scales of heterogeneity of an outcropping leveed channel system, cretaceous dad sandstone member,...
DESCRIPTION
The Cretaceous Lewis Shale-Fox Hills formations in southern Wyoming comprise a 3rd order highstand systems tract, comprised of several 4th order lowstand-highstand cycles. One of these 4th order cycles is well exposed in three outcrops--- called Spine 1, Spine 2, and Rattlesnake Ridge--- over a 2 mi (3.2km) 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 Lewis 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, deepwater (turbidite) sytems 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 non-erosive, flat bases, and with rare flame and dish structures and convolute beds. Sheet sandstones can be traced in outcrop for a distance of 3.5-4mi (5.6-6.4km), and for several more miles (km) into the subsurface. Sheet sandstone deposits can be subdivided into individual splay sandstones, and further subdivided into shingles with erosive bases and separated by shales. Channel sandstones have been studied in more detail than the sheet sandstones, using geophysical techniques such as high-resolution shallow seismic, ground-penetrating radar, and electromagnetic induction, drilling and logging wells behind the outcrop, and traditional measured sections and photomosaics. Spines 1 and 2 are spaced approximately a km apart. Outcrop and shallow geophysics indicate the area between the two ridges consists mainly of thin-bedded levee deposits, with occasional thicker-bedded splay sandstones. Together the spines form the largest of a three-fold scale of heterogeneity, a leveed-channel system (1-2km wide and 100-200m thick). Within this system are the two channel-complexes (0.5km wide; 50-60m thick) comprising Spines 1 and 2. Each complex is composed of individual channel deposits (100-200m wide and 3-12m thick). Facies of individual deposits include interbedded turbidite sandstones and shale-clast conglomerates; the facies distribution is analogous to fluvial meandering river deposits, with a steep cutbank side and a shallow, point-bar side. On Spine 1 there are 10 such channel sandstones; all but two are separated by thin-bedded, inner levee deposits. On Spine 2, there are 5 channel sandstones with similar facies. Stacking patterns of the channel sandstones, their internally 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 0.5km wide. Several of the features seen on outcrop and with the shallow geophysical images bear striking resemblance to features seen on subsurface seismic reflection profiles/horizons and on well log cross sections. Even though some of the outcrop features are smaller in scale than those in the subsurface, similar origins are possible. Applications of this research to exploration include (1) the knowledge that 4th order lowstand sheet sands that may be hydrocarbon-productive can occur within 3rd order highstand deposits, so that sequence or seismic stratigraphic analysis for exploration should be conducted at the 4th order scale, and (2) the Lewis Shale is an excellent outcrop analog of delta-fed, mud-prone, progradational, deepwater reservoirs. Applications to reservoir develop include: (1) the knowledge that sheet sandstones can be internally compartmentalized by shale breaks, (2) channel sandstones are internally heterogenous at a variety of scales---in this case, into (I) leveed-channel system, (II) leveed-cTRANSCRIPT
