Secondary Porosity in Sandstones and Acid-Producing Reactions in Adjacent Shales, Oligocene, South Texas

David A. McMahon

Frio and Vicksburg sandstones in the McAllen Ranch and Hinde fields in south Texas show two depth-related zones of secondary porosity. Anomolously high porosity in the upper zone, from 4,000 to 6,500 ft (1,220 to 1,980 m), is related to the dissolution of analcime cement by acidic pore fluids released from adjacent shales. Much of this acid may be produced by the reduction of octahedral iron in smectite clay. Sodium released by analcime dissolution may be responsible for abnormally high salinities calculated for pore fluids in these sandstones. Calcite cementation below the upper zone may be related to fluids rich in calcium carbonate that have migrated up fault zones from great depth.

The lower zone of secondary porosity extends from 8,400 to 10,000 ft (2,560 to 3,050 m) at Hinde and from 8,400 to 14,000 ft (2,560 to 4,270 m) at McAllen Ranch. The top of this zone also corresponds to the clay dehydration depth and the top of the abnormally pressured zone. At this depth and temperature, randomly interstratified illite and/or smectite clay becomes ordered and a large amount of water is expelled from the clay structure by potassium fixation. Smectite illitization produces minor amounts of kaolinite and chlorite and, more importantly, releases silica and acidic pore water. The acidic pore water migrates into adjacent sandstones and enhances the dissolution of feldspar grains and calcite, creating secondary porosity. The shales at McAllen Ranch have undergone more exten ive illitization than those at Hinde, and more secondary porosity is observed in the McAllen Ranch sandstones.

Data from detailed chemical and mineralogical analyses were used in mass-balance calculations to confirm that the amounts of acid produced by these reactions can account for the observed secondary porosity in these sandstones.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.