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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 243, part 1
Sedimentology and Ichnology of Paleozoic Estuarine and Shoreface Reservoirs, Morrow Sandstone, Lower Pennsylvanian of Southwest Kansas, USA--page 12 of 14

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Discussion (continued)

Implications for Evaluation of Reservoir Heterogeneity

Recognition of valley-fill sandstones in the lower Morrow has profound implications for hydrocarbon exploration and subsequent production because reservoir quality is largely influenced by external geometry and distribution of depositional facies. Our integrated ichnologic, sedimentologic, and stratigraphic approach shows that the lower Morrow Sandstone displays more complex types of heterogeneity than previously envisaged by the open-marine model. In these complex tracts of depositional systems, heterogeneity is created at different scales by facies and facies-assemblage distributions and by spatial partitioning within sandstone bodies. The petrophysical characteristics of the different sedimentary facies were documented in detail by Byrnes et al. (1999).

Three basic styles of heterogeneity have been recognized by Weber and van Geuns (1990): layer cake, jigsaw puzzle, and labyrinth. The open-marine model predicts a layer-cake style of facies distribution as a consequence of strandline-shoreline progradation. On the other hand, recognition of valley-fill sequences points to more compartmentalized reservoirs--either the jigsaw puzzle style (with complex crosscutting relationships of sandstone bodies) or labyrinth style (consisting of isolated reservoir sandstones)--due to heterogeneity created by valley incision and subsequent infill.

Galloway and Hobday (1996) identified five levels of heterogeneity: gigascopic, megascopic, macroscopic, mesoscopic, and microscopic. Gigascopic heterogeneity is shown at the scale of depositional systems, while megascopic heterogeneity deals with the geometry of permeable and impermeable units. The architecture of depositional systems in the lower Morrow of southwest Kansas is more complex than previously believed, and valley, interfluves, and open-marine areas are now recognized. A clear example of controls on reservoir characteristics at the scale of depositional systems is shown in the Arroyo field, where distribution of fluvio-estuarine, valley-fill deposits is roughly coincident with the contours of the reservoir. In interfluve areas, such as those exemplified in the Fretz core, nonproductive paleosol facies are the stratigraphic lateral equivalent of reservoir valley sands.

Macroscopic heterogeneity is expressed at the facies scale. Sedimentologic and ichnologic analyses indicate a high variability in sedimentary facies, which governs fluid behavior, and porosity and permeability heterogeneities. The proposed facies scheme for the Arroyo and Gentzler fields provides a way to analyze heterogeneity at the macroscopic scale (cf. Byrnes et al., 1999). For example, the distinction between fluvial (facies A) and estuarine (facies C) channel sands in Arroyo field is based on the presence of bioturbation and mud drapes (as well as other tidal structures) in the transgressive estuarine sands. Although both channel fills contain good-quality reservoir sands, permeability is higher in the fluvial facies because mud-baffles restrict the flow in the estuarine sands. Shoreface sandstone packages are laterally continuous in Gentzler field. However, facies subdivision of these sandstone bodies in upper-, middle-, and proximal lower-, and distal lower-shoreface facies provides evidence of reservoir heterogeneity. Finer-grained sediments and mud drapes in the proximal and distal lower shoreface commonly create permeability barriers within reservoir shoreface sandstones.

Mesoscopic heterogeneity occurs at the scale of lithofacies and stratification, while microscopic heterogeneity is expressed at the scale of individual grains and pores. In the lower Morrow, mesoscopic and microscopic heterogeneities are reflected by the styles of bedding and lamination, presence of mud drapes, biogenic disruption of primary fabric, and diagenetic overprint. Although the current assumption is that bioturbation reduces porosity and permeability, this is not necessarily the case. Deposit-feeders that backfill their burrows may damage pore connectivity in certain situations, but open structures produced by suspension-feeders and passive carnivores do not reduce porosity and permeability and may even act as conduits for fluid migration (cf. Muñoz, 1994; Gingras et al., 1997). This is shown by high values of porosity and permeability in estuarine-channel facies and upper- to middle-shoreface facies, which contain a suite dominated by traces of suspension-feeders and carnivores.

Compartmentalization also results in a very complex mosaic of flow units. While the open-marine model essentially envisages layer-cake shoreface sandstones, albeit with distinct internal permeability barriers as shown by our facies zonation, the valley-fill model involves a more complex pattern of external geometries of flow units, including ribbons, pods, prisms, belts, lobes, and sheets.

Integration of sedimentologic, petrographic, ichnologic, and stratigraphic data with petrophysic information allows recognition of different reservoir zones. Fluvial sandstones (facies A), encountered at the base of Kendrick, are the highest quality reservoirs. These sandstones contain scarce calcite cement, are mainly unbioturbated, and have very high values of porosity and permeability. A similar reservoir facies has been recognized in the upper Morrow by Al-Shaieb et al. (1994). Although slightly less permeable, bay-head delta sandstones (facies C) also represent very good reservoirs. A suite of dwelling traces of suspension-feeders of the Skolithos ichnofacies is present in these deposits, but bioturbation does not seem to have obliterated porosity and permeability. In fact, the presence of burrows probably channelled sediment fluids (e.g., Muñoz, 1994; Gingras et al., 1997). In their discussion of estuarine sands in the upper Morrow, Al-Shaieb et al. (1994) attributed low porosity to clay dispersion by burrowing animals. In the present case, however, the decrease in permeability is most likely related to the presence of mud drapes, which act as local barriers for fluid migration. Secondary porosity was created by dissolution of glauconite grains. Estuary-mouth, coarse-grained sandstones (facies H) also include good-quality reservoirs. These sandstones have porosity values similar to those of fluvio-estuarine channel sandstones, but permeability is remarkably lower. According to Zaitlin and Shultz (1990), estuary-mouth bar sands are typically better reservoir facies than inner-estuary sands. In the present case, however, original permeability may have been reduced by cementation due to calcite precipitation derived from dissolution of skeletal fragments. Central-bay shales (facies D) do not include reservoirs because of their finer grain size, but they may have acted as effective seals for reservoirs in fluvio-estuarine sands (cf. Zaitlin and Shultz, 1990). Associated tidal-flat and channel heterolithic deposits (facies E and F) are also poor reservoirs because of their very fine grained sandstones, intense soft-sediment deformation, and abundant clay drapes.

In the open-marine system, middle- to upper-shoreface sandstones (facies I and J) include reservoirs of high to moderate quality. As in the case of estuarine channel sands, suspension-feeder burrows do not obliterate porosity. However, porosity was reduced due to partial cementation by calcite and dolomite, derived from dissolution of the abundant skeletal fragments present in these facies (cf. Al-Shaieb et al., 1994). Lower-shoreface sandstones (facies K and L) contain relatively poor reservoirs because of their finer grain size and the intense and disruptive bioturbation by deposit-feeding infauna of the Cruziana ichnofacies, particularly in the distal lower shoreface. Activity of deposit-feeding animals that backfilled their burrows and dispersed clay sediment throughout the matrix typically obliterates porosity and permeability. Finally, offshore transition, offshore and shelfal deposits (facies M, N, and O) are very fine grained and locally totally bioturbated; therefore, they do not include reservoirs. In summary, the emerging picture is one of a heterogeneous and compartmentalized reservoir, displaying a complex pattern in distribution and connectivity of reservoir sandstones, and impermeable and semi-permeable vertical and lateral seals at different scales.

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Kansas Geological Survey
Web version November 9,1999