Temperature From Seismic Data: Application in the Lüderitz Basin, Offshore Namibia

Abstract

In the absence of wells, thermal modelling to ascertain source maturity in frontier basins becomes difficult, as temperature measurements at wells have till date proven to be the most reliable source of such data. This study examines the possibility of using a non-invasive approach reliant on much more readily available seismic data in frontier offshore basins. Previous work has recognised bottom simulating reflectors, representing the base of gas hydrate stability above the free gas zone, to provide the basis for a near-surface thermal gradient calculation and heat flow estimation. This paper seeks to further refine the workflow for present day thermal modelling through the use of heat flow derived from BSRs and a bulk P-wave velocity to thermal conductivity transform based on seismic processing velocities converted to interval velocities. Initial modelling was conducted on a high resolution 3D seismic cube from the Lüderitz Basin offshore Namibia, where the source maturity remains in question due to limited well data. A lone exploration well 2513/8-1 and an ODP borehole Site 1084 provide the proximal and distal control respectively, for the physical characteristics of the sediments and the temperature estimations produced. A bulk velocity to thermal conductivity transform has been utilised as part of the modelling workflow, with ODP core data utilised to develop a bespoke transform applicable for the Lüderitz Basin. Operating under seawater and pure methane phase hydrate stability conditions as defined by Dickens & Quinby-Hunt (1994), gives a mean shallow heat flow of 65 mWm^-2. Coupled with a mean thermal conductivity of 1.55 Wm^-1K^-1 (using the bulk transform) gives a geothermal gradient down to the key Hauterivian Top B prospect depth of 42.1 °Ckm^-1. Thus the temperature estimate for this level is 122 °C, which suggests that the key Aptian source in this area is likely in the oil window. Future work to test the validity of the methodology developed during this study, including the bulk transform, will involve application to a new case study in an area such as the North Sea where subsurface temperature conditions are well constrained through abundant borehole and seismic data.

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