Bio-Pipeline Networks in the Enhancement of Fluid Flow in Bioturbated Reservoirs

S. George Pemberton

Department of Earth & Atmospheric Sciences, University of Alberta

Edmonton , Alberta , Canada , T6G 2E3

George Pemberton is a world-recognized authority on trace fossil identification and its application in reservoir description and high resolution sequence stratigraphy. He has published extensively on this topic. His research as taken him from studies of modern tracks and trails from non-marine to deep water environments. He has pioneered correlation techniques and an understanding of facies from trace fossils.

George has done many lectures and short-courses for AAPG and has been a member of AAPG for many years. TNK-BP is glad to have the opportunity to sponsor his presentation and share with you new ideas and techniques that can be used to better understand reservoir to regional scale geology and reservoir management.

Abstract:

In the past, trace fossil research in hydrocarbon reservoir rocks was almost always confined to exploration geology; however, recent research is showing that ichnology has significant applications in production geology. Bioturbation is generally considered to be detrimental to the permeability of sedimentary deposits. This is because biogenic churning of laminated sediment may lower sorting characteristics, thereby reducing the sediment's permeability. Current research indicates that substrate controlled ichnofossil assemblages can enhance the permeability and vertical transmissivity of a relatively impermeable matrix. Permeability enhancement develops when burrows into a firm ground are filled with sediment from the overlying strata. If the lithology contrasts with the encapsulating firm ground substrate, anisotropic porosity and permeability is developed. The same concept can be applied to carbonate reservoirs where the burrows are subjected to different diagenetic phases. This again results in anisotropic permeability that can have dramatic effects on reserve calculations. Understanding the flow dynamics of the anisotropic permeability provides a potentially powerful reservoir development tool. The implications of such understanding are far reaching, particularly pertaining to calculations of reserves and deliverability.

Biogenic permeability enhancement can be grouped into five general categories. The first and second include surface-constrained and non-constrained, discretely packaged textural heterogeneities. The third consists of selectively sorted, weakly defined textural heterogeneities. Diagenetic heterogeneity broadly describes the fourth major group; this is most important where bioturbate textures in carbonates are considered. Finally, the fifth classification of burrow permeability enhancement is proposed to comprise cryptically bioturbated sandstones.

Just as important is the recognition that all of the previously outlined categories of burrow-associated heterogeneity generate dual porosity/permeability networks in the flow media. Multiphase transport, and therefore hydrocarbon production, can be complex in such cases. Because burrow fabrics are commonly chaotic, flow paths through burrow-related flow conduits are comparatively tortuous, further complicating geological models. Tortuous, heterogeneous media presents a notable complication for reservoir development. In that burrows may provide flow conduits that interact extensively with the surrounding matrix. Their tortuous nature implies that dead ends and cut-offs may be common. An understanding of how burrow-associated heterogeneities control fluid flow within sedimentary units is necessary if production from bioturbated reservoirs is to be optimized.

In order to illustrate these concepts specific examples will be drawn from reservoirs including the Ordovician Yeoman Fm. in Saskatchewan, the Devonian Wabamun Fm. in Alberta, the Triassic Sag River Fm. in Alaska, the Jurassic Arab D in Saudi Arabia, the Jurassic Bruce Field in the North Sea, the Cretaceous Ferron Fm in Utah, the Cretaceous Milk River Fm in Alberta and the Miocene Mirador Fm. in Columbia.