Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Wilson, M. E. J. Articles by Thirlwall, M. Search for Related Content GeoRef GeoRef Citation

Reservoir quality, textural evolution, and origin of fault-associated dolomites

¹ Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, United Kingdom; present address: Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA6865, Australia; moyra.wilson{at}durham.ac.uk
² Maersk Oil, 50 Esplanaden, DK-1263, Copenhagen K, Denmark; present address: Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, Texas 77380; martin.evans{at}anadarko.com
³ FI^a, 41 Oaken Lane, Claygate, Esher, Surrey, KT10 0RG, United Kingdom; oxtoby{at}dial.pipex.com
⁴ Geological Research and Development Centre, Bandung, Indonesia; dharma{at}grdc.esdm.go.id
⁵ Scottish Universities Environmental Research Centre, East Kilbride GT5 0QF, United Kingdom; and School of Geosciences, Edinburgh University, Edinburgh EH935W, United Kingdom; T.Donnelly{at}suerc.gla.ac.uk
⁶ Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom; matthewt{at}gl.rhul.ac.uk

Moyra has a B.A. degree (geology) from Cambridge University and a Ph.D. in carbonate sedimentology from London University. From 1995 to 1999, she was a postdoctoral research assistant at London and, since then, has been a lecturer at Durham University, United Kingdom. Moyra's research focuses on understanding equatorial carbonates, their reservoir development, and factors influencing their evolution. She works regularly with the industry on applied projects.

Martin received his B.Sc. degree and his Ph.D. from the University of Wales and began his industry career with Amoco (1985). Since then, he has worked in international exploration and production with BP, Maersk Oil, and Anadarko Petroleum Corporation. Martin joined Anadarko in 1997 and has held a variety of exploration and development positions in London and Houston. He is currently the chief geologist.

Norman has an M.A. degree in geology from Oxford University and an M.Sc. degree in petroleum geology from Imperial College, London. From 1976 to 1994, he was a geologist with BP, where he researched fluid inclusions to evaluate reservoir development and fluid prediction. Since 1994, he has been a freelance consultant in applied fluid-inclusion studies and has an honorary research associateship at Royal Holloway, United Kingdom.

Dharma is a senior geologist with the Geological Research and Development Centre, Bandung, Indonesia. He has worked on geological survey mapping projects and collaborative international university and industry projects. Dharma has considerable experience in fieldwork throughout Indonesia and has also worked in the Middle East.

Terry is a laboratory research manager at the Scottish Universities Environmental Research Centre in East Kilbride, United Kingdom. His responsibilities are in stable isotope studies and analytical techniques, and his position is joint with Edinburgh University.

Matthew received his Ph.D. in igneous geochemistry from Edinburgh and then did a postdoctoral research assistantship in isotope geochemistry at Leeds University. Since 1984, he has been a lecturer and is now professor in isotope geochemistry at Royal Holloway, London. He is responsible for multicollector inductively coupled plasma–mass spectroscopy, thermal infrared mapping spectrometry, and x-ray fluorescence laboratories. His research interests are mantle geochemistry, depleted uranium, Sr isotope stratigraphy, and analytical techniques.

Fault-associated dolomitized carbonates are proven hydrocarbon reservoirs in the subsurface; yet their origins and spatial variability in reservoir quality are poorly understood. Fault-associated dolomitization has affected a 4–8-km (2.5–5-mi)-wide strip of inner-platform carbonates of the Oligocene–Miocene Taballar Limestone, exposed onshore northeastern Borneo, where they are juxtaposed against deep-marine shales of the Eocene Maliu Mudstone. On textural grounds (planar dolomites postdating compaction) and from the predominance of monophase aqueous inclusions, replacive dolomite rhombs and later dolomite cements appear to have formed at about 50–60°C and at shallow burial depths up to 0.5–1 km (0.3–0.6 mi). Isotopic and inclusion data indicate that the most likely dolomitizing fluid was dominated by Neogene seawater (for dolomites, ¹⁸O_V-PDB = –5.9 to –10.3; [consistent with the precipitation from southeast Asian Oligocene–Miocene seawater {¹⁸O = –1.5 to –4.2} at temperatures of 40°C up to a maximum of about 50–75°C], ⁸⁷Sr/⁸⁶Sr = 0.708566–0.708697; from T_m = 2.1–3.4 wt.% NaCl). However, replacement and remobilization of the precursor limestone also contributed to the isotopic signature of the dolomites (⁸⁷Sr/⁸⁶Sr = 0.708115–0.708197), whereas an evolved formation water component cannot be ruled out. Dolomitizing fluids used faults and fractures as conduits to move into and alter the limestone, migrating farthest into the most permeable strata. The likely driver for fluid movement was convective flow caused by nearby Neogene igneous activity, perhaps in combination with tectonically induced hydrologic drive related to fault reactivation.

Close to the main fault, late-stage dolomite cements occlude pore throats, reducing porosity (<5%) and permeability (<5 md). The best reservoir quality occurs in medium- to coarsely crystalline idiotopic mosaics of dolomite that have completely replaced the limestone 0.5–2 km (0.3–1.2 mi) away from the main fault where the late-stage cements did not form. These dolomites have 12–20% intercrystalline porosity and moderate to good permeability (tens of millidarcys). Fracturing has had variable impact on reservoir quality. Where late fractures remained open, permeability is enhanced (tens to hundreds of millidarcys), whereas brecciation and fault gouges result in sealing and reduced permeability. The textures, reservoir quality characteristics, and mechanisms of dolomitization show similarities to the few other detailed studies in the literature. This study refines the types of predictive reservoir models used in hydrocarbon exploration for subsurface faulted and dolomitized carbonates and increases the understanding of dolomitizing mechanisms of equatorial carbonates.