Bioturbation: Reworking Sediments for Better Or Worse

Bioturbation: Reworking Sediments for Better or Worse

Murray K. Gingras Petroleum geologists are interested in bioturbation because it reveals clues S. George Pemberton University of Alberta about the depositional environment. Bioturbation can also destroy or enhance Edmonton, Alberta, Canada porosity and permeability, thereby affecting reservoir quality, reserves calculations Michael Smith and flow dynamics. Maturín, Venezuela

Oilfield Review Winter 2014/2015: 26, no. 4. Copyright © 2015 Schlumberger. FMI is a mark of Schlumberger. Sediments undergo several modifications to Bioturbation is typically a small-scale but 1. Ali SA, Clark WJ, Moore WR and Dribus JR: “Diagenesis become the source rocks, reservoirs and seals potentially significant geologic process that may and Reservoir Quality,” Oilfield Review 22, no. 2 (Summer 2010): 14–27. that generate and contain petroleum reserves. occur wherever plants or animals live. It can take 2. Al-Hajeri MM, Al Saeed M, Derks J, Fuchs T, Hantschel T, The changes that occur between deposition and several forms, including displacement of soil by Kauerauf A, Neumaier M, Schenk O, Swientek O, Tessen N, Welte D, Wygrala B, Kornpihl D and lithification, collectively known as diagenesis, plant roots, tunnels created by burrowing ani- Peters K: “Basin and Petroleum System Modeling,” include the processes of compaction, cementa- mals and footprints left by dinosaurs (next page). Oilfield Review 21, no. 2 (Summer 2009): 14–29. tion, dissolution and recrystallization.1 But before Of most interest to the oil and gas industry any of these occur, another process can consider- are the changes brought about by organisms ably affect rock properties. As soon as they are that are active near the water/sediment inter- deposited, sediments can be altered by biotur- face in marine settings. Such activities are typi- bation: the disruption of sediment and soil by cally limited to a meter or so in depth but may living things. 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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Oilfield Review AUTUMN 14 Bioturbation Fig. 2 ORAUT14-BIOT 2 > 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 Recently, however, geologists have expanded the This article describes ways in which animal animals helps geologists characterize the envi- application of bioturbation to address production activity can affect sedimentary deposits and ronmental conditions prevalent during a brief geology challenges. focuses on reservoir rocks. Examples from both interval of geologic time: after the sediments Animal activity in sediments disrupts layering, siliciclastic and carbonate formations show how were deposited, but while they were still soft creates flow pathways, enables exchange of min- geologists use this information to infer ancient enough to deform. erals and fluids between sedimentary layers, environmental conditions and characterize pres- For many years, bioturbation studies found changes pore fluid chemistry and adds or removes ent-day formation properties. application mainly in exploration geology—in organic matter. These changes can facilitate or estimating paleobathymetry, assessing deposi- impede mobility of diagenetic fluids, increase or Life Just Under the Surface tional environment and identifying key strati- decrease porosity and permeability and alter per- Animals that live near the water/sediment graphic surfaces. These are all important inputs meability homogeneityOilfield Reviewand isotropy. Recognizing interface often leave evidence of their life- to the geologic models used for determining these effects andAUTUMN including 14 them in reservoir styles. For example, surface expressions of sub- potential source rock and reservoir quality and simulation modelsBioturbation can improve Fig. 1production pre- surface bioturbation can be discerned in the ORAUT14-BIOT 1 for modeling basins and petroleum systems.2 dictions and enhanced oil recovery operations. intertidal zone of a beach (previous page). In

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72917schD7R1.indd 2 1/20/15 2:36 AM > 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 Dwelling Crawling (Fugichnia) (Domichnia) (Repichnia)

