Membrane seal and tertiary migration pathways in the Bodalla South oilfield, Eromanga Basin, Australia

Papers Membrane seal and tertiary migration pathways in the Bodalla South oilfield, Eromanga Basin, Australia* P. J. Boult Department of Applied Geology, Gartrell School of Mining, Metallurgy and Applied Geology, University of South Australia, The Levels Campus, South Australia 5095, Australia

Received 17July 1991; revised 3 December 1991; accepted 15 December 1991 The Middle Jurassic Hutton Sandstone oil accumulation at the Bodalla South oilfield in the Eromanga Basin, Australia is primarily a structural trap with stratigraphic control. In common with many others in this basin it does not appear to be full to the structural spill point. Analysis of mercury injection curves has revealed that the caprock of the Hutton Sandstone (i.e. the Birkhead Formation, which is predominantly a lithic sandstone) is a membrane type seal. The calculated maximum size of oil column that will accumulate beneath this caprock is 12.64 m and the maximum oil column encountered to date is 11.25 m. There is no waste zone above the Hutton Sandstone reservoir at Bodalla South. Keywords: caprocks; Bodalla South oilfield, Australia; structural traps; membrane seals

Introduction

Overview of seals and secondary migration

This paper presents a model for the petroleum trapping mechanism within the Middle Jurassic Hutton Sandstone at the Bodaila South oilfield in the Eromanga Basin of south-western Queensland, Australia (Figure 1). Leakage or tertiary migration pathways above the reservoir are discussed. The Jurassic to Cretaceous Eromanga Basin and underlying Permian Cooper Basin are both intracratonic. The Eromanga Basin covers an area of approximately 1 x 106 km z. The Permian to Lower Cretaceous is characterized by repeated sequences of fluvial to lacustrine siliciclastic sediments, as shown in Figure 2. The Upper Cretaceous has a substantial marine influence. Petroleum has been produced from most of the potential reservoir horizons from the Early Permian Patchawarra Formation to the Early Cretaceous Cadna-Owie Formation. However, the Hutton Sandstone is the principal producing formation in the Bodalla South area. Most of the oil accumulations of the Eromanga Basin occur in the vicinity of the underlying Cooper Basin Permian and Triassic zero edges (Salomon et al., 1990). Fields within the Eromanga Basin are characterized by multiple, vertically stacked pools (Heath et al., 1989). Typical oil columns are less than 20 m, irrespective of structural closure.

Seals

*Presented at the meeting 'Caprocks', London, UK, 14 June ]990

Caprock seals have been divided genetically into two types by N. L. Watts (1987). (a) Hydraulic: those in which capillary entry pressures are so high (typically several thousand psia) that seal failure occurs preferentially by hydraulically or tectonically propagated fractures totally penetrating the sealing layer. The greater the seal thickness the less likely that fractures will breach it and thus the seal integrity is primarily a function of seal thickness for a given rock strength. (b) Membrane: those that have lower capillary entry pressures (typically less than one thousand psia) and fail by capillary leakage thus acting as 'automatic valves that release excess oil through them when the maximum oil column is exceeded' (Jennings, 1987). Capillary breakthrough pressures are much lower than for hydraulic type seals and seal integrity is thus less related to thickness.

The weak link The key to understanding the parameters that control seal capacity is to determine which rock unit is forming the 'weak link' and thus which generic seal type is forming the effective seal. If the 'weak link' at Bodalla South is of the membrane seal type, then use of the algorithm of Berg (1975) for predicting a maximum petroleum column is applicable and this should model the Hutton Sandstone oil pool beneath the Birkhead Formation.

0264-8172/93/010003-11 © 1993 Butterworth-Heineman n Ltd

Marine and Petroleum Geology, 1993, Vol 10, February

3

Membrane seal and tertiary migration, Bodalla South: P. J. Boult WINDORAH • .."

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Figure 1 Location m a p of the Bodalla South oilfield in the Eromanga Basin showing the juxtaposition of other hydrocarbon fields and the Permian zero edge [after Salomon et a l (1990)]

Secondary migration It is considered by the author that petroleum migrates within the Eromanga Basin predominantly as a separate liquid phase. The main controls on the vertical migration of oil in the Eromanga Basin are thus capillary pressures and the buoyancy of the petroleum phase as hydrodynamic effects are considered to be negligible (Bowering, 1982; Muller, 1989).

