Understanding zonal contribution is imperative in a well with comingled production from multiple stimulated zones. This has even more importance in shale plays where a primary appraisal objective is to identify the prolific zones with a vertical well to decide which layers of the formation will be targeted for future development. The use of measured inflow over time of each targeted zone provides an excellent methodology to establish zonal contribution and wellbore evaluation.

The Monterey Formation in the southern San Joaquin valley of California produces from three silica phases. While the industry has been pretty successful in producing from the shallow Diatomite rock via cyclic steam operations, the deeper silica-based resources available in the Antelope Shale have yet to be commercially established. Extracting hydrocarbons locked in these silica phases requires unconventional technologies to access the high porosity / low permeability network. Unconventional completions require greater front-end cost and carry an increased risk to return on investment. To minimize the risk, each pay zone needs to be effectively described using both theoretical and empirical data.

In this paper, we present and discuss a case study where chemical-based tracer technology was used to develop an accurate identification of the distinct pay zones within the different silica phases of the Antelope Shale. Unique oil-phase chemical tracers were applied during the permitted hydraulic fracturing on four vertical wellbores that gave an insight into the zonal production contribution and helped narrow potential zones for future development. In comparison to tool based zonal contribution measurement methods, which provide a snapshot in time measurement, the chemical tracers provided the capability to monitor and measure inflow over a longer period indicating that zonal contribution varied with time. Some formation layers produced immediately and then declined, while others took longer to cleanup and start contributing to total production. These results helped narrow the uncertainty around the potential target zones.

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