Shale Oil: From Deposition to Extraction and Analysis of

Modern Productive Shale Oil Plays.

Preston Cook Brigham Young University Department of Geological Sciences

ABSTRACT

Recently, advances in technology and higher oil and gas prices have led to

exploration and exploitation of unconventional oil and gas. Shale oil and gas are perhaps

the most economically important unconventional hydrocarbons. They represent the largest

on-shore deposits of oil and gas in the United States. As more effort is dedicated to the

exploration of hydrocarbon rich shale plays, it will become increasingly important for

exploration geologists to familiarize themselves with the depositional process and

geochemistry of organic shale.

Several depositional factors heavily influence the economic viability of shale. First,

they must have high total organic carbon. Second, they must also be thermally mature for

the kerogen to be converted into oil. Finally, it is important that they be brittle, so that they

can be hydraulically fractured efficiently.

Many hydrocarbon rich plays are already producing oil and gas, such as the

Barnett, Bakken and Niobrara formations. These formations will be discussed in more

detail to give a basic understanding of the nature of hydrocarbon rich plays.

INTRODUCTION

As unconventional oil and gas become more important for hydrocarbon production, there

will be higher demand for geologists that have a thorough understanding of shale oil plays. The

information presented here is not intended to be an all-encompassing reference for shale oil;

rather, it is intended to be an introduction to the subject.

Deposition of Organic Shales and Conversion into Shale Oil

Deposition of Shale. The exact definition of shale varies widely but it serves our

purposes to define it simply as a fine grained (50% of the grains silt sized or smaller)

sedimentary rock. The fine grained texture is indicative of a certain depositional environment.

According to Potter, Maynard and Depetris (2005), the majority of clay size particles

originate from the chemical weathering of rocks exposed at the Earth’s surface, with some

contribution from volcanic ash and mechanical weathering by glaciers. Silt sized particles are

produced mainly through mechanical weathering processes. These fine grains are easily

suspended in rivers and consequently are quickly transported downstream towards lakes or

oceans. In the ocean or lake, some of the coarser sediments settle out, but due to the high energy

associated with waves, silt and clay size sediments stay suspended until they are transported

below the storm wave base. Here, the energy is no longer high enough to keep the clay and silt

particles suspended. At this point the allogenic sediments fall out of suspension and are

deposited on the ocean or lake floor.

Deposition of Organics and Carbon Preservation. While the allogenic siliclastic

sediments are being deposited, microscopic and macroscopic organisms die and accumulate on

the sea floor intermixed with the sediment. Under normal conditions, these organisms

decompose and the majority of the carbon is lost, resulting in organic poor shale. Carbon

preservation is absolutely critical for shale to produce oil.

One method of carbon preservation occurs when anoxic conditions exist at the sediment-

water interface. If organic rich sediments are deposited in an anoxic environment, the organic

materials are preserved. Various factors can contribute to creating an anoxic environment. For

example, anoxic conditions can be observed in restricted marine or lacustrine environments

where water is not affected by currents or wave action and becomes stagnant (Figure 1). A good

modern example of a restricted marine environment where organic rich sediment is being

deposited is the Black Sea (Heckel 1977).

Well defined density stratification in the water column can also give rise to anoxic

conditions. During warm periods in Earth’s history, a strong oceanic thermocline forms and as

temperature goes up, it becomes more and more defined. This results in strong vertical density

stratification. This strong stratification in the water column prevents oxygen rich water from the

deep ocean from mixing with the warmer layers closer to the surface of the ocean (Heckel 1977).

The layer of water that lies below the wave base and above a strong thermocline becomes

oxygen depleted and is called an oxygen minimum zone. During certain periods in Earth’s

history, temperatures were high enough to cause what is known as an oceanic anoxic event

(OAE). During these OAEs, the oxygen minimum zone was well defined and very oxygen

deficient. Any organics that were deposited within the oxygen minimum zone were preserved

(Figure 1).

Figure 1. Cross section of hypothetical oceanic system during an oceanic anoxic event.

Notice the carbon preservation occurring in both restricted marine environments and in

the oxygen minimum zone. Modified from Schlanger and Jenkyns, 1976.

Accumulation rate is also a critical factor in determining how much carbon will be

preserved. As organic materials fall through the water column, they pass through a near-surface

zone where microbes responsible for decomposition are highly active. When accumulation rates

are low, most of the organic carbon decomposes as it falls through this zone. As sedimentation

rates increase, more organics and oil prone constituents are preserved (Lynne and Ibach, 1982).

If sedimentation rates are high enough, it is suspected that carbon could be preserved even if

conditions aren’t anoxic (Cook, 2012 pers. comm.).

Ultimately, a high total organic content (TOC), is vital for the petroleum generation

potential of shale. Carbonaceous shale is defined by having more than 0.5% TOC, but most of

the shale that is producing today has a TOC of four percent or more. That being said, a high TOC

does not always result in an oil shale producing rock. Organic shale must be exposed to

temperatures around 65° C for the kerogen to convert to oil. Thermal maturity is achieved when

significant amounts of oil are being generated (Dutton P. 1980).

