Using Structural Dip Modeling to Determine Structure and Stratigraphic Position
As we explore for and develop more complex reservoirs the impact of borehole position becomes critical. When the bed dips are highly variable and complex, the interpretation of the dip and strike can be difficult. In addition, a horizontal or vertical well may pass through multiple structural domains within the reservoir horizon. While drilling, structural dip modeling (SDM) offers the ability to interpret the stratigraphic position of the borehole before a dip log or image log has been run. After drilling, SDM provides a means to fine tune both structure and stratigraphic position. In existing wells where dip data are not available, SDM enables one to determine both structure and stratigraphic position with only a correlation log and directional survey.
The modeling process involves adding dips and faults until a correlation log from near offset called a “stratigraphic template” matches the correlation log of the well being modeled. The modeling process generates a modeled template log and a vector section™. The final result of the modeling includes formation tops as well as a structural interpretation.
One of the difficulties in dip interpretation in general is the visualization of the dips in 3D space. Many techniques have been created for this purpose, but nearly all of them involve first projecting the 3D information onto 2D entities. At first glance, a vector section appears to be a cross section, but it is a fully 3D object comprised of vectors perpendicular to structural dip whose length is based on true stratigraphic thickness (TST). Vector sections can be projected onto cross sections or viewed directly in three dimensions. They can make realistic-looking cross sections with little interpretation.
The stratigraphic template is a log with depths in cumulative TST instead of measured depth. Templates can be created directly from measured depth in vertical wells with low dip, but otherwise TST must be calculated from dips and deviations to create a template. The modeled template log is calculated by first adding or subtracting a constant TST to the template such that it is aligned at a point common to both the template log and the correlation log in the modeled well. The modeled template log is created by moving log values from the template to match TST calculated in the borehole.
Examples are given of horizontal and vertical structural modeling in the Canadian Rockies Foothills province using RDA Dip Interpretation Suite.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009