Ann: Preliminary Wireline results shows Multiple Potential Zones, page-829

The "skin" on the sidewall core. More correctly called mud filtercake....formed when the fluid component of a drilling mud invades the formation in the bore hole wall. The liquids penetrate into the formation while the solids get deposited as filter cake on the borehole wall. This is what was seen on the side wall core.

When a Mud Engineer carries out a filtrate and filter cake check he puts a sample of mud into a sealed container and the mud is forced through a filter paper. The filter paper simulates the borehole wall. The fluid that passes through the filter paper is the filtrate and the solids left on the filter paper are the filter cake. The rate at which the filtrate collects is measured over a fixed period of time and recorded as "fluid loss" on the mud report. The thickness of the filter cake is likewise recorded on the same report. ON a high pressure/high pressure temperature well (HPHT) the tests would be carried out with the sample heated and pressurized to simulate downhole conditions.

Skin is a term used when describing formation damage.....ie a reduction in permeability due to invasion of drilling mud solids into the formation or alternately swelling of clay matrix in the pore spaces due to fluid invasion. The reduction in permeability decreases as you move out from the borehole.