AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Natural Fractures in Shales: Timing, Sealing, Mechanisms of Formation, and Relevance for Shale-Gas Reservoirs
(1) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
Natural fracture systems are important for production in shale-gas reservoirs in two ways. They may reactivate during hydraulic fracture treatments or they may be partly open, contributing to permeability without reactivation. Degree of openness and fracture plane strength are related in part to the specific structural-diagenetic history of each fracture set and shale host rock. Several possible mechanisms control fracture formation. A key variable is the depth of burial, and thereby the temperature, pore-fluid pressure and effective stress at the time of fracture development. Examples exist across the spectrum; from veins developing before host-rock compaction is complete, to veins forming at maximum burial due to hydrocarbon generation or other mineral reactions, to late, shallow veins of gypsum formed due to pyrite oxidation in the weathering zone. This study examines examples that illustrate these mechanisms from several US shales, including the Devonian New Albany Shale in the Illinois Basin, the Mississippian Barnett Shale from the Delaware Basin, west Texas, and the Pennsylvanian Smithwick Shale from central Texas.
Analysis of fracture-filling cements in three fracture sets in a core from the Barnett Shale in the Delaware Basin, West Texas is combined with a burial history model in order to determine the timing and mechanism of each fracture set. In our example, an early fracture set mostly sealed with carbonate cement, but with small remaining pores, has been folded during compaction. A second set of fractures includes both horizontal and irregular subvertical fractures that are sealed with fibrous barite containing primary, liquid-rich hydrocarbon and aqueous fluid inclusions. We interpret these primary inclusions as forming during cracking of kerogen to oil. This conversion caused an overpressure, thus providing a mechanism for fracturing. In addition, this second set of fractures contains secondary, vapor-rich hydrocarbon inclusions with condensate rims, trapped in barite along cross-cutting planes that are sub-parallel to the fibers. We interpret these secondary inclusions as having formed during secondary gas generation. A third set of fractures contains quartz bridges with crack seal structure indicative of quartz cementation during fracture opening. Aqueous inclusions in these quartz bridges yield higher temperatures than aqueous inclusions in the second set. The third set is the most common and likely the most important for production.