... rock bits for well drilling: 1, Very soft with low compressive strength, such as soft marl, clay, unconsolidated sandstorm, salt, etc. Recommended Bit ... speed, 300~60 r/m.2, Soft with low comprssive strength, such as shale, clay, sadstne, soft limestone, etc. Recommended Bit type: ... 250~60 r/m.3, Medium soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. Recommended Bit ...