Diagenesis of the Amposta Offshore Oil Reservoir (Amposta Marino C2 Well, Lower Cretaceous, Valencia Trough, Spain)

Geoﬂuids (2010) 10, 314–333 doi: 10.1111/j.1468-8123.2009.00266.x

Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well, Lower Cretaceous, Valencia Trough, Spain)

E. PLAYA` 1 , A. TRAVE´ 1 ,M.A.CAJA2 ,R.SALAS1 ANDJ.D.MARTI´ N-MARTI´ N 1 1Departament de Geoquı´mica, Petrologia i Prospeccio´ Geolo`gica, Facultat de Geologia, Universitat de Barcelona, C ⁄ Martı´ i Franque`ss⁄ n, Barcelona, Spain; 2Departamento de Petrologı´a y Geoquı´mica, Facultad de Ciencias Geolo´gicas, Universidad Complutense de Madrid, Avda. Jose Antonio Novais, Madrid, Spain

ABSTRACT

Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre-Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene-early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host-rock, suggesting a high degree of fluid-rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher 87Sr ⁄ 86Sr ratios than the host-limestones. These CS3 karstic infillings recrystallized early within evolved-meteoric waters having very little interaction with the host-rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic 87Sr ⁄ 86Sr values, indicating low fluid-rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm).

Key words: limestones, calcite cements, fracture porosity, isotopes, Valencia Trough

Received 29 June 2009; accepted 20 October 2009

Corresponding author: E. Playa` , Departament de Geoquı´mica, Petrologia i Prospeccio´ Geolo` gica, Facultat de Geologia, Universitat de Barcelona, C ⁄ Martı´ i Franque` s, s ⁄ n, 08028 Barcelona, Spain. Email: [email protected]. Tel: 34 3 4021401. Fax: 34 3 4021340.

Geoﬂuids (2010) 10, 314–333

been also made in Miocene clastics and limestones (Merten INTRODUCTION 2006). The Amposta oil field was discovery by Shell Espanã The Valencia Trough is a Neogene oil-producing basin in 1970, and began producing oil in 1973. where accumulations are in Mesozoic and Neogene rocks. Excellent secondary porosity was formed by deep karstif- Main reservoir rocks are fractured and karstified Jurassic ication during the Palaeogene exposure; intense fracturing and Lower Cretaceous limestones located at the top of brecciation and dolomitization also promoted porosity tilted fault blocks overlain by Miocene sediments; these enhancement (Orlopp 1988). In the Mediterranean region, palaeo-highs (buried hills) developed during a period of the Mesozoic-Tertiary boundary developed as a posttectonic Neogene rifting. Smaller subeconomic discoveries have unconformity after the main phase of the Palaeogene

2010 Blackwell Publishing Ltd Diagenesis of the Amposta offshore oil reservoir 315

Alpine orogeny. It is a multiple, composite unconformity (1) How many fracturing events have affected these car- formed by superposition, or stacking, of several Palaeogene bonate reservoirs and when did they occur in relation and Miocene unconformities that truncate the fractured to the tectonic evolution of the basin? Mesozoic carbonate sequences at various depths. Upper (2) How many generations of fracture-filling cements can Jurassic and Lower Cretaceous carbonate sequences are be identified and how do they relate to the fracturing well exposed onshore and varied degrees of karstification events? are common in outcrops, depending on their subaerial (3) Were the fracture-filling cements internally- or exter- exposure period. These carbonate beds have caves, solu- nally-derived (open or closed-system diagenesis)? tion-enlarged fractures, vuggy porosity and large caverns, (4) What are the sources of formation water that facilitated very similar to those documented in the nearby Amposta cementation of fractures; meteoric water, deep basin field (Watson 1982). Several palaeokarst horizons devel- water, etc.? oped from presumed Lower Eocene (Esteban & Klappa 1983) and related to sedimentary discontinuities have been GEOLOGICAL FRAMEWORK recognized onshore (Esteban 1973; Esteban & Julia` 1973), only slightly modified by present-day karstic pro- The Amposta oil field is located in the Valencia Trough cesses. Karst is as old as late Cretaceous in the offshore (northern Gulf of Valencia, NW Mediterranean), stretching zone (Esteban 1991). Well-preserved examples of karstic between the eastern Iberian Peninsula and the Balearic dissolution have been described in the neighbouring Casa- Islands, at 22 km offshore south of the Ebro delta blanca oil field (Esteban 1991; Lomando et al. 1993) and (Fig. 1). The Catalan Coastal Ranges constitute the wes- in the onshore Catalan Coastal Ranges (Esteban 1973; tern boundary of the Valencia Trough. The regional struc- Calvet et al. 1983; Klimowitz et al. 2005). ture of the western Mediterranean area is a direct In this paper, the different cements, fractures and stylo- consequence of the Neogene (late Oligocene-early Mio- lites and karstic sediments are described for the main oil cene) extensional tectonic activity. The Valencia Trough reservoir rock of Amposta Marino C2 well from the displays a well-developed horst and graben structure with a Amposta field. The fluid flow history before, during and general NE-SW trend. Pre-Neogene tectonic structures are after oil migration is inferred from petrological and geo- difficult to recognize in the Gulf of Valencia. The NE-SW chemical characterization of the different cements. The rift units are limited by transverse NW-SE faults with a main questions being addressed are thus: marked strike-slip component (high-relief accommodation

Fig. 1. Simplified geological map of the Catalan Costal Ranges and location of the main oil fields in the Valencia Trough (western Mediterranean Sea), showing the detail of the Amposta oil field and drilled wells. Location of the studied well (C-2, Amposta Marino well) (adapted from Losantos et al. 1989).

2010 Blackwell Publishing Ltd, Geofluids, 10, 314–333 316 E. PLAYA` et al. zones). The exposed up-thrown ridges in the Catalan chromatograph operating at 40 kv and 30 mA) was used Coastal Ranges are very similar to the Valencia Trough for identification of the main mineral phases. graben system buried beneath the Neogene sediments off- The samples were carefully selected following the petro- shore (Roca et al. 1999; Vegas & de Vicente 2004). The logical study in order to characterize the geochemical com- Amposta oil field is located in an elongated tilted bloc of position of the different cement phases. Carbon-coated thin Mesozoic carbonates (25 km long), dipping toward the sections were analysed using a CAMECA SX-50 microprobe ESE at 20 (Fig. 2A). The western margin is bounded by a equipped with four vertically-displayed WD X-ray spectrom- major extensional fault, which has a NE-SW direction par- eters and operating at 20 kV of excitation potential, 10 lm allel to the shoreline, and with a throw in excess of of beam diameter and 10 nA of current intensity for the Ca 1000 m (Seemann et al. 1990). and Mg analysis and 50 nA for Mn, Fe, Sr and Na. The A thick sedimentary succession is recorded above the detection limits are 440 ppm for Ca, 380 ppm for Mg, Hercynian basement; this constitutes a largely deformed 350 ppm for Mn, 340 ppm for Fe, 209 ppm for Sr and Palaeozoic–Mesozoic sequence and Neogene-Quaternary 240 ppm for Na. Analytical precision of the major elements cover, separated by an important erosive surface (Fig. 2B). averaged 6% standard error at 3r confidence level. Two major rifting stages occurred during the Mesozoic For carbon and oxygen stable isotope determinations, (Salas & Casas 1993; Salas et al. 2001). The first stage 100–500 lg of 66 host-limestones and calcite cements affected the Upper Permian and Triassic units (red beds, were obtained using a dental microdrill. Samples were siliciclastics, carbonates and evaporites), whereas the second reacted with 103% H3PO4 for 2 min in vacuum at 70C. stage affected the Jurassic to Lower Cretaceous limestones. The CO2 was analysed using an carbonate Kiel device con- The Upper Cretaceous units are not preserved in the study nected on-line to a Finnigan MAT252 mass spectrometer. area. The values, expressed in per mil with respect to the Vienna Part of the Valencia Trough and the present Catalan Pee Dee Belemnite (VPDB) standard, had a precision of Coastal Ranges were uplifted during the regional Palaeo- ±0.02& VPDB for d13C and ±0.08& VPDB for d18O. gene contraction (Gaspar-Escribano et al. 2004). During For 87Sr ⁄ 86Sr ratio analyses, 14 samples were micro- this period (late Eocene-early Oligocene), Mesozoic car- drilled and analysed in the CAI of Universidad Complu- bonates were uplifted and intensively eroded. Later, during tense de Madrid. The powdered samples were converted to the late Oligocene-early Miocene, the compressional struc- chlorides by leaching them several times in HCl 2.5N. The tures were overprinted by extensional faulting related to a final liquid samples were loaded into a cation exchange col- regional rifting stage, giving rise to the NE-SW trending umn with DOVEX 52Wx12 200 ⁄ 400 mesh resin and the structures. Although some areas (e.g. Barcelona offshore gra- strontium separates were analysed with a VG Sector 54 ben) contain basal Palaeogene continental facies (Martinez TIMS, previously loaded onto a Ta filament. The standards del Olmo & Esteban 1983), the transitional and marine and reference samples used, NBS-987, averaged 0.710255 Neogene sediments (synrift and postrift) were mostly (n = 9), indicating a weighted reproducibility within the ) deposited directly and unconformably up on the Mesozoic range of ±4 · 10 5. The analytical accurancy was 0.01%. basement in the Valencia Trough. Compressive inversion All the results were normalized to 87Sr ⁄ 86Sr = 0.1194. of previous normal faults together with the Mediterranean Four pyrite samples, 150–350 lg, were analysed for their 34 Sea level dropping during the Messinian resulted in a d S composition. The SO2 gasses produced by the new phase of extensive erosion. The Plio-Quaternary sand- sulphides were analysed on a continuous-flow elemental- stones and clays from the Ebro Group were deposited analyser Finnigan DELTA plus XP mass spectrometer with unconformably over the Miocene clastics of the Castellon Finnigan Mat CHN 1108 analyser for sulphur. The values Group (Fig. 2). are given relative to the VCDT (Vienna Canõn Diablo Troilite) reference, offering precision of ±0.4&. METHODS LOWER CRETACEOUS HOST-LIMESTONES About 23 samples from Amposta Marino C-2 well, at true vertical depth (TVD) between 1874 and 1957 m below The host-limestones (usually brecciated) are wackestones sea level (Table 1, Fig. 3) were collected for petrological to packstones with dull-red luminescence and red blue flu- and geochemical studies of the host-limestones and orescence. They are mainly composed of marine fauna, cements filling fractures and stylolites. Standard thin sec- such as benthic foraminifera, silicified sponge spicules, and tions were examined using optical, cathodoluminescence echinoderm fragments. Foraminifera consist principally of (Technosyn Cold Model 8200 MkII; 16–19 kv and Orbitolinopsis, Nautiloculina, Pseudocyclamina and Choffa- 350 lA) and fluorescence microscopes (Nikon equipped tella, and miliolids, as well as green algae. The uppermost with a blue-violet incident light excitation filter). X-ray dif- part of the sampled interval (sample 1874) is a wacke- fraction (Siemens D-500 with secondary graphite-mono- stone characterized by freshwater facies (girogonites of

