Seismic Detection Of Juvenile Reservoir Character Under Leaky Fluid Cushions

Abstract

ABSTRACT Fluid layers (such as salt which are largest incompressible and which support overburden in The subsurface most often represent difficult failing Owing both to their alibility to flow as well as widely Exhibiting higher then what might be considered Normal temperatures and pressures. The incentive For penetrating such formations are typicily high uality reservoirs which lie below them. We now begin to understand that if the fluid layer Can move laterally or "leak" (i.e. low, & form or ven dewater for the case of a shale section) the effective result sheiks 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 rich 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 Fecundating technical endemic, the most immediate Piece of practical information we might wish to Have would be a measure - however crude - of the Retardation of reservoir 'batten?s relative to Anticipated properties. Of course we wish to have Such information in aduance of drilling as wail us know quite clearly from the Guy of Mexico sub salt Penetrations that the differences may be great1,2. Yet a direct calculation would involve the particular Judi (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 unknown leads us to suggest That our best current hopes for a success approach to determining the information we seek rests with measurement of relative Sana/shale reflective state. We may use both well logs as well As seismic abate in attempts to develop such Reactions. Our knowledge generality 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 likeiypracticai Value in order to ashes the problem during that Time in which our knowledge base empanel. Emphasis here wily fm on this measurement Procedure rather than on the physics and mechanics Of the low and deformation processes involved Clearly these are worthy of stud as well, but in our View at some later date. INTRODUCTION Recent work we performed in the German North Sea showed that the Rotliegendes, a Permian aged Aeolian Sandstone had porosities and permeability?s Which were unusually large for a rock of this age? And at depths of 18-24,000 ft. additionally, the Sand to shale relative consolidation state, much less Compacted, is more typical of what is usually seen in Miocene classic sequences. The presence of an Overlying bedded largely salt section of about 9, OOO Ft. thickness pointed to the salt as being responsible.