Application of an Integrated Geochemical Approach to Assess Hydrocarbon Presence and its Composition Below the Hitherto Free Water Level. A Case Study from Flank Appraisal Wells of a Producing Field in the Onshore UAE

Abstract

Recent appraisal campaign on a producing field in onshore Abu Dhabi has found evidence of oil presence 60-170 ft below hitherto known Free Water Level (FWL). The Oil water contact (OWC) was identified by electrical and mud logs, geochemistry analyses on core plugs and well cuttings, core stain, core UV, thin sections and Dean Stark saturation. The calculated volume of the hydrocarbon (HC) below the FWL is significant. However, below FWL, one well tested traces of oil. A comprehensive study including geochemistry was commissioned to understand hydrocarbon composition and its mobility for reserves evaluation and future development plan Total of 1600 ft of core was acquired from three appraisal wells and cuttings from two wells. Pyrolysis was conducted to evaluate hydrocarbon presence and its composition. Subsequently, a subset of sample from pyrolysis data was selected for solvent extraction and detailed analysis for oil characterization. Saturates, Aromatics, Resins and Asphaltene (SARA) fractions were measured on few samples. Finally, Gas Chromatography and Compound Specific Isotope Analysis (CSIA) were performed to compare the extract with the produced oil from the crestal area to assess the variation of HC composition Parameters derived from mud log have provided a good understanding of hydrocarbon distribution. Thin section and Scan Electron Microscope (SEM) observation showed presence of black, solid carbon particles (Solid bitumen) in the pore spaces despite solvent cleaning. Oil Saturation Index (OSI) and solvent extraction show lower and higher values below and above the OWC, respectively. S1 and S2 yields provided a detailed understanding of hydrocarbon distribution. Saturate composition was highest at reservoir top where the only MDT sample (22% of oil) was successfully collected, whereas it was low where only water on MDT and traces of oil (>99% BSW) were collected during testing. The pyrolysis data, typically applied to assess source rock quality and maturity, has been utilized successfully in a novel approach to assess the oil distribution and its composition in a conventional reservoir. It is now understood that, in addition to reservoir rock facies, oil facies should also be established to achieve a comprehensive reservoir assessment in this challenging reservoir that has witnessed multiple phases of hydrocarbon charge as indicated by solid bitumen and liquid hydrocarbon. These dispersed bitumen particles are likely to provide large adsorption surface areas for any oil that might otherwise would have been available for flow upon testing.