Chemostratigraphy of the Mississipian- age Barnett Formation, Fort Worth Basin, Wise County, Texas USA.

摘要

The Mississippian-age Barnett Formation is a shale-gas system dominated by fine grained clay- to silt-size particles deposited in the Fort Worth Basin, a peripheral foreland basin that formed during the late Paleozoic as a result of continental collision between Laurasia and Gondwana. A detailed assessment of the chemostratigraphy and depositional environment of the Barnett Formation in the northern end of Fort Worth Basin, Texas will be studied using a variety of geochemical methods. One drill core located in the south-eastern part of Wise County (Texas, USA) was scanned at high resolution (∼ 2 inch interval) using a hand-held X-ray fluorescence (ED-XRF) spectrometer in order to provide a quantitative analysis of its major ( e.g. Si, Ca, Al) and trace (e.g. MO, U, V) element geochemistry. Furthermore, total organic carbon (TOC), total inorganic carbon (TIC) and total nitrogen (TN) data were collected at one-foot sample spacing. Major element geochemistry (Si/Al) suggests a silica rich mudstone; however relative proportions of % silica (SiO2) and Zircon (Zr) suggest that most of this excess silica in the Barnett Formation is biogenic in origin. Trace element relationships reveal that the Barnett Formation in the northern Fort Worth Basin was deposited under anoxic/euxinic conditions with relatively high total organic carbon concentration ranging from 2.0 to approximately 8 %. The organic matter provenance was determined to be primarily of marine origin. Changes in the stratigraphy using EFFe/Al together with DOPT also confirm that the Barnett Formation (lower interval) contains abundant iron relative to normal gray shale.