Large-Scale Carbonate Slope Gravity Failures: From Stratigraphic Evolution to Numerical Failure Prediction

Abstract

Carbonate margin -and slope systems exhibit massive scale gravity failures controlling the export of large quantity of carbonate production from the shelf and slope to the deep parts of the basin. Understanding the triggering of these events, and integrating their occurrence into the stratigraphic evolution of a carbonate system is decisive for prospect evaluation at the basin scale. Gravity-driven mass transport can induce and control the location of calciclastic reservoir facies.

Gravity failures are mechanical destabilizations of sediment accumulated in a stable configuration, but influenced by pre-conditioning mechanisms (e.g. over steepening, fluid overpressure build-up by overloading or fluid flow…) that induce failure during a triggering event (e.g. seismic ground acceleration, cyclic loading, fluid escape…). This contribution shows the integrated investigation of this genetic chain at each time step of the system evolution by forward stratigraphic and basin numerical modelling tools.

The first step is a forward stratigraphic modelling using DionisosFlow software in order to determine the geometrical and facies evolution through time. The second step is the computation of the « mechanical stratigraphy » that is the time-dependent state of sediments density and associated mechanical load, fluid pressure and resulting stress state. It is determined with IFPEN basin modelling tools used for hydro-mechanical calculations. The third step is the computation with a limit analysis approach of the most probable failure geometry and associated load factor, using the OptumG2 software (U. Newcastle, Aus).

This innovative workflow is applied on 2D sections of the well-described Plio-Quaternary Western and Eastern slopes of the Great Bahama Bank. They exhibit recurrent and various types of mass failure events. The simulations are carried out using mechanical characterization of the sedimentary material derived from dedicated oedometric and triaxial tests and from literature. A great attention is paid to the effects of early cementation on the mechanical stratigraphy.

The results shows how the internal stratigraphic and mechanical evolution of the system is a first-order control of slope failures in response to external trigger events. These results provide us with new and mechanically tested insights on the time and spatial occurrences of slope failures. These processes can be integrated into a forward basin exploration workflow.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018