Idea Transcript
Bioturbation: Reworking Sediments for Better or Worse
Murray K. Gingras S. George Pemberton University of Alberta Edmonton, Alberta, Canada
Petroleum geologists are interested in bioturbation because it reveals clues
Michael Smith Maturín, Venezuela
and flow dynamics.
Oilfield Review Winter 2014/2015: 26, no. 4. Copyright © 2015 Schlumberger. FMI is a mark of Schlumberger. 1. Ali SA, Clark WJ, Moore WR and Dribus JR: “Diagenesis and Reservoir Quality,” Oilfield Review 22, no. 2 (Summer 2010): 14–27. 2. Al-Hajeri MM, Al Saeed M, Derks J, Fuchs T, Hantschel T, Kauerauf A, Neumaier M, Schenk O, Swientek O, Tessen N, Welte D, Wygrala B, Kornpihl D and Peters K: “Basin and Petroleum System Modeling,” Oilfield Review 21, no. 2 (Summer 2009): 14–29.
about the depositional environment. Bioturbation can also destroy or enhance porosity and permeability, thereby affecting reservoir quality, reserves calculations
Sediments undergo several modifications to become the source rocks, reservoirs and seals that generate and contain petroleum reserves. The changes that occur between deposition and lithification, collectively known as diagenesis, include the processes of compaction, cementation, dissolution and recrystallization.1 But before any of these occur, another process can considerably affect rock properties. As soon as they are deposited, sediments can be altered by bioturbation: the disruption of sediment and soil by living things.
Bioturbation is typically a small-scale but potentially significant geologic process that may occur wherever plants or animals live. It can take several forms, including displacement of soil by plant roots, tunnels created by burrowing animals and footprints left by dinosaurs (next page). Of most interest to the oil and gas industry are the changes brought about by organisms that are active near the water/sediment interface in marine settings. Such activities are typically limited to a meter or so in depth but may cover an area of tens to hundreds of square kilo-
> Surface expressions of burrows under the surface. As the tide retreats at the Bay of Vallay, North Uist, Scotland, small wormlike animals burrow into the soft, silty sand searching for food. By the thousands, they create shallow tunnels but leave waste on the surface (left). In this example, the fecal piles cover an area of at least 5 km2 [2 mi2] (right).
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> Bioturbation on the surface and in the subsurface. Bioturbation includes animal imprints and tunnels created by burrowing animals. The photographs of the crab burrow (left ) and the ant nest (middle) are from the sandy backshore of beaches near Savannah, Georgia, USA. (Photographs courtesy of Murray K. Gingras.) The photograph of the dinosaur footprint (right ) is from Dinosaur State Park, Connecticut, USA.
meters. Understanding the behaviors of these animals helps geologists characterize the environmental conditions prevalent during a brief interval of geologic time: after the sediments were deposited, but while they were still soft enough to deform. For many years, bioturbation studies found application mainly in exploration geology—in estimating paleobathymetry, assessing depositional environment and identifying key stratigraphic surfaces. These are all important inputs to the geologic models used for determining potential source rock and reservoir quality and for modeling basins and petroleum systems.2
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Recently, however, geologists have expanded the application of bioturbation to address production geology challenges. Animal activity in sediments disrupts layering, creates flow pathways, enables exchange of minerals and fluids between sedimentary layers, changes pore fluid chemistry and adds or removes organic matter. These changes can facilitate or impede mobility of diagenetic fluids, increase or decrease porosity and permeability and alter permeability homogeneity and isotropy. Recognizing Oilfield Review AUTUMN 14 them in reservoir these effects and including Bioturbation Fig. 1production presimulation models can improve ORAUT14-BIOT 1 dictions and enhanced oil recovery operations.
This article describes ways in which animal activity can affect sedimentary deposits and focuses on reservoir rocks. Examples from both siliciclastic and carbonate formations show how geologists use this information to infer ancient environmental conditions and characterize present-day formation properties. Life Just Under the Surface Animals that live near the water/sediment interface often leave evidence of their lifestyles. For example, surface expressions of subsurface bioturbation can be discerned in the intertidal zone of a beach (previous page). In
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> Traces, shafts and tunnels. Marine animals that live at or near the sediment/water interface leave traces of various shapes, sizes and complexity. (Adapted from Gingras et al, reference 3.)
Higher Energy Dynamic Habitats Escaping (Fugichnia)
Dwelling (Domichnia)
Crawling (Repichnia)
Lower Energy Stable Habitats Feeding (Fodichnia)
Farming (Agrichnia)
Grazing (Pascichnia)
Oilfield Review AUTUMN 14 Bioturbation Fig. 3 ORAUT14-BIOT 3
> Traces of animal behavior. Ichnologists interpret traces to indicate animal activities such as escaping, dwelling, crawling, feeding, farming and grazing, among others. Traces may be variations or combinations of these. The behaviors are loosely associated with depositional settings of higher energy (top) and lower energy (bottom) and may be considered a continuum. A variety of species might produce similar structures if their activities are similar. A single species can create several kinds of traces while performing different activities and the traces may vary if made in different substrates. (Adapted from Gingras et al, reference 3.)
