Chapter 44 The Role of Heavy Mineral Analysis as a Geosteering Tool During Drilling of High-Angle Wells

Chapter 44

THE ROLE OF HEAVY MINERAL ANALYSIS AS A GEOSTEERING TOOL DURING DRILLING OF HIGH-ANGLE WELLS ANDREW C. MORTONa,b a

HM Research Associates, 2 Clive Road, Balsall Common, West Midlands CV7 7DW, UK b CASP, University of Cambridge, 181a Huntingdon Road, Cambridge CB3 0DH, UK ABSTRACT The use of heavy mineral analysis on a ‘real-time’ basis at well site is a logical extension of the application of the method as a non-biostratigraphic correlation tool. To date, the method has been used to monitor the stratigraphy during drilling of horizontal wells in three fields on the UK continental shelf (Clair, Ross and Hannay), and in several cases heavy mineral data have been integral in deciding to amend well bore trajectories in order to remain within hydrocarbon-bearing zones, a process known as ‘geosteering’. Heavy mineral analysis now has a positive track record for geosteering horizontal wells in a variety of depositional environments, from fluvial/aeolian to shallow and deep marine, and can therefore be genuinely considered as an alternative geosteering tool in circumstances where biostratigraphic methods have inadequate resolution. Heavy mineral data have been used in the decision-making process in a variety of situations, including whether to maintain angle, to steer up or to steer down, to sidetrack and to terminate drilling. Application of heavy mineral analysis at the well site requires establishment of a robust correlation scheme prior to drilling horizontal wells, using samples from offset wells adjacent to the proposed well track. Parameters used to monitor the geology during horizontal drilling include those conventionally used for reservoir correlation (principally, ratios of stable minerals with similar hydrodynamic behaviour), but other parameters have also proved useful in the particular circumstance associated with individual fields. For example, the abundance of unstable minerals is locally useful in the Clair Field, and ratios of minerals with contrasting hydrodynamic behaviour are useful in the Ross and Hannay Fields. Keywords: heavy minerals; stratigraphy; correlation; geosteering; North Sea

Developments in Sedimentology, Vol. 58, 1123–1142 r 2007 Elsevier B.V. All rights reserved. ISSN: 0070-4571/doi:10.1016/S0070-4571(07)58044-1 1123

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Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

1. INTRODUCTION Heavy mineral analysis is one of the most widely used of the various nonbiostratigraphic methods available for sandstone correlation (Dunay and Hailwood, 1995), especially in the hydrocarbon industry. Correlation by heavy mineral means is crucially dependent on the existence of stratigraphically significant variations in provenance and/or transport history during deposition of the succession under investigation, a prerequisite to the successful application of any provenance-based correlation method (Morton et al., 2002). Heavy minerals are especially well suited for sandstone correlation since they are highly sensitive indicators of provenance and sediment transport history (Mange and Maurer, 1992; Morton and Hallsworth, 1999). The first applications date back to the early part of the last century, the work of Milner (1923) being of seminal importance in establishing the value of the technique. Since then, heavy minerals have been used for correlation purposes in many sedimentary basins worldwide, examples including the Pliocene of the San Joaquim Valley, California (Reed and Bailey, 1927), the Cretaceous-Tertiary of Venezuela (Feo-Codecido, 1956) and the Paleozoic-Mesozoic of the Middle East (Weissbrod and Nachmias, 1986). The technique has proved especially useful in the hydrocarbon basins of the UK and Norway, with a wide range of applications through a wide range of depositional environments, from fluvial and aeolian through paralic, shallow marine and deep marine settings, and across a large part of the geological column, from Devonian to Tertiary (Allen and Mange-Rajetzky, 1992; Morton and Berge, 1995; Mange et al., 1999; Morton et al., 2002). In recent years, the correlative value of heavy mineral analysis has been taken a step further, with the technique being used ‘real-time’ at the well site to monitor the stratigraphy encountered during drilling of high-angle wells. High-angle (horizontal or near-horizontal) wells are increasingly being used to facilitate economic exploitation of hydrocarbon reserves, since a large number of high-angle wells can be drilled from a single production facility. In many cases, successful exploitation depends on ensuring the well bore remains in the productive parts of the reservoir (the so-called ‘pay’ zone). The use of geological parameters to alter the trajectory of wells during drilling, thereby ensuring the well bore remains within the pay zone, is known as geosteering. Geosteering therefore requires continual monitoring of the geology during drilling operations. In most cases, this is achieved using a combination of cuttings description by the well site geologist, geophysical logging while drilling (LWD) and high-resolution biostratigraphy. However, in some reservoirs, the biostratigraphic component of this ‘geosteering suite’ may not be available, for example, when biostratigraphic events lack sufficient resolution, or when the succession is entirely biostratigraphically barren. Under such circumstances, heavy mineral analysis can play an important role. To date, applications of heavy mineral analysis at well site to monitor the geology encountered in high-angle wells have been confined to the UK continental shelf. This probably reflects the widespread application of heavy mineral analysis as a correlation tool in the hydrocarbon basins of the UK. Heavy mineral analysis at well site was pioneered on appraisal wells in the Clair Field, west of Shetland, in 1996–1997, but its first application in a field development programme was on the Ross Field in the North Sea. It has subsequently been applied in a number of other field developments, including Hannay, Clyde and Murchison.

