Rock physics model of glauconitic greensand from the North Sea

Abstract

The objective of this study was to establish a rock physics model of North Sea Paleogene greensand. The Hertz-Mindlin contact model is widely used to calculate elastic velocities of sandstone as well as to calculate the initial sand-pack modulus of the soft-sand, stiff-sand, and intermediate-stiff-sand models. When mixed minerals in rock are quite different, e.g., mixtures of quartz and glauconite in greensand, the Hertz-Mindlin contact model of single type of grain may not be enough to predict elastic velocity. Our approach is first to develop a Hertz-Mindlin contact model for a mixture of quartz and glauconite. Next, we use this Hertz-Mindlin contact model of two types of grains as the initial modulus for a soft-sand model and a stiff-sand model. By using these rock physics models, we examine the relationship between elastic modulus and porosity in laboratory and logging data and link rock-physics properties to greensand diagenesis. Calculated velocity for mixtures of quartz and glauconite from the Hertz-Mindlin contact model for two types of grains are higher than velocity calculated from the HertzMindlin single mineral model using the effective mineral moduli predicted from the Hill's average. Results of rock-physics modeling and thin-section observations indicate that variations in the elastic properties of greensand can be explained by two main diagenetic phases: silica cementation and berthierine cementation. These diagenetic phases dominate the elastic properties of greensand reservoir. Initially, greensand is a mixture of mainly quartz and glauconite; when weakly cemented, it has relatively low elastic modulus and can be modeled by a Hertz-Mindlin contact model of two types of grains. Silica-cemented greensand has a relatively high elastic modulus and can be modeled by an intermediate-stiff-sand or a stiff-sand model. Berthierine cement has different growth patterns in different parts of the greensand, resulting in a soft-sand model and an intermediate-stiff-sand model.

Petrophysics and rock physics modeling of diagenetically altered sandstone

“…We used three laboratory measured data sets: glauconite-bearing greensand from the North Sea (Hossain, 2011), Fontainebleau sandstone (Gomez, 2009), and clay-bearing sandstone (Han, 2010). Each data set contains measurements of porosity, permeability, resistivity, velocity, and clay content.…”

“…Acoustic properties of rocks are also a function of saturation (Wyllie et al, 1958), pressure (Wyllie et al, 1958), pore type (Nur and Simmons, 1969), and pore fluid distribution (Mavko and Mukerji, 1995). The acoustic velocity of rocks also depends on the diagenetic properties of those rocks (Hossain et al, 2011b). Figure 4 shows the rock physics relations between seismic velocity and porosity based on modified Hashin-Shtrikman bounds.…”

“…There are other factors that may influence this relationship. Elastic velocities are influenced by diagenetic processes in rocks (Hossain et al, 2011b). However, the effects of diagenetic processes on resistivity are not yet well described.…”

Permeability-velocity model for clay bearing and cemented sandstone

“…We used three laboratory measured data sets: glauconite bearing greensand from the North Sea (Hossain et al, 2011); Fontainbleau sandstone (Gomez, 2009) and clay bearing sandstone (Han, 2010). The glauconite bearing Paleocene greensand is dominated by quartz but weakly cemented to quartz cemented.…”

“…However, sandstones are often found to have undergone diagenetic processes. Clay bearing and cemented sandstones are common place in (Gomez, 2009), clay bearing sandstone (Han, 2010) and cemented sandstone (Hossain et al, 2011). The reference lines represent published porosity-permeability relations.…”