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Abstract

The geometry of sandstone bodies involves their shape, size, and orientation. The original geometry is subject to later modification by erosion, faulting, folding, tilting, compaction of underlying sediment, and internal compaction.

Although orientation or “trend” has been, and will continue to be, used successfully in some cases without knowledge of the origin of the sandstone bodies, greater predictability should be possible if the origin can be determined—provided that the distributional patterns of sediments of various origins are known. Insofar as geometry is concerned, three major problems are (1) to reconstruct the geometry correctly, (2) to know what it implies regarding origin, and (3) to know the distributional pattern of sediment of that origin in an analogous depositional situation.

For reconstruction of sandstone-body geometry, total sand thicknesses or sand/shale ratios for thick sedimentary sequences are of limited value. Isopach maps of individual sand bodies define their size and orientation but only partially define their shape; cross sections “hung” on a closely related underlying or overlying bed whose original attitude relative to the sandstone body is known or can be reasonably assumed are required to define shape. Possible modification of original shape by compaction or other processes must also be considered.

The plan dimensions of present-day deltas, barrier bars, and other sedimentary types are rather well known, but three-dimensional data are scarce. Too commonly, three-dimensional data from ancient sediments are misleading, because the origin has been incorrectly determined.

Internal features such as cross-bedding, flow markings, grain orientation, and bed or grain-size sequences and the relationship of a sandstone body to beds above, below, and laterally, are important for interpreting origin, particularly where control is too sparse to define the geometry.

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