Obstacles for Oil Production

As clay particles settle, they orient preferentially. The long axis settles perpendicular to

the settling surface, resulting in lamination. The preferential orientation of clay particles results

in a low permeability for all laminated shale. Historically, the low permeability of shales made

them useful as impermeable oil reservoir seals. They were also important petroleum source

rocks. Only recently have advances in technology opened up the possibility of extracting oil

directly from the oil saturated shale.

The process of extracting oil from low permeability shale begins by drilling a horizontal

well (Figure 2). Along the well at multiple points called "frack stages", the shale is hydraulically

fractured. The fracture is created when water or a viscous gel is pumped into the rock. The added

pressure breaks the rock, dramatically increasing permeability. After the rock is fractured, sand is

pumped in to hold the fractures open. Drilling operations usually pump 1000 pounds of sand per

foot of lateral well bore, with an average well using from one to four million pounds of sand.

After the sand is in place, oil flow is facilitated and oil can be extracted on a large scale (Cook,

2012 pers. comm.).

Figure 2. Depiction of horizontal drilling and extensive hydraulic fracturing. This method

allows for the exploitation of the entire shale play. It can also be useful to extract oil from

underneath developed areas where vertical drilling might be impossible.

This method of oil extraction applies only to shale that is brittle. If the shale is too rich in

clay, hydraulic fracturing is ineffective. Abundance of organisms with silica or carbonate based

skeletal structures can be a determining factor in the brittleness of shale. As temperature,

pressure and interstitial fluid composition changes, the silica skeletal structures recrystallize,

forming a network. Shale with a strong silica network behaves in a more brittle manner than

those without (Cook, 2012 pers. comm.).

Examples of Hydrocarbon Producing Shale

The Barnett Shale. The Mississippian Barnett Shale of the Fort Worth Basin of eastern

Texas could have the largest exploitable reserves of on shore gas in the United States. The Fort

Worth Basin is a foreland basin associated with the Mississippian Ouachita orogeny

(Montgomery et al. 2005). During this period, the basin was part of a sea associated with a

subduction zone on the southeastern margin of Laurentia (Figure 3). The Barnett’s TOC has an

average value of about four percent and radiolarian skeletons provide the silica to make it brittle

enough for hydraulic fracturing (Cook, 2012 pers. comm.). Horizontal wells drilled in the

Newark East field are producing two to three times as much as vertical wells.

Figure 3. Paleogeographical map of the central United States

during the early Mississippian. The depositional area for the

Barnett Shale has been highlighted in red. Courtesy of Ron

Blakey (2011).

The Bakken Shale. The Devonian age Bakken Shale of the Williston Basin in North

Dakota and Montana is an extremely carbon rich shale. The Bakken’s TOC averages around

17%, much higher than the Barnett (Cook, 2012 pers. comm.). This and other Devonian age

organic shale formations such as the Marcellus and Woodford were deposited during a global

oceanic anoxic event, allowing for high percentages of preserved carbon. Deposition occurred in

an epeiric sea that covered much of Laurentia (Figure 4). The Bakken formation consists of a

middle siltstone member sandwiched between upper and lower organic rich shale. The middle

siltstone member acts as a reservoir for the upper and lower shale members. Most wells in the

Bakken are horizontally drilled through the middle siltstone member and hydraulically fractured

(Figure 2). The fractures propagate through the siltstone member and through the shale bounding

it on either side, allowing for oil extraction from both the siltstone and oil rich shale.

Figure 4. Paleogeographical map of the United

States during the late Devonian. The depositional

area for the Bakken Shale has been highlighted in

red. Courtesy of Blakey (2011).

The Niobrara Shale. The Niobrara formation of Colorado, Wyoming, Nebraska and the

northwest corner of Kansas was deposited during the late Cretaceous in what was the Western

Interior Seaway (Figure 5). During this time, the Western Interior Seaway stretched from the

Arctic Ocean to the Gulf of Mexico. The warm waters from the Gulf of Mexico made deposition

of carbonate favorable. Accordingly, the Niobrara formation is intermixed strata of organic rich

shale and chalk. The chalk of the Niobrara is what is known as a “tight” reservoir meaning it has

an extremely low permeability (Cook, 2012 pers. comm.).

CONCLUSIONS

Figure 5. Paleogeographical map of the United

States during the Late Cretaceous (85 Ma). The

depostional area for the Niobrara shale is

highlighted in red. Courtesy of Ron Blakey (2011).

Certain environments favor the deposition of carbon enriched shale, such as low energy

marine or lacustrine environments, high accumulation rate of organics and anoxic conditions at

the sediment-water interface. After deposition, the sediment must reach thermal maturity to

begin producing oil. Shale has very low permeability, making traditional methods of extracting

oil ineffective. Where it was once impractical to extract oil from oil bearing shale, it has now

become economically viable and technologically feasible.

Economic exploitation of shale oil is a new and growing field. The largest on shore

deposits of oil in the United States are shale oil. For exploration geologists it is becoming

increasingly important to understand carbonaceous shale plays. The formations described here

are just a few of the oil bearing shale plays on the North American continent but worldwide there

are many more. Shale oil and other unconventional sources of oil are becoming increasingly

important as availability of technology and oil prices continue to rise. Undoubtedly, the majority

of new exploration geologists will work with oil rich shale plays at some point in their career.

REFERENCES CITED

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Lynne, E., and Ibach, J., 1982, Relationship Between Sedimentation Rate and Total Organic Carbon Content in

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