Mr. Akbar Sheriff, Area Development Geologist
Unocal 0il & Gas, California District

EVALUATION OF DEVILWATER SHALE

Dear Akbar:

This letter reports my initial findings from the examination of twelve samples of Devilwater Shale in the interval 4811-4828' in the McKittrick Front No.415 well to determine the probable effect of mud acid (12% HCl, 3% HF) on the formation.

SUMMARY

Reservoir quality in the proposed completion interval 4800-4950' is judged as fair to good. Analyses of cores and logs indicate that the dominant lithology is quartz-phase calcareous/dolomitic siliceous shale. The rock is naturally fractured and possesses matrix porosity of about 25% as well as minor oil saturation. Total clay content is relatively low and consists primarily of kaolinite with subordinate smectite, illite, chlorite, and vermiculite, Fine-grained dolomite is potentially the best reservoir rock. Total clay content is low and the rock is naturally fractured although the fractures are largely cemented.

The siliceous shale possesses matrix porosity and permeability that might be damaged by acid treatment if certaln precautions are not taken. Unlike "classic" Monterey slllceous rocks, the Devilwater reservoir probably contains a significant amount of oil in matrix porosity that feeds into fracture porosity. Invasion of acid into matrix porosity and reaction with clay and carbonate minerals can significantly reduce permeability .

The major lithologic factors influencing the success of acid stimulation are:

1. Clay Mineralogy

The siliceous shale contains a moderate (10-20%) amount of clay including kaolinite (3-10%), smectite (2-4%), vermiculite (0-3%), and chlorite (0-5%). To minimize potential damage an iron chelating agent and pH buffer should be included in the acid system, and the spent acid must be completely recovered from the formation.

2. Carbonate Mineralogy

Calcite and dolomite are significant components of the siliceous shale and dolomite reservoir rocks. The major risks posed by abundant carbonate are rapid depletion of the acid and precipitation of insoluble calcium fluoride. These effects can be controlled by proper acid system design.

3. Formation Integrity

The HCl-HF acid should not seriously disaggregate the siliceous shale In the wellbore. The dolomite may be strongly etched and might be expected to loose mechanical strength in response to acid treatment.

PROCEDURE

In the interest of saving time, I used twelve core plugs that had already been drilled by Core Lab, Bakersfield as part of their core analysis program. Sample depths and corresponding analytical tests are listed below:

Depth (ft)	Thin Section	Clay XRD	Bulk XRD	% Carbonate	Acid Reaction
4811-12'	X	X	X	X	X
4813-14'	X	X	X	X	---
4814-15'	X	X	---	X	---
4815-16'	X	X	X	X	X
4817-18'	X	X	X	X	X
4818-19'	X	X	---	---	---
4819-20'	---	---	---	---	---
4822-23'	X	X	X	---	---
4823-24'	X	X	X	X	---
4825-26'	X	X	X	X	X
4826-27'	---	---	---	---	---
48Z7-28'	X	X	X	X	---

Petrographic thin sections were prepared from ten of the core plugs. The same ten plugs were also analyzed for clay mineralogy by powder X-ray diffraction (XRD). Six plugs were subjected to bulk rock XRD to determine whole-rock mineralogy.

Eight of the cores have been analyzed for total carbonate utilizing a gasometric technique wherein powdered rock is reacted with 6M HCl in a closed vessel. The partial pressure of C0-2 generated by the reaction (measured by a calibrated pressure gauge) is proportional to the amount of carbonate in the rock.

Four samples were tested for their reaction to a mixture of 12% HCl and 3% HF by the simple method of placing a 1-2 gram piece of rock in the acid solution for 18 hours.

Two of the cores were subjected to acid flow testing by Leonard Kalfayan. No geologic analyses have been done on thsse plugs. The results of Kalfayan's work has been to you reported separately.

FINDINGS

Lithology

The cored interval 4808-4828' is described by Shiflett and Pitts as laminated siliceous shale with local calcareous laminations. My examination of the core plugs generally confirms this description with the exception that the core plugs appear to contain more carbonate and clay than described by Schiflett and Pitts. Individual macroscopic core plug descriptions are listed in TABLE 1.

