Influence of Laumontite and Chlorite Mineral Characteristics on Reservoir Physical Properties in Junggar Basin

To deepen the understanding of chlorite and laumontite mineral characteristics within the Lower Wuerhe Formation in the Madong area of the Junggar Basin, a detailed investigation was carried out using thin sections, electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Through this multi-method approach, their chemical compositions, occurrences within thin sections, and the impacts on the physical properties of the reservoir were illustrated. The results indicate that the chlorite minerals exhibit a trioctahedral crystal structure and are found in four distinct forms: as pore linings, grain coatings, pore fillings and chlorite pseudomorph. They belong to the iron-magnesium transitional type with a magnesium-rich composition. The octahedral sites primarily feature Fe substituting for Mg, with an Al/(Al + Mg + Fe) ratio ranging from 0.25 to 0.37. Chlorite formation is attributed to the alteration of argillaceous rocks and mafic rock transformations, with significant material input from the hydrolytic dissolution of tuffaceous volcanic materials and interconversions among clay minerals. Chlorite has been shown to significantly enhance reservoir physical properties, contributing to the development of high-quality reservoirs. Laumontite occurs in three states: crystalline aggregates, filling materials, and through replacement processes. Laumontite crystalline aggregates are often surrounded by debris, which promotes their formation. In its filling state, laumontite coexists with chlorite, calcite, and other minerals, competing for material sources, which can partially inhibit its formation. Replacement-type laumontite forms mainly by replacing feldspar and rock fragments, leading to a high Si/Al ratio and acid resistance, making it less susceptible to dissolution. However, the effect of laumontite on reservoir properties is relatively limited compared to that of chlorite. The diagenetic environment of the Lower Wuerhe Formation varies with increasing burial depth, transitioning from alkaline to acidic and then back to alkaline conditions.