--> --> Abstract: Dating Diagenetic Events in the Barnett and Marcellus Shale: Evidence from Paleomagnetic, Petrographic, and Geochemical Analysis, by John C. Deng, Earl B. Manning, Devin P. Dennie, S. Johari Pannalal, and R. Douglas Elmore; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Dating Diagenetic Events in the Barnett and Marcellus Shale: Evidence from Paleomagnetic, Petrographic, and Geochemical Analysis

John C. Deng1; Earl B. Manning1; Devin P. Dennie2; S. Johari Pannalal1; R. Douglas Elmore1

(1) School of Geology and Geophysics, The University of Oklahoma, Norman, OK.

(2) Devon Energy Corporation, Oklahoma City, OK.

The Mississippian Barnett and Devonian Marcellus Shale are the source and reservoir for hydrocarbons. To understand the nature and timing of diagenetic events in these prominent gas shales, integrated paleomagnetic and diagenetic studies have been undertaken. A key focus of these studies is to test if orogenic fluids from the Appalachian-Ouachitas entered and altered these units.

Previous studies on the Barnett suggest that orogenic fluids caused localized increases in thermal maturity along the Ouachita thrust zone. A paleomagnetic analysis of the Barnett indicates the presence of stable chemical remanent magnetizations (CRMs) residing in magnetite. The calculated paleopoles for the CRMs fall along the North American apparent polar wander path (NA-APWP) between the Pennsylvanian and Triassic. The Pennsylvanian CRM is interpreted to be the result of burial diagenetic processes such as maturation of organic matter and/or clay diagenesis. Also, the late Permian-Early Triassic CRM is found in samples that have radiogenic 87Sr/86Sr ratios indicating an external fluid source; all samples from the closest well to the Ouachita thrust have radiogenic values. These results suggest that this CRM was caused by fluids from the Ouachita thrust zone. Other diagenetic features include late veining and cementation by sulfates (e.g. barite and celestine) and silicate minerals such as plagioclase that could have affected the mechanical stratigraphy.

Two CRMs were identified in the Marcellus in Pennsylvania and West Virginia. An intermediate temperature CRM resides in pyrrhotite, whereas a high temperature CRM resides in magnetite. Tilt test results suggest a syntilting CRM for the two folds sampled. The calculated paleopoles for both CRMs plot on the early Permian part of the NA-APWP. Unlike the Barnett, vein tests do not indicate a connection between vein forming fluids and the CRMs. The magnetization intensity, however, is directly related to the proximity of the sample site to the Marcellus and the underlying units. This suggests that the magnetic minerals, carrying the CRMs, may have formed during the migration of orogenic fluids in the underlying units and at least partially altered the lower Marcellus. The paleomagnetic, petrographic and geochemical evidence suggest that both the Barnett and the Marcellus gas shales were locally altered by externally derived fluids in the Permian. The resulting diagenetic mineralization may have altered the mechanical stratigraphy.