Top of Structure Versus Top of Porosity
How does Top of Structure vs. Top of Porosity
Impact you, your prospect and your volumetric?

By: Daniel J. Tearpock

Subsurface structure maps are drawn on specific stratigraphic units to depict the three-dimensional geometric shape of the geologic structures being mapped. Once the geometry of the structure has been determined, the primary effort is focused on the mapping of all hydrocarbon-bearing stratigraphic units.

At times, for various reasons, a structure map is prepared on a good seismic event or resistivity marker that is correlatable on seismic data or in all or most of the wells in a region or field, instead of mapping an actual hydrocarbon-bearing unit. In some cases this may be done because the hydrocarbon-bearing unit is discontinuous or has great vertical variation not reflecting the true shape of the structure. Therefore, it is necessary to prepare a structure contour map first on a stratigraphically equivalent marker in order to construct a map that conforms to the true structure of the field or region. This marker may be a few feet or several hundred feet above the actual hydrocarbon-bearing unit(s). Once the structural framework is prepared by contouring the data from the stratigraphically equivalent marker, a second map, called a Porosity Top Map, is required on the top of any hydrocarbon-bearing reservoir rock for the purpose of delineating the actual configuration and limits of the productive unit(s) (Fig. 1).

Figure 1 Electric logs from three wells. The upper stratigraphic marker conforms to true structure and is used to construct a map representing the true structural framework of the area. The top of the thick productive sand member does not conform to structure, but it represents a porosity top. It must be mapped separately to delineate the actual reservoir configuration.

It is also common for the upper portion of a particular stratigraphic unit to be composed of nonreservoir-quality rock. This nonreservoir-quality rock is often referred to as a tight zone or tight streak. Although the top of the unit may represent the actual stratigraphically-equivalent horizon, or the marker defined from seismic or well log data, it is underlain everywhere by impermeable, nonreservoir-quality rock. Therefore, the structure maps prepared to interpret the true structure commonly cannot be used to evaluate the reservoir itself.

Once a structure map is completed, the next step is to prepare a top of porosity map for accurate delineation of the reservoir, and for later use in the construction of net hydrocarbon isochore maps. Two parameters are considered in evaluating the importance of separately mapping the top of porosity: (1) the thickness of the zone between the correlative marker and the top of the reservoir unit, and (2) the relief of the structure. A thick zone has a greater effect than one that is thin. Low-relief structures introduce greater error in delineating the limits of a reservoir than steeply dipping structures, particularly if the low-relief structure contains a bottom water reservoir.

Figure 2a shows a structure map and cross section for the 6000-ft Reservoir. This unit consists of nonreservoir-quality rock in the upper 75 ft. The same reservoir is mapped on the top of the porous rock or porosity top in Fig. 2b. Notice in cross section A-A' that by mapping on the top of the unit, in which the upper 75 ft consists of nonreservoir quality rock, the limit of the reservoir (gas/water contact) is extended beyond the true gas/water contact as mapped on the top of porosity. Even though no net pay is assigned to the tight zone, the productive area of the reservoir mapped on the top of the non-productive portion of the unit is larger. In turn, the volume of the reservoir is also larger than that mapped on the porosity top. In this case, the volume, based on net gas isochore maps, is larger by 32 percent. This added reservoir area (Fig. 2c) created by mapping on the top of the stratigraphic unit does not contain hydrocarbons and therefore is not productive; consequently, the volume of recoverable hydrocarbons based on this map is overestimated.