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this case, thousands of piles of sand-rich fecal coils dot the floor of a shallow bay. These fecal strands are produced by burrowing, wormlike creatures that take in the bulk sediment, ingest nutrients and excrete the indigestible rock grains. Their subsurface burrows may be tens of centimeters deep, and an assemblage or community of these organisms can affect an area of several square kilometers. Infauna, or animals that live in sediments—clams, tubeworms, crabs and shrimp, for example—can disrupt sediments in many ways (previous page, top). They may create tubelike tunnels and shafts of varying inclination. These burrows may be simple, shallow unlined holes or may have compacted walls, be lined with contrasting material or have multiple openings. The burrows may remain open for a period of time, collapse or be filled immediately with similar or contrasting sediments (right). Tunnels in somewhat consolidated sediments have a better chance of staying open than those in softer sediments. Some infaunal activity can cause complete mixing of a volume of sediment but leave no detectable traces. For example, animals foraging in layered sediments may disrupt the substrate so completely that the layering is no longer visible, causing the sediment to appear to be one massive, homogeneous interval. Aquatic animals that live on the sediment surface, epifauna, can also leave traces of their activity. Although these animals—mussels, sea stars, flounder and some crabs—may not burrow or modify the sediments to a great degree, they may leave evidence in the form of furrows and other tracks. In the rock record, bioturbation manifests itself mainly as fossilized traces of animal activity: fossilized imprints, tracks, excavations, dwellings or waste products. The study of these traces is the field of ichnology. This specialty focuses on using trace fossils, or ichnofossils, to decipher paleoecological aspects of sedimentary environments. The types, number and variety of traces may help geologists determine aspects of the depositional environment such as whether sediments were deposited quickly or slowly or in shallow or deep marine or nonmarine waters. 3. Gingras MK, Bann KL, MacEachern JA and Pemberton SG: “A Conceptual Framework for the Application of Trace Fossils,” in MacEachern JA, Bann KL, Gingras MK and Pemberton SG (eds): Applied Ichnology. Tulsa: Society for Sedimentary Geology, Short Course Notes 52 (2009): 1–26. The Latin words for these trace fossils—fodichnia, domichnia, fugichnia, cubichnia, repichnia, pascichnia and agrichnia—are used to classify them according to behavior.
Ichnofossils are interpreted to be related to animal survival strategies associated with sedimentary and environmental conditions. They are different from body fossils in that they represent a behavior or activity, not a particular organism. Only infrequently, such as in the case of some dinosaur footprints, can ichnologists identify the animal species that created an ichnofossil. Instead, they attempt to deduce what the animal was doing when it created the trace.
By studying trace fossils, ichnologists have identified several types of animal behavior, including feeding, dwelling, fleeing, resting, crawling, grazing and farming (previous page, bottom).3 Depending on the activity, the associated traces may be found on the sediment surface—which eventually becomes the interface between two layers—or within a sediment layer. Ichnologists use the evidence of these behaviors to characterize the paleoenvironment of a rock layer.
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> Contrasting fill. This burrow in fine-grained sediment is filled with coarse-grained material. This U-shaped trace is interpreted to be the dwelling burrow of an annelid or a crustacean in a low-energy shoreface or sandy tidal flat environment. (Photograph copyright S. George Pemberton.)
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M Mottled or M Massive Appearing
Burrowed
Pervasively burrowed
Mud
Sand
Unburrowed or Laminated
Freshwater or Oxygen depleted
Cross-bedded
Lacustrine, quiescent bay or deep marine
High sedimentation rate
Laminated
Load-casted bedding contacts and abundant organic detritus
Quiescent bay or lagoon, possibly tidal flat
Freshwater or High sedimentation rate
Penecontemporaneous deformation observed in association with massive media
Moderate to rare, evenly distributed
Sand
Burrowed contacts at top or bottom of massive-appearing sediment, or vestigial burrows evident locally
Large, diverse
Distal prodelta open bay
Small
River influenced delta, restricted bay or estuary
Moderate to rare Sporadically distributed
Mud
Probably shallow marine or marginal marine Probably inner shelf fine-grained intertidal flat with low tide range or (less likely) shallow lacustrine
Laminated to scrambled
High sedimentation rates and variable depositional conditions
Small
Sediment gravity flow
or
High sedimentation rates, good food resource and generally consistent conditions
Large, diverse
Downward hydraulic jump
or
or Fluvial, fluviolacustrine or deltaic
True cryptic bioturbation bedding
or
Inner shelf offshore
Large, diverse ichnofossils Small ichnofossils, low diversity
Freshwater
Planar
Unimodal distribution of grains, no mineralogic diversity of grains
Low sedimentation rates and good food resource
Event sedimentation generally dominated (temporally) by fair-weather processes Proximal prodelta or delta front bay mouth complex Inner estuary tidal channel
Large, diverse
Lower shoreface
Small
Bays or deltas Rarely point bars
> Interpreting depositional conditions from bioturbation texture. Classifying sedimentary textures into three types—unburrowed or laminated, burrowed and mottled or massive appearing—helps ichnologists infer depositional environment. (Adapted from Gingras et al, reference 3.)
