Abstract: Reservoir Potential of Volcanogenic Sandstones

David K. Davies, William R. Almon

The recent upsurge in frontier drilling activity has spurred interest in volcanogenic sandstones as potential hydrocarbon reservoirs. Successful techniques for exploration necessitate an understanding of the rates and causes of permeability reduction and enhancement.

Studies in Guatemala reveal rapid rates of permeability loss owing to diagenesis in volcanogenic sands. Clay pore linings may be developed on sand-sized grains within 2 years of deposition. Lithification and significant porosity occlusion may occur within 70 years of deposition in both marine and nonmarine environments. Our data suggest that the original permeability of a volcanogenic sand can be reduced more than 90% within 2,000 years of deposition, chiefly by pore linings and pore fills of hematite-goethite, smectite, and zeolite. Production and completion problems can be expected to be numerous in such sandstones. The presence of hematite-goethite and smectite cements introduces the potential for high irreducible water saturations, and extreme acid and freshwater sensitivity. Prec pitation of these cements can occur within 2 m of the surface, and at atmospheric temperatures and pressures. Thus, they do not require elevated temperatures, significant depths of burial, or long time spans for their formation. The potential for the development of secondary, dissolution porosity is high in these sandstones, if some fluid pathways remain open. Many of the hydrocarbon reservoirs which we anticipate will be discovered in volcanogenic sandstones amy owe their origin and quality to the presence of secondary, not primary porosity.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma