R. Marfil

Facies-Related Diagenesis and Multiphase Siderite Cementation and Dissolution in the Reservoir Sandstones of the Khatatba Formation, Egypt's Western Desert

In the fluvio-deltaic, quartz-arenitic sandstones of the Jurassic Khatatba Formation in the Salam field (Egypt9s Western Desert), diagenesis and sedimentary facies control reservoir quality. Fluvial channel sandstones have the highest porosities (10-15%) and permeabilities (100-600 md), in part because of siderite cementation, which (1) inhibited compaction and quartz cement and (2) was later dissolved, creating intergranular secondary porosity (1/4 of total porosity). Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive, shallow-burial Fe-dolomite cement, respectively. Siderite precipitation was multiphase and separated by distinct dissolution events. The earliest siderite precipitated near surface, within suboxic tropical coastal swamps containing predominantly meteoric waters. Some influence of marine waters is indicated by local enrichments in Mg and Ca. The next major siderite generation shows a trend to decreasing Mn and Ca contents, and is of shallow-burial origin. The last major siderite phase is Mg rich and interpreted as deeper-burial in origin. Some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities. The most important dissolution, however, occurred during deep burial, resulting in (1) a major corrosion surface predating the last Mg-rich zone, (2) selective dissolution of some earlier zones, and (3) secondary porosity. This burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults. Other diagenetic phases observed include early-diagenetic pyrite, kaolin, quartz, bitumen, and late-diagenetic barite and illite. Kaolinite precipitated at shallow depths ( 130°C) during and/or after rapid Late Cretaceous burial. Quartz developed in two phases, separated by oil migration. This paper illustrates that, in fluvio-deltaic quartz arenites deposited under the influence of humid tropical climate, reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies. This paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones, and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages, including early, shallow-burial, and deep-burial diagenesis. The present study also shows that, in multiphase siderite cements, the earlier growth zones can be selectively dissolved and replaced by later siderite zones (i.e., recrystallized) during burial diagenesis. These findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier-formed siderites are essentially stable during diagenesis.

The role of mixing-zone dolomitization in sandstone cementation: evidence from the Triassic Buntsandstein, the Iberian Range, Spain

Abstract Petrographic and geochemical data suggest that dolomite cement in shallow-marine sandstones of the Triassic Buntsandstein in the Ateca horst of the Iberian Range, Spain, has precipitated in the zone of mixing between phreatic meteoric and marine waters. Dolomite or rhombohedric crystals consisting of concentric bands of alternating syntaxial dolomite and calcite have filled isolated voids (350 μ–12 mm in diameter). Subsequently, limpid dolomite followed by sparry calcite precipitated in void center as the latest cements. The carbonates have low δ 13 C PDB (−9.5 to −6.0%) and δ 18 O PDB (−8.1 to −3.2%.) values and are enriched in radiogenic strontium ( 87 Sr / 86 Sr = 0.710112 to 0.720293) indicating a dominantly meteoric component in the porewaters. The overall sequence of carbonate cementation suggests precipitation from porewaters that evolved in the mixing zone with successively increased meteoric influence, and finally to fully meteoric probably due to a major sea-level fall.

Diagenesis of carbonate cements in Permo-Triassic sandstones from the Iberian Range, Spain: evidence from chemical composition and stable isotopes

Petrographic, chemical and stable isotope investigations have been made on calcite, dolomite, and Fe-dolomite/ankerite from Permian-Triassic clastic sequences from the Iberian Range (Spain). Calcite and dolomite cements have precipitated before the Fe-dolomite/ankerite. Calcite, however, has also formed by calcitization of dolomite due to uplift. The negative δ13CPDB values (−9.51 to −2.13%) in these carbonates suggest derivation of carbon from oxidation of organic matter in the early carbonates and from decarboxylation of organic matter in the late Fe-dolomite/ankerite. Early diagenetic dolomitic cements have heavier δ13C(> −3.54%), higher Mn/(Mn + Fe) and Mn/(Mn + Mg) and lower Fe/(Fe + Mg) ratios than later diagenetic dolomitic cements. The latters are also characterized by lighter δ18OPDB (> −10.64%) than the early cements due to increase in temperature and to pore water-mineral reactions.

