Ámbito educativo en la población gitana

Porosity vs. Depth

High- and low-porosity zones are stratigraphically correlatable in wells from the Gyda area (Figure 5A).

High-quality reservoir zones are seen in units C1₂and B₂, whereas nonporous sandstones are seen in units C1₄, C1₁, B₁, and A (Figure 5A). The transgressive

sandstone in unit A₁, cored in well 2/1-2, has moderate porosity compared to its burial depth (3340 m RKB [rel-ative to Kelly Bushing]). Low porosities in the muddy intervals of units C1₄and B₁are probably related to the high clay content (e. g., high gamma-ray signals) of these rocks. The low porosity in the cleaner interval of unit C1₁, however, contrasts the good porosity in the apparently analogous sandstones of unit C1₂and the upper parts of unit B.

The porosity is more uniformly distributed throughout the cored sections in the sandstones from the Ula area (Figure 5B). Low porosities are observed within muddy intervals and in carbonate cemented zones. The wells from the central part of the Ula field (i.e., wells 7/12-2, 7/12-4, and 7/12-6) have porosities of ~20% at ~3400 m RKB. The more deeply buried sec-tions of well 7/12-7 and 7/12-5, however, have porosi-ties of only 15% and 12% at 3800 m and 3900 m, respectively. Wells 7/11-5 and 7/11-6 also show large porosity variations within apparently homogeneous sandstones. A reversed relationship between porosity and clay content is indicated in units A and B in well 7/11-5. Here the porosity is higher in unit B than in unit A, although the gamma-ray log signals indicate cleaner sandstones in upper parts of A than in B.

The lithostratigraphical units are divided into three

“reservoir-quality facies” with respect to the relation between porosity and depth. Facies 1, the high-porosity outliers, is characterized by high porosity (>20%) at depth >4 km and represents units B₂and C1₂. Facies 2, the normal-porosity sandstones, are those characterized by low gamma-ray signals and low porosity at great burial depth (>4000 m) (i.e., units A₁, A₃, and C1₁). Facies 3, the poor reservoir quality sandstones, are those with Figure 3. Burial depths for oil and gas fields off Norway.

high gamma-ray signals and poor reservoir quality at all depths (Table 1). Least-squares regression lines based on the porosity vs. depth relationship for the three reser-voir-quality facies are shown in Figure 6. Near 3500 m, the porosity is approximately the same in facies 1 and 2, where the porosity in both clean, low gamma-ray sand-stone facies is ~25%. Below 3500 m, however, the poros-ity in the two facies differentiates. In the facies 2 sandstones, the porosity–depth gradient is steep, and at 4500 m these sandstones have porosities <10%. To the contrary, the facies 1 sandstones lose porosity more gen-tly with depth, and porosities near 20% are preserved to 4500 m. The muddy facies 3 sandstones have porosities

<15% at all depths, and these units are in essence unprospective at depths below 3000 m.

Porosity vs. Pore Fluid Composition

The porosities of facies 1 and 2 sandstone units from oil zones and from the water legs/dry wells are plotted vs. depth in Figure 7. The figure depicts no sig-nificant and systematic difference in porosity between water- and hydrocarbon-saturated reservoirs.

The porosity difference across the oil-water contact is particularly large in well 7/11-5. In the oil-filled unit B and the water-filled unit A, the 75% porosities are 23.6% and 12.3%, respectively. In well 1/3-3, however,

the oil-water contact is located about 10 m below the boundary between the low-porosity unit C1₃and the high-porosity unit C1₂. However, a minor decrease in porosity is observed across the oil-water contact at 4221 m RKB. The average porosity between 4213–4221 m is 23.7% (standard deviation 2.0%), whereas between 4221 and 4247.5, it is 22.1% (standard deviation 2.3%). The standard deviations in the two subpopu-lations are approximately equal, and a simple one-way analysis of variance can be used to reject, at the 95% confidence level, the hypothesis that the poros-ity difference above and below the water zone (1.6%) is not statistically different (the calculated φvalue for the data is 9.6, with 1 and 102 degrees of freedom, which is larger than φ(0.05;1102)= 3.9). However, this does not necessarily mean that it is the difference in the pore-fluid composition that causes the porosity difference.