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 317

Fig. 2. (A) Geological cross-section of the Amposta field. (B) Stratigraphy of the Amposta oil field. Slightly modified from Seemann et al. (1990).

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 318 E. PLAYA` et al.

Table 1 Observed fracture, cement and stylolite types and presence of kaolinite, pyrite (in fractures type C), barite and oil in the studied samples of the Amposta Marino C-2 well.

Fracture type Randomly Cement ⁄ sediment type Sample oriented (Depth, m.) A B Cstylolites CC1 CC2 CS3r CS3g CC4 Barite Kaolinite Pyrite (in fractures) Oil

1874 x x x x x x x x x

1893.3 x x x x (CC4) x

1895.5 x x x x x

1896 x x x x

1906 x x x x (CC4) x

1908.8 x x x

1909.5 x x x

1910.3 x x x x x x x

1910.8 x x x x x x (CC4) x

1942 x x x x x x

1943.5 x x x (CC4)

1944.6 x x x x x x

1945.8 x x x x

1951.4 x x x x x x (CC4)

1952.6 x x x x x x x x x (CS3) x

1952.7 x x x x x x (CS3, CC4) x

1954.3 x x x x x x x

1957 x x x x x x

CC1, non-luminescent calcite cement 1; CC2, blocky non to dull-red luminescent calcite cement 2; CS3r, reddish microsparite calcite sediment 3; CS3g, greenish microsparite calcite sediment 3; CC4, blocky calcite cement 4.

charophytes –Atoporochara- and ostracods), and shallow The 87Sr ⁄ 86Sr ratios of the limestones range from 0.70748 marine dasycladacean algae. Small framboidal pyrites are to 0.70763 (mean value of 0.70755; Table 4; Fig. 6). disseminated in the host-limestones or concentrated in the stylolitic planes, as insoluble residue. FRACTURE TYPES, CEMENTS AND The reservoir interval is dated as Valanginian to Aptian STYLOLITES (Seemann et al. 1990); the presence of Atoporochara and Orbitolinopsis indicates a Barremian–Aptian age (Hardenbol Three main types of fractures, related to specific calcite et al. 1998). cement or sediment infillings (Table 1, Fig. 7) have been The marine limestones and the uppermost brackish car- identified. bonate facies do not reveal significant differences in their minor and trace element contents, except for the Fe con- Fracture type A and calcite cement 1 (CC1) tents (Table 2, Fig. 4). The host-limestones are characterized by Mg, Sr and Na contents from 1015 to 4910 ppm, Fractures of type A are part of the oldest fracture system. between 355 and 1410 and up to 435 ppm, respectively; These fractures are vertical to subvertical, thin fractures Mn contents are below detection limit in all cases. Fe con- (1–3 mm thick) with straight walls, totally occluded by tent display higher values in the uppermost brackish facies calcite cement 1 (CC1) (Fig. 8A,B). than in the marine limestones (from 875 to 1815 ppm, The CC1 constitute anhedral non-luminescent calcite and up to 560, respectively; Table 2). Oxygen isotopic crystals, from 15 to 120 lm in size (Fig. 8G,H). These compositions range from )4.2 to )2.2 & VPDB, and calcite cements are characterized by Mg from 3625 to d13C between +0.9 and +1.5 & VPDB (Table 3, Fig. 5). 6320 ppm, Sr from 345 to 1400 ppm, Fe up to

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 319

and )5.9& VPDB for oxygen and from )4.4 to +1.3& VPDB for carbon), depending on the samples and evidence that several generations of cements (and possible deforma- tion events) are recorded in these fractures (Table 3, Fig. 5).

Fracture type B and calcite cement 2 (CC2)

Fractures of type B are horizontal to subhorizontal fractures from less than 1 mm to 3 cm width. Subhorizontal stylolite planes affect the walls of these fractures (the walls of the fractures are the later location of the stylolites; Fig. 8E). They are totally occluded by the calcite cement 2 (CC2) (Fig. 8A,E). The CC2 is made of white to brownish, anhedral to subhedral, blocky calcite crystals, non to dull-red luminescent and red blue ﬂuorescent (as is the host-rock). Calcite crystals show cloudy appearance due to solid inclusions with marked cleavage planes and sizes ranging between 300 lm and 2 mm. CC2 is characterized by Mg from 710 to 2730 ppm, Sr from 555 to 1480 ppm, Fe between 340 and 1660 ppm, and Na and Mn up to 455 and 385 ppm, respectively (Table 2, Fig. 4). The oxygen and carbon isotope compositions of these calcite cements range from )8.9 to )5.1 & and from +0.1 to +1.1 & VPDB, respectively (Table 3, Fig. 5). The 87Sr ⁄ 86Sr ratios range from 0.70744 to 0.70762 (average value of 0.70751; Table 4; Fig. 6).

Fracture type C, calcite sediment 3 (CS3) and calcite cement 4 (CC4)

Fractures of type C are vertical to subvertical and crosscut both fractures A and B. The fracture type C displays sharp and undulating walls, locally enlarged by dissolution, and are totally or partially filled by calcite sediment 3 (CS3) and calcite cement 4 (CC4) (Fig. 8A,B). Locally these fractures are wedge-shaped (with a maximum width of 2 cm; Fig. 8B). Minor porosity remains even after CC4 precipitation. The CS3 is constituted of reddish (CS3r) and greenish (CS3g) microsparite calcite sediment and clay minerals. Individual calcite crystal sizes ranges between 8 and 40 lm. This generation shows dull orange to bright orange luminescence and red blue fluorescence. The CS3r partially filled the original fracture porosity; it displays a geopetal distribution locally, where the upper part of the fracture is occluded by CC4 (Fig. 8B,F). The reddish CS3r records Mg and Sr from 2620 to 4040 ppm and 640 to Fig. 3. Schematic section of the Amposta Marino C2 well showing the position of the studied samples, the oil water contact (OWC; after 1055 ppm, respectively. Recorded Fe and Na are up to Seemann et al. 1990), the Gamma Ray log and the location of the total 720 and 385 ppm, respectivlely, while Mn is below detec- losses (Courtesy of Escal UGS s.l.). tion limit (Table 2, Fig. 4). The oxygen and carbon isotope compositions range from )9.9 to )5.6 & VPDB and 3995 ppm and Na up to 255 ppm (Mn content is below from )1.4 to +0.3 & VPDB, respectively (Table 3, detection limit) (Table 2). Oxygen and carbon isotopic Fig. 5). The strontium isotope compositions range from compositions offer a wide range of values (between )9.4 0.70857 to 0.70888 (average value of 0.70871), which are

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 320 E. PLAYA` et al.

