Journal of Petroleum Geology, vol. 16(3), July 1993, pp. 313-322 313

PETROPHYSICAL CHARACTERISTICS OF THE MIDDLE EOCENE LAUMONTITE TUFF RESERVOIR,

SAMGORI FIELD, REPUBLIC OF GEORGIA

M.E. Grynberg", D. Papava", M. Shengelia", A. Takaishvili", A. Nanadze* and D.K. Patton""

I n the Samgori ,field, necir Tbilisi in the Republic o f Georgia, changes in reservoir properties ciiustd by ciltrration of miiCc/le Eocene volcanogenic rocks control the amount ofoil and gas these rocks will yidd. These changes. which include,frncturing, development o f vugs. and laumontite ziwlitizotion. impart n vcrtical and lateral variirbility. thus giving rise to irregular petrophysical propflies of the rock such cis jiltrational capacity, elastic deformational properties, and productivity. Indiceitivr qf such d(ffLrences is the large variation in initiol recovery.from one well to unother

Accwrding to tht. reservoir model proposed here, productivity at Samgori is dominantly cmtrolled by ,frocturcJ porosity and permeability. Fractured bodies of oil-bearing laurnontized tufts (ire enclosed Mithin ci thick layer ofandesitc+hasalt tufs and tuffites. which act as a seal. As ci

resi4lt, thP. frncturcd tuff.\ are relativt+ isolated-from the action ofgeostatic and geotectonic loads. which would othmvise result in closure o f the,frcictures and microfractures. In addition, the.fluid prmure of the reservoir tends to maintain open ,fractures. As jluid pressure declines over the producing llfe of the ,field. effLctive stress incream ond the .fractures close. This results in the isolation of producing zones and the consequent development o f reservoir heterogeneity. A n understanding qf this effect i s essmricil to proper managc.ment o f the reservoir so as to ensure optimum rc.c.ovLvy of oil.

The contribution of mcitrii porusity to productivity is olso recognized. Fluid injection analysis has establishPd thcit matrix porosity and permeability vary according to lithology, with the highest vrilui)s in laurnontized tuffi: lairmontite inter-crystalline voids can also be observed, using sccinning electron microscopy.

INTRODUCTION

Production of more than 165 MM brl of oil at Samgori from a laumontite tuff. an atypical oil reservoir. has stimulated reservoir characterization work to determine the effect of various parameters on reservoir performance. Patton (1993) presented the regional geological setting for the field and proposed a model for the generation. migration and entrapmcnt of thc oil and gas. Vernik (1990) has presented data on

* Georgiiin Oil, 65 Kostrivo St.? Tbilisi 380015, Republic of Georgia. ** Eanh Sciences find Resourcw Institutes Universit-v o f South Carolina, Columbia. SC 29208,

USA. Current trddress: Kolkhis Oil cind Gas, 7537 S. University Blvd.. No. 206, Littleton. Co. 80122, USA.

petrophysical properties, pore morphology, pore volume. sonic-wave propagation. and strength properties of the reservoir. He offered methodology to estimate in-siru stress using downhole logging data. and discussed the significance of borehole strength on productivity. Grynberg et al., (1991) discussed variability of production in the field. the significance of matrix porosity and the relationship between formation pressure and fracture porosity. The present paper reviews a proposed model for the Sumgori field reservoir (Grynberg er 01.. 1991) and incorporates new data from SEM examination and X-ray diffraction analysis of reservoir rock samples. SEM photographs depict the crystal habit of the laumontite in the productive horizon, confirm the presence of microfractures. and provide observations of volume and morphology of the matrix porosity.

RESERVOIR LITHOLOGIES

Reservoir rocks at Samgori can be grouped according to filtrational capacity properties determined by analysis of cores from deep wells. The significance of each group depends upon the development of properties favourable to hydrocarbon production. and also upon the extent to which the group occurs in the section.

Properties considered are related to genetic features. including post-sedimentary changes. and productivity. which has been affected by secondary processes. Implicit in the rationale for establishing the groups is the need to derive quantitative criteria for identifying petrophysically similar intervals. The work of determining these criteria is still in progress. To the extent that quantitative criteria have been formulated, they are reflected in the petrophysical and lithologic contrasts displayed by the five groups listed in Table 1.