Lower Energy Stable Habitats

Feeding Farming Grazing (Fodichnia) (Agrichnia) (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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Oilfield Review AUTUMN 14 Bioturbation Fig. 5 ORAUT14-BIOT 5 this case, thousands of piles of sand-rich fecal Ichnofossils are interpreted to be related to By studying trace fossils, ichnologists have coils dot the floor of a shallow bay. These fecal animal survival strategies associated with sedi- identified several types of animal behavior, includ- strands are produced by burrowing, wormlike mentary and environmental conditions. They are ing feeding, dwelling, fleeing, resting, crawling, creatures that take in the bulk sediment, ingest different from body fossils in that they represent grazing and farming (previous page, bottom).3 nutrients and excrete the indigestible rock a behavior or activity, not a particular organism. Depending on the activity, the associated traces grains. Their subsurface burrows may be tens of Only infrequently, such as in the case of some may be found on the sediment surface—which centimeters deep, and an assemblage or com- dinosaur footprints, can ichnologists identify the eventually becomes the interface between two munity of these organisms can affect an area of animal species that created an ichnofossil. layers—or within a sediment layer. Ichnologists several square kilometers. Instead, they attempt to deduce what the animal use the evidence of these behaviors to character- Infauna, or animals that live in sedi- was doing when it created the trace. ize the paleoenvironment of a rock layer. ments—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 inclina- tion. These burrows may be simple, shallow unlined holes or may have compacted walls, be lined with contrasting material or have multi- ple openings. The burrows may remain open for a period of time, collapse or be filled imme- diately 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 mas- sive, homogeneous interval. Aquatic animals that live on the sediment sur- face, 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 paleoeco- logical 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 3 cm marine or nonmarine waters. > 3. Gingras MK, Bann KL, MacEachern JA and Pemberton SG: Contrasting fill. This burrow in fine-grained sediment is filled with “A Conceptual Framework for the Application of Trace coarse-grained material. This U-shaped trace is interpreted to be the Fossils,” in MacEachern JA, Bann KL, Gingras MK and dwelling burrow of an annelid or a crustacean in a low-energy shoreface or Pemberton SG (eds): Applied Ichnology. Tulsa: Society for sandy tidal flat environment. (Photograph copyright S. George Pemberton.) 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.

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Oilfield Review AUTUMN 14 72917schD7R1.indd 4 Bioturbation Fig. 4 1/20/15 2:36 AM ORAUT14-BIOT 4 M Unburrowed or Burrowed Mottled or Laminated MassiveM Appearing

Pervasively burrowed Mud Sand Unimodal distribution of grains, True cryptic Low sedimentation rates bioturbation Freshwater and good food resource no mineralogic diversity of grains bedding or Lacustrine, quiescent bay or deep marine Large, diverse Inner shelf or Oxygen depleted ichnofossils offshore Load-casted bedding Downward contacts and abundant hydraulic jump Small Quiescent bay organic detritus ichnofossils, or lagoon, possibly High sedimentation rate or Cross-bedded low diversity tidal flat or Penecontemporaneous Freshwater Moderate to rare, deformation observed Sediment evenly distributed in association with gravity flow Fluvial, fluvio- massive media lacustrine or or deltaic High sedimentation rates, Sand Probably shallow marine or marginal marine good food resource and Burrowed contacts Freshwater generally consistent conditions Planar at top or bottom of or massive-appearing Probably inner shelf Laminated Large, Distal prodelta sediment, or vestigial High sedimentation rate diverse open bay fine-grained intertidal burrows evident locally flat with low tide range Mud or (less likely) shallow Small River influenced delta, lacustrine restricted bay or estuary

Moderate to rare Sporadically distributed Laminated to scrambled

High sedimentation Event sedimentation rates and variable generally dominated (temporally) depositional conditions by fair-weather processes Proximal prodelta Large, or delta front Large, Lower shoreface diverse bay mouth complex diverse

Inner estuary Small Small Bays or deltas tidal channel 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 Unburrowed—Sediments that are relatively • anoxic settings (poorly oxygenated) is to divide them into three main types of lithified undisturbed, such as those with original layering • constantly shifting sediments on the seafloor sediment: unburrowed, burrowed and massive intact and with little or no evidence of bioturba- • high sedimentation rates appearing (above).4 Classification of these types tion, are usually ascribed to one or more of the • arid or frozen locales. serves as the starting point for interpreting the following depositionalOilfield environments:Review Unburrowed sandy sediments usually indicate AUTUMN 14 depositional conditions under which such sedi- • freshwater, whereBioturbation there are Fig. few 6 deeply burrow- freshwater deposition or shifting sedimentation. ments formed. ing organismsORAUT14-BIOT 6 However, many continental environments do