Interpretation of mercury injection curves Using the Laplace equation and field units relating capillary pressure to pore throat diameter, adapted for tabular pore throats (Wardlaw, 1976), it is possible to relate mercury injection pressures and pore throat sizes within test rocks by the equation 107 Cnem/a

-

(1)

d where d = diameter of pore throat in micrometres and CBPm/a = capillary breakthrough pressure for the mercury air system in psia. The size and shape of pores within tested rocks can be further confirmed by use of the scanning electron microscope. However, this is a qualitative confirmation only. The mercury injection technique not only has the advantage of being quantitative but because of its non-wetting qualities is also a close approximation to the secondary migration of petroleum in water-wet rocks. Thus the two can be related and the following useful equation developed by Berg (1975) can be used to determine the column of oil trapped beneath a membrane type seal.

4

T. =

I FTp/w (CB Pm/a(seal)

--

CB Pm/a(reservoir))

(2)

526 (Pw - Pp) where Th = height of oil column above the oil-water level (metres); CBPm/a(seal) = capillary breakthrough pressure of the seal (psia); CBPm/a(reservoir) capillary breakthrough pressure of the reservoir (psia); IFTp/w = interracial tension of petroleum/water (dynes/cm); pp = density of petroleum (g/cm 3); and Pw = density of water (g/cm3). CBP~a(~aO and CBPm/a(reservoir) a r e determined from mercury injection curves by either: (a) determining the pressure at which mercury freely enters the rock - this is normally interpreted as a plateau in the curve which can be extrapolated back to the pressure axis to obtain a pore entry pressure; or (b) in the case of a curve that has no definite plateau, it can be assumed that a certain critical saturation is needed for breakthrough to occur. Breakthrough is defined as the existence of a continuous filament of the non-wetting fluid that allows its passage through the rock. The value of critical saturation in terms of a percentage is then used to read the breakthrough pressure from the mercury injection curve. Prediction of petroleum-water interracial tensions (IFTp/w) at reservoir conditions is the subject of considerable controversy, of which Jennings (1987) gives a good account. Schowalter (1979) summarized the situation by stating that the effect of confining pressure on crude oil-formation water interracial tension appears to be small enough that it can be considered negligible. However, he considered temperature to be a significant parameter. Subsurface fluid densities are readily calculated from

Marine and Petroleum Geology, 1993, Vol 10, February

=

M e m b r a n e seal a n d tertiary migration, Bodalla South: P. J. Boult surface conditions using the formation factor, which Basin, being trapped in multiple stacked pools beneath takes account of the reservoir pressure and leaky seals. temperature. The Hutton Sandstone produces the bulk of the oil within the Eromanga Basin and the Birkhead Formation is the main seal. Production mainly comes Regional setting for the study from four-way dip closured anticlines with variable A model for the migration of oil within the Cooper and stratigraphic influence on reservoir geometry caused by Eromanga Basins has been put forward by Heath et al. the transitional nature of the Hutton Sandstone to (1989) and is depicted in Figure 3. They postulate that Birkhead Formation contact. The Hutton Sandstone is most of the oil was generated within the Cooper Basin. a craton-derived braided fluvial deposit with waning This migrated updip to the edge of the Cooper Basin energy point bar deposits towards the top. It is and from there on vertically through the Eromanga regionally consistent in terms of its distribution and

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Marine and Petroleum Geology, 1993, Vol 10, February

5

Membrane seal and tertiary migration, Bodalla South: P. J. Boult quality. The Birkhead Formation conformably overlays the Hutton Sandstone but with some erosion of the oil pool

formation C a d n a - o w i e Fm Namur Sat W e s t b o u r n e Fm

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Hutton by Birkhead channels (Hill, 1985). The transition to the Birkhead Formation represents a variable waning energy depositional environment accompanied by a provenance switch from craton derived sediment to volcanic arc derived sediment (Watts, K. J., 1987). In some areas the transition from Hutton to Birkhead is predominantly a provenance change with similar sized Birkhead channels replacing Hutton channels. In other areas the Hutton Sandstone is conformably overlain by low energy overbank siltstone and shales. In the Bodalla South area the change in depositional environment is minimal but the provenance change is distinct. Regionally the Birkhead Formation is of variable thickness from over 100 m in several depocentres within the basin to absent at its southern and western edge. The volume of shale within the Birkhead Formation, calculated from core and log analysis, varies from 2 to 57% with an average of 29% (Smyth et al., 1984).

