The Eagle Ford Shale is a black calcareous shale with a high organic carbon content that
underlies much of southeastern Texas.
In some parts of the area underlain by the Eagle
Ford Shale, heat and pressure have converted organic material within the shale
into oil and natural gas. Between 2008 and the present, the Eagle Ford Shale
has become one of the most heavily drilled rock units in the United States.
The eagle ford was known to contain significant
amounts of hydrocarbon and was thought to be the source rock for much of the
oil and natural gas produced from rock units above it such as the Austin Chalk.
However, the Eagle Ford itself was not known as an oil or natural gas producer.
The rock unit had such a low permeability that oil and natural gas could not
flow through the rock into a production well.
This changed in 2008 when Petrohawk drilled an
Eagle Ford well in La Salle County that had an initial flow rate of 7.6 million
cubic feet of natural gas per day. This well demonstrated that hydraulic fracturing and horizontal drilling could
be used to produce gas from the Eagle Ford Shale.
the Eagle Ford formation :
Three lithologies comprise the bulk of the Eagle Ford Shale in this
1-argillaceous mud rock (shale)
2-calcareous mud rock (marl)
The marls consist mainly of
coccoliths and contain more total organic carbon (TOC) than the other
The strength and rigidity of the
rocks increase with calcite content, the limestones are stronger and more rigid
than the marls.
The shales are divided up into black
calcareous shales and dark gray, very calcareous silty shales that have thin
laminae of forams.
The three main lithologies can be identified using the logs obtained
from different logs as shown below:
The value of Pe of the litho-density log at depth 10000 ft is about
3.3 indicating the presence of a shale formation, while at depths 10500-10850
the value is about 5 indicating that the formation is limestone, that is more
obvious in the figure below:
represents the presence of the lithologies in the Eagle Ford Formation.
The thickness of the Eagle Ford formation and the productive zone
The productive area of the Eagle ford formation is of a thickness
that ranges from 200-300 ft, where the producing formations are the hydrocarbon
bearing shale and some fractured limestones the given data proved that the
producing zone is located at depth starting from 10800 to 11100 ft in some
This can be
identified using data below:
We can notice here that at the intervals from 10800 to 11100
(thickness of 300 ft):
The data shows that that interval may contain hydrocarbon (gas) due
to the following reasons:
resistivity log shows an increase in the resistivity of that interval,
indicating that the lithology has changed from shale (due to low resistivity)
to a permeable formation.
cross over between porosity neutron and density logs show the presence of gas
in that interval.
density log show a sudden decrease in the average bulk density indicating that
the formation is a gas bearing formation.
The log below
shows ensures the presence of gas in the mentioned interval :
The log show an increase in the volumes of gas in the formation in
the intervals mentioned.
From the data given above we can conclude that:
productive formation is of a thickness ranges from 200-300.
hydrocarbon present in the formation is dominantly gas or high GOR oil.
properties and evaluation of the Eagle Ford formation:
The Eagle Ford is typically a laminated, black,
calcareous, organic-rich shale with a very low permeability, that’s why the
hydraulic fracturing is used to induce fractures in the Eagle Ford Shale at
many locations, together with horizontal wells, where the fractures facilitate
the flow of natural gas and oil out of the rock and into the well.
Depending on the core data, The Eagle Ford Shale
and associated rock units were considered to be one of the most successful oil
and gas areas of the United States.
Although the core data analysis showed that the
average permeability of the formation was very low making the reservoir in the
Eagle Ford formation an unconventional reservoir, but it showed that the
reservoir had a very high Total Organic Carbon (TOC) , Giving the reservoir a
high potential to produce large amounts of hydrocarbon using stimulation and
horizontal wells techniques .
The core data were obtained at intervals of
10850-11020 ft, these data were used to evaluate the reservoir and to obtain
the geological properties of the rock such: porosity, permeability &
analysis showed that the Geological properties are:
Average TOC = 3.2 wt%
The average Porosity of the shale samples = 8.21%
Average Permeability of shale =1.644 E-5 mD
Average oil saturation in shale = 39 %
Average gas saturation in shale = 37.4 %
Average water saturation in shale = 23.5 %
The Eagle Ford Formation is considered to be a
very successful oil and gas area , where it is composed of three main
lithologies (shale ,Marl and Limestone) , with a high content of total organic
carbon TOC (more than 3 wt% ) representing a high potential of the formation to
produce oil and gas with the use of stimulation and horizontal wells to
increase the ability of the formation to produce , where the producing shale
formation are of very low permeability that requires fracturing jobs to enhance
the production .