Diagenesis and favourable diagenetic facies zones of the Chang-6 oil reservoirs in the Binxian-Changwu region, Ordos Basin

The fine-grained sandstones associated with deep-lake mudstones predominate in the Chang-6 oil reservoirs as the favourable area for the heavy oil exploration in the Binxian-Changwu region, Ordos Basin. In the light of core observation, thin section examination and analytical data, the emphasis in this study is placed on porosity types and other physical properties, diagenetic evolution, classification of diagenetic facies and delineation of the favourable exploration areas based on the planar distribution of diagenetic facies zones. The diagenesis in the Chang-6 oil reservoirs is now developed into A to B stages of late diagenesis. The primary porosities have been lost due to compaction, and the permeabilities continued to be decreased by the kaolinite cementation. However, late dissolution of feldspar and lithic clastics led to the redistribution of the porosities and improvement of the physical properties. The diagenetic facies zones comprise strong compaction, strong dissolution, weak dissolution, strong carbonate cementation and microfracture diagenetic facies zones, of which the strong dissolution diagenetic facies sandstones are interpreted as the prolific zone in the Chang-6 oil reservoirs.