Application of spectroscopy and sigma measurements to enhance petrophysical evaluations and geosteering in the mineralogical complex, thin bedded Nahr Umr sandstone reservoir

Abstract Advanced LWD measurements are used to improve the mineralogical understanding and volumetric interpretation in the complex, thin bedded Cretaceous Nahr Umr iron-rich sandstone. The added information in the form of spectroscopy and sigma combined with a high resolution sequence stratigraphic framework enables a probabilistic interpretation model to be developed more closely honoring the vertical and lateral heterogeneity. Utilization of azimuthal sensors is also fundamental to the ability to resolve the individual layers as effectively as possible and therefore reduce the uncertainly further in the final volumetric evaluation. There is however still some remaining uncertainties associated with this due to mixture of averaging and azimuthal sensors and the different depth of investigation of the individual sensors. The combination of the advanced LWD measurements and the improved understanding of the expected log responses in the complex mineralogy provide a vital building block for effective geosteering decisions to be made ensuring optimum well placement for effective reservoir drainage. Introduction Maersk Oil Qatar is developing the complex, iron-rich (glauconite, Fe-oolite, siderite) thin bedded Cretaceous Nahr Umr sandstone reservoir in the Al Shaheen field (ref. 1), Qatar, using extended reach horizontal wells up to 31,000 ft long targeting 2–10 ft thick pay zones. The multifaceted reservoir architecture with lateral heterogeneity and vertical layering presents considerable challenges for evaluation and geosteering due to practical limitations with measurement resolution in the 3D domain and the number of unknowns which can be resolved by typical acquisition programs. The main objective of this work was to utilize the additional information from spectroscopy and sigma to provide an improved interpretation of porosity and saturations. These measurements relate primarily to the matrix component and secondarily to the fluid content, hence are well suited to reduce the evaluation uncertainties in this setting. Conventional Interpretation Methodologies Petrophysical interpretations in long reach horizontal wells are typically based on data sets including conventional gamma ray, resistivity, density, neutron and photo electric effect. The evaluations are typically conventional deterministic techniques. This is adequate in (thick) homogeneous reservoir with well known matrix and fluid properties. The Nahr Umr clastic reservoir in Block 5 in comparison is thinly laminated sand and argillaceous units with a heterogeneous complex mineralogy resulting in significant interpretation challenges. With the basic LWD data sets available the main objective has been focused on aligning the matrix density and the clay volume with core evaluations to reduce the uncertainty in the effective porosity and water saturation determination. This has initially been achieved by applying additional coefficients on the clay volume component to reduce the impact on the effective porosity.