The results of the thin-section study are summar1zed in TABLE 2. In general there is fair agreement between the above hand sample identification and the composition of the rock as determined by petrographic analysis. Four rock types are recognized in thin section: (1) shale, (2) siliceous shale, (3) porcelanite, and (4) dolomlte.

A brief description of each rock type follows:

1. Shale 4811-12', 4815-16'

A laminated, clay- and organic-rich rock containing abundant calcareous foraminifers and dispersed very finely crystalline carbonate. Detrital silt is negilgible (<1%). Layered phosphatic(?) pellets are common and very finely crystalline pyrite is scattered throughout the rock. Macroporosity is confined to intragranular porosity within foraminifer tests. This porosity is partiaily filled by authigenic quartz and coarsely crystalline carbonate. Microporosity within the clayey matrix is probably significant but it is too fine to measure in thin section. Irregular vertical fractures cut sample 4811-12'. The fractures are closed (no fracture porosity) and stained by oil.

1. 4811-12' cal/dol shale dark gray, speckled white with calcareous forams, laminated with thin concentrations of forams, moderately hard.

2. 4813-14' dolomite or calc/dol siliceous shale light gray, laminated, "sugary" texture with patchs of coarsely crystalline calcite, hard, sratched with difficulty by probe.

3. 4814-15' calc/dol siliceous shale AA, scattered forams and relic diatoms

4. 4815-16' dolomite or calc/dol siliceous shale medium gray, laminated, "sugary" texture.

5. 4817-18' calc/dol siliceous shale medium gray-green, "sugary" texture, contains small shale rip-ups, moderately hard, more easily scratched by probe than above samples.

6. 4818-19' calc/dol siliceous sha1e light-medium gray, speckled white with forams, laminated, moderately hard.

7. 4819-20' calc/dol siliceous shale light gray, laminated, moderately hard.

8. 4822-23' siliceous shale dark gray, 1aminated, moderately hard, possibly oil-saturated or rich in organic matter

9. 4823-24' calc/dol siliceous shale light-medium gray, low-moerate density, abundant forams, moderately hard

10. 4825-26' limestone/dolomite medium-dark gray, "sugary" texture, dense, healed fractures,very hard, scratched with much difficulty .

11. 4826-27' limestone/dolomite medium-dark gray with white fracture filling, Brecciated, dense, very hard

12. 4827-28' limestone/dolomite medium gray, "sugary" texture, dense, hard

[Table 1. Hand sample description of Devilwater core plugs.]

2. Siliceous Shale 4813-14', 4814-l5', 4817-18', 4818-19', 4822-23'

A laminated rock generally similar to "shale" but containing less clay and more authigenic quartz. Calcareous foraminifers and duspersed very finely crystalline carbonate total more than l0% of the rock in four of the five samples. Detrital silt constituents less than 1% of the rock except in sample 4822-23' which contains 3-5% detrital chert. Authigenic pyrite is a common accessory mineral occurring as dispersed very fine grains and aggregates of grains. Rare glauconite and phosphate are also present in nearly every sample. Fossils are mainly calcareous and arenaceous foraminifers but also include rare fish scales and bones, diatom fragments (replaced by carbonate), and phosphatic shell fragments. Isolated pores inside calcareous foraminifer tests constitute the only macroporosity. Microporosity was not measured but is probably substantial. Samples 4817-18' and 4822-23' are significantly fractured. The fractures are closed and oil stained.

3. Porcelanite 4823-24'

A silica-rich laminated rock distinguished from "siliceous shale" by its very low clay content but otherwise similar to siliceous shale with respect to carbonate content, detrital silt, fossils, and accessory minerals. Intragranular macroporosity is estimated at 3-5% and microporosity within the siliceous matrix appears very high. Incl1ned vertical fractures cut the rock. The fractures are closed and possibly oi1 stained.

