Fracture Development and Characterization from Field Data in a Dolomite-Limestone Sequence

Abstract Open fractures are an important contributor to reservoir quality in carbonate formations. Herein, their development and relevant parameters are controlled by various factors, among which lithology is considered the most critical. With this in mind, we investigated differences in open-fracture parameters between dolomite and calcite intervals to understand how lithology influences fracturing. To this end, several wells within a sequence of dolomitic and calcitic intervals from the major oil producing Asmari Formation (southwest Iran) which is a major fractured reservoir were chosen. Detailed petrographic and petrophysical analyses were carried out to characterize the fracture intervals in this reservoir. Then, fracture parameters such as density, aperture, porosity, and dip as well as reservoir heterogeneity were quantified using high-quality electrical image logs and core data. Finally, fracture parameters were compared between the two lithologies to delineate their variations. Furthermore, sonic waveforms and well-test data from the selected wells confirmed the presence of fractures and their estimated parameters. Our results indicate that all dolomites in this reservoir are diagenetic and range from fine- to coarse-crystalline. Image logs confirmed that all detected fractures are structural (due to folding and faulting) and show larger apertures and better connectivity in the dolomite intervals, particularly in the coarser sections, than in calcites. Moreover, open fractures with large apertures appear to be the main source of heterogeneity within the dolomite intervals. Therefore, crystal size and structural features were identified as two additional governing factors in fracture development. This enhanced fracture network was distinguished by fluctuations in sonic waveforms within the coarser dolomite intervals (due to a better fracture network). Finally, the role of effective fracture parameters on reservoir heterogeneity is discussed. These findings provide valuable insights into fracture characterization in heterogeneous carbonate systems and can support more accurate reservoir modeling and improved development strategies in similar fractured reservoirs worldwide.