Triassic Dolomitic Sand
This example is Exercise 2 from a Video Course for CEOs, geoscientists, engineers, and petrophysicists looking to improve their oil-finding skills.

This example points out several pitfalls that can trap inexperienced analysts. First, the high resistivity at the top of the log segment suggests a great hydrocarbon show. But this is an anhydrite cap rock with zero porosity. The density neutron log separation quickly show this. You need resistive AND porous to have a reservoir.

Second, the pay zones underneath the cap consists of three thin beds (each about 1.5 to 2 meters thick). It is tempting to average the porosity over the entire 5 meters. While this would give a reasonable average porosity and reserves estimate, it would underestimate the productive capacity of the well by a factor of about 10. That's because permeability is a function of porosity to the power 6 and the high porosity streak in the middle of the reservoir must be analyzed separately to see the tremendous impact it has on productivity.

Third, this is not a pure quartz sand and the density porosity by itself underestimates the effective porosity by 2 to 4% porosity, depending on the amount of heavy minerals in the sandstone. The quicklook rules given elsewhere in this Handbook can handle all these problems, but you have to apply them carefully. Traditional, over-simplified methods will give extremely pessimistic results, so don't use them.

Finally, there is no low resistivity on this log segment, so there is no obvious water zone. RW must come from another well down dip or from a catalog value.

rAW LOGS and PETROPHYSICAL RESULTS for EXERCISE 2

Raw log  data depth plots for Dolomitic Sand Example. Note how porosity is picked on the peaks because the beds are thin (2 meters – 6 feet or less). Even then, the log analysis will underestimate porosity compared to core.

Visual log analysis for Exercise 2, based on Crain's Rules

Depth plot for Dolomitic Sand Example. Black dots are core analysis data. The log does not match the core as well as we would like, but this is mainly due to very thin porosity laminations,
thinner than the logging tool resolution.

This example is sandstone with complex mineralogy. The porosity resistivity overlay will help find the zone of interest but the high resistivity of the anhydrite cap and lower shale may be confusing at first. Some people like simple rules for defining pay zones, such as a resistivity cutoff. This would not work in this case, or in many other situations either. The SP also suffers badly from the high resistivity.

"META/KWIK" QUICKLOOK SPREADSHEET  RESULTS for EXERCISE 2

The  best tool for quicklook log analysis is a spreadsheet. Pick parameters and log data from the raw logs shown above. When data entry is complete, the answers are instantly available.

Download and use these spreadsheets:
SPR-02 META/KWIK Log Analysis Conventional Oil Gas Bitumenn USA
Conventional Oil, Gas,Bitumen -- shale, porosity, saturation, permeability,  net pay, productivity, reserves.

META/KWIK data and results for Exercise 2

"META/LOG ESP" CROSSPLOTS and RESULTS for EXERCISE 2

The following crossplots were made using META/LOG ESP, an expert system on a spreadsheet.

1.

Crossplots for Dolomitic Sand Example

Crossplots help define the lithology and the lack of a water or transition zone.

1.       Porosity vs Resistivity - shows no water.

2.       Porosity vs Saturation - shows constant water volume lines. Data follows one hyperbolic line – no transition or water zones. Shale data falls to the bottom of the graph.

3.       Density vs Neutron - shows all data below sandstone line, indicating either no perfectly clean and or mixed lithology sand (GR suggests clean sand). Shale data falls towards bottom and right.

4.       Core porosity vs core permeability - shows a data distribution that can be used to derive a regression line mathematically.

The listings for final results and hydrocarbon summary are shown below. The log analysis porosity is a little low and could be increased to match core analysis better by adjusting the target DENSMA parameter in the META/LOG program.

Results and raw data for Dolomitic Sand Example

Summary results of log and core data for Dolomitic Sand Example

Notice the very close comparison between the core porosity (black dots) and log analysis porosity (smooth line) on the computed log analysis depth plot shown above.

This is a thin bed problem as much as a heavy mineral problem, so data in the porous interval must be picked at very close intervals. The “peaks and valleys” rule applies. Any form of averaging the data will mask the contribution of the central porous “hot spot”. Ninety percent of the productivity comes from this 1.5 meter interval (2056.5 –2058.0 meters), but it contains only half the oil in place.

Notice the excellent permeability versus porosity relationship shown in the graph at the right.

