Eagle Ford TOC -- October 26, 2017

From: AAPG, posted May 8, 2017:

The source of the Cenomanian to Santonian petroleum systems across East and South Texas has been attributed to the Eagle Ford Shale play.

However, little effort has been made to distinguish the relationship between the depositional settings, organic facies, oil families, and lithostratigraphic characteristics of the source rock.

This study finds that there are significant variations in stratigraphy, reservoir type, and produced hydrocarbon chemistry between the South Texas Eagle Ford and the so-called East Texas Eagle Ford.

The South Texas Lower Eagle Ford Shale reservoir facies is dominated by organic-rich, relatively low clay, foraminfera-rich, coccolith mudstones/marlstones, whereas the superficially equivalent source rocks in East Texas have a much more dominant terrestrial influence.

In a regional reservoir modeling study at the confluence of East Texas and South Texas on the San Marcos Arch, the interplay of these depositional systems had to be accounted for to achieve reliable results. The model included analysis of cores from multiple counties combining detailed stratigraphic facies descriptions and petrophysical data from the base of the Austin Chalk to the Buda Formation.

Source rock data is available from approximately 118 wells throughout this interval between the Austin Chalk and Buda. The dataset includes TOC, pyrolysis, and vitrinite reflectance data.

Based on TOC analyses across the entire trend, the average TOC of the East Texas and South Texas Eagle Ford is 3.43%.

Pyrolysis data and visual kerogen descriptions clearly show the South Texas Eagle Ford contains primarily Type II algae-rich oil prone kerogen. In contrast, the East Texas Eagle Ford contains type II kerogen with terrestrially derived mixed kerogen from the northeast.

Thermal maturity in the Eagle Ford play area varies systematically with structure independent of the depositional systems from early oil generation to dry gas trending northwest to southeast. Produced oil geochemistry data from 70 oils include bulk molecular compositions, Pristane/nC17, Phytane/nC18, Pristane/Phytane, C13 to C20 isoprenoids, saturate and aromatic carbon isotope compositions, sterane and hopane ratios.

The geochemical data suggest that the oils from the South Texas Eagle Ford and East Texas Eagle Ford plays are generated from two distinct types of organofacies. One type is dominantly carbonate mudstone sourced in South Texas, and the other type is siliciclastic marine shale sourced in East Texas.

See also: researchgate, January 6, 2016. TOC varies across the region, from 2% to 12%. The TOC increases from northeast to southwest.

API of crude oil across the US. Look how uniform the crude oil is in North Dakota compared to Texas; note how "heavy" California oil is compare to Texas and North Dakota.

OBSERVATION 1: Random Observations Regarding the USGS 2013 Assessment of the Bakken: TOC

Newbies should go back and read this post of November 1, 2011.

There are numerous factors affecting the quality of an oil basin. Total organic content (TOC) is one of the Big 5.

A bit more about TOC at this link. Now, how does the Bakken stack up against, let's say, Saudi Arabia?

There are five areas in the world where "world class" source rock exists:

• The Bakken
• Norwegian Sea (North) and North Sea
• Venezuela
• Saudi Arabia
• Norwegian Sea (South) and The Netherlands

For comparison, TOCs:

• Bakken: as high as 11%
• United Arab Emirates: 0.3 to 5.5%
• Saudi Arabia: 2 to 5%
• Trinidad: 2.2%
• North Sea: 5.6% 

When I first came across this information, I about fell off my chair: the Bakken has a TOC average of 11 percent, compared to Saudi with varying reports of 2 - 5 percent. Wow.

So, that was from the earlier post, back in 2011.

Did the USGS 2013 Survey of the Bakken-Three Forks have anything to say about the TOC of the Bakken? Yes, based on new information, and I quote from the first page:

"The upper and lower shale members are the primary source rocks for the Bakken TPS, with present-day total organic carbon (TOC) values from less than 1 weight percent to 35 weight percent (Lillis, 2013)."

