FE 02-16J Chemical Rock Description

•Very hard (very slow ROP, bit bouncing, vibration)

•Glass-like brittleness

•Bedded cherts are usually even bedded, thinly laminated to massive

•Color could be indicative of the environment of deposition

•Cuttings: large, elongate, blade-shaped, fresh conchoidal fracture surfaces, cryptocrystalline or microcrystalline, very hard

•Possible abundant metal shavings in the sample

13 trang xuanthi 28/12/2022 2460

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Chemical Rocks • Chert • Halite • Anhydrite and Gypsum • Carbonaceous rocks
Chemical Rocks: Chert: Color • Diatomaceous and radiolarian chert is black to dark grey due to clay impurities • Spiculiferous cherts are light to medium grey with a brown to green tinge due to large amounts of calcite
Chemical Rocks: Anhydrite and Gypsum • Determination between anhydrite and gypsum is not always possible at the wellsite, but an attempt should be made
Checking For the Presence of Anhydrite and Gypsum Barium Chloride Test 1. Place several cuttings in a bottle and fill with distilled water 2. Agitate and pour off water. Refill and repeat 3. Fill bottle half full with distilled water and add 3 drops of HCl and agitate 4. Add 2 drops of Barium Chloride 5. A pearly white discoloration will confirm the presence of gypsum or anhydrite
Carbonaceous Rocks • Coal beds are useful marker beds • Can be inferred from ROP • Give well defined methane peaks • Show up quite well in the GR, Density-Neutron logs • Unusual to encounter coal beds > 6 ft (2 meters) thick • In geologically young deposits, lignite (brown coal) is found • There should be signs of vegetal matter in the lignite
Sapropelic Coal • Oil-prone source rock • Non-woody, comprises of spores, algae and macerated plant material • Massive unlaminated glassy appearance, conchoidal fracture, firm rather than hard