Accumulation of oil and gas in a sandstone body depends on several factors including the state of generation and time of migration of hydrocarbons or their precursors, directional variations in porosity and permeability, the existence of stratigraphic or structural closure with a suitable seal, and the geometry of the sandstone body. Many holes have been drilled on the basis of geophysical inteqretations that indicated structural closure within a prospective section, only to find the section lacking in suitable source beds for hydrocarbons, or with no impermeable seal above the potential sandstone reservoir. The sandstone itself may be locally tight. Further, the spatial relation- ships of depositional trends and geometry to permeable zones within the sandstone body are commonly unknown. To complicate our understanding of the situation, the depositional trends and geometry of the sandstone body itself may not be known. With these possibilities in mind, the following comments are offered on the classification of sandstone bodies.
A sheet or blanket sandstone body may be designated as a mappable stratigraphic unit, such as a member or formation, and yet lack continuity and homogeneity. At one locality it may consist of a single sandstone unit, and at another it may comprise two or more sandstone beds that have individual depositional trends, shapes, and petrophysical characteristics. At a particular location oil or gas may be encountered in Sandstone "A", where it occurs below the up-dip edge, but not in adjacent Sandstone "B" that pinches out elsewhere. This type of situation is common in alluvial point bar and channel-fill sands, in anastomosing delta distributary sands, and in off-lapoing marine shoreline sands.
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