Sequential Indicator Simulation

Facies Without an Order

Many facies have no natural order: a tidal sand, a crevasse splay, and a lagoonal mud can all sit side by side in any arrangement. Truncated Gaussian cannot represent that, so we need a method where any facies may border any other. Sequential indicator simulation (SIS) is that method.

Indicators and Probabilities

SIS codes each facies as an indicator, one where the facies is present and zero elsewhere, and gives each its own indicator variogram. Visiting the cells one at a time, it estimates the probability of each facies at the cell from the nearby data and those variograms, draws a facies from the probabilities, and freezes it. The proportions and the indicator ranges control the result, and nothing forces an order.

The Two-Point Ceiling

SIS is flexible and conditions well, but like every variogram method it knows only two-point statistics: how one pair of points relates. That is enough for patchy, blocky facies but not for long, sinuous, connected shapes. Ask SIS for channels and it returns disconnected blobs. Breaking that ceiling needs multiple-point statistics, next.