porosities and seismic velocities of mudstones from wairarapa and oil wells of north island, new...

New Zealandlournal of Geology and Geophysics, 1990, Vol. 33: 29-39 0028-8306/90/3301-0029 $2.50/0 © Crown copyright 1990

29

Porosities and seismic velocities of mudstones from Wairarapa and oil wells of North Island, New Zealand, and their use in determining burial history

PATRICIA E.WELLS Research School of Earth Sciences, Victoria University of Wellington P.O. Box 600 Wellington, New Zea1and*

*Present address: 48 Blackman Cres., Macquarie,Canberra, ACT, 2614, Australia

Abstract The seismic velocities and porosities of mudstones and other fine-grained sedimentary rocks can be used to estimate burial history. The porosity of exhumed mudstone sequences of Wairarapa has been derived from density measurements, and can be used to estimate prior depth of greatest burial, to within about 100m, by the linear regression function, greatest depth of burial = -111 porosity + 4975, or the exponential function, greatest depth of burial = -8636 log porosity + 14384.

The velocity trend of the exhumed mudstones of southeastern Wairarapa is not as good an indicator of burial history, as it has only a poor fit to the regression function, greatest depth = 2666 velocity - 3331. However, fully loaded, fme-grainedsedimentaryrocks (mainly mudstones) still under lithostatic pressure in North Island oil wells have a good fit to the velocity-depth regression functions, depth = 1026 velocity -1419 or, depth = 6617 log velocity -1403. This relationship can be used to calculate the thickness of sediment missing from the stratigraphic column.

Comparison of the velocity-depth trend of a partially unloaded, fme-grained sedimentary rock sequence in western Wairarapa with that of several other partially unloaded New Zealand sequences with a known history of vertical uplift (as shown by oil wells) indicates that 1100-1500 m of sediment has been unloaded by erosion.

Keywords burial history; velocity-depth trends; porosityd~pth trends; exhumed mudstone; Wairarapa; unconformity; oil-well sequences

INTRODUCTION

A large proportion of the upper Cenozoic rocks ofWairarapa are mudstone and other fine-grained sedimentary rocks. The physical properties of these rocks have been studied (Wells 1989a) to investigate whether the rocks have retained some imprint of their burial history and to see if this can be used to estimate the thickness of sediment that is missing through erosion at unconformities.

G89018 Received 18 April 1989; accepted 2 October 1989

Almost all sedimentary sequences compact and dewater on burial. The degree of compaction is usually gauged in terms of porosity, which can be determined from dry and saturated rock densities. Similarly, the seismic velocity of mudstone and other fine-grained sedimentary rocks appears to increase with increasing depth of burial. If change in porosity and/or velocity can be shown to be a function of ~epth for ~ given rock type, the relationship can be used to infer preVIous depth of burial of uplifted rocks.

Sonic log data are available for North Island oil wells that passed through thick sequences of mudstone and other finegrained sedimentary rocks similar to the upper Cenozoic rocks exposed onshore in Wairarapa, enabling some comparisons to be made between the velocity trends of these rocks when they are fully loaded, partially unloaded and fully unloaded.

An exponential relationship between porosity and depth of burial has been established for arange of sedimentary rocks in the Northef!1 ~emisphere (e.g., North Sea Basin study by Sclater & Christte 1980) but there have not been equivalent, comprehensive studies of the porosity-depth or velocitydepth trends for NewZealandfme-grained sedimentary rocks. Some pioneering studies of the physical properties of North Island rocks were conducted by Garrick in the 1960s (e.g., Garrick 1969).

MATERIALS AND METHODS

Selection of fteld samples

A sequence exposed at the land surface that is suitable for examining porosity, density, and sonic velocity trends with depth of burial should have the following characteristics: (1) One lithotype shouldbepresentover a large stratigraphic

thickness so that any trends observed are independent of changes in lithology;

(2) The section should provide exposure sufficient to enable sampling at c. 150 m intervals;

(3) Samples must be fresh and, if weakly consolidated, should be water saturated to avoid disaggregation during subsequent laboratory treatment;

(4) The location should have a complete stratigraphic section with no faults or unconformities so that the stratigraphic position of a sample can be related to its depth of greatest burial prior to exhumation.

The depth of greatest burial of a surface sample can be estimated using simple trigonometrical techniques if a "layercake" stratigraphy can be assumed (this assumption is not always justified for the upper Cenozoic sedimentsofWairarapa which accumulated in a fold and thrust belt adjacent to the ancient Hikurangi margin (Wells 1989a».

Only two sections in Wairarapa satisfied most of these conditions: the Hinakura Road and Mangaopari Stream areas of southeastern Wairarapa (Fig. 1).

30 New Zealand Journal of Geology and Geophysics, 1990, Vol. 33

eTane-1 39°

co f .. ~ .... 41 .... ~ .... ~ f s41 Q. Maui -3 e

Maui-2 e) Maui-1e .t<

e Kiwa-1 ;< I

;. . \- e MaUl -4

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Fig. 1 Location of study areas in Wairarapa (circled stars) and oil wells of North Island (filled circles) used for comparison of data. Inset map A: location of the vertical sections (circled stars) on either side of the Wairarapa Fault on the seismic reflection line of Cape et al. (in press), within sheet NZMS 260 S26. Inset 11U1p B (adapted from Vella & Briggs (1971): location of sampling sites of exhumed mudstone along Mangaopari Stream. Inset map C (adapted from Vella & Collen 1984): location of sampling sites of exhumed mudstone along Spooner Stream and Hinakura Road. Maps B and C are from NZMS 260 S27.

