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Sandstone-Framework Instability As a Function of Burial Diagenesis

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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 suscepti- ble 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/135/1/101/4885769/gsjgs.135.1.0101.pdf by guest on 24 September 2021 102 P. J. C. Nagtegaal 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 sand- stones 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/135/1/101/4885769/gsjgs.135.1.0101.pdf by guest on 24 September 2021 Sandstone-]'ramework instability as [unction o[ burial diagenesis 103 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.
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