Sedimentology and diagenesis of "dense zones" above and below the Thamama B reservoir, Kharaib Formation (lower cretaceous) in Sahil field, Abu Dhabi

  • Qiong Wu

Student thesis: Master's Thesis

Abstract

The Thamama-B reservoir zone (having porosity of mostly 15-30%) is bounded above and below by intervals of very low porosity (mostly 0.2-1.0%), including the "dense A zone" (DA) above and the "dense B zone" (DB) below. These regionally extensive limestone layers have been studied in cores from the Sahil oilfield in order to describe their depositional and diagenetic characteristics and interpret the reasons for their low porosity. Both DA and DB have higher gamma ray (GR) activity (generally 30-60 API units) than the B reservoir zone (mostly 5-10 APIU), which spectral-GR data indicate to reflect higher siliciclastic-fines content (K and Th) associated with a subordinate contribution from higher U content. The main GR peaks in each dense zone (5 in DA and 11 in DB) are correlative throughout the field area (27 km N-S and 8 km E-W), reflecting cyclic fluctuations in clay content. In the cores studied, cyclicity is expressed by alternating darker and lighter color, with darker intervals having higher contents of disseminated pyrite, "blackened" (pyritized) grains, wispy argillaceous laminations, and horizontal burrows. In terms of depositional textures, the two dense zones are very different. The DA, also known as the Hawar Member of the Kharaib Formation, is 6-7 m thick and consists of orbitolinid-rich skeletal-peloidal packstone with minor rudist-intraclast floatstone. Cycle tops are marked by firmground surfaces capping Glossifungites-burrowed lighter-colored intervals with gradational downward transition into darker packstone. The DB is 14-15 m thick and consists of mudstone with minor orbitolinid wackestone. Bedding is poorly defined by thinner intervals of darker color with gradational contacts into surrounding lighter-colored mudstone. Evidence for subaerial exposure is observed in neither DA nor DB. The uniformly low porosity, absence of oil staining, and absence of crest/flank thickness variations indicate that the nearly total destruction of porosity in the dense zones must have preceded emplacement of oil in the B zone, where oil is interpreted as having preserved higher porosity in the crest of the field by inhibiting chemical compaction. Corresponding with the major textural difference between DA and DB, porosity loss has been accomplished to greater degree by calcite cementation of former macropores in DA and to greater degree by mechanical compaction of mud matrix in DB. The main process responsible for the very low porosity of both units, however, is interpreted to be calcite cementation of micropores between neomorphosed (microrhombic) micrite particles, a process that is much less advanced throughout most of the B reservoir zone. The more intense calcite cementation of the dense zones is suggested to result from chemical compaction along the abundant argillaceous laminations present throughout both DA and DB.
Date of Award2016
Original languageAmerican English
SupervisorStephen Ehrenberg (Supervisor)

Keywords

  • Applied sciences
  • Cementation
  • Clay
  • Dense zones
  • Porosity
  • Petroleum engineering
  • 0765:Petroleum engineering

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