Tectonic influence on axial-transverse sediment routing in the Denver Basin

ABSTRACT Detrital zircon U-Pb and (U-Th)/He ages from latest Cretaceous–Eocene strata of the Denver Basin provide novel insights into evolving sediment sourcing, recycling, and dispersal patterns during deposition in an intracontinental foreland basin. In total, 2464 U-Pb and 78 (U-Th)/He analyses of detrital zircons from 21 sandstone samples are presented from outcrop and drill core in the proximal and distal portions of the Denver Basin. Upper Cretaceous samples that predate uplift of the southern Front Range during the Laramide orogeny (Pierre Shale, Fox Hills Sandstone, and Laramie Formation) contain prominent Late Cretaceous (84–77 Ma), Jurassic (169–163 Ma), and Proterozoic (1.69–1.68 Ga) U-Pb ages, along with less abundant Paleozoic through Archean zircon grain ages. These grain ages are consistent with sources in the western U.S. Cordillera, including the Mesozoic Cordilleran magmatic arc and Yavapai-Mazatzal basement, with lesser contributions of Grenville and Appalachian zircon recycled from older sedimentary sequences. Mesozoic zircon (U-Th)/He ages confirm Cordilleran sources and/or recycling from the Sevier orogenic hinterland. Five of the 11 samples from syn-Laramide basin fill (latest Cretaceous–Paleocene D1 Sequence) and all five samples from the overlying Eocene D2 Sequence are dominated by 1.1–1.05 Ga zircon ages that are interpreted to reflect local derivation from the ca. 1.1 Ga Pikes Peak batholith. Corresponding late Mesoproterozoic to early Neoproterozoic zircon (U-Th)/He ages are consistent with local sourcing from the southern Front Range that underwent limited Mesozoic–Cenozoic unroofing. The other six samples from the D1 Sequence yielded detrital zircon U-Pb ages similar to pre-Laramide units, with major U-Pb age peaks at ca. 1.7 and 1.4 Ga but lacking the 1.1 Ga age peak found in the other syn-Laramide samples. One of these samples yielded abundant Mesozoic and Paleozoic (U-Th)/He ages, including prominent Early and Late Cretaceous peaks. We propose that fill of the Denver Basin represents the interplay between locally derived sediment delivered by transverse drainages that emanated from the southern Front Range and a previously unrecognized, possibly extraregional, axial-fluvial system. Transverse alluvial-fluvial fans, preserved in proximal basin fill, record progressive unroofing of southern Front Range basement during D1 and D2 Sequence deposition. Deposits of the upper and lower D1 Sequence across the basin were derived from these fans that emanated from the southern Front Range. However, the finer-grained, middle portion of the D1 Sequence that spans the Cretaceous-Paleogene boundary was deposited by both transverse (proximal basin fill) and axial (distal basin fill) fluvial systems that exhibit contrasting provenance signatures. Although both tectonic and climatic controls likely influenced the stratigraphic development of the Denver Basin, the migration of locally derived fans toward and then away from the thrust front suggests that uplift of the southern Front Range may have peaked at approximately the Cretaceous-Paleogene boundary.

Supplemental Material: Tectonic influence on axial-transverse sediment routing in the Denver Basin

Measured sections from Bijou Creek, U-Pb ages of detrital zircons, U-Pb secondary standard results, (U-Th[Sm])/He analytical results, non-negative matrix factorization results, sample groups for end-member comparison, and sample lithologic information.

Supplemental Material: Tectonic influence on axial-transverse sediment routing in the Denver Basin

Measured sections from Bijou Creek, U-Pb ages of detrital zircons, U-Pb secondary standard results, (U-Th[Sm])/He analytical results, non-negative matrix factorization results, sample groups for end-member comparison, and sample lithologic information.

