During burial and heating, maturation of source rock organic matter resulted in the expulsion of hydrocarbons (oil and gas). Consequently, the redox state of the shale and hydrocarbon system became reducing as indicated by the ubiquitous presence of H₂S. Both anhydrite and barite are unstable in the presence of H₂S and were consumed by thermochemical sulfate reduction (TSR). At peak burial, liquid hydrocarbons cracked to methane gas and remnant solid pyrobitumen which typically occupies the median suture zone of the fibrous calcite layers. It was along this median suture that calcite replacement of anhydrite and barite was initiated, proceeding to replace sulfate minerals from the center of the layer out towards the shale contact.

We estimated the in-situ methane pressure attending TSR in the Haynesville shale by using micro–Laser Raman Spectroscopy to directly measure the density of methane gas inclusions in BPCLs. Average fluid pressure gradients preserved within the fibrous calcite are ~0.87 ± 0.03 psi/ft (± 1σ; n=4), considerably above hydrostatic but below both overburden and shale fracture gradients. We find no evidence to suggest that fluid pressures exceeded lithostatic or that fibrous calcite grew in dilated vein systems from their margins towards the center of the BPCLs.

The replacement of primary bedded anhydrite and barite by calcite preserves the original orientation of the precursor sulfate minerals i.e., their fibrous “beef” texture is an inherited feature. The replacement of sulfate by calcite results in a solid volume loss of ~20 to 30 vol. %. Collapse of the layers due to loss of volume and overburden stress results in the minimum horizontal stress being parallel to bedding. Pyrobitumen layers were compressed and disaggregated due to tensile failure. Both late-stage calcite and disaggregated pyrobitumen grew in the direction of minimum horizontal stress, i.e., parallel to bedding.