Abstract: Liquid Hydrocarbon Migration Paths in Thick, Rapidly Deposited Clastic Sediments

Robert J. Cordell

In basins with thick, rapidly deposited clastic sediments, migration paths of source-bed fluids are strongly dependent on shale-sandstone facies relationships.

Sandstones interbedded with shales receive source-bed fluids from both above and below. Fluid movement is caused by the fluid-pressure differential between middle parts of shale units and shale-sandstone contacts. Source-bed fluids entering the sandstones migrate updip and/or upstructure where mechanisms for oil formation and concentration are present.

A thick, predominantly sandstone section in a flanking or marginal basin position may receive large quantities of source-bed fluids from below, or laterally from shalier downdip section with which it interfingers. Unfortunately, the contained hydrocarbons are likely to be lost unless an overlying transgressive shale or other sealing bed is deposited prior to the main migration stages, or unless the oil being generated is sufficiently asphaltic to provide its own seal.

An extremely thick shale facies tends to develop geopressure conditions with increasing burial. Sandstones directly overlying or a short distance above the geopressure zone should receive abundant upwardly migrating source-bed fluids. However, migration models seem untenable that involve upward liquid-hydrocarbon migration through thousands of feet of section above the top of a geopressured or other deep source.

In shale section containing isolated, lenticular, highly productive sandstones, migration and accumulation process appears to defy rational technical explanation.

In all the above facies settings, several factors play a key role. Higher temperature with increasing depth of burial is responsible for the generation of bitumen, including liquid hydrocarbons, from the source-bed organic matter. The higher temperature also facilitates desorption of the bitumen and its incorporation in migrating source-bed water. A concomitant process is clay-mineral transformation which enhances the release of bitumen and furnishes at least a part of the migrating water. The accompanying increase in fluid pressure promotes migration.

APG Search and Discovery Article #90974©1975 AAPG Mid-Continent Section Meeting, Wichita, Kansas