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84, no. 11 (November 2000), pp. 1719–1742. 1719. Depositional and diagenetic controls on the reservoir quality of Lower Cretaceous. Pendeˆncia sandstones,.
Depositional and diagenetic controls on the reservoir quality of Lower Cretaceous Pendeˆncia sandstones, Potiguar rift basin, Brazil

AUTHORS Sylvia M. C. dos Anjos ⬃ Petrobra´s Research Center, Ilha do Funda˜o, Cidade Universita´ria, Q. 7, 21949–900, Rio de Janeiro, RJ, Brazil; [email protected]

Sylvia M. C. dos Anjos, Luiz F. De Ros, Roge´rio Schiffer de Souza, Carlos Manuel de Assis Silva, and Cristiano L. Sombra

ABSTRACT The quality of the reservoirs of the Lower Cretaceous Pendeˆncia Formation of the Potiguar basin, northeastern Brazil, is directly controlled by depositional facies-related carbonate cementation and compaction. The study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and, in particular, the role of carbonate cementation in poorly understood continental systems. The Pendeˆncia Formation is a thick sequence of fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine sandstones, conglomerates, and shales deposited during the rift stage of the basin. The sandstones are predominantly arkoses (average Q49F40L11), with subordinate plutonic and volcaniclastic feldspathic litharenites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbidites and of compaction in the fluvial deposits. Carbonate cementation in Pendeˆncia reservoirs increases progressively with depth and is facies controlled. Eodiagenetic, nonferroan calcite I (d18OPDB ⳮ10.7 to ⳮ4.0‰; d13CPDB ⳮ17.5 to Ⳮ8.5‰), mesodiagenetic rhombohedral ferroan dolomite/ankerite (d18OPDB ⳮ9.3 to ⳮ3.9‰; d13CPDB, ⳮ1.7 to Ⳮ1.1‰), and ferroan calcite II (d18OPDB ⳮ17.2 to ⳮ6.8‰; d13CPDB ⳮ13.6 to Ⳮ2.3‰) were precipitated at three distinct temperature intervals calculated from the d18O values: 21 to 58⬚C, 70 to 79⬚C, and 85 to 150⬚C, respectively. According to the d13C values, dissolved carbonate for calcite I was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation. Ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter. The shales were a major source of dissolved

Copyright 䉷2000. The American Association of Petroleum Geologists. All rights reserved. Manuscript received February 23, 1999; revised manuscript received March 15, 2000; final acceptance April 15, 2000.

AAPG Bulletin, v. 84, no. 11 (November 2000), pp. 1719–1742

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Presently at the head of the Reservoir Section of Petrobra´s Research Center, Sylvia dos Anjos has developed research on the x-ray diffraction and electron microscopy characterization of clay minerals and on the construction of predictive models of reservoir quality. She graduated in geology at the Federal University of Rio de Janeiro in 1978 and received an M.Sc. degree in shale petrology in 1984 and a Ph.D. in reservoir geology in 1986 from the University of Illinois in Urbana. Luiz F. De Ros ⬃ Universidade Federal do Rio Grande do Sul, Instituto de Geocieˆncias, Av. Bento Gonc¸alves, 9500, Agronomia, 91501–970, Porto Alegre–RS, Brazil; [email protected] Luiz De Ros worked for nine years on reservoir geology and clastic diagenesis in Petrobra´s before joining the Rio Grande do Sul Federal University in 1990. He received his B.Sc. degree from the same university, his M.Sc. degree in reservoir geology from Ouro Preto University, Brazil, and his Ph.D. in sedimentary petrology from Uppsala University, Sweden. He has published on the characterization and quality modeling of clastic reservoirs from several basins in Brazil, Norway, Tunisia, and other countries. Roge´rio Schiffer de Souza ⬃ Petrobra´s Research Center, Ilha do Funda˜o, Cidade Universita´ria, Q. 7, 21949–900, Rio de Janeiro, RJ, Brazil; [email protected] Roge´rio Schiffer de Souza received his B.Sc. degree in geology from Vale dos Sinos University, southern Brazil, in 1980. He worked as teacher and researcher at the Federal University of Rio de Janeiro and received his M.Sc. degree there in 1989. Since 1987 he has worked on sandstone diagenesis and petrologic reservoir characterization at Petrobra´s Research Center, and he received a Ph.D. on predictive modeling of reservoir

quality from the University of Texas at Austin in 2000. Carlos Manuel de Assis Silva ⬃ Petrobra´s Research Center, Ilha do Funda˜o, Cidade Universita´ria, Q. 7, 21949–900, Rio de Janeiro, RJ, Brazil; [email protected] Carlos Manuel de Assis Silva received his B.Sc. degree in geology from the Federal University of Minas Gerais, Brazil, in 1987 and an M.Sc. degree in reservoir geology from Ouro Preto University, Brazil, in 1991. He has worked at Petrobra´s Research Center since 1993, where his main activities deal with the sedimentologic and petrographic characterization of sandstone reservoirs and their application to integrated geological modeling. Cristiano L. Sombra ⬃ Petrobra´s Research Center, Ilha do Funda˜o, Cidade Universita´ria, Q. 7, 21949–900, Rio de Janeiro, RJ, Brazil; [email protected] Cristiano Sombra graduated in geology in 1977 from the Federal University of Bahia, Brazil, and worked from 1978 to 1985 for Petrobra´s as well-site and development geologist. After receiving an M.Sc. degree in reservoir geology from the Federal University of Ouro Preto in 1987, he moved to Petrobra´s Research Center, where he works on statistical and geological methods of reservoir quality prediction and on the combination of sedimentologic, log, and petrophysical characterization of clastic reservoirs. ACKNOWLEDGEMENTS We thank Petrobra´s for access to data and samples and permission to publish this work. LFDR acknowledges the support of the Brazil National Research Council (CNPq) and Rio Grande do Sul State Research Foundation (Fapergs). This article was substantially improved by the suggestions of S. Morad and of AAPG reviewers J. P. Hendry and L. Sanchez-Barreda, as well as of editors N. Hurley and M. Longman.

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carbonate, as indicated by the isotopic similarity between their calcite (d13CPDB ⳮ0.2 to Ⳮ1.8‰; 87Sr/86Sr ⬇ 0.719) and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones. As a result of this cementation pattern, thin turbiditic and deltaic sandstone beds are pervasively cemented. The best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths. Deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales. Alluvial-fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity. However, porosity enhancement was very limited, due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework.

INTRODUCTION Carbonate cements are volumetrically the most abundant and detrimental to porosity of sandstone reservoirs, in which they commonly create barriers to fluid flow and pressure compartments. The recognition of their important influence on reservoir quality has lately attracted the attention of substantial research (e.g., Ortoleva, 1994; Crossey et al., 1996; Morad, 1998). However, knowledge of the geochemical conditions of precipitation, sources, and evolution of these cements is still limited, particularly in continental sequences. The Lower Cretaceous sandstones of the Potiguar rift basin, northeastern Brazil, present an opportunity to unravel the role of carbonate cementation in continental reservoirs and to understand the diagenetic patterns of clastic sequences in interior rifts. Adequate conditions for such a study are provided by a wide and representative coring of the sandstones that have well-defined depositional facies and burial history at different depths and positions within the basin. The Lower Cretaceous Pendeˆncia Formation is comprised of a thick sequence of sandstones, conglomerates, and shales deposited in fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine environments in the Potiguar basin. Oil reservoirs in this unit experienced a variety of diagenetic processes that strongly affected their porosity and permeability. Carbonates are the most abundant cements and, together with compaction, the main factors controlling reservoir quality. The objective of this article is to characterize the patterns of distribution of the carbonate cements, from bed to basin scale, as well as their mineralogical, chemical, and isotopic composition. This characterization is applied to elucidate the conditions of carbonate precipitation, its relationship to compaction, and their combined control on the quality of the Pendeˆncia reservoirs. The results of this article yield significant implications for the exploration of the Pendeˆncia reservoirs and for the general understanding of car-

Rift Reservoirs Quality, Potiguar Basin, Brazil

bonate cementation in sandstones from continental rifts and similar extension basins.

