Angola is one of the regions considered a global scale Hydrocarbon Province in West Africa. The majority of hydrocarbon accumulations in West Africa are located in Angola and almost of oil production comes from offshore fields. Current oil production as of 2013 accounts for 1.8 million barrels of oil per day (source: EIA)
(source: U.S. Energy Information Administration EIA)
The most prolific sedimentary basins are respectively the:
- Lower Congo Basin and,
- Kwanza Basin
Before
providing a high level overview of the Offshore Angolan Petroleum Systems, let’s beforehand understand
from a generic stand point what a Petroleum System concept is, and how the
ingredients are combined together to generate hydrocarbons.
Magoon and Beaumont define the Petroleum Systems as a unifying concept that encompasses a pod of active source rocks and all genetically related oil and gas accumulations. It includes all the geologic elements and processes that are essential for hydrocarbon accumulation.
The
main essential ingredients of a Petroleum Systems are:
- Source rock
- Reservoir rock
- Seal rock
And the processes of a Petroleum Systems are:
- Trap formation and
- Generation and migration of hydrocarbons
Let
us now provide an overview of how the Angolan margins have evolved.
Geological Settings
The Angolan basin were formed during the opening
of South Atlantic Ocean (Bartolomeu et al.
2010, cited by Martoon et al. 2000; Rosendahl et al. 2005).
(Courtesy
of AAPG Memoir 73)
The
offshore basins (Lower Congo & Kwanza) evolved from an active continental
rift during the early cretaceous into a passive margin that culminated in the
drift stage, which separated African plates from the South Atlantic plates
(Miguel 2012; cited by Machado 2007). They belong to a Mesozoic series of
basins that developed during the late Jurassic to Neocomian times (Bartolomeu
et al 2010).
The
rifting of Angolan margins started around 144 – 140 Ma and continued with the
separation of continents by 127 – 117 Ma.
The tectonic regime associated with the continental separation has been divided into 4 stages:
- Pre-rift
- Synrift
- Transitional
- Early and late drift
units
The
Synrift stage is characterized by graben developments along the early rifted
margin that formed the depocenters for organic rich lacustrine shales. The lake
systems created within the grabens were filled by lacustrine turbidities
arranged laterally and upward into organic rich shales.
The following sections provides from a practical point how the 5 ingredients are encompassed to
generate hydrocarbon accumulations.Source rock
- Rocks rich in organic material mainly derived from algae, bacterial and land plant. Usually evaluated by the high concentration in organic matter, expressed as percentage of TOC (Total Organic Carbon). For more detailed comprehensive information about different types of source rocks please refer to (Miguel 2012).
- Bucomazi Formation source rocks contains a TOC>2% and an excellent hydrocarbon potential S>5
- Hydrogen Index HI>500 and very low Oxygen indices OI<100>
- In the Lower Bucomazi Formation occurs most of the maturity levels with vitrinite reflactance and pyrolysis maturity Ro>0.6
- Migration is the process whereby the hydrocarbons moves
from the source rocks as a result of high pressure and temperature moving
towards a more permeable strata until it gets arrested by a trap seal.
- It is commonly defined into two mechanisms
Secondary migration which is the journey from source rock until it gets trapped.
- Normally the expulsion takes place at vitrinite reflactance R0 of 0.6%.
- The migration along
Reservoir rock
- A reservoir rock needs to possess the following intrinsic properties:
- Closed pore
- Cul-de-sac pore (dead – end pores)
- Catenary
pore
Net to gross which is a measure of the potentially productive part of a reservoir. It is expressed as a percentage of producible (net) reservoir within the overall (gross) reservoir package.
Hydrocarbon saturation which defines the saturation of water, oil and gas within a reservoir. Very commonly pore systems contain both oil and water. The proportions of each phase are usually expressed as percentages.
- Reservoir rocks can be:
Carbonate composed of chemical precipitates. Carbonate reservoirs appear in the form of Limestone or Dolomite.
Reservoir rocks in the shallower areas of the
Most porosity in sandstone reservoirs is preserved primary interparticle porosity. Sandstone reservoirs possess excellent porosities 20-30% and good permeability up to 1000 mD.
- Is the fundamental part of
the trap that prevents the hydrocarbons from migrating onwards through the
rock.
- Seals are commonly
impermeable and porous.
- The commonest types of seals
are shales and evaporites.
- The seals are further sub-divided into membrane seals – those in which hydrocarbons are unable to force its way through the largest pores and, hydraulic seals - those in which hydrocarbons can escape only by creating fractures.
The seals across Lower Congo basin are
transgressive marine shales of Cenomanian-Eocene age.
Trap
- There are several types of
traps where the simplest type is the anticlinal trap
- Traps formed in the
All of the above ingredients played a vital role during generation and maturation of hydrocarbons accumulations in the
References
Angola Report 2014 [Online]. Available at:http://www.eia.gov/countries/cab.cfm?fips=ao
Bartolomeu et al. 2010. “The Combined Effect of Sedimentation Rate and Salt Tectonics on the Angolan Margin. ISBN: 978-989-96923-1-2
Bartolomeu et al. 2010. “The Combined Effect of Sedimentation Rate and Salt Tectonics on the Angolan Margin. ISBN: 978-989-96923-1-2
Gluyas. J. and Swarbrick. R. 2004. “Petroleum Geoscience”. Blackwell
Publishing. ISBN 0-632-03767-9
Magoon. L. B. and Beaumont.
E. A. “Petroleum Systems”. AAPG.
Miguel. A. 2012. “Petroleum Systems Offshore Cabinda .
[Online]. Available at: http://www.grin.com/en/e-book/270708/petroleum-geology-offshore-Cabinda. ISBN 978-3-656-62306-9
Selley, R. C., 1998. Elements of Petroleum Geology, 2nd ed. London
