Formation mechanism of the Donghe Sandstone residual oil zone in the Tarim Basin, NW China

The residual oil zone (ROZ) with near-residual oil saturation is a naturally water-flooded interval adjacent to an oil-bearing reservoir. It has become a promising development target for the technical progress of enhanced oil recovery. The hydrocarbon show, bitumen distribution and grains containing oil inclusions all indicate the presence of ROZs in the Donghe Sandstone Member of the Tazhong4 and Hade Oilfields in the Tarim Basin. Analysis of the two ROZ examples reveals that a stable paleoreservoir and structural adjustment are significant elements of the ROZ-forming process. The ROZ of the Tazhong4 Oilfield is defined as a reactivated fault ROZ (RFROZ) lying between the paleo oil-water contact (OWC) and producing-OWC. Paleo-hydrocarbons leaked through the reactivated faults, and the current oil saturation has a positive proportion to baffle frequency. The ROZ in the paleo trap northwest of the current reservoir in the Hade Oilfield is defined as a migratory trap ROZ (MTROZ). Paleo-hydrocarbons migrated through connected sand bodies. The current oil saturation in the MTROZ has a positive proportion to baffle frequency and negative proportion to petrophysical properties. The microscopic occurrence state of the residual oil in ROZs was analyzed with direct computed tomography (CT) scanning. The displacement experiments coupled with CT scanning aimed to reveal the different occurrence states of the residual oil in the major stage of natural water flooding in the ROZ-forming process. In the microscopic view, the formation of both RFROZ and MTROZ is a process of clustered flow converting to discontinuous flow, and the clustered flow can be scattered to a larger extent in a high permeability zone. The ROZ development feasibility was preliminarily researched. Low-salinity water flooding after the displacement of the formation water in the etching model provided experimental corroboration for remobilizing the residual oil.