ABSTRACT

The Louisiana State Lease 5706 No. 2 well, operated by Pioneer Natural Resources USA, Incorporated, located within 0.4 km (0.25 mile) of the Atchafalaya River in the Myette Point, South Field, St. Mary Parish, Louisiana, lost pressure control during re-completion operations in November and December 1996, and sprayed approximately 756,682 liters (4,767 barrels) of natural gas condensate and saline formation water, during a five-day interval, over approximately 180 acres of the surrounding hardwood bottomland swamp forest. The condensate spray severely damaged the forest and covered the regolith in the affected area.

To determine the extent of coverage and the mobility of the condensate in the impacted area, sediment control cores were collected randomly from the impacted areas and adjoining unaffected areas. Following clear-cutting of the impacted area, 0.61 m (2 ft) soil cores were collected in a grid pattern to assess long term affect of the spill. The sediment around the well consists of a lacustrine delta in Grand Lake. Well-sorted, fine-grained fluvial sands compose the basal section of the cores. The upper 8 to 20 cm (3.15 to 7.9 in) consists of very fine-grained silts and sands. Condensate was identified in the cores with the use of ultraviolet fluorescent analysis.

During normal flow of the Atchafalaya River, the porous and permeable fine-grained clastic sediments of the overbank and distributary channel deposits within the study area allowed the natural gas condensate to migrate downward through the vadose zone to the water table. The annual flood event of the Atchafalaya River appears to have enhanced removal of the natural gas condensate from

Figure 1. Location of the State Lease 5706 No. 2 and former gas condensate spill, Myette Point, South Field on the Louisiana Oil Spill Coordinator's Office (LOSCO) oil spill contingency planning map.

Figure 2. Outline of the former spill study area within the Myette Point, South Field (adapted from the U. S. Geological Survey Centerville and Centerville, NW 7.5-minute topographic quadrangle maps).

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the soil. Ultraviolet inspection of study-area sediment cores indicated minimal fluorescence in the clear-cut area versus bright yellow fluorescence at the capillary fringe in an impacted-area control sediment core. Clear cutting of the affected forest seems to have enhanced remediation of the most-heavily-impacted arzeas of the swamp. The comparison of cores from the clear-cut area to the cores in the control area verified the success of the remedial design chosen for this site.