Abstract:

The Chang 8 sandstones of the Yanchang Formation in Zhenjing area of Ordos Basin, occurring as one of the major
reservoirs, mainly consist of feldspathic litharenite and lithic arkose. They belong to extra-low porosity and permeability reservoirs,
with dissolved intergranular pores as the chief accumulation space, and the reduced-neck, lamellar and curved lamellar pore
throat as the main throat type. Based on the pattern of capillary pressure curve, porosity, permeability, displacement pressure
and pore throat radius, the Chang 8 sandstones can be classified as type I, II and III, in which the type II are the main reservoirs.
There are four factors affecting the reservoir characteristics of Chang 8 sandstones: sedimentary processes, diagenesis, fractures,
and hydrocarbon emplacement. The best reservoir quality is developed in the subaqueous distributary channels of braided river
delta fronts, and the one of medium sandstones better than that of fine sandstones. Otherwise, the reservoir qualities of sandstones
rich in quartz, feldspars and metamorphic rock fragments are better than those of sandstones with high contents of sedimentary
and igneous rock fragments, as well as mica. The diagenetic processes have exerted dual impacts on the reservoir quality. The
constructive one is characterized by dissolution and pore-lining chlorite cementation, and the sandstone intervals with them
occurring together are the optimum zones for the oil and gas accumulation. The destructive one involves mechanical compactionand the late stage carbonate cementation. The fractures, with the NE-SW tectonic ones formed in the late stage of Cretaceous-
Paleogene as the main type, contribute more to permeability than storage, making the heterogeneity of reservoirs intensified. The
effects of hydrocarbon emplacement on reservoir quality are characterized by the reconstruction and conservation of porosity
dissolved by organic acid in hydrocarbon.

Key words: reservoir characteristics, controlling factors, Chang 8 sandstones, Zhenjing area, Ordos Basin