Differential mechanism of tight oil enrichment and reservoir controlling model of source and reservoir combinations: a case study of Qingxi Sag, Jiuquan Basin

The quality of source and reservoir rocks as well as their spatial assemblage types are crucial factors that controlling the charging, accumulation, and enrichment of tight oil resource. However, there are still deficiencies in the understanding about these factors, especially regarding differences in the mechanism of charging, accumulation, and enrichment of tight oil between different source-reservoir assemblage types, which restricts the formation of theory as well as the process of exploration and development. Taking the Qingxi Sag of the Jiuquan Basin as an example, we have drawn conclusions by field observations, core descriptions, and drilling analyses that there may be eight types of source-reservoir assemblages. Among these, there are three basic types, namely, the lower-generating and upper-reserving type, upper-generating and lower-reserving type, and sandwich-like type. In addition, there are two special types, the self-generating and self-reserving type and source-reservoir interbedding type. Moreover, the other three types are composite types, which are the lowergenerating and upper-reserving type Ⅱ, upper-generating and lower-reserving type Ⅱ, and sandwich-like type Ⅱ. Among the above mentioned eight types, the first five types are common. The results of statistical drilling and nuclear magnetic resonance experiments mutually verify the corresponding relation between source-reservoir assemblage type and tight oil enrichment. The assemblage types can be ranked according to their positive correlation with enrichment in the following order, from the highest to the lowest: self-generating and self-reservoiring type, sandwich-like type, upper-generating and lower-reservoiring type, lower-generating and upper-reservoiring type, and source-reservoir interbedding type. Differences in the mechanism of charging, accumulation, and enrichment of tight oil among the five common source-reservoir assemblage types are revealed and explained. In addition, their accumulation and enrichment patterns are determined and principal controlling factors are proposed, namely, accumulation dynamics, charging patterns, and storage conditions, which are in fact closely related to the source-reservoir assemblage types.