About This Item
Share This Item
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.
End_of_Article - Last_Page 427------------