Determination of the depositional environment for sandstone bodies generally improves the accuracy in estimating sandstone reservoir trends. Depositional framework is best understood by comparing the geometric and internal features of each sandstone to those of models, which are sand and sandstone deposits of known depositional environment. The different environments are distinguishable most commonly, not by a unique property, but by diagnostic combinations of the various features, such as trend, width, thickness, nature of contacts, sequence of sedimentary structures, textural sequence, and constituents. These characteristics are catalogued for 18 different environments, ranging from eolian to deep-marine basin floor, and models of alluvial valley, alluvial plain, and de taic environments are illustrated.

The Pennsylvanian Kisinger Sandstone, which crops out in north-central Texas, was deposited in a deep and narrow valley by a westerly flowing river. Each textural sequence of massive-bedded conglomerate, crossbedded conglomeratic sandstone, and convolute-bedded sandstone represents a genetic unit formed at one position of the river, which was 200-300 ft wide. The fine-grained sandstone and carbonaceous shale in the upper 30 ft of the 150-ft section were deposited under near coastal conditions as the valley filled in response to a continued rise of sea level.

The Pennsylvanian Robinson Sandstone in southern Illinois was deposited by a river flowing west-southwest on an alluvial plain. Unidirectional medium-scale and small-scale crossbedding, upward decrease in grain size, and sharp basal and lateral contacts indicate stream deposition. The absence of allochthonous pebbles suggests deposition on a plain rather than in a valley. The absence of marine indicators suggests alluvial rather than deltaic conditions.

The Davis sand (Yegua) in the Hardin field of southeast Texas was deposited by a deltaic distributary. A width-thickness ratio of approximately 30, and abrupt lower and lateral contacts, together with interstratification and the microfauna of equivalent beds, are suggestive of deposition near the mouth. The narrow width of 1,250 ft suggests further that the sand represents a genetic unit, with insignificant lateral migration.

Most of the Cretaceous Newcastle Sandstone in North Dakota represents deposition on a broad, slowly subsiding deltaic plain which formed as streams advanced west-northwestward from South Dakota, and southward from Saskatchewan. In the deltaic complex Skull Creek prodeltaic clays underlie delta-margin

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coastal sands and distributary sands, which together are thought to represent most of the sandstone in the Newcastle. Very gentle slopes, shallow sea floor, and slow rate of subsidence were primary reasons for the widespread distribution of a relatively thin sandstone section and the wide variation in sandstone trends.

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