Isotopic exchange between thermal fluids and

Water-Rock Interaction – Birkle & Torres-Alvarado (eds) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-60426-0

Isotopic exchange between thermal fluids and carbonates at offshore Campeche oil fields, Gulf of Mexico P. Birkle Instituto de Investigaciones Eléctricas, Gerencia de Geotermia, Cuernavaca, Morelos, Mexico

M.A. Lozada Aguilar, E. Soriano Mercado & J.J. Torres Villaseñor PEMEX-PEP, Activo Integral Cantarell, Cd. del Carmen, Campeche, Mexico

ABSTRACT: Formation water and core samples (dolomite, breccia, halite) from the Cantarell and Ku-Maloob-Zaap oil reservoirs, Gulf of Mexico, have been analyzed for 11B/10B, δ13C, and 87Sr/86Sr. Differences in 87Sr/86Sr-ratios between formation water (0.70796–0.70903) and adjacent salt domes (0.70707) exclude halite dissolution as major process to explain the chemical heterogeneity (TDS = 12–290 g/L) and partial hypersaline composition of formation water. Low Cl/Br ratios of fluids suggest sub-aerial evaporation of seawater as a primary mineralization process. Dolomitization and clay desorption represent the dominant processes altering the primary geochemistry and mineralogy of the carbonate host rock. Similar 11B/10B and 87Sr/86Sr ratios of dolomite and breccia host rock and reservoir fluids, as well as a short residence time (late Pleistocene) for formation water within the reservoir, suggest a relatively fast and ongoing chemical-isotopic exchange between reservoir fluids and host rock, also reflected by similar 87Sr/86Sr-ratios for fracture and matrix dolomite. 1

INTRODUCTION

concepts for the hydrogeological reservoir model. In the present paper, a variety of isotopic methods (11B/10B, δ13C, 87Sr/86Sr) are applied to reconstruct the origin of chemical-isotopic diversity of brine composition at the Cantarell and Ku-Maloob-Zaap oil reservoirs. Samples from formation water are compared with carbonate reservoir rocks and adjacent halite domes to define the type and degree of reservoir alteration by formation fluids.

The Cantarell complex, located 85 km offshore in the Bay of Campeche, Gulf of Mexico, is one of the largest oilfields in the world. Upper Cretaceous carbonate breccia and highly dolomitized Jurassic limestone form the main host rock type of the highly fractured reservoir; upper Paleocene and middle Eocene calcarenite and sandstone formations are less important. Discovered in 1976, Cantarell´s production peaked with 2.1 million barrels per day (330,000 m3/d) in 2003. Production declined rapidly after that, and by 2009 had fallen to 772,000 barrels per day (123,000 m3/d). Due to the Cantarell decline, production at the adjacent Ku, Maloob and Zaap fields, which produce from Kimmeridgian, Lower Paleocene-Upper Cretaceous and Middle Eocene carbonates, was boosted in 2007. With an oil production of 802,002 barrels/ day in November 2009, Ku-Maloob-Zaap recently became Mexico’s most productive oil field. In the last few years, the oil-water contact in the Cantarell field has been advancing, and this has considerably reduced the oil window zone. One likely explanation is that the natural fracture network, which provides most of the permeability in the field, favors production of water and gas over oil (Cruz & Sheridan 2009). In order to reconstruct the enhanced trend of water invasion into production wells, hydrochemicalisotopic studies were performed to provide actualized

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METHODS

A total of 92 water samples from Upper Jurassic to Eocene reservoir units from 43 oil production wells were taken from the Cantarell and Ku-Maloob-Zaap oilfields during three sample periods from March to November 2008. Partially low water contents of the samples emulsion (< 5%) and low API (American Petroleum Institute gravity) values from 14° to 22° of the hydrocarbons required the partial use of an organic de-emulsifier and centrifuge technique to separate the water phase from the oil phase. Thermal Ionization Mass Spectrometry (TIMS) was applied at the Saskatchewan Isotope Laboratory (University of Saskatchewan, Canada) and at Geochemical Technologies Corp. (Waco, Texas, U.S.) to measure 87Sr/86Sr and 11B/10B ratios, respectively. Sr isotope ratios were adjusted relative to a fixed value of 0.710250 for the NIST SRM 987 standard, and with a 1σ analytical uncertainty

