Formation of bedding-parallel, fibrous calcite veins in laminated source rocks of the Eocene Dongying Depression: A growth model based on petrographic observations

Abstract

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression of Bohai Bay Basin, East China. Understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks. This study investigates the mechanism(s) of growth of bedding-parallel veins in hydrocarbon source rocks of the Es4 upper and the Es3 lower units in the Dongying Depression, based on systematic observations of macro- and micro-scale features both in veins and their wall-rocks, combined with X-ray diffraction and analyses of geochemical indicators.The source rocks are heterogeneous in both composition and structure. The beefs mainly occur in laminated rocks consisting of organic-rich laminae, which occur together with horizontally-continuous banded lamalginites and rare carbonate-rich laminae, overall corresponding to high TOC, low carbonate contents, and low maturity (average Ro = 0.57%). Most beefs are embedded in the neighboring lamalginites of organic-rich laminae with low clay-mineral contents. The beefs are products of diagenesis accompanying burial, and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion. The presence of bedding-parallel bituminite veins and the median plane (or “parting”) is evidence for horizontally oriented hydrofracturing due to fluid overpressure. In turn, fluid overpressure reflects the generation and primary migration of hydrocarbon-bearing fluids. Hydrocarbon generation (organic matter evolution) and clay-mineral dehydration (illitization of smectite) were responsible for the fluid overpressure. Crystal growth within the beefs was a continuous process with an antitaxial growth direction, and there was no growth competition between adjacent fibres. These observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited. The force exerted by crystallizing vein fill contributed to the high fluid pressure, suggesting that fibrous vein minerals actively promoted further dilation as the veins grew. Vein dilation accompanied the migration of hydrocarbon-bearing fluids, evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites. Therefore, the hydrocarbon-bearing fluids were present at the time of horizontal fluid flow, from the start of fracture opening to the end of the dilation.