Systematic Assessment of the Influence of Albitization and Secondary Mineralization on the Mineralogy, Fracturing, and Reservoir Properties of Tight Reservoir Rocks in the J-III Horizon of the Karazhanbas Oil Field

A comprehensive petrophysical analysis of 232 core samples from the J-III productive horizon (wells 182, 1136, and 8096), supported by routine laboratory analyses and X-ray diffraction (XRD) data from 70 samples, was carried out. Integration of reservoir-property parameters and mineralogical characteristics made it possible to establish genetic relationships between material composition, post-sedimentary transformations, and the formation of reservoir properties. The rock-forming framework is dominated by quartz, albite, and chlorite, while calcite—primarily of secondary origin—is confined to pore spaces, having precipitated during the diagenetic and catagenetic stages. Minor phases, including nacrite, kaolinite, chalcopyrite, molybdenite, and graphite, record superimposed hydrothermal events, with graphite indicating episodic exposure to elevated temperatures during the petrogenetic evolution of the rocks. Mineralogical heterogeneity is pronounced: quartz is ubiquitous and albite widely distributed, yet the abundances of calcite and chlorite show considerable variability. Statistical analysis reveals modal populations of albite and calcite, alongside a near-lognormal distribution of chlorite. Examination of paired mineral associations distinguishes clay-rich from clay-poor varieties and confirms the genetic independence of albitization, chloritization, and calcitization, as well as the secondary nature of carbonate mineralization. The J-III productive horizon is characterized by extremely poor reservoir properties: modal porosity is approximately 1%, more than 95% of the values are below 2%, and permeability is predominantly below 0.01 mD. These rocks therefore belong to the class of tight, low-porosity, and low-permeability reservoirs. Local storage anomalies are largely controlled by the development of microfractures. The lack of a consistent correlation between porosity and the extent of carbonation or dolomitization suggests that these processes exert only a subordinate effect on reservoir properties. Dolomitization is frequently accompanied by additional mineralization and compaction. However, when pore-space volume is preserved, it can lead to an increase in void ratio, as dolomite is denser than calcite while the total pore volume remains nearly unchanged. The reconstructed petrogenetic model involves the deposition of sandy-clayey material containing plagioclase and organic matter, followed by diagenetic and catagenetic transformations—particularly albitization and calcitization—that resulted in a dense, secondarily mineralized rock mass. Late tectono-hydrothermal reactivation led to the development of a fracture system, which governs present-day reservoir properties and serves as the main conduit for hydrocarbon migration and accumulation. Mineralization along these fractures confirms their fluid-conducting role. Experimental acid treatment demonstrated that permeability can increase by up to four orders of magnitude, revealing the presence of hidden storage capacity and mobilizable micropore systems. The J-III horizon is thus interpreted as a fractured reservoir, with a development strategy focused on identifying and mapping fracture zones and enhancing their connectivity through horizontal drilling and stimulation techniques.