A review of techniques used to determine geological and thermal history in the Southern North Sea

Abstract

The Southern North Sea has been a prolific source of gas to the UK economy over the last 30 years. As many of the producing fields are now becoming depleted, attention has switched to exploration in more marginal areas of the basin. In order to evaluate suitable structures in these areas, an understanding of the complex structural and thermal history of the Southern North Sea Basin is required. Particular emphasis must be placed upon the prediction of source rock maturity, the timing of generation and migration of hydrocarbons, and the thermal controls exerted on potential reservoir sand units.Over large parts of the basin the Carboniferous source rocks are not at their maximum depth of burial at present day. A variety of techniques utilizing sonic velocity, vitrinite reflectance and apatite fission track analysis have been used to quantify the magnitude of uplift that the Carboniferous section has undergone through geological time. With the exception of apatite fission track analysis, these techniques provide only an estimate of 'net uplift', i.e. the amount of uplift relative to maximum depth of burial or maximum temperature. They record neither the timing and effects of individual uplift events nor the timing of maximum burial prior to net uplift. In order to accurately reconstruct the burial and thermal history of the basin these techniques must be integrated with a detailed knowledge of the geological history of the Southern North Sea.The use of these techniques in isolation, together with a lack of appreciation of the limitations of the data has in the past resulted in spurious results, leading to criticism and mistrust of the techniques. This paper attempts to show that when correctly applied and in appropriate geological circumstances, each analytical technique can provide useful data for burial and thermal history reconstruction. Examples are shown to highlight the effects of incomplete data interpretation and the impact of inaccurate determination of present day temperatures. Particular attention is paid to determining the burial and thermal history of the economically important Carboniferous source and Permian reservoir units, and the interpretation of uplift data is restricted to the post-Zechstein section.