Some Aspects of High Pressures in the D-7 Zone of the Ventura Avenue Field

Abstract

Abstract The D-7 zone of the Ventura Avenue field is of special interest because theinitial reservoir pressure at 9200'Ft nearly equaled the pressure exerted bythe overburden. While the phenomenon has been observed elsewhere in the world, there is no record of a previous case in California. Such high pressures havebeen attributed to sealing of the reservoir, followed either by emergence ofthe beds or by compaction of the sediments in various ways. Oil in the D-7 zoneis undersaturated. At pressures above the bubble point, oil is recovered by theslight expansibility of the reservoir framework and its liquid contents. Morethan 40 pct as much oil probably will be recovered by this mechanism as will berecovered by internal gas drive after pressures pass below the bubble point.Operations are handicapped by sand and pipe troubles. Introduction The Ventura Avenue field has been under development since 1915. It has longbeen considered a "high-pressure" field, but interest was heightened bythe extreme conditions encountered in the recent development of the so-calledD-7 zone. The original reservoir pressure in this pool is estimated to havebeen 8300 psi at a depth of 9000'Ft below sea level, or 9200 ? ft below thesurface. A fluid pressure approaching that exerted by the overlying sediments has beenencountered in other parts of the world, but never before in California to thedegree observed in the D-7 pool. With few exceptions, the original pressure in California fields is aboutequivalent to the static head of water below the water table in theoutcrop. Structure The Ventura anticline is a prominent feature of the Ventura basin, beingtraceable on the surface about 17 miles. The Ventura Avenue field is at theapex.of the anticline. It is about one mile wide and five miles long, themajor axis lying in an east-west direction. The productive limits cover an areaof about 2300 acres. The entire anticline has been severely folded and faulted. The productiveportions in particular are broken up by three major thrust faults having ageneral east-west strike. The resultant four fault blocks are termed the A, B, C, and D blocks, respectively. Certain electric-log intervals are assignednumbers. Fig 1 is a diagrammatic north-south section of the field near thecrest, looking west, and shows three of the four fault blocks. To the west, theupper fault divides into two branches, which define the B block. Wells penetrating a fault may duplicate over 1000'Ft of formation. However, thegreater fault movement is the strike shift, which is believed to exceed 2500'Ftboth between the A and C blocks and between the C and D blocks. The location of water is irregular and unpredictable. A few intermediate watersexist, while some strata have more than 2000'Ft of productive closure. T.P. 2204