In situ oil shale retort with variations in surface area corresponding to kerogen content of formation within retort site

摘要

An in situ oil shale retort is formed in a subterranean formation containing oil shale. The formation comprises at least one stratum of relatively higher average kerogen content which is included in formation of relatively lower average kerogen content. A void is excavated in a retort site in the formation, leaving a remaining portion of unfragmented formation within the retort site adjacent the void. The portion of unfragmented formation within the retort site is explosively expanded toward the void to form a fragmented permeable mass of formation particles containing oil shale in an in situ retort in which fragmented formation particles from the stratum of higher kerogen content have a lower surface area per unit volume than the average surface area per unit volume of the fragmented mass. This can be accomplished when the higher kerogen content portion has a larger particle size than the average particle in the fragmented mass, or when it has a larger void fraction than the average void fraction of the fragmented mass. When the fragmented mass is retorted, there is no substantial increase in resistance to flow of gas through the fragmented mass due to the relatively higher thermal expansion of fragmented formation particles having the higher kerogen content.