Gravity interpretation of shale sections, with specific reference to reefs, similar folds, salt domes, buried ridges, and faults

The objectives of this investigation are (a) to find a way of making realistic assumptions as to the density distribution in the sedimentary subsurface, and, in particular, in shale sections, (b) to simplify certain subsurface models prior to gravity calculations, within allowable limits, so as to render them feasible for rigorous calculations, otherwise there would be no alternative to making relaxation computations, and (c) to develop mathematical equations for computing gravity due to certain subsurface models. A density analysis of the sedimentary subsurface was made. Since about 50 per cent of all the sedimentary rock in the world is shale, the outcome of gravity interpretation of sedimentary structures primarily depends on the correctness of assumptions concerning densities in shale sections. Effects of gravitational compaction, rock deformation, and sand content on shale density were investigated. A relationship between shale density on one hand and the maximum depth of burial and sand content on the other hand was established. This relationship was applied to the interpretation of gravity anomalies hue to isolated "true reefs"; an explanation for the hitherto unexplained reef anomalies was obtained. The relationship between shale density and depth of burial' was broken into straight-line segments, each corresponding to a constant rate of increase of density with depth. Mathematical equations were developed for calculating gravity anomalies due to structures in shale sections on the basis of linear density-depth functions.