gravity gradiometry: technology and feasibility for

VIII INGEPET 2014 (GEO EX)
GRAVITY GRADIOMETRY: TECHNOLOGY AND FEASIBILITY FOR HYDROCARBON
EXPLORATION
Vsevolod Egorov, Feargal Murphy, Paul Versnel, Jessica Rands (ARKeX)
ABSTRACT
The search for new hydrocarbons is becoming more challenging and difficult as many “easy-to-find”
fields have been discovered and exploited. There is a quest for cost-effective and environmentally
responsible exploration technologies.
Airborne gravity gradiometry has emerged as one of these developments, proving itself in a number
of petroleum basins. Over the past few years, gravity gradiometry played a prominent role in recent
major discoveries within the East African Rift System. Gravity gradiometry offers a high resolution,
wide bandwidth gravity signal and has attracted attention of both the hydrocarbon and mining
exploration industries.
The technology is based on measuring directional gradients of the gravity field, with a considerable
signal-to-noise ratio improvement over conventional (scalar) airborne gravity. As a result, more
detailed variations in the subsurface density distribution are measured and thereby geological
structures are mapped in greater detail with all the benefits in efficiency of airborne acquisition.
Gravity gradiometry data and the resultant interpretation will high grade areas for further exploration
by assisting in the correlation of structures away from existing seismic coverage and optimal
positioning of new 2D and 3D seismic surveys. Gravity gradiometry also provides additional
constraint for integrated interpretation of geophysical and geological data, thereby reducing
interpretational ambiguity often associated with interpretation of data in isolation.
This work introduces the fundamental aspects of the technology and provides model responses for
geological targets as might be expected in western South America.
The feasibility work will demonstrate the applicability of gravity gradiometry in resolving such
geological features as shallow basement relief, intra-sedimentary folds, thrust belt architecture and
other structures of exploration interest.
GRAVITY GRADIOMETRY
The gravity method has been part of mineral and petroleum exploration since the early twentieth
century. Although, and especially in petroleum exploration, gravity technology forms a fraction of
the overall program cost, it plays a vital role at its different stages, from regional geological
assessment of the area and designing the follow-up seismic surveys to integrated interpretation of
data leading to prospects identification.
Since the first application of gravity method, instrumentation, processing and interpretation
techniques have been in constant development providing significant improvements of data quality
and information. Nabighan et al. (2005) provide an excellent review of the method’s history.
The introduction of gravity gradiometry to hydrocarbon exploration in the mid-1990s was a major
milestone in the method’s history. Today two commercially available gravity gradiometry systems
are developed by Lockheed Martin, with a number of other groups working on developing new
VIII INGEPET 2014
2
instruments. The Lockheed Martin FTG (full-tensor gradient) system is utilized for land, marine and
airborne surveys with most of these conducted as airborne (with the first airborne survey initiated in
the early 2000s).
The most-important advantage of gravity gradiometry over conventional gravity is that due to the
instrument design, the system is relatively insensitive to aircraft accelerations. In a nutshell,
whereas conventional gravity measures only one vertical component of the gravity (Gz), Full
Tensor gravity gradiometry measurements result in 6 independent curvature components from
which a Cartesian tensor can be derived. As gravity is also measured directly, FTG delivery
includes Gz, Gzz, Gzx, Gzy, Gxx, Gxy, Gyy, Gyz (Figure 1), increasing the resolution of data (e.g.
Saad, 2006). Another implication of the full tensor measurements is that anomalous sources can be
more accurately mapped even when they are located away from the survey flight line (Barnes et al.,
2012).
The time and cost-effectiveness of airborne FTG surveys is a key benefit. Gravity gradiometry has
been successfully applied in various geological settings, notably the East African Rift System where
it has been instrumental in recent major discoveries. In the appropriate geological setting, FTG
offers a significant acceleration of the exploration timeline at a much reduced cost. (e.g., Doherty et
al., 2013; and Price et al., 2013). The method has also provided value in other geological settings,
for example allowing mapping of salt distribution in Gabon and offshore NE Greenland. The method
has also been applied in more complex areas such as thrust and fold belts constraining poorly
imaged structures interpreted from seismic in the Oman Thrust Belt (e.g., Protacio et al., 2010).
GEOLOGICAL SETTING
The majority of the western South American sedimentary basins are located within two distinct
zones: Pacific margin and sub-Andean foreland, east of the Andes. From the tectonic setting point
of view these basins could be divided into the following categories (from west to east): trench slope
basins, accretionary prism, forearc-, intermontane- and foreland basins as illustrated in the
generalized section of Peru (Figure 2).