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypts Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

Diagenetic processes influencing porosity in sandstones from the Triassic Buntsandstein of the Iberian Range, Spain

Abstract Sandstones of the Triassic Buntsandstein of Spain were studied in an attempt to quantify effects of diagenetic processes such as compaction, cementation and leaching on reservoir properties. Samples were taken from Shell well Siguenza 44-3 some 110 km northeast of Madrid, drilled on the Siguenza High in the Iberian Range. The sandstones are arkoses to sublitharenites of a typical Triassic redbed facies characteristic of the whole region. The present-day depth of the base Buntsandstein is 538 m, whilst a maximum burial depth of ca. 2600 m was estimated from vitrinite reflectance (0.94 to 1.14), kaolinite temperature (76°C) and illite crystallinity (Kuebler index 4.9). From an estimated average Initial Porosity of 38%, porosity was reduced to the present value of 13%. The strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15%, followed by chemical compaction (pressure solution) with a loss of about 6%. The cement (11.4% R.V.) apparently reduced the Initial Porosity by about only 5%. Early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution. This is supported by an increase in sutured grain contacts where early cements are absent or rare. The later quartz cement, interpreted to be largely the product from pressure solution, has reduced porosity in a ratio of 0.5:1. This suggests that compaction continued after the formation of the late quartz cement. Although leaching was observed especially in feldspar grains, clay and K-feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity. Permeability in the Buntsandstein is generally low (average 17.4 mD) and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0.11 to 73.0 mD.

Provenance of Triassic Feldspathic Sandstones in the Iberian Range (Spain): Significance of Quartz Types

ABSTRACT The base of the Triassic in the Iberian Range is represented by detrital sediments (Buntsandstein facies) deposited initially in a continental environment, finally evolving into a marine environment that is represented at the top of the sequence. The lithology of this facies is dominated by arkosic sandstones. The aim of this study is to reconstruct the nature and position of the source areas of these sandstones. Provenance research was carried out by quartz-grain typology. Eleven stratigraphic sections were sampled. The amount of interstitital matrix has been considered in selecting samples because mechanical compaction suffered by sandstones with little matrix may give rise to a significant increase in the undulosity of monocrystalline quartz. The Ollo de Sapo gneissic formation, located in the Hesperian Massif, has been mentioned as source rocks of feldspathic sandstones in previous works. In order to verify the provenance of feldspathic sandstones, in artificial sands derived by grinding gneisses, and sand samples collected at stream heads that drain the gneiss outcrops, we followed the methodology of Basu et al. (1975). Analytical results indicate that two different areas within the Triassic basin were notably influenced by different source areas: a) a western zone, the nearest to the gneissic source rocks, where monocrystalline, nonundulatory quartz grains predominate (Qm 5°) increase. Sediment evolution during transport processes is markedly reflected by the increase in Qm 5°) ratios in the westernmost zone, away from the source area. Low values in the above-mentioned ratios in the eastern zone are interpreted as results of local influence by low-ranking metamorphic source areas. Finally, this ethod also allows for the monitoring of the evolution of sediment maturity throughout the basin.

Different degree of albitization in the syn-rift Lower Cretaceous sandstones, Maestrat Basin, Iberian Range: Burial temperature or composition/provenance influence?

a Departamento de Petrologia y Geoquimica, Facultad C.C. Geologicas, Universidad Complutense de Madrid, Avda. Jose Antonio Novais s/n, 28040-Madrid, Spain b Departament de Geoquimica, Petrologia i Prospeccio Geologica, Facultat de Geologia, Universitat de Barcelona, Marti i Franques, s/n, 08028-Barcelona, Spain c Departamento de Ciencias de la Tierra, Area de Petrologia y Geoquimica. Facultad de CC. Geologicas, Universidad de Zaragoza, 50.009-Zaragoza, Spain

Multiphase carbonate cementation related tofractures in the Upper Jurassic limestones, Maestrat Basin (Iberian Range, Spain)

Abstract In the western part of the Penyagolosa subbasin (Maestrat Basin, Spain), carbonate cementation occludes fractures and infills stylolites in Tithonian-Berriasian limestones. Field relationships, petrography, cathodoluminesence and geochemical analyses (microprobe, fluid inclusions, oxygen, carbon and strontium isotopes) of the carbonate cements show that the paragenetic sequence includes (A) calcite cements in echelon tension gashes (– 11.37 δ 18 O VPDB). (B) Scarce isolated rhombic dolomite replacement cement