As oil price remains high, the focus in source rock exploration has moved from gas-to liquid-rich plays and warrants revisiting "bypassed" hydrocarbon charged source rocks, which were deemed uneconomic when first drilled. In these oil-rich source rock fields, in North America there are thousands of wells with different vintages of nuclear and electrical logs, yet these wells generally lack any advanced logs (nuclear magnetic resonance, elemental spectroscopy, etc.) beyond the traditional triple combo (electrical resistivity, bulk density, and neutron logs). Hence, the need for practical petrophysical models based on basic suites of logs. Such models can best predict petrophysical properties (grain density, porosity, and water saturation), when reliable laboratory measurements on rock samples are available for calibration purposes.
This paper describes a workflow that utilizes a considerable amount of laboratory measurements made on crushed-rock to upscale a petrophysical model based on a triple combo logging suite only. The model was developed by dividing the field (laterally) in oil-prone and gas-prone fairways, and (vertically) in petrophysical units. The hydrocarbon generation potential is based on geochemical measurements such as thermal maturity and total organic carbon content (TOC) from core and cuttings in the area. The petrophysical units reflect major geological intervals with similar porosity, and clay content. The workflow was sequentially built by correlating triple combo logs to core-measurements, using TOC and maturity for organic matter, x-ray diffraction for mineralogy and grain density, porosity, and water saturation from Dean-Stark extraction, for volumetrics.
The model was developed for the Mancos Shale (New Mexico), a Cretaceous age source rock, which includes the Niobrara Formation. The Mancos Shale has been penetrated in various fields while developing conventional sandstone reservoirs. Our petrophysical model was validated with measurements on a core that was recently acquired in the anticipated high-hydrocarbon-yield window. Petrophysical properties predicted from logs agree well with core measurements in blind tests, demonstrating the robustness of the model despite it being based on a basic suite of logs and a simple deterministic approach.
The Mancos shale petrophysical model has since been deployed to the asset team as an automated workflow. Over a thousand wells have been processed to generate fairway maps, locate sweet spots, and for landing lateral wells, all at maximizing resource production.