About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 17 (1933)

Issue: 7. (July)

First Page: 827

Last Page: 842

Title: Preservation of Oil During Erosion of Reservoir Rocks

Author(s): Geo. Edwin Dorsey (2)

Abstract:

Some of the most prolific oil fields in the United States are productive from porous rocks below unconformities of major importance. The oil is found trapped at the unconformity in porous rocks of pre-unconformity age sealed by non-porous rocks of post-unconformity age.

If it is assumed that the oil formed and accumulated in the primary uplifts, it becomes necessary to explain how this oil was preserved during the long erosional interval when even the reservoir rocks were exposed and eroded. This problem is avoided if it be assumed that the oil was either not formed until after the deposition of the post-unconformity impervious rocks, or was introduced into the pre-unconformity reservoir rocks in post-unconformity time along the unconformity or in some other manner.

It is believed that an impartial survey of the geological data affords no evidence either that the oil was formed in post-unconformity time or was introduced at a later date. Comparisons of the rock sections containing the unconformities with sections of identical age and lithology without great unconformities show the oil occurrences in both to be much alike. It seems just as reasonable, therefore, to assume the formation of oil to have occurred by the time the folding began which enabled accumulation to start.

If this belief is accepted the problem of preserving the oil during exposure and erosion of its reservoir rocks must be squarely faced. The writer points out a protective mechanism. The action of gravity alone will protect the oil because (1) by definition an uplift lies above its surroundings, is an area of run-off of surface waters, and the exposed porous rocks constitute areas of intake, and (2) the underground fluid level of a major uplift never lies at the surface of erosion, but at appreciable depths below the surface. This protective process is latent in all uplifts undergoing erosion. Other factors necessary before this potentially protective mechanism becomes effective are (1) depth to underground fluid level when submergence occurs, (2) rate of deposition of impervious post- nconformity beds compared to rise of oil level toward eroded ends of reservoir rocks, (3) efficacy of post-unconformity sediments as impervious seals.

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24

AAPG Member?

Please login with your Member username and password.

Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].