Porosity Related to Lithology

The high-porosity zones in units C1₂and B₂in well 2/1-6 contain extensively bioturbated, fine-grained sandstones. The low-porosity unit C1₄comprises muddy graywackes and siltstones, whereas the low-porosity sandstone in unit C1₁is a relatively clean, medium-grained sandstone. The coarsening/cleaning-upward sequence in unit B indicates a clear inverse Figure 4. Upper Jurassic stratigraphy in the Cod Terrace area, showing the chronostratigraphic and

sequence stratigraphic division of the Upper Jurassic strata and distribution of shales and sandstones in the Feda Graben and Cod Terrace area (modified from Forsberg et al., 1994). Letters refer to

sequences discussed in the text.

Figure 5. Well correlation between wells from the Gyda area (A) and Mime-Ula area (B) showing gamma-ray log and core-porosity panels for the individual wells. The range in the porosity panels is 0%–30%. Location of wells is shown in Figure 1.

correlation between clay content and porosity. How-ever, this correlation is disrupted by the clean, low-porosity sandstones in unit C1₁.

Inverse correlation between clay content and poros-ity is also observed in units B and C1_1–3of well 7/11-5.

The gamma-ray logs from this well indicate a slightly higher clay content in the low-porosity unit C1_1–3than

in the high-porosity unit B (Figure 5). The upper part of unit A, however, resembles unit C1₁in well 2/1-6 and has very low porosity, in spite of a very low clay content, as indicated by the gamma-ray signal.

In unit A in well 7/12-4, a strong negative correla-tion between clay content and porosity is indicated and, as such, the unit resembles unit B of wells 7/11-5 Table 1. Characteristics of Wells.^*

Reservoir Hydro- Depth Mean Burial

Quality carbon Interval Thickness Depth 75-Percentile Number Well Unit Facies Present (m RKB^***) (m) (m RSF^***) Porosity of Plugs

1/3-3 C1₄ 3 4138–4148 10 4049 8.2 24

C1₃ 2 + 4181–4200 19 4098 14.6 61

C1₂ 1 +/- 4210–4248 38 4135 23.8 115

2/1-2 A₂ 3 3318–3329 11 3230 9.0 33

A₁ 2 - 3330–3336 9 3239 14.8 19

2/1-3 C1₁ 2 + 3823–3832 9 3731 15.3 24

B₂ 1 + 3840–3862 22 3755 23.7 53

B₁ 3 3880–3888 8 3788 5.4 18

2/1-4 C1₁ 2 + 4036–4090 54 3972 13.2 163

B₂ 1 + 4095–4125 30 4021 18.4 79

B₁ 3 4132–4138 6 4044 5.9 20

2/1-6 C1₄ 3 4171–4200 29 4094 8.9 65

C1₂ 1 - 4202–4245 43 4133 21.6 126

C1₁ 2 - 4250–4315 65 4201 9.2 49

B₂ 1 - 4320–4350 30 4244 16.7 90

B₁ 3 4361–4376 15 4277 8.5 16

2/1-8 B₂ 1 + 3898–3923 25 3807 19.8 60

B₁ 3 3931–3955 24 3840 4.1 63

A 3 3955–3981 26 3865 3.4 65

7/8-3 Ula Fm^** (2) + 3731–3768 37 3618 13.7 113

7/11-5 C1_1-3 2 + 4159–4171 22 4060 16.9 26

B 1 + 4171–4191 20 4083 23.6 67

A 2 - 4205–4238 33 4117 12.3 112

7/11-7 C1_1-3 2 - 4100–4111 11 4005 13.5 46

B 1 - 4110–4131 21 4020 17.8 74

A 2 - 4131–4145 14 4037 11.7 55

7/11-7 C2 (1) - 4549–4557 8 4435 18.0 25

C2 (2) - 4558–4565 7 4443 8.6 15

7/11-9 A? (2) - 4172–4177 5 4068 13.9 16

7/12-2 C1_1-3 2 + 3385–3410 25 3302 21.6 63

B 1 + 3410–3476 66 3347 23.7 138

7/12-4 B 1 + 3450–3492 42 3375 20.5 102

A₃ 2 + 3492–3510 18 3406 19.0 48

A₂ 3 3511–3525 14 3423 12.5 30

7/12-5 A 2 +/- 3850–3900 50 3772 12.2 139

7/12-6 C1_1-3 2 + 3407–3434 27 3327 20.9 62

B 1 + 3434–3474 40 3361 21.0 106

A₃ 2 + 3474–3507 33 3397 22.0 93

712-7 A-B 2 +/- 3800–3842 42 3737 16.8 112

23/27-3 Ula Fm (1) + 4010–4047 37 3920 22.1 96

23/27-4 Ula Fm (1) - 3405–3425 20 3309 24.2 63

23/27-6 Ula Fm (1) - 3869–3909 40 3778 25.2 126

*Measured depth, burial depth, presence of hydrocarbons, and porosity of individual sandstone units.

**Ula Fm = Ula Formation.

***m RKB = depth relative to Kelly Bushing; m RSF = depth relative to sea floor.

Figure 6. Porosity in individual sandstone units vs. depth. The sandstone units are divided into three reservoir-quality facies showing different porosity depth relationships (Table 1). Facies 1 (high-porosity sandstones): φ= 45 ×e^{(–0.196 ×}Z^/1000); Facies 2 (normal-porosity sandstones): φ= 47.3-0.0085 × Z;

Facies 3 (poor reservoir quality mudstone): φ= 45 ×e(–0.490 ×Z/1000). The exponential regression lines for the high- and poor-porosity facies are obtained by locking the pre-exponential factor to 45%.

Figure 7. Porosity in facies 1 (high-porosity) sandstones and 2 (normal-porosity) sandstones replot-ted with respect to the presence or absence of hydrocarbons.

and 2/1-6. Within unit B, however, no such clear corre-lation between porosity and clay content is indicated.

Except for the low-porosity levels corresponding to carbonate cemented layers and the reduced porosity in the lower part of unit A, the porosity is rather homoge-neously distributed throughout the cored section in this well.

Porosity vs. Bulk Mineralogy

Eighty-eight samples from wells 7/11-5, 7/11-6, 7/12-4, and 2/1-6 have been analyzed for their bulk mineralogy composition by quantitative XRD (X-ray diffraction) measurements (methods used are docu-mented by Ramm, 1991). To test the correlation between bulk mineralogy and porosity, correlation coefficients between porosity and mineral content from the com-plete XRD data set and a number of sample subsets were examined (Table 2).

When all samples were considered (Table 2, column 1), there is a significant positive correlation between porosity, quartz, and feldspar content, but there is a neg-ative correlation between porosity, clay, and calcite con-tent. The positive correlation between quartz content and porosity is influenced by the low quartz contents in seven carbonate cemented samples and in four mud-stones samples, all having low porosity.

When the mudstones and the carbonate cemented samples (>10% total carbonate) are excluded, porosity is negatively correlated to depth and clay content but positively correlated with feldspar content. In this sample subset, there is no positive correlation between quartz content and porosity (Table 2, column 2).

The correlation between porosity and depth is mainly caused by differences in porosity between samples from well 7/12-4 (3450–3525 m) and the sam-ples from the three other wells (4100–4355 m). Hence, considering the data from the three deeper wells gives a possibility to assess the mineralogical influence on the porosity variations at deep burial (Table 2, column 3). Although significant negative correlation between porosity, clay, and calcite content, and positive corre-lations between porosity and feldspar content, are indicated, the correlation coefficients are low, and few statistically significant values are found.

