Prospect Model in Petroleum Geology

Muddy Sandstone Model

The eastern flank of the Powder River basin in the western United States has very slight local structural deformation. For the most part, fields are in stratigraphic traps. Regional distribution and regional structure of Muddy channel sandstones are illustrated in Figure 1 and Figure 2.

Prospect generation at the level of the Early Cretaceous Muddy Sandstone entails making paleotopographic and other kinds of stratigraphic maps, based on a regional depositional model.

The depositional model of the Muddy (Figure 3)

is roughly analogous to the complex of modern depositional environments displayed along the Texas Gulf Coast (Figure 4). By mapping depositional environments at the stratigraphic levels of all prospective reservoir sands, it is easier to interpret fragmentary log data in terms of shapes and trends of sand bodies.

In recent years, geologists have recognized that tangible anomalies within these depositional models may be identified as local prospects by using seismic, gravimetric, and aerial photographic data. In some cases, though, paleotectonic (and hence perhaps paleodepositional) trends can be recognized indirectly on gravimetric residual or derivative maps. During the late 1960s, several exploration companies integrated their stratigraphic studies on gravimetric residual maps in the very competitive Muddy sandstone play. This followed the discoveries of Recluse field, Wyoming, and Bell Creek field, Montana. The supposition is that ancient basement structural trends, which are evident on the residual gravimetric maps, are periodically rejuvenated through time. Linear residual minima might be expected to contain locally subsiding depositional features (stream channel sands, shoreline sands, offshore sand bars) during Muddy time.

Some explorationists regard such a use of gravimetric data as either absurd or futile. Others, however, have taken the position that since many dry holes are going to be drilled before finding such a field (Bell Creek field was a major Muddy discovery following a string of 34 dry holes) prospecting in subsiding tectonic-depositional trends that are based on gravimetric minima may find reservoir sand bodies more efficiently than drilling “stratigraphic tests” in a simple grid pattern.