ABSTRACT

Block 25 field, Chandeleur Sound, St. Bernard Parish, Louisiana, was discovered by Atlantic Richfield Company through the drilling of its State Lease 4542, no. 1 well in October of 1965. Drilling had been preceded by a Common Depth Point seismic survey that revealed the presence of a subtle, east-west elongated, rollover structure situated on the downthrown side of a down-to-the-coast growth fault.

Present water depths in the area of the field are on the order of 10-12 ft.

Two pay sands occur on the structure, the 4,800 ft Sand with a total original hydrocarbon column of approximately 30 ft and the 5,200 ft or BB Sand with a total, original hydrocarbon column of approximately 55 ft. The 5,200 ft Sand is the primary producer in the field. Total production is in excess of 18.5 million barrels of oil. 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.

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 rate of 55° and an upthrown-to-downthrown displacement which increases from 60 ft at a depth of 3507 ft (subsea) to 650 ft at a depth of 7295 ft (subsea).

The geometry of the BB Sand suggests that it is a bar type deposit. Rapid thickening of the sand interval occurs 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. Some channeling or breaching of the bar has taken place.

Apparently, hydrocarbons present in the pay sands have not been derived from the sediments directly above or below the reservoirs. These sediments have been analyzed as immature. It is postulated that the oil accumulated in the 4,800 ft Sand and BB Sand reservoirs probably migrated into the Block 25 structure from peripheral areas.

The 4,800 ft Sand is Bigenerina b. Upper Miocene in age and the 5,200 ft Sand or BB Sand is Textularia l. Upper Miocene in age.