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Discovery of highly porous and permeable sandstones at great depths and temperatures has clearly demonstrated that porosity reduction with depth is not monotonic. Recognition that porosity can be created in sandstones at depth has spurred tremendous interest in developing predictive models of porosity evolution and distribution in sedimentary basins. Instead of predicting economic basement, emphasis has shifted to prediction of porosity "windows" in the subsurface.
Historically, diagenesis has been considered a function of sandstone composition and temperature. However, it has become increasingly clear that this view is too simplistic. Factors such as pore fluid composition, flow rate, organic maturation, and time may significantly alter the course of diagenesis. Development of predictive models that provide for these parameters will, when coupled with structural-stratigraphic and hydrocarbon generation models, permit the relative timing of porosity evolution, hydrocarbon generation, and trap formation to be determined.
In order to simulate the diagenetic evolution of basins and predict porosity distribution, the processes that lead to creation and destruction of porosity must be understood. Many of the important porosity-producing processes have been identified through petrologic studies: dissolution of carbonates, feldspars, and rock fragments. Formation of deep porosity in a variety of basins is commonly associated with precipitation of kaolinite and iron-rich carbonate, suggesting that, although the paths of diagenesis may be diverse, common trends exist. Development of predictive diagenetic models will require continued accumulation of petrologic data and case studies more fully using presently available technology (e.g., electron and ion microprobe, stable isotope geochemistry, age-dating techn ques). This will better document the time, temperature, and chemical environment of formation of diagenetic materials.
Areas of research requiring attention are the following. (1) Fluid flow and heat transfer in sedimentary basins. What are the volumes of fluid, rates of flow and flow paths? How do these change as a basin evolves? (2) Geochemistry of subsurface fluids. Reliable analyses are required to identify compositional trends of subsurface fluids. What controls the pH of subsurface fluids? These questions will require further research on shale diagenesis, fluid diagenesis, fluid expulsion, and clay membrane filtration. (3) Diagenesis of organic matter in sediments. Recent studies have shown that by-products of petroleum generation (e.g., CO2, H2S, organic acids) may be an important factor in sandstone diagenesis. (4) Computer models simulating the chemical consequences of r ck-fluid interaction are restricted by the lack of reliable thermodynamic data for many common diagenetic minerals (e.g., clays, zeolites). We need additional information concerning rates of dissolution and precipitation of common minerals under various conditions. (5) Physical compaction of sandstones. (6) Relationship between depositional environment and subsequent diagenetic events.
Because porosity prediction requires an understanding of many related disciplines, an integrated approach is required. By combining the talents and expertise of petrologists, organic and inorganic geochemists, fluid mechanicists, and structural geologists, not only will we be able to develop powerful models for porosity prediction, but we will also be better able to place porosity development in its proper context as one aspect of basin evolution and hydrocarbon accumulation.
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