The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16-18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206.
Current estimates of gas in place in the Devonian Shales of the Appalachian Basin range up to 2,500 TCF. For the most part, the majority of this resource is tied up in the shale matrix and the source and reservoir of this gas is the same as the low permeability and porosity of the shale prevents migration of the gas to a more classical reservoir rock. Where there is fracturing of the shales, the fracture network serves as a reservoir and wells drilled in the area of fracturing tend to be good producers. producers. For exploration purposes, the most probable areas for drilling of the shales are those that have a high density of natural fracturing. Aerial photography has been used to locate potential drilling sites in the Basin. photography has been used to locate potential drilling sites in the Basin. The use of this technique is based upon the premise that fractures evident in surface sediments are often useful indicators of subsurface fracturing favorable for the accumulation of hydrocarbons.
Buried fault traps, anticlines, and salt domes are frequently revealed by surface fault configurations or locally anomalous joint orientations. Moreover, since fractures are frequently conduits and collection sites for hydrocarbons, a concentration of surface fractures may indicate a potential productive reservoir at depth. productive reservoir at depth. Aerial photography allows fractures to be recognized and mapped in almost every type of terrain and soil cover. A photogeological study designed to aid in the location of specific prospects typically begins with a study of LANDSAT photos to spot large scale features. This is followed by a study of stereoscopic photo pairs and photomosaics of larger scale. For pinpointing drilling sites in such cases, the greatest weight is placed on the interpretations from stereoscopic photo pairs. geology, the techniques used in applying aerial photography as an exploration tool and finally the application of photogeology in identifying specific well sites in the Devonian Shales of the Appalachian Basin.
Structures responsible for conventional hydrocarbon traps such as anticlines, fault closures, and salt domes are frequently indicated by surface fault configurations or locally anomalous joint orientations. Joint patterns are sometimes also useful in the diagnosis of stress forces that produce folds, wrench faults, etc. Furthermore, since fractures are frequently conduits for both hydrocarbons and metallic minerals, a concentration of surface fractures may sometimes indicate fracture networks that serve as the reservoirs for hydrocarbons at depth, or vein-type mineral lodes. This paper describes the use of aerial photographs to locate fractures, and subsequently to locate drilling sites in Erie County, Pennsylvania, where the traps in the Devonian shale and the underlying Pennsylvania, where the traps in the Devonian shale and the underlying sands are stratigraphic. In the shales, porosity for gas is provided mainly by intersecting networks of natural fractures into which gases migrate from the very impermeable shales.
In a study such as this, it is important for the geologist to identify and map surface traces of fractures as accurately as possible because such fractures are remarkably ubiquitous. However, in some areas man-made artifacts, that are easily confused with fracture traces, are almost as ubiquitous as well. Careful analysis backed by field experience is essential to discriminate between the two; and field verification is recommended in the most detailed work.