Variation in style of overpressure in Scotian Shelf wells, Scotian Basin

Abstract

Overpressure is a phenomenon where pressures greatly exceed normal hydrostatic pressure and occurs in many wells within the Scotian Basin. Due to this area being actively explored for oil and gas over the last five decades, it is very important to understand where and what is causing overpressure. The main causes of overpressure are disequilibrium compaction, clay diagenesis, and hydrocarbon generation, although, the relative importance of these processes in the Scotian Basin is uncertain. To assess and interpret the causes of variability in the style of overpressure in different wells in the Scotian Basin, velocity and density data from wireline data logs were used to produce velocity vs. density cross plots. These plots allowed the possible secondary mechanisms of overpressure generation to be visualized. XRD of < 2 μm clays from shales within overpressured wells were analyzed based on clay mineralogy to possibly find a link between overpressure and diagenesis occurring in the studied samples. Down-well variation in velocity vs. density of shales based on wireline logs showed a wide range of velocity vs. density patterns in overpressured sections. There was an apparent regular distribution of different types observed based on velocity-density patterns. Fractures and cementation may have an influence on velocity and density downwell. The fractures may be due to the buildup of overpressure and its eventual release. The opening of fractures would cause a decrease in velocity and that would be observable in velocity-density plots. The appearance and disappearance of smectite at certain depths in wells may be due to: a) downhole contamination by smectite cuttings from the upper sections of the wells. b) Local increase of temperature by circulating fluids causing these diagenetic transformations and c) changes in the supply of smectite by rivers that were climatically controlled.