Case Study 7:PENNSYLVANIAN CANYON LIMESTONE, NEW MEXICO, U.S.A.:

The Problem.

This case study illustrates the advantage of havinga measurement of lithology that is largely independentof porosity and fluid saturation: the photoelectriceffect (Pe) from the spectral density log. Because thiscase study is the final one of the sequence, the inter-pretation is left largely up to the reader to plan andcomplete. As with the other case studies, solutions areprovided for validation of the reader’s efforts.

If you decide to make an invasion correction of thedual laterolog, this equation will provide accuracy thatis sufficient for this case study:

10.43

Background

In the late 1990s an independent oil companydrilled a 7150-ft well in southeastern New Mexico that

penetrated Precambrian basement. The well wasdrilled on a small faulted anticline. After the well wasat total depth (TD), it was logged with this saltwater-mud logging suite: Dual Laterolog-Rxo and GammaRay-Neutron-Spectral Density (Figure 10.35). Yourtask is to determine whether the zone is potentiallyproductive, and if so, how much hydrocarbon is inplace.

Well site information and other pertinent information

These facts pertain to the porous Canyon zone from6758 to 6772 ft (Figure 10.35):

• The neutron and density porosities (PHIN andPHID, respectively) are quantified with refer-ence to limestone.

• From samples of bit cuttings, the zone wasdescribed as light tan to gray ooid grainstonewith abundant oomoldic porosity.

• Rw = 0.03 ohm-m at formation temperature.• Rmf = 0.0406 ohm-m at formation temperature.The Work Table, Table 10.13, shows depths of

zones suggested for interpretation; entries in the Solu-tion Table (Table 10.14) are based on these depths.

Log Interpretation Case Studies 235

Table 10.13. Canyon Sandstone, New Mexico, U.S.A.: Work table. The symbol “v/v” indicates volume-for-volume decimal fraction.

Parameters

Rw (measured): 0.03 ohm-m @ Tf Rmf @ fm. temp.: 0.041 ohm-m @ Tf

a: 1 m: 2 n: 2

Data

Depth LLD LLS MSFL Pe PHID PHINfeet ohm-m ohm-m ohm-m b/electron v/v decimal v/v decimal

6760

6763

6768

Calculations

Depth Rt PhiNDgas Swa Sxo Swrfeet ohm-m v/v decimal v/v decimal v/v decimal v/v decimal MHI BVW

6760

6763

6768

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236 ASQUITH AND KRYGOWSKI

Figure 10.35. Dual laterolog-Rxo. Canyon Limestone, New Mexico, U.S.A. Vertical scale: 2 ft per chart division. Gamma ray curve scaled in API units.Resistivity curves scaled in ohm-m. Density porosity and neutron porosity scaled in porosity units, 0.04 per chart division.

Dual laterolog-Rxo. Canyon Limestone, New Mexico, U.S.A.

In the interval 6758 to 6772 ft note the following facts:

1. The dual laterolog exhibits a salt-mud hydrocarbon invasion profile. (See Chapter 1, Figure 1.5 for an example.)

2. The neutron and density porosity curves show crossover (φ d>φ n) in the zone described from cutting samples as limestone; the crossover is indicative ofthe presence of gas.

3. Average value of the Pe curve is about 5, which confirms that the zone is limestone.

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Case Study 7:PENNSYLVANIAN CANYON LIMESTONE, NEW MEXICO, U.S.A.:

The Solution.

From Figure 10.35 the following attributes of theCanyon interval (6758 to 6772 ft) can be discerned:

• The dual laterolog shows a profile consistentwith invasion in a hydrocarbon-bearing zone.

• Neutron porosity is less than the density porosi-ty (“crossover”). Because the formation isdescribed as a limestone, and because the poros-ity curves are referenced to limestone, onewould expect the porosity curves to be overlaindirectly if the formation were liquid-filled (oil orwater). The crossover indicates gas in the for-mation.

