Carbonate facies, diagenesis and sequence stratigraphy of an eocene nummulitic seservoir interval (jdeir formation), offshore NW Libya

Abstract

This study investigates the sedimentology, petrology and depositional environments of a major Early Eocene nummulitic reservoir unit: the Jdeir Formation, from offshore NW Libya in the Mediterranean Sea. This formation is a prolific hydrocarbon-producing unit that was deposited as part of the Mesozoic-Cenozoic stratigraphic fill of the Sabratah Basin. The Sabratah Basin is an elongate ESE/WNW trending fault-bounded basin that originated as a left lateral pull-apart basin during the Late Triassic-Early Jurassic. Presented in this thesis is a review of the tectonostratigraphic setting of the Jdeir Formation, an evaluation of the facies and an interpretation of the depositional environment of the platform. During this Master project, diagenesis was also evaluated with the aim of better understanding reservoir development of the Jdeir Formation. On the basis of detailed core description and petrographic study eight facies have been distinguished. These are: (1) Planktonic Foraminifera Fades, (2) Discocyclina-Nummulitc Facies, (3) Nummulite Facies, (4) Alveolina Facies, (5) Peloidal-Bioclastic Facies, (6) Mollusc Facies, (7) Echinoderm Facies and (8) Sandy- Bioclastic Facies. These are interpreted as having been deposited in open-marine, fore-bank, bank, lagoonal (back-bank) and restricted lagoonal environments. Nummulitic rudstones, dominated by B-forms with minor A-forms, comprise the upper part of bank, and float/rudstones form the lower part of the bank. Abrasion and fragmentation of bioclasts resulted from the transport of sediment from palaeohighs and their reaccumulation into intra-, or back-bank environments. Discocyclina and planktonic foraminifera-rich facies formed in open-marine environments, with the former accumulating towards the base of the photic zone. The back-bank or lagoonal deposits are highly variable, were sometimes affected by siliciclastic influx, and may be dominated by molluscs, echinoderm debris or imperforate foraminifera. Facies and thickness variations between the three wells were controlled by the palaeotopography and palaeoenvironments of the Sabratah Basin. It is appears that the depositional settings of the shallow platform deposits were more variable than has-been previously documented. The carbonates of the Jdeir Formation have been altered by a variety of diagenetic processes as inferred from petrography, cathodoluminescence, SEM and stable isotope analysis. The diagenetic sequence involved initial marine diagenesis including micritization, which has obliterated much of the original fabric of many skeletal grains, and the formation of micrite envelopes also occurred at this time. Leaching of bioclasts and the matrix has created vuggy and mouldic porosity, which is now commonly partially or completely filled by drusy and minor blocky calcite cements and rare kaolinite cement. Recrystallization of micrite to microspar and pseudospar is attributed to meteoric diagenesis, which has occluded some of the original porosity. The final diagenetic features include development of dissolution seams, stylolites and fractures, coarse calcite spar, dolomitization, silicification and pyrite. All these later diagenetic features are inferred to have occurred in a burial environment. As a result of these diagenetic processes, Nummulitic limestones of the Jdeir Formation have lost most of their primary porosity. Secondary porosity formed through the dissolution of aragonitle skeletons and micritic matrix during meteoric diagenesis, and possible also late burial diagenesis. The overall sequence stratigraphic interpretation of the Jdeir Formation would most likely be transgressive deposits (transgressive system tract) based on underlying early dolomite which is associated with tidal flat deposits and overlying deep marine deposits. Intra-formation smaller-scale trends most likely formed under transgressive, stillstand and regressive conditions. The incomplete well data through the Jdeir Formation and different numbers of transgressive and regressive cycles in each well makes formation-wide correlation problematic.