khuff formation, abu dhabi - Geology

THE INFLUENCE OF ROCK FABRIC AND MINERALOGY ON THERMOCHEMICAL SULFATE REDUCTION: KHUFF FORMATION, ABU DHABI R.H. WORDEN1*, P.C. SMALLEY 2, AND M.M. CROSS3 1

School of Geosciences, The Queen’s University, Belfast BT7 1NN, U.K. e-mail: [email protected] 2 BP-Amoco, Chertsey Road, Sunbury on Thames, Middlesex TW17 7LN, U.K. 3 Department of Earth Sciences, The University of Manchester, Manchester M13 9PL, U.K.

ABSTRACT: Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and calcite. In this study of the dolomite-hosted hydrocarbon gas reservoirs in the Permo-Triassic Khuff Formation, Abu Dhabi, a geochemically well constrained rock–gas system, we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of TSR reactions and thus on H2S concentration in the gas phase. The controls on the rate on H2S accumulation were: (1) TSR reaction kinetics. TSR became significant as temperature exceeded ; 1408C, a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane. (2) Anhydrite dissolution rate. Once initiated, the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site. Sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide (i.e., suggesting total reaction for each unit of sulfate that dissolves). Also, finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite (likely a result of relative surface area), suggesting that anhydrite dissolution was the rate-limiting step. Transport rates are unlikely to have been rate-limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals. (3) Transport rate. Later, as TSR proceeded, it became transport controlled, as calcite, growing on the surface of anhydrite crystals, began to isolate them from dissolved methane. TSR ceased once calcite had effectively armor-plated (totally isolated) the remaining anhydrite. Finely crystalline anhydrite underwent more extensive and more rapid TSR than coarser anhydrite crystals because these had a greater ratio of surface area to volume, allowing more and faster dissolution and requiring more calcite to isolate them from methane. Localized loss of H2S occurred in the reservoir by reaction with indigenous Fe-bearing clays. Consequently, reservoirs with a relatively high siliciclastic content have less H2S than would be expected from the advanced state of anhydrite replacement by calcite. In order to predict TSR-related H2S concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum-emplacement history.

INTRODUCTION

Thermochemical sulfate reduction (TSR) occurs when sulfate is chemically reduced by organic compounds from petroleum fluids at elevated temperature (conventionally assumed to be . 1008C). The products of this process include reduced forms of sulfur (H2S, elemental sulfur) and oxidized forms of carbon (calcite, CO2), as well as water and a variety of organo-sulfur compounds. TSR has been studied in a number of sedimentary basins because it can have a profound impact on the commercial viability of petroleum resources, with H2S typically being undesirable. Most TSR-related studies have focused on the organic geochemistry of the pro* Present address: Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, U.K. JOURNAL OF SEDIMENTARY RESEARCH, VOL. 70, NO. 5, SEPTEMBER, 2000, P. 1210–1221 Copyright q 2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-1210/$03.00

cess, although several have looked at the effect of TSR on rock properties such as (secondary) porosity and the growth of carbonate minerals at the expense of anhydrite (e.g., Heydari and Moore 1989; Krouse et al. 1988; Machel 1988; Heydari 1997). One of the defining criteria in the study and analysis of TSR is the rate of the process, which would enable prediction of the amount of H2S generated for a given thermal history. Much effort has been spent on investigating the role of the organic reactants (e.g., Orr 1974; Sassen 1988; Manzano et al. 1997) and the roles of water and preexisting H2S (Worden et al. 1996; Hutcheon et al. 1997; Machel 1998; Worden et al. 1998). However, TSR studies have so far not really addressed two key issues: (1) the influence of rock fabric on reaction rate, and (2) the crystallographic mechanism of the replacement of anhydrite by calcite. This paper addresses the petrographic evidence for the mechanism of TSR in the rocks of the Khuff Formation, Abu Dhabi, a system that has been carefully characterized geochemically (Worden et al. 1995, 1996; Worden et al. 1997; Worden and Smalley 1996). It will be demonstrated that attempts to predict quantitatively the rate of TSR demand an understanding of the physical habit of anhydrite in the reservoir. It will also be shown that the growth of calcite at the expense of anhydrite is facilitated by the existence of a favorable carbonate mineral substrate. Furthermore, we will demonstrate that the amounts of H2S in the TSR-influenced reservoirs of the Khuff Formation are not only a function of the extent of TSR but also a function of the local reservoir’s ability to remove or ‘‘scrub’’ H2S from the gas via reaction with Fe minerals. GEOLOGICAL SETTING OF THE KHUFF FORMATION

The Upper Permian to Lower Triassic Khuff Formation comprises an eastsoutheast-dipping series of carbonates and evaporites deposited under coastal sabkha and shallow marine conditions (Figs. 1, 2; El Bishlawy 1985; Alsharan 1993; Alsharan and Nairn 1994). The Khuff Formation is presently buried to depths ranging from 2500 m to greater than 6000 m, representing present-day temperatures from 1008C to greater than 2208C (Fig. 2; Gumati 1993). The Khuff Formation is part of a subsiding passive margin that has experienced a relatively simple burial history, reaching local maximum temperatures at the present day (Gumati 1993; El Bishlawy 1985). The Khuff Formation is composed of dolomitized micrites and grainstones with interbedded evaporites (Alsharan 1993). It has been subdivided stratigraphically into the Upper and Lower Khuff Formations (Fig. 2; El Bishlawy 1985). Between the two is the Middle Anhydrite, a laterally extensive layer of massive anhydrite of several meters thickness (Alsharan and Nairn 1994). The Middle Anhydrite is thought to have kept the Upper and Lower Khuff hydrodynamically separate during much of their diagenetic histories (El Bishlawy 1985). Geochemical and petrographic data from this study will be subdivided and averaged for each well and for the Upper and Lower Khuff Formations; these formations represent hydrodynamically isolated units within the same reservoir. Dolomitization probably occurred soon after burial commenced (Fig. 3, after El Bishlawy 1985) and was accompanied by the displacive growth of gypsum nodules (Patterson and Kinsman 1982; Kendall 1984). Gypsum (CaSO4.2H2O) underwent dehydration and was replaced by anhydrite (CaSO4) at greater burial depths (approximately 1000 m; Jowett et al. 1993). The Khuff Formation contains

