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Approximately 24 million bbl of recoverable oil have been found in stratigraphic traps in the Lower Pennsylvanian Tyler Formation at the Dickinson, South Heart, and eastern Green River fields, Stark County, North Dakota. Production is from a multiple sequence of quartzose sandstones 5 to 18 ft (1.5 to 5 m) thick deposited as barrier islands along regressive shorelines. Where a shoreline sandstone is fully developed, a typical vertical sequence consists, in ascending order, of the following.
1. 1 to 6 ft (0.3 to 2 m) of black to greenish-gray, sparsely fossiliferous shale. Thin interbeds of fossiliferous carbonate mudstone may be present (shallow-neritic environment).
2. 1 to 6 ft (0.3 to 2 m) of very fine to fine-grained sandstone containing small, deposit-feeding burrow structures. Stratification is finely laminated to ripple cross-stratified. Thin interbeds of siltstone and shale are common (lower-shoreface environment).
3. 3 to 12 ft (1 to 3.6 m) of fine to medium-grained, well-sorted sandstone which commonly exhibits medium to low-angle sets of cross-stratification. These genetic units are the principal reservoir rocks (upper shoreface environment).
4. 1 to 2 ft (0.3 to 0.6 m) of fine to medium-grained, well-sorted sandstone which commonly appears massive, but in a few cores exhibits parallel stratification. The upper few inches are clayey and mottled by root structures (foreshore environment).
5. 0.5 to 3 in. (2 to 7 cm) of coal (marsh environment).
In a landward direction (south) the shoreline sandstones interfinger with thin fossiliferous limestones, black shales, and oxidized mudstones which are interpreted to be lagoon, marsh, and mudflat deposits.
Throughout much of the subject area, porosity and permeability in the sandstones have been greatly reduced or completely destroyed by development of caliche paleosols. In the western part, the caliche consists of gray to brown limestone nodules or nodular layers of limestone in the sandstones and contains abundant pyrite. In the eastern part, the caliche has been strongly oxidized, and nodular to brecciated limestone in the sandstones is associated with reddish-brown to white clay, iron oxide cement, and scattered anhydrite nodules. It is estimated that the caliche destroys as much as 50% of the potential reservoir rock in the area and is an essential factor in the stratigraphic entrapment of the petroleum accumulations by providing an eastern (updip) barrier to migration.
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