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The big news in petroleum geology could very well sprout from research at ExxonMobil
that could help provide a new foundation for understanding the process of how
sedimentary bodies evolve. This understanding is key for predicting where oil
reservoirs might be found. The work could also give new impetus to the concept
of fractals in geoscience.
Hydrocarbon reservoir performance is controlled by the spatial distribution
of properties ranging from pore network aspects seen at the scale of a core
plug to the distribution and connectivity of sand bodies that are only resolvable
at large seismic scales. Studies of modern environments and outcrops have long
been used to create analogs that span this range of scales. But always, uncertainty
remains over how applicable a modern analog is to a specific subsurface dataset.
Thus, emerging exploration and production technology has been focused on reducing
uncertainty in describing a reservoir and in developing the clearest possible
picture of reservoir geometry and connectivity.
During the annual convention of the American Association of Petroleum Geologists
in May, the ExxonMobil researchers reported results from a multiyear study of
an integrated suite of datasets that included modern and ancient (outcrop and
subsurface) depositional systems, high-resolution 3-D seismic, laboratory experiments
and numerical modeling. Their work could be a major step forward in understanding
reservoir geology.
They propose that the clastic sedimentary rock record is built of scale-invariant
hierarchies of sedimentary bodies. These bodies are deposited by jet-plume pairs
as they dissipate excess kinetic energy and minimize gradients. If the team
is are correct, the findings could provide a fundamental understanding for describing,
interpreting and predicting the types and distributions of sedimentary bodies
within a unifying framework. Such a framework would be significantly more useful
than the narrower perspective of depositional environment or scale.
These results also have important implications for increasing the ease with
which reservoir models can be built and for reducing the uncertainty in the
prediction and geologic modeling of hydrocarbon reservoir properties. This is
especially true if a physics-based jet-plume pair numerical model can be used
to model sand body shape and to understand the link between 3-D-sediment body
shape and the grain-size distribution and related parameters such as permeability.
They presented the hypothesis that most coarse-grained siliciclastic deposits
are composed of bodies with a strong commonality in shape, internal structure
and property distribution and that these similarities extend over a range of
depositional environments, depositional styles and dimensions. The commonality
arises from the fact that they all arise from turbulent jet deposition (D.C.
J. D. Hoyal et al. 2003 AAPG Annual Convention).If this hypothesis is correct,
then reservoir models can be built of such objects without reliance on any specific
analog.
Combining detailed measurements on several hundred sedimentary-body shapes,
from a variety of depositional environments, perimeter/arithmetic length cross
plots and box counts of shape parameters, shows that the shapes are self-affine.
Area/geometric length cross plots and principal component analyses show that
these shapes are statistically similar. The measured bodies range in length
from 1,000 kilometers to less than 10 centimeters and span 14 orders of magnitude
of area (John C. Van Wagoner, et al.). A three-component model including length,
and two parameters which characterize width variation along the central axis,
was able to fit 88 percent of body shape variability from the population (Paul
A. Dunn et al.).
These data clearly suggest that something more fundamental than depositional
environment and scale may be acting as a first-order control on the properties
of coarse-grained siliciclastic bodies. The self-affine nature of the sand body
shapes further indicates that siliciclastic strata are arranged into bundles
of nested, hierarchical bodies or deposits. The shape of these deposits is independent
of scale and place of deposition because they are all deposited by common physical
processes (D.C. J. D. Hoyal et al.).
According to the ExxonMobil investigators, non-equilibrium open systems (i.
e. systems through which energy and matter are transmitted) evolve toward increasing
complexity as they attempt to reach equilibrium. They evolve by creating dissipative
structures that rid the system of excess kinetic energy in order to minimize
gradients. A tree or leaf shape, for example, is most efficient in transferring
the entropy that dissipation transfers to the surrounding environment.
The most common mechanism to decelerate a flow {emdash} spatial expansion {emdash}
creates flow separation, shear layers and a turbulent jet. The jet (or jet-plume
pair) is a branching structure defined as "an internally driven flow from
an orifice or region of flow constriction that expands and decelerates through
turbulent flow entrainment into a body of the same or similar fluid," the
team reported. According to these investigators, turbulent jet deposits may
be the most fundamental and ubiquitous identifiable bodies in the clastic sedimentary
record. Jets produce sedimentary bodies such as current rippled beds, troughs
and planar cross beds. They also play a major role in forming bars in rivers,
as well as delta mouths and deepwater fans. Jet deposits, regardless of scale,
exhibit one or more of the following properties: 1) thickness and grain size
decay approximately exponentially in the direction of flow and a Gaussian distribution
across the body; 2) a region of erosion and bypass bounded by levees near the
orifice; 3) a proximal region of bed-form deposition and a distal region of
suspended load deposition; 4) the distribution of bedding types changes predictably
with the radial distance from the orifice (John C. Van Wagoner, et al.).
Apart from the expansion angle, jet-plume pair deposits exhibit a large degree
of similarity in all environments where jets dominate over ambient flow and
where re-working by other processes has not obscured them. The properties of
these jet-plume pair deposits are strongly correlated to their shape through
the turbulent flow field and depositional process (D.C. J. D. Hoyal et al.).
Physics-based models of jet-plume pair deposition can be used to understand
the link between 3-D-sediment body shape and the grain-size distribution, a
link that which appears to be fundamental to the turbulent deposition process
(R.T. Beaubouef et al.).
Over time, jet deposits evolve into more complex sedimentary bodies in which
the jets migrate to the periphery of the body where they can most effectively
dissipate energy. Depositional experiments suggest that sedimentary bodies pass
through three evolutionary phases. The initial phase is a single jet and jet
deposit. The jet deposit evolves into a deposit that mimics a leaf pattern and
finally a tree-like deposit. This evolution is reflected in the scale of different
deposits. Small features, such as current ripples and cross beds, are deposited
by single jets of brief duration. Bars and other intermediate scale features
form from more persistent jets. Larger scale sedimentary bodies, such as deltas
or fans, evolve in response to multiple flows from a persistent primary orifice
(John C. Van Wagoner, et al.).
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