The Hinakura Road section (Fig. 1 C) consists of a thick sequence of Upper Miocene to Pliocene mudstone. The structure of the mudstone is very simple (Vella & Collen 1984) with uniformly dipping strata, no faulting, and no detectable unconformities. Good exposure enables fresh samples to be collected at regular intervals throughout the

1500 m section. The lack of facies change and the deep-water character of the mudstone suggest that it accumulated near the centre of a rapidly subsiding basin for which a layer-cake stratigraphy can reasonably be assumed. Therefore, the previous maximum depth of burial of the Hinakura Road mudstone samples has been estimated by simple

Wells-Porosity & velocity trends, N.Z. mudstones

Table 1 Seismic velocity data for exhumed mudstones of south-eastern Wairarapa. Burial depths are estimated depth of greatest burial (discussed in text). Seismic velocity is derived from transit time data. Data in brackets are considered to be unrepresentative. All Hinakura samples are mudstones except Hl4a (siltstone) and H15a (silty, very fme sandstone). Huangarua samples are mudstones sampled from either Barytellina Mudstone (Hg1; a muddy, fine sand-stone) or the Te Muna Formation (Collen & Vella 1984), with Hg2 and lowest lignite from mudstone members 2 , and Hg3 & 4 from mudstone member 6. Mangaopari samples M15-17* are from Mangaopari Mudstone (Vella & Briggs 1971); M15 is from 20 m above Hikawera Tuff, and M13 & 14* are from Bells Creek Mudstone, 60 m and 1 m above Hikawera Tuff, respectively. (*In Table 5.)

Sample no. Burial depth Transit time Seismic velocity (m) ijI.s/ft) (km/s)

Hinakura Road H1Ba 1890 207.8 (1.47) H1Bb 1890 169.5 1.80 HI 1880 171 1.78 H2 1650 168 1.81 H3a 1360 181.6 1.68 H3b 1360 172.9 1.76 H3c 1360 183.3 1.67 H3d 1360 183.1 1.67 H4 1320 181.5 1.63 H5a 1300 189.3 1.61 H5b 1300 216.3 (1.41) H6a 1030 183.6 1.66 H6b 1030 182.6 1.67 H8a 980 185.3 1.65 H8b 980 192.4 1.59 H9a 910 193.5 1.58 H9b 910 191.1 1.59 H10a. 870 185.0 1.65 HlOb 870 190.7 1.60 Hlla 830 178.9 1.70 Hllb 830 180.7 1.69 H12a 680 183.5 1.65 H12b 680 189.9 1.60 H13a 670 180.6 1.69 H13b 670 184.5 1.65 H14a 370 248.0 (1.23) H14b 370 237.6 1.28 H15a 330 220.0 138

Huangarua River Hgl 380 166.5 1.83 Hg2 368 182.5 1.67 Hg3 350 180.6 1.69 Hg4 200 185.9 1.64

Mangaopari Stream M8a 258 186.9 1.63 M8b 258 203.1 1.50 M9 328 202.4 1.51 M15 578 186.0 1.64 M13 703 354.6 (0.86) M12 723 181.4 1.68 M12Ba 743 166.7 1.83 M12Bb 743 180.6 1.69

31

trigonometrical methods, although their stratigraphic positions could vary by as much as 30 m, which is as accurate as grid references and scale drawing allow. Supplementary samples were taken from a section through Bells Creek Mudstone and Mangaopari Mudstone exposed in Mangaopari Stream 14 km southwest of Hinakura Road (Fig. IB). Also, samples of mudstone and other fine-grained sedimentary rocks from the Hautotara and Te Muna Formations (Collen & Vella 1984), overlying Mangaopari Mudstone, were collected along the HuangaruaRiver,c.3.5kmduenorthofMangaopariStream.

The stratigraphic relationship of these samples is indicated in Table 1 and Fig. 2. The Mangaopari Stream section is at the northern end of the Aorangi Range (Fig. 1), in an area characterised by rapid facies change, unconformities, and shallow-water facies (Vella & Briggs 1971, fig. 2), and these features suggest that the Upper Miocene and Pliocene sediments in the vicinity of Mangaopari Stream accumulated near the edge of a sedimentary basin for which a layer-cake stratigraphy is not appropriate and a "lensoid" stratigraphy is more applicable. The original geometry of this basin is not known, so, for the sake of simplicity, the depths of burial of the samples in the Mangaopari Stream area have been calculated by simple trigonometrical techniques, assuming that there was no significant thickness of sediment overlying Pukenui Limestone Formation. This simplistic approach introduces an even greater error range (i.e., >±30 m) into the depth calculations for these samples compared to that inferred for the Hinakura samples, although their relative stratigraphic positions are fairly well constrained because they are sampled over a short distance (c. 1 km).

Seismic velocity data

Seismic velocity data were obtained from several different sources: direct measurement of the velocity of exhumed (fully unloaded) mudstone sequences in eastern Wairarapa using high frequency transducers over short distances; the use of seismic prospecting data from partially unloaded stratigraphic sequences in western Wairarapa; and use of electric logs of several North Island oil wells (Fig. 1,2). There arenodataavailable to enable cross-correlation of the velocities obtained from these three different sources, and in this study it is assumed that the velocity data are comparable. In an overview of the question of the correlation of velocities derived from sonic well logs and ultrasonic techniques, Barrett & Froggatt (1978) concluded that the predicted differences are not large «5%).

The measurements of seismic compressional wave velocity (hereafter termed velocity) were made on block samples of mudstone using the PUNDIT (portable Ultrasonic NonDestructive Digital Indicating Tester), which measures the travel time of an 82 kHz pulse between two transducers pressed firmly against samples of measured length. The method of using the PUNDIT and estimating error limits for the measurements (3%) follows that of Barrett & Froggatt (1978).

