Title : Salt basins exploration risks: The good, bad and ugly
Abstract:
For the last three decades the conventional exploration for oil and gas has mostly engaged with sedimentary columns associated with rock-salt basins build up. The Gulf of Mexico’s subsalt exploration success was an intriguing endeavor for opening new plays in South America, West Africa, Mediterranean etc. In the last two decades, the subsalt and pre-salt exploration prospects have yielded prolific hydrocarbon traps especially in Deepwater worldwide. For example, in the Gulf of Mexico, frontier overthrust subsalt Deepwater added 2 million barrels / day of oil to the USA energy needs. The presence of the salt intrusions in the sedimentary columns has a great impact on creating potential hydrocarbon traps of different large forms in the host stratigraphic column and inversely causing several exploration and drilling challenges.The unique petrophysical properties of the salt and its impermeable lithology , relative to the surrounding rocks , create optimum habitat for hydrocarbon generation, migration, entrapment in sealed containments.The salt’s lower density relative to the host rocks creates a vertical buoyancy effect. Moreover, it’s exceptional density reduction with burial generates extra up-lift with increasing depth. Salt’s ductile nature becomes more pronounced with increasing depth and temperature.The emplacement and displacement of salt within the host beds plays a crucial role in trapping and breaching of hydrocarbons. Emplacement can be simultaneous with the host rock sedimentation process or post sediments lithification as intrusive mass. The intrusive salt chances of breaching the trap are high and consequently increase the risk of exploration success. Some of the challenges during the exploration phase is the subsurface seismic imaging processing and interpretations. The drastic seismic velocity contrast between the salt and the host sediments exacerbates the seismic driven imaging results of the subsurface prospective trap configuration. An additional challenge in the frontier areas is increasing water depth. This results in a narrowing safe pressure drilling window between the subsurface formation and the fracture Pressures profiles. Potential drilling hazards such as shallow water flow (SWF) and hard kicks can turn to blowouts, and setting extra casing strings are common. The perturbating of stress fields within these structural traps leads to relative complex geomechanical settings. Therefore, most of the conventional normal geopressure modeling driven by overburden has to be adapted to the altered new geomechanical setting created by the salt intrusion. In a nutshell, finding and producing oil and gas from salt basins is a double-edged sword . It is lucrative , however some of the wildcats can turn to a cost burden for the operators. Life threatening and serious property damages can result as well if the technical safety procedures are not followed. Building the geological framework and salt emplacement and displacement history of the potential trap during the prospect generation phase can help predict the exploration risk and the possible drilling hazard. The costs of Deepwater exploring and producing facilities have increased exponentially. However, the potential of finding a highly rewarding oil reserves is worth taking the big gamble.
