Abstracts

INTRODUCTION

Petrographic studies in sedimentary Brazilian basins are still scarce when compared to sedimentologic and stratigraphic ones, and the Sergipe-Alagoas Basin is not an exception. Previous works in the rift section regarding hydrocarbon exploration have focused on the sequence stratigraphy, sedimentology and interpretation of depositional systems (e.g. Barreiro 2004Barreiro C.B. 2004. Integração de dados de afloramento, perfis eletrorradioativos e de georadar para análise de análogos aos reservatórios pré-rifte/rifte: um exemplo de aplicação nas formações Serraria, Barra de Itiúba e Penedo, Bacia de Sergipe-Alagoas. MS Dissertation, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, 160 p.; Chiossi 2005Chiossi D.S.N. 2005. Seqüências deposicionais de 3ª ordem em riftes continentais: um modelo de tratos de sistemas para grábens assimétricos aplicado ao Cretáceo Inferior da Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 13(2):181-203.; Borba et al. 2011Borba C., Paim P.S.G., Garcia A.J.V. 2011. Estratigrafia dos depósitos iniciais do rifte no Campo de Furado, região de São Miguel dos Campos, Bacia de Sergipe-Alagoas. Revista Brasileira de Geociências, 41(1):18-36.). These tools are very important; however, they provide only macroscopic notions of reservoir qualities. Understanding the distribution of diagenetic alterations and their impact on the reservoir quality is a keystone in a petroleum field producer (Bloch 1994Bloch S. 1994. Effect of detrital mineral composition on reservoir quality. In: Wilson M.D. (ed.) Reservoir quality assessment and prediction in clastic rocks. Society of Economic Paleontologists and Mineralogists Short Course 30, p. 161-182.; Bloch & Helmold 1995Bloch S., Helmond K.P. 1995. Approaches to predicting reservoir quality in sandstones. American Association of Petroleum Geologists Bulletin, 79:97-115.; Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.; Ketzer et al. 2003Ketzer J.M., Morad S., Amorosi A. 2003. Predictive diagenetic clay-mineral distribution in siliciclastic rocks within a sequence stratigraphic framework. In: Worden R.H., Morad S. (eds.) Clay Mineral Cements in Sandstones. International Association of Sedimentologists Special Publication 34, Oxford, Blackwell Science, p. 42-59.; Morad et al. 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.). According to Morad et al. (2010)Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309., variations in the pathways of diagenetic evolution are linked to: depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; detrital sand composition; rate of deposition (controlling residence time of sediments at specific near-surface and geochemical conditions); and burial thermal history of the basin. It is evident that diagenesis causes a complex deviation from the single progressive trend of declining porosity and permeability versus depth (Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.).

The aim of this study is the petrologic characterization of the primary texture, diagenetic processes, and pores types. Relations and intensity of these constituents have direct impact on porosity. Petrological data will allow the definition of reservoir petrofacies and inferences on quality of reservoir of the Barra de Itiúba Formation.

GEOLOGICAL SETTING

The Sergipe-Alagoas Basin is located in Brazil Northeastern (Fig. 1), along homonym states, with a NE elongated direction along 350 km of extension, between 9° and 11°30' South (Campos Neto et al. 2007Campos Neto O.P.A., Lima W.S., Gomes Cruz F.E. 2007. Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 15(2):405-415.). It is limited with the Parnaíba-Pernambuco Basin at North by the Maragogi High and with the Jacuípe Basin at South by the Vaza-Barris fault system (Feijó 1994Feijó F.J. 1994. Bacias de Sergipe e Alagoas. Boletim de Geociências da Petrobras, 8(1):149-160.). Its origin is associated with the breakup of Gondwana supercontinent in the Late Jurassic/Early Cretaceous, which resulted in the formation of dozens of sedimentary basins in the Brazilian and African margins. The Sergipe-Alagoas Basin recorded all the rift phases, from pre-rift to drift strata and a thick sedimentary column (up to 10,000 m in the depocenter) deposited from Carboniferous to Quaternary.

Figure 1.
Structural map of the Sergipe-Alagoas Basin showing the configuration of the main normal faults, tectonic compartments, and localization of studies wells in the Atalaia High (modified from Garcia et al. 1990Garcia A.J.V., De Ros L.F., Souza R.S., Bruhn C.H.L. 1990. Potencial de reservatórios profundos na Formação Serraria, Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 4(4):467-488.).

