PORE-LINING CHLORITES IN SILICICLASTIC RESERVOIR

Clay Minerals (1994) 29, 665679

P O R E - L I N I N G C H L O R I T E S IN S I L I C I C L A S T I C R E S E R V O I R S A N D S T O N E S " E L E C T R O N M I C R O P R O B E , SEM A N D X R D D A T A , A N D I M P L I C A T I O N S FOR T H E I R

O R I G I N

S. HILLIER*

Universitiit Bern, Geologisches lnstitut, Baltzerstrasse 1, Ch-3012 Bern, Switzerland

(Received 21 July 1993; revised 11 Aplql 1994)

A B S T R A C T : Pore-lining chlorites are often responsible for the preservation of porosity in deeply buried sandstones because they inhibit the formation of quartz overgrowths, but little is understood about when and how they form. Chemical analyses and XRD data indicate that there are at least two common types: Fe-rich, and Mg-rich. The Fe-rich examples occur as individual euhedral crystals, are invariably interstratified with 7 ~ layers (probably berthierine) and form as the Ib g = 90 ~ potytype at low temperatures. With increasing temperature, their chemistry changes, 7 ~ layers are lost, crystal size increases, and eventually they transform to the high temperature llb f3 = 97 ~ polytype. The Mg-rich examples occur as boxwork arrangements of crystals, are not interstratified with 7 fi, layers and are exclusively the lib g = 97 ~ polytype. The Fe-rich examples occur most frequently in sandstones that were deposited at the transition between marine and non-marine environments and the presence of Fe-rich oolites in many samples suggests a link to the ironstone facies. They probably formed originally at surface or near-surface conditions as a 7 ~ mineral, such as berthierine or even odinite, in a fresh water/marine water mixing zone in tropical regions. The Mg- rich varieties tend to be found in aeolian or sabkha sandstones in close association with evaporites. They are probably replacements of Mg-rich smectites via the intermediate mineral corrensite. Precursor Mg-rich smectites formed originally from evaporite brines at near-surface conditions; chlorite itself was not formed until temperatures were high enough to crystallize the lib 13 = 97 ~ polytype.

One of the most common occurrences of chlorite in sandstones is as a uniform mat of crystals coating f ramework grains where they prot rude into open pore-space (Wilson & Pitman, 1977). Many studies of such pore-lining chlorites have noted that they tend to inhibit the formation of quartz overgrowths and that this can result in the

preservation of primary intergranular porosity in amounts that are anomalously high for the depths to which the sandstones are buried (Heald & Larese, 1974; Pit tman etal . , 1992; Ehrenberg , 1993). The pore-lining ar rangement of these chlorites has suggested to most workers that chlorite formation must have been an early diagenetic event , probably occurring within the first few hundred metres of burial. Occurrences of these chlorites, however , have not yet been

* Present address: Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2Q J, UK.

documented from such shallow depths and little is known about the diagenetic conditions that

induce their formation. The simplest classification of the chlorites is

into Fe-rich types, known as chamosite , and Mg- rich types, known as clinochlore (Bayliss, 1975). Both types have been described occurring as

pore-linings in siliciclastic sandstones , although, judging from the li terature, Fe-rich examples appear to be more common. | n the present study, examples of both Fe- and Mg-rich varieties, have been examined by electron microprobe analysis,

scanning electron microscopy (SEM)~ and X-ray diffraction (XRD) , and the data are discussed in relation to other examples in the li terature. The aim was to de termine the constraints that morphology, structure and chemistry might place on the origin of pore-lining chlorites in siliciclastic sandstones. Pore-lining chlorites are also common in volcaniclastic sandstones but these are

not considered here.

�9 1994 The Mineralogical Society

666 S. Hillier

M A T E R I A L S A N D M E T H O D S

Sands tone samples conta in ing Fe-rich pore- l in ing chlori tes were ob ta ined from the B h u b a n Forma- t ion of the Surma Group , Bangladesh ; the Garn , Tofte , In t r a -Dunl in , Tilje and Statf jord Forma- t ions, offshore Norway; the Dogger be ta Haupt - sandste in , G e r m a n y ; and the Spiro Sands tone , O k l a h o m a ; fu r the r details can be found in Table 1. Magnes ium-r ich examples were

ob ta ined f rom the Rot l i egend of no r the rn Ger- many, f rom the area descr ibed by G a u p p et al .

(1993) and Plat t (1993). For SEM, small freshly f rac tured samples were p repa red by osmium sa tura t ion to reduce charging, gold coated, and examined in a Camscan $4 ins t rument . For X R D analysis, rock samples were c rushed with a h a m m e r and the < 2 lam fract ion separa ted by gravity settl ing. These were Mg-sa tura ted , then redispersed in deionized wate r and centr i fuged

TABLE 1. Examples of Fe-rich pore-lining chlorites examined in this study and from the literature.

Stratigraphy Age Location Depositional environment References

Surma Group Neogene Bengal Basin, Deltaic Imam & Shaw (1985, Bangladesh 1987)

Texas & Louisiana Deltaic Sullivan & McBride (1991)

Deltaic: especially Stonecipher & May distributary mouth bars (1990)

Deltaic: distributary channel Longstaffe (1986), mouth bar, and Ayalon & Longstaffe interdistributary (1988)

L. Cretaceous Near shore shallow marine Porter & Weimer (1982), Campanian Pittman (1988)

L. Cretaceous Reworked offshore marine Tillman & Alman (1979) Turonian bar

L. Cretaceous Fluvial to near-shore marine Thompson (1979), Hearn Cenomanian & Lock (1986),

Hamlin & Cameron (1987)

Ehrenberg (1991, 1993)

