Porosity Prediction in Sandstones Using Erosional Unconformities

G. Shanmugam

Erosional unconformities of subaerial origin are created by tectonic uplifts and eustatic sea level fall. Most erosional unconformities developed on sandstones are planes of increased porosity because uplifted sandstones are exposed to undersaturated CO₂-charged meteoric waters that result in dissolution of unstable framework grains and cements. The chemical weathering of sandstones is intensified in humid regions by heavy rainfall, lush vegetation, soil zones, and the voluminous production of inorganic and organic acids. Erosional unconformities are considered hydrologically "open" systems because of abundant supply of fresh meteoric water and relatively unrestricted transport of dissolved constituents away from the site of dissolution. Thus, porosity in sands ones commonly increases toward overlying unconformities. Empirical models have been developed on the basis of the observed relationship between erosional unconformities and porosity in the underlying sandstones in the North Sea (Middle Jurassic Brent Group) and in the Alaskan North Slope (Triassic Ivishak Formation). An important practical attribute of these models is that they allow for the prediction of porosity in frontier areas by recognizing erosional unconformities in seismic reflection profiles and by constructing subcrop maps for underlying sandstones. Hydrocarbon-bearing sandstone reservoirs in Alaska, Algeria, Australia, China, Libya, Netherlands, North Sea, Norwegian Sea, and Texas occur immediately beneath major erosional unconformities.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.