Have a read of these Seaace....

Have a read of these Seaace.

http://www.wlu.edu/geology-departme...schultz-13-interprets-seismic-in-artic-alaska

https://repository.wlu.edu/bitstream/handle/11021/23861/Schultz_theses_2013.pdf?sequence=1

GEOLOGY OF THE STUDY AREA

Overview

The study area (the National Petroleum Reserve in Alaska) is located in northwestern
Arctic Alaska, which is situated on what is known as the Arctic Alaskan microplate (Hubbard et
al., 1987). The region has undergone a complex tectonic history, including several distinct
orogenic events and significant anticlockwise rotation relative to the North American craton
(Embry, 1990). The Brooks Range orogeny was initiated in the Jurassic, with sediments from
the North filling the accommodation space in the foreland to form the Kingak shale during this
time (Martin, 1970; Houseknecht and Bird, 2004). The Arctic microplate then rotated counter
clockwise into its current position and the Colville Basin was formed as a result of loading from
the Brooks Range and rifting in the Beaufort. The Torok and Nanushuk were then deposited in
the Aptian to Cenomanian, predominantly from sediments coming off the Chukotka Belt, as
well as the Brooks Range, which were then deposited into shelf, slope and basin floor
depositional environments (Houseknecht et al., 2009). Arctic Alaska has been divided into
several physiographic provinces, including the Arctic Coastal Plain, the Northern Foothills, the
Southern Foothills and the Brooks Range, all of which make up parts of the NPRA (Gryc, 1970)
(Figure 10).

Shublik Formation

The Shublik Formation is a member of a Carboniferous to Lower Cretaceous package of
rocks known as the Ellesmerian sequence (Parrish et al., 2001), consisting of rock ranging in
composition from limestone and calcareous shale to siltstones and sandstone, as well as some
phosphatic layers (Jones and Speers, 1976). Deposition of the Shublik Formation likely occurred
relatively slowly in a low-energy marine environment at water depths of about 200-1000 feet,
with relatively restricted circulation but sufficient nutrient levels to allow for abundant fossils
(Detterman, 1970). The Shublik Formation generally ranges in thickness from about 90 to 510
feet and is continuous across the NPRA and most of the North Slope, except for some regions in
northeastern Alaska and near Point Barrow. Deposition of the Shublik Formation occurred on
the gently sloping shelf created by the underlying Sadlerochit Group with a sediment source
located to the North providing mud to silt sized sediment, as well as some sand (Bird, 1987).
The Shublik Formation is the dominant source rock for many petroleum systems in the North
Slope, with geochemical analysis suggesting an organic carbon content averaging about 1.7 wt
% and a majority of the formation situated in the catagenic or metagenic stages for petroleum
generation based on vitrinite reflectance data (Peters et al., 2006; Magoon and Bird, 1987).

Kingak and Pebble Shales

The Kingak Shale is a rock unit consisting largely of marine shale with some interbedded
siltstone and sandstone that overlies the Shublik Formation (Houseknecht and Bird, 2004). The
Kingak Shale ranges in thickness from 0 feet in the Northeast where it has been eroded away to
up to 3,450 feet in the South, and is likely a contributing source rock for oil in the North Slope
with an organic carbon content ranging from 0.5 to 2.0 wt % and a mixture of marine and
terrestrial kerogen (Magoon and Bird, 1987; Houseknecht and Bird, 2011). The Kingak Shale is
Jurassic to Lower Cretaceous in age, and is bounded by strong seismic reflectors that represent
the Triassic Shublik and the Cretaceous Pebble Shale (Bird, 1987). Interpretation of seismic
data from the NPRA shows a pattern of soutward downlapping clinoforms (Bird, 1987;
Houseknecht and Bird, 2004), which were interpreted in much of the NPRA by a previous
Washington and Lee Honors Thesis by Natalie Stier (2012). The Pebble Shale is a marine shale
with interbedded sandstones that overlies the Lower Cretaceous Unconformity (LCU) surface
(Schenk and Bird, 1993). The unit is about 160 to 520 feet in thickness, with a radioactive shale
layer in approximately the top 100 feet of the unit, which is designated as the gamma ray zone
(GRZ) due to the peak that it produces in the gamma ray curve. The Pebble Shale is another
potential source rock with a relatively high average organic carbon content of 2.4 wt %, though
the unit may be immature in much of the NPRA (Magoon and Bird, 1987).


Torok and Nanushuk Formations

The Aptian-Cenomanian aged Torok Formation and time equivalent deltaic Nanushuk
Formation consist of moderate- to deep-water shales, siltstones and sandstones (Weimer,
1987). The slope and deep-water facies were likely deposited at depths of at least 1,000 feet
and formation as a whole ranges in thickness from 1,200 feet to up to 20,000 feet in the Colville
trough (Bird, 1987). The organic carbon content of the Torok is relatively low, averaging 1.2 wt
%, and the Torok trends towards terrestrial (type III) kerogen, though the thermal maturity as
per the vitrinite reflectance suggests that it is mature enough in some locations to generate
hydrocarbons, though it is likely dominated by gas production along with some potential for oil
(Magoon and Bird, 1987). The Torok Formation is the fine-grained marine slope and basin
floor facies that is age equivalent to the Nanushuk Formation, which is composed primarily of
shallow marine sandstone (Bird, 1987). The Torok and the Nanushuk have been identified as
deposited through several phases of deposition, including regression, transgression,
aggradation and progradation (Houseknecht and Schenk, 2001), with the later producing
significant eastward migration of the Torok shelf margin.

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As far as your last question is concerned, my current thoughts are that with the 88E/Bex rock mechs annmnt this clearly states that the bottom seal is a good one, that is the pebble shale unit or Lower Cretaceous Unconformity (LCU), and that gas kicks/shows were exhibited in the Kuparuk sands which indicates that NO seepage has PROBABLY occurred thru the LCU into the HRZ from the Kuparuk.

This may or may not be the case.

We may find out with the upcoming PSH annmnt as it may apportion migrational characteristics to each of these layers thereby giving a Super Highway migrational route into the Barrow Arch and to the North West Kuparuk Basin

I havnt read the Shultz thesis fully tho Seaace. She may come to other conclusions re these tectonic movements and progradation sequences that may change my assumptions.

The fact of the matter is that the stratigraphic development of the slope was quite complex BUT what really matters is what is there today !!!

The whole basis of the BPMS is that one can effectively track back from the current reservoirs via seismic and log data to give the migrational paths and the original source rocks.


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