Burial History and Its Impact from the Petroleum

Systems of Rio Muni Basin, Equatorial Guinea:

Delivery, Capture and Degradation

The Rio Muni Basin, offshore Equatorial Guinea, is a volumetrically significant petroleum system sourced primarily from Lower Cretaceous source rocks and reservoired primarily in Senonian deep marine clastics. The thermal evolution of the source intervals, including the impact of Miocene uplift, erosion and re-burial, effected the timing of expulsion of oil and gas volumes into the system. Differences in source, reservoir and trap histories relating to uplift in-board, vs. little to no uplift out-board, lead to differing fluid properties. These are exemplified in the Elon and Ceiba pool, respectively.

The primary controls on the timing and volume of hydrocarbons expelled are present day crust type and thickness, and overburden thickness at time of maximum burial. Much of the outboard area lies on oceanic crust, while the inboard area contains a structurally complex and variable continental crust. Crustal thickness derived from 3-D gravity inversion, temperature data, and a series of 1-D basin models to map and predict lateral variation in heat flow were all used to determine the thermal evolution of the basin. The eroded section was restored beneath the Miocene unconformity using e-logs and biomarker thermal stress indicators. Uplift and erosion was quantified using both maximum thermal stress indicators with 1D models, and geophysical log responses. From the resulting points, a map of net erosion was created and used in two and a half dimension map-based thermal history modeling.

The resultant quantitative forward model of expelled fluid volumes from the source rocks, linked with the burial and thermal history of the reservoirs, explains differences in fluid properties. This model explains the presence of biodegraded residues in oils reservoired presently at high temperatures in the Ceiba f ield, and the non-biodegraded oil in the presently cool reservoir of the Elon pool.