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Block 25 field, Chandeleur Sound, St. Bernard Parish, Louisiana, was discovered by Atlantic Richfield Co. through the drilling of its State Lease 4542 No. 1 well in October 1965. Drilling had been preceded by a common-depth-point seismic survey that revealed a subtle, east-west elongated, rollover structure on the downthrown side of a down-to-the-coast growth fault.
Present water depths in the field area are 10 to 12 ft (3 to 3.7 m).
Two pay sands occur on the structure, the 4,800^prime Sand with a total, original hydrocarbon column of approximately 30 ft (9.1 m), and the 5,200^prime or BB Sand with a total, original hydrocarbon column of approximately 55 ft (17 m). The 5,200^prime Sand is the primary producer in the field. Total oil production from the field to date is more than 18.5 million bbl. The gravity of the oil is 27/27.1° API, and several of the wells are being gas-lifted. Thirty-seven wells are currently producing in the field.
Relief on the structure at BB Sand depth is approximately 65 ft (20 m), which is based on the difference by subtracting the depth of the last closing contour on the BB Sand and the highest point of penetration.
The down-to-the-coast fault which trends east-west and lies on the northern side of the field apparently has contributed to the trapping of hydrocarbons in the structure. The fault has a dip of 55° and an upthrown-to-downthrown displacement which increases from 60 ft (18.3 m) at a depth of -3,507 ft (-1,069 m), to 650 ft (198 m) at a depth of -7,295 ft (-2,224 m).
The geometry of the BB Sand suggests that it is a bar-type deposit. The sand interval thickens abruptly on the southern side of the field. Accumulation has occurred in the structurally high sand area to the north, toward the shore, and in the upper, back-bar members of the BB Sand. The bar has some channeling or breaching.
The source of the hydrocarbons in the pay sands apparently has not been from the sediments above or below the reservoirs. These sediments have been analyzed as immature. It is postulated that the oil accumulated in the 4,800^prime Sand and BB Sand reservoirs probably migrated into the Block 25 structure from peripheral areas.
The 4,800^prime Sand is Bigenerina B, upper Miocene in age, and the 5,200^prime Sand or BB Sand is Textularia L, upper Miocene in age.
The preceding discussion includes numerous studies done by ARCO geologists, geophysicists, and engineers who have worked on the Block 25 field.
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