From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, January 6, 2006
Fractures to Caves to Fractures: the Touching-Vug Pore System
Pore space in carbonate reservoirs can be characterized by two pore systems—one composed of interparticle and separate-vug porosity referred to as the matrix pore system and the other composed of an interconnected vug system referred to as the touching-vug pore system. Whereas much is known about the matrix pore system and how to model its flow properties, little useful information is available for modeling the flow properties of the touching-vug system. One reason is that matrix pore systems are small scale, tend to be facies related, and can be characterized from core material. Touching-vug systems, however, are often large scale, are unrelated to depositional systems, and cannot be effectively characterized from core material. Instead, they seem to be related to (1) tectonic and compactional stresses that form fractures and faults at a wide range of scales, (2) dissolution along fractures and within the matrix controlled by lithology, fracture and matrix permeability, and the geochemistry and hydrology of groundwater, and (3) cementation that partially or completely occludes pore space. This pore system is commonly referred to as “fractures” because production data show the permeability to be much higher than matrix permeability. However, I group the resulting pore space into the term “touching vugs” because fracture porosity is only one part of this highly complicated pore system.
Because this pore system is on a large scale, carbonate outcrops provide the best opportunity to develop methods of characterizing its origin and flow properties. Large solution-collapse features found in the Ordovician of the Franklin Mountains have been studied in the past and are an ongoing study of the Reservoir Characterization Research Laboratory (RCRL). More recently, the RCRL has been investigating vug systems found in Cretaceous outcrops of Central Texas. The locations include the well-known rudist buildup at Pipe Creek, solution-enlarged fractures at Pipeline Quarry and at Canyon Lake spillway, and caves and vugs at the Lake Medina road cut. These outcrops illustrate a range of pore types from simple fracture porosity to solution-enlarged fractures, vugs, caves, and collapsed caves that produce breccia porosity and more fractures. They also illustrate how fractures, stratigraphy, and hydrogeochemistry combine to form a complex pore system that is not simply fracture porosity. Therefore, I have coined the term “touching vugs” for this pore system.