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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 11. (November)

First Page: 2465

Last Page: 2466

Title: Petroleum Source Beds: Environment of Deposition and Stratigraphy: ABSTRACT

Author(s): Gerard Demaison

Article Type: Meeting abstract


Measurement of organic carbon content, alone, is insufficient to identify potential oil source beds because terrestrial OM (Organic Matter), oxidixzed planktonic OM, or reworked OM from a previous sedimentary cycle can create misleadingly high levels of organic carbon in marine sediments. Consequently, the presence of an oil-prone organic facies, as identified

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by kerogen typing, is essential to establish oil source rock character. Kerogen type is mainly dependent on the origin of the precursor plant remains (whether planktonic or terrestrial) and on the oxidizing (oxic) or reducing (anoxic) character of the early depositional environment.

Oxygen depletion, sufficient to arrest or minimize bioturbation at the benthic boundary, enhances oil source bed deposition, because it leads to almost entirely anaerobic microbial reworking of planktonic remains. This type of bacterial reworking favors the preservation and concentration of lipids in the residual OM, leading to the formation of "oil-prone" koergens (types I and II).

Marine oil source bed deposition and occurrence are controlled mainly by factors relevant to qualitative organic matter preservation during early sedimentation, rather than by planktonic productivity in the shallow euphotic zone. Many areas of high planktonic productivity, today, do not correspond with zones of high organic enrichment in bottom sediments (e.g., Grand Banks of Newfoundland, Antarctica, Australia, Northwest Shelf, Northeast Brazilian Shelf) because of oxic conditions, commonly in combination with low sedimentation rates. Conversely, whenever zones of high productivity, such as those present in some coastal upwellings, are underlain by anoxic water layers, then prolific oil source bed deposition does occur.

Zones where deep ventilation and thus oxic conditions prevail at sea bottom are much more common than zones of oxygen depletion. Persistent oxic conditions at the benthic boundary lead to deposition of "gas-prone," "type III," to "non-source," "type IV" organic facies, depending on sedimentation rate and amount of terrestrial organic matter input. Such unfavorable organic facies, resulting from past oxic conditions, have been commonly recognized as stratigraphically widespread under continental margins and in cratonic basins, regardless of past water depth. Thus, prolific oil source beds, in terms of relative rock volumes, are the exception in most sedimentary basins.

These observations are compatible with the functioning of the carbon cycle: efficient organic matter recycling through mineralization, rather than enhanced preservation, is the most common and most probable fate of dead organic matter in the environment.

Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows:

1. Large anoxic lakes:
Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition.

2. Anoxic silled basins:
Only those landlocked silled basins with positive water balance tend to become anoxic. The Baltic and Black Seas are examples. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds.

3. Anoxic layers caused by upwelling:
These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is commonly sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression.

4. Open-ocean anoxic layers:
These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a smaller scale, to worldwide "oceanic anoxic events" which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia.

Recognition of the proposed anoxic models in ancient sedimentary basins helps in regional stratigraphic mapping of oil shale and oil source beds. Furthermore, explanation and prediction of the most favorable zones for widespread and prolific oil source bed occurrence can be achieved by paleogeographic reconstructions (plate tectonics, paleoclimate, and paleo-oceanography) conducted in conjunction with seismic stratigraphy and regional geochemical studies.

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