Figure 8 depicts the variations in the Clay Index (the ratio of total clays to quartz plus feldspar con-tent) and quartz content vs. porosity. It is observed that most samples follow a trend of reducing porosity with increasing Clay Index. Some samples with little clay have very low porosity, however. All of these samples are extensively cemented either with carbon-ate or quartz cement. When these samples are excluded, systematic and highly significant relation-ships between porosity bulk mineralogy are observed (Table 2, column 4). Significant and nega-tive correlation between porosity and clay content is particularly apparent.

The quartz cemented intervals in unit C1₁in well 2/1-6, and upper parts of unit A in well 7/11-5 do not follow the same trend between porosity and clay

content as do the other samples. The samples from the low gamma-ray interval 4201–4345 m in well 2/1-6, representing units B₂, C1₁, and C1₂, show distinctly dif-ferent relations between porosity and bulk mineralogy (Table 2, column 5). These samples all contain little clay material, but the porosity is very variable. Within this group of samples, the quartz content shows a strong neg-ative correlation with porosity. This relation is illustrated in Figure 8B, where the quartz cemented samples from unit C1₁in well 2/1-6 and those from the upper part of unit A in well 7/11-5 cluster in the lower right corner of the diagram and are characterized by having low poros-ity and very high contents of quartz plus feldspar.

Petrographic Observations from Thin Sections Thin sections from wells 7/11-5, 7/11-6, 7/11-10S, 7/12-4, and 2/1-6 have been studied and point counted.

The point counting was done with emphasis on esti-mating the amount of intergranular (primary) and intragranular (secondary) porosity and the amount of intergranular cements (mainly quartz and carbonate).

The petrographical characteristics of the different strati-graphical units are indicated through a brief description of samples from well 2/1-6 in Appendix B.

Intergranular volume (IGV) vs. cement diagrams including data from well 2/1-6 are shown in Figure 9.

The compactional and cementational porosity loss (COPL and CEPL, respectively) are estimated by equa-tions 1 and 2, which are modified versions of those presented by Ehrenberg (1989)

(1)

(2) where φ₀ is the original porosity (which here is assumed to be 45%), TC is the total cement, and IP is the intergranular porosity. All parameters are expressed as volume percentages of total rock volume.

Most samples (e.g., those from units B, C1₂, and C1₄) have little quartz cement and have lost most of their intergranular porosity by compaction. Their present intergranular porosity is inversely correlated with the matrix content. Substantial amounts of quartz cement are observed in the samples from unit C1₁. The samples from well 2/1-6 may be divided into three groups. The well-sorted arenites from units C1₂ and B₂ are characterized by low contents of matrix and quartz cement and by high porosity.

These sandstones contain 25%–30% intergranular porosity plus cement and ~7% cement. According to equations 1 and 2, they have lost ~21%–27% porosity (~50% of the original porosity) by compaction and 5%–6% (~10% of the original porosity) by cementa-tion. The fine-grained graywackes from units C1₄and B₁are characterized by low porosity and low con-tents of quartz cement. On the average, they contain 17% intergranular porosity plus cement and 5%

CEPL COPL TC

cement and have, according to equations 1 and 2, lost

~34% porosity (75% of their original porosity) by compaction and 3% (~7% of the original) by cementa-tion. The quartz cemented arenites from unit C1₁are characterized by low porosity and matrix content and high content of quartz cement. On the average, they contain ~20% intergranular porosity and 15% cement and have, according to equations 1 and 2, lost ~30%

(67% of the original) porosity by compaction and 10%

(25% of the original) by cementation.

Petrographic Observations Using Scanning Electron Microscopy

The petrographic observations from bulk mineral-ogy analyses by XRD and from thin sections revealed that much porosity variation can be related to varia-tions in the clay content. However, the samples from unit C1₁in well 2/1-6 and the uppermost part of unit A in well 7/11-5 have low porosity in spite of low clay content, and this is due to the extensive quartz cemen-tation. Sample chips from wells 2/1-6 and 7/11-5 have been examined in SEM in order to describe textures that might explain why some of the clean samples are extensively quartz cemented, whereas others are not.