4. Dolomite 4825-26', 4827-28'

A dense, vaguely laminated rock composed largely of finely to very finely crystallline dolomite. No clay is present and detrital silt makes up less than 1% of the total rock. Authigenlc chert fills some pores. Recrystallized calcareous foraminifers and cemented moldic porosity result in crystal-size variation across the thin section. Macroporosity in the form of isolated moldic pores (after foraminifers and diatoms) is negligible; intercrystalline microporosity could not be measured but it is likely significant. Sample 4825-26' is cut by a complex re-cemented (largely closed) fracture system. Some narrow fractures are open.

Bulk Mineralogy and Silica Phase

TABLE 3 dlsplays the whole-rock mineralogy of six Devilwater sanmples determined by powder XRD analysis. Listed below is a comparison of thin-section lithology with the quartz/feldspar ratio and carbonate content determined by bulk XRD.

Depth (ft)	Thin-Section Lithology	XRD Qtz/Fds	XRD Calcite	XRD Dolomite
4813-14	calc/dol siliceous shale	41	49%	4%
4815-16	calc shale	51	16%	--
4817-18	calc siliceous shale	46	7%	--
4822-23	siliceous shale	35	2%	--
4823-24	calc/dol porcelanite	very high	5%	--
4827-28	dolomite	--	--	80%

The XRD data shows a slight difference between shale (4815-16') and siliceous shale. The shale contains the most pyrite (3%) and [sic] well as measurable kaolinite (3%) and mica (2%). But the shale is not distinctly different - the siliceous shale at 4822-23 also contains kaolinite and mica.

The very high ratio of quartz to feldspar is indicative of quartz-phase silicca diagenisis, as is the absence of opal-CT silica. The location of the opal-CT to quartz transition is approximately 3770' based on log analysis.

Carbonate Mineralogy

Displayed below is a tabulation of total carbonate abundance and mineralogy determined by three separate techniques: (1) thin-section petrography, (2) gasometric measurement, and (~) powder X-ray diffraction (XRD).

Depth (ft)	Thin Section	Evolution of CO-2 (% total carbonate)	Calcite (%)	Dolomite (%)
4811-12	10-15+	3	-NR-	-NR-
4813-14	27-35	31	49	4
4814-15	10-30	33	-NR-	-NR-
4815-16	10	12	16	0
4817-18	5-10	1	7	0
4818-19	7-11	-NR-	-NR-	-NR-
4822-23	2-5	-NR-	2	0
4823-24	10	0	5	0
4825-26	91+	82	-NR-	-NR-
4827-28	95+	76	0	80

Correspondonce between the three measurements is good considering the experimental error associated with each technique. The measurements show that the Devilwater section contains a significant amount of calcite and dolomite.

Clay Mineralogy

TABLE 4 displays the clay mlneralogy of the ten Devilwater samples as determined by powder XRD. Shown below as a tabulation of total clay content from the XRD analysis compared to the semi-quantitative thin-section measurements.

Depth (ft)	XRD (%)	Thin Section (est. abundance)
4811-12	18	>75
4813-14	14	25-50
4814-15	18	25-50
4815-16	27	>75
4817-18	16	25-50
4818-19	10	25-50
4822-23	12	5-25
4823-24	4	<5
4825-26	9	0
4827-28	19	0

The apparent lack of agreement between the two sets of measurements can be explained by the common effect of visually overestimating clay volume in thin section and the tendency for XRD analysis to measure a low clay content if there is a significant amount of ''coarse-grained (>4um grain size) clay. The latter effect is due to the technique of sample preparation for XRD analysis wherein only the clay-size (<4um) fraction is analyzed. With the exception of sample 4827-28', high/low values of XRD clay abundance correlate with high/low estimates from thin-section study. The discrepancy between measurements for sample 4827-28' probably reflects variability in sample lithology; the thin section and clay XRD samples are different rock types.

The clay mineral suite is dominated by kaolinite with subordinate (but locally abundant) smectite, illite, chlorite, and vermiculite. The prevalence of kaolinite in these Devilwater samples is markedly different from the dominantly illite/smectite and smectite clay assemblages in Reef Ridge and Antelope rocks analyzed by us.

[TABLE 4 missing here.]

