Finding and Characterizing Sedimentary Basins – Using Offshore Gravity Data

Abstract:

Marine gravity data acquisition and interpolation methods are undergoing a resurgence of interest spurred by continued improved understanding of passive and active tectonic margin settings. The data and modelling are concerned with the interaction of mantel heat flow, and a broader framework is now available for understanding the context of where and why sedimentary basins form.

The early stage exploration activity is most economically achieved by conducting marine gravity surveys. The benefit of acquiring gravity on sparse 2D lines is that realistic estimates of the regional density variations can be made by interpolation – And yet, many continental margins around the world remain without the necessary gravity survey data that would provide the primary method for finding and characterizing sedimentary basins.

In unconventional geological settings, gravity is also enjoying a resurgence of interest as part of the cycle of building better velocity models to improve the structural geology interpretation.

Historically the most dominant marine gravity survey instruments derive from the original La Coste design, involving the zero-length spring. This is designed to sense wavelengths as short as 3 kms. The latest version, which adopts the benefits of digital electronics, is the MG6. Recent developments in gravity instrumentation systems allow acquisition teams to produce progressive Free Air gravity maps.

When there are several overlapping surveys of various vintages, a best fit merge and levelling effort is required. Next, a marine bathymetry correction can follow. Marine sediment is often found to have a density of around 2 g/cc or less. A bathymetry grid is therefore essential and should be sampled at, at least 4 times the spatial resolution of the gravity. This is due to the variations in short wave length gravity caused by active/passive tectonics, tidal loadings, and tilts.

Case Study – South East Australia

In the period from 1999 to 2004 the Otway Basin, Great Australian Bight, and Gippsland Basin, all underwent new marine gravity surveys (Figure 1). This was followed by an effort to create comprehensive maps of the margins, and has resulted in new and expanded oil and gas production from areas that were previously under-explored.