The stratigraphy chart of Sergipe-Alagoas Basin was firstly organized by Schaller (1969)Schaller H. 1969. Revisão estratigráfica da Bacia Sergipe-Alagoas. Boletim Técnico da Petrobras, 12(1):21-86., and later reviewed by Feijó (1994)Feijó F.J. 1994. Bacias de Sergipe e Alagoas. Boletim de Geociências da Petrobras, 8(1):149-160. and Campos Neto et al. (2007)Campos Neto O.P.A., Lima W.S., Gomes Cruz F.E. 2007. Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 15(2):405-415.. Barra de Itiúba Formation (K34-K36 sequences, correlated with Aratu until Lower Jiquiá local stage, sensu Campos Neto et al. 2007Campos Neto O.P.A., Lima W.S., Gomes Cruz F.E. 2007. Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 15(2):405-415.) was defined by the authors as deposits constituted of greenish-gray and black shales, interbedded with fine-grained sandstones and gravel-grained limestones. Moreover, the Barra de Itiúba Formation is characterized by lacustrine-deltaic deposits, while their proximal equivalents, gravel-grained sandstones of Penedo Formation, would be fluvial systems. Both are formed in the rift phase of the Sergipe-Alagoas Basin under a very active tectonic regime, and comprise the main reservoirs there. Galm and Santos (1994)Galm P.C., Santos D.F. 1994. Caracterização de uma discordância de idade Pré-Aratu (Eocretáceo) na Bacia de Sergipe-Alagoas. Acta Geológica Leopoldensia, 39(2):555-562. recognized the pre-Aratu unconformity (DPA) within Barra de Itiúba Formation, a hiatus up to 5 My. Campos Neto et al. (2007)Campos Neto O.P.A., Lima W.S., Gomes Cruz F.E. 2007. Bacia de Sergipe-Alagoas. Boletim de Geociências da Petrobras, 15(2):405-415. separated sediments under this unconformity (DPA) from Barra de Itiúba Formation and called them Feliz Deserto Formation (K10-K20 sequences, Rio da Serra stage). The studied wells belong to a petroleum offshore field located in Atalaia High, which was formerly encompassed in Aracaju High.

METHODS

Detailed petrographic analyses were carried out in 40 thin sections of two wells, provided by Petrobras UO-SEAL. The thin sections were prepared with blue epoxy-impregnated samples, and the carbonate compositions stained with potassium ferricyanide plus Alizarin solution (Tucker 1988Tucker M.E. 1988. Techniques in Sedimentology. Oxford, Blackwell Science Publishing, 391 p.). Quantification of primary and diagenetic constituents and pore types was obtained by counting 300 points per section, using the Gazzi-Dickinson quantification method (cf. Zuffa 1985Zuffa G.G. 1985. Optical analyses of arenites: influence of methodology on compositional results. In: Zuffa G.G. (ed.) Provenance of Arenites. NATO-ASI Series C. D. Reidel Pub. Co., 148, Dordrecht, The Netherlands, p. 165-168.). Data were stored and processed in Petroledge^(r) software (De Ros et al. 2007De Ros L.F., Goldberg K. 2007. Reservoir petrofacies: a tool for quality characterization and prediction. In: AAPG, Annual Convention and Exhibition, Long Beach, Abstracts Volume, p. 1.).

The terms eogenesis, mesogenesis, and telogenesis are applied to the diagenetic stages sensu Morad et al. (2000)Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.. Eogenesis includes processes that happen under the influence of depositional fluids, at shallow depths and low temperatures (between 30 and 70°C, about 2 km depth). Mesogenesis includes processes and reactions involving water formation under effective burial conditions and high relative temperature (70 to ~200°C). Shallow mesogenesis corresponds to depths between 2 and 3 km and temperatures between 70 and 100°C, and the deep mesogenesis extends from depths around 3 km and temperatures around 100°C to the limit of metamorphism (temperatures higher than 200 - 250°C), and to highly variable depths, depending on the thermal gradient of the area. Telogenesis refers to those processes related to the uplift and new exposure of sedimentary rocks to superficial conditions (near surface meteoric conditions). This causes a destabilizing of mesogenetic minerals and a new adaptation of the rock constituents in the new conditions.

The concept of reservoir petrofacies used in this paper is the same from De Ros and Goldberg (2007)De Ros L.F., Goldberg K. 2007. Reservoir petrofacies: a tool for quality characterization and prediction. In: AAPG, Annual Convention and Exhibition, Long Beach, Abstracts Volume, p. 1.. This method consists into grouping samples according to the petrographic attributes that affect in reservoirs quality: texture, structure, types and bulk of primary and diagenetic constituents, and porosity.

RESULTS

Structures, textures, and fabric

The sandstones are very fine- to medium-grained (Fig. 2A), massive or with plane-parallel lamination defined by axis orientation of low-grade metamorphic fragments and mud intraclasts (Fig. 2B). Deformational structures include localized bioturbation and stylolites along carbonaceous fragments. Localized concretions and fractures, sometimes filled by dolomite or pyrite, may be also present. They are very poorly- to well-sorted, dominantly poorly sorted (Fig. 2C); angular to rounded, mainly sub-rounded; with low sphericity. Presence of quartz overgrowths and compaction of soft clasts (low-grade metamorphic rock fragments and mud intraclasts) may modify the roundness in some samples. Grain orientation is chaotic or sub-parallel.

Figure 2.
Structural, textural, and compositional aspects of the studied samples: (A) Very fine-grained, well-sorted, massive sandstone (crossed polarizers, XP); (B) Plane-parallel lamination defined by axis orientation of low-grade metamorphic fragments (arrow, XP); (C) Very fine-grained, very poorly-sorted, massive sandstone (uncrossed polarizers, //P); (D) Phyllite (arrow), gneiss (1) and meta-sandstone (2, XP) fragments; (E) Muscovite (XP); (F) Mud argillaceous intraclasts (arrow, //P).