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FIG. 1.—Map of petroleum accumulations in the Khuff Formation, Abu Dhabi (after El Bishlawy 1985).

effective secondary porosity in sucrosic dolomites and some primary porosity in the grainstones. There are very large gas pools in the Khuff reservoirs, all being approximately normally pressured and containing dry hydrocarbon gases with variable amounts of H2S (from 2 to 50% H2S by volume; Alsharan and Nairn 1994). Note that there are no oil-bearing Khuff structures in this study; the petroleum is entirely in the gas phase and is methane-dominated. METHODS

Forty-three gas samples were collected from 21 exploration and appraisal wells from a wide range of depths within the Khuff Formation (2600–6100 m). Methods of gas sampling and analysis are described in Worden et al. (1995). Core samples (221) were collected from a subgroup of five wells spanning a range of depths (3700–6200 m). These were examined petrographically. Optical petrography was performed on thin sections stained to discriminate ferroan and nonferroan calcite and dolomite (Dickson 1965). All 221 thin sections were examined to assess qualitatively the presence and textural types of anhydrite, the replacement textures of calcite after anhydrite,

FIG. 2.—Schematic cross section of the geology of the Khuff Formation showing the range of burial depths, the geometry of the dipping strata, the Upper and Lower Khuff Formations, and the intermediate Middle Anhydrite. The local temperature minimum required for TSR (1408C) is also shown.

and the presence of elemental sulfur. The different textural types of anhydrite in each section were quantified by point counting on 200 points per thin section. Quantification of reaction progress of the replacement of anhydrite by calcite was achieved using polished thin sections and SEM with back-scattered electron microscopy followed by image analysis. This procedure permitted measurement of the fraction of anhydrite that had been replaced by calcite. Clay minerals were characterized chemically using energy-dispersive X-ray analysis (EDAX) and mineralogically using XRD. Cathodoluminescence imaging was performed on polished thin sections to examine growth zones in the replacive calcite. Carbonate minerals and anhydrite dominate the Khuff Formation. In order to quantify the small amounts of silicate minerals with any degree of accuracy it was necessary to dissolve the carbonate and sulfate minerals from a weighed portion of crushed core in dilute hydrochloric acid. The residue was weighed and then standard XRD analyses and quantification routines were performed. The original quantities of silicate minerals were recalculated by reference to the weight loss during acid digestion. Thermometry of fluid inclusions in calcite was performed using a Linkam THM600 heating–cooling stage with 0.18C precision. Phase transition temperatures were determined by temperature cycling; heating experiments were conducted before freezing to prevent inclusion deformation by ice growth, which would affect homogenization temperatures. Carbon isotope analysis of the calcite reaction product after anhydrite was performed using a Nd–YAG laser sampling device linked to a conventional mass spectrometer for analysis of the generated CO2 (Smalley et al. 1992). This system has a resolution of about 50 mm with analytical precision of 6 0.2‰ for carbon and 6 0.5‰ for oxygen. The laser was used to sample calcite within anhydrite nodules, without risk of contamination from dolomite matrix. Intergrown anhydrite and calcite were sampled by this method, the anhydrite having no influence upon the analysis and measurement of carbon and oxygen isotopes. Carbon and oxygen isotope data are reported relative to the standard Peedee Formation belemnite (PDB) using conventional delta (d13C and d18O) notations. Carbon isotopes of gases were determined using routine techniques (Worden and Smalley 1996). Elemental sulfur samples for sulfur isotope analysis were separated from core by soxhlet extraction of crushed, weighed core sample using dichlo-

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FIG. 3.—Paragenetic sequence showing the main stages in the diagenetic evolution of the Khuff Formation (adapted from description in El Bishlawy 1985). Dolomitization occurred in a sabkha environment and occurred at the same time as the growth of gypsum nodules. Dolomitized facies had minimal growth of calcite and dolomite cements during burial diagenesis prior to the onset of TSR and the replacement of anhydrite by calcite. This paper concentrates exclusively on the last phase of the paragenesis (TSR).

romethane. Elemental sulfur was precipitated as CuS using weighed, activated copper placed at the bottom of the soxhlet flask. The quantity of extracted elemental sulfur was determined by noting the weight added to the copper powder and cross-checking this with the weight lost by the crushed core sample. There was no significant difference between the measured elemental sulfur using the two approaches, showing that the elemental sulfur was the only material in the core that was soluble in organic solvents. The extracted powders were analyzed by conventional mass spectrometry (Worden et al. 1997). Anhydrite samples for sulfur isotope analysis were extracted from rock hand specimens by drilling out the cores of large centimeter-scale anhydrite nodules with a dentist’s drill. Anhydrite was analyzed for sulfur isotopes using conventional techniques and has a precision of 6 0.1‰. Sulfur isotope data are reported relative to the standard Can˜on Diablo Troilite (CDT) using conventional delta (d34S) notation. RESULTS

Anhydrite Fabrics Anhydrite in the Khuff Formation occurs in four principal forms. In core samples, anhydrite nodules of up to 1–2 cm diameter were plainly visible. Anhydrite nodules were originally gypsum nodules that underwent dehydration during burial (Fig. 3), although other forms of anhydrite (veins and poikilotopic crystals) may have formed directly as anhydrite during burial. The nodules displayed typical evaporite textures, including chicken-wire fabrics (cf. Kendall 1984) and isolated nodules in a matrix of fine-grained dolomite. However, thin-section analysis of these nodules revealed two varieties with distinct crystal fabrics (Fig. 4). Some of the nodules were composed of extremely finely crystalline, felted anhydrite crystals (cf. Murray 1964; Maiklem et al. 1969). These finely crystalline nodules contained lath-shaped crystals of less than ; 50 mm length. A second variety of anhydrite nodule was composed of much larger crystals that were typically up to about 250 to 600 mm in length. These will be hereafter referred to as fine and coarse nodules, respectively, which refers to crystal size, and not nodule size.

