Dolomitization of Jurassic carbonates in the Western High Atlas of Morocco: processes and implications for reservoir properties

Abstract

Excellent exposures of Jurassic carbonate platforms crop out in the Essaouira-Agadir Basin, Western High Atlas, Morocco. These platforms have been extensively dolomitized, with spatial and stratigraphic variation in the style and extent of dolomitization. Jurassic dolomitized carbonates are important reservoirs along the Central Atlantic Margin, but a key challenge is predicting porosity generation and preservation for future exploration in the subsurface. Dolomitization of the Jurassic carbonate platforms is stratabound, following facies organization of the host limestones, and locally non-stratabound. In all studied carbonate platforms, three main phases of dolomitization are identified: i) replacive, fabric preserving dolomite with euhedral and subhedral crystal morphology, selectively replacing carbonate mud supported facies; ii) replacive fabric destructive dolomite with anhedral and saddle crystal textures, dominating grain supported facies, recognized in close proximity to faults; and iii) dolomite cement with saddle crystal morphology occluding porosity and fractures, found in damage zones of faults and recognized as the final phase of dolomite in the Essaouira-Agadir Basin. The oxygen and carbon stable isotopic signatures of the fabric preserving dolomites are comparable to those of Jurassic marine carbonates suggesting its formation by seawater. The depleted oxygen stable isotopic values of the replacive fabric destructive dolomites and the dolomite cements suggests precipitation from hot fluids. This is supported by clumped isotope analysis, that calculates elevated fluid temperatures compared to the host limestone implying precipitation from a hydrothermal fluid that was transported onto the platforms along faults and fractures. Radiogenic 87Sr/86Sr ratios of the dolomites and the co-existence of diagenetic quartz, muscovite and albite suggest fluid circulation via a basal clastic aquifer. The fluids responsible for the formation of the hydrothermal dolomites could have originated from: i) seawater; ii) formational brines; and/or iii) meteoric waters which underwent fluid-rock interaction with: i) Triassic sandstones; ii) Triassic salt; and iii) the Central Atlantic Magmatic Province basalts. The dolomites are occasionally dedolomitized and post-dated by calcite cements filling fractures and pore space, exhibiting geochemical signatures consistent with precipitation from meteoric fluids. Meteoric water could have migrated downward through the Jurassic carbonate succession during basin uplift, exposure and karstification. Intergrowth of the late calcite with bitumen suggests that hydrocarbon emplacement occurred during the latest stages of basin evolution.

Date of Award	15 Nov 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor), Catherine Hollis (Supervisor) & Stefan Schroeder (Supervisor)