4. Gingras et al, reference 3. 6. Hickey JJ and Henk B: “Lithofacies Summary of the 7. Taylor AM and Goldring R: “Description and Analysis of 5. Buatois LA and Mángano MG: “Animal-Substrate Mississippian Barnett Shale, Mitchell 2 T.P. Sims Well, Bioturbation and Ichnofabric,” Journal of the Geological Interaction in Freshwater Environments: Applications of Wise County, Texas,” AAPG Bulletin 91, no. 4 Society 150, no. 1 (February 1993): 141–148. Ichnology in Facies and Sequence Stratigraphic Analysis (April 2007): 437–443. 8. A colonization event occurs when one or more species of Fluvio-Lacustrine Successions,” in McIlroy D (ed): Loucks RG and Ruppel SC: “Mississippian Barnett Shale: spread to a new area. The Application of Ichnology to Palaeoenvironmental and Lithofacies and Depositional Setting of a Deep-Water 9. Pemberton SG, MacEachern JA, Gingras MK and Stratigraphic Analysis. London: The Geological Society, Shale-Gas Succession in the Fort Worth Basin, Texas,” Saunders TDA: “Biogenic Chaos: Cryptobioturbation and Special Publication 228 (2004): 311–333. AAPG Bulletin 91, no. 4 (April 2007): 579–601. the Work of Sedimentologically Friendly Organisms,” O’Brien NR: “The Effects of Bioturbation on the Fabric Palaeogeography, Palaeoclimatology, Palaeoecology of Shale,” Journal of Sedimentary Petrology 57, no. 3 270, no. 3–4 (December 15, 2008): 273–279. (May 1987): 449–455. 10. Gingras et al, reference 3.

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72917schD7R1.indd 5 1/20/15 2:36 AM exhibit trace fossils.5 Unburrowed fine-grained Bioturbation Percent laminated sediments rich in clay and silt are typi- Index Bioturbated Classification cally interpreted to result from sedimentation in 0 0 No bioturbation freshwater or anoxic conditions, although high 1 1 to 4 Sparse bioturbation, bedding distinct and few discrete sedimentation rates might yield the same result. traces or escape structures Many organic-rich source rocks, some of which 2 5 to 30 Low bioturbation, bedding distinct, low trace density are targets of tight oil and gas shale plays, are and escape structures often common 3 31 to 60 Moderate bioturbation, bedding boundaries sharp, examples of fine-grained sediments deposited in traces discrete and overlap rare environments with a low oxygen supply. Because 4 61 to 90 High bioturbation, bedding boundaries indistinct and such depositional environments are not welcom- high trace density with overlap common ing to many animals, the sediments may exhibit 5 91 to 99 Intense bioturbation, bedding completely disturbed layering and ordered clay grains and show little (just visible), limited reworking and later burrows discrete or no bioturbation.6 6 100 Complete bioturbation and sediment reworking because of repeated overprinting Burrowed—Categorization of burrowed media is based on the distribution of ichnofossils > Bioturbation index. The bioturbation index is a scheme for quantifying the and characteristics—primarily size and diver- degree of sediment bioturbation. The index grades trace abundance and overlap and the resultant loss of primary sedimentary fabric. (Adapted from sity—of the ichnological assemblage. Ichnologists Taylor and Goldring, reference 7.) 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 ents. Complete obliteration of layering is the characterized by erosion, lack of deposition or of trace overlap (right). The BI is related to the highest degree of cryptobioturbation; layering abrupt changes in depositional environment. duration of colonization events and, through may be disrupted to lesser degrees and still be Identifying the key bounding surfaces and cor- them, to rates of sedimentation.8 bioturbated. Cryptobioturbation in sand usually relating them with data from wells and seismic Highly to completely burrowed sediments are indicates a marine depositional environment, but surveys form the basis of the sequence strati- evidence of both a significant infaunal biomass in fine-grained sediment it may be produced in graphic approach. In creating an integrated and conditions of slow sediment accumulation. marine or freshwater environments.10 interpretation, geologists use trace fossils along Moderate to sparse bioturbation, characterized by with sedimentological analysis, core measure- evenly distributed trace fossils, indicates a lower Sequence Stratigraphic Interpretation ments and well logs to characterize sediments infaunal biomass and higher sedimentation rate. Through sequence stratigraphy, geologists iden- within each sequence and identify the deposi- The size and diversity of ichnofossils in burrowed tify sequences, or sedimentary deposits that are tional surfaces and discontinuities that separate media reflect the chemical aspects of the deposi- bounded by unconformities, which are surfaces sedimentary sequences. tional waters. For example, in marine deposits, large trace fossils are indicative of high dissolved oxygen content and stable ocean salinity. A pre- ponderance of small trace fossils suggests salinity- or oxygen-stressed environments. High diversity of fossil types is related to oxygen content and Oilfield Review salinity and also indicates abundant nutrients. AUTUMN 14 Massive—Sediments that appear to be mas- Bioturbation Fig. 7 sive, or homogeneous in texture, can result from ORAUT14-BIOT 7 any of the following: • lack of sufficient grain-size variation to define sedimentary lamination • sedimentation rate high enough that no grain- size segregation occurs • mechanical mixing from soft-sediment defor- mation during gravity flows • high degrees of biogenic churning. 3 cm 3 cm Only the last of these is caused by bioturba- tion, and recognizing it as such is not always easy > Cryptic bioturbation. Some biogenic activity leaves no distinct traces but instead results in because the rock may appear homogeneous widespread, subtle disruption of the original sedimentary fabric. In an outcrop-derived core from the (right).9 It has therefore been given the name 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), cryptobioturbation or cryptic bioturbation. The has destroyed much of the original layering. In a wellbore core from the Middle Jurassic Bruce field homogeneous texture is caused by rapid rework- in the North Sea (right), it has obliterated any sign of layering. (Adapted from Pemberton et al, ing of sediments by organisms in search of nutri- reference 9.)

