Genetic relationship between shell fossils and shale oil: A case study of Jurassic shale reservoir in the northeast Sichuan Basin

ABSTRACT: Benthic bivalves, the most widely distributed mollusks since the Mesozoic era, often inhabited environments where their fossilized remains are found adjacent to or intermingled with organic-rich shale. Recent Jurassic shale oil exploration in the Sichuan Basin has revealed that bioclastic layers, composed of abundant fossil bivalves and closely associated with shales and, exhibit significant hydrocarbon potentials. However, the microscopic structures of these bivalve fossils and their role in hydrocarbon storage and migration remain poorly understood. In this study, we characterized the microporosity of bivalve shells within the Middle-Lower Jurassic bioclastic shale in the northeastern Sichuan Basin using a combination of 2D imaging (thin section, SEM), 3D reconstruction (FIB-SEM), and permeability simulation. The micropores within the shell fossils range from 100 to 1000 nm in radius and are uniformly distributed in a grid-like pattern within the shell interior, where they host liquid hydrocarbons. The bioclastic carbonate layers exhibit an overall porosity of approximately 0.8%. Comparative analysis with extant bivalve shells suggests that these micropores represent residual pores from the nacreous brick wall structure. Due to the regular orientation of the shells and their microporous nacres, permeability coefficients along the long bivalve fossil axes are three to five times higher than those along the short axes. These residual micropores within the bioclastic fossil shells have a positive influence on both the storage and migration of shale oil and gas, making bioclastic fossil-bearing shalespromising sweet spots for shale oil and gas exploration in similar sedimentary environments.