A basic way of interpreting sedimentary rocks is to divide them into three main types of lithified sediment: unburrowed, burrowed and massive appearing (above).4 Classification of these types serves as the starting point for interpreting the depositional conditions under which such sediments formed.
Unburrowed—Sediments that are relatively undisturbed, such as those with original layering intact and with little or no evidence of bioturbation, are usually ascribed to one or more of the Oilfield environments: Review following depositional AUTUMN 14 • freshwater, where there areFig. few6 deeply burrowBioturbation ing organismsORAUT14-BIOT 6
• anoxic settings (poorly oxygenated) • constantly shifting sediments on the seafloor • high sedimentation rates • arid or frozen locales. Unburrowed sandy sediments usually indicate freshwater deposition or shifting sedimentation. However, many continental environments do
4. Gingras et al, reference 3. 5. Buatois LA and Mángano MG: “Animal-Substrate Interaction in Freshwater Environments: Applications of Ichnology in Facies and Sequence Stratigraphic Analysis of Fluvio-Lacustrine Successions,” in McIlroy D (ed): The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. London: The Geological Society, Special Publication 228 (2004): 311–333.
6. Hickey JJ and Henk B: “Lithofacies Summary of the Mississippian Barnett Shale, Mitchell 2 T.P. Sims Well, Wise County, Texas,” AAPG Bulletin 91, no. 4 (April 2007): 437–443. Loucks RG and Ruppel SC: “Mississippian Barnett Shale: Lithofacies and Depositional Setting of a Deep-Water Shale-Gas Succession in the Fort Worth Basin, Texas,” AAPG Bulletin 91, no. 4 (April 2007): 579–601. O’Brien NR: “The Effects of Bioturbation on the Fabric of Shale,” Journal of Sedimentary Petrology 57, no. 3 (May 1987): 449–455.
7. Taylor AM and Goldring R: “Description and Analysis of Bioturbation and Ichnofabric,” Journal of the Geological Society 150, no. 1 (February 1993): 141–148. 8. A colonization event occurs when one or more species spread to a new area. 9. Pemberton SG, MacEachern JA, Gingras MK and Saunders TDA: “Biogenic Chaos: Cryptobioturbation and the Work of Sedimentologically Friendly Organisms,” Palaeogeography, Palaeoclimatology, Palaeoecology 270, no. 3–4 (December 15, 2008): 273–279. 10. Gingras et al, reference 3.
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exhibit trace fossils.5 Unburrowed fine-grained laminated sediments rich in clay and silt are typically interpreted to result from sedimentation in freshwater or anoxic conditions, although high sedimentation rates might yield the same result. Many organic-rich source rocks, some of which are targets of tight oil and gas shale plays, are examples of fine-grained sediments deposited in environments with a low oxygen supply. Because such depositional environments are not welcoming to many animals, the sediments may exhibit layering and ordered clay grains and show little or no bioturbation.6 Burrowed—Categorization of burrowed media is based on the distribution of ichnofossils and characteristics—primarily size and diversity—of the ichnological assemblage. Ichnologists have developed a bioturbation index (BI) to describe the degree to which sediments exhibit bioturbation.7 The index classifies, on a scale of zero to six, the abundance of traces and amount of trace overlap (right). The BI is related to the duration of colonization events and, through them, to rates of sedimentation.8 Highly to completely burrowed sediments are evidence of both a significant infaunal biomass and conditions of slow sediment accumulation. Moderate to sparse bioturbation, characterized by evenly distributed trace fossils, indicates a lower infaunal biomass and higher sedimentation rate. The size and diversity of ichnofossils in burrowed media reflect the chemical aspects of the depositional waters. For example, in marine deposits, large trace fossils are indicative of high dissolved oxygen content and stable ocean salinity. A preponderance of small trace fossils suggests salinityor oxygen-stressed environments. High diversity of fossil types is related to oxygen content and salinity and also indicates abundant nutrients. Massive—Sediments that appear to be massive, or homogeneous in texture, can result from any of the following: • lack of sufficient grain-size variation to define sedimentary lamination • sedimentation rate high enough that no grainsize segregation occurs • mechanical mixing from soft-sediment deformation during gravity flows • high degrees of biogenic churning. Only the last of these is caused by bioturbation, and recognizing it as such is not always easy because the rock may appear homogeneous (right).9 It has therefore been given the name cryptobioturbation or cryptic bioturbation. The homogeneous texture is caused by rapid reworking of sediments by organisms in search of nutri-
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Bioturbation Index
Percent Bioturbated
Classification
0
0
1
1 to 4
Sparse bioturbation, bedding distinct and few discrete traces or escape structures
2
5 to 30
Low bioturbation, bedding distinct, low trace density and escape structures often common
3
31 to 60
Moderate bioturbation, bedding boundaries sharp, traces discrete and overlap rare
4
61 to 90
High bioturbation, bedding boundaries indistinct and high trace density with overlap common
5
91 to 99
Intense bioturbation, bedding completely disturbed (just visible), limited reworking and later burrows discrete
6
100
No bioturbation
Complete bioturbation and sediment reworking because of repeated overprinting
> Bioturbation index. The bioturbation index is a scheme for quantifying the degree of sediment bioturbation. The index grades trace abundance and overlap and the resultant loss of primary sedimentary fabric. (Adapted from Taylor and Goldring, reference 7.)
