Overthickening of sedimentary sequences by igneous intrusions

Abstract

The identification of extensive intrusive igneous complexes both in subsurface data and in field studies has resulted in quantification of the volumes of igneous material. Despite this research there is still little connection established between the amount of igneous material intruded into a basin and its effect on subsequent basin evolution in terms of burial and loading. To understand how additional igneous material may influence basin evolution we investigate igneous intrusions from the Faroe-Shetland Basin (FSB) utilising subsurface data. This study highlights that the total estimated thickness of sediment during Cretaceous is likely an overestimate as the sedimentary fill consists of significant quantities of igneous material which was emplaced during the Paleocene (56-54 Ma). Previously this additional igneous material has not been accounted for in estimates of sedimentation rates and the burial history of the FSB. Importantly petroleum system modelling to understand generation and expulsion of hydrocarbons benefits from correct estimates of basin fill. The overthickening of basins by igneous material will affect the timing of hydrocarbon generation and subsequently the proper evaluation of exploration targets. In order to fully understand basin evolution the volumes of igneous material and when this material was emplaced must be acknowledged and considered. INTRODUCTION text file Click here to access/download;text file;Revisions_Overthickening by Igneous Intrusions.docx

Modeling petroleum expulsion in sedimentary basins: The importance of igneous intrusion timing and basement composition

“…2B). However, with emplacement of the 1190 m thickness of Paleogene intrusions predicted in this location by Mark et al (2018) at the correct time (58-55 Ma), the thinner overburden during the Upper Cretaceous above the KCF would reduce the source rock paleotemperature, with the predicted onset of petroleum generation in the late Campanian (73 Ma), ∼17 m.y. closer to the present day than in previous models (Fig.…”

“…However, as demonstrated by Mark et al (2018), the presence and thickness of Paleogene intrusions are highly variable across the FSB, from as much as 2 km in the Nuevo sub-basin to 200 m locally in the Flett sub-basin, ∼30 km away. We therefore argue that no one unifying mechanism across the basin can solely explain the time discrepancy between basin models and the actual timing of petroleum charge.…”

“…Because ∼91% of measured intrusions in FSB wells are <40 m in thickness and typically basaltic in nature (Mark et al, 2018), our model suggests geologically rapid cooling of intrusions (on the order of 10 2 to 10 4 yr) due to the large contact area with surrounding country rock sediments and the relatively rapid heat conduction associated with thin intrusions (Peace et al, 2017).…”

“…In addition, the magnitude of overpressure varies markedly across the FSB (Iliffe et al, 1999), calling into question the viability of employing this model regionally. Schofield et al (2017) and Mark et al (2018) both suggested that removal of igneous intrusions and restoration of the original basin sedimentary thickness may lead to later burial and onset of oil generation within the FSB than previously assumed, but this was not quantified.…”

“…Detailed modeling inputs, outputs, and rationale are provided in the supporting material in the GSA Data Repository 1 . Mark et al (2018) showed that the Cretaceous thickness within the FSB, previously assumed to be of sedimentary origin, is commonly a combined thickness of both Cretaceous sedimentary material and a substantial thickness of Paleogene-aged sills, producing an "overthickening" of the Cretaceous sequences post-deposition. Crucially, this forms a significant proportion of the overburden above the Jurassic source rocks (Schofield et al, 2017).…”