GEOLOGICAL SETTING The Potiguar basin is located in northeastern Brazil (Figure 1). The sedimentary fill of the basin (Figure 2) accumulated during rift, transitional, and drift tectonic stages (Matos et al., 1987; Bertani et al., 1989). The onshore basin is divided by internal highs into the Apodi, the Boa Vista, and the Umbuzeiro grabens. Rapid subsidence during the Early Cretaceous rift stage resulted in the deposition of up to 5000 m of sediments in asymmetric grabens (Figure 3). In the onshore part of the basin, rift phase shales and sandstones of the Pendeˆncia Formation comprise most of the basin fill. The rift stage was followed by a period of uplift and erosion that truncated the top of the Pendeˆncia Formation along basin margins and internal highs, resulting in a regional unconformity (Figure 4). During the transitional stage, marine incursions took place, and lagoonal sediments of the Alagamar Formation were deposited. In the onshore part of the basin, the drift stage was marked by the deposition of a transgressive depositional sequence that had fluvial sediments of the Ac¸u Formation overlain by platform carbonates of the Jandaı´ra Formation. During the same stage in the offshore part of the basin, a regressive sequence (the Tibau, Guamare´, and Ubarana formations) was deposited (Figure 2) (Araripe and Feijo´, 1994). The petroleum reservoirs of the basin are sandstones of the Ac¸u and Alagamar formations, and sandstones and conglomerates of the Pendeˆncia Formation. The Pendeˆncia reservoirs produce in 36 oil fields, hav-

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Figure 1. Location map of onshore Potiguar basin, northeastern Brazil, showing the main structural features and oil fields producing from the Pendeˆncia Formation.

ing an accumulated production of around 2 ⳯ 109 m3 (12.6 ⳯ 109 bbl) of oil and 600 ⳯ 109 m3 of gas and containing a reserve of approximately 5 ⳯ 109 m3 (31.5 ⳯ 109 bbl) of oil and 2500 ⳯ 109 m3 of gas. The main hydrocarbon source rocks in the basin are the lacustrine shales of the Pendeˆncia Formation.

DEPOSITIONAL ENVIRONMENTS In the onshore part of the Potiguar basin, the Pendeˆncia Formation consists of fan-deltaic sandstones and conglomerates deposited along the basin margins and fluvial, deltaic, and turbiditic sandstones, interbedded with lacustrine shales, in the central parts of the basin (Figure 5). Hydrocarbons occur in all these facies at a depth range from 700 to 4030 m. Based on biostratigraphic boundaries, the formation has been informally subdivided into units I to V, from base to top (Figure 3). The lithofacies boundaries are in general diachronous. The fan-delta deposits are composed of very poorly to poorly sorted (average r ⳱ 1.5␾) massive (unstratified in a bed scale) conglomerates and conglomeratic sandstones and medium-grained to finegrained sandstone with decimetric to metric planar stratification (average grain size 0.13␾). Vertical grain-size variations are abrupt and chaotic, and there is no continuity of depositional bodies in a oil-field scale. The fluvial deposits are comprised of mediumgrained to very coarse-grained (average grain size 1.25␾), moderately sorted (average r ⳱ 0.9␾) sandstones in sequences with trough decimetric crossbedding and erosional base. The lateral continuity of the fluvial sand bodies in a scale of hundreds of meters and the low sand-mud ratio of these deposits indicate deposition by braided systems. The deltaic sandstones are fine grained to medium grained (average grain size 2.25␾), moderately well sorted (average r ⳱ 0.6␾), having decimetric planar cross-bedding. The turbiditic deposits are predominantly composed of fine-grained to very fine-grained (average grain size 2.1␾), moderately to well-sorted (average r 0.8␾), massive to parallel-laminated sandstones. Turbidite deposits commonly lack the A interval of the Bouma cycle and show good lateral continuity, indicative of middle to distal lobe deposits. The basal, contrasting unit I is composed of thinly bedded medium-grained to coarse-grained (average grain size 1.1␾) green volcaniclastic sandstones, Anjos et al.

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Figure 2. Stratigraphic column of the onshore and offshore parts of the Potiguar basin (adapted from Araripe and Feijo´, 1994).

probably deposited by alluvial-fluvial systems under a protorift setting. The grains of Pendeˆncia sandstones are subrounded to angular and show shape modifications related to diagenetic processes such as dissolution, compaction, carbonate replacement, and quartz overgrowth cementation. 1722

Rift Reservoirs Quality, Potiguar Basin, Brazil

PETROLOGIC METHODS Cores from 42 oil wells covering the entire onshore part of the basin were sampled in a depth range of 700 to 4300 m. Altogether, 398 blue epoxy resinimpregnated thin sections from the Pendeˆncia sandstones were petrographically described, and composi-

Figure 3. Burial history of an onshore area of the Potiguar basin that was unaffected by the postrift uplift. Decompaction calculated from sonic and density logs. Rift model having nonuniform, two-layer crustal stretching (dcrust ⳱ 1.4; dsubcrust ⳱ 1.0). Thermal conductivity from about 200 measurements in representative lithologies of the basin. Note the rapid subsidence during the first millions of years.

Figure 4. Burial history of an onshore area of the Potiguar basin that was affected by the postrift uplift, which resulted in exposure and erosion of the top of the Pendeˆncia Formation. Procedures as in Figure 3.

tional modal analyses of 248 thin sections were performed by counting 300 points per thin section. Sorting was estimated by comparison with the standard charts of Beard and Weyl (1973). Packing proximity index was determined following Kahn (1956) procedures. Carbonate cements were stained for identification with an acid solution of alizarin red and potassium ferrocyanide. Microporosity was determined as the difference between petrophysical porosity and porosity measured by petrographic modal analysis. The percentage of types of detrital and diagenetic constit-

uents and of porosity is expressed in relation to bulk rock volume. Carbon and oxygen isotopes of the carbonate cements were analyzed in 40 representative samples, preferentially selected for containing only one type of cement. A few samples having bioclasts and more than one carbonate cement were analyzed for comparison. The samples were reacted with 100% phosphoric acid for 1 hour at 25⬚C for calcite and for 24 hours at 50⬚C for dolomite. The released CO2 was analyzed using a Deltae Finnigan MAT mass spectrometer. The phosAnjos et al.

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Figure 5. Schematic structural section of the Pendeˆncia rift, which is divided into asymmetrical grabens separated by internal highs and filled by fandeltaic, fluvial, deltaic, turbidite, and lacustrine deposits and covered by shallow marine carbonates. Section corresponds to line AA⬘ in Figure 1.

phoric acid fractionation factors used were 1.01025 for calcite and 1.01060 for dolomite/ankerite (Friedman and O’Neil, 1977). Stable isotope data are presented in the normal d notation relative to PDB (Craig, 1957). Precision (1r) was better than Ⳳ0.05‰ for both d13C and d18O. The equations of Irwin et al. (1977) were used to calculate the precipitation temperature of calcite and dolomite/ankerite. To identify the clay minerals present in the sandstones, x-ray diffraction analyses of the ⬍2 lm fraction were performed in a Rigaku RU 200 diffractometer in 250 oriented samples. The samples were air-dried, ethylene glycol–saturated, and heated at 490⬚C for 4 hours. Twenty-eight samples were analyzed in a Jeol JXA-840A scanning electron microscope (SEM), having a backscattered electron (BSE) detector, coupled with a Tracor-Northern energy-dispersive x-ray (EDX) analyzer, to characterize the major chemical composition of carbonate cements, the morphology of authigenic clay minerals, and the paragenetic relationships among diagenetic constituents.

