Determining well-bore pathways during multilateral drilling campaigns in shale resource plays: an example using chemostratigraphy from the Horn River Formation, British Columbia, Canada

K.T. Ratcliffe1, J. Woods2, C. Rice3

Keywords: chemostratigraphy, shale resources, Horn River Formation, NE British Colombia

Figure 1. Location of Horn River Basin and generalised stratigraphy of the basin. Red box in the stratigraphy column highlights study interval.

Summary Introduction

This paper demonstrates how the technique of The Horn River Formation in north east British Colombia is chemostratigraphy is used to define a regional stratigraphic estimated to have 500 Tcf gas in place, which makes it the third framework for the Horn River Formation, a shale resource play largest North American natural gas accumulation discovered prior in north eastern British Colombia, Canada. Initially, a regional to 2010. Lithologically, it is composed of dark grey to black, stratigraphic framework is defined and subsequently the technique variably organic rich, siliceous and calcareous mudrocks. In the is used in multilateral wells drilled from a central pad. The latter area of this study it is approximately 200m thick and is further enables the well-bore pathway in to be placed relative to the subdivided into the Evie, Otter Park, and Muskwa members stratigraphy of the pilot hole. (Figure 1). The Evie and Muskwa members are notably organic- rich and siliceous.

Typically, when exploiting the Horn River Formation, a series of lateral wells are drilled from a single central pad. In all 1 Chemostrat Ltd., Unit 1 Ravenscroft Court, Buttington Cross Enterprise such drilling campaigns, knowing where the well-bore pathway Park, Welshpool, Powys, SY21 8SL, UK travelled relative to the regional stratigraphy and being able to [email protected] relate the well-bore pathway back to the pilot hole is imperative 2 Chemostrat Australia, Suite 17, 44 Kings park Road, West Perth, WA 6005 (Schmidt et al., 2010). However, hitting and remaining in the 3 Apache Corporation, 700 9th Avenue SW, Calgary, Alberta, Canada “sweet spot” when drilling in shale resource plays is particularly challenging. A recent study conducted by Halliburton concluded

Brisbane, QLD, 10–14 September, 2012 Eastern Australasian Basins Symposium IV 143 K.T. Ratcliffe, J. Woods, C. Rice

Figure 2. Chemostratigraphic characterisation and correlation of two vertical wells. Package 1 equates approximately to the Evie Member, Package 2 to the Otter Park Member and the Package 4/Package 5 boundary correlates approximately to the top of the Muskwa Member. Refer to Figure 1 for lithostratigraphic nomenclature.

that “approximately 50% of wells geosteered using the Results conventional gamma ray geosteering methods within an area of the Haynesville were misplaced for more than 50% of their lateral length.” (http://www.epmag.com/Production-Drilling/ Regional chemostratigraphy Geosteering-Unconventional-Shale-Reservoirs-Potential_80771). Here, the technique of chemostratigraphy is applied to a series of The elements and element ratios plotted as chemical logs for lateral wells to demonstrate how the technique can help improve two vertical study wells in Figure 2 allow the study interval to time spent in a given zone. be divided into 5 chemostratigraphic packages and 9 geochemical units, the main features of which are:

Keg River Carbonates: this formation underlies the Horn Methodology River Formation and is geochemically distinctive due to is high CaO values, reflecting its clean limestone lithology. Chemostratigraphy involves the characterization and correlation of sediments using variations in their elemental Package 1 is characterized by high values of EFV and is also compositions (Ratcliffe et al., 2010 and references cited therein). differentiated from the overlying package by its low RTi and high It is widely applied to help with stratigraphic correlations in SiO2/Zr values. This package is approximately equivalent to the petroleum provinces (Pearce et al., 2005a, Ratcliffe et al, 2006, Evie Member. Hildred et al., 2010, Wright et al., 2010). Elemental data have been acquired here using Inductively Coupled Plasma Optical Emission Package 2 is differentiated from those below and above by Spectrometry and Mass spectrometry (ICP OES MS) (Jarvis and its high RTi and low EFV values. It also typically has upward Jarvis 1995), which results in data for 50 elements being acquired; increasing Th/U values. This package is approximately equivalent 10 major elements, 26 trace elements and 14 rare earth elements. to the Otter Park Member.

The dataset acquired from the Horn River Formation for this Package 3 is differentiated from the packages above and below study to date is from 4 pad locations, with data from 4 vertical by its low Th/U and high EFV and SiO2/Zr values. holes and over 30 lateral wells. Total sample numbers exceed 1000 and all data have been acquired from cuttings samples.

144 Eastern Australasian Basins Symposium IV Brisbane, QLD, 10–14 September, 2012 Determining well-bore pathways during multilateral drilling campaigns in shale resource plays: an example using chemostratigraphy from the Horn River Formation, British Columbia, Canada

Package 4 is somewhat transitional between Package 3 and well-bore. Typically, however, the regional bedding dip is already Package 5, with upward increasing RTi and Th/U values. Packages known prior to drilling and can be accounted for while interpreting 3 and 4 are approximately equivalent to the Muskwa Member. the lateral pathways.

Package 5 is typically poorly samples since it is the top of the Figure 3 displays the vertical, build and lateral section of well zone of interest. It has the highest RTi values of the study interval. d-F52-L/094-O-08. In the build section, the Package 4 / Package 3 and Package 3 / Package 2 boundaries are clearly recognizable By comparing these data with mineralogy and TOC data and by comparing the changing values of RTi, SiO2/Zr, EFV and Th/U by using multivariate statistical analysis and graphical plots in the in this well with the wells displayed on Figure 2. Between 2755- manner described in Pearce et al., (2005b), Svendsen et al., (2007), 2760mTVD in the build section, the high Th/U spike that defines Ellwood et al., (2008) and Pe-Piper et al., (2008) it is possible to Unit 2.3 is seen, indicating that the heel of this well lies within make the following interpretations of the controlling factors of the Unit 2.2 or 2.1. In the lateral section from 2900-3620mMD the key element ratios used in Figure 2. Th/U values are sufficiently low to indicate that this section of the well is within Unit 2.1. At 3620mMD, the Th/U increases CaO: directly proportional to the amount of calcite in the to levels more typical of Unit 2.2, indicating that the well-bore samples. was travelling up stratigraphy. From 3620 – 4600mMD, the chemistry is relatively consistent and typical of Unit 2.2, apart U: directly proportional to the amount of preserved organic from the sample at 4500mMD which has high CaO values. This matter in the samples. sample almost certainly represents a calcite stringer that was not seen in vertical sections due to sample resolution; when drilling at RTi: Calculated by summing Al2O3, TiO2, Fe2O3 and K2O low angle to bedding, even stringers a few centimeters thick are values provides an indication of the amount of terrigenous material likely to be resolved and these can provide invaluable information in the sediment. on more indurate layers that will affect subsequent fraccing. At 4600mMD, where the well-bore starts to increase TVD notably, Th/U: Here this ratio is modeling the amount or clay versus the Th/U values decrease indicating that this drop in TVD within organic content. the wellbore resulted in it re-entering Unit 2.1. The toe of the well remains within Unit 2.1. EFV: Calculated using EFVsample = V/Al2O3sample / V/ Al2O3standard shale. Tribovillard et al., (2006) demonstrated Well d-O52-L/094-O-08 is on the opposite side to the central that where EFV =1 the depositional environment was oxic. High pad to well d-F52-L/094-O-08 (Figures 3 and 4). Therefore, values of EFV indicate that the depositional environment was while the lateral section in well d-F52-L/094-O-08 had to “chase” dysoxic, anoxic or euxinic. bedding up-dip, the lateral section in well d-O52-L/094-O-08 is “chasing” the bedding down-dip. In well d-O52-L/094-O-08, the SiO2/Zr: This ratio highlighted areas of the sequence that Package 4 / Package 3 and Package 3 / Package 2 boundaries are contain biogenic silica, which has been demonstrated by others clearly recognizable by comparing the changing values of RTi, (Wright et al., 2010b; Ratcliffe et al., 2012). SiO2/Zr, EFV and Th/U in this well with the wells displayed on Figure 2. The high Th/U values at 2760mTVD in the build Based on the interpretations of elements above, it can section indicate that the heel of the well lies within Unit 2.3. Th/U be concluded that during deposition of Packages 1 and 3 the values remain in the lateral section until 3250mMD when they depositional environment was anoxic, terrigenous input was low drop, indicating that the well penetrated Unit 2.2 at that depth. and that a high portion of the quartz being deposited was biogenic At 3550mMD, the Th/U increases in association with a local in origin. The higher CaO values in Package 1 than Package 3 TVD maximum in the well-bore, suggesting that the Unit 2.2 / (Figure 2) could indicate a shallower depositional environment 2.1 boundary was penetrated and the well-bore was within Unit (above the CCD ?) or the carbonate could be in the form of detrital 2.1 between 3550-3800mMD. At 3800mMD, a slight decrease in carbonate material. Packages 2 and 4 were both deposited in TVD of the well-bore results in return to a Unit 2.2 geochemical relatively oxic conditions, with increasing amounts of terrigenous signature. At 4550mMD, the well-bore rapidly starts to increase material entering the system. The Th/U spikes in Package 2 are TVD which initially results in penetration of Unit 2.1, before somewhat enigmatic since they do not have a corresponding the toe of the well T.D.’s in Package 1, as indicated by the sharp increase in RTi or decrease in U. They probably indicate a increase in EFV values in the last two samples analysed. distinctive composition of terrigenous material (volcanogenic ?) was supplied to basin toward the top of Package 2. Conclusions Placement of lateral wells By using chemostratigraphy, regional correlations of shale Figures 3 and 4 show the interpretation of two lateral wells reservoirs can be achieved, which help understand broad, basinal from the d-52-L/094-O-08 pad, one drilled south east from the stratigraphies and events. However, a potentially more important pad, the other northwest from the pad. The key to interpreting well- application is the ability to use changes in elemental compositions bore pathways in highly deviated wells is a) having knowledge of to determine well-bore pathways in multilateral drilling campaigns. the regional dips and b) being able to accurately determine where The data presented here have been acquired in a laboratory, but the heel of the well was relative to the stratigraphy. The dip of they can be largely acquired while drilling at wellsite within, c. 20 beds can be determined from the chemostratigraphy, where a minutes of a cuttings sample arriving at surface. Therefore, this unit boundary is crossed more than once in a well, for example application of chemostratigraphy has large potential in real-time in Figure 3, the top of Unit 2.1 is crossed at least twice, joining geosteering, not only in the Horn River Basin, but in any shale those points indicates the direction of bedding dip relative to the resource play of any age from around the World.

Brisbane, QLD, 10–14 September, 2012 Eastern Australasian Basins Symposium IV 145 K.T. Ratcliffe, J. Woods, C. Rice

Figure 3. Chemostratigraphic interpretation of the build and lateral section of well d-F52-L/094-O-08. Lower left inset shows the well-bore pathways in plan view, with the well in this figure highlighted in red.

146 Eastern Australasian Basins Symposium IV Brisbane, QLD, 10–14 September, 2012 Determining well-bore pathways during multilateral drilling campaigns in shale resource plays: an example using chemostratigraphy from the Horn River Formation, British Columbia, Canada

Figure 4. Chemostratigraphic interpretation of the build and lateral section of well d-O52-L/094-O-08. Lower left inset shows the well-bore pathways in plan view, with the well in this figure highlighted in red.

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Carboniferous Schooner Formation, southern North Sea” In Tribovillard, N., Bout-Roumazeilles, V., Algeo, T., Lyons, “Carboniferous hydrocarbon geology: the southern North Sea T.W.; Sionneau, T., Montero-Serrano, J.C., Riboulleau, A. and and surrounding onshore areas” In Collinson, J.D., Evans, D.J., Baudin, F. 2008. Paleodepositional conditions in the Orca Basin Holliday, D.W. and Jones N.S. (eds) Yorkshire Geological Society, as inferred from organic matter and trace metal contents. Marine Occasional Publications series, v.7, p.147–64. Geology, Vol. 254, Issue 1-2, pp. 62-72.

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