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Petroleum Geology
William Almon

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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Almon works for ChevronTexaco Exploration & Production Technology Company in Bellaire, Texas, and is a ChevronTexaco Fellow.

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