This drilled stratum has plenty of clay minerals, when wet in mud, the hole wall’s clay will absorb water, and
swelling, dispersing. For water sensitive and stripping stratums, the penetrating hydration should be minimized,
which is reducing water loss and enhancing mud salinity. Being different to chlorite stratum, carbonaceous slate is
more fractured, high oxidizability and lower coefficient of linear expansion. Its drilling problem is mainly hydration
stripping, poor cementation, cementation and hole wall protective of drilling fluid must be improved.
According to the above requirements, test mud formulas are shown as followings: