2019 AAPG Annual Convention and Exhibition:

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Using Stratigraphic Modelling to Ascertain the Sensitivities and Uncertainties in Computing the Gravity Responses of Sedimentary Basins

Abstract

Most geophysical tools and techniques are sensitive to heterogeneities (e.g. density, magnetic susceptibility) and to the architecture (e.g. thickness) of sedimentary basins. As gravity anomalies can be related to heterogeneities in the basement and also in the sedimentary cover, these tools only allow for a partial imaging of the structure and distribution of potential resource hosts either under or in the sedimentary deposits. An alternative approach to these traditional modelling tools is to use methods to predict the expected gravity responses of basins independently of the basement responses, especially in order to compute an accurate depth to basement or in order to discuss heterogeneities in the basement.

Common gravity forward modelling of sedimentary basins integrates density gradients to reflect the evolution of the porosity with depth linked to compaction and diagenesis. We aim to compute a range of probable gravity response of basins integrating density heterogeneities related to sedimentary processes and stratigraphic architecture. Using stratigraphic and gravity modelling we can separate gravity anomalies due to heterogeneities in the cover from anomalies located in the basement. Thus, we are able to accurately model the geophysical footprint of the sedimentary cover. We will present the first phase of this project: (1) the development of a workflow that computes the gravity response of sedimentary cover, (2) the understanding of the workflow sensitivity, and (3) the uncertainties on the gravity footprint as a result from the integration of stratigraphic modelling and gravity modelling.

The sensitivity analysis provides a ranking of the main stratigraphic and sedimentological controls on sedimentary basin’s gravity footprint together with a stratigraphic model parameter reduction. Of relevant stratigraphic modelling parameters, only the sediment supply and the subsidence control the gravity response over long periods (~10Ma). In addition, the sensitivity analysis shows that over short periods (~1Ma), variations in the gravity response are controlled by the sediment supply, the type of sediments and their transport parameters. We have applied our approach on a 2D model of the Northern Carnarvon Basin, Australia, and we compare the results and the uncertainties with common gravity forward modelling workflow.