Initially, measurements were made in the field by pressing the transducer and receiver against fresh outcrops and recording the time for the signal to travel the measured distance between them. The same results were obtained more conveniently by collecting fresh (unweathered and unfractured), watersaturated blocks of mudstone and measuring their velocity in the laboratory. Samples at least 10 cm long were collected from streambeds (where possible) , and velocity measurements


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FIg.2 Stratigraphic columns for North Island oil wells and Wairarapa sections (locations given in Fig.l). The western Wairarapa columns are from my interpretation of the seismic reflection proftle of Cape et aI. (in press). Arrows indicate the relative stratigraphic position of exhumed mudstone samples of Hinakura and Mangaopari; HT = Hikawera Tuff.

were made while the sample was considered to be fully water saturated. Because velocity decreases with 4rYing, samples not collected from streambeds were fully immersed in water for at least two weeks before velocity measurements were made. Samples treated in this way were found to have velocities close to that of whole, unfractured blocks of fresh (water-saturated) mudstone.

Table 2 Interval velocities of partially Wlloaded, fine-grained sedimentary sequences (mostly Upper Miocene and Pliocene mudstones and siltstones) adjacentto the Wairarapa Faultin western Wairarapa. Data for A and B were calculated from the seismic reflection proftle through western W airarapa byC. Cape (Geophysics Division, DSIR)

Depth interval (m below surface) Interval velocity (kmIs)

A: Section 1100 m west of Wairarapa Fauh 0-53 1.75

53-346 2.67 346 - 583 3.16 583 - 1029 4.05

B: Section 800 m east of Wairarapa Fauh 172-334 2.31 334 - 539 2.73 539 -1044 2.81

C: Comparison of interval velocities of A and B to those in oil-well sequences (Fig. 3)

200 400 600

Sectioo A 2.2-3.0

2.88-3.24 3.4->3.8

SectimB 2.1-2.4 2.5-2.9 2.7-2.8

NZwells 1.48 1.67 1.85

The velocities of 41 samples from 3 locations in southeast Wairarapa (Hinakura, Huangarua River and Mangaopari Stream, Fig. 1) were measured, and the results are presented in Table 1.

The only velocity data available for buried rocks in Wairarapa (Table 2) are interval velocities calculated from the seismic reflection profile of Cape etal. (in press). Velocity data were selected from vertical sections immediately adjacent to the Wairarapa Fault (Fig.1A) where the Upper Miocene and Pliocene mudstones to siltstones are lithologically similar to those of the same age range in southeastern Wairarapa (Hinakura and Mangaopari). These velocities have been calculated using the assumptions that the stacking velocities are equal to r.m.s. (root mean square) velocities and that the strata are horizontal. Because stacking velocities are usually slightly higher than r.m.s. velocities and the strata at the western end of the seismic proftle are known to be tilted and folded, it is likely that the interval velocities deduced using these assumptions are slightly exaggerated (C. Cape pers. comm.1988).

Seismic velocity data from well logs are usually given as "interval transit time" (At) which is the time a compressional sound wave takes to move through 1 ft of formation. The unit of measurement is microseconds per foot {J.Ls/ft), and this parameter is inversely proportional to seismic velocity by the function, velocity -1 = interval transit time x 0.003281 (velocity in km/s and interval transit time (At) in J.1S/ft).

As there are no sonic logs available for fully loaded sequences in Wairarapa, electric-log data from Hawke Bay-l (BP Shell Aquitaine & Todd Petroleum Development Ltd


1976), Maui-l, Maui-3, Tane-l, and Kiwa-l (Shell BP & Todd Oil Services Ltd. 1969, 1970a, 1977, 1982) oil wells (Table 3) were selected to investigate the velocity-depth trend for fine-grained lithologies of the North Island of New Zealand. The location of these wells is shown in Fig. 1. All these wells (except for Hawke Bay-I) occur on the stable "western platform", and do not have unconformities or any other indications of vertical movement; they penetrated large thicknesses of relatively homogeneous, fine-grained lithologies. Sonic velocity logs from Maui-l, Kiwa-l, and Hawke Bay-l were read at c. 30 m intervals and corrected for height of the oil rig above the sea floor. Similar data from Tane-l and Maui-3 are available as sonic transit time versus depth plots (Tane-l well completion report, Shell BP & Todd Oil Services Ltd 1977).

In addition, the sonic logs ofMaui-4 (Shell BP & Todd Oil Services Ltd 1970b), Tasman-l (BPShell Aquitaine & Todd Petroleum Development Ltd 1976), and North Tasman-l (Creevey 1976) oil wells (Table 4) were used to establish the velocity-depth trend of sequences with a known history of

33

uplift. They were also used for comparison with the seismic velocity trend of sequences of partially uplifted, fine-grained sedimentary rocks adjacent to the Wairarapa Fault Zone in western Wairarapa (discussed further below).

Laboratory measurement of porosity Porosity values of mudstones can be obtained from density measurements (Hatherton & Leopard 1964). Disaggregation of weakly consolidated mudstone samples was avoided by collecting wet samples and carrying out all wet processing before drying them. The method was as follows:

(1) Wherever possible, fresh mudstone samples of 20-30 g were collected from streambeds. At least two independent samples were collected from each site (Table 5) and kept wet;

(2) Wet samples were cut into cuboids (c. 16 cm3);

(3) All samples were immersed in water and saturated under vacuum;

(4) Saturated samples were weighed while submerged

Table 3 Interval transit time and derived seismic velocity for fully loaded oil-well sequences. All depths are corrected to "below sea floor". Sonic transit time (6t) in J.1s/ft; velocity (U, derived from 6t ) in km/s.

Depth At U Depth At U Depth At U

Maui-1 Kiwa-1 Tane-1 277 190 1.60 325 160 1.91 315 160 1.91 338 185 1.65 765 150 2.03 625 150 2.03 429 165 1.85 815 130 2.35 815 140 2.18 444 175 1.74 1165 120 2.54 1070 130 2.35 460 170 1.79 1440 110 2.77 1315 120 2.54 490 163 1.87 1890 100 3.05 1965 100 3.05 521 160 1.91 1915 97 3.14 2265 90 3.39 551 173 1.76 2090 95 3.21 582 167 1.83 2365 85 3.59 612 162 1.88 2665 83 3.67 Maui-3 643 157 1.94 2765 80 3.81 296 160 1.91 673 152 2.01 456 150 2.03 704 147 2.07 656 140 2.18 734 146 2.09 Hawke Bay-1 856 130 2.35 765 145 2.10 333 150 2.03 1036 120 2.54 795 144 2.12 433 160 1.91 1286 110 2.77 826 143 2.13 483 150 2.03 1676 100 3.05 887 130 2.35 583 155 1.97 1906 90 3.39 917 130 2.35 633 145 2.90 948 128 2.38 683 142 2.15 978 126 2.42 733 140 2.18 1009 122 2.50 783 138 2.21 1039 120 2.54 833 140 2.18 1070 115 2.65 933 135 2.26 1100 112 2.72 983 130 2.35 1252 108 2.82 1033 125 244 1344 105 2.90 1433 125 2.44 1405 103 2.96 1533 125 2.44 1435 102 2.99 1583 122 2.50 1527 100 3.05 1633 122 2.50 1679 95 3.21 1683 115 2.65 1786 90 3.39 1733 108 2.82 1862 95 3.21 1783 108 2.82 1969 90 3.39 2015 85 3.59 2381 80 3.81 2411 78 3.91 2594 80 3.81 2686 79 3.56 2777 78 3.91 2822 75 4.06 2884 70 4.35


in water, then surface-dried, water-saturated samples were weighed in air;

(5) Samples were air dried for at least two weeks, then oven-dried overnight, before dry sample weight was measured.

Saturation of samples: Full saturation of mudstone samples was obtained under vacuum by replacement of air in pore spaces by water. Cuboids of fresh mudstone were immersed in water in small beakers, and several beakers at a time were placed in a large desiccation cylinder attached to a waterpump vacuum. Vacuum pressure was maintained for at least 48 h or until no further bubbles appeared on the surface of the mudstone samples when the glassware was gently tapped.

Submerged saturated sample weight: Samples which had been saturated under vacuum were placed on a hanger suspended beneath an electronic balance and immersed in a beaker of water. Care was taken to ensure that no bubbles adhered to the sample or to the hanger, and weighing was delayed until the watertwbulence produced during immersion had dissipated.

Water-saturated, surface-dry weight: Surface moisture on saturated samples was absorbed by paper towels, then the saturated, surface-dried samples were weighed on an electronic balance.

Dry sample weight: Saturated samples were left to air dry at room temperature for at least two weeks, then, because the mudstone was found to be hydroscopic over this period of time, samples were oven dried overnight, and their dry weight was measured within an hour of removal from the oven.

Volume: Calculation of volume from the linear dimensions of the cut cuboids was found to be only suitable as an

approximation, because of the difficulty in cutting smooth and perpendicular surfaces. An alternative method using Archimedes' principle was adopted, enabling the volume of irregularly shaped samples to be determined by buoyancy. The volume of the sample = (submerged, saturated weightsaturated, surface-dried weight in air) / water density.

Density and porosity calculations: Calculation of porosity can be derived from the ratio of dry and particle density (Hatherton & Leopard 1964) where: the porosity of the sample = I-(dry weight/particle density); dry density = dry sample weight / volume; wet density = saturated (surface dried) weight / volume; and particle density = dry sample weight/(dry weight-submerged, saturated weight).

The maximum probable error for each density value determined here is estimated to be 2% (after Barrett & Froggatt 1978).

RESULTS AND DISCUSSION

Velocity trends

Regression functions were derived for the exhumed mudstone sequence of eastern Wairarapa (Hinakura data); the partially unloaded oil-well sequences of Taranaki Basin (Maui-4, Tasman-I, and North Tasman-I); and the fully loaded oilwell sequences of Taranaki Basin and Hawke's Bay (Maui-1, Maui-3, Kiwa-I, Tane-I, and Hawke-Bay-I). It was not possible to derive regression functions from the seismic prospecting velocity data for sections adjacentto the Wairarapa FaultZonein western Wairarapa but the data can be compared with seismic velocity data from other areas in graphical form (see Fig. 4).

The velocity measurements of exhumed mudstone at Hinakura and Mangaopari in southeast Wairarapa (Fig. 1,

Table 4 Seismic velocity data of partially unloaded oil-well sequences. All depths are corrected to "below sea floor". Velocity (U, in krn/s) is derived from interval transit time (~t, in ~/ft) of oil-well sonic logs.

Depth l!J U Depth l!J U Depth l!J U

Maui-4 Maui-4 (eft!) North Tasman-1 285.9 135 2.26 929.0 95 3.21 257.5 125 2.44 319.4 135 2.26 959.5 97 3.14 303.5 125 2.44 349.9 135 2.26 1020.4 90 3.39 541.5 105 2.90 380.4 135 2.26 1142.4 90 3.39 573.5 100 3.00 410.8 130 2.35 1172.8 89 3.43 791.5 90 3.39 441.3 122 2.50 1203.3 90 3.39 1080.5 80 3.81 471.8 115 2.65 1233.8 89 3.43 1163.5 82 3.72 502.3 115 2.65 1264.3 89 3.43 1212.5 80 3.81 532.8 110 2.77 1294.8 87 3.50 1275.5 78 3.91 563.2 110 2.77 1325.2 85 3.59 Tasman-1 593.7 110 2.71 1355.7 80 3.81 223.2 130 2.35 624.2 115 2.65 1386.2 80 3.81 268.2 120 2.54 654.7 113 2.77 1416.7 80 3.81 313.2 115 2.65 684.1 13 2.58 1447.2 81 3.76 358.2 110 2.71 715.6 108 2.82 1477.6 81 3.76 418 105 2.90 746.1 107 2.85 1508.2 81 3.76 493.2 100 3.05 716.6 100 3.05 1538.6 80 3.81 673.2 95 3.21 807.1 102 2.99 1569.1 80 3.81 697.2 90 3.39 837.6 98 3.05 1599.6 80 3.81 853.2 85 3.59 868.0 98 3.11 1630.0 78 3.91 1093.2 80 3.81 898.5 98 3.11 1660.5 76 4.01 1273 80 3.81