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Oilfield Review AUTUMN 14 Bioturbation Fig. 8 ORAUT14-BIOT 8 Depth, Depth, An important factor in the distribution of ft ft organisms is the surface they inhabit.11 Ichnolo- X,X06 X,X06 gists characterize sedimentary surface environ- ments according to consistency of the ground and

X,X07 X,X07 have developed a classification of surface types in terms of stiffness: • soupground—water-saturated mudrocks X,X08 X,X08 • softground—muddy sediment with some dewatering X,X09 X,X09 • looseground—sandy • stiffground—stabilized

X,X10 X,X10 • firmground—dewatered and compacted • hardground—lithified. Only with adequate stiffness can these media X,X11 X,X11 support traces that can be preserved in the fossil record. Therefore, ichnofossils are usually dis- X,X12 X,X12 cernable only in stiffground and firmground sur- faces (although backfilled and lined burrows may 3 cm X,X13 X,X13 be discernible in softgrounds); hardground sur- faces are too hard for most organisms to pene- trate. Firmgrounds in marine environments may X,X14 X,X14 be attractive to animal colonization. Their firm- ness offers the animal protection; they tend to occur in areas of slow sediment accumulation, > Orinoco wellbore image. A feature in an FMI image (left) may be interpreted (middle) as a U-shaped and the firm sediment does not require constant 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 burrow maintenance. However, for a surface to be dip; the yellow lines are fractures. (Photograph copyright S. George Pemberton.) both firm and populated, it must have been deposited, dewatered and somewhat compacted before serving as a habitat. In clastic settings, Depth, Depth, these requirements often are associated with ft ft erosionally exhumed substrates, and the result- ing surfaces correspond to erosional discontinui- X,X03 X,X03 ties.12 Identifying erosional discontinuities is 3 cm important because they form the bounding sur- X,X04 X,X04 faces of sedimentary sequences. Geologists have incorporated ichnological

X,X05 X,X05 information in sequence stratigraphic studies in a wide range of environments, including Jurassic marine sequences of the North Sea, Permian flu- X,X06 X,X06 vio-lacustrine facies of Argentina, Jurassic car- bonates in Saudi Arabia and Cretaceous marine X,X07 X,X07 sequences in Canada.13 Most such studies make use of ichnofossils identified in outcrops and cores, but visual indications of bioturbation may X,X08 X,X08 Oilfield Review AUTUMN 14 come from well logs. Bioturbation Fig. 9 X,X09 X,X09 ORAUT14-BIOT 9 Imaging Ichnofossils If burrows and other traces are large enough and X,X10 X,X10 filled with material that has resistivity of suffi- cient contrast to that of the host rock, they may appear in borehole resistivity images. Examples X,X11 X,X11 of resistivity images from wells in clastic forma- tions in the Orinoco heavy oil belt in Venezuela > Wellbore image of possible bioturbation. The FMI image (left) has high-resistivity (light colored) show a range of features that may be interpreted features that may be interpreted (middle) to be burrows resembling the ichnofossil Thalassinoides as evidence of bioturbation. (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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Oilfield Review AUTUMN 14 Bioturbation Fig. 10 ORAUT14-BIOT 10 There, an operating company is developing a Depth, Depth, heavy oil field with multiple horizontal wells and ft ft wants to place the wells in the best reservoir sands. To this end, the operator commissioned an integrated study that combined lithostrati- X,X04 X,X04 graphic, biostratigraphic, sedimentological and petrophysical analyses of cores and log data in X,X05 X,X05 the four main reservoir units to create a deposi-

