Sandstone-Framework Instability As a Function of Burial Diagenesis

II geol. Soc. Lond. Vol. 135, 1978, pp. 101-105, 2 figs., 1 plate. Printed in Northern Ireland.

Sandstone-framework instability as a function of burial diagenesis

P. J. C. Nagtegaal

SUMMARY: The effect of sandstone-framework instability during diagenesis on porosity and permeability is compared for quartz, arkosic and lithic arenites. Of the cases investigated, quartz frameworks are the most stable and suffer only from mechanical compaction and pressure solution. Arkosic frameworks have variable stability involving potential widespread alteration of feldspars. The least stable are the lithic sandstone frameworks which are susceptible to all four main porosity and permeability-reducing processes: mechanical compaction, plastic deformation, pressure solution, and mineralogical alteration of framework constituents.

Sandstone burial diagenesis normally results in partial and sorting mixes as experimentally determined by framework collapse through mechanical compaction Beard & Weyl (1973). This matching procedure was (geometrical rearrangement of grains), plastic defor- facilitated by converting the tabulated data of Beard & mation, pressure solution, and mineralogical alteration Weyl into a graphical form, which was a simplification of framework constituents. The effect of these proces- only to the extent that the curves for coarse, medium, ses on sandstones of varying mineralogical composi- etc. sands were constructed by averaging the values for tion in the form of porosity and permeability decrease respectively coarse upper and lower, and medium forms the subject of the present study. The influence upper and lower etc. as originally published (Fig. 2A). of cementation originating from outside sources is thus A field on the porosity/permeability plot was thus left aside. arrived at which serves as a standard of reference in Apart from diagenesis, sandstone porosity and per- the illustrations, allowing ready comparison of the meability are controlled by grain size and sorting. The extent of diagenetic change in standstones of varying influence of these factors was investigated experimen- mineralogical composition (Figs. 2B--D). tally in detail by Beard & Weyl (1973) and their data are taken as a reference set for estimated initial (pre- diagenesis) values. Quartz arenites It was found" that even small amounts of feldspars, phyllite, and volcanic rock fragments may exert a Quartz is the most stable, clastic component gener- marked influence on the type and rate of diagenetic ally present in sandstones and the course of diagenesis change. To arrive at the desired lithological differenti- in quartz arenites may therefore serve as a standard ation of sandstone types, the terms 'arkosic' and reference in studies of sandstones of other 'lithic' are therefore used here for a 10 per cent level mineralogies. of feldspars and lithics, respectively. A sandstone The diagenesis of quartz sandstones was studied, grouping according to these criteria is given in Fig. 1. amongst others, by Maxwell (1964) and Ffichtbauer To effectively isolate the role of the sandstone (1967). Conclusions relevant to the present study are framework, only clean arenites (less than 5 per cent that mechanical compaction (geometrical re- detrital clayey matrix) essentially free of allochthonous arrangement of grains) takes place initially; coarse- cements were selected and compared. The depth fig- grained sandstones compact (and lose porosity) more ures mentioned in text and figures all correspond readily than fine-grained ones because of the generally approximately to maximum palaeo-burial depths. greater roundness, which facilitates mechanical slip between grains. Below approximately 1500 m, pressure solution, the effect of which is stronger in fine-grained Depositional texture sandstones than in coarse-grained ones, may take over To evaluate the effect of diagenesis on the porosity (Fiichtbauer 1967). Maxwell's work indicates that the and permeability of sandstones, comparisons were geothermal gradient and residence time at depth are made between the measured values and estimated factors of first-order importance in the pressure- initial (pre-burial) values. The measured values re- solution process and composition of pore fluids. ported were all determined in the Shell laboratories in The quartz arenites selected come from a Miocene Rijswijk, The Netherlands. The estimated initial val- regressive coastal sequence at a depth of 11800- ues were obtained by determining grain size and sort- 12000ft (3596-3657m) from a well in the Niger ing in thin section and by fitting these into the data set Delta. Of a large number of samples of measured for porosity and permeability of artificial grain size porosity and permeability, a representative set of 12