Table 1) have been fitted to the linear regression functions greatest depth of burial = 2666 velocity - 3331 (Hinakum), and greatest depth of burial = 1639 velocity - 2170 (Mangaopari).

However, the data do not have a good fit, with the coefficients of determination, r (where r = correlation coefficient) only equal to 54.5% for the Hinakura data, and 64.8% for the Mangaopari data, with the result that the estimated depth of greatest burial of these samples can vary from that predicted by these functions by as much as ± 400 m. When the data from both sampling sites are combined, the fit is poorer still (r'- = 40.2%), suggesting that the velocity of exhumed mudstones has only limited use in determining burial history.

In contrast, velocity data from oil wells is much more useful in determining burial history. Plots of sonic velocity versus present depth of burial from the five New Zealand oil wells still under full litho static pressure are spread about a line of best fit which shows steady increase in velocity with increasing depth of burial (Fig. 3). The relationship between the two variables can be approximated either by the linear regression function, depth = 1026 velocity - 1419 ; r'-= 93.4%

Table 5 Density and porosity data for exhumed mudstones of southeast Wairarapa. Burial depth refers to estimated depth of greatest burial (discussed in text). Samples are the same as in Table 1.

Burial Wet Dry Particle depth density density densi~ Porosity

Sample (m) Wcm3) Wcm3

) Wcm) (%)

Hinakura Road HIB 1890 2.21 1.93 2.65 27 HI 1880 2.23 1.97 2.68 28 H2 1650 2.17 1.86 2.67 30 H3 1360 2.10 1.76 2.68 34 H4 1320 2.11 1.78 2.66 33 H5 1300 2.08 1.71 2.65 35 H6 1030 2.05 1.67 2.70 38 H8 980 2.09 1.74 2.68 36 H9 910 2.07 1.71 2.69 36 HI0 870 2.03 1.65 2.68 38 Hll 830 2.06 1.69 2.68 37 H12 680 2.06 1.70 2.66 36 H13 670 2.01 1.64 2.64 38 H14 370 2.00 1.61 2.67 40

Huangarua River Hgl 380 2.06 1.69 2.69 38 Hg2 368 1.61 1.11 2.22 50 Hg3 350 1.98 1.56 2.69 42 Hg4 200 2.14 1.83 2.67 31

Mangaopari Stream M13a 703 2.02 1.63 2.71 40 M13b 703 2.05 1.67 2.70 38 M14a 644 2.05 1.67 2.71 38 M14b 644 2.05 1.66 2.70 38 M15 578 2.05 1.64 2.76 41 M16a 528 2.03 1.63 2.72 40 M16b 528 2.04 1.64 2.73 40

M17a 408 2.02 1.62 2.72 40

M17b 408 2.03 1.62 2.72 40

35

(depth in metres, velocity inkm/s) , or the exponential function, depth = 6617 log velocity - 1403 ; r'-= 91.2%. There are no data available to predict whether or not the trend would become exponential at greater depth.

Comparison of the velocity-depth trends of exhumed mudstone to mudstone still under fulllithostatic pressure in New Zealand oil wells (Fig. 3) indicates that the exhumed mudstone has physical properties different to that still under lithostatic pressure. This suggests that the exhumed rocks have undergone geomechanical change during uplift and unloading. The exact nature of this change is uncertain. It does not appear to be due to the development of micro fractures because the mudrock samples are well consolidated and did not disintegrate while being saturated under vacuum. The change may be related to reorganisation of microfabric.

In a recent study of the mechanical behaviour of New Zealand sedimentary rocks, Huppert (1986) interpreted a quasi-elastic response of mudstones to slow loading and unloading and found (pers. comm. 1987) thatgeomechanical tests and microfabric studies both suggest that most of the soft rocks exhibited considerable stress relief that followed unloading. Huppert determined that, during their loading history, soft Tertiary sedimentary rocks of central North Island can adjust to changing loading conditions by geomechanical rearrangement of fabric elements (e.g., clay particles, clastic grains, various aggregates). These rocks develop an evolutionary series of fabric types, from an initial "open matrix" fabric to a "turbostatic" then "skeletal" microfabric. Huppert proposed that, during burial, there is a differential collapse of the fabric, resulting in a denser arrangement of the fabric elements and changes in interparticle bonding. Although the rocks exhibit a quasi-elastic response to loading and slow unloading (e.g., associated with regional uplift), some of the changes are irreversible and result in the production of a chaotic matrix microfabric (cf. the initial open fabric) upon unloading and uplift

Fine-grained sedimentary sequences which have been partially uplifted (partially unloaded) exhibit a different velocity-depth trend to either that of fully loaded sequences or those which have been fully unloaded. Figure 4 compares the velocity-depth trend of exhumed mudstones of southeastern Wairarapa to that of fully loaded oil-well sequences, and that of fine-grained sedimentary sequences with a known history of partial uplift (Le., Maui-4, Tasman-I, and North Tasman-l oil-well sequences, and sequences adjacent to the Wairarapa Fault in western Wairarapa).

