Medium hard formation with high compressive strength. Such as hard shale, limestone, sandstone, dolomite, anhydrite and marble.

高抗压强度的中硬地层，如硬页岩。石灰岩。砂岩。白云岩。和硬石膏。大理石。

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Low compressive strength of soft to medium formations. Such as soft shale, soft limestone, soft sandstone, hard and soft abrasive interbedded medium formation.

低抗压强度的软到中等地层，如中软页岩。中软石灰岩。中软砂岩。和软硬研磨性夹层的中等地层。

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Chevron, for example, has invested heavily in shale production in the Marcellus field in Pennsylvania, while ConocoPhillips has been working hard at the Eagle Ford formation in Texas.

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