Geometry of Producing Mesaverde Sandstones, San Juan Basin1
Published:January 01, 1961
Within the San Juan basin the sandstone zones that occur at the top and bottom of the Mesaverde group were not deposited as a continuous blanket sand. In some areas thick, relatively clean sandstone units occur. In other areas thin, poorly sorted sandstone beds are found. These sandstone units exhibit a definite geometric pattern of distribution. Sandstone beds of the Point Lookout formation (lower Mesaverde) were deposited as a shoreline phase of a sea regressing northeastward. Sandstone bodies of the Cliff House formation (upper Mesaverde) represent the shoreline deposits of a sea transgressing south-westward at a later date. The shoreline along which these sands were deposited moved rapidly across some areas. In other areas it remained stationary for relatively long periods of time. The thicker sands correspond to places where the shoreline remained stationary, within a narrow belt, for the longer periods of time.
The successive vertical and lateral positions of the various Cliff House and Point Lookout shorelines have been established and are demonstrated on cross sections and maps. Those positions where the shoreline stabilized for relatively long periods of time are apparent in the form of “steps” that can be traced across the central part of the San Juan basin. The relatively thick, well-sorted sandstone units that correspond to the positions where the shoreline stabilized have been divided into a series of sandstone “benches” of varying widths.
Excellent examples of major “steps” in the Cliff House shoreline can be seen in surface exposures in the southeast and northwest parts of the San Juan basin. Those exposed at the surface in the northwest part of the basin exhibit a similar strand-line trend and in general correlate with the “steps” found in the subsurface.
Figures & Tables
Geometry of Sandstone Bodies
This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included.
The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants.
The dictionary definition of the word “geometry” is the science of magnitudes in space. In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume—
Geometry of Sandstone Bodies—Spatial relationships of sandstone deposits within the sedimentary framework.
As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.