Heidemann Field Report

Kimball County, Nebraska

EAS 424

Lori McDonald

Introduction

The J Sandstone in this field of study is located in the northern and western portions of

Nebraska in the Denver Basin. The area covers a 250 square mile area in the western part of Nebraska.

The J sandstone is Lower Cretaceous in age and is a middle member of the Dakota Group. It is further

subdivided into three distinct members, the J2, J1, and valley fill J. J2, the oldest sediment at the bottom

of the section, is characterized as a lenticular marine bar sandstone that is interfingered with mudstone

and shale units. A regression and eventual transgression of the shallow water sea marks an

unconformity between the J2 and J1. J1sandstone is also a shallow water marine sediment laid on top of

the J2. Another regression of the sea is seen as another erosional unconformity between the J1and

valley-fill that was deposited to the NE of the J2and J1. Valley fill sequences are characterized by

meandering sandstone beds that overlies the J1and J2 sequences, and entrench themselves within the

older sequences. Laramide folding of the area and anticlinal structures present caused the entrapment

of the most productive fields of oil, which are located in the upper stream beds of the valley fill deposits.

Stratigraphy is a more important factor in oil trapping in lower members of the J sandstone.

The J Sandstone is on average less than 25 feet thick, and is a medium grained, well sorted

friable sandstone with good porosity and permeability. (Gabarini, 1964) Sedimentary structures

present, textures and fossils are all used to help characterize the depositional environment in this area.

Oil accumulation in the J Sandstone is controlled by the shape of the sandstone body, such as structural

high areas. It is also controlled by stratigraphic changes from sandstone to shale, as well as local

anticlines and J Sandstone pinch out structures. A bentonite layer is deposited on top of the “J”

sandstone sequences, giving a specific age timeline for when these sediments were deposited.

Previous Investigations

J2 sediments are characterized as being formed from an influx of coarse clastics from the West

and deposition of sands in a shallow water near shore or shoreline environment. This unit covers a large

area in Western Nebraska. (Garbarini, 1964) Because it mostly consists of almost planar, horizontal

laminae less than a half-millimeter thick; this portion of the “J” is interpreted as being laid down in very

shallow water or subaerial conditions. These fine graded laminations indicate a broad, relatively

current-free environment. Oil trapping in this section of the “J” sandstone is mostly stratigraphic, with

structure not being much of a factor. (Etheridge, 1989) The top of this sequence is marked by a root

burrow zone and coal deposits. This indicates a regression of the sea, and is recorded in the rock record

as an unconformity. The J2 interval thins from more than 40 ft thick in the SE to less than 20 ft thick in

the N. (Harms, 1966) This thinning is thought to be a result of erosion by stream channeling after the

deposition of stream deposits.

The deposition of the J1 sandstone is interpreted as the re-entry of the sea with the deposition of

sand, silt and clay derived from the eastern in near shore or offshore environments. Mineralogy

indicates that J1 was derived from an eastern provenance. (Etheridge, 1989) The coarser material and

small scale burrows indicate a marine environment that was fairly shallow. The J1 shallow water marine

consist largely of shale, interfingered with elongated lenticular sandstone bodies. (Exum, 1968) Both the

J1 and J2 are relatively thin, widespread units of sandstone. The producing members of the J1 have

elongate, lenticular sandstone bodies that interfinger with the adjacent mudstone and shale facies.

(Harms, 1966) The clean sandstone in the central bar facies forms the important petroleum reservoirs.

The sandstone bodies in the J1show a distinct depositional trend and are elongated NW-SE. (Harms,

1966) The position of the oil fields is not specifically related to structural configuration, so therefore the

trapping of the oil must be largely stratigraphic. (Etheridge, 1989) The mudstone and shale members

that encase the sandstone serve as good oil trapping barriers. The majority of oil reservoirs in the J1

units are within stratigraphically isolated sandstone. Shale and sandstone of J1and J2 members provide

up dip barriers for entrapment. (Harms, 1966)