The type section of the Codell Sandstone

We formally assign, describe and interpret a principal reference section for the middle Turonian Codell Sandstone Member of the Carlile Shale near Codell, Kansas. This section, at the informally named Pumpjack Road, provides the thickest surface expression (9 m, ~30 ft) of the unit in Ellis County. The outcrop exposes features that typify the Codell throughout the southern Denver Basin and vicinity. At this reference section, the Codell conformably overlies the Blue Hill Shale Member of the Carlile Shale and is unconformably overlain by the Fort Hays Limestone Member of the Niobrara Formation or locally by a thin (<0.9 m, <3 ft) discontinuous mudstone known as the Antonino facies. The top contact of the Codell is slightly undulatory with possible compaction features or narrow (<30.5 m, <100 ft), low-relief (0.3-0.6 m, 1-2 ft) scours, all of which hint that the Codell is a depositional remnant, even at the type section. At Pumpjack Road, the Codell coarsens upward from a recessive-weathering argillaceous medium-grained siltstone with interbedded mudstone at its base to a more indurated cliff-forming muddy, highly bioturbated, very fine-grained sandstone at its top. The unit contains three informal gradational packages: a lower Codell of medium to coarse siltstone and mudstone, a middle Codell of muddy coarse siltstone, and an upper muddy Codell dominated by well-sorted very fine-grained sandstone. The largest grain fractions, all <120 mm in size, are mostly quartz (40-80%), potassium feldspar (7-12%), and albite (1-2%), with some chert (<15%), zircon, and other constituents such as abraded phosphatic skeletal debris. Rare fossil fish teeth and bones also occur. Detrital and authigenic clays make up 9 to 42% of the Codell at the reference section. Detrital illite and mixed layer illite/smectite are common, along with omnipresent kaolinite as grain coatings or cement. As is typical for the Codell, the sandstone at the type section has been pervasively bioturbated. Most primary structures and bedding are obscured, particularly toward the top of the unit where burrows are larger, deeper and more diverse than at its base. This bioturbation has created a textural inversion in which the larger silt and sand grains are very well sorted but are mixed with mud. Detrital zircons from the upper Codell are unusual in that they are mostly prismatic to acicular, euhedral, colorless, unpitted, and unabraded, and have a near-unimodal age peak centered at ~94 Ma. These characteristics suggest they were reworked mainly from Cenomanian bentonites; their ultimate source was likely from the Cordilleran orogenic belt to the west and northwest.

Sedimentology, petrography, and deposition of the Upper Cretaceous Codell Sandstone in the Denver Basin

We integrate new and previous stratigraphic and petrographic data for the mid-Turonian Codell Sandstone to interpret its provenance, depositional characteristics, and environments. Our focus is on sedimentologic, X-ray diffraction, and X-ray fluorescence analyses of cores and thin sections spread throughout the Denver Basin, augmented by interpretation and correlation of well logs, isopach maps, outcrops, and provenance data. Although we treat the Codell as a single mappable unit, it actually consists of two geographically disjunct sandstone packages separated by a southwest-northeast-trending gap, the NoCoZo, short for No Codell Zone. The Codell is everywhere capped by a significant unconformity and across much of the northern Denver Basin rests unconformably on the underlying shales of the Carlile Shale. In the southern Denver Basin, the Codell commonly contains two parasequences, each of which becomes less muddy upward. Biostratigraphic and geochonologic data suggest that the unit represents deposition over a relatively brief time, spanning ~0.4 Ma from ~91.7 to ~91.3 Ma. The Codell is predominantly a thin (<50 ft) sheet-like package of pervasively bioturbated coarse siltstone and very fine-grained sandstone dominated by quartz and chert grains 50 to 100 μm in diameter. The unit is more phosphatic than the underlying members of the Carlile Shale, and its grain size coarsens to medium-grained in the northern part of the basin. An unusual aspect of the Codell across our study area is the generally excellent grain sorting despite the presence of an intermixed clay matrix. This duality of well sorted grains in a detrital clay matrix is due to the bioturbation that dominates the unit. Such burrowing created a textural inversion that obscures most of the unit’s primary sedimentary structures, except for thin intervals dominated by interlaminated silty shale and very fine sandstone. A relatively widespread and unburrowed example of this bedded facies is preserved in a thin (<10 ft) interval that extends across most of the northern Denver Basin where it is informally called the middle Codell bedded to laminated lithofacies. Sparse beds with hummocky or swaley cross-stratified and ripple cross-laminated fine-grained sandstone are present locally in this bedded facies. We hypothesize that Codell sediments were derived from a major deltaic source extending into the Western Interior Seaway from northwestern Wyoming, and that the Codell was deposited and reworked southward on the relatively flat floor of the Seaway by waxing and waning shelf currents as well as storms and waves. Codell sediments were spread across an area of more than 100,000 mi2 in this epeiric shelf system that spans eastern Colorado, southeastern Wyoming, western Kansas, parts of Nebraska and beyond.

Codell carrier-bed play, Denver Basin

Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.

Codell continuous oil accumulation in the northern Denver and SP logs Basin as defined by resistivity, density, and SP logs

A continuous Codell Sandstone oil accumulation is present in the northern Denver Basin downdip from water-wet Codell. The Codell oil accumulation can be defined by resistivity, spontaneous potential (SP), and density logs. Updip from the oil accumulation, average deep resistivity of the Codell decreases to below 4 ohm-m, SP response increases, and density porosity increases. Codell sandstones are continuous across the transition from downdip oil to updip water, so the updip seal does not seem to be caused by a stratigraphic trap. The transition corresponds to a change in thermal history; the area of the oil accumulation was subject to much higher heat flow than the updip wet area. This thermal maturity may have had an impact on clay diagenesis resulting in reduced porosity in the more thermally mature part of the Codell. This paper presents a wireline log-based workflow that can be used to identify and map regional changes in thermal maturity that control hydrocarbon accumulations and sweet-spots in low-permeability rocks such as the argillaceous Codell Sandstone.

Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.