Primary composition

The original composition is mostly lithic, with sublitharenites, litharenites, subarkoses, and rarely quartzarenites (sensu Folk 1968Folk R.L. 1968. Petrology of sedimentary rocks. Austin, Hemphill's, 182 p.; Fig. 3). Primary composition was slightly modified by diagenetic processes, such as the kaolinization of feldspar and low-grade metamorphic fragments, resulting in more quartzose compositions. The two studied wells do not show substantial differences in composition (Well A=Q₆₆F₆L₂₈ and B=Q₆₉F₆L₂₅), although Well B has a tendency for subarkose to feldspathic composition.

Figure 3.
Detrital composition of sandstones plotted in Folk (1968)Folk R.L. 1968. Petrology of sedimentary rocks. Austin, Hemphill's, 182 p. diagram. White circles are samples from Well A and gray circles from B.

Quartz grains are dominantly monocrystalline (average: 34%; maximum: 61%, Fig. 2C) and polycrystalline (average: 2%; maximum: 10%). Orthoclase is the only feldspar present (average: 2%; maximum: 6%), and it is commonly replaced by kaolinite.

Lithic fragments are abundant, mainly of low-grade metamorphic rocks (Fig. 2D) like slate (average: 3%; maximum: 12%), phyllite (average: 1%; maximum: 6%), meta-siltstone (average: 5%; maximum: 17%), and meta-sandstones (average: 1%; maximum: 7%). Sedimentary rock fragments are less common, including chert (average: 2%; maximum: 7%), mudstones (average: < 1%; maximum: 1%), sandstones (average: < 1%; maximum: 2%), and siltstones (average: ≤ 1%; maximum: 3%). Plutonic rock fragments are rare (< 1%).

Muscovite (average: < 1%; maximum: 3%, Fig. 2E) occurs as monocrystalline grains and in sedimentary and metamorphic fragments. Zircon, tourmaline, and rutile are less common (together, average: < 1%; maximum: 2%).

Intrabasinal constituents are mud and argillaceous intraclasts (average: 2%; maximum: 11%, Fig. 2F). They are generally deformed by mechanical compaction and sometimes replaced by kaolinite. Carbonate bioclasts are rare, replaced and recrystallized by dolomite/ferrous dolomite and ankerite. An ostracod-rich layer (5%) was found only at the depth of 1872.60 m in Well B. Phosphatic and siliceous bioclasts rarely occur. In fine-grained sandstones and lutites, may occur some argillaceous bioturbation matrix (average: 2%; maximum: 58%).

Diagenetic constituents

The diagenetic processes and their products identified in the studied rocks are: compaction of metamorphic rocks fragments and intraclasts forming pseudomatrix; quartz and feldspar (albite and K-feldspar) overgrowth and outgrowth; cementation and replacement by kaolinite, and precipitation of dolomite and ferrous dolomite to ankerite; pyrite; iron oxides and hydroxides; and titanium diagenetic minerals. Wells A and B have similar characteristics, thus the results will be showed together. Nevertheless, there are differences in the sequence and intensity of diagenetic processes. Therefore, two diagenetic models will be proposed (see "Discussion" item).

The origin of pseudomatrix is related to compaction of mud intraclasts (argillaceous pseudomatrix, average: 2%; maximum: 13%, Fig. 4A) and low-grade metamorphic fragments (lithic pseudomatrix, average: 2%; maximum: 18%), as in Fig. 4B.

Figure 4.
Diagenetic aspects from the Barra de Itiúba Formation: (A) Argillaceous pseudomatrix (1) and quartz overgrowths (2, plane polarizers //P); (B) Lithic pseudomatrix (arrow, crossed polarizers XP); (C) Discontinuous quartz overgrowths engulfing kaolinite (XP); (D) Kaolinite booklets replacing indeterminate primary constituent (//P); (E) Crystal zonation with ankerite in the core and dolomite in rhomb margins (XP); (F) Framboidal pyrite replacing blocky dolomite (arrow, XP).

Quartz occurs as continuous and discontinuous overgrowths and outgrowths on monocrystalline and polycrystalline quartz grains (average: 4%; maximum: 13%). It engulfs kaolinite (Fig. 4C), dolomite, pseudomatrix and diagenetic clay. There are rare discrete quartz crystals (< 1%).

Discontinuous (sometimes continuous) albite and K-feldspar overgrowths are rare (< 1%). Albite overgrowths cover orthoclase and plagioclase, and microcrystalline albite replaces orthoclase. Discrete albite crystals were found only at the depth of 2094.30 m in Well A. The K-feldspar overgrowths cover only orthoclase.

Kaolinite is present in all studied samples as booklets, lamellar and vermicular forms (Fig. 4D), replacing orthoclase (average: 1%; maximum: 6%), chert, muscovite (average: 1%; maximum: 6%), low-grade metamorphic rock fragments (average: 3%; maximum: 12%), lithic pseudomatrix (average: < 1%; maximum: 2%), and argillaceous matrix (average: < 1%; maximum: 5%). Furthermore, booklets and lamellar aggregates replace argillaceous intraclasts (average: 3%; maximum: 12%) and mud pseudomatrix (average: 3%; maximum: 7%). Lamellar kaolinite generally expanded and replaced muscovite, indicating early precipitation. Kaolinite also fills primary pores (average: 1%; maximum: 5%), causing porosity reduction.

