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Depositional facies and platform architecture of microbialite-dominated carbonate reservoirs, Ediacaran–Cambrian Ara Group, Sultanate of Oman

Grotzinger, John and Al-Rawahi, Zuwaina (2014) Depositional facies and platform architecture of microbialite-dominated carbonate reservoirs, Ediacaran–Cambrian Ara Group, Sultanate of Oman. AAPG Bulletin, 98 (8). pp. 1453-1494. ISSN 0149-1423. doi:10.1306/02271412063.

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Intrasalt carbonates of the Ediacaran–Cambrian Ara Group constitute a significant reservoir element of the intrasalt “stringer” play in Oman, in which dolomitic carbonates are encased in salt at depths of 3 to 7 km (1.9 to 4.3 mi). These reservoir carbonates have significant microbial influences. Although Ara Group reservoirs are mostly latest Precambrian, the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded, in lacustrine settings where calcified invertebrates are not a significant source of carbonate, or after periods of mass extinction before faunal recovery. A broad range of carbonate facies provides the context in which to understand the origin of the microbialite-dominated reservoirs developed across both ramp and rimmed shelf profiles. Major facies associations include carbonate-evaporite transition zone, deep ramp and slope, subtidal microbialites, clastic-textured carbonates, and restricted peritidal carbonates. Microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties, and that help in predicting the location of reservoir fairways. Microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities (>50%), shallow subtidal pustular laminites with cm-scale variability of lamina morphology, deeper subtidal crinkly laminites that show mm-scale variability of lamina morphology, and intertidal tufted laminates that show mm- to cm-scale tufted textures. Other reservoir facies are more conventional grainy carbonates including ripple cross-stratified grainstone–packstone, hummocky cross-stratified grainstone–packstone, flat pebble conglomerate, ooid and intraclast grainstone–packstone, and Cloudina grainstone–packstone. These facies are almost invariably dolomitized and all have moderate to excellent reservoir quality. These facies comprise carbonate platforms, broken up during salt tectonics, that range up to 160 m (525 ft) in thickness and extended laterally, prior to halokinesis, for tens to over 50 km (31 mi). The distribution of reservoir facies follows sequence stratigraphic predictions, with microbialites occurring in every accommodation profile. Late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build-ups, whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies. Pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods. Crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate.

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Grotzinger, John0000-0001-9324-1257
Additional Information:© 2014 The American Association of Petroleum Geologists. Manuscript received April 30, 2012; provisional acceptance February 07, 2012; revised manuscript received December 30, 2013; final acceptance February 27, 2014. We thank the Ministry of Oil and Gas and the Exploration Management of Petroleum Development Oman (PDO) L.L.C. for their permission to publish this paper. We gratefully acknowledge all of the PDO explorers, past and present, which have helped to understand the stringer play. In particular, we thank Joachim Amthor, Jim Beer, Jean-Michel Larroque, Bruce Levell, Gideon Lopez Cardozo, Jeroen Peters, Mark Newall, Joachim Rheinhardt, Hisham Al-Siyabi, Steven van Rossen, and Harry Soek for their significant efforts in helping to develop the models presented here and in supporting the extensive coring program that underpins these models. Jim Beer and Jen Griffes helped with construction of Figure 3. We enjoyed productive and open exchange of ideas with Nigel Cross, Imelda Johnson, Cathy Hollis, Gordon Coy, and Paul Milroy in the core shed. The manuscript was improved by very helpful review provided by Brian Coffey, Mark Longman, Gideon Lopez Cardozo, Jim Markello, and Taury Smith.
Issue or Number:8
Record Number:CaltechAUTHORS:20140911-082136898
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Official Citation:John Grotzinger and Zuwaina Al-Rawahi Depositional facies and platform architecture of microbialite-dominated carbonate reservoirs, Ediacaran–Cambrian Ara Group, Sultanate of Oman AAPG Bulletin, August 2014, v. 98, p. 1453-1494, doi:10.1306/02271412063
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49571
Deposited By: Tony Diaz
Deposited On:11 Sep 2014 22:11
Last Modified:10 Nov 2021 18:45

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