Diagenetic fluid controls chemical compositions of authigenic chlorite in clastic reservoirs

Authigenic chlorite is widely developed in clastic reservoirs, and its chemical compositions should reflect diagenetic fluid and reservoir properties, although this has been little studied. Here we carried out an investigation of sandy conglomerate reservoirs in the upper Permian upper-Wuerhe Formation in the Zhongguai Uplift of the Junggar Basin, northwestern China. Results show that authigenic chlorite in the sandy conglomerate reservoirs can be divided into grain-coating and pore-filling types, and the former can be further divided into grain-coating with cement and without cement. The chlorite coats are mainly brunsvigite and diabanite, in which calcite, laumontite, and quartz cements are typically developed. The presence of cements in chlorite coats had little effect on the chlorite geochemistry. In the early diagenetic stage, grain-coating chlorite was derived from trioctahedral smectite by solid-state transformation in an alkaline environment with the presence of large numbers of Fe and Mg ions. During mesodiagenesis, with increasing burial depth and temperature, the grain-coating chlorite was transformed into pore-filling chlorite, resulting in a higher Fe content and the formation of brunsvigite. At this stage, the chlorite crystal structure became more stable. The grain-coating chlorite partially preserved primary pores by inhibiting quartz cementation and limiting compaction; however, the pore-filling chlorite was not conducive to reservoir development. The clay coats in the study area are indicative of high-quality reservoirs, which contain mainly dissolution pores after the removal of soluble cements. Therefore, reservoirs far from their sediment source and with chlorite coats are of relatively high-quality, which may be a common feature of clastic reservoirs.