Secondary electron images of characteristic samples from high-porosity zones in well 2/1-6 are presented in Figure 10. The samples are characterized by little pore-occluding cement and high preserved primary intergranular porosity. It is observed, however, that

clean quartz grain surfaces are not present. All grains are coated with clay minerals, and more commonly by small (1 mm) microquartz crystals. Occasionally 10- to 50-mm-large euhedral overgrowths stand out from the coated framework grains.

Larger euhedral quartz overgrowths are extremely rare in the high-porosity samples, and it appears that there is a close relationship between the occurrence of the microquartz coating and amount of euhedral quartz cement. Hence, minor porosity loss caused by quartz cementation may be due to efficient inhibition of late diagenetic growth of quartz cement by the clay and microcrystalline quartz coating on framework grains.

Thus, the high porosity in the clean sandstones of unit B in well 7/11-5 and units C1₂and B₂of well 2/1-6 appears to be related to inhibited quartz cementation by the coating. In unit C1₁of well 2/1-6 and the upper part of unit A in well 7/11-5, quartz precipitation has not been inhibited; much porosity is destroyed by chemical compaction and quartz cementation in these sandstones.

Constraints on the Quartz Precipitation from Fluid Inclusion Homogenization Temperatures Quartz cement is the volumetrically most important cement in the deeply buried, clean sandstones with low porosity, but nearly absent in the good-porosity sand-stones. Quartz cement accounts for ~15% in unit C1₁in well 2/1-6, 20% in unit A in well 7/11-5 (Walderhaug, 1994b), and 5% in well 7/12-6 (Nedkvitne et al., 1993).

Table 2. Correlation Coefficients Between Porosity and Mineral Content.

All Samples–– Quartz-Cemented Only Interval

Mudstones Only Samples from 4201-4345

and Carbonate Wells 2/1-6 + 7/11-5 + 2/1-6 mKB in All Samples Cemented 7/11-5 + 7/11-6 Excluded Well 2/1-6

(n = 88) (n = 77) (n = 51) (n = 41) (n = 11)

Depth –0.3031* –0.3914^† –0.2069 –0.0018 –0.4115

Chlorite/Quartz –0.3504^† –0.3668* –0.3041^‡ –0.5321* –0.1920

Chlorite –0.3787^† –0.3819^† –0.3107^‡ –0.5521^† –0.2095

Illite/Quartz –0.3813^† –0.3143* –0.1950 –0.4826* 0.2682

Illite –0.3881^† –0.3394* –0.2027 –0.5274^† 0.2572

Clay Index –0.3894^† –0.4891^† –0.3925^‡ –0.8490^† 0.3224

Quartz 0.3259 –0.1650 –0.1524 0.0765 –0.6529^‡

K-Feldspar/Quartz 0.0835 0.3380^‡ 0.2458^‡ 0.2795^‡ 0.6568^‡

K-Feldspar 0.3951^† 0.3361* 0.2135 0.3980 0.1021

Albite/Quartz 0.0121 0.2525^‡ 0.2645^‡ 0.0944 0.4392

Albite 0.4083^† 0.3234* 0.3717* 0.2849^‡ 0.3306

Calcite/Quartz –0.4371^† –0.1327 –0.2808^‡ –0.2733^‡ –0.2900

Calcite –0.4537^† –0.1443 –0.3541 –0.2890^‡ –0.5252^‡

Ankerite/Quartz –0.0699 –0.1297 0.0434 –0.1410 0.6065^‡

Ankerite –0.0160 –0.1665 0.0023 –0.1608 0.5382^‡

Siderite/Quartz –0.1313 –0.0450 –0.0469 –0.0688 0.2321

Siderite –0.0242 –0.0466 –0.0896 –0.0656 –0.1048

Pyrite/Quartz –0.0785 –0.1195 –0.0361 –0.1655 –0.0937

Pyrite –0.0392 –0.1284 –0.0467 –0.1894 –0.1689

†Significance level > 99.9%.

* Significance level > 99%.

‡Significance level > 90%.