Introduction

The Maastrichtian Lewis Shale comprises nearshore- to deep-marine sandstone, siltstone, and mudstone, and is located in the eastern part of the Southwestern Wyoming Province (also referred to as the Greater Green River Basin) in southern Wyoming and northwestern Colorado (fig. 1 on map sheet). Regionally, the Lewis produces abundant natural gas and lesser amounts of oil from low porosity, low permeability clastic reservoirs. The Lewis interfingers with the marginal marine Fox Hills Sandstone, which in turn, interfingers with the coastal plain Lance Formation, and represents time-equivalent, genetically related, marine strata.

The Lewis Shale has been extensively studied over the last half-century due to the prolific amounts of natural gas it has produced in the Southwestern Wyoming Province. The province formed during the Laramide Orogeny and was subdivided into several subbasins by intrabasin uplift (Roehler, 1992). Specifically, the Lewis has produced gas from the Great Divide, Washakie, and Sand Wash Basins, and associated Wamsutter arch and Cherokee ridge (fig. 1). As of 2022, the Lewis has produced approximately 1.5 trillion cubic feet of gas and 22.4 million barrels of oil (IHS Markit, 2022). Numerous studies suggest that the Lewis and related strata are contained in an overpressured, basin-centered gas system, that is self-sourced and produces from very tight (<0.1 millidarcies) clastic reservoirs (see Law, 2002, and references therein). Additionally, several stratigraphic cross sections, of various vintages, have shown that the Lewis Shale system represents a series of prograding clinothems deposited in shelf, slope, and basin-floor environments (for example, see figs. 3.4 and 3.8 in Pasternack, 2007; Carvajal and Steele, 2009). The cross sections and associated data release (Hearon, 2023) presented in this report build upon the previous research of the Lewis Shale and highlight the complex stratigraphy of the Lewis Shale-Fox Hills Sandstone-Lance Formation system. The cross sections emphasize that there were at least three sediment sources active during Lewis deposition, and they illustrate how flooding surfaces can be correlated over long distances between the different sediment input areas.

Data and Methods

Cross sections A–A' (north to southeast, fig. 2 on map sheet), B–B' (west to east, fig. 3 on map sheet), and C–C' (southwest to northeast, fig. 4 on map sheet) were created using gamma-ray (GR) and resistivity (RES) logs from 105 wells throughout the eastern part of the Southwestern Wyoming Province. The cross sections transect a broad area underlain by the Lewis Shale, which is bounded to the north and south by fault zones, to the east by uplift, and to the west by the Lewis stratigraphic pinch-out. Cross section A–A' comprises 48 wells, cross section B–B' comprises 25 wells, and cross section C–C' comprises 36 wells. Wells 19*, 26*, 69*, and 70* are intersection points between the three cross sections and their locations are shown on the map in figure 1. The interpreted cross sections are shown both with and without wells logs for enhanced visualization of the system’s geometry (figs. 2–4).

Tops for the Almond Formation of Mesaverde Group (hereafter, Almond Formation), Lewis Shale, and Fox Hills Sandstone were interpreted across the three cross sections (figs. 2–4). Additionally, 15 flooding surfaces (numbered 1–15 in figs. 2–4) were correlated from the log data and were used to define the tops of 15 clinothems. The flooding surfaces were identified where the GR intensity was relatively high and often wrapped around the log display, and where the RES was relatively low. Resistivity logs are not displayed on the cross sections due to space constraints.

All well logs are from the Colorado Oil and Gas Conservation Commission and Wyoming Oil and Gas Conservation Commission websites (Colorado Oil and Gas Conservation Commission, 2022; Wyoming Oil and Gas Conservation Commission, 2022). Logs that were not available in a digital format were digitized from a raster image. Well header information and depths to key stratigraphic surfaces are listed in the associated data release (Hearon, 2023).

Paleogeography and Stratigraphy

The Lewis Shale and related strata were deposited in a shallow marine embayment along the western shoreline of the Western Interior Seaway (Roehler, 1993) prior to the formation of the subbasins and uplifts within the eastern Southwestern Wyoming Province shown in figure 1. The oldest stratigraphic unit in the cross sections is the Upper Cretaceous Almond Formation, which is the uppermost formation of the Mesaverde Group. The Almond Formation is a mix of shallow marine and nonmarine coastal sediments. It consists primarily of sandstones, mudstones, and coal beds, and records the overall landward migration of the coastline during relative sea-level rise. In general, when compared to the Lewis Shale, the Almond has lower GR values, higher RES values, and a highly serrated signature in both sets of logs (GR logs shown in figs. 2–4). There is an abrupt and distinct change in log character at the top of the Almond and the base of the overlying Lewis, marking a notable change from sandstone and coal to mudstone. At this contact, the GR intensity and RES in the Lewis shift to much higher and lower values, respectively, and the range in values for both logs is much narrower. Additionally, the contact between the Almond and Lewis shows stratigraphic rise to the north (fig. 2, cross section A–A') and to the west (fig. 3, cross section B–B') toward the Rock Springs uplift (fig. 1), recording the last major sea-level transgression in the area (McMillen and Winn, 1991).

