The Detailed Palaeogeomorphologic Restoration of the Ordovician Weathering Crust in the Northern Jingbian Gas Field: Methods and Applications into Reservoir Properties and Gas Accumulation

Abstract

Palaeogeomorphology plays a crucial role in the development of sedimentary facies and the reservoir distribution in the late period of basin evolution, and to a certain extent, controls the assemblage of the late hydrocarbon reservoirs and caprock formations. Therefore, it has a function in controlling the enrichment of oil and gas. Caledonian orogeny uplifted the Ordovician Majiagou Formation in the Ordos basin and thus led to the formation of the weathering crust reservoirs in the process of weathering, erosion and leaching for over 130 Ma. Such palaeogeomorphology represents the background of the palaeokarst, which has an essential influence on the formation of reservoir rocks. Based on Pre-carboniferous paleogeologic map plotted by 305 wells, and the thickness of bauxite in 272 wells, methods of residual thickness and moulage are applied to reconstruct the palaeogeomorphology in the studied area. In combination with the thickness of horizon to the weathering crust, the criteria for the second order palaeogeomorphologic units were proposed. They are karst highland, karst slope and karst basin, which can be further subdivided into the third order palaeogeomorphologic units of remnant hill, depression, gentle hill, shallow depression and erosional grooves. According to the criteria and refined identify results of ancient grooves using geological data of 272 wells, palaeogeomorphology of the Ordovician weathering crust in the northern Jingbian gas field has been restored accurately. The results show that karst highland developed in the northwest part of the studied area, with remnant hill and depression occurring locally. Karst slope was present in the central and east, predominatly in the form of gentle hills and shallow depressions, with 5 nearly EW-trending ancient erosional grooves developed locally. Finally, integrated data of reservoir property with gas production test implied that the remnant hill in the karst highland, the gentle hill in the karst slope and the two sides of the erosional grooves were favorable positions for gas accumulation.