2. Principles and Practice

1125

2. PRINCIPLES AND PRACTICE In order for heavy mineral analysis to be successfully applied at well site, the pay zone must be mineralogically different to the overlying and underlying units. Ideally, the zones underlying and overlying the pay zone should also differ from one another. If such mineralogical differences exist, it should be possible to establish whether the drill bit is drilling productive reservoir, and if not, how the hole trajectory should be altered in order to return to the pay zone. Prior to application of heavy mineral analysis at well site, it is therefore necessary to establish whether the productive stratigraphic unit has a distinctive mineralogical signature that distinguishes it from the overlying and underlying units. This is achieved by undertaking heavy mineral analysis of the reservoir succession in previously drilled wells adjacent to the proposed well trajectory. Ideally, this template should be established using core material, since—although the well site analysis is conducted on ditch cuttings—cores have greater integrity, being significantly less subject to contamination and other forms of alteration that may occur during the drilling process. Although heavy mineral templates are best established using core data, it is important to recognise that there may be differences in mineral parameter measurements made on core and ditch cuttings samples from the same succession, caused by the drilling process and by downhole contamination. Abundances of mechanically unstable heavy minerals may be depleted in ditch cuttings compared with core, owing to the action of the drill bit. This problem is likely to be exacerbated in wells drilled with polycrystalline diamond compact (PDC) bits, which essentially grind through the formation, compared with tricone bits, which have a cutting action. Identification of stratigraphic events at well site should therefore be based on the along-hole patterns shown by the mineral parameters, rather than on their actual measured values. The range of heavy mineral parameters that can be analysed at well site is more limited than that in conventional laboratory-based correlation studies. The establishment of a valid correlation framework critically depends on acquisition of provenance-sensitive parameters. These parameters reflect changes in sediment provenance and transport history, but are unaffected by other processes that operate during the sedimentary cycle (Morton et al., 2002), such as hydraulic fractionation and diagenesis. Provenance-sensitive criteria fall into two categories: ratio data and varietal data (Morton and Hallsworth, 1994). Ratio data determine the relative abundance of minerals with similar hydraulic and diagenetic behaviour, such as apatite and tourmaline, and can be readily acquired at well site using conventional optical microscopy. By contrast, varietal data, which quantify differences shown by individual mineral populations, are routinely acquired by single-grain electron microprobe analysis, for instance on garnet (Morton, 1985) or tourmaline (Henry and Guidotti, 1985), but this approach is not applicable at the well site since the technology is not available offshore. However, it is possible to undertake varietal studies using conventional optical microscopy, subdividing mineral populations on the basis of properties such as colour, habit or internal structure (Mange-Rajetzky, 1995), and such data can be readily acquired at well site.

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Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

The application of heavy mineral analysis at well site is illustrated using three examples, all from the UK continental shelf, covering a range of ages and depositional settings. Example 1 is from the fluvial/aeolian/lacustrine Devonian-Carboniferous Clair Group in the Clair Field, west of Shetland; example 2 is from the shallow marine Jurassic Ross Formation of the Ross Field, outer Moray Firth; and example 3 is from the deep marine Cretaceous Britannia Formation of the Hannay Field, central North Sea. 3. CLAIR FIELD The Devonian-Carboniferous non-marine clastic succession that forms the reservoir in the Clair Field (Fig. 1) is essentially devoid of biostratigraphic markers. A combination of heavy mineral and sedimentological data enabled the establishment of a stratigraphic breakdown of the succession into 10 units, labelled I–X from base to top (Allen and Mange-Rajetzky, 1992). Units I–VI comprise the Lower Clair Group, and VII–X comprise the Upper Clair Group (Fig. 2). Fluid flow from the main part of the Clair Field reservoir is dependent on the combined presence of permeable sandstones and open fractures (Coney et al., 1993). In the Core Area of the Clair Field, these requirements are met by Unit V, which has the best reservoir quality and is the locus for an open fracture system. Successful exploitation of the Clair Field is therefore dependent on drilling horizontal wells targeted on Unit V, in order to optimise hydrocarbon production. In view of the variations in mineralogy recognised by Allen and Mange-Rajetzky (1992), heavy mineral analysis was considered a possible option to assist with geosteering of these high-angle wells (Morton et al., 2003). The possible application of heavy mineral analysis for geosteering high-angle wells in the Clair Field was tested by undertaking a high-resolution study of the Units IV–VI interval in the key well 206/8-8 (Fig. 3). Within this interval, important variations in several parameters were recognised, enabling identification of three heavy mineral units, Units VIm, Vm and IVm (Morton et al., 2003). The VIm–Vm boundary is close to the boundary between lithostratigraphic Units VI and V, but the Vm–IVm boundary occurs some 20 m below the boundary between lithostratigraphic Units V and IV. The parameters that are useful for discrimination of Units VIm, Vm and IVm are as follows: (1) apatite/tourmaline ratio (ATi): ATi is high (>90) in both Units VIm and IVm, but generally lower in Unit Vm; (2) garnet/zircon ratio (GZi): the GZi pattern is similar to that of ATi, high (>90) in both Units VIm and IVm, but generally lower in Unit Vm; (3) abundance of unstable minerals: Unit VIm is characterised by common unstable minerals throughout (principally epidote and titanite); by contrast, these minerals are generally absent in Units Vm and IVm, although occasional samples contain amounts similar to Unit VIm; the abundance of unstable minerals is measured in two ways, first as a frequency % determination and second as a ratio against tourmaline (denoted as UTi); (4) apatite roundness (ARi): apatite morphology was recognised as a key parameter for Clair Field correlation by Allen and Mange-Rajetzky (1992). Apatite tends to

3. Clair Field

1127

204 205

207

206

1

2

CLAIR

206/7

206/8

Shetland Islands

206/9

6

9

8

7

206/8-8 206/8-11Z

0 0 206/12

20

8

11

60

kms

km 206/14

206/8

40

12 13

14

15

Orkney Islands

ROSS 17

18

HANNAY

19

20

206/8

14/30b-3

13/29

13/28

14/30b 20/5c

2 km

k Fau

lt

94 00

0

940

97 00

s Ban

95 95 00 00

Bosie

950 0

20/5c-6 N

13/29a-A4

0

8 kms

Fig. 1. Location of Clair, Ross and Hannay Fields on the UK continental shelf. Inset maps show relevant well locations (adapted from Coney et al., 1993; Law et al., 1999; Morton et al., 2001). Contours on the Hannay inset map are depth to top Britannia sandstone (in feet).

Marginal marine and distributary channel sandstone

Unit IX

Proximal braidplain or fan sandstones

Unit VIII

Fluvial sandstones including point bar deposits

Unit VIIIB

Fluvial sandstones, overbank fines Fluvial sandstones

Unit VIIIA Unit VI

Upper Unit VI LKB Lower Unit VI

Lower Clair Group

Devonian

Unit V Unit IV

Fluvial sandstones, thin lacustrine mudrocks LKB = Lacustrine key Bed; Lacustrine mudrocks Fluvial sandstones, thin lacustrine mudrocks Fluvial sandstones, possibly wind-modified Fluvial sandstones, possibly wind-modified

Unit III

Fluvial sandstones and sandflats

Unit II

Fluvial sandstones and conglomerates

Unit I

Fan conglomerates, lacustrine mudrocks

Fig. 2. Clair Field stratigraphy (after Allen and Mange-Rajetzky, 1992; Morton et al., 2003).

Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

Upper Clair Group

?Carboniferous

1128

Unit X

3. Clair Field

1129

1820 m

top Vm top V

top IV

top IVm

1970 m 40 60 80 10040 60 80 100 0 ATi GZi

50 ARi

100 0 50 100 unstables %, UTi

Fig. 3. Downhole heavy mineral profile across the lower part of Unit VI, Unit V and into top Unit IV in the cored well 206/8-8. VIm, Vm and IVm are the mineralogical zones that approximately equate to lithostratigraphic Units VI, V and IV (from Morton et al., 2003). ATi ¼ apatite/tourmaline index; GZi ¼ garnet/zircon index; ARi ¼ apatite roundness index (% rounded apatite in apatite population); unstables (open diamonds) ¼ % epidote+titanite; UTi (filled diamonds) ¼ unstables-tourmaline index (% unstables in total unstables+tourmaline).

become rounded more rapidly than many of the other clastic components due to its relatively low hardness, and apatite roundness is therefore a sensitive indicator of prolonged transport, including the extent of aeolian activity. In 206/8-8 (Fig. 3), ARi values are low in Units VIm and IVm, and high in Unit Vm. However, the pattern is gradational, and unit boundaries cannot be specified on the basis of this parameter alone. In addition, changes in RuZi (rutile/zircon ratio) are useful for discrimination of the Upper Clair Group from the Lower Clair Group, with the former having high RuZi and the latter low RuZi. This change reflects a significant change in provenance across the Lower Clair/Upper Clair boundary (Allen and Mange-Rajetzky, 1992). The variations in heavy mineral parameters over the Units IVm–VIm interval result from the interplay of provenance, transport history and diagenesis. Differences in ATi and GZi are ascribed to provenance, and the greater apatite roundness in Unit Vm reflects more prolonged transport. Since variations in ATi, GZi and apatite roundness reflect changes in provenance and transport history, they are likely to be correlatable on a field-wide basis. By contrast, the presence of unstable minerals in Unit VIm is due to lower porosity in this part of the succession (McKie and Garden, 1996), inhibiting pore fluid movement and leading to the preservation of relatively unstable minerals. The abundance of epidote- and titanite-bearing sandstones at specific levels in Unit Vm (Fig. 3) reflects the presence of similar low

Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

1130

porosity/permeability sandstones lower in the succession. Since the distribution of unstable minerals is controlled by diagenesis, this parameter is unlikely to be a reliable discriminator between Unit VIm and Unit Vm over the field as a whole, although it appears to be laterally consistent in the Core Area. The pilot study on 206/8-8 established that heavy mineral analysis provides a sound basis for distinguishing the good quality reservoir Unit Vm from the poorer quality Units VIm and IVm. Additional studies, using both core and cuttings, showed that similar heavy mineral events could be identified in adjacent wells (Morton et al., 2003), thereby establishing that the events can be traced across the Core Area of the field and that they could be identified in cuttings as well as core samples. Given these encouraging results, it was decided that the technique would be used at well site during drilling of subsequent horizontal wells. 3.1. Clair Field Example: Well 206/8-11Z One of the horizontal wells where well site heavy mineral analysis was pioneered is 206/8-11Z (Morton et al., 2003). The well track for 206/8-11Z is shown in Fig. 4, with the heavy mineral parameters acquired at well site shown in Fig. 5. Four key heavy mineral events (nos. 1–4) were identified during drilling. The upper part of the drilled Clair Group succession (down to point no. 1) is characterised by extremely high GZi, low ATi, low ARi, relatively high RuZi, moderate abundances of unstable minerals and high ratios of unstable minerals to tourmaline (UTi). These features are typical of the Upper Clair Group. At point no. 1, there is a marked change in mineralogy, with ATi values becoming consistently high, GZi and RuZi showing marked decreases, and unstable minerals showing a significant increase. This marks a fundamental change in provenance within the Clair Group, and indicates penetration of Unit VIm. The increased abundance of unstable minerals in Unit VIm NW

SE

1400

Base Cretaceous unconformity X

depth (m subsea)

1600

IX

Shetland Group

11Z - as drilled 11Z - plan profile 206/8-11A

VI

Upper Clair Group (undifferentiated)

VIII 1800

VIIB VIIA

2000

IV

1

VI

LKB V

2

4

3 IV

2200 200 m

Fig. 4. Interpreted cross-section along trace of wells 206/8-11A and -11Z, showing locations of heavy mineral events nos. 1–4 (from Morton et al., 2003). LKB ¼ lacustrine key bed (an important stratigraphic marker horizon in mid Unit VI).

3. Clair Field

#1

2500

#2 #3

3000

measured depth (m)

2000

1131

#4

0

25 50

ATi

75 100

60

70 80

GZi

90 100

0 10 20 30 40 50

RuZi

0

25

50

ARi

75

0

25 50

75 100

unstables %, UTi

Fig. 5. Variations in heavy mineral parameters in 206/8-11Z, showing heavy mineral events nos. 1–4. ATi ¼ apatite/tourmaline index; GZi ¼ garnet/zircon index; RuZi ¼ rutile/zircon index; ARi ¼ apatite roundness index (% rounded apatite in apatite population); unstables (open diamonds) ¼ % epidote+titanite; UTi (filled diamonds) ¼ unstables-tourmaline index (% unstables in total unstables+tourmaline).

compared with the Upper Clair Group may be due to the change in provenance, but could also result from greater preservation in the poor-quality Unit VIm succession. Unit VIm characteristics persist as far as point no. 2, where there is a decrease in UTi, an increase in ARi and a decrease in ATi. This combination of parameters indicates entry into the good-quality reservoir Unit Vm. Unit Vm characteristics continue to point no. 3 (Fig. 5), where there is a change to lower ARi, higher ATi and higher GZi, suggesting penetration of Unit IVm. This was unexpected, since the well trajectory was essentially horizontal and nearly parallel to bedding (Fig. 4), and the normal full vertical thickness of Unit Vm had not been penetrated. The original well prognosis was therefore reviewed in the light of these changes in heavy mineral data. Morton et al. (2003) considered that at this point, the well bore crossed a subseismic fault, thereby crossing from the main part of Unit Vm into sediments belonging to Unit IVm.

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Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

Between points nos. 3 and 4, the succession alternates between two mineralogical types, one with relatively low ARi, high ATi and high GZi (similar to Unit IVm), the other with higher ARi, lower ATi and lower GZi (similar to Unit Vm type). Morton et al. (2003) suggested that the well bore was tracking close to the Unit Vm/Unit IVm boundary. Since this position was sub-optimal, it was decided to steer the well upsection to try to regain a position nearer to the mid-point of Unit Vm. This demonstrated the value of the HM data, since this conclusion could not have been made on the basis of the cuttings and LWD data alone. There is a major change in mineralogy at point no. 4 (Fig. 5), with ATi and ARi dropping dramatically, GZi rising to nearly 100, and RuZi also showing a significant increase. These features all indicate renewed penetration of the Upper Clair Group, implying that the well trajectory had intersected a major fault (Fig. 4). Unstable mineral abundances are slightly lower than previously seen in the Upper Clair Group, probably because of dilution by material from the stables-dominated highangle section. This is probably also the reason why the UTi value levels out at
80, compared with >90 for the equivalent section penetrated prior to point no. 1. The well was terminated shortly after this event was recognised. 4. ROSS FIELD The main reservoir in the Ross Field (Moray Firth, North Sea, Fig. 1) consists of shoreface sandstones of the Ross Formation, with significant reserves in the underlying alluvial/fluvial Parry Formation (both Late Jurassic). The Ross Formation, which was deposited during a progressive marine transgression, is informally subdivided into four units, termed R1, R2, R3 and R4, in ascending order (Morton et al., 2001). Each unit is approximately 6 m thick, and represents a basal transgressive event followed by shoreface progradation. Reservoir quality in the Ross Formation is strongly dependent on depositional facies, the best quality being associated with the more proximal marine facies (middle/upper shoreface) and the poorest quality with the most distal (offshore) facies. The distribution of good-quality reservoir across the field is therefore dependent on both stratigraphy and paleogeographic setting. Exploitation of the Ross Field requires drilling horizontal wells that target the relatively thin R1–R4 pay zones. Since biostratigraphy lacks sufficient resolution to distinguish R1, R2, R3 and R4, heavy mineral analysis was considered as an alternative geosteering tool. As with the Clair Field case study, prior to drilling horizontal wells, the potential value of heavy mineral analysis as a geosteering tool in the Ross Field was investigated by analysing cores of the Ross Formation from adjacent offset wells. These studies showed the existence of stratigraphic variations in a number of heavy mineral parameters of various types, including overall detrital heavy mineral recovery, mineral ratios and mineral grain attributes. The most useful parameters in the Ross Field proved to be: (1) Heavy mineral recovery (relative abundance of detrital non-opaque heavy minerals compared with other components of the heavy mineral residues), which is very low in distal offshore sediments, but increases with proximity to the paleoshoreline; the poor recovery in the more distal facies is related to the reduction in

4. Ross Field

1133

clastic sediment supply, with authigenic pyrite being the main phase; recovery also tends to be relatively low in paralic sediments (Parry Formation and R1 coastal plain facies); (2) Zircon/tourmaline ratio (ZTi), which varies on a high-frequency basis throughout both Ross and Parry due to a combination of changes in sediment provenance and hydrodynamic conditions during deposition; (3) Apatite/tourmaline ratio (ATi), which reflects the extent to which sediment has been weathered during the sedimentation cycle; ATi is high throughout in the marine parts of the Ross Formation (R1–R4) but very low in paralic sediments (Parry Formation and R1 coastal plain facies); (4) Abundance of euhedral and brown zoned zircons, which are indicators of sediment derived directly from the Ross granite of the adjacent hinterland. Such zircons are scarce in the Ross Formation but are common in the underlying Parry Formation. An example of the stratigraphic variations in these parameters is shown for well 13/29a-A3 (Fig. 6). The existence of stratigraphic variations in heavy mineral data provided confidence that the method would have sufficient resolution to identify and distinguish the various stratigraphic units during drilling of horizontal wells. Consequently, the technique was used at well site during drilling of nine horizontal wells on the Ross Field (13/29a-A3Y, 13/29a-A4, 13/28a-B2, 13/28a-B3, 13/28a-C1Z, 13/ 28a-C2, 13/29a-D1, 13/29a-D2 and 13/29a-E1). Heavy mineral analysis is particularly effective in the Ross Field because of the relatively slow penetration rates (typically 3–6 m/h) and because the logging tools are placed at considerable distance behind the drill bit. For example, the gamma ray tool is approximately 16 m behind the bit, and with a typical rate of penetration of 5 m/h, a change in formation can be identified by heavy mineral analysis around 2 h before the gamma ray log, taking into account cuttings lag time and heavy mineral extraction. 4.1. Ross Field Example: Well 13/29a-A4 Well 13/29a-A4 (Fig. 1) targeted the R3 unit, which has the best reservoir quality in this part of the Ross Field. In this well, heavy mineral data provide a clear distinction between R3 and the overlying non-reservoir R4 unit, and also differentiate the better-quality upper R3 from the poorer-quality lower R3 (Fig. 7). R4 is distinctive in having a combination of very low detrital heavy mineral recovery and low ZTi. Detrital heavy mineral recovery and ZTi are both high in upper R3, but lower R3 has high recovery in conjunction with low ZTi. Consequently, when heavy mineral analysis indicated that the well track had penetrated lower R3, for example at 4337 m and subsequently at 4788 m, it was decided to build angle, thus enabling the well track to re-enter better-quality upper R3. Similarly, when heavy mineral analysis indicated that the well track had penetrated R4 at 4615 m and subsequently at 4996 m, decisions to drop angle were taken. This was successfully achieved after the first penetration of R4 between 4615 and 4706 m. However, after the second R4 section between 4996 and 5029 m, R3 sediment was encountered only until 5052 m, at which point penetration of R4 was renewed. This indicated that it was no longer possible to steer the well, and since the target of drilling 760 m of high-quality reservoir had already been achieved, the decision to terminate drilling was made.

1134

3160

R3

R2 3180 R1

R1 (paralic)

3190

P3 3200

0 20 40 60 80 20 heavy mineral recovery

40

60 ZTi

80

100

0

25

50 ATi

75

100 0

20 40 60 % euhedral zircon

Fig. 6. Variations in key heavy mineral parameters from part of the core in Ross Field well 13/29a-A3. These parameters were used to build up a template of mineralogical characteristics of the Ross and Parry Formations across the field to help calibrate and interpret the parameters determined subsequently at well site (adapted from Morton et al., 2001).

Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

measured depth (m)

3170

4. Ross Field

1135

13400

4100

13500 R4

13600 13700

4200

13800 13900 R3U

14000 14100

4300

14200 14300 14400

4400

14500

R3L

14600 4500

14800 14900 15000

R3U 4600

15100 15200 15300

measured depth (m)

measured depth (ft)

14700

R4

15400

4700

15500 R3U

15600 15700

4800

15800

R3L

15900 16000 4900

16100 R3U

16200 16300 16400

R4

16500

R3U

16600

R4

16700 0 100 200 gamma (API)

0 10 20 30 40 50 Heavy Mineral Recovery

50 60 70 80 90 100 ZTi

Fig. 7. Along-hole variations in two key parameters determined at well site during drilling of Ross Field well 13/29a-A4, shown against the LWD gamma ray log. The mineralogy clearly identifies penetrations of R4, upper R3 (R3U) and lower R3 (R3L), on the basis of variations in heavy mineral recovery (detrital heavy minerals/detrital heavy minerals plus opaques) and ZTi (zircon/tourmaline index) (adapted from Morton et al., 2001).

1136

Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

5. HANNAY FIELD Deep-water Aptian-Albian sandstones form reservoirs for a number of hydrocarbon accumulations in the Moray Firth and central North Sea, including the Captain and Britannia Fields (Garrett et al., 2000). One area of particular interest in recent years has been the so-called ‘Kopervik fairway’, lying to the south of the Halibut Horst in the Moray Firth, UK (Law et al., 2000). Several accumulations have been found along this trend, notably Atlantic, Blake, Cromarty, Goldeneye and Hannay (Garrett et al., 2000; Law et al., 2000). The Hannay Field (Fig. 1), which was discovered in 1996 by well 20/5c-6 (Law et al., 2000), has an oil column of approximately 30 m, with exploitation achieved by a horizontal well, 20/5c-8, drilled in 2001–2002. The sandstones that comprise the reservoir belong to the Britannia Sandstone Formation (Johnson and Lott, 1993), and underlie mudstones of the Carrack Formation. Owing to the sand-rich nature of the reservoir succession in the Hannay Field, heavy mineral analysis was considered as a potential geosteering tool. Core from the discovery well 20/5c-6 was used to establish a heavy mineral template for the subsequent horizontal well. The results of the core study were promising, showing the presence of variations in a number of heavy mineral parameters that established the existence of four heavy minerals units, H1–H4 (Fig. 8). The key parameters used in the subdivision of the 20/5c-6 succession are: (1) Heavy mineral recovery (relative abundance of detrital non-opaque heavy minerals compared with other components of the heavy mineral residues), which distinguishes good-quality, sand-rich intervals from poor-quality, sand-poor units; (2) Garnet/zircon (GZi), apatite/tourmaline (ATi) and to a lesser extent rutile/zircon (RuZi), all of which reflect changes in sand provenance; (3) Zircon/apatite (ZAi) and zircon/tourmaline (ZTi), which reflect a combination of changes in provenance and hydraulic conditions at the time of deposition; (4) TiO2 minerals/zircon (RZi), which responds mainly to local diagenetic conditions, variations being due to the extent of authigenic anatase development. The most distinctive unit in the 20/5c-6 succession is H2 (Fig. 8), which has lower heavy mineral recovery, lower ZAi and lower ZTi, together with higher ATi, higher GZi and slightly higher RuZi than the rest of the Britannia Sand. The other units (H1, H3 and H4) all have relatively high recovery, ZAi and ZTi values, in conjunction with low ATi and GZi. There are subtle differences in these parameters between Units H1, H3 and H4, with H3 having lower recovery, lower ZTi and lower ZAi than H1 and H4. The most well defined variations in zones H1, H3 and H4, however, are shown by the RZi value, which varies from
20 to
60 (Fig. 8). These variations are essentially due to differences in abundance of authigenic anatase, which is scarce in H1, moderately abundant in H3 and very abundant in H4, especially at the top of the unit. The heavy mineral variations within the succession are due to a combination of differences in provenance, depositional facies and diagenetic history. The markedly higher GZi, ATi and slightly higher RuZi in Unit H2 indicate that this unit has a different provenance than the rest of the succession. The difference in provenance is also associated with a facies change, H2 being less sandy, as manifested by the higher gamma ray response and lower heavy mineral recovery (Fig. 8). The lower ZTi and

9490

9500 H2 9510

5. Hannay Field

H1

2900

H3

9530 measured depth (m)

measured depth (ft)

9520

9540

9550

H4

9560

9570

2920

9580

9590 0 25 50 0 20 40 60 60 80 100 60 80 100 0 ZAi ZTi core gamma recovery %

40 80 0 ATi

40 GZi

80

0 20 40 60 RZi, RuZi 1137

Fig. 8. Heavy mineral profile in the cored Britannia Sand of Hannay Field well 20/5c-6, showing definition of Units H1–H4. Recovery ¼ % detrital heavy minerals in the heavy mineral residue; ZAi ¼ zircon/apatite index; ZTi ¼ zircon/tourmaline index; ATi ¼ apatite/tourmaline index; GZi ¼ garnet/zircon index; RZi ¼ TiO2 minerals/zircon index (filled diamonds); RuZi ¼ rutile/zircon index (open diamonds).

Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

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ZAi values in Unit H2 could be due to hydraulic conditions, but might also be due, at least in part, to the different provenance. The more subtle variations in ZAi and ZTi between Unit H3 and Units H1 and H4 reflect different hydraulic conditions, since other parameters (GZi, ATi and RuZi) show no variation, indicating a uniform provenance. The variations in abundance of authigenic anatase are a diagenetic effect, and it is therefore questionable whether the RZi value is correlatable on a field-wide basis. 5.1. Hannay Field Example: Wells 20/5c-8Z and 20/5c-8Y Development of the Hannay Field reservoir was achieved by the horizontal well 20/ 5c-8. The first penetration of the Britannia Sand was made by well 20/5c-8Z (Fig. 9). Four heavy mineral units (Z1–Z4) were identified in this well. (1) Unit Z1 is characterised by good heavy mineral recovery, extremely high ZAi (close to 100), high ZTi (c. 90), very low ATi (o4) and low GZi (
20). These parameters are consistent with penetration of the upper part of the Britannia Sand, equivalent to Unit H1 in the template well 20/5c-6. One sample within Unit Z1 has relatively low recovery, but other parameters remain unchanged, and the low recovery is therefore ascribed to the presence of mud-rich lithologies rather than penetration of sandstones equivalent to Unit H2. (2) Unit Z2 is characterised by relatively poor heavy mineral recovery, a decrease in ZAi and ZTi, an increase in ATi and GZi, and a slight increase in RuZi. These parameters indicate penetration of strata equivalent to Unit H2. (3) Unit Z3 is marked by a return to parameters closely comparable to those in Z1 (Fig. 9), with an increase in heavy mineral recovery, ZAi and ZTi, together with a decrease in ATi, GZi, RuZi and mica abundance. Unit Z3 represents renewed penetration of sandstones equivalent to Unit H1 in 20/5c-6. (4) Unit Z4 has an unusual mineralogical composition, with very low heavy mineral recovery and a high abundance of carbonate. These parameters cannot be matched with any previously seen, higher either in 20/5c-8Z or in the template well 20/5c-6. Unit Z4 is therefore interpreted as indicating penetration of the overlying stratigraphic unit, the Carrack Formation.

Z1

Z2 Z3 Z4 0 20 40 60 80 recovery

60 70 80 90 100 ZAi

60 70 80 90 100 ZTi

0 20 40 60 80 ATi

0 20 40 60 80 GZi

0 5 10 15 20 25 RZi, RuZi

Fig. 9. Along-hole variations in key heavy mineral parameters from Hannay Field well 20/8c8Z, showing definition of four units, Z1–Z4. Legend same as in Fig. 8, except ‘recovery’ ( ¼ % detrital heavy minerals/detrital heavy minerals plus opaques).

5. Hannay Field

1139

Penetration of the Carrack Formation indicated that the well trajectory required major amendment. This could have been achieved by altering the angle to enable downcutting back into the Britannia Sand reservoir. However, this was not considered feasible, and therefore a decision was taken to sidetrack the well within Unit Z2. Four heavy mineral units (Y1–Y4) were recognised in the sidetrack well 20/5c-8Y (Fig. 10): (1) The first of these, Unit Y1, is identical to Unit Z1, having been drilled in 20/8c-Z. (2) Unit Y2 is characterised by relatively low recovery, relatively low ZAi (
70–90), relatively low ZTi (
70–80), relatively high ATi (>20) relatively high GZi (>40) and increased RuZi. This combination of parameters indicates penetration of strata equivalent to Unit H2. (3) Unit Y3 has increased heavy mineral recovery, higher ZAi and ZTi, and lower ATi, GZi and RuZi. ZAi and ZTi values are intermediate between those seen in Y2 and the subsequent Unit Y4, suggesting that Unit Y3 is equivalent to H3 in 20/5c-6. (4) Unit Y4 is characterised by good heavy mineral recovery, very high ZAi, high ZTi, low ATi low GZi and low RuZi. These parameters indicate penetration of strata equivalent to Unit H4 in 20/5c-6. The decision to terminate drilling was made after the well had penetrated sufficient good-quality Britannia Sand reservoir for production purposes.

Y1

Y2

Y3

Y4

0

20 40 60 80 60 70 80 90 100 60 70 80 90 100 0

recovery

ZAi

ZTi

20 40 60 80 0

ATi

20 40 60 80 0

GZi

5 10 15 20 25

RZi, RuZi

Fig. 10. Along-hole variations in key heavy mineral parameters from Hannay Field well 20/ 8c-8Y, showing definition of four units, Y1–Y4. Legend same as in Figs. 8 and 9.

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Chapter 44: The Role of Heavy Mineral Analysis as a Geosteering Tool

All the parameters that established the reservoir breakdown in the Hannay Field template well 20/5c-6 proved to be useful in the horizontal well, with the notable exception of the RZi value. Consequently, parameters that reflect differences in provenance and hydrodynamic conditions appear to be laterally consistent across the Hannay Field. By contrast, the parameter most strongly affected by diagenetic conditions is laterally variable, with no increase seen in Units Y3 and Y4.

6. CONCLUSIONS As a result of its application in the Clair, Ross and Hannay Fields, heavy mineral analysis now has a positive track record for geosteering horizontal wells during field appraisal and development programmes in a variety of depositional environments, including fluvial/aeolian, shallow marine and deep marine. The technique can therefore be genuinely considered as an alternative geosteering tool in circumstances where biostratigraphic methods have inadequate resolution. Heavy mineral data have been used in the decision-making process in a variety of situations, including whether to maintain angle, to steer up or to steer down, to sidetrack, and to decide when to terminate drilling. Parameters used to monitor the geology during horizontal drilling include those conventionally used for reservoir correlation (Morton et al., 2002), but other parameters have also proved useful in the particular circumstance associated with individual fields. For example, the abundance of unstable minerals in the Clair Field is an important parameter for distinguishing Unit VIm from Vm and IVm, whereas detrital heavy mineral recovery proved useful in both the Ross and Hannay Fields. Hydrodynamically controlled parameters such as zircon/apatite and zircon/tourmaline ratios also proved useful in Ross and Hannay. Consequently, a wide variety of parameters should be considered when undertaking feasibility studies prior to application at well site. Caution is always required, however, when using parameters that are not only controlled by provenance, since lateral variations in hydrodynamic and diagenetic conditions are likely to occur across individual fields. This is exemplified by the RZi parameter in the Hannay Field. Authigenic anatase is abundant in certain zones in the template well 20/5c-6, causing marked variations in RZi. However, this feature is absent from the horizontal wells 20/5c-8Z and 20/5c-8Y, indicating that authigenic anatase development is highly localised and that variations in RZi cannot be used for correlation purposes in the Hannay Field. Application of heavy mineral analysis at well site requires establishment of a robust correlation scheme prior to drilling horizontal wells, using samples from offset wells adjacent to the proposed well track. Each individual field must be considered as a separate case, since there is no guarantee that correlatable variations exist, and there is no consistency to the parameters that permit stratigraphic subdivision in different reservoir successions. In several cases, there are differences in absolute values between data acquired from core and cuttings, due to the mechanical action of the drill bit, and emphasis should therefore be placed on trends rather than absolute values when undertaking analysis at well site. Despite the potential for discrepancy between core and cuttings data, the technique has advantages over other non-biostratigraphic well site

References

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methods, notably ‘chemostratigraphic’ methods, since the method deals with samples in terms of their constituent components rather than in bulk, enabling contamination from drilling additives and caving to be filtered out. Limitations of the application of heavy mineral analysis at well site are threefold. First, the method is applicable only to clastic reservoir successions. Second, for the technique to be successful at well site, the succession under evaluation must show stratigraphically significant variations in heavy mineralogy. This requires investigation of the heavy mineral signatures of existing offset wells prior to drilling horizontal wells. Finally, data production takes approximately 2 h from receipt of sample, and thus the rate of penetration is a limiting factor. In formations that can be drilled rapidly, heavy mineral data production may be too slow to allow decisionmaking. Nevertheless, there are many fields in which the technique may prove to be a valuable component of the geosteering suite, and with the increasing emphasis on field development using horizontal wells, heavy mineral analysis has an important potential role to play in the future. ACKNOWLEDGEMENTS I am grateful to BP and the Clair Field partners for permission to present the Clair Field example, and to Talisman for permission to use the Ross and Hannay examples. REFERENCES Allen, P.A., Mange-Rajetzky, M.A., 1992. Sedimentary evolution of the Devonian-Carboniferous Clair Field, offshore northwestern UK: impact of changing provenance. Marine and Petroleum Geology 9, 29–52. Coney, D., Fyfe, T.B., Retail, P., Smith, P.J., 1993. Clair appraisal: the benefits of a cooperative approach. In: Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe. Proceedings of the 4th Conference. Geological Society, London, pp. 1409–1420. Dunay, R.E., Hailwood, E.A. (Eds.), 1995. Dating and Correlating Biostratigraphically-Barren Strata. Geological Society of London Special Publication, vol. 89. Feo-Codecido, G., 1956. Heavy mineral techniques and their application to Venezuelan stratigraphy. Bulletin of the American Association of Petroleum Geologists 40, 984–1000. Garrett, S.W., Atherton, T., Hurst, A., 2000. Lower Cretaceous deep-water sandstone reservoirs of the UK central North Sea. Petroleum Geoscience 6, 231–240. Henry, D.J., Guidotti, C.V., 1985. Tourmaline as a petrogenetic indicator mineral: an example from the staurolite-grade metapelites of NW Maine. American Mineralogist 70, 1–15. Johnson, H., Lott, G.K., 1993. Cretaceous of the central and northern North Sea. In: Knox, R.W.O’B., Cordey, W.G. (Eds.), Lithostratigraphic Nomenclature of the UK North Sea. British Geological Survey, Nottingham. Law, A., Raymond, A., White, G., Atkinson, A., Clifton, M., Atherton, T., Dawes, I., Robertson, E., Melvin, A., Brayley, S., 2000. The Kopervik fairway, Moray Firth, UK. Petroleum Geoscience 6, 265–274. Mange, M.A., Maurer, H.F.W., 1992. Heavy Minerals in Colour. Chapman and Hall, London, 147pp. Mange, M.A., Turner, P., Ince, D., Pugh, J., Wright, D., 1999. A new perspective on the zonation and correlation of barren strata: an integrated heavy mineral and palaeomagnetic

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study of the Sherwood Sandstone Group, East Irish Sea Basin and surrounding areas. Journal of Petroleum Geology 22, 325–348. Mange-Rajetzky, M.A., 1995. Subdivision and correlation of monotonous sandstone sequences using high resolution heavy mineral analysis, a case study: the Triassic of the Central Graben. In: Dunay, R.E., Hailwood, E.A. (Eds.), Dating and Correlating Biostratigraphically-Barren Strata, Geological Society of London Special Publication, vol. 89, pp. 23–30. McKie, T., Garden, I.R., 1996. Hierarchical cycles in the non-marine Clair Group (Devonian), UKCS. In: Howell, J.A., Aitken, J.F. (Eds.), High Resolution Sequence Stratigraphy: Innovations and Applications, Geological Society of London Special Publication, vol. 104, pp. 139–157. Milner, H.B., 1923. The study and correlation of sediments by petrographic methods. Mining Magazine (London) 28, 80–92. Morton, A.C., 1985. A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea. Sedimentology 32, 553–566. Morton, A.C., Berge, C., 1995. Heavy mineral suites in the Statfjord and Nansen Formations of the Brent Field, North Sea: a new tool for reservoir subdivision and correlation. Petroleum Geoscience 1, 355–364. Morton, A.C., Hallsworth, C.R., 1994. Identifying provenance-specific features of detrital heavy mineral assemblages in sandstones. Sedimentary Geology 90, 241–256. Morton, A.C., Hallsworth, C.R., 1999. Processes controlling the composition of heavy mineral assemblages in sandstones. Sedimentary Geology 124, 3–29. Morton, A.C., Knox, R.W.O’B., Hallsworth, C.R., 2002. Correlation of hydrocarbon reservoirs using quantitative heavy mineral analysis. Petroleum Geoscience 8, 251–262. Morton, A.C., Safton, K., Mundy, D.J.C., Passingham, B., Sargent, M., 2001. Geosteering high-angle wells in the Ross Field, North Sea: the application of heavy mineral analysis. Offshore Magazine, April Issue, 64–66. Morton, A.C., Spicer, P.J., Ewen, D.F., 2003. Geosteering of high-angle wells using heavy mineral analysis: the Clair Field, West of Shetland. In: Carr, T.R., Mason, E.P., Feazel, C.T. (Eds.), Horizontal Wells: Focus on the Reservoir. American Association of Petroleum Geologists, Methods in Exploration, vol. 14, pp. 249–260. Reed, R.D., Bailey, J.P., 1927. Surface correlation by means of heavy minerals. Bulletin of the American Association of Petroleum Geologists 11, 359–368. Weissbrod, T., Nachmias, J., 1986. Stratigraphic significance of heavy minerals in the late Precambrian-Mesozoic clastic sequence (‘Nubian Sandstone’) in the Near East. Sedimentary Geology 47, 263–291.