In an interbedded sequence of sand and shale, most compaction water would move horizontally from the basin center to its edges. The cumulative volume of water that has been expelled from, and moved through, sediments increases with the horizontal distance of migration. This volume also increases for the same horizontal distance with increase of burial depth or compaction.
In thick shales unbroken by sandy zones, the main direction of water movement is vertically upward. In a vertical-movement model, the cumulative-water volume gradually increases with depth to a maximum at intermediate depths, then gradually decreases toward the base of the sedimentary column. The cumulative-water volume in this case generally is smaller than in the horizontal modelwith relatively long migration distance.
The Tertiary sequence in the Gulf Coast area may resemble a third model, which combines features of the other two in that it consists of an upper interbedded sand-shale sequence (horizontal migration) and a lower monotonous shale sequence (vertical migration). The depth of the boundary between these two zones varies, ranging probably from 3,000 to 15,000 ft (900 to 4600 m) over all. A combinationplot of the water volumes corresponding to the horizontal- and vertical-migration models would, therefore, simulate the volumes of water movement in this area; the cumulative-water volume increases rapidly toward the base of the interbedded zone, and then decreases in the underlying shale zone. It is interesting to note that these expelled-water volume plots resemble the oil-production frequency plots constructed by Burst for this area. The similarity may suggest that explusion and movement of water in sediments is of primary importance in hydrocarbon occurrence.