- Copyright © 2003 Society of Exploration Geophysicists
Glauconite is an iron rich variety of clay that can be found as individual pellets, composite grains, and intergranular cement. Its density ranges between 2.4 g/cm3 and 2.95 g/cm3, averaging 2.67 g/cm3. It has a Moh's scale hardness of 2. Authigenic glauconite is formed under a limited range of geologic and geochemical conditions; it typically develops on the outer margins of continental shelves, in areas of low sediment input (Odin, 1980), and its presence is valuable as an indicator of transgressive sequences.
Identifying glauconite in the subsurface is important for depositional environment interpretation, stratigraphic correlation, dating, tracing of unconformities, and geochemical exploration for source and reservoir rocks (Srivastava, 1986). A number of commercial hydrocarbon reservoirs are glauconitic sandstones—for example in Colombia, Ecuador, Peru, Venezuela, Australia, Eastern China, North Sea, United States, Canada, Saudi Arabia, and Ireland.
Although glauconite tends to exist as grains and as such is part of the rock framework, under moderate overburden pressure, these grains are easily compacted (Figure 1) and may form a pseudomatrix that occludes the original primary porosity. This behavior is in contrast to that observed in clay minerals. This problem, and the fact that there are no published studies about the elastic properties of glauconite and glauconitic sandstones, motivated this research to understand their rock physics properties. We present analyses of data from five lithologies containing varying amounts of glauconite and identify the best seismic attributes to evaluate its presence and the reservoir quality.
Samples and lithology
The samples in this study come from Caballos Formation in Putumayo and Upper Magdalena Basins, Colombia (Figure 2), which is described as a marine transgressive blanket sandstone deposited in a shallow …