Training

carbonate-sedimentology;unconventional-reservoirs;reservoir-quality

Acknowledgements
The authors gratefully acknowledge the management of the Al Hosn Gas for permission to publish this manuscript.

1: Abstract

• Detailed core description of multiple wells within a giant gas field in the United Arab Emirates (UAE) has enhanced the understanding of the depositional make-up and possible controls on depositional style for the Arab Formation. In this field, the Arab Formation differs from other areas such as Qatar in that anhydrite and dolomite are scarce in the lower units of the Arab Formation and recognition of any
sequence-stratigraphic boundaries is limited. In summary, the limestone-dominated reservoir units combine to reflect a large-scale shallowing-upwards trend from basinal/outer, mid-ramp to inner-ramp depositional settings, based on core-derived facies association analysis. The facies associations are defined according to sedimentological and faunal characteristics and form genetically related, larger-scale units reflecting a low-energy, mid- to outer-ramp depositional setting, which grades sharply into an inner-ramp shoal and shoal complex depositional setting.

• Within the low-energy units of the lowest Arab Formation reservoir, mudstones are interbedded with bioclast-rich accumulations of variable thickness and an uncertain origin. The vertical spacing of these units may be a function of random/non-random cyclicity (?Milankovitch) and reflect variation in depositional slope angle and hence slump/debrite deposits or deeper-water faunal communities. The transition from the distal depositional setting to the proximal depositional setting occurs over a relatively narrow zone into oolitic grainstones as the system progrades. The grainstones form a thick sequence of bedded units reflecting both shoal and intershoal areas within an inner-ramp depositional setting, but cyclicity is not apparent in this interval.

• The small-scale cyclicity present in the lower units of the Arab Formation may potentially be equal to those that are defined by dolomite-anhydrite cycles in Qatar, but in this area of Abu Dhabi, the overall distal setting has negated the development of these mixed evaporite-carbonate cycles. The implications of this includes a lack of intraformational seals and a uniformity of lithofacies (i.e. limited variation), which is also expressed in the more proximal inner ramp deposits and provides for good reservoir potential.

2: Overview

• Within the Arab D unit, shell-rich beds composed of disarticulated bivalves, comminuted bivalve debris, benthic foraminifera and gastropods form sub-deci-feet to rarely deci-feet scale beds of wackestone to packstone to floatstone that are interbedded with dominant mudstones.
• These beds are observed to have a variable spacing between each other vertically, which may be attributed to non-random cyclicity (?Milankovitch), random cyclicity (autocyclic/allocyclic processes) or are non-cyclic.
   - Autocyclic (autogenic) model invokes localised controls on depositional stacking patterns to control cyclicity (Ginsburg, 1971, Osleger, 1991).
   - Allocyclic (allogenic) mechanisms require extrinsic factors, such as episodic regional subsidence, to generate cyclicity (Fischer, 1964; Grotzinger, 1986).
   - Milankovitch cycles reflect climatic variations associated with orbital forcing (Crevello, 1991; Fischer 1988).
• Points that will be considered in this poster include:
   - Consideration of evidence for cyclic origins of the shell-rich beds versus non-cyclic origins
   - An evaluation of the dataset and its limitations for making such interpretations.

3: Scales of Cyclicity

• In order to determine whether the shell-rich beds were deposited under the influence of cyclic processes, an understanding of the various types/scales of cycles is needed. These are briefly summarised here:
• Supersequences (1st -2nd Order cycles)
   - Icehouse-greenhouse influences
• Composite sequences (3rd Order cycles)
   - Basis of Exxon global cycle chart (Vail et al. 1977)
   - 1-3my (Haq et al. 1987)
   - 1-10my (Goldhammer et al. 1991)
   - Climatic/glacio-eustatic variation
• High-frequency Sequences (4th-5th Order cycles)
   - 20-400ky sea level cycles reflecting:
   - Milankovitch-band glacio-eustacy (Koershner and Read, 1989)
   - Or productivity/oxygenation (Fischer and Bottjer, 1991)
• Milankovitch cyclicity describes collective effects of changes in Earth orbital movements summarised in Hays et al. (1976).
• A large number of papers and books now infer orbital forcing (Milankovitch cycles) on sedimentary sequences that show cyclicity including De Boer and Smith (1994), but not everyone agrees ..... e.g. Drummond and Wilkinson (1993).
Thin shell-beds within mudstones of the Arab D interval cored in one well from a giant oilfield in the U.A.E., give the appearance of cyclical-driven sedimentation, and it is this, which has led to the review of the mechanisms for determining cyclicity addressed in this poster.

 

Can cyclicity really be determined in thin bed-dominated sequences?