ents. Complete obliteration of layering is the highest degree of cryptobioturbation; layering may be disrupted to lesser degrees and still be bioturbated. Cryptobioturbation in sand usually indicates a marine depositional environment, but in fine-grained sediment it may be produced in marine or freshwater environments.10 Sequence Stratigraphic Interpretation Through sequence stratigraphy, geologists identify sequences, or sedimentary deposits that are bounded by unconformities, which are surfaces
characterized by erosion, lack of deposition or abrupt changes in depositional environment. Identifying the key bounding surfaces and correlating them with data from wells and seismic surveys form the basis of the sequence stratigraphic approach. In creating an integrated interpretation, geologists use trace fossils along with sedimentological analysis, core measurements and well logs to characterize sediments within each sequence and identify the depositional surfaces and discontinuities that separate sedimentary sequences.
Oilfield Review AUTUMN 14 Bioturbation Fig. 7 ORAUT14-BIOT 7
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> Cryptic bioturbation. Some biogenic activity leaves no distinct traces but instead results in widespread, subtle disruption of the original sedimentary fabric. In an outcrop-derived core from the Cretaceous Ferron Sandstone, Utah, USA (left ), bioturbation is extensive, but some layering is still intact. Cryptic bioturbation in a wellbore core from the Eocene Mirador Formation, Colombia (middle), has destroyed much of the original layering. In a wellbore core from the Middle Jurassic Bruce field in the North Sea (right), it has obliterated any sign of layering. (Adapted from Pemberton et al, reference 9.)
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> Orinoco wellbore image. A feature in an FMI image (left) may be interpreted (middle) as a U-shaped burrow. A photograph from an unrelated core (right ) shows a burrow of the type (an ichnofossil known as Arenicolites) that may be present in the FMI image. The green lines represent formation structural dip; the yellow lines are fractures. (Photograph copyright S. George Pemberton.)
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Oilfield Review AUTUMN 14 Bioturbation Fig. 9 ORAUT14-BIOT 9
> Wellbore image of possible bioturbation. The FMI image (left) has high-resistivity (light colored) features that may be interpreted (middle) to be burrows resembling the ichnofossil Thalassinoides (right ) in an unrelated core. The structures are classified as dwelling and feeding burrows of a deposit-feeding crustacean living in lower shoreface to offshore environments. (Photograph copyright S. George Pemberton.)
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An important factor in the distribution of organisms is the surface they inhabit.11 Ichnologists characterize sedimentary surface environments according to consistency of the ground and have developed a classification of surface types in terms of stiffness: • soupground—water-saturated mudrocks • softground—muddy sediment with some dewatering • looseground—sandy • stiffground—stabilized • firmground—dewatered and compacted • hardground—lithified. Only with adequate stiffness can these media support traces that can be preserved in the fossil record. Therefore, ichnofossils are usually discernable only in stiffground and firmground surfaces (although backfilled and lined burrows may be discernible in softgrounds); hardground surfaces are too hard for most organisms to penetrate. Firmgrounds in marine environments may be attractive to animal colonization. Their firmness offers the animal protection; they tend to occur in areas of slow sediment accumulation, and the firm sediment does not require constant burrow maintenance. However, for a surface to be both firm and populated, it must have been deposited, dewatered and somewhat compacted before serving as a habitat. In clastic settings, these requirements often are associated with erosionally exhumed substrates, and the resulting surfaces correspond to erosional discontinuities.12 Identifying erosional discontinuities is important because they form the bounding surfaces of sedimentary sequences. Geologists have incorporated ichnological information in sequence stratigraphic studies in a wide range of environments, including Jurassic marine sequences of the North Sea, Permian fluvio-lacustrine facies of Argentina, Jurassic carbonates in Saudi Arabia and Cretaceous marine sequences in Canada.13 Most such studies make use of ichnofossils identified in outcrops and cores, but visual indications of bioturbation may come from well logs. Imaging Ichnofossils If burrows and other traces are large enough and filled with material that has resistivity of sufficient contrast to that of the host rock, they may appear in borehole resistivity images. Examples of resistivity images from wells in clastic formations in the Orinoco heavy oil belt in Venezuela show a range of features that may be interpreted as evidence of bioturbation.