The interval velocity versus depth plot for Upper Miocene fme-grained lithologies in the section west of the Wairarapa Fault (Table 2A) shows a steady increase in sonic velocity with present depth of burial. It also has a velocity-depth trend that is subparallel to the velocity-depth curve of fine-grained lithologies still under full lithostatic pressure (i.e., fully loaded) in New Zealand oil wells (Fig. 4) but has significantly higher velocities (Table 2C) at comparable depths. Data for the section east of the WairarapaFault (Table 2B) do not show the same simple trend with depth, but do show consistently higher velocities compared to the fully loaded oil-well sequences (Table 2C and Fig. 4).

I interpret the higher velocities of the mudstones in western Wairarapa as due to "overcompaction" of the Upper Miocene-Pliocene strata, and that the higher velocities of the sequence west of the W airarapaFault indicate that these have previously been buried to greater depths than the section east

36

' .. •

E .>t -:2 ,t :0 J:I

'0

z: D-e Q

3

1·5

New Zealand Journal of Geology and Geophysics, 1990, Vol. 33

".

". • ". ". 0 ". ".

".

* • Exhumed mudstone .Hinakura AMangaopari

2·0 2·5 3·0 Sonic velocity (kms-1)

• ........... •

1>: • 1>:

3·5

Oil wells

• Maui-l ". Hawke Bay-l

.>;< Kiwa-1 o Maui-3

• Tane-1

Fig.3 Velocity versus depth of burial plot for fme-grained sedimentary rocks in five North Island oil wells. A best fit line for the data has the equation, depth = 1026 velocity -1419. Velocities of exhwned mudstone from Hinakura and Mangaopari in southeastern Wairarapa are plotted against depth of greatest burial on the left-hand side of the figure_ The exhumed mudstone plots have a different trend to that of the fully loaded oil-well sequences, which suggests that the physical properties of the exhumed rocks have changed during their unloading and uplift.

i :!!. z: a. e " .. c • .. ~

Do

+

-~I

2·5 4-0 Sonic velocity (kms-I )

Fig. 4 Velocity versus depth trends of partially tmloaded mudstones of western Wairarapa. Interval velocities for sections west and east of the Wairarapa Fault are plotted against present depth, and shown as black and white bars, respectively. The velocity-depth trend of partially tmloadedmudstonesinoil wells is shown as squares (Maui-4), stars (fasman-l), and dots (North Tasman-I). The closeness of fit of the two western Wairarapa sequences to the oil-well plots suggests that the western Wairarapa sections have had 1100-1500 m of sediment eroded from them.


of the Wairarapa Fault (i.e., the Upper Miocene and Pliocene rocks in these western Wairarapa sections have been uplifted into their present stratigraphic position but have retained some of the compaction characteristics (increased density and velocity values) that they acquired during their previous burial). The geology of the area (Wells 1989b) indicates that the area west of the Wairarapa Fault has been uplifted about 750-900 m higher than the section to the east. Consequently, at a given horizon, rocks in the western section would be expected to have a higher sonic velocity than rocks in eastern section because they have been previously buried to greater depths and therefore have been compacted to a greater degree; this is reflected in the sonic velocity of these sections.

Estimation of the thickness of sediment removed from part of western Wairarapa The thickness of sediment unloaded from the UpperCenozoic sequence in westem Wairarapa can be estimatedbycomparing their velocity-depth trend to that of North Island oil-well sequences with a known history of uplift (Maui4, Tasman-I, and North Tasman-1 oil wells).

The unloading history of these wells has been interpreted from two independent sources. (1) Sonic transit time versus depth plots ofMaui-4 andNorth

Tasman-1 oil wells have been interpreted by Shell BP & Todd Oil Services (1970b) to indicate that, at the level of the Oligocene limestone equivalent, there has been erosion of at least 1200 m of sediment from theMaui-4 sequence, and of c. 1520 m of sediment from the Tasman-1 well sequence.

(2) Seismic interpretation of the (faulted) offset of Oligocene limestone in the vicinity of Maui-4, North Tasman-I, and Tasman-1 oil wells indicates uplift and erosion of c. 1100 m in the vicinity ofMaui-4 and North Tasman-1 oil wells, and erosion of c. 1500 m in the vicinity of Tasman-1 oil well (G. Thrasher, DSIR, pers. comm. 1988).

The velocity-depth plot of the wells can be fitted to the regression lines

depth =781 velocity - 1497 Maui-4 (n = 42, r'- = 96.9%)

depth = 663 velocity - 1448 Tasman-1 (n = 11, r'-= 93.43%)

depth = 666 velocity - 1384 North Tasman-1 (n =9, r'- = 97.5%).

The interval velocity-depth plot of these oil wells is shown in Fig. 4, as well as the velocity-depth plot of the western Wairarapa sections. The western Wairarapa plot is close to that of the unloaded oil-well sequences, suggesting they have been unloaded by a similar amount (i.e., c.1100-1500 m). If the interval velocities for western Wairarapa are lowered slightly to make allowance for the fact that they are probably slightly exaggerated (discussed in previous section), the fit of the western Wairarapa data to the Maui-4, Tasman-I, and North Tasman-1 data is improved The interval velocity data of the section east of the Wairarapa Fault generally plot lower than the velocity data for the section west of the fault zone. This is consistent with the interpreted geological history of the area, and suggests that c. 1 km of sediment has been removed from there as well.

The present geometric relationship of the sections on either side of the Wairarapa Fault (Wells 1989a, fig. 3) indicates that 200--600 m of sediment has been removed from the vicinity of the Wairarapa Fault since the beginning of late

37

Quaternary time. However, the velocity-depth trends of these sections indicate that a thickness of c. 1100-1500 m of sediment has been removed since the end of Late Miocene time, suggesting that c. 1 km of sediment has been eroded during the development of unconformities during the Pliocene to the end of the middle Quaternary time interval.

