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Beef (Bedding-Parallel Fibrous Veins): Worldwide Occurrence and Significance in Terms of Fluid Overpressure, Hydrocarbon Generation and Mineralization (Including Sulfides, Emerald, Uranium or Gold)

Cobbold, Peter R.*1; Zanella, Alain1; Ruffet, Gilles1; Rodrigues, Nuno2; Loseth, Helge2
(1) Geosciences, University of Rennes, Rennes, France.
(2) Statoil Research Centre, Statoil, Trondheim, Norway.

We follow the original definition of “beef”: bedding-parallel veins of fibrous minerals, where fibers form quasi-vertically. Such veins are common in shale. They result from tensile fracturing and vertical dilation, coeval with fiber growth. They imply fluid migration and enough overpressure to lift the overburden, while fractures propagate laterally. The tectonic context is typically either static or of thrust faulting. We have reproduced such processes in sandbox models incorporating pore fluids. Darcy flow in response to an overpressure gradient diminishes the vertical stress, until it more than compensates for the weight of overburden. Thus the vertical stress may become tensile.

Over the last few years, we have compiled a worldwide catalogue of beef. Typically, veins consist of white gangue minerals (for example, halite, gypsum, calcite, other carbonates, or quartz), but they may also contain accessory minerals of economic interest (for example, bitumen, sulfides, emerald, pitchblende or gold). Aqueous inclusions may contain oil or gas. In our catalogue, there is close correlation between the composition of beef and the composition and age of its host rock. Thus (1) gypsum beef is common in strata of continental origin (evaporitic or lacustrine), especially of Permo-Triassic or Tertiary ages, (2) calcite beef is common in organic-rich marine-carbonate shale, especially of Jurassic or Early Cretaceous ages, and (3) quartz beef is common within marine-turbidite sequences, especially of Paleozoic or Proterozoic ages. There is also a correlation between composition and thermal maturity, typical temperatures of formation being (1) up to 60° C for gypsum beef, (2) 70° C to 120° C for calcite beef, and (3) 200°C to 350° C for quartz beef. Thus hydrocarbon-bearing calcite beef may be a good indicator of a petroleum system, in which oil or gas migrates together with aqueous solutions, in response to overpressure gradients. In some examples at least, both carbonates and hydrocarbons result from chemical compaction of source rock.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California