Oilfield Review
There, an operating company is developing a heavy oil field with multiple horizontal wells and wants to place the wells in the best reservoir sands. To this end, the operator commissioned an integrated study that combined lithostratigraphic, biostratigraphic, sedimentological and petrophysical analyses of cores and log data in the four main reservoir units to create a depositional model. The model helped geologists identify the locations and orientations of stacked channel sands and plan development wells with increased confidence. In several cases, burrows in low-resistivity, shaly intervals were filled with resistive sediment. An FMI fullbore formation microimager log from one of the deeper formations revealed a low-resistivity layer with a large, U-shaped burrow filled with resistive material (previous page, top). This ichnofossil is typically associated with low-energy shoreface or sandy tidal flat environments. In the same well, a borehole image from a shallower formation showed circular features that could be interpreted as cross-sectional cuts through horizontally oriented burrows. The high-resistivity features were in a low-resistivity layer near its interface with an overlying resistive layer (previous page, bottom). Burrows of this type are common in lower shoreface to shelfal environments. Possible ichnofossils that have the opposite resistivity contrast can also be seen in FMI images in this field. In a different well, geologists identified a low-resistivity conical burrow in a layered interval of higher resistivity (right). Ichnofossils of this type are vertically oriented, single-entrance burrows with an opening that expands to create a funnel shape. They are commonly filled with sediment that is of finer grain than that of the host layer. These are feeding or dwelling burrows of deposit feeders and are indicators of lower shoreface to proximal shelf settings. While identification of these ichnofossils did not drive the interpretation of the depositional sequences, it corroborated the analysis of the lithostratigraphic, biostratigraphic, sedimentological and petrophysical properties derived from cores and log data, thus reinforcing the integrated interpretation. Geologists were able to identify maximum flooding surfaces and correlate them between wells in the field and were also able to extend this interpretation to neighboring fields.
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> A low-resistivity conical feature. An FMI image (left ) from a well in Venezuela exhibits a lowresistivity (dark) conical structure (middle) that resembles the vertical burrow ichnofossil Rosselia (right ), although the scales are quite different. An Ichnofossil of this type is a vertically oriented single-entrance burrow that has an opening that expands to create a funnel shape. These burrows are commonly filled with sediment that is of finer grain than that of the host layer. These are feeding or dwelling burrows of deposit feeders and are indicators of lower shoreface to full marine settings. The yellow lines represent formation dip; the blue lines may be flooding surfaces. (Photograph copyright S. George Pemberton.)
Oilfield Review AUTUMN 14 13. Taylor AM and Gawthorpe RL: “Application of Sequence 11. Grain size, organic content, local energy and sediment Bioturbation Fig. 11 and Trace Fossil Analysis to Reservoir Stratigraphy cohesiveness are other factors that may influence Description: Examples from the Jurassic of the North colonization patterns. ORAUT14-BIOT 11
Pemberton SG, MacEachern JA and Saunders T: “Stratigraphic Applications of Substrate-Specific Ichnofacies: Delineating Discontinuities in the Rock Record,” in McIlroy D (ed): The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. London: The Geological Society, Special Publication 228 (2004): 29–62. Taylor and Goldring, reference 7. 12. Pemberton et al, reference 11.
Sea,” in Parker JR (ed): Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. London: The Geological Society (1993): 317–335. Buatois and Mángano, reference 5. Pemberton et al, reference 11. MacEachern JA, Pemberton SG, Gingras MK, Bann KL and Dafoe LT: “Uses of Trace Fossils in Genetic Stratigraphy,” in Miller W III (ed): Trace Fossils: Concepts, Problems, Prospects. Amsterdam: Elsevier (2007): 110–134.
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Over the past few decades, oil company geologists have used trace fossil input mainly in exploration and development efforts such as those in the Orinoco example. Recently, they have begun to incorporate this information in productionrelated studies. Bioturbation Effects on Production Bioturbation can destroy or enhance permeability. Geologists generally consider bioturbation detrimental to permeability; biogenic churning tends to undo grain sorting, and redistribution of fine clay grains can reduce overall permeability of layered media. However, evidence in recent
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> Enhancing permeability. Burrows filled with coarse-grained sediments create highpermeability channels in fine-grained, lowpermeability host rock. Burrows of this type, known as Glossifungites, may have population densities as high as 2,500 burrows/m2 [250 burrows/ft2]. (Photograph copyright S. George Pemberton.)
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soils and sediments shows that in some cases, bioturbation enhances porosity and permeability by creating new pathways for fluid movement. Porosity and permeability increase when holes burrowed into a firmground are filled with contrasting, usually coarse-grained, sediment.14 These ichnofossils can add porosity and permeability to an otherwise low-porosity, impermeable matrix. If the burrows are aligned—many will be vertically oriented—the resulting permeability is anisotropic: greater in the vertical direction than in any horizontal directions. In some instances, the burrows constitute the reservoir porosity and permeability. In others, the burrows may be filled with material that later becomes impermeable. In yet other instances, enhanced permeability lies in a diagenetic zone around the burrow. Failing to detect or ignoring the presence of biogenically modified porosity can lead to errors in estimates of hydrocarbon reserves; if the burrows are filled with high-porosity material, reserve calculations that do not take them into account will be too low, and if the burrows are tight, reserve calculations could be too high. Identifying and quantifying the effects of enhanced permeability in reservoir zones are crucial for successful well completions and accurate production simulations. Researchers at the University of Alberta in Edmonton, Canada, have studied the porosity and permeability effects of bioturbation.15 They see the greatest effects when burrows in dewatered, firmground substrate are filled with coarsegrained sediment (left). Burrows of this type can reach areal densities of 2,500 burrows/m2 [250 burrows/ft2]. The effects on permeability depend on burrow connectivity, depth of penetration and permeability contrast between matrix and burrow fill. The permeability-enhanced zone may be up to 3 m [10 ft] thick and is generally limited to areal extents of 1 km2 [0.4 mi2]. Layers exhibiting this type of bioturbation have been recognized in several oil fields. The Ghawar oil field in Saudi Arabia, the world’s largest, is one such example. The oil is contained in carbonate rocks of the Jurassic Arab-D Formation. Production logging has detected thin, superhigh-permeability zones called “super-k” zones that contribute a large proportion of the total flow. In some of the super-k zones, the permeability appears to be related to faults and fractures, while in others, the high permeability is attributed to dolomitization and leaching.16
The University of Alberta geologists examined cores of one super-k layer and reported the presence of a geologic surface of burrow-enhanced permeability. They hypothesized that the surface developed when a firmground, low-porosity micritic calcite layer was exposed during regional erosion that occurred during a rise in sea level (next page). Epifaunal organisms excavated burrows about 1 to 2 cm [0.4 to 0.8 in.] in diameter that penetrated up to 2 m [7 ft] below the surface. Many burrows filled with sucrosic dolomite, which is more porous and permeable than the micrite matrix. Flowmeter measurements indicate that in some wells, 70% of the production comes from this single unit. Although the high permeability of this layer is beneficial to oil production, it can cause difficulties when water is drawn into it from the underlying aquifer. The burrows may act as pathways for some of the 1 million m3 [6 million bbl] of water produced daily in the Ghawar wells. In some instances, burrowing may be present but fail to add effective porosity. One example of this phenomenon comes from the Natih Formation of Oman, which was deposited on a shallow marine carbonate platform in the middle Cretaceous.17 The E Member of the Natih Formation is a reservoir of heavy oil in the Al Ghubar field, and as of 2003 had produced less than 5% of its estimated oil in place. Original estimates of reserves incorporated neutron and density log–based porosities of 20% to 45%. To determine the causes of the production underperformance, geologists and engineers scrutinized core and log porosity measurements. Analysis of thin sections from the various carbonate rock types penetrated by a 60-m [200-ft] core revealed five types of porosity, four of which may be ineffective, meaning they do not contribute to production. The effective porosity type—solution-enhanced interparticle poros14. Pemberton SG and Gingras MK: “Classification and Characterizations of Biogenically Enhanced Permeability,” AAPG Bulletin 89, no. 11 (November 2005): 1493–1517. 15. Pemberton and Gingras, reference 14. 16. For more on dolomitization: Al-Awadi M, Clark WJ, Moore WR, Herron M, Zhang T, Zhao W, Hurley N, Kho D, Montaron B and Sadooni F: “Dolomite: Perspectives on a Perplexing Mineral,” Oilfield Review 21, no. 3 (Autumn 2009): 32–45. 17. Smith LB, Eberli GP, Masaferro JL and Al-Dhahab S: “Discrimination of Effective from Ineffective Porosity in Heterogeneous Cretaceous Carbonates, Al Ghubar Field, Oman,” AAPG Bulletin 87, no. 9 (September 2003): 1509–1529.
Oilfield Review
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D
> Development of a super-k layer in the Ghawar field, Saudi Arabia. Geologists propose that the superpermeability in the Jurassic Arab-D interval developed when regionally extensive erosion exposed a low-porosity micritic calcite firmground (A). Crustaceans colonized this firm sediment, creating a dense network of burrows (B). The burrows filled with detrital sucrosic dolomite (C), which is more porous and permeable than the micritic calcite that contains the burrows. Oil (gold) flows freely though the resulting super-k layer (D). (Adapted from Pemberton and Gingras, reference 14.)
ity—creates effective reservoir intervals in the grainstone facies that make up 20% of the total thickness of the Natih E reservoir. In some zones, leaching of cement has left the grainstones with carbonate grains held together only by the viscous oil. The remaining 80% of the Natih E reservoir contains packstone and wackestone exhibiting the other four types of porosity, which are for the most part ineffective. These rocks have abundant
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0.5- to 2.0-cm [0.2- to 0.8-in.] burrows filled with partially dolomitized grainstone, creating interparticle porosity. The burrow fillings make up between 10% and 50% of the rock volume. However, the burrows are not sufficiently connected to produce significant amounts of oil. Similarly, the other porosity types—microporosity, moldic porosity and intragranular porosity— are not effective in this reservoir. Unfortunately, the neutron and density porosity logs cannot dis-
tinguish between effective and ineffective porosity, leading to inaccurate calculations of reserves. To determine if other logs would be more suitable for assessing effective porosity and permeability, the geologists correlated the rock and pore types identified in the core with other wireline log responses; they began by matching core gamma ray responses with well log gamma ray readings. Of the available well logs—gamma ray, resistivity, sonic, density porosity and neutron porosity—
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only the resistivity logs showed clear correlations with core plug permeability (below). The high-permeability oil-stained zones seen in cores correlate with intervals that exhibit high values of deep resistivity. These zones also correspond to separation between medium and deep resistivity curves, indicating invasion of drilling fluid into the formation, which occurs only in permeable units. The resistivity curves have little or no separation in the burrowed wackestone layers, indicating low permeability and ineffective porosity.
0
The results of the study suggest that because the grainstones that have interparticle porosity make up only 20% of the total thickness of the porous oil-prone interval, the estimate of recoverable oil in place should be decreased by 80%. If this reduction is taken into account, about 25% of the recoverable oil in place has been produced, which the operator considered acceptable for this carbonate reservoir.
Burrow Porosity and Permeability in a Gas Field In carbonate formations, burrows filled with dolomite can act as primary or secondary conduits for fluid movement. Flow behavior in burrowed formations depends on the amount of bioturbation, the connectivity of burrows and the contrast in porosity and permeability between the dolomite fill and the carbonate matrix. Bioturbated carbonate mudstones make up part of the producing interval in the Pine Creek gas field of Alberta, Canada. From depths exceed-
Gamma Ray
Density Porosity
Neutron Porosity
Sonic Porosity
Plug Porosity
Plug Permeability
gAPI 25 50
% 50 40 30 20 10 0
% 50 40 30 20 10 0
% 50 40 30 20 10 0
% 50 40 30 20 10 0
mD
75
0.1
10,000
Deep Resistivity 0.1
ohm.m
10,000
Deep Resistivity Minus Medium Resistivity 0.1
ohm.m
10,000
Reservoir top
Oil/water contact
> Well logs and core data from the E Member of the Natih Formation, Al Ghubar field, Oman. Reservoir underperformance led geologists to reevaluate log and core measurements to determine the best indicators of effective porosity and permeability. Only the logs of deep resistivity (Track 7) and of the difference between medium and deep resistivity (Track 8) showed clear correlations with core plug permeability (Track 6). High-permeability zones are shown by yellow shading. (Adapted from Smith et al, reference 17.)
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0 to 1 mD 1 to 10 mD 10 to 100 mD > 100 mD
2 cm
1
1 cm
2
3
4
5
1 cm
> Spot permeametry and microCT analysis of samples from the Wabamun Group in Alberta, Canada. In this formation, permeability is increased where burrows are associated with localized bioturbation. A core sample (top left ) exhibits dolomiteassociated trace fossils (light brown) and a nondolomitized lime mudstone matrix (light gray). Results of spot permeametry measurements (top middle) can be contoured to produce a permeability map (top right ). The highest permeability values are up to 340 mD and correspond to the dolomitized trace fossils. MicroCT scans in 3D (bottom, top row) at 34-μm resolution reveal mineral phases in five cross sections of a core sample. The dolomite-filled burrows are represented as shades of blue, lime mudstone matrix as light gray and vugs as unfilled holes (demarcated by black arrows). The 2D cross-sectional images at the bottom were used to constrain the attenuation phases within the core sample. In these images, the dolomite-filled burrows appear in light gray, limestone matrix in dark gray and vugs in black.