The amount of sediment that was removed during individual erosional episodes associated with these unconfonnities is uncertain. For example, a Middle Pliocene unconfonnity is widespread throughout western Wairarapa (Wells 1989a), and the associated period of nondeposition may have spanned "at least a few tens of thousands of years" and produced "18 m deep channels" in Lower Pliocene sediments in the Eketahuna area (Neef 1984, pp. 44 and 55). In western Wairarapa, large thicknesses of sediment were eroded from the Tertiary sequence during Early Pliocene time (e.g.,c.900mofUpperMiocenesedimentwasremovedfrom the crest of an anticline in the subsurface c. 2 km east of the Wairarapa Fault (Wells 1989a». This example does not actually constrain how much sediment was removed from individual sections closer to the Wairarapa Fault, but it does indicate the magnitude of the erosional event associated with the development of the Early Pliocene unconfonnity in this area.

Density and porosity of exhumed mudstones of southeast Wairarapa

Porosity values of exhumed mudstone of southeast Wairarapa(Table5) were obtained from density measurements using the ratio of dry and particle density (Hatherton & Leopard 1964). The data showed that the particle density of upperCenozoicmudstoneatHinakuraremainsapproximately constant throughout the 1600 m section, with a mean particle density of 2.67 g/cm', and that wet and dry density show a progressive increase with increasing depth of (prior) burial (allowing for uncertainties in establishing the true depth of previous maximum burial discussed earlier), which is reflected in the progressive decreases in porosity with increasing depth of greatest burial shown in Fig. 5.

The relationship between porosity and depth of burial of exhumed mudstones sampled from Hinakuraand Mangaopari Stream (Fig. 5) shows a progressive change that can be related to the previous depth of greatest burial. The porosity-depth relationship is approximated by theregression function, depth = - 111 porosity + 4975 (r'- =91.1 % ) (depth in metres, porosity %) or by the exponential regression function, depth = - 8636 log porosity + 14384 (r'-= 90.3%), and these functions are accurate in predicting depth of greatest burial to within 100 m. For example, a sample of mudstone from Huangarua River (Hg3, Table 5) has a porosity value of 42%, which, using the linearporosity-depth function above, indicates it has previously been buried to depths of 313 (± 100 m) which is in fair agreement with the predicted overburden of 400 m from published stratigraphy (Collen & Vella 1984). The porositydepth relationship applies only to mudstone lithologies; plots of sandstones and lignite sampled from Huangarua River do not lie close to the regression line (Fig. 5).

The relationship between porosity and depth has been investigated elsewhere (e.g., Athey 1930; Ruby & Hubbert 1960; Magara 1976; Sclater & Christie 1980). Using the data of Athey (1930) and others, Ruby & Hubbert (1960) showed that for nonnal pressures the porosity (P) of mudstones can be represented by the relationship P = P" .-cz where P" = surface


0·5

-E = .. ~

::I .Q

1·0 .. .. ! to

~ III

"0

~

a. 1'5

• Q

2·0

20 Porosity ("/0)

Fig. 5 Porosity versus depth of greatest burial plot for exhwned mudstoneofWairarapa. Circles, Hinakuradata; squares, Mangaopari data ; stars, Huangarua River data. The data fit the linear regression fimction, depth of greatest burial = - 111 porosity + 4975. Only mudstones plot close to the line (e.g., the Huangaruadata away from the line are sandstone and lignite samples).

porosity, c = compaction factor, z = depth of~urial. Sclater & Christie (1980) investigated the porosity-depth ttendofNorth Sea mudstone using the sonic 10ginformationofSchlumberger (1974) and the porosity-sonic log conversion of Magara (1976), and confirmed that normal mudstones show an exponential decrease of porosity with depth of burial. The porosity-depth trends of the exhumed mudstone ofWairarapa are consistent with this.

The porosity of exhumed Hinakura mudstones is related to interval transit time by the linear regression function, porosity = 0.149 at + 6.85. This function does not have a very good fit (r = 43.2%), which greatly limits its usefulness in predicting porosity values (and also, therefore, in indirectly predicting depth of burial through the use of the porositydepth functions discussed earlier).

The porosity-interval transit time relationship for these exhumed mudstones is similar to that derived by Magara (1973, 1976) from formation density logs of fully loaded mudstone sequences in North American oil wells. He derived the functions, porosity = 0.00472 at - 0.362 (Magara 1973), andporosity=0.466at-31.7(MagaraI976),assuminggrain density for mudstones of 2.65 and 2.72 g/cm3

, respectively. The difference in the North American porosity-transit time relationship compared to that of southeast Wairarapais likely to be due to a number of factors, including: (1) difference in grain density (the mean grain density of the Hinakura mudstones of southeastern Wairarapa is 2.67 g/cm3); (2) the different methods used to derive the variables (formation density logs versus density mea" '~ments and ultrasonic

equipment) for the fully loaded and unloaded mudstone sequences, respectively; and (3) to irreversible changes in the microfabricofthe Wairarapamudstonesduringtheirunloading (discussed earlier).

CONCLUSIONS

1. The porosity-depth relationship of exhumed mudstone at Hinakura and Mangaopari in southeastern Wairarapa can be used as a guide to burial history. The variables are related by the linear regression function, greatest depth of burial = -111porosity +4975 (r= 91.1 %), and nearly as well by the exponential function, greatest depth of burial = - 8636 log porosity + 14384 (r = 90.3%), and can predict depths to within ± 100 m ..

2. The seismic velocity of exhumed mudstone of southeast Wairarapa is much less useful in determining burial history. It bears a relationship to the estimated previous depth of maximum burial which is approximated by the function, depth of greatest burial = 2666 velocity - 3330, but has a poor fit (r = 54.5%), and predicted depths could be out by as much as± 400 m.

