Reservoir quality, structural architecture, fluid evolution and their controls on reservoir performance in block 9, F-O gas field, Bredasdorp Basin, offshore South Africa
Fadipe, Oluwaseun Adejuwon
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The use of integrated approach to evaluate the quality of reservoir rocks is increasingly becoming vital in petroleum geoscience. This approach was employed to unravel the reason for the erratic reservoir quality of sandstones of the F-O gas field with the aim of predicting reservoir quality, evaluate the samples for presence, distribution and character of hydrocarbon inclusions so as to gain a better understanding of the fluid history. Information on the chemical conditions of diagenetic processes is commonly preserved in aqueous and oil fluid inclusion occurring in petroleum reservoir cements. Diagenesis plays a vital role in preserving, creating, or destroying porosity and permeability, while the awareness of the type of trap(s) prior to drilling serves as input for appropriate drilling designs. Thus an in-depth understanding of diagenetic histories and trap mechanisms of potential reservoirs are of paramount interest during exploration stage.This research work focused on the F-O tract located in the eastern part of Block 9 on the north-eastern flank of the Bredasdorp Basin, a sub-basin of Outeniqua Basin on the southern continental shelf, offshore South Africa. The Bredasdorp Basin experienced an onset of rifting during the Middle-Late Jurassic as a result of dextral trans-tensional stress produced by the breakup of Gondwanaland that occurred in the east of the Falkland Plateau and the Mozambique Ridge. This phenomenon initiated a normal faulting, north of the Agulhas-Falkland fracture zone followed by a widespread uplift of major bounding arches within the horst blocks in the region that enhanced an erosion of lower Valanginian drift to onset second order unconformity.This study considered 52 selected reservoir core samples from six wells(F-O1, F-O2, F-O3, F-O4, F-R1 and F-S1) in the F-O field of Bredasdorp Basin with attention on the Valanginian age sandstone. An integrated approach incorporating detailed core descriptions, wireline log analysis (using Interactive petrophysics), structural interpretation from 2D seismic lines (using SMT software) cutting across all the six wells, multi-mineral (thin section, SEM,XRD) analyses, geochemical (immobile fluid and XRF) and fluid inclusion(fluid inclusion petrography and bulk volatile) analyses were deployed for the execution of this study. Core description revealed six facies from the six wells grading from pure shale (Facies 1), through progressively coarsening interbedded sand-shale “heterolithic facies (Facies 2 - 4), to cross bedded and minor massive sandstone (Facies 5 - 6). Sedimentary structures and mineral patches varies from well to well with bioturbation, synaeresis crack, echinoid fragments, fossil burrow, foreset mudrapes, glauconite and siderite as the main observed features. All these indicate that the Valanginian reservoir section in the studied wells was deposited in the upper shallow marine settings. A combination of wireline logs were used to delineate the reservoir zone prior to core description. The principal reservoirs are tight, highly faulted Valanginian shallow-marine sandstones beneath the drift-onset unconformity, 1At1 and were deposited as an extensive sandstone “sheet” within a tidal setting. The top and base of the reservoir are defined by the 13At1 and 1At1 seismic events,respectively. This heterogeneous reservoir sandstones present low-fair porosity of between 2 to 18 % and a low-fair permeability value greater than 0.1 to 10 mD. The evolution of the F-O field was found to be controlled by extensional events owing to series of interpreted listric normal faults and rifting or graben generated possibly by the opening of the Atlantic. The field is on a well-defined structural high at the level of the regional drift-onset unconformity, 1At1.Multi-mineral analysis reveals the presence of quartz and kaolinite as the major porosity and permeability constraint respectively along with micaceous phases. The distribution of quartz and feldspar overgrowth and crystals vary from formation to formation and from bed to bed within the same structure. The increase in temperature that led to kaolinite formation could have triggered the low-porosity observed. Three types of kaolinite were recognized in the sandstone, (1) kaolinite growing in between expanded mica flakes; (2)vermiform kaolinite; and (3) euhedral kaolinite crystals forming matrix.Compositional study of the upper shallow marine sandstones in the Valanginian age indicates that the sandstones are geochemically classified as majorly litharenite having few F-O2 samples as subarkose with all F-O1 samples classified as sub-litharenite sandstone.Most of the studied wells are more of wet gas, characterized by strong response of C2 – C5 with F-O1 well showing more of gas condensate with oil shows (C7 – C11) based on the number of carbon atom present. In some cases,sulphur species (characterized by the presence of H2S, S2, CS2 and SO2) of probably thermal origin were identified while some log signatures revealed aromatic enriched sandstones possibly detecting nearby gas charges. The studied wells in the F-O field, based on fluid inclusion bulk volatile analysis are classified as gas discoveries except for F-O1 with gas condensate and oil shows.The integration of multi-mineral results and fluid inclusion studies show a dead oil stain with no visible liquid petroleum inclusion in the samples indicating the presence of quartz, kaolinite and stylolite as a major poro-perm constraint.