Sonic properties as a signature of overpressure in the Marcellus gas shale of the Appalachian Basin

Abstract

The Marcellus Formation, a Devonian gas shale in the Appalachian Basin, is a heterogeneous rock as the result of a complex depositional, diagenetic, and deformational history. Although it is overpressured over a large portion of its economic area, the origin and distribution of pore pressure within the gas shale are not well-understood. We have used the sonic properties of the Marcellus and statistical analyses to tackle this problem. The sonic data come from a suite of 53 wells including a calibration well in the Appalachian Basin. We first analyze the influence of various extrinsic and intrinsic parameters on sonic velocities with univariate regression analyses. The sonic velocities of the Marcellus in the calibration well generally decrease with an increase in gamma-ray american petroleum institute (API) and increase with density and effective stress. Basin-wide median sonic velocities generally decrease with an increase in median gamma-ray API and pore pressure and increase with burial depth (equivalent confining stress), effective stress, and median density. Abnormal pore pressure is verified by a stronger correlation between the median sonic properties and effective stress using an effective stress coefficient of approximately 0.7 relative to the correlation between the median sonic properties and depth. The relatively small effective stress coefficient may be related to the fact that natural gas, a “soft” fluid, is responsible for a basin-wide overpressure of the Marcellus. Following the univariate regression analyses, we adopt a multiple linear regression model to predict the median sonic velocities in the Marcellus based on median gamma-ray intensity, median density, thickness of the Marcellus, confining pressure, and an inferred pore pressure. Finally, we predict the pore pressure in the Marcellus based on median sonic velocities, median gamma-ray intensity, median density, thickness of the Marcellus, and confining pressure.

Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review

“…As one of the few countries in the world to achieve commercial development of shale gas, the shale gas production formations in the United States are mainly Devonian, Carboniferous, Jurassic and Permian-all having a relatively low thermal maturity. For example, the Ro of Mississippi Barnet shale in the Fort Worth basin is between 1% and 1.3%, the Ro of Devonian Marcellus shale in the Appalachian basin is between 1.2% and 3.5%, the Ro of Devonian Woodford shale in the Anadarko basin is between 1.2% and 4%, and the Ro of Devonian Antrim shale in the Michigan basin ranges from 0.4% to 1.6% [4][5][6][7][8][9][10][11][12]123]. Compared with the United States, the four sets of organic-rich shale in southern China have a relatively high thermal maturity.…”

“…1.3%, the Ro of Devonian Marcellus shale in the Appalachian basin is between 1.2% and 3.5%, the Ro of Devonian Woodford shale in the Anadarko basin is between 1.2% and 4%, and the Ro of Devonian Antrim shale in the Michigan basin ranges from 0.4% to 1.6% [4][5][6][7][8][9][10][11][12]123]. Compared with the United States, the four sets of organic-rich shale in southern China have a relatively high thermal maturity.…”

“…(2) There is only one economic shale gas formation. At least 30 sets of economic shale gas formations have been found in 29 basins in the United States [4][5][6][7][8][9][10][11][12]136]. However, China has only found one shale formation (Ordovician-Silurian shale) with commercial exploitation value in one basin (the Sichuan Basin and its surrounding areas) [136].…”

“…Due to the development of horizontal well and fracturing technology, shale gas exploration and development in the United States has been successful and energy independence has been realized. At present, the commercially exploited shale in North America is mainly marine shale (mainly Devonian shale, Carboniferous shale, Permian shale and Jurassic shale), such as Devonian Marcellus shale in the Appalachian basin, Devonian Antrim shale in the Michigan basin, Devonian New Albany shale in the Illinois basin, Carboniferous Barnett in the Fort Worth basin, and Jurassic Haynesville shale in the East Texas basin and North Louisiana basin [4][5][6][7][8][9][10][11][12]. Following the United States and Canada, China has also made breakthroughs in shale gas exploration and development.…”