Seismic detection of juvenile reservoir character under leaky fluid cushions (including subsalt)

Abstract

Fluid layers (such as salt) which are largely incompressible and which support overburden in the subsurface most often represent difficult drilling owing both to their ability to flow as well as usually exhibiting higher than what might be considered normal temperatures and pressures. The incentive for penetrating such formations are typically high quality reservoirs which lie below them. We now begin to understand that if the fluid layer can move laterally or “leak” (i.e. flow, deform or even dewater for the case of a shale section) the effective result shields deeper units from some measure of the overburden induced compaction processes. Of course we know from the higher conductivity of the fluid layer and temperature profile that thermal maturation of source rocks below this fluid zone would be retarded as well. Hence we can anticipate that target reservoirs under “leaky” fluid layers or “cushions” will exhibit juvenile reservoir character (less compacted or altered) relative to their actual geologic age and depth. While the physics of such phenomena and its modeling and representation constitute a fascinating technical endeavor, the most immediate piece of practical information we might wish to have would be a measure however crude of the retardation of reservoir “state” relative to anticipated properties. Of course we wish to have such information in advance of drilling as well. We know quite clearly from the Gulf of Mexico subsalt penetrations that the differences may be great. Yet a direct calculation would involve the particular fluid (salt or shale for example), rates of leakage, absolute depths, temperatures and the chemical constitution and depositional styles of the sediments as well as other factors many unknown. This daunting number of unknowns leads us to suggest that our best current hope for a successful approach to determining the information we seek rests with measurement of relative sand/shale reflectivity state (Figures 1 and 2). We may use both well logs as well as seismic data in attempts to develop such relations. Our knowledge generally concerning this phenomenon and indeed the proposed method of measurement is quite primitive at this time. Nevertheless, we feel it important to document a potentially promising avenue having likely practical value in order to address the problem during that time in which our knowledge base expands. Emphasis here will focus on this measurement procedure rather than on the physics and mechanics of the flow and deformation processes involved. Clearly these are worthy of study as well, but in our view at