Source of quartz cement and its impact on reservoir quality in Jurassic Shaximiao Formation in central Sichuan Basin, China

Quartz cement is an essential authigenic mineral in the tight sandstones of the Shaximiao Formation in the Sichuan Basin, China. Accurately identifying its sources and investigating its impact on reservoir quality is important for understanding diagenetic fluid evolution and pore development in these reservoirs. To address this, we conducted a systematic analysis of the silicon sources in quartz cement employing mineralogical, fluid in-clusion, and geochemical data. Quartz cement is characterized by two phases of quartz overgrowth and pore-filling authigenic quartz. Homogenization temperatures of fluid inclusions range from 60 to 150 degrees C, indicating that quartz cementation occurred from the early diagenetic phase (B stage) to the late mesodiagenetic phase. During the early diagenetic stage, volcanic material was dissolved by meteoric water, resulting in the formation of the earliest quartz cement and smectite. At temperatures of approximately 70-100 degrees C, smectite illitization, accompanied by K-feldspar consumption and SiO2 precipitation. In the mesodiagenetic stage (temperature approximately 100-130 degrees C), organic acids led to the dissolution of minerals like feldspar and laumontite, releasing SiO2. In the deep burial stage, clastic quartz grains experienced dissolution due to high stress, providing a limited source of SiO2. Smectite alteration and the dissolution of aluminosilicate minerals represent the primary sources of silicon for quartz cementation, while contributions from volcanic material hydrolysis and pressure solution are relatively minor. The presence of chlorite films inhibits the development of quartz overgrowths, and the limited precipitation of authigenic quartz within the primary porosity has a minimal impact on reservoir quality. Consequently, sandstones rich in chlorite films represent a favorable lithology for the development of high-quality reservoirs.

Automated summary

Quartz cement is an important authigenic mineral in the tight sandstones of the Shaximiao Formation in the Sichuan Basin. This study analyzed the silicon sources of the quartz cement using mineralogical, fluid inclusion, and geochemical data. The results showed that smectite alteration and dissolution of aluminosilicate minerals were the primary sources of silicon for quartz cementation. Contributions from volcanic material hydrolysis and pressure solution were relatively minor. The presence of chlorite films inhibited the development of quartz overgrowths and had minimal impact on reservoir quality.