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Porosity Prediction in Sandstones Using Erosional Unconformities

By
G. Shanmugam
G. Shanmugam
Mobil Research and Development Corporation DallasTexas, U.S.A.
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Published:
January 01, 1990

Abstract

Erosional unconformities of subaerial origin are created by tectonic uplifts and eustatic sea-level falls. Porosity increases below most erosional unconformities developed on sandstones, because uplifted sandstones are exposed to undersaturated CO2-charged meteoric waters, resulting in dissolution of unstable framework grains and cements. The chemical weathering of sandstones is intensified in humid regions by the heavy rainfall, soil zones, lush vegetation, and the accompanying voluminous production of organic and inorganic acids. Erosional unconformities are considered hydrologically and geochemically “open” systems because of the abundant supply of fresh meteoric water and relatively unrestricted transport of dissolved constituents away from the site of dissolution, causing a net gain in porosity near unconformities. Consequently, porosity in sandstones tends to increase toward overlying unconformities. Such porosity trends have been observed in hydrocarbon-bearing sandstone reservoirs in Alaska, Algeria, Australia, China, Libya, the Netherlands, Norwegian North Sea, Norwegian Sea, and Texas. A common attribute of these reservoirs is that they were all subaerially exposed under warm and heavy rainfall conditions.

An empirical model has been developed for the Triassic and Jurassic sandstone reservoirs in the Norwegian North Sea on the basis of the observed relationship that shows an increase in porosity in these reservoirs with increasing proximity to the overlying Base-Cretaceous unconformity. An important practical attribute of this model is its capability of predicting porosity in the neighboring undrilled areas by recognizing the Base-Cretaceous unconformity in seismic reflection profiles and by constructing subcrop maps of stratigraphic units susceptible to porosity formation by meteoric waters. Caution must be exercised in developing predictive models using unconformities, because porosity reduction due to cementation may also occur beneath some erosional unconformities.

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Contents

AAPG Memoir

Prediction of Reservoir Quality Through Chemical Modeling

Indu D. Meshri
Indu D. Meshri
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Peter J. Ortoleva
Peter J. Ortoleva
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American Association of Petroleum Geologists
Volume
49
ISBN electronic:
9781629811239
Publication date:
January 01, 1990

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