Table 1 provides examples and classifies basins based on their tectonic type. Notice that the same
basin could fit in more than one category due to its tectonic history and size. They could be subclassified further based on the tectonic details and factors affecting their creation, for example, pullapart basins along the strike-slip faults, salt movement and structural controls of pre-existing
features. Some of the basins have experienced more than one stage of tectonic deformations,
which leads to diversity of geological structures and plays. For example, some of the intermontane
basins and thrust fold belts in the Andes preserve remnant features of the older back-arc basins
(e.g., Late Jurassic – Early Cretaceous Rocas Verdes Basin of the southern South America). It is
also well-documented that pre-Cretaceous (pre-Andean) extensional grabens are preserved within
the Sub-Andean foreland basins and filled with Paleozoic and Early Mesozoic sediments (e.g.,
Bolaños, 2013; Stewart et al., 2013). Among other publications, Klein et al. (2011) and Zúñiga y
Rivero et al. (2010) provide excellent overview of the Peruvian basins and their petroleum potential.
VIII INGEPET 2014
3
FEASIBILTY MODELING
One of the most efficient methods to assess applicability of the technology is by constructing a
feasibility model. At the first stage, geological targets and their physical properties are determined
and the earth model is built. Ideally, the model is based on an accurate depth-converted and
interpreted seismic section or derived from seismic and well data combined with geological crosssection(s). If these data are available, a density model can be constructed based on direct well
measurements or derived from velocity data. Most-commonly the 2D section is a starting point of
the model. It can be extended to a pseudo-3D model to evaluate model response in map view.
Another important factor to be tested by the feasibility modeling is survey design parameters.
Ultimate target of this exercise is to design the survey parameters that allow mapping of the
geological features of interest at the lowest possible cost. Thus, besides geological model, line
spacing and orientation, flight altitude, speed of aircraft and instrumental noise are all part of the
calculations and tests.
During this work two models were constructed based on published geological cross-sections.
Hypothetical strike-slip (transfer) faults were introduced to both models to illustrate and model the
response to possible and geologically-common offsets of structural features. The first model (Figure
3) captures the main features of the Pacific margin basins, i.e. forearc and trench slope basins. The
geological cross-section along seismic line 93-75 located south of the Lima basin (Bolaños, 2013)
was selected for this exercise. Modeled FTG response (Gzz) clearly shows that FTG data allow
resolution of proposed Cretaceous depositional centers (Gzz low), suggested to contain mature
source rocks in the coastal basins. The Cenozoic depocenter is also clearly mapped (Gzz low). If
significant thickness of sediments was accumulated in these depocenters, then they could also be
within the maturation and hydrocarbon generation window. Gzz highs are observed over basement
highs separating sedimentary depocenters. Mapping of these highs has direct implication for
hydrocarbon plays identification. Commonly petroleum deposits are found in the structural and
stratigraphic traps associated with the basement highs. In the overlying section, development of
possible reefs could be also related to these highs.
The second model was built to investigate the application of FTG within the foreland basin and,
particular, its proximal part. This model is based on the regional cross-section across the Ucayali
basin (Le Vot and Froute, 1999). The fold and thrust belts (FTB’s) and compression-formed
structures of the foreland’s proximal part are proven areas of petroleum discoveries within many
basins of Sub-Andean foothills. The mapping of these structures is one of the first exploration
objectives in the foreland basins. Though many of the largest structures have been found,
understanding of the structural detail and exact extent could be improved by the addition of FTG.
The feasibility modeling across the Ucayali basin confirms the capability of FTG to map the
structural detail in this setting.
CONCLUSIONS
The application of the FTG technology is a time- and cost-effective exploration tool. This is
confirmed in a number of sedimentary basins around the world with the East African Rift system
being the most remarkable example. The FTG principles demonstrate improvement of data
resolution in comparison to conventional gravity, though understanding of geology and careful
feasibility modeling are required to understand its applicability in any particular setting. As
demonstrated in this work, use of FTG may allow mapping of geological structures of exploration
interest. This is encouraging. However, it is important to understand that FTG in itself is not an
VIII INGEPET 2014
4
ultimate answer, but it is another, often very powerful, piece of information to be integrated in the
exploration process.
REFERENCES
Doherty, J.T., S. Bennett, F. Martini, G. Melady, E. Rogers and K. Simpson, 2013, Integrated
geophysical exploration in the frontier East Africa Rift system, in Exploration of continental rifts:
from regional to prospect level, SEG Annual Meeting, Gravity and Magnetics Workshop.
