The staged growth of bedding-parallel fibrous calcite veins, from synsedimentary period to oil-generative window

Abstract

Bedding-parallel fibrous calcite veins are commonly found in low-permeability strata rich in organic matter from various geological periods. Despite their importance in examining the fluid flow in sedimentary basins, dynamics of crustal permeability and mineralization, microbial mineralization, detection of early life signatures, and hydrocarbon production and accumulation, the growth processes of these veins remain inconclusive. Here we show a two-stage model to explain the growth of bedding-parallel fibrous calcite veins, by analyzing a wide range of organic geochemical, elemental, isotopic, and petrographic data from both new and published sources. This model consists of the early diagenetic stage and oil-generative window, with a focus on samples from the middle Eocene strata of the Bohai Bay Basin and the Cavan Bluff Limestone. This study emphasizes the crucial role of early diagenetic processes, such as anaerobic microbial activities, in initiating the formation of bedding-parallel fibrous calcite veins at the sediment-water interface. Additionally, the further modification and secondary enlargement of these veins by fluids generated from hydrocarbon production and clay evolution are described. While this model may not be applicable to all bedding-parallel fibrous calcite veins, it provided valuable insights and helped reconcile seemingly contradictory findings regarding these veins. Furthermore, the study indicates the potential of bedding-parallel fibrous calcite veins in paleoenvironmental reconstruction and emphasizes the significance of reevaluating certain existing knowledge within the fields that employ these veins as specimens.