Valley fill sediments are limited to areas where the J1and J2 are greatly thinned through erosion,

or absent. These sediments are light grey in color where they are unstained by oil, with fine to very fine

grain size. (Etheridge, 1989) Valley fill sediments are limited to a narrow, N-S trending band. The

sediments in the valley fill are determined to be fluvial because they occur in a steep-walled, long

narrow depression cut into older sedimentary structures. (Gabrini, 1964) Sandstone that has burrowed

into sandstone and mudrock bodies of the J1 and J2 serves as an effective oil trapping barrier. (Harms,

1966) The major oil reservoirs in this study area of the “J” reservoirs are in the stream deposited

sandstone. This is the area with the most successful oil production, producing over 5 million barrels of

oil. (Harms, 1966) The valley fill sediments are determined to be low gradient meandering streams.

(Exum, 1968) After deposition of the valley fill sediments, the area underwent folding during the

Laramide Orogeny. (Etheridge, 1989) Oil staining in the rocks indicates that oil was trapped in other

parts of the valley and was redistributed after the folding. Now the oil reservoirs are situated in the

sand stream deposits at structural high areas, and structure is essential for the entrapment of oil in this

specific reservoir. Escape of the oil to the north or south along the valley fill trend is prevented by local

plunging anticlines of overlying shale. (Etheridge, 1989) Entrapment of oil in this valley fill section

requires a combination of stratigraphy and structure.

Stratigraphy

The sandstone in the J2is relatively all the same lithology, and is very light grey color with very

fine grain size. It is well sorted with a white clay matrix. It consists of almost horizontal laminae that are

less than a half millimeter thick. (Harms, 1966) Between the J2and J1 members, there are burrows made

by organisms, which causes some intermixing between the two layers. The dominant mineralogy of the

J2sandstone layer is quartz, chert, and feldspar. It is distinguished from the J1 and valley fill sandstone

deposits by having a lower quartz to chert ratio, with a lower feldspar content. (Harms, 1966) Heavy

minerals in the J2 include tourmaline, rutile, zircon, and leuxcoxene. There is almost a complete absence

of chloritiod (brittle mica) in the J2, which is seen in larger quantities in the J1 and valley fill sediments.

(Harms, 1966) It also has a larger percentage of kaolinite than other J sandstone beds. The laminae of

the J2is marked by concentrations of fine silt sized black grains that have higher concentrations in lower

parts of the section. Sand sized clay pellets are seen in nearly all thin sections. They are composed of

aggregates of different clays, and are derived from the reworking of partly consolidated silt and mud

that was shaped during sedimentary processes. Kaolinite The only fossils found in this part of the J

sandstone are microscopic such as dinoflagellates, pollen or spores. These fossils also indicate a marine

depositional environment. (Exum, 1968)

The J1 sandstone is composed of very fine to fine sand particles with clay sized materials present

in the matrix. (Harms, 1966) This unit contains more sandstone, and has sedimentary structures such as

ripple marks, small scale cross stratification. The mineralogy of the J1sandstone is mostly quartz, chert,

and feldspar. There is a higher percentage of chert and feldspar in thin section because many of these

grains are smaller than 049mm in diameter. (Harms, 1966) This unit is medium grey in color, with a high

silt content at the top. It has very thin graded laminae at the top as well, but is otherwise unbedded in

appearance. (Etheridge, 1989) Much of the shaley portions of the unit have a speckled appearance.

Fossils contained in this unit include many foraminifera, and abundant microfossil assemblage.

Carbonized plant fragments are also visible in the lower shaley unit. The thickness of this unit is

approximately 5-10 feet. Burrows present between the J1 and J2 sandstone members illustrate the

unconformity between the two layers where the strata were exposed when the sea regressed. (Harms,

1966)

The upper member of the J sandstone unit is composed of sandstone, siltstone, and shale. It is a

very fine grained, light grey sandstone with dark grey siltstone and shale. (Harms, 1966) Sedimentary

structures in the upper layers of the J valley fill are complex, with cleaner, thicker bedded sandstone as

the most common upper part of the sequence. (Etheridge, 1989) It is composed of largely siderite and

grey shale pebbles, with coarse cross laminae that have dip angles of greater than 20 degrees. (Harms,

1966) The texture of the valley fill sections contains sandstone with interbedded siltstone and shale.

