Abstract

Primary migration is initiated by compaction fluid pressures in muds which are usually much higher than fluid pressures in associated carrier-reservoir systems. This pressure differential causes the primary fluid movement from muds to carrier-reservoir systems, and also secondary migration to trapping sites.

The significant vertical (upward and downward) primary migration of liquids is generally short-distant because of interbedded sands and other coarse-grained sediments which intercept the fluids and carry them updip or upstructure. Trapping is typically controlled by fine-grained seals which fractionate the fluid, allowing water to pass slowly through, but restricting passage of organic matter and dissolved salts. This semipermeable effect establishes conditions for the continually increasing concentration and aggregation of the organic matter and its transformation into crude oil.

Based largely on these principles, migration models are proposed for each of seven clastic geological settings: (1) basins with major basinal anticlines and thick alternating sands and mudstones; (2) basins with sand and mudstone section and large anticlines near basin margins; (3) basin flanks with rich source beds overlying regionally extensive carrier-reservoir section; (4) major deltas; (5) provinces with complex depositional controls and only moderate subsidence; (6) turbidites in southern California intermontane basins; and (7) turbidites or density current deposits in a slow-depositional regime. Models 6 and 7 necessarily have some unique features because of special aspects of their geological histories.