Changes

Normal to overpressured gradients are proposed for much of the remaining parts of the Marcellus Shale play in north-central West Virginia and northward into Pennsylvania and the southern tier of New York. Pressure gradients in these areas are projected to range from approximately 0.43 to more than 0.80 psi/ft. It is also noted that a decline in the pressure gradient may exist along the eastern margins of the play, near the structural front, possibly caused by a combination of a lack of overlying seal integrity and/or a decrease in preserved organic material approaching the structural front. Based on the established successes of recent Marcellus Shale horizontal and vertical wells in northeastern Pennsylvania, southwestern Pennsylvania, and northern West Virginia, it is proposed that consistently high production volumes and high ultimate reserves in the Marcellus Shale will be mainly found in the areas that are normal to overpressured. The position of the Rome trough system is closely related to the areas of highest observed pressure gradients in the Marcellus Shale.

Normal to overpressured gradients are proposed for much of the remaining parts of the Marcellus Shale play in north-central West Virginia and northward into Pennsylvania and the southern tier of New York. Pressure gradients in these areas are projected to range from approximately 0.43 to more than 0.80 psi/ft. It is also noted that a decline in the pressure gradient may exist along the eastern margins of the play, near the structural front, possibly caused by a combination of a lack of overlying seal integrity and/or a decrease in preserved organic material approaching the structural front. Based on the established successes of recent Marcellus Shale horizontal and vertical wells in northeastern Pennsylvania, southwestern Pennsylvania, and northern West Virginia, it is proposed that consistently high production volumes and high ultimate reserves in the Marcellus Shale will be mainly found in the areas that are normal to overpressured. The position of the Rome trough system is closely related to the areas of highest observed pressure gradients in the Marcellus Shale.

The cause of the decrease in pressure gradient in central and southern West Virginia in the Marcellus Shale is likely related to an inadequately developed seal or degradation of seal integrity by natural fracturing. Conversely, the normal and overpressured Marcellus Shale areas of the basin likely will have a well-developed and relatively noncompromised seal or seals, which served to retain most of the natural gas generated during thermal maturation. The seal integrity in northern West Virginia and western Pennsylvania was still not complete, as the Marcellus Shale and possibly other Devonian black shales were the source rock for most of the oil and gas produced in these areas in the Lower Devonian and younger reservoirs.<ref name=M&S2006 />

The cause of the decrease in pressure gradient in central and southern West Virginia in the Marcellus Shale is likely related to an inadequately developed seal or degradation of seal integrity by natural fracturing. Conversely, the normal and overpressured Marcellus Shale areas of the basin likely will have a well-developed and relatively noncompromised seal or seals, which served to retain most of the natural gas generated during [[thermal maturation]]. The seal integrity in northern West Virginia and western Pennsylvania was still not complete, as the Marcellus Shale and possibly other Devonian black shales were the source rock for most of the oil and gas produced in these areas in the Lower Devonian and younger reservoirs.<ref name=M&S2006 />

Two possible theories are proposed to explain the pressure gradient changes. These theories reflect a close correlation between both the Middle Devonian Tully Limestone (C. Patterson, 2009, personal communication) and the deeper Silurian Salina Salt compared with the regional pressure profile. [[:File:M97Ch4FG8.jpg|Figure 8]] shows the outline of the limits of the Silurian Salina Salt overlain onto the regional pressure gradient map and exhibits a good correlation between the presence of thick Salina Salt and the normal- to high-pressure areas. The presence of a regional decollement in the Salina Group evaporites has been proposed<ref>Gwinn, V. E., 1964, Thin-skinned tectonics in the Plateau and northwestern Valley and Ridge provinces of the central Appalachians: Geological Society of America Bulletin, v. 75, no. 9, p. 863–900, doi:10.1130/0016-7606(1964)75[863:TTITPA]2.0.CO;2.</ref><ref name=Shmkr1996 /><ref name=M&S2006 /> for the area underlain by the thick Salina Salt. This Salina decollement created an allochthonous block that was transported generally westward along low-angle detachment thrust faults. Shumaker<ref name=Shmkr1996 /> also proposed that south of the Salina pinch-out, the basal detachment zone occurred within the Devonian interval. It is proposed that this detachment and extensive movement in the Devonian section led to severe compromise of seals above the Marcellus Shale and subsequent loss of hydrocarbons and pressure.

Two possible theories are proposed to explain the pressure gradient changes. These theories reflect a close correlation between both the Middle Devonian Tully Limestone (C. Patterson, 2009, personal communication) and the deeper Silurian Salina Salt compared with the regional pressure profile. [[:File:M97Ch4FG8.jpg|Figure 8]] shows the outline of the limits of the Silurian Salina Salt overlain onto the regional pressure gradient map and exhibits a good correlation between the presence of thick Salina Salt and the normal- to high-pressure areas. The presence of a regional decollement in the Salina Group evaporites has been proposed<ref>Gwinn, V. E., 1964, Thin-skinned tectonics in the Plateau and northwestern Valley and Ridge provinces of the central Appalachians: Geological Society of America Bulletin, v. 75, no. 9, p. 863–900, doi:10.1130/0016-7606(1964)75[863:TTITPA]2.0.CO;2.</ref><ref name=Shmkr1996 /><ref name=M&S2006 /> for the area underlain by the thick Salina Salt. This Salina decollement created an allochthonous block that was transported generally westward along low-angle detachment thrust faults. Shumaker<ref name=Shmkr1996 /> also proposed that south of the Salina pinch-out, the basal detachment zone occurred within the Devonian interval. It is proposed that this detachment and extensive movement in the Devonian section led to severe compromise of seals above the Marcellus Shale and subsequent loss of hydrocarbons and pressure.