Subsurface Databases: Graphical Display and Error Detection for Stratigraphic Interpretation in the Michigan Basin
William D. Everham and Jacqueline Huntoon
email : wdeverha@mtu.edu
phone : (906) 487-2826

Presented at the 1996 Geological Society of America Annual Meeting in Denver, CO.

Presentation Figures

Abstract

Subsurface mapping and stratigraphic interpretation is greatly facilitated by the use of digital data that can be used to generate graphic displays. Graphic displays of lithologic sample descriptions (LSD) obtained from driller's logs constitute a source of information that can be used to rapidly identify potential inaccuracies in commercially available subsurface databases, subsurface facies changes, and display sequence stratigraphic interpretations. Generation of pseudologs that contain depth vs. lithology information in Log ASCII Standard (LAS) or Log Binary Standard (LBS) format allows use of log analysis software to display information and correlate wells.

Prior to use, LSD data from driller's logs must be converted to digital form. This can be accomplished in-house or through the purchase of a commercial database. For our study a FORTRAN program was written to extract the LSD data from a commercial database and output the data in LAS format. Commercial log interpretation software was utilized to convert the LAS file to LBS format. A log template was created to graphically display the lithology data. Cross-sections that display lithology data and top subsea picks were then generated.

Examples from the Michigan Basin demonstrate that the display of data in a graphical fashion provides the user with a visual method to compare top subsea picks with log and lithology data. Potentially incorrect top picks can be easily identified using this method, and the database information corrected. Regional cross-sections made using these data define large-scale sediment patterns and permit identification of stratigraphic sequences with a greater degree of confidence.



Development

Lithologic Sample Wells Extraction Procedure Original Database Format Las Format Lithologic Log Display
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The subsurface database used in this project was purchased from Aangstrom Precision Corporation. It contains information on 51,275 oil and gas wells in Michigan's Lower Peninsula. Of those, 11,474 wells contain Lithologic Sample Description (LSD) data. The figure titled Lithologic Sample Description Wells shows the distribution of the LSD wells. The first step in the creation of the pseudolog lithologic displays was to convert these data to the digital form used by our log interpretation software. This was accomplished utilizing a FORTRAN program written to extract the depth and dominant lithology information from the database and output a Log ASCII Standard (LAS) file. The figure titled Process shows a flow chart of the extraction procedure. The figure titled Database Format shows the data in its original format. The figure titled LAS File shows the data in the program output format. (The data extracted from the database to be used in the LAS file is in red.) When the lines containing the LSD data are reached, the program reads the depth range and lithology as a character string. From this the start and end depths are determined and a value of one is assigned to the lithology of that depth range. A value of zero is assigned to the other possible lithologies. Currently the program recognizes ten different lithologies; those most frequently occurring in the basin.

A subroutine converts the character depths to integer values. For example, in the samples below it can be seen that for the depth range 605 to 665 the lithology is shale (SH) and for 845 to 875 it is gypsum (GY). This process creates a log "curve" file where the "curve" represents the lithology.

The next step is to convert the LAS file to the Log Binary Standard (LBS) format. This is easily done by applying GeoGraphix-Schlumberger QLA 2 Log Interpretation Program's log conversion utility. QLA 2's Log Display module is used to create the pseudolog lithology display template. The figure titled Lithologic Log Display with Gamma Ray and Neutron Density Curves shows the lithology "curve" (with the curve area filled in with a color and/or pattern depending on the lithology), gamma ray, and neutron density curves as well as the top subsea picks for several formations. These log display templates are used in conjunction with GeoGraphix Exploration System's Cross Section and Wellbase modules to create the pseudolog lithologic cross-sections seen on the other section of this display.


Stratigraphic Sequences

Stratigraphic Sequences Plan View Map of Michigan Shale Influx
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The cross-section titled Stratigraphic Sequences consists of four wells, starting in Missaukee County and running through Clare and Gladwin Counties to Bay County. (See Plan View Map of Michigan for the cross-section location, labeled A-A'.) It is hung on the top of the Traverse Formation which contains a highly radioactive layer, providing a good time line. The cross-section consists of the lithology log "curve" and the top picks for several formations. The top picks were used to determine the location of the Sloss sequence boundaries3 (magenta lines). The lithology log "curve" was employed to create the relative sea level curves. These curves were drawn showing relative sea level changes based on the solubility of the carbonates and evaporites present in the basin (least to most soluble: limestone-dolomite-gypsum-anhydrite-halite). High relative sea levels were associated with deposition of limestone, the low levels with halite deposition. Where halite is overlying limestone a rapid sea level fall was assumed. These relative sea level lows were then correlated between wells, creating the 3rd or higher order sequence boundaries (dark green & dark red lines). Previous work done in the basin4 has recognized the existence of two of these sequence boundaries in the lower Silurian: one at the top of the Clinton Group and the other at the top of the Niagara Group. The sequence boundaries in the cross-section below labeled 1 and 2 match those boundaries. The visualization utility of the lithologic logs greatly facilitated the identification of these sequence boundaries.

The cartoon titled Shale Influx shows the probable mechanisms of shale deposition in the basin. The cross-section reveals several episodes of shale influx. The shale layers surrounded by halite are assumed to be deposited at relative sea level lows (bottom section of cartoon). Conversely, those associated with limestone are assumed to be deposited during relative sea level highs (top section of cartoon).





Error Detection

Error Detection Plan View Map of Michigan
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The error detection utility of the pseudolog lithologic display can be seen in the cross-section titled Error Detection. It consists of three wells in northern Osceola County. (See Plan View Map of Michigan for cross-section location, labeled B-B'.) The cross-section contains the lithology "curve", as well as the gamma ray, neutron density, and caliper curves. The first well shows an obvious error in the top picks for the Bell shale and Dundee limestone (light blue rectangle). Making use of the lithology "curve" and the other curves we can determine better top subsea values for these two formations (new top subsea lines in red, new picks in blue). Another apparent error is the top pick for the Sylvania sandstone in the third well (magenta rectangle #1). A slight drop in the radioactivity and density is an indicator of the presence of the Sylvania sandstone2 and the lithology "curve" shows a sandstone. Based on this information the Sylvania top pick should be adjusted upward. We can also use the available information to determine a Syvania pick for the second well (magenta rectangle #2). (New top subsea line in red, new pick in blue.) A somewhat less obvious error appears toward the bottom of all three wells. There is a large drop in the density and an increase in the caliper at the top of a halite unit which is apparently the top of the Salina F-unit salt (dark green line).

In addition to allowing quick detection of errors, the lithologic display is an effective teaching tool. The display permits easy visualization of the log curves and their responses in different lithologies. This utility could prove very useful in general and especially to those not familiar with the lithology of a particular area. For example, the "unique" density and caliper log responses in the Detroit River Group evaporites is easily seen (light green rectangles).



Lithology and Temperature

Lithology and Temperature Plan View Map of Michigan
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The figure titled Lithology and Temperature is made up of the lithologic log and the digitized temperature log for a well in Oceana County. (See Plan View Map of Michigan for well location.) The curve represents the third of three temperature log runs. These data were used to estimate the thermal conductivity (k) for the penetrated units. The first unit is a dolomite extending from 5000 to 5240 ft. (red lines). The second is a sandstone unit extending from 5240 to 7150 ft. (blue lines). The calculations were made using Fourier's Law: q = k(dT/dz). Where, q = heat flow, k = thermal conductivity, and dT/dz = thermal gradient. Heat flow values in the Michigan Basin range from 33-58 mW/m2. A heat flow of 1.1 HFU or about 46 mW/m2 was measured5 in a well near this one and is used in these calculations. The thermal gradients were found to be 11.3 C/km and 12.4 C/km, for the dolomite and sandstone, respectively. The calculated conductivity for the dolomite is 3.5 W/mK. This value is consistent with values obtained from a conductivity vs. temperature curve for dolomite6. The sandstone conductivity was found to be 3.8 W/mK. This value is consistent with values obtained from a conductivity vs. temperature curve for a dense, water saturated sandstone6 (the water content of our sandstone is unknown).


References

  1. Ages from AAPG (1985). Correlation of Stratigraphic Units of North America (COSUNA) Project.
  2. Lilienthal, R.T. (1978). Report of Investigation 19. Stratigraphic Cross-sections of The Michigan Basin. MI DNR.
  3. Fisher, J.H., et al. (1988). Michigan Basin Chapter 13. The Geology of North America Vol. D-2. GSA & Sloss, L.L. (1963). Sequences in the Cratonic Interior of North America. GSA Bulletin. Vol. 74.
  4. Howell, P.D. (1993). Styles of Susidence in the Michigan Basin. Ph. D. Dissertation. University of Michigan.
  5. Combs, J. & Simmons, G. (1973). Terrestrial Heat Flow Determinations in the North Central US. Journal of Geophysical Research. Vol. 78.
  6. Robertson, E.C. (1988). OFR 88-441. Thermal Properties of Rocks. US Dept. of Interior. Geological Survey.




Acknowledgments

Project funding provided by the US Department of Energy (Cooperative Agreement #: DE-FC22-94BC14983).

Computer facilities provided by the Department of Geological Engineering and Sciences Subsurface Visualization Lab; Dr. James R. Wood, Director.

All figures were produced using GeoGraphix Exploration System (GES) 7.7 software. The software was provided by GeoGraphix, Inc. 1860 Blake St., Suite 900, Denver, CO 80202; (303) 296-0596; geog@geographix.com.

Well data is from the Michigan Oil and Gas Well Database purchased from Aangstrom Precision Corp., 5805 E. Pickard Ave. Mt. Pleasant, MI 48858; (517) 772-2232.

Cartographic data was downloaded from the USGS web site. http://www-nmd.usgs.gov/