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for the samples of 0.005%. In the case of 11B, a precision 1σ of less than +/– 0.5 ‰ for replicates is given by the standard deviation from analysis of the NBS (NIST) Standard Reference Material SRM 951. δ13C-ratios were determined at the NSFArizona AMS Facility (University of Arizona, Tucson). Similarly, ten core samples from Jurassic and Cretaceous dolomitized grainstone, dolomite and breccia, as well as one sample from an adjacent Jurassic salt dome were analyzed for Sr, B and C isotopic composition. Prior to the analytical procedure, core material was crushed, and matrix and fracture minerals were manually separated.

0.7095

Seawater (late Pleistocene - present) Fluid alteration

0.709

WATER SAMPLES: Seawater

0.708

87

Sr/86Sr

0.7085

BTP-KS Rock alteration

0.7075

CORE SAMPLES: Dolomite + breccia

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Halite dome

JSK

KI

Fossil seawater (Kimmeridgian - Paleocene)

0.707

Fossil seawater (Callovian)

0.7065 0

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400

600

800

1000

1200

1400

1600

Sr [mg/L]

3 3.1

RESULTS Figure 1. Converging trends of 87Sr/86Sr-ratios between formation water and reservoir host rock (Ku-MaloobZaap oilfield) by secondary dolomitization, initiating from a primary Kimmeridgian to Paleocene seawater composition of the reservoir carbonates, and a late Pleistocene isotopic seawater composition for formation fluids. Legend: BTP-KS = late Cenozoic (Paleocene) – late Cretaceous (Turonian); KM = late to early Cretaceous (Albian – Cenomanian), KI = early Cretaceous (Berriasian – Aptian), JSK = late Jurassic (Kimmeridgian). Also shown is Callovian seawater composition in comparison with local Jurassic halite dome.

Primary origin of brine salinity

The origin of salinity in fluids from sedimentary basins has historically been attributed to sub-aerial evaporation of seawater (Carpenter 1978), shale membrane filtration (Graf 1982), and the dissolution of evaporates (Land & Prezbindowksi 1981). Subaerial evaporation of seawater in particular has been attributed to explain the partial hypersaline composition of brines in several US (Kharaka & Hanor 2004) and Mexican (Birkle et al. 2009, Birkle & Angulo 2005) oil reservoirs of the Gulf of Mexico. For the present Cantarell reservoir, the comparison of the isotopic composition of a halite core sample (87Sr/86Sr = 0.70707; δ11B = 27.3‰, δ13C = 5.4‰), representative for the regional salt bodies and extracted from the well Ek-63 at a depth of 4,320 m.b.s.l., with reservoir formation water (87Sr/86Sr = 0.70796– 0.70903; δ11B = 18.6‰ to 43.4‰, δ13C = −12.8‰ to 9.0‰), showed significant isotopic differences between these phases. This suggests that halite dissolution does not explain water mineralization (Fig. 1). Additionally, low Cl/Br (135–560) and Na/Br (116–477) ratios confirm that evaporation of seawater beyond halite precipitation is the principal process to explain the hypersaline brine composition, as the brines reach TDS concentrations of up to 290,000 mg/L (well C-317). Increasing fluid salinity with reservoir depth, as shown for the Sr-trend of groundwater from shallow Paleocene horizons towards deeper Jurassic layers in Figure 1, confirms the gravity-related infiltration of the most saline water towards the deepest horizons of the reservoir. 3.2

exchange processes. Formation water with a maximum residence time of 35,000 years within the reservoir (determined by the 14C method) has slightly lower 87Sr/86Sr ratios in comparison to Pleistocenerecent seawater (descending arrows in Fig. 1), whereas secondary dolomite and altered breccia host rock have greater 87Sr/86Sr in comparison to the original Kimmeridgian-Paleocene seawater composition (McArthur et al. 2001; 87Sr/86Sr = 0.7069–0.7072) (ascending arrows in Fig. 1). Considering the relatively short residence time of formation water within the reservoir, the observed isotopic converging trends between both phases imply a relatively fast (since Late Pleistocene) and ongoing chemical exchange process between reservoir fluids and host rock. Moreover, the ion balance between calcium enrichment and magnesium depletion in the present water samples supports the hypothesis of dolomitization as the principal secondary alteration process. 3.3

