Dissolution of Plagioclase as Hydrocarbon Reservoirs: ABSTRACT

Plagioclase feldspar is an important contributor to reservoir porosity as indicated by studies of first-cycle Miocene basins of south Texas (Frio Formation) and southern California (Stevens Formation). In the former case study, volcanic plagioclase (An₃₂) exhibits secondary porosity with associated partial albitization. In the latter case, plutonic plagioclase (An₃₀) exhibits secondary porosity without albitization, although petrographically albite was stable with respect to the leach fluids. In both examples, dissolution was highly selective proceeding 2 to 3 times faster along cleavage planes than normal to them.

The dissolution by-product is chiefly kaolinite such that the overall reaction is 2H⁺ + H₂O + CaAl₂Si₂O₈ = Al₂Si₂O₅(OH)₄ + Ca⁺⁺. Formation water compositions indicate the leach reactions are occurring today at about 100°C (212°F).

Petrographic, microprobe, and mass balance calculations indicate that alumina is conserved on the scale of a thin section. Calcium is particularly mobile on a scale of tens of meters.