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was studied petrographically. Quartz and polycrystal- The response of the framework of these quartz line quartz average 95 per cent, feldspars 3 per cent arenites to burial diagenesis has been mainly mechani- and other constituents each make up less than 1 per cal compaction, and pressure solution to a minor cent of solid-rock volume, which classifies these rocks extent only. In the coarse-grained sandstones, break- as very pure quartz arenites (Fig. 1). On the age of grains at maximum stress points and partial porosity/permeability plot, the samples differentiate healing have also been observed. Although generally well according to depositional environment, which is present, this has not led to significant loss of porosity mainly the result of grain size differences (Fig. 2). and permeability. Pressure solution, present in all samples, is de- veloped to a limited degree only. Considering the Arkosic arenites appreciable depth of burial, this may be mainly due to the relatively young age and a very low geothermal The potential unstable framework constituents in gradient (approx. 1.3°C/100 m). arkosic sandstones are the feldspars. Feldspars tend to With reference to the estimated initial porosities, be stable in basic solutions of high sodium, potassium, the very fine to fine-grained sandstones show a slightly and silica concentration and unstable in neutral to acid larger diagenetic shift than the medium- to coarse- solutions of low concentration of the same elements grained sandstones (Fig. 2B), in accordance with the (Garrels & Howard 1959, Wollast 1967, Helgeson et fact that pressure solution tends to have a more severe al. 1969). The first set of conditions occurs in saline effect on finer-grained sands (Fiichtbauer 1967). solutions which form in evaporitic basins, and which

TEXTURAL MATURITY

QUARTZ (F <10%, L <10%)

QUARTZ WACKE QUARTZ PSAMMITE QUARTZ ARENITE .. . . r FT~_&t~..j,:~al--'_c~ k \36 ARKOSIC (F>tO , L< IO)

ARK0$1C- LITHIC (F>L, L> tO) ARKOSIC WACKE ARKOSIC PSAMMITE ARKOSIC ARENITE ARKOSIC- LITHIC WACKE ARKOSIC- LITHIC PSAMMITE -=- i" ,r, . • LITHIC (L>tO, F< tO)

LITHIC - ARKO$1C ( L>F, F>tO) LITHIC WACKE LITHIC PSAMMITE LITHIC ARENITE ] LITHIC-ARKOSIC WACKE I LITHIC-ARKOSIC PSAMMITE l LITHIC-ARKOSIC ARENITE [ 'M'NERA~_ I WACKE [ PSAMMITE I ARENITE I / ' tx'~I (greywocke) l -- 1 J = "P Poorly to extremely "P Moderately to well ~ Well to extremely Z O0%SANDY MUDSTONE75.% Poorly sorted sst. MATRIX sorted sst. , '7. well sorted sst. O°q cMUADySsrOoNESsAND MATRIX SUPPORTED 2%0RE5~° SAND GRAIN SUPPORTED / • I f /

Fro. 1. Sandstone nomenclature utilised in this study. The classification follows generally accepted lithological subdivisions but classes arkosic and lithic sandstones at 10 per cent of feldspars and lithics, respectively, because of the significance of these components in diagenesis. Texturally, 'psammites' are recog- nised as an intermediate class with regard to matrix content.