Barnes, G.J., 2012, interpolating the gravity field using full tensor gradients measurements, First
Break, vol. 30, 97-101.
Protacio, J.A., J. Watson, F. Van Kleef and D. Jackson, 2010, The value of integration of gravity
gradiometry with seismic and well data – an example from a frontal thrust zone of the UAE-Oman
Fold Belt, EAGE Workshop, Barcelona.
Bolaños, R., 2013, Licitación pública internacional para otorgar contratos de licencia offshore,
Perupetro, Información Técnica, Bidding Round Peru 2013.
Klein, G.D., F.J. Zúñiga y Rivero, H. Hay-Roe and E. Alvarez-Calderon, 2011, A reappraisal of the
Mesozoic/Cenozoic tectonics and sedimentary basins of Peru, AAPG, Search and Discovery Article
#10332.
Le Vot, M. and Froute, J-Y., 1999, Peruvian foothills: Exploration in a frontier area, INGEPET ’99,
EXPR-1-MV-18.
Nabighian, M., Ander, M., Grauch, V., Hansen, R., LaFehr, T., Li, Y., Pearson, W., Peirce, J.,
Phillips, J., and Ruder, M., 2005, Historical development of the gravity method in exploration,
Geophysics, 70(6), 63ND–89ND. doi: 10.1190/1.2133785.
Price, A., A. Cacheux, P.E. Lardin, P.R. Chowdhury, G. Shields, J. Weber and R.Yalamanchili,
2013, Airborne gravity gradient for oil exploration in Uganda, in Exploration of continental rifts: from
regional to prospect level, SEG Annual Meeting, Gravity and Magnetics Workshop.
Saad, A.H., 2006, Understanding gravity gradients – a tutorial, The Leading Edge, August, 942949.
Stewart, M., A. Mantilla-Pimiento, S.Mazur, C. Macellary, S.Campbell, 2013, pre-Andean orogeny
rift basins and pre-Cretaceous sediment thickness, in Exploration of continental rifts: from regional
to prospect level, SEG Annual Meeting, Gravity and Magnetics Workshop.
Zúñiga y Rivero, F.J., G.D. Klein, H. Hay-Roe, and E. Álvarez-Calderon, 2010, The hydrocarbon
potential of Peru: Lima, Peru: BPZ Exploración & Producción S.R.L., 338 p.
VIII INGEPET 2014
5
G zz
z
G xz
G zz
G yz
z
G xz
G yz
G zy
gz
G zx
gy
gx
x
y
G zy
gz
G yy
G zx
G xy
x
G yx
gy
gx
y
G yy
G xy
G yx
G xx
G xx
Figure 1. Gravity field and its components. Conventional gravity measures one vertical component
(Gz). Full Tensor gravity gradiometry measurements result in 6 independent curvature components
from which a Cartesian tensor can be derived. As gravity is also measured directly, FTG delivery
includes Gz, Gzz, Gzx, Gzy, Gxx, Gxy, Gyy, Gyz, increasing the resolution of data.
Accretionary
Complex
Forearc
Basins
Intermontane
Basins
Foreland Basins
Trench Slope
Basins
Volcanic
Arc
Continental Crust
Figure 2. Types of the Mesozoic/Cenozoic sedimentary basins in the western South America.
VIII INGEPET 2014
6
BASIN TECTONIC TYPE
BASIN NAME
TRENCH SLOPE
PIMENTEL, PARACAS
FOREARC / TRENCH SLOPE
PROGRESSO, TALARA, LIMA
FOREARC
TUMBE, LANCONES, SECHURA, SALAVERRY,
TRUJILLO, HUACHO, PISCO, MOLLENDO,
MOQUEGUA
INTERMONTANE / FTB AND FORELAND
BAGUA, SANTIAGO, HULLAGA, ENE, TITICACA
FTB AND FORELAND
MARAÑON, UCAYALLI, MADRE DE DIOS
Table 1. Tectonic types of sedimentary basins in the western South America and their examples in
Peru (based on Bolaños, 2013, and Zúñiga y Rivero et al., 2010).
Gravity High over
Basement Highs
Gravity Low over
Cretaceous Depocenter
Gravity Signal over
Basement/Sediments Faults
Gravity Low over
Cenozoic Depocenter
Offset of Anomalies along
Strike-slip Faults
EO-OG
MI
Pε-EO
K
Basement
Figure 3. FTG feasibility model of the Pacific margin basins. Pseudo-3D model built based on westeast seismic line 93-75, southern Lima Basin.

Download Report