tional model. The model helped geologists iden- X,X06 X,X06 3 cm tify the locations and orientations of stacked channel sands and plan development wells with X,X07 X,X07 increased confidence. In several cases, burrows in low-resistivity, X,X08 X,X08 shaly intervals were filled with resistive sediment. An FMI fullbore formation microimager log from one of the deeper formations revealed a low-resis- X,X09 X,X09 tivity layer with a large, U-shaped burrow filled with resistive material (previous page, top). This X,X10 X,X10 ichnofossil is typically associated with low-energy shoreface or sandy tidal flat environments. In the X,X11 X,X11 same well, a borehole image from a shallower for-

mation showed circular features that could be X,X12 X,X12 interpreted as cross-sectional cuts through hori- zontally oriented burrows. The high-resistivity fea- X,X13 X,X13 tures were in a low-resistivity layer near its interface with an overlying resistive layer (previ- ous page, bottom). Burrows of this type are com- X,X14 X,X14 mon in lower shoreface to shelfal environments. Possible ichnofossils that have the opposite X,X15 X,X15 resistivity contrast can also be seen in FMI images in this field. In a different well, geologists identi- X,X16 X,X16 fied a low-resistivity conical burrow in a layered

interval of higher resistivity (right). Ichnofossils of X,X17 X,X17 this type are vertically oriented, single-entrance burrows with an opening that expands to create a funnel shape. They are commonly filled with sedi- > A low-resistivity conical feature. An FMI image (left) from a well in Venezuela exhibits a low- ment that is of finer grain than that of the host resistivity (dark) conical structure (middle) that resembles the vertical burrow ichnofossil Rosselia layer. These are feeding or dwelling burrows of (right), although the scales are quite different. An Ichnofossil of this type is a vertically oriented deposit feeders and are indicators of lower shore- single-entrance burrow that has an opening that expands to create a funnel shape. These burrows face to proximal shelf settings. 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. While identification of these ichnofossils did The yellow lines represent formation dip; the blue lines may be flooding surfaces. (Photograph not drive the interpretation of the depositional copyright S. George Pemberton.) sequences, it corroborated the analysis of the lithostratigraphic, biostratigraphic, sedimento- Oilfield Review logical and petrophysical properties derived from 11. Grain size, organic content, local energy and sedimentAUTUMN 1413. Taylor AM and Gawthorpe RL: “Application of Sequence cores and log data, thus reinforcing the inte- cohesiveness are other factors that may influenceBioturbation Fig.Stratigraphy 11 and Trace Fossil Analysis to Reservoir grated interpretation. Geologists were able to colonization patterns. ORAUT14-BIOTDescription: 11 Examples from the Jurassic of the North Pemberton SG, MacEachern JA and Saunders T: Sea,” in Parker JR (ed): Petroleum Geology of Northwest identify maximum flooding surfaces and corre- “Stratigraphic Applications of Substrate-Specific Europe: Proceedings of the 4th Conference. London: late them between wells in the field and were Ichnofacies: Delineating Discontinuities in the Rock The Geological Society (1993): 317–335. also able to extend this interpretation to neigh- Record,” in McIlroy D (ed): The Application of Ichnology Buatois and Mángano, reference 5. to Palaeoenvironmental and Stratigraphic Analysis. Pemberton et al, reference 11. London: The Geological Society, Special Publication 228 boring fields. MacEachern JA, Pemberton SG, Gingras MK, Bann KL (2004): 29–62. and Dafoe LT: “Uses of Trace Fossils in Genetic Taylor and Goldring, reference 7. Stratigraphy,” in Miller W III (ed): Trace Fossils: 12. Pemberton et al, reference 11. Concepts, Problems, Prospects. Amsterdam: Elsevier (2007): 110–134.

