Characterisation and Origin of Anhydrite-Rich ‘Lateral Caprock' Adjacent to Halite-Cored Salt Diapirs; Implications for Prospectivity in Salt Basins

Jackson, Christopher A.; Lewis, Matthew M.; Mannie, Aruna

Mapping of salinity levels in groundwater adjacent to salt structures commonly indicates the presence of saline ‘plumes', which are inferred to form in response to the migration of relatively warm fluids up, and dissolution of halite from, the margins of these structures. To date, the products of this process have not been directly sampled in the deep subsurface. Determining the potential for halite dissolution in the deep subsurface is important because density-driven convection of groundwater, which can drive large-scale fluid, heat and solute transport, is dependent on dissolution of halite from the margins of salt structures. Furthermore, uncertainties related to the geometry and composition of salt structures has implications for hydrocarbon exploration and production; for example, hydrocarbon traps in many basins rely on updip pinch-out of reservoirs against impermeable, halite-dominated ‘salt' structures. In this study we use 3-D seismic reflection and borehole data to: (i) document the geometry and lithology of a salt diapir; (ii) assess the feasibility that large volumes of halite can be dissolved from the margins of steep-sided salt structures; and (iii) consider the implications that salt dissolution may have on hydrocarbon prospectivity in salt-influenced basins. Seismic data suggest that a weld is developed along the diapir stem, although borehole data indicate that the stem consists of an inner, c. 1500 m thick, halite-dominated zone, and an outer, c. 250 m thick, anhydrite-dominated ‘sheath'. We interpret that the anhydrite represents ‘lateral caprock', which formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir, and dissolution of halite. The possibility that caprock may form on the flanks of salt structures indicates that lithological variations should therefore be taken into account when risking salt-related pinch-out traps in salt-influenced sedimentary basins. Furthermore, the dissolution of large volumes of halite from the margins of steep-sided salt structures by deep, warm, basinal brines has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins and the location of economically-important metals. Our study also shows that the margins of steep-sided salt structures may be misidentified by several hundred metres if time-migrated seismic reflection data are used.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013