Dolomite is the main diagenetic constituent, replacing primary constituents (average: 10%; maximum: 41%), kaolinite (average: 3%; maximum: 11%), authigenic quartz (average: < 1%; maximum: 3%), and mud pseudomatrix (average: < 1%; maximum: 4%). Dolomite also fills primary and fracture pores (average: 1%; maximum: 10%). Dolomite habits include macrocrystalline, fine and coarse mosaic, poikilotopic crystals, as well as large and small rhombs and saddle dolomite. The chemical composition varies between dolomite, ferrous dolomite, and ankerite. Crystal zonation is common in Well A, with ankerite in the core and dolomite in rhomb margins (Fig. 4E).

Pyrite occurs as framboids, macrocrystalline and blocky crystals (Fig. 4F), replacing primary constituents (average: < 1%; maximum: 3%), pseudomatrix (< 1%), and dolomite and ferrous dolomite/ankerite (average: < 1%; maximum: 4%). Rarely pyrite fills primary and fracture pores (< 1%).

Microcrystalline iron oxides and hydroxides coatings delimit the grain boundaries regarding quartz overgrowths (Fig. 4A). Their occurrence is inexpressive (< 1%), except for one sample in Well B at 1825.70 m (3%). They replace detrital constituents (quartz, slate fragments, and mud intraclasts) and diagenetic constituents (kaolinite, dolomite and argillaceous pseudomatrix), and fill primary pores.

Diagenetic titanium minerals (< 1%) display blocky, macrocrystalline, prismatic, and microcrystalline forms. Those constituents replace detrital quartz, phyllite fragments, kaolinite, dolomite, and argillaceous pseudomatrix.

Jarosite, gypsum, bitumen and paraffinic hydrocarbon are present in the studied rocks. Gypsum fills pores and it is partially dissolved (only in Well B at 1853.85 m). Bitumen covers monocrystalline quartz and several fragments. Paraffinic hydrocarbon fills primary pores. All these constituents occur in low expression (< 1%).

Porosity and compaction

The macroporosity is mainly primary intergranular (average: 4%; maximum: 19%, Fig. 5A), but there is secondary porosity by dissolution and rock and/or grain fractures. The primary constituents dissolved (Figs. 5B to 5D) are low-grade metamorphic rock fragments (slate, phyllite, meta-siltstone and meta-sandstones), chert, mud-argillaceous intraclasts, monocrystalline quartz and orthoclase, and siliciclastic matrix of bioturbation. The dissolved diagenetic constituents include argillaceous pseudomatrix and kaolinitized grains. Fracture porosity is more common in mud argillaceous intraclasts (average: < 1%; maximum: 3%). There are excavation pores in some samples due to a located bioturbation (Fig. 5E).

Figure 5.
Types of macroporosity: (A) Primary porosity widely preserved; (B) Porosity of pseudomatrix dissolution (1) and moldic (2); (C) Moldic porosity (arrow); (D) Secondary porosity of quartz dissolution (arrow); (E) Excavation pore (arrow); (F) Primary porosity is obliterated by dolomite rhomb (1) and argillaceous pseudomatrix (2). All photomicrographs were taken with uncrossed polarizers (//P).

The fabric of most samples are normal, rarely tight, or loose. Sandstones exhibit mechanical compaction, and locally chemical compaction (e.g. levels of stylolitization). The percentage of primary porosity loss due to compaction versus cementation may be represented in Ehrenberg's diagram (1989Ehrenberg S.N. 1989. Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones: discussion; compaction and porosity evolution of Pliocene sandstones, Ventura Basin, California: discussion. American Association of Petroleum Geologists Bulletin, 73(10):1274-1276., Fig. 6), which reports the intergranular volume (IGV) with cement percentage, accepting 40% of the initial porosity (depositional; Worden & Burley 2003Worden R.H., Burley S.D. 2003. Sandstone diagenesis: the evolution of sand to stone. In: Burley S.D., Worden R.H. (eds.) Sandstone diagenesis: recent and ancient. Reprint Series of the International Association of Sedimentologists, New York, Blackwell Publishing Ltd., p. 3-44.). Intergranular porosity values in the diagram are overestimated for the samples analyzed because initial porosity is smaller than 40% due to poor particle selection.

Figure 6.
Ehrenberg (1989)Ehrenberg S.N. 1989. Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones: discussion; compaction and porosity evolution of Pliocene sandstones, Ventura Basin, California: discussion. American Association of Petroleum Geologists Bulletin, 73(10):1274-1276. diagram showing the relationship between intergranular volume and total cement, correlating the primary porosity reduction with cementation and/or compaction. White circles are samples from Well A, and gray ones from B.