The Lewis Shale conformably overlies the Almond Formation and is subdivided into three members, in ascending order (figs. 2–4): (1) an informal lower member; (2) a middle, formally named Dad Sandstone Member; and (3) an informal upper member (Gill and others, 1970; Pyles and Slatt, 2000). The lower member of the Lewis is predominantly mudstone and has consistently higher GR values when compared to the rest of the Lewis. The lower member is interpreted as a third-order transgressive systems tract and may be an internal source of gas for sandstone accumulations within the Lewis (McMillen and Winn, 1991; Pyles and Slatt, 2000; Pasternack, 2007).

The top of the lower member is marked by the highest GR zone in the interval and is informally referred to as the Asquith marker (figs. 2–4; Pasternack, 2005, after Asquith, 1970). The Asquith marker is an organic-rich black shale that can be correlated in well logs throughout the study area and is the datum for all three cross sections. The Asquith marker is interpreted as a third-order condensed section that represents the maximum flooding surface of the Lewis Shale and records the turn-around from transgression to regression of the Cretaceous Western Interior Seaway (McMillen and Winn, 1991; Pyles and Slatt, 2000). The Asquith marker has been studied extensively in the Champlin 276 Amoco D-1 well (API number 4900720477), which cored the entire interval. Pyrolysis data indicate that the Asquith marker has an average total-organic-carbon content of 3.1 percent and is composed of mixed Type II/III kerogen (Pyles, 2000; Rigoris, 2005). The top and bottom of the Asquith marker can be mapped throughout the eastern part of the province, and flooding surface 1 merges with and downlaps onto the top of the Asquith marker as illustrated in wells 17, 53, and 93 (figs. 2–4). The Lewis Shale is often modeled as a self-sourced petroleum system (Law, 2002), where the organic-rich Asquith marker is a source for oil- and gas-charged sandstone reservoirs.

The Dad Sandstone Member of the Lewis Shale overlies the lower member and is markedly sand rich. Sandstone intervals within the Dad Member are interpreted as deltaic, slope, and basin floor channels and fan deposits resulting from turbidity currents and other sediment gravity flows (Hettinger and Roberts, 2005, and references therein). In logs, the sandstones of the Dad Sandstone Member frequently have a low-GR, blocky signature, and are interbedded with thin, high-GR flooding surfaces that often define the tops of each individual clinothem. The locus of sand deposition in the Dad Sandstone Member is in the eastern part of the Washakie Basin (fig. 2, well 31), where it is nearly 1,400 feet (ft) thick. The sandy intervals of the Dad Sandstone Member are the primary gas reservoirs in the Lewis.

The upper member of the Lewis Shale is dominated by siltstone and mudstone, but also contains some isolated, shoreface sandstone reservoirs. The upper member interfingers with the overlying Fox Hills Sandstone on a regional scale.

The uppermost stratigraphic units shown in the cross sections are the Fox Hills Sandstone and Lance Formation, which were deposited in shallow marine and coastal plain environments, respectively. The Fox Hills and Lance are time-correlative with the Lewis, and represent the proximal, landward equivalents to the offshore marine Lewis. In the GR logs (figs. 2–4), the Fox Hills often has a distinct coarsening-upward signature, which is common in prograding shelfal environments, while the Lance shows a thinly bedded, serrated signature, often interbedded with coal. The complex interfingering of these formations is indicative of the multiple high-order regressions and transgressions of the Western Interior Seaway.

Discussion

The three cross sections A–A', B–B', and C–C' record deposition of the Lewis Shale-Fox Hills Sandstone-Lance Formation system as it prograded and infilled an embayment in the Western Interior Seaway during the Maastrichtian. The stratigraphy of this system has been extensively studied in the past; however, these cross sections illustrate, for the first time, three different sediment input directions that were active at the same time. The main sediment source was from the northern margins of the embayment, whereas the southeast and southwest sources contributed far less sediment into the study area. It is likely that additional sediment accumulation from the two southern sources occurred outside of the present-day study area but has been removed by post-depositional uplift and erosion.

Hamzah (2002) details onlap of the Lewis Shale onto a paleo-high, similar to what is displayed in cross section A–A', however, the author never discusses this beyond its occurrence. Here, I interpret the onlap of the northerly source onto a high created by the distal edges of a second set of clinothems likely sourced from the southeast (wells 40–48). It is apparent that the southeastern source either shuts off at various times during basin infill, or more likely is depositing sediment outside of the current study area. Specifically, the northern source is depositing sediment the entire time between flooding surfaces 1 to 15, however, the southeastern source appears to deposit sediment in the area only between flooding surfaces 1–2, 3–5, 6–7, and after 15. Additionally, GR logs from wells 40–48 indicate that deposition in this southern area was dominated by mudstone and siltstone as opposed to the sandier accumulations from the north.

Summary

Overall, deposition of the Lewis Shale system records both the transgression (lower member and Asquith marker) and regression (Dad Sandstone Member and upper member) of the Cretaceous Western Interior Seaway during the Maastrichtian in Wyoming and Colorado. Flooding surfaces identified in well logs (figs. 2–4) can be correlated over long distances and provide evidence that multiple source areas were active at the same time. Progradation of the Lewis Shale system into the paleobasin resulted in deposition of nearly 1,400 feet of cumulative sandstone beds, which are now productive gas reservoirs.