Classification of Nonmarine Sandstones

(adsbygoogle = window.adsbygoogle || []).push({}); In this course, exploration concepts of nonmarine sandstones reservoirs are examined and analyzed with tools to determine the reservoir environment. General Considerations of Nonmarine Sandstones https://youtu.be/G37g-a1ybOA?si=p-FYWUD3ivXzeAhh Figure 1 outlines a classification of the major continental environments in which sand is being actively deposited. FIGURE 1 Figure 2 offers a generalized illustration of these environments. FIGURE 2 (adsbygoogle = window.adsbygoogle || []).push({}); The first three of these are mainly deposited by streams and can be placed in an order that roughly corresponds to increasing distance from an upland sediment source area. They represent environments in which progressively finer, more water-worked sediment is moved and deposited. In contrast, eolian dune sands exist in a unique, nonaqueous setting, while lacustrine environments often combine sedimentation regimes of many types, from deltas to submarine fans. Alluvial fans and braided streams are generally the higher energy aqueous environments, where lateral and vertical changes in lithology can occur suddenly and often. A number of interrelated factors determine whether channels meander or braid. In general, braided rivers form where gradient is relatively steep, discharge is occasional and heavy, and sediment supply is high. Meandering rivers are more characteristic where gradients are lower, discharge more perennial, and sediment is more fine-grained, with a higher proportion of silt and clay. (adsbygoogle = window.adsbygoogle || []).push({}); Rules of Thumb and Their Limitations (adsbygoogle = window.adsbygoogle || []).push({}); Usually, there are no quick and easy methods that distinguish nonmarine from marine rocks. A few general rules of thumb should be mentioned, however. In the sedimentary record, many non-marine rocks are associated closely with unconformities, often lying immediately over, or being sandwiched between, major erosional surfaces. Sediments deposited above the water table have their pores filled with air and intermittently percolating groundwater, which is acidic and oxidizing. (Figure 1, Sketch showing relationship between local water table and oxidation. Iron-bearing sediments within the vadose zone have a much higher chance of taking on red coloration.) FIGURE 1 (adsbygoogle = window.adsbygoogle || []).push({}); They are, therefore, often orange or reddish in color, due to the presence of oxidized (ferric) iron. Below the water table, however, pores are saturated with water that is often stagnant and reducing. Consequently, iron tends to occur as a sulfide (pyrite) or, more commonly, in the drab gray-green ferrous state. Thus, upland eolian, alluvial fan, and braided stream deposits are often reddish in color, while lowland flood plain and lacustrine sediments more closely resemble the drab color of most marine rocks. Due to lateral lithologic variations in many nonmarine facies, seismic profiles through these rocks are sometimes characterized by discontinuous reflections. (Figure 2, Seismic section to illustrate the discontinuous reflectors which often characterize nonmarine deposits. FIGURE 2 Part a is a close-up of the type of reflections commonly seen. Part b is a seismic profile of a preserved river channel. Note discontinuity of reflections within the channel.) This, however, is a generalization and often not the case for nonmarine sequences that represent considerable portions of geologic time. Moreover, lacustrine sediments provide a conspicuous and important exception. Other potential indicators are nonmarine fossils (e.g., vertebrates, freshwater mollusks and gastropods, insect tracts, rootlet horizons) and exposed-surface sedimentary structures such as raindrop imprints and mudcracks. Nearly all these may occur in transitional marine (e.g., deltaic) environments as well, how-ever. In addition, these types of evidence are all distinctly rare in outcrops, and therefore extremely so in cores. As a whole, therefore, even in combination, these nonmarine indicators are suspect. In fact, none of them may be present in a section which is determined by detailed analysis to be terrestrial. At the same time, their presence can be used as guiding evidence to tip-off the geologist to the possibility of a nonmarine facies. Assessment about Nonmarine Sandstones (adsbygoogle = window.adsbygoogle || []).push({}); 1. Of the five non-marine sandstone environments the four that have yielded the most in petroleum reserves are: A .alluvial fan, braided stream, meandering stream, and eolianB .braided stream, meandering stream, eolian and lacustrineC .alluvial fan, lacustrine, braided stream, and eolian 2. Non-marine sandstones are generally deposited in an oxidizing environment and may exhibit discontinuous seismic reflections. A .TrueB .False (adsbygoogle = window.adsbygoogle || []).push({});