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during diagenesis may derive from compacting feldspar stability, were treated by Sommer (1975). A evaporitic deposits. The second set of conditions is total of 50 samples from two localities were analysed optimally realised at surface in the humid tropics and petrographically, and all classified as arkosic arenites. may occur during diagenesis in sediments rich in vege- In the samples from the first locality, the feldspar tation debris owing to the release of CO2 in the course content (nearly all K-feldspars) averages 20 per cent. of the coalification process [mainly in the low-rank The feldspars are fresh and show ubiquitous over- lignite (40 per cent fixed C) to medium-rank bitumin- growths, which is attributed to the influence of saline ous (65 per cent fixed C) stage (Mott 1943, Patijn K-rich solutions. Additional evidence of the presence 1964)]. of such solutions has been found in the form of Examples of varying feldspar stability during numerous, scattered, small dolomite rhombs and the diagenesis are given by Fiichtbauer (1974, p. 149) occurrence of anhydrite inclusions in authigenic from Triassic sandstones in Germany. quartz. The arkosic arenites taken as an illustrative case In the samples from the second locality, the feldspar here are from the Jurassic of the North Sea. Some content (again nearly all K-feldspars) averages 7.8 per aspects of the diagenesis of these sandstones, including cent but must have been higher before diagenesis.

,•t'#C. t05 131

t0 4 rz ,/~ CHANNE L FILLS AN D "X CREVAS SE t0 3 P-.Pl AYN I G RAIN S IZE ~ COAS FAL BARR IER t0 2 ,7 l ""I t0 ,,, ,,. ESTIMATE[ i/J s// INITIAL R )SITION t FIG. 2. ® DIAGENES IS ® / A. Reference grid for porosity and ~- 0.t .._1 permeability values of various grain size and sorting mixes of unconsoli- u..i dated clay-free sands. Drawn on the ,,-- t05 (:a.. , i '¢M. , M. basis of experimental data of Beard & Weyl (1973). ~-F. /,//-7,'. 104 iARK~iC ARENI1,ES i a. Porosity/permeability plots for UNAFFE CTED B~ Av, Miocene quartz arenites at FELDSP ~.R ALTE R- t05 RATION 11 800-12 000 ft (3596-3657 m) burial, (12 samples) Niger delta.

t02 c. Porosity/permeability plots for ar- ARKOSK : ARENI-A ~'/[ MIOCENI voL-.//1 / kosic arenites with and without TES AFF ECTED/ CANIC-L ITHIC/ feldspar kaolinisation, at 9000- BY FELl >_PAR/ AR EN I T ( dAPAN ) t0 KAOI I1~ 10000ft (2743-3047 m) (20 sam- SA 60oo ') / pies) and 10000-11 000ft (3048- -C/ 3352 m) (34 samples) burial respectively. Jurassic sands, North Sea.

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Many feldspars and even muscovite are altered to components were not differentiated. Further samples varying degrees, and there is abundant authigenic have been taken from Westphalian D exposed strata interstitial kaolinite, both finely crystalline and in the in the South Central Pyrenees, Spain. These sand- form of large vermicules. Common local large masses stones contain on average 14 per cent quartz, 2 per of kaolinite testify to the former presence of feldspar cent feldspar, the rest being made up of non-volcanic grains (P1. 1, fig. A). This suggests that there has been lithics, mainly phyllite (Nagtegaal 1969). no further mechanical readjustment to the areas The diagenesis in the case of the Japanese samples weakened by feldspar alteration and that mechanical is similar to that described by Galloway (1974) for compaction had therefore been completed, which syn- volcanic sandstones from the NE Pacific. It consists of chronises well with the period of increased CO2 re- early formation of clay rims, development of pore- lease during diagenesis. filling clay minerals and subordinate amounts of The porosity/permeability plots for both sets (Fig. laumontite, and minor quartz and feldspar over- 2c) overlap to a certain extent but also show an extra growths. The authigenic clay minerals are chiefly field of low permeabilities for the arkosic arenites with montmorillonite and some chlorite. There is petro- kaolinised feldspars. This effect can be directly attri- graphic evidence of devitrification of pumiceous and buted to the widespread authigenesis of kaolinite other glassy fragments and marked plastic deformation which now forms a diagenetic matrix. The wide range of the soft lithics. The porosity/permeability plot for in permeabilities results from the fact that the degree these rocks is given in Fig. 2D. At comparable depth, of feldspar alteration varies from sample to sample, the porosities are of the same order as reported by being nearly complete in the samples of lowest permea- Galloway, although at a much lesser depth of burial bility. The porosities are somewhat lower than those the values are considerably lower than in the case of of the samples with unaltered feldspars, probably due the arkosic arenites with kaolinised feldspars (Fig. 2c). to an initial wider sorting range (Fig. 2c). This is thought to be due to the fact that in these The arkosic arenites with unaltered feldspars plot in lithic-arkosic arenites, in addition to widespread approximately the same field as the quartz arenites of mineralogical alteration, plastic deformation of soft comparable grain size and sorting range (burial depths lithics is a major porosity- and permeability-reducing are of the same order of magnitude, Figs. 2a, c), process. The clay-mineral growth habit in the clay indicating that at the depth range investigated such rims, which have clay flakes perpendicular to grain arkosic arenites follow the same trend as the quartz surfaces, had a further strongly deteriorating effect on arenites. permeability. Although widespread kaolinisation of feldspars In the Spanish samples, no other diagenesis was strongly affects permeability, drastic permeability de- observed than extreme plastic deformation of phyUite crease owing to clay-mineral authigenesis is achieved grains (P1. 1, fig. B). Porosities are less than 10 per only on deeper burial, when the kaolinite loses its cent and permeabilities less than 0.1 roD. These rocks stability and, in the presence of potassium, illite is have undergone considerable burial (interbedded coals generated in a common clay rim and pore-bridging have a mean fixed carbon content of 91 per cent, growth habit (Stalder 1973). which in the case of a geothermal gradient of 3°C and 4°C/100 m, would correspond to approximately 6000 Lithic arenites and 4500 m burial respectively). However, the absence of authigenic minerals, except some late replacive calcite Lithic arenites constitute a highly variable group of suggests that plastic deformation of the soft lithics set sandstones due to the variety in composition of rock in very early and proceeded rapidly. fragments. Rock fragments are commonly classed as volcanic, metamorphic, and sedimentary (Pettijohn et PLATE 1 (facing) al. 1973, p. 187). However, the effect of the presence FIG. A. Local mass of kaolinite and some sericite of rock fragments on diagenetic sandstone-framework in fine-grained, moderately sorted arkosic are- stability is more readily appreciated when they are nite interpreted as the in situ alteration pro- duct of a feldspar grain. Crossed nicols. Juras- classed as 'hard lithics' (quartzite, chert, carbonate-- sic, North Sea. x 200. resisting plastic deformation), 'soft lithics' (phyllite, micaceous schist, many volcaniclastics--easily yielding FIG. B. Extreme plastic deformation of phyllite to plastic deformation) and those susceptible to grains, resulting in nearly complete loss of mineralogical alteration such as volcaniclastics and porosity and permeability in a coarse-grained, carbonates. moderately sorted lithic arenite. Owing to The examples discussed here include Miocene lithic- differences in texture and cleavage, the indi- vidual phyllite grains are still recognisable. arkosic arenites from Japan occurring at a depth of Lightest gray are quartz grains, black is ferric 5000-6000 ft (1524-1828 m). The lithics (55 per cent) oxide introduced by weathering. Single polar- are predominantly of andesitic volcanic origin. Quartz ised light. Westphalian D sandstone, Pyrenees, averages 20 per cent and feldspars 13 per cent; other Spain. x 60.

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104 PLATE 1

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Conclusions feldspars. Mechanical framework collapse due to this process was not observed but is possi- The limited number of cases studied does not yet ble at very high feldspar percentages. allow conclusions of general applicability, but the ob- 3. Lithic arenites (those with soft and volcanic servations are consistent with the following state- lithics) range lowest in framework stability be- meats. cause of their susceptibility to plastic deforma- 1. Of the samples studied, quartz arenites show the tion and mineralogical alteration of the highest framework stability. Coarse-grained, framework constituents, in addition to the well sorted pure quartz arenites have the optimal mechanical compaction and pressure solution po- potential of preserving high porosity and per- tentially operative in nearly all sandstones. These meability during burial diagenesis. sandstones have the least chances of retaining 2. Arkosic arenites show a similar behaviour to porosity and permeability at depth. quartz arenites down to a depth of at least 11 000 ft (3352 m) under the influence of basic- ACKNOWLEDGEMENTS. I would like to thank my colleagues Susan H. Ritchie, Uli Seemann and Nico C. Rossel for reacting saline solutions. The framework loses its thought-provoking discussions on the subject and the analyti- stability, with a resulting decrease mainly in per- cal data provided. The text benefited from critical reading by meability, in an acid-reacting interstitial solution Roland C. Murat. I am indebted to Shell Internationale as a result of widespread kaolinisation of Research Mij. B. V. for their permission to publish this work.

References

BEARD, D. C. & WERE, P. K. 1973. Influence of texture on geologic age on porosity of quartzose sandstone. Bull. porosity and permeability of unconsolidated sand. Bull. Am. Ass. Petrol. Geol. 48, 697-709. Am. Ass. Petrol. Geol. 57, 349-69. MoTr, R. A. 1943. The origin and composition of coals. F(/CHTBAUER, H. 1967. Influence of different types of Fuel, 22, 20-6. diagenesis on sandstone porosity. Proc. 7th World Petrol. NAGTEGAAL, P. J. C. 1969. Sedimentology, paleoclimatology, congr. Mexico, 2, 353-69. and diagenesis of post-Hercynian continental deposits in -- 1974. Sediments and sedimentary rocks I. E. Schweizer- the South Central Pyrenees, Spain. Leid. geol. Meded. bart'sche Verlagsbuchhandlung (N~igele u. Obermil- 42, 143-238. ler), Stuttgart, 464 pp. PATIJN, R. J. H. 1964. Die Entstehung von Erdgas infolge GALLOWAY, W. E. 1974. Deposition and diagenetic altera- der Nachinkohlung im Nordosten der Niederlande. Erdfl tion of sandstone in Northeast Pacific arc-related basins: Kohle Erdgas Petrochem. 17, 2-9. implications for graywacke genesis. Bull. geol. Soc. Am. PETrIJOI-rN, F. J., POTTER, P. E. & SIEVER, R. 1973. Sand 85, 379-90. and sandstone. Springer-Verlag, New York, 618 pp. GARRELS, R. M. & HOWARD, P. 1959. Reaction of feldspar SOMMER, F. 1975. Histoire diag6n6tique d'une s6rie gr6seuse and mica with water at low temperature and pressure. de Mer du Nord. Datation de l'introduction des hyd- Clays Clay Miner. 6, 68-88. rocarbures. Revue. Inst. fr. P~trole 30, 729-41. HELGESON, H. C., BROWN, T. H. & LEEPER, R. H. 1969. STALDER, P. J. 1973. Influence of crystallographic habit and Handbook of theoretical activity diagrams depicting aggregate structure of authigenic clay minerals on sand- chemical equilibria in geologic systems involving an stone permeability. Geologie Mijnb. 52, 217-9. aqueous phase at one atm and 0 ° to 300° C. Freeman, WOLLAS'r, R. 1967. Kinetics of the alteration of K-feldspar in Cooper & Co., San Francisco, 253 pp. buffered solutions at low temperature. Geochim. cos- MAXWELL, J. C. 1964. Influence of depth, temperature, and mochim. Acta 31, 635-48.

Received 3 March 1977; read 9 March 1977; revised typescript received 25 April 1977.

PETER J. C. NAGTEGAAL Koninklijke/Shell exploratie en Produktie Labora- torium, Shell Research B. V., Rijswijk, The Netherlands.

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