WELL HISTORY INFO

Pancanadian Beaverlodge 11-36-72-08W6

KB Elev: 689.4 m      Logs: DIL-SP, FDC-CNL-GR, BHCS-GR

Log depths in METERS.

Testing Record 11-36

DST #2: 2044.0 – 2070.0 m  Inflate Straddle
VO: 10.0/45.0 SI: 150.0/249.0 min

FP: 1937.0/2877.0 psi  SIP: 3124.4/3101.5 psi  HP: 3614.2/3614.2 psi

PMAX: 21686 kPa; BHTEMP: 72 C; Relatively high permeability; no formation damage;

Blow description: test run with closed chamber, oil to surface in 30 minutes after blowdown on final flow.  Recovery: 323.6 m clean uncontaminated oil; maximum gas to surface 42793 m3/d (1.511 mmcf/d)

PRODUCTION HISTORY

This well had an initial rate of 200 bbl/day and declined gracefully until year 22 when pump rate

was increased, causing water breakthrough.

Cumulative production was 429 000 bbl oil and

Over 1 million bbl water. On a logarithmic production scale, the exponential decline is a straight line. The log analysis predictions are a little high on
recoverable oil and a little low on initial production
rate. Only a minor amount of tuning is required to get both results to match ground truth.

Linear production scale (above) shows exponential decline, logarithmic scale (right) shows a straight line, also demonstrating exponential decline

Listing for Dolomitic Sand Example

Compare your results with the core data on the next page, especially in the thin high porosity streak 2056.8 to 2058 meters. You will also have to adjust the CPERM parameter in the permeability equation to get a good match to core. Note that a conventional shaly sand analysis will under-estimate porosity by as much as 4% porosity, so the only correct approach is the shale corrected complex lithology model recommended in this book. If heavy minerals are not a problem, the complex lithology model works just as well as the shaly sand model, so there is no reason to ever use the shaly sand approach..