Nevertheless, Ehrenberg's (1989)Ehrenberg S.N. 1989. Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones: discussion; compaction and porosity evolution of Pliocene sandstones, Ventura Basin, California: discussion. American Association of Petroleum Geologists Bulletin, 73(10):1274-1276. diagram indicates that original porosity was obliterated mainly by mechanical compaction, not by cementation. Sandstones from Well A were more affected by mechanical compaction than the ones from Well B, due to the greater amount of ductile fragments, such as intraclasts and low-grade metamorphic fragments. This resulted in a lower intergranular porosity in Well A (up to 10%, except for two samples). Samples from Well B are more porous (up to 20%). In addition to mechanical compaction, intergranular porosity loss also happened due to kaolinite and dolomite cementation, as well as pseudomatrix generation (Fig. 5F).

Reservoir petrofacies associations

Thirteen reservoir petrofacies, identified based on petrography data, were summarized in four reservoir petrofacies associations (Tab. 1). They possibly reflect the real reservoir quality in microscale: good-, medium- and low-quality and non-reservoir. Fig. 7 shows a representative set of photomicrographs from reservoir groups.

Figure 7.
Reservoir petrofacies associations identified in the Barra de Itiúba Formation, based on petrological data. Photomicrographs of each association were taken with uncrossed polarizers. (A) Good-quality; (B) Medium-quality; (C) Low-quality; (D) Non-reservoir.

The good-quality petrofacies association (Fig. 7A) consists of fine- to medium-grained sandstones, well to moderately sorted, with a quartzose primary composition (sublitharenites, quartzarenites and subarkoses), and rare mud intraclasts (< 2%) and pseudomatrix (< 3%). The total cementation in these rocks are up to 20%, composed mainly of quartz (3 to 13%), kaolinite (2 to 17%), and pyrite (< 1%). Kaolinite and dolomite replace primary and diagenetic constituents. The primary intergranular porosity is preserved, and secondary one was generated by grain dissolutions (total porosity, from 14 to 17%).

The medium-quality petrofacies association (Fig. 7B) comprises fine- and muddy fine-grained sandstones, well to very poorly sorted, with a feldspar-lithic primary composition (subarkoses, sublitharenites, feldspathic litharenites). Presence of intraclasts (up to 4%) and pseudomatrix (up to 6%) is significant. Total cement is up to 19%, with quartz (4 to 15%), kaolinite (1 to 8%), and dolomite (0 to 10%). Kaolinite and dolomite may also replace primary and diagenetic constituents. Primary intergranular porosity is partially preserved, and secondary porosity happens due to grain dissolution. Average total porosity is 8%, with a maximum of 11%.

The low-quality petrofacies association (Fig. 7C) consists of siltstones and very fine- to medium-grained sandstones, well to poorly sorted, with a feldspar-lithic primary composition (litharenites, sublitharenites, and subarkoses), with abundant mud intraclasts (up to 5%) and pseudomatrix (up to 13%). Total cement is up to 15%, with quartz (2 to 4%), kaolinite (0 to 5%) and dolomite (0 to 14%). Kaolinite and dolomite replace primary and diagenetic constituents. Primary porosity is rare, largely obliterated by pseudomatrix and dolomite. Minor secondary porosity resulted from grain dissolution. Average total porosity is 4%, with a maximum of 6%.

The non-reservoir petrofacies association (Fig. 7D) consists of very fine- to medium-grained sandstones and lutites, very poor or poorly sorted, with a feldspar-lithic primary composition (litharenites, subarkoses and sublitharenites) sometimes with abundant matrix (up to 57%), stylolites, incipient bioturbation, argillaceous intraclasts (up to 7%), and pseudomatrix (up to 23%). Total cement is up to 22%, with quartz (1 - 9%) and dolomite (1 - 14%). Kaolinite and dolomite replace primary and diagenetic constituents. All the porosity is secondary, from grain dissolution and fracturing (average total porosity is of 1%, with a maximum of 5%).

The good-quality reservoir petrofacies association is characterized by average total porosity above 15%, whereas the medium association presents average total porosity above 7%. In these associations, the dominant porosity is intergranular (intergranular porosity/total porosity > 0.80). The low-quality petrofacies associations display average total porosity between 1 and 4%, and the non-reservoir < 1%. Fig. 8 presents the relationship between macroporosity and depth for the studied samples, according to the reservoir quality. The reservoir petrofacies associations retain their porosities with increasing depth. Only in Well A, samples classified as non-reservoir display a small increase in porosity with depth, probably due to an increase in the secondary porosity. Overall, there is not a difference in porosity between Wells A and B as to depth.

Figure 8.
Relationship between macroporosity (%) and depth (m). The macroporosity is preserved with increasing depth. In Well A there is an increase in the secondary porosity due to dissolution of constituents and fractures. The samples are presented in their respective reservoir petrofacies associations.

DISCUSSION

The paragenetic sequences were constructed from textural relations between primary and diagenetic constituents using a conventional optical microscopy. Two diagrams (Fig. 9) were presented for litharenites/sublitharenites and subarkoses, since the intensity of diagenetic processes are different for these compositions (De Ros 1996De Ros L.F. 1996. Compositional controls on sandstone diagenesis. Comprehensive summaries of Uppsala dissertations from the Faculty of Science and Technology, 198:1-24.). These diagrams illustrate the paragenetic sequences and main processes in each diagenetic stage.

