PRINCIPLES OF NET PAY ISOCHORE MAPPING

Daniel J. Tearpock, Subsurface Consultants & Associates

The construction of net pay isochore maps is often considered by geoscientists and engineers as a mechanical process with little need for interpretation or analysis. After all, the geoscientist has used all of his or her geological knowledge, experience, skills, techniques and imagination to interpret seismic data, correlate well logs, and build a three dimensionally valid interpretation of a region, prospect, field or reservoir. Through this process the geoscientist has structurally defined perhaps a given reservoir that is under study. Now the remaining process is simple. With a structure or top of porosity map and a spreadsheet of net pay values, a net pay map can be constructed. The net pay values for each well within the reservoir are posted. Now by using a net sand map and a mechanical method of contouring a net pay isochore map can be generated. With a relatively simple structure, uncomplicated stratigraphy (in general, a uniform vertical distribution of reservoir quality rock), a single phase hydrocarbon (gas or oil) and a reservoir with a small water wedge, the process of making a net pay isochore map is truly not all that difficult. In fact, one can boil the steps down to a cookbook approach.

However, this is not the case when confronted with a reservoir that is characterized by variations or an erratic vertical distribution of reservoir vs. nonreservoir quality rock. Such cases require geological analyses, the review of actual well log data and special mapping techniques in order to generate an accurate net pay isochore map used for a deterministic estimate of In-Place and recoverable hydrocarbons. The primary method for generating a net pay isochore map by hand in such a reservoir requires the use of a technique known as “Walking Wells”.

The “Walking Wells” method greatly improves the accuracy of the estimated hydrocarbon volume in stratigraphically complex reservoirs. The “Walking Wells” method allows the geoscientist or engineer to define more accurately the net pay values at selected points within the wedge edge of a reservoir. Key parameters required for walking wells are the well logs themselves, a structure map on the top and base of the reservoir and a net sand map. With the assumption that the variation in the vertical distribution of reservoir quality rock and the amount of reservoir quality rock is constant in a direction parallel to the net sand contours, these net sand contours are used as a guide to define the direction in which to walk a required number of wells within the wedge zone in order to post additional selected points of control for net pay.

The use of the method of walking wells results in a more accurate net pay isochore map within the wedge zone of a reservoir, thereby providing a more accurate estimate for the volume of In-Place and recoverable hydrocarbons. As engineers often state, “if the geoscientists give us the wrong container size and volume, our reserves will be wrong, we will not be able to conduct an accurate history match and the reservoir or field development can have more uncertainty than it should.”

AAPG Search and Discovery Article #90098©2009 AAPG Education Department, Houston, Texas 9-11 September 2009