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72917schD7R1.indd 8 1/20/15 2:37 AM Over the past few decades, oil company geolo- soils and sediments shows that in some cases, The University of Alberta geologists examined gists have used trace fossil input mainly in explo- bioturbation enhances porosity and permeability cores of one super-k layer and reported the pres- ration and development efforts such as those in by creating new pathways for fluid movement. ence of a geologic surface of burrow-enhanced the Orinoco example. Recently, they have begun Porosity and permeability increase when permeability. They hypothesized that the surface to incorporate this information in production- holes burrowed into a firmground are filled with developed when a firmground, low-porosity related studies. contrasting, usually coarse-grained, sediment.14 micritic calcite layer was exposed during regional These ichnofossils can add porosity and permea- erosion that occurred during a rise in sea level Bioturbation Effects on Production bility to an otherwise low-porosity, impermeable (next page). Epifaunal organisms excavated bur- Bioturbation can destroy or enhance permeabil- matrix. If the burrows are aligned—many will be rows about 1 to 2 cm [0.4 to 0.8 in.] in diameter ity. Geologists generally consider bioturbation vertically oriented—the resulting permeability is that penetrated up to 2 m [7 ft] below the sur- detrimental to permeability; biogenic churning anisotropic: greater in the vertical direction than face. Many burrows filled with sucrosic dolomite, tends to undo grain sorting, and redistribution of in any horizontal directions. In some instances, which is more porous and permeable than the fine clay grains can reduce overall permeability the burrows constitute the reservoir porosity and micrite matrix. Flowmeter measurements indi- of layered media. However, evidence in recent permeability. In others, the burrows may be filled cate that in some wells, 70% of the production with material that later becomes impermeable. comes from this single unit. In yet other instances, enhanced permeability Although the high permeability of this layer is lies in a diagenetic zone around the burrow. beneficial to oil production, it can cause difficul- Failing to detect or ignoring the presence of ties when water is drawn into it from the underly- biogenically modified porosity can lead to errors ing aquifer. The burrows may act as pathways for 3 cm in estimates of hydrocarbon reserves; if the bur- some of the 1 million m3 [6 million bbl] of water rows are filled with high-porosity material, produced daily in the Ghawar wells. reserve calculations that do not take them into In some instances, burrowing may be present account will be too low, and if the burrows are but fail to add effective porosity. One example tight, reserve calculations could be too high. of this phenomenon comes from the Natih Identifying and quantifying the effects of Formation of Oman, which was deposited on a enhanced permeability in reservoir zones are shallow marine carbonate platform in the middle crucial for successful well completions and accu- Cretaceous.17 The E Member of the Natih rate production simulations. Formation is a reservoir of heavy oil in the Al Researchers at the University of Alberta in Ghubar field, and as of 2003 had produced less Edmonton, Canada, have studied the porosity than 5% of its estimated oil in place. Original esti- and permeability effects of bioturbation.15 They mates of reserves incorporated neutron and den- see the greatest effects when burrows in dewa- sity log–based porosities of 20% to 45%. To tered, firmground substrate are filled with coarse- determine the causes of the production under- grained sediment (left). Burrows of this type can performance, geologists and engineers scruti- reach areal densities of 2,500 burrows/m2 nized core and log porosity measurements. [250 burrows/ft2]. The effects on permeability Analysis of thin sections from the various depend on burrow connectivity, depth of penetra- carbonate rock types penetrated by a 60-m tion and permeability contrast between matrix [200-ft] core revealed five types of porosity, and burrow fill. The permeability-enhanced zone four of which may be ineffective, meaning they do may be up to 3 m [10 ft] thick and is generally not contribute to production. The effective poros- limited to areal extents of 1 km2 [0.4 mi2]. Layers ity type—solution-enhanced interparticle poros- exhibiting this type of bioturbation have been 14. Pemberton SG and Gingras MK: “Classification and recognized in several oil fields. Characterizations of Biogenically Enhanced The Ghawar oil field in Saudi Arabia, the Permeability,” AAPG Bulletin 89, no. 11 (November 2005): 1493–1517. world’s largest, is one such example. The oil is 15. Pemberton and Gingras, reference 14. contained in carbonate rocks of the Jurassic 16. For more on dolomitization: Al-Awadi M, Clark WJ, > Arab-D Formation. Production logging has Moore WR, Herron M, Zhang T, Zhao W, Hurley N, Kho D, Enhancing permeability. Burrows filled with Montaron B and Sadooni F: “Dolomite: Perspectives on coarse-grained sediments create high- detected thin, superhigh-permeability zones a Perplexing Mineral,” Oilfield Review 21, no. 3 permeability channels in fine-grained, low- called “super-k” zones that contribute a large (Autumn 2009): 32–45. permeability host rock. Burrows of this type, proportion of the total flow. In some of the 17. Smith LB, Eberli GP, Masaferro JL and Al-Dhahab S: known as Glossifungites, may have population “Discrimination of Effective from Ineffective Porosity in 2 super-k zones, the permeability appears to be Heterogeneous Cretaceous Carbonates, Al Ghubar Field, densities as high as 2,500 burrows/m Oman,” AAPG Bulletin 87, no. 9 (September 2003): 2 related to faults and fractures, while in others, [250 burrows/ft ]. (Photograph copyright 1509–1529. S. George Pemberton.) the high permeability is attributed to dolomitiza- tion and leaching.16