Stages for fracture filling

In order to reconstruct the sequential stages for dolomitization, secondary dolomite from host rock matrix and fracture fillings, extracted from one single drilling core (Well Zaap-8, Core No. 3, Early Cretaceous; Depth 3521–3526 m b.s.l.), was analyzed for 11B/10B, 13C/12C, and 87Sr/86Sr ratios (Fig. 2). The hypothesis of a complete isotopic homogenization of the mineral phases by co-genetic, secondary

Dolomitization of the reservoir

In general, marine carbonates preserve the 87Sr/86Srsignature of seawater present during their sedimentation. In the present case, alteration of the primary isotopic composition of formation water and reservoir host rock (from the initial seawater composition) was caused by post-depositional

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Dolom ite fracture filling: δ13 C = 1.9 ‰ δ11 B = n.m . 87 Sr/86Sr = 0.70861

Dolom ite m atrix: δ13C = 2.2 ‰ δ11B = 24.6 ‰ 87 Sr/86Sr = 0.70834

n

tio

rp

so

Ad

Seaw ater evaporation

Form ation w ater Zaap reservoir: δ13C = -2.5 to 6.1‰ 11 δ11 B = 23.4 to 29.4 ‰ 87 Sr/86Sr = 0.70810-0.70889

Interaction with clay minerals

δ11B [‰ ]

40

EK63

20

Clay desorption

Form ation w ater (JSK) Form ation w ater (KI) Form ation w ater (KM ) Form ation w ater (BTP-K S) Seaw ater W ell cores (Dolo m ite, breccia) H alite dom e (w ell Ek-63)

0 0

1

2

3

4

1/B [m m ol/kg]

Figure 2. δ11B, δ13C and 87Sr/86Sr isotopic composition of dolomite matrix and fracture filling from the well Zaap-8, and isotopic value range for formation water from the Zaap reservoir.

Figure 3. Boron and δ11B composition of formation water from the Cantarell oilfield with present seawater and reservoir core samples (dolomite, breccia and halite). Legend details in Fig. 1.

dolomitization of the host rock—matrix as well as fracture fillings—by contact with infiltrating surface water during Late Pleistocene seems feasible. The vertical chemical-isotopic stratification from low-saline groundwater towards hypersaline aquifer units explains the wide isotopic range for these groundwater samples from the Zaap reservoir (value range is given in Fig. 2). 3.4

for modern marine carbonates of +22.1 ± 3‰, Hemming & Hanson 1992), but rather adsorbed-B on clay surfaces. Spivack et al. (1987) showed that adsorbed-B in marine sediments has an average δ11B of +14‰, but it accounts for only 10 to 20% of the total-B in the sediments. The primary boron isotopic composition is not known for the Mexican reservoir sediments, but Williams et al. (2001) reported δ11Bvalues between -2‰ and +2‰ for pore filling clays from organic mudstone (with interbedded sandstones) from the Wilcox Formation in south-central Louisiana. Total clay concentrations of the analyzed carbonate cores (dolomite, breccias) reach up to 16.9% (M-456, N-2), whereas dolomitization degree ranges between 54.5% (M-456: N-2) and 99.4% (Z-36: N-2). Isotopic variations observed in formation water and carbonate rock are interpreted to result from the interaction between fluid B and surface adsorbed-B. δ11B values of formation water decreased from its initial seawater composition for the Gulf of Mexico (+40.6‰) to the current values between +18.6 and +30.4‰. Elevated B/Cl ratios (0.001–0.025) for the Cantarell and Ku-Maloop-Zaap formation waters are beyond the evaporation trend of seawater, indicating secondary enrichment of reservoir fluids by an additional source other than evaporated seawater. In the case of U.S. sedimentary basins from the Gulf of Mexico, 11B depletion accompanied by B-enrichment of formation water is explained by the progressive dissolution of 11B-depleted crustal silicate minerals (Land & Macpherson 1992). As for the 87Sr/86Sr-ratios, the wide range of δ11B values for formation water (18.6–34.0‰) and core samples (11.5–24.6‰) suggest spatial variations for the degree of isotopic exchange at the Cantarell reservoir. Interaction of brines with clay minerals controls their B content and isotopic composition.

