CPH | Dolomitic Sand With Oil, Anhydrite Cap

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Triassic Dolomitic Sand
This example is Exercise 2 from Crain's Practical Quantitative Log Analysis for Conventional Reservoirs - 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-01 META/KWIK Log Analysis Conventional Oil Gas Bitumenn Metric
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.

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

THIS PAGE

Triassic Dolomitic Sand
Raw Logs and Results
META/KWIK Results
META/LOG ESP Results
Well History
Core Data Versions 1 and 2

THIS CHAPTER

Beginner's Guide
Integrated Petrophysics
Reservoir Description
What Is A Log?
Who Uses Logs?
Quantitative Analysis Basics
Near Wellbore Invasion
Rock-Fluid Model Definitions
The Log Response Equation

Reading / Picking Logs
Quicklook Visual Analysis
Crain's Rules - Visual Analysis
Basic Shale Volume Methods
Basic Porosity Methods
Basic Lithology Methods
Water Saturation Methods
Basic Permeability Methods
Net Pay and Productivity
Course Exercises

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