Bedding-parallel calcite layers (BPCLs) preserving a fibrous “beef” texture are common throughout the overmature Haynesville Formation shale. Their interfaces with the host shale contain radiating splays of anhydrite pseudomorphs after gypsum rosettes, suggesting either a primary evaporitic or an early burial origin. In places, the calcite layers contain remnant barite or anhydrite in crystallographic alignment with its host calcite, indicating that the calcite formed by replacing a fibrous sulfate precursor phase.
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. At peak burial, liquid hydrocarbons cracked to CH4 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 toward the shale contact.
We estimated the in situ CH4 pressure attending thermochemical sulfate reduction in the Haynesville shale by using microlaser Raman spectroscopy to directly measure the density of CH4 gas inclusions in BPCLs. Average fluid pressure gradients preserved within the fibrous calcite are approximately 0.87 ± 0.03 psi/ft (±1σ; n = 4), considerably above hydrostatic but below both overburden and shale fracture gradients. We found no evidence to suggest that fluid pressures exceeded lithostatic or that fibrous calcite grew in dilated vein systems from their margins toward the center of the BPCLs.
The replacement of primary bedded anhydrite and barite by calcite preserves the original orientation of the precursor sulfate minerals—in other words, their fibrous beef texture is an inherited feature. The replacement of sulfate by calcite results in a solid volume loss of approximately 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 subsequently grew in the direction of minimum horizontal stress (i.e., parallel to bedding).