ing 3,000 m [10,000 ft], the field has produced more than 550 MMcf [15.6 million m3] of gas. In a study using slabbed core samples from 11 wells in the field, University of Alberta geologists examined the sedimentological, ichnological and petrophysical properties of facies in the Wabamun Group—the primary reservoir facies in the Pine Creek field.18 They also imaged the core
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samples using Oilfield X-ray micro–computed tomograReview phy (microCT) AUTUMN and helical14 computed tomography Bioturbation to obtain 2D and 3D images Fig. and15 performed spot ORAUT14-BIOT 15 permeability tests to analyze permeability distributions in the samples. In the four reservoir facies, the amount of burrow-associated dolomite ranged from 0% to about 80% to 100%. MicroCT scans of a core from
the most heavily bioturbated facies revealed the complexity of the burrow distribution (above). The dolomitized burrows represent a mixture of 18. Baniak GM, Gingras MK and Pemberton SG: “Reservoir Characterization of Burrow-Associated Dolomites in the Upper Devonian Wabamun Group, Pine Creek Gas Field, Central Alberta, Canada,” Marine and Petroleum Geology 48 (December 2013): 275–292.
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Pine Creek Well
Monthly gas production, Mcf
Production from burrows
Diffusion from matrix into burrows
100
10
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Year
> Production history in an ichnofossil-hosted tight gas reservoir. Monthly gas production from a well in the Pine Creek field shows early production from gas-filled burrows in the first 15 or 20 years. Gas production then declines because the gas must diffuse from the low-permeability matrix into the burrows to be produced.
tubular structures ranging in diameter from millimeters to centimeters. The difference in lithology between the burrows and the nondolomitized limestone mud matrix makes the burrows easy to image. Spot-permeability tests quantified the permeability of the cores on a 0.5-cm [0.2-in.] grid. Permeability of the matrix is less than 1 mD, whereas permeability of the dolomitized burrows is greater than 100 mD. The large contrast in permeability between burrows and matrix gives rise to a distinctive production history for wells in the field (above). For the first 15 years in the life of a well, the formation produces gas from the burrows. After the easy gas has been extracted, the declining production is interpreted to be of gas that has diffused from the matrix into the burrows. Geologists studying this field have coined a new term— ichnofossil-hosted tight gas—to describe this burrow-matrix association. Burrow Significance Biologic disturbance of sediments can have many effects, for better or worse, on reservoirs. By recognizing burrows and other trace fossils, ichnologists gain knowledge they can incorporate with other information to infer a formation’s 19. Pemberton and Gingras, reference 14. 20. Aplin AC and Macquaker JHS: “Mudstone Diversity: Origin and Implications for Source, Seal, and Reservoir Properties in Petroleum Systems,” AAPG Bulletin 95, no. 12 (December 2011): 2031–2059.
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depositional environment and hydrocarbon potential. This information helps them guide exploration activities. Bioturbation alters the physical properties of a rock as it is being formed. The process can increase or decrease porosity and permeability and can modify permeability anisotropy, sometimes to a significant degree. Quantifying these effects and including them in reservoir simulation models can improve production predictions and enhanced oil recovery operations. Bioturbation can have the same effects on fine-grained layers as it has on reservoir rocks. Shales and mudstones may lose their capability to act as reservoir seals if bioturbation causes a large increase in vertical permeability. In the Sirasun and Terang gas fields in Indonesia, the marly caprock was found to have burrows that were filled with hollow foraminifera. The lowOilfield Review permeability formation had certifiable reserves AUTUMN 14 3 of 500 Bioturbation Bcf [14 billion Fig.m16] of gas. The burrows causedORAUT14-BIOT it to acquire reservoir characteristics, 16 making for a leaky seal.19 Recent activity in gas- and oil-prone mudstone and shale formations—called unconventional reservoirs because they act as both source rock and reservoir—may benefit from more studies of bioturbation. Evidence of bioturbation has been documented in several low-permeability, fine-grained rocks.20 Ichnofossils have been identified in the Woodford Formation and the Lower Marcellus Shale in the US and in the
Bakken Shale and the Montney Shale in Canada. As in the example from the Pine Creek field, extensive zones of trace fossils in these formations may improve gas storativity and the connectivity of porosity with induced fractures. Bioturbation may also affect rock mechanical properties, potentially influencing the outcome of hydraulic fracturing. In a manner of speaking, many human activities qualify as bioturbation. The wells we drill and the tunnels we bore are on scales far surpassing those of burrows by sea creatures, but we can still learn from the effects of their smallscale efforts. By recognizing bioturbation and appreciating its consequences, geoscientists are likely to improve their understanding of reservoirs and do a better job recovering hydro carbon resources. —LS
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