Frio or Vicksburg Oligocene Fm

Wilcox Group Palaeocene to Eocene

Basal Belly River L. Cretaceous Campanian

Terry sandstone

Texas Gulf Coast

Western Canada Sedimentary Basin, Alberta

Denver Basin, Colorado

Frontier Powder River Basin, Formation Wyoming

Tuscaloosa or Louisiana & Mississippi Woodbine Fm

Garn Formation M. Jurassic Haltenbanken, offshore Bathonian Norway

Dogger beta M. Jurassic Schleswig-Holstein, Haupsandstein Aalenian Germany

Tofte Fm Toarcian Haltenbanken, offshore Norway

Intra Dunlin Sand E. Jurassic Veselefrikk Field, Pliensbachian offshore Norway

Tilje Fm E. Jurassic Haltenbanken, offshore Pliensbachian Norway

Halse Formation E. Jurassic Bornholm, Denmark Pliensbachian

Statfjord Rhaetian- Veselefrikk Field, Formation Sinemurian offshore Norway

Gray Sandstone Pennsylvanian Texas, West Tuscola Strawn series Field

Spiro Sandstone Pennsylvanian Arkoma Basin, Oklahoma and Arkansas

Braid delta, merging with foreshore and shoreface deposits

Shallow marine

Shallow marine, deltaic

Horn (1965), Zimmerle (1963)

Ehrenberg (1991, 1993)

Near shore marine, ebb tidal Ehrenberg (1991, 1993) delta shoal, or mouth bar

Tidally influenced shallow Ehrenberg (1991, 1993) marine deltaic

Shallow marine with Larsen & Friis (1991) ironstone layers; preceded and succeeded by deltaic sediments

Shallow marine middle to Ehrenberg (1991, 1993) upper shore face

Deltaic: river dominated delta lobe

Variety marine and non marine fluvial and tidal channels

Dutton (1977), Land & Dutton (1978)

Lumsden et al. (1971), Houseknecht (1987)

Pore-lining chlorites

on to 3.5 cm 2 unglazed ceramic tiles to give highly o r ien ta ted prepara t ions . R a n d o m powder samples were p repa red from freeze dried < 2 ~tm suspensions. The X R D data were recorded from a powder d i f f rac tometer by count ing for 3 s at steps of 0.02~ f rom 2 to 35~ for o r ien ted p repara t ions , and f rom 30 to 65~ for r a n d o m powders .

Mic roprobe analyses of chlori tes were made on pol ished thin-sect ions of vacuum impregna ted samples using a Cameca Camebax SX50 microprobe. The accelerat ing voltage was 15 kV, the b e a m cur ren t 5 hA, the spot size --1 ~tm, and the count ing t ime was 10 s per e l emen t and 5 s on b o t h sides of the peak for the background . With L1F, P E T and two T A P crystals, Fe, Si, K and AI were analysed first, Mn, Na, Ca and Mg, second and Ti, third, so tha t analysis of the four main cat ions in chlori te was comple te af ter 40s . Potass ium was analysed in the first group and Si before Na on T A P because con tamina t ion of analyses by K-bear ing illites and il l i te-smectites is a more c o m m o n prob lem than con tamina t ion with Na-bear ing minerals . Count ing relat ive s tandard devia t ions were typically Si 1 .3%, AI

667

1.6%, Fe 2.6% and Mg 2 .7%. S tandards were or thoclase (Si, A1, K), albite (Na), anor th i te (Ca) , fors ter i te (Mg), fayalite (Fe), t ephr i te (Mn) and i lmeni te (Ti). ' G o o d ' chlori te analyses were chosen on the arbi t rary basis of <0 .5 wt% total Ca O + N a 2 0 + KzO and s t ructural fo rmulae calculated by normal iz ing to 56 negat ive charges, assuming all Fe to be Fe z+, and ignoring trace quant i t ies of Ti, Ca, Na and K which for the most par t are close to or less than de tec t ion limits.

R E S U L T S

Microprobe analysis

Microprobe analyses of bo th Fe-rich and Mg- rich po req in ing chlori tes (Tables 2, 3; Figs. 1 ,2 ) , show tha t it was not possible to ob ta in analyses whose oxide totals are close to what they should be for chlori te (i.e. 85-88%) . This was a lmost invariably the case and is bel ieved to be due to micro-porosi ty be tween the chlori te grains, the analysed volume being bigger than individual crystals. Nonethe less , the oxide da ta are presented because for pore- l in ing chlori tes the prob-

TABLE 2. Electron microprobe analyses (wt%) and structural formulae of Fe-rich pore-lining chlorites. N - number of analyses.

Spiro 3 Torte 7 Statfjord 6

N = 18 N = 2 7 N = 13 SiO2 22.11 19.89 19.77 Ti02 0.04 0.03 0.04 A1203 21.39 16.66 6.89 FeO 31.53 24.47 24.79 MgO 4.92 4.55 3.90 MnO 0.13 0.03 0.04 CaO 0.03 0.11 0.12 Na20 0.06 0.04 0.11 K20 0.08 0.12 0.12

80.29 65.91 65.77

Si 5.35 0.14 5.77 0.11 5.77 AI 6.09 0.11 5.70 0.10 5.81 AI(IV) 2.65 0.14 2.23 0.11 2.23 AI(VI) 3.43 0.t0 3.47 0.10 3.58 Fe 6.38 0.17 5.94 0.15 6.04 Mg 1.77 0.09 1.97 0.10 1.69 Mn 0.03 (/.02 0.01 0.01 0.01 Sum(VI) 11.61 0.11 11.38 0.09 11.33 Fe/(Fe + Mg) 0.78 0.01 0.75 0.01 0.78

Garn 9 Statfjord 4 Dunlin 1 Dunlin 10 Haupts l

N = 14 N = 18 N = 2 4 N - 9 N - 10 19.73 17.51 20.43 19.79 18.76 0.04 0.04 0.04 0.06 0.08

17.33 14.92 16.96 14.99 15.78 23.70 21.31 21.57 18.83 20.57 4.08 3.56 5.09 4.41 2.86 0.04 0.04 0.(13 0,03 0.02 0.05 0.08 0.05 0.08 0.05 0.19 0.18 0.05 0.11 0.18 0.14 0.07 0.11 0.29 0.27

