Inversion and propagation of the Late Paleozoic Porjianghaizi fault (North Ordos Basin, China): Controls on sedimentation and gas accumulations

摘要

In contrast to typical intracratonic basins, the Ordos Basin comprises marginal deformation belts and complex fault systems. This study uses 3D seismic data tied to sonic and stratigraphic data from exploration wells, to document the geographic extend of the Porjianghaizi fault and its control on sedimentation and gas accumulation in the northern Ordos Basin. Our results show that the geometry of large Porjianghaizi fault is controlled by five small fault segments. The growth history of the Porjianghaizi fault composes four stages: (1) an initiation stage (Late Carboniferous to Early Permian); (2) a reactivation stage during the Late Triassic; (3) an inversion stage during the Middle Jurassic; and (4) an interaction and linkage stage during the Late Jurassic. Detailed studies show that the Porjianghaizi fault had significant control on sedimentation and gas accumulation. During the initiation stage (Late Carboniferous to Early Permian), five fault segments and associated relay ramps controlled the sedimentation in the study area. The relay ramps formed transport pathways in which sediment extended from north basin margin to the basin center. In contrast, sedimentation along the fault was more localized. Based on Shale Gouge Ratios (SGR) and formation water salinity, the fault sealing capacity of the Porjianghaizi fault is characterised by "horizontal segmentation", which means the fault composes laterally sealed areas and laterally connected areas along the fault strike. As a corollary, we propose that gas resources were mainly generated from the south where there are thick source rocks, migrating vertically through interbedded open fractures first, and then laterally through the junction zone of fault (relay ramps). It eventually accumulated in structural traps found in the northern areas. Understanding the evolution of the Porjianghaizi fault is important for predicting the distribution of sedimentary fades in Upper Carboniferous and Lower Permian units. As well as understanding the migration and distribution of gas resources in the north Ordos Basin, these results can further help in hydrocarbon exploration in the north Ordos Basin.