Harris (1989) concluded that the Middle Jurassic Brent sandstones in the North Sea were initially feldspar rich but became diagenetically quartz- ose with ...
Comment and Reply on "Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe" COMMENT
tion at Lyell has yet to be addressed. In addition, what happened to dissolved feldspars at Lyell? Harris dismissed the importance of meteoric water diagenesis in the Brent Group. The Brent Group, irrespective of its depth of burial, shows uniformly high core porosity when it occurs immediately beneath the Cimmerian unconformity in the Statfjord field area (Fig. 1). Petrographic observations show that secondary porosity caused by feldspar dissolution in the Brent Group tends to increase toward the unconformity (Shanmugam, 1988). This trend has been attributed to feldspar dissolution by percolating meteoric waters during the Cimmerian uplift and erosion (Shanmugam, 1988). Other authors (Bjorlykke, 1983; Bjorlykke and Brendsdal, 1986) proposed two periods of feldspar dissolution in the Brent Group by meteoric waters: the first during the Middle Jurassic deposition, and the second during the Cimmerian uplift. Because there is an empirical relation between feldspar dissolution and proximity to the unconformity in the Statfjord field, samples will exhibit different degrees of feldspar dissolution depending on their stratigraphic position (i.e., updip vs. downdip to subcrop). Therefore, the position of samples with reference to unconformities is an important factor that should be considered, irrespective of whether Harris believes in meteoric diagenesis.
G. Shanmugam, Dallas Research Laboratory, Mobil Research and Development Corporation, P.O. Box 819047, Dallas, Texas 75381 Harris (1989) concluded that the Middle Jurassic Brent sandstones in the North Sea were initially feldspar rich but became diagenetically quartzose with increasing burial depth. Perhaps because of space limitations, Harris did not include critical data on depositional composition and compaction. This Comment addresses these and other important issues. Establishing the initial (depositional) composition of the Brent Group is critical to Harris's study. Conventional point-count techniques (i.e., counting grains as grains and pores as pores) are not appropriate for the Brent Group because it contains dissolved framework grains. In these cases, dissolved portions of a grain should be counted as grain, not as porosity (Shanmugam, 1985). However, Harris (1989, p. 362) used a method in which "points falling on a pore within a feldspar were treated as porosity." In short, his technique did not account for dissolved grains, and therefore his point-count data (Harris, 1989, Fig. 2) represent the diagenetic composition, not the initial depositional composition. According to Harris (1989, p. 363), the apparent initial composition of the Brent Group was established on the basis of grain remnants. He estimated that nearly 80% of the feldspar (20% of the solid rock volume) was dissolved at Lyell field, but his point-count data show that secondary porosity caused by feldspar dissolution constitutes an average of 3% of the rock volume at Lyell. Does this mean that Harris gathered two sets of point-count data (i.e., one on depositional composition and another on diagenetic composition)? If so, both sets of data should be presented for evaluation. In short, what is the evidence that suggests 20% of the rock volume at Lyell was originally composed of feldspars?
Harris (1989, Fig. 2 caption) suggested that all Brent formations show similar patterns of increasingly quartzose composition with increasing burial depth. This is not justified, because the Tarbert, Etive, Rannoch, and Broom Formations were not adequately sampled in all three fields (Table 1). This leaves the Ness Formation, which is a complex unit in terms of depositional facies. In the Statfjord field, the Lower Ness Formation was deposited in a saline bay-lagoonal environment, and the Upper Ness Formation was deposited in a delta-top environment with associated swamp, lagoon, distributary channel, levee, and washover fans (Roberts et al., 1987). I would expect the depositional composition of washover sands to be more quartzose than fluvial sands. Given the variability in depositional facies combined with Harris's point-counting techniques, it is not clear what the Ness trend means. In addition to inadequate sampling, Harris's data points are sufficiently scattered to allow for alternate interpretations. For example, in the absence of Rannoch samples from the critical Lyell field, one could argue that the samples with the highest quartz content come from the shallowest Statfjordfield(cf. Rannoch composition between Statfjord and Hutton fields; Harris, 1989, Fig. 2), a trend that is the opposite of what Harris claimed for all formations. In summary, Harris's conclusions seem to me to be drawn from inadequate sampling and not enough critical data on depositional composition and compaction.
Harris suggested that more feldspars were dissolved at Lyell (i.e., 20% of the rock volume) than at Hutton or Statfjord. If so, why do all three fields show constant amounts of secondary porosity? If the answer is compaction, then Harris should present the compaction data from all three fields. For example, did he quantify the relative importance of compactional processes (see Houseknecht, 1987) to porosity reduction? Harris's qualitative statement "sandstones at Lyell are also highly compacted" (1989, p. 362) does not justify his quantitative claims. Dissolution of large amounts of feldspar at deep burial implies that generation of secondary porosity was related to carbon dioxide from maturing kerogen. Bjorlykke (1983) showed from his calculations that in most basins the carbon dioxide produced from kerogen is insufficient to create significant secondary porosities because the carbon dioxide released will be neutralized in the source rocks. Thus, the mechanism of feldspar dissolu-
ACKNOWLEDGMENTS I thank M. E. Mathisen and J. E. Welton for their comments on the manuscript, and Mobil Research and Development Corporation for permission to publish this Comment.
TABLE 1. NUMBER OF SAMPLES USED IN HARRIS'S (1989) FIGURE 2 Figure 1. Brent Group showing uniformly high average core porosity values (28%-30%) immediately beneath unconformity. Four wells are located in transect of approximately 20 km. Note that porosity remains high despite differences in burial depths. Statfjord field and adjacent areas, North Sea. K.B. = kelly bushing.
GEOLOGY, March 1990
CRETACEOUS/
Formation
TERTIARY
-l
JURASSIC (BRENT GROUP)
JURASSIC/TRIASSIC L (DUNLIN GROUP, [ S T A T F J O R D FM., I A N D L U N D E FM.)
Tarbert Ness Etive Rannoch Broom Total
statfjord field
Hutton field
—
—
5 2 16 3 26
9 9 2 6 26
Lyell field 1 8 2 — —
11
Note: — = no sample. Lyell field, where diagenetic quartzarenites are presumably prevalent, is not sampled adequately from all five formations.
287
REPLY
Figure 1. Dip-oriented profiles from Statfjord and Hutton fields, showing relation of secondary porosity in well samples to position of unconformity. Jagged line indicates point where Brent is totally truncated; down-dip direction is to left. Numbers in parentheses are numbers of samples analyzed. Data show that secondary porosity does not i n c r e a s e t o w a r d unconformity.
Nicholas B. Harris, Exploration Research and Services Division, Conoco Inc., P.O. Box 1267, Ponca City, Oklahoma 74603 In my paper on secondary porosity in the Brent sandstones (Harris, 1989), I demonstrated that sandstone compositions shift from lithic arkosic, arkosic, and subarkosic at Statfjord field (2500 m) to sub-arkosic at Hutton field (3050 m), to quartzarenitic at Lyell field (3500 m). I showed that the volume of secondary porosity due to grain dissolution is relatively minor, averaging 2.5% to 3.0% in the three fields studied. The sandstones in the three fields probably had similar initial compositions, on the basis of their proximity, regionally similar source terranes, contemporaneity of deposition, and similarity of depositional environments. I concluded that significant amounts of feldspar were dissolved in the sandstones during burial diagenesis, far more than is recorded as secondary porosity, and I suggested that the mechanical strength of the sandstones may limit the amount of secondary porosity that can be preserved. Shanmugam disagrees with several points in my paper. I address the most significant of these in turn. 1. Shanmugam states "His [point-count] technique did not account for dissolved grains, and therefore his point-count data (Harris, 1989, Fig. 2) represent the diagenetic composition, not the initial depositional composition." I collected two sets of point-count data. My Figure 2 (Harris, 1989), a QFL plot, represents compositions following feldspar dissolution, as Shanmugam states. My Figure 4 (Harris, 1989), a quartz-plagioclase-Kfeldspar plot, shows both present and reconstructed compositions. Figure 4 clearly shows that reconstructed compositions shift almost uniformly away from the quartz pole in comparison to present compositions. Had reconstructed compositions been used in Figure 2, the overall trend in these compositions from Statfjord to Lyell fields would have been virtually identical to the trend in present compositions. This follows because the abundance of secondary porosity is similar in the three fields and because dissolved feldspar accounts for most of the secondary porosity. 2. Shanmugam states "Harris should present the compaction data from all three fields." Such data are shown in Table 1 here; they show that intergranular volume, a standard measure of compaction, decreases systematically from Statfjord to Hutton to Lyell. 3. Shanmugam states "Harris dismissed the importance of meteoric water diagenesis..." and the "position of samples with reference to unconformities is an important factor that should be considered." In his Comment he presents bulk porosity data but no petrographic data in support of his contention that secondary porosity is enhanced near the unconformity. In a previous paper (Shanmugam, 1988), he presented petrographic data from a single well; the data are based on visual estimates of secondary porosity rather than point counts. Figure 1 here shows the abundance of secondary porosity in three dip-oriented profiles from Statfjord and Hutton fields, based on petrographic point counts (details in Harris, 1989). The data clearly show that secondary porosity is not enhanced near the Cimmerian unconformity, but rather is nearly constant. 4. Shanmugam comments that my conclusions are "not justified, because the Tarbert, Etive, Rannoch, and Broom Formations were not adequately sampled in all three fields. . . .This leaves the Ness Formation, which is a complex unit in terms of depositional fades." Feldspar abundance most clearly varies with grain size (Odom et al., 1976) where single suites of sandstones are at similar burial depth. PreTABLE 1. INTRAGRANULAR VOLUME IN BRENT SAMPLES
Formation
Statfjord Vol. No. of samples (%)
Ness Etive Rannoch
33.3 28 .6 33.9
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5 9 9
Hutton Vol. No. of samples (%) 29.5 28.7 27.3
20 24 5
Lyell Vol. No. of (%) samples 22.0 20.0
8 1
C Downdip
sumably, Shanmugam's comment regarding depositional complexity alludes to the relation of grain size to depositional facies. Among the Ness samples, the average grain size for each of the fields is medium-grained sand. Because sandstones of similar grain size are compared, the comparison remains valid. Shanmugam ignores the fact that the average composition of each formation shifts in similar fashion for all three fields (see Harris, 1989, Fig. 2). Whereas the sample base in some cases is small, the uniformity in compositional variation is compelling. Were the sample base too small for valid comparison, some degree of randomness would be expected in the compositional variation. In summary, Shanmugam's criticisms are based largely on misunderstanding of my work. He does not realize that point-count data were collected such that apparent detrital compositions could be reconstructed, nor that a figure portraying such compositions was part of my article. He suggests that my work be discounted because it does not fit current models of secondary porosity formation, though it may be that the models, not the data, are inadequate. He contends that his unconformity model for feldspar dissolution was treated inadequately; my data directly contradict his model and show that abundance of secondary porosity is unrelated to the unconformity. Finally, he contends that my sample base is inadequate, ignoring the fact that all formations exhibit parallel compositional changes. ACKNOWLEDGMENTS I thank my colleagues R. W. Lahann and D. F. Toomey for reviewing this Reply and Conoco Inc. for permission to publish it. Maria Skaggs drafted the figure.
COMBINED REFERENCES CITED Bjorlykke, K., 1983, Diagenetic reactions in sandstones, in Parker, A., and Sellwood, B.W., eds., Sediment diagenesis: Dordrecht, Netherlands, D. Reidel, p. 169-213. Bjorlykke, K., and Brendsdal, A., 1986, Diagenesis of the Brent sandstone in the Statjford Field, North Sea, in Gautier, D.L., ed., Role of organic matter in sediment diagenesis: Society of Economic Paleontologists and Mineralogists Special Publication 38, p. 157-167. Harris, N.B., 1989, Diagenetic quartzarenite and destruction of secondary porosity: An example from the Middle Jurassic Brent sandstone of northwest Europe: Geology, v. 17, p. 361-364. Houseknecht, D.W., 1987, Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones: American Association of Petroleum Geologists Bulletin, v. 71, p. 633-642. Odom, I.E., Doe, T.W., and Dott, R.H., Jr., 1976, Nature of feldspar-grain-size relations in some quartz-rich sandstones: Journal of Sedimentary Petrology, v. 46, p. 862-870. Roberts, J.D., Mathieson, A.S., and Hampson, J.M., 1987, Statfjord, in Spencer, A.M., et al., eds., Geology of the Norwegian oil and gas fields: London, Graham & Trotman, p. 319-340. Shanmugam, G., 1985, Significance of secondary porosity in interpreting sandstone composition: American Association of Petroleum Geologists Bulletin, v. 69, p. 378-384. 1988, Origin, recognition and importance of erosional unconformities in sedimentary basins, in Kleinspehn, K.L., and Paola, C., eds., N e w perspectives in basin analysis: N e w York, Springer-Verlag, p. 83-108. GEOLOGY, March 1990 287