Thin-section analysis also revealed that anhydrite occurs in what appear to be fractures representing postdepositional, very early diagenetic shrinkage (e.g., dehydration) cracks that were subsequently filled with anhydrite (Fig. 4). This form of anhydrite tends to comprise crystals of anhydrite (up to about 1000 mm in length, although more typically about 500 mm). The fourth form of anhydrite consists of poikilotopic crystals that enclose detrital bioclastic grains and ooids (Fig. 4). These poikilotopic crystals fill primary pore spaces between the detrital components. Poikilotopic anhydrite crystals are typically 500 mm in length but locally may reach up to 3 mm. Gas Geochemistry and Sulfur Isotopes There is a strong link between the reservoir depth (and therefore temperature) and the H2S concentration in the reservoir (Fig. 5A), with TSR kicking in at 4300 m, equivalent to 1408C (Worden et al. 1995). Shallow Khuff reservoirs contain no evidence of TSR; low levels of H2S therein (and their sulfur isotope signature; see below) indicate a contribution of H2S from an organic source or other low-temperature source. H2S increases in the gas phase at the expense of methane-dominated hydrocarbon gases (Fig. 5B). The high-H2S and low-H2S reservoirs have similar values of P/T (pressure divided by temperature), because the presence of H2S is not associated with any overpressure development or thermal anomaly. According to a rearranged Boyle’s Law (P/Ta number of moles of gas per unit volume), the souring is not associated with a net increase in the number of moles of gas per unit volume. The increase in H2S must therefore be associated with an absolute decrease in the amount of total hydrocarbons (Fig 5B). If the soured reservoirs were overpressured (higher value of P), then the number of moles of gas per unit volume would be greater and a plot like Figure 5B could be due to net addition of H2S to a conserved quantity of hydrocarbons. The absence of overpressure in the high H2S reservoirs of the Khuff Formation proves that hydrocarbons are decreasing in absolute volume while the H2S is increasing in concentration. Anhydrite in the Triassic section of the Khuff Formation has a d34S


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FIG. 4.—Photomicrographs showing pre–thermochemical sulfate reaction anhydrite textures. (‘‘a’’ represents anhydrite; ‘‘d’’ represents dolomite): A) finely crystalline nodules (transmitted light, scale bar 5 0.1 mm), B) finely crystalline nodules (back-scattered electron image), C) coarsely crystalline nodules and poikilotopic anhydrite (transmitted light, scale bar 0.2 mm), D) fracture fills (transmitted light, scale bar 5 0.5 mm).

value of 118 to 120‰ CDT. Anhydrite in the Permian section has a d34S value of 110 to 112‰ CDT (Worden and Smalley 1996; Worden et al. 1997). The d34S value of H2S in the low-H2S-concentration shallow reservoirs varies from 0.0 to 13.0‰ CDT and were thus not TSR-related, because TSR typically produces H2S with the same sulfur isotope signature as the parent anhydrite (i.e., there is no sulfur isotope fractionation associated with natural TSR: Machel et al. 1995). Such low sulfur isotope ratios for the shallower H2S might suggest either breakdown of S-rich kerogen or S-rich petroleum components or bacterial sulfate reduction (although this cannot be substantiated). The d34S value of H2S in the deeper reservoirs that contain the higher levels of H2S is about 118‰ CDT for the Triassic part of the section and about 112‰ CDT for the Permian part of the section. Elemental sulfur, a minor by-product of TSR, has a d34S value of 110 to 112‰ CDT in the Permian part of the Khuff Formation and a d34S value of 118 to 120‰ CDT in the Triassic part of the Khuff Formation (Worden et al. 1997). The stratigraphically defined sulfur isotope data show that these reduced forms of sulfur (H2S and elemental sulfur) were derived from anhydrite. The d34S data confirm that shallower (lowertemperature) reservoirs have low concentrations of H2S that had a different origin from the H2S in the deeper (higher-temperature) reservoirs.

Calcite Replacement of Anhydrite At present-day depths greater than a threshold of approximately 4300 m, calcite crystals, visible in hand specimen, can be found within anhydrite nodules, especially at their edges (Fig. 6). Thin-section analysis showed that where calcite is present in small quantities within anhydrite nodules, it occurs as replacive rinds around the surfaces of the nodules. For samples with greater amounts of calcite in the anhydrite, calcite extends progressively towards the core of the anhydrite. The anhydrite reaction clearly occurred from the outside towards the inside (Fig. 6A, B). Calcite crystals within (fine crystalline) anhydrite nodules have a crystal length between 200 mm and several millimeters; i.e., orders of magnitude greater than the anhydrite crystals in the finely crystalline nodules. Replacive calcite at the edges of anhydrite nodules contains innumerable tiny remnant anhydrite inclusions (Fig. 6A–C). This is true even in cases where there is only a small quantity of calcite at the very edge of the anhydrite nodules as well as in examples that have significant calcite growth extending towards the core of the nodule. Such included anhydrite crystals have irregular and corroded outlines. Patches of neighboring anhydrite inclusions typically have identical crystallographic orientation, as indicated by their common positions of optical extinction.

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FIG. 5.—Gas chemistry data. A) Depth versus gas souring index (CH4 /(CH4 1 H2S) for the Khuff Formation. Highly sour wells occur only at depths greater than about 4300 m, though some deep, hot wells have relatively low amounts of H2S. B) Hydrocarbon gas content versus H2S content.

