Spatial and Temporal Characterization of Mechnical Rock Properties From West Caicos, BWI

Abstract

Early brittle deformation is a key component in the structural, stratigraphic and diagenetic evolution of steep-rimmed carbonate platforms common to many major petroleum systems (e.g., pre-Caspian, Stuart City, and Scotian Abenaki). The goal of this study is to gain an understanding of the early response of carbonates to loading, deformation, and potential margin collapse or failure and to incorporate the variability of mechanical properties that are critical to the early deformation process. Samples were collected and unconfined compressive strength (UCS) field measurements were taken on West Caicos, where the eustatic history is tightly constrained, and detailed facies successions and fractures have been mapped. Key locations for sampling and UCS field measurements on West Caicos were chosen based upon the exposure of reef and grainstone facies and their relationship to the four major glacial-interglacial sea-level fluctuations. Facies successions include a base that is characterized by reef deposits abruptly transitioning into grainstone facies (e.g. foreshore, upper shore face, eolianite) and is often capped by an exposure surface. Three different analyses were completed: (1) detailed petrographic descriptions of grain type, texture, sorting, and cementation; (2) Porosity and Permeability measurements were completed on 1″ diameter plugs; and (3) triaxial compression testing to ascertain mechanical rock behavior of samples and compare against in situ UCS readings. A Schmidt hammer was used to take 295 field rock hardness measurements to further constrain the mechanical behavior of facies within a stratigraphic framework. Thin section analysis and porosity and permeability data from 125ka and 400ka eolianite ooid grainstones reveals that cementation increases through time, indicating UCS is a function of porosity and age. Initial triaxial compression testing reveals a 125ka eolianite ooid grainstone has UCS of 4 MPa, similar to, but slightly lower than the in situ UCS of 9 MPa. Results from in situ UCS measurements illustrate that with increasing age, overall rock strength increases. Measurements collected from West Caicos also demonstrate that rock strength is dependent upon proximity to exposure surfaces, where UCS increases. The temporal and spatial variation in UCS across West Caicos illustrates that rock strength of young carbonates must be considered when evaluating syndepositional deformation of ancient steep-rimmed carbonate platforms.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016