Exploration Insights March 2020

22 | Halliburton Landmark Exploration Insights | 23 Permian–Triassic Carboniferous Late Devonian Early Devonian Silurian New Red Sandstone R S? Pennine Coal Measures Group and Warwickshire Group Old Red Sandstone Much Wenlock Limestone Coalbrookdale Shales S? Early Ordovician Cambrian Neoproterozoic Acadian Deformation. Collison of Western Avalonia with Laurentia (closure of western Iapetus Ocean) Shelvian Deformation. Collison of Eastern Avalonia with Baltica (closure of Tornquist Ocean) volcanic rocks Ordovician Variscan Deformation (closure of the Rheic Ocean) volcanic rocks S? Shineton Shales Wrekin Quartzite and Lower Comley Sandstone S? Uriconian Volcanics and Intrusives Break-up unconformity. Opening of the Rheic Ocean Cadomian Orogeny (Northern Gondwana Assembly) © 2020 Halliburton Figure 1> A tectono-stratigraphic overview of the Shropshire Hills. Modified from Earp and Hains, 1971, Figure 3. EXCURSION ITINERARY The Hunt for Old Oil: A Short Field Excursion to the Shropshire Hills, England by: David Ray The following five stops represent a full field day. Their order presents a logical linkage between the geology encountered, while minimizing the travel between stops (Figure 2). Public conveniences, shops, and places to eat can be found at Coalport, Ironbridge, and Much Wenlock. Age Triassic Geologists making their way through Ercall Quarry, in the Shropshire Hills of the United Kingdom. Permian Carboniferous Devonian The Shropshire Hills occupy one of the most geologically diverse areas in the United Kingdom. Late Precambrian to Triassic sedimentary successions and igneous rocks crop out across the region. It is also a type area for the middle Silurian (Wenlock Epoch), containing two of the stage-defining global boundary stratotype sections and points (GSSPs). The proximity of the Shropshire Hills to ancient ocean basins and the presence of the terrane- bounding Church Stretton Fault system have further contributed to the stratigraphic architecture, such that the succession is divisible into several unconformity-bounded tectonostratigraphic packages (Figure 1). The area contains many surface petroleum indications (Selley 1992). While the Shropshire Hills are unprospective for hydrocarbon exploration today, in the later part of the eighteenth century, oil from seeps, such as that at the Tar Tunnel near Ironbridge, was used, for example, in preserving timber, treating ropes, and treating rheumatic and skin complaints (Craig et al. 2018). Here, we present a one-day excursion, searching for the likely source of the Tar Tunnel oil. The excursion explores Precambrian to Carboniferous rocks, and investigates how plate tectonics and sea-level change controlled the rock record and the eventual accumulation of oil. We provide an overview of the geological history of the Shropshire Hills, and highlight how this geographically small, but geologically diverse area, can be used to demonstrate global processes and aspects of petroleum geology. ACCESS, HEALTH, AND SAFETY The geological sites described, herein, are on both private and public land, and as such, access permission must be sought where necessary. Some of the sites are subject to conservation orders, such as Sites of Special Scientific Interest (SSSI), which restrict collecting and hammering. As with any field excursion, the organizers must undertake appropriate health and safety planning and precautions. + Shrewsbury Silurian Ordovician Cambrian Precambrian n tto tre e S n h o urc lt Z Ch Fau en W l k oc N Stop 3 + Telford in ek Stop 4 r W Stop 2 Coalbrookdale Coal Field Stop 1 Much + Wenlock ge Ed Stop 5 5 km © 2020 Halliburton Figure 2> Simplified geology map of the study area, showing the location of stops. Modified from Earp and Hains, 1971, Figure 2. Stop 1. The Tar Tunnel, Coalport (UK grid reference SJ 6943 0259) » Parking: YHA Coalport carpark, followed by a short walk to the stop. » Suggested stop duration: 35 minutes (an overview is best given outside the Tar Tunnel; include an extra 15 minutes to tour the Tar Tunnel, itself). » Geology: An oil seep in the Coalbrookdale Coal field, setting the scene for the source of oil seeps within Shropshire Hills. » Travel time to next stop: Approximately 10 minutes. The Tar Tunnel, part of the Ironbridge Gorge Museums, is an eighteenth century tunnel whose brickwork still oozes tar. The oil was accidentally discovered in 1787, when Quaker ironmaster, William Reynolds, began a tunneling project to link the River Severn, then an important trading route, with the Blists Hill Coal Mine, situated above the Ironbridge Gorge and River Severn. After approximately 300 m of tunneling, the workers struck oil (Didyk et al. 1983; Craig et al. 2018). Realizing the commercial potential of this discovery, Reynolds set about collecting the oil, much of which was boiled to convert it into pitch. Initially, some 10 barrels of oil per day were collected, although this soon reduced to 4 barrels a day; and 35 years after its discovery, the seep ran-dry. The house you see today was built over the tunnel entrance soon after. The tunnel continued to be used for the ventilation of the mine until the 1930s, after which it was lost, until its rediscovery in 1965. It has subsequently become a tourist attraction. The Tar Tunnel, situated in the Coalbrookdale Coalfield, provides an opportunity to set out the aims and objectives for the field excursion. The tunnel and reservoir are in late Carboniferous (Moscovian; Warwickshire Group) fluvial sandstones (Halesowen Formation), as seen high on the flanks of the gorge. The reservoir unconformably overlies a number of likely source rocks, including: marine shale bands in the Pennine (Lower and Middle) Coal Measures Group (Bashkirian); several hundred meters of Silurian graptolitic shales (e.g. Coalbrookdale Shale); and Early Ordovician black shales (e.g. Shineton Shale) (Figure 3).

Exploration Insights March 2020 | Page 22