DETRITAL COMPOSITION OF THE SANDSTONES The average modal detrital composition of the Pendeˆncia sandstones (Table 1) indicates that the sandstones are predominantly arkoses with an average framework composition of Q49F40L11 (Figure 6). The coarse fan-delta deposits with abundant plutonic fragments, and the sandstones of unit I, rich in volcanic fragments, are feldspathic litharenites. 1724

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Quartz grains (average 35%) are predominantly monocrystalline in the fine-grained to mediumgrained sandstones and polycrystalline in the coarsegrained sandstones. They commonly show corroded borders due to replacement by carbonate cements. Potassium feldspar (average 23%) is four times more abundant than plagioclase, with a predominance of orthoclase over microcline. Feldspars are relatively less abundant in the fine-grained deltaic and turbiditic sandstones. The intense alteration (sericitization, vacuolization, and saussuritization) of feldspar grains that are mixed with fresh feldspar grains is likely to have occurred in the source rocks. In situ, diagenetic alteration processes include dissolution, replacement by carbonates, albitization, and kaolinization. Plutonic and/or high grade metamorphic rock fragments are more common (average 6%) (Table 1) than low-grade to middle-grade micaceous and hornfelsic metamorphic rock fragments (average 1%). Metamorphic carbonate rock fragments (marbles and hornfels) are relatively common in the fan-deltaic reservoirs. Chloritized basic volcanic rock fragments (average ⬍1%) occur essentially in the sandstones of unit I, where they average 12% (Anjos et al., 1990a). Muscovite and biotite average 3% and are most common in the deltaic and turbiditic sandstones. Heavy minerals include garnet, titanite, epidote, zircon, opaques, rutile, tourmaline, amphiboles, and pyroxenes, indicating source rocks dominantly plutonic and metamorphic of the amphibolite facies. Tremolite/actinolite, pargasite, and diopside are derived from hornfelsic contact-metamorphic rocks dominantly along the southern rift margin. Mud in-

Table 1. Average Detrital Compositon and Average and Maxima (Bulk Rock Vol.%) of the Main Diagenetic Constituents in the Depositional Facies and Stratigraphic Units of the Pendeˆncia Formation Unit Facies Detrital constituents Quartz K-feldspar Plagioclase Plutonic rock fragments Metamorphic rock fragments Volcanic rock fragments Carbonate grains Mud intraclasts Micas Heavy minerals Diagenetic constituents Infiltrated clays Quartz overgrowths Feldspar overgrowths Calcite Fe-dolomite/ankerite Chlorite Kaolinite Pyrite Grain diss. por./total

Unit I

Units II, III, and IV

Alluvial ? (11 samples)

Fan delta (18 samples)

Fluvial (90 samples)

Delta front (86 samples)

Turbidite (47 samples)

45.1 11.7 6.7 3.6 – 12.3 1.3 3.1 3.2 2.0

26.1 18.0 1.0 19.8 6.7 0.1 1.8 0.7 2.1 0.6

37.5 25.9 5.1 6.0 1.0 0.3 0.1 3.1 2.2 0.8

33.2 23.6 5.8 4.1 0.5 0.1 0.1 1.9 4.8 0.7

36.7 20.4 6.9 4.0 0.5 0.3 0.7 5.8 3.3 0.7

– 1.0/3.0 tr/0.1 5.5/7.0 – tr/0.1 – 1.3/3.0 –

0.8/11.0 0.1/1.0 0.5/2.5 7.2/18.0 1.4/5.0 1.0/10.0 9.3/21.0 0.1/0.1 0.4

0.1/3.0 0.7/4.0 0.5/3.0 1.5/8.0 0.4/6.0 1.0/7.0 0.1/1.0 tr/2.0 0.2

– 0.8/5.0 0.9/4.5 3.0/14.0 1.0/17.0 1.2/13.0 0.1/3.0 tr/1.0 0.2

– 0.5/3.0 0.8/4.0 7.7/33.0 1.6/15.0 1.8/18.0 – 0.2/2.0 0.2

traclasts (average 3%) are most abundant in the turbiditic and fluvial deposits (Table 1) and are extensively compacted to pseudomatrix and chloritized. Carbonate bioclasts (mollusks, ostracods), oncoliths, peloids, and intraclasts are minor constituents, except in localized interdeltaic hybrid deposits. The preservation of this immature detrital assemblage indicates a provenance from the uplifted basement blocks along the rift margins, with sediments rapidly eroded under arid climate and transported into the basin by fan-delta systems.

DIAGENESIS The Pendeˆncia sandstones were subjected to diagenetic processes that intensely affected their reservoir characteristics, as previously recognized by Alves (1985), Farias (1987), Carrasco (1989), and Anjos et al., (1990b; 1991). In this section, we describe these processes in terms of eogenetic modifications, occur-

ring relatively soon after deposition and under influence of the surface conditions; mesogenetic processes developed during effective burial; and telogenetic modifications related to the uplift and exposure along the rift margins and internal highs (sensu Schmidt and McDonald, 1979). The definition of a precise paragenetic timing relationship was not possible for all the diagenetic phases observed, owing to the very rapid rift-phase subsidence (implying accelerated and compressed eogenetic and early mesogenetic modifications) and the scarcity of geothermometric and geochronologic data. Based on the paragenetic relationships observed in optical and electronic microscopy, elemental and isotopic composition, and x-ray diffractometry, separate paragenetic sequences were defined for the Pendeˆncia reservoirs in areas affected and unaffected by postrift exposure (Figure 7). The relative abundance of cement types does not vary consistently with depth, suggesting a complex interplay of eogenetic, facies-related, structural, and stratigraphic controls for their distribution. Anjos et al.

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Figure 6. Detrital composition of the sandstones from the main depositional facies and basal, volcaniclastic unit I of Pendeˆncia Formation, plotted in McBride (1963) diagrams. The sandstones are mainly arkoses. The coarse, fan-deltaic sandstones with high amounts of plutonic/high-grade metamorphic rock fragments and the volcaniclastic unit I sandstones are dominantly feldspathic litharenites.

Mechanical clay infiltration affected the fan-delta and fluvial deposits subjected to episodic flooding under semiarid climate (Table 1) (Moraes and De Ros, 1990). Infiltrated clays occur as thin, irregular, discontinuous coatings of mixed-layer illite-smectite and chlorite-smectite, evolved from detrital smectites (Figure 8a). These coatings show partial dissolution and kaolinization in some fan-delta deposits, due to meteoric water influx. Quartz cementation was widespread but volumetrically limited. Quartz overgrowths and prismatic outgrowths average about 0.5% of the bulk rock volume, reaching 5% in some deltaic and fluvial sandstones (Table 1). Quartz cementation does not show any trend of increase with depth or with depositional facies and varies highly among closely spaced samples. Quartz precipitation was recurrent during diagenesis. This is suggested by the occurrence of (1) continuous overgrowths around the grains of some samples, indicating a shallow, precompactional or syncompactional precipitation; (2) prismatic quartz outgrowths associated with late chlorite, albite, and ferroan carbonates in deep samples; and (3) multiple overgrowths in deep samples (Figure 8b). Authigenic feldspar has a distribution and paragenetic relationship similar to that of quartz, but it oc1726

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curs in smaller amounts (Table 1). The scarce potassium feldspar overgrowths and outgrowths were covered by precompactional calcite. The formation of albite overgrowths and prismatic outgrowths and ingrowths and the albitization of detrital feldspars are associated with ferroan postcompactional carbonates and were most intense in the deep graben-center sandstones (Figure 8c). Optical petrographic and BSE criteria (cf. Morad, 1986; Morad et al., 1990), and EDX analyses indicate that in these rocks both detrital Kfeldspars and plagioclases were albitized to a similar degree. The carbonate cements are the most abundant diagenetic constituents in all the depositional facies throughout most of the studied depth range (Figure 9) and comprise calcite, ferroan calcite, ferroan dolomite, and ankerite (Figure 10). Calcite predominates over dolomite/ankerite, averaging ⬇5% and reaching up to 33% (Table 1). Carbonate cement content shows no systematic variation with depth (Figures 10, 11). Among the diverse depositional facies, fluvial sandstones have the smaller amounts of calcite cement at any depth interval, whereas the turbidites show the larger amounts (Figure 9). In a metric to centimetric scale, carbonate cements show four main patterns of distribution: (1) irregular, randomly scattered patches; (2) alternation of cemented, coarser grained laminae and uncemented, finer grained laminae; (3) pervasively cemented beds; and (4) peripheral cementation along the contacts with interbedded shales (Moraes and Surdam, 1993). Within the sand bodies, calcite displays preferential cementation of the coarser, originally more permeable beds or laminae, whereas dolomite occurs preferentially along micaceous or intraclastic layers. Three main compositional/textural types of carbonate cements were recognized in the reservoirs: • Calcite I is characterized by nonferroan composition (FeCO3 up to 1.1 mol %; average 0.6 mol %) (Table 2, Figure 10) and by the relatively loose packing of the cemented areas, which show dominantly point and long intergranular contacts. This indicates that calcite I is syncompactional, precipitated at relatively shallow depths. Calcite I dominates in shallow reservoirs, at less than 1800 m of present depth. Up to 30% of calcite occurs in pervasively cemented sandstone layers and along shale contacts (Figure 8d). Calcite I crystals (⬇ 0.2 to 5 mm) show mosaic to poikilotopic habits, marginally replacing the framework grains. Calcite I is also commonly present

Figure 7. Diagrams showing the paragenetic sequence of the main diagenetic features in Pendeˆncia Formation reservoirs of areas unaffected (a) and affected (b) by postrift exposure.

along the cleavage planes of mica (mostly biotite), causing expansion of the mica flakes indicative of shallow precipitation (Figure 8e). In this location, calcite I shows its highest observed iron contents (⬇ 1 mol % FeCO3), possibly due to local iron derivation from the biotite. Calcite I d18OPDB values

range from ⳮ10.7 to ⳮ4.0‰, and d13CPDB, from ⳮ17.5 to Ⳮ8.5‰ (Table 3). • Calcite II is consistently ferroan (1.3 to 6.1 mol % FeCO3; average 2.6 mol %); (Table 2, Figure 10), without visible iron zonation in individual crystals. It cements areas with tight grain packing (domiAnjos et al.

