Study on a Prediction Method of Hydraulic Units Using Logging Curves Constrained by Complex Lithofacies: A Case Study of the Tidal Estuarine Reservoir in Napo Formation, Oriente Basin

Hydraulic (Flow) unit is a type of reservoir genetic unit that integrates the petrophysical characteristics and fluid flow features, and its variability is controlled by both lithofacies types and diagenetic effects. The distribution of HU reflects the macroscopic heterogeneity of the underground reservoir. Due to the regional change of hydrodynamic conditions, such as the influence of the relative strength variation of tidal and fluvial energy, a considerable number of facies are formed in tidal estuarine reservoirs. However, distinct lithofacies exhibit unique hydraulic unit features with respect to their different bedding types. Thus, the complexity of lithofacies and their development patterns makes it highly difficult to predict hydraulic units precisely. Firstly, we have identified fifteen types of lithofacies marked by tidal rhythmites, bidirectional cross-bedding, and double mud layers among others. Further, by means of the porosity and permeability data obtained from 493 core plug samples of 10 core wells, reservoir quality index(RQI) and normalized porosity index(φz) are calculated to reflect the variation in reservoir quality. And we propose the introduction of flow unit index(FZI) to classify hydraulic units. In addition, we classified into six categories of hydraulic units with separate intervals of FZI index, and two additional classification schemes artificially designed are established for the purpose of comparison, a four-classification scheme and an eight-classification scheme. Finally, the attributes were selected from three logging curves(i.e., GR, RESD, and DT) and the support vector machine(SVM) model was applied to predict hydraulic units of un-cored intervals. The result is that the scheme with lithofacies constraints is the most prominent which represents the optimal number of classifications. In summary, lithofacies plays a decisive role in the original physical properties of reservoirs and has a significant impact on the distribution of hydraulic units in the tidal estuarine reservoir. This study provides a geological basis for the reasonable prediction of hydraulic units and serves as a fundamental guideline for the identification of the remaining oil areas.