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PSReservoir Heterogeneity Caused by Diagenesis in Tensleep Sandstones, Teapot Dome, Wyoming*

By

Peigui Yin1, Dag Nummedal2, and Qingsheng Zhang3

 

Search and Discovery Article #50016 (2005)

Posted September 27, 2005

 

*Adapted by authors from poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005

 

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       Poster 1       Poster 2       Poster 3

 

1IER, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071

2Colorado Energy Research Institute, Colorado School of Mines, Golden, CO 80401

3Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401.

 

Abstract 

Potential reservoir lithofacies in the Teapot Dome Tensleep Sandstone include eolian dune, interdune, and sand sheet, all of which are composed of fine- to very fine-grained, quartz arenites, with local concentration of dolomite sand grains. Lamination and grain size variation inherited from depositional environments caused the variation of porosity and permeability in different depositional units, whereas cementation and compaction further increased the reservoir heterogeneity.

 

The major diagenetic events identified in the Tensleep Sandstone are listed in Figure 1. Microcrystalline dolomite is concentrated along finer-grained laminae, reducing porosity and causing permeability directionality. The dolomite was formed by direct precipitation from hypersaline solutions or dolomitization of lime mud and carbonate grains. Incipient quartz overgrowth was developed after the formation of aphanocrystalline dolomite crystals. Anhydrite cement exists as nodules, patches, or zones, precipitated along the remaining coarser-grained laminae. Presence of the anhydrite cement led permeable eolian sandstones heterogeneous. Mechanical compaction distorted the ductile dolomite sand grains, damaging porosity and permeability due to filling of the intergranular space.

 

In the cross-bedded strata of Tensleep eolian sandstones, the thin laminae are usually cemented by aphanocrystalline dolomite, whereas the thick laminae are cemented by anhydrite (Figure 2). This cementation is expressed as pin stripes (Fryberger and Schenk, 1988) on outcrops and the alteration of dark and light lamination in subsurface cores (see Figure 2). The feature is a distinctive in eolian sandstones. Precipitation of the dolomite and anhydrite requires special depositional and diagenetic conditions.

 

Formation of the dolomite and anhydrite commonly occur in the coastal areas under arid conditions (McKENZIE et al., 1980). In these areas, recharge from the oceanic water can provide calcium and magnesium cations to the eolian dunes, and intensive evaporation will facilitate precipitation of carbonate and sulfate minerals (Figure 2).

 

After the oceanic water flooding, the ground water table tends to drop rapidly under arid conditions. However, The water column can be much high within the fine-grained laminae than within the coarse-grain laminae due to capillary pressure (Fetter, 1994), and the dolomite could be precipitated from these fluids with evaporation (Figure 3). The fine-grained laminae had been cemented by aphanocrystalline dolomite when anhydrite was precipitated from with continuous evaporation. Therefore, the anhydrite precipitation filled the coarse-grained laminae (Figure 2).

 

After the diagenetic modification, the eolian dune, interdune, and sand sheet lithofacies in the Teapot Dome area can be classified into six petrophysical facies:

  • Uncemented, dolomite-sand-free, dune sandstone facies (DUC).

  • Cemented, dolomite-sand-free, dune sandstone facies (DC).

  • Cemented, dolomite-sand-rich, dune sandstone facies (DDC).

  • Uncemented, dolomite-sand-rich, interdune sandstone facies (IDUC).

  • Cemented, dolomite-sand-rich, interdune sandstone facies (IDC).

  • Cemented, dolomite-sand-rich sand sheet facies (SSht).

Each petrophysical facies exhibits different reservoir qualities (Figure 4).

 

Precipitation of dolomite and anhydrite cements and richness of dolomite sand grains are related to depositional environments and ground water tables. Therefore, reservoir quality of the Tensleep sandstones can be predicted based on the integrated study of depositional systems and diagenetic history.

 

Figure Captions

 

Figure 1. Diagenetic events identified in Tensleep Sandstone.

Figure 2. Distribution of dolomite and anhydrite in cross-bedded eolian sandstones.

Figure 3. Oceanic water soaks the eolian sands during the high tide flooding (above). The concentrated fluids will remain in the fine-grained laminae, while the coarse-grained laminae have dried up with extensive evaporation (below).

Figure 4. Plots of porosity versus permeability for each petrophysical facies.

 

 

 

 

 

References 

Fryberger, S.G., and Schenk, C.J., 1988, Pin stripe lamination: a distinctive feature of modern and ancient eolian sediments: Sedimentary Geology, v. 55, p. 1-15.

McKenzie, J.A., Hsu, K.J., and Schneider, J.F., 1980, Movement of subsurface waters under the sabkha, Abu Dhabi, U.A.E., and its relation to evaporative dolomite genesis, in Zenger, D.H., Dunham, J.B., and Ethington, R.L., eds., Concepts and Models of Dolomitization: SEPM Special Publication 28, p. 175-189.