Abstract
A large unexplored tectonic basin with the potential for significant hydrocarbon accumulations was identified in north-central Oregon using a variety of geophysical techniques. The basin, informally named after the local town of Heppner, is covered by several thousand feet of Miocene Columbia River Basalt Group (CRBG) but is readily identified by a gravity low against the Blue Mountains Uplift. The Paleocene/ Eocene Herren Formation (Pigg, 1961), which outcrops on the Blue Mountains Uplift south of the Heppner Basin, offered good source and reservoir potential. Based on lateral extent, thickness and paleocurrent structures in the Herren Formation, the unit was expected to be present in the basin.Gravity modeling produced nonunique interpretations, thus magnetotelluric (MT) information was used to constrain the CRBG thickness. Static shifts in the MT data were removed using transient electromagnetic (TEM) data before MT data inversion. After extensive experimentation, adequate seismic data were obtained for structural mapping, but the seismic data were interpretable with confidence only after MT determinations of the CRBG thickness.As a result of the favorable geologic and geophysical information, the ARCO Hanna well was drilled to 9100 ft (2800 m) near Heppner, Oregon in section 23, T2S, R27E in 1988. The thickness of the CRBG and Oligocene John Day Formation were accurately predicted by the geophysical interpretations. An unanticipated thickness of Eocene Clarno Formation was encountered and drilling ceased in this unit. No Herren Formation was penetrated during drilling.Geophysical well logs indicate the Clarno Formation has densities and resistivities sufficient to account for the gravity and electrical anomalies defining the prospect. Poor seismic quality was explained by the heterogeneous nature of the pre-CRBG volcanic section encountered in the well.