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Figure 8. (a) Anisopachous coatings of mechanically infiltrated clays; red-stained detrital carbonate grain; crossed polarizers (hereafter XP); (b) multiple quartz overgrowths (indicated by arrows) postdated by ferroan calcite II (stained purple) that extensively replaces feldspar grains (indicated by F); uncrossed polarizers (hereafter //P); (c) partially albitized K-feldspar grain (albite having darker gray tone) with prismatic albite outgrowths (indicated by arrows); BSE image; (d) poikilotopic nonferroan calcite I (stained red) cementing an incipiently compacted sandstone; (XP); (e) calcite I expanding biotite flakes; (XP Ⳮ 1/4 k plate); (f) postcompactional, poikilotopic calcite II corroding the grains margins; (XP).

nantly concavo-convex and sutured intergranular contacts) as crystals 50 to 1000 lm across, uncommonly showing poikilotopic habit. Calcite II covers and surrounds, and thus postpones, quartz overgrowths. It extensively replaces feldspar and quartz 1728

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grains (Figures 8b, 8f) and partially fills intragranular pores within dissolved detrital feldspars. Calcite II dominates in reservoirs below 1300 m. The d18OPDB values range between ⳮ17.2 and ⳮ6.8‰, and d13CPDB values from ⳮ13.6 to Ⳮ2.3‰ (Table 3).

Figure 9. Average amounts of carbonate, kaolinite, and other cements in sandstones of the main depositional systems.

• Ferroan dolomite/ankerite (FeCO3 10.4 to 31.9 mol %, average 22.7 mol %) (Table 4, Figure 10) is present in reservoirs deeper than 1300 m and is most abundant in the micaceous delta-front and turbidite sandstones (up to 17%; average ⬇ 1%) (Table 1). Ferroan dolomite/ankerite appears as intergranular rhombohedral crystals (Figure 12a), commonly zoned with iron content increasing outward. It also replaces feldspar and quartz grains and calcite I cement and expands and replaces micas (Figure 12b). Its preferential precipitation along micaceous levels enhances the permeability anisotropy of deltaic and turbiditic sandstones (cf. Moraes and Surdam, 1993). Ferroan dolomite/ankerite is locally engulfed and replaced by, and thus postdated by calcite II.

Figure 11. Intergranular volume, petrographic macroporosity, total cement content, and calcite content as a function of depth and relative to each depositional system. A general trend of porosity loss due to compaction with increasing depth is dominant in the first 2 km of depth. Early carbonate cementation prevented compaction of most turbidite reservoirs.

The d18OPDB values of ferroan dolomite/ankerite range from ⳮ9.3 to ⳮ3.9‰, and d13CPDB, from ⳮ1.7 to Ⳮ1.1‰ (Table 3).

Figure 10. Major chemical composition (EDX) of the carbonate cements in the Pendeˆncia reservoirs. Detail of the calcite cements (right) shows a minimum overlap between calcite I and calcite II. Anjos et al.

1729

Table 2. Composition from EDX Analyses of Representative Calcite Cements* Well

Depth

CaCO3

MgCO3

FeCO3

SrCO3

MnCO3

3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 7-LV-10-RN 7-LV-10-RN 7-LV-10-RN 7-LV-10-RN 7-LV-10-RN 7-LV-10-RN 7-SE-19-RN 7-SE-19-RN

1371.3 1371.3 1371.3 1371.3 1371.3 1371.3 1371.3 1371.8 1371.8 1371.8 697.4 697.4 697.4 697.4 697.4 697.4 1309.7 1309.7

99.3 99.2 100.0 97.8 100.0 97.6 96.5 96.1 98.1 98.2 98.4 98.1 97.4 98.2 97.5 97.5 93.4 99.1

0.1 bdl bdl 0.3 bdl 0.9 0.2 0.7 bdl bdl bdl bdl 1.3 bdl bdl bdl 2.2 0.2

0.7 bdl bdl 1.1 bdl 0.7 1.0 0.2 0.7 0.5 1.1 0.2 bdl 0.3 0.3 bdl 1.0 bdl

bdl bdl bdl 0.5 bdl 0.5 bdl 2.7 bdl bdl 0.1 bdl bdl bdl bdl bdl 0.7 bdl

bdl 0.8 bdl 0.3 bdl 0.4 2.4 0.3 1.2 1.3 0.4 1.7 1.3 1.5 2.3 2.5 2.7 0.8

Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I

97.9

0.7

0.6

0.9

1.3

Calcite I

95.7 95.4 96.0 98.3 96.9 94.2 95.9 94.9 87.2

0.4 0.8 0.3 bdl 0.6 0.3 1.0 bdl 2.1

1.3 2.1 2.2 1.5 2.2 3.8 2.2 2.1 6.1

1.4 0.6 0.3 bdl 0.2 bdl bdl bdl bdl

1.2 1.1 1.2 0.2 0.1 1.8 0.9 3.0 4.7

Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II

94.9

0.8

2.6

0.6

1.6

Calcite II

Calcite I average 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 7-SE-19-RN 7-SE-19-RN

1371.8 1371.8 1371.8 1371.8 1371.8 1371.8 1371.8 1309.7 1309.7

Calcite II average

Carbonate

*bdl ⳱ below detection limit.

Other minor types of diagenetic carbonate include displacive nodules of microcrystalline calcite (calcrete) locally precipitated in fluvial and deltaic fine sediments, and microcrystalline to cryptocrystalline calcite cementing interdeltaic bioclastic sediments. Chlorite is the most common clay mineral (average ⬇ 1%; up to 18%) (Table 1) and occurs as pore-lining rims (Figure 12c) and replacing mud intraclasts, micas, volcanic rock fragments, heavy minerals, and mechanically infiltrated clay coatings. In unit 1 sandstones with abundant volcanic fragments, chlorite rims are thick and continuous around the grains. These precompac1730

Rift Reservoirs Quality, Potiguar Basin, Brazil

tional chlorite rims were generated by the transformation of eogenetic smectite rims, as indicated by the presence of chlorite/smectite mixed with the chlorite (cf. Humphreys et al., 1994). In these sandstones, basic volcanic rock fragments were extensively altered to smectite and chlorite/smectite (corrensite) (Figure 12d). Fan-delta and fluvial sandstones contain discontinuous, irregular chlorite and chlorite/smectite coatings, which morphology indicates a derivation from the replacement of mechanically infiltrated smectitic coatings. Conversely, the deltaic and turbiditic reservoirs have delicate, thin (⬍ 10 lm), and discontinuous post-

Table 3. Isotopic Ratios and Calculated Precipitation Temperatures of Carbonates in Pendeˆncia Formation Well 7-LV-23 7-LV-25 7-BR-6 7-BR-6 1-CRB-1 1-DR-2 1-GO-1 1-GO-1 7-JD-10 3-LV-5 7-LV-10 7-LV-11 7-LV-11 7-LV-25 7-LV-25 1-SGR-1 4-LOR-2 4-LOR-2 4-LOR-2 4-LOR-2 7-JD-10 4-LOR-2 3-UPN-2 1-LV-2 1-PX-1 1-RFQ-1 1-RFQ-1 1-RFQ-1 1-RFQ-1 1-RAP-3 1-LU-1 1-LU-1 1-LU-1 1-PX-1 1-RAP-3 1-RDC-1 1-LV-2 1-LG-1 1-LG-1 1-LO-1 1-LO-1 1-LO-1 1-LG-1 1-LO-1 1-LO-1 7-SE-19 7-SE-37 3-UPN-2