65.30 57.72 64.34 58.59 58.58

0.12 5.75 (I.07 5.80 0.13 5.93 0.16 6.27 0.14 6.05 0.16 0.(19 5.96 0.11 5.82 0.17 5.80 0.21 5.59 0.17 5.99 0.13 0.12 2.25 0.07 2.20 0.13 2.07 0.16 1.73 0.14 1.95 0.16 0.09 3.71 0.09 3.62 0.12 3.73 0.11 3.86 0.14 4.03 0.11 0.18 5.77 0.13 5.90 0.18 5.24 0.17 4.99 0.23 5.54 0.20 0.06 1.77 0.04 1.76 0.07 2.19 0.20 2.07 0.2l 1.38 0.06 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.10 11.27 0.06 11.29 0.09 11.17 0.09 10.94 0.11 10.96 0.12 0.01 0.77 0.01 0.77 0.01 0.71 0.02 0.71 0.03 0.80 0.01

668 S. Hillier

TABLE 3. Electron microprobe analyses (wt%) and structural formulae of Mg-rich pore-lining chlorites, and the standard chlorite CCa-1 prepared as a normal polished thin-section and as a 2-5 p.m powder to obtain low oxide totals. N = number of

analyses.

Rotl 95 Rotl 31 Rotl 69

N - 5 N = 3 N = 3 SiO2 24.74 24.79 24.76 TiO2 0.02 0.02 0.05 A1203 17.28 18.34 18.45 FeO 17.21 11.53 16.03 MgO 13.62 15.43 14.27 MnO 0.52 0.28 0.16 CaO 0.06 0.11 0.06 Na20 0.20 0.02 0.07 K20 0.11 0.09 0.14

73.75 70.60 73.99

Si 6.02 0 . 1 0 6 .04 0 .12 5.94 AI 4.95 0 . 0 9 5 .27 0 .07 5.20 AI(IV) 1.98 0.10 1.96 0 .12 2.06 AI(VI) 2.97 0.08 3.31 0.11 3.14 Fe 3.52 0 .33 2 .35 0 .38 3.18 Mg 4.91 0.31 5.60 0.18 5.11 Mn 0.11 0 .04 0 .06 0 .02 0.03 Sum(VI) 11.51 0 .08 11.32 0.ll 11.46 Fe/(Fe +Mg) 0.42 0 .04 0 .29 0 .04 0.36

Rotl 79 Rotl 66 CCal CCal 2-5 ~tm

N = 6 N = 3 2 N = 3 5 o N = 3 5 18.22 19.61 26.33 0 .38 16.61 0.03 0.02 0.08 0 .05 0.07

13.50 14.71 20.01 0 .54 12.94 11.64 10.30 21.64 0 .44 13.02 10.68 12.33 17.86 0 .38 11.35 0.18 0.19 0.07 0 .05 0.06 0.03 0.09 0.03 0 .03 0.02 0.02 0.05 0.02 0 .02 0.04 0.11 0.06 0.01 0 .01 0.02

54.42 57.35 86.05 0.83 54.12

0.10 5 .94 0.31 5.93 0 .22 5 .55 0 .07 5.53 0.05 5 .16 0 .28 5 .25 0 .19 4 .97 0 .12 5.09 0.10 2 .06 0.31 2.07 0 .22 2 .45 0 .07 2.47 0.14 3 .10 0 .10 3 .18 0 .14 2 .52 0 .08 2.61 0.50 3 .18 0 .22 2 .64 0 .28 3 .81 0 .06 3.63 0.24 5 .15 0 .30 5 .57 0 .28 5 .61 0.11 5.67 0.01 0 .05 0 .02 0 .05 0 .02 0 .01 0 .01 0.02 0.12 11.48 0.18 11.44 0 .16 11.96 0.04 11.93 0.05 0 . 3 6 0 .03 0 .32 0 .03 0 . 4 0 0.01 0.39

O

0.20 0.17 0.20 0.20 0.25 0.22 0.02 0.18 0.02

lem appears to be largely unavoidable and because the potential effects of the low totals on

the calculated structural formulae are believed to be insignificant. To test this latter assumption, analyses were also made of the s tandard metamorphic chlorite CCa-1, available from the Clay Minerals Society, using the same analytical conditions as for the diagenetic chlorites. Chlorite CCa-1 was prepared both as a pol ished thin- section for reference, and as a polished 2-5 ~tm

powder and epoxy resin mixture. Analyses of the powder were made purposely in areas of low grain density to obtain low oxide totals. The average structural formula derived from the low oxide total data is effectively identical to that derived from the normal rock thin-section data, al though the dispersion about the mean increases considerably (Table 3, Fig. 2). This suggests that the errors that may be associated with low oxide totals are largely r andom and that the mean values would provide the most reliable basis for genetic in terpreta t ions of the data. The data for the 2-5 ~tm powder of CCa-1 included many totals be tween 20 and 50%, much lower than those accepted for the diagenetic chlorites suggesting

that analytical artifacts due to low totals should not be a major problem. For the diagenetic

chlorite data, dispersion about the mean of the normalized structural formulae is in all cases small. Since scatter amongst the individual analyses is due to the sum of exper imenta l errors and be tween point variation, the latter must be small, indicative of a single equil ibrium phase. Similar results were obta ined previously by Curtis et al.

(1985) for petrographically identical pore-l ining chlorites from sandstones and the use of mean composit ional data to describe them can be justified on this basis.

All of the Fe-rich chlorites have very similar Fe/(Fe + Mg) values be tween 0.7 and 0.8.

However , the samples M34, M36B and M37 (Surma Group, Bangladesh) , did show lower values of --0.6. They are not included in Table 1 because no analyses met the criteria for selection,

the best containing --0.5% K20 indicating unavoidable illite contaminat ion. This was also a problem for several of the o ther samples for which no microprobe data are presented and arises due to the somet imes very close association of illite and chlorite. Overall , the analyses of the

Pore-lining chlorites 669

8

7 -

S i 6~

4

/ /

/ -, /

Sum(VI;=10 " ' " " . , , /

/ /

' S u d o i t e

/ /

/ /

, /

5

/ . . z ~ S e r p e n t i n e / ' / / - . / /

" . . / AI=2 / - . / - . /

. / - . , / ./. - /

/ / / ". Sum(VI)=12

"-. / AI=4 /

" " " ' - i I '" - @ ' ~ ' / " . " ' " / / /

Sum(VI)=11 "'" / 220~ �9 Spiro3N=18 . ~ . / 155~ [] Torte 7 N=27

/ / " - 140~ �9 Statfjord 6 N=13 / ~, /

" "- / . . '7, 140~ • Garn 9 N=14 / . . / 125~ �9 Statfjord 4 N=18

AI=8 / 120~ 0 Duniin 1 N=24 . /

- . . / 120oc �9 Dunlin 10 N=9

"" - . / l m e s i t e 100oc o Haupts 1 N=10 - J

! i T �9 i i

6 7 8 9 10 11 12

R 2+

FIG. t. Chemical compositions of Fe-rich pore-lining chlorites plotted in the vector representation of chlorite compositions of Wiewiora & Weiss (1990), as used by Hillier & Velde (1991) showing apparent chemical evolution of compositions with temperature, parallel to lines of constant A1 content. Error bars indicate +_lm Temperatures are estimated maximum

temperatures.

Fe-rich chlorites are typical of diagenetic chlorites being more siliceous, less rich in (Fe + Mg), and with lower octahedral totals comapred to metamorphic chlorites of similar A1 content (Hillier & Velde, 1991). The relatively constant Fe/(Fe + Mg) ratio of these chlorites allows them to be compared meaningfully in terms of changes in composit ion with temperature . As shown previously (Hillier & Velde, 1991) the compositional trend is parallel to lines of constant total A1 content, with the highest temperature samples plotting closest to the fully trioctahedral line between amesite and serpentine (Fig. 1).

For the Mg-rich chlorites Fe/(Fe + Mg) values range be tween 0.42 and 0.29. In comparison to the Fe-rich examples, A | contents are significantly lower. Again they show the typical differences from metamorphic chlorites of similar total A1 content, such as the sample CCa-1 for example (Table 3, Fig. 2). No composit ional t rend is apparent (Fig. 2), consistent with the fact that maximum temperatures were probably similar for all samples.

Scanning electron microscopy

Both Fe-rich and Mg-rich chlorites show similar pore-lining occurrences as coatings on framework grains where they extend into pore-space and are absent at points where framework grains were in contact (Fig. 3). However , on a finer scale, the similarity ends here, both the morphology and the arrangement of the Fe-rich chlorites being very different to the Mg-rich ones. All of the Fe-rich examples show well developed euhedral crystals that are oriented perpendicular to f ramework grain surfaces and arranged individually in all orientations, more or less at random Fig. 3A, B, C, D). In contrast, the Mg-rich examples are typically more anhedral and are characterized by an organized arrangement of crystals into a honeycomb or boxwork pattern (Fig. 3E ,G ,H) . At high magnifications the boxwork is seen to be composed of many packets of individual crystals lying with their basal crystal faces in contact and connected to other such packets edge to edge. A further notable difference is that small rosettes of

670 S. Hillier

Si 6

/ / " ' - .

/ /

Sum(VI)=10 "" ..

/ /

/

( Sudo i te " 4

/ /

/ /

,/

4 5 6 7 8 9 10 11 12

/ "- / " Serpent ine / / /

. . / AI=2 /

, / " . . / -/, - /

Sum(VI)=l I ", Sum(VI)=l 2 "" -... / AI=4 /

--.. / " - ~ /

/ --... _ _ ~ / / �9 Rot195 N=5

/ ' -........ El Rotl 31 N=3 / �9 Rot169 N=3

/ / -/. ~ ' / zx Rotl 79 N=6

/ . .. / �9 Rotl 66 N=32

AI=8 / �9 CCal thin section N=35 /

... / o CCal 2-5pm variable totals N=35

" . / Ames i te i | " l l I / �9 i i

R 2+

FIG. 2. Chemical compositions of Mg-rich pore-lining chlorites from the Rotliegend of Germany, and chlorite CCa-1 from the Clay Minerals Society, source clays repository. Analyses of CCa-1 show that it plots very close to the fully trioctahedral line between amesite and serpentine even when analysed as a 2-5 ~tm powder with purposely lowered oxide totals. Error

bars indicate +lo.

chlori te crystals sitting on top of the chlori te coatings are very c o m m o n for the Fe-rich examples but were never observed for the Mg-rich ones.

As indicated in Fig. 1 and Table 4, the maximum tempera tu res to which the Fe-rich chlori te samples were subjected range from --90 to 220~ and over this t empe ra t u r e range it is ev ident f rom the SEM images tha t the crystal size of these chlorites increases considerably (Fig. 3B, C, D). This change appears to be principally in the thickness of the crystals in the c* direct ion which increases f rom a fract ion of a micron for the lowest t empe ra tu r e examples to more than 1 ~m thick for the highest t em pe r a t u r e ones.

X-ray diffraction

The X R D pa t te rns of the o r i en ta t ed < 2 ~tm fract ion of two Fe-rich and one Mg-rich chlori te are shown in Fig. 4A. For a lmost all of the Fe-rich chlori tes, the odd-orde r basal reflections are b roade r than the even-o rde r ones, indicat ing that

they are interstrat if ied with 7 A layers (Reynolds , 1992). The only except ions were the chlori tes f rom the Spiro Sands tone which show bo th odd- and even-o rde r peaks tha t are nar row and of more or less identical width at half peak height . In all cases the Mg-rich examples showed no evidence of odd-orde r b roaden ing . Four examples of Fe-rich chlori tes with different amount s of odd- order peak b roaden ing are shown in Fig. 4B and all peak width measu remen t s are given in Table 4. Also included in Table 4 are qual i ta t ive indicat ions of the sample clay minera logy and es t imates of the p ropor t ion of 7 ~ layers in these minerals based on the me thods of Reynolds (1992) and Hill ier & Velde (1992). Bo th me thods indicate tha t the highest p ropor t ion of 7 interstrat i f icat ion occurs in samples f rom the Dogger be ta Haup t sands te in , the m e t h o d of Hill ier & Velde (1992) giving 26% and the m e t h o d of Reynolds , 22%. Overal l , the values ob ta ined by the more accurate me thod of Rey- nolds (1992) indicate less 7 A interstrat i f icat ion than those ob ta ined with the me thod of Hill ier &

Pore-lining chlorites 671

FIG. 3. Examples of Fe-rich (A, B, C, D) and Mg-rich (E, F, G, H) pore-lining chlorites showing difference in morphology and arrangement and illustrating the increase in grain size of Fe-rich examples with increasing temperature (4B, C, D). A = Tilje 3, showing uniform pore-lining chlorite absent at grain contacts, also note coating outlining dissolved feldspar grain with quartz overgrowths growing into it; B = Dunlin 1, C = Torte 7, D = Spiro 1, estimated maximum temperatures of B, C and D are 120, 155 and 220~ respectively; E = Rotl 66, low magnification view of Mg-rich chlorite to contrast with Fe-rich chlorite shown in A~ F = Y1, pore-lining corrensite from the Permian Yates formation; G - Rot166, note similarity

between boxwork texture of this sample and the corrensite shown in F; H = Rotl 66, close up of boxwork texture.

672 S. Hillier

TABLE 4. Clay mineralogy, estimated maximum temperature, per cent 7 A mineral interstratified in chlorite, and peak- width measurements (A~

Peak width Sample No. Clay mineralogy measurements (A~

I + I-S Chlorite Kaolinite T~ %7 •(1) %7 A(2) 001 002 003 004 005

M34 * ********* 90 16 8 M36b * ********* 100 17 12 M37 * ********* 100 17 12 Haupts 1 * ****** *** 100 26 22 Haupts 2 * ****** *** 100 Haupts 3 * *** ****** 100 Dunlin 1 * **** ***** 120 20 12 Dunlin 10 * ** ******* 120 Statfijord 4 * ***** **** 125 25 15 Garn 5 * **** ***** 135 16 13 Staffjord 6 * **** ***** 140 23 17 Garn 9 * ***** **** 140 22 15 Tilje 3 * ********* 140 Torte 7 * ********* 155 11 7 Torte 8 * ********* 155 9 7 Tilje 2 *** ******* 165 Spiro 1 ********** 220 0 0 Spiro 2 ********** 220 0 0 Spiro 3 * ********* 220 0 1

0.73 0.45 0.74 0.47 0.74 0.76 0.45 0.74 0.45 0.87 0.76 0.42 0.8 0.4 0.82 1.09 0.26 1.07 0.24 1.07

0.87 0.28 0.85 0.26 0.72

1.05 0.40 0.91 0.36 0.91 0.72 0.31 0.72 0.23 0.72 0.96 0.35 0.91 0.25 0.91 0.92 0.36 0.94 0.25 0.82

0.56 0.25 0.53 0.25 0.51 0.49 0.26 0.49 0.26 0.49

0.15 0.15 0.14 0.17 0.18 0.12 0.14 0.14 0.19 0.18 0.21 0.17 0.18 0.17 0.21

(1) method of Hillier & Velde (1992) (2) method of Reynolds (1992)

V e l d e (1992). H o w e v e r , it is l ikely t h a t t h e

p r e s e n c e o f kao l in i t e in m a n y s a m p l e s a n d t he low

in t ens i t y o f t he 005 ch lo r i t e re f lec t ion m a d e s o m e

o f t he p e a k w i d t h m e a s u r e m e n t s n e c e s s a r y for t h e

R e y n o l d s (1992) m e t h o d m o r e s u s c e p t i b l e to

e r ro r , a l t h o u g h e v e r y e f fo r t was m a d e to t ry to

m e a s u r e p e a k w i d t h s as a c c u r a t e l y as poss ib le .

E x a m i n a t i o n o f r a n d o m p o w d e r d i f f r ac t ion

p a t t e r n s s h o w e d t h a t all o f t h e F e - r i ch ch lo r i t e s

a r e t he lb g = 90 ~ p o l y t y p e , t h e on l y e x c e p t i o n

b e i n g t h o s e f r o m t h e Spi ro S a n d s t o n e w h i c h a re

m i x t u r e s o f t h e Ib B = 90 ~ p o l y t y p e a n d t h e l i b 13 = 97 ~ p o l y t y p e (Fig. 5). In c o n t r a s t , all o f t h e

M g - r i c h ch lo r i t e s w e r e t he l i b g = 97 ~ p o l y t y p e

(Fig. 5).

