Detailed Layer Studies in the East Wilmington Oil Field

CLARKE, DONALD D., City of Long Beach, Long Beach, CA

Recent detailed studies have been made of the Pliocene and upper Miocene sequences in the East Wilmington oil field. Roughly 1200 vertical ft of section (Ranger Zone) were evaluated in fault blocks VI and VII. The sequence was broken into 40 to 50 correlatable units (sub-subzones) based on electric logs. Twelve horizons (subzones) were used to guide the correlation work and aid in the construction of true stratigraphic logs. The TST logs were used to make many north-south and east-west cross sections. This provided good loop control for correlations. Over eight hundred wells were evaluated in this way. Structure maps, isochore maps, and percent sand maps were constructed. The primary purpose of this work was to determine the net pay for the Ranger zone. This work has found excellent s condary applications including definition of flow units, clarification of stratigraphic sequencing, and depositional history.

The depositional history involved the sedimentation and transport of lobed deposits and sheet or draped deposits. The Upper Miocene sands are intercalated with shales and siltstone. These thin turbidites are reasonably wide spread and show some lenticularity but not the lobate lenticularity that is common in the Pliocene sands. Many of the units thin on the crest. This could be because of the compaction or washover along the crest of a growing anticline. Displacement along some of the faults increases down section, suggesting continued activity during deposition.

The Pliocene section is dominated by large lobate fans. These lenticular depositional units show much about sediment transport. The lobes lie laterally between each other with each successive lob filling the low area adjacent to or between the earlier lobes. Balancing of the deposited sand results. Any two given sequences of sand determined from electric-logs of wells will look similar but they are not the same. This is significant since nonspecific injection into sands has produced less than optimal sweep efficiencies. Knowledge of the lobed nature of the sands and specific localized characteristics helps in the design of injection programs that will greatly improve the sweep efficiencies.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)