Figure 9.
Paragenetic models to the Barra de Itiúba Formation with paragenetic order of diagenetic processes for each primary composition (Litharenites/Sublitharenites and Subarkoses).

Summary model of diagenetic evolution

Eogenetic alterations exert a substantial control on the deep-burial, mesogenetic evolution of the sandstones (Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.; 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.), because burial diagenesis will be constrained by the extent of fluid flow through the remaining porosity and influenced by the presence of earlier diagenetic products to affect. Eogenetic processes observed in this study include the dissolution of primary constituents, replacement and cementation by kaolinite, ferrous oxides and hydroxides, quartz overgrowths, dolomite and pseudomatrix generation.

Under warm, subhumid to humid conditions (precipitation > 1,000 mm yr^-1) and in proximal fluvial sediments, active percolation of undersaturated meteoric waters cause the dissolution of detrital silicates (primarily lithic grains, feldspar and mica), and precipitation of kaolinite (Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.). Kaolinized micas generally show expanded textures (Ketzer et al. 2003Ketzer J.M., Morad S., Amorosi A. 2003. Predictive diagenetic clay-mineral distribution in siliciclastic rocks within a sequence stratigraphic framework. In: Worden R.H., Morad S. (eds.) Clay Mineral Cements in Sandstones. International Association of Sedimentologists Special Publication 34, Oxford, Blackwell Science, p. 42-59.; Morad et al. 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.), which is an indication of happening before significant compaction. Ferrous oxides and hydroxides precipitated as coatings over this diagenetic phase, while there was in contact with oxidizing surface waters, typical of continental, humid environments. The silica enrichment in solution due to the dissolution of feldspar and lithic fragments allowed the precipitation of quartz as overgrowths. Authigenic quartz began to precipitate in eogenesis (continuous overgrowths, McBride 1989McBride E.F. 1989. Quartz cementation in sandstones - a review. Earth-science Reviews, 26:69-112.; Bjørlykke & Egeberg 1993Bjørlykke K., Egeberg P.K. 1993. Quartz cementation in sedimentary basins. AAPG Bulletin, 77:1538-1548.; Giles et al. 2000Giles M.R., Indrelid S.L., Beynon G.V., Amthor J. 2000. The origin of large-scale quartz cementation: Evidence from large data sets and coupled heat-fluid mass transport modeling. In: Worden R.H., Morad S. (eds.) Quartz cementation in sandstones. International Association of Sedimentologists Special Publication, 29, p. 21-38.; Milliken 2005Milliken K.L. 2005. Late Diagenesis and Mass Transfer in Sandstone-Shale Sequences. In: Mackenzie F.T. (ed.) Sediments, Diagenesis, and Sedimentary Rocks. Oxford, Elsevier, UK, p. 159-190.), proceeding during mesogenesis as prismatic projections and discontinuous overgrowths (Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.).

Mechanical compaction was more intense in lithic/sublithic sandstones because of the abundance of ductile fragments, mostly low-grade metamorphic rock fragments, generating argillaceous pseudomatrix. Mechanical compaction was not significant in the subarkose sandstones due to fabric stabilization given by precipitation of quartz overgrowths and by the minor quantity of low-grade metamorphic rock fragments and intraclasts.

Two phases of dolomitization were observed, one in eogenesis and another in mesogenesis. The first, less expressive, was interpreted from the paragenetic relation with quartz overgrowths, since the latter engulfs and then is replaced by dolomite. Extensive cementation by dolomite as eogenetic processes in lacustrine deltas is reported by Pitman et al. (1986)Pitman J.K., Anders D.E., Fouch T.D., Nichols D.J. 1986. Hydrocarbon potential of nonmarine Upper Cretaceous and Lower Tertiary Rocks, eastern Uinta Basin, Utah. In: Spencer R.W., Mast R.F. (eds.) Geology of tight gas reservoirs. AAPG Studies in Geology, 24:235-252., Anjos et al. (2000)Anjos S.M.C., De Ros L.F., Souza R.S., Silva C.M.A., Sombra C.L. 2000. Depositional and diagenetic controls on the reservoir quality of Lower Cretaceous Pendência sandstones, Potiguar rift basin, Brazil. AAPG Bulletin, 84:1719-1742., and Luo et al. (2009)Luo J., Hlal O., Morad S., Ketzer J.M. 2009. Diagenetic and reservoir-quality evolution of fluvial and lacustrine deltaic sandstones: Evidence from Jurassic and Triassic sandstones of the Ordos Basin, northwestern China. Journal of Petroleum Geology, 32:79-102.. The second stage of dolomitization is marked through the cementation and replacement of primary constituents by ferrous dolomite (sometimes ankerite), and non-ferrous dolomite occurred due to a change in the interstitial fluid composition to more alkaline and reduced terms. This appeared in the lithic/sublithic sandstones as a zonation in dolomite rhombs, with display ankeritic core and magnesium-rich rims.

Mesogenesis is strongly influenced by induced eogenetic alterations, as well as by temperature, pressure, and composition in basinal brines (Morad et al. 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.). Continuous burial led to mechanical and chemical compaction in advanced stages, evidenced by levels of stylolitization, false contacts between muscovite and other primaries constituents, and sutured contacts (Morad et al. 2000Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.). The instability between interstitial fluids and primary and diagenetic constituents resulted in several dissolution phases during every diagenetic stage.

