ALBERT

Description: The Lower Cretaceous Goldeneye gas field lies in the Captain turbidite fairway of the Moray Firth and has recently ceased production. Its situation and dimensions have made it an excellent candidate for CO 2 sequestration. The field was extensively modelled for the original development planning when uncertainty was assessed from the perspective of volumetrics and field behaviour. The subsequent need to assess its suitability as a CO 2 store has given the opportunity for a look back at an uncertainty analysis with the benefit of full-field performance, and to perform a new analysis aimed at different issues concerning behaviour during CO 2 injection. Both sets of analyses required coupled static–dynamic modelling runs in which the key static parameter ranges of the field were varied, including depth conversion, internal geometries and aquifer properties. For the field development work the parameter ranges were explored to assess in-place volumes and field behaviour under natural aquifer and depletion drive; for the CO 2 uncertainty work, however, parameter ranges were explored to demonstrate storage capacity and CO 2 containment. The look-back showed that the field volumes indicated by production data landed in the upper part of the original uncertainty range and that there was definitive spare capacity in the field relative to the planned injection volume.

Description: Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different early diagenesis shortly after deposition. Transmitted-light, cathodoluminescence and backscattered scanning-electron microscopy, as well as stable-isotopic, X-ray diffraction and total organic carbon (TOC) analyses were employed to delineate the major controls on the cyclic pattern of early diagenesis and hydrocarbon source potential.The Natih-B intrashelf basinal carbonates are composed of pelagic sediments that exhibit high-frequency cyclicity marked by decimetre-thick lithofacies alternations, mainly between: Lithofacies A compacted, partially bioturbated, skeletal, organic carbon-rich mudstone to wackestone; and Lithofacies B uncompacted, extensively bioturbated, skeletal, sparry-calcite rich wackestone to packstone. Individual units are composed variously of authigenic and biogenic calcite (58.1–97.6%, average 78.5%) and organic carbon (0.3–13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite, dolomite and phosphatic material (fish debris). Lithofacies A contains relatively more organic carbon, clay, pyrite and dolomite than Lithofacies B and constitutes an excellent source rock. Diagenetic textures of Lithofacies A are dominated by compactional deformation of burrow fabrics, faecal pellets and solution seams, in addition to zoned/bright luminescent, non-ferroan sparry and isopachous calcite cement in and around uncompacted foraminifer tests, 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, non-ferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, non-ferroan, sparry calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus-style cementation, together with geopetal structures and mictritic peloids. Stable-isotopic compositions of both lithofacies were determined from whole-rock 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‰); all results being relative to Vienna Pee Dee Belemnite.These petrographic and isotopic characteristics suggest that the Natih-B abundant calcite cements and replacements were precipitated early, prior to compaction, mainly from ‘normal’ (open, oxic) seawater at slightly elevated depositional temperatures. Some of the slightly negative δ13C values, however, indicate an addition of isotopically light carbon, probably derived from organic-matter oxidation by organisms living in marine pore waters. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B (wackestones to packstones) are interpreted as ‘discontinuity surfaces’ that cap shallowing-upward, fifth-order cycles, formed as a function of sediment starvation and increased bottom-current activity during relative sea-level stillstand/turnaround. In contrast, Lithofacies A (mudstones to wackestones) is believed to reflect high organic production coupled with high sedimentation rate and rapid burial. These conditions limited total infaunal colonization and extensive calcite precipitation, and preserved organic matter together with some escape burrows and in-place fauna, suggesting episodic sediment influx when more accommodation was available and seafloor diagenesis was minimized during relative sea-level rises. The relatively higher amounts of pyrite and dolomite in Lithofacies A likely indicate organic-matter degradation by bacterial sulphate reduction in anoxic pore waters during shallow burial.

Description: The Lower Cretaceous Goldeneye gas field lies in the Captain turbidite fairway of the Moray Firth and has recently ceased production. Its situation and dimensions have made it an excellent candidate for CO 2 sequestration. The field was extensively modelled for the original development planning when uncertainty was assessed from the perspective of volumetrics and field behaviour. The subsequent need to assess its suitability as a CO 2 store has given the opportunity for a look back at an uncertainty analysis with the benefit of full-field performance, and to perform a new analysis aimed at different issues concerning behaviour during CO 2 injection. Both sets of analyses required coupled static–dynamic modelling runs in which the key static parameter ranges of the field were varied, including depth conversion, internal geometries and aquifer properties. For the field development work the parameter ranges were explored to assess in-place volumes and field behaviour under natural aquifer and depletion drive; for the CO 2 uncertainty work, however, parameter ranges were explored to demonstrate storage capacity and CO 2 containment. The look-back showed that the field volumes indicated by production data landed in the upper part of the original uncertainty range and that there was definitive spare capacity in the field relative to the planned injection volume.