OIL GENERATION INDUCES SPARRY CALCITE FORMATION IN LACUSTRINE CALCITIC MUDROCKS, EOCENE OF EAST CHINA

Sparry calcite is common in mudrocks; it has been reported to occur as veins in bedding-parallel, oblique, and vertical fractures. Tectonic forces were traditionally suggested to be responsible for calcite vein formation. Hydrocarbon generation and overpressure could be responsible for forming bedding-parallel calcite veins of fibrous sparry calcite; however, the process of sparry calcite formation by hydrocarbon generation is not well understood.

Sparry calcite is common in the Eocene mudrocks of the Zhanhua and Dongying Sags of the Bohai Bay Basin and the Biyang Sag of the Nanxiang Basin, east China. In this study, the petrology, carbon and oxygen isotopes, minor geochemistry, fluid inclusion, and drill stem testing characteristics of sparry calcite and/or sparry calcite-bearing intervals in these Eocene mudrocks were investigated to establish the conditions for sparry calcite formation. Oil generation could lead to overpressure, inducing accommodation space for the precipitation of sparry calcite, while organic acids, a by-product of oil generation, could promote dissolution of micrite within these calcitic mudrocks, providing an ion source for diagenetic sparry calcite growth. Laminated shale easily reaches the yield point for fracturing because of its anisotropic nature. An upward seepage force by fluid overpressure can induce bedding-parallel fractures, resulting in three stages of sparry calcite formation characterized by: 1) discrete and equant sparry calcite, 2) horizontally intergrown and equant sparry calcite, and 3) horizontally intergrown and fibrous sparry calcite. Structureless mudstone can experience elastic deformation before fracture generation, forming equant and dispersed sparry calcite as well. Thus oil generation can be responsible for sparry calcite formation in calcitic mudrocks, establishing a new tool to confirm a thermally mature oil window for exploration purposes.