ACID REACTION

Four samples (shales 4811-12' & 4815-16'; siliceous shale 4817-18'; and dolomite 4825-26') were placed in 12% HCl and 3% HF at room temperature for 18 hours to determine the effect on the various rock types. The purpose of this simple test was to see if the rock samples maintained their physical integrity in the acid solution.

The three shaley samples were found to be little affected by the acid. There was minor "fizzing" and surface disaggregation due to the dissolution of carbonate but the samples maintained their shape and strength. The dolomite sample maintained its shape after the acid bath but it was weakened considerably and could be disaggregated with the fingers.

This test suggests that the formation will not be severely disaggregated by acid treatment although minor disaggregation and fines production should be expected.

LOG INTERPREATION

The log character of the zone 4800-4950' indicates a mixed lithology of quartz-phase siliceous shale, dolomite, and clay shale. The top of the zone at 48OO' marks the transition into a thick overlylng sequence of shale and dolomite. The section below 4800' is significantly more siliceous than the overlying strata. The most siliceous intervals within the zone appear to be 4800-08' and 4837-4867'. The cored interval (4808-4828') represents the "poorer" reservoir rock in the zone, assuming an uncorrected core depth.

RESERVOIR QUALITY

Reservoir quality in the proposed completion interval 4800-4950' is judged as fair to good. Analyses of cores and logs indicate that the dominant lithology is quartz-phase calcareous/dolomitic siliceous shale. The rock is naturally fractured and possesses good matrix porosity (approx. 25%) as well as minor matrix oil saturation. Total clay content is relatively low and consists primarily of kaolinite with subordinate smectite, illite, chlorlte, and vermiculite. Fine-grained dolomite, represented by two samples is potentially the best reservoir rock. Total clay content is low and the rock is naturally fractured although the fractures are largely cemented.

The siliceous shale possesses matrix porosity (25%) and permeability (0.5md) that might be damaged by acid treatment. Unlike "classic" Monterey siliceous rocks, the Devilwater reservoir probably contains a significant amount of oil in matrix porosity that feeds into fracture porosity. Invasion of acid into matrix porosity and reaction with clay and carbonate minerals can significantly reduce permeability.

Mud acidizing (HCL-HF) to remove invaded mud solids from perforations and near-wellbore fracture porosity should not cause formation damage if certain precautions are taken. The major lithologic factors influencing the response to acid stimulation are:

1. Clay Mineralogy

The siliceous shale contalns a moderate (10-20%) amount of clay. Non-expanslve kaolinite (3-10%) is the dominant clay mineral and it is only slightly effected by mud acid. Expans1ve smectite (2-4% and vermiculite (0-3%) are water sensitive and would be expected to reduce permeablility if contacted by fresh water or acid mixed in fresh water. Chlorite (0-5%) is soluble in HCl and poses a risk of precipitating insoluble iron hydroxide unless the acid solution contains an iron chelating agent. To minimize potential damage the spent acid must be completely removed from the formation.

The dolomite contains negligible clay and there is little danger of clay damage.

2. Carbonate Mineralogy

Calcite and dolomite are significant components of the siliceous shale and dolomite reservoir rocks. One negative effect of abundant carbonate 15 that the injected acid is spent comsuming carbonate rather than dissolving the invaded mud solids. This is why a HCl "pre-flush" typically precededs [sic] the HCl-HF slug. Another danger posed by treating a carbonate-rich rock with HCl-HF is the precipitation of insoluble calcium-fluoride from the reaction of calcite and HF. This risk can be minimized by buffering the HCl-HF solution with acidic or citric acid to maintain a low pH, and by removing the HCl-HF before it is spent .

3. Matrix porosity/permeability

The siliceous shale possesses matrix porosity (25%) and permeability (0.5md) that might be damaged by acid treatment. Unlike "classic" Monterey siliceous rocks, the Devilwater reservoir probably contains a significant amount of oil in matrix porosity that feeds into fracture porosity. Invasion of acid into matrix porosity and reaction with clay and carbonate minerals can significantly reduce permeability.

4. Formation Integrity

The HCl-HF acid did not seriously disaggregate the siliceous shale in a simple laboratory test. The dolomite was strongly etched and might be expected to loose mechanical strength in response to acid treatment.