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72917schD7R1.indd 9 Oilfield Review 1/20/15 2:37 AM AUTUMN 14 Bioturbation Fig. 12 ORAUT14-BIOT 12 A BCD A B CD AB C D ABC 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 0.5- to 2.0-cm [0.2- to 0.8-in.] burrows filled with tinguish between effective and ineffective poros- grainstone facies that make up 20% of the total partially dolomitized grainstone, creating inter- ity, leading to inaccurate calculations of reserves. thickness of the Natih E reservoir. In some zones, particle porosity. The burrow fillings make up To determine if other logs would be more suit- leaching of cement has left the grainstones with between 10% and 50% of the rock volume. able for assessing effective porosity and permea- carbonate grains held together only by the However, the burrows are not sufficiently con- bility, the geologists correlated the rock and pore viscous oil. nected to produce significant amounts of oil. types identified in the core with other wireline log The remaining 80% of the Natih E reservoir Similarly, the other porosity types—microporos- responses; they began by matching core gamma contains packstone and wackestone exhibiting ity, moldic porosity and intragranular porosity— ray responses with well log gamma ray readings. the other four types of porosity, which are for the are not effective in this reservoir. Unfortunately, Of the available well logs—gamma ray, resistivity, most part ineffective. These rocks have abundant the neutron and density porosity logs cannot dis- sonic, density porosity and neutron porosity—

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72917schD7R1.indd 10 1/20/15 2:37 AM only the resistivity logs showed clear correlations The results of the study suggest that because Burrow Porosity and Permeability with core plug permeability (below). the grainstones that have interparticle porosity in a Gas Field The high-permeability oil-stained zones seen make up only 20% of the total thickness of the In carbonate formations, burrows filled with in cores correlate with intervals that exhibit porous oil-prone interval, the estimate of recov- dolomite can act as primary or secondary con- high values of deep resistivity. These zones also erable oil in place should be decreased by 80%. If duits for fluid movement. Flow behavior in bur- correspond to separation between medium and this reduction is taken into account, about 25% of rowed formations depends on the amount of deep resistivity curves, indicating invasion of the recoverable oil in place has been produced, bioturbation, the connectivity of burrows and the drilling fluid into the formation, which occurs which the operator considered acceptable for this contrast in porosity and permeability between only in permeable units. The resistivity curves carbonate reservoir. the dolomite fill and the carbonate matrix. have little or no separation in the burrowed Bioturbated carbonate mudstones make up wackestone layers, indicating low permeability part of the producing interval in the Pine Creek and ineffective porosity. gas field of Alberta, Canada. From depths exceed-

Deep Resistivity Minus Medium Gamma Ray Density Porosity Neutron Porosity Sonic Porosity Plug Porosity Plug Permeability Deep Resistivity Resistivity gAPI % % % % mD ohm.m ohm.m 0025 5075 50 40 30 20 10 50 40 30 20 10 0 50 40 30 20 10 0 50 40 30 20 10 0 0.1 10,000 0.1 10,000 0.1 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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Oilfield Review AUTUMN 14 Bioturbation Fig. 14 ORAUT14-BIOT 14 0 to 1 mD