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Figure4 North-east to south-west seismic line HQ83-99 through wells 1 and 4 [after LASMO Energy (1985), Bodalla South Field Evaluation Study, unpublished]. The J horizon is the top of the regional seal, the Birkhead Formation. The structure consists of a late four way dip closure over a reverse faulted basement block

6

Marine and Petroleum Geology, 1993, Vol 10, February

Membrane seal and tertiary migration, Bodalla South: P. J. Boult Description or the Bodalla South field

-_2

The Bodalla South field was discovered in 1984 (Dolan et al., 1988) and up to the time of writing seven wells have been drilled. The oilfield comprises two main reservoirs, the Hutton Sandstone and the Basal Jurassic Sandstone (Figure 2). Original oil in-place for the Hutton Sandstone reservoir was proved at 3 mbbls of a very light crude (48 ° API) with a very low gas-oil ratio (5 SCF/STB); average porosity is 18.3% and permeabilities in excess of 500 mD are common. The seal to the Hutton Sandstone is the Birkhead Formation, the top of which is a good seismic reflector, which is shown in Figure 4 as the 'J' horizon. The depth map created from seismic interpretation that is tied into drilled wells is shown in Figure 5. Structural closure at Bodalla South is estimated to be 3 0 - 4 0 m. The structural well log correlation of Figure 6 shows the Hutton S a n d s t o n e - B i r k h e a d Formation transition. This correlation is based on detailed core, DST, dipmeter and wireline log data. The producible oil at the top of the Hutton Sandstone at Bodalla South is pooled in at least three separate sand bodies as follows: P1, occurring in wells 1 and 4, containing 1.287 x 106 bbls (all the oil columns could be greater than quoted by a few metres as each well did not necessarily drill through the top of the structure) - - pool 1 (P1) has an oil column of at least 8 - 1 0 m; P2, occurring in well 2 only, containing 1.270 x 106 bbls - - pool 2 (P2) has an oil column of at least 11.25 m; and P3, occurring in well 3 only, containing 0.510 x 106 bbls - - pool 3

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Figure 5 Depth map to t o p Birkhead Formation [after LASMO Energy (1985), Bodalla South Field Evaluation Study, unpublished] based on well depths, seismic and Io,'al velocity variations

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Figure 6 Structural well log correlation of the upper Hutton Sandstone and lower Birkhead Formation at Bodalla South showing core intervals, capillary sample sites, major oil pools, permeability streak and cuttings shows and the distribution of sedimentary facies based on core data, test data, d i p m e t e r and wireline log motifs. Superimposed on this is the provenance/diagenetic boundary representing the change from 'Hutton' to 'Birkhead' times

