Superposition relationships of sand body and their influence on oil-water distribution: A case study of Chang 4+5² of Yanchang Formation in Nan 212 well block of Nanniwan Oilfield

To clarify the relationship between sand body superposition type and oil-water distribution of the Chang 4+5² from Yanchang Formation in the Nan 212 well block of the study area, the sedimentary microfacies type, vertical superposition and lateral contact style of the Chang 4+5² were analyzed through core observations and logging interpretations, and the oil-bearing characteristics of different types of sand body superposition were discussed in combination with well test data. The results indicate that the Chang 4+5² in the Nan 212 well block of the study area was deposited in a deltaplain sedimentary environment, that primarily generates distributary channels and interchannel. On the basis of vertical superposition, sand body types may be separated into massive, segmented interbedded, thin interbedded and thin sand bodies, and according to horizontal contact relationships, it can be divided into isolated, vertical superposition, horizontal overlapping, and cutting superposition sand bodies. Thin interbedded and segmented interbedded sand bodies make up most of Chang 4+5² vertical superposition relationship sand body, and segmented interbedded sand bodies are better at holding oil than thin interbedded sand bodies. The horizontal contact relationship of Chang 4+5^2-1 sand body consists mostly of vertical superposition, cutting superposition, and horizontal overlap sand body, with cutting superposition sand body having the highest oil-bearing property. The Chang 4+5^2-2 mainly develops a horizontal overlap and an isolated sand body with a relatively worse oil-containing property.