Origin and distribution of irregular oil-water contacts in tight sandstones, Chang 81 Member, Longdong area, Ordos Basin, China

In recent years, understanding the distribution of oil and water in tight sandstones has been a challenge in the exploration and development of the Chang 8(1) Member in the Longdong area of the Ordos Basin. The spatial pattern of the oil/water contacts pattern varies widely over a distance of 160 km from south to north. The origin of these patterns is unknown. The present-day oil-water distribution in the reservoirs reflects the superimposed effects of hydrocarbon migration and accumulation. An integrated analysis of the characteristics of the present-day oil-water distribution in the Chang8(1) Member revealed that the a typical water-oil relationships are primarily located in the northeastern oil migration zone, and nearly normal water-oil relationships are located in the southwestern oil accumulation zone. Large-scale oil accumulation occurred at the end of the Early Cretaceous in response to strong fluid dynamics. The oil migrated from northeast to southwest in the slightly oil-wet carrier beds, and nearly normal water-oil relationships and larger accumulations formed in the Xifeng and Heshui oilfields in the southwest. The fluid dynamics subsequently weakened, and reservoir heterogeneity dominated, causing the oil to be trapped more easily in the tight reservoirs in the deeper parts of the northeastern oil migration zone. A typical oil-water distributions, such as inverted oil-water distributions, gradational oil-water interfaces, and low-abundance oil reservoirs, developed in such locations as the Huachi oilfield. Strong reservoir heterogeneity existed in both stages, but the fluid dynamics were significantly different, which controlled the distribution of oil and water in the tight reservoirs.