In examples where calcite growth into anhydrite nodules is well developed, which is generally restricted to the finer nodules, the entire edge of a nodule may have been replaced by calcite (albeit with anhydrite inclusions). The style of calcite growth in the coarser varieties of anhydrite is somewhat different. In these cases, calcite tends to grow as a thin, semicontinuous, replacive rind on the edge of the anhydrite yet still occurs at progressively greater depths into the anhydrite single crystals as the amount of calcite increases. The key difference to the style of replacement of the finer crystalline nodules is that this type of anhydrite replacement is typically free of the corroded inclusions found in the fine anhydrite nodules. TSR in the Khuff Formation produced calcite and elemental sulfur (Fig. 6D) as well as H2S (Worden and Smalley 1996; Worden et al. 1997). In order to balance the stoichiometry of the reaction, water must also have been produced. Evidence for the generation of water is present in the record of fluid inclusions and oxygen stable isotopes within the rock (Worden et al. 1996; Worden et al. 1997). Cathodoluminescence Fabrics Cathodoluminescence (CL) images of subhedral calcite crystals within anhydrite nodules display locally extensive concentric zones of variably nonluminescent and dull (orange) luminescent layers (Fig. 7). CL zones in the newly formed calcite appear to follow strictly the outlines of dolomite crystals at the interface between the matrix dolomite and the former anhydrite nodule. The conformity of the calcite CL zones to the outline of neighboring dolomite crystals suggests that the crystallographic orientations of the matrix dolomite and calcite resulting from TSR are similar; i.e., the dolomite is a templating substrate. Fluid-Inclusion Temperatures from Replacive Calcite In the more advanced stages of calcite growth into anhydrite crystals and nodules, calcite contains primary two-phase aqueous inclusions as well as complex multiphase inclusions. Microthermometric analyses of these inclusions produced a dataset of 222 individual measurements of homogenization temperature from five wells. These data show a strikingly unimodal profile (Fig. 8). Data from individual wells or samples can be viewed as simple subsets of the overall profile; there is no evidence to suggest that

different wells or samples have undergone different histories of calcite growth. The lowest homogenization temperature measured was 132.28C. Only three out of 222 measurements are below 1408C. The histogram reveals a mean value of 1658C, although the lowest temperatures are in the range of 135–1458C. Experimental data indicate that fluid inclusions in carbonate minerals are sometimes prone to reequilibration when heated beyond their homogenization temperatures, causing an increase in inclusion volume, a decrease in bulk density, and consequently an increase in homogenization temperature (Prezbindowski and Larese 1987; Goldstein 1986; Meunier 1989). Homogenization temperatures measured on reequilibrated inclusions therefore do not reflect true mineral precipitation temperatures. In replacive calcite, however, several three-phase inclusions were found containing predominantly liquid water, a vapor bubble, and a globule of H2S (or less likely solid elemental sulfur) attached to the vapor bubble. These inclusions bear a strong resemblance to the three-phase inclusions in Roedder (1984) which were reported as probably being H2S-rich. Upon heating, the vapor and the liquid globule shrank simultaneously and ultimately the three phases homogenized at the same temperature. Results of three-phase homogenization range from 140 to 1808C and conform to the distribution defined by twophase inclusion homogenization (Fig. 8). If reequilibration had occurred, the phase behavior of two-phase and three-phase inclusions would be affected differently, and they would give different homogenization temperatures. The conformity between the two datasets (Fig. 8) is important evidence that the data on fluid-inclusion temperatures represent the actual temperature of calcite growth and thermochemical sulfate reduction. Other evidence against resetting was presented by Worden et al. (1995), including the lack of positive skew in the distribution and the lack of correlation between homogenization temperature and inclusion size. Mean fluid-inclusion temperatures from TSR calcite are similar to present-day temperatures, showing that TSR is likely to have been a recent phenomenon. It is thus reasonable to assume that the hydrocarbon gas currently present in the Khuff Formation was the hydrocarbon gas that was present during TSR. Carbon Isotope Data from Replacive Calcite, Methane, and CO2 Calcite intergrown with remaining anhydrite at the edges of nodules (Fig. 6A) and calcite from advanced stages of anhydrite replacement display a


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FIG. 6.—Photographs showing the way in which calcite replaces anhydrite. (‘‘a’’ represents anhydrite; ‘‘d’’ represents dolomite; ‘‘a&c’’ represents intergrown anhydrite and replacive calcite; ‘‘c’’ represents calcite; ‘‘s’’ represents elemental sulfur.) Mixture of back-scatter and ordinary light images. A) Partially reacted white, finely crystalline anhydrite, intergrown TSR calcite with remnant anhydrite appears as a dark rind at the edge of the clear anhydrite nodule. Dolomite matrix is the dark, fine-grained material in the bottom left of the micrograph (transmitted light, scale bar 5 0.1 mm), B) Partially reacted anhydrite; TSR calcite appears as a mid-gray poikilotopic rind at the edge of the bright (strongly electron back-scattering) anhydrite; dolomite is the dark matrix to the bottom right of the image (back-scattered electron image). C) Partially reacted, coarsely crystalline anhydrite; TSR calcite appears as a slightly darker rind (arrowed) at the edge of the snowy-white anhydrite crystal (transmitted light, scale bar 5 0.5 mm). D) Extensively reacted, finely crystalline anhydrite. TSR calcite occurs as large crystals that grew out from the edge of the original anhydrite nodule. The center of the image contains pore-filling epoxy with remnants of anhydrite crystallites. (transmitted light; scale bar 0.4 mm),

wide range of oxygen and carbon isotope data (approximately 0 to 214‰ and 15 to 234‰, respectively; Fig. 9A). There is a positive correlation between d13C and d18O, indicating that there are probably two end-member sources contributing to the replacive calcite. The data have been averaged for the Upper and Lower Khuff for each well (where the well penetrated both the Upper and Lower Khuff). There is a correlation between d13C and d18O for the averaged data (Fig. 9A). Also, the average extent of reaction for each hydrodynamic unit from each well correlates well with the average d13C (Fig. 9B), suggesting progressive change of calcite d13C as TSR advanced. Methane in shallow Khuff Formation reservoirs has a d13C value of about 243‰ PDB (this value being typical of thermogenic gas sources; Hunt 1995). In deep sour Khuff Formation reservoirs, methane has a d13C of about 225‰ PDB (Fig. 9C). CO2 has a d13C of about 26‰ PDB in shallow Khuff Formation reservoirs whereas in deep sour Khuff Formation

reservoirs CO2 has a d13C of about 214‰ PDB. These data suggest that (residual) methane became relatively enriched in 13C while product CO2 became depleted in 13C as TSR advanced. Role of Anhydrite Type on Extent of Reaction The amounts of replacive calcite were measured petrographically for each type of anhydrite in each sample. These values were averaged for each 100 m depth interval, and the data for the four different forms of anhydrite were plotted as a function of depth (Fig. 10). This figure contains much key information about the TSR process: ● The TSR process starts at ; 4300 m depth (corresponding to a present-day temperature of 1408C) in all of the anhydrite types apart from the coarsest (the poikilotopic variety), which shows signs of reaction only below ; 4500 m.

