Removal of overburden velocity anomaly effects for depth conversion

Abstract

A method of compensating for the presence of discrete overburden velocity anomalies during depth conversion of time horizons interpreted on conventional, post‐stack time‐migrated seismic data is presented. Positive and negative time delays are estimated either from the push‐down or pull‐up of reflectors directly beneath the anomalies or from interpreted time thickness of the anomalous body and interval velocities estimated from well data. The critical steps are pre‐stack simulation of seismic acquisition across the velocity anomalies, incorporating the effects of a Fresnel volume which changes its width as a function of depth, and simulation of common‐midpoint (CMP) stacking using a linear regression of time delay, Δt, versus offset‐squared, X2. The time‐correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time. Depth conversion is then performed using a background velocity function. The final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon. The method is implemented by manipulating time grids within an industry‐standard mapping package. The final average velocity map shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…Hydrocarbon-bearing glaciogenic deposits are most commonly imaged using reflection seismic methods both onshore and offshore, but refraction, shear-reflection and velocity modelling techniques have also been applied to unravel the fluid content and velocity effects of tunnel valleys (e.g. Armstrong et al 2002;Pugin et al 2004;Eiriksson et al 2006;Ahmad et al 2009;Kristensen & Huuse 2012). Ground-penetrating radar studies of eskers, push moraines and outwash deposits yield amazingly detailed images on an outcrop scale (Bakker & van der Meer 2003;Winsemann et al 2009).…”

“…Although electromagnetic acquisition is efficient relative to seismic acquisition onshore, the reverse is true offshore, where electromagnetic imaging is typically only used for monitoring production or injection of fluids or for derisking exploration well targets. The Pleistocene glacial successions offshore NW Europe often contain significant volumes of methane that poses a risk to exploration drilling and an impediment to seismic imaging (Armstrong et al 2002;Eiriksson et al 2006;Buckley 2012;Kristensen & Huuse 2012). As the North Sea hydrocarbon province is maturing, gas-bearing glaciogenic deposits are currently being viewed as potential resources including, most notably, the giant Peon gas reservoir hosted in glaciogenic outwash fan sandstones and sealed by diamicton ( Fig.…”

“…Buckley 2012), while seismic modelling efforts have been focused on eliminating the often adverse effects of shallow gas on deeper seismic imaging ( Fig. 9; Armstrong et al 2002;Kristensen & Huuse 2012). In mature basins, such as the North Sea, rising oil and gas prices have increasingly focused exploration towards previously uneconomical hydrocarbon accumulations, including some that were previously considered shallow hazards, most notably the 1 TCF Peon gas field, which is hosted in a submarine glacial outwash fan of middle Pleistocene age ( Fig.…”

Stacking velocities in the presence of overburden velocity anomalies

“…The word ‘error’ appears in the title (Blackburn 1980) or in the text of papers. For example, ‘(i)n the presence of velocity anomalies, stacking velocities show systematic errors’ (Armstrong, McAteer and Connolly 2001, p. 81). In both these cases, the term ‘error’ was used in the context of time to depth conversion rather than seismic imaging, for which purpose the stacking velocity values were entirely appropriate.…”

“… Stacking velocity in the presence of a shallow overburden velocity anomaly (from Armstrong et al 2001). …”