The most widespread class are andesite-basalt tuffs and tuffites (60-70% of section) which. despite wide textural differences (grain size varies from very-fine sand to granule), can be identified as a class of low-porosity tuffogenes (Groups I , 3). The rocks of this class are characterized by secondary processes that decrease primary porosity through chloritization. increase in carbon content, development of illite, sericitization. and analcitization. Connected porosity i n these rocks is approximately 5%. with effective porosity of 0.6%. and a permeability of 0.05 md. In the subsurface. the matrix is totally saturated by water. Thin sections reveal that the effective porosity depends upon the existence of fractures. Fractures in the oil-bearing intervals of these rocks are saturated by oil.

The second class in order of frequency of occurrence (20-25% of section) is andesite- basalt tuffs and calcareous tuffites (Groups 2 and 4). which are predominantly silty to sandy, with relatively high connected porosity ( 1 1 - 12%). an effective porosity of0.4-0.85%. and permeability of 0.01-0.1 md. These rocks are concentrated at the top of the section. Laterally, in the SE part of the region, the degree of alteration within this class is lcss than in the preceding class. Vugs are commonly developed along microfractures. Andesite- basalt tuffs and tuffites from the Southern Dome of Sumgori show patchy mineralization and micro-fracturing produced by metasomatic processes. Effective porosity reaches 2.5- 3%. and permeability is 10-15 md.

Least widespread. but important as a reservoir, is a class of partially or completely laumontized tuffs and zeolites that have been studied in the Sumgori, Gleri, Ninorsmindcr. Rusruvi and Munavi fields. This class includes porous. fractured reservoir rocks of Group 5 and part of Group 2. accounting for I O - l S % of the total section in the Samgori and Eleti fields and i n single occurrences as much as 20%.

For example, within a 300-m interval of deep middle Eocene in Well 121 in the Sumgori field, there are 60 m of partially and completely laumontized tuffs. including a 30-m zone of fractured rocks. These tuffs are sometimes well-stratified and often form lens-like bodies. stocks. and vein-like bodies (referred to. i n the Russian literature. as "aposomes")

Table 1. Parameters of petrophysical groups of middle Eocene volcanogenic-sedimentary rocks, near Tbilisi. Republic of Georgia. From Grynberg et a/. (1991).

P- Wave Volcanogenic Density Density Porosity Porosity Permeability Content Formation Velocity

Grain Bulk Connected Effective Carbonate

Group Sedimentary Rocks (lO-'kg/m.'l (IO-'kgh+j (%) (%) (mD) (%I Factor (IO'm/sec)

I Low-porosity andesite- basalt tuffs. tuffites and tuff mark

2 High-porosity andesite- basalt tuffs and tuffites

3 Low-porosity analcime tuffs

4 High-porosity analcime tuffs

5 Laumontite tuffs

2.68 (725)* 2.61-2.91

2.68 ( 189) 2.56-2.88

2.52 (140) 2.36-2.62

2.51 (41) 2.36-2.61

2.57 (77) 2.4-2.58

2.58 ( 1506) 2 4-2.78

2.47 (298) 2.35-2.67

2.44 (258) 2.28-2.6

2.34 (41) 2.2 1-2.42

2.41 (127) 2.06-2.46

5.4 (1625) 0.2- 10.8

11.2 (298) 6-23.3

5 (258) 0.3-10.8

12 (41) 8-17

12 (132) 6.3-27

0.56 (640) 0.056 (1050) II (972) 151 (545) 4.95 (641) 0- I .05 0-0.644 0-59.5 18-2140 3.03-6.33

0.85 (I 16) 0.102 (242) 8 (207) 77 (161) 4.34 (161) 0-2.5 0.001-44 0-44 13-25 1 2.92-5.96

0.38 (135) 0.005 (174) 3.5 (161) 305 (83) 5 (90) 0.1-1.6 0-0.173 0-29 40-944 38.6-64.8

0.42 (40) 0.015 (40) 3 (41) I1 1 (40 4.23 (40) 0.1-1.5 0.001-0.14 0.3-7.9 2 1-248 3.47-5

3.7 (28) 14.8 (81) 3.9 (1 1 I) 41.2 (65) 3.25 (53) 1.14-7.2 0.14-464 0-9.2 12-97 23.2-44.1

* Mean value (number of samples)

Minimum and maximum values

Fig. 1. SEM-photo depicting blocky crystal habit of laumontite, typical of Ssmgori field reservoir. Energy-dispersive X-ray spectrum is from probe at white arrow near lower-centre of photo, and indicates

major elements of Ca, Si and Al present in laumontite.

that serve as lithologic traps for oil. They each contain their own system of irregular microfractures. Together with the host rocks. these bodies arc cut into blocks by subvertical fractures and form a single hydrodynamic system.

Laumontization of tuffs decreases to the east and N E of the region (Wrll4. Ninotstnitidci) where intensely zeolitized tuffs and laumontized tuffs make up only 2-3% of the section. Such variability affects field development as well as reserve estimates for undrilled prospects. A suitable reservoir model must provide for the effects of variable zeolitization, because zeolitization is characteristic of post-depositional alteration of volcanic rocks in general.

X-ray diffraction analysis of sample 196 from the 2,520-2,525 ni interval in Samgori Well I21 confirms a high percentage of laumontite in this reservoir, with 47% of the minerals in

M. E. Gtytiherg et al. 317

Fig. 2. SEM-photo of intercrystalline matrix porosity developed within laumontite in NW quadrant of photo. Energy-dispersive X-ray spectrum is from probe at white arrow near left-centre of photo, and indicates major elements of Ca, Si and A1 present in laumontite. Mineralogy of smaller crystals in SE

quadrant is less certain, and illite and chlorite may be present.

the rock identified as laumontite. Fig. 1 depicts the laumontite crystal habit typically observed here.

It should be noted that when connected porosity in the partially and completely laumontized tuffs reaches values close to those of the high-porosity tuffogenics (12%). there is also an increase in effective porosity (3.7-7%) and permeability (14.8-460 md) in the laurnontized tuffs. Samples with a permeability of less than 0.1 md can be considered to possess matrix permeability only, whereas higher permeability measurements commonly associated with intensive invasion of drilling mud is characteristic of micro- fractures.

Fig. 3. SEM-photo of void space created by microfracture trending through centre of photo. Energy- dispersive X-ray spectrum is from probe at white arrow in left-centre of photo, and indicates major

elements of Ca, Si and Al present in laumontite.

The existence of permeable matrix in these rocks. even when microfractures are absent. is established by study of the morphology of the micropores by fluid injection. These fluid-injection data indicate the prevalence of micropores with diameters greater than IO-' m. the thickness of the physically-bound water film. Vernik (1990) also reports this porc- size distribution. Partially and completely laumontized tuffs, as well as highly porous silty and sandy tuffs and tuffites. are productive reservoirs of the fracture-porosity type. but where the capacity of matrix in these rocks is significantly greater than the capacity of fractures, production is then due to both pores and fractures.

Both matrix porosity and rnicrofracture porosity can be observed in the sample of laumontite reservoir which was analyzed by X-ray diffraction as above. Connected

3 19

Rosc c l i a p i n indicates fracture trends

Fig. 4. Model of middle Eocene volcanogenic-sedimentary reservoir, Samgori field. From Grynberg et a/. (1991).

porosity of the sample from kerosene injection is 9.80% and effective porosity is 7.46%. Permeability to gas i 4 reported a s 5.54 md and 14.51 md measured at 90°C. These values ;ire typical of the laumontite reservoir at Strnigori (Table I ) . Significant porosity attributed to intercrystalline voids can be observed by SEM. confirming the presence of matrix porosity resulting froin laumontization (Fig. 2). Microfractures also contribute to perinenbility (Fig. 3 ) and are evident in thin-section.

