--> --> Abstract: Evolution of Ooid Porosity in the Arab Formation, Onshore Abu Dhabi (UAE), by Govert Buijs, Ray W. Mitchell, John L. Whitworth, and Mohamed J. Al Mansouri; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Evolution of Ooid Porosity in the Arab Formation, Onshore Abu Dhabi (UAE)

Govert Buijs1; Ray W. Mitchell2; John L. Whitworth3; Mohamed J. Al Mansouri4

(1) Subsurface Technology, ConocoPhillips, Houston, TX.

(2) Subsurface Technology, ConocoPhillips, Bartlesville, OK.

(3) E&P Alaska, ConocoPhillips, Anchorage, AK.

(4) Abu Dhabi Gas Development Company Ltd, Abu Dhabi, United Arab Emirates.

Samples of the oolitic grainstones from the Arab formation in the subsurface of onshore Abu Dhabi reveal the importance of early-stage dissolution and preservation of early porosity, and present evidence that porosity off-structure is the result of enhanced compaction, pressure solution and associated cementation facilitated by high water saturation. The field studied is an anticlinal structure with steep flanks. Much of the structural closure displays relatively high seismic amplitude corresponding with good porosity in the Arab grainstones. Deep on the flank the seismic amplitude decreases markedly and the grainstone interval has essentially no porosity.

The microporous ooid cortices are comprised of euhedral calcite microcrystals ~ 2 microns in size. Most of the porosity in the oolitic grainstones is microporosity in the finely crystalline cortices of ooids, with lesser amounts in other micritic grains and some remnant interparticle porosity. Petrographic relationships indicate that microporosity is the result of partial dissolution during early diagenesis and recrystallization of primary microporosity in the ooid coatings. Similar porosity in the deeply buried off-structure well is occluded by continued growth of the constituent microcrystals.

Stable isotopes of bulk-rock samples indicate that most calcite precipitated from fluids with a composition similar to altered seawater. A marked shift to more negative δ13C values at the top of the Arab grainstone interval may indicate early diagenetic calcite precipitated in a soil zone during exposure of the Arab. At this top a sharp erosive surface and conglomeratic lag indicate a significant period of exposure.

Porosity reduction of the grainstones in the flank was likely facilitated by higher water saturation that enhanced Ostwald ripening and mechanical and chemical compaction due to significantly greater burial depths. Additional compaction and cementation may have been caused by water-weakening providing the calcium carbonate that migrated into the micropore network and precipitated as calcite. At the crest, low water saturation in hydrocarbon saturated zones inhibited late burial cementation.