Episodic Regional Burial and Subsequent Exhumation Exert a Major Influence on Hydrocarbon Prospectivity

Abstract

In basins with complex histories, conventional basin modelling invariably combines burial histories based on the preserved stratigraphic section with variable heat flow to match calibration data (usually vitrinite reflectance). However, numerous studies involving application of low temperature thermochronology (principally apatite fission track analysis) combined with VR have shown that thermal history is often controlled not by variation in heat flow but by deeper burial and subsequent exhumation, in which km-scale section is deposited and subsequently removed typically within 30 Myr or less. These episodes are not restricted to basins, but also affect adjacent basement regions across areas of several 10 4 km 2 . The origin of such events has long been debated. Due to the regional extent, the resulting unconformities are often very low angle, and the corresponding time intervals are often erroneously interpreted as periods of stability and non-deposition. The notion of deeper burial and subsequent exhumation during these intervals is sometimes questioned because of the lack of an accepted mechanism. Nevertheless, a large body of data points to the reality of these events. It has also become clear that these exhumation episodes are broadly synchronous over vast regions. Three phases of Cenozoic exhumation are recorded in areas from Alaska to Greenland, Norway and Svalbard, while exhumation episodes ranging in time from Carboniferous to Cenozoic appear to be broadly synchronous in Brazil, South Africa, and Australia. The lack of attention to missing section contrasts starkly with the effort devoted to the preserved section. Yet in many basins hydrocarbon prospectivity is controlled by deposition and removal of the missing section rather than elevated heat flow. Deeper burial leads to enhanced maturity levels while mistaking the effects of deeper burial for elevated heat flow can lead to incorrect discounting of deep source potential. Exhumation can lead to remigration and loss of charge due to seal breach and phase change. Understanding the controlling processes is essential in order to accurately define the variation of maturity in time and space so as to identify more prospective regions, and geodynamic models are required that can provide a mechanism. Meanwhile, basin modelling studies should focus more on deposition and removal of km-scale thicknesses of section instead of elevated heat flow.