A TOC of 35% is the highest I have ever seen reported for the Bakken. As noted above, the 30-second sound bite: the average TOC for the Bakken is 11 percent. Most sources quote TOC as high as 16 percent in the Bakken, and there is one source that quotes a TOC as high as 20%.

Again, more recent data suggests the TOC in the Bakken can be as high as 35%. That must be some kind of record.

The four-page USGS 2013 assessment does not provide more detail regarding TOC. It does not matter. But it does put the Bakken into perspective for some folks. Like me. [Ryder Scott, June-August 2011, Vol 14, No 2, states that "a total organic carbon (TOC) of 2 percent is considered a sufficient screening criterion for oil shale plays. However, both the Bakken and Arthur Creek ["Northern Territory, Australia] have been reported to contain much higher TOCs. Greater TOC and shale thicknesses are correlated to higher production." -- Ryder Scott. See comments below.]

Just How Good Is The Bakken -- A Reminder and a Utica Update -- The Bakken, North Dakota, USA

This link about the Bakken TOC was posted May 24, 2010. By then, the Bakken boom had begun, but investors and entrepreneurs still had the opportunity to get in near the ground floor, as they say.

The higher the total organic carbon (TOC) in the source rock, the better the formation.

TOC: total organic content -- this has to do with the "source rock" -- the rock that generates the oil. Look at this definition of TOC from the Schlumberger website:

A rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Preservation of organic matter without degradation is critical to creating a good source rock, and necessary for a complete petroleum system.

Note that the Bakken source rock has 11% TOC. [If you want to see this data for yourself, go to this link: https://www.dmr.nd.gov/ndgs/bakken/bakkenthree.asp; then, scroll down to GI-2. The fifth presentation "Bakken TOCS" is what you are looking for; click on "PDF 52 Meg" -- due to the size it will take a few moments to download. Incredible numbers; even an amateur can see the TOCs -- but there is a lot more than TOC, but it's a start.]

So, what's the TOC in the Utica? Verbatim excerpts from this link (pay particular attention to Point Pleasant and Utica references):

The final isolith map includes the thickness of all organic-rich beds, shale beds and interbedded limestones from the top of the Utica Shale to the bottom of the Logana Member of the Lexington Formation in Ohio; the Logana Member in eastern Kentucky and West Virginia where the Point Pleasant and Utica are not present; and the interval from the top to the bottom of the Utica Shale where only the Utica is present in Pennsylvania, Ontario and New York ...

Data for total organic carbon (TOC) were overlain ...

Adding these data defined areas .. indicates that nearly the entire assessmetn area is in the good to very good range. Exceptions are thin, linear, northeast-southwest trends of fair to poor TOC values along the Ohio River between West Virginia and Ohio and smaller, parallel trends farther west in Ohio; and ...

The map of minimum TOC values presents a more pessimistic picture. Small areas in Ohio, Pennsylvania and New York are deemed to be very good and occur within a much larger area of these same three states that isin the good range for TOC. However, southern Ohio and most of West Virginia is mapped as having mostly poor potential.

... TOC data from 28 deep wells in the basin indicate that in 11 of the 28 wells, TOC values were greater than 0.5% [not a typo; 0.5%; compare to 11% for the Bakken] in the Beekmantown Formation and deeper formations. The highest values are 4.4% from the Rogersville Shale ...2.83% and 1.12% from the Conasauga Formation ... Other TOC values range from 0.57% to 0.88%. [Again, compare these numbers with the Bakken].

When you get to the maps, the "very good" areas generally have a minimum TOC of 4% and less.

The relatively lower TOC's compared to the Bakken, are counterbalanced by the very thick Utica, measured at 100 to 150 feet, and as thick as 500 feet at places. An undated article at Geology.com states that not much is known about the true TOC of the Utica.

Having said that, I believe the Lodgeple to the Bakken (three sub-formations) and the Three Forks (several sub-formations) is also 100 - 150 feet thick in the better Bakken.

The TOC in the Bakken is quite simply, incredible.