• The Pe curve averages about 5 units, confirmingthat the zone is limestone, and that the neutron-density crossover is a pore-fluid effect (i.e. gas).

The following conclusions should be drawn fromyour calculations:

• The zone is porous, with gas-zone, neutron-den-sity porosity (Equation 10.18) showing from0.10 to 0.18 (10 to 18 %) (Table 10.14).

• Archie water saturations (Equation 10.1) rangefrom 0.07 to 0.12 (7 to 12 %).

• Estimates of bulk volume water, BVW (Figure10.36, and Table 10.14) range between 0.011and 0.013; they are below the carbonate cutoffvalue of 0.015 (Chapter 7, Table 7.1), indicating

that the formation should produce water-free.• The values for moveable hydrocarbon index

(MHI) are under the carbonate cutoff of 0.6, alsoindicating that the Canyon should producehydrocarbons.

• The Archie water saturation values (Swa) are lessthan the ratio water saturation values (Swr)—which is expected—because porosity of theCanyon zone is oomoldic (see pg. 117), asdescribed from inspection of the bit-cuttingsamples.

The Canyon was perforated from 6760 to 6770 ft;initial potential flowing (IPF) was 918 mcfgpd plus 3bopd and NO water. If you assume that this well willdrain 160 acres, the original gas in place (OGIP), esti-mated from the equation below (from Equation 10.12),is 1.74 BCF (billion cubic feet).

OGIP = 43,560 � PhiNDgas � (1 – Sw) � thick-ness � ((0.43 � depth)/14.7) � area

where:• PhiNDgas = 0.14 (average)• Sw = 0.10 (average)• thickness = 10 ft• depth = 6766 ft (average)• area = 160 acresWith the average recovery factor for this formation

in this area of 0.70 (70%), the well could produce 1.21bcf of gas. At a price of $2.00 per mcf, the productionwould be worth approximately $2,400,000.

See the sample calculations in Table 10.14, andresults computed from samples spaced at one-half-footintervals, shown in Figure 10.37.

Log Interpretation Case Studies 237

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238 ASQUITH AND KRYGOWSKI

Table 10.14. Case Study 7: Canyon Sandstone, New Mexico, U.S.A.: Solution table. The symbol “v/v” indicates volume-for-volume decimal fraction.

Parameters

Rw (measured): 0.03 ohm-m @ Tf Rmf @ fm. temp.: 0.041 ohm-m @ Tf

a: 1 m: 2 n: 2

Data

Depth LLD LLS MSFL Pe PHID PHINfeet ohm-m ohm-m ohm-m b/electron v/v decimal v/v decimal

6760 170 150 80 5.0 0.135 0.050

6763 200 130 40 4.6 0.125 0.065

6768 130 95 35 4.9 0.230 0.115

Calculations

Depth Rt PhiNDgas Swa Sxo Swrfeet ohm-m v/v decimal v/v decimal v/v decimal v/v decimal MHI BVW

6760 190 0.102 0.123 0.222 0.479 0.555 0.013

6763 270 0.100 0.106 0.321 0.249 0.329 0.011

6768 165 0.182 0.074 0.188 0.312 0.394 0.013

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Log Interpretation Case Studies 239

Figure 10.36. Bulk-volume-water (BVW) plot, CanyonLimestone, New Mexico, U.S.A.

Figure 10.37. Canyon Limestone, New Mexico, U.S.A. Computer-processed log.

Note:

1. True formation resistivity, Rt, calculated from LLD and LLS, is shown in Track 2 with the deep laterolog, LLD.

2. Archie water saturation, Swa, is shown as the light-shaded area from Swa = 1.0, on the left-hand side of Track 3.

3. Hydrocarbon-filled pore space is shown as the dark shaded area on the right-hand side of Track 3, between the porosity, PhiNDgas, and thebulk volume water, BVW. Water-filled pore space is shown as the light shaded area between BVW and the right-hand margin of the track(scale value of zero).

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