Marine and Petroleum Geology, 1993, Vol 10, February

7

Membrane seal and tertiary migration, Bodalla South: P. J. Boult (P3) has an oil column of between 7.5 and 12.8 m. boundary is indicated in Figure 6 by a continuous and Pools 1 and 2 are in pressure communication through broken line with a quartz content less than 60% on the the water leg and pool 3 is a bounded or closed system. broken side. It should be noted that this boundary is An understanding of the nature of capillary pressures not at the same stratigraphic horizon throughout the within the seals to these pools helps in determining the field taking into account the 'early Birkhead' channel local or tertiary migration pathways of the oil and its (seen in wells 1 and 2) with its concomitant overbank probable downdip extent for pools 1 and 3. deposits (seen in wells 4, 2, 3 and 6) and the late 'late Oil-water contacts are illustrated in Figure 6 with Hutton' channel seen in all wells of Figure 6 except the maximum oil column encountered being in well 2 at well 2. 11.25 m. The Birkhead Formation at Bodalla South is The boundary between the Hutton Sandstone and between 70 and 80 m thick, comprising predominantly Birkhead Formation in the Bodalla South area has fluvial channel and point bar deposits with minor levee traditionally been picked at the first upward bank, crevasse splay, floodplain and coal swamp appearance of lithic or mixed type sandstone (Watts, deposits. No true lacustrine or marine shales occur K. J., 1987). However, their boundary at Bodalla within the Birkhead Formation at Bodalla South. South is transitional, caused by a repeated provenance switch from the 'Hutton' craton derived sediment to the Methodology 'Birkhead' volcanic arc derived sediment. Petrological studies can easily differentiate between Hutton and Graphic core logs for wells 2 and 3 are presented in Birkhead lithologies. As a rule of thumb [Salomon Figures 7 and 8, illustrating oil shows, sedimentology, (1989) Bodalla Block Stratigraphic Study, Hutton gamma ray log, permeability and porosity. Potential Sandstone, Geological Aspects, LASMO Energy, seals are indicated. Mercury injection samples were Brisbane, unpublished data], the Birkhead comprises selected from potential seals on the basis of whole rock 40% or less of quartz and the Hutton comprises 60% or availability. Most were selected from the immediate more of quartz with a transition zone between these seal seen in the conventional 4" cores but three sidewall values. To simplify this division for the purposes of this cores from higher in the Birkhead Formation were study the Birkhead is taken as including the transition also selected. Mercury-air capillary breakthrough zone. The Birkhead is a very labile sandstone that has pressures (CBPm/a) were measured using a semiundergone extensive diagenesis. This provenance automatic Micromeritics Autopore 9200. The

BODALLA SOUTH 2 SEDIMENTOLOGY 'Planar Cross-Bedding

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8 Marine and Petroleum Geology, 1993, Vol 10, February

Membrane seal and tertiary migration, Bodalla South: P. J. Boult BODALLA

SOUTH

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SEDIMENTOLOGY

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Figure 8 Bodalla South 3 core log showing the relationship between sedimentology, core analysis data, oil shows and mercury injection sample sites [modified from LASMO Energy (1985) Bodalla South Field Evaluation Study (unpublished) and LASMO Energy (1984-5) Bodalla South Well Completion reports 1 - 7 (unpublished)]

Autopore operates to a pressure of 60 000 psia and measures mercury intrusions as small as 0.0001 cm3/g sample. Samples were cut into small, less than 1 cm, cubes which typically weighed 5 - 1 0 g. Forty-five pressure points were used allowing 12 seconds for equilibration to take place once the pressure had stabilized. The data were recorded electronically and plotted as cumulative mercury injection against pressure as shown in Figures 9 and 10. Comparison of the mercury injection data with the 100- I BODALLA SOUTH 2

q Birkhec~d / q Hutton

o

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mineralogy and morphology of pores was investigated using a scanning electron microscope. Selected photographs are shown in Figures 11 and 12. The results were compared with an incremental plot of mercury injection against pore throat size [using the equation of Wardlaw (1976) for tabular pore throats] which is shown in Figure 13. Correlation of mineralogy was possible with the use of X-ray diffraction plots and optical slides.

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Figure 10 Bodalla South t, 3, 4 and 5 combined cumulative capillary pressure curves, showing the distribution of curves. See text for explanation and Petroleum

Geology,

1 9 9 3 , V o l 10, F e b r u a r y

9

M e m b r a n e seal a n d t e r t i a r y m i g r a t i o n , Bodalla S o u t h : P. J. B o u l t

BODALLA SOUTH 2 TYPICAL POROSITY DIS TRILl U ~17ON CUR VE5'

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Figure 11 Scanning electron micrograph of Bodalla South 2 1456.45 m (DD). This photo shows some of the mineralogy of the clays in a typical membrane seal type rock. Stacked plates of pseudohexagonal kaolinite can be seen in the centre. They are probably neoformative and represent the occlusion of the original primary porosity. Probable stratified smectite-illite grain coating or pore lining can be seen on the right where the impression of a plucked grain has been left. The other side of this pore is seen on the left bounded by a feldspar grain that has a smooth surface representing the area of contact with an adjacent grain that has been removed during sample preparation

Results A total of 19 samples from the potential caprock of the main oil pools, where there were whole rock samples, were injected with mercury. Pressure curves were generated and are presented in Figures 9 and 10. In each figure it is evident that the curves fall into two distinct types. These appertain to the two generic types of seal referred to by N. L. Watts (1987) as membrane

Figure 12 Scanning electron micrograpn of Bodalla South 2 1456.45 m (DD). This is an enlargement of the centre part of

Figure 11 showing the pore throat sizes. Kaolinite books can be seen adjacent to probable authigenic quartz or feldspar. The size of the probable interconnecting pore throats left between these minerals and the space between the 'pages' of the books is typically less than 1 /~m. This is the predominant pore throat size that is preventing mercury injecting into the sample until a pressure of approximately 150 to 200 psia has been attained. It is ultimately this, the typical pore throat size, that is creating the 'resisting' capillary pressure within the membrane seal and preventing the upward migration of petroleum 10

Marine and Petroleum

Geology,

01 RADIUS

micro

1 rweLres

10

Figure 13 Two Bodalla South 2 incremental capillary pressure curves converted to radii of pore throats, showing the pore throat size distribution in both hydraulic and membrane type seals. Of note is the correlation of pore throats with diameters between 0.5 and 2/~m in the membrane type seal with those seen in the scanning electron micrographs of Figures 11 and 12 and hydraulic seals. Eleven membrane, six hydraulic and two borderline type seals were defined. Positioning the occurrence of these on the correlation diagram of Figure 6 (labelled m for membrane, h for hydraulic and m/h for borderline) and core logs gave an indication of which type of seal was likely to be effective. Although several hydraulic type seals were encountered within potential seal-forming horizons they were considered either to be not laterally continuous or thick enough to be effective. For example, in well 2, shown in Figure 7, two shaly horizons within the overbank deposits at 1455.0 m [drilled depth (DD)] and 1458.12m (DD), give hydraulic type mercury injection curves with a very high CBPm/a over 3000 psia (Figure 9). However, although in the vicinity of well 2 the bottom shaly horizon at 1458.12 m (DD) might even be acting as a local seal, thus preventing migration up into a very coarse lag sand with 15% porosity at 1457.75 m (DD), this shale is very thin. It is thus probably not laterally extensive enough to be the overall effective seal. Another type of hydraulic seal with a very high CBPm/a was found within the Birkhead point bar sequence (see Figure 6) in the samples of well 3 at 1456.8 and 1459.05 m (DD) and well 5 at 1454.30 m (DD). Optical slides revealed that these are lithic to arkosic sandstones that have been subjected to intense local diagenesis in the form of feldspar and rock fragment alteration and the deposition of authigenic clays. In fact, their diagenesis was so intense that their frameworks have collapsed, thus forming a compacted sandy authigenic clay similar to that described by Harris (1989) from the Brent Sandstone in the North Sea. In the Birkhead Formation at Bodalla South they occur where the borehole gamma ray log and permeability log have a serrated motif. These motifs are not laterally extensive across to well 2. This fact, coupled with the occurrence of membrane type CBPm/a measured in the same formation at well 2, means the 'collapsed' sandstones are not considered laterally extensive enough to be the effective seal. The effective seal or 'weak link' appears to be related to the 'uncollapsed' lithic sandstone that is typical of the Birkhead Formation in this area and is the most extensive lithofacies type. This lithofacies had enough

1993, V o l 10, F e b r u a r y

Membrane seal and tertiary migration, Bodalla South: P. J. Boult at Bodalla South is between 12.64 and 10.86 m. This is in close agreement with the actual oil columns encountered, although it is noted that the wells are not necessarily drilled through the top of the structure.

resistant framework grains so that when diagenesis took place compaction was minimal. Primary and secondary pores, caused by the dissolution of labile grains, were maintained intact long enough for authigenic clays to form. Even some of the sidewall cores from much higher in the Birkhead Formation (the capillary pressure curves of which are displayed in Figures 9 and 10 although some of their sample sites are located off the top of Figure 6) are in fact siltstones and have a membrane seal type of mercury injection curve with a CBPm/a of less than 200 psia. (Fractures due to the sampling techniques used in taking sidewall cores have consistently been found to be an order of magnitude greater than those that are significant in membrane type seals.) This is despite the fact that these sidewall cores were taken on gamma ray peaks and thus represent some of the most shaly beds of the Birkhead! A close look at the membrane seal type rocks under the scanning electron microscope (Figures 11 and 12) reveals that the plateaux in the mercury injection curves of the membrane seal type rocks are due to abundant intergranular authigenic kaolinites and smectites (Wilson and Pittman, 1977), which have pore throats between them from 0.5 to 1.0 ~ m as revealed by the incremental mercury injection plot (Figure 13) related to pore size using Equation (1).

