Sammendrag
Estimation of reservoir heterogeneity from well logs is an important but yet difficult task encountered in geophysical formation evaluation and reservoir engineering. The understanding of cemented intervals within a reservoir is important as cementation affects the reservoir drainage. Diagenetic effects, such as cementation, are formed at different time intervals during deposition creating several cement types. Cemented intervals are easily interpreted from wireline logs with its high density and low porosity peaks. The classification of cement types and their inter-correlation between wells influence optimal drainage solution. This paper presents a workflow for mapping and interpret different cement intervals through a structured multivariate analysis of wells from the Upper Triassic to Lower Jurassic fluviodeltaic Åre fm., offshore mid-Norway. Traditionally, principal component analysis (PCA) is run by analyzing the entire wireline log and using PCA scores to characterize variability within and between lithologies. In this study, characterization of cemented intervals has been addressed through the use of three separate PCAs with the goal to portray reservoir heterogeneity. PC loadings and scores are calculated from: (1) a total analysis of all lithologies and all log variables (2) a subset analysis including only cemented intervals and finally (3) a refined reservoir heterogeneity analysis portraying the effects of secondary sources of petro-variability is quantified through the use of the Eckart-Young theorem. The two first PCAs are used to specify signatures indicating cemented intervals and the third PCA focus on the internal variations within the cemented zones. The PCA weighting of the standardized wireline log responses within cemented zones are reused on the same formation in a new well to automatically identify cemented zones with their internal variations. Biplots of these cemented zones allow for differentiation of calcite and siderite cement, as well as total thickness of each zone. This paper demonstrates that reconstruction of the log variables can be used to portray within-lithology heterogeneities and refine the more standard characterization of reservoir heterogeneity. The use of the Eckart-Young theorem for inverse PCA enhances our ability to characterize reservoir heterogeneity compared to the more traditional workflows. The application of this procedure provided additional insight into the complex fluviodeltaic characteristics of a heterolithic sequence characterized by cemented intervals that poses great challenges to development.
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