1 to 10 mD

10 to 100 mD

> 100 mD

2 cm 1 cm

12345

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 dolomite- associated 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 samples using OilfieldX-ray micro–computed Review tomogra- the most heavily bioturbated facies revealed the more than 550 MMcf [15.6 million m3] of gas. phy (microCT) AUTUMNand helical 14 computed tomography complexity of the burrow distribution (above). In a study using slabbed core samples from to obtain 2D andBioturbation 3D images Fig.and 15performed spot The dolomitized burrows represent a mixture of ORAUT14-BIOT 15 11 wells in the field, University of Alberta geolo- permeability tests to analyze permeability distri- 18. Baniak GM, Gingras MK and Pemberton SG: “Reservoir gists examined the sedimentological, ichnologi- butions in the samples. Characterization of Burrow-Associated Dolomites in the cal and petrophysical properties of facies in the In the four reservoir facies, the amount of Upper Devonian Wabamun Group, Pine Creek Gas Field, Central Alberta, Canada,” Marine and Petroleum Wabamun Group—the primary reservoir facies in burrow-associated dolomite ranged from 0% to Geology 48 (December 2013): 275–292. the Pine Creek field.18 They also imaged the core about 80% to 100%. MicroCT scans of a core from

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Production from burrows Diffusion from matrix into burrows

100

10 Monthly gas production, Mcf

1965 1970 1975 1980198519901995200020052010 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 depositional environment and hydrocarbon Bakken Shale and the Montney Shale in Canada. millimeters to centimeters. The difference in potential. This information helps them guide As in the example from the Pine Creek field, lithology between the burrows and the nondolo- exploration activities. extensive zones of trace fossils in these forma- mitized limestone mud matrix makes the bur- Bioturbation alters the physical properties of tions may improve gas storativity and the con- rows easy to image. a rock as it is being formed. The process can nectivity of porosity with induced fractures. Spot-permeability tests quantified the perme- increase or decrease porosity and permeability Bioturbation may also affect rock mechanical ability of the cores on a 0.5-cm [0.2-in.] grid. and can modify permeability anisotropy, some- properties, potentially influencing the outcome Permeability of the matrix is less than 1 mD, times to a significant degree. Quantifying these of hydraulic fracturing. whereas permeability of the dolomitized burrows effects and including them in reservoir simula- In a manner of speaking, many human activi- is greater than 100 mD. tion models can improve production predictions ties qualify as bioturbation. The wells we drill The large contrast in permeability between and enhanced oil recovery operations. and the tunnels we bore are on scales far sur- burrows and matrix gives rise to a distinctive pro- Bioturbation can have the same effects on passing those of burrows by sea creatures, but duction history for wells in the field(above) . For fine-grained layers as it has on reservoir rocks. we can still learn from the effects of their small- the first 15 years in the life of a well, the forma- Shales and mudstones may lose their capability scale efforts. By recognizing bioturbation and tion produces gas from the burrows. After the to act as reservoir seals if bioturbation causes a appreciating its consequences, geoscientists easy gas has been extracted, the declining pro- large increase in vertical permeability. In the are likely to improve their understanding of res- duction is interpreted to be of gas that has dif- Sirasun and Terang gas fields in Indonesia, the ervoirs and do a better job recovering hydro­ fused from the matrix into the burrows. Geologists marly caprock was found to have burrows that carbon resources. —LS studying this field have coined a new term— were filled with hollow foraminifera. The low- ichnofossil-hosted tight gas—to describe this permeabilityOilfield formation Review had certifiable reserves AUTUMN 14 3 burrow-matrix association. of 500 BioturbationBcf [14 billion Fig. m 16] of gas. The burrows causedORAUT14-BIOT it to acquire reservoir16 characteristics, Burrow Significance making for a leaky seal.19 Biologic disturbance of sediments can have Recent activity in gas- and oil-prone mud- many effects, for better or worse, on reservoirs. stone and shale formations—called unconven- By recognizing burrows and other trace fossils, tional reservoirs because they act as both source ichnologists gain knowledge they can incorpo- rock and reservoir—may benefit from more rate with other information to infer a formation’s studies of bioturbation. Evidence of bioturbation has been documented in several low-permeabil- 19. Pemberton and Gingras, reference 14. 20 20. Aplin AC and Macquaker JHS: “Mudstone Diversity: ity, fine-grained rocks. Ichnofossils have been Origin and Implications for Source, Seal, and Reservoir identified in the Woodford Formation and the Properties in Petroleum Systems,” AAPG Bulletin 95, no. 12 (December 2011): 2031–2059. Lower Marcellus Shale in the US and in the

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