Abstract
Fluid inclusion microthermometric analysis of multiple vein sets from the Middle Devonian shales provides insight into deformation conditions and fluid evolution in the Appalachian Plateau during, and since, the Alleghanian orogeny. The data illustrate a dynamic system in which deformation conditions and fluid compositions changed continually during basin subsidence in response to orogeny and postorogenic uplift and denudation.
Two types of inclusions are present in the veins: (1) hydrocarbon inclusions that contain CH4, C2H6, CO2, higher hydrocarbons (more than two carbon atoms), and bitumen and (2) two-phase aqueous inclusions that contain very saline (up to 27 wt% NaCl equivalent) NaCl-CaCl2 brines. The veins formed in response to stresses associated with the Alleghanian orogeny during subsidence of the Middle Devonian shale section in the central Plateau province. Each vein set contains inclusions of hydrocarbons that are thermally more mature than those in the previously formed set. The earliest vein set, CFJV1 (cross-fold joints and veins 1), formed during the onset of hydrocarbon generation at a depth of ∼2.3 km and contains inclusions that are composed of degraded bitumen. The next youngest vein set, CFJV2a, formed under lithostatic pore-fluid pressure conditions at a depth of at least 2.3 km and contains primary inclusions with a condensate-like composition. CFJV2b also formed under lithostatic conditions, at a depth of 3.0 km, and contains primary CH4-C2H6-CO2 inclusions. The younger CFJV3a formed at at least 3.9 km depth under lithostatic conditions and contains primary CH4-CO2 inclusions.
The youngest vein set, strike-parallel SJV (strike joints and veins), formed during maximum burial, or early during uplift, of the Middle Devonian shale section. SJV, along with reopened cross-fold joints and veins, contains barite with nearly pure CH4 inclusions and two-phase aqueous inclusions. The presence of barite in all of the vein sets is a result of the breaching of the Middle Devonian shale overpressure seal in response to relaxation of Alleghanian compressive stresses and the influx of surrounding brines into the shale section. The trapping of fluids in the barite continued throughout the uplift history because of microfracturing.