Table 2 Summary of trace element composition of host-limestones, calcite cements and calcite inﬁllings in the Amposta Marino C2 well.

Ca Mg Sr Fe Na Mn Mg ⁄ Ca molar Sr ⁄ Ca molar Ca ⁄ Fe molar Generation (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) ratio (fluid)*, Ratio (fluid)à ratio (fluid)§

Brackish host-limestones m 377858 1013 658 875

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 321

The CC4 generation is the most volumetrically-significant (1) Isopachous-rim cement generation made of subhedral to of all fracturing-filling cements. In general, it occurs as euhedral calcite crystals, 15 to 80 lm in size and with limpid white calcite crystals, filling the fractures orange-bright luminescence. The larger crystals display a (Figs 8B, 9D). CC4 is composed of two cement generations non-luminescent nucleus, dark in plane light, with an (Fig. 8J): orange to yellow-bright luminescent outer zone.

Table 3 Oxygen and carbon isotopic compositions of the host-limestones Table 3 (Continued). and the calcite cements. Depth d18O & d13C & Generation Depth (m) Label d18O & VPDB d13C & VPDB Generation (m) Label VPDB VPDB

Marine host-limestones 1942 6d -2.22 +1.14 CC4 1895.5 4a -11.23 +0.61 1942 6a -13.25 +0.50 1942 6e -2.32 +1.05 1942 6b -12.99 +0.73 1910.3 8h -2.92 +1.37 1942 6c -13.01 +0.45 1942 10b -3.57 +1.36 1909.5 7a -13.33 +0.31 1944.6 12f -3.27 +0.88 1910.3 8c -11.73 +0.31 1944.6 12g -3.59 +0.88 1910.3 8d -13.17 +0.47

1951.4 14c -3.93 +1.50 1910.3 8f -12.86 +0.49

1952.6 15g -2.72 +1.42 1910.3 8g -13.55 +0.22

1957 18d -4.21 +0.96 1910.8 9a -13.04 +0.46

1957 18e -3.46 +1.23 1910.8 10a -12.77 +0.43

CC1 1952.7 16c -5.87 -3.49 1910.8 10c -13.15 +0.41

1952.7 16d -6.36 -4.38 1943.5 11a -11.21 +1.22

1952.7 16e -9.37 +1.26 1943.5 11b -12.45 +0.80

CC2 1874 1a -7.69 +0.47 1944.6 12c -12.29 +0.28

1874 1b -5.15 +0.53 1944.6 12d -11.87 +0.83

1874 1c -7.26 +0.55 1944.6 12e -12.77 +0.68

1874 1d -7.77 +0.13 1952.6 15d -12.68 +0.67

1874 1f -8.94 +0.31 1952.6 15e -12.68 +0.54

1952.6 15b -8.31 +0.72 1952.7 16a -12.69 -0.20

1952.6 15c -8.90 +1.14 1952.7 16f -12.04 +0.38

CS3r 1954.3 17a -9.85 +0.32 1952.7 16g -13.13 +0.38

1957 18b -8.38 -1.40 1954.3 17f -12.04 +0.27

1957 18c -9.91 -0.77 1954.3 17b -12.71 +0.62

1957 19a -5.64 -0.72 1954.3 17c -13.01 +0.47

1957 19b -5.77 -0.35 1954.3 17d -12.44 +0.36

1957 19c -8.87 +0.34 1954.3 17e -11.99 +0.91

CS3g 1910.3 8i -5.83 -0.08 1957 19d -11.95 +0.69 1910.3 8j -8.66 -0.59 1957 19e -12.34 +0.46 1944.6 12a -8.59 +0.01 1910.3 21a -12.75 +0.65 1952.6 15a -5.87 -1.20 1910.3 21b -12.68 +0.71 1952.6 15f -4.07 -0.58 1910.3 21c -12.09 +0.56 1952.6 15h -4.09 -0.27 1910.3 21d -10.24 +1.11 1952.7 16b -4.41 +0.45 See legend in Table 1 for CC1, CC2, CS3 and CC4.

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 322 E. PLAYA` et al.

VPDB, and from )0.2 to +1.2 & VPDB, respectively. This cement generation records the lowest oxygen isotope ratios, being about 5& lower than the d18O values of the rest of the calcite cements and about 8& lower than the host-rock (Table 3, Fig. 5). The 87Sr ⁄ 86Sr ratios vary between 0.70899 and 0.70927, with an average value of 0.70913; these cements are somewhat more radiogenic than the CS3, and significantly more radiogenic than the host-rock and the CC2 (Table 4, Fig. 6). Kaolinite, barite and pyrite are frequently associated with CC4. Kaolinite occurs with a booklet texture and has bright blue luminescence patterns and bright white fluorescence (Fig. 8G,H). Kaolinite patches are pore-filling after CC4 precipitation, thus postdating CC4 (Fig. 9C), but it is locally engulfed by overgowths of CC4 (Fig. 9D). Kaoli- nite is commonly impregnated by oil (Fig. 9C). Barite Fig. 5. d18O and d13C values of the host-limestones and calcite cements- sediments. Range of values for Cretaceous seawater after Veizer & Hoefs occurs very locally as prismatic crystals forming aggregates (1976) and Jenkyns et al. (2002). See legend in Table 1 for CC1, CC2, CS3 (Fig. 9F). Pyrite occurs as millimetric crystal aggregates, and CC4. 1–2 mm in size (Fig. 8A); in some cases, the pyrite crystals show pseudo-skeletal habit. Sulphur isotope compositions of aggregates of euhedral crystals (fracture-related) oscillate from )16.7& to )12.9& VCDT (Table 5). Both pyrite Table 4 Strontium isotopic compositions of the host-limestones and the calcite cements. See legend in Table 1 for CC1, CC2, CS3 and CC4. and barite postdate CC4.

Generation Depth (m) Label 87Sr ⁄ 86Sr Stylolites 1908.8 6A 0.707633±5 1910.3 8B 0.707524±5 At least two generations of stylotites occur in the studied Marine host-limestones 1957 18A 0.707483±5 samples. The oldest stylotite system cross-cuts fracture type 1874 1A 0.707483±5 A. The second, and most abundant, stylolite system are 1874 1B 0.707436±5 constituted by mostly-open and randomly-oriented stylo- CC2 1952.6 15C 0.707616±6 lites (from subhorizontal to subvertical), giving the rock a 1954.3 17B 0.708699±5 pseudo-nodular texture. This second generation of stylo- CS3r 1957 19A 0.708576±5 lites postdates CC1 and CC2 cements, but display ambigu- ous relationships with fractures type C, thus tentatively CS3g 1952.6 15A 0.708615±5 indicating a coeval origin. Subhorizontally oriented stylo- CC4 1910.3 8A 0.708995±6 lites, which can occur as the walls of the fracture type B 1943.5 11C 0.709273±5 (Fig. 8E), have also been attributed to this pseudo-nodular 1944.6 12B 0.709129±5 1952.7 16A 0.709166±5 texture between the randomly oriented stylolites. Subhori- 1954.3 17A 0.709100±5 zontal and randomly oriented stylolites were open but CC4 cement, together with oil, has partially filled this open stylolitic porosity (Fig. 8C). (2) Blocky anhedral to subhedral calcite crystals with dull- red luminescence and red blue fluorescence, up to OIL, CEMENT AND POROSITY 1 mm in size, locally zoned with a dark nucleus (dull- RELATIONSHIPS red luminescent) and a dull orange to yellow luminescent outer zone. Outer borders of calcite crystals are Non-luminescent and brown-fluorescent oil occurs in par- rounded and appear to be corroded, indicating a disso- tially-occluded type C fractures and stylolites and in the lution event postdating CC4 precipitation (Figs 8I, host-limestones. Thus, oil emplacement was post-CC4 pre- 9C,E). cipitation. Where the fractures of type C are more-or-less CC4 is characterized by Mg from 390 to 10450 ppm totally occluded by CC4, oil is only found in the middle of and Sr between 475 and 1580 ppm; recorded Fe, Na and the fracture, following the intercrystalline contacts and ⁄ or Mn lower than 3030 ppm, 435 ppm and 470 ppm, respec- along cleavage planes (Fig. 9B). Locally, oil impregnates tively (Table 2, Fig. 4). The d18O and d13C values of the the host-limestones; in such cases, oil is present only blocky calcite cement vary between )13.5 and )10.2 & close to fractures or stylolites (which also contain oil;

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 323

Fig. 6. 87Sr ⁄ 86Sr ratio of the host-limestones and calcite cements-sediments. Range of values for Upper Jurassic-Cretaceous seawater after Veizer & Compston (1974), Burke et al. (1982), Koepnick et al. (1985), Jones et al. (1994), Veizer et al. (1997) and Jenkyns et al. (2002). See legend in Table 1 for CC1, CC2, CS3 and CC4.

