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AAPG Bulletin, V.
Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection
1Departamento de Petrologia y Geoquimica, Facultad de Ciencias Geologicas, Universidad Complutense, 28040, Madrid, Spain; email: [email protected]
2University of Kansas, Department of Geology, 1475 Jayhawk Boulevard, Room 120, Lawrence, Kansas, 66045-7613; email: [email protected]
3Dipartimento di Scienze della Terra, Universita degli Studi di Pavia, Pavia, Italy, 27100; email: [email protected]
4Departamento de Petrologia y Geoquimica, Facultad de Ciencias Geologicas, Universidad Complutense, 28040 Madrid, Spain; email: [email protected]
Carlos Rossi is a professor in the Department of Petrology and Geochemistry at Complutense University (Madrid). He received a Ph.D. in geology from Complutense University in 1993. His main research interests are sequence stratigraphy and diagenesis of carbonates, Quaternary karst, and diagenesis of sandstone petroleum reservoirs, especially from north Africa.
Robert H. Goldstein is the Haas Distinguished Professor at the University of Kansas. He received a B.S. degree from Juniata College, Huntingdon, Pennsylvania, and an M.S. degree and Ph.D. from the University of Wisconsin. His current research includes development of the fluid inclusion tool for diagenetic research, integration of diagenesis with sequence stratigraphy and tectonic setting, and evaluation of controls on the sequence stratigraphic character of carbonate rocks.
Andrea Ceriani is a postdoctoral fellow at the University of Pavia (Italy). He received his M.S. degree (1995) and Ph.D. (2001) from the University of Pavia. His current research includes diagenesis of carbonates and siliciclastics, with particular attention to fluid-inclusion studies, and provenance of modern sands and ancient arenites.
Rafaela Marfil obtained her Ph.D. in geology from Complutense University (Madrid). She is currently a professor of sedimentary petrology and clastic diagenesis, as well as senior research scientist in charge of several research projects about reservoir diagenesis. She has collaborated with Shell Spain and Repsol-YPF, publishing several articles on the characterization and diagenesis of clastic reservoirs from Spain and Egypt.
This work was funded by a grant of the Vicerrectorado de Investigacion, Universidad Complutense de Madrid. Additional funding was provided by the Direccion General de Ciencia y Tecnologia, project PB96-1236-C02-02. Khalda Petroleum Co. and Repsol Exploracion Egipto facilitated sampling and provided geological and well data. Kitty Milliken and Rob Reed are thanked for their assistance with cathodoluminescence work and helpful comments. Reviews by S. Ehrenberg, J. G. Gluyas, and J. Kupecz greatly improved the original manuscript.
A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypt's 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.
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