Hydrocarbon Findings in Challenging Shilaif in Western UAE Core Analysis Approach
Authors: Gonzalez David , Al Jallad Osama , Vican Kresimir , Al Blooshi Abdula , Al Shehhi Maryam , Shashanka Ashis , Mishra Anoop 
Published: November 2020

Cenomanian carbonate deposits of Shilaif Formation, located west of Abu Dhabi, exhibit a high degree of heterogeneity at multi scales. To characterize this formation and explore its hydrocarbon potential, several geological, petrophysical, and geochemical analyses, including digital rock analysis (DRA), were applied and integrated.

A hundred ft of whole core were logged at the surface using dual-energy X-ray CT (DECT) and Spectral Gamma (SGR) to generate bulk density, photoelectric factor, and gamma logs. Based on integration between DECT, SGR, and wireline logs, high-resolution total organic carbon (TOC), brittleness index, and mineralogy logs were generated at a millimeter scale, and representative core samples were extracted. Inorganic and organic geochemical analyses represented by X-ray diffraction, LECO TOC, HAWK pyrolysis, vitrinite reflectance, and hydrocarbons chromatography (SARA) were performed on selected samples to define rock mineralogy, type, and degree of organic matter maturity and chemical composition of the hydrocarbons. Nuclear magnetic resonance (NMR) at native, dry and full-brine saturation conditions, mercury injection capillary pressure (MICP), and crushed rock analysis (CRA) were conducted on selected samples to determine fluid saturations, porosity, permeability, and pore-throat-size distribution. The pore-scale analysis was also performed using argon-ion-milled SEM images to quantify organic matter contents and total, effective, and organic matter porosities. All core data obtained at multiscale were integrated with the wireline and generated high-resolution logs to validate and select optimal horizontal leg landing zones.

DECT and SGR logs showed the top of Shilaif is mainly made of calcite, while its bottom is calcitic with minor concentrations of clay minerals and pyrite. Integration between DECT and SGR logs showed the mid and bottom of the formation have high radioactivity attributed to the presence of organic matter in intermediate concentrations and the presence of clay minerals. Pyrolysis analysis indicated a kerogen type I to II with an average Tmax equal to 431°C. Measured and calculated vitrinite reflectance (Ro) was, on average, 0.59, confirming that the kerogen of Shilaif in the area of study falls within the immature to the early mature oil window. Measurements such as CRA, MICP, NMR, and 2D SEM analyses showed that Shilaif has low porosity of approximately 3% on average and very low permeability averaging 0.00057 mD. The 2D SEM images and NMR data confirmed this and revealed that it lacks the porosity associated with the organic matter resulting from its low degree of transformation. Lab data and upscaled petrophysical logs showed that Shiliaf in this field has a low degree of thermal maturity and fall within the early oil window.

Integration between core analysis results and wireline data helped understand the Shilaif Formation characteristics and determine its hydrocarbon potential. It also provided additional calibration to the wireline data.