3. Mudstones and other fine-grained sedimentary rocks still under lithostatic pressure in North Island oil wells exhibit a distinct velocity-depth relationship that is best described by the linear function, depth = 1026 velocity - 1419 (r = 93.38%; depth in metres, sonic velocity in km/s), and nearly as well described by the exponential function, depth = 6617 log velocity - 1403 (r = 93.04%).

4. The velocity-depth relationship in partially unloaded sequences of mudstone in western Wairarapa indicates that those rocks have retained an imprint of their burial history. Comparison of their velocity-depth trends to that of several New Zealand oil wells with a known history of vertical uplift indicates that 1100-1500 m of sediment has been unloaded from the stratigraphic sequences. Only about 200-600 m of eroded sediment can be accounted for by late Quaternary erosion and about 1 km has been removed by erosion during the Pliocene to the end of the middle Quaternary time interval.

ACKNOWLEDGMENTS

This manuscript has benefitted from the useful comments of Dick Walcott, Paul Vella, and Peter Barrett (Victoria University of Wellington), and the constructive comments of Stephen Hicks and another reviewer. Field expenses were partly met by a University Grants Committee Postgraduate Scholarship and by a grant from the Internal Research Committee of the Victoria University of Wellington.

REFERENCES

Athy, L. F.1930: Density, porosity and compaction of sedimentary rocks. American Association of Petroleum Geologists bulletin 14: 1-24.

Barrett, P. J.; Froggatt, P. C.1978: Densities, porosities and seismic velocities of some rocks from Victoria Land, Antarctica. New Zealand journal of geology and geophysics 21: 175-187.


BP Shell Aquitaine& Todd Petroleum Development Ltd 1976: Well completionreport-HawkeBay-l.NewZealandGeological Survey unpublished open-file petroleum report 667.

Cape, C.; Lamb,S. H.; Vella,P.; Wells,P.E.;Woodward,D. in press: Geological structure of Wairarapa Valley, New Zealand, from seismic reflection profiles. Journal of the Royal Society of New Zealand.

Collen, J. D.; Vella, P. 1984: Hautotara, Te Muna and Ahiaruhe Formations, Middle to Late Pleistocene, Wairarapa, New Zealand. Journal of the Royal Society of New Zealand 14: 297-317.

Creevey, K. 1976: North Tasman-l well implantation report. Aquitaine (Australia and New Zealand Ltd) New Zealand Geological Survey unpublished open-file petroleum report 686.

Garrick, R. A. 1969: Some physical properties ofrocks in the East Cape-MahiaPeninsularegion, North Island, New Zealand. New Zealand journal of geology and geophysics 12: 738-760.

Hatherton, T.; Leopard, A. E. 1964: The densities of New Zealand rocks. New Zealand journal of geology and geophysics 7: 605-625.

Huppert, F. 1986: Petrology of soft sedimentary rocks and its relationship to geomechanical behaviour, central North Island, New Zealand. Unpublished Ph.D. thesis, lodged in the Library, University of Auckland, New Zealand.

Magara, K. 1973: Compaction and fluid migration in Cretaceous shales of western Canada. Canada Geological Survey paper 72-18: 1-3

---1976: Thickness of removed sedimentary rocks, paleo pore pressure, and paleotemperature, southwestern part of Western CanadaBasin.AmericanAssociationofPetroleum Geologists bulletin 60: 554-565.

Neef, G. 1984: Late Cenozoic and early Quatemary stratigraphy of the Eketahuna district (N153). New Zealand Geological Survey bulletin 96.

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New Zealand Aquitaine Petroleum Limited 1970: Well completion report. Tasman-I. New Zealand Geological Survey unpublished open-file petroleum report 512.

Ruby, W. W.; Hubbert, M. K. 1960: Role of fluid pressure in the mechanics of overthrust faulting. Geological Society of America bulletin 70: 167-206.

Schlumberger. 1974: Chapter 4, pp. 66--72 in: Cambell et al. ed. Well evaluation conference, North Sea. London, Schlumberger.

Sclater, 1. G.; Christie, P. A. 1980: Continental stretching: an explanation of the post-mid-Cretaceous subsidence of the central North Sea Basin. Journal of geophysical research 85: 3711-3739.

Shell B.P. & Todd Oil Services Ltd 1969: Maui-I. New Zealand Geological Survey unpublished open-file petroleum report. 540.

---1970a: Maui-3. New Zealand Geological Survey unpublished open-file petroleum report 542.

---1970b: Maui-4. New Zealand Geological Survey unpublished open-file petroleum report 543.

---.1977: Well resume. Tane-l (offshore) PPL 38007, sub area of Taranaki. New Zealand Geological Survey unpublished open-file petroleum report 698.

---1982: Well resume. Kiwa-l. New Zealand Geological Survey unpublished open-file petroleum report 880.

Vella, P.; Briggs, W. M. 1971: Lithostratigraphic names, Upper Miocene to Lower Pleistocene, Northern Aorangi Range, Wairarapa. New Zealandjournal of geology and geophysics 14: 253-274.

Vella, P.; Collen, J. D. 1984: Four rhyolitic tuff marker beds, Lower Pliocene, Wairarapa, New Zealand. Journal of the Royal Society of New Zealand 14: 133-138.

Wells, P. E. 1989a: Late Neogene vertical tectonic movements in western Wairarapa, New Zealand. Unpublished Ph.D. thesis, lodged in the Library, Victoria University, Wellington, New Zealand.

---1989b: Late Neogene stratigraphy of the Carrington area, western Wairarapa, North Island, New Zealand. Journal of the Royal Society of New Zealand 19: 283-303.

porosities and seismic velocities of mudstones from wairarapa and oil wells of north island, new...

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