Abstract
Tight oil is usually accumulated near source rocks after short-distance migration. For the Chang 8 member tight oil reservoir in the Longdong area of the Ordos Basin, however, the tight oil accumulation does not completely follow the source control theory, which states that richer tight oil is found in zones closer to the source rock. The causes behind such differential accumulation need to be investigated. On the basis of previous studies, this paper accurately restores the oil migration dynamics in the main reservoir-forming period by using the latest paleo-hydrodynamic restoration technology through mudstone compaction analysis. The oil migration dynamics are then compared with the start-up pressure gradient of the transport layer along the potential oil migration path during the main reservoir-forming period. Finally, combined with the paleo-structural characteristics during the main reservoir-forming period, the causes for the differential accumulation of tight oil in the Chang 8 member of the Longdong area are analyzed. The results show that: 1) during the main reservoir-forming period, tight oil in the Chang 8 member tended to and could migrate from the north and northeast to the southwest and has accumulated in the nose-like paleo-structure in the southwest; and 2) following the main reservoir-forming period, as the reservoir formation was compacted and the fluid pressure and hydrocarbon generation capacity reduced, oil accumulated nearer the source rock, and to a lesser extent than it did during the reservoir-forming period. In these two stages, changes in the intensity of oil migration dynamics dominated the differential accumulation of tight oil. This study provides a new perspective for similar efforts.
Highlights
Tight oil is diversely defined (Clarkson et al, 2011; Zhao, et al, 2012; Zou et al, 2015; Du et al, 2016; Zhao et al, 2017)
Research spanning 20 years confirms that tight oil flow in a reservoir typically follows the low-velocity non-Darcy flow pattern, that is, oil starts to flow at a certain start-up pressure gradient (Zeng et al, 2014)
Multiple core flow experiments for tight reservoirs in the Ordos Basin have suggested that oil could migrate only when the start-up pressure gradient reaches a certain level (Zeng et al, 2014)
Summary
Tight oil is diversely defined (Clarkson et al, 2011; Zhao, et al, 2012; Zou et al, 2015; Du et al, 2016; Zhao et al, 2017). Additional previous studies on Chang 8 in the Longdong area have suggested three stages of oil accumulation and the Late Jurassic sand bodies with good physical properties and lipophilic pores in sandstones that have developed since the Early Cretaceous as oil migration paths (Luo et al, 2010).
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