VBA's Log function is the natural logarithm.

From ACCESS Help File:

Log Function
     Returns a Double specifying the natural logarithm of a number.

Syntax
   Log(number)

The required number argument is a Double or any valid numeric expression
greater than zero.

Remarks
   The natural logarithm is the logarithm to the base e. The constant e is
approximately 2.718282.
   You can calculate base-n logarithms for any number x by dividing the
natural logarithm of x by the natural logarithm of n as follows:
           Logn(x) = Log(x) / Log(n)

   The following example illustrates a custom Function that calculates
base-10 logarithms:
           Static Function Log10(X)
               Log10 = Log(X) / Log(10#)
           End Function