Abstract

The Cretaceous Lewis Shale-Fox Hills Formations in southern Wyoming comprise a third-order transgressive–highstand systems tract, comprised of several fourth-order lowstand–highstand cycles. One of these fourth-order cycles is well exposed in three outcrops — informally named Spine 1, Spine 2, and Rattlesnake Ridge — over a 3.2 km² (2 mi²) area. The strata within these outcrops are important for two reasons: (1) they provide good outcrop exposures of the Lewis Shale in an area of active exploration for gas, and (2) the Dad Sandstone Member of the Lewis Shale here is considered to be an excellent, scaled analog to delta-fed, mud-dominated, progradational, deep-water (turbidite) systems that produce oil and gas in areas such as the Gulf of Mexico and offshore west Africa.

Sheet sandstones underlie leveed-channel sandstones at these outcrops. The sheet sandstones are upper fine-grained, massive to parallel-laminated, Bouma Ta, Tb, and Ta-b beds, mainly with nonerosive, flat bases, and with uncommon flame and dish structures and convolute beds. Sheet sandstones can be traced in outcrop for a distance of 5.6–6.4 km (3.5–4 mi), and for several more miles (km) into the subsurface. Sheet sandstone deposits can be subdivided into individual frontal-splay sandstones, and further subdivided into shingles with erosive bases and separated by shales.

Channel sandstones have been studied in more detail than these sheet sandstones. The outcrops have been studied using geophysical techniques, such as high-resolution shallow seismic, ground-penetrating radar, electromagnetic induction, and drilling and logging wells behind the outcrop, as well as traditional measured sections and photomosaics. Spines 1 and 2 are spaced approximately 1 km (0.5 mi) apart. Outcrop and shallow geophysics indicate the area between the two ridges consists mainly of thin-bedded levee deposits, with local thicker bedded splay sandstones. Together the spines form the largest of a threefold scale of heterogeneity: a leveed-channel system (1–2 km [0.6–1.2 mi] wide and 100–200 m [328–657 ft] thick), a leveed-channel complex (<0.5 km [<0.3 mi] wide and 50–70 m [164–230 ft] thick), and a leveed-channel deposit (<0.5 km [0.3 mi] wide and 3–12 m [9.8–39 ft] thick).

Two channel complexes comprise Spines 1 and 2. Each complex is composed of individual channel deposits. Facies of individual deposits include interbedded turbidite sandstones and shale-clast conglomerates; the facies distribution is analogous to meandering river deposits, with a steep cut-bank side and a less steep point-bar side. On Spine 1 there are ten such channel sandstones; all but two are separated by thin-bedded inner-levee deposits. On Spine 2, there are five channel sandstones with similar facies. Stacking patterns of the channel sandstones, their complex stratigraphy, and their geometry and orientation (as determined by shallow geophysics) indicate the channel sandstones are sinuous, with the sinuosity extending into the outcrop. At Spine 1, a master channel has been detected geophysically, which apparently focused the channel sandstones into an interval about 0.5 km (0.3 mi) wide.

Several of the features seen both in outcrop and with the shallow geophysical images bear striking resemblance to features seen on conventional exploration seismic reflection profiles and horizons, and on well-log cross sections. Although some of the outcrop features are smaller in scale than those in the subsurface, similar origins are possible.

Applications of this research to exploration and production include (1) the knowledge that fourth-order lowstand sheet sands that may be hydrocarbon-productive can occur within third-order highstand deposits, so that sequence- or seismic-stratigraphic analysis for exploration should be conducted at the fourth-order scale, and (2) the fact that the Lewis Shale is an excellent outcrop analog of delta-fed, mud-rich, progradational, deep-water reservoirs. Applications to reservoir development include: (1) the knowledge that sheet sandstones can be internally compartmentalized by shale breaks; (2) the observation that channel sandstones are internally heterogenous at a variety of scales — in this case, into (I) leveed-channel system, (II) leveed-channel complex, and (III) leveed-channel deposit; and (3) the fact that the Lewis Shale in the Spine 1/Spine 2/Rattlesnake Ridge area provides an excellent outcrop analog for subsurface leveed-channel reservoirs, and at the same scale.