Depth (m)

d13C (PDB)

d18O (PDB)

d18Owater

Tprecip. ⬚C

804.40 941.20 1230.50 1241.35 1745.75 608.30 1356.60 1363.35 703.80 716.80 936.10 922.90 937.60 939.70 951.50 1525.50 679.45 686.90 694.40 750.40 791.20 696.40 1500.80 2241.20 1788.85 2584.65 2896.40 2899.10 3257.50 3501.52 1774.80 1789.30 2597.90 1641.40 3489.20 1912.80 2231.60 2251.50 2258.20 1224.35 2438.45 2809.80 2828.15 3001.90 2812.75 1320.20 1309.20 1468.55

ⳮ0.4 ⳮ1.2 1.5 0.9 ⳮ0.4 1.8 ⳮ0.8 0.9 ⳮ17.5 1.8 1.4 1.2 0.7 ⳮ0.9 ⳮ0.4 ⳮ6.7 2.2 ⳮ2.3 2.2 8.5 8.3 ⳮ4.0 0.9 1.1 ⳮ0.8 ⳮ0.6 ⳮ3.3 ⳮ4.1 0.8 ⳮ13.6 2.3 2.2 2.3 ⳮ0.7 ⳮ12.6 ⳮ1.6 ⳮ1.6 0.0 ⳮ0.6 ⳮ10.5 ⳮ5.3 ⳮ4.0 ⳮ2.4 ⳮ5.5 ⳮ4.2 ⳮ1.7 ⳮ0.8 1.2

ⳮ9.1 ⳮ8.4 ⳮ7.8 ⳮ9.5 ⳮ10.2 ⳮ7.0 ⳮ6.5 ⳮ10.7 ⳮ8.3 ⳮ10.5 ⳮ7.3 ⳮ7.0 ⳮ7.4 ⳮ6.2 ⳮ9.7 ⳮ4.0 ⳮ7.2 ⳮ5.9 ⳮ7.3 ⳮ6.7 ⳮ8.5 ⳮ8.3 ⳮ7.4 ⳮ10.8 ⳮ13.7 ⳮ13.2 ⳮ11.7 ⳮ12.9 ⳮ12.2 ⳮ12.7 ⳮ12.3 ⳮ11.4 ⳮ11.6 ⳮ12.5 ⳮ13.3 ⳮ12.2 ⳮ14.2 ⳮ9.6 ⳮ12.3 ⳮ10.3 ⳮ14.4 ⳮ13.3 ⳮ10.8 ⳮ17.2 ⳮ15.0 ⳮ3.9 ⳮ4.8 ⳮ4.8

ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

48 44 40 50 54 36 34 58 43 57 37 36 38 32 51 21 37 30 37 34 44 43 38 94 117 113 101 110 105 109 106 99 100 108 114 105 121 85 106 90 124 114 94 150 129 70 78 78

87

Sr/86Sr

0.719488 0.718996

0.718583 0.712987

0.707732

0.71817

0.714052

0.722349 0.717537 0.720265

Carbonate Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite I Calcite IⳭooids Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Calcite II Fe-dolomite Fe-dolomite Fe-dolomite

Anjos et al.

1731

Table 3. Continued Well 3-UPN-2 3-UPN-2 3-UPN-2 1-LG-1 1-LG-1 1-PX-1 1-PX-1 1-UPN-1 1-UPN-1

Depth (m)

d13C (PDB)

d18O (PDB)

d18Owater

Tprecip. ⬚C

1500.00 1490.95 1493.25 1745.00 1812.00 1068.00 1128.00 1116.00 1314.00

ⳮ0.5 1.0 1.1 0.8 ⳮ0.2 0.0 0.5 ⳮ0.0 1.8

ⳮ5.0 ⳮ9.3 ⳮ8.4 ⳮ7.7 ⳮ7.8 ⳮ3.3 ⳮ4.8 ⳮ2.8 ⳮ7.0

2

79 116 108 40 40 18 25 16 36

2 2 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3 ⳮ3

87

Sr/86Sr

0.71906 0.718735

Carbonate Fe-dolomite Fe-dolomite Fe-dol. (Ⳮcal. II) Calcite in shale Calcite in shale Calcite in shale Calcite in shale Calcite in shale Calcite in shale

Table 4. Composition from EDX Analyses of Representative Fe-Dolomite/Ankerite Cements* Well

Depth

CaCO3

MgCO3

FeCO3

SrCO3

MnCO3

Carbonate

3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 3-CAC-3-RN 7-SE-19-RN 7-SE-19-RN 7-SE-19-RN 7-SE-19-RN 7-SE-19-RN 7-SE-19-RN 7-UPN-12 7-UPN-12 7-UPN-12 7-UPN-12 7-UPN-12 7-UPN-12 7-UPN-12 1-SE-22-RN 1-SE-22-RN 1-SE-22-RN 1-SE-22-RN 1-SE-22-RN 1-SE-22-RN

1371.3 1371.3 1371.8 1371.8 1371.8 1371.8 1371.8 1309.7 1309.7 1309.7 1309.7 1309.7 1309.7 1545.5 1545.5 1545.5 1545.5 1545.5 1545.5 1545.5 1449.5 1449.5 1449.5 1449.5 1449.5 1449.5

63.5 63.2 57.2 65.4 67.1 65.8 64.5 58.9 55.1 55.1 57.7 58.0 66.0 61.5 60.5 59.7 56.7 60.5 55.7 61.3 54.7 59.1 59.8 58.0 62.8 59.0

15.9 16.8 11.4 14.7 16.2 17.0 16.0 14.7 12.5 13.2 9.5 10.0 19.7 12.2 13.5 13.4 16.1 12.4 10.8 11.5 13.8 17.9 16.2 17.5 14.9 16.9

16.8 15.6 26.2 17.5 13.1 14.1 15.9 24.8 30.8 31.2 26.4 30.6 10.4 24.8 25.8 26.3 25.9 25.6 31.9 25.3 29.1 16.6 18.8 23.0 19.1 21.3

bdl bdl bdl bdl 0.1 0.9 bdl 1.2 bdl bdl 4.0 0.2 bdl bdl bdl bdl bdl 0.9 0.9 0.7 bdl bdl 0.7 0.4 bdl 1.4

3.8 4.4 5.2 2.4 3.6 2.2 3.6 0.4 1.7 0.5 2.4 1.2 3.9 1.5 0.3 0.6 1.3 0.7 0.7 1.2 2.3 6.4 4.5 1.1 3.3 1.4

Fe-dolomite Fe-dolomite Ankerite Fe-dolomite Fe-dolomite Fe-dolomite Fe-dolomite Ankerite Ankerite Ankerite Ankerite Ankerite Fe-dolomite Ankerite Ankerite Ankerite Ankerite Ankerite Ankerite Ankerite Ankerite Fe-dolomite Fe-dolomite Ankerite Fe-dolomite Ankerite

60.0

14.5

22.7

0.5

2.3

Ankerite

Compositional average *bdl ⳱ below detection limit.

1732

Rift Reservoirs Quality, Potiguar Basin, Brazil

Figure 12. (a) Blocky ferroan dolomite/ankerite in deltaic reservoir with incipient feldspar dissolution; outer, dark-blue stained zones and outgrowths are ankerite; (//P); (b) ankerite (stained blue) expanding and replacing biotite; (//P); (c) dissolved rock fragment rimmed internally and externally by chlorite and partially filled by kaolinite (K); (//P); (d) volcanic fragments altered to smectite and chlorite/smectite (corrensite) in volcaniclastic unit I; (XP); (e) intense kaolinization of feldspars and mud pseudomatrix in fan-deltaic conglomerate; (//P); (f) pyroxene (diopside) grain overgrown and marginally replaced by authigenic Na-Fe-amphibole (arfvedsonite) (brown); replacive poikilotopic calcite cement (top left) and partially dissolved, microcrystalline carbonate rock fragment (bottom); (XP Ⳮ 1/4 kmica plate). See Figure 8 for descriptions of abbreviations.

compactional chlorite rims that cover quartz and feldspar overgrowths and dolomite rhombs and that are engulfed by late prismatic quartz outgrowths and albite. In deep reservoirs, massive pore-filling aggregates

of chlorite containing dark impurities of Ti-oxides and organic matter are interpreted to be the product of pervasive chloritization of pseudomatrix derived from the compaction of mud intraclasts, altered volcanic Anjos et al.

