Tracing forming mechanism of the sparry calcite growth in the lacustrine shale of east China: A glimpse into the role of organic matter in calcite transformation

Abstract

The formation of sparry calcite is very common in fine‐grained sedimentary rocks worldwide. Sparry calcite is formed from the diagenetic calcite crystals having a grain size > 50 μm. It is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Depression, Bohai Bay Basin. The detailed investigations of the Es3x shale show that the sparite crystals start appearing at a burial depth > 3,000 m and the total organic carbon content (TOC) is >3 wt%. It indicates that the sparry calcite formed at high depths and in the most organic‐rich intervals of the studied shale. The presence of organic matter (OM) laminae parallel to the sparry calcite laminae, and sparry calcite along the edges of the OM show a direct physical relationship between sparry calcite and OM. It suggests that the diagenetic system was not influenced by hydrothermal fluids and only the OM is possibly responsible for the formation of sparry calcite in the studied shale. Sparry calcite forms in the thermally mature oil window, and oil generation in the Es3x shale causes the dissolution of micrite, providing the ions source for sparry calcite formation. Therefore, the geothermal gradient, as well as oil generation and kerogen maturation, are responsible for sparry calcite growth in the studied shale. The proposed model shows that the sparry calcite can form in the pores formed by elastic deformation as well as in the microfractures in the study area. In laminated shale, sparry calcite followed three stages of its formation, including (a) formation of laterally discrete calcite crystals, (b) lateral intergrowth of equant calcite crystals and (c) lateral intergrowth of fibrous calcite. Likewise, the genesis of sparry calcite in the non‐laminated shale also followed the same three phases; however, elastic deformation played a dominant and an important role in this variety of shale. Based on the detailed interpretations, we suggest that the OM, microfractures and elastic deformation act as major contributors in the formation and maturation of sparry calcite. We expect that this study also highlights the growth mechanism and maturation of sparry calcite in the Zhanhua Depression as well as other similar rift‐related basins around the globe.