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FIG. 7.—CL micrographs of growth zones in calcite following the crystallographic orientation of the dolomite crystal substrate. (‘‘d’’ represents dolomite; ‘‘c’’ represents calcite, ‘‘Ø’’ represents porosity). Calcite replacement after anhydrite in a dolomite matrix. Calcite has mostly dull luminescence with zoning illustrated by slight variation in luminescence. The edges of calcite crystals have bright luminescence adjacent to the dolomite matrix. Arrows indicate CL growth zones in the TSR calcite that lie parallel to dolomite crystals at the center of the growth zones.

● Degree of anhydrite alteration increases with depth below 4300 m. The degree of alteration at any given depth is clearly related to anhydrite crystal size. The fine nodules have more calcite within them than the coarse nodules (cf. Fig. 6). The coarse nodules in turn appear to contain more calcite than the veins of anhydrite or the poikilotopic form of anhydrite, which contains the smallest amount of calcite. In cases where different forms of anhydrite coexist in a single sample, the form with the smallest crystals of anhydrite always has the greatest amount of calcite. ● The extent of TSR, as reflected by the amount of replacive calcite, increases during burial up to a maximum at about 5000 m. Thereafter the extent of reaction with depth appears to remain approximately constant. This suggests that TSR effectively ceased once rocks were buried to 5000 m. This suggests that there must be a mechanism for inhibiting further TSR after a certain extent of the reaction. Note that, when reaction ceases, the finer anhydrite crystal sizes show a higher total degree of reaction than the coarser varieties. Clay-Mineral Volumes and Textures The microscale fabric of the Khuff Formation was assessed using backscattered electron imaging. The Khuff Formation is dominated by dolomite that forms a matrix hosting the nodules and other patches of anhydrite. The dolomite matrix has up to about 10% porosity developed between the micron-scale rhombic dolomite crystals. There is a regionally variable (on the well scale) silicate mineral assemblage of K-feldspar, quartz, and illite within the dolomite matrix in certain wells in the Khuff Formation (Fig. 11). Semiquantitative analysis of secondary X-rays revealed that illite in the shallow Khuff Formation (, 4300 m) has a significant iron component. In the deeper Khuff Formation (. 4300 m), this iron peak is generally missing from the illite. However, small flakes of a bright (efficient electron back-scattering) mineral occur in and on the illite fibers. Microprobe analysis confirmed that this mineral was pyrite (FeS2). Many samples had very small quantities of illite, but several samples displayed up to about 4 percent. The bulk-rock quantities of illite and pyrite were determined by recalculation using the pre- and post-acid-digestion

FIG. 8.—Data on fluid-inclusion homogenization temperatures from two-phase and three-phase aqueous inclusions from thermochemical sulfate-reduction calcite, which has replaced anhydrite. Both three-phase and two-phase inclusions have a similar range of homogenization temperatures. The lowest temperature is 1328C (one inclusion). There is significant calcite growth resulting from thermochemical sulfate reduction only at temperatures greater than 1408C.

masses. The quantities of these minerals were averaged for each hydrodynamic zone in each well (i.e., Upper and Lower Khuff for each well where both were penetrated; Fig. 11). DISCUSSION

Origin of Calcite in Anhydrite Nodules and Crystals Calcite formed within anhydrite nodules is clearly the result of in situ thermochemical sulfate reduction. The reaction commenced during burial once temperatures had exceeded 1408C, as illustrated by (1) the depth (temperature) at which hydrocarbon gases (methane) starts to be replaced by H2S (Fig. 5), (2) the depth (temperature) at which anhydrite starts to be replaced by calcite (Fig. 10), and (3) the fluid-inclusion homogenization temperatures (Fig. 8). In the case of the Khuff Formation, the reducing agent was predominantly methane gas, as proven by (1) the negative correlation between amounts of methane and H2S (Fig. 5B), (2) the progressively increasing d13C of remaining methane as TSR proceeds (Fig. 9C), (3) the absence of liquid-phase petroleum compounds and solid-phase organic matter, and (4) the progressively decreasing d13C of carbon in the replacive calcite (Fig. 9B, C) and CO2 gas (Fig. 9C). The spread of carbon isotope values in the replacive calcite shows that at the earliest stages of TSR the ambient formation water was dominated by bicarbonate derived from dissolution of the matrix dolomite. As TSR commenced, the first-formed aqueous carbonate species plus the newly dissolved calcium from the anhydrite led to supersaturation with respect to calcite. However, this calcite was initially dominated by the dolomite-derived carbonate and thus inherited the dolomite’s heavier carbon isotope signature. As TSR proceeded, the methane-derived carbonate came to dominate progressively, thus leading to ever more negative d13C values in the resulting calcite (Fig. 9A, B).


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FIG. 9.—Stable-isotope data from TSR calcite. A) Plot of carbon against oxygen isotope data. B) Plot of carbon isotopes versus (petrographically defined) extent of reaction. All the data are from TSR calcite. The data show that the carbon isotope composition of the system evolved towards 13C-depleted values during TSR. This reflects the gradual domination of methane-derived aqueous carbonate over matrix dolomite-derived bicarbonate. C) Plot of carbon isotopes for gases and calcite as a function of the extent of anhydrite alteration (note that the scale is different from that in Part B). Methane carbon became isotopically heavier while CO2 and calcite became isotopically lighter during TSR.

