Depositional Environment and Petrographic and Seismic Interpretation of B Sandstone (A Primary, Waterflood, and Enhanced Recovery Target), Olive Field, Southwestern Mississippi
Roger D. Shew
Olive field is oil productive from a thin stratigraphically trapped sandstone within the Upper Cretaceous lower Tuscaloosa Formation. The discovery of Olive in 1981 led to renewed interest in the Mid-Dip Tuscaloosa Trend that continues today. Discovery and development was based on the correlation of the "B" sandstone to a mappable "soft" seismic event at a depth of 10,500 ft. The field was developed by 25 wells on 40-acre spacing. Olive has been waterflooded since 1983 and is a target for enhanced recovery using carbon dioxide. A total of 7.5 million stock tank bbl or approximately 75% of the original oil in place is estimated to be recoverable by conventional and enhanced recovery methods.
The B sandstone was deposited by a northwest to southeast-flowing high-sinuosity meandering river system. Point bar, channel, and channel abandonment deposits are present. The point bar formed from the lateral migration of the river system from west to east, and channel abandonment is evident at the eastern edge of the field. Sandstone lateral continuity is high. Well log and seismic data indicate that this river system supplied sediments to McComb field 2 mi south of Olive.
Reservoir quality and continuity are controlled by depositional and diagenetic events. The most important controls are compaction, chlorite grain coatings, quartz and ankerite cements, and local thin mudstone layers that reduce vertical permeability. The best reservoir quality (^phgr = 26% and k = 124 md) is associated with those zones having pervasive grain-coating chlorite. The chlorite is interpreted to have partly prevented compaction and reduced quartz cementation. Although high-water saturations (Sw = 68%) have been calculated, clean oil was still produced from this reservoir because the water is bound in the microporous chlorite coatings. Reservoir quality is reduced in intervals with matrix clays and quartz and ankerite cements. Local ankerite-cemented layers are ef ective vertical permeability barriers.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.