Igneous Intrusions in Porous Sandstone Sequences--Widespread Thermal Effects Measured by Fission Track Annealing and Vitrinite Reflectance: ABSTRACT

Abstract

Current literature suggests that igneous bodies have only minor thermal effects on intruded sedimentary rocks, increasing the maturity of a thickness of adjacent strata approximately twice the width of the intrusion. This study shows that this is not always true. In the Canning basin of Western Australia, Permian dikes, sills, and laccoliths have intruded porous and permeable Carboniferous and Permian sandstones. Efficient vertical and lateral heat transfer has occurred by movement of hot waters through the sedimentary rocks over large distances away from the igneous bodies. This heat transfer is recorded by the resetting of fission tracks in detrital Precambrian apatites, which now have apparent ages similar to those of the igneous intrusions. In some instances, a signif cant increase in vitrinite reflectance within the sediments is also evident, but vitrinite appears to be less sensitive to heat pulses of short duration, even though temperatures greater than 110°C have developed. Fission-track studies suggest that temperatures of at least 110°C to 130°C have occurred up to 3 km from thin doleritic dikes and sills in porous sandstones where pre-intrusion temperatures were around 40°C. Some evidence of increased temperature is also apparent 26 km from the nearest mapped intrusion, although this has not been sufficient to totally anneal fission tracks. End_Page 299------------------------------ Vitrinite reflectance readings are significantly higher in wells that penetrated thin intrusions, and this increase in vitrinite reflectance to values of around 1% is evident in one well at least 500 m above a 156-m thick doleritic dike where fission tracks have also been reset. The intrusions have thus heated a considerable volume of regionally immature rocks to temperatures equating to the oil window for a short period of time. Whether this relatively short-lived temperature increase has led to significant generation of hydrocarbons is unknown; however, oil recovered from one well that penetrated a doleritic dike had a sterane:aromatic sterane ratio suggestive of generation during a rapidly cooling heat pulse. End_of_Article - Last_Page 300------------