Pyrite precipitation occurred associated with dolomite, also as a grain-replacive constituent. It was related to fluids enriched in H₂S derived from thermal sulfate reduction (Machel 1987Machel H.G. 1987. Some aspects of diagenetic sulphate-hydrocarbon redox reactions. In: Marshall J.D. (ed.) Diagenesis of Sedimentary Sequences. Geological Society (London) Special Publication 36, Oxford, Blackwell Scientific, p. 15-28.). The formation of diagenetic titanium minerals occurred as a result of Ti-enrichment in the interstitial solutions by dissolution of detrital heavy minerals. There was also precipitation of discontinuous K-feldspar and albite overgrowths (Saigal et al. 1988Saigal G.C., Morad S., Bjørlykke K., Egeberg P.K., Aagaard P. 1988. Diagenetic albitization of detrital K-feldspars in Jurassic, Lower Cretaceous, and Tertiary clastic reservoirs from offshore Norway, I. Textures and origin. Journal of Sedimentary Petrology, 58:1003-1013.; Morad et al. 1990Morad S., Ketzer J.M., De Ros L.F. 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology, 47(1):95-120.) in this stage.

The exposure of rocks to conditions of surface waters during telogenesis destabilized the mineral assemblage, resulting in dissolution of primary and diagenetic constituents. Local precipitation of iron oxides and hydroxides, kaolinite, jarosite and gypsum was a product of fluid percolation. Jarosite (KFe₃ ⁺³(SO₄)²(OH)₆) is a product of hydration and oxidation of pyrite in conditions of local hidrothermalism or telogenesis.

Differential diagenetic patterns

The main difference between diagenetic patterns for subarkoses and litharenites/sublitharenites is that the subarkoses display a larger quantity of quartz overgrowths. Consequently, mechanical compaction for subarkoses was not significant due to fabric stabilization given by precipitation of quartz overgrowths and minor quantity of low-grade metamorphic rock fragments and intraclasts. This compositional distinction leads to a different intensity of diagenetic processes. A reflection of this is the replacement and cementation by dolomite to ferrous dolomite/ankerite considerably less intense than in the litharenites and sublitharenites.

Provenance

The original feldspar-lithic detrital composition indicates a provenance from orogenic recycling (sensu Dickinson 1985Dickinson W.R. 1985. Interpreting provenance relations from detrital modes of sandstones. In: Zuffa G.G. (ed.) Provenance of Arenites. The Netherlands, NATO-ASI Series C. D. Reidel Pub. Co., Dordrecht, 148, p. 333-361.; Fig. 10). The more quartz-feldspathic composition in Well B may reflect a change of provenance, with a contribution of cratonic interior source. The low-grade metamorphic lithic fragments may come from Proterozoic basement of the Miaba and Vaza-Barris Group (Moraes Rego 1933Moraes Rego L.F. 1933. Notas sobre a geologia, a geomorfologia e os recursos minerais de Sergipe. Anais da Escola de Minas de Ouro Preto, 24:31-84.). The dolomite fragments that were founded possibly come from Cambrian meta-sedimentary rocks from the Estância Group (Silva et al. 1978Silva F.M.A., Santana A.C., Bonfim L.F.C. 1978. Evolução tectono-sedimentar do Grupo Estância: suas correlações. In: 30º Congresso Brasileiro de Geologia, Recife, Anais, v. 2, p. 685-699.), basement of the Sergipe Sub-Basin.

Figure 10.
The provenance Dickinson I diagram (Dickinson 1985Dickinson W.R. 1985. Interpreting provenance relations from detrital modes of sandstones. In: Zuffa G.G. (ed.) Provenance of Arenites. The Netherlands, NATO-ASI Series C. D. Reidel Pub. Co., Dordrecht, 148, p. 333-361.), showing the main contribution of recycled orogen provenance and subordinate contribution of craton interior to the Well B. White circles are samples from Well A, gray ones from B.

Some thin sections suggested a cratonic interior provenance. It was not possible to identify a compositional pattern with systematic variation in provenance either in Wells A or B. Therefore, the presence of more quartzose terms may be related to autocyclic controls in the depositional environment, related to a longer-distance transport (and consequent deterioration of lithic fragments).

In compositional terms, the studied samples are very different from those of Becker (1987)Becker A.P. 1987. Estudo petrológico dos reservatórios da Fm. Barra de Itiúba no Campo de Furado, Bacia de Sergipe Alagoas. MS Dissertation, Instituto de Geociências, Universidade Federal de Ouro Preto, Ouro Preto, 111 p.. The average composition of Becker (1987)Becker A.P. 1987. Estudo petrológico dos reservatórios da Fm. Barra de Itiúba no Campo de Furado, Bacia de Sergipe Alagoas. MS Dissertation, Instituto de Geociências, Universidade Federal de Ouro Preto, Ouro Preto, 111 p. is arkoses; while of the studied samples are lithic sandstones (Q₆₆F₆L₂₈ and Q₆₉F₆L₂₅, in Wells A and B, respectively). The analyzed samples are similar in terms of detrital and diagenetic composition to the Carmópolis Formation (Souza et al. 1995Souza R.S., De Ros L.F., Morad S. 1995. Dolomite diagenesis and porosity preservation in lithic reservoirs: Carmópolis Member, Sergipe- Alagoas Basin, Northeastern Brazil. AAPG Bulletin, 79(5):725-748.). However, the studied interval was positioned stratigraphically and correlated through ostracod biozones (data from Petrobras), which should provide a good biostratigraphic position to studied interval.

