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.
Figures & Tables
Prediction of reservoir quality ahead of the drill is one of the most complex problems facing exploration geologists, especially when they are exploring in frontier basins, where rock and water data are minimal or non existent. Although useful descriptive models of diagenesis have existed in the past, they cannot be applied in the areas where rock and water data do not exist. This volume comes out of a 1987 conference oand contains 10 chapters that document the substantial progress made toward the goal of modeling reservoir quality. One facet of chemical modeling, namely porosity prediction, is the thrust of this book. However, chemical modeling has contributed heavily in the field of environmental geochemistry, nuclear waste disposal, and in the thermal recovery of heavy oil and the like, thus one such chapter is included in this memoir.