D I S C U S S I O N

Chemica l c o m p o s i t i o n a n d s tructure

T h e r e su l t s o f m i c r o p r o b e a n a l y s e s o f t h e p o r e -

l in ing ch lo r i t e s a re s imi la r to t h o s e r e p o r t e d in

p r e v i o u s i n v e s t i g a t i o n s t h a t s h o w t ha t d i a g e n e t i c

ch lo r i t e s a re m o r e s i l i ceous , h a v e less (Fe + M g ) ,

a n d m o r e o c t a h e d r a l v a c a n c i e s c o m p a r e d to h igh

t e m p e r a t u r e m e t a m o r p h i c ch lo r i t e s o f s imi l a r

to ta l AI c o n t e n t (Cu r t i s etal . , 1985; J a h r e n &

A a g a a r d , 1989; Hi l l i e r & V e l d e , 1991; J a h r e n &

A a g a a r d , 1992). H o w e v e r , t he c a u s e o f t h e s e

a p p a r e n t d i f f e r e n c e s h a s n o t b e e n e s t a b l i s h e d

wi th c e r t a i n t y a n d cou ld be d u e to o n e o r a

c o m b i n a t i o n o f ana ly t ica l p r o b l e m s , c o n t a m i n a -

t ion o f a n a l y s e s by o t h e r p h a s e s , in te r s t ra t i f i ca -

t ion wi th d i o c t a h e d r a l phy l lo s i l i ca t e s , d i o c t a h e d -

ral s u b s t i t u t i o n s , o r a n i o n n o n - s t o i c h i o m e t r y .

T h e a n a l y s e s o f t he s t a n d a r d m e t a m o r p h i c

ch lo r i t e C C a - 1 s u g g e s t s t h a t ana ly t i ca l a r t i f ac t s

d u e to low ox ide to ta l s a r e un l ike ly . In t h e

d i a g e n e t i c e x a m p l e s t h e c o m p o s i t i o n a l t r e n d

a p p e a r s to be a f u n c t i o n o f t e m p e r a t u r e (Fig. 1)

so t h a t c o n t a m i n a t i o n also s e e m s an un l i ke ly

c a u s e b e c a u s e t h e p o t e n t i a l a m o u n t s o f con-

t a m i n a t i o n a re c o m p a r a b l e t h r o u g h o u t .

A l t h o u g h m i n o r a m o u n t s o f p o t e n t i a l c o n t a m i -

n a n t s a r e i n d i c a t e d a n d will pa r t l y a f fec t t h e

c a l c u l a t e d s t o i c h i o m e t r y , t h e y do n o t a p p e a r

'002'

'001'


Statfjord 4, Fe/(Fe+Mg) = 0.77

'003'

'004'

Spiro 3, Fe/(Fe+Mg) = 0.78

Rotl 66, Fe/(Fe+Mg) = 0.32

| I i I

2 6 10 14 18

/ A I I i |

22 26 30 34

A

673

'001' '005' B 1.09

~ ~ Statfjord 4 (125~

0 , 9 1 & o 2 0

0.49 Toffe 8 (155~

0.49A~

2 6

Spiro 3 (220~

o 0.214~

2 |

10 33

"20 "20

F1G. 4 (A) XRD patterns of two Fe-rich and one Mg-rich pore-lining chlorites. The Fe-rich examples have almost identical Fe/(Fe + Mg) ratios but different peak height ratios due to interstratification with 7 iX layers in the Statfjord 4 example which broadens its odd-order peaks. Peak ratios based on integrated areas are comparable. Note also presence of kaolinite in Statfjord 4 evident from the inflection on the low-angle side of the peak at 25~ Mg-rich example Rot 66 shows no evidence for 7 ,/~ interstratification. (B) Four examples of Fe-rich pore-lining chlorites showing the change in width (A~ of the 00l and 005 peaks due to variable amounts of interstratified 7/~ layers. Generally, the decreasing proportion of 7 layers correlates with increasing temeprature (Table 4). Note also the concurrent decrease in the proportion of expandable layers in the illite-smectite mineral as shown by the changing sha?e of the peak at about 8~ Patterns are air-dried Mg-

saturated preparations. Cu-Ko: radiation.

674 S. Hillier

tsl

Tofte 8

lb 13=90 ~

I , I , I , i

2

/ 6

| | f | I I �9

30 35 40 45 50 55 60 65

~ FIG. 5. Examples of random powder XRD patterns of pore-lining chlorites. The relatively low temperature Fe-rich examples (Haupts 1, Tofte 8) are exclusively the lb 13 = 90 ~ polytype, whilst those that have been to higher temperatures (Spiro 2) are mixtures of Ib 13 = 90 ~ and the lib 13 = 97 ~ polytype. The Mg-rich examples (Rot 66) are exclusively the lib 13 = 97 ~ polytype. Calculated positions and intensities of hOl peaks for polytype identification are shown for comparison. Cu-

Kcr radiation.

suff ic ient to expla in the da t a comple t e ly . T h e p r e s e n t da t a s e e m to conf i rm tha t t he c o m p o s i - t ional d i f f e r ence b e t w e e n d i agene t i c and m e t a -

m o r p h i c ch lor i t es is, at least in pa r t , a crystal chemica l d i f f e r e n c e o f s o m e kind.

Hil l ier & V e l d e (1992) p r e s e n t e d m i c r o p r o b e

and X R D da ta fo r an Fe - r i ch po re - l i n ing ch lor i te s imilar to t h o s e d e s c r i b e d h e r e a n d c o n c l u d e d tha t it was in te rs t ra t i f i ed wi th 7 A layers . It was s u g g e s t e d tha t if t he se 7 ,~ layers w e r e d ioc ta - hedra[ they migh t expla in its typical d i agene t i c chemica l c o m p o s i t i o n . Judg i ng f rom the da t a in


Tables 2, 3 and 4, however , there does not appear to be a good systematic correlat ion be tween the amount of 7 A interstratification in the Fe-rich chlorites and any of Si, (Fe + Mg), or octahedral occupancy, as there should be if the 7 A layers were dioctahedral. Fur thermore , both the Fe-rich samples from the Spiro Sandstone and the Mg- rich chlorites from the Rot l iegend show no evidence of 7 A interstratification, yet they still show the chemical characteristics of low-temperature diagenetic chlorites. In all probability therefore, the 7 A layers interstratified with the Fe-rich chlorities are berthierine.

