JOLLEY, BRYAN D., Shell Western E&P Inc., Bakersfield, CA

ABSTRACT: Lithology of a Thief Zone, C-Block East Waterflood, Ventura Field

Ventura field produces from a sequence of thin-bedded turbidite sandstones. Stratigraphic thickness of the producing interval exceeds 6000 ft. Thickness of individual flow units within this interval vary from a few inches to 10 ft; 1-2 ft is typical.

Studies of subsurface data and of genetically similar sandstones outcropping in the field suggested that (1) sand packages (30-50 ft thick) were continuous across the field, (2) continuity of individual flow units across a flood pattern was only 50% in the depositional strike direction but much better in the dip direction, and (3) fluid movement through the sands was not

greatly affected by stratigraphic barriers.

The Ventura waterfloods have not performed up to expectation. Achieving good injection profiles has been very difficult due to long injection intervals (1000-3000 ft), Water cycling through thief zones has resulted in poor vertical sweep. Although the thief zones are not well understood, their control is a key to prolonging the field's life.

A core was acquired in an effort to characterize a major thief zone. Core data indicates that layering is an important control on injectivity. The thief zone consists of a thick package of amalgamated turbidite flow units. It is sandwiched between nonamalgamated flow units into which water injectivity is very poor. Measured reservoir properties (porosity, permeability, etc.) do not explain the observed injectivity contrast. Continuity of the flow units in the nonamalgamated intervals is likely poorer than originally predicted. Amalgamation of the flow units composing the thief zone apparently results in their much improved transmissibility.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.