Observing maturing source rocks on seismic reflection data

Abstract

Hydrocarbon generation in a source rock is a complex, irreversible phase change that occurs when a source rock is heated during burial to change the phase to a fluid. The fluid density is less than the kerogen density; therefore, in a closed or partially closed system, the volume of the pore space occupied by fluids increases. Burial also increases the effective stress, which leads to compaction and a significant reduction in porosity. The challenge of identifying source rocks on seismic data then becomes differentiating the smaller porosity increase due to hydrocarbon formation from the larger porosity decrease associated with burial. We have used a calibrated rock-physics model to indicate that [Formula: see text] and porosity data can be used to predict the P- and S-wave velocities and the density in wells over large sedimentary sections, including a source rock of variable maturity. These well data and models indicate that the difference between an immature and mature source rock is an increase in porosity (lower density) relative to compacting, nonsource rock sediments. We use these results to identify a potential source interval in the Orphan Basin in Eastern Canada on 2D regional seismic data. We find that the full stack amplitude response of a maturing source rock is significant during the main phase of generation (0.2 < transformation ratio < 0.8) relative to the surrounding sediments. Regional scale consistency of the amplitude response with the kerogen maturity model from an integrated basin simulator reduces exploration risk because of the independence of the thermal model from the seismic amplitude response. Finally, combining the seismic response with the source rock maturity model provides insight into the likely kerogen kinetics. Most of applications require regional data sets to capture the maturity window; however, applications are also possible around allochthonous salt where geometries can lead to local changes in the heat flow.