In summary, it appears that neither a single one nor a combinat ion of artifacts, contamination and interstratification, can fully explain the chemical composit ion of these low-temperature chlorites, and that ei ther dioctahedral substitution or anion non-stoichiometry is to some degree responsible for the way in which the composit ions deviate from those of high temperature metamorphic chlorites. However , that is not to say that this is always the explanation for non-stoichiometry and unrecognized problems such as analysis of mixtures are undoubtedly common place.

Origin and evolution of pore-lining chlorites

Except for the highest temperature examples from the Spiro Sandstone, all of the Fe-rich pore- lining chlorites examined in this study are interstratified with 7 ~ layers. Fur thermore , it is evident that there is a general, though erratic, decrease in the proport ion of 7 ,~ interstratification with increasing maximum temperature (Table 4). Such a correlat ion suggests that these Fe-rich pore-lining chlorites may have originated from a fully 7 A precursor formed at lower temperatures. The attraction of this explanation is that the petrographic data indicate that these chlorites form very early on in the burial history, and by implication at low temperatures where, rather than chlorite, berthierine or even odinite is likely to form. Such an explanation requires that there has been an evolution from a 7 A to a 14 A mineral via a progressive loss of 7 / ~ layers. This may occur by recrystallization or by a dissolution and growth process such as Ostwald ripening (Jahren, 1991). Indeed, there is direct evidence from the SEM (Fig. 3) that the crystal size of the Fe-rich chlorites increases with temperature ,

675

particularly in the c* direction. Such evidence indicates clearly that these minerals must continue to grow and evolve long after the initial precipitation 'event ' , as argued previously by Jahren (1991).

All except for the highest temperature Fe-rich chlorites were the Ib B = 90 ~ polytype. Petro- graphically, the occurrence of the highest temperature samples from the Spiro Sandstone is identical to those from lower temperatures , and chemically their Fe/(Fe + Mg) ratios are comparable. Therefore , the occurrence of a mixture of both the Ib B = 90 ~ and lib B = 97 ~ polytypes in these samples suggests that an originally Ib B = 90 ~ chlorite has been partially t ransformed to the more stable llb B = 97 ~ polytype as a result of the high temperatures that these samples have experienced. However , it should also be noted that the composit ion has changed significantly (Fig. 1) with tempera ture so that to ascribe the polytype transformation to temperature alone may not be wholly justified.

Many other occurrences of pore-lining Fe-rich chlorites documented in the li terature are listed in Table 1. Probably the most striking feature is that all of these examples occur in near-shore marine or deltaic sediments, as has been pointed out by Ehrenberg (1993). In addition, many occur in close association with oolitic ironstones and often iron-rich ooids are found as f ramework grains in the sandstones themselves (Ehrenberg, 1993). These associations suggest that the geochemical regime and palaeogeographical location that favoured Fe-rich ooid deposition are probably similar to those that favour the precipitation of early Fe-rich pore-lining clays. Al though berthierine does not appear to be forming in present- day sediments, the related minerals of the verdine facies are known to form just offshore from many tropical deltas (Odin, 1990). A 7 A precursor of the Fe-rich pore-lining chlorites may well have formed close to the surface in such tropical fresh water/marine water mixing zones, or perhaps somewhat later as meteor ic groundwaters mixed with connate marine ones as a result of delta progradation or sea-level fall. The potential importance of mixing was also suggested by Stonecipher & May (1990) and is supported by the oxygen isotopic data of Longstaffe (1986) that indicate chlorite precipitation from brackish waters. Al though the studies listed in Table 1 are diverse in their sedimentological descriptions,

676 S. Hillier

there is a suggestion that many of the host sediments were deposited during regressions and it would be interesting to examine if this early diagenetic phenomena could be related to, and perhaps predicted from, sequence stratigraphy.

In contrast to the Fe-rich pore-lining chlorites, the Mg-rich chlorites from the Rot l iegend do not show any evidence of 7 / ~ interstratification and are exclusively the high temperature l i b B =97 ~ polytype. In addition, the morphology and arrangement of the Mg-rich pore-lining clays is very different to the Fe-rich varieties. The boxwork-like arrangement is more reminiscent of a swelling clay, such as the pore-lining corrensite from the Permian Yates Format ion shown in Fig. 3F for comparison. Several examples of pore-lining corrensites have been described in the li terature (Tompkins, 1981; Purvis, 1990; Janks et al. , 1992), but as far as the author is aware, the only other detailed descriptions of Mg-rich pore- lining chlorites are from the deeply buried, aeolian Norphlet Format ion of the US Gulf Coast (Dixon et al. , 1989; Kugler & McHugh, 1990). Interestingly, the morphology and boxwork-l ike arrangement of the Norphle t examples appears identical to the Rot l iegend ones described here, and it is also reported to be the l i b , high- temperature polytype (Dixon et al. , 1989).

Compared to the Fe-rich chlorites, such structural and morphological differences suggest that the origin of the Mg-rich examples is very different. The burial / temperature related

sequence of Mg-smectite, followed by corrensite, followed by Mg-rich chlorite is well known from many diagenetic and hydrothermal environments (Hillier, 1993) and it is suggested here that Mg- rich grain-coating chlorites originate via such a sequence because this would explain both their boxwork-l ike arrangement and their high temperature I lb B = 97 ~ structure. The boxwork morphology is inherited from the swelling clay precursors and they are the l i b B = 97 ~ type because chlorite itself does not form from corrensite until temperatures are already relatively high, say 100~ or more (Hillier, 1993). Additionally, the Mg-rich chemical composit ion might have been of some importance in determining the formation of the l i b B = 97 ~ polytype.

