SCHEMA:
CREATE TABLE table_name_15 (field VARCHAR, geological_trend VARCHAR, discovery VARCHAR, operator_s_ VARCHAR)
USER:
Which Field has a Discovery of na, and an Operator(s) of woc, and a Geological Trend of western

QUERY:
SELECT field FROM table_name_15 WHERE discovery = "na" AND operator_s_ = "woc" AND geological_trend = "western"