Quantitative assessment of the sweet spot in marine shale oil and gas based on geology, engineering, and economics: A case study from the Eagle Ford Shale, USA

Shale oil and gas have become important areas of petroleum exploration and development. Successful shale oil and gas exploration depends on the accurate evaluation of sweet spots, which are the commercial targets. This paper proposes a comprehensive new method for sweet spot evaluation based on average estimated ultimate recovery in barrels of oil equivalent (EUR_BOE) in conjunction with key geological factors, engineering and technical conditions, and economic and technical indicators. Research results from 72 systematically cored wells, 991 sets of core analyses, and production data from 1317 horizontal wells in the lower Eagle Ford Shale (EFS) of the U.S. Gulf Basin show that marine shale oil and gas sweet spots depend primarily on four key factors: oil and gas resources, oil and gas storage capacity, fluidity, and fracability. These conclusions are based on quantitative evaluation models relating to EUR_BOE and key geological parameters. Model outcomes are controlled by inputs for total organic carbon (TOC), vitrinite reflectance (%R-o), original formation and hydrostatic pressure difference (Delta Pf-h) or formation pressure gradient (P-index), effective shale thickness (He-shale), clay volume content (V-clay), fracture porosity (Phi(f))and other geological parameters pertaining to EUR_BOE. Shale oil and gas sweet spots in non-fractured zones must satisfy certain conditions including TOC >= 2.3 wt%, R-o >= 0.9%, He-shale >= 15 m, V-clay <= 25 vol%, and Delta Pf-h between similar to 3 and 35 MPa or P-index between similar to 1.15 and 1.85 MPa/100 m. Defining shale oil and gas sweet spots in fractured zones is more complex, because developing fractures and the application of advanced engineering technology will increase EUR_BOE; sweet spots are impacted by sealing conditions, fracture and fault development, and technical engineering conditions. The proposed method for evaluating shale oil and gas sweet spots is validated through the determination of sweet spots in the lower EFS, and provides a basis for the evaluation of shale oil and gas sweet spots in similar marine deposits.

Automated summary

Shale oil and gas sweet spots assessment relies on a new comprehensive method incorporating average estimated ultimate recovery and key geological factors, engineering conditions, and economic indicators. Research data from the lower Eagle Ford Shale in the US Gulf Basin suggest that marine shale oil and gas sweet spots are primarily influenced by oil and gas resources, storage capacity, fluidity, and fracability. The proposed evaluation approach has been validated and can serve as a basis for similar marine shale oil and gas deposits.