50. West-east stratigraphic cross section B-B' through TI 2N of the Paleocene Fort Union Formation, Sand Wash Basin, illustrating operationally defined stratigraphic units and coal occurrence of the lower coal-bearing unit
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51. West-east stratigraphic cross section C-C' through T1ON of the Paleocene Fort Union Formation, Sand Wash Basin, illustrating operationally defined stratigraphic units and coal occurrence of the lower coal-bearing unit
58. Detailed north-south stratigraphic cross section E-E' between R92W and R93W of the lower coal-bearing unit, Paleocene Fort Union Formation, Sand Wash Basin, illustrating occurrence of coal packages 1 and 2
Expedition 337 of the Integrated Ocean Drilling Program (IODP) was conducted by the riser drilling vessel Chikyu at Site C0020 near the Western Pacific margin off Shimokita Peninsula, Japan. The primary scientific objective of Expedition 337 was to investigate deeply buried microbial communities in lignite coal-bearing sediments down to a depth of 2466 m below the seafloor (mbsf), and to characterize their biogeochemical role in carbon cycling (Inagaki et al. 2012, 2015). The microbial cell abundance decreased sharply in deep sediment layers below 1200 mbsf, with most sediment samples containing a maximum of several hundred microbial cells per cubic centimeter (Inagaki et al. 2015). However, a larger microbial biomass was observed in lignite coal layers. Physical and chemical characteristics of the cored sediment samples were determined at onboard and offshore laboratories (Gross et al. 2015; Glombitza et al. 2016; Tanikawa et al. 2016; Ijiri et al. 2017; Trembath-Reichert et al. 2017), but the key factors responsible for constraining the vertical distribution of the microbial populations in the deep sedimentary biosphere have not yet been well clarified (Hinrichs and Inagaki 2012). While permeability and pore characteristics potentially govern not only microbial biomass but also the reservoir capacity of microbially produced coalbed methane (Gamson et al. 1993; Clarkson and Bustin 1996; Strąpoć et al. 2011), the transport dynamics associated with these characteristics have not yet been reported.
In this study, to determine the key physical properties that influence the habitability for microbial life of deep subseafloor environments, we present the permeability and pore size distribution of core samples obtained from the coal-bearing sedimentary basin off Shimokita Peninsula, Japan (Sanriku-oki subbasin), obtained during IODP Expedition 337. The variations in the permeability and pore characteristics of sediments were investigated to clarify whether the decreasing trend in microbial cell abundance with increasing sediment depth could be explained, at least in part, by depth-dependent physical properties (Rebata-Landa and Santamarina 2006). The relationship among permeability, lithology, porosity, and pore size distribution in the sediment core samples was examined in order to clarify the factors that affect fluid transport and microbial cell abundance in the approximately 2.5-km-deep subseafloor sedimentary biosphere.
At Site C0020, the coal-bearing sections in unit III and the lower portion of unit IV had relatively high microbial cell concentrations. These units/sections were characterized by having relatively permeable coal and sandstone layers with a relatively large pore size (Fig. 5b, c). The changes that occur with increasing depth, which are mainly due to variations in the lithology, are larger for the permeability and the pore size than for the porosity. Consequently, because of the large variation in lithology at Site C0020, there was no clear correlation between the porosity and the permeability below 1000 mbsf (Fig. 7a).
The large pore size determined by MICP for coal likely reflects the existence of voids in the fractures (cleats). Consequently, the high permeability of coal at low effective pressure likely has a marked influence on both vertical and bedding parallel flow through cleats. The permeability of coal is very sensitive to the effective pressure, and the in-situ coal permeability can be very different to the initial permeability. Therefore, the in-situ pore size (aperture size) for coal is expected to be much smaller than the measured pore size (Fig. 7b) Nevertheless, voids in fractures may form an important pathway for fluid flow through coalbeds, because the in-situ permeability of fractured coal is much higher than that of the intact coal matrix or neighboring siltstone/shale formations (Fig. 5b, Ijiri et al. 2017). Therefore, it is considered that nutrients and energy sources were mainly released from the coal layers through the cleats, where large amounts of energy sources are stored, and that these compounds seeped into the overlying permeable sandy sediments. However, the impermeable shale and siltstone layers, which overlie the permeable sandstone layers, may act as barriers to vertical fluid flow. As shown in Fig. 5b, the permeability change between permeable (sandstone, coal) and impermeable (siltstone, shale) layers was two to four orders of magnitude in coal-bearing units. Since almost no nutrient flow occurs through the impermeable layer, the very low cell densities at 1500 to 1800 mbsf and from 2000 to 2400 mbsf (
During the early Bajocian, a conspicuous coal-bearing siliciclastic succession was deposited in the northern Tabas Bock, which is important for understanding the regional geodynamics of the Central-East Iranian Microcontinent (CEIM) as well as for the Jurassic coal genesis in this part of Laurasia. Sedimentary facies analysis in a well-exposed section of the lower Bajocian Hojedk Formation (Kalshaneh area, northern Tabas Block) led to the recognition of ten characteristic sedimentary facies and three facies associations, representing channels with point bars and floodplains of a Bajocian meandering river system. Modal analysis indicates that the mature quartz arenites and quartzo-lithic sandstones of the Hojedk Formation originated from the erosion and recycling of older, supracrustal sedimentary rocks on the Yazd Block to the west. The coal petrography and maturity show an advanced maturation stage, whereas the great thickness of these continental strata points to a pronounced extension-related subsidence in the northern Tabas Block. The rapid rate of differential subsidence can be explained by accelerated normal block-faulting in the back-arc extensional basin of the CEIM, facing the Neotethys to the south. Compared to the thick Jurassic, the post-Jurassic strata are relatively thin and played a limited role in the thermal history of the coal in the northern Tabas Block. A relatively high geothermal gradient in the tectonically highly mobile area of the northern Tabas Block and/or heating by regionally widespread Palaeogene intrusions were most probably the key drivers of the thermal maturation of the Middle Jurassic coals.
New research on the Lance and Fort Union formations has allowed geologists to better define the extent of the coal-bearing strata and potential natural gas reservoir rock in the Great Divide Basin of Wyoming, according to a recent study by the Wyoming State Geological Survey (WSGS).
The subsurface correlations were based on well logs from oil, gas, and coal exploration efforts throughout the Great Divide Basin, a sub-basin of the Greater Green River Basin. The findings are presented in WSGS Open File Report 15-3, Stratigraphic Cross Sections and Subsurface Model of the Lance and Fort Union Formations, Great Divide Basin, Wyoming, by Ranie M. Lynds and Christopher J. Carroll. This report is available in a large poster format for $25 via the agency website or as a free pdf download.
This WSGS Open File Report (large poster format) includes a series of six cross sections, three structure contour maps, and two isochore (vertical thickness) maps. It also includes a description of the geologic history and stratigraphy as well as references. The cross sections indicate two main coal intervals within the Fort Union Formation. The lower coal-bearing interval reaches a maximum thickness of approximately 2,500 feet (60 feet of net coal thickness) in the northeast portions of the Great Divide Basin, and thins to less than 600 feet in the west (23 feet net coal), near Point of Rocks. The upper coal-bearing interval is nearly 1,000 feet thick (37 feet net coal) in the central part of the basin and is entirely absent along the western margin. Most of the natural gas shows in the region are from the lower part of the Lance Formation, which generally thickens to the north and east. 2ff7e9595c
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