Subsurface Temperature Prediction From Seismic Measurements: A 3-D Seismic Case Study From The Lüderitz Basin, Offshore Namibia

Abstract

In frontier petroleum provinces significant risk is associated with source maturity. Explorers typically rely on basin modelling to de-risk charge prior to drilling. The traditional bottom up approach to understanding the thermal history takes into consideration factors such as the tectonic evolution and lithospheric thickness of the area. There are several assumptions in this method that can significantly impact the final temperature estimates. Borehole temperature data has till date been the best and most reliable source of thermal information. However well data is scarce in frontier basins. This paper proposes an alternative approach underpinned solely using more readily available high-quality reflection seismic and reflection seismic-velocity data to predict subsurface temperatures. Previous work has shown that it is possible to derive heat flow through the interpretation of a bottom simulating reflector following the top of free gas at the base of gas hydrate stability. Existing ODP literature has demonstrated a link between P-wave velocity and thermal conductivity, enabling the development of a modified bulk P-wave velocity to thermal conductivity transform applicable to passive margin settings. An integrated workflow has been developed to allow full 3D temperature estimation based on seismic volumes and their associated interval velocity model, property cubes of thermal conductivity, spatially varying geothermal gradient and heatflow. This paper reports the application of this new workflow to a frontier exploration 3D seismic volume in the Lüderitz Basin, offshore Namibia. This is a frontier basin with limited activity till date, the only existing drill sites including an ODP borehole (Site 1084) and a shallow water exploration well (2513/8-1). Working petroleum systems exist to the north (Walvis Basin) and south (Orange Basin) of the study area. Thus, there exists potential in the Lüderitz Basin for hydrocarbon generation. The exploration well provides the proximal well calibration along strike but mainly samples Mesozoic sediments, thus yielding relatively high thermal conductivities compared with the Cenozoic overburden in most of the exploration license. A modelled pseudo well provides a temperature profile within the basin. Temperature of 112°C at Barremian carbonate target depth of 3821.3m sub-sea (referred to as Prospect B) is predicted, suggesting that present-day conditions are right for oil and gas condensate generation.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018