VEIN EVOLUTION DUE TO THERMAL MATURATION OF KEROGEN IN THE MARCELLUS SHALE

The morphological evolution of abundant mineralized fractures and associated vein fill was investigated in Middle Devonian Marcellus Shale cores with thermal maturity ranging from 1.3 to 2.0 % Ro across the Appalachian Basin. One of the Marcellus Shale cores is within the dry gas window, at a present-day depth of 7,447 to 7,557 ft (2,270 to 2,303 m), has numerous calcite-filled fractures, and from top to bottom has an increasing total organic carbon content (TOC) of 2 to 15%. Fracture formation and mineralization in the organic-rich core are shown to be mainly self-induced as kerogen thermally matured in the shale. Petrographic microscopy on thin sections is used to explain the relationship between the fracture and its vein cement morphology along with the number of generations of calcite fill. Oxygen and carbon stable isotope analysis will be performed on the cement fill to validate the paragenetic sequence order of calcite fill and the source of vein-forming fluid.

Preliminary data shows that at least two fracturing and filling events occurred. Crosscutting relationship shows that the first event formed relatively thick (~1 to 3.5 mm) mineralized calcite-filled veins followed by relatively thin (<1 mm) mineralized veinlets of assorted fill. Fracture morphologies were grouped based on angle to bedding and include horizontal, vertical (continuous and offset), oblique, and secondary veinlets. Calcite-fill morphology and the number of generations of fill varied. Horizontal veins formed first during kerogen catagenesis and contained only one generation of bladed-fibrous calcite. Vertical fractures formed later and contained up to five generations of mineralization made up of acicular-fibrous, large blocky, and small blocky crystals. Bitumen, of varying thickness (<1 to 5 mm) shows a relationship with fracture morphology.

Individual groups of fracture morphologies have their own unique combinations of vein morphology, number of mineralization episodes, and bitumen thickness, which provide insight into the evolution of self-induced fractures during maturation. Calcite-filled fracture intensity and morphology can have a strong influence on development of complex geometry during hydraulic fracture stimulation of horizontal laterals.