Impact of diagenetic constituents in reservoir quality

Petrographic analysis has showed that reservoir quality is mostly impacted by diagenetic processes (Morad et al. 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.). Depositional texture does not vary significantly, since all studied rocks are fine-grained (siltstones and fine-grained sandstones). Loss of porosity occurred due to pseudomatrix generation (a product of mechanical compaction), and kaolinite and dolomite cementation. Thus, detrital composition has a considerable influence on porosity reduction: sandstones with more contents of lithic fragments displayed high porosity reduction by mechanical compaction. On the other hand, sandstones with more feldspar and intraclasts showed generation of secondary porosity by dissolution of these constituents. In some petrofacies (low-quality and non-reservoir), there was an important creation of secondary porosity by dissolution in different diagenetic phases. Quartz overgrowths were important for the maintenance of primary porosity because they sustained the framework, limiting mechanical compaction. The relationship between the main diagenetic constituents and porosity is illustrated in Fig. 11.

Figure 11.
(A) Negative correlation between total porosity and total dolomite for the two studied wells. Dolomite does not interfere significantly with porosity, since it is dominantly substitutive. (B) Relationship between total porosity and kaolinite, showing wide data dispersion. Some samples in Well B present that kaolinite affects negatively the porosity. (C) Relationship between total porosity and authigenic quartz. The presence of quartz overgrowths impact positively on the total porosity. (D) Relationship between total porosity and pseudomatrix. Generation of pseudomatrix had a significant impact in intergranular porosity especially in Well B. (E) Inverse relation between porosity and quantity of ductile grains.

Fig. 11A shows the inverse relation between total porosity and total dolomite, mainly in Well B. Well A shows dispersion similar to B; however, in some samples, the quantity of dolomite does not significantly interfere with porosity, since dolomite is dominantly substitutive.

In relation to kaolinite and total porosity, there is wide data dispersion (Fig. 11B). This occurs mainly because kaolinite replaces low-grade metamorphic rock fragments, feldspar, micas, and mud intraclasts. Despite the dispersion, some samples in Well B show that kaolinite affects negatively in the total porosity, which may occlude pores and pore throats to variable extents depending on their morphology (Lander & Bonnell 2010Lander R.H., Bonnell L.M. 2010. A model for fibrous illite nucleation and growth in sandstones. AAPG Bulletin, 94:1161-1187.; Morad et al. 2010Morad S., Al-Ramadan K., Ketzer J.M., De Ros L.F. 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bulletin, 94(8):1267-1309.).

The authigenic quartz (Fig. 11C) has a positive impact on total porosity. The precipitation of quartz overgrowths inhibited mechanical compaction and generation of pseudomatrix that obliterates primary porosity (Bloch et al. 2002Bloch S., Lander R.H., Bonell L. 2002. Anomalously high porosity and permeability in deeply buried sandstones reservoirs: origin and predictability. AAPG Bulletin, 86:301-328.).

Pseudomatrix generation had a significant impact on intergranular porosity, especially in Well B (Fig. 11D). This was the main process of primary porosity destruction. There is a strong inverse relation between porosity and quantity of ductile grains (Fig. 11E). These constituents and their diagenetic products (lithic and argillaceous pseudomatrix) are responsible for porosity loss (Pittman & Larese 1991Pittman E.D., Larese R.E. 1991. Compaction of lithic sands: Experimental results and applications. AAPG Bulletin, 75:1279-1299.; Bloch 1994Bloch S. 1994. Effect of detrital mineral composition on reservoir quality. In: Wilson M.D. (ed.) Reservoir quality assessment and prediction in clastic rocks. Society of Economic Paleontologists and Mineralogists Short Course 30, p. 161-182.; De Ros 1996De Ros L.F. 1996. Compositional controls on sandstone diagenesis. Comprehensive summaries of Uppsala dissertations from the Faculty of Science and Technology, 198:1-24.), which is evident in Well A, where ductile grains are abundant.

CONCLUSIONS

The petrographic analyses of 40 thin-sections in two wells located in Atalaia High indicated that:

ACKNOWLEDGEMENTS

The authors would like to thank UO-SEAL Petrobras for the access to the studied thin sections and for the permission to publish this work, especially M.Sc. Maria Rosilene Menezes. Special acknowledgements are paid to the support of the Brazilian National Petroleum Agency - ANP (in the form of a fellowship and research funds for A.G. Rodrigues), to Jaques Soares Schmidt and Christiano Ng for suggestions and final review, and to LHB-Endeeper for access to the Petroledge^(r) software. The authors wish to thank an anonymous reviewer for the pertinent suggestions.

REFERENCES

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