed. On the whole, the ancient karst landform on the top of Maokou formation in the study area belongs to the initial stage of the formation and evolution of karst landform. The relative elevation difference of the regional terrain which is slightly undulating is generally less than 120 m. The relative elevation difference of hills (peaks) and depressions is generally less than 10-30 m, which belongs to the initial stage of tectonic uplift and denudation and is characterized by coastal (island) karst landform, and the surface water system is not fully formed. The karstification time of the study area is relatively short, belonging to the initial stage of epigenetically exposed karst which is characterized by the joint karstification of atmospheric fresh water and seawater. The karstification mode is mainly the leaching and infiltration of atmospheric fresh water, which is difficult to form concentrated lateral runoff. Karstification mainly occurs in the shallow part, and karst is dominated by dissolution holes. Large-scale karst fractures and caves have not been formed. There are great differences in karst development in different geomorphic locations, among which the karst platform belongs to the groundwater recharge area; the atmospheric precipitation is mainly vertical infiltration; and the lateral runoff of groundwater is slow. The karst plain belongs to groundwater runoff and discharge area, where groundwater runoff is slow, and the intensity of karstification is relatively weak. The karst basin belongs to groundwater drainage area with long water-rock interaction cycle and weak karstification intensity. The gentle karst slope belongs to groundwater runoff area with the strongest hydrodynamic force and the most developed holes, which is suitable for the future reservoir exploration.