| Abstrakt: |
The paleokarst zones developed in the Ordovician Yingshan Formation carbonates represent significant hydrocarbon reservoirs in the Tahe area, western China. In this type of reservoir, the identification of pay zones is problematic. The challenge is due to high uncertainty regarding spatial dimensions, sorting, lithological heterogeneity, and the texture of paleokarst fillings. Therefore, we focus on the physical characterization of these reservoirs using an integrated multidisciplinary approach applied to each filling type. Our methodology includes scanning electron microscopy (SEM) and micro-macroscopic sedimentological analyses, as well as petrophysical evaluations using the borehole image logs (FMI) and dipole sonic imager (DSI). The study specifically focused on three filling types: (i) clastics, (ii) breccia, and (iii) chemical deposits (i.e., chert and calcite cement). We generally recognized these fillings in well core samples recovered from the study area. Our results revealed that the lithofacies with the best features for oil recovery are related to fine-grained sandstones. This lithofacies is characterized by an average effective porosity and permeability range from 5 to 12% and from 5 to 50 mD, respectively. The unfilled crackle breccia and empty cavities also show an average effective porosity and permeability range from 4 to 9% and from 10 to 20 mD, respectively. Although the chert deposits interbedded with carbonates exhibit substantial fracture density, their detection using well logs remains a challenge because of the deposits' thin thicknesses (around 10–15 cm in vertical scale). Moreover, we highlight how the textural immaturity and high calcite precipitation in the matrix significantly impact the reduction of storage capacity in the chaotic breccia. As a validation method, we compare rock typing results obtained from well logs and core sample data. The outputs show an excellent general correlation between the determined rock types. Finally, we propose a classification scheme for the paleokarst fillings exclusive to the Tahe area. [ABSTRACT FROM AUTHOR] |
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