ABSTRACT

Production results from the Natih B oil-bearing clean (<10% clay) Cretaceous carbonate rich source rock are correlated to an integrated interpretation of measurements and postproduction geochemical analysis on oil sample. The full picture from pilot well location selection, based on an unconventional resource assessment, to sidetrack horizontal well design and optimization to the findings from the post-multistage fracturing production test is presented and discussed. A full logging suite including while-drilling surface measurements (diffuse reflectance infrared Fourier transform spectroscopy - DRIFTS, and the advanced mud gas logs - AMGL) was acquired in the pilot vertical well. Reservoir Quality (RQ) and Completion Quality (CQ) assessments were made by integrating all available measurements, and the while-drilling cuttings data served as calibration for log-derived TOC and minerology, comparison with in-situ fluid type and maturity (recent while-drilling measurements on source rocks). The RQ assessment indicated the reservoir to be a low-porosity (3 to 7 p.u.) laminated carbonate with a high resistivity (>10,000 ohm.m), kerogen-hosted-porosity layer that alternates with a kerogen-lean layer with conventional pores. While-drilling two independent proxies were applied to assess the thermal maturity. Kerogen maturity from DRIFTS was 0.6 to 0.7% vitrinite reflectance equivalence (VRE), and maturity computation from mud gas ratios using AMGL indicated ~0.5–0.8% VRE, both implying early-to-peak oil maturity level. The pilot well could not be tested due to operational issues and the best landing-point selection for the lateral was based on integrated RQ-CQ analysis in the pilot well. The lateral was geo-steered using logging-while-drilling measurements (distance to boundary tool and high resolution image logs), DRIFTS TOC, and AMGL logs. After drilling the lateral, a wireline triple combo with dipole sonic was acquired. A RQ-CQ evaluation was performed for completions and hydraulic fracture design optimization. The hydraulic fracturing operation was successful with three stages propped as per engineered design. The most significant new finding is the good correlation between the recent advanced while-drilling (available in real-time) proxies for kerogen and fluid maturity with the fluid maturity level from the geochemical fingerprinting (SARA and biomarker analysis) of the produced oil sample and the good agreement of the same with the production performance of the Natih B in the lateral. The workflows and lessons learnt during the full cycle of the Natih B exploration in Oman can be applied in parallel scenarios (exploration of source rocks in general or exploration of the Natih B source rock or equivalents elsewhere in the region).

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