Reactions with the initially small amounts of ethane and propane gases have also been invoked for TSR in the Khuff Formation (Worden and Smalley 1996), although the low percentages of these gases in the lowH2S fields testifies that that they were only a relatively minor part of the TSR story. The kinetics of a direct gas–solid reaction would probably be prohibitively slow (e.g., Lasaga and Kirkpatrick 1981). This, together with carbon isotope evidence (discussed above and in Worden and Smalley 1996), suggests that reaction of aqueous sulfate and hydrocarbon gas occurred in aqueous solution. TSR between methane and anhydrite and ethane and anhydrite can thus be written as a series of processes including anhydrite

dissolution (R1), redox reaction between dissolved methane and sulfate (R2), and then precipitation of calcite (R3): 1 22 CaSO4(s) → Ca 2(aq) 1 SO4(aq) 1 2 22 1 CH SO4(aq) 4(aq) 1 H (aq) → HCO3(aq) 1 H2S (g) 1 H2O 1 21 1 HCO 2 Ca (aq) 3(aq) → CaCO3(s) 1 H (aq)

(R1) (R2) (R3)

For the analogous reaction series between other alkanes (such as ethane) and anhydrite the processes are similar but the reducing agent has a lower

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FIG. 10.—Depth versus the average extent of anhydrite replacement by calcite for the different types of anhydrite. The anhydrite types with the smaller crystal size are more reacted at any given burial depth.

C:H ratio and thus results in elemental sulfur (R4), which can then react with methane to produce H2S and CO2 (R5): 1 22 2CaSO4(s) → 2Ca 2(aq) 1 2SO4(aq)

(R1)

1 2 22 1 C H 2SO4(aq) 4 6(aq) 1 2H (aq) → 2HCO3(aq) 1 S8

1 H2S (g) 1 2H2O 21 1 2HCO 2 1 2Ca(aq) 3(aq) → 2CaCO3(s) 1 2H (aq)

4S8 1 CH4(aq) 12H2O → CO2(g) 1 4H2S (g)

FIG. 11.—Extent of anhydrite replacement by calcite versus gas souring index (CH4 /(CH4 1 H2S) for the Khuff Formation. If H2S concentration had been controlled only by the degree of anhydrite alteration, then all highly reacted wells would have abundant H2S, but some wells have advanced anhydrite alteration yet relatively low souring. The average quantities of illite for these fields are represented as the percentage figures on the diagram. The anomalous field has the highest quantity of illite, suggesting that the reservoir has scrubbed H2S.

(R4) (R3) (R5)

The strongest lines of direct evidence supporting the occurrence of reactions of this type in the Khuff Formation include the carbon isotope evidence from calcite and CO2 (e.g., Worden and Smalley 1996), the generation of water in the reservoir (evidenced by data on oxygen isotopes and fluid-inclusion salinity; e.g., Worden et al. 1996), and the loss of methane and accumulation of H2S simultaneous with anhydrite replacement by calcite (Worden et al. 1995). Note that these conclusions from empirical observations of natural TSR reactions differ somewhat from laboratory work, which has thus far failed to achieve reaction between methane and aqueous sulfate. This is most likely to be due to the geologically unrealistic conditions (fluid–rock ratio, temperature, reactant chemistry, time scale) of typical laboratory simulations (e.g., see Rubie and Thompson 1985 for a review of the generic problems of relating data on experimental rates and mechanisms to real systems). The data from the Khuff Sour Gas Province lead to the inevitable conclusion that, over geological time scales in real rocks, the reaction of light hydrocarbon gases (e.g., methane) and anhydrite does actually occur in high-temperature reservoirs despite the prevailing lack of experimental evidence. Wettability and the Site of Reaction: Gas Zone, Transition Zone, or Water Zone? In a gas field, water is clearly the dominant fluid in the water zone but is always present as a residual phase in the gas zone (i.e., water remains in the pores as a residuum even after petroleum gas has entered the struc-

ture). Between the two is a transition zone of intermediate water saturation. The transition zone is usually thin, being on the scale of meters to tens of meters, depending largely on rock fabric. Machel et al. (1995) concluded that TSR occurred predominantly in the transition zone in Nisku Reef reservoirs in Canada. We have observed no such spatial control on TSR for the Khuff Formation. Anhydrite is equally altered to calcite throughout the present gas zone. There is also no pattern or trend reflecting increased TSR through the current transition zone. We thus deduce that even the low water saturations in the gas zone are sufficient to allow TSR reactions. Minerals have different surface affinities for different types of fluids. Some mineral surfaces have a higher affinity for petroleum compounds than for water (e.g., many carbonate minerals, some clay minerals, some Fe-rich minerals; Barclay and Worden 2000). Polar (nitrogen-, oxygen-, or sulfurrich) petroleum compounds tend to have a greater affinity for mineral surfaces than do petroleum compounds such as alkanes. Anhydrite has a greater surface affinity for water than hydrocarbon gases, and methane is nonpolar. Consequently, in a methane–anhydrite system, exposed anhydrite crystals have a continuous coating of water even in gas fields (the system would be described as ‘‘water-wet’’ by petrophysicists; cf. Barclay and Worden 2000). Thus, even at the top of the Khuff gas column, there will be a water film of finite thickness on mineral surfaces. Reaction between methane and dissolved anhydrite probably happened on the exposed surfaces of mineral grains within the residual water film clinging to grain surfaces. Possible Rate-Controlling Steps during TSR The overall process can be considered to occur in five steps (Fig. 12): (1) dissolution of reactants, (2) transport to the site of reaction, (3) chemical redox reaction, (4) transport to the site of precipitation, and (5) nucleation and precipitation of the solid products of reaction. The length scales over which transport occurs may be millimeters or even micrometers. Growth of the solid reaction product, calcite, may be rate-limiting, because retarded


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FIG. 12.—Schematic representation of the rate-controlling steps involving dissolution and supply, reaction, and then transport away from the site of reaction followed by nucleation and growth of solid reaction products. The slowest step controls the overall rate of reaction.

growth of calcite would lead to massively elevated bicarbonate concentrations (see reaction 3 listed above). This elevated bicarbonate, in turn, would reduce ultimately the rate of sulfate–methane reaction, because the concentrations of the products of reaction 2 (listed above) would increase, leading to a pseudo-equilibrium state (in a closed system). In other words, the chemical driving force for reaction of sulfate with methane could be reduced if the aqueous reaction products achieved high concentrations. The rate of the overall TSR/calcite precipitation process will be limited by the slowest of the five steps (Fig. 12). Chemical reaction between anhydrite and methane is thermodynamically favorable (i.e., it leads to reduced Gibbs free energy) under practically all diagenetic conditions (see Worden and Smalley 1996), yet seems to occur in nature only at elevated temperatures (Worden et al. 1995). This suggests that at least one of the five steps, listed above, is prohibitively slow at low temperatures. The rate of the chemical reaction between hydrocarbons (but not methane, as discussed earlier) and aqueous sulfate (i.e., step 3) has been assessed experimentally (e.g., Goldhaber and Orr 1995) and is typically assumed to be the rate-limiting step for natural TSR at low temperatures, i.e., slow reaction kinetics inhibit low-temperature TSR. In the following sections we investigate which steps control TSR rate during various stages of reaction at high temperature. Evidence from Sulfur Isotopes According to experimental data, sulfur isotopes should fractionate between sulfate and sulfide during TSR, with the lighter isotopes preferentially entering the reduced sulfur compounds (e.g., H2S and elemental sulfur). This should lead to the gaseous H2S being of the order of 15‰ lighter than the source sulfate (see experimental data summarized by Machel et al. 1995). However, this is not found in the Khuff Formation, where the H2S has the same sulfur isotope ratio as the local anhydrite (Fig. 5; Worden and Smalley 1996; Worden et al. 1997). The sulfur isotope data suggest that every batch of sulfate that dissolved in water reacted totally with hydrocarbon gases. If more sulfate had dissolved than could instantaneously react, the produced H2S would have been isotopically lighter than the sulfate. The isotopic data therefore suggest that supply of sulfate to the site of reaction (i.e., either of steps 1 or 2 above), and not the rate of chemical reaction, may be the rate-controlling step once TSR had started. The isotopic similarity between the sulfate source and reduced forms of sulfur (sulfides) in TSR systems has been noted previously for several sourgas provinces (e.g., review by Machel et al. 1995). TSR rate being controlled by the rate of anhydrite supply to the site of reaction may thus be a common factor in many TSR provinces. Nucleation of Calcite on Dolomite Substrate A significant kinetic barrier to many geochemical reactions is the activation energy of nucleation of solid reaction products (the initial part of

the precipitation step 5, discussed above). In some reactions, the solid products grow directly in the pore fluid; this is known as homogeneous nucleation. Other reactions manage to overcome the barrier to progress by epitaxial growth (e.g., quartz overgrowths on detrital quartz, calcite overgrowths on shell detritus); in these cases the need to nucleate is overcome by simply growing the new material on a mineralogically similar substrate. An alternative mechanism, known as topotaxial nucleation, is for the reaction product to grow directly on a mineralogically different substrate: a new mineral forces a crystallographic fit to another (preexisting) mineral. Topotaxy is reasonably well understood in metamorphic terranes (e.g., Kfeldspar replacing white micas during heating; Worden et al. 1992) but is less well known for diagenetic processes (e.g., triclinic microcline growing on monoclinic sanidine; Worden and Rushton 1992). Reactant calcite seems to have nucleated topotaxially onto dolomite during TSR in the Khuff Formation. CL micrographs (e.g., Fig. 7) show subtle growth zones in the TSR calcite; these closely mimic the outlines of discrete dolomite crystals at the very edges of the anhydrite nodules, strongly suggesting continuity of the crystal structure despite the different crystallographic characteristics of calcite and dolomite. It is likely that calcite nucleated on matrix dolomite crystals directly adjacent to anhydrite and adopted the general crystallographic orientation of the dolomite substrate. It must have been energetically easier for calcite to nucleate on dolomite than to produce new nuclei. Further calcite growth was effectively epitaxial, inasmuch as the active substrate was then calcite. The key to this process is the first topotaxial phase of calcite growth, which will have reduced the overall activation energy of the TSR process relative to homogeneous nucleation. In the Khuff Formation, a dolomite substrate is equally present in all parts of the reservoir. It is thus unlikely that nucleation or growth of reaction products was at any time the rate-controlling step for TSR. It is conceivable, however, that direct replacement of anhydrite by calcite could be energetically difficult in other reservoirs lacking a carbonate substrate (e.g., sandstones) and could thus be rate-controlling in these circumstances. Anhydrite Crystal Size and the Extent of TSR Figure 10 shows the variable degree of alteration of the four types (crystal sizes) of anhydrite. These patterns show that anhydrite crystal size exerts a strong control on the degree of TSR experienced by different types of anhydrite; the degree of reaction decreases for larger crystal sizes for any depth of burial (Fig. 10). The textural control on anhydrite reaction is probably a simple function of the accessibility of fluid reactants to the anhydrite. Smaller crystals provide a greater ratio of surface area to volume, thus facilitating the dissolution step (1) discussed above. The smaller crystals also provide a more extensive network of grain boundaries along which reactants can move, thus facilitating the transport step (2) discussed above. The key consequence of this is that coarsely crystalline anhydrite-rich fa-

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cies will undergo less TSR and H2S generation (before calcite isolates anhydrite) than finely crystalline anhydrite-rich facies. Calcite Armoring Limits Extent of Reaction Clusters of anhydrite crystals in the Khuff Formation have no intergranular porosity and tend to form interlocking mosaics that would have negligible permeability to water or methane. The centers of anhydrite nodules and the cores of individual anhydrite crystals were initially more isolated from reactant methane than the outer surfaces of crystals or nodules where hydrocarbon gases could gain easy access to anhydrite by way of the porous dolomite matrix. The result was the initial growth of replacive calcite only at the very edges of anhydrite nodules or crystals. Petrographic examination revealed that, at the early stages of anhydrite replacement, little new porosity appears to have been created within the anhydrite. Space created by anhydrite dissolution and reaction was largely filled by replacive calcite. This suggests that aqueous calcium (from dissolved anhydrite) and newly formed bicarbonate (from TSR) did not travel far from the site of the redox reaction. TSR led to the corrosion of the anhydrite nodules along crystal surfaces and, before entirely replacing individual crystals of anhydrite, it went on to attack the boundaries of other anhydrite crystals immediately in front of the zone of TSR deeper inside the anhydrite nodule. By this mechanism, calcite began to penetrate into anhydrite nodules, leaving behind severely corroded anhydrite relict crystals isolated within crystalline calcite. This texture (Fig. 6) strongly suggests that the replacive calcite is effective in isolating remnant anhydrite from reactive hydrocarbon gases. The precipitation of calcite reaction product close to the site of anhydrite dissolution and subsequent TSR provided a mechanism for eventually slowing down and arresting the TSR process, as the data on the degree of reaction versus depth indicate (Fig. 10). As the calcite armoring built up, the cores of anhydrite nodules (and individual crystals) were effectively isolated from petroleum compounds. Thus, although the kinetics of the chemical reaction between dissolved sulfate and hydrocarbon gases are clearly favorable, the reaction progress is ultimately limited by the inability of one or more of the reactants to gain access to the site of reaction. Evolution of Rate-Controlling Steps during TSR A critical part of any scheme for prediction is to understand what step controls the process. In the case of TSR in the Khuff Formation, there seem to have been several rate-controlling steps operating at different stages in the process: ● The reaction started at about 1408C presumably because the rate of chemical reaction between aqueous sulfate and dissolved methane was too slow at lower temperatures. The rate of reaction must have been rate-limiting prior to and at the onset of reaction. ● Once the reaction started, however, it must have been controlled by the rate of arrival of aqueous sulfate to the reaction site. If it had been controlled by the rate of chemical reaction there would have been fractionation of sulfur isotopes between the sulfate and the sulfide. The rate of supply was probably controlled by the rate of dissolution of anhydrite (as opposed to the rate of transport) because the reaction occurred very close to the point of dissolution. Finely crystalline anhydrite undergoes TSR at a lower temperature than coarsely crystalline anhydrite (Fig. 10), also suggesting a control by surface area, and thus anhydrite dissolution. ● As TSR proceeded during burial and heating, the reaction became transport controlled as calcite began to isolate the remaining anhydrite from reactive dissolved methane. Ultimately, calcite sealed off anhydrite from methane. The transition from dissolution to transport control is probably reflected by the inflection point on the curves in Figure 10 (e.g., a depth of about 4700 m for the finely crystalline nodules). This suggests a sudden rather than a progressive change in the rate-controlling step.

H2S Scrubbing in the Khuff Formation Removal of H2S in sedimentary systems can be an important control on the overall concentration of H2S in oil and gas fields (Orr 1977). The typical absence of H2S in clastic reservoirs is likely to be a function of the removal of H2S from the gaseous phase by reaction with iron-bearing minerals (e.g., Hunt 1995). Removal of H2S has also been proposed for carbonate oil and gas fields (e.g., Orr 1977; Wade et al. 1989) despite the lower total iron contents of carbonates relative to clastics. An important observation in the Khuff reservoirs is that the degree of anhydrite alteration to calcite is not always matched by the amount of H2S in the gas (Fig. 5). The places where this disparity is most obvious also contain the greatest quantities of illite. The growth of pyrite from the iron component within illite presumably drives the removal of H2S from the gas (Fig. 11). Thus, the present-day content of H2S is a function not only of the degree of anhydrite replacement but also of the reactivity of the rock to H2S. One well with petrographic evidence of advanced TSR should have a geochemical gas souring index (GSI 5 CH4 /[CH41 H2S]) of about 0.70, on the basis of the degree of reaction, but in fact has an index of 0.20 because of the abundance of illite (Fig. 11). This suggests that indigenous clay minerals (or other Fe-bearing minerals) can be effective in maintaining the H2S at relatively low levels even in carbonate reservoirs. The relatively large quantity of pyrite found in this well corroborates the notion that H2S loss was due to illite abundance (Fig. 11). Previous work on H2S removal in carbonate reservoirs by reaction with clay minerals in shales showed a trend of decreasing H2S with increasing proximity to the shale (Wade et al. 1989). No such pattern has been observed in the Khuff Formation, because the removal of H2S was the result of reaction with dispersed indigenous clay minerals (and not those in neighboring strata). CONCLUSIONS

(1) Nucleation of calcite during thermochemical sulfate reduction has occurred via topotaxial nucleation on dolomite matrix crystals. This process presumably lowers the overall activation energy and thus facilitates reaction. (2) The crystal size of the reactant anhydrite played a critical part in controlling the rate and final extent of reaction. Nodules consisting of smaller anhydrite crystals tend to be more reactive than are those comprising larger crystals. (3) Armoring of the anhydrite occurs as the calcite reaction product precipitated around the reacting anhydrite. The remaining anhydrite becomes effectively isolated from the reactive system (i.e., methane) once a calcite rind of sufficient thickness has enclosed it. (4) The ability of the reservoir to retain H2S-rich gas is a function of the Fe-bearing siliciclastic content of the carbonate reservoir. Minor claybearing units may provide a mechanism for scrubbing H2S from the gas phase by precipitating the sulfide gas as pyrite (as indicated by the greater amounts of Fe-bearing illite and pyrite in these units). (5) Prediction of H2S contents of natural hydrocarbon gases demands more than a simple understanding of TSR thermodynamics and kinetics. It is also important to consider the texture of the anhydrite in the reservoir and the possible presence of Fe-bearing minerals that could provide local sinks for H2S. ACKNOWLEDGMENTS

This work was funded and supported by the Abu Dhabi National Oil Company and BP. Norman Oxtoby is acknowledged for his excellent fluid inclusion work. Journal reviewers Marty Goldhaber, Hans Machel, and Bruce Fouke are thanked for their critical input. Associate editor Timothy Lyons is thanked for all his helpful work (and for posing some interesting questions about earlier versions of this paper).

THERMOCHEMICAL SULFATE REDUCTION Emma Heasley is also thanked for careful and critical appraisal of the manuscript at an advanced stage. REFERENCES

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