Effects of Pore Geometry Changes on Chalks Velocity Anisotropy

Saberi, Mohammad Reza ¹; Barone, Mirko ¹; van Heiningen, Pieter ¹
(1)Fugro Robertson BV, Leidschendam, Netherlands.

Chalks are deep-water pelagic sediments consisting largely of stable low-magnesium calcite with high critical porosity. Progressive diagenesis reduces their initial porosity and makes changes in their pore geometry. Reduction in the porosity implies velocity increment, while changes in the pore structure may cause the velocity to increase or decrease. Pore geometry changes toward stiffer pores (intra-particle porosity) results in higher velocities compared with the changes toward compliant pores (inter-particle porosity). Pore-model stiffness (PMS value) is a parameter which was introduced by Saberi et al. (2009) and was used to incorporate porosity and pore types into velocity interpretation. They self-consistent approach (SCA) to make a link between velocity, porosity and lithology changes in chalks. The PMS value is an attribute of this link which is governed by both velocity and porosity. In theory, for a given porosity, velocity increases with increasing PMS value. Therefore, PMS values contain information about velocity, porosity and pore types, and may be assigned for different lithofacies (ooze, chalk, and limestone). We investigated the relationship between PMS value and Observed Velocity anisotropy at some drilled wells during deep sea drilling program (DSDP) and ocean drilling program (ODP).

Many authors analyzed and modeled chalks physical properties (porosity, density, velocity anisotropy, pore geometry etc.), using these programs in different diagenetic phases. A dataset from some sites of the DSDP/ODP located in the Ontong Java Plateau (western equatorial Pacific) were selected and used to interpret the horizontal and vertical velocities using PMS values to analyze the effects of pore geometry changes on velocity anisotropy.

Our results show that velocity anisotropy is very small for ooze intervals, while it increases with depth by passing through chalk and limestone intervals. The PMS value follows almost the same trend for different lithofacies and decreases from ooze to limestone intervals. This indicates the role of compliant pores (intercrystalline porosity) on velocity anisotropy: rocks with soft pore system (lower PMS values) are more susceptible for velocity anisotropy although they may have gone through digenesis and become indurate. However, soft rocks with a stiff pore system (high PMS value) do not necessary show an anisotropic velocity.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.