ABSTRACT

The size of a fault cut as determined by log correlation is defined as the True Vertical Thickness of the missing or repeated section. Likewise, sand thicknesses determined from well logs, and used for net sand or net hydrocarbon isopach mapping are also defined in terms of True Vertical Thickness. Therefore, the size of a fault or the thickness of a sand determined by log correlation with a deviated well, exhibiting a measured log thickness, must be converted to True Vertical Thickness.

We have derived a three-dimensional correction factor equation applicable to deviated wells. The equation is used to correct the size of fault cuts for use in fault surface mapping and structure map integration, and the thickness of sand applicable to isopach mapping. This equation is more accurate than any previously published two or three-dimensional equation.

Although two-dimensional equations are simple to use, they lack application in three dimensions. We consider the three-dimensional equation preferable because: 1) this one equation can be used to calculate a correction factor regardless of the direction of wellbore deviation, and 2) the true dip of the beds is used in the equation instead of the apparent dip, which is used in the two-dimensional equations.

We introduce a three-dimensional coordinate system, derive the new equation, and illustrate the application of the equation to several deviated wells.