Horizontal Detachments, Planes of Weaknesses and Layer Parallel Shortening in Shale — Potential Impact on Unconventional Shale Development
Shale units are commonly the place of predilection for horizontal detachments. Their occurrence during geological time or induced by human operations will be reviewed. Their expression and our understanding of the mechanisms involved will be addressed using a series of examples from Canada, South America and South-East Asia.
Analogues will include outcrops from the La Borracha Island (Venezuela), Miri anticline (Sarawak), Quito Road Cut and cored shales from the Santa Barbara and Lama fields (Venezuela) as well as from the Utica shale from Quebec
Horizontal detachments can disrupt and misalign faults and fractures by displacing, even slightly, the rock units or layers located above and below the detachment. These displacements may have effects on sealing capacity, migration paths, fracture density, fracture orientation and on fraccability. Each of these issues will be reviewed with examples.
Thus in Alberta, some oil is locally produced from the Kiskatinaw Formation among otherwise gas producing fields. These anomalies are associated with detachments within the Golata shale that displace normal faults and create pathways for oil from the Exshaw shale.
Diagenesis associated with pressure solution and slickensides includes quartz cement precipitation in rocks units surrounding horizontal detachments; these highly cemented zones, especially in hybrid shales can create hydraulic frac barriers.
High density tensile fractures are commonly associated with layer parallel shortening along shale bed interface. These fractures may influence the frac placement and will have some bearing on the gas deliverability of the stimulated zone.
Pre-existing shear fractures located in the neighbourhood of some planes of weaknesses may be reactivated and create horses or duplex structures that may have some bearing on a well completion (productivity, stability…). Such mechanisms are best observed and illustrated in cores and with image logs.
Pore pressure increase associated with hydraulic fracturing can change the local stress regime and initiate horizontal frac propagation along specific bedding planes. These bed boundaries in shale behave as micro-detachments that may have positive or negative effects on a stimulated well productivity.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013