FRACTURES

Kcgional fracturcs i n volcanogenic sequences in the Adjara-Trialeti folded zone observed at the surface maintain their orientation at the depth of the middle Eocene reservoir rocks. Most common directions are NW (310-330"). NE (40-70") and east-west (100-1 lo"). The bulk of the fractures trend close to the direction of the fold axes (310-330"). but some fractures also trend 0-20" (Fig. 4). The direction of fractures intersected by well bores coincides with the directions listed above. I n addition to regional fractures. localized schistose cleavage is also present.

Open subvertical fractures provide the hydrodynamic connection for the reservoir. Fluid prcssure tends to maintain open fractures. because i t relieves the effects of lithostatic pressure and reduces effective stress. As fluid pressure declines over the producing life of wells i n the field. effective stress increases and the fractures close.

When the vertical component of average effective stress. the distribution of the measured Poisson coefficient. the thickness of the bed. and the value of the current

320 Samgori Jeld. Republic of Georgia, Part 11: Prfroph-vsical charactrristics

tormation pressure are known, it is possible to predict vertical-fracture porosity during production of the field (Grynberg ef al., 1991).

RESERVOIR MODEL

An understanding of fractured, laumontized reservoirs and their distribution enables us to develop a more precise model for the middle Eocene volcanogenic-sedimentary reservoir in the Tbilisi Region. Previously, the reservoir was considered to be completely saturated by water, impermeable, with a low matrix porosity (3-5%). and cut by a system of fractures. The fracture system caused good hydrodynamic communication in the oilfield, resulting in a reservoir comparable in thickness to the entire middle Eocene section. In this assessment, oil was supposed to be concentrated only in fractures and vugs, but as communication was present throughout the reservoir, high productivity resulted. Heterogeneity of the reservoir was considered to be based on variations in density and openness of the fractures and microfractures, and the possibility of matrix porosity was not considered.

The current view is that bodies of partially and completely laumontized tuffs are enclosed within a thick layer of andesite-basalt tuffs and tuffites, which acts as a kind of seal (Fig. 4). As a result, the altered tuffs are relatively isolated from the action of geostatic and geotectonic tension. This promotes the preservation of high-productivity characteristics over a wide range of tensional conditions. Without the seal and resulting isolation, such tensional conditions would reduce the opening of the fractures and microfractures which cut the rocks. Vernik (1990) also recognized this shielding effect.

DISCUSSION

The volcanogenic massif is composed of rocks whose Poisson and borehole-strength coefficients differ significantly. Where the vertical gravitational component increases slowly and uniformly with depth, the horizontal components vary from layer to layer. As a result, subvertical fractures cutting layers with different horizontal components are differentially opened. Consequently, the fractures are open to varying extents, with potential permeability and productivity higher in those sections that are affected by lesser horizontal components of the tensional field.

The most significant characteristic influencing productivity is fracture permeability. Orientation and width of subvertical open fractures (Fig. 4) result in anisotropy of fractured reservoir rock and control productivity. The fractures control the morphology of the middle Eocene reservoirs, their relationship with structures of the area, and the distribution of hydrocarbons.

Relationships between changes in distribution of the fractures as a percentage of the oil-saturated section, and changes in formation pressure and borehole strength, are shown on Fig. 5, together with the change in formation pressure over time during development of the Samgori field. This figure illustrates the closure of subvertical fractures as formation pressure in the field (defined by "a", which is formation pressure divided by normal hydrostatic pressure) decreases. For example, when a=0.89, which is characteristic for the middle Eocene reservoir in the initial stages of development, the open subvertical fracturing is distributed over an average of 6570% of the oil-saturated section (Line 1). The maximum decrease in formation pressure during the production period was 2.8 megapascals. This was equal to an "a" value of 0.80, and was reached in 1984. As a result of the change in tensional conditions, distribution of subvertical fracturing decreased to an average value of 4040% of the oil-saturated sequence (Line 2). The result is an increase in reservoir heterogeneity and a decrease in production.

