A recent geology/mineralogy research group project focusing on carbonate source rock repeatedly observed unique pattern of microfractures. Observations indicated that diagenesis-led mineral distribution predisposes rock to a certain fracture pattern. If this was correct, then next question was whether dolomitization preceded fractures or vice versa. The research effort used advanced microscopy (optical and scanning electron microscopy-SEM) along with high-resolution mineral mapping on many samples to answer these questions. The project established a structural-diagenetic sequence of a dolomitic limestone source rock reservoir and fracturing. Impacts of dolomite influence on fracturing in limestone (mainly composed of calcite minerals) were studied using the quantitative evaluation of minerals obtained from a scanning electron microscope (QEMSCAN) mineral mapping technique. The relationship of other minerals to fracturing such as clay minerals within same rock was also analyzed.
In all observations, dolomitization preceded fractures. This was inferred from the absence of dolomite mineral precipitates within the fracture veins. Other observations noted fractures that cut through the host calcite minerals but were deflected away or around dolomite minerals. In a few instances the deflections created multiple fractures that continued the propagation. Fractures also cut through the clay minerals. This observation implied that the degree of dolomitization and dolomite distribution within the calcite matrix impacted fracture propagation in a carbonate source rock reservoir. A well-dispersed dolomite minerals structure in a limestone source rock reservoir contributed to rock toughness, resisted fracture propagation, and possibly generated multiple fractures, because the dolomite grains acted as stress concentrators. This scenario resulted in a larger surface area created by fractures than the stimulated reservoir volume. The energy required to propagate these fractures was high, compared to fractures that were not affected by dolomite and did not need to change direction. These fractures were expected to remain open to some degree, even after the load was removed, due to the teething created by the need for the fractures to deflect.
These observations have the potential for a better understanding of fracture architecture in relation to mineral distribution within a given rock. This information can provide additional input for consideration when determining fraccability index based on rock properties.
