The character, distribution, and probable genesis of a typical lenticular oil-bearing sand body of the Gulf Coast Tertiary, namely, Zone 19B, an Oligocene sand of the Seeligson field, Texas, were determined for the purpose of developing and defining criteria by which such sand bodies can be readily identified and delimited in the subsurface.

The stratigraphic framework was specified by cross sections, fence diagrams, isopach maps, and facies maps, prepared from electric logs and core descriptions, and the sediments were characterized by size, shape, thin-section, X-ray, differential thermal, and chemical analyses.

Principal results are: (1) the isopachal trend of the sand body is nearly normal to the regional depositional strike; (2) the sand grades from medium grain at the base to fine at the top and can be divided into three zones on maximum grain size; (3) a typical specimen is a mixture of (a) well sorted sand, the framework size fraction, (b) interstitial clay, the tail of the size-distribution curve, and (c) interstitial calcite cement; (4) the mineral composition is similar to that of a Recent river sand, but dissimilar to that of beaches, of the same geological province.

The sand body is probably an Oligocene alluvial-plain deposit as indicated by its similarity to such Recent deposits in external form, vertical textural zonation, colors of associated shales, mineralogy, presence of mud fragments, and absence of indigenous fauna.

The often stated opinion that oil-field trends of the Gulf Coast, such as the Frio-Vicksburg, are "shore-line" trends has led many geologists to infer that the reservoir sands of these oil fields are strand-line deposits. The conclusions of the present study emphasize the risk of such deductions. The trends may be indirectly related to shore lines by reason of marine source beds, but many, or possibly most, of the reservoir sands may be of continental origin. The trapping in this zone arises from updip normal faulting and gentle doming of a lenticular alluvial sand body which is oriented in the direction of regional dip.

The greatest value of these findings lies in the extrapolation and extension of this knowledge of local genesis to other parts of the section. In such a fashion a regional depositional and tectonic history can be outlined, centers of deposition for various geologic ages can be delimited, and areas favorable for the presence of permeable beds and oil accumulation can be approached.