Exploration Insights September 2019 | Page 16

Source rock
volume Extent of
petroleum
generation
process Volume of
generated
hydrocarbons Volume of
in-place
hydrocarbons
Calculated from
lateral and vertical
extent of source
rock Calculated from the
remaining versus
original source rock
generative potential Calculated from
source rock volume
and extent of
petroleum
generation process Calculated from
application of
generation
accumulation
efficiency range
Exploration Insights | 17
16 | Halliburton Landmark
Figure 1 > The principal workflow steps and outputs to calculate the volume of hydrocarbons generated, and an estimation of the in-place
volumes accumulated in reservoirs.
A Geochemical Approach to
Forecasting the Volume of
Preserved Hydrocarbons
By: Rachel Round
Well pads along the Little Missouri River. Photo © Chris Boyer, Kestrel Aerial Services, Inc.
An ongoing challenge in exploration is the
assessment of the hydrocarbon resources
in a basin. A first step in this process is the
calculation of the volumes of hydrocarbons
generated. This article presents a workflow
that uses organic geochemical and geological
data to estimate the volume of hydrocarbons
generated without the need to build a complex
3D basin model. A secondary aim is to estimate
the amount of in-place petroleum likely to
be accumulated in reservoirs. This provides
insights that reduce the charge risk present at
all levels of exploration maturity, allowing basins
to be ranked in terms of their potential. The
methodology is illustrated by a case study of
the Bakken Petroleum System, Williston Basin.
Neftex ® Insights portfolio, principally the Neftex
Organic Geochemistry Database, to generate a
suite of maps that inform on the source rocks
within a petroleum system on a basin scale.
These maps are derived from widely available,
or inferable, data in agreement with the mindset
of Schmoker (1994), who highlighted that
calculations such as this should be applicable
to a diverse set of petroleum systems, with
variable data availability. The individual source
rock maps created are:
» » Preservation extent
» » Isopach
» » Organic matter variation via kerogen
type
ESTIMATING THE VOLUME OF
HYDROCARBONS » » Present-day total organic carbon (TOC)
Workflow Overview » » Original TOC
A workflow (Figure 1) suitable for non-specialists
has been developed that pulls together public
domain data and understanding held across the » » Original HI
» » Present-day hydrogen index (HI)
» » Basal maturity
The mapped data are combined in a series of
simple equations to calculate the source rock
volume, the extent of the petroleum generation
process, the volume of hydrocarbons generated,
and ultimately the amount of in-place petroleum
preserved in commercial accumulations in
reservoirs.
Source Rock Volume
Calculating the source rock volume (from lateral
and vertical extents) is the first step in the
workflow. Source rocks with facies variation may
need to be dealt with as more than one unit.
The lateral distribution of the source rock is
mapped by compiling the organic-rich facies
in all relevant Neftex Gross Depositional
Environment (GDE) maps, and reducing these
to honor observations from data held across the
Neftex portfolio; such as source rock occurrence
data, depth analysis maps, well and outcrop
picks, surface geology, tectonic elements, cross
sections, seismic, and isopachs. Additionally,
Neftex Depth Analysis frameworks map can be
created for the specific source rocks.
The vertical distribution of the source rock is
presented as an isopach map. This is generated
from thickness data, such as well and outcrop
data, cross sections, seismic data, isopachs, and
data with depths tied to the source rock in the
Neftex Organic Geochemistry database.
The calculated source rock volume can be
integrated with the maturity map to define the
volume of mature or post-mature source rock.
Extent of Petroleum Generation
Process
Assessing the extent of the petroleum
generation process is critical to estimating
the volume of hydrocarbons generated. This is
achieved by assessing the amount of kerogen
that has been converted to petroleum (kerogen
decomposition). The difference between the
original (pre-maturation) and remaining (present-
day) generative potential of the source rock is
calculated in the following steps:
1. Data processing: Geochemical data are
compiled, principally from the Neftex
Organic Geochemistry database, and
screened to determine whether they
are applicable. If the data coverage
is sparse or patchy, inferences can
be made from knowledge of the
depositional environment and geological
history of the basin. Statistical analysis
is carried out on the data as numerous
pieces of geochemical data can be
recorded at any one location.
2. Remaining source rock generative
potential: The variation in the present-
day quantity and quality of organic
matter in the source rock is assessed
by mapping the average present-day
TOC and the average present-day HI.
This enables any variations across the
source rock to be identified that will
affect its generative potential.
3. Original source rock generative
potential: The variation in the original
quantity and quality of organic matter in
the source rock is assessed by mapping
the average original TOC and the average
original HI. Restoration of the original
TOC and HI requires present-day TOC,
present-day HI, kerogen type, and
maturity of the source rock to be known,
and the following steps to be followed:
a. The relationship between maturity