Fig. 8C,D,E), suggesting that oil circulated preferentially along these planes and that fracture porosity and stylolite porosity were interconnected.

DISCUSSION Fig. 7. Relationships of fractures, cements–sediments, stylolites and oil. See legend in Table 1 for CC1, CC2, CS3 and CC4. Diagenesis of the host-limestones

When precipitation occurs in equilibrium, the chemical com- slightly lower than the expected values for limestones pre- position of the fluid from which calcite precipitates is related cipitated from Cretaceous seawater (from )2.5 to +1 & to the chemical composition of the precipitate, and can be VPDB), and the carbon isotope compositions match well determined by using the distribution coefficient equation of with those of the Cretaceous seawater (ranging from )2 McIntire (1963) (i.e., Meyers & Lohmann 1985; Banner & and +3& VPDB; Veizer & Hoefs 1976; Jenkyns et al. Hanson 1990). Although some studies (Reeder & Grams 2002) (no data are available for the brackish limestones; 1987; Paquette & Reeder 1995) have shown that minor ele- Fig. 5). Strontium ratios are slightly more radiogenic than ment distribution in natural carbonates does not always those of the Upper Jurassic-Lower Cretaceous seawater reflect equilibrium partitioning; in this study, we assume that (0.7068–0.7074; after Veizer & Compston 1974; Burke precipitation occurred in equilibrium. et al. 1982; Koepnick et al. 1985; Jones et al. 1994; Veizer The molar ratios of Sr ⁄ Ca and Ca ⁄ Fe of the host marine et al. 1997; Jenkyns et al. 2002; Fig. 6). These data show limestone are consistent with precipitation from formation that the original marine and brackish signals have been waters whereas the molar ratios of Mg ⁄ Ca are consistent overprinted during diagenesis. with precipitation from either meteoric or formation waters (Table 2; Skougstad & Horr 1963; Kinsman 1969; Relationships between fracturing events and palaeofluids McIntire 1963; Tucker & Howson et al. 1987; Tucker & Wright 1990). Formation waters are here defined as Pre-Alpine fractures and calcite cement 1 (CC1). waters initially trapped during sedimentation that Pre-Alpine fractures (fracture type A) occur as vertical to changed their composition during basin evolution due to subvertical thin fractures, which represent several, undif- diagenetic reactions (after Lawrence & Cornford 1995). ferentiated, fracturing events. A large range of variation The molar ratios of Sr ⁄ Ca of the uppermost brackish host- in the oxygen isotope compositions of the CC1 supports rock are characteristic of being precipitated from formation this multistage interpretation (Fig. 5). waters, whereas the molar ratios of Ca ⁄ Fe indicate the influence of meteoric waters; the molar ratios of Mg ⁄ Ca indicate Palaeogene compressional fractures and calcite cement 2 either precipitation from meteoric or formation waters. (CC2). The analysis of minor and trace elements in the marine During the Palaeogene Alpine compression, the horizon- and in the uppermost brackish host-limestones does not tal fractures (type B) were developed (Figs 10 and 11). seem to record unequivocal evidence of their marine and Fractures are totally occluded by blocky calcite cement brackish origins (Table 2). The d18O of the limestones are (CC2).

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 324 E. PLAYA` et al.

The molar ratios of Mg ⁄ Ca, Sr ⁄ Ca and Ca ⁄ Fe of CC2 the open stylolites (formed by compaction during the in fractures type B are consistent either with precipitation pre-Neogene burial stage and opened during the uplift and from meteoric or formation waters. unloading during mesodiagenesis). At this stage, corrosion The similarity between the d13C values of CC2 and those produced irregular walls by enlarging fracture porosity. of the host-rock (Fig. 5) is probably due to the buffering of Extensive karstification of the host Lower Cretaceous lime- the pore-water carbon isotopic composition by extensive stones, including large caverns, responsible for the reser- dissolution of the host-limestones (Marshall 1992). The voir, probably occurred at this time, after development of presence of abundant stylolite surfaces within the host-lime- type C fractures and prior to CS3 infilling. stones provides evidence of this dissolution process. The CS3 infilling, comprised of microsparite and clay The similarity between the 87Sr ⁄ 86Sr ratios of the CC2 minerals, partially fills these vertical enlarged fractures, usu- and those of the host-rock (Cretaceous host marine lime- ally in a geopetal disposition. This CS3 generation displays stones; Fig. 6), together with the d13C values, indicate a petrological characteristics similar to the detrital karstic car- high degree of fluid-rock interaction in a closed palaeohy- bonates described in Trave´ et al. (1998). According to drogeological system. Oxygen isotopic compositions are these observations, CS3 is interpreted as originally being a the most similar to host-limestones with respect to the residual-detrital sediment with a degree of recrystallization other cements, thus also supporting this interpretation. which increases the crystal size from mud to fine-sand. The Thermal modelling from the Amposta Marino C3 well isotopic (O, C, Sr) compositions of CS3 do not match (location in Fig. 1) indicates that the maximum subsidence with the underlying Triassic–Jurassic limestones and the attained by the Lower Cretaceous limestones during the Lower Cretaceous host-limestones (Veizer & Compston Palaeogene was <1300 m; considering a geothermal gradi- 1974; Burke et al. 1982; Koepnick et al. 1985; Pirrie & ) ent of 30Ckm 1 (Salas & Permanyer 2003; Permanyer & Marshall 1990; Jones et al. 1994; Gro¨tsch & Vahrenkamp Salas 2005), an assumed temperature of 25C at the sur- 1995; Veizer et al. 1997; Jenkyns et al. 2002; Figs 5 and face, and ruling out the inflow of externally sourced hotter 6); thus, recrystallization would have modifed the original fluids, the maximum precipitation temperature was, there- isotopic signal. Evolution to lower oxygen isotopic compo- fore, probably less than 60C. Thus, assuming this rela- sitions and more radiogenic 87Sr ⁄ 86Sr ratios are expected tively low temperature of precipitation for CC2 and for recrystallization processes in carbonates (Kupecz & applying the temperature equation of Craig (1965), calcu- Land 1994; Malone et al. 1994; Reinhold 1998), in accor- lated d18O of fluid is between )5.7 and 0& VSMOW dance with the observed trends in the studied CS3 infil- (Table 6); this measurements are also consistent with lings. Assuming recrystallization from 25 to 50C and meteoric or formation waters. applying the temperature equation of Craig (1965), it In summary, precipitation of CC2 in type B fractures would have precipitated from waters with d18O values from occurred from a fluid that interacted strongly with the )6.4 to +1.4 & SMOW (Table 6). These oxygen composi- rock, approaching a closed-system, equilibrium compositions for parental fluids, the d13C compositions for CS3, tion. Similar fractures and infilling calcite cements have which are close to 0&, the molar ratios of Mg ⁄ Ca, Sr ⁄ Ca been observed in the Palaeogene compressive structures and Ca ⁄ Fe which are consistent with recrystallization from from the Catalan Coastal Ranges (type 2 fractures, see formation waters, and the 87Sr ⁄ 86Sr ratios of the CS3 Trave´ et al. 1998). which are higher than those of the host-limestones, indicates a certain degree of interaction between the fluid and Neogene extensional fractures, calcite sediment 3 (CS3) more radiogenic sediments. Input of similar fluids during and calcite cement 4 (CC4). the Neogene extension was interpreted in the Catalan The vertical fractures (type C) formed during the late Oli- Coastal Ranges by Trave´ & Calvet (2001), where the gocene-Miocene extension after the opening of the meteoric fluid was a more evolved fluid due to significant randomly- and subhorizontally-oriented stylolites (Figs 10 interaction with sediments coupled with an increase of the and 11). Circulation of corrosive (or at least undersatu- residence time in the aquifer and the reservoir effect. rated with respect to calcite) fluids along type C fractures Precipitation of CC4 would have reduced, but not in the Amposta area enlarged the fracture walls and totally occluded, the remaining porosity after CS3 stage.

Fig. 8. (A), (B), (C), (D) and (E) photographs from the most representative samples of the studied C2-Amposta Marino core. These photographs illustrate the relationships among fracture types, cements and stylolites. The white label indicates the sample depth (in metres). (F), Photomicrograph (plane polarized light) of fracture type C occluded by CS3r (in geopetal disposition) and CC4; note the irregular and sharp contact, enlarged by dissolution, between host- limestones and fracture type C. (G) and (H), Photomicrograph (plane polarized light) and cathodoluminescence image, respectively, showing fracture type A ﬁlled with non-luminescent CC1 cut by fracture type C ﬁlled with the dull-red to non-luminescent blocky CC4; note the occurrence of kaolinite postdating CC4. (I), Photomicrograph (plane polarized light) showing CC2, CC4 and porosity; note that the CC4 crystals are not idiomorphic and slightly corroded; (J), Photomicrograph (cross polarized light) showing the two stages of CC4 cementation (1, rim; 2, blocky). P, pyrite. See legend in Table 1 for CC1, CC2, CS3r, CS3g and CC4.

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 325

(A) (B) (C) (D)

(E) (F)

(G) (H)

(I) (J)

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 326 E. PLAYA` et al.

(A) (B)

(E) (F)

Fig. 9. Photomicrograph (cross polarized light; A, D, E, F, and plane polarized light B and C) showing: (A) fracture type C occluded by clasts of host-limestones, CS3g and CC4; (B) fracture type C cemented by blocky CC4. Oil (brownish colour) is emplaced in the intercrystalline porosity and cleavage planes of CC4; (C) and (D) patches of kaolinite postdating CC4 inﬁlling; note the irregular and corroded borders of the calcite crystals (white arrows in C); and the engulfed kaolinite within the CC4; (E) ﬂoating calcite crystals (CC4) in the kaolinite groundmass; (F), Barite postdating CC4 in fracture type C. See legend in Table 1 for CC1, CC2, CS3r, CS3g and CC4.

Fluid inclusion within CC4 indicates that this cement pre- that the parental ﬂuids had variable composition. Consider- cipitated at temperature between 96 and 115C (C. Rossi, ing temperatures higher than 90C, the molar ratios of pers. com.). The great dispersion of the Ca, Mg, Sr, Na, Fe Mg ⁄ Ca, Sr ⁄ Ca and Ca ⁄ Fe are consistent with formation and Mn contents in the CC4 (Table 2, Fig. 4) indicates water rather than meteoric water (Table 2). Applying the

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 327

Table 5 Sulfur isotopic compositions of the pyrite in fracture-fills. ids (i.e. external fluids) into fracture type C (Marfil et al. 2005). CC4 precipitation pre-dates the main oil emplace- Depth (m) Label d34S & VCDT ment in a buried karstic terrain, dominated by an increas- 1910.3 8a )14.1 ingly open palaeohydrogeological system; fluids were hot, ) 1910.3 8b 15.4 high d18O basinal and brines indicating negligible interac- 1910.3 8b(rep) )16.7 tion with the Lower Cretaceous host-limestones (and 1957 18a )13.0 pre-existing cements). Thus, progressive opening of the hydrogeological system within the Cretaceous limestones isotope equilibrium temperature equation of Craig (1965), of the Amposta reservoir up until the main petroleum precipitation of CC4 took place from waters with d18O val- charge has been proven (Fig. 11). ues between +6.4 and +11.2& VSMOW (Table 6), which is in accordance with the formation water origin for the Precipitation of exotic minerals parental fluid. These heavier values are consistent with residual evaporitic brines or deep saline basinal fluids (or Kaolinite, barite and pyrite aggregates are found after CC4 oil-field brines) (Montan˜ez 1994). The occurrence of evap- precipitation. The term exotic (Neilson & Oxtoby 2008) orites has not been described in the Jurassic to Cretaceous has been applied to this mineral paragenesis, which is com- sequence (Seemann et al. 1990; Clavell & Berastegui monly interpreted as being precipitated prior, or during, 1991; Fig. 2). Additionally, taking into account that Juras- arrival of petroleum (Rossi et al. 2001; Esteban & Tabern- sic to Cretaceous limestones from the Valencia Trough er 2003; Salas et al. 2007). Exotic cementation in the were mainly deposited under marine conditions, it is neces- Amposta samples was not massive and kaolinite, which is the sary to consider that marine-derived brines are carbonate- most significant volumetrically, is usually oil-impregnated. poor (carbonates precipitated before the sulphate stage). Pore-filling kaolinite patches most likely crystallized after Therefore, it is unlikely that significant quantity of CC4 corrosion. Engulfed kaolinite within the calcite crys- (marine) residual evaporitic brines could be responsible for tals (Fig. 9D) points to a co-precipitation, but contrasting CC4 precipitation, leaving deep basinal brines (i.e. high pH conditions are probably required to precipitate kaoli- salinity formation water) as the remaining option. nite and calcite, thus supporting the post-CC4 kaolinite Sr isotope ratios are more radiogenic values than the formation. Irregular calcite borders that are in contact with host-limestones and even higher than the underlying Trias- kaolinite cement patches suggest that the fluids from which sic evaporites (Fig. 6), thus also excluding the influence of kaolinite precipitated were acidic (as well as bearing Si and residual evaporitic brines but suggesting a significant Al) and thus caused partial dissolution of CC4 calcite. source of radiogenic siliciclastic-derived allochthonous flu- When kaolinite is present as pore-filling mineral in lime-

Fig. 10. Diagenetic evolution of the Amposta oil reservoir rocks and possible relationships with the main tectonic events. See legend in Table 1 for CC1, CC2, CS3 and CC4. CS3r*-CS3g*: this event includes detrital (karstic) deposition and recrystallization of CS3.

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 328 E. PLAYA` et al.

(A) (B1)

(B2) (C)

Fig. 11. Evolution of ﬂuid migration pathways during the Palaeogene and Neogene main tectonic events.

Table 6 Calculated d18O of the precipitating ﬂuid for each calcite cement at different temperatures (T).

Measured d13C & Measured d18O & VPDB VPDB Calculated d18O & VSMOW

Generation MSDMSDT = 25ºC T = 50ºC T = 95ºC T = 115ºC

Marine +1.2 0.2 )3.2 0.6 –––– host-limestones CC2 +0.6 0.3 )7.7 1.2 )5.7 +0.3 +11.0 +15.7 CS3r )1.7 2.4 )8.5 1.7 )6.4 )0.5 +10.3 +15.0 CS3g )0.3 0.5 )6.6 2.5 )4.6 +1.4 +12.1 +16.8 CC4 +0.5 0.3 )12.4 0.9 )10.2 )4.3 +6.4 +11.2

See legend in Table 1 for CC1, CC2, CS3 and CC4. Bold d18O VSMOW values: calculated oxygen isotopic compositions of fluids for the most probable calcite precipitation temperatures. stones, it has been suggested that direct precipitation in Pyrite is the second most abundant exotic mineral in the pore spaces occurred (Maliva et al. 1999; Marfil et al. studied samples. Many sulphur bearing minerals have been 2003), but specific processes involving kaolinite cementa- described as part of the exotic mineral assemblage in carbon- tion have not been geochemically and thermodynamically ate reservoirs, with anhydrite being the most volumetrically- explained. Presence of relicts of calcite cement within the significant; precipitation of anhydrite is generally associated kaolinite patches in the studied samples (Fig. 9E) suggests with occurrence of evaporite units in the basin (Neilson & a replacive origin for the kaolinite. Oxtoby 2008). Nevertheless, anhydrite has not been

2010 Blackwell Publishing Ltd, Geofluids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 329 observed in the Amposta samples. Precipitation of pyrite in in agreement with the expected values after BSR. Thus, type C fractures requires an external sulphur source, as either expelled fluids from the petroleum source rock (Ascla Fm) a sulphate or sulphide species. Sulphate species could be are considered as being responsible for the sulphur (pyrite) related to dissolved marine-derived sulphate, residual evapo- contribution into the Amposta reservoir rock. On the rite brines or dissolution of sulphate rocks. Evidence of sea- other hand, low H2S contents of the Amposta crude sup- water circulation (or residual pore-trapped seawater within port this interpretation, rather than having been originated the limestones) has not been observed in the whole diage- from thermochemical sulphate reduction (Machel et al. netic sequence from the Amposta rocks. Similarly, signifi- 1995; Worden & Smalley 1996; Cai et al. 2001; Worden cant amount of enclosing sulphate minerals have not been et al. 2003). Available Fe in the Amposta rocks, possibly encountered in the Jurassic-Cretaceous rocks of the studied from dissolution of host-limestones and calcite cements area. Thick successions of evaporites occur in the under- during kaolinite formation under acidic conditions, par- lying Triassic units; however, all effective porosity in tially inhibited H2S accumulation and favoured pyrite pre- sulphate and chloride deposits was occluded earlier, at very cipitation. Cai et al. (2005) also interpreted H2S shallow depths (by 100 m of burial, halite units are tight accumulation due to escape of free H2S from the source and impervious; Casas & Lowenstein 1989; Warren 1999). rock to the reservoir and prior to pyrite formation. On the other hand, the gypsum to anhydrite transformation As petroleum emplacement seemingly does not entirely of Triassic sulphates, which implies expulsion of large quanti- stop precipitation of other exotic minerals (Neilson & ties of brine, most probably occurred from few metres to Oxtoby 2008), sulphur-rich fluids could circulate prior or about one kilometre depth (Warren 2006), and therefore also during petroleum charge in the reservoir (Fig. 11) produc- much before pyrite precipitation. Thus it is unlikely that ing pyrite precipitation. marine- or evaporite-derived sulphate was important during burial diagenesis. Oil generation and emplacement Other sources of sulphur as reduced suphide (mainly

H2S) species can be invoked; main sources of reactive sulp- Most oil in the Valencia Trough fields was sourced from hides are thermal decomposition or organic sulphur com- the lower and middle Miocene organic-rich, dark brown, pounds in kerogen or oil or sulphate reduction (bacterial hemi-pelagic marls of the Alcanar Group, also named Casa- sulphate reduction (BSR), or thermochemical sulphate blanca Formation (Albaige´s et al. 1986; Clavell & Beraste- reduction (TSR); Worden & Smalley 1996). Temperatures gui 1991; Fig. 2). In these rocks the oil generation and of more than 175C are thought to be required to cause migration began in the late Miocene toearly Pliocene the decomposition of organic matter (Cai et al. 2001). (Clavell & Berastegui 1991; Varela et al. 2005). Temperatures in source rocks did not exceed 111C In the adjacent onshore Maestrat basin, the Jurassic Ascla (Permanyer & Salas 2005), and assuming that tempera- Formation has been identified as an alternative oil source tures in petroleum fluids during pyrite precipitation where rock; petroleum generation started when the Ascla shales from 96C (temperature for CC4 precipitation) to 126C entered the oil window at 90–100 Myr (Turonian; Perman- (present oil field temperature; Seemann et al. 1990), the yer & Salas 2005). Oil migration and petroleum charge in necessary high temperatures for this process were not the Maestrat basin occurred during the Late Cretaceous- attained. The Amposta crude is sulphur-rich (5–7% in early Tertiary (Rossi et al. 2001; Salas & Permanyer 2003; weight; Seemann et al. 1990), but with low H2S contents Permanyer & Salas 2005), before the Alpine compressive (lower than 0.2%; R. del Potro-Escal UGS, pers. comm.). tectonism that uplifted the onshore area promoting karstifi- Seemann et al. (1990) interpreted that this crude was cation and erosion of the potential onshore reservoirs. The expelled from a carbonate-rich source rock with high sul- Amposta oil, although being in the Valencia Trough, was phur content. The Ascla Formation source rock was depos- sourced from the Jurassic Ascla Formation (Seemann et al. ited in relatively reducing conditions favouring BSR 1990; Salas & Permanyer 2003; Permanyer & Salas 2005; processes (Salas & Permanyer 2003; Permanyer & Salas Varela et al. 2005; Fig. 2B), as in the neighbouring onshore 2005). BSR results in a large isotopic difference between Maestrat basin. Nevertheless, migration of oil from the Ascla the parental sulphate and the resulting H2S, from )25 to Formation towards the Cretaceous rocks occurred necessar- )50& (Kemp & Thode 1968; Fritz et al. 1989), but neg- ily after deposition of the Lower Miocene clastic sediments ligible isotopic changes seem to occur between H2S gas that seal the reservoir (Fig. 3) and, therefore, later than in and resulting sulphide minerals (Nakai & Jensen 1964). the Maestrat onshore basin. Sulphur isotopic composition of Lower Cretaceous marine- Burial and thermal model of the Marino C3 well (location derived sulphate is around +13 or +16& (Claypool et al. in Fig. 1) suggested two-stage oil generation from the Ascla 1980; Kampschulte & Strauss 2004; Paz et al. 2005); thus, marls in the Amposta reservoir (Permanyer & Salas 2005): the low d34S values of the analysed pyrites in fractures in i) a first stage during the late Cretaceous to early Tertiary, the Amposta rocks (from )16.7& to )13.0& VCDT) are analogous to that in the onshore setting, and, ii) a second

2010 Blackwell Publishing Ltd, Geofluids, 10, 314–333 330 E. PLAYA` et al. stage of generation when the Ascla Formation was at, or waters were responsible of CS3 recrystallization, after sedi- close to, the maximum burial depth, induced by the overbur- mentation of detrital CS3 in the karstic terrain. Progressive den Neogene synrift sediments, during the Miocene. The re-burial was recorded by precipitation of CC4 as a result diagenetic sequence observed in the present work on the well of migration of hot and d18O-enriched brines. Amposta Marino C-2 indicates that the main oil emplace- The main oil migration postdates CC4 precipitation. Oil ment took place after precipitation of CC4, probably during circulated through partially cemented fractures, open stylo- a late stage of the Neogene extensional regime (Fig. 10). lites, intercrystalline porosity and in the cleavage planes of Oil circulation in the Amposta reservoir occurred in par- the CC4 crystals. Burial dissolution of CC4 occurred prior tially uncemented fractures, open stylolites, the intercrystal- to, or during, oil charge, possibly related to kaolinite pre- line porosity and apparently along the cleavage planes of cipitation (i.e. acid fluids). The sulphur source associated the CC4 crystals. Esteban & Taberner (2003) proposed a with the late precipitation of pyrite was likely related to paragenetic scheme that concurs with the observations isotopically-light sulphur expelled, e.g. as sulphide, from reported here of burial-related corrosion of CC4 prior to the petroleum source rock (Ascla Fm). (or during) oil migration. These authors compiled data from productive carbonate rocks from different geologic ACKNOWLEDGEMENTS provinces and concluded that major and minor corrosion stages (and porosity generation) occurred during burial The present work was funded by the Escal UGS s.l. com- diagenesis of carbonate reservoirs, leading or preceding the pany, the Spanish Government Projects CGL2006-04860, roll-front of petroleum migration. However, the studied and the ‘Grup Consolidat de Recerca Geologia Sedimenta`ria’ samples from Amposta Marino C2 well do not provide 2009SGR-1451. Drs. M. Esteban (Carbonates Interna- clear evidence of major porosity enhancement in burial tional Iberia S.L., Mallorca-Spain), C. Martin-Closas conditions, thus suggesting that major secondary porosity (Universitat de Barcelona) and C. Rossi (Universidad was produced instead by meteoric diagenesis (Orlopp Complutense de Madrid) are thanked for their valuable 1988; Seemann et al. 1990; Clavell & Berastegui 1991). comments and additional scientific information. S. Morad The original oil water contact (OWC) in the Amposta (Uppsala University, Sweden, and Petroleum Institute of oil field was at 1940 m TVD below sea level (data Abu Dabhi, United Arab Emirates) and Prof. Richard obtained in 1970 in the Amposta discovery well; Seemann Worden are also thanked, for their accurate revision and et al. 1990; Fig. 3). Petrographical observations in the valuable suggestions. We appreciate the support and facili- Amposta Marino C-2 well are in accordance with the ties of R. del Potro and C. Barat (Escal UGS S.L., Madrid, OWC; the remaining porosity and macroscopic oil impreg- Spain), and the review comments of an earlier manuscript. nation occur in the rocks above 1910.8 sample (Table 1), XRD determinations, microprobe analyses and oxygen, car- while the original porosity is nearly totally obliterated by bon and sulphur isotope analyses were performed in the cementation in the limestones below 1943.5 metres (there Serveis Cientificote`cnics (Universitat de Barcelona). Stron- are no available data between these samples). tium isotope analyses were done in the CAI of the Uni- versidad Complutense de Madrid. CONCLUSIONS The main oil migration in the Amposta oil reservoir is REFERENCES related in time to Neogene extensional fractures. Three Albaige´s J, Algaba J, Clavell E, Grimalt J (1986) Petroleum geo- fracturing stages have been identified in the Barremian– chemistry of the Tarragona Basin (Spanish mediterranean off- Aptian reservoir rocks of the Amposta field. Fracture types shore). Organic Geochemistry, 10, 441–50. A and B are attributed to early fractures developed prior Banner JL, Hanson GN (1990) Calculations of simultaneous isotopic and trace element variations during water-rock interaction to, and during the Alpine compression and are totally filled with applications to carbonate diagenesis. Geochimica et Cosmo- by calcite cement 1 (CC1) and 2 (CC2), respectively. Frac- chimica Acta, 54, 3123–37. ture type C developed during Neogene extension, and is Burke WH, Denison RE, Hetherington EA, Koepnick RB, Nelson 87 86 filled by reddish and greenish microspar calcite sediment HF, Otto JB (1982) Variation of seawater Sr ⁄ Sr throughout (CS3r and CS3g), blocky calcite cement 4 (CC4), kaoli- Phanerozoic time. Geology, 10, 516–9. Burton EA, Walter LM (1991) The effects of P and temperature nite, pyrite, barite and oil. CO2 on magnesium incorporation in calcite in seawater and MgCl2- The entire diagenetic sequence indicates a closed pala- CaCl2 solutions. Geochimica et Cosmochimica Acta, 55, 777–85. eohydrological system and a high degree of fluid-rock Cai C, Hu W, Worden RH (2001) Thermochemical sulphate interaction during precipitation of CC2 (Alpine compres- reduction in Cambro-Ordovician carbonates in Central Tarim. sion). During the Tertiary extension, the system became Marine and Petroleum Geology, 18, 729–41. Cai C, Worden RH, Wolff GA, Bottrell SH, Wang D, Li X (2005) open to more radiogenic external fluids. Type C fractures Origin of sulphur rich oils and H2S in Tertiary lacustrine were enlarged by subaerial exposure. Evolved-meteoric

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 331

sections of the Jinxiang Sag, Bohai Bay Basin, China. Applied limitations and implications for palaeoceanography. Journal of Geochemistry, 20, 1427–44. Geological Society of London, 159, 351–78. Calvet F, Permanyer A, Vaquer R (1983) El paleokarst del Jones CE, Jenkyns HC, Coe AL, Hesselbo SP (1994) Strontium contacto Mesozoico-Mioceno en el Penede´s y Camp de Tarrag- isotope variations in Jurassic and Cretaceous seawater. Geochimica ona. X Congreso Espanõl de Sedimentologıá. Menorca, 173–175, et Cosmochimica Acta, 58, 3061–74. communication. Kampschulte A, Strauss H (2004) The sulphur isotopic evolution of Casas E, Lowenstein TK (1989) Diagenesis of saline pan halite; Phanerozoic seawater based on the analysis of structurally comparison of petrographic features of modern, Quaternary and substituted sulfate in carbonates. Chemical Geology, 204, 255– Permian halites. Journal of Sedimentary Petrology, 59, 724–39. 86. Clavell E, Berastegui X (1991) Petroleoum geology of the Gulf of Katz A (1973) The interaction of magnesium with calcite during Vale`ncia. In: Generation, Accumulation, and Production of crystal growth at 25-90C and one atmosphere. Geochimica et Europe’s Hydrocarbons (ed. Spencer MA), pp. 355–68. Oxford Cosmochimica Acta, 37, 1563–86. University Press, Oxford. Special Publication of the European Katz A, Sass E, Starinsky A, Holland HD (1972) Strontium Association of Petroleoum Geoscientists, 1. behavior in the aragonite-calcite transformation: An experimen- Claypool GE, Holser WT, Kaplan YR, Sakai H, Zak I (1980) The tal study at 40–98C. Geochimica et Cosmochimica Acta, 36, age of sulfur and oxygen isotopes in marine sulfate and their 481–96. mutual interpretation. Chemical Geology, 28, 199–260. Kemp AL, Thode HG (1968) The mechanism of the bacterial Craig H (1965) The measurement of oxygen isotope paleotemper- reduction of sulphate and of sulphite from isotope fraction- atures. In: Stable Isotopes in Oceanographic Studies and Paleotem- ation studies. Geochimicat et Cosmochimica Acta, 32, 71– peratures (ed Tongiorgi E), pp. 161–82. Consiglio Nazionale 91. delle Ricerche, Laboratario di Geologia Nucleare, Pisa. Kinsman DJJ (1969) Interpretation of Sr+2 concentrations in car- Dromgoole EL, Walter LM (1990) Iron and manganese incorpo- bonate minerals and rocks. Journal of Sedimentary Petrology, 39, ration into calcite: effects of growth kinetics, temperature and 486–508. solution chemistry. Chemical Geology, 81, 311–36. Klimowitz J, Hernańdez E, Serrano A (2005) A field trip guide Esteban M (1973) Petrologıá de las Calizas Cretaćicas del Sector book. The Mediterranean Basin (Catalan costal range onshore Central de los Catalańides (Prov. de Tarragona y Barcelona). analogues). In: Asociacioń de Geo´logos y Geofı´sicos Espanõles del PhD Thesis, Universitat de Barcelona, Barcelona, unpublished. Petro´leo (AGGEP)-XXV Aniversario (ed. Martıńez del Olmo W), Esteban M (1991) Palaeokarst: case histories. In: Palaeokarsts and 187–208. Museo de Ciencias Naturales, Madrid. Palaeokarstic Reservoirs (eds Wright VP, Esteban M, Smart PL), Koepnick RB, Burke WH, Denison RE, Hetherington EA, Nelson pp. 120–46. Postgraduate Research Institute for Sedimentology, HF, Otto JB, Waite LE (1985) Construction of the seawater University of Reading, Reading. 152. 87Sr ⁄ 86Sr curve for the Cenozoic and Cretaceous: supporting Esteban M, Julia` R (1973) Discordancias erosivas intrajura´sicas en data. Chemical Geology, 58, 55–81. los Catalańides. Acta Geolo´gica Hispańica, 8, 153–7. Kupecz JA, Land LS (1994) Progressive recrystallization and sta- Esteban M, Klappa CF (1983) Subaerial Exposure Environment. bilization of early-stage dolomite: lower Ordovician Ellenburger In: Carbonate Depositional Environments (eds. Scholle PA, Beb- Group, West Texas. Special Publication of the International Asso- out DG, Moore CH), pp. 2–54. American Association of Petro- ciation of Sedimentologists, 21, 255–79. leum Geologists, Tulsa. 33. Lawrence SR, Cornford C (1995) Basin geofluids. Basin Research, Esteban N, Taberner C (2003) Secondary porosity development 7, 1–7. during late burial in carbonate reservoirs as a result of mixing Lomando AJ, Harris PM, Orlopp DE (1993) Casablanca and ⁄ or cooling of brines. Journal of Geochemical Exploration, field, Tarragona Basin, offshore Spain: a karsted carbonate res- 78-79, 355–9. ervoir. In: Paleokarst Related Hydrocarbon Reservoirs (eds. Fritz P, Basharnal GM, Drimmie RJ, Ibsen J, Qureshi RM Fritz RD, Wilson JL, Yurewicz DA) Core Workshop, 18, in (1989) Oxygen isotope exchange between sulphate and water New Orleans, SEPM 201–25, Tulsa. during bacterial reduction of sulphate. Chemical Geology, 79, Losantos M, Aragone`s E, Bera´stegui X, Palau J, Puigdefa`bregues 99–105. C (1989) Mapa Geolo`gic de Catalunya, scale 1:25000. Servei Gaspar-Escribano JM, Garcıá-Castellanos D, Roca E, Cloetingh S Geolo`gic de Catalunya, Barcelona. (2004) Cenozoic vertical motions of the Catalan Coastal Ranges Machel HG, Krouse HR, Sassen R (1995) Products and distin- (NE Spain): The role of tectonics, isostasy, and surface trans- guishing criteria of bacterial and thermochemical sulfate reduc- port. Tectonics, 23, TC1004. doi: 10.1029 ⁄ 2003TC001511. tion. Applied Geochemistry, 10, 373–89. Gro¨tsch J, Vahrenkamp V (1995) Can carbon isotopes be used as Maliva RG, Dickson JAD, Fallick AE (1999) Kaolin cements in a stratigraphic tool in Mid-Cretaceous shallow water carbonates? limestones: potential indicators of organic-rich pore waters AAPG Bulletin, 79, 1218. during diagenesis. Journal of Sedimentary Research, 69, 158– Hardenbol J, Thierry J, Farley MB, Jacquin T, Graciansky PC, Vail 63. PR (1998) Mesozoic and Cenozoic Sequence Chronostrati- Malone MJ, Baker PA, Burns SJ (1994) Recrystallization of dolo- graphic Framework of European basins (Cretaceous Biochronos- mite: evidence from the Monterey Formation (Miocene) Cali- tratigraphy, Chart 5). In: Mesozoic and Cenozoic Sequence fornia. Sedimentology, 41, 1223–39. Stratigraphy of European Basins (eds. De Graciansky PC, Marfil R, Delgado A, Rossi C, La Iglesia A, Ramseyer K (2003) Hardenbol J, Jacquin Th, Vail PR) ), pp. 3–13. SEPM Special Origin and diagenetic evolution of kaolin in reservoir sandstones Publication 60, Tulsa. ans associated shales of the Jurassic and Cretaceous, Salam Howson MR, Pethybridge AD, House WA (1987) Synthesis and Field, Western Desert (Egypt). IAS Special Publication, 34, distribution coefficient of low-magnesium calcites. Chemical 319–42. Geology, 64, 79–87. Marfil R, Caja MA, Tsige M, Al-Aasm IS, Martıń-Crespo T, Salas Jenkyns HC, Jones CE, Gro¨cke DR, Hesselbo SP, Parkinson DN R (2005) Carbonate-cemented stylolites and fractures in the (2002) Chemostratigraphy of the Jurassic System: applications, Upper Jurassic limestones of the Eastern Iberian Range, Spain:

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 332 E. PLAYA` et al.

A record of palaeofluids composition and thermal history. Sedi- Roca E, Sans L, Cabrera L, Marzo M (1999) Oligocene to middle mentary Geology, 178, 237–57. Miocene evolution of the central Catalan margin (northwestern Marshall JD (1992) Climatic and oceanographic isotopic signals Mediterranean). Tectonophysics, 315, 209–29. from the carbonate rock record and their preservation. Geologi- Rossi C, Goldstein RH, Marfil R, Salas R, Benito MI, Perman- cal Magazine, 129, 143–60. yer A, de la Penã JA, Caja MA (2001) Diagenetic and oil Martinez del Olmo W, Esteban M (1983) Paleokart Development. migration of the Kimmeridgian Ascla formation, Maestrat In: Carbonate Depositional Environments (eds. Scholle PA, Basin, Spain. Marine and Petroleum Geology, 18, 287–306. Bebout DG, Moore CH) American Association of Petroleum Salas R, Casas A (1993) Mesozoic extensional tectonics, stratigra- Geologists, 33, 93–5. AAPG, Tulsa. phy, and crustal evolution during the Alpine cycle of the eastern McIntire WL (1963) Trace element partition coefficients — a Iberian basin. Tectonophysics, 228, 33–55. review of theory and applications to geology. Geochimica et Salas R, Permanyer A (2003) Evidencias de generacioń de hidro- Cosmochimica Acta, 27, 1209–64. carburos en la formacioń de margas del Mas d’Ascla (Jura´sico Merten R (2006) Petroleoum exploration and production in superior, Cadena Ibe´rica oriental) y su relacioń con el campo de Spain. In: Contributions to the Geology of Spain (eds. Schroe- Amposta de la Cuenca de Tarragona. Boletıń Geolo´gico y Minero, der R, PerejońA)Zeitschrift der Deutschen Gesellschaft fu¨r 114, 75–86. Geowissenschaften. 157 ⁄ 4, 717–32. Verlagsbuchhandlung, Salas R, Guimera` J, Mas R, Martıń-Closas C, Meleńdez A, Stuttgart. Alonso A (2001) Evolution of the Mesozoic Central Iberian Meyers WJ, Lohmann KC (1985) Isotope geochemistry of region- Rift System and its Cainozoic inversion (Iberian Chain). In: ally extensive calcite cement zones and marine components in Peri-Tethys Memoir 6: Per-Tethyan Rift ⁄ Wrench Basins and Mississippian limestones, New Mexico. In: Carbonate Cements Passive Margins (eds. Ziegles PA, Cavazza W, Robertson AHF (eds. Schneidermann N, Harris PM). SEPM Special Publication, & Crasquin-Soleau S) Me´moires du Museum National d’His- 36, 223–39. toire Naturelle, 186, 145–85. Museúm National d’Histoire Montan˜ez IP (1994) Late diagenetic dolomitization of Lower Naturelle, Paris. Ordovician, Upper Knox Carbonates: a record of the hydrody- Salas J, Taberner C, Esteban M, Ayora C (2007) Hydrothermal namic evolution of the southern Appalachian Basin. AAPG Bul- dolomitization, mixing corrosion and deep burial porosity for- letin, 78, 1210–39. mation: numerical results from 1-D reactive transport model. Mucci A (1987) Influence of temperature on the composition of Geofluids, 7, 99–111. magnesian calcite overgrowths precipitated from seawater. Geo- Seemann U, Pu¨mpin VF, Casson VF (1990) Amposta oil field. chimica et Cosmochimica Acta, 51, 1977–84. AAPG Treatise of Petroleum Geology, Atlas of oil and gas fields, Mucci A, Morse JW (1983) The incorporation of Mg2+ and A-017, 1–20. Sr2+ into calcite overgrowths: influences of growth rate and Skougstad MW, Horr CA (1963) Occurrence and distribution of solution composition. Geochimica et Cosmochimica Acta, 47, strontium in natural waters. US Geological Survey Water Supply 217–33. Paper, 1496-D, 55–97. Nakai N, Jensen ML (1964) The kinetic isotope effect in the bac- Stoessell RK, Klimentidis RE, Prezbindowski DR (1987) Dedo- terial reduction and oxidation of sulfur. Geochimicat et Cosmo- lomitization in Na–Ca–Cl brines from 100 to 200C at 300 chimica Acta, 28, 1893–912. bars. Geochimica et Cosmochimica Acta, 51, 847–55. Neilson JE, Oxtoby NH (2008) The relationship between petro- Trave´ A, Calvet F (2001) Syn-rift geofluids in fractures related to leum, exotic cements and reservoir quality in carbonates – a th early-middle Miocene evolution of the Valle`s-Penede`s half- review. Marine and Petroleum Geology, 25, 778–90. graben (NE Spain). Tectonophysics, 336, 101–20. Orlopp DE (1988) Casablanca oil field, Spain: a karsted Carbonate Trave´ A, Calvet F, Soler A, Labaume P (1998) Fracturing and trap at the shelf edge. Proceedings of the Offshore Technology fluid migration during Palaeogene compression and Neogene Conference, OTC Publisher (Offshore Technology Conference), extension in the Catalan Coastal Ranges, Spain. Sedimentology, Houston. 45, 1063–82. Paquette J, Reeder RJ (1995) Relationship between surface struc- Tucker ME, Wright VP (1990) Carbonate Sedimentology. Black- ture, growth mechanism, and trace element incorporation in cal- well Scientific Publications, Oxford. cite. Geochimica et Cosmochimica Acta, 59, 735–49. Varela J, Vicente-Bravo JC, Navarro J, Esteban M, Martıńez del Paz JDS, Rossetti DF, Macambira MJB (2005) An Upper Olmo W (2005)The oil fields in the Spanish Mediterranean Aptian saline pan ⁄ lake system from the Brazilian equatorial Sea. In: Asociacioń de Geo´logos y Geofı´sicos Espanõles del Petro´- margin: integration of facies and isotopes. Sedimentology, 52, leo (AGGEP)-XXV Aniversario (ed. Martıńez del Olmo W), 1303–21. 121–30. Museo de Ciencias Naturales, Madrid. Permanyer A, Salas R (2005) Integrated thermal model, diagenetic Vegas R, de Vicente G (2004) El surco de Valencia. In: Geologıá history and oil correlation in western Mediterranean, Spain. IV de Espanã (ed. Vera JA) p. 615. Sociedad Geolo´gica de Espanã- ALAGO Workshop on Basin Modeling, Buenos Aires, Communi- Instituto Geolo´gico y Minero de Espanã, Madrid. cation. Veizer J, Compston W (1974) 87Sr ⁄ 86Sr composition of seawater Pirrie D, Marshall JD (1990) High-paleolatitude Late Cretaceous during the Phanerozoic. Geochimica et Cosmochimica Acta, 38, paleotemperatures: new data from James Ross Island, Antarctica. 1460–84. Geology, 18, 31–4. Veizer J, Hoefs J (1976) The nature of O18 ⁄ O16 and C13 ⁄ C12 Reeder RJ, Grams JC (1987) Sector zoning in calcite cement crys- secular trends in sedimentary carbonate rocks. Geochimica et tals: implications for trace element distributions in carbonates. Cosmochimica Acta, 40, 1387–95. Geochimica et Cosmochimica Acta, 51, 187–94. Veizer J, Buhl D, Diener A, Ebneth S, Podlaha OG, Bruckschen Reinhold C (1998) Multiple episodes of dolomitization and dolo- P, Jasper T, Korte C, Schaaf M, Ala D, Azmy K (1997) mite recrystallization during shallow burial in Upper Jurassic Strontium isotope stratigraphy: potential resolution and event shelf carbonates; eastern Swabian Alb, southern Germany. Sedi- correlation. Palaeogeography, Palaeoclimatology, Palaeoecology, mentary Geology, 121, 71–95. 132, 65–77.

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333 Diagenesis of the Amposta offshore oil reservoir 333

Warren J (1999) Evaporites. Their Evolution and Economics. Black- Worden RH, Smalley PC (1996) H2S-producing reactions in deep well Science, Great Britain. carbonate gas reservoirs: khuff formation, Abu Dhabi. Chemical Warren J (2006) Evaporites. Sediments, Resources and Hydrocar- Geology, 133, 157–71. bons. Springer, Germany. Worden RH, Smalley PC, Barclay SA (2003) H2S and diagenetic Watson HJ (1982) Casablanca ﬁeld offshore, a paleogeomorphic pyrite in North Sea sandstones: due to TSR or organic sulphur trap. In: The Deliberate Search for the Subtle Trap, AAPG compound cracking? Journal of Geochemical Exploration, 78-79, Memoir, 32, 237–50. 487–91.

2010 Blackwell Publishing Ltd, Geoﬂuids, 10, 314–333