Exploration Insights September 2019 | Page 18

18 | Halliburton Landmark
and present-day TOC and HI is evaluated
to determine at what level of maturity
petroleum generation initiated.
b. The maximum maturity at the base
of the source rock is mapped and the
immature, mature, or post-mature areas
of the source rock are identified.
c. A kerogen type map is constructed
from analysis of geochemical data from
immature samples. Depending on the
inferred uniformity of the source rock,
this can be a great guide to the original
TOC and HI of mature or post-mature
samples.
d. Average TOC and HI values measured
from mature and post-mature samples
are restored to their original levels,
following an empirical method based
on the observed regression of these
parameters across the oil window with
maturation and generation (adapted
from Cornford, 1998). It is assumed that
the present-day TOC and HI measured
from immature samples will be the
same as the original values. If data are
sparse, assumptions can be made from
the inferred kerogen type and source
rock analogues. Data are combined and
orginal TOC and HI maps are created.
e. The original TOC and HI values must
be equal to or greater than the present-
day TOC and HI values across the
entirety of the mapped area. Spatial
analysis of the maps is carried out and
adjustment made, as required.
Volume of Generated Hydrocarbons
To calculate the volume of hydrocarbons
generated, the method defined by Schmoker
(1994) was selected, which uses the data
discussed in the previous steps. The present-
day and original masses of organic carbon in the
source rock are calculated first; this combines
Mass of organic carbon (M) = (TOC/100) x Density x Volume
Units: M = g TOC, TOC = wt%, density = g/cm 3 , volume = cm 3
Total mass of hydrocarbons generated (HC) =
-6
[(HI original x M original ) - (HI present day x M present day )] x 10
Units: HC = kg hydrocarbons, HI = mg HC/g TOC, M = g TOC
Exploration Insights | 19
the source rock volume and TOC data with
formation density. The mass of hydrocarbons
that could still potentially be generated from the
source rock is then calculated by multiplying
the present-day mass of organic carbon with
the present-day HI values across the map.
This is subtracted from the ultimate mass of
hydrocarbons that could potentially be generated
(if the entirety of the source rock reaches its
generative potential). Conversions from the
mass of hydrocarbons to surface volumes
of oil or gas per unit area can then be made.
These calculated volumes should be taken as a
minimum estimate.
Volume of In-Place Hydrocarbons
The volume of hydrocarbons generated is often
huge compared with what is accumulated, as
<10% of oil generated is typically preserved. The
amount of in-place petroleum accumulated in
the reservoir is the result of numerous factors,
such as: expulsion efficiency; the timings of
generation, trap and seal formation; migration;
and other post-accumulation processes.
The generation accumulation efficiency (GAE)
is the ratio of the volume of petroleum in-
place for the petroleum system to the total
volume of petroleum generated (Magoon and
Dow, 1994). GAEs from analogous known
petroleum systems are applied to the volume of
hydrocarbons generated, in a Monte Carlo-type
simulation. This results in a range of volumes of
petroleum in-place in commercial accumulations,
with probabilities of occurrence. In highly
effective unconventional resource plays any loss
of hydrocarbons may be relatively negligible
and, therefore, a high GAE applied; whereas, for
conventional plays, GAEs may be very low, in
some cases <1%, even for world-class source
rocks (Magoon and Dow, 1994).
THE IMPORTANCE OF CARRYING
OUT VOLUMETRIC CALCULATIONS
Petroleum System Ranking
Systematic application of this workflow to
petroleum systems in different basins will
enable the quantitative comparison of source
rock generative potential and petroleum systems
effectiveness. This ranking will inherently take
the thickness and lateral distribution of the
source rock and the quantity and quality of the
organic matter into consideration. This aids
understanding of the factors that affect the
estimation of volume of hydrocarbons generated
and the subsurface risks associated with the
generation of hydrocarbons.
Additional Benefits
The workflow produces standardized maps of
measured and derived data that are insightful in
themselves and can be incorporated into other
workflows, such as source rock mapping, basin
modeling, unconventional resource calculations,
and play fairway mapping.
The calculations applied in the workflow,
particularly the restoration of the original TOC
and HI, should be considered in any workflow
requiring a source rock assessment.
TESTING THE WORKFLOW
Case Study
The workflow was developed and tested on
the Late Devonian to Early Mississipian Bakken
Petroleum System, Williston Basin, central
North America. The Bakken source rocks and
the equivalent Exshaw/Banff Formation in
Canada have been extensively studied and
geochemically evaluated.
The Williston Basin is a mature basin for
conventional petroleum exploration. More
recently, it has become synonymous with
unconventional exploration due to the Middle
Bakken resource play, which is sourced by the
world-class Lower and Upper Bakken Shale
Member source rocks.
Data from over 780 wells across the Williston
Basin were evaluated. The majority of the data
fall in north-eastern Montana and western
North Dakota as these coincide with the
basin depocenter, the most heavily explored
part of the basin (Figure 2). For the Lower
and Upper Bakken source rocks, over 700
pieces of thickness data, 2,300 TOC results,
2,000 HI results and 1,900 pieces of maturity
data (mainly Rock-Eval pyrolysis Tmax) were
compiled, screened, and statistically reviewed.
A constant formation density of 2.6 g/cm 3 was
used.
The map suite required for the calculation of the
volume of hydrocarbons generated was created
(examples shown in Figure 2). The type of
organic material varies laterally across the basin
in both the Lower and Upper Bakken, due to
changes in the paleo depositional environment
and the origin of the kerogen (Figure 2). Both
source rocks predominantly comprise oil-
prone, marine derived kerogen (Kerogen Type
II), with gas-prone terrestrially derived kerogen
(Kerogen Type III) in the northwest of the basin.
This affects the generative potential of the
source rocks across the basin; however, in the
main depocenter, where the source rocks are
mature to post-mature, the source rocks are
principally Kerogen Type II.
The Lower and Upper Bakken source rocks
have average present-day TOC values of
13.6 and 14.2 wt%, and average present-
day HI values of 305 and 316 mg HC/g TOC,
respectively. Both source rocks are mature
in the depocenter of the basin. Through
restoration of the original source rock
generative potential, it is predicted that the
Data Inputs
Isopach and Preservational
(Areal) Extent
Original Total Organic Carbon
and Kerogen Type
© 2019 Halliburton
Thermal Maturity
Figure 2 > Examples of the data used and maps created to
calculate the volume of hydrocarbons generated from the Upper
Bakken source rock.