Impact of Lithologic Heterogeneity on Brittleness of Cenozoic Unconventional Reservoirs (Fine-Grained) in Western Qaidam Basin

Abstract

In order to understand the impact on reservoir brittleness of lithologic heterogeneity in the continental mixed fine-grained sedimentary rocks in the western Qaidam Basin, the mechanical properties of the rocks and their correlation with mineral composition and petrographic characteristics were studied. A total of 20 samples from two parallel groups (10 samples in each group) were analyzed by triaxial stress test mineralogy, and morphology. The results show that the reservoir rocks can be divided into five different types according to the mechanical properties of the reservoir (characterized by stress–strain curves), among which Types I and III belong to a similar elastoplastic failure model, Type II shows a special pulse failure mode for plastic material, Type IV shows a failure mode of mixed characteristics, and Type V exhibits a typical plastic failure model. The correlation between minerals and mechanical properties indicates that quartz and feldspar, which are often considered brittle minerals, do not contribute much to the brittleness of these continental fine-grained sedimentary rocks. The main minerals affecting the reservoir brittleness are dolomite and clay minerals, contributing positively and negatively to it, respectively. The petrographic analysis results prove that the abnormal correlation between rock mechanical properties and quartz and feldspar is caused by the different rock fabrics. When dolomite forms a rock skeleton, it typically exhibits greater strength, brittleness and physical properties than other minerals. Based on the results, a brittleness evaluation standard for continental unconventional reservoirs (fine-grained) is proposed, and the validity of the standard is verified by the spatial correlation between the lithology probability model and the micro-seismic monitoring data. This indicated that the spatial heterogeneity of the dolomite-rich rock is the main controlling factor for the effective development of the Cenozoic continental unconventional reservoirs (fine-grained) in the Western Qaidam Basin.