Use of capillary pressure curves to determine oil column height From cumulative capillary pressure diagrams for wells 2, 3, 4 and 5 (Figures 9 and 10) it is evident that the membrane seal type rocks have an average capillary breakthrough pressure (CBPm/a(seal)) of between 200 and 150 psia. The average capillary breakthrough pressure for the reservoir (CBPm/a(reservoir)) is 21 psia (calculated from an average of porous plate measurements [ L A S M O Energy (1985) Bodalla South Field Evaluation Study, unpublished]. Subsurface fluid densities are calculated to be Pw = 0.9661 g/cm 3 and Ph = 0.7469 g/cm 3. Interfacial tension between crude oil and formation water from the nearby Kenmore field, which can be located on Figure 1, was measured at several temperatures at atmospheric pressure. Experimental results show that interfacial tension decreases significantly with an increase in temperature and time. The decrease is faster and larger with increasing temperature. This appears to be associated with an unidentified colloid in the formation water that, after about one day, rises to the interface and acts as a natural surfactant that is more efficient at higher temperatures. The initial interfacial tension of a fresh o i l - f o r m a t i o n water mix is between 19 and 21 dynes/cm for temperatures between 20 and 37°C, respectively. After the formation water has cleared, the interracial tension decreases to between 11.3 and 7 dynes/cm. For petroleum production purposes it is the fresh or dynamic interfacial tension that is important in determining relative permeabilities. However, for the trapping situation it is considered that the static or steady state interfacial tensions are more applicable. Substituting into Equation (2) using the parameters IFTp/w = 7 - 1 1 . 3 dynes/cm, CBPm/a(seal ) 150-200 psia, CBPm/a(. . . . . . Jr) = 21 psia, Pp = 0.7469 g/cm 3 and Pw = 0.9661 g/cm , the maximum oil column Th that can be held by the base of the Birkhead Formation =

Discussion The overall closure on the Bodalla South Hutton Sandstone oil pool is estimated from seismic data to be between 30 and 40 m. The maximum oil column encountered to date is 11.25 m in well 2. The structure thus appears not to be filled to spill point. There are five possibilities that could account for this: ( 1 ) i n a d e q u a t e oil charge; ( 2 ) e a r l y migration followed by continued structural development; (3) the structure size has been incorrectly mapped; ( 4 ) t h e structure is full to structural spill point and a large waste zone exists above the main oil pools; or (5) the structure is not full to spill point because the Birkhead Formation is not capable of retaining more than an 11.25 m column of oil. The first possibility, of inadequate oil charge, is questionable as there are significant oil shows in the above horizons. The empirical relationship of most Eromanga Basin oil accumulations lying near to the Cooper Basin zero edge and the similarity of Cooper Basin source rocks and oil to those in the Eromanga Basin has led one group of workers (Heath et al., 1989) to postulate a Cooper Basin origin for the oil. The presence of stratigraphically shallow oil in the Bodalla South area suggests that deeper structures such as the Hutton Sandstone accumulation ought to be full to structural spill point or that oil may possibly be generated from within the shallower source rocks of the Eromanga Basin. The second possibility, of early migration followed by continued structural development, cannot be refuted because details of oil migration timing and structural growth are as yet inconclusive (Salomon et al., 1990). The third possibility, that the structure size has been incorrectly mapped, is not considered likely as the dense seismic grid of 500 m spacing allows good structural control. The top of the Birkhead Formation is a good seismic reflector providing a reliable time structure map. The mapped closure takes into account the average velocity variation to the top of the Birkhead Formation determined from check shots at wells 1-5. The fourth possibility, that a large waste zone exists above the main oil pools, is addressed in the following discussion by putting forward the argument for the existence of a waste zone and then subsequently refuting it. It could be that a stratigraphic horizon 2 0 - 3 0 m above the pools is acting as a field-wide non-leaky seal. The field is effectively full to structural spill point and a waste zone exists within the relatively tight rocks at the base of the Birkhead Formation as shown in Figure 14. If this were so, we would expect to see shows down the flanks of the structure within all high permeability and porosity streaks that occur above the o i l - w a t e r contact, e.g. at point B. This is, however, not so as can be seen in the well 3 core log (Figure 8). It can also be seen in Figure 14 at point A that where well 7 encounters the Hutton Sandstone it is above the o i l - w a t e r contact predicted for the spill point based on that in well 2. However, this well did not encounter

Marine and Petroleum Geology, 1993, Vol 10, February

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Membrane seal and tertiary migration, Bodalla South: P. J. Boult 714

O / W contact

2

3

6 possible spill point

(from well 2 ) ~

A Figure 14 Schematic representation of the problem of whether the base of the Birkhead Formation is either a waste zone or acting as a leaky seal (see text for discussion)

petroleum within the top porous Hutton channel facies, which can be seen in Figure 6. The last option, that the Birkhead Formation is a leaky membrane type seal, is the preferred option by this author when the following facts are taken into consideration: (a) the three oil pools within the main structure have similar oil column heights but oil-water contacts differ by several metres; (b) the occurrence of oil shows in horizons above the Birkhead Formation, both here and in many nearby oilfields have very similar geochemical signatures (Heath et al., 1989); and (c) the geometry of observed sedimentary facies and their associated capillary pressures. When comparing the distribution of oil and provenance/diagenetic boundaries it can be seen that it as probably the latter that is controlling the former. The diagenetic/provenance boundaries in Figure 6 have been simplified as there are occasional thin beds of cleaner 'reservoir quality' sandstone within what is essentially 'Birkhead' material. These are considered not to be part of the main oil pools. Thus their oil accumulations do not have significant buoyancy to potentially breach their respective local seals. It is worth considering at this point the distribution of minor shows above and between the oil pools shown. The following question might be asked: 'If leakage is taking place why do we only see shows where there are permeability streaks?' The answer to this lies in the nature of leakage through a membrane type seal. Leakage occurs only very near the top of a structure where the oil column building up beneath the seal is at its maximum height. The oil then leaks upward in a 'conduit' from the point where the 'weak link' is breached, as predicted by Dembicki and Anderson (1989). Once the seal is breached at this point by just a few millimetres, the column here has even more buoyancy and thus leakage is self-perpetuating at this point. Where permeability streaks are encountered the oil spreads out and may be encountered by drilling. Onward migration above a permeability streak can only continue if either (a) the buoyancy of the oil column within such a streak is sufficient to overcome the CBPh/w of its seal or (b)the permeability streak is confined within seal type rocks and the streak becomes completely full. Upward migration from a permeability streak may or may not occur above the position that the oil made its way into it. The route that it takes is dependent on the geometry of the permeability streak. Not all

permeability streaks will be accessed by the leakage or tertiary migration path of the oil. This further explains the lack of shows in some permeability streaks above the main oil pools in wells 2 and 3.

Conclusions 1. Secondary and tertiary oil migration within the Eromanga Basin takes place predominantly as a separate phase. 2. The Birkhead Formation is a membrane type seal to the underlying Hutton Sandstone and there is no significant waste zone above the reservoir at Bodalla South. . The measured oil column in the Bodalla South Hutton oil pool (11.25 m) is in good agreement with the predicted oil column (10.86-12.64 m) from mercury injection data and analysis of subsurface fluids. . It is important to assess seal continuity and distinguish between hydraulic and membrane type seals when determining which is effectively trapping a petroleum column. . Mercury injection techniques are a reliable method of determining pore throat size distributions of small rock samples from conventional and sidewall cores. . The probability of encountering large oil columns within the Hutton Sandstone, where it is overlain by lithic sandstones/arkoses, similar to those at the base of the Birkhead Formation at Bodalla South, is small whatever the vertical closure of the trap may be. Larger oil accumulations may occur where the Birkhead is generally less quartz-rich or finer grained, but realistic figures for interpolation/ extrapolation into adjacent unexplored areas can only be arrived at by further study.

Acknowledgements Thanks to George Carman of Austin Oil N.L. for the initial idea of looking at seal integrity in the Eromanga Basin, to Rob Willink formerly of National Center for Petroleum Geology and Geophysics for the suggestion to apply the theory to the Bodalla South oilfield, to Bob East of AMDEL Core Services for consultation on the handling of capillary pressure techniques, to Jim Jago of the University of South Australia and to Joe Salomon of LASMO Energy for technical advice. Also thanks to all the staff at LASMO Energy, SANTOS,

12 Marine and Petroleum Geology, 1993, Vol 10, February

Membrane seal and tertiary migration, Bodalla South: P. J. Boult Jennings, J. B. (1987)Capillary pressure techniques: applications ADMEL Core Services and the University of South to exploration and development geology Am. Assoc. Petrol Australia for their co-operation in providing relevant GeoL Bull. 71, 1196-1209 data. Muller, P. J. (1989) Aspects of the hydrology of the southern

References Berg, R. R. (1975) Capillary pressures in stratigraphic traps Am. Assoc. Petrol GeoL Bull. 59, 939-956 Bowering, O. J. W. (1982) Hydrodynamics and hydrocarbon migration - - a model for the Eromanga Basin Austral PetroL ExpL Assoc. J. 22, 227-236 Dembicki, H. Jr. and Anderson, M. J. (1989) Secondary migration of oil: experiments supporting efficient movement of separate, buoyant oil phase along limited conduits Am. Assoc. Petrol GeoL Bull. 73, 1018-1021 Dolan, P., Griffiths, P. D. and Welton, S. R. (1988) The successful recovery of well productivity in the Bodalla South Field Austral Petrol ExpL Assoc. J. 28, 7-18 Harris, N. B. (1989) Diagenetic quartzarenite and the destruction of secondary porosity: an example from the Middle Jurassic Brent Sandstone of north west Europe Geology 17, 361-364 Heath, R. S., Mclntyre, S. and Gibbins, N. (1989) A Permian origin for Jurassic oil in the Eromanga Basin Proceedings of the Cooper Eromanga Conference, Adelaide, June 1989 (Ed. B. J. O'Neil), pp. 405-416 Hill, L. V. (1985) Environmental analysis of the Hutton Sandstone to Birkhead Formation transition within the south-western Eromanga basin, Queensland Unpublished BSc (Hons) Thesis, Adelaide University

Eromanga Basin, Queensland Proceedings of the Cooper and Eromanga Basins Conference, Adelaide, June 1989 (Ed. B. J. O'Neil), pp. 493-505 Salomon, J. A., Keenihan, A. P. and Calcraft, A. P. (1990) Bodalla South Field - - Australia, Queensland, Eromanga Field. In: Structural Traps 1, Tectonic Fold Traps (Eds E. A. Beaumont and N. A. Foster), Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, American Association of Petroleum Geologists, Tulsa, pp. 129-155 Schowalter, T. T. (1979) Mechanics of secondary hydrocarbon migration and entrapment Am. Assoc. Petrol GeoL Bull. 63, 723-760 Smyth, M., Cook, A. C. and Philp, R. P. (1984) Birkhead revisited: petrological and geochemical studies of the Birkhead Formation, Eromanga Basin Austral Petrol ExpL Assoc. J. 24, 230-242 Watts, K. J. (1987) The Hutton s a n d s t o n e - Birkhead Formation transition, ATP 269(1 ), Eromanga Basin Austral Petrol ExpL Assoc. J. 27, 215-228 Watts, N. L. (1987) Theoretical aspects of cap-rock and fault seals for two phase hydrocarbon columns Mar. Petrol GeoL 4, 274-307 Wardlaw, N. C. (1976) Pore geometry of carbonate rocks as revealed by pore casts and capillary pressure Am. Assoc. Petrol GeoL Bull. 60, 245-257 Wilson, D. M. and Pittman, E. D. (1977) Authigenic clays in sandstones: recognition and influence on reservoir properties and paleoenvironmental analysis J. Sedim. Petrol. 47, 3-31

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