Thermal Maturity in the Gulf of Cadiz Due to Mantle Plumes, Triple Junctions, and Plate Tectonics
Authors: Lowrie Allen , Somoza Luis , Moffett Susan 
Published: October 2008

Petroleum exploration in the Gulf of Cadiz began in the 1970's, and by 1999, 26 commercial wells have been drilled. Several 2-D, multi-channel seismic grids at spacings of approximately 4 and 6.5 km have been shot across the continental shelf, upper and middle slope in waters depths up to 1000 m (Fig. 1). Recent drilling by Repsol (1996 - 1998) on the continental shelf off the city of Cadiz are producing natural gas, much of it biogenic. The gas is being transported to nearby Cadiz. The geothermal gradient measured in these few wells on the central and inner shelf is 2.4 degrees C/100 m. If that value should characterize the entire Gulf of Cadiz and adjacent continental margin, then it is likely, given observed sediment thicknesses, that the area might not be commercially petroliferous. In this article, geologic arguments are presented that the regional geothermal gradient, especially along the shelf-break and continental slope, may be appreciably higher, locally up to 3.1 degrees C/100 m.

INTRODUCTION

With the initiation of sea-floor spreading in the North Atlantic at approximately 180 – 170 million years ago, the beginning of Iberia as a continental margin commenced. The very first initiation of extension in the Triassic apparently resulted in shallow seas intermittently filled with salty marine waters, desiccated repeatedly, and with each desiccation, a scum of evaporite was left behind to accumulate into appreciable thicknesses. Dimensions of that original thickness are difficult to decipher given the subsequent geologic history of that area (Maldonado et al., 1999). Information derived from commercial exploration wells show salt as individual layers underlying crust when both former and latter were extruded from east to west by "escape tectonics" caused by convergence between Iberia and Africa from Middle Eocene to the present (Maldonado et al., 1999).

Interpreted industry seismic data reveal individual thrusts that appear to have moved laterally 5 – 10 km, perhaps as much as 15 km (Maldonado et al., 1999) (Figs. 2 and 3). There is the real probability that the rugged basement (showing individual and local interpreted thrusts) itself has been moved laterally to the west as the Gibraltar Arc, the most southwest extension of the Alpine Orogeny, was translated up to 100+ km to the west (Maldonado, et al., 1999). With such a dynamic evolution, original characteristics of salt are modified beyond recognition.

The Alboran Domain, in essence a micro-plate between the African and Iberian Plates, has moved west, causing "escape tectonics". Crustal blocks up to 15 – 25 km deep were converted into thrusts. Temperatures at such depths may approximate 400 – 500 degrees C. With the crustal thrusts encased in salt, which is characterized by an ability to transmit heat double or triple that of sediments, heat can be disseminated in intricate patterns along the continental margin. Heat is transported by radiation, conduction and convection. Once heat is radiated to the surrounding sediments, conduction and convection become the principal heat transfer mechanisms.