Macro-heterogeneity evaluation based on different sand body structures: a case study of Chang 8 reservoir group in Heshui area, Ordos Basin

Abstract

Macro-heterogeneity is one of the important factors affecting oilfield development, especially for deltaic sedimentary environments with complex sand body structures. The structure type and characteristics of sand bodies were analyzed for Chang 8 reservoir group (the eighth member of Upper Triassic Yanchang Formation) in the Heshui area of the Ordos Basin. An evaluation method for reservoir macro-heterogeneity was established based on outcrop and core observation, and logging and production performance data. Four sand body structures in the Chang 8 shallow water delta were identified, including continuous superposition, interval superposition, lateral single layer and sand-mud interbedded types. The interlayers and restraining barriers lead to the decrease of the spatial continuity of sand body. Generally, the development degree of interlayers and permeability heterogeneity of the continuous superimposed type is the weakest, and the macro-heterogeneity is the weakest, followed by the interval superposition and the lateral single layer types, while the macro-heterogeneity of sand-mud interbedding type is the strongest. The interlayer density (DK), interlayer frequency (Pk) and reservoir quality coefficient (RQI) were selected to construct the evaluation index (N) of intraformational heterogeneity. The N index can reflect the fluid displacement characteristics and water content change of single sand body within different sand body structures. The evaluation index J of interlayer heterogeneity was constructed by selecting parameters such as the percentage of sandstone (Sn) and the number of interlayer sand layers per unit thickness (T). The J index is a good measure to evaluate and predict the fluid production strength of oil wells. Therefore, the intraformational heterogeneity index (N) and interlayer heterogeneity index (J) can be used to evaluate the macro-heterogeneity of reservoirs.

Keywords

• macro-heterogeneity
• interlayers
• restraining barriers
• sand body structure
• ordos basin