AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Reservoir Facies Distribution and Pore Types of Dolomitized Microbialite Reservoirs of the Ara Group, Sultanate of Oman
(1) Geological and Planetary Sciences, Caltech, Pasadena CA, CA.
(2) Petroleum Development Oman, Muscat, Oman.
Microbially-influenced sediments of the Ara Group are a significant reservoir of the intrasalt “stringer” play in Oman. These dolomitized carbonates are encased in salt at depths of 3-7 km. Microbialites are subdivisible into a number of facies of significance in reconstructing environmental history used to predict the location of reservoir fairways. Microbially-influenced subtidal facies include thrombolites with massive clotted textures likely formed by a consortium of microbes and higher algae, and with very high initial porosities (>50%); pustular laminites with cm-scale variability of lamina morphology, creating bumpy textures that likely represent well-developed benthic mats; deeper crinkly laminites that show mm-scale variability of lamina morphology and could represent either benthic or pelagic organic accretion processes; and intertidal tufted laminates that show mm- to cm-scale tufted textures which likely represent phototaxis in benthic microbial mats. These facies form part of 100m thick carbonates platforms that extended laterally, prior to halokinesis for tens of kilometers and occur in distinct stratigraphic intervals (A1-A4) each separated by salt. Facies distribution follow principles of sequence stratigraphy, with microbialites occurring in every accommodation profile. Late highstand/early transgressive systems tracts favor greater lateral extent of thrombolite build ups, whereas later transgressive to early highstand system tracts favors greater lateral discontinuity of microbial buildups. 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 and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagic derived organics to accumulate.
In evaluating pore systems in microbialites, a broad range of textures are possible. These are controlled by sedimentologic, biologic, diagenetic, and stratigraphic processes. The models developed here may be applicable to younger microbially-influenced 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.