Oil was initially recovered from the most permeable zones, which have well-developed fracturing. During production, as formation pressure decreased, some of the fractures

M. E. Gryrrherg et al. 32 1

60 -

80 &,Ye

Fig. 5. Relationship between formation pressure (p), borehole strength (A), and distribution o f open fractures (0) during production, Samgori field. From Grynberg et a/. (1991).

began to close. This resulted in isolation of oil-bearing zones, rapid encroachment of water into the fractures. increased reservoir heterogeneity, massive water flooding, and a sharp decrease in oil recovery. In 1984. formation pressure reached the lowest point during the development of the field (Fig. 5) . This resulted in irreversibly increased reservoir heterogeneity and isolation of pockets of oil. For example, Well 137 was drilled between totally-watered Wells 7 and 45, but produced oil with no water for two years. Of two neighbouring wells. 39 and 117, the first went to water after 18 months of production, but after a long shut-in period returned to producing pure oil. Well 117 continues to flow oil. Well 120 was completed near wells which were producing a high percentage of water, but it was completed as a flowing oilwell and continues to flow.

There are two main types of wells in the Sumgori field: wells that are highly productive (600-1200 tons/day) and wells that are not (30-60 tondday). Wells of the first group are productive for 1.5- 2 years before they begin producing with a high water-cut ( W l l s 3, 7. 8. 13. 15, etc.). Wells of the other group remain productive and stable, with levels close to their initial potentials and low water cuts (Wells 12, 68. 76. 120, etc.). Highly-productive wells drain high-permeable zones of the field; wells of lower productivity drain less- permeable zones. Isolation of wells is the result of increasing heterogeneity of the reservoir, which develops during production. The Samgori field is divided into isolated, seemingly randomly-occurring areas of varying permeability that include zones of unrecovered oil reserves.

CONCLUSIONS

( 1 ) During drilling and development involving complicated geological-geophysical targets in volcanogenic sedimentary sections, i t is necessary to consider the existence of fractures and vuggy porosity in reservoir rocks. Predictions which incorporate the effect of variable tensional conditions on filtrational capacity are necessary to develop appropriate drilling, testing and producing programs.

(2) Technology and development planning must provide for equal drainage of all parts of the field by balancing well locations in order to maintain stable formation

pressure. I f production results in a n abrupt decrease in formation pressure, ii part of the oil-in-place will not be recovered due to reservoir heterogeneity. and total oil recovery will decrease accordingly. Fracture permeability. which varies over thc producing lifc of t he wells and the irregular distribution of laumontite matrix porosity at S~/mguri. prcclucles precise estimates of total oil recovery by the statistical methods norrnally used for calculating reserves d 11 r i n g ti el d d cve I o p men t .

( 3 )

ACKNOWLEDGMENTS

T h e Authors a re grateful to Georgian Oil for iiccess to data a n d [or permission to publish these rcsults. Appreciation is cxtcntlctl to Anadorko Petrolcum Corpn.. Kerr- McGee Corpn.. Louisiana Land a n d Exploration Co.. and Norcen International Ltd for linancial support a n d for permission to publish. Professors A. Nuirn atid M. Gipson. Jr. made many hcl p ful sugges tions. Ko t i st a ti t i n Ak hvlctliii ni ki ntl l y p rovitlccl t rii t i slii t io 115

from the Russian language.

REFERENCES

GKYNBERG. M . E.. PAPAVA. D.. S H F . N G t L I A . M.. 'I 'AKAISHVILI. A,. and NANAIIZL. A,. 1991 ( 1 1 1 R I ~ . v . Y ~ w ~ ) . The morphology of the Middle Eocene rewrvoir and p~irticii l~irs ol' the exploitation of Srrtiigori Field. Grolo~i , (! / 'Oi l trt1rl G"/.\. March. 1901. 23-25,

PATTON. D. K.. 1993. .%ti~jywi I'ielcl. Republic ol' Cieorgiii: critical rebieu 01' i\lancl-arc oil and p i s . Joiirti. Am)/. G d , 16 (2). 153- I6X.

VEKNIK. L w . 1990. A i icw type o f reservoir rock in \olciiiiicla\tic. wcliiciices. . L 4 K Ri t l l . . 74, X 3 0 - X30.