Hitzman, Murray W.¹ 
(1) Colorado School of Mines, Golden, CO

ABSTRACT: Hydrothermal Dolomitization in the Carboniferous of the Irish Midlands — Analogues and Implications for New Reservoir Types

The transgressive marine carbonate sequence of the Carboniferous in the Irish Midlands underwent minor extension with accompanying thermal maturation during, and immediately following, sedimentation. These hydrothermal events produced a number of hydrothermally dolomitized zones which are concentrated in several stratigraphic intervals, principally the Waulsortian mudbank limestones. The different styles of hydrothermal dolomite bodies in the Irish Midlands have been extremely well characterized due to their association with sulfide mineralization. Hydrothermal dolomitization occurred along normal faults at several different periods, forming bodies which are analogous to the Albion-Scipio field. Normal fault-related hydrothermal dolomitization also produced stratabound dolostone bodies which extend hundreds to thousands of meters from the fault fluid pathways. These bodies may display reduced permeabilities relative to unaltered carbonate wall rocks, resulting in the formation of potential stratigraphic traps (Navan deposit), or enhanced permeabilities leading to the formation of potential dolostone reservoirs (Lisheen deposit). Hydrothermal fluid movement in southern Ireland has also produced a regional zone of hydrothermal dolomitization (~7000 km²) with greatly enhance permeability and porosity that could significantly influence hydrocarbon migration. Future exploration for hydrothermal dolostone reservoirs can be guided by the understanding gained from studies of well-constrained examples such as those in the Irish Carboniferous.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.