They lack foraminifera, but have abundant plant fossils and pollen. (Harms, 1966) Siltstone and shaley

units in the valley fill sediments are more complex, being only a few millimeters thick in some areas.

The heavy mineral content is not distinctive except for the presence of the brittle mica chlorotiod. For

fossil content it has some foraminifera, with a significantly less percentage of dinoflagellates than other J

units. (Harms, 1966) It also contains more cypress pollen and carbonized plant material is common in

this upper layer of the J unit. Porosity ranges from 2-17 percent. (Harms, 1966)

Methods

These sediments of the “J” Unit have been investigated and recorded through well logging,

mineralogical investigation, provenance studies, seismic interpretation, and stratigraphic correlation of

known units. The thickness of the units present has also been analyzed by creating an isopach and

structure map of the drill sites. The structure and isopach contour maps illustrate the tops of the

bentonite and J sand to find any structures that may be conducive to oil entrapment and would promote

further exploration. Structure maps are created by subtracting the value of the Kelly bushing from the

top of the bentonite and top of the J sandstone. The isopach map calculates the thickness of the

bentonite layer between the top of the bentonite and top of the J sandstone, and the thickness of the

net J sand.

Discussion

Based upon the structural and stratigraphic isopach maps of the cross sectional area that was

investigated; there are both structural and stratigraphic traps that are present. The structure map

illustrates how different structural highs and lows are capable of trapping oil and gas. The stratigraphic

map also illustrates areas of different thicknesses that would serve as effective traps for oil and gas. As

seen from previous investigations and the maps produced, the J sand unit has long lenticular sand

bodies associated with it. It also has structural highs and lows that could create a dome-shaped trap.

The isopach map is a representation of the thickness of both the bentonite and net sand. The thickness

of the net sand is not uniform across the section and is thicker in some areas. Different rates of

deposition and different environments of deposition have lead to the specific depositional pattern of the

net sand. The Bentonite layer is a thick volcanic ash layer that was deposited on top of the upper

member of the J sand. This clay-rich layer is an effective barrier to aid in the trapping of oil and gas

reservoirs.

Conclusions

Looking at the structural and stratigraphic maps of this area, the structural history of this area is

complex and has varied over time. The transgression and regression cycle of the sea has left deposits of

very unique sand silt and clay formations that are long, lenticular sand bodies. Oil and gas in this region

is both trapped through structure and Stratigraphy. Both structural and stratigraphic factors influence

the discovery well.

Exploration

Based on the maps produced through this project, areas that would be suggested for additional

drilling would be the SW ¼ NW ¼ of Section 26, NW ¼ SW ¼ of Section 26, and the SW ¼ SW ¼ of section

26. These sites would be recommended for additional drilling because of their close proximity to other

oil producing wells in the area, and also because of the structural features of the sandstone reservoir

seen in the structure map. Also, looking at the cross section of the area, it is likely that this area would

contain a structural high spot, providing for another factor in trapping oil and gas reserves.

References Cited

Dolson and others, 1991, Regional paleogeographic trends and production, Muddy Sandstone (Lower Cretaceous), Central and northern Rocky Mountains: AAPG Bull., v. 75, p. 186-206.

Etheridge, FG, and Dolson, JC, 1989, Unconformities and valley-fill sequences – key to understanding “J” Sandstone (Lower Cretaceous) reservoirs at Lonetree and Poncho Fields, D-J Basin, Colorado, Rky. Mtn. Assoc. of Geologist Gdbk, p. 221-223.

Exum, FA, and Harms, JC, 1968, Comparison of marine-bar with valley-fill stratigraphic traps, western Nebraska: AAPG Bull., v. 52, p. 1851-1868.

Garbarini, GS, 1964, Cedar Valley and Canal Fields, Scotts Bluff County, Nebraska: The Mountain Geologist, v. 1, p. 47-53.

Harms, JC, 1996, Stratigraphic traps in valley fill, western Nebraska: AAPG Bull., v. 50, p. 2119-2149.