Clay desorption

Boron concentrations of Cantarell formation water are elevated (46.1–192 mg/L) in comparison to standard meteoric water (< 1.0 mg/L) and seawater (4.5 mg/L) (Fig. 3). In contrast, δ11B values between +18.6‰ (well 432) and +34.0‰ (well 2072) are less than that of dissolved boron from present-day seawater from the Gulf of Mexico with a δ11B-value of +40.6‰. The most probable mechanisms that can affect both the B concentration and B isotope signature in formation fluids could be a) desorption of exchangeable B in clays, resulting in elevated B with low 11B (in fluids), and/or b) a potential involvement of deep generated fluids. The second option can be excluded, as elevated 14C-concentrations of the present reservoir fluids (1.6–30.0 pmC) and more than 10 km of accumulated sedimentary column exclude major contributions of deep, 14C-free magmatic fluids. Because B partitions preferentially into vapor and δ11B vapor-liquid values are small for geothermal systems, ranging from ∼3‰ to 1‰ at ∼140 to 300°C (Leeman et al. 1992), effects by liquid-vapor separation would be negligible for Cantarell fluids that have average temperatures of 110 to 120°C. δ11B-values between +11.5‰ and +24.6‰ for dolomite and breccias from the Jurassic-Cretaceous reservoir probably do not represent the total rock value for carbonate rock (with a global value range

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3.5

Origin of salt domes 87

petroleum reservoir, Gulf of Mexico, Mexico. Applied Geochemistry 20: 1077–1098. Birkle, P., Martínez, B.G. & Milland, C.P. 2009. Origin and evolution of formation water at the Jujo-Tecominoacán oil reservoir, Gulf of Mexico. Part 1: Chemical evolution and water-rock interaction. Applied Geochemistry 24: 543–554. Carpenter, A.B. 1978. Origin and chemical evolution of brines in sedimentary basins. Oklah. Geol. Surv. Circ. 79: 60–77. Cruz, L. & Sheridan, J. 2009. Relative contribution to fluid flow from natural fractures in the Cantarell field, Mexico. SPE Latin American and Caribbean Petroleum Engineering Conference, 31 May-3 June 2009, Cartagena de Indias, Colombia, Paper 122182-MS. Graf, D.L. 1982. Chemical osmosis, reverse chemical osmosis, and the origin of subsurface brines. Geochimica et Cosmochimica Acta 46: 1431–1448. Hemming, N.G. & Hanson, G.N. 1992. Boron isotopic composition and concentration in modern marine carbonates. Geochimica et Cosmochimica Acta 56: 537–543. Kharaka, Y.K. & Hanor, J.S. 2004. Deep fluids in the continents: I. Sedimentary Basins. In H.D. Holland & K.K. Turekian (eds.), Treatise in Geochemistry 5: 499–540, Elsevier. Land, L.S. & Prezbindowksi, D.R. 1981. The origin and evolution of saline formation water, Lower Cretaceous carbonates, south-central Texas, U.S.A. Journal of Hydrology 54: 51–74. Land, L.S. & Macpherson, G.L. 1992. Origin of saline formation waters, Cenozoic section, Gulf of Mexico sedimentary basin. AAPG Bulletin 76(9): 1344–1362. Leeman, W.P., Vocke, R.D. & Mc Kibben, M.A. 2002. Boron isotopic fractionation between coexisting vapor and liquid in natural geothermal systems. In Y.K. Kharaka & A.S. Maest (eds.) Proc. 7th Int. Symp. WRI-7: 1007–1010. Rotterdam: Balkema. Martínez R.M. 2009. Dolomitzation and generation of vugular porosity in the K/T breccia of the Cantarell field, Marina-Campeche zone. Ph.D. thesis, U.N.A.M., Mexico City, 224 p. (original in Spanish). McArthur, J.M., Howarth, R.J. & Bailey, T.R. 2001. Strontium isotope stratigraphy: LOWESS version 3: Best fit to the marine Sr-isotope curve for 0–509 Ma and accompanying look-up table for deriving numerical age. The Journal of Geology 109: 155–70. Mitra, S., Duran J.A.G., Garcia J.H., Hernandez, S.G. & Subhotosh, B. 2006. Structural geometry and evolution of the Ku, Zaap, and Maloob structures, Campeche Bay, Mexico. AAPG Bulletin 90(10): 1565–1584. Salvador, A. 1991. Chapter 8: Triassic-Jurassic. In A. Salvador (ed.), The Gulf of Mexico Basin. The Geology of America, Vol. J: 131–180. Boulder, CO: The Geological Society of America. Spivack, A.J., Palmer, M.R. & Edmond, J.M. 1987. The sedimentary cycle of boron isotopes. Geochimica et Cosmochimica Acta 51: 1939–1949. Williams, L.B., Hervig, R.L., Wieser, M.E. & Hutcheon, I. 2001. The influence of organic matter on the boron isotope geochemistry of the gulf coast sedimentary basin, USA. Chemical Geology 174: 445–461.

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In general, evaporates preserve the Sr/ Sr-signature of the respective marine water composition during the period of sedimentary deposition (McArthur et al. 2001). In southeastern U.S. and Mexican sedimentary basins, the evaporite deposits are known as Louann Salts and Istmo Salts, respectively (Salvador 1991). Most authors assigned a Middle Jurassic age (Callovian) for the saline column as a unique and extensive event for seawater evaporation and salt precipitation. Comparing the known strontium isotopic composition of seawater during Callovian (87Sr/86Sr = 0.70687–0.70696; Salvador 1991) with halite core material from the well EK-63 (depth = 4,315– 4,324 mb.l.s.; 87Sr/86Sr = 0.70707), the close relation between both values supports the Middle Jurassic as the principal depositional period for halite evaporates at the specific Ek reservoir site (Fig. 1). 4

CONCLUSIONS

Understanding the types and degree of water-rock interaction processes in oil reservoirs is essential to reconstruct ongoing groundwater mobilization by active petroleum extraction. The present study on secondary alteration processes in the Cantarell and Ku-Maloob-Zaap oil reservoirs postulates several geochemical reactions to explain abrupt changes in current aquifer mobility and pathway reactivity. An improved fracture permeability by extensional tectonic episodes from Pliocene to Holocene (Mitra et al. 2006), as well as the generation of porosity by active dolomitization (petrographic studies by Martínez 2009) could provide enhanced conditions for hydraulic conductivity between groundwater horizons. The abrupt invasion of oil reservoir intervals by formation water is explained by the formation of preferential fracture-related conduits, allowing accelerated dynamics for groundwater migration. An elevated degree of alteration for reservoir dolomitization, as well as the abundance of secondary clay minerals (montmorillonite, smectite, illite) allow us to postulate desorption of exchangeable boron as a principal process that alters the primary isotopic composition of reservoir fluids. Probably, due to the lack of suitable thermal conditions (T = 110–120°C), the water-rock interaction process maintained partial equilibrium, reflected by the partial degree of isotopic homogenization between host rock (δ11BCarbonates = +11.5 to +24.6‰; δ11BSalt = +27.3 to +34.3 ‰) and Cantarell formation water (δ11BWater = +18.6‰ to +34.0‰). REFERENCES Birkle, P. & Angulo, M. 2005. Hydrogeological model of Late Pleistocene aquifers at the Samaria-Sitio-Grande

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