Online Journal for E&P Geoscientists

Al Balushi, Said A.^*1; Macquaker, Joe ²; Hollis, Cathy ³; Marshall, Jim D.⁴
(1) Study Centre, Petroleum Development Oman, Muscat, Oman. (2) Department of Earth Sciences, Memorial University of Newfoundland, St John’s, NF, Canada. (3) School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom. (4) Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.

Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different diagenesis shortly post-deposition. Petrographic, total organic-carbon (TOC), X-ray diffraction and stable-isotopic analyses have been employed to delineate the controls on early diagenesis and hydrocarbon source potential variability.

The Natih-B intrashelf-basinal carbonates are composed of pelagic sediments marked by dm-thick lithofacies alternations, mainly between Lithofacies A: compacted, partially-bioturbated, skeletal, organic-carbon-rich mudstone-wackestone and Lithofacies B: uncompacted, extensively-bioturbated, skeletal, sparry-calcite-rich wackestone-packstone. Individual units are composed variously of authigenic and biogenic calcite (58-98%, average 79%) and organic carbon (0.3-13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite and dolomite. Diagenetic textures of Lithofacies A are dominated by compacted burrows, faecal pellets and solution seams, in addition to zoned/bright-luminescent, nonferroan sparry and isopachous calcite cement in and around uncompacted foraminifers, in an uncemented matrix. In contrast, Lithofacies B does not show any signs of compaction other than microstylolites, and is dominated by zoned/dull-luminescent, nonferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, nonferroan, sparry-calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus cementation, geopetal structures and micritisation. Stable-isotopic compositions (relative to Vienna Pee Dee Belemnite) of both lithofacies were determined from bulk samples (δ13C = −0.9 to 0.9‰, average 0.3‰; δ18O = −5.6 to −3.7‰, average −4.8‰) and sparry calcite (both cement and matrix) subsamples (δ13C = −0.6 to 1.2‰, average 0.6‰; δ18O = −5.7 to −3.7‰, average −4.3‰).

These petrographic and isotopic characteristics suggest that the abundant authigenic calcite was precipitated early, pre-compaction, mainly from normal (open, oxic) seawater at slightly elevated depositional temperatures. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B are interpreted as stratal surfaces that cap shallowing-up cycles. In contrast, Lithofacies A is believed to reflect high organic production, coupled with high sedimentation rate and rapid burial.