ABSTRACT

Organic and mineral geochemistry can be of invaluable importance for petroleum system assessment and reservoir characterization, especially when integrated with other disciplines, such as formation evaluation and sequence stratigraphy. Identifying key geochemical changes within lithology refines what traditional approaches to reservoir characterization are not able to provide.

The following study shows how the integration of several formation evaluation technologies from Advanced Surface Logging (ASL) to Logging While Drilling (LWD) and Wireline (WL) leads to the characterization of a complex reservoir system compartmentalized by an active structural setting. The formation consists of interstratified dolomite, limestone and sandstone with strong diagenesis overprint overlain by shale deposits that play the double role of source rock and seal. The pay zones, within the carbonates, contains a complex fluid column governed by multiple paleo-contacts with a thick imbibition zone.

ASL organic geochemistry (TOC and Pyrolysis) has enabled the source rocks zonation in terms of quality and maturity; furthermore, when integrated with NMR logs, it identified the best candidate layers for the generation of hydrocarbons.

ASL inorganic (XRF/XRD) and organic (Thermal Desorption Gas Chromatography, TD-GC) geochemistry along with mud gas analyses, have enabled a fine zonation of the reservoir interval in four sections. The combination with LWD logs revealed the mechanisms governing such geochemical differences: the main hydrocarbon accumulation shows perfect vertical continuity with segregation while the oil-water transition reveals residual saturation and biodegradation trend until the water zone.

Furthermore, the joint interpretation of rock textures with elemental patterns is a good driver to identify different diagenetic degrees. In carbonates, the vicarious elements of Ca (Calcium), Na (Sodium) and Strontium (Sr), such as Magnesium (Mg), Manganese (Mn), Zinc (Zn) and Iron (Fe) help to identify the different types of cement and matrix (sparite and micrite). The correlation of these elemental diagenetic proxies, with XRD, TD-GC, resistivity and porosity logs refined the petrophysical assessment and helped the identification of the most productive layers for an optimal well completion.

This paper demonstrates how ASL geochemistry contributes in various manners to the fine description of reservoir geology. When integrated with LWD and WL, it allows the characterization of organic facies, the evaluation of rock petrophysics with its diagenetic overprint, the zonation of an entire fluid column along with the governing processes of saturation and, ultimately, the identification of productive layers.

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