Occurrences of Mg-rich chlorite, corrensite and Mg-smectite are well known from various facies in evapori te basins worldwide, and a close association with evaporites is also true for both the Rotl iegend and the Norphlet Format ion sandstones. Such an association suggests that the origin of Mg-rich grain-coating chlorites, or more accurately their swelling clay precursors, is related to the presence of the evaporites themselves, possibly as evapori te brines invade associated sandstones shortly after deposition or during early burial.

The various differences between Fe-rich and Mg-rich pore-lining chlorites in siliciclastic sandstones are summarized in Table 5, together with their proposed different origins. However , it

TABLE 5. Summary of differences between Fe-rich and Mg-rich pore-lining chlorites.

Fe-rich Mg-rich

Morphology Arrangement

XRD

Polytype

Occurrence

Facies associations Common diagenetic minerals/

events Origin

Pseudo-hexagonal Individual plates and rosettes Edge to face contacts Interstratified with 7 A layers at <200~

lb 13 = 90 ~ transforming to lib 13 = 97 ~ above 200~

Near-shore marine sandstones in tropical climates, frequently deltaic

Oolitic ironstones Siderite, quartz, calcite, kaolinite, felds-

par dissolution From 7 ~. mineral probably berthierine

possibly odinite

Cornflake-like Boxwork Face to face contacts Often associated and/or interstratified

with corrensite Exclusively lib 13 = 97 ~ ?

Coastal aeolian dunes and sandy sabkhas plus any facies in close contact with evaporite brines

Evaporites FeO rims, anhydrite, K-spar, calcite,

quartz, dolomite, fibrous illite From Mg-smectite to corrensite sequence


should be emphas ized tha t a l though these two origins are bel ieved to be the mos t c o m m o n in siliciclastic rocks, undoub ted ly pore- l in ing chlorites may be fo rmed in o the r ways, such as in volcaniclastics or sands tones rich in volcanic clasts. Indeed , it has f requent ly been suggested tha t the a l te ra t ion of volcanic or lithic grains is responsible for chlori te fo rmat ion in many of the examples reviewed here , for example T h o m s o n (1979). In many instances the a l tera t ion of lithic clasts may provide the local source of Fe and Mg but o the r sources of these c o m p o n e n t s are possible so tha t the presence of lithic grains is p robab ly not the only, or even an essential , prerequis i te , Indeed the associat ion with specific sed imen ta ry facies suggests tha t a par t icular set of geochemical condi t ions is the main control . It is hoped tha t the scheme p resen ted here provides someth ing of a s tar t ing poin t for compar i son and order ing of observa t ions on these kinds of chlorites, and tha t upda te and revision will eventual ly lead to a be t t e r unders t and ing of the factors tha t are necessary for the i r fo rmat ion in the first place.

C O N C L U S I O N S

The re are two c o m m o n types of pore- l in ing chlori tes, Fe-r ich chamosi te and Mg-rich clinochlore. The chamosi tes occur as individual euhedral crystals and are invariably interstrat i f ied with a 7 A mineral , p robab ly ber th ie r ine . With increasing bur i a l / t empera tu re the chamosi tes lose the i r 7/~, componen t , increase in crystal size, and eventual ly begin to t r ans form from the low- t empe ra tu r e Ib 13 = 90 ~ polytype to the high- t e m p e r a t u r e l lb 13 = 97 ~ polytype. They p robab ly or ig inated f rom a 7 A precursor such as ber th ier - ine, or possibly odini te , fo rmed at near-surface condi t ions in sands tones associated with mar ine and meteor ic wate r mixing zones in tropical regions. In contras t , the cl inochlores show textures tha t are p robab ly inher i t ed f rom smecti- tic precursors . The original pore- l in ing clay minera l was p robab ly a low- tempera tu re Mg-rich smect i te which was replaced by correns i te and then chlori te , the whole process preserving the boxwork or h o n e y c o m b a r r a n g e m e n t tha t is commonly character is t ic of swelling clays. The pore- l in ing cl inochlores are the l ib 13 = 97 ~ polytype because chlor i te fo rmat ion in this minera l sequence did not occur until t empera - tures were relatively high, i.e. >100~

677

The pe t rographic evidence for the origin of bo th Fe-r ich and Mg-rich pore- l in ing chlori tes indicates tha t they fo rmed early in the diagenet ic history at surface or near-surface condi t ions. The mineralogical ev idence p resen ted suggests tha t chlori te was not in fact the original minera l tha t formed. Ber th ie r ine (or odini te) was the precursor of the pore- l in ing chamosi tes and Mg- smecti te the precursor of the pore- l in ing clino- chlores, bo th minerals be ing more compat ib le with an early near-surface fo rmat ion as indicated by the pe t rograph ic data.

A C K N O W L E D G M E N T S

Many thanks are due to Prof. Albert Matter for making this study possible, to Frank David, Paul Nadeau, Christoph Sprtl, Harry Shaw, Earle McBride, and Konrad Rocken- bauch for provision of samples, and to Prof. Tjerk Peters for use of XRD equipment, The manuscript was improved by the comments of Bruce Velde, an anonymous referee and Harry Shaw as editor. Thanks are due to BEB Erdgas und Erd01 GMBH for both financial support and permission to publish. Statoil and its license partners (Norsk Hydro, Saga, Neste Petroleum, Mobil, Conoco, Total, Nosk Agip, Enterprise Oil, Deminex, Svenska Petroleum) and RWE- DEA A.G. are also acknowledged for permission to publish the results. Financial support (Grant Nos. 20-30854.91 and 20-37363.93), and funding for the electron microprobe (Grant No. 21-264789.89) at the University of Bern was provided by the Swiss National Science Foundation.

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PORE-LINING CHLORITES IN SILICICLASTIC RESERVOIR