David K. Davies¹, Richard K. Vessell¹

(1) GeoSystems, Kingwood, TX

ABSTRACT: Sequence Stratigraphic Control on Seal Extent, Quality and Log Response Characteristics: Incised Valley Systems

Analysis of conventional cores and wireline logs from non-fractured, incised valley systems in various parts of the world reveals that the sedimentary and petrophysical properties of the seals (shales and tight limes) vary as a function of position in the stratigraphic sequence. These variations have important implications in terms of the efficacy of the shales and limes as barriers to fluid migration (seals, sensu stricto) or as intervals of low permeability that allow fluid migration (baffles).

Incised valley fill sequences commonly contain multiple sand reservoirs arranged both vertically and laterally. Many of these reservoirs are separated (but not necessarily isolated) by shales. Variations in the position of the oil/water contacts among the various reservoirs indicate variations in 1) seal quality -- a function of capillary entry pressure, and 2) seal integrity -- a function of the lateral extent and the thickness of the seal interval. Seal quality varies with changes in seal composition and texture which, in any one valley fill system, are related to relative geographic location ("segments" of Zaitlin et al, 1994). Similarly seal integrity, in non-faulted areas, varies with relative geographic location. Seals deposited in different segments of the incised valley have different log response characteristics. Precise identification of the distribution of segments of the incised valley, using log characteristics from seals and from associated sand bodies, can be critical to the prediction of the type of fluid content within any specific reservoir. In large oil and gas fields, the success or failure of reservoir development projects is directly related to the position, extent and effectiveness of the seals and baffles that occur within the vertical sequence of incised valley fill deposits.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado