New indicators from bedding-parallel beef veins for the fault valve mechanism

Abstract

Beef structures (bedding-parallel veins of fibrous calcite) are widespread within the Lower Triassic carbonate rocks in the Sichuan Basin of China, especially within clay-rich strata of low permeability. In the veins, fibrous calcite occurs in the outer zones, and coarse equant calcite in the inner zones. At least two generations of calcite crystallization took place during aqueous alteration, at the same time as deformation recorded by the calcite. The first-generation calcite fibers are at steep angles to the hydrocarbon-bearing host beds, and they grew vertically against the force of gravity at a time when the source rocks were maturing. Second-generation calcite occurs as coarse equant grains that sealed pores via localized fluid flow during horizontal tectonic compression, so that shear stresses acted at the fracture margins. Shearing of the host rock was accommodated in part by dissolution-precipitation creep (DPC), grain rotation, and grain slippage, recorded in crystallographic preferred orientations (CPOs) of the host calcite grains beside the crack walls. The beef veins formed during high pore-fluid overpressures along hydrofractures. We propose that the bedding-parallel veins with beef structures are evidence of a “crack-seal slip” fault valve process during hydrocarbon generation. The hydrocarbon-bearing calcite beef structure may be a good indicator of oil or gas migration, and of the flow direction of aqueous solutions.