CORE ANALYSIS DATA FOR 11-36-72-08W6

 11367208W6 S# Top Base Len Kmax K90 Kvert Porosi GrDen BkDen Soil Swtr Lithology meters meters meter mD mD mD frac kg/m3 kg/m3 frac frac 1 2054.35 2054.54 0.19 0.32 0.30 0.08 0.045 2881 2796 0.138 0.138 DOL INTRANHYARGL 2 2054.54 2054.74 0.20 2.45 2.38 0.41 0.100 2737 2563 0.152 0.237 SS  F   DOL 3 2054.74 2054.91 0.17 20.40 20.00 0.34 0.116 2689 2493 0.147 0.118 SS  F   CALC 4 2054.91 2055.07 0.16 16.40 16.40 0.83 0.103 2706 2530 0.136 0.136 SS  F   CALC 5 2055.07 2055.26 0.19 64.50 57.70 40.30 0.145 2683 2439 0.117 0.183 SS  F   CALC 6 2055.26 2055.48 0.22 60.30 58.80 37.30 0.148 2679 2431 0.124 0.198 SS  F 7 2055.48 2055.58 0.10 84.20 80.00 0.01 0.145 2700 2454 0.116 0.206 SS  F 8 2055.58 2055.74 0.16 1.77 0.31 0.03 0.037 2736 2672 0.104 0.363 SS  F   DOL 9 2055.74 2055.89 0.15 10.00 10.00 4.86 0.124 2694 2484 0.156 0.208 SS  F   DOL 10 2055.89 2056.02 0.13 15.00 14.20 0.36 0.119 2695 2493 0.145 0.232 SS  F   DOL 11 2056.02 2056.21 0.19 25.40 19.10 0.07 0.099 2721 2551 0.000 0.142 SS  F   CALC 12 2056.21 2056.30 0.09 15.00 0.01 0.01 0.107 2700 2518 0.188 0.263 SS  F 13 2056.30 2056.47 0.17 99.80 98.60 54.70 0.147 2696 2447 0.107 0.246 SS  F 14 2056.47 2056.75 0.28 230.00 225.00 164.00 0.158 2679 2414 0.101 0.251 SS  F 15 2056.75 2056.93 0.18 189.00 170.00 67.00 0.168 2691 2407 0.098 0.245 SS  F   CALC 16 2056.93 2057.13 0.20 206.00 198.00 175.00 0.171 2678 2391 0.088 0.296 SS  F 17 2057.13 2057.37 0.24 108.00 104.00 94.10 0.166 2658 2383 0.120 0.361 SS  F 18 2057.37 2057.55 0.18 152.00 141.20 82.00 0.196 2663 2337 0.115 0.298 SS  F 19 2057.55 2057.73 0.18 135.00 135.00 80.00 0.191 2672 2353 0.162 0.246 SS  F   V/F 20 2057.73 2058.01 0.28 186.00 186.00 80.00 0.207 2659 2316 0.099 0.262 SS  F   V/F 21 2058.01 2058.26 0.25 37.10 36.50 0.55 0.129 2701 2482 0.095 0.219 SS  F   DOL 22 2058.26 2058.44 0.18 207.00 181.00 28.60 0.197 2683 2351 0.086 0.224 SS  F 23 2058.44 2058.62 0.18 0.90 0.29 0.01 0.022 2737 2699 0.276 0.331 SS  F   CALC 24 2058.62 2058.77 0.15 271.00 237.00 5.88 0.150 2678 2426 0.081 0.359 SS  F   CALC 25 2058.77 2058.99 0.22 7.45 7.33 0.12 0.091 2701 2546 0.109 0.146 SS  F   DOL 26 2058.99 2059.20 0.21 15.70 14.00 0.06 0.098 2700 2533 0.163 0.163 SS  F   DOL 27 2059.20 2059.42 0.22 27.80 18.89 4.35 0.139 2697 2461 0.162 0.223 SS  F   DOL 28 2059.42 2059.59 0.17 12.80 12.80 0.05 0.104 2710 2532 0.183 0.160 SS  F   DOL 29 2059.59 2059.76 0.17 30.90 29.60 0.01 0.075 2720 2591 0.145 0.181 SS  F   DOL 30 2059.76 2059.88 0.12 77.90 77.10 68.10 0.145 2647 2408 0.086 0.205 SS  F 31 2059.88 2060.14 0.26 76.20 72.90 25.50 0.160 2666 2399 0.096 0.221 SS  F 32 2060.14 2060.34 0.20 21.50 20.30 0.10 0.185 2777 2448 0.132 0.205 SS  F   DOL 33 2060.34 2060.47 0.13 12.60 11.80 0.38 0.102 2719 2544 0.177 0.155 SS  F   DOL 34 2060.47 2060.77 0.30 0.08 0.08 0.01 0.047 2716 2635 0.138 0.255 SS  F   DOL ARGL 35 2060.77 2060.95 0.18 0.13 0.07 0.01 0.055 2712 2618 0.000 0.535 SS  F   DOL 36 2060.95 2061.10 0.15 0.01 0.01 SHALE 37 2061.10 2061.26 0.16 0.02 0.01 0.01 0.031 2752 2698 0.000 0.504 SS  F   DOL ARGL 38 2061.26 2061.52 0.26 0.03 0.01 0.01 0.047 2743 2661 0.000 0.505 SS  F   DOL 39 2061.52 2061.71 0.19 0.05 0.01 0.01 0.012 2720 2699 0.000 0.622 SS  F   DOL 40 2061.71 2061.94 0.23 0.01 0.01 SHALE 41 2061.94 2062.07 0.13 0.02 0.01 0.01 