Abstract

Erosional unconformities of subaerial origin are created by tectonic uplifts and eustatic sea-level fall. Most subaerial unconformities developed on sandstones are planes of increased porosity because uplifted sandstones are exposed to undersaturaded 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 the heavy rainfall, soil zones, lush vegetation, and the accompanying voluminous production of organic and inorganic 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; causing a net gain in porosity near unconformities. Thus, 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 commonality of these reservoirs is that they were all subaerially exposed under heavy rainfall conditions. The porosity development beneath erosional unconformities has important practical applications in exploration because porosity in the undrilled areas may be predicted by recognizing erosional unconformities in seismic reflection profiles and by constructing subcrop maps.