The Pennsylvanian Atoka bank carbonates were deposited on the northwest shelf of the Delaware Basin in Eddy County, New Mexico, forming a stratigraphic trap for natural gas. Brecciation is common in the core samples of some wells. This paper describes the brecciation in the phylloid algal limestones and discusses its origin. In the core samples, brecciated dark areas, consisting of irregularly shaped fragments, are seen together with internal sediment-filled pores, characterized by light gray coloured areas. Detailed examination of the core samples reveals that the brecciated dark areas correspond to open space areas between phylloid algal colonies, on the basis of comparison with the well-preserved primary rock texture in the cores. Brecciation is mainly due to the selective dissolution of phylloid algae that produced phylloid algal moulds, later filled by sandy internal sediments under subaerial conditions. The subsequent compaction of the limestone caused the rock to break up and formed a breccia in situ. The sandy internal sediment prevented the moulds from collapse as well as breccia formation in the pore-filling area, owing to its loose character. Overall, the brecciation process, including the dissolution of phylloid algae and breakage of the rocks, significantly improved the reservoir quality, whereas the internal sediment reduced the reservoir quality, for it reduced the porosity.