0.027 2773 2725 0.000 0.534 SS  F   DOL 42 2062.07 2062.31 0.24 3.31 3.20 1.32 0.084 2707 2564 0.155 0.217 SS  F   DOL 43 2062.31 2062.54 0.23 8.80 8.38 3.62 0.121 2735 2525 0.096 0.119 SS  F   DOL 44 2062.54 2062.67 0.13 12.90 11.30 6.41 0.132 2800 2562 0.103 0.144 SS  F   DOL ANHY 45 2062.67 2062.92 0.25 0.90 0.80 0.01 0.077 2737 2603 0.108 0.108 SS  F   DOL 46 2062.92 2063.09 0.17 0.37 0.37 0.04 0.057 2726 2628 0.167 0.232 SS  F   DOL 47 2063.09 2063.26 0.17 0.54 0.53 0.13 0.080 2731 2593 0.107 0.215 SS  F   DOL 48 2063.26 2063.39 0.13 1.51 1.40 0.62 0.077 2721 2588 0.134 0.205 SS  F   DOL 49 2063.39 2063.64 0.25 0.42 0.37 0.17 0.074 2717 2590 0.116 0.143 SS  F   DOL 50 2063.64 2063.89 0.25 0.29 0.28 0.03 0.074 2741 2612 0.149 0.275 SS  F   DOL 51 2063.89 2064.00 0.11 0.18 0.16 0.02 0.050 2701 2616 0.084 0.189 SS  F   DOL 52 2064.00 2064.17 0.17 0.26 0.23 0.09 0.061 2720 2615 0.048 0.240 SS  F   DOL 53 2064.17 2064.31 0.14 0.16 0.04 0.01 0.031 2772 2717 0.123 0.197 SS  F   DOL ARGL 54 2064.31 2064.46 0.15 0.13 0.09 0.03 0.059 2743 2640 0.138 0.277 SS  F   DOL 55 2064.46 2064.64 0.18 0.02 0.02 0.01 0.039 2755 2687 0.000 0.314 SS  F   DOL ARGL 56 2064.64 2064.80 0.16 0.06 0.05 0.01 0.028 2736 2687 0.000 0.406 SS  F   DOL 57 2064.80 2064.96 0.16 0.12 0.09 0.01 0.046 2740 2660 0.000 0.270 SS  F   DOL 58 2064.96 2065.19 0.23 0.01 0.01 SHALE 59 2065.19 2065.39 0.20 0.08 0.05 0.01 0.061 2782 2673 0.000 0.622 DOL P/S ANHYSD 60 2065.39 2065.53 0.14 1.27 1.18 0.10 0.081 2783 2639 0.089 0.221 DOL P/S ANHYSD 61 2065.53 2065.71 0.18 1.78 1.56 0.29 0.074 2802 2669 0.087 0.134 DOL P/S ANHYSD 62 2065.71 2065.87 0.16 0.32 0.32 0.02 0.060 2783 2676 0.000 0.565 DOL P/S ARGLSD 63 2065.87 2066.08 0.21 0.28 0.24 0.04 0.065 2791 2675 0.054 0.108 DOL P/S ANHYSD 64 2066.08 2066.15 0.07 0.18 0.01 0.01 0.077 2780 2643 0.000 0.054 DOL P/S SD  CHT 65 2066.15 2066.54 0.39 0.18 0.15 0.01 0.041 2849 2773 0.100 0.125 DOL P/S ANHYSD 66 2066.54 2066.67 0.13 30.90 28.10 2.28 0.156 2815 2532 0.119 0.089 DOL P/S ANHYSD 67 2066.67 2066.82 0.15 13.30 10.20 2.18 0.108 2774 2582 0.115 0.064 DOL P/S SD 68 2066.82 2067.10 0.28 4.62 2.34 1.52 0.092 2787 2623 0.108 0.108 DOL P/S ANHYSD 69 2067.10 2067.34 0.24 0.36 0.19 0.01 0.039 2831 2760 0.047 0.047 DOL P/S H/F ANHYSD 70 2067.34 2067.57 0.23 0.06 0.04 0.01 0.033 2804 2744 0.096 0.479 DOL PP/VANHYSD 71 2067.57 2067.59 0.02 0.01 0.01 SHALE 72 2067.59 2067.86 0.27 0.07 0.07 0.01 0.040 2758 2688 0.139 0.312 SS  VF  DOL ARGL 73 2067.86 2067.87 0.01 0.01 0.01 SHALE 74 2067.87 2068.04 0.17 0.05 0.05 0.01 0.033 2752 2694 0.092 0.459 SS  VF  DOL ARGL 75 2068.04 2068.29 0.25 0.16 0.15 0.01 0.067 2761 2643 0.000 0.610 SS  F   DOL 76 2068.29 2068.54 0.25 0.04 0.04 0.01 0.025 2782 2737 0.000 0.396 SS  F   DOL 77 2068.54 2068.76 0.22 0.05 0.04 0.01 0.031 2754 2700 0.000 0.284 SS  F   DOL 78 2068.76 2068.91 0.15 0.02 0.02 0.01 0.027 2795 2747 0.000 0.360 SS  F   DOL 79 2068.91 2069.10 0.19 0.11 0.03 0.01 0.016 2753 2725 0.000 0.427 SS  F   DOL 80 2069.10 2069.39 0.29 0.04 0.02 0.01 0.024 2803 2760 0.000 0.600 SS  F   DOL ARGLPRY 81 2069.39 2069.68 0.29 0.02 0.02 0.01 0.019 2794 2760 0.000 0.382 SS  F   DOL ANHY Arithmetic Averages 0.19 33.0 28.8 12.8 0.089 2735 2582 0.091 0.268

Core data listing for Dolomitic Sand Exercise 2

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