1733

rock fragments, and ferromagnesian heavy mineral grains. Kaolinite is limited to the borders of the basin and down to depths around 600 m below the postrift (pre– Alagamar Formation) unconformity. Up to 21% kaolinite is present in the marginal fan-delta deposits (average 9.3%), but it is scarce in the deltaic and fluvial reservoirs and absent in the turbiditic and basal volcaniclastic unit I deposits (Table 1). In the fan-deltaic deposits, kaolinite extensively replaces feldspar grains, micas, and mud intraclasts (Figure 12e) and covers chlorite rims (Figure 12c) and dissolved ferroan dolomite/ankerite rhombs. The distribution and the paragenetic setting of this kaolinite suggests a late, telogenetic origin by meteoric water infiltration. Besides grain rearrangement, mechanical compaction resulted in the deformation of ductile fragments such as mud intraclasts and metamorphic and volcanic rock fragments and in the fracturing of rigid grains. Samples close to major fault zones show extensive grain fracturing. Mechanical compaction is dominant in the interval down to 1200 m of present depth, where the original porosity was reduced from an estimated average of about 40% to 20–25% (Figure 11). Beyond that depth, chemical compaction through intergranular pressure dissolution predominates. Chemical compaction through intergranular pressure dissolution promoted the development of concavo-convex and sutured intergranular contacts, and it is most intense in the uncemented or poorly cemented, fine, micaceous sandstones of delta-front and turbidite deposits. This intense chemical compaction is related to enhanced dissolution of quartz grains along the contacts with the micas. Bjørkum (1996) suggested that the enhanced dissolution of quartz in contact with micas and illitic clays is not mainly controlled by pressure but a product of a chemically catalyzed reaction along the phyllosilicate surfaces. In Pendeˆncia sandstones, however, the deformation of mica flakes in contact with the quartz grains indicates that pressure is an important component in this process. Minor diagenetic constituents include anatase, which occurs mostly as residual rims left from the dissolution of detrital heavy minerals such as ilmenite and amphibole but is also associated with chloritized micas and volcanic rock fragments (cf. Morad, 1988). It is consequently most abundant (up to 2%) along laminae with concentrations of these detrital constituents. Microcrystalline pyrite is scarce, but some deep sandstones show coarsely crystalline pyrite filling intergranular pores and extensively replacing detrital and 1734

Rift Reservoirs Quality, Potiguar Basin, Brazil

diagenetic constituents such as feldspar and heavy mineral grains, biotite flakes, volcanic rock fragments, mud intraclasts, and chlorite and ferroan carbonate cements. Authigenic Na-Fe-amphibole (arfvedsonite) occurs in some fan-delta-front arkoses and litharenites along the southern basin margin as overgrowths, fracture healing, and replacement of detrital amphibole and pyroxene (Figure 12f). It is interpreted to have precipitated from K-Na-Ca-Fe-Mg-Ti-chloride brines that ascended along regional faults from deep basinal and underlying basement sources (De Ros et al., 1994). Arfvedsonite pre-dates calcite II cement and is associated with small authigenic titanite crystals and with chlorite. Poikilotopic laumontite occurs locally in the volcaniclastic sandstones of unit I, extensively replacing detrital feldspars and heavy minerals.

DISCUSSION Porosity Evolution of the Reservoirs The sharp decline of intergranular volume and porosity with depth and their currently low average values indicate that compaction was intense in Pendeˆncia sandstones (Figure 11). Compaction was limited by early calcite I cementation, mostly in the turbiditic reservoirs. Compaction and cementation were factors of similar importance in the destruction of porosity, with a dominance of cementation in the turbiditic and of compaction in the fluvial sandstones, as indicated by the plot of cement volume percent vs. intergranular volume percent (Figure 13; cf. Houseknecht, 1987). Both intergranular and intragranular macroporosity decrease steeply with depth (Figure 14a). This is a result of the immature detrital composition of the reservoirs and of a burial history characterized by a rapid rift stage (Early Cretaceous) subsidence followed by a long residence at the present depths (Figure 3). Trends of porosity decrease vary, however, for each depositional facies (Figure 14b). Fan-delta reservoirs have the steepest porosity decrease with depth because of their poor sorting, mineralogical immaturity (abundant rock fragments), and high amounts of diagenetic kaolinite and calcite. Conversely, the fluvial reservoirs display the lowest gradient of porosity decrease with depth, owing to their coarse grain size, moderate to good sorting, and smaller cement volume (Figure 14b). The turbiditic reservoirs display an intermediate behavior. Despite their high content of ductile mica and mud

Figure 13. Plot of cement volume percent vs. intergranular volume percent of representative Pendeˆncia sandstones (cf. Houseknecht, 1987). Compaction and cementation had in general a similar importance in the destruction of porosity, with a dominance of cementation in the turbidite and of compaction in the fluvial deposits.

Figure 14. (a) Total petrographic macroporosity (dashed line), and grain dissolution porosity (intragranular and moldic; continuous line) as a function of depth in sandstones of diverse depositional systems; (b) petrophysical porosity vs. depth for reservoirs from the main depositional systems of the Pendeˆncia Formation.

intraclasts, compaction was constrained by their good sorting and moderate to high cement contents (Figure 14b).

The generation of secondary porosity by grain dissolution (feldspars, volcanic rock fragments, intraclasts, and heavy minerals) was volumetrically subordinate in relation to total porosity. Evaluation of the amounts of intergranular secondary porosity generated through cement dissolution is extremely subjective and imprecise (Giles, 1987). Therefore, the intergranular pores were quantified without any attempt to discriminate between primary and secondary pores The selective distribution of calcite cement in the coarser-grained laminae within the sandstones and the euhedral calcite crystal outlines commonly observed in Pendeˆncia sandstones are characteristic of partial cementation rather than of partial dissolution. Therefore, intergranular porosity seems to be largely primary and not secondary. However, in places calcite I cement shows irregular crystal borders indicative of dissolution, particularly closer to the basin margins and to the postrift unconformity. Dissolution of late ferroan carbonates was of minor importance and limited to a partial dissolution of ankerite and ferroan calcite II. In areas affected by postrift exposure, the occurrence of both postcompactional ferroan calcite corroded and covered by kaolinite and uncorroded engulfing and replacing kaolinite suggests two time intervals of calcite II precipitation. The restriction of kaolinite to the basin margins and its proximity to the postrift unconformity is attributed to meteoric water influx related to postrift uplift and erosion (Anjos et al., 1990b, 1991, 1992) (Figure 4). The infiltration of meteoric water was more efficient along the rift margins owing to (1) the high sand/ mud ratio of the marginal fan-deltaic systems, (2) the hydraulic head provided by the uplifted rift shoulders, and (3) the major marginal faults that provided enhanced permeability pathways. Some of the basin-center, delta-front, and turbidite reservoirs (e.g., Serraria [SE] and Upanema [UPN] oil fields) (Figure 1) show minor dissolution of feldspar grains and dolomite/ankerite cements but no signs of telogenetic meteoric influx such as kaolinization. This suggests that this dissolution occurred during mesodiagenesis, possibly due to solvents coming from the enclosing shales, such as organic carboxylic acids generated by the thermal maturation of organic matter, a process proposed mainly by Surdam and co-workers (1989a, b). However, formation-water analyses from six Pendeˆncia Formation oil fields reveal that the present concentrations of organic acids are fairly low (Table 5). Oil fields close to the basin margins (Cachoeirinha [CAC], Livramento [LV], and Treˆs Marias [TM]) Anjos et al.

1735

Table 5. Organic Acids Concentrations (ppm) in Formation Waters from Pendeˆncia Formation Oil Fields Wells Depth (m) Formate Acetate Propionate Butanate Pentanate Salicilate

CAC-6 1849–1858

LV-21/26 750–765 950–975

LOR-11 856–859 868–871

UPN-4/14 1377–1380 1488–1498

SE-33 1362–1364 1377–1380

1165–1166

trace 22.8 47.6 not detected not detected trace

trace 87.2 26.9 not detected not detected not detected

trace 976 32.1 trace trace not detected

37.1 310 29.3 trace trace not detected

40.5 981 97.2 16.8 12.1 not detected

trace 26.8 42.5 not detected not detected 6.25

(Figure 1) show evidence of dilution of the organic acids by meteoric waters, including the comparatively high concentration of propionate, which is commonly the by-product of partial meteoric degradation of dicarboxylic solvents, particularly malonic acid. This evidence suggests that organic solvent concentration may have been higher in the past and could account for the minor observed mesodiagenetic dissolution (Anjos et al., 1991). Conditions of Carbonate Cementation The sources and geochemical conditions of the carbonate cementation in the Pendeˆncia reservoirs are indicated by aspects of the chemical and isotopic composition of the cements and their spatial distribution. Distribution Patterns The distribution patterns of the carbonate cements indicate that the main source was located outside the reservoirs. The preferential cementation of coarser grain size laminae suggests a selective advection of fluids bringing dissolved carbonate along the conduits having originally higher permeability. The common equivalence in detrital composition between the preferentially cemented coarse-grained laminae and the fine-grained laminae indicates that differential pore size and permeability, and not composition, are responsible for this pattern. The scarcity of detrital carbonate, represented by marble and calc-hornfels rock fragments in some fan-deltaic lithic conglomerates and by bioclasts in interdeltaic hybrid arenites, indicates that internal sources were of very subordinate importance for carbonate cementation. The peripheral cementation of the sandstones along the contacts with interbedded shales (Moraes and Surdam, 1993) suggests that the shales were a ma1736

Rift Reservoirs Quality, Potiguar Basin, Brazil

TM-6

jor source of dissolved carbonate. This is also supported by (1) the large amount of cement in the turbidite reservoirs (sequences having the highest shale/sandstone ratios) (Figure 9), (2) the high carbonate content of the shales (up to 36% micrite), and (3) the isotopic similarity between calcite from the shales and most of the carbonate cements in the sandstones. The diffusion of dissolved carbonate from shales to sandstones is related to the dissolution of microcrystalline carbonate and to the maturation of organic matter in the lacustrine shales, which are the main oil source rocks. The implication of this cement distribution is that the amount of calcite cement is strongly controlled by the sand/mud ratio and by the thickness of the sandstone bodies in the turbiditic and deltaic sandstones interbedded with shales. Log data from two oil fields having dominantly peripheral calcite cementation show that sandstone bodies less than 20 cm thick are substantially less porous, owing to extensive calcite cementation (Figure 15). Peripheral cementation by calcite I and dolomite/ankerite predominates in turbiditic and deltaic reservoirs. Fluvial reservoirs display complex patterns of cementation by calcite II or calcite I as irregular patches and pervasively cemented layers and laminae related to advection through higher permeability pathways. Large-scale mass transfer during carbonate cementation in the comparatively mud-poor fluvial systems along the basin center indicates that both diffusion and advection were active in these areas. Because the basement includes metamorphic carbonate rocks (marbles and calc-hornfels), another potential source for dissolved carbonate involves the deep pressure dissolution and metamorphic/metassomatic reactions involving these lithologies. The precipitation of sodic authigenic amphibole (arfvedsonite) (Figure 12f) intercalated in the normal paragenetic sequence in some sandstones along the southern margin of the

Figure 15. Relation between the thickness of turbidite layers and the average porosity in the Upanema and Lorena fields.

basin suggests an episodic migration of fluids from deep basement during rifting (De Ros et al., 1994). Compositional Patterns The compositional patterns of the carbonate cements indicate their sources and the conditions of their precipitation. The pattern of early cementation by nonferroan carbonates followed by cementation and replacement by ferroan carbonates that occurs in Pendeˆncia reservoirs is observed in many sandstones from rifts or other extension basins, such as Tertiary U.S. Gulf Coast (Boles and Franks, 1979; Franks and Forester, 1984; Land and Fisher, 1987; Taylor, 1990), Jurassic North Sea (Blanche and Whitaker, 1978; Burley et al., 1989; Macaulay et al., 1993), Paleozoic U.S. interior (Land and Dutton, 1978; Dutton and Land, 1985), and Permian/Triassic North Sea (Pye and Krinsley, 1986; Sullivan et al., 1990; Purvis, 1992). This trend of increasing iron content with progressive diagenesis in the carbonate cements of sandstones is commonly ascribed to diagenetic reactions in asso-

ciated mudrocks, such as the illitization of smectitic clay minerals (Boles and Franks, 1979; Kantorowicz, 1985; Lynch, 1997), or the thermal destabilization of iron organometallic complexes derived from reactions involving smectitic clay minerals, iron oxides, and organic matter (Surdam et al., 1989a). Evidence for a connection between the illitization of detrital smectites and the precipitation of ferroan dolomite/ankerite is provided by their observed association with mud intraclasts and pseudomatrix. Another common habit of ferroan dolomite/ankerite and Fe-calcite is within the cleavages of biotite (Figure 12b), which was apparently favored by the iron availability and consumption of HⳭ by the mica surfaces (cf. Boles and Johnson, 1984). For the precipitation of late ferroan calcite cement in Pendeˆncia sandstones, an additional source of iron was supplied by the partial dissolution of ferroan dolomite/ankerite. The reverse trend of CaCO3 vs. FeCO3 observed in the carbonate cements (Figure 16) indicates that the substitution of iron for calcium occurs not only in the structure of calcite but also of ferroan dolomite/ ankerite (Figure 16), where iron apparently occupies the positions of both magnesium and calcium. This evidence and the pattern of increasing iron content with the precipitation of ferroan dolomite/ankerite (Figure 12a) and Fe-calcite (Figure 8b) indicate a gradual increase in iron activity over time. Isotopic Patterns The trends of carbon and oxygen isotopic composition provide important evidence for the conditions of precipitation of Pendeˆncia carbonate cements (Figure 17, Table 3). The d13C values of calcite I plotted vs. depth (Figure 18) show a wide scatter of d13C values in the stratigraphic interval from 700 to 1500m. This suggests that within this shallow zone a variety of processes were involved in the precipitation of the early calcite I cement. The high d13C values of some calcite I cements (⬇ Ⳮ8‰) (Figure 17, Table 3) indicate that methanogenic fermentation of organic matter was a source for part of the dissolved HCO3ⳮ (Irwin et al., 1977; Clayton, 1994). Moderately low d13C values of calcite I samples (⬇ ⳮ6 to ⳮ2‰) (Figure 17, Table 3) could be related to the oxidation of organic matter. However, as methanogenesis can release both isotopically heavy and light HCO3ⳮ (Clayton, 1994), these values could also be related to fermentation. Fermentation can also be the carbon source for the ferroan dolomite/ankerite and calcite II that have positive d13C values and high Anjos et al.

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7 6 5 FeCO3

4 35 30

2 1

25 FeCO3

Calcite I Calcite II Dolo/Ankerite

3

0

20

-1

15

86

88

90

10

92 94 CaCO3

96

98

100

5 0 -5 50

60

70 80 CaCO3

90

100

Figure 16. CaCO3 vs. FeCO3 diagram of the carbonate cements, indicating a general iron for calcium substitution, as shown by the reverse trend. Detail of calcite cements suggests that iron for calcium substitution occurs also in the calcites.

Calcite I Calcite II Dolomite/ ankerite Calcite in shale 0

δ18 O

-5

-10

-15

-20 -20

-15

-10

-5

0

5

10

δ13C Figure 17. Stable carbon and oxygen isotopic compositions of diagenetic carbonates in the Pendeˆncia reservoirs and of calcite in associated shales.

d18O values (indicating lower precipitation temperatures; discussed in following paragraphs) (Figure 17, Table 3). Considering that sulfate reduction is insignificant in continental settings as a consequence of the very low activity of SO42ⳮ in meteoric waters, the extremely low d13C calcite I value of ⳮ17.5‰ is likely to be related to methane oxidation (Clayton, 1994) (Figure 17, Table 3). 1738

Rift Reservoirs Quality, Potiguar Basin, Brazil

Figure 18. Cross-plot of carbon isotopic ratios vs. depth for the carbonate phases analyzed.

The clustering of most d13C values around 0‰ would also indicate the dissolution of micritic carbonate and bioclasts within the associated shales (d13C ⳱ ⳮ0.2 to Ⳮ1.8‰) (Figure 17, Table 3) as a major source of dissolved HCO3ⳮ for the precipitation of carbonate cements in the sandstones. Because bioclasts are scarce in the sandstones, internal sources are believed to have a minor influence on these d13C values. A derivation of carbonate from the mudrocks to the sandstones is also supported by the similarity of 87 Sr/86Sr values of the calcite in shales and of representative samples of the carbonate cements (Table 3). The similarity between the d13C values of ferroan dolomite/ankerite in the sandstones and of the calcite in the shales corroborates a source from the mudrocks. The values of 87Sr/86Sr of two ferroan dolomite/ ankerite samples are the highest, most radiogenic among the analyzed set (Table 3) yet still relatively close to the shale calcites values. This suggests that rock-fluid interactions, most likely clay minerals reactions, were more intense during the derivation of carbonate to the ferroan dolomite/ankerite than to calcite I precipitation. Nevertheless, the range of the d13C values of ferroan calcite II (ⳮ5 to Ⳮ5‰) indicates an input of dissolved carbonate from both methanogenic fermentation and the thermal decarboxylation of organic matter (Figure 17, Table 3), and a possible contribution from the recycling, that is, dissolution and reprecipitation,

Figure 19. Estimated precipitation temperatures of calcite I, dolomite/ankerite, and calcite II calculated from d18O values (cf. equations in Irwin et al., 1977).

500 1000 1500 Depth (m)

of previously precipitated calcite I and dolomite/ ankerite cements. Below the present-day depth of 2000 m, calcite II samples display a trend of decrease of d13C values from ⬇ 0‰ to ⬇ ⳮ15‰ (Figure 18), suggesting an increasing input of HCO3ⳮ from the progressive thermal decarboxylation of organic matter (Morad et al., 1998). The similar decrease in d18O values, indicating elevated precipitation temperatures, supports this postulation (Table 3, Figure 17). The temperature of precipitation of calcite I cements was estimated by assuming a d18O value of ⳮ3‰ for meteoric waters at the ⬇ 10⬚S paleolatitude of the Potiguar basin during the Early Cretaceous (Lloyd, 1982). These waters included both depositional and connate fluids expelled from the muddy sediments during early burial. Because no isotopic analyses of Pendeˆncia formation waters are available, a d18O value of Ⳮ2‰ was assumed for the fluids that precipitated dolomite/ankerite and calcite II cements. This value is compatible with mesogenetic fluids moderately evolved through fluid-rock interactions during the progressive transformation of smectitic clay minerals in the shales (Land and Fisher, 1987; Fisher and Boles, 1990). However, the value of Ⳮ2‰ is obviously an oversimplification, because this value is likely to have varied from area to area within the basin, as a function of depth and the extent of fluid-rock interaction. The precipitation temperatures calculated using the assumptions given previously and equations from Irwin et al. (1977) configure three distinct populations (Table 3, Figure 19). Calcite I ranges from 21 to 58⬚C, having an average of 41⬚C. Dolomite precipitated mostly from 70 to 79⬚C, with one single high temperature sample of 116⬚C (Table 3, Figure 19). Calcite II ranges from 85 to 150⬚C, having an average of 109⬚C. Despite the uncertainties derived from the assumptions involved in d18Owater, the calculated temperatures are in line with the petrographic interpretation of paragenetic sequence among the carbonate types (Figure 7). The petrographic and isotopic interpretation of the paragenetic relationship between calcite I and calcite II is further supported by the distribution of the two phases in relation to the present depth, which shows a reverse trend with the overall d18O values of the carbonates (Figure 20). The relationship between the timing of carbonate precipitation and the d18O values is also documented by the covariation with the intergranular volume of the cemented sandstones (Figure

2000 2500 3000

Calcite I Calcite II

3500 4000 -18

-16

-14

-12 -10 -8 δ18O (PDB)

-6

-4

-2

Figure 20. Plot of the d18O values of calcite I and calcite II vs. depth.

21). Both plots suggest a precipitation of calcite cements into two main stages during increasing burial depth and compaction.

CONCLUSIONS • Carbonate cements are the most abundant diagenetic constituents in Pendeˆncia reservoirs and, together with compaction, the main control on reservoir quality. Carbonate cementation is progressive with depth and facies controlled. The reservoir quality implication is that fluvial sandstones at moderate depths present the best potential quality. Deltaic Anjos et al.

1739



Figure 21. Plot of the d18O values of the carbonate phases analyzed vs. the intergranular volume (IGV %) of host sandstones. • and turbiditic sandstones are significantly more cemented, owing to carbonate diffusion from the interbedded lacustrine shales, and show permeability anisotropy accentuated by dolomite/ankerite precipitation along micaceous and clay laminae. Alluvial-fan conglomerates and coarse sandstones were flushed by meteoric waters during postrift telodiagenesis, without, however, considerable porosity enhancement. This is due to kaolinite precipitation and the intense compaction related to their immature detrital framework. • The sandstones are predominantly arkoses (average Q49F40L11). The coarse fan-deltaic deposits, with abundant plutonic fragments, and the basal sandstones (unit I), rich in volcanic fragments, are feldspathic litharenites. Provenance is from basement blocks uplifted along the rift margins, with sediments rapidly eroded under arid climate, transported into the basin by fan-deltaic systems, and distributed through fluvial-deltaic and turbiditic systems. • During eodiagenesis, the reservoirs were affected by precipitation of nonferroan calcite I (d13CPDB ⳮ17.5 to Ⳮ8.5‰; d18OPDB ⳮ10.7 to ⳮ4.0‰; Tprecip. 21 to 58⬚C), K-feldspar, and mechanical clay infiltration. Mechanical compaction was intense, involving rearrangement and fracturing of quartz and feldspar grains and deformation of ductile mud intraclasts and metamorphic and volcanic rock fragments. • Mesogenetic processes included the precipitation of ferroan dolomite/ankerite (d13CPDB, ⳮ1.7 to 1740

Rift Reservoirs Quality, Potiguar Basin, Brazil





Ⳮ1.1‰; d18OPDB ⳮ9.3 to ⳮ3.9‰; Tprecip. 70 to 79⬚C), ferroan calcite II (d13CPDB ⳮ13.6 to Ⳮ2.3‰; d18OPDB ⳮ17.2 to ⳮ6.8‰; Tprecip. 85 to 150⬚C), quartz, albite and chlorite cements, the albitization of detrital feldspars, and the chloritization of volcanic rock fragments, eogenetic smectite, and intraclastic pseudomatrix. Chemical compaction was widespread but more intense in micaceous, poorly cemented delta-front and turbidite sandstones. At the end of the rift phase, the top of the Pendeˆncia Formation was eroded, resulting in telogenetic dissolution and kaolinization restricted to the borders of the basin and to the proximity to the postrift unconformity. This occurred because of active meteoric water influx along enhanced permeability pathways represented by the marginal fan-deltaic systems and faults, with hydraulic head provided by the uplifted rift shoulders. The intensity of compaction is indicated by the generally reduced intergranular volume, which shows a steep decline in the first 2000 m of present depth, except where limited by early calcite cementation, as in the turbidites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbiditic and of compaction in the fluvial deposits. The sharp porosity decrease with depth is related to the immature detrital composition and the rapid rift subsidence followed by a long residence time at approximately the present depths. The peripheral cementation along the contacts having interbedded shales suggests that diffusion from the shales was a major source of dissolved carbonate for cementation. This is also supported by the large amount of cement in the turbidites (highest shale/ sandstone ratios) and by the similarity of isotopic values between the calcite from the shales (d13CPDB ⬇ 0‰; 87Sr/86Sr ⬇ 0.719) and most cements in the sandstones. Consequently, the amount of cement is strongly controlled by the thickness of the sandstone layers interbedded with the shales: thin sandstones are pervasively cemented. The d13C values of calcite I show a wide scatter from 700 to 1500 m, suggesting a variety of precipitation processes. High d13CPDB values (⬇ Ⳮ8‰) indicate a methanogenic fermentation source. Moderately low d13C values (ⳮ6.7 to ⳮ2.2‰) are related to the oxidation of organic matter. Extremely low d13C calcite I values (ⳮ17.5‰) are ascribed to methane oxidation. However, the clustering of d13C values

around 0‰ indicates that the calcite from the shales was a major source of HCO3ⳮ for calcite I and dolomite/ankerite cements. • Below the present depth of 2000 m, the d13C values of calcite II display a trend from ⬇ 0‰ to ⬇ ⳮ15‰, suggesting a derivation of HCO3ⳮ from the progressive thermal decarboxylation of organic matter. • The petrographic and isotopic interpretation of timing and temperatures of precipitation of calcite I and calcite II is supported by the segregation of the two phases into distinct present depth intervals and by the covariance between intergranular volume and d18O, which is consistent with increasing burial depth and compaction.

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