Possible Continuous-Type (Unconventional) Gas Accumulation in Lower Silurian "Clinton" Sands, Medina Group, and the Tuscarora Sandstone in the Appalachian Basin: A Progress Report of 1995 project activities

Robert T. Ryder, Kerry L. Aggen, Robert D. Hettinger, Ben E. Law, John J. Miller, Vito F. Nuccio, William J. Perry, Jr., Stephen E. Prensky, John R. SanFilipo, and Craig J. Wandrey

Open-File Report 96-42

STRUCTURE-CONTOUR AND DRILLING-DEPTH MAPS

Because of the large number of wells in the Clinton/Medina/ Tuscarora play, the initial problem to be addressed is to determine the feasibility of constructing structure-contour and drilling-depth maps by machine from existing databases. Approximately 78,000 of the 211,000 wells in PI WHCS data files for the Appalachian basin penetrate the Clinton/Medina/Tuscarora interval. Although the regional structure of the producing interval is characterized by gentle, eastward homoclinal dip, with low and fairly uniform structural relief, enhanced gas production has been attributed to local structures by some workers (for example, Osten, 1982; Zagorski, 1991). These structures may include anticlines, anticlinal noses, terraces, and fault zones, all of which may influence hydrocarbon entrapment and(or) fracturing in Clinton/Medina/Tuscarora sandstone reservoirs. In order to identify such features it will be necessary to utilize as many data points as possible, preferably with various contouring packages and the WHCS database. Use of the WHCS database is, however, complicated by the use of inconsistent operator terminology to identify stratal units.

A test map showing structural contours has been constructed for the study area at a scale of 1:500,000 using the Dynamic Graphics Inc. Interactive Surface Modeling (ISM) system and WHCS data files. The top of the Packer Shell or the Reynales Limestone (fig. 10) was chosen as the datum because they are well represented in the database and assumed to be approximately equivalent. The north-central Ohio portion of the map is shown at a reduced scale in figure 13. The entire map utilizes approximately 58,000 data points and covers most of the play areas. Regional structure is adequately shown on the map, but there are a number of artifacts that may be a function of data distribution or miscorrelation. The Packer Shell and Reynales Limestone are convenient datum horizons from a geological standpoint, in that they are widespread, easily picked on geophysical logs, and probably reasonably approximate time lines. Subsequent correlations (fig. 10) show that, in fact, the Packer Shell and Reynales Limestone are not equivalent. In southern Ohio, for example, the Dayton Limestone (Packer Shell) (fig. 10) is typically more than 30 ft below the Reynales Limestone. Furthermore, there are a number of differences as to the usage of the terms Packer Shell and Reynales within the WHCS database and these differences affect the structure contours to some degree. In New York and Pennsylvania, for example, the Irondequoit Limestone (fig. 10) is commonly identified as Packer Shell in the WHCS database and, where identified as such, ranges from about 15 ft higher than the Reynales Limestone datum in New York to over 30 ft higher in Pennsylvania.

Such discrepancies may be tolerable on a regional level, particularly, at the original map scale of 1:500,000 and at the contour interval of 250 ft shown in figure 13.. However, because the Packer Shell has been inconsistently picked in the WHCS database, it is possible that use of this datum, without editing, would be unsatisfactory at larger scales. Also, "Clinton" sands and sandstones of the Medina Group are inconsistently picked in the WHCS database. The database is now being examined for other possible datums. This work is still in progress, but initial indications are that other structural datums may be more suitable for searching the WHCS database, such as, the top of the Medina Group in New York and Pennsylvania and the top of the "Clinton" sands in Ohio.

In addition to preliminary work with the WHCS database, a fairly comprehensive search of the literature on structure below the Upper Silurian Salina Group salt in the play area has been made. Most of the literature is in the form of graduate-school theses. Twenty six such theses were obtained through inter-library loans and were searched for structural information, as well as other information pertinent to the project. Other sources of existing sub-salt structural information are mostly limited to State geological survey reports that are typically generalized or cover only small areas. The literature search has identified several articles where cross-strike discontinuites (CSD's) of Wheeler, (1980) may have influenced hydrocarbon production (Rodgers and Anderson, 1984; Coogan